metamorphism, geochemistry and origin of magnesian volcanic rocks, klamath mountains...

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J. metamorphicGeo/., 1992,10, 55-69 Metamorphism, geochemistry and origin of magnesian volcanic rocks, Klamath Mountains, California B. R. HACKER AND W. G. ERNST . Department of Geology, School of Earth Sciences, Stanford University, Stanford, CA 94305-2115, USA M. D. BARTON* Department of Earthand SpaceSciences, University of California, Los Angeles, CA 90024-1567, USA :,;\ c ;,.,;. , ABSTRACT Metabasaltic rocks in the Klamath Mountains of California with 'komatiitic' major element concentrations were investigated in order to elucidate the origin of the magnesian signature. Trace-element concentra- tions preserve relict igneous trends and suggestthat the rocks are not komatiitic basalts, but im~at~re arc rocks and within-plate alkalic lavas. Correlation of 'excess' MgO with the volume per cent hornblende (:t:clinopyroxene) suggests that the presenceof cumulus phasescontributes to the MgO-rich compositions. Early submarine alteration produced regional 6180 values of + 10:t: 1.5%0 and shifts in AlzO3,.NazO, and KzO concentrations. Regional metamorphic grade in the study area varies from biotite-zone greenschist facies (350-550° C, c. 3 kbar) southward to prehnite-actinolite facies (200-400° C, ~3 kbar) , but little isotopic or elemental change occurred during the regional recrystallization. The greenschist facies assemblage is actinolitic hornblende + phengite + epidote + sodic plagioclas~+ microcline + chlorite + titanite + hematite + quartz in Ti-poor metabasaltic rocks; in addition to these hasesbiotite is present in Ti-rich analogues. Lower grade greenstones contain prehnite and more nearl stoichiometric actinolite. The moderate to low pressures of regional metamorphism are compatibl~ with P-T conditions in a magmatic arc. Later contact metamorphism at 2-2.9:t: 0.5 kbar and at peaf temperatures approaching 600°C around the English Peak and Russian Peak granodiorites produced 3... -km-wide aureoles typified by gradual, systematic increases in the pargasite content of amphibole, m~ covite content of potassic white mica, and anorthite content of plagioclase compositions. Metasomatisrtt :during contact metamorph- ism produced further increasesin bulk-rock 6180SMOW of as much as +6%0.Thus, the unusually MgO-rich nature of the Sawyers Bar rocks may be attributed at least partly to metas~ratism and the presence of magnesian cumulus phases. : Ii Key words: Klamath Mountains; komatiitic basalt; meta.-mati=; oxygen ir""'. . INTRODUCTION AND REGIONAL as post-accretIon contact !J.etam~rph.lsm. ThIs paper G E 0 LOG Y reports. the result~ o~ ou~ investIgatIon of. t.he post- magmatIc recrystallIZatIon textures and composItIons, and Ernst (1987) reported that polymetamorphosed Permian to discusses how metamorphic changes can be stripped away Jurassic(?) mafic volcanic rocks (greenstones) in the to reveal original igneous bulk-rock compositions. A Sawyers Bar area of the Klamath Mountains, California companion paper (Ernst et al., 1991) describes the igneous (Fig. 1), are compositionally similar to komatiitic basalts. petrogenesis of these rocks., A third report (Ernst, 1990) Phanerozoic komatiitic basalts are unusual, and their deals with regional terrane analysis. occurrence in the Klamath Mountains might be of special The Klamath Mountains a..e a collage of magmatic arc, tectonic significance, so an investigation was initiated to accretionary wedge, ocean b~sin, and other eugeosynclinal determine whether the highly magnesian compositions rocks formed during outboard growth of the western North were the result of primary igneous processes,metamorph- American margin. This stu~y, focuseson greenstonesin the ism, or some combination of the two. Moreover, the Sawyers Bar area that are intercalated with limestone, distinctive composition might be a powerful basis for argillite, and chert containimg radiolaria of late Permian correlating or differentiating these rocks from those of through early Jurassic age: (Blome & Irwin, 1983). The nearby terranes, if it could be shown that the composition cross-cutting mid-Jurassic e.lilglish Peak and Russian Peak was not related to some local secondary phenomenon such plutons (V-Pb zircon ages of ~164 and 159Ma, respectively; Wright & Fahan, 1988) bracket the end of . Now at Department of Geosciences, University of Arizona, greenstone eruption; ophiolitic plagiogranite from the Tucson, AZ85721,USA. region south of Fig. 1 yielded a strongly discordant V-Pb 55 -

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Page 1: Metamorphism, geochemistry and origin of magnesian volcanic rocks, Klamath Mountains ...hacker.faculty.geol.ucsb.edu/viz/Hacker92_Magnesian... · 2003-02-07 · petrogenesis of these

J. metamorphic Geo/., 1992, 10, 55-69

Metamorphism, geochemistry and origin of magnesianvolcanic rocks, Klamath Mountains, CaliforniaB. R. HACKER AND W. G. ERNST. Department of Geology, School of Earth Sciences, Stanford University, Stanford, CA 94305-2115, USA

M. D. BARTON*Department of Earth and Space Sciences, University of California, Los Angeles, CA 90024-1567, USA

:,;\c ;,.,;.

,ABSTRACT Metabasaltic rocks in the Klamath Mountains of California with 'komatiitic' major element concentrations

were investigated in order to elucidate the origin of the magnesian signature. Trace-element concentra-tions preserve relict igneous trends and suggest that the rocks are not komatiitic basalts, but im~at~re arcrocks and within-plate alkalic lavas. Correlation of 'excess' MgO with the volume per cent hornblende(:t:clinopyroxene) suggests that the presence of cumulus phases contributes to the MgO-rich compositions.Early submarine alteration produced regional 6180 values of + 10:t: 1.5%0 and shifts in AlzO3,. NazO, andKzO concentrations. Regional metamorphic grade in the study area varies from biotite-zone greenschistfacies (350-550° C, c. 3 kbar) southward to prehnite-actinolite facies (200-400° C, ~3 kbar) , but littleisotopic or elemental change occurred during the regional recrystallization. The greenschist faciesassemblage is actinolitic hornblende + phengite + epidote + sodic plagioclas~ + microcline + chlorite +titanite + hematite + quartz in Ti-poor metabasaltic rocks; in addition to these hases biotite is present inTi-rich analogues. Lower grade greenstones contain prehnite and more nearl stoichiometric actinolite.The moderate to low pressures of regional metamorphism are compatibl~ with P-T conditions in amagmatic arc. Later contact metamorphism at 2-2.9:t: 0.5 kbar and at peaf temperatures approaching600° C around the English Peak and Russian Peak granodiorites produced 3... -km-wide aureoles typifiedby gradual, systematic increases in the pargasite content of amphibole, m~ covite content of potassicwhite mica, and anorthite content of plagioclase compositions. Metasomatisrtt :during contact metamorph-ism produced further increases in bulk-rock 6180SMOW of as much as +6%0. Thus, the unusually MgO-richnature of the Sawyers Bar rocks may be attributed at least partly to metas~ratism and the presence ofmagnesian cumulus phases. :

IiKey words: Klamath Mountains; komatiitic basalt; meta.-mati=; oxygen ir""'. .

INTRODUCTION AND REGIONAL as post-accretIon contact !J.etam~rph.lsm. ThIs paperG E 0 LOG Y reports. the result~ o~ ou~ investIgatIon of. t.he post-

magmatIc recrystallIZatIon textures and composItIons, andErnst (1987) reported that polymetamorphosed Permian to discusses how metamorphic changes can be stripped awayJurassic(?) mafic volcanic rocks (greenstones) in the to reveal original igneous bulk-rock compositions. ASawyers Bar area of the Klamath Mountains, California companion paper (Ernst et al., 1991) describes the igneous(Fig. 1), are compositionally similar to komatiitic basalts. petrogenesis of these rocks., A third report (Ernst, 1990)Phanerozoic komatiitic basalts are unusual, and their deals with regional terrane analysis.occurrence in the Klamath Mountains might be of special The Klamath Mountains a..e a collage of magmatic arc,tectonic significance, so an investigation was initiated to accretionary wedge, ocean b~sin, and other eugeosynclinaldetermine whether the highly magnesian compositions rocks formed during outboard growth of the western Northwere the result of primary igneous processes, metamorph- American margin. This stu~y, focuses on greenstones in theism, or some combination of the two. Moreover, the Sawyers Bar area that are intercalated with limestone,distinctive composition might be a powerful basis for argillite, and chert containimg radiolaria of late Permiancorrelating or differentiating these rocks from those of through early Jurassic age: (Blome & Irwin, 1983). Thenearby terranes, if it could be shown that the composition cross-cutting mid-Jurassic e.lilglish Peak and Russian Peakwas not related to some local secondary phenomenon such plutons (V-Pb zircon ages of ~164 and 159 Ma,

respectively; Wright & Fahan, 1988) bracket the end of. Now at Department of Geosciences, University of Arizona, greenstone eruption; ophiolitic plagiogranite from theTucson, AZ85721, USA. region south of Fig. 1 yielded a strongly discordant V-Pb

55

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56 B. R. HACKER, W. G. ERNST & M. D. BARTON

116"I

]4.-

Fig. 1. Geology of the Sawyers Bar area, showing locations of samples. A preliminary version of a portion of this map, including across-section, was presented by Ernst (1987). Traces of axial planes are highly interpretive. Sawyers Bar is indicated by a black star.Sample symbols for this illustration (and for Figs 3-11) are as follows: triangles = darker coloured, Ti-rich metavolcanic rocks;circles = lighter coloured, Ti-poor metavolcanic rocks; squares = hypabyssal rocks; filled symbols = Mg-rich ('komatiitic') meta-igneousrocks; unfilled symbols = normal (basaltic) meta-igneous rocks.

zircon age of 310-265 Ma, indicating that eruption may Much of the study area (Fig. 1) consists of massive tohave begun in Permo-Triassic time (Ando et al., 1983). weakly foliated, sparsely pillowed flows and hypabyssalThese rocks were thrust eastward beneath the Stuart Fork bodies of highly magnesian to tholeiitic basalt compositionterrane, a late Triassic subduction complex (Goodge, (Ernst, 1987). They are divided into two groups: darker1989) prior to the emplacement of the mid-Jurassic coloured, high- Ti (>2.0 wt% TiO2) rocks and lighterplutons. coloured, low-Ti «1.5 wt% TiO2) rocks. Those of the first

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METAMORPHISM OF MAGNESIAN VOLCANIC ROCKS 57

group are enriched in light rare-earth elements (LREE),P, Hf, Zr, Ta, Nb, Fe, and Ti, and compositionallyresemble alkalic ocean-island basalts, whereas the secondgroup shares trace-element affinities with immature arctholeiites (Ernst et at., 1991). A significant portion of boththe Ti-rich and Ti-poor samples have major-elementconcentrations appropriate for komatiitic basalts (Arndt &Nisbet, 1982a,b)-up to 18wt% MgO and CaO/AI2O3ratios ~1 (Ernst, 1987). Flows interfinger with quartzo-feldspathic detrital strata and chert interpreted as distalturbidites. The related hypabyssal rocks intrude the flowsand sedimentary rocks. Because the metamorphosed dykesand sills are chemically indistinguishable from the Ti-poorextrusive rocks, the entire Ti-poor sequence is permissiblycoeval.

The structure of the region is interpreted as several,kilometre-scale or larger, west-vergent synforms andantiforms (Ernst, 1987). Indistinct axial-planar foliationdips steeply to the ENE. Compositional layering in themetasedimentary strata parallels contacts with intercalatedmetavolcanic rocks. A preliminary geological map ispresented as Fig. 1.

The greenstones are texturally complex, having under-gone eruption, possible submarine alteration, regionalmetamorphism, and local contact metamorphism. 'Sub-marine' alteration is used throughout this paper to indicateany kind of chemical change in the submarine realm,regardless of whether it is associated with ridge-cresthydrothermal activity, the construction of a magmatic arc,or the formation of an ocean island. Evidence forsubmarine alteration comes primarily from considerationof the bulk-rock compositions and oxygen isotopes. Aregionally extensive biotite-zone greenschist facies toprehnite-actinolite facies metamorphism is recognized, byboth textures and compositions, in foliated rocks distantfrom plutons. For example, prismatic igneous amphibolesare. ov~rgrown ~y nematoblastic a.mphibole zoned toward Fig. 2. Back-scattered electron micrographs of regionallyactInolIte. Regional metamorphism was followed by metamorphosed metavolcanic rocks. 57M: intergrowth ofcontact metamorphism to amphibolite facies during actino!ite (c), Fe.-chlorite (F), Mg~chlorite (M), ~lbite .(a),intrusion of the mid-Jurassic English Peak and Russian ~n~esl~e (p), el?ldot~ ~e)., mu.scovlte (m), and mlc.rocl.lne (~); .

. indicatIons of dlsequlllbnum Include patchy zonation In actinolite,Peak calc-alkalIne ~lutons. Contact. metamorphosed two kinds of chlorite, and two kinds of plagioclase. 351M: twosamples are recognIzed by hornfelslc textures and kinds of igneous amphibole, tschermakitic hornblende (t) anddistinctive compositional zoning of amphibole and magnesio-hastingsite (h), overgrown by actinolitic hornblende (a)plagioclase. Amphiboles in contact aureoles consist of in a dark matrix of plagioclase, microcline, biotite, muscovite,

.. .. . I. h h d chlorite and quartz.pnsmatlc pargaslte nms on actmo Ite cores t at a '

developed during the regional metamorphism. Three tofour kilometres from the plutons, contact metamorphism isnot discernible in the mineral parageneses or composi- xene crystals. Relict igneous phases compose 0-70 vol%tions, and regional metamorphic minerals are well of each sample, indicating that some rocks may have beenpreserved. Scattered local crystallization of pumpellyite nearly holocrystalline, others perhaps glassy. Igneousand zeolite minerals was associated with or followed crystals preserved in the metavolcanic rocks includecontact metamorphism. A description and discussion of clinopyroxene, hornblende, plagioclase, spinel, apatite,these features forms the bulk of this contribution. and possibly anorthoclase. Only relict clinopyroxene,

During metamorphism, primary textures were partially hornblende, and plagioclase are abundant. This mineralto completely recrystallized, and samples containing relict assemblage is similar to Archaean komatiitic basalts, whichigneous crystals are sporadically distributed throughout the contain predominantly clinopyroxene and minor olivinestudy area (Fig. 2). The best preserved textures in (Cameron & Nisbet, 1982), but the presence of

I hypabyssal rocks are subophitic to diabasic, with c. 100-,um hornblende, apatite, and anorthoclase is quite unlike

elongate plagioclase laths embedded in larger clinopyro- komatiitic basalts.

,

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58 B. R. HACKER, W. G. ERNST & M. D. BARTON

METASOMATISM AND CRYSTAL exchange -:vith metasedimentary rocks during contactACCUMULATION metamorphIsm. . . '.

Bulk-rock composItIons were presented and dIscussed InSeveral different approaches were used to identify the detail by Ernst (1987) and Ernst et al. (1991); only thoseorigin of the 'komatiitic' geochemical signature of the aspects relevant to evaluating metasomatism and crystalSawyers Bar greenstones: (i) correlation of oxygen isotopic accumulation are presented here. Most trace elementsvalues with metamorphic mineral modes and bulk-rock within the metavolcanic rocks show systematic relation-composition; (ii) identification of elemental variations that ships with each other (Fig. 3). The trace elementare atypical of igneous rocks; and (iii) determination of concentrations are probably related to igneous processes,original bulk-rock compositions based on compositions and but the igneous major elemental ratios may not have beenmodes of relict igneous minerals. If such changes can be preserved during metamorphism. The Ti-rich rocks haveidentified, then it should be possible to 'see through' any trace element concentrations typical of alkalic within-platemetasomatic veil back to the igneous bulk compositions lavas, including elevated Ta and Nb contents, and theand clarify the igneous petrogenesis of these unusually Ti-poor rocks are similar to immature arc lavas, includingMgO-rich rocks. We interpret the variable chemical and having reduced Ta and Nb values (Ernst et al., 1991).isotopic compositions of the metavolcanic rocks to reflect a Trace element concentrations are unlike komatiites orcombination of three distinct processes: (i) limited igneous komatiitic basalts, which, for example, contain <16 ppmcompositional variation mainly involving cumulus hornbl- Ce (Beswick, 1982) and <140 ppm Zr (Arndt & Nesbitt,ende and/or augite; (ii) minor metasomatism accompany- 1982). Moreover, the trace element concentrations areing submarine alteration and regional metamorphism; and incompatible with back-arc basin basalts, boninites, or(iii) oxygen isotopic exchange and very limited chemical mid-ocean ridge basalts (Ernst et al., 1991). The

80 ,,"":'.

0.0 . to Zr. Typical concentrations of P, Hf, Ce,

0 50 100 150 200 250 300 and Zr in basaltic rocks are shaded (B. R.

Z ( ) Hacker literature survey, unpublished).r ppm Symbols as in Fig. 1.

- I' '! i

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METAMORPHISM OF MAGNESIAN VOLCANIC ROCKS 59

concentrations of all minor elements that were measured Cr contents of 400-1000 ppm, Ni contents of 130-500, andare comparable to alkalic within-plate lavas (Ti-rich rocks) Mg/(Mg + Fe) ratios of 0.55-0.65, for the most magnesianand immature arc lavas (Ti-poor rocks), as detailed by lavas (Ernst et al., 1991). Without unaltered rocks fromErnst et al. (1991). the same eruptive suite to serve as a basis for comparison,

Whereas minor and trace element concentrations show means for assessing major element metasomatism aresystematic variation with respect to fractionation indices limited. Compared to worldwide volcanic rocks, some ofsuch as Zr (Fig. 3) (Ernst et al., 1991), most of the major the Sawyers Bar volcanic rocks have unusually lowelement concentrations, although within the range of amounts of AlzO3 or NazO, elevated MgO, and variableunaltered worldwide volcanic rocks, show considerably KzO concentrations (Fig. 4). Zr contents are clearlymore scatter (Fig. 4) (Ernst et al., 1991). Some of the correlated with other 'immobile' trace elements (Fig. 3)scatter could be due to fractionation or assimilation; and are probably igneous, hence the unusual AlzO3, MgO,however, these greenstones are fairly primitive, and NazO and KzO contents may reflect metasomatism or thevariation is more likely the result of metasomatism or presence of cumulus phases. Figure 4 shows that somecrystal accumulation. The primitive nature is indicated by samples may be depleted by as much as 3 wt% AlzO3,

1.5 wt% NazO and 0.5 wt% KzO, and elevated by as much-. 18 as 6 wt% MgO and O.~ wt% KzO. ~o other. major.Q .vCABoC.. elements are present In concentrations atypical of~ 16 cc, ."'"'«:-co"'«««»»»:-,:-:- Id .d I . k Alth h h d I d AI 0oW cc cc "'«"'co"'"'"',.:"'"'co"' co",",",",",",",",",",:",:.:",",",.:", wor WI e vo camc roc s. oug t e ep ete z 3

~ 14 and elevated MgO contents may have contributed to the~ 'komatiitic' character of these basalts, many of the samples0 12 (including some with high MgO contents) are not

N demonstrably different from worldwide volcanic rocks.

< 10. Thus, the 'komatiitic' character of these rocks, including8 the elevated MgO, Ni, and Cr contents, may be the result

-. 20 of some igneous process.~ . . altered? Bulk-!ock Cr contents de~rease with increasing Zr and! 16 ., """,;;;ili::::;::;:ili:::::::;::::::;:::::ili;:::i;\ ~~~~~~~::e f~~~~na:~:e~~In:ccu~~~ati~~n~~~:~:~~~ t~;

0 12 compositional evolution (Ernst et al., 1991). Similar~ behaviour of Ni also suggests that olivine, clinopyroxene,

~ 8 or hornblende fractionation or accumulation was impor-tant. Element-ratio diagrams rule out the participation of

4 olivine and plagioclase, but support the contention that-. 4 clinopyroxene or hornblende were fractionating or

~ accumulating phases (Ernst et al., 1991).~ 3 ;;::::;;,,;,,;;;;;;:=;,;;;,,;;;;; To evalute whether the elevated MgO contents could~ ;;[:;;::;\::;;;;;:;;\;;:':-1;;111=:::: also have been affected by clinopyroxene or hornblende0 2 CO ""::"""""""::,,'»CO,,,,,,»,,», fractionation or accumulation, it is necessary to determine

N ~ whether the excess MgO contents of Sawyers Bar

~ 1 ~ ~ greenstones are correlated with the volume per cents ofe . igneous hornblende or pyroxene. In worldwide volcanic

0 arc rocks, MgO is correlated with CrzO3 (Fig. 5a). At a-. 3 given CrzO3 content, the Sawyers Bar rocks are displaced~ l. d? to higher MgO contents-toward the compositions of~' atere. ' I' h bl d d ' .oW re Ict om en e an pyroxene -by comparIson with~ 2 worldwide volcanic rocks. Using the relationship between-- MgO and CrzO3 shown by the diagonal line in Fig. 5(a),

~ 1 'excess' MgO for a given CrzO3 content was estimated for~ each sample (Fig. 5b). Figure 5(b) shows that the

magnitude of 'excess' MgO is correlated with the0 abundance of igneous hornblende. Moreover, the mag-

0 50 100 150 200 250 300 nitude of the excess is qualitatively similar to thatZr (ppm) suggested in Fig. 4. To avoid some of the potential

circularity in Fig. 5(a, b) (circular because hornblendeFig. 4. Comparison of Sawyers Bar greenstone major-element contains substantial CrzO3 as well as MgO), the 'excess'concentrations with worldwide volcanic rocks. Shaded fields MgO derivation has been repeated using Hf in place oflabelled 'VCAB' and 'O1B' show typical values of intraoceanic Cr 0 . F' 5() th It' . .and continental arc rocks and ocean-island basalts, respectively . z 3 In Ig. c; e res~ s are, In agr,eement. ~Imllar(B. R. Hacker literature survey, unpublished). Symbols as in diagrams based on correlation of excess MgO with theFig. 1. volume per cent of pyroxene yield similar results,

, ,':\. I

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,- I~.. B R HACkER, W G ERNST' M D BARTON

(a) suggesting that bulk-rock compositions are not quenched20 liquid compositions, but contain a contribution from18 cumulus amphibole and/or pyroxene. Moreover, there is

no need to invoke metasomatism to account for the high16 MgO contents of these rocks, although metasomatism may

0 be responsible for the unusual Alz03, NazO, and KzOon 14 contents.

~ 12.Q Oxygen isotopes

~ 10 0 . blk k .. d .~ xygen Isotope u -roc compositions were measure In~ 8 worldwide 21 samples to assess the degree of metasomatism in the

volcanic metavolcanic rocks. Analyses were done by fluorination6 arc rocks with ClF3 (Bothwick & Harmon, 1982) and conventional4 mass spectrometry; reproducibility was :i:0.2%.0 0 Oxygen isotope systematics are disturbed from plausible. 0.1 0.2 0.3 0.4 0.5 igneous compositions in all analysed metavolcanic rocks

wt % Cr 0 (Table 1). Measured 6180SMOW values increase toward2 3 granitoid plutons (Fig. 6). Samples more than 3 km from

the plutons have 6180 values of +8.7 to + 11.4%0, whereas(b) 6 samples closer to these late intrusions are as heavy as

~ ~~~l~==) + 15.9%0. The English Peak pluton yields bulk rock 61800 Cr 2°3 basis. values of +11.3%0 (nearly unaltered sample) to +10.4%0on 5 (sample adjacent to and contaminated by wall rocks);

~ samples of metasedimentary rocks intercalated with the4 metavolcanic rocks have 6180 values ranging from + 12.4

~ to +17.9%0. Hence, it may be concluded that the heavy~ 3 oxygen values noted in the greenstones adjacent to the~ plutons were a consequence of exchange with fluids~ 2 derived from the interlayered 180-rich metasedimentary~ C rocks. In spite of the observation that 6180 values are

~ 1 dependent on the proximity of samples to plutons, we arer~ . unable to demonstrate that other elements behave in a~ similar fashion. This suggests that any shifts in bulk

0 compositions away from probable igneous values are0 10 20 30 40 50 60 70 unrelated to contact metamorphsim.

Although there are many uncertainties, low- T ( <2500 C)Vol % Volcanic Hornblende ~ubmarine hydrothermal alteration typically ~nriches 180

In shallow «2-3 km) mafic rocks from protollth values of+5 to +7%0 (Taylor, 1968; Beaty & Taylor, 1982) to final

(c) values of +7 to +12%0 (e.g. Bowers & Taylor, 1985;12 Muehlenbachs, 1986). Deeper level (hotter) hydrothermal

0 I Dr h!:a~i~ ~ exchange should produce 180 depletions at moderate toon 10 I'::~=~:J . high fluid-to-rock values. The +8 to +11%0 range in the

~ metavolcanic rocks away from the granitic plutons is~ 8 compatible with equilibration with seawater (0%0) in the~ . 100 to 200° C range. An alternative origin with higher

~ 6 ~ temperature exchange would require a considerably large

~~ 4 <> Fig. 5. (a) Relationship of MgO to CrZO3 content in volcanic~ C <> rocks. Compositions of worldwide arc volcanic rocks (B. R.~ 2 <> Hacker literature survey, unpublished), and Sawyers Bar relict~ igneous hornblende and pyroxene crystals are shaded. Diagonal~ C line shows approximate relationship between MgO and CrZO3. (b)

0 C 'Excess' MgO for a given CrZO3 content in hornblende-bearingsamples is correlated with the volume per cent of igneous

0 10 20 30 40 50 60 70 hornblende. The scale of the vertical axis is related to the slope ofthe diagonal line in Fig. 5(a). (c) 'Excess' MgO for a given HfV I 01 V I . H content in hornblende-bearing samples is correlated with the

0 -/0 0 camc ornblende volume percent of igneous hornblende. Symbols as in Fig. 1.

(

II

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'1111- ..',;...~ '- -~, - ~ METAMO.P~ VOLCANIC ROCKS 61 j

Table 1. 6180 bulk-rock values for metamorphosed igneous irregular, but substantial MgO enrichment in therocks, associated sedimentary rocks, and the English Peak pluton. metavolcanic rocks and the considerable variabilit y in K 02

Rock type Sample 6180 (%0) and Na20, attributed earlier to crystal accumulation, couldalso have been influenced by the submarine hydrothermal

Ti02-rich volcanic 91Mt 15.89 fluids that caused the regional 180 enrichment (e.g. Alt et~~~~ ~~.: al., 1986; Seyfried et al., 1988).207M 12:10* The heterogeneous but marked enrichment in 6180354Mt 12.85 near the Russian Peak and English Peak plutons is most372M 11.32 consistent with a contact metamorphic origin. The large386M 10.80 180 enrichment contrasts with the typical large 180

T ' O 1 . 46M 13 81 depletions observed in the aureoles of most epizonalI 2-poor vo camc . . . .86M 8.69 intrusions (Cnss & Taylor, 1986). Calculated waters would92Mt 14.22 have had 6180 = +9 to +12%0 at the conditions of

117M 11.15 metamorphism, much heavier than any plausible surface-~~~ 1::~~* derived. water: The only reason~ble source of heavy374M 13.02 oxygen IS the Intercalated metasedimentary sequence (see516M 10.58* Table 1). Metasedimentary rocks are more abundant in the

areas near the intrusive contacts, the general areas of 180Hypabyssal 220M 11.37 * enrichment in the greenstones. Local evidence for

285M 11.46 t d. . h (d .b d b )me ase Iment-metalgneous exc ange escn e a ove

Sedimentary 134M 16.63 and the lack of other adequate sources of heavy oxygen166M 14.67 require re-equilibration on the scale of 101-103 m. This178M 14.87 exchange was probably facilitated by local fluid circulation~~~~ ~~.~~ driven by emplacement of the English Peak and Russian355M 13:28 Peak plutons; however, the volume of introduced fluid

must have been small relative to the volume of the altered! English Peak pluton 24M 11.27 (quartz = 13.53) rocks because of the are ally limited 180 enrichment.I 26M 9.0* (quartz = 13.98) Given the subequal amounts of exposed metasedimen-

326M ]0.4* t d t I . k h. I d hary an me avo camc roc s, t IS postu ate exc ange. Average of two analyses. should have decreased the 6180 of the metasedimentaryt 'Komatiitic' sample. rocks. Such a decrease is difficult to document given the

substantial variation in sedimentary oxygen isotopevolume of water isotopically heavier than seawater. No compositions; however, it is possible that much of theadequate source for such a fluid is apparent. This suggests metasedimentary section could have had original composi-that oxygen isotopic exchange during regional metamorph- tions in excess of +20%0 as is typical of cherty, calcareous,ism was minor. Integrated fluid-to-rock ratios during and clay-rich sediments (Hoefs, 1987) and has now beensubmarine hydrothermal exchange must have been >c. 5 shifted to c. + 15%0 (Table 1) (Ernst et al., 1991).in order to achieve the observed isotopic shifts. The

16 y METAMORPHIC MINERALS

151 Thirty-seven low-variance, mineralogically representative.0 samples were selected for back-scattered electron micros-

-. 14 . copy and electron-probe microanalysis to evaluatee 0 metamorphism of these unusually magnesian metavolcanic

~ 13 v y 0 rocks.* Sample localities are indicated on the geological'-' 12 y map of Fig. I. Mineral analyses were made with a

0 four-spectrometer Cameca Camebax electron-probe~ 11 v v 0 [] microanalyser using well-characterized natural and syn-

c..o 10 0 thetic mineral standards whose major element concentra-tions are reproducible to better than :i:2% for major

9 0 elements and :i: 10% for minor elements. Textural analysis0 and spot selection were performed in back-scattered-

8 electron mode. The electron beam diameter was 2IJm at0 1 2 3 4 5 6 7 15-kV accelerating voltage and 10-15-nA sample currentDistance from Pluton (km) on minerals. Counting periods of 20s for peak intensities

Fig. 6. Relationship between bulk-rock oxygen isotopecompositions of metavolcanic rocks and proximity to calc-alkaline * Detailed chemical analyses of minerals from the Sawyer's BarEnglish Peak and Russian Peak plutons. Symbols as in Fig. 1. Area are available, on request, from the first author.

1 I,

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62 B. R. HACKER, W. G. ERNST & M. D. BARTON

and 10 s for background intensities were made for each of MgO contents of amphiboles in the greenstones are related10 elements. Detection limits at these conditions are a to bulk-rock magnesia contents. No other elements infunction of mineral composition, but conservative limits amphiboles are correlated with bulk-rock composition.are 0.02 wt% for SiOz, AlzO3, TiOz, CrzO3, and MnO, Biotite is present in fewer than half the metabasaltic0.03 wt% for FeO, MgO, and CaO, 0.04 wt% for KzO, samples studied, primarily in Ti-rich metamorphosedand 0.05 wt% for NazO. Analyses of inhomogeneous extrusive rocks. The biotite occurs as subhedral platesgrains are presented individually. All mineral formulae 20-80 /lm in length. Biotite ranges in compositionwere calculated by the method of Laird & Albee from K\oMg\.3Fe\3Cro.o TiozAlo.z(AI\.zSiz.s)O\o(OH)z to(1981a, b), except as noted. Amphibole names follow the K\oMg\7Feo.8Cro.\ Tio.oAlo.4(AI\\Siz9)O\o(OH)z. MnO,convention of Rock & Leake (1984). CaO, and NazO are minor constituents, whereas CrzO3

Regional metamorphic assemblages span upper green- accounts for as much as 1.1 wt% in biotite. There are tooschist to prehnite-actinolite facies. The low-variance few biotite analyses to evaluate whether there is anyregional greenschist facies assemblage is actinolitic systematic regional variation in the composition.hornblende + phengite + epidote + chlorite + albite + micro- Phengite occurs in most greenstone samples as 10-50-/lm-cline + quartz + titanite + hematite in low- Ti rocks; biotite sized subhedral platelets produced by recrystallization ofis also present in low-variance assemblages developed in feldspar. White mica ranges in composition fromhigh- Ti rocks. The low-variance, regionally developed KosMgo.3Feo\AI\s(Alo6Si34)O\o(OHh to Ko9Mgo..Feo.o-prehnite-actinolite facies assemblage is actinolite + AI\.8(Alo9Si3JO\o(OHh. TiOz, CrZO3, MnO, CaO, andphengite + epidote + chlorite + albite +prehnite + titanite + NazO are all minor constituents of the phengite platelets.quartz. Contact metamorphic assemblages nearest to The MgO contents of white micas (0.6-4.3 wt%) areplutons contain pargasitic hornblende:t clinopyroxene + weakly correlated with bulk-rock MgO contents. Phengitescalcic plagioclase + epidote + muscovite + biotite + titanite. near granitic plutons in the contact aureoles contain more

Detailed microprobe traverses from amphibole cores to TiOz and less SiOz than those generated during the earlierrims reveal that three stages of amphibole growth are regional metamorphism (Fig. 11).represented in the greenstones. Some amphiboles contain Chlorite is present in every sample studied, typically asan igneous pargasitic hornblende core overgrown by a subhedral flakes 20-80 /lm in length. Chlorite TiOz, Cr303regional metamorphic rim zoned outward to actinolite, in and MgO contents depend on bulk-rock composition. Noturn overgrown by a contact metamorphic rim zoned systematic regional variation in chlorite composition wasoutward to pargasite. Amphiboles that grew during detected, and the range in composition of chlorite plateletsregional metamorphism consist of discrete fibrous neo- from a single sample is comparable to the variation inblasts in the groundmass and rims on stout prisms of compositions of all analysed crystals. Much of the chemicalvolcanic hornblende and pyroxene. Most are subhedral to variation can be accounted for by substitution ofeuhedral, lath-shaped to equant crystals l00-300/lm in (Mg,Fez+,Mn)Si for (Cr,AIV\)ApV, producing AIV\ varia-length, and are principally actinolite. Most amphiboles tion of 1.2-1.5 atoms per formula unit (p.f.u.). MnO andgrown during regional metamorphism display decreases in CrZO3 are present in minor quantities.NaM4, NaA + K, AIV\ + Fe3+ + Ti, and Apv, from core to Plagioclase crystals interpreted as volcanic, regionalrim (Fig. 7a). This zonation may have resulted from some metamorphic, and contact metamorphic are all present.combination of two processes: continuous equilibration of Igneous plagioclase crystals, AnZ~50 (Ernst et al., 1991),the amphibole rims during decreasing temperature, or are typically long and slender, oscillatorilly zoned, anddisequilibrium growth. intimately intergrown with pyroxene or amphibole.

Contact metamorphic amphiboles form overgrowths on Metamorphic plagioclase occurs in nearly all samplesregional metamorphic amphiboles and are zoned smoothly within healed cracks in relict igneous laths and as neoblastsfrom actinolitic cores to paragasitic hornblende rims (Fig. within or adjacent to precursor igneous crystals. Most7b). One contact metamorphosed sample (46M) close to plagioclase crystals are 20-60 /lm in diameter, anhedral,English Peak pluton contains the texturally equilibrated and equant. The compositions of plagioclase grains do notpair magnesio-cummingtonite + magnesio-hornblende as- depend on bulk-rock compositions, except in contactsociated with andesine and Mg-rich chlorite. Most calcic metamorphosed rocks lacking quartz where plagioclaseamphiboles produced during contact metamorphism CaO correlates with bulk rock CaO. Individual albitedisplay continuous increases in NaA + K, AIV\ + Fe3+ + Ti, (ADo\-09)' individual oligoclase (An\U-20)' and albite +Apv, Na/Ca, and AI/Si, from core to rim (Figs 7 & oligoclase pairs are found in regional metamorphic8)-exactly opposite to the regional metamorphic trend. assemblages (Fig. IOc). Albite + oligoclase pairs in twoApproaching the granitoids, SiOz and CaO contents of samples have compositions appropriate for the peristeriteregional actinolitic amphibole rims begin to decline sharply gap (Maruyama et al., 1982): ADo3%OO + An\s%o\ andbeginning about 3-4 km from plutons as these phases are Afi04%oo + An\s%o\ (Fig. lOc). In contact metamorphosedoverprinted; AlzO3. TiOz, NazO, and KzO contents rocks within 2 km of calc-alkaline plutons, plagioclaseconcomitantly increase (Figs 9 & lOa, b). The mechanism ranges from Anzo to An98 (Fig. 10c). Contact metamorphicfor the incorporation of the AlzO3 is principally through plagioclase crystals contain minor but measurable amountsthe tschermak and edenite substitutions (Fig. 7b), of TiOz, CrzO3, and MnO; FezO3, MgO, and KzO arecompatible with higher temperatures near the plutons. The more abundant.

, ,\, I I I !

,

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METAMORPHISM OF MAGNESIAN VOLCANIC ROCKS 63

(0)04 .25. IoGl IoGllltoEd , ,.".toPg

.200.3 . "2.

a U. +.15 a~~ 0.2 . a ~ .

Z a ~ .10-= .

0.1 Z .05 .

.toEd, Pg to Ts

0.0 .000.0 0.1 0.2 0.3 OA 05 0.6 .00 .05 .10 .15 .20 .25 .30

NaA +K AI/(AI+Si)

15 OAtoGl .. toTs IoGlE=. 0.3.of 1.0 a a+ n a .

... # ~ . ~ a ~ 0.2 a a

~ 0.5 . aa . to Pg Z ~ 8 a 8

>- '" B ~ a 0.1 a ".,8" 8 a

-< . 8Ed a P toTs

0.0 0.0 'F.d0.0 0.5 1.0 1.5 0.0 05 1.0

AI IV Al VI +Fe3+ + Ti

(b)0.4 toGl .25 toGllltoEd a~ a~IoPg a --- .20 a

0.3 8 ~ /U. a +.15 a~~ 0.2 a a a Z IQ aZ a a - .10 a

a ~ ,,;,~a a -= a .0.1 a e" a Z .05 a

a a a a 'to Ed, Pg a 10 Ts0.0 .00

0.0 0.1 0.2 0.3 0.4 0.5 0.6 .00 .05 .10 .15 .20 .25 .30

NaA +K AI/(AI+Si)

1.5 - 0.4IoGlto Gl to Ts ,."

.- a aE-c

~ ~~a ~a a a 0.3 8 aof 10 . a

"' ..~~a + . a ... a aa. oS'

~ I a ,;..o; a ~ 02 aa~ 8 D a (V . Baa+ IoPg Z 8a~ 0.5 . . ~8 a- 0.1:' a a a-< a BEd a a a a 10 Ts

0.0 0.0 Ed0.0 05 1.0 1.5 0.0 05 1.0

All V AI VI +Fe3+ + Ti

Fig. 7. Compositions of amphiboles (atoms per 23 oxygens) developed during (a) regional and (b) contact metamorphism plotted ondiscriminant diagrams of Laird & Albee (1981b). Tschermakite substitution produces a 1: 1 slope on the Alvl + Fe3+ + Ti versus Arvdiagram, and glaucophane substitution produces a 1: 1 slope on the NaM4 versus Alvl + Fe3+ + Ti diagram. Typical core-to-rim zonationsare shown by arrows. Filled symbols indicate amphiboles that coexist with a Ca-AI silicate, chlorite, albite, quartz, and a Ti-phase;unfilled symbols indicate amphiboles from higher variance assemblages. End-member amphibole compositions shown are Ed = edenite,Gl = glaucophane, Pg = pargasite, and Ts = tschermakite.

II

,-, j, I "I,

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64 B. R. HACKER, W. G. ERNST & M. D. BARTON

18 metamorphic' igneous . metamorphic

14 [!~~] \ .,.'.'.\ ~J.~~.'.'.'.'" i. , . .1.~ .', '..J \~' . . . ./ '.. ,'. ,'...'10 . \ . . '" '. / \~. 1 1 ".,. i . .FeO*'

6. '.-. . ..'"."'.'" .' I '.' '.'.' '.' ..' .' '.T'~ \2 l""L .'.'...'.. . ,' "'.' N 0 /..a .. 18 metamorphic. A igneous metamorphic

14 []~EI t"'\,.,! \. ~!O3 ,1"\'" \ ,..'. \ ,"." .., \~ .. .~' .'", ... . .- ..10 . ", .' " I . FeO* . ,.,..,.' ., ""-'". I '. .'j . -- '. '.'. '. .,..", .,.'.' \ I~ . , .,.' " .' .,.~ .' .'

6T'~'. l""L .'. . . .'.' .2 '.' ' , '... Na 0 ,'.' '.-.' -".'

2 ...,'.. Fig, 8, Compositional profiles of single

amphibole crystals from two samples. The0 50 100 150 200 amphibole cores formed by crystallization

I I from a magma and the rims formed duringrm regional metamorphism.

Metamorphic microcline (AfiooOr97-98Ab02--O3) is present with XFe = Fe/(AI + Fe) = 0.00-0.07, was found in 20in more than half the samples studied, and is found in all samples, including hornfelsic greenstones bordering thesamples with ~1.5 wt% K2O. It typically occurs adjacent English Peak pluton; in one sample it forms pseudomorphsto phengite crystals in 10-15-.um-diameter, anhedral, after epidote. Zeolite minerals occur in two samples. Theequant crystals, and consequently was identified only by principal constituents are SiO2, AIP3, CaO, and Na20,back-scattered electron microscopy. Fe203 (~0.5 wt%) and the different crystals within single samples are quiteand MgO (~0.3 wt%) are abundant minor elements, variable in composition.whereas TiO2, Cr203, and MnO are less important. Titanite and ilmenite are widespread titaniferous phases

Epidotes are widespread euhedral prismatic to subhedral forming 20-50-.um anhedral, equant crystals; titanite isequant minerals 50-100.um in diameter. Their composi- typically an alteration product of ilmenite. Hematite,tions are not related to bulk-rock compositions, and show chalcopyrite, and pyrite are scattered trace phases in mostno systematic spatial variation. Epidotes with cores that samples.contain 12-31 mol% pistacite, Ca2Fe3Si3O'2(OH), arez~ned. to slightly more .aluminous rims (~1-3?mol% CONDITIONS OF METAMORPHISM ANDplstaClte), and cores WIth .3-13mo.l% plstaCite are REGIONAL SIGNIFICANCEovergrown by more ferrugInous nms (12-19 mol%pistacite). TiO2, Cr2O3, MnO, MgO, Na20 and K2O are Textures and mineral compositions suggest the followingpresent in minor or insignificant amounts. sequence of events: (i) the metavolcanic rocks were

Pumpellyite was found in three samples, filling cracks in extruded and altered in a submarine environmenthornblende in one contact metamorphosed rock, and in coincident with deposition of the intercalated deep-waterveins in two regionally metamorphosed rocks-hence it sedimentary rocks (Ernst et at., 1991); (ii) high-gradeclearly grew later than or during waning stages of those greenschist (north) to prehnite-actinolite (south) faciesmetamorphic events. The composition varies from regional metamorphism overprinted the entire sectionCa37Mgo.sFeosAIs1Si6P22(OH)s to Ca3sMgosFeo2AIso- during deformation; (iii) the English Peak and RussianS~OO22(OH)s' In this study, pumpellyite analyses were Peak calc-alkaline plutons intruded the area, causingnormalized to 24.5 oxygen atoms pfu (after Yoshiasa & hornfelsic contact metamorphism and shifts in oxygenMatsumoto, 1985). Prehnite, Ca'.9(Fe, AlhoSi3.oO1o(OHh isotopic composition; and (iv) pumpellyite and zeolite

II ,I

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METAMORPHISM OF MAGNESIAN VOLCANIC ROCKS 65

0.06 . compositions and assemblages (Fig. 12). The presence of0 OS . . epidote + actinolite and the absence of glaucophane. ... . suggest that the regionally developed greenschist facies

0.04 j, 1 assemblages formed at temperatures between 350 and~ 0.03: . . i . 5500 C. Note, however, that reactions (1) and (2) in Fig. 12. . .. . . ~ . are unreversed and experiments delimiting reaction (2)

002 .. . 1. .. .. :. . . were unbuffered with respect to t0 2 as well. Moreover,. . t t ... . .0.01 . .. lii . . :! 1. experiments on reaction (1) were conducted in the. . iJ . . '. . (0 !. . . . . t ; CaO-MgO-AlzO3-SIOz-HP CMASH) system, whereas.00 those on reaction (6) were conducted in the CaO-FeO-2.0 .. .. I i: FezO3-AlzO3-SiOz-HzO (CFASH) system. Epidote +

I !: I . . i! ; ! i -: pargasite in contact metamorphic assemblages suggests1.9 . : L. .. . i I . i ~ peak metamorphic conditions between reactions (4) and

- ..., .. (6) . F. 12~ I.. . 10 Ig. .~ U 1.8 = i = Laird & Albee (1981b) showed that compositions ofoS 1.7:. metamorphic amphiboles from mafic rocks containing= . chlorite + epidote + plagioclase + quartz + a Ti-phase are~ 1.6 systematic functions of pressure and temperature. The

"3 0.6 c regional and contact metamorphic amphiboles in thee 0.5 c 8 c c Sawyers Bar area all fall within the medium-P

~ c 8 c 8 (kyanite-sillimanite) to 10w-P (andalusite-sillimanite)Q 0.4 ~ 8 facies series trends as delineated by Laird & Albee

~ ~ 0.3 [C B C (1981b), Hynes (1982), and Laird et at. (1984). The~ 0.2 cg B C C C C presence of oligoclase + actinolite in the greenstones (Fig.

=- 0.1 a ~~ C C G D ~: ~ 11) is also co~pati.ble with moderate to low metamorphicrI) 00 C C 8 C g' B 8 B C C ~ ~ pressures (MIyashIro, 1973, p. 288; Maruyama et at.,e. 0 1982).Q 0.08 0 : Liou et at. (1985) proposed the existence of the~ g 2 prehnite-actinolite facies to describe assemblages con--< 0.06 taining prehnite + actinolite + epidote + chlorite + albite +

i= 0: 0 . quartz + titanite. This facies is transitional between zeolite0.04 9 gOo 0 or prehnite-pumpellyite to greenschist facies at low0 02 0 0: I. 0 pressures, and may correspond to metamorphic tempera-. 0 ~ y 0

0 ~ ~ ~ u 0 "8 0 02 II ~ 0 ~~ tures of 200-4000 C and pressures of ~3 kbar (Liou et at.,0.00 v - 0 - 0 ~ 0 I. 2 0 00 1987). Rocks containing this assemblage are present in the

20 ::. south-west of the Sawyers Bar area (Fig. 10c).. . . The Al content in igneous hornblende coexisting with1.5 \.' . : . potassium feldspar, quartz, plagioclase, biotite, titanite,

'< 10 ',:. . . ilmenite or magnetite, and silicate liquid can be used to. f; t..; t ~ estimate the depth of emplacement of the English Peak. eo. . t . t .'0.5 . ... ... .! . ;.:. pluton. Hornblende cores contain 1.3-1.4 Al atoms p.f.u.,.. . . .. . .. ~~00' . . . t . : .. andrimshave1.1-1.6Alatomsp.f.u.Althoughthetotal. 0 1 2 3 4 5 6 7 distribution of rim compositions is large, the mean and

D' t fit (k ) standard deviation are 1.4:l: 0.14 Al atoms p.f.u. EmpiricalIS ante rom p u on m (Hollister et at., 1987) and experimental calibrations

'.' (Johnson & Rutherford, 1989) indicate pressures ofFig, 9. Changes in compositions of amphiboles (atoms per 23 .oxygens) produced by mid-Jurassic English Peak and Russian 2.6-3.7.:l: ~ kbar and 2-2.9:l: 0.5. kbar, respectIvely, forPeak calc-alkaline plutons; each symbol represents a single crystallIZation of hornblende with 1.4:l: 0.14 Al atomsanalysis and columns of symbols at a given distance from plutons p.f.u. The experiments were conducted at several oxygenare from a single sample. fugacities at pressures as low as 2 kbar and are reversed;

the preferred pressure of crystallization is 2-2.9:l: 0.5 kbar(bar 7 in Fig. 12).

growth occurred locally during late-stage retrograde In summary, regional greenschist facies metamorphismrecrystallization. Based on radiometric and fossil ages occurred at temperatures of 350-5500 C and moderate tomentioned in the introduction, the regional metamorphism low pressures (c. 3 kbar); in contrast, slightly later contacttook place after or during early Jurassic time, and the metamorphism occurred at temperatures as high as c.contact metamorphism is mid-Jurassic in age. 6000 C, coincident with crystallization of the English Peak

Estimates of physical conditions during metamor- granodiorite at 2-2.9 :l: 0.5 kbar. Prehnite-actinolite faciesphism(s) are possible, based on the observed phase assemblages developed at lower temperatures (200-

I i I 'i

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66 B. R. HACKER, W. G. ERNST & M. D. BARTON

a b !. . i

.. 1-.. ..-11\ ..;~; ..' i

I.,,

Fig. 10. (a) Contours of wt% TiO2 in amphibole cores. (b)Contours of wt% Al2O3 in amphibole cores. (c) Distribution ofmetamorphic plagioclase and prehnite. Regional metamorphism:circles indicate albite, squares oligoclase, diamonds albite of anycomposition + oligoclase of any composition and trianglesalbite + oligoclase peristerite pairs; shaded area indicatesregionally metamorphosed samples containing the lower gradeequilibrium pair prehnite + actinolite. Contact metamorphism:solid lines delineate areas near plutons containing plagioclase

0 kin 2 more anorthitic than An2o' Other symbols and contacts after~ Fig. I.

4000 C) and pressures (~3 kbar) (Liou et at., 1987). suggest that regional metamorphism in the Sawyers BarBecause these estimates are based on experiments area is correlative with the Siskiyou event: (i) early toconducted under non-ideal conditions (e.g. some reactions mid-Jurassic age; (ii) moderate to low metamorphicare unreversed, unbuffered, or conducted on simplified pressures; and (iii) the southward decrease in metamorphicsynthetic systems), they should be viewed as only pressures and temperatures. .approximate. Using thermal modelling, Barton & Hanson (1989)

A mid-Jurassic metamorphism termed the Siskiyou demonstrated that regional low-P metamorphism onlyevent (Coleman et at., 1988) occurred during and after occurs in areas with a large fraction of relatively evenlywidespread plutonism in the central Klamath Mountains distributed igneous intrusions--otherwise sufficient heat is(Wright & Fahan, 1988; Donato, 1989). Several features not available at shallow depths. It is possible that regional

i, ' , ,I i ,

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METAMORPHISM OF MAGNESIAN VOLCANIC ROCKS 67

3.40 . I /= NNO /~ 3.30 :--; ':' I I ~

e .'.. - QFM,...~ :.../ NNO 0 . . c ... .c~ 3.20 . ~.- .

00 ... Q,. T

7.3.10 . oJ..

~ 0.008 QFM

Co 0~ 0.006 200 300 400 500 600 700e T (OC)0 0.004 .~ Fig. 12. P- T conditions for regional (horizontal lines) and contact.- (vertical lines) metamorphism of the metavolcanic rocks.E-4 0.002 . Reactions indicated are: (1) pumpellyite + chlorite + quartz =. . . clinozoisite + tremolite + H2O (Liou et al., 1985); (2)

0.000 pumpellyite + chlorite + quartz = epidote + actinolite + H2O0 1 2 3 4 5 6 7 (Nitsch, 1971); (3) maximum glaucophane stability (Maresch,

. 1977); (4) actinolitic hornblende + epidote + chlorite =DIstance from pluton (km) tschermakitic hornblende + epidote (Liou et al., 1974; Apted &

Liou, 1983); (5) albite + epidote = plagioclase (Apted & Liou,Fig. 11. Relationship between maximum Si and Ti contents 1983); ~nd (6) epidote. + quartz = !?irossular + anort~ite +(atoms per 11 oxygens) of phengites and proximity to the English mag~etlte (Llou: 1973, Apted & LIOU,.1~83). Q~. qu~rtz-Peak and Russian Peak calc-alkaline plutons. Phengites that fayalite-magnetlte-.H2O buffer, NNO. mckel-:mckeloxlde-H2Ocoexist with microcline + quartz + chlorite :i: biotite are shown buffer. .Als? shown IS (7) the pressure. bracket In~erred forwith larger symbols. crystallization of hornblende crystals In the English Peak pluton

(see text).

metamorphism in the Sawyers Bar area, which pre-dated differentiated by obtaining radiometric ages on igneouscontact metamorphism, was caused by heat advected to and metamorphic minerals, and by obtaining bettershallow crustal levels by plutons like those that caused the constraints on the pressures during regional metamor-contact metamorphism. The regional and contact meta- phism.morphism may thus be genetically linked, and nearly Phengites near plutons contain more TiO2 and less SiO2coeval. Heat for both events may have come from the than those that grew during the regional metamorphismsame volcano-plutonic edifice that erupted the volcanic (Fig. 11). The magnitude of the SiO2 decrease toward therocks. As a less likely alternative, the volcanism, regional plutons has implications for barometric estimationsmetamorphism, contact metamorphism, and pluton em- involving phengite. For example, Massone & Schreyerplacement may be unrelated. These possibilities might be (1978) used the results of unreversed experiments in the

Table 2. Barometry of contact metamor- . . . .phic rocks. Muscovite Chlonte Biotite

Si AI Fe Mg K Si AI Fe Mg Si AI Fe Mg K

91M 3.11 2.70 0.09 0.10 0.93 2.88 2.49 1.19 3.42 2.85 1.59 0.81 1.67 0.91351M 3.19 2.66 0.05 0.10 0.84 2.80 2.47 1.28 3.37 2.88 1.51 0.78 1.72 0.95

Bucher-Nurminen Massone & SchreyerPowell & Evans (1983) (1987) (1987)

In (Kd) P (kbar) loglo (Kd) P (kbar) P (kbar)

91M 10.2 2-3 4.6 1-2 <1.5-2.5351M 10.2 2-3 4.5 1-2 4-5

Both samples contain the assemblage muscovite + biotite + chlorite + quartz; 351Malso contains microcline. Pressures are calculated for temperatures of 400-5000 C, fromfigs 1 & 2 of Powell & Evans (1983), figs 1 & 2 of Bucher-Nurminen (1987), and fig. 3 ofMassone & Schreyer (1987). Muscovite and biotite compositions are given as cations per11 oxygens; chlorite composition as cations per 14 oxygens.

, c' , I " ~I

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68 B. R. HACKER, W. G. ERNST & M. D. BARTON

K2O-MgO-AI2O3-SiO2-H2O (KMASH) system con- Contact metamorphism at c. 2-2.9 kbar and at peakducted in the temperature range 350-6000 C to quantify temperatures of c. 6000 C around the English Peak andchanges in the Si content of phengite coexisting with Russian Peak plutons produced 3-4-km-wide aureolesbiotite, quartz, and potassium feldspar, as a function of containing predominantly magnesio-hornblende +pressure and temperature. Two brackets for this same oligoclase + epidote + titanite + quartz. Igneous intrusionreaction were ,previously obtained by Velde (1965). also increased {)180 rock values by an additional 5%0Discrepancies between the two data sets prompted within 3-4 km of plutons, probably by exchange withadditional reformulations of this barometer by Powell & intercalated metasedimentary rocks. The regional meta-Evans (1983) using Velde's data, and by Bucher-Nurminen morphism may have been caused by heat advected to(1987) using Massone & Schreyer's data. For one sample shallow crustal levels by plutons like those that caused thecontaining muscovite + biotite + chlorite + quartz + micro- contact metamorphism.cline, we calculate contact metamorphic pressures of1-2, 2-3, and 4-5 kbar, for Bucher-Nurminen's (1987), ACKNOWLEDGEMENTSPowell & Evans' (1983) and Massone & Schreyer's(1987) formulations, respectively. For one sample con- This research was supported by the University oftaining muscovite + biotite + chlorite + quartz, Bucher- California, Los Angeles, by Stanford University and by theNurminen's (1987) and Powell & Evans' (1983) formu- Department of Energy through grant DE-FG03-lations yield contact metamorphic pressures of 1-2 and 87ER13806 to W.G.E. Thanks are due to UCLA2-3 kbar, respectively. Because the crystallization pressure colleagues R. Jones for microprobe assistance and R.calculated for the English Peak pluton is 2-2.9 :i:0.5 kbar, Alkaly for polished sections. We also thank R. E. Bevins,we prefer the contact metamorphic pressures calculated M. Cho, H. Day, J. Schumacher, G. Sen, and F. Spear forfrom Bucher-Nurminen's (1987) and Powell & Evans' helpful reviews. The stable isotope analyses were(1983) calibrations (Table 2). supported by a grant from the Petroleum Research Fund

of the American Chemical Society to M.D.B.

CONCLUSIONS

Highly magnesian volcanic rocks in the Sawyers Bar area REF ERE N C E Sof the Klamath Mountains show metasomatic and texturaleffects of submarine metamorphism regional metamorph- Alt, J. C., Honnorez, J., Laverne, C. & Emmermann, R., 1986.. . ' .. Hydrothermal alteration of a 1 km section through the upperIsm, ~nd. contact metamo~hlsm .~~ll,owln.g Igneous oceanic crust, Deep Sea Drilling Project hole 504B: mineralogy,crystallIZation. In general, the komatlltlc major-element chemistry, and evolution of seawater-basalt interactions.affinities are incompatible with trace-element concentra- Journal of Geophysical Research, 91, 10,309-10,355.tions, which indicate eruption as oceanic intraplate, mildly Ando, C. ~.,.I~in, W. P., Jones, D: L. & Saleeby,~. B., 1~83.alkalic basalts and immature arc tholeiites (Ernst et al., The ophIolitic North For~ terran~ In ~he Salmon. RIver r.eglon,1991) Th I . k . h d . M 0 ( central Klamath Mountains, CalifornIa. Geological Society of

. e metavo camc roc s are ennc e In g as America Bulletin, 94, 236-252.

much as 6 wt%) and depleted in Al2O3 and Na20 relative Apted, M. J. & Liou, J. G., 1983. Phase relations amongto worldwide volcanic rocks. Correlation of 'excess' MgO greenschist, epidote-amphibolite, and amphibolite in a basalticwith the volume per cent of hornblende (:i:clinopyroxene) system. American Journal of Science, 283-A, 328-354 (Orville

volume).suggests that the presence of. cu~ulus phases may have Arndt, N. T. & Nesbitt, R. W., 1982a. Geochemistry of Munroproduced much of the magnesian signature. Township basalts. In: Komatiites (eds Arndt, N. T. & Nisbet,

Substantial, relatively 10w-T submarine fluid-rock E. G.), pp. 309-329. George Allen & Unwin, London.interaction caused enrichment of {)180 from probable Arndt, N.: T. & Nisbet, E. G., 1982b: What is a komatiite? In:igneous values of c. 6%0 to c. 10%0. Bulk compositional Komatlltes (eds Arn~t, N. T. & Nisbet, E. G.), pp. 19-28.

. George Allen & Unwin, London.shifts (e.g. A12O3, Na20, and K2O) may have occurred Barton, M. D. & Hanson, R. B., 1989. Magmatism and theduring submarine metamorphism as well. development of low-pressure metamorphic belts: implications

Subsequent regional metamorphism (probably the from the western United States and thermal modeling.mid-Jurassic Siskiyou event of Coleman et al., 1988) Geological Society of America Bulletin, 101,1051-1065.

t h . I d l.ttl h . I h Beaty, D. W. & Taylor, H. P., Jr, 1982. The oxygen isotopeappears 0 ave InVO ve I e c emlca exc ang e. . ... .

.. . geochemIstry of komatlltes: evIdence for water-rock Interac-

Regional metamorphic grade decreases from greenschist tion. In: Komatiites (eds Arndt, N. T. & Nisbet, E. G.), pp.facies (350-5500 C, c. 3 kbar) southward to prehnite- 267-280. George Allen & Unwin, London.actinolite facies (200-4000 C, ~3 kbar). The greenschist Blome, C.. D. & Irwi?, W..P., ~983. Tectonic significance of Latefacies assemblages are actinolitic hornblende + biotite PaleozoIc to Juras~lc radlo!ana~s from the North ~ork terrane,

. .. Klamath Mountains, CalifornIa. In: Pre-JurassIc Rocks In(only In ~I-nc~ rocks) + ~hengl~e +. epidote + ~odlc plagw- Western North American Suspect Terranes (ed. Stevens, C.H.),

clase + mlcrochne + chlonte + tItanIte + hematite + quartz. pp. 77-87. Pacific Section of Economic Paleontologists andLess intensely recrystallized metabasaltic rocks in the Mineralogists, Los Angeles, California.south carry actinolite and prehnite. The moderate to low Bothwick, J. & Harmon, R. S., 1982. A note regarding ClF3 as an

of g' onal metamorph.s e t .bl . th alternative to BrFs for oxygen isotope analysis. Geochimica etpressures re I I m ar compa 1 e WI C h .. A . 46 166c

1668... osmoc Imlca c.a, , .'- .metamorphism In a magmatic arc and may be related to Bowers, T. S. & Taylor, H. P., Jr, 1985. An integrated chemicalthe heat sources causing eruption of the volcanic rocks. and stable isotope model of the origin of midocean ridge hot

I I ,

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""'-',- - -- - -METAMORPHISM OF MAGNESIAN VOLCANIC ROCKS 69

spring systems. Journal of Geophysical Research, 90, 12,583- Science, 281, 127-175. . . .12,606. Laird, J., Lanphere, M. A. & Albee, A. L., 1984. Dlstnbutlon of

Bucher-Nurminen, K., 1987. A recalibration of the chlorite- Ordovician and Devonian metamorphism in mafic and peliticbiotite-muscovite geobarometer. Contributions to Mineralogy schists from northern Vermont. American Journal of Science,and Petrology, 96, 519-522. 284,376-413.

Cameron, W. E. & Nisbet, E. G., 1982. Phanerozoic analogues of Liou, J. G., 1973. Synthesis and stability relations of epidote,komatiitic basalts. In: Komatiites (eds Arndt, N. T. & Nisbet, C~AI2FeSi3O12(OH). Journal of Petrology, 14,381-413.E. G.), pp. 29-49. George Allen & Unwin, London. Liou, J. G., KuniYo.shi, S. & Ito, K., 197~. Experime~tal ~tu?ies

Coleman, R. G., Manning, C. E., Mortimer, N., Donato, M. M. of the phase relations between greenschIst and amphIbolIte m a& Hill, L. B. 1988. Tectonic and regional metamorphic basaltic system. American Journal of Science, 274,613-632.framework of the Klamath Mountains and adjacent Coast Liou, J. G., Maruyama, S. & Cho, M., 1985. Phase equilibria andRanges, California and Oregon. In: Metamorphism and Crustal mineral parageneses of metabasites in low-grade metamorph-Evolution of the Western United States (ed. Ernst, W. G.) ism. Mineralogical Magazine, 49,321-333.Rubey Vol. VII, p. 1061-1096. Prentice Hall, Englewood Liou, J. G., Maruyama, S'. & Cho, M.,.1987: Very low-~radeCliffs, NJ. metamorphism of volcanIc and volcanIclastIc rocks--mmeral

Criss, R. E. & Taylor, H. P., Jr, 1986. Meteoric-hydrothermal assemblages and mineral facies. In: Low. Temperaturesystems. In: Stable Isotopes in High Temperature Geologic Metamorphism (ed. Frey, ~.), pp. 59-1~2. Blackle, Glasgow.Processes, Reviews in Mineralogy, Vol. 16 (eds Valley, J. W., Maresch, W. V., 1977. Expenmental studIes on glaucophane: anTaylor, Jr. H. P., & O'Neil, J. R.), pp. 373-424. analysis of present knowledge. Tectonophysics, 43, 109-1.25. .

Donato, M. M., 1989. Metamorphism of an ophiolitic tectonic Maruyama, S., Liou, J. G. & Suzuki, K., 1982. The penstentemelange, northern California Klamath Mountains, USA. gap in low-grade metamorphic rocks. Contributions toJournal of Metamorphic Geology, 7,515-528. Mineralogy and Petrology, 81,268-276.

Ernst, W. G., 1987. Mafic meta-igneous arc rocks of apparent Massone, H.-J. & Schreyer, W. 1987. Phengite geobarometrykomatiitic affinities, Sawyers Bar area, central Klamath based on the limiting assemblage with K-feldspar, phlogopite,Mountains, northern California. Special Publication of the and quartz. Contributions to Mineralogy and Petrology, 96,Geochemical Society, 1, 191-208. 212-224.

Ernst, W. G., 1990. Accretionary terrane in the Sawyers Bar area Miyashiro, A., 1973. Metamorphism and Metamorphic Belts.of the Western Triassic and Paleozoic belt, central Klamath George Allen & Unwin, London, 492 pp.Mountains, northern California. Geological Society of America Muehlenbachs, K., 1986. Alteration of the oceanic crust and theSpecial Paper, 2SS, 297-306. 180 history of seawater. In: Stable Isotopes in High Temperature

Ernst, W. G., Hacker, B. R., Barton, M. D. & Sen, G., 1991. Geologic Processes (eds Valley, J. W., Taylor, H. P. & O'Neil,Occurrence, geochemistry and igneous petrogenesis of mag- J. R.), Reviews in Mineralogy, Vol. 16, pp. 425-444.nesian metavolcanic rocks from the WTrPz belt, central Nitsch, K.-H., 1971. Stabilitiitsbeziehungen von Prehnite-undKlamath Mountains, northern California. Geological Society of Pumpellyite-haltigen Paragenesen. Contribution.~ to MineralogyAmerica Bulletin, 103,56-72. and Petrology, 30,240-260.

Goodge, J. W., 1989. Evolving early Mesozoic convergent margin Powell, R. & Evans, J. A., 1983. A new geobarometer for thedeformation, central Klamath Mountains, northern California. assemblage biotite-muscovite-chlorite-quartz. Journal of Met-Tectonics, 8,845-864. amorphic Geology, 1, 331-336.

Hacker, B. R. & Ernst, W. G., 1991. Permian to Jurassic tectonic Rock, N. M. S. & Leake, B. E., 1984. The Internationalevolution of the Hayfork, Salmon River, North Fork, and Mineralogical Association amphibole nomenclature scheme:Stuart Fork terranes, Klamath Mountains. Geological Society of computerization and its consequences. Mineralogical Magazine,America Abstracts with Programs, 23,32. 48,211-227.

Hill, L. B., 1985. Metamorphic, deformational and temporal Seyfried, W. E., Jr, Berndt, M. E. & Seewald, J. S., 1988.constraints on terrane assembly, northern Klamath Mountains, Hydrothermal alteration processes at mid-ocean ridges:California. In: Tectonostratigraphic Terranes of the Circum- constraints from diabase alteration experiments, hot-springPacific Region (ed. Howell, D. G.), Earth Science Series, Vol. fluids, and composition of the oceanic crust. Canadian1, pp. 173-186. Circum-Pacific Council for Energy & Mineral Mineralogist, 26,787-804.Resources, Houston, TX. Silberling, N. J., Jones, D. L., Blake, M. C., Jr & Howell, D. G.,

Hoefs, J., 1987. Stable Isotope Geochemistry. Springer-Verlag, 1987. Lithotectonic terrane map of the western conterminousBerlin, 241 pp. United States. Miscellaneous Field Studies Map MF-1874-C,

Hollister, L. S., Grissom, G. C., Peters, E. K., Stowell, H. H. & scale 1:2,500,fXXJ.Sisson, V. B., 1987. Confirmation of the empirical correlation Taylor, H. P., Jr, 1968. The 180 geochemistry of igneous rocks.of Al in hornblende with pressure of solidification of Contributions to Mineralogy and Petrology, 19, 1-71.calc-alkaline plutons. American Mineralogist, 72,231-239. Velde, B., 1965, Phengite micas: synthesis, stability, and natural

Hynes, A., 1982. A comparison of amphiboles from medium- and occurrence. American Journal of Science, 263, 886-913.low-pressure metabasites. Contributions to Mineralogy and Wright, J. E. & Fahan, M. R., 1988. An expanded view ofPetrology, 81, 119-125. Jurassic orogenesis in the western United States Cordillera:

Johnson, M. C. & Rutherford, M. J., 1989. Experimental Middle Jurassic (pre-Nevadan) regional metamorphism andcalibration of the aluminium-in-hornblende geobarometer with thrust faulting within an active arc environment, Klamathapplication to Long Valley caldera (California) volcanic rocks. Mountains, California. Geological Society of America Bulletin,Geology, 17,837-841. 100,859-876.

Laird, J. & Albee, A. L., 1981a. High-pressure metamorphism in Yoshiasa, A. & Mat'lumoto, T., 1985. Crystal structuremafic schist from northern Vermont. American Journal of refinement and crystal chemistry of pumpellyite. AmericanScience, 281,97-126. Mineralogist, 70, 1011-1019.

Laird, J. & Albee, A. L., 1981b. Pressure, temperature, and timeindicators in mafic schist: their application to reconstructing thepolymetamorphic history of Vermont. American Journal of Received 7 April 1990; revision accepted 27 April 1991.

II