a new perspective on metamorphism and metamorphic...

Gondwana Research 18 (2010) 106–137

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A new perspective on metamorphism and metamorphic belts☆

Shigenori Maruyama a,⁎, H. Masago b, I. Katayama c, Y. Iwase d, M. Toriumi e, S. Omori a, K., Aoki a

a Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Japanb Center for Deep Earth Exploration, Japan Agency for Marine-Earth Science and Technology, Japanc Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, Japand Department of Earth and Ocean Sciences, National Defense Academy of Japan, Japane Department of Earth and Planetary Science, Graduate School of Frontier Sciences, The University of Tokyo, Japan

☆ Adapted and modified from the article by MaruyamIwase, Y., Toriumi, M., 2004. A revolutionary newmetamorphism, its exhumation and consequent mGeography, 113, 727–768 (in Japanese).⁎ Corresponding author.

E-mail addresses: [email protected] (S. [email protected] (S. Omori).

1342-937X/$ – see front matter © 2010 International Adoi:10.1016/j.gr.2010.03.007

a b s t r a c t
a r t i c l e i n f o
Article history:Received 8 February 2010Received in revised form 22 March 2010Accepted 23 March 2010Available online 30 March 2010

Keywords:MetamorphismFacies seriesIsogradBarrovian hydrationExhumationTectonics

The discovery of ultrahigh-pressure rocks from collision-type orogenic belts has revolutionized the classicinterpretation of (1) progressive and retrogressive metamorphism recorded on surface exposures of regionalmetamorphic belts, (2) geochronology of the various stages of metamorphism, (3) origin of metamorphictextures, (4) P–T–t path, (5) metamorphic facies series, (6) exhumation model, and (7) role of fluids duringregional metamorphism. Based mainly on our recent studies of the Kokchetav, Dabie Shan, Indonesia,Franciscan and Sanbagawa belts, we suggest the following revolutionary paradigm shifts.The so-called mineral isograds defined on the maps of regional metamorphic belts were a misunderstandingof the progressive dehydration reaction during subduction because extensive late-stage hydration hasmostly obliterated the progressive minerals in pelitic–psammitic and metabasic rocks. Progressively zonedgarnet has survived as the sole progressive mineral that was unstable with the majority of matrix-formingminerals. The classic Barrovian isograds should therefore be carefully re-examined. The well-documentedSHRIMP chronology of spot-dating zoned zircons with index minerals from low-P in the core, through HP–UHP in the mantle to low-P on the rim clearly shows that the slow exhumation speed of 23–40 My frommantle depth to mid-crustal level was followed by mountain building with doming at latest stage. Extensivehydration of the UHP–HP unit occurred due to fluid infiltration underneath, when the UHP–HP unit intrudedthe low-grade to unmetamorphosed unit at a mid-crustal level. Most deformation textures such as minerallineations, porphyroblasts, pull-apart, or boudinaged amphiboles, formed during extensive hydration at thelate stage do not constrain the progressive stress regime. The P–T–time path determined by thermo-barometry using mineral inclusions in garnet and forward modeling of garnet zoning, independent of thematrix minerals, indicates an anticlockwise trend in the P–T space, and follows an independent P–T change inthe metamorphic facies series. This is consistent with the numerically calculated geotherm along theWadati–Benioff plane. Collision-type orogenic belts have long been regarded as being characterized by theintermediate-pressure type metamorphic facies series. The kyanite–sillimanite is an apparent type faciesseries formed by the late-stage extensive hydration. In contrast, the original high-P to ultrahigh-P type faciesseries with an anticlockwise kink-point at around 10 kb is a progressive type. A collision-type regionalmetamorphic belt crops out as a very thin unit sandwiched between overlying and underlying low-P orweakly metamorphosed units. The metamorphic belt has an aspect ratio (thickness vs width) of 1:100, and itextends for several hundreds to a thousand km. It resembles a thin mylonitic intrusion from the mantleextending from 100 to 200 km depth into the crustal rock unit. The underlying unit is thermallymetamorphosed in the andalusite–sillimanite type facies series. The major reason for the misunderstandingof the progressive metamorphism in collision-type orogenic belts is the underestimation of the role of fluidsderived from the underlying low-grade metamorphic unit, when juxtaposed at a mid-crustal level. Thecirculation of fluids along the plate boundaries is more important than a P–T change.

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a, S., Masago, H., Katayama, I.,interpretation of a regionalountain building. Journal of

ruyama),

ssociation for Gondwana Research.

1. Introduction

Our understanding of regional metamorphism is pivoted on infor-mation from experimental petrology combined with thermodynam-ics, structural and petrological studies of regional metamorphic beltsfrommicroscopic to map scale, and the paradigm established by platetectonics (e.g. recent works by Brown 2010-this issue, Omori et al.,

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107S. Maruyama et al. / Gondwana Research 18 (2010) 106–137

2009; Santosh and Kusky, 2010, Zhang et al., 2009, among others). Theempirical and conceptual developments during the period 1965 to1980 led to various advances in this field and laid the basis for amodern approach (Miyashiro 1998). Thermodynamic database andexperimental data on subsolidus phase relations became available inthe late 70s (Helgeson et al., 1978) which was further improved bylater workers. Analyses of pressure–temperature–time (P–T–t) pathsbecame possible through the application of electronmicroprobe in thestudy of rock-forming minerals in regional metamorphic belts. These,combinedwith radiogenic isotope geochronology and thermal historyby numerical simulation, led to the proposal of tectonic models for theexhumation of regional metamorphic belts such as the benchmarkwork of England and Thompson (1984). It was an epoch makingsuccess which demonstrated the potential of metamorphic petrologyin modeling mountain building processes along consuming plateboundaries by combining information from geology and petrology(e.g., Spear et al., 1990). Thus, systematization of metamorphicpetrology under Earth Science was generally believed to have beensuccessfully achieved.

However, it was at this period that ultrahigh-pressure (UHP)metamorphic rocks were discovered from some orogenic belts. Thisdiscovery was a trigger to reconsider the existing concepts of regionalmetamorphism from the narrow realm of metamorphic petrology to amuch larger canvas, particularly incorporating processes in consum-ing plate boundaries as well as the information from geophysics.Although the first finding of coesite-bearing of UHP rocks from Alps(Chopin, 1984) was a major breakthrough, it soon became apparentthat this is not an isolate case, and that UHP rocks occur in manyregional metamorphic belts formed by continent–continent collisionas revealed by reports in the last 20 years (e.g. Zhang et al., 2009 andreferences therein; Masago et al., 2009). Traditionally, and empiri-cally, intermediate-pressure type metamorphism was considered tobe a characteristic for the collision-type orogeny, and was regarded tobe a progressive metamorphism. However, this was proved wrong bysubsequent studies as progressive metamorphism ranged from low-Thigh-P type facies series covering blueschist through eclogite tocoesite and or diamond facies metamorphism. Recently, similarresults were also obtained from the Pacific-type regional metamor-phic belts (Aoki, 2009). Therefore, metamorphic petrology systema-tized through the generally accepted models of England andThompson (1984) and Spear et al. (1990) faced new challenges. Thenew discoveries warranted a restructuring of the basic framework ofmetamorphic petrology in terms of the geodynamics and tectonics ofregional metamorphic belts. A paradigm shift in this field should beable to address: (1) progressive regional metamorphism; (2)retrogressive and probably hydration metamorphism; (3) geochro-nology; (4) metamorphic textures and structures and the estimationof stress; (5) P–T–t path; (6) metamorphic facies series; (7)exhumation tectonics and (8) the behavior of metamorphic fluid.Most of these themes are now facing several new challenges anddrastic revision of previous concepts based on technological andconceptual advancements. In this paper we synthesize the traditionalideas and concepts of metamorphic petrology and evaluate the recentobservations, finally leading to the proposal of some new concepts.

2. A brief historical review of regional metamorphism

In this section we provide a brief overview of the existing majorconcepts including salient assumptions on regional metamorphism.

2.1. Metamorphic rocks exposed on the surface indicate absence of fluids

If excess fluid enriched in water is always available during meta-morphism, a chemical equilibrium must prevail, and hence high-grade metamorphic rocks cannot be present on the surface of theEarth. Zeolite facies minerals such as clays are the stable assemblages

under the surface, at temperature and pressure conditions prevailingin the wet region. However, exposed regional metamorphic belts suchas those of Alps, Himalaya and other areas show the presence of hightemperature minerals such as plagioclase and pyroxene on a regionalscale. The common presence of anhydrous high-T minerals in region-ally exposed metamorphic belts suggests disequilibrium undersurface environment (Fig. 1). A possible explanation for this phe-nomenon is the absence of fluid during the exhumation from deepcrust to the surface.

Mineral isograds have been traditionally constructed to explainthe progressive nature of regional metamorphic belts (see exampleshown in Fig. 2). From low- to high-grade of metamorphism, thechange in pelitic mineral assemblages define the isograds, such as forexample the chlorite, biotite, garnet, staurolite, sillimanite andkyanite isograds in the collision-type Scottish Highland (Barrow,1893; Fettes, 1979) (Fig. 2), and chlorite, garnet, biotite and oligoclasein the Pacific-type Sanbagawa belt, Japan (Higashino, 1990). Theseisograds can be explained by P and T increase, which promote dehy-dration reactions at given chemical composition. Interpretation wasthat excess fluid was present during the P–T increase leading to thecompletion of chemical equilibria promoting the progressive meta-morphism (Fig. 1a,b). However, after the thermal peak (P–Tmaximum) the recrystallizedmetamorphic belts return to the surface.If there is no fluid, the mineral assemblages formed at specific P–Tconditions should remain frozen and will not undergo any change(Fig. 1c). The presence of high-grade mineral assemblages in theexposed metamorphic belts has been taken to indicate the absence offluids after the thermal peak, a speculation that has been welldemonstrated in petrology and supported by empirical considera-tions. Therefore, even though the logic is somewhat circular, mostpetrologists endorsed this concept.

2.2. Meaning of mineral isograd

As shown typically in the case of Scottish and Appalachian meta-morphic belts, the appearance of anhydrous minerals seems to becommon with increasing metamorphic grade. Therefore, in a givenrock system, such as pelitic or mafic, themineral assemblages indicatedehydration reactions at the high-T side to expelling fluids at thehigher-grade side. This process is envisioned as progressive meta-morphism and records a P–T increase during regional metamorphism.Therefore, the mineral isograds preserve the P–T structure during themaximum-P–T conditions.

However, if we consider the texture of metamorphic rocks, themargin of amphibole or garnet is often replaced by low-temperatureminerals, which indicate minor hydration and recrystallizationfollowing the peak metamorphic conditions. Thus, hydration recrys-tallization was considered to be minor during the exhumation ofregional metamorphic belts. It was thus considered that high-grademetamorphic rocks could be transported to the surface without anysignificant hydration–recrystallization.

As will be discussed in detail later, this notionwas wrong. The late-stage retrogression was extensive to obliterate almost all progressivemineralogy at high grades, hence we need to reconsider the basicconcept of metamorphism and metamorphic belts.

2.3. Exhumation model

Almost all regional metamorphic belts were considered to begenerally stable within the stability field of plagioclase. Plagioclaseoccupies more than 50% modal content among the mineral assem-blages in psammitic to pelitic sedimentary rocks and metabasites. Theupper stability limit of plagioclase in both metabasite and metasedi-mentary rocks is up to 12 kbar (Miyashiro, 1973, 1994a,b). Therefore,transportation depth of regional metamorphic belts was considered tohave never exceeded the bottom of continental crust (ca. 35 km).

Fig. 1. Schematic diagram of regional metamorphism in subduction zones. (a) The formation of protolith. Accretionary complex formed at trench and gradually decreased its watercontents during subduction (shown with change in density of dots in the metamorphic belts in the diagram from a and b). As most prograde metamorphic reactions are dehydrationwith excess aqueous fluid, chemical equilibration is achieved at prograde to peak stages. (b) On the contrary, most retrograde reactions during the exhumation stage are hydrationreactions, which cannot proceed without sufficient water being supplied from outside the system. For this reason, it has long been believed that metamorphic rocks exposed on thesurface preserve near peak stage of the metamorphic mineral assemblages (c). However, recent studies have clearly revealed that metamorphic belts have undergonerecrystallization with a large amount of water infiltrating at the mid-crustal level (d).

108 S. Maruyama et al. / Gondwana Research 18 (2010) 106–137

Based on this assumption, it was generally held that continental crustcannot subduct into mantle and would only underplate leading tocrustal duplication as in the case of the model proposed for Himalayas(England and Thompson, 1984). Advances in experimental petrologyled to the compilation of an exhaustive database on the thermody-namic properties of most of the rock-forming minerals (e.g. Helgesonet al., 1978, Holland and Powell, 1998). Using this database,metamorphic P–T paths have been traced by the continuous changein the composition of minerals such as garnet that are assumed to bein equilibrium with the matrix minerals. If chemical equilibria wereperfectly maintained, then the radiometric ages derived from keyminerals provide valuable insights into the exhumationmechanism ofregional metamorphic belts.

One of the best examples for widely cited exhumation model is theHimalayan metamorphic belt proposed by England and Thompson(1984). We will now consider the tectonic history of this belt. Theestimated horizontal movement is about 70 to 80 km/My fromcomputations based on the northward migration of Indian continentat the rate of 7 to 8 cm/year (England and Thompson, 1984). If thesubduction angle is 30°, the leading edge of the continent would reachMoho depths within 1 million year (Fig. 3a). Therefore, thermalcalculation justifies the instantaneous overlapping of two continentalcrusts (Fig. 3b). In this calculation, it was assumed that the temperatureat the surface of the subducted continental crust was zero degreecentigrade. The surface of the subducted crust is heated up through timeby the overlying lower crust of Asia, to an estimated temperature of500 °C. The geotherm on the top surface of the Indian continent is thusgradually heated up to attain a steady state geothermal gradient whichis a function of time (Fig. 3b). In this case, a time span of only 5 millionyears is sufficient to attain such a state.

Because the subducted continental crust is expected to have largebuoyancy, this crust must uplift right after the duplication of the twoblocks following isostatic requirements (Fig. 3a, right upper panel).The present day surface exposure of the Himalayan regionalmetamorphic belts exhibit a continuous sequence from very low

grade (300 °C) to high grade (700 °C), suggesting that the beltexposes the subducted Indian crust from surface to the lowerstructural level. The protoliths of Himalayan metamorphic rocksstill preserve some of the lithostratigraphy derived from theGondwana continental assembly, and the paleontological recordsare also consistent with this inference (Gansser, 1964; Lefort, 1975).Therefore, the Asian continental crust, which constitutes the upperpart of duplicated continents, seems to have been significantlyeroded, the debris fromwhich now constitute the voluminous Bengalfan deposits within the Indian Ocean. Judging from Fig. 3b, thesurface regions which recrystallized below 300 °C were not heatedup above by the high-temperature Asian lower crust, and theexhumation must have been extremely fast and short that occurredwithin a few million years.

We now consider the P–T architecture of the subducted Indian crust.The Indiancrustwas transported to adepthof ca. 45 kmwithin1 millionyear. The fast rate of subduction means that during the initial stage ofsubduction, only the pressure must have increased with temperatureremaining nearly constant (Fig. 3c). At the depth of 45 km, thesubducted crust stays for a considerable time when Pmust be constantwhereas T must gradually increase. This is due to the fact that heattransfer occurs only by conduction which is a very slow process ascompared to the speed of the platemovement (England and Thompson,1984). It should be noted that due to isostatic rebound, metamorphicbelts slowly uplift leading to temperature increase vis-à-vis pressuredecrease to exhume themetamorphic belts to the surface by buoyancy.Simultaneously, extensive erosion occurs on the surface (Fig. 3c). Asexpected in such a model, a clockwise rotation of the P–T path occurswhich matches well with the P–T time path deduced from chemicallyzoned garnets (Spear, 1993). This model has been supported by severalpetrological studies and Ganguly et al. (1998) estimated the averageexhumation speed of metamorphic belts in the Himalaya based on thediffusion velocity of trace elements from zoned garnet (see also TironeandGanguly, 2010-this issue for recentmodels on garnet diffusion). Theresults were also consistent with an extremely rapid exhumation.

Fig. 2. (a) Map view of metamorphic zoning and mineral isograd on the Scottish Highland. (b) The appearance and disappearance of index minerals in pelitic rocks with increasing metamorphic grade (Fettes, 1979). Intermediate-pressuretype metamorphic facies series in Scottish Highland has been called as Barrovian and regarded to be the world standard of progressive metamorphism in the regional metamorphic belts. (c) AFM ternary expression of the progressivedisappearance of staurolite. Staurolite-bearing assemblages change to an assemblage of sillimanite+garnet+biotite (right) with the disappearance of staurolite due to temperature increase (center). (d) The appearance and disappearance ofindex minerals in a model pelitic rock with increasing metamorphic grade along prograde P–T path of the Kokchetav UHPmetamorphic rock (after Masago et al., 2010-this issue). Compare (b) and (d) to evaluate the late stage of hydration tomask the stability field of progressive HP minerals at highest grades. 109

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Fig. 3. Classic model for continent collision orogeny of England and Thompson (1984). (a) Schematic cross section of the collisional orogen showing crustal duplication by collision(left) and subsequent isostatic uplift and erosion (right). (b) Thermal profiles along the depth of the collision zone at various stages after collision. The discontinuous thermal profileformed by double thickened crust (0 Ma) recovers to a steady state geotherm with time. (c) Pressure–temperature paths for the hanging wall and footwall of the orogen. Both showclockwise P–T time path through the orogeny.


3. A revolutionary change in the concepts of metamorphism

3.1. Discovery of UHP metamorphic rocks

The attractivemodels proposed for the formation and exhumationofthe Himalayan regional metamorphic belts provide a typical case of thesystematization of metamorphic petrology and illustrate the consisten-cy between theoretically predicted model and the field and petrologicdata. Hence, metamorphic petrologists and geologists widely appliedthe Himalayan model for other regional metamorphic belts formed bycontinent–continent collision employing a similar petrological, tectonicgeochronological method. This template study thus became a commoncurrency over the world and similar models for different regionalmetamorphic belts in space and time were proposed.

However, the discovery of UHP metamorphic rocks from Alps(Chopin, 1984) marked a historic change in our concepts on regionalmetamorphism and cast the first shadow of doubt on the Himalayanmodels. Chopin's (1984) report of coesite-bearingwhite schist from thehighest grade of the Penninic nappe in the Italian Alps was followed byfurther similarfindings such as the discovery of coesite fromNorwegianCaledonides (Smith, 1984). Thereafter, ultrahigh-pressure metamor-phic rocks have been reported from a number of collisional orogenicbelts over theworld (see Zhang et al., 2009 and references therein), andnewreports continue even todayadding to thenumber ofUHP localities,which already exceed 25 (Fig. 4). Although there is a general consensus

that UHP metamorphic rocks are remarkably exotic in terms of P–Tcondition,manypeople still consider theEnglandandThompson (1984)model for Himalayan metamorphic belts as an acceptable model.

3.2. Mode of occurrence: coherent, allochthonous or autochthonous?

Following the discovery of UHP rocks, one of the main topics ofdebate was the mode of occurrence of the metamorphic belts. Thepressure conditions of UHP rocks range from 30 to 70 kbar, which isclearly outside the stability field of plagioclase, the stability of whichdenotes pressures of only less than12 kbar. This led to a contradiction inthe conceptual framework of regionalmetamorphism. One possibility isthat UHP rocks were caught up within low-pressure regional meta-morphic belts as tectonic blocks. The second view is that it is similar togravel within conglomerates or olistostrome (Wang et al., 1992). Thethird possibility is that the regional metamorphic belt itself wassubjected to high-pressure to ultrahigh-pressure metamorphismfollowed by hydration–recrystallization along intermediate-P seriesbelow12 kbar (see a summarybyWanget al., 1992).Among these threeinterpretations, most people believed the former two cases, possiblybecause UHP rocks were derived from eclogites which were thought tobe derived from ophiolites. Another reason for this contention was dueto the fact that the major portion of regional metamorphic belts wasderived from psammitic to pelitic or orthogneiss which do not exhibitthe HP–UHP metamorphic conditions.

Fig. 4. Global distribution of UHP metamorphic rocks (partly modified after Parkinson et al., 2002). Numbers below the localities represent ages in Ma.


However, continued studies on UHP metamorphism revealed thatthe possibility (3) mentioned above is the most viable scenario. Adetailed case study was reported by Katayama et al. (2000) whoanalyzed zircon inclusions separated from pelitic and psammitic rocksfrom diamond-grade Kokchetav UHP regional metamorphic belt usinglaser Raman spectrometry. More than 20 minerals were identifiedwithin the zircons. Metamorphic mineral zoning was establishedbased on the mineral inclusions in zircons, and the results clearlydemonstrated that the psammitic and pelitic metamorphic units ofKokchetav must have undergone regional metamorphism under HP–UHP conditions. These metasedimentary rocks comprise more than90% of the belt, and therefore the metamorphic belt itself must havebeen subjected first to HP–UHP facies series and was later affected byintermediate-pressuremetamorphismwithinMoho depth (Katayamaet al., 2001; Masago et al., 2010-this issue). This study, and similarother works were a turning point on the concept of collision-typeregional metamorphism.

3.3. Concept of regional metamorphism faces serious challenge

Following the Kokchetav research, similar results were obtainedfrom the Dabie–Sulu Triassic collisional orogenic belts in South China(Liu et al., 2002; see Zhang et al., 2009 for a recent review). Moreover,coesite was also discovered in granitic gneiss from the western part ofHimalayan belt, the Pakistan and Indian Himalayas (Kaneko et al.,2003). This led to a drastic change in concept and it became apparentthat most of the collisional metamorphic belts must have suffered firstHP–UHP metamorphism during the prograde stage (e.g. Nishimiya

et al., 2010; Santosh and Kusky, 2010), followed by exhumation up tomid-crustal level when hydration obliterated part of the progressivemetamorphic history through replacement by intermediate faciesseries recrystallization.

The prograde and retrograde metamorphic history and the timegap between these two episodes were clearly identified through spotanalysis of zircons from UHP rocks using SHRIMP method pioneeredby Katayama et al. (2001). The history recorded from the core andmargin of single zircon grains provided new insights into themarkedly different P–T conditions; for example, a diamond-bearingcore against graphite and plagioclase bearing rim. Thus, twoindependent metamorphic episodes were distinguished with ca. 20to 30 million years difference between the two events. This has beenconfirmed not only in Kokchetav, but also in other UHP terranesincluding the Dabie–Sulu and Himalayas (Wang et al., 1992; Tabataet al., 1998; Liu et al., 2002; Kaneko et al., 2003; Sachan et al., 2004;Zhang et al., 2009). The time gap reaching 20 to 30 million yearsindicate extremely slow exhumation rate.

The Kokchetav and Dabie–Sulu examples, among other terranes,finally led to the understanding that HP–UHP metamorphism was notan exotic phenomenon and represents part of a progressive metamor-phic cycle association with the deep subduction of continental marginsfollowed by late-stage hydration–recrystallization during exhumation.The information so far available indicates that UHP metamorphic rocksare generally restricted to Phanerozoic orogenic belts, and no typicalArchean and Proterozoic examples have been reported, except minorexceptions for latest Proterozoic examples from Brazil and Africa (Caby,1994; Campos Neto, 2000; Parkinson et al., 2002).

Fig. 5. Metamorphic facies series and the representative metamorphic belts (modifiedafter Miyashiro, 1965). Three metamorphic facies series (high-P/T, medium-P/T andlow-P/T) and two intermediate series were proposed. Medium- and high-P/T serieswere regarded to characterize collision- and Pacific-type metamorphism respectively.Zeo: zeolite facies; PA: Pumpellyite–actinolite facies; PrA: Prehnite–actinolite facies;GS: Greenschist facies; EA: Epidote–amphibolite facies; AM: Amphibolite facies; BS:Blueschist facies; EC: Eclogite facies; HGR: High-P granulite facies; LGR: Low-pressuregranulite facies; Px-HR: Pyroxene-hornfels facies.


3.4. Impact on the Pacific-type orogenic belts

The new findings from collision-type regional metamorphic beltssuch as those of Kokchetav were soon extended to the Pacific-typeorogenic belts such as the Sambagawametamorphic belt in Japan (Otaet al., 2004; Aoki et al., 2007, 2008, 2009). At the highest metamorphicgrade for Sambagawa belt in the Shikoku island of SW Japan, anamphibolite mass of approximately 10×4 km2 is exposed with a relictmineral assemblage corresponding to eclogite facies, the pressureconditions of which are markedly higher than the already describedsurrounding amphibolite facies rocks (below 12 kbar). However,these eclogites were considered as tectonic blocks occurring alongfault zones (Takasu 1989). More recently, eclogite facies mineralassemblages were widely mapped over the entire mass (Ota et al.,2004). Moreover, small eclogite bodies were also reported from thesurrounding Sambagawametamorphic rocks (Aoya andWallis, 1999).These findings radically revised the traditional concepts of progres-sive Sambagawa metamorphism along HP intermediate facies seriesbelow 12 kbar, which represents only a later stage hydration–recrystallization after exhumation and emplacement at mid-crustallevel. Thus, the high-pressure progressive metamorphismmight haveoccurred at an earlier stage.

4. New paradigm

The framework of concepts on the dynamics of regional meta-morphism underwent rapid changes in the recent years and in thissection we summarize some of the crucial aspects.

4.1. Intermediate facies series

One of the aspects to be evaluated is whether or not the kyanite–sillimanite type characterizes metamorphism in collisional orogenicbelts. Miyashiro (1961) defined three independent metamorphicfacies series and two intermediate groups between the three. The typelocalities listed by Miyashiro (1961) are the Franciscan, Sambagawa,Scottish Highland, New Zealand and the Ryoke belt (Miyashiro, 1973).Based on the description of regional metamorphic belts, it waspointed out that the intermediate-pressure type metamorphic beltscharacterize the collision type. On the contrary, high-P to high-pressure-intermediate variety denotes the Pacific-type subduction,also low-pressure type paired with HP type. Furthermore, themetamorphic facies series were employed to define a clockwise P–Tpath (Fig. 5). The finding of UHP metamorphic rocks from severalregional metamorphic belts proved that Miyashiro's generalizationsdo not hold true. Regardless of whether the orogeny is collision-typeor Pacific-type, the regional metamorphic belts display progressivemetamorphism under HP to UHP conditions. Also, the P–T–time pathis not clockwise, but anticlockwise as will be shown in a later section.

4.2. Re-examination of mineral isograd

We will now examine the question whether it is possible toreconstruct the progressive metamorphism during subduction. Majorminerals of pelitic and psammiticmetamorphic rocks in collisional beltsare plagioclase, quartz, biotite, muscovite and amphibole which areobviously not the products of the prograde high-pressure meta-morphism during subduction as discussed before. If so, what were theminerals and their composition, the stable mineral assemblages andreactions during the progressive stage? Can these be retrieved?Compared to pelitic and psammitic schists, metabasites are massiveand more competent and therefore, the central portion of a metabasitebody tend to preserve the progressive stage of metamorphism even ifthe unit witnessed late-stage hydration–recrystallization. Thus carefulfield investigations, mapping and detailed sampling is essential to tracethe signature of progressive metamorphism as illustrated in the case of

KokchetavandDabie–Sulu.However, thevolumeofmetabasites is smalland generally less than 10% of the total volume of rocks exposed in suchbelts. The remaining 90% is composed of para- or orthogneiss. Thematrix minerals of metasedimentary rocks have recrystallized undermid-crustal level. The rare metabasites, even though occurring as thinunits, locally preserve jadeite+coesite+garnet, as in the case wherethey are enclosed within metacarbonates (Ogasawara et al., 2002). Insome cases, metamorphosed sandstones surrounded by metacarbo-nates preserve similar UHP assemblages. The stable HP and UHPminerals are preserved within zircon crystals and include coesite,diamond, garnet, jadeite and silica-rich phengite. These minerals andmineral assemblages are different from the matrix-forming minerals inpsammitic and pelitic rocks. In this case, UHP metamorphic belts weregenerated by subduction of passive margin sediments down to 100 oreven 200 km depth, dehydrated, and dried at maximum depths.Thereafter they exhume along the Benioff plane up to mid-crustallevels of 10 to 15 km depth, where extensive hydration and recrystal-lization occurred when most of HP–UHP minerals disappeared exceptthose occurring as inclusions within robustminerals such as garnet andzircon (Masago et al., 2010-this issue). This critical observation refutedthe generally accepted notion that the composition and assemblages ofmatrix minerals are the basic starting point (e.g., Spear, 1993) tounderstand regionalmetamorphism. Someof the fundamental conceptsthus collapsed warranting a new framework of metamorphism.

4.3. Tracing prograde metamorphism from zoned garnet

Garnet in pelitic and psammiticmetamorphic rocks is the onlymajorrock-forming mineral, which may have been stable during progrademetamorphism.Nevertheless garnetwasnot inequilibriumwithmatrixminerals. Since zoned garnet has been regarded as the ideal mineral todraw pressure–temperature change through time, garnet solid solutionchemistry has been used most frequently to estimate P–T–t path and alarge number of papers have been published. However, some of theunderlying concepts were erroneous.We cannot usematrixminerals atall, because of the late-stage extensive hydration–recrystallization. Is it


possible to reconstruct prograde change of mineral assemblages frompelitic and psammitic regional metamorphic rocks? There are twowaysto estimate the prograde P–T path from the zoning of garnet.

The inversion using Gibb's method (e.g., Spear et al., 1990;Okamoto and Toriumi, 2001; Inui and Toriumi, 2002; Okamoto andToriumi, 2004) starts from a provided initial condition and traces solidsolution chemistry of garnet and other equilibrium minerals toreproduce the prograde P–T change. This method is a feasibleapproach, although if the prograde garnet and other minerals havebeen lost during retrogressive metamorphism this method cannot beused. On the other hand, Gibb's method does not include enthalpy forthe calculation and uncertainties derived are minimal. Anothermethod is a forwardmodeling of themineral assemblage by extensivethermodynamic calculation using an internally consistent thermody-namic database of the minerals such as Holland and Powell (1998).The calculation predicts equilibrium mineral assemblages in P–T–Xspace. Even though the matrix minerals, except garnet, are totallyretrograded, the prograde P–T path can be estimated from mineralinclusions in garnet grain and composition of garnet near by theinclusion. If some geothermobarometer could be applied to theinclusion and garnet, the estimate will become more quantitative(Omori and Masago, 2004; Masago et al., 2009). However, such aforward calculation needs enthalpies of phases, which is a parameterwith a large uncertainty. Both the methods have advantages anddisadvantages and therefore if these are used in combination, arealistic P–T path can be obtained.

4.4. The Sambagawa metamorphism

With increasing metamorphic grade, the minerals in pelitic schistrange from chlorite, garnet and biotite to oligoclase. These mineralshave long been believed to be the products of progressive metamor-phic reactions. That is, theseminerals were formed by the dehydrationreactions by plate subduction with resultant pressure–temperatureincrease to reach P–Tmaximumunderwhich oligoclasewas formed atmaximum-P and T (Fig. 6). The estimated P–T conditionswere 12 kbar,600–700 °C. The contradiction then was that oligoclase is not stableunder these conditions.

If all minerals except garnet were formed during hydrationrecrystallization at later stage, the progressive metamorphism in theSambagawa belt needs to be re-examined, particularly using zonedgarnet and its inclusions at different metamorphic grades. Studies onthese lines by Okamoto and Toriumi (2001, 2004), Inui and Toriumi(2002), and Aoya et al. (2003) led to the identification anticlockwiseP–T path instead of the previously believed clockwise P–T path for theSambagawa belt.

Quartz schist and calcareous schist, which are impervious to fluidinfiltration, tend to preserve the history of progressive metamorphicminerals. The identification of talc, phengite, paragonite and jadeitefrom pelitic schist shows that although these rocks are minorcomponents in HP–UHP belts, they provide valuable information onthe progressive nature of metamorphism. The studies also indicatethat plagioclase was not stable at the highest pressure andtemperature conditions (Aoki, 2009).

4.5. A re-examination of pressure–temperature path

4.5.1. Drastic shift in pressure estimate and P–T–t pathP–T path of metamorphic belts have been calculated by zoned

garnet in many cases, although in most examples plagioclase ispresent as a stable matrix mineral and all the minerals stable below12 kbar are assumed to be in chemical equilibrium. The results alwaysshow clockwise P–T paths below 12 kbar. On the contrary, the actualpressures of prograde metamorphism were 3 to 7 times higher (30 to70 kbar) and the nature of P–T path could also have been different.Because plagioclase is unstable at the highest grade during regional

metamorphism, some of the fundamental assumptions in studies,which traced clockwise P–T paths, were erroneous. A more realisticapproach is based on mineral inclusions within garnet and zircon,which show progressive compositional change during progressivemetamorphism. The results indicate a remarkable difference from theprevious approach. Thus, (1) maximum pressure estimates changedfrom less than 12 kbar up to 70 kbar; (2) clockwise path in P–T spaceturned to anticlockwise; and (3) the P–T time pathwas identical to themetamorphic facies series (Masago et al., 2009) as shown in a latersection.

4.5.2. Low-grade metamorphismThe discovery of UHP–HP metamorphism radically revised the

concepts of high-grade metamorphism, although the finding wasrestricted to the highest-grade metamorphic rock such as eclogite. Asimilar misunderstanding was prevalent in the case of low-grademetamorphic rocks. Metabasites under low-grade metamorphic condi-tions of pumpellyite–actinolite facies sometimes show the texture ofglaucophaneenclosedbyactinolite (Fig. 7) (Maruyamaet al., 1986). Thistexture indicates early high-pressure conditions followed by tempera-ture increase at constant pressure to stabilize actinolite because thereaction between glaucophane and actinolite has a gentle, positiveslope. The idea of a clockwise P–T path was derived from the concept ofunderplating where an accretionary complex is transported to about15 km depth by descending oceanic plate and underplated against thehanging wall. In this case, the transportation to 15 km depth takes only0.2 million year if the subduction speed is 10 cm/year. Since this timespan is geologically very short, the resultant scenario is only pressureincreases at constant temperature. However, once accreted to thehanging wall, then temperature increases at constant depth since thehanging wall is always of higher temperature being part of the mantlewedge.

However, the reaction mentioned above is not dehydration, but ahydration reaction and substantial volume of water is necessary togenerate actinolite through the consumption of glaucophane. Meta-morphic belts require the supply of large amounts of water fromoutside. Therefore, this texture can be explained by pressure decreaseat constant temperature instead of isobaric temperature increase(Fig. 7). Important process here is the infiltration of water-rich fluids,similar to the case of eclogite at highest grade. The lower grade unitsunderlying the regional metamorphic belts such as those of theShimanto belt contain large amounts of hydrous minerals, indicatingthe infiltration of large volumes of hydrous fluids during theexhumation from depths to shallow levels. The structurally interme-diate units of the Sambagawa belt contain eclogite facies rocks thatshow the highest P–T grade (Banno et al., 1978; Banno and Sakai,1989; Ota et al., 2004) (Fig. 8). These are remarkably hydrated andrecrystallized to amphibolite. Below the structural intermediate level,lower pressure and temperature rocks are present which are morehydrated and recrystallized than those in the highest-grade meta-morphic core at the structurally intermediate level (Fig. 8). The sourceof fluids would have been the underlying low-grade metamorphicrocks of the Shimanto belt (Fig. 8).

4.5.3. Re-examination of metamorphic ageIf continent–collision-type regional metamorphic belts were

formed by overlapped continental crust, the clockwise P–T–texhumation of regional metamorphic belt must have occurred withinan extremely short time span. From the beginning of subductionthrough transport to great depths and final exhumation to the shallowlevels must have occurred within a few million years as calculated bythe numerical simulation of overlapping continental crust (see Fig. 3).

On the contrary, zircon U–Pb SHRIMP ages clearly indicate a muchlonger time span than that predicted above. For example the SHRIMPage of zircon,which contains coesite inclusions, shows 48 Ma at 100 kmdepth in the Pakistan Himalaya (Kaneko et al., 2003). After return from

Fig. 6. (a) Metamorphic zoning on the map view. (b) and (c) Mineral appearance/disappearance diagram for Sambagawa pelitic schist in central Shikoku (Higashino, 1975, 1990;Enami, 1982), (d) Interpretation for progressive mineral change during subduction. Higher-grade index minerals indicate higher P and T, hence oligoclase must be highest P and Tminerals during subduction. (e) Progressive change of mineral assemblage in pelitic system where oligoclase is not stable at highest grade (Aoki, 2009). Late-stage extensivehydration caused the birth of Barrovian minerals such as biotite and oligoclase during the return pass to the surface.


this depth to the mid-crustal level (15 to 10 km depth), the extensivehydration recrystallization occurred at 25 Ma, as plagioclase occurs asinclusions the zone developed in the zircons at this stage. These resultsshow that it took about 23 million years for the return journey fromUHPdepths to the Barrovian recrystallization depths.Moreover, thedrill coredata of Bengal fan deposit in the Indian Ocean (ODP Leg 116) show thatthe mountain building in the Himalayas began since 8 Ma (Derry andFrance-Lanord, 1996). This is grossly inconsistent with the exhumationrate of regional metamorphic belts from mantle depths to mid-crustallevel. Therefore, mountain building of the Himalayas seems to beunrelated to the event of regional metamorphism.

Similarly, the time span for transport frommantle depths of 100 to200 km to the mid-crustal level took about 30 million years(Katayama et al., 2001) (Fig. 9). The time span for the subductionand exhumation is one order of magnitude higher than that envisaged

in the crustal duplication model. Obviously, the field observationsrefute the model of continent duplication as discussed earlier in Fig. 3.

In the case of Himalayan metamorphic rocks, most of the radio-metric ages of K–Ar and Ar–Ar concentrate at about 25 Ma derivedfrom phengite or muscovite (Searle, and Freyer, 1986; Hubbard andHarrison, 1989; Copeland et al., 1991; Guillot et al., 1994). Accordingto the previous speculation, 25 Ma is the timing when the hightemperature regional metamorphic belts pass through the depthequivalent to 400 °C by the exhumation of deep-seated regionalmetamorphic belt to the mid-crustal depth. Therefore, peak meta-morphism must be much older than 25 Ma. Recent observationssuggest that the concept of closure temperature might be wrong, andthat the K–Ar dates may represent recrystallization ages at the abovedepths by extensive fluid infiltration underneath. One way to test theclosure temperature is the spot analysis of white mica formed during

Fig. 7. P–Tpath estimation from the growth texture of amphibole (Maruyama et al., 1986).Attached figure shows crossite mantled by actinolite. The texture can be explained byeither isobaric heating (a) or isothermal decompression (b). In either case extensivehydration is necessary. Crs — crossite; Act — actinolite; Chl — chlorite; Ab — albite.

Fig. 9. P–T-time path for the Kokchetav massif (Katayama et al., 2001). Inset figureshows a zoned zircon with a core which is as old as 1.1 to 1.4 Ga mantled bymetamorphic overgrowth with coesite, diamond and jadeite. The outermost marginincludes quartz, graphite and albite. These three zones were dated by SHRIMP yieldingthe ages shown in the figure. Extremely slow average exhumation speed is estimated bythis method. (d) Shown on the bottom of the figure are the P–T conditions when hightemperature solid intrusion of HP–UHP belt occurred generating a contact metamor-phic aureole (D) at 501±10 Ma with temperatures of 500–650 °C and pressures of 3 to4 kbar.


the progressive stage and which now occurs as mineral inclusionwithin garnet or zircon and compare with the spot analysis of whitemica frommatrix in the same rocks. In the case of Himalayas, if matrixwhite mica yields 25 Ma ages from eclogite facies rocks, thenthe white mica inclusions within zircon and garnet must be 48 Ma.If such is the result, then the popular interpretations based on closuretemperature would be wrong. This idea needs to be tested in futurestudies.

In the case of Sambagawa metamorphic belt, a similar problemexists as that in the Himalayas. The age of prograde eclogite faciesmetamorphism is SHRIMP date of 120 Ma (Okamoto et al., 2004). Onthe contrary, the ages obtained fromwhitemica are 60 to 80 Ma by K–Ar and Ar–Ar ages (Itaya and Takasugi, 1988; Takasu and Dallmeyer,1990). Whether the 60 to 80 Ma age represents the time of hydrationrecrystallization at mid-crustal level or the much older white micaunder UHP conditions at 120 Ma was reset at 400 °C due to closuretemperature effects need to be examined by spot analyses of micainclusions in garnet.

5. Re-examination of metamorphic facies series

It has been previously believed that onemetamorphic rock recordsone metamorphic facies. The concept of metamorphic facies serieswas formulated by integrating the distribution of low to high-grade

Fig. 8. Hydration effect on the Sambagawa belt from the underlying Shimanto belt. The crosShimanto belt is very low grade under PA (Pumpellyite–actinolite facies) to PrA (Prehnite–acfacies rocks formed under 20 kbar and 800 °C (Ota et al., 2004). Peak mineral assemblages anamphibolite due to extensive fluid infiltration from underneath (Ota et al., 2004).

facies rocks in regional metamorphic belts (Miyashiro 1961, 1965)(Fig. 10a).

In general terms, metamorphism begins with increasing pressureand temperature from the surface condition to P–T maximum at depthand thereafter the return to surface. Thus, the rocks in regionalmetamorphic belts could have been subjected to several metamorphicgrades (Fig. 10b). deRoever andNijhuis (1963)proposed thepioneeringconcept of plurifacial metamorphism based on the observation that onemetamorphic rock preserves several facies under the microscope such

s section along the N–S in central Shikoku is schematically shown. Note the underlyingtinolite facies). The structurally intermediate unit of Sambagawa is composed of eclogited texture are only preserved in the core of large mafic bodies such as the Iratsu eclogite

Fig. 10. Definition of the metamorphic facies series by Miyashiro (1965). (a) Miyashirodefined the metamorphic facies series as the curve that connects the P–T conditionsshown as thick solid lines. (b) An individual rock may have experienced severalmetamorphic facies in a single metamorphism. The highest-grade rock in the figureexperienced 5 metamorphic facies as shown as circled numbers 1 to 5. This was earlierdefined as plurifacial metamorphism (de Roever and Nijhuis, 1963). Spear et al. (1990)redefined the metamorphic facies series as a curve that connects the thermal peakconditions of the P–T path of individual rock (thin solid line) within ametamorphic belt.See text for discussion.


as zonedCa–Naamphibole andCa–Napyroxene. The clockwiseP–T timepath connecting severalmetamorphic facies has become a fashion since1980 from the analysis of regional metamorphic belts over the world.Since one rock specimen preserves several metamorphic facies in caseswhere high-grademetamorphismhas operated,Miyashiro's proposal ofmetamorphic facies series needed redefinition. Thus to avoid confusion,the highest T at given P condition for each rock was connected to drawthe P–T path and was defined as the metamorphic facies series (Spearet al., 1990) (Fig. 10b).

Themetamorphic facies series thus defined is again clockwise in P–T space; with increasing depths the temperature gradient tends toincrease. Ifwe consider the facies series characterized by clockwise P–Tpath at depths over 10 kb along consuming plate boundary, thetemperature must increase beyond the solidus of basalt or peridotites,and slabmust be partiallymelted. Thus, the subductedmaterials aboveoceanic crust together with the sediments on the top will be meltedalmost completely. However, observation from natural examplesalong themodern subduction zones does not support this, because arcmagmatism behind the trench is not in conformity with the idea ofslab-melting in the case of old slab N25 Ma (Defant and Drummond,1990; Drummond and Defant, 1990). Seismological observations alsodo not indicate any slab-melting at great depths.

The P–T change along the descending slab has been calculatednumerically (for example, Peacock 1996) (Fig. 11a). The results clearlyindicate an anticlockwise trend, instead of a clockwise path. Such atrend is commonly recognized by geophysicists who consider plateboundary as a thermal boundary layer and it is well recognized thatgeothermal gradient is not constant with increasing depth. Forexample, the Kokchetav HP–UHP units show an anticlockwise path(e.g. Masago et al., 2009) (Fig. 11b) and the slope of the estimated P–T

curve changes at about 13 kbar and 700 °C to a much straight pathwith no significant increase in temperature up to 40 kbar. This trendappears to be common not only in continent collision zones but also inPacific-type orogenic belts (Inui and Toriumi, 2002, Aoya et al., 2003;Okamoto and Toriumi, 2004; Masago et al., 2009).

6. Re-examination of textures

6.1. Metamorphic texture

Regionalmetamorphic rocks contain a variety of textures related toprograde metamorphism, recrystallization and deformation. Forexample, albite porphyroblasts often include a variety of mineralsdefining internal deformation fabric. Elongate minerals such asamphiboles define mineral lineations. Boudin or pull-apart texturesare also common which indicate extension and breakdown of theelongatedminerals into several fragments, all of which have long beenconsidered to have been formed during subduction, i.e., progressivetextures. Also common are snowball garnetswhich entrail the rotationand growth of inclusion minerals, as well as pressure shadows. Othermicrostructures include sheath folds, deformation of radiolarians androunded cobbles and crenulation foldwithinmica-rich domains. Thesetextures offer information on the direction of stress and its relativemagnitude under the assumption of boundary conditions and initialstate.

In the Sambagawa belt, mineral lineation and crenulation fold axisof mica, boudin or pull-apart texture of mineral grain or elongatedminerals, and radiolarian deformation, among other features indicateE–W extension, which is parallel to the orogenic belt (e.g., Toriumi,1982). This elongation coincides with the direction of metamorphicfluid flow in which the elongated minerals have grown. Thesestructures have long been considered to form during progressivemetamorphism through subduction during the Cretaceous time andrelative plate motion. However, all of these mineral textures anddeformation were not formed during progressive recrystallization,but were generated at a later stage after the emplacement at mid-crustal level by the addition of fluids underneath (Toriumi, 1982, 1985and Toriumi and Noda, 1986).

Albite porphyroblast appears at the highest grade in the Sambagawabelt and thezone inwhichabundant albite is presenthas been termedasthe spotted zone. Empirically, this zone corresponds to the biotite zoneat the highest grade. The timing of formation of the porphyroblast inrelation to the progressive or retrogressivemetamorphism is importantto discuss the origin of the metamorphic texture. This texture appearsrestricted only to the highest-grade portion because its developmenthas been traditionally linked to progressive metamorphism at thehighest, and therefore, the deepest conditions. However, the albiteporphyroblasts include even retrogressive minerals developed at lowerpressure conditions during hydration (Toriumi, 1975). Therefore, thetiming of the formation of these porphyroblasts must be after theemplacement of the Sambagawa belt into the mid-crustal level.Presumably, large amounts of fluids infiltrated into the Sambagawabelt, which promoted extensive scale of recrystallization at 80 to 70 Ma.The texture and deformation of the rocks in this belt also indicate lack ofpreservation of the progressive stage. An E–W extension strain isderived from the deformation pattern of gravels in metaconglomerateschist and the recrystallized age of white mica by K–Ar method. Theconglomerate schist in Oboke area has been considered to be a part ofthe Sambagawa metamorphic belt from deformational patterns.However, LA-ICPMS U–Pb detrital zircon age separated from quartzporphyry cobble in the conglomerate indicates 92±4 Ma (Aoki et al.,2007). This age represents the timing of consolidation of the acidicmagma for the protoliths of gravels. Therefore, the sedimentary age ofconglomerate must be younger than 92 Ma. On the other hand, theSambagawa progressive metamorphism is dated as 120 Ma (Okamotoet al., 2004) clearly indicating that Oboke conglomerate is not a part of

Fig. 11. (a) Numerically simulated temperature profile along the Benioff plane of a subducting slab at 10 cm/yr (Peacock, 1996). Geotherm is gentler at shallow level usually less than50 km and turns to steep at deeper with anticlockwise sense of P–T path. In the case of a young slab, it would be gentler than the older, and where the slab is old, it is steeper. Dottedline C shows the generally estimated metamorphic P–T path with a clockwise sense. If the clockwise path C is correct, subducted slab must have been melted. In the bottom figure,line A shows the geothermal gradient within plate shown below. Line B shows the geothermal gradient along Benioff plane, corresponding to those paths on the top figure. Alsoshown are wet solidus of MORB and anhydrous solidus of MORB. Numbers of 2, 5, 10, and 50 denote the age of subducted slab. (b) P–T estimate of the Kokchetav massif coveringepidote amphibolite facies grade through amphibolite to high pressure granulite, amphiobole– to zoisite–eclogite and dry eclogite (see Masago et al., 2010-this issue). Metamorphicfacies series shows an anticlockwise path, which is different from the traditional clockwise path. Progressive P–T path follows an isothermal decompression during exhumation.


the Sambagawa metamorphic belt. Hence, the so-called Sambagawabelt must be divided into two different metamorphic belts. When theSambagawa metamorphic rocks were emplaced at mid-crustal level,above the Shimanto belt, both Sambagawa and Shimanto shared thesame history of recrystallization and deformation with E–W extension.Themicrofabrics of the Sambagawa schist are remarkably similar to thedeformation patterns associated with low-grade metamorphism in theShimanto belt. This shows that the metamorphic texture of theSambagawa schist does not correspond to the progressive stage, but isrelated to retrogressive hydration stage. The major texture of theSambagawa schist was formed during the retrogressive hydration stageat 80 to 70 Ma when Sambagawa was juxtaposed to the Shimanto belt.The metamorphism of basic and pelitic schist in the Shimanto beltreached up to blueschist facies. The zircon U–Pb chronology yielded84 Ma for the protolith age, and the abundant grained white micayielded K–Ar ages around 60–65 Ma for themetamorphism (Aoki et al.,2008). It is difficult unequivocally define the textures or fabric formedduring progressive stage of metamorphism, and to differentiate theearlier stage of exhumation frommantle depths to themid-crustal levelwith the presently available information except in specific cases. Onesuch example is a relict eclogite formed at the highest grade of theSambagawa metamorphism. The elongation axis of the omphaciteand related textures show orogen-vertical N–S slide. The disposition ofthe Sambagawa belt shows a structurally intermediate position, verticalto the strike of the orogenic belt, and pre-dates the development ofthe E–W lineation (Toriumi and Kohsaka, 1995; Yamamoto et al.,2004). There areminor rocks in outcrop scale ormicroscale,which showthe texture or deformation during the exhumation from the mantledepths. However, all of those do not preserve the progressive stage of

subduction. The deformation fabric during progressive metamorphismmust be restricted to mineral inclusions within garnet and zircon. Thesnowball garnets may provide one of the tools to reconstruct the three-dimensional movement of the progressive deformation.

7. Discovery of top and bottom boundary of regionalmetamorphic belts

One of the fundamental requirements is to define the top andbottom boundaries of the regional metamorphic belt, although thishas not been resolved in most cases. Metamorphic petrologists haveoften neglected the presence of discontinuous boundary by simplyassigning a continuous gradation into the lower grade rocks graduallywithout considering a discontinuous gap above and below themetamorphic units, although structural geologists had predicted thepresence of a clear boundary. On outcrop scale, a sharp boundarycannot be identified between the two units because of mode ofdeformation and metamorphic grade do not show any markeddifference. The maximum thickness of regional metamorphic unitssuch as the Sambagawa reaches up to 2 km. The pressure gradientwithin the belt remained unresolved. Among the models proposed forthe Sambagawa, a structural break is commonly noted along theboundary of the metamorphic zone such as in the Kanto Mountains(Hashimoto et al., 1992).

UHP–HP belts often exhibit remarkable pressure difference withinthe belts. Detailed studies in the Kokchetav unit covering 250 km×20 km helped in identifying nearly horizontal top and bottom boundary(Kaneko et al., 2000). Locally, tectonicwindowsare present revealing theunderlying unmetamorphosed units because the bottom boundary is


nearly flat (Kaneko et al., 2000). And more interestingly, a contactmetamorphic aureole in the unit underlying theKokchetavUHP–HPunitwasalso identified. This contact aureole is divided into four zones and themetamorphic grade progressively increases towards the boundary.Metamorphic facies series belong to andalusite–sillimanite type andcontact aureole reaches up to 600 °C at 2 kbar (Terabayashi et al., 2002).Metamorphic zircon separated from contact zone yielded SHRIMP U–Pbage of 501±10 Ma which coincides with the age of the extensivehydration and recrystallization stage within the HP–UHP units(Katayama et al., 2001). This observation indicates that the contactmetamorphism was a result of the emplacement of the HP–UHP unitsinto upper crustal level at 6 km depth. Thus, a new concept of theexhumation of diamond-grade metamorphic rocks and their hightemperature solid intrusion emerged, similar to magma intrusion. Thisidea clearly demonstrated that a radical change from the traditionalconcept of buoyancy-drivenupward exhumation and erosion of regionalmetamorphic belts is undoubtedly wrong.

A suite of 7000 samples were collected from the Kokchetav fromwhich 350 were analyzed by electron microprobe and zirconsseparation performed from 250 samples to evaluate the detailed P–Tstructure. The study yields maximum pressures of 60–70 kbar at 900–1000 °C (see a summary by Maruyama et al., 2002). Metamorphicunits within the 2 km thick belt were sub-divided into five mineralzones with the structurally intermediate zone showing the highestgrade. Themetamorphic grade decreases both upward and downwardwith a pronounced pressure gradient with only 5–6 kbar at theboundaries. However, the boundaries of mineral zones are continuouswithout any sharp structural breaks, indicating ductile deformation toemplace those mineral zones upwards after the thermal peak. Overallthermobaric structures show symmetric pattern with an axis at thestructural intermediate. However, the thickness of each mineral zoneis not symmetric above and below. The deformation structureindicates that the structurally intermediate zone moved from southto north (Kaneko et al., 2000; Fig. 12c).

In the case of Pacific-type metamorphic belts such as theSambagawa, the method to define top and bottom boundaries hadalready been proposed by Isozaki and Itaya (1990). The precise ageswere measured from radiolarian microfossils and K–Ar dating of fine-grained white mica separated from low-grade metamorphic rocksalong the outcrop scale. Later, Nishimura (1990) used the crystalli-zation index of carbonaceous material in the outcrop scale to preciselyfix the boundary in meter scale. The importance of defining top andbottom boundaries had been pointed out in earlier studies with theboundary defined either top or bottom, or both for forty examples ofregional metamorphic belts from different regions of the world(Maruyama et al., 1996).

8. Chaotic state arising from the concept of nappe tectonics

The Penninic nappe of the Alpine metamorphic belt is the bestexample where the top and bottom boundaries are well defined withthe intermediate zone enclosed within overlying and underlyingunmetamorphosed units (Fig. 12a). However, the origin of UHP–HPunit has not been well understood largely due to the confusion arisingfrom the concept of nappe tectonics (Fig. 13). In the following sectionwe briefly address this problem.

In this metamorphic belt, there are three tectonic slices demarcatedbyhorizontal faults (CowardandDietrich, 1989;Maruyamaet al., 1996).The structurally lowest unit, the Helvetic nappe which constitutes thebasement of Europe has been classified as an allochthon that has nevermoved after its formation. The overlying unit (Penninic nappe UHP–HPunit), which was metamorphosed in the Eocene to Miocene, movedfrom south to north and was thrust over the allochthon. Thereafter, thetopmost unit, the Austro-Alpine unit or nappe which is considered as abasement of Africa, was again thrust over on the top. Although this hasbeen the prevailing view, it is more reasonable to consider that the

structurally intermediate unit, the Penninic nappe, was tectonicallyinserted into the overlying and underlying units because this structuralintermediate unit is the only unit that was subjected to the Eocene–Miocene HP–UHP conditions of metamorphism. In the prevailingconcept, the reconstructed three units are assumed to have possesseda horizontal relationship as shown in Fig. 13. However, a prominent gapis evident between units H and P in Fig. 13, a marked discontinuity interms of lithology, stratigraphy and geochronology. These three are notcontinuous each other and lithostratigraphically different before nape-stacking (Fig. 13).

Underneath the Kokchetav HP–UHP unit, a well-developed contactmetamorphic aureole has been defined with andalusite–sillimanitetype facies series (Terabayashi et al., 2002), clearly demonstratingthat the exhumation of regional metamorphic belts was not buoyantuplift as proposed by England and Thompson (1984) but a hightemperature intrusion into shallow crustal level as discussed in theprevious section. Moreover, the emplacement depths were shallowerthan 12 km (4 kbar) and the highest temperature recorded from themargin of metamorphic belts is 600–650 °C. This indicates that theinnermost domain of the metamorphic belts must have witnessedmuch higher temperatures, albeit lower than 1000 °C, when theserocks were at nearly 200 km depth. Due to this reason, when themetamorphic belt was emplaced, large volume of fluids must haveinfiltrated with the local development of migmatite.

From the structural analysis of the interior of the Kokchetavmetamorphic belt, the root zone was estimated to the south, with anorthward movement as a sub-horizontal tectonic slice during exhu-mation. The average exhumation speedwas extremely slow, 5 mm/year,as estimated byKatayama et al. (2001). Therewas nomountain buildingon the surface until the metamorphic unit was emplaced at depthsshallower than 12 km. The major period of formation of sedimentarybasin was after the doming stage, and developed by high angle normalfaults after extensive hydration, equivalent to the post-orogenicmountain building stage. Post-collision orogenic granites intruded intothe metamorphic belt (Katayama et al., 2001). Traditionally, the mainstage of orogenywas regarded as the exhumation or uplifting of regionalmetamorphic belt, which was also regarded to be the major mountainbuilding stage. However, we consider that this concept is incorrect, asexhumation of regionalmetamorphic belt to themid-crustal level occursmuch earlier and without mountain building. The mountain buildingstage is a post metamorphic event through doming as first proposedMaruyama (1990) and Maruyama et al. (1996) based on wedgeextrusion model. A similar scenario was also recently proposed bySantosh et al. (2009a) to explain themetamorphic belts in the Cambriancollisional orogen of Gondwana in southern India.

9. A new model of exhumation

9.1. “Intrusion” of regional metamorphic belts

The model proposed by England and Thompson (1984) marks thepioneering tectonic synthesis to explain the formation of continentcollisional type orogenic belts and their exhumation employingmetamorphic petrology as a powerful tool. If the double crustal modelthey envisaged is valid, the structural bottom of the regional meta-morphic belts must continue to higher-grade metamorphic rocks up totheMoho depths, and underlain bymantle peridotites. On the contrary,the units underlying the regional metamorphic belts are composed ofweakly metamorphosed or unmetamorphosed rocks. Moreover, thethermobaric structure of the interior of the regional metamorphic beltsdoes not support the model, which predicts unidirectional continuouspressure temperature increase to the bottom. Asmentioned in previoussections, the well-examined metamorphic belts such as Kokchetavalways show structural intermediate position for the highest-gradeunits aswell as theHimalaya and Alps. The three-dimensional structureindicates marked thermal and baric gradients within the thin (2–3 km)

Fig. 12. Cross section of representative orogenic belts. (a) Alps (Maruyama et al., 1996); (b) Himalayas (Kaneko, 1997); (c) Kokchetav (Kaneko et al., 2000); (d) Sanbagawa (Masagoet al., 2004). Note the presence of sandwiched structure of all regional metamorphic belts enveloped by low-grade or weakly metamorphosed units above and below. Also note thelarge vertical exaggeration to show the internal structure of HP–UHP belts. If there is not vertical exaggeration, the UHP–HP belt will appear as an extremely thin line with aspectratio of 1:100.


high-grade unit without any fault boundaries inside, suggesting thatductile deformation prevailed after the peak metamorphic conditionsand the high-grade metamorphic unit ‘intruded’ into the weakly meta-

morphosed or unmetamorphosed geological units. The average exhu-mation speedwas extremely slow frommantle depths of 200 to 100 kmto themid-crustal levels of 10 to 15 km, at about 4 mm/year (Katayama

Fig. 13. Schematic diagram to show nappe tectonics in the Alpine orogeny. Thebasement allochthon by H (Helvetic nappe) was overthrust by P (Penninic nappe)which in turn is overthrust by A (Austro-Alpine unit).


et al., 2001). The present day ongoinguplift ratemeasured inHimalayas,Taiwan and outer Indonesian non-volcanic arc shows only a few cm/year. The exhumation rate of HP–UHP units up to themid-crustal depthis one order of magnitude slower.

9.2. Mountain building

The mountain building stage is the second step triggered bydoming which occurred after the emplacement of regional metamor-phic belts into the mid-crustal level and the development of highangle normal faults surrounding the dome generating large sedimen-tary basins. The best-documented example is the Himalayan moun-tain building caused by Indian collision against Eurasia (Fig. 14). Theinitiation of collision of India against Eurasia started at 50 Ma and UHPmetamorphism occurred at 48 Ma (Kaneko et al., 2003). The UHP–HPunit was emplaced from mantle depth to the mid-crustal level at

Fig. 14. (a)Map view of Indian collision and formation of Indus and Bengal fans, whichwere dboth fans, which have maximum thickness about 5000 m, is similar to the mountain belts inthrough time since 20 Ma for the drill core sediments at ODP site 717–719 shown on (a) (Dethe rapid increase of detrital material in the fan, indicating uplift of the Himalayan mountainUHP metamorphism started ca. 50 Ma followed by retrogressive hydration around 25 Ma.reactivation probably 1 Ma ago.

25 Ma but themountain building stage has not yet begun. At 8 Ma, theHimalayan metamorphic belt uplifted by doming and extensivedevelopment of secondary normal faults together with correspondingformation of Indus–Bengal submarine fan deposit (Fig. 14). Thus, theformation of large folded mountain belts occurs as the final stage ofexhumation and is unrelated to the main stage of exhumation of themetamorphic belts from mantle depths to the mid-crustal level. TheHimalayan mountain belt was reactivated since the last 1 million yearpossibly in response to the southward jump of plate boundary andbuoyant continental crust was underplated beneath the higherHimalayas to push up the Himalayan mountain chain (Sakai, 2002).

9.3. Dynamics of extrusion

The aspect ratio of regionalmetamorphic belts shows ca. 1:100 in thecase of the Himalayanmetamorphic belt, the thickness in only a few kmwhereas the belt continues over 1000s of km along the strike with 200–300 km width. Thus, regional metamorphic belts are thin and platywhich are cut by paired faults: normal fault on the top and reverse faulton the bottom, and the belt is extruded above the Benioff thrust on theconsuming plate boundary. The exhumation velocity is one order ofmagnitude smaller than the plate movement, which has been proposedbyMaruyama (1990) andMaruyama et al. (1994, 1996). These featurescan be attributed to the following three processes. (1) Subduction anglechanges from deep to shallow through time. The subducted wedge-shaped continental material at depths was extruded by shallowing ofthe subduction angle. (2) The rheology shows marked contrast in the

erived from themountain building associatedwith Himalayan orogeny. Total volume ofthe Himalayas above sea level. (b) Modal change of clay minerals and Sr-isotope valuesrry and France-Lanord, 1996). The figure clearly shows that 8 Ma is the turning point forbelt. (c) Elevation of Himalayanmountain belt as a function of time back to 50 Ma. HP toExtensive mountain building started around 8 Ma and gradually dropped with recent


units above and below the regional metamorphic belts. The regionalmetamorphic belt is composed of quartzofeldspathic material whichturns ductile at temperatures above 350 °C, whereas the overlyingwedge mantle is composed dominantly of olivine and the underlyingunit is eclogitic oceanic crust and peridotites which behave as brittlebelow 1000 °C. (3) Rheological change during exhumation of quartzo-feldspathic regional metamorphic belts. The regional metamorphic beltmoves along the Benioff thrust up to mid-crustal level and becomesstagnant at depths of 10 to 15 km because the quartzofeldspathicmaterials transform from ductile to brittle by the temperature decreaseat this stage, becoming harder. The 350 °C temperature divide corre-sponds to the brittle–ductile transition for the quartzofeldspathicmaterial at mid-crustal depths of 10 to 15 km. Thus, regional meta-morphic belts, which behave ductile fluid, are inserted between theoverlying rigid mantle wedge and the underlying rigid slab. When thesubduction angle turns to shallower, then the stress field changes totrigger the wedge to extrude into mid-crustal level. This is the mostessential dynamic process controlling the exhumation of regionalmeta-morphic belts.

9.4. Regional metamorphic belts brought up by buoyancy uplift?

The density of quartzofeldspathic rocks is about 2.8 g/cc which islower than that of peridotites (3.2 to 3.3 g/cc for depths up to 50 to60 km at which stage amphibolite transforms to eclogite with adensity 3.3 to 3.4 g/cc) (Komiya et al., 2002; 2004). These valuescontinue to a depth of 250 kmwith amaximum of 5% variation. Below250 km depth, K-hollandite (density 4.7 g/cc) and stishovite (density4.3 g/cc) become stable. Above 90 kbar, the density of a quartzofelds-pathic rock is larger than the mantle material (Irifune et al., 1994) andtherefore buoyancy does not aid in the exhumation of regionalmetamorphic belts from depth. On the other hand, it turns to becomea strong slab-pull force.

Between 0 and 70 kbar, a quartzofeldspathic rock is buoyant thanmantle peridotites or eclogites. However, the small density differencewould not help the uplift of regional metamorphic belts as a plumewithin overlying mantle wedge (Fig. 15). More difficulties for thebuoyancy uplift mechanism include the size of regional metamorphicbelts and three-dimensional shape which is extremely different fromthe mushroom shape predicted in buoyancy-driven models. Moreover,as discussed in a previous section, the aspect ratio of the metamorphicbelt is around 1:100, and is quite different from the plume shape, andplaty. If size is too small, even though density difference is large enough,buoyancy uplift is not effective. Thickness of about 1 to 3 km isextremely small compared to the whole crust in space. Simplecalculation never supports that it can penetrate into mantle wedgebelow1000 °C.Modeof occurrence of regionalmetamorphic belts in thefield clearly refutes the idea of plume because of the shape.

9.5. The wedge extrusion model

The wedge extrusion model proposes a wedge-shaped viscousfluid (regional metamorphic belt), sandwiched by the underlying slaband overlying mantle wedge on both sides, that extrudes through theshallowing of the subduction angle (Maruyama et al., 1996). Thewedge region A or B on the corner of the triangle (Fig. 15) is assumedto be filled by a non-compressive fluid. The rigid wall OA movesaround the center of point O and extrude the fluid AOB.With regard toboundary conditions, OA and OB behave as either as a free slip orfrictional boundary surface. In a typical case of angle AOB shown inFig. 15, the initial conditions are alpha equals 30°, beta equals 5°, anddepth of the wedge is 200 km. If the angle of the wedge beta turns to0.5°, it would lead to extrusion of material from 60 km depth to thesurface either along a free slip boundary or frictional boundary.

Tomovematerial from a depth of 120 km to the surface, around 80%deformation is required at the wedge triangular region. The average

exhumation speed would depend on the deformation speed in thewedge region. Deformation speed within the wedge is assumed to beconstant. Exhumation speed must be faster at the deeper regions andtends to be slower near the surface. In the above example, theexhumation speed at 120 km depth reduces to one third near thesurface and the material during the exhumation is stretched five times(free slip) to 20 times (frictional slip) along the exhumation direction. Inthe laboratory experimental studies performed by Chemenda et al.(2000) to understand the exhumation mechanism of Himalayanregional metamorphic belts, paired faults were clearly reproduced andthe model of wedge extrusion well illustrated.

9.6. Second stage uplift and mountain building

Wedgeextrusionmechanismcanexhume the regionalmetamorphicbelts along Benioff thrust from mantle depths to the mid-crustal level.However, another mechanism is necessary from there to move thematerial to the surface. This is the second stage process of doming andmountain building by domal uplift of the unit, which is sandwiched asthe core of the regional metamorphic belt. The typical example is theHimalayan mountain building, which started since 8 Ma triggered bythe southward jump of plate boundary. Along the new plate boundary,continental crust began to subduct underneath the Himalayas andjacked up part of higher Himalayas (Takada and Matsuura, 2002)(Fig. 16).

9.7. Modern analogue of regional metamorphic belts and exhumationmechanism

It is interesting to examine why regional metamorphic belts weregenerated episodically and exhumed to the surface to through time.During the last 500 million years along the circum Pacific region suchas Japan and California, glaucophane-bearing regional metamorphicbelts were formed and exhumed to the surface every 100 My on anaverage (Maruyama et al., 1996). New orogenic belts were formedunderneath and/or oceanward through time episodically. In the caseof continent collision-type regional metamorphic belts, although theoccurrence and timing vary among different cases, our synthesisshows that about 30 collision-type orogenic belts were formed on thePhanerozoic earth by continent collision (Fig. 4).

In the case of continent collision, the formation and exhumation ofregional metamorphic belts appear to be related to the size ofcontinent. For example, the size of continents to form UHP unitsshows a wide range from India, North China, and Western Europe tosmall continents such as Madagascar and Java Island in Indonesia(Liou et al., 2000, 2002; Parkinson et al., 2002). In the case of thecontinents smaller than the size of Madagascar or arc, we do notnormally see any example of UHP metamorphic belts. We examinethis aspect in more detail below.

Whether the subducted continental crust is accreted to thehanging wall without subduction into deep mantle, or returnedback to the surface depends on a number of factors including size(buoyancy) of the continent and the degree of coupling between thesubducting slab and the overlying continental crust. The descendingPacific plate immediately before the trench outer wall of the JapanTrench in NE Japan is bent to generate horst and graben structures.The grabens have a maximum depth of 500 m and are a few kmwide,which are then filled by quartzofeldspathic trench turbidites. Thesesediments are being subducted into the deep mantle together withPacificMORB crust (Miura et al., 2003). Buoyancy derived from grabenfilled sediments would not be effective because of the relatively smallsize or volume. However, in the case of intraoceanic arc subduction,such as the Kyushu–Palau paleo-arc and Amami plateau (paleo-arc)with thickness less than 20 km, subduction proceeds into deepmantlewithout any accretion (Kodaira et al., 2000; Yamamoto et al., 2009). Inthe case of Izu Mariana arc with a thickness of continental crust of

Fig. 15. Dynamic process of exhumation of regional metamorphic belt. (a) Buoyancy-driven plumemodel. It seems to be unrealistic that plume-like metamorphic unit penetrates therigid mantle wedge and reach finally to the surface. (b)Wedge extrusion model driven by narrowing of the wedge angle between the subducting slab and mantle wedge. (c) Wedgeextrusion model proposed by Maruyama (1990) and Maruyama et al. (1996) that a ductile metamorphic unit intrudes into shallow-level brittle units through two-steps.


about 30 km, at least some of the intraoceanic arc tends to be accretedwhich indicates adequate buoyancy to overcome the coupling forceexerted by descending slab. Note that buoyancy depends on thevolume, hence if the collision of arc is not vertical or tangential to the

Fig. 16. Jack-up model with ramping of the subducted continental crust, which explainsthe second stage uplift of the Himalayan orogen (Takada and Matsuura, 2002). HFT —

Himalayan Frontal Thrust, MBT — Main Boundary Thrust; MCT — Main Central Thrust.Note the large vertical vector around MCT for the uplift of MCT by the ramping ofbuoyant continental crust underneath, shown by a broken line.

trench, but parallel to the trench, even 20 km thick arc crust can beaccreted to the hanging wall, a process that was presumably commonin the Archean.

We now consider the stress system that promotes the exhumationof UHP–HP metamorphic belts using modern analogues. Regions ofour interest are shown in Fig. 17 which is a world map to show thedistribution of plate boundary. The ongoing exhumation area ofregional metamorphic belt must be along the active consuming plateboundary. In the case of collision-type regional metamorphic belts,the best modern analogue is the non-volcanic outer arc in Timor–Tanimbar region of Indonesia (Fig. 18). Along the non-volcanic arcfrom Timor through Tanimbar to Salem, a U-shaped non-volcanicouter arc continues over 2000 km. Miocene glaucophane-bearingregional metamorphic belts are exposed in southwestern part of thisregion for about 700 km long and in the northwestern part for overanother 600 km, although not in the central part (Ishikawa et al.,2007; Kaneko et al., 2007). To the west of the N–S trending non-volcanic arc, a forearc basin (Weber Deep) is developed with a depthof up to 7000 m. The region itself is underlain by strong extensionalstate (Charlton et al., 1991). On the contrary, the domains to the northand south are underlain by strong N–S compressional state (Fig. 18).

The metamorphic belts towards the SW part, particularly thesouthwestern end, correspond to the late stage of collision orogeny

Fig. 17.World map showing ongoing orogeny along the active consuming plate boundaries of the Earth. Six examples are marked, the details of which are explained in later figures.


and the island itself is uplifted as a dome showing the development ofa number of normal faults on the island to uplift Quaternary reeflimestone above 1300 m. The top of the folded mountain in the Timorreaches 2960 m and shows the highest elevation speed over the world,0.4 to 0.6 cm/year for the last 0.5 million years on an average. To the eastof the outer arc, the elevation rate tends to be small. In the case ofLaibobar Island, 700 km away from Timor, the top boundary ofglaucophane-bearing metamorphic belt is exposed above the sea level(which is a normal fault) and the Quaternary reef has not been upliftedfor the last 1 million year. The difference in terms of uplift clearly showsthe time difference in the stage of orogenic evolution, and confirms thatdomal uplift has not been initiated to the east. On Laibobar Island,minornormal faults were identified which indicate that the domal stage mayhave started very recently. However, the Timor Island has already beenuplifted vertically over 1300 m.Under the Tanimbar Island,which is theeasternmost portion of the non-volcanic arc, domal uplift has not yetbegun and the leading edge of the subducted Australian continentalplate is being broken off from the foregoing descending oceaniclithosphere (Osada and Abe, 1981) (Fig. 18b). The boundary of break-off between continental lithosphere and oceanic lithosphere ischaracterized by high seismicity, vertical to the seismicity in the Benioffplane (Osada and Abe, 1981). The seismological characteristics suggestslab break-off and that the buoyant continental lithosphere cannotsubduct deeper than the present, a constraint posed by the more denseolder oceanic lithospherewhich exerted a higher slab-pull force leadingto the slab break-off. To thewest under the Timor Island, the continentallithosphere is already released from oceanic slab and hence the freebuoyant subducted continental slab promotes exhumation of regionalmetamorphic belts as shown in Fig. 18.

Thus, the process of exhumation is related to slab break-off andresultant change in the subduction angle from deep to shallowthereby promoting the subducted wedge to remove to arc–trench gap(Fig. 18b) (Maruyama et al., 1996). Tanimbar illustrates the stage justafter slab break-off and complete release of slab. On the other hand,the case of Timor shows a further advanced stage of second domaluplift. On the contrary, 100 km north of Timor, the volcanic arc hasalready shut down its magmatic factory to the north of volcanic arc,and a south dipping back thrust is developed (Hamilton, 1979; Harris,2006; Nugroho et al., 2009). The active back thrust disappears to thenortheastern end of Timor indicating that the entire region from the

north of the volcanic arc to the trench is underlain by the strong N–Scompressional state (Fig. 18).

On the other hand, to the west of Timor, the Indo-Australian plateis completely separated from the descending oceanic slab, which issinking down into deep mantle. The subduction angle of theAustralian continental lithosphere has turned shallower therebypushing up the entire Timor Island. The continued uplift of TimorIsland starting from 0.5 Ma has resulted in the present day peak of themountains reaching over 2960 m.

We now consider ongoing example from the Pacific-type orogenicbelts. Whether continent collision-type or Pacific-type regionalmetamorphic belts fundamentally record the same structural pattern;that is, a sandwich structure and two-step exhumation processfollowed by extensive mountain building at the latest stage. Thissuggests that a similar mechanism operates to exhume the regionalmetamorphic belts regardless of their category as collision or Pacific-type. As mentioned in an earlier section, the fundamental control isexerted by the change in subduction angle to shallow values. In thecase of Pacific-type, the shallowing subduction angle and theapproach of mid-oceanic ridge to the trench are well-studied. It isconsidered that in the circum Pacific orogenic belts, mid-oceanic ridgesubduction has occurred once every one hundred million years on anaverage (Engebretson et al., 1985).

Some of the youngest Pacific-type orogenic belts occur in theOlympic Peninsula, southwest of Seattle near Vancouver (Fig. 19a)(Maruyama et al., 1996). This is the region of Quaternary uplift cut tothe east by active normal fault, and to the west by active reverse fault.These two faults seem to be paired and sandwich the thin ‘platy’geologic unit 20 km×100 km in space. The core of this wedge-shapedunit contains low-grade lawsonite-bearing schists formed at about11 km depth (Brandon and Calderwood, 1990). The wedge extrusionhere started in the Miocene to expose the deep-seated portion of anaccretionary complex. Apatite fission track age shows 7 to 11 Ma(Brandon et al., 1988; Brandon and Calderwood, 1990).

The Juan de Fuca plate subducting underneath the Mt. Olympus isvery young, ca. 5 Ma (Engebretson et al., 1985). To the west, the N–Strending active Juan de Fuca ridge is present whichmoves eastward atthe rate of 3 cm/year and which will finally subduct after few millionyears. Considering the last 20 million year movement of the mid-oceanic ridge, the age of subducting slab underneath is gradually

Fig. 18. Ongoing exhumation of continent collision type metamorphic belt in Timor–Tanimbar region of Indonesia (partly modified from Maruyama et al., 1996). (a) An outline oftectonic environment of the Indonesian region. Note the contrasting stress field between the west (compressional) and the east (extensional). (b) Cross section along A–B in theextensional environment. Forearc region is underlain by the compressional stress field whereas the back arc side is dominated by extension to form the 7000 m deep Weber Basin.Beneath the arc, slab break-off is ongoing. (c) Western side N–S cross section of the Timor Island. Note the location shown on (a). The entire region is underlain by strong N–Scompressional stress field and the back thrust is developed on the back arc side. Oceanic slab has been disconnected with the resultant buoyancy of continental lithosphere. Theregional metamorphic belt exposed on the surface is in the second stage of exhumation by doming associated with a number of high angle normal faults.


younging through time. The subducted oceanic slab bends rathersuddenly at 40 km depth of the Benioff plane, which wouldconsiderably promote the wedge to extrude upward as shown in apresent day cross section in Fig. 19.

History of plate subduction in this region supports an approachingridge through time to remove the wedge to the surface. In the case ofthe well-studied Pacific-type blueschist belts of Sambagawa belt inJapan, the emplacement of deep-seated eclogite blueschist belt to themid-crustal level occurred at 70 to 80 Ma which is almost identical tothe ridge subduction of the Kula-Pacific. The transport to the surfaceoccurred afterwards, at about 50 Ma. The example of the Sambagawaindicates that the transport to shallow surface level was after theevent of ridge subduction. The Juan de Fuca plate has only a 20 to30 km thick lithosphere because of the very young age. Therefore atdepths of 25 to 50 km, a number of small earthquakes occur withinthe slab mantle. These earthquakes are derived by dehydrationreaction, similar to the well-studied case in Japan (Hasegawa et al.,2009; Maruyama et al., 2009; Omori et al., 2009). The regionalmetamorphic belts emplaced at mid-crustal level under the forearcregion could have been infiltrated by abundant dehydrated fluid —

both deep subducted sediments and from the downgoing slab.We now examine the case of SW Japan. The age of the Philippine

Sea Plate subducting underneath SW Japan arc is highly variabledepending on the place (Fig. 20). For example, under Kyushu, a very

old portion of the oceanic plate (older than 100 Ma) together with 4remnant island arcs are being subducted (Seno and Maruyama, 1984;Maruyama et al., 2009). On the other hand, in the central Honshu andKanto regions, the oceanic lithospheric is older than 48 Ma as deducedfrom the age of boninite in the Ogasawara for arc (Seno andMaruyama, 1984). The Philippine Sea plate subducts at the interme-diate region from Kanto to Kyushu and the Shikoku Palece–Vera Basinhas an age of 8 to 30 Ma. The youngest part is the region marked byKinan Sea Mount chain, which is being subducted just under the KiiPeninsula. Kii Peninsula is the region where most remarkable uplift isongoing since the Quaternary. Therefore, the central part of the KiiPeninsula has been severely eroded exposing the structural bottomunder the Sambagawa belt. Thus, the Shimanto belt is in direct contactwith the Median Tectonic Line to the north, with the Sambagawa beltmissing in the Central Kii Peninsula. An evaluation of the uplift rate inSW Japan shows that the eastern and western parts were less erodedand the central part of the Kii has been most uplifted. Therefore,apparently, the age of subducting plate is critical; if it is young, thehanging wall seems to be selectively uplifted and eroded.

In the case of NE Japan, the Cretaceous Pacific plate is beingsubducted under the arc. The E–W cross section of NE Japan arc atSendai brings out subducted sediments from high-resolution ofseismic tomography made by Hasegawa et al. (2009). The subductedsediments are present below the Benioff plane at 40–80 km depth,

Fig. 19. Ongoing Pacific-type orogeny in the Cascade region in the western North America (partly modified by Brandon and Calderwood, 1990). (a) Tectonic environment of theCascade region showing the distribution of subduction zones and subducting young Juan de Fuca plate. Note the location of the youngest lawsonite-bearing schist at Mt. Olympus. (b)E–W cross section showing the uplift of Mt. Olympus which is cut on the top by normal fault and on the bottom by reverse fault. Extensive hydration could be present underneathshown by the nest of frequent seismicity from the underlying Juan de Fuca plate.


together with hanging wall of already-accreted sediments. Thehanging wall of metamorphosed unit under BS–EC conditions maybe promoted to uplift along the Benioff plane carrying fragments ofmantle wedge by anticlockwise convection in the wedge cornertriangle (Fig. 21). If this process is ongoing, the exhumation of UHP–HP belt may occur in the case of old-slab subduction.

In the area where younger plates are being subducted in SW Japan,local differences of uplift are also observed. For example, along thestrike of orogenic belts, topographic depression appears periodicallyfrom east to the west. The Ise Bay, Kii Strait and Bungo Strait weretectonically subsided whereas the intermediate region in Kii Penin-sula and Shikoku were elevated.

Recently, microearthquakes termed as tremors have been preciselyrecordedby the recently established seismic network in Japan. This hashelped to carefullymonitor the fluid derived fromMoho depths and totrace the ‘day-to-day’ movement of fluids (Obara, 2002; 2009). Theresults show two regions where no tremors were observed beneaththe Kii Strait and the Ise Bay (Fig. 22a). In spite of the fact thatsubduction-related dehydration occurs commonly underneath shownby slab-seismicity, the fluid has not come out above the Moho depth.The following is a speculation to explain this enigma. Above the Benioffplane in these two regions, high-pressure–low-temperature regional

metamorphic belts are being extruded and emplaced at mid-crustallevel. These units absorb the dehydrated fluid released fromunderneath to produce hydrous minerals. On the contrary, in theintermediate regions such as Kii Peninsula and the central part ofShikoku, hydration completed to extract excessfluids above the depth,with the second stage of doming to form high angle secondary normalfaults. In future, the youngest blueschist belt may appear in both Kiiand Shikoku if this interpretation is correct. We compare two crosssections across eastern Shikoku with no tremors, and central KiiPeninsula with frequent tremors (Fig. 22b). Along the eastern Shikokusection, a topographic anomaly is seen in the forearc called Tosabae.The origin of this topographic anomaly (high) close to the sea level, hasbeen speculated to be due to the presence of subducted seamountunderneath (Kodaira et al., 2000). An alternate interpretation is thatthe subducted accretionary complex was tectonically extruded afterblueschist faciesmetamorphism up tomid-crustal level to push up theforegoing portion, which is now close to the sea level (Fig. 22). The KiiPeninsula and Central Shikoku are now underlain by second stage ofdoming as supported by the tectonic uplift above 350 m for the last0.17 Ma at Muroto cape (Yoshikawa et al., 1964). Average uplift rate is2 mm/year. The sharp V-shaped notch at the south of MurotoPeninsula probably was formed by high angle normal fault along the

Fig. 20. (a) Index map showing the age variation of Philippine Sea plate. Western Philippine Sea on the north is as old as 100–40 Ma. The eastern margin, which subducts under theKanto region, is older than 48 Ma. The central part of Philippine Sea that subducts underneath SW Japan is as young as 30–17 Ma. (b) Cross section along A–A’ (modified after Parket al., 2002). Regional metamorphic belt seems to be exhumed along the splay faults, which may explain the recent remarkable uplift in the Kii Peninsula. Also shown is the topboundary of subducting Philippine Sea plate. The presence of cold seep indicates low-temperature springs enriched in nutrients. Cold seep is the surface expression of splay fault, andthe abundant nutrients feed biological communities.


Fig. 21. A schematic illustration of 3 domains of metamorphic–metasomatic factory (MMF), subduction zone magma factory (SZMF) and big mantle wedge (BMW) under NE Japanacross Sendai (Maruyama et al., 2009, based on seismic tomographywork by Hasegawa et al., 2009). The double seismic planes (hatched) cause a large pulse of dehydrated fluid fromthe descending Pacific-slab leading to viscosity contrast among MMF, SZMF and BMW. Counter-flow convection in the mantle wedge begins immediately above 200 km depth andmoves toward a volcanic front obliquely to release an arc magma under the volcanic front. The boundary between MMF and SZMF corresponds to the culmination point of frequentseismicity at 60 km depth. The enlarged figure shows a contrasting convection flow for MMF and SZMF, the former promoting exhumation of subducted sediments as shown byyellow color including fragments of wedge mantle peridotites (Alpine peridotites).


coastal lines, supporting the idea of domal uplift. Such domal uplift cutby high angle normal faults suggests the underplating of a buoyantmass of accretionary complex below to push up the above. Thebuoyantmass could be younger spreading ridge, or rising large volumeof quartzofeldspathic accretionary complex. A similar process seems tobe ongoing under the Kii Peninsula. Thus, SW Japan might expose theyoungest blueschist belt in future.

10. Relationship to orogeny

Regional metamorphic belts occupy the orogenic core and arehence the most important element of an orogen. The process toproduce regionalmetamorphic belts is complex including generationof the metamorphic belt, subsequent exhumation, emplacement atmid-crustal levels, and finally mountain building which exposes theregional metamorphic units to the surface. Large-scale orogenic beltsregionally affect the already existing continental crust causingwidespread sedimentation in the surrounding regions, accompaniedby arc magmatism to increase the volume of continental crust. Thus,regional metamorphism is one of the most important processes inorogeny.

Orogeny is not an instantaneous phenomenon, but a process thatlasts over a long time of the order of a hundred million years. In thecase of Himalayas, the subduction started at 50 Ma. The beginning ofUHP to HP metamorphism started at 48 Ma (Kaneko et al., 2003),which is very close to the timing of subduction initiation, and it took23 million years from UHP metamorphism at mantle depths to themid-crustal level exhumation at 25 Ma where extensive hydration–recrystallization occurred. Furthermore, dome like uplift and moun-tain building started only after 8 Ma with a large time gap reaching

15 million years. From the time of initiation of metamorphism tomountain building, it took a total of 40 million years. Subsequentlythe plate boundary shifted leading to the rise of the Himalayanmountains since 1 Ma ago. Mountain building is still going on atpresent (Fig. 23).

In the case of Pacific, taking the Sambagawa belt as an example,subduction and underplating of accretionary complex from trench upto 60–70 km depth took only 1 million year, based on the computa-tions of relative platemotion during the Cretaceous time (Engebretsonet al., 1985; Maruyama and Seno, 1986). Regional metamorphism hasits peak at 120 to 125 Ma as demonstrated by zircon SHRIMP age(Okamoto et al., 2004). Exhumation and emplacement at mid-crustallevel occurred between 80 and 60 Ma, where extensive hydrationoccurred. Therefore, it took 40 to 45 million years frommantle depthsto the mid-crustal level. Thereafter, dome-like second step mountainbuilding began to expose the high-grade regional metamorphic rocksto the surface by 50 Ma (Isozaki and Maruyama, 1990). From thesecond stage mountain building, the uplift rate is estimated as 4 mm/year, converting 10 to 12 km uplift during 20 to 30 million years. Thus,the Sambagawa orogeny has taken 70 million years from its beginningto the surface exposure of regional metamorphic rocks.

Pacific-type orogeny in the Sambagawa metamorphic belt duringthe Cretaceous is yet another process. In this case, the formation of thefolded belts may have occurred not only at late stage but also at theinitial stage. To record the earliest Cretaceous event, information fromAsian continents is critical. In the Japanese Islands, information islimited to the presence of large batholith belts. The northern limit ofthe batholith belt is missingwhich is now observed in China. Aswill bediscussed later, in S. America and N. America the initial stage ofCretaceous orogeny is marked by a series of thrust belts developed on

Fig. 23. Tectonic evolution of the Himalayan region from 60Ma to the present (modified after Kaneko, 1997). Before the Indian collision, Pacific-type orogeny prevailed to form paired belts,Cretaceous huge batholith belt on the north associated with contemporaneous BS belt on the south. The Indian continent subducted underneath the southern margin of Eurasia at 50 Ma, theregionalmetamorphism started from48Ma andproceeded to progressiveHP–UHPmetamorphism, followedby slab break-off to trigger the exhumationofUHP–HPbelt intomid-crustal depthby 25 Ma bywedge extrusion process. Asthenospheric upwelling by delaminated slab heated the bottom of thickened continental crust under Tibet, caused eclogitemelting at theMoho depthand erupted adakite lava-flows. Also shown is the top boundary of the UHP–HP unit called STDF (South Tibetan Detachment Fault). The bottom boundary is MCT (Main Central Thrust). Anextensive hydration occurred at 25Ma atmid-crustal level. Second stage doming started around 8Ma to causemountain building to transport huge amounts of sediments to the Indian Ocean.


the northernmost foothill of the mountain belts consisting a majorportion of the batholith belts. If this is the case, Pacific-type orogeny

Fig. 22. (a) Distribution of tremors at the Moho depths in SW Japan (Obara, 2002). Note thenear Ise Bay. Also shown is the isodepth contour of the top of the Philippine Sea plate. (b) Scregion whereas C–D is the tremor present region. Absence of tremor can be explained by thregional metamorphic unit. Tremor is observed after the saturation of exhumed regional m

may have had two stages of mountain building at the initial stage andat the final stage.

absence of tremor in Kii Strait and easternmost part of Shikoku and in the small regionhematic cross section along A–B and C–D. A–B is the cross section for the tremor-absente dehydrated fluids were all absorbed to hydrate the exhumed and water unsaturatedetamorphic belt shown along C–D cross section.


11. Himalayas and Andes: a comparison

In the Himalayas, a 50 million years long history exists prior to thefinal collision-type orogeny (Fig. 23). At present, within the IndianOcean near the spreading ridge, prominent seismicity recordscompressional stress. Therefore, the plate boundary is delineatednot along Main Frontal Thrust in the Himalayas, but within the IndianOcean (Engdahl et al., 1998), which corresponds to the northernboundary of the Indo-Australian plate. If the plate boundary shiftscompletely from MFT to Indian Ocean, the Himalayan MountainRange would not be a mobile orogenic belt, which means the end ofcollision orogeny.

On the other hand, in the central part of the AndeanMountain Belt,mountain building has definitely begun since Miocene at about 25 Ma(Ramos and Aleman, 2000). This was caused by continent subductionof SouthAmerica underneath theAndeanmountain range knownasA-type subduction to jack-up the Andean mountain. This processcorresponds to the mountain building at the initial stage of Pacific-type orogeny (Fig. 24). However, regional metamorphic belt has notyet been emplaced into the core of the Andean mountain, andextensive scale of felsic volcano-plutonism has not yet occurred.Moreover, subduction zone volcanism itself is absent in about onethird of the entire Andean region (Gutscher et al., 1999). Theseobservations suggest that theAndeanmountain range is underlain by amountain building stage, but has not yet reached the activated timedominated by extensive volcano-plutonism and exhumation ofregional metamorphic belts in the forearc region. Presumably, theAndean belt is still in the initial stage over the entire 70 million yearslong Pacific-type orogeny. However, Andean mountain range is wideenough so that a regional provincialismmight be present. For example,at the southern end of the Andean belt, mid-oceanic ridge began tosubduct underneath where TTG granite intruded near the trench only50 km from the trench. Therefore, this local area may have been in thestage of a peak Pacific-type orogeny (Fig. 24c). But the regional meta-morphic belt has not yet been exposed on the surface. Comparing theSambagawa orogeny, the stage at the southern end of the Andeanbelt is the stage before the doming uplift and after the exhumation ofregional metamorphic belts at mid-crustal level. Nevertheless, theoverall stage of the whole Andean belt has not yet reached the secondmountain building stage. In future, a few mid-oceanic ridges wouldreach to the trench to promote the orogenic activity (Fig. 24c).

11.1. Mountain building is unrelated to the exhumation of HP–UHP belt

In collision orogeny, formation of huge mountain belt is restrictedto the final stage. On the other hand, the Pacific-type is marked by twostages of mountain building — the first stage is characterized by thetectonic uplift of volcanic front with simultaneous HP–UHP meta-morphism by A-subduction from the backside down to 60 to 100 kmdepth. The second mountain building occurs at the end of orogeny asdiscussed before.

Regardless of collision-type or Pacific-type, the mountain buildingis unrelated to exhumation of regional metamorphic belts. Mountainbuilding to expose the orogenic core occurs by the domal uplift of thesandwiched units with the core of regional metamorphic belts at theend of orogeny. However, at this period, tectonically eroded portionreaches only 10 km. Formation of folded mountain accompanied byactive erosion generates the remarkable topography of mountainbelts, but corresponding to only about 3 kbar change in metamorphicpressure. It is important to note the extremely large pressure gradientfrom 25–70 kbar down to 3–5 kbar recorded in regional metamorphicrocks, are absent in terms of vertical movement on the surface. This isdue to extremely slow exhumation of regional metamorphic rocksfrom mantle depths to mid-crustal level and because of the very thinsheet-like solid intrusion of the metamorphic belt along Benioff

thrust. Therefore, there is no manifestation on the surface in terms ofvertical movements.

There is an essential difference between collision-type and Pacific-type orogens (Fig. 25). In the case of collision reflecting the differentmechanism of uplift, only the pressure drops at near-constanttemperature at temperature after the metamorphic pressure reachesthe peak. On the other hand, both pressure and temperature decreasetogether in the case of Pacific-type. This may be due to the fact thatoceanic plate subduction was simultaneously ongoing when the HP–UHP units are being removed tectonically to the mid-crust, althoughneed to verify in future (Fig. 25).

12. Concept of paired metamorphic belts

In 1961, Miyashiro proposed metamorphic facies series and pairedmetamorphism based on the geology of Japan. These two conceptshelped the establishment of plate tectonics in 1968. However, theconcept of paired metamorphic belts has been considerably modifiedsince then (Santosh and Kusky, 2010; see also Brown, 2010-thisissue). For example, collision-type regional metamorphic belts are notpaired at all. No counterpart of low P high T metamorphic belt ispresent. Also, granitic batholith belt is absent in general. In the case ofAlps and Himalayas, minor post-orogenic granites intruded at the endof the orogeny. It should be noted that oceanic plate subduction is anecessary phenomenon prior to the final collision of continents in thecase of collision-type orogeny. Therefore, Pacific-type accretionarycomplexmust have been formed to the north of collision type orogenyin the case of Himalayas. In this case, proportional to the total lengthof subducted oceanic plate, size of accretionary complex and size ofbatholith belts must vary. To the north of the Himalayan MountainRange, a huge batholith belt and small glaucophane-bearing meta-morphic belts are observed both of which are ca. 100 Ma old andformed prior to the Himalayan collision (Maruyama et al., 1996).These paired rocks would belong to Pacific-type orogen, and not to thecollision type. On the other hand, there is no batholith belt in the Alps.In the case of collision type orogen, there is no heat source to producehigh-T low-P type metamorphic belts.

In the case of Pacific-type orogenic belts, the presence of low-P andhigh-T regional metamorphic belts is rather exceptional. If present,both belts were not exhumed to the surface at the same period. Forexample, in the Sambagawa–Ryoke paired metamorphic belt,regarded as the world standard, the Ryoke belt runs side by sidewith the Median Tectonic Line (MTL) on the map view, with Ryoke inthe north and Sambagawa in the south, in general. But in the threedimensions, the Ryoke belt rests above the Sambagawa with a sub-horizontal fault called as paleo-MTL (Isozaki and Maruyama, 1991).The boundary fault was formed at 15 Ma when Japan Sea opened andthe upper Ryoke belt was thrust over the Shimanto–Sambagawa andJurassic accretionary complex from north to southward. Thereafter, inKii Peninsula and Shikoku, particularly the regions towards theeastern part were preferentially uplifted to erode out the upper Ryokebelt. As a result, the apparent distribution of the Sambagawa to thesouth and Ryoke to the north appear to give an impression of pairedbelts. Yet, on the westernmargin of Shikoku, whole Kyushu and Kantoplain, top and bottom relationship of the Ryoke and Sambagawa arepreserved very well. The apparent arrangement of paired belt is alsorecognized in New Zealand and Celebes. Hida, Sangun and Hidaka arealso exceptional examples of paired belts over theworld. In the case ofPacific-type orogenic belts, huge TTG belts on the continent side arepaired with low-temperature high-pressure type regional metamor-phic belts oceanward. In this case, batholith belt is paired with high-pressure low-temperature regional metamorphic belts, and not withlow-pressure high-temperature metamorphic belts. Finally we con-clude that Miyashiro's concept of pairedmetamorphism is correct, butincorrect for the simultaneous uplift to expose paired belts.

Fig. 24. Tectonic map of Andean orogenic belt showing E–W cross section (A–B, C–D). Also shown is the distribution of thrust on the eastern foothill of the Andean mountain belts(Ramos, 1999). Subducting mid-oceanic ridge is present only at the southern margin of the Andean mountain. As shown in A–B cross section (Ramos et al., 1996), a series of thrustspush up the Andean belts by A-subduction of South American continent underneath the Andean belt. Regional metamorphic belts may have been formed along the southern marginof Andean belt, but have not yet been exposed to the surface (shown along cross section C–D). Red triangles denote volcanoes.


13. Fluids and regional metamorphism

The major cause of regional metamorphism has long been con-sidered as an effect of the increase of P and T. However, the evolutionand migration of fluids have been highlighted in recent studies and

perhaps played the most crucial role (e.g., Santosh et al., 2009b). Thezircon crystals formed during metamorphism include high-T low-Pminerals such as magmatic origin at the core e.g., quartz, albite whichare mantled by the newly grown zircon during the increased P–Tconditions. This in turn includes coesite, jadeite and diamond in the

Fig. 25. Schematic diagram to show the slightly different exhumation processesbetween Pacific- and collision-types. The fundamental process of two-step exhumationis identical for both, but P–T time path is systematically different. There is much deepersubduction in the case of continent-collision compared to the Pacific-type. The nearlyisothermal decompression for the collision type is slightly different from the Pacific-type. The difference may be explained by subduction and refrigeration in the case ofPacific-type.


case of UHP rocks. The outermost margin includes low-P mineralsagain such as plagioclase. This observation demonstrates that thequartz and feldspar at the core were not converted to the high-pressure and high-temperature minerals even though extremelyhigh-P and high-T conditions prevailed over geologic time. Moreover,UHT and UHP minerals included within zircon crystal have notchanged to low-P and low-T minerals even after their transport toshallow level. Shielding of those minerals within robust minerals likezircons prevented their phase transformation in spite of the drasticchange of P–T conditions over geologic time.

On the other hand, matrix minerals in UHP rocks have almost allcompletely transformed to low-pressure minerals. All of theseobservations support the critical role of fluids in mineral phasetransformation. Absence of fluids was the major cause for the absenceof phase transformations of the inclusions within zircon crystals. Asimilar phenomenon has also been observed in the case of mineralinclusions in garnet and omphacite. These minerals appear to beshielded and protected from the fluid flow along grain boundaries orcleavages and cracks within UHP rocks.

We will now examine the outcrop scale of fluid infiltration in UHPterranes. In the case of UHP rocks, whichwere taken down to depths of

100 to 200 km depths and witnessed temperatures of 1000 °C, someigneous protoliths composed of anhydrous minerals preserve mag-matic texture and minerals in spite of the UHP and UHT conditions(Fig. 26). Some gabbroic rocks in HP–UHP belts in China and Alpspreserve olivine and plagioclase despite their conversion to eclogiticassemblages, because of absence of fluids in gabbro.

Garnetite transforms from gabbro and is completely recrys-tallized under UHP conditions. At mid-crustal levels of 10–15 kmdepths and at temperature of 400–650 °C, these rocks recrystallizeto amphibolite through external fluid infiltration. If the garnetiteis large enough, the core of the unit would remain as garnetitesuccessively mantled by garnet amphibolite and amphibolite.However, if fractures cut through, the central garnetite domain isalso is amphibolitized. Similar examples are also seen in the caseof quartzofeldspathic gneisses surrounding eclogite where ex-treme ductile deformation and completely hydration obliteratingthe UHP event can be seen. All of these observations clearlyindicate that fluids play a more critical role in controlling mineralstability and ductility for quartzofeldspathic country rocks than Pand T increase.

14. Earthquake at subduction zones

The discovery of UHP metamorphic rocks and their petrologic andtectonic characteristics have revolutionized the concepts of regionalmetamorphism, particularly focusing on the evolution and migrationof fluids. The link between earthquakes and metamorphism in con-suming plate boundaries became increasingly apparent. In thesezones, subducted hydrated slab dehydrates with increasing pressureand temperature and the fluid liberated would remarkably influencethe rheological properties and stress fields (Peacock, 1993, Omori etal., 2002, 2009; Hacker et al., 2003 etc.). In other words, the birth offluid and its transportation translates to triggering of earthquake,based on the concept of dehydration embrittlement (e.g., Meade andJeanloz, 1991; Omori et al., 2009). Metamorphic petrologists canstudy the fossil evidence for earthquake within regional metamorphicbelts and evaluate the dynamics of orogenic movement. Subductionzones along which frequent earthquakes occur are in fact the locationof ongoing progressive metamorphic reactions (Maruyama et al.,2004; Omori et al., 2009).

15. Fluid circulation characterizes the plate boundary

Distribution of earthquakes is confined only to the plateboundary (Fig. 27a). Fluid circulation is restricted only to theplate margin of consuming plate boundary from which fluid movesdeep into the earth's interior at least covering the whole uppermantle down to 660 km depth. In the divergent boundary, fluidscome up from mid ocean ridge magma, which in turn originated at410 km through a curtain-like mantle upwelling (Zhao, 2004).Fluids may have been derived from the Earth's interior to thesurface through superplumes (see Maruyama et al., 2009; Santoshet al., 2009b). This observation indicates that regional metamor-phism occurs only along the plate boundary where earthquakesoccur. For the other regions such as under the continents andwithin ocean plate, no free fluids are available except hotspots andhence no recrystallization proceeds. Even under the continents inthe lower crust or below the Moho, even if temperature conditionsexceed 700 °C, no metamorphic recrystallization must prevail iffluid is absent. A contrasting picture emerges in the distribution ofearthquakes between mid-oceanic ridge and continental rift. In theformer case, earthquakes are present down to 660 km depth, but inthe latter, only at depths shallower than 30 km along the transformfault boundary, earthquakes are generated at depths shallowerthan 15 km. These contrasting differences in depths correspond to

Fig. 26. Partially eclogitized metagabbro, which preserves igneous mineralogy and textures which survived UHP metamorphism (after Mork, 1985). (a) Sketch of the domain inwhich igneous mineralogy and texture are relatively well preserved. (b) Sketch of more advanced stage of metamorphic recrystallization. (c) Schematic diagram to show the gradualchange from gabbro to eclogite in mineralogy and texture with progressive metamorphism and eclogitization.


the depths of circulating fluids. The physical differences of fluidcirculation depend on the plate boundary.

16. Metamorphic recrystallization in the whole mantle

If the circulating fluid is the only factor to cause regional meta-morphism as documented clearly in UHP–HP metamorphic rocks andzoned zircon which contains quartz, graphite, albite inclusions in thecore and rim, with mantle including diamond and coesite and/orjadeite, the site of ongoing recrystallization in the whole mantle scalecould be envisaged as shown in Fig. 27. The cross section of the Earthis schematically shown cross-cutting Pacific and African superplumetogether with the western Pacific triangular zone with double-sidedsubduction zones, to make it clear for circulating fluids to recrystallizethe mantle and related rocks. According to the double-sidedsubduction, the upper mantle in the western Pacific triangular zoneis being recrystallized most predominantly among whole uppermantle region, and by the same reason the lowermost mantle couldpreserve the lowest temperature of ca. 2300 °C at CMB (core–mantleboundary) (Maruyama et al., 2007).

In addition, the two sites of rising superplume could bring recrys-tallization from the bottom to the top by the presence of fluids derivedfrom liquid outer core, because of the presence of light elements suchas C, H, O, and S, which occupy ca. 10% of the outer core, and areoversaturated in the growing solid inner core (Maruyama, 1994;Maruyama et al., 2007). Below the mid-oceanic ridge down to ca.30–40 km depth, rising mantle flow would release magmatic melts tocause the recrystallization to reset the isotope equilibration.

On the other hand, the fluid-absent zone such as the bottom ofcratonic continents would not be recrystallized even at T over 700 °C,nor will there be any isotopic re-equilibration. Hence the radiometric

ages remain as before. If this is true, the orogenic peridotites may retainold mantle convection history even at CMB level, preserved in withinmineral inclusions of zircons.

17. Subduction zone geotherm through geologic history and theshrinking forbidden zone

The subduction zone geotherm must have changed through time,according to the cooling Earth since its birth at 4.6 Ga in the planetaryspace. This issue has long been considered by several authors based onthe P–T estimates of regionalmetamorphic belts over theworld, eitherPacific-type or collision-type. Here we summarize the concept basedon the results presented inMaruyama et al. (1996) andMaruyama andLiou (2005).

Since the pioneering work by de Roever (1957), several authorshave tried to confirm the geothermal gradients at subduction zonethrough time, and all of them recognized a clear trend of secularvariation with time (Fig. 28). The maximum T limit for regionalmetamorphism has been less than 900–1000 °C throughout Earthhistory. However, the maximum-P condition increased from 2.0 GPabefore 1000 Ma to N6.0 GPa after 630 Ma. Most metamorphic terranesolder than 1000 Ma show maximum-P conditions less than 1.0 GPa,indicating that Precambrian metamorphism occurred within crustaldepths and slab-melting to produce TTG (tonalite–trondhjemite–granodiorite) rocks that dominate the continents (Martin, 1993). Thisis because awide P–T field of slab-melting lies above thewet solidus ofhydrated MORB (Fig. 28).

All recognized UHP belts are younger than 630 Ma, and thesubduction zone metamorphism to form UHP belt became possibleonly after the temperatures became cold enough to inhibit slab-melting (Fig. 28). It should be pointed out that the subduction zone

Fig. 27. a): Global distribution of earthquakes (shown by dots) and plate boundaries (modified after Seno, 1995). The distribution of earthquakes shows an excellent correlation withthe plate boundary such as mid-oceanic ridges, continental rifts and transform faults. The upper mantle beneath the western Pacific shown as shaded region is dominated by watersupplied by double-sided subduction zones from the east and from the south (see Maruyama et al., 2007). b): The cross section of the Earth along the solid curve in a), indicatingongoing recrystallization of mantle by fluid circulation (blue color). Recrystallization, which corresponds to ongoing regional metamorphism, may be highly restricted as shown bythe cross section. Even in the deep mantle recrystallization may be possible only by fluid migration from the liquid outer core. See more details in the text.


geotherm is nearly parallel to the wet solidus of MORB (Maruyamaand Liou, 1998). This probably caused the sudden appearance of UHP–HP belts after 630 Ma.

Following the secular change of subduction zone geotherm, asrecorded in regional metamorphic rocks shown in Fig. 28, the

forbidden zone in P–T space at low-T and high-P side has reducedthrough time. The region was wide enough to cover a major part of P–Tspace belowMORBwet solidus before 1000Ma, but got rapidly reducedtowards the low-T side after 630 Ma. This change allowed lawsoniteeclogite to appear first since 520 Ma along the eastern margin of

Fig. 28. P–T diagram showing the decrease of subduction zone geotherm throughgeologic time (modified after Maruyama and Liou, 2005). The summary of P–Tconditions are estimated from about 300 metamorphic belts over the world throughtime back to the Archean (ca. 4.3 Ga) (summarized after Maruyama and Liou, 2005).Subduction zone is the coldest region in the mantle at anytime in the geologic past. Thecoldest region shown in the P–T space has reduced towards the lower-T side throughtime, as shown by the shrinkage of the forbidden zone. The Archean-Proterozoicforbidden zone was widest with an extensive slab-melting. The reference frames of P–Tgrids such as coesite-quartz, diamond-graphite and jadeite+quartz=low-albite orhigh-albite, MORB solidus under dry or wet condition, andmetamorphic facies grids areafter Maruyama et al. (1996).


Australia (Watanabe et al., 1997). Another example is from Guatemala(Tsujimori et al., 2006).

18. Postscript

It has already passed nearly 30 years since coesite-bearing UHProcks were first reported from collisional belts in Europe, but severaldebates continue. The UHP belts constitute only a very minorcomponent among the various metamorphic belts of the world.However, this trivial “noise” has radically revised many of theconcepts and framework of metamorphic petrology established overthe last 100 years and expanded the realm of metamorphic petrologyto include many subjects including deep Earth geophysics. One of themajor advancements in metamorphic petrology was the understand-ing of the role fluids and the critical role they play in mineraltransformation. Indeed, crustal rocks subjected to extreme metamor-phic conditions were the key for this paradigm shift, with newerinformation on ongoing processes in active plate boundaries addingmore fuel to the debate.

Acknowledgments

Prof. A. Miyashiro was a pioneering worker who systematizedmetamorphic petrology, combined with tectonic settings, back in late1950s. He was born in Japan, and contributed to Earth Science in Japan.

Unfortunately the Japanese scientific community lost him from Japan in1965ashe spent the latterhalf of his academic years inU.S.A, developinghis field to cover whole Earth, particularly ocean-floor petrology. WeJapanese would like to appreciate his highly intelligent and thoughtfulworks written in Japanese first before the articles in English werepublished. All of these contributions educated Japanese, and developedunderstanding of the consuming plate boundary around Japan and thewhole Earth.

We would like to thank Prof. M. Santosh for his valuable helpduring the preparation and revision of this manuscript. We also thankan anonymous referee for thoughtful comments.

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