introduction to special issue on granites and rhyolites: a commentary for the nonspecialist

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 86, NO. BII, PAGES 10131-10135, NOVEMBER 10, 1981

Introduction to Special Issue on Granites and Rhyolites' A Commentary for the Nonspecialist

FRED BARKER

U.S. Geological Survey, Denver Federal Center, Denver. Colorado 80225

WHY GRANITES AND RHYOLITES?

The siliceous, potassic igneous rocks that contain more than ~69% SiO2 and typically 3.5-5.5% K20--the granites and rhyolites--now follow basalts in their importance to earth scientists. Though the 1970's could best be termed a 'Decade of Ba- salts' [Hart and Allegre, 1980], an increasing number of earth scientists are now turning their attention to the more siliceous rocks. The present collection of papers on granites and rhyo-

containing 25% of other minerals need contain only 15% quartz to fall into the granite field (Figure 1). This petrogra- phic definition of granite thus precludes exact chemical definitions.

Rhyolites pose a different problem in terminology because most are partly to largely glassy and so cannot be classified by proportions of mineral phases. A chemical classification is necessary. A useful yet simple one, shown here as Figure 2, is

lites includes geological and geophysical field studies, physical based on SiO2 and K20 abundances and was proposed by and chemical experimental studies, analysis of both rocks and . Peccerillo and Taylor [1976] and modified by Ewart [1979]. As

the reader would judge, the correspondence of chemically de- the constituent minerals for major and minor elements and various isotopic ratios, and consideration of pertinent plate tectonic processes and environments. Processes Which generate granitic-rhyolitic melts and mechanisms of emplacement of these melts in the upper crust or at the earth's surface are of paramount interest here. This special issue is a sample of modern research and concepts about granites and rhyolites.

NOMENCLATURE

Because the definitions of granite and rhyolite have been changed recently, terminology is discussed first.

AlmOst all granites are intrusive and most rhyolites are ex- trusive. However, a classification that is independent of mode of occurrence is preferred and so granite is defined petro- graphically. It is a wholly crystalline rock whose major mineral constituents are 1-25 mm in average dimension (finer- grained varieties are termed felsite or aplite; coatset varieties are pegmatite) and it contains essential alkali feldspar, quartz, and plagiøclase. Commonly it also contains one or more varietal minerals such as biotite, hornblende, muscovite, sodic, amphibole, or others and accessory phases •uch • magnetite, ilmenite, apatite, zircon, allanitc, garnet, or others. The history of rock classifications, definitions of a multitude of pet- rographic terms, and a now little-used but internally consis- tent system of classification are given by Johannsen [1939, 1932, 1937, 1938]. Many modem workers, however, use the more recent International Union of Geological Sciences (IUGS) classification [Streckeisen, 1976]. The IUGS definition of granite and similar holocrystalline, SiO2-saturated rocks is shown in Figure 1.

The IUGS classification allows granite to contain as much as 90% of minerals other than quartz and feldspars. Few granites, though, contain more than about 20% of other minerals. The reader should keep in mind that in this scheme the pro- poRions of the three defining minerals are relative and that their absolute contents in a given granite vary inversely with total content of other, nonessential (varietal and accessory) minerals. Thus a granite comprised of only the two feldspars and quartz must contain at least 20% quartz, whereas a granite

fined rhyolite and of granite defined by proportions of minerals is haphazard. Classification of these rocks remains a minor problem.

Many petrologists and geochemists further separate granites, rhyolites and related rocks into the following three groups by molar proportions of several major constituents [Shand, 1951]:

peraluminous: A1203 > Na20 + K20 + CaO metaluminous: Na•O + K20 < AI•O3 < Na•O + K•O

+ CaO

peralkaline: AI•O• < Na20 + K•O

Two other classification schemes of igneous rock suites deserve mention. The CIPW rock norm classification [Cross et al., 1902; Johannsen, 1939] is based on the calculation (by weight proportions) of a complete set of hypothetical normative 'minerals' that include quartz (SiOn), orthoclase (KA1Si•Os), albite (NaAISi•Os), anorthite (CaAl•Si•Os), corundum (AI20•), enstatite (MgSiO3), and others. The CIPW classification generally is better applied to volatile-poor basaltic systems [see Morse, 1980] than to most granite or rhyolite suites, but it has proven very useful to experimental petrologists in describing relatively simple artificial systems, such as the classic investigation by Tuttle and Bowen [1958] of the granite-H•O system at low to moderate water pressures. Some workers also use the CIPW scheme to describe granites or rhyolites as corundum-normative or diopside-normative (i.e., showing corundum or diopside as a calculated normative 'mineral').

Another notable descriptive scheme is that of Peacock [1931], which is used to classify suites of igneous rocks that range from mafic or intermediate to siliceous compositions. Plots of Na•O + K•O versus SiO• and of CaO versus SiO• are superposed. The SiO2 content at which the two lines intersect is termed the alkali-lime index (ALI). Peacock distinguishes four groups of rock series on this index: ALI< 51 --'alkali½ (alkaline), 51 <ALI < 56 alkali-calcic, 56 <ALI < 61 = calc- alkalic (ca•lc-alkaline), and ALI> 61 calcic. Peacock's classification is more applicable to rock suites formed by closed-sys-

This paper is not subject to U.S. copyright. Published in 1981 by tem crystal-liquid fractionation of a basaltic or andesitic pa- the American Geophysical Union. rental magma. The terms 'calc-alkaline' and 'alkaline' still are

Paper number IBI015. 10131

10132 BARKER: INTRODUCTION TO GRANITES AND RHYOLITES

Quartz

6O

ALKALI-FELDSPAR

GRA E

20/ / ! \ \ •,.QUARTZ

10 35 65 90 Alkali feldspar Plagioclase

Fig;. 1. Ternary diag;ram in volumetric proportions after $treckei- sen [1976] showing; compositional fields of g;ranit½ and other rock types. Proportions of other minerals, not shown here, may range from 0 to 90% of the rock.

widely used. These terms and others are used in the extended chemical classifications in the papers of Irvine and Baragar [1971] and Miyashiro [1974].

GENERATION OF GRANITIC-RHYOLITIC MAGMAS

Granitic-rhyolitic magmas typically are generated in re- sponse to thermal and tectonic processes in the mantle. To date no one has demonstrated that such magmas may form in the mantle. Indeed, much experimental and geochemical work indicates that such siliceous liquids cannot originate there (e.g., Stern and Wyllie, Huang and Wyllie, this issue). Therefore, such magmas must be generated in the crust, al- though some may contain material of immediate mantle derivation (see, e.g., DePaolo, this issue). Furthermore, though rhyolite does occur in oceanic environments, such as the low- K rhyolites of some island arcs and the normal to alkaline rhyolites located near some oceanic spreading centers ($i- gurdsson [1977] makes an excellent case for generation of Ice- landic rhyolite by partial melting of plagiogranite of the oceanic crust)--they are miniscule in amount as compared to the granites and rhyolites of the continents. The latter occur in environments ranging from accretionary prisms at continental maxgins to continental margin magmatic arcs and fold belts to extensional regions and hotspots in continental interiors.

The origin of granite and rhyolite has been debated for the last two centuries. Three general processes have been suggested, as shown in Table 1. These are: (1) partial melting of pre-existing solid rocks; (2) closed-system crystal-liquid fractionation; and (3) coupled fractionation-melting, or open-system fractionation in which a mafic or intermediate liquid may extensively react with, or incorporate, the immediate wallrocks or roof. A subsidiary process•liquid state differentiation-- may operate to produce significant compositional gradients in many magma chambers (Hildreth, Bacon et al., Mahood, this issue).

Partial Melting

The concept of partial to complete melting of crustal rocks to produce granitic-rhyolitic magmas was extensively dis-

cussed early in this century. It was approached from two very different points of view. One considered the Precambrian crystalline complexes of granitic to tonalitic gneisses, mica schists, amphobolites, and other intensely deformed and metamorphosed rocks intruded by siliceous magmas ranging from quartz diorite to granite. Sealerholm [1907] gave us a classic example of such a complex in the mid-Proterozoic rocks of southern Finland and coined the word 'anatexis' for

the partial to complete (?) melting of a siliceous gneiss to produce granitic magma. The second approach to partial melting involved the geosyncline or fold belt. Loewinson-Lessing [1911] described folding of a 10- to 15-kin-thick filling of a large sedimentary basin wherein sufficient crustal thickening occurred that the lower parts of the basin reached melting temperatures. Fusion of shale, sandstone, and other sediments produced granitic magmas. These rise diapirically or stope their way into the overlying rocks.

Modem studies of crustal melting use the pressure-temper- ature constraints established by experimental petrology. Hart and •lllegre [1980] summarize much recent work, emphasizing the use of O, St, Nd, and Pb isotopes in evaluating both source rocks and processes. Partial melting in this issue is discussed by Bickford et al., Cullers et al., Czamanske et al., Hill et al., Le Fort, Shaw, and Flood, and Wyborn et al.

Australian workers led by Chappell and White [1974] have given a large impetus to consideration of partial melting processes. In the Lachian fold belt, southeastern Australia, they distinguish two major types of granite by source material---S type and I type. The S type is derived from sedimentary rocks (actually metamorphosed shale or pelitic schist for all Austra- lian S type granites), and the I type granites are derived from igneous rocks that may range from gabbroic to granitic compositions. Both types are intruded as magma or mush con- sisting of various proportions of melt and residuum (or 'res- tite'). S type granites are biotite-muscovite-bearing and chemically are strongly peraluminous. I types, by contrast, are metaluminous, biotite or biotite-hornblende granites. The S type designation applies to similar granites in other parts of the world (see, e.g., Kistler et al., Le Fort, Lee et al., this issue), typically with only minor changes in Chappell and White's [1974] original definition. However, there are other, far more voluminous granites of sedimentary origin, mostly in the Precambrian shields, that do not fit Chappell and White's classification. These granites are derived from graywackes--

6 i

(BANAKITE)

-- HIGH-K ANDESITE

ANDESITE

L'•--K ANDESITE' I

HIGH-K DACITE

HIGH-K RHYOLITE

DACITE

(talc-alkaline)

LOW-K DACITE

I 63 65 69 70

SiO 2 (WEIGHT PERCENT)

RHYOLITE

LOW-K RHYOLITE

I I 75

Fig. 2. Classification of andesite, dacite, and rhyohte by K20 and SiO2 contents [after Ewart, 1979].

BARKER: INTRODUCTION TO GRANITES AND RHYOLITES 10133

TABLE 1. A Hypothetical Scheme for the Generation of Granitic-Rhyolitic Magmas

General Process

Partial Melting Crystal-Liquid Fractionation Combined Melting-Fractionation

Geological environment Fold belts, accretionary prisms(?), Midocean ridges, oceanic and Continental margin magmatic arcs, Precambrian greenstone belts, continental intraplate magmatism continental. rifts, continental old metamorphic and plutohie or hotspots, island arcs, oceanic hotspots, locally in accretionary complexes. and continental rifts, other prisms, probably in other

environments. environments.

Basaltic liquid of olivine- or Marie melt plus crustal melt less quartz-tholeiitic type, or precipitated liquidus phases and andesitic liquid in some less precipitated refractory environments. crustal material; crustal melt

from any crustal rock containing K•_O, Na•_O, SiOn_ (other components are incidental).

Heats of crystallization of magmatic (liquidus) phases of mantle- derived marie magma, locally (or commonly?) superheat of mantle- derived magma, crustal rocks initially about 200ø-600øC.

Mantle-derived marie magma pools in crust at deep to shallow levels and (1) while precipitating liquidus phases reacts directly with crustal rocks, partly melting and incorporating them, partly reacting with and precipitating their refractory parts, or (2) simply heating and melti• nearby crustal rocks, or (3) mixing of liquids produced in 1 and2.

Metaluminous (approaching peral - kaline) and peralkaline types of high Fe?Mg, low H•_O activity from 1 and 3 above; 2 above produces types as under 'partial melting' at left.

Source materials

Heat source

Derivation of liquid

Types of rocks produced

Graywacke, pelite (S type liquids), volcanic rocks, metamorphic complexes, plutonic complexes.

Depression of source rocks into higher geotherms.

Partial melting of source rock and then (1) separation of liquid from refractory residue or (2) ascent of liquid-residue mush.

S type from pelite only; trondhjemite from metabasalt; minimum melt to calc-alkaline-trend type from other sources.

(Self-contained.)

Separation of olivine, pyroxenes, plagioclase, and other phases from increasingly siliceous liquid: e.g., 100 parts basaltic liquid yield 8-10 parts granitic liquid.

Granophyric-textured, high Fe/Mg types common; syenitic-trachytic types also common.

See text for details and references.

often of volcanogenie type•and so yield partial melts of met- quartz in diabase dikes and sills; and the soda rhyolites that aluminous to mildly peraluminous character. An excellent ex- are found with trachyte and other moderately alkaline lavas ample is given by Arth and Hanson [1975] in their chemical of some near-ridge oceanic islands (e.g., as at Easter Island modelling of the late Archean Vermilion and Giants Range [Baker et al., 1974]). Many years ago Grout [1926] calculated Granites of northeastern Minnesota. The I type designation of that a moderately potassic basalt (K20 -- 1.52%) could yiel d a Chappell and White, however, has not met with similar wide- maximum of about 10% granite by crystal-liquid fractiona- spread application (see discussion in Czamanske et al., this is- tion. sue). Controversy arises as to the scale of the fractionation of ba-

salt to granite and rhyolite. Can granite stocks and batholiths Closed-System Crystal-Liquid Fractionation several to many cubic kilometers in volume or continental

The second major mode of origin of siliceous magmas is by rhyolitic ash flows of similar volume form by the closed-sys- crystal-liquid fractionation or differentiation in which the tem fractionation of basaltic liquid? Bowen [1922a, 1928] pro- magma does not interact with any of the enclosing rocks or posed a 'reaction series' of minerals that successively crystal- fluids. In this process minerals that are less siliceous than the lized and separated from a primary or mantle-derived basaltic liquid crystallize and either settle or float out of the magma liquid. This series commenced with olivine and calcic plagio- and thereby cause the remaining liquid to become more si- ½lase and terminated with potash feldspar, muscovite and liceous. This mechanism was first suggested by Darwin [1851] quartz, and led to production of calc-alkaline granitic-rhyo- and has since been applied extensively to many crystal-liquid litic liquid. Bowen [1948] further suggested that most granites systems of marie to intermediate composition. There is little were produced by such fractionation and that assimilation of doubt that basaltic liquids fractionate to yield granitic ½ompo- crustal rocks by the liquid was of minor extent. Bowen's influ- sitions: examples include the interstitial rhyolitic glasses (SiO2 ence was pervasive and a generation of geologists•except for 75.0-75.9%, K20 5.5-6.0%) formed by crystallization of basalt the quasi-rational granitizers who contended that fluids or (SiO2 50-51%, K20 -- 0.5-0.6%) of the Makaopuhi and Alae clouds of ions formed granites•tacitly agreed with him. To lava lakes, Hawaii [Wright and Okamura, 1977]; the world- date, however, no one has demonstrated that any granite or wide occurrence of a few percent of scattered dikelets and rhyolite mass of several cubic kilometers or larger has formed pods of granophyre or intergrowths of alkali feldspar and by simple fractionation. There are several problems posed by

10134 BARKER: INTRODUCTION TO GRANITES AND RHYOLITES

fractionation on such a scale that have not yet been fully re- solved: one involves the small yield of siliceous liquid from basaltic liquid and the mechanics of its aggregation into large magma bodies [Holmes, 1936]. Another involves maintaining reservoirs of basaltic liquid in the crust of sutiicient size to generate granitic batholiths 2 to 8 km thick and 5000-10,000 km 2 in area, such as the Pikes Peak batholith and the Wiborg granite massif of southern Finland and Russian Karelia. The intermediate to ultramafic residual material from such a large fractionation event would be of 25- to 100-km aggregate thickness. If such residua existed, have they all sunk back into the mantle? Lastly, isotopic studies of some granitic rocks long thought to be generated by simple fractionation indicate that they contain moderate to large fractions of wallrock material. A prominent example is the late-stage granophyre of the Skaergard intrusion, East Greenland [Leemah and Dasch, 19781.

Coupled Fractionation-Melting

The third major mode of origin of granite and rhyolite is a coupling or combination of crystal-liquid fractionation and melting of the enclosing crustal rocks (Table 1). As long ago as 1914, Daly speculated that superheated basaltic liquid might assimilate large fractions of crustal rocks (his 'abyssal assimilation'). In notable papers a few years later, Bowen [1922b, 1928] set forth the principles controlling reaction of magmas with inclusions. In particular, he pointed out that heats of crystallization of liquidus minerals--rather than superheat-would be the major energy source for the melting of siliceous to intermediate crustal inclusions in basaltic or ande-

sitic liquids. Thus much of the heat originally supplied by a mantle reservoir in heating ultramafic mantle material to produce basalt is transferred by injection of that basaltic liquid into the crust and it becomes available for the melting of crustal rocks. Bowen, however, did not assign a major role to this mechanism for the production of granitic-rhyolitic liquids, as mentioned above. Holmes, however, did this in a notable but neglected paper of 1931, using as examples the largely bimodal gabbroic and dioritic-granitic Hebredean subvolcanic complexes of northwestern Scotland. Wager et al. [1965] re- vived Holmes' model for the shallow granites of the Western Red Hills complex, Skye. Strontium isotopic ratios indicate that these granites, indeed, were derived largely from Pre- cambrian basement rocks [Moorbath and Bell, 1965].

The Hebredean granite bodies are roughly 10-100 km 3 in volume. Serious application of mantle-derived, basaltic heat sources to generation of larger batholiths include: (1) Har- graves' [1962] and Ashwals' [1978] discussion of anorthositic gabbro and the associated granite and syenite of the Adiron- dack massif; (2) Presnall and Bateman's [1973] and Brown's [1973] conclusion that andesitic or basaltic liquid generated at or near subduction zones migrated upward into the crust, causing melting of wallrocks and the extensive continental margin calc-alkaline magmatism of Sierra Nevadan type; and (3) Hodge's [1974] modelling of melting and other thermal relations around magma chambers. Barker et al. [1975] inde- pendently suggested that the widespread continental gabbro- anorthosite-syenite-potassic granite suites were formed by coupled fractionation-melting. Smith and Shaw [1975], Duf- field et al. [1980], Bacon (this issue), and Hildreth (this issue) point out that basaltic liquid is a necessary heat source in the generation of continental rhyolites. The writer emphasizes that the plutonic suites of gabbro-anorthosite-syenite-granite

type (e.g., the rapakivi massifs of southern Finland, and the Pikes Peak batholith) originally may have graded upward from deep anorthosite-gabbro complexes to syenite-granite complexes to rapakivitic batholiths to overlying subvolcanic- volcanic ring complexes (such as the Younger Granites and associated rhyolites and other rocks of Nigeria and nearby regions), and at the surface to largely bimodal basalt-rhyolite fields such as Yellowstone.

Taylor [1980] has considered the role of oxygen and strontium isotopes in coupled fractionation-melting. DePaolo [1981] has derived equations describing the behavior of trace elements and isotopes in such systems, and in this issue he considers the origin of the Sierra Nevada and Peninsular Ranges batholiths in this context.

Many aspects of coupled fractionation-melting need eluci- dation. Especially, do we have a complete range of interaction from an endmember in which the thermal source magma simply supplies the heat of partial melting but does not mix with or incorporate crustal material to situations in which the thermal source magma incorporates very high fractions of crustal melt?

FUTURE WORK

As seen in this issue, the currently active areas of research on granites and rhyolites are in the disciplines of geochemistry and petrology, and in volcanic processes. We need further studies on the magmatic source regions in the lower and intermediate crust. These would include modelling studies on thermal aspects of melting, such as that of Yoder [1980]; theoreti- cal consideration of all possible source rocks, e.g., that of pelites by Grant [1973] and Thompson and Algor [1977]; study of geothermometers and geobarometers of xenoliths to give direct information on conditions of magma generation, like that of Wyborn et al. in this issue; geophysical studies that will give us physical information on regions where magnaas are generated, such as that by the COCORP project of a magma body in the Rio Grande rift, New Mexico (see Reilinger et al. [1980] for references); and especially studies relating tecton- ism to the chemical style of magmatism.

Acknowledgments. I extend my thanks to many people for their guidance and cooperation with this special issue. Robert Coleman suggested JGR-Red as its vehicle, pointing out JGR's rare combination of worldwide distribution and modest cost. Allan Cox enthusias-

tically backed Coleman's suggestion. Editor Thomas Ahrens guided several aspects of the review process with care and good judgment and was helpful in many other ways. Ruth Ridenour gave exemplary editorial assistance and her expeditious handling of manuscripts and reviews made our present publication date possible. Lastly, the more than 50 earth scientists who reviewed the papers herein deserve our thanks for their effective commentary.

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BARKER: INTRODUCTION TO GRANITES AND RHYOLITES 10135

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(Received March 12, 1981; revised May 21, 1981;

accepted June 16, 1981.)

introduction to special issue on granites and rhyolites: a commentary for the nonspecialist

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