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Home , Metamorphism

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

Metamorphism, and origin of magnesian volcanic rocks, , California B. R. HACKER AND W. G. ERNST . Department of , School of Earth Sciences, Stanford University, Stanford, CA 94305-2115, USA M. D. BARTON* Department of Earthand Space Sciences, University of California, Los Angeles, CA 90024-1567,USA

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, 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 , but im~at~re arc rocks and within-plate alkalic lavas. Correlation of 'excess' MgO with the volume per cent (:t:clinopyroxene) suggeststhat the presence of cumulus phasescontributes to the MgO-rich compositions. Early submarine alteration produced regional 6180 values of + 10:t: 1.5%0and shifts in AlzO3,.NazO, and KzO concentrations. Regional metamorphic grade in the study area varies from -zone (350-550° C, c. 3 kbar) southward to prehnite- facies (200-400° C, ~3 kbar) , but little isotopic or elemental change occurred during the regional recrystallization. The greenschist facies assemblage is actinolitic hornblende + + + sodic plagioclas~ + microcline + chlorite + titanite + hematite + in Ti-poor metabasaltic rocks; in addition to these hases biotite is present in Ti-rich analogues. Lower grade greenstones contain prehnite and more nearl stoichiometric actinolite. The moderate to low pressures of regional are compatibl~ with P-T conditions in a magmatic arc. Later 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 content of , m~ covite content of potassic white , and anorthite content of compositions. Metasomatisrtt :during contact metamorph- ism produced further increasesin bulk- 6180SMOWof as much as +6%0. Thus, the unusually MgO-rich of the Sawyers Bar rocks may be attributed at least partly to metas~ratism and the presence of

magnesiancumulus phases. : Ii Key words: Klamath Mountains; komatiitic ; meta.-mati=; 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 discusseshow metamorphic changes can be stripped away Jurassic(?) 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 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,focuses on greenstonesin the ism, or some combination of the two. Moreover, the Sawyers Bar area that are intercalated with , distinctive composition might be a powerful basis for , and containimg radiolaria of late Permian correlating or differentiating these rocks from those of through early Jurassic age: (Blome & Irwin, 1983). The nearby , 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 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 the Tucson,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

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Fig. 1. Geology of the Sawyers Bar area, showing locations of samples. A preliminary version of a portion of this map, including a cross-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-igneous rocks; 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 to have begun in Permo-Triassic time (Ando et al., 1983). weakly foliated, sparsely pillowed flows and hypabyssal These rocks were thrust eastward beneath the Stuart Fork bodies of highly magnesian to tholeiitic basalt composition terrane, a late Triassic complex (Goodge, (Ernst, 1987). They are divided into two groups: darker 1989) prior to the emplacement of the mid-Jurassic coloured, high-Ti (>2.0 wt% TiO2) rocks and lighter plutons. coloured, low-Ti «1.5 wt% TiO2) rocks. Those of the first METAMORPHISM OF MAGNESIAN VOLCANIC ROCKS 57

group are enriched in light rare-earth elements (LREE), P, Hf, Zr, Ta, Nb, Fe, and Ti, and compositionally resemble alkalic ocean-island basalts, whereas the second group shares trace-element affinities with immature arc tholeiites (Ernst et at., 1991). A significant portion of both the Ti-rich and Ti-poor samples have major-element concentrations appropriate for komatiitic basalts (Arndt & Nisbet, 1982a,b)-up to 18wt% MgO and CaO/AI2O3 ratios ~1 (Ernst, 1987). Flows interfinger with quartzo- feldspathic detrital strata and chert interpreted as distal turbidites. The related hypabyssal rocks intrude the flows and sedimentary rocks. Because the metamorphosed dykes and sills are chemically indistinguishable from the Ti-poor extrusive rocks, the entire Ti-poor sequence is permissibly coeval. The structure of the region is interpreted as several, kilometre-scale or larger, west-vergent synforms and antiforms (Ernst, 1987). Indistinct axial-planar dips steeply to the ENE. Compositional layering in the metasedimentary strata parallels contacts with intercalated metavolcanic rocks. A preliminary geological map is presented as Fig. 1. The greenstones are texturally complex, having under- gone eruption, possible submarine alteration, regional metamorphism, and local contact metamorphism. 'Sub- marine' alteration is used throughout this paper to indicate any kind of chemical change in the submarine realm, regardless of whether it is associated with ridge-crest hydrothermal activity, the construction of a magmatic arc, or the formation of an ocean island. Evidence for submarine alteration comes primarily from consideration of the bulk-rock compositions and oxygen isotopes. A regionally extensive biotite-zone greenschist facies to prehnite-actinolite facies metamorphism is recognized, by both textures and compositions, in foliated rocks distant from plutons. For example, prismatic igneous are. ov~rgrown ~y nematoblastic a.mphibole zoned toward Fig. 2. Back-scatteredelectron micrographs of regionally actInolIte. Regional metamorphism was followed by metamorphosedmetavolcanic rocks. 57M: intergrowthof contact metamorphism to 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(~); . . indicatIonsof dlsequlllbnumInclude patchy zonation In actinolite, Peak calc-alkalIne ~lutons. Contact. metamorphosed two kindsof chlorite, and two kindsof plagioclase.351M: two samples are recognIzed by hornfelslc textures and kindsof igneousamphibole, tschermakitic hornblende (t) and distinctive compositional zoning of amphibole and magnesio-hastingsite(h), overgrownby actinolitichornblende (a) plagioclase. Amphiboles in contact aureoles consist of in a dark matrix of plagioclase,microcline, biotite, , pnsmatlc.. pargaslte.. nms on actmo. I Ite. cores t hat had chlorite' andquartz. developed during the regional metamorphism. Three to four kilometres from the plutons, contact metamorphism is not discernible in the parageneses or composi- xene . Relict igneous phases compose 0-70 vol% tions, and regional metamorphic are well of each sample, indicating that some rocks may have been preserved. Scattered local crystallization of nearly holocrystalline, others perhaps glassy. Igneous and minerals was associated with or followed crystals preserved in the metavolcanic rocks include contact 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 mineral to completely recrystallized, and samples containing relict assemblageis similar to Archaean komatiitic basalts, which igneous crystals are sporadically distributed throughout the contain predominantly clinopyroxene and minor study 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.

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

METASOMATISM AND exchange -:vith metasedimentary rocks during contact ACCUMULATION metamorphIsm. . . '. Bulk-rock composItIons were presented and dIscussedIn Several different approaches were used to identify the detail by Ernst (1987) and Ernst et al. (1991); only those origin of the 'komatiitic' geochemical signature of the aspects relevant to evaluating and crystal SawyersBar greenstones: (i) correlation of oxygen isotopic accumulation are presented here. Most trace elements values 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 element are 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 been modes of relict igneous minerals. If such changes can be preserved during metamorphism. The Ti-rich rocks have identified, then it should be possible to 'see through' any trace element concentrations typical of alkalic within-plate metasomatic veil back to the igneous bulk compositions lavas, including elevated Ta and Nb contents, and the and clarify the igneous petrogenesis of these unusually Ti-poor rocks are similar to immature arc lavas, including MgO-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 or combination of three distinct processes:(i) limited igneous komatiitic basalts, which, for example, contain <16 ppm compositional 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 are ing 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. Typicalconcentrations 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 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, and are comparable to alkalic within-plate lavas (Ti-rich rocks) Mg/(Mg + Fe) ratios of 0.55-0.65, for the most magnesian and immature arc lavas (Ti-poor rocks), as detailed by lavas (Ernst et al., 1991). Without unaltered rocks from Ernst 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 are systematic variation with respect to fractionation indices limited. Compared to worldwide volcanic rocks, some of such as Zr (Fig. 3) (Ernst et al., 1991), most of the major the Sawyers Bar volcanic rocks have unusually low element concentrations, although within the range of amounts of AlzO3 or NazO, elevated MgO, and variable unaltered worldwide volcanic rocks, show considerably KzO concentrations (Fig. 4). Zr contents are clearly more 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 the variation is more likely the result of metasomatism or presence of cumulus phases. Figure 4 shows that some crystal 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 ~oW 16 cc cccc, "'«"'co"'"'"',.:"'"'co"'."'"'«:-co"'«««»»»:-,:-:- co",",",",",",",",",",:",:.:",",",.:", wor IdWI.d e vo Icamc. rock s. Alth ough t he depI eted AI z0 3 ~ 14 and elevated MgO contents may have contributed to the ~ 'komatiitic' character of these basalts, many of the samples 0 12 (including some with high MgO contents) are not N demonstrably different from worldwide volcanic rocks. < 10. Thus, the 'komatiitic' characterof these rocks, including 8 the elevatedMgO, Ni, and Cr contents,may be the result -. 20 of some igneous process. ~ . . altered? Bulk-!ock Cr contentsde~rease with increasingZr and ! 16 ., """,;;;ili::::;::;:ili:::::::;::::::;:::::ili;:::i;\ ~~~~~~~::ef~~~~na:~:e~~In:ccu~~~ati~~n~~~:~:~~~ t~;

0 12 compositional evolution (Ernst et al., 1991). Similar ~ behaviour of Ni also suggeststhat 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 hornblende 0 2 CO""::"""""""::,,'»CO,,,,,,»,,», fractionationor accumulation,it is necessaryto determine N ~ whether the excess MgO contents of Sawyers Bar ~ 1 ~ ~ greenstonesare correlated with the volume per cents of e . igneous hornblende or . In worldwide volcanic 0 arc rocks, MgO is correlatedwith CrzO3(Fig. 5a). At a -. 3 given CrzO3content, the SawyersBar rocks are displaced ~ l. d? to higher MgO contents-toward the compositions of ~' atere. ' I' oW re Ict hom bl en de an d 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 the 0 abundance of igneous hornblende. Moreover, the mag- 0 50 100 150 200 250 300 nitude of the excess is qualitatively similar to that Zr (ppm) suggested in Fig. 4. To avoid some of the potential circularity in Fig. 5(a, b) (circular because hornblende Fig. 4. Comparisonof SawyersBar greenstonemajor-element contains substantial CrzO3 as well as MgO), the 'excess' concentrationswith worldwidevolcanic rocks. Shaded fields MgO derivation has been repeated using Hf in place of labelled'VCAB' and 'O1B' showtypical values of intraoceanic Cr 0 . F' 5() th It' . . andcontinental arc rocksand ocean-islandbasalts, respectively . z 3 In Ig. c; e res~ s are, In agr,eement. ~Imllar (B. R. Hackerliterature survey, unpublished). Symbols as in diagrams based on correlation of excess MgO with the Fig. 1. volume per cent of pyroxene yield similar results,

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

(a) suggesting that bulk-rock compositions are not quenched 20 liquid compositions, but contain a contribution from 18 cumulus amphibole and/or pyroxene. Moreover, there is no need to invoke metasomatism to account for the high 16 MgO contents of these rocks, although metasomatism may 0 be responsible for the unusual Alz03, NazO, and KzO on 14 contents. ~ 12 .Q Oxygen isotopes 0 . blk k .. d . ~ 10 xygenIsotope u -roc compositionswere measure In ~ 8 worldwide 21 samples to assessthe degree of metasomatism in the volcanic metavolcanicrocks. Analyseswere done by fluorination 6 arc rocks with ClF3 (Bothwick & Harmon, 1982)and conventional 4 massspectrometry; reproducibility was :i:0.2%. 0 0 Oxygenisotope systematics are disturbedfrom plausible . 0.1 0.2 0.3 0.4 0.5 igneous compositions in all analysed metavolcanic rocks wt % Cr 0 (Table 1). Measured 6180SMOWvalues increase toward 2 3 granitoidplutons (Fig. 6). Samplesmore 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 6180 0 Cr 2°3 basis. valuesof +11.3%0(nearly unaltered sample) to +10.4%0 on 5 (sample adjacent to and contaminated by wall rocks); ~ samples of metasedimentary rocks intercalated with the 4 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 are r~ . unableto demonstratethat other elementsbehave in a ~ similar fashion. This suggeststhat any shifts in bulk 0 compositions away from probable igneous values are 0 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 valuesof +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 to on 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 CrZO3content 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 CrZO3content in hornblende-bearing samplesis 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 of the diagonal line in Fig. 5(a). (c) 'Excess' MgO for a given Hf V I 01 V I . H content in hornblende-bearing samples is correlated with the 0 -/0 0 camc ornblende volumepercent of igneoushornblende. Symbols as in Fig. 1.

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

Table1. 6180bulk-rock values for metamorphosedigneous irregular, but substantial MgO enrichment in the rocks,associated sedimentary rocks, and the EnglishPeak pluton. metavolcanic rocks and the considerable variabilit y in K 20 Rock type Sample 6180 (%0) and Na20, attributed earlier to crystal accumulation, could also 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 6180 354Mt 12.85 near the Russian Peak and English Peak plutons is most 372M 11.32 consistent with a contact metamorphic origin. The large 386M 10.80 180 enrichment contrasts with the typical large 180

' O 1 . 46M 13 81 depletions observed in the aureoles of most epizonal TI 2-poorvo camc . . . . 86M 8.69 intrusions(Cnss & Taylor, 1986).Calculated waters would 92Mt 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 heavy 374M 13.02 oxygen IS the Intercalated metasedimentary sequence (see 516M 10.58* Table 1). Metasedimentary rocks are more abundant in the areas near the intrusive contacts, the general areas of 180 Hypabyssal 220M 11.37* enrichment in the greenstones. Local evidence for 285M 11.46 met ased Iment-metalgneous. . exch ange (descn.b ed ab ove) Sedimentary 134M 16.63 and the lack of other adequate sources of heavy oxygen 166M 14.67 require re-equilibration on the scale of 101-103m. This 178M 14.87 exchangewas probably facilitated by local fluid circulation ~~~~ ~~.~~ driven by emplacement of the English Peak and Russian 355M 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 becauseof the areally limited 180 enrichment. I 26M 9.0* (quartz = 13.98) Given the subequal amounts of exposed metasedimen- t d t I . k h. 326M ]0.4* ary an me avo camc roc s, t IS postuI ated exch ange . Averageof two analyses. should have decreased the 6180 of the metasedimentary t 'Komatiitic' sample. rocks. Such a decrease is difficult to document given the substantial variation in sedimentary oxygen isotope volume of water isotopically heavier than seawater. No compositions; however, it is possible that much of the adequate 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 excessof +20%0as is typical of cherty, calcareous, ism was minor. Integrated fluid-to-rock ratios during and clay-rich sediments (Hoefs, 1987) and has now been submarine 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 evaluate e 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 standardswhose major element concentra- tions are reproducible to better than :i:2% for major 9 0 elements and :i: 10% for minor elements. Textural analysis 0 and spot selection were performed in back-scattered- 8 electron mode. The electron beam diameter was 2IJm at 0 1 2 3 4 5 6 7 15-kV accelerating voltage and 10-15-nA sample current Distance from Pluton (km) on minerals. Counting periods of 20s for peak intensities

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

1 I , 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 greenstonesare related 10 elements. Detection limits at these conditions are a to bulk-rock magnesia contents. No other elements in function 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 metabasaltic 0.03 wt% for FeO, MgO, and CaO, 0.04 wt% for KzO, samples studied, primarily in Ti-rich metamorphosed and 0.05 wt% for NazO. Analyses of inhomogeneous extrusive rocks. The biotite occurs as subhedral plates grains are presented individually. All mineral formulae 20-80 /lm in length. Biotite ranges in composition were calculated by the method of Laird & Albee from K\oMg\.3Fe\3Cro.oTiozAlo.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 assemblagesspan upper green- accounts for as much as 1.1 wt% in biotite. There are too to prehnite-actinolite facies. The low-variance few biotite analyses to evaluate whether there is any regional greenschist facies assemblage is actinolitic systematic regional variation in the composition. hornblende + phengite + epidote + chlorite + albite + micro- Phengite occurs in most greenstone samplesas 10-50-/lm- cline + quartz + titanite + hematite in low- Ti rocks; biotite sized subhedral platelets produced by recrystallization of is also present in low-variance assemblagesdeveloped in . White mica ranges in composition from high-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, and phengite + 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 (0.6-4.3 wt%) are plutons contain pargasitic hornblende:t clinopyroxene + weakly correlated with bulk-rock MgO contents. Phengites calcic 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 earlier rims 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 as an igneous pargasitic hornblende core overgrown by a subhedral flakes 20-80 /lm in length. Chlorite TiOz, Cr303 regional metamorphic rim zoned outward to actinolite, in and MgO contents depend on bulk-rock composition. No turn overgrown by a contact metamorphic rim zoned systematic regional variation in chlorite composition was outward to pargasite. Amphiboles that grew during detected, and the range in composition of chlorite platelets regional metamorphism consist of discrete fibrous neo- from a single sample is comparable to the variation in blasts in the groundmass and rims on stout prisms of compositions of all analysed crystals. Much of the chemical volcanic hornblende and pyroxene. Most are subhedral to variation can be accounted for by substitution of euhedral, 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 per formula unit (p.f.u.). MnO and grown during regional metamorphism display decreasesin CrZO3are present in minor quantities. NaM4, NaA + K, AIV\ + Fe3++ Ti, and Apv, from core to Plagioclase crystals interpreted as volcanic, regional rim (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, and disequilibrium growth. intimately intergrown with pyroxene or amphibole. Contact metamorphic amphiboles form overgrowths on Metamorphic plagioclase occurs in nearly all samples regional metamorphic amphiboles and are zoned smoothly within healed cracks in relict igneous laths and as neoblasts from actinolitic cores to paragasitic hornblende rims (Fig. within or adjacent to precursor igneous crystals. Most 7b). 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 not pair magnesio-cummingtonite + magnesio-hornblende as- depend on bulk-rock compositions, except in contact sociated with andesine and Mg-rich chlorite. Most calcic metamorphosed rocks lacking quartz where plagioclase amphiboles produced during contact metamorphism CaO correlates with bulk rock CaO. Individual albite display 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 metamorphic 8)-exactly opposite to the regional metamorphic trend. assemblages(Fig. IOc). Albite + oligoclase pairs in two Approaching the granitoids, SiOz and CaO contents of samples have compositions appropriate for the peristerite regional actinolitic amphibole rims begin to decline sharply gap (Maruyama et al., 1982): ADo3%OO+ An\s%o\ and beginning about 3-4 km from plutons as these phases are Afi04%oo+ An\s%o\ (Fig. lOc). In contact metamorphosed overprinted; AlzO3. TiOz, NazO, and KzO contents rocks within 2 km of calc-alkaline plutons, plagioclase concomitantly increase (Figs 9 & lOa, b). The mechanism ranges from Anzo to An98 (Fig. 10c). Contact metamorphic for the incorporation of the AlzO3 is principally through plagioclase crystals contain minor but measurable amounts the tschermak and edenite substitutions (Fig. 7b), of TiOz, CrzO3, and MnO; FezO3, MgO, and KzO are compatible with higher temperatures near the plutons. The more abundant.

, ,\ , I I I !

, METAMORPHISM OF MAGNESIAN VOLCANIC ROCKS 63

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

.20 0.3 . "2. a . U+.15 a ~~ 0.2 . a ~ .

a .10 Z -=~ . 0.1 Z .05 . . toEd, Pg to Ts 0.0 .00 0.0 0.1 0.2 0.3 OA 05 0.6 .00 .05 .10 .15 .20 .25 .30 NaA +K AI/(AI+Si)

15 toGl .. toTs OA IoGl E=. 0.3. of 1.0 a a + n a . ...~ a ~~ 0.2 a a . 0.5 #. aa . 8 a 8 ~ to Pg Z ~ >- '" B ~ a 0.1 a ".,8" 8 a -< . 8 Ed a P toTs 0.0 0.0 'F.d 0.0 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 --- .20 ~a a~IoPg 0.3 8 ~ U . a +.15 / a ~~ 0.2 a a a Z IQ a Z a a - .10 a a ~ ,,;,~a a -= a . 0.1 a e" a Z .05 a a a a a 'to Ed, Pg a 10Ts 0.0 .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 - 0.4 to Gl to Ts ,." IoGl .- a a ~ 0.3 8 E-c ~~a ~a a a a of 10 . a ...+ . a aa. a "' ..~~a 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 Ed a a a a B 10Ts 0.0 0.0 Ed 0.0 05 1.0 1.5 0.0 05 1.0 All V AI VI +Fe3+ +Ti Fig. 7. Compositions of amphiboles (atoms per 23 ) developedduring (a) regionaland (b) contactmetamorphism plotted on discriminant diagrams of Laird & Albee (1981b). Tschermakite substitution produces a 1: 1 slope on the Alvl + Fe3+ + Ti versus Arv diagram, and substitution produces a 1: 1 slope on the NaM4versus Alvl + Fe3+ + Ti diagram. Typical core-to-rim zonations are 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.

I I ,-, j , I "I, 64 B. R. HACKER, W. G. ERNST & M. D. BARTON

18 metamorphic' igneous . metamorphic

14 [!~~] .,.'.'.\ ~J.~~.'.'.'.'" i \. , . .1. ~'~ .',.'..J . . \ . 10 ./ '.. \ .,'. .,'...' '" '. / \ i . 1 .1 ".,..FeO*' ~ 6. ' "."'.'" .' I .-.'.' . ' .' ..' .' .' '.' .' .'. 2 . . T'~l""L \ .'.'...'. , 18 metamorphic.' "'.' A igneous Na 0 metamorphic.. /..

14 []~EI t"'\,.,!\. ~!O3 ,1"\ '" \ ,..'.\ ,"." .., ~'~ .. .' . \ 10 ", . ", .'... I. . FeO* . ,.,..,.'.- ., ..""-'" --. "'. '.'. I '. '. .,..", .,.'.'.' j \ I. ~ , .,.' " .' .,. ~ . .' .' 6 '. T'~l""L . 2 '.'.' .' , '... Na 0 ,'.' .'.-.' .'.' .-".' 2 ...,'.. Fig, 8, Compositional profiles of single amphibole crystals from two samples. The 0 50 100 150 200 amphibole cores formed by crystallization I I from a and the rims formed during rm regionalmetamorphism.

Metamorphic microcline (AfiooOr97-98Ab02--O3)is present with XFe = Fe/(AI + Fe) = 0.00-0.07, was found in 20 in more than half the samples studied, and is found in all samples, including hornfelsic greenstones bordering the samples with ~1.5 wt% K2O. It typically occurs adjacent English Peak pluton; in one sample it forms pseudomorphs to phengite crystals in 10-15-.um-diameter, anhedral, after epidote. Zeolite minerals occur in two samples. The equant 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 quite and 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 are widespread euhedral prismatic to subhedral forming 20-50-.um anhedral, equant crystals; titanite is equant 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 are scattered trace phases in most no systematic spatial variation. Epidotes with cores that samples. contain 12-31 mol% pistacite, Ca2Fe3Si3O'2(OH), are z~ned. to slightly more .aluminous rims (~1-3?mol% CONDITIONS OF METAMORPHISM AND plstaClte), and cores WIth .3-13mo.l% plstaCite are REGIONAL SIGNIFICANCE overgrown by more ferrugInous nms (12-19 mol% pistacite). TiO2, Cr2O3, MnO, MgO, Na20 and K2O are Textures and mineral compositions suggest the following present 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 environment hornblende in one contact metamorphosed rock, and in coincident with deposition of the intercalated deep-water veins in two regionally metamorphosed rocks-hence it sedimentary rocks (Ernst et at., 1991); (ii) high-grade clearly grew later than or during waning stages of those greenschist (north) to prehnite-actinolite (south) facies metamorphic events. The composition varies from regional metamorphism overprinted the entire section Ca37Mgo.sFeosAIs1Si6P22(OH)s to Ca3sMgosFeo2AIso- during deformation; (iii) the English Peak and Russian S~OO22(OH)s' In this study, pumpellyite analyses were Peak calc-alkaline plutons intruded the area, causing normalized to 24.5 oxygen atoms pfu (after Yoshiasa & hornfelsic contact metamorphism and shifts in oxygen Matsumoto, 1985). Prehnite, Ca'.9(Fe, AlhoSi3.oO1o(OHh isotopic composition; and (iv) pumpellyite and zeolite

I I ,I METAMORPHISM OF MAGNESIAN VOLCANIC ROCKS 65

0.06 . compositions and assemblages(Fig. 12). The presence of 0 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 . 5500C. 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, 0.01. .. . liit t .... . :!. 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 -: pargasitein contact metamorphic assemblagessuggests 1.9 . : L. .. . i I . i ~ peak metamorphic conditions between reactions (4) and ~ - I....., . .. (6) 10. F Ig.. 12 . ~ U 1.8 = i = Laird & Albee (1981b) showed that compositions of oS 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 the e 0.5 c 8 c c Sawyers Bar area all fall within the medium-P ~ c 8 c 8 (-) to 10w-P (-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 metamorphic rI) 00 C C 8 C g' B 8B C C ~ ~ pressures(MIyashIro, 1973, p. 288; Maruyamaet 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 zeolite 0.04 9 gOo 0 or prehnite-pumpellyite to greenschist facies at low 0 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 assemblageare present in the 20 ::. south-westof the SawyersBar area (Fig. 10c). . . . The Al content in igneous hornblende coexisting with 1.5 \.' . : . potassium feldspar, quartz, plagioclase, biotite, titanite, '< 10 ',:. . . ilmeniteor magnetite,and silicate liquid can be usedto . f; t..; t ~ estimatethe depth of emplacementof the English Peak 0.5 . ...eo...... ! .t ;.:.. t .' pluton.Hornblende cores contain 1.3-1.4 Al atomsp.f.u., 00' ...... t ...... : ~~.. andrimshave1.1-1.6Alatomsp.f.u.Althoughthetotal . 0 1 2 3 4 5 6 7 distribution of rim compositionsis large, the mean and D' t fit (k ) standarddeviation are 1.4:l: 0.14 Al atomsp.f.u. Empirical IS ante rom p u on m (Hollister et at., 1987) and experimental calibrations

'.' (Johnson & Rutherford, 1989) indicate pressures of Fig, 9. Changesin compositions of amphiboles (atoms per 23 . oxygens)produced by mid-JurassicEnglish Peak and Russian 2.6-3.7.:l: ~ kbar and 2-2.9:l: 0.5.kbar, respectIvely, for Peak calc-alkalineplutons; each symbol represents a single crystallIZationof hornblende with 1.4:l: 0.14Al atoms analysisand columns of symbolsat a givendistance from plutons p.f.u. The experiments were conducted at several oxygen are from a singlesample. 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 metamorphism recrystallization. Based on radiometric and fossil ages occurred at temperatures of 350-5500C and moderate to mentioned in the introduction, the regional metamorphism low pressures (c. 3 kbar); in contrast, slightly later contact took place after or during early Jurassic time, and the metamorphism occurred at temperatures as high as c. contact metamorphism is mid-Jurassic in age. 6000C, coincident with crystallization of the English Peak Estimates of physical conditions during metamor- granodiorite at 2-2.9 :l: 0.5 kbar. Prehnite-actinolite facies phism(s) are possible, based on the observed phase assemblages developed at lower temperatures (200-

I i I 'i 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 of metamorphic plagioclase and prehnite. Regional metamorphism: circles indicate albite, squaresoligoclase, diamonds albite of any composition + oligoclase of any composition and triangles albite + oligoclase peristerite pairs; shaded area indicates regionally metamorphosed samplescontaining the lower grade equilibrium pair prehnite + actinolite. Contact metamorphism: lines delineate areas near plutons containing plagioclase 0 kin 2 more anorthitic than An2o' Other symbols and contacts after ~ Fig. I.

4000C) and pressures (~3 kbar) (Liou et at., 1987). suggest that regional metamorphism in the Sawyers Bar Because these estimates are based on experiments area is correlative with the Siskiyou event: (i) early to conducted under non-ideal conditions (e.g. some reactions mid-Jurassic age; (ii) moderate to low metamorphic are unreversed, unbuffered, or conducted on simplified pressures;and (iii) the southward decreasein metamorphic synthetic systems), they should be viewed as only pressuresand temperatures. . approximate. Using thermal modelling, Barton & Hanson (1989) A mid-Jurassic metamorphism termed the Siskiyou demonstrated that regional low-P metamorphism only event (Coleman et at., 1988) occurred during and after occurs in areas with a large fraction of relatively evenly widespread 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

, ' , ,Ii i , METAMORPHISM OF MAGNESIAN VOLCANIC ROCKS 67

3.40 . I = / NNO / ~ 3.30 :--;':' I I ~ e .'.. - QFM ...0 ,...~ :.../. . .cc NNO .-~ 3.20 . . ~ 00 ... Q,. T 7 3.10 . . oJ..

~ 0.008 QFM Co 0 ~ 0.006 200 300 400 500 600 700 0e 0.004 . T (OC) ~ 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 = . . . + + H2O (Liou et al., 1985);(2) 0.000 pumpellyite + chlorite + quartz = epidote + actinolite + H2O 0 1 2 3 4 5 6 7 (Nitsch,1971); (3) maximumglaucophane stability (Maresch, . 1977);(4) actinolitichornblende + 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-H2O coexist with microcline + quartz + chlorite :i: biotite are shown buffer. .Als? shown IS (7) the pressure.bracket In~erred for with 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 igneous contact metamorphism, was caused by heat advected to and metamorphic minerals, and by obtaining better shallow 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 SiO2 coeval. Heat for both events may have come from the than those that grew during the regional metamorphism same volcano-plutonic edifice that erupted the volcanic (Fig. 11). The magnitude of the SiO2 decrease toward the rocks. As a less likely alternative, the , regional plutons has implications for barometric estimations metamorphism, contact metamorphism, and pluton em- involving phengite. For example, Massone & Schreyer placement 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 0.91 351M 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 & Schreyer Powell & Evans (1983) (1987) (1987)

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

91M 10.2 2-3 4.6 1-2 <1.5-2.5 351M 10.2 2-3 4.5 1-2 4-5 Both samples contain the assemblage muscovite + biotite + chlorite + quartz; 351M also contains microcline. Pressuresare calculated for temperatures of 400-5000C, from figs 1 & 2 of Powell & Evans (1983), figs 1 & 2 of Bucher-Nurminen (1987), and fig. 3 of Massone & Schreyer (1987). Muscovite and biotite compositions are given as cations per 11 oxygens; chlorite composition as cations per 14 oxygens.

, c' , I " ~I 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 peak ducted in the temperature range 350-6000C to quantify temperatures of c. 6000C around the English Peak and changes in the Si content of phengite coexisting with Russian Peak plutons produced 3-4-km-wide aureoles biotite, quartz, and potassium feldspar, as a function of containing predominantly magnesio-hornblende + pressure and temperature. Two brackets for this same oligoclase + epidote + titanite + quartz. reaction were ,previously obtained by Velde (1965). also increased {)180 rock values by an additional 5%0 Discrepancies between the two data sets prompted within 3-4 km of plutons, probably by exchange with additional 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 the containing muscovite + biotite + chlorite + quartz + micro- contact metamorphism. cline, we calculate contact metamorphic pressures of 1-2, 2-3, and 4-5 kbar, for Bucher-Nurminen's (1987), ACKNOWLEDGEMENTS Powell & Evans' (1983) and Massone & Schreyer's (1987) formulations, respectively. For one sample con- This research was supported by the University of taining muscovite + biotite + chlorite + quartz, Bucher- California, Los Angeles, by Stanford University and by the Nurminen'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 UCLA 2-3 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 for from 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 ES of the Klamath Mountains show metasomatic and textural effects. of submarine metamorphism. ' regional ..metamorph- Alt,Hydrothermal J. C., Honnorez,alteration J., Laverne, of a 1 kmC. section& Emmermann,through theR., upper1986. Ism, ~nd. contact metamo~hlsm .~~ll,owln.g Igneous oceaniccrust, Deep SeaDrilling Projecthole 504B:, crystallIZation. In general, the komatlltlc major-element chemistry, and evolution of seawater-basaltinteractions. affinities are incompatible with trace-element concentra- Journalof GeophysicalResearch, 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 ~heSalmon. 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 among to worldwide volcanic rocks. Correlation of 'excess' MgO greenschist,epidote-amphibolite, and amphibolitein a basaltic with the volume per cent of hornblende (:i:clinopyroxene) system.American Journal of Science,283-A, 328-354(Orville suggests that the presence of. cu~ulus phases may have Arndt,volume). N. T. & Nesbitt, R. W., 1982a.Geochemistry of Munro produced much of the magnesian signature. Townshipbasalts. 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 ?In: igneous values of c. 6%0to c. 10%0.Bulk compositional Komatlltes(eds Arn~t, N. T. & Nisbet, E. G.), pp. 19-28. . GeorgeAllen & Unwin, London. shifts (e.g. A12O3, Na20, and K2O) may have occurred Barton, M. D. & Hanson, R. B., 1989. Magmatismand the during submarine metamorphism as well. developmentof low-pressuremetamorphic belts: implications Subsequent regional metamorphism (probably the from the western United States and thermal modeling. mid-Jurassic Siskiyou event of Coleman et al., 1988) GeologicalSociety of AmericaBulletin, 101,1051-1065. t h . I d l.ttl h . I h Beaty, D. W. & Taylor, H. P., Jr, 1982.The oxygen isotope appears 0 ave InVOve 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 (200-4000C, ~3 kbar). The greenschist Blome, C..D. & Irwi?, W..P., ~983.Tectonic significance of Late facies assemblages are actinolitic hornblende + biotite PaleozoIcto Juras~lcradlo!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 Paleontologistsand Less 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 regardingClF3 as an of g' onal metamorph.s e t.bl . th alternative to BrFs for oxygen isotope analysis. Geochimica et pressures re I I m ar compa 1 e WI C h .. A 46 166c ... osmocImlca c.a,. , .'- 1668. metamorphism In a magmatic arc and may be related to Bowers,T. S. & Taylor, H. P., Jr, 1985.An integratedchemical the heat sources causing eruption of the volcanic rocks. and stableisotope model of the origin of midoceanridge hot

I I , ""'-',------

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 pelitic biotite-muscovite geobarometer. Contributions to Mineralogy from northern Vermont. American Journal of Science, and , 96, 519-522. 284,376-413. Cameron, W. E. & Nisbet, E. G., 1982. Phanerozoic analoguesof 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 and Ranges, California and Oregon. In: Metamorphism and Crustal mineral paragenesesof 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-~rade Cliffs, NJ. metamorphism of volcanIc and volcanIclastIc rocks--mmeral Criss, R. E. & Taylor, H. P., Jr, 1986. Meteoric-hydrothermal assemblages and mineral facies. In: Low. Temperature systems. 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: an Taylor, Jr. H. P., & O'Neil, J. R.), pp. 373-424. analysis of present knowledge. 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American Journal of Received 7 April 1990; revision accepted27 April 1991.

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