hot and deep: rock record of subduction initiation and

Lithos xxx (2009) xxx–xxx

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Hot and deep: Rock record of subduction initiation and exhumation ofhigh-temperature, high-pressure metamorphic rocks, Feather Riverultramafic belt, California

Christopher M. Smart, John Wakabayashi ⁎California State University, Fresno, Department of Earth and Environmental Sciences, 2576 E. San Ramon Avenue, Mail Stop ST-24, Fresno, CA 93740, USA

⁎ Corresponding author.E-mail address: [email protected] (J. Wak

0024-4937/$ – see front matter © 2009 Published by Edoi:10.1016/j.lithos.2009.06.012

Please cite this article as: Smart, C.M., Wtemperature, high-pressure metamorphic r

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Article history:Received 23 July 2008Accepted 6 June 2009Available online xxxx

Keywords:Subduction initiationMetamorphic solesOphiolitesHigh-pressure rock exhumation

Studies of a 10 to 300-m-thick unit of high-grade metamorphic rock (“external schists”) that crops out alongthe western border of the Feather River ultramafic belt (FRB), northern California, yield new insights intosubduction initiation and ophiolite emplacement processes. The high-temperature (T) foliation of theexternal schists dip moderately to steeply eastward beneath the ultramafic rocks of the FRB, a 150-km-longslab of suboceanic upper mantle and the high-T fabric shows a tops-to-the-west (FRB-side-up) sense ofshear. The structurally highest external schists record peak metamorphic conditions of 650–760 °C at 1.3–2.2 GPa. In contrast, sheeted dikes of the Devil's Gate ophiolite that overlie the ultramafic rocks yieldmetamorphic conditions of 710–730 °C at about 0.3–0.7 GPa. A km-scale lens of amphibolite withinultramafic rocks yields somewhat lower pressures than the structurally highest external schist, as do thestructurally lower rocks within the external schists. Significant exhumation of the external schists relative tothe structurally overlying ophiolitic rocks occurred along at least two major zones and the most significantexhumation was accommodated at least 1.5 km structurally above the ultramafic-external schist contact.Based on available geochronology, intraoceanic subduction may have initiated at approximately 240 Ma, andexposure of the external schist occurred prior to the deposition of rocks in the structurally highest part of theCalaveras Complex (minimum 177 Ma), a subduction complex that structurally underlies the external schists.High-T metamorphism of the Devil's Gate ophiolite may have resulted from partial (failed) ridge subduction.

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1. Introduction

Mechanisms of subduction initiation are the subject of consider-able debate, butmost authors agree subduction initiation exploits pre-existing weaknesses and material contrasts in the oceanic lithosphere(Casey and Dewey, 1984; Mueller and Phillips, 1991; Stern andBloomer, 1992; Wakabayashi and Dilek, 2003). Ophiolites are on-landremnants of oceanic crust and many of these ophiolites structurallyoverlie the position of former subduction zones (e.g., Moores, 1970).Structurally beneath many ophiolites are thin (b500 m) units of high-grade metamorphic rocks called metamorphic or dynamothermalsoles. These soles are thought to have formed during subductioninitiation beneath young oceanic lithosphere (hot subduction initia-tion), based primarily on the high temperature of metamorphismrecorded in them (peak temperatures in the 700–900 °C range), theirlithologies (primarily metabasite with meta-pelagic sediments), andtheir structural and chronologic relationships with the ophiolite thatdirectly overlies them (Williams and Smyth, 1973; Spray, 1984;Jamieson, 1986; Hacker, 1990). Metamorphic soles commonly have

abayashi).

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akabayashi, J., Hot and deeocks, Feather River ultramafi

inverted metamorphic gradients due to tectonic underplating duringsubduction of progressively older (and colder) oceanic lithosphere(Peacock, 1987; 1988; Hacker, 1990; 1994; Gnos, 1998), and showanticlockwise pressure–temperature–time (P–T–t) paths (P on posi-tive y-axis) (Wakabayashi, 1990; Dilek and Whitney, 1997; Önen andHall, 2000; Guilmette et al., 2008). Because metamorphic solesapparently formed during inception of subduction, their geology,and the geology of adjacent rocks, provide insight into the setting andmechanisms associated with hot subduction initiation (i.e. Jamieson,1986; Hacker, 1990; Guilmette et al., 2008).

Although many studies have been conducted on metamorphicsoles, some critical aspects of metamorphic sole development havereceived little attention. For example, metamorphic soles were onceassumed to have been “welded” to the base of ophiolites after theywere underplated (scraped off the downgoing plate) as subductionbegan beneath the ophiolite (Williams and Smyth, 1973; Malpas,1979; Searle and Malpas, 1980). Such a model assumed that noexhumation of the sole relative to the ophiolite occurred aftermetamorphism. As new geobarometric methods became available,studies showed metamorphic pressures for soles that vastly exceededthat which could be explained by the structural thickness of theophiolite above the sole, indicating significant exhumation of the sole

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Fig. 1. Location map. Modified from Edelman and Sharp (1989). Abbreviations areCC: Calaveras Complex, DGO: Devil's Gate ophiolite, RAS: Red Ant schist, SFU: Shoo FlyComplex and other rocks bordering the east side of the Feather River ultramafic belt,WU: Undifferentiated Mesozoic (primarily) and Paleozoic metamorphic and plutonicrocks.

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relative to the ophiolite (summarized in Wakabayashi and Dilek(2000, 2003). For example, metamorphic pressures estimates rangefrom about 0.95 GPa to 1.8 GPa for different parts of the sole beneaththe Semail ophiolite of Oman (Gnos, 1998; Searle and Cox, 2002; Grayand Gregory, 2003), probably the world's most thoroughly studiedmetamorphic sole. The thickness of the overlying ophiolite can onlyaccount for burial pressures of about 0.5 to 0.6 GPa (e.g., Searle andMalpas, 1980). Many ophiolites are much thinner than the Semailophiolite, and the disparity between pressure estimates associatedwith metamorphic sole pressures (of about 0.5 to 1.5 GPa) and thepotential burial pressure associated with the ophiolite thickness(about 0.1 to 0.4 GPa) may be much greater (e.g., Jamieson, 1986;Guilmette et al., 2008). In addition, studies have identified inverted Pgradients (structurally high parts with pressure estimates of about 1.0to 1.8 GPa to structurally low parts of about 0.3 to 0.4 GPa) withinmetamorphic soles, indicating major internal imbrication within thesole (Jamieson, 1980; Jamieson, 1986; Gnos, 1998). These meta-morphic pressure contrasts have not been addressed in detail inmodels of subduction initiation (andmetamorphic sole development)and ophiolite emplacement (Wakabayashi and Dilek, 2003). Anotherproblem that has introduced complexity into the study of meta-morphic soles and subduction initiation processes has been theidentification of high-grade metamorphic rocks that are spatiallyassociated with ophiolite belts but do not appear to be classicmetamorphic soles as defined above. These include units of

Table 1Feather River ultramafic belt geochronology.

Location Age (Ma)

Alleghany schist 322 ±27, 345±9; 343.7±.6Oriental Mine granite (intrudes Alleghany schist) 388±22/−12Metagabbro intruding FRB in Yuba River area 285±8Devil's Gate ophiolite 248; 272±6;Gabbro dike intruding FRB north of Devil's Gate 387±7Red Ant schist N174Metaplagiogranite interlayered with “internal schist” 306–324“External schist” 236±4

Please cite this article as: Smart, C.M., Wakabayashi, J., Hot and deetemperature, high-pressure metamorphic rocks, Feather River ultramafi

amphibolite beneath ophiolites that are too thick (kilometers) tohave been generated by conductive heating beneath a hot mantlehanging wall (Harper et al., 1996; Barrow and Metcalf, 2006), high-grade rocks that may have been derived from the base of a magmaticarc instead of from the top of the downgoing plate (Grove et al., 2008),and high-grade metamorphic rocks that structurally overlie, ratherthan underlie an ophiolite (Dilek et al., 2008).

This paper presents structural, lithologic, and metamorphic P–Testimates for an amphibolite unit bordering the Feather Riverultramafic belt in northern California. We will review the regionalframework of these rocks, then present new field, petrographic, andmetamorphic petrologic data that bear on the origin and evolution ofthese rocks. We will show that structural, lithologic, and petrologicevidence supports a metamorphic sole model for these rocks and thespecific field and petrologic relationships give new insight into theexhumation of such rocks and the importance of such exhumation inmodels of subduction initiation and ophiolite emplacement.

2. Regional setting

The 150-km-long by 1–8 km wide Feather River ultramafic belt(FRB) of the northern Sierran Nevada, California (Fig. 1), comprisesvariably serpentinized ultramafic rocks, with lesser amounts ofmetagabbro, metadiabase, and metabasalt; collectively these rockshave been considered an ophiolite (Ehrenberg, 1975; Sharp, 1988;Edelman et al., 1989, Saleeby et al., 1989; Edelman and Sharp, 1989).All rocks of the FRB appear to have undergone peak metamorphism atamphibolite grade, with locally variable retrogression, although thereare significant internal differences in peak metamorphic conditions aswewill show. The FRB has yielded a rather large range in igneous (twodates of 385±10 and 314+10/−8 Ma, U/Pb zircon; Saleeby et al.,1989) and metamorphic ages (about 234 to 387 Ma, Ar/Ar and K/Arhornblende; Weisenberg and Avé Lallemant, 1977; Standlee, 1978;Hietanen, 1981; Böhlke and McKee, 1984) and it has been called apolygenetic ophiolite (Saleeby et al., 1989) (geochronology summar-ized in Table 1).

In the headwaters of the South Fork Feather River, and Slate Creek,pillow basalts, sheeted dikes, and gabbros crop out structurally aboveultramafic rocks of the FRB. These mafic igneous rocks and thesubjacent ultramafic rocks have been called the Devil's Gate ophiolite(Edelman et al., 1989), so the Devil's Gate ophiolite may be considereda subunit of the FRB (Fig.1). Metamorphic age dates obtained from theDevil's Gate ophiolite are 276±6 Ma (Ar/Ar hornblende; Standlee,1978) and 248 Ma (K/Ar hornblende, Hietanen, 1981).

The FRB is faulted along both eastern and western boundariesagainst rocks of dramatically different age and lithology (Sharp, 1988;Saleeby et al., 1989). East of the FRB is the Shoo Fly complex whichconsists of Ordivician to Devonian continentally-derived metasand-stone and chert deposited that are structurally overlain by a tectonicmélange (Varga and Moores, 1981; Hannah and Moores, 1986). ADevonian to Permian volcanic sequence overlies the Shoo Fly complexat an angular unconformity (Durrell and d'Allura, 1977; Harwood,1983; Hannah and Moores, 1986). The Shoo Fly Complex has

Method Reference

K/Ar, hornblende; Ar/Ar, hornblende Böhlke and McKee (1984); Hacker (1993)U/Pb, zircon Saleeby et al. (1989)K/Ar, hornblende Hietanen (1981)K/Ar, hornblende; Ar/Ar, hornblende Hietanen (1981); Standlee (1978)Ar/Ar hornblende Standlee (1978)K/Ar, muscovite Schweickert et al. (1980)U/Pb, zircon Saleeby et al. (1989)Ar/Ar hornblende Weisenberg and Avé Lallemant (1977)



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undergone pumpellyite–actinolite grade metamorphism in theregions flanking the FRB, (Day et al., 1988; Hacker, 1993).

The FRB is faulted against the Calaveras Complex on its west and,locally, the Red Ant schist. The Calaveras Complex is considered asubduction complex composed mainly of phyllite and metachert withblocks of volcanic rocks (Hietanen, 1981; Sharp, 1988; Edelman et al.,1989). In-situ conodonts and fusulinids found in the metasedimentsindicate that they were deposited at least as late as the Permian andthat the unit youngs westward (Hietanen, 1981; Bateman et al., 1985).The subduction–accretion or assembly of the Calaveras Complex wasunderway by 177 Ma, based on the U/Pb zircon age of a pluton thatcross cuts some of the earlier structures within the Calaveras Complex(Sharp, 1988). In the Calaveras Complex is mainly of pumpellyite–actinolite grade in regions adjacent to the FRB (Day et al., 1988;Hacker, 1993).

Fig. 2. Geologic map of the western borde


The Red Ant schist consists of quartz-rich schists (metachert andmetaclastic rocks) and metavolcanic rocks that underwent blueschistfacies metamorphism (Schweickert et al., 1980; Hietanen, 1981;Edelman et al., 1989). The Red Ant schist crops out structurallybeneath and west of the Devil's Gate ophiolite, whereas 10 km to thesouth it occurs east of and structurally beneath the FRB in the NorthYuba River area (Edelman et al., 1989). In the North Yuba River area anamphibolite-grade unit, known as the Alleghany schist, is foundstructurally beneath FRB ultramafic rocks and structurally above RedAnt schist. Radiometric dates on the Alleghany schist are 322±27 and345±9 Ma (K/Ar, hornblende; Böhlke and McKee, 1984) and 343.7±0.5 Ma (Ar/Ar, hornblende; Hacker, 1993). A K/Ar age on awhite micataken from the Red Ant schist indicates that the metamorphic age ofthe Red Ant schist is at least 174 Ma (Schweickert et al., 1980). Theactual metamorphic age of the Red Ant schist is difficult to interpret

r of the Feather River ultramafic belt.




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because the closure temperature for the white mica may be close to(or have been exceeded by) the temperature of the ubiquitouspumpellyite–actinolite overprint of all of the metamorphic terranes inthis part of the northern Sierra Nevada, and because excess argoncannot be directly interpreted in conventional K/Ar results (Hacker,1993).

A thin unit (10 to 300 m thick) of amphibolite, locally garnet-bearing, crops out along the west border of the FRB in the North ForkFeather River area (Fig. 2). Williams and Smyth (1973) and Ehrenberg(1975) proposed that these high-grade metamorphic rocks may be ametamorphic sole. Our study focuses on this unit and its structuraland metamorphic relationships to adjacent units.

3. Field relationships and structural geology

3.1. Geologic units and lithologies

The study area covers ~15 km2 of area near the confluence of theNorth Fork Feather River and the East Branch North Fork Feather River(Fig. 2) and spans the western border of the FRB, where it is faultedagainst the eastern Calaveras Complex. The Calaveras Complex andFRB units in this area generally strike northwesterly and dip steeply tothe east and are intruded by gabbroic and dioritic bodies that cross cutthe major foliation in both the FRB and Calaveras Complex (one of thelatter is shown on the northwestern part of Fig. 2).

The variably serpentinized ultramafic rocks of the Feather Riverultramafic body make up the easternmost and structurally highestunit in the area. In this region, the ultramafic belt is 3–5 km wide.These ultramafic rocks are metamorphosed in amphibolite faciesconditions with characteristic minerals such as tremolite, talc, andlocally anthophyllite, with antigorite (Ehrenberg, 1975, this study).The tremolite reaches a centimeter in length and is commonly severalmm long. These rocks have a metamorphic foliation defined by planarlayering of amphibole long-axes. Although foliated, many of theserocks form massive outcrops with relatively sparse fractures. Incontrast some outcrops exhibit closely spaced (cm scale or less)fractures or a brittle foliation, and some of these fractured rocks tendto have been retrograded to lower grade serpentinite mineralogy(lizardite-dominated). In this area, crustal rocks are rare within theultramafic body and this is generally representative of the FRB as awhole (Ehrenberg, 1975). Crustal rocks within the ultramafic body

Fig. 3. Photo of melt segregations in external schist. The black arrows point to areaswhere zones that appear to have been melt rich (felsic material between small pieces ofamphibolite restite) restite) feed leucosomes. This location about 15m north of samplelocation FR6.


consist of small (less than a hundred meters in long dimension) post-metamorphic dioritic or gabbroic intrusions, and lenses up to 3 km inthe long dimension of amphibolite that have been called the internalschists (Ehrenberg, 1975). Saleeby et al. (1989) obtained a U/Pb dateof 306–324 Ma for a plagiogranite (tonalite) that intrudes internalschist and Weisenberg and Avé Lallemant (1977) report an Ar/Arhornblende age of 236±4 Ma from the same unit.

The western contact of the ultramafic body, the Rich Bar fault, dipsmoderately to steeply eastward and appears to steepen in dip orbecome overturned in the southernmost part of the field area (Fig. 2).This contact is offset by late brittle faults (Fig. 2). Directly west of thecontact is a b300 meter thick unit of amphibolite facies metamorphicrocks, the external schist of Ehrenberg (1975), the rocks that havebeen proposed as a possible metamorphic sole (Williams and Smyth,1973; Ehrenberg, 1975). The external schist consist primarily ofamphibolite, with lesser amounts of metachert (nearly pure quartzwith very small amounts of garnet, whitemica, and other phases), andsomewhat intermediate rock, that we call “quartz-bearing amphibo-lite” that may have been mafic rock with cm-scale (or less) chertinterlayers or lenses. The structurally highest part of the externalschist includes garnet amphibolites (Fig. 2). Plagioclase amphiboliteor epidote amphibolite make upmost of the structurally lower parts ofthe external schist. The metamorphic grain sizes of most mineralsrange from tenths of a mm, to about 2 mm for most these rocks. Theexternal schist shows abundant partial melting textures (Fig. 3),indicative of peak metamorphic temperatures above the wet basaltsolidus. Some of the external schist in the Rich Bar area (southern partof Fig. 2 along East Branch Feather River) appears bluish in outcrop,but microprobe analyses (see below) show that these rocks lack sodicamphibole or other blueschist facies minerals.

West of, and structurally beneath, the external schist crop outslates/phyllites, cherts, andminor metavolcanic rocks of the CalaverasComplex. This unit has the aspect of a melange with a slate/phyllitematrix and chert blocks up to tens of meters or so in long dimension.The structurally highest part of Calaveras Complex rocks exposed nearthe Beldon Siphon (Fig. 2) appear to have a coarsermetamorphic grainsize (metamorphic white mica and actinolite to several tenths of amm) than the very fine grained (hundredths of mm metamorphicgrain sizes) rocks that characterize the remainder of the CalaverasComplex in this area. The Beldon Siphon exposures also include whatappear to be metamorphosed breccias with clasts of amphibolite andmafic volcanic rocks set in a phyllite or fine white mica quartz schistmatrix.

A hornblende gabbro dike or small pluton intrudes the CalaverasComplex and cuts the northern section of the external schist in YellowCreek canyon (“gb” in the northwestern part of Fig. 2). This dike lacksthe foliation seen in the Calaveras Complex and external schist andlacks high-grade metamorphism.

Additional samples were collected from the Devil's Gate ophiolite,about 40 km southeast of the study area (Fig. 1), for comparison withsamples from the field area. Here sheeted dikes and pillow basalts arerecognizable despite having been metamorphosed at amphibolitegrade (Edelman et al., 1989).

3.2. Structural geology

The ultramafic rocks, internal schist and external schist exhibit afoliation defined by the planar alignment high-temperature meta-morphic minerals that strikes northwest and dips northeast (Fig. 2).Foliations in the southeast portion of the area tend to dip vertically orto the west. The steepening is apparent in the map patterns exhibitedby the external and internal schist contacts. Throughout the area thefoliations are subparallel to the contact between the external schistand the ultramafic unit. The external schist has a stretching lineation,most easily recognized by the alignment of the long axes ofamphiboles. This lineation orientation shows much scatter. Lineations



Fig. 4. Photomicrograph of garnet amphibolite sample 92-2. Plane polarized light.Abbreviations: grt: garnet, hbl: hornblende; rt: rutile.

Fig. 5. Photomicrograph of sample FR6, a quartz-rich garnet–clinopyroxene amphibo-lite. Plane polarized light. Abbreviations as for Fig. 4 and: cpx: clinopyroxene, qtz:quartz, ttn: titanite, phen: phengite.

Fig. 6. Back scattered electron (BSE) image of FR6. garnet (grt), phengite (phen),clinopyroxene (cpx) with albite (ab), and quartz (qtz). The rims of the phengites arevery slightly brighter than the cores, probably reflecting higher Fe concentration andcorrespondingly higher Si substitution; this subtle difference is best viewed on theupper of the two phengites in the view.


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plunge direction is tied to the foliation dip, so east-plunging lineationsare associated with east-dipping foliation. C and s surfaces, shearbands, and asymmetric porphryclast tails, and asymmetric small-scale(generally centimeter to meter scale) folds in the foliation, consis-tently ultramafic-side-up sense-of-shear, which is tops-to-the-westfor east-dipping foliation. Isoclinal folds are common in the externalschist, although these are most easily observed only in outcrops thatshow pronounced compositional layering. Amplitudes of these foldsvary from centimeter scale to at least tens of meters. Axes of thesefolds appear to be subparallel to the foliation strike. The consistentshear sense orientation in the external shear sense indicates that thissense of shear predates the folds, rather than being associated withhigh-temperature passive flow or flexural slip folding which wouldresult in opposite senses of shear on opposing limbs of folds. The earlyisoclinal folds are themselves folded by at least one generation ofmoreopen folds at the scale of meters or larger, and these later generationsof folds are responsible for the scatter in the foliation and lineationorientations.

Foliation within the Calaveras Complex is subparallel to thefoliation in the external schist and ultramafic rocks. Calaveras complexfolding, and shear sense within the Calaveras Complex were notevaluated in this study.

4. Petrography

Mineral abbreviations in the following sections are from Kretz(1983).

4.1. External schist

We have divided the external schist into three main rock types,metabasites, rocks that appear to reflect fine interlayering ofmetacherts and metabasite in varying proportions, and metacherts.

The metabasites of the external schist can be divided into garnetamphibolite (Hbl+Grt±Ep±Ab±Qtz+Rt), plagioclase amphibolite(Hbl+Pl (entirely or nearly entirely replaced by Ab)±Cpx±Qtz+Rtor Ttn) or epidote amphibolite (Hbl+Ab+Ep±Qtz±Chl with eitherRt or Ttn). Some amphibolite from the Rich Bar area also have rarebluish rims on green or green-brown hornblende. Apparent blueamphibole rims on hornblende from amphibolite associated with theFRB have been previously identified by Ferguson and Gannett (1932)from the Alleghany district in the Yuba River region, about 60 kmsoutheast of the field area.We did not find fresh plagioclase (excludingalbitic plagioclase) in the external schist. The (inferred) plagioclase has


been heavily retrogressed or altered, with a fine-grained growth ofwhite mica, and Ca-silicate minerals. Epidote tends to show a higherpistachite content (higher birefringence and deeper yellow pleochro-ism) in epidote amphibolite than the garnet-amphibolite. Rutile iscommonly rimmed and in some cases nearly entirely replaced bytitanit. Garnet is rare in the metabasite and most are badly retrogradeand fractured, and consist of fragments generally less than 0.1 mm, butsome reach4 mm. The grain size ofmostminerals in amphibolite rangefrom 0.1 to 5 mmwith the tendency for somewhat finer grain sizes inepidote amphibolite. Hornblendes ranges in size from 1 to 5 mm.Hornblendes exhibit variable retrogression and commonly havepatchy brownish regions with surrounding areas of brownish green,green and bluish green amphibole. Epidote found in the garnetamphibolite is usually 1–2 mm while those found in the epidoteamphibolite ~0.1 mm in size. Pale green clinopyroxene up to 0.5 mm insize occurs in some plagioclase amphibolite. We did not findclinopyroxene in mafic garnet amphibolite (Fig. 4), whereas clinopyr-oxene does occur in the quartz-bearing garnet amphibolite.

Hornblende shows a strong preferred orientation with the longaxes lying in the foliation planes. Quartz shows evidence of plastic



Fig. 7. Photomicrograph showing titanite (ttn) rimming relics of rutile (ru) in sampleFR6. Plane polarized light. Other abbreviations same as Figs. 4 and 5.

Fig. 8. Photomicrograph of internal schist sample FR16. Plane polarized light. Most ofthis view shows amphiboles that are zoned from pale actinolitic cores to darkerhornblende rims. Rutile grain is shown. Most of the rutile in this sample has beenreplaced by titanite.


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strain with ribbon grains and subgrain development. Asymmetricshear fabrics with shear bands and c and s surfaces are common.

Quartz-bearing amphibolite, which may have been derived fromvariable proportions of finely interleaved metachert and metabasite,contains 10–50% quartz. Those rocks with the highest quartz contentsmay be impure (perhaps tuffaceous) metacherts or siliceous meta-tuffs, whereas those with the lowest quartz contents may representmetabasites with limited intercalated metachert. The primary (high-grade) assemblage is Hbl+Qtz+Cpx+Grt+Phen±Pl±Chl±Rt±Zo. Biotite has been reported from these rocks (Ehrenberg, 1975;Hacker and Peacock, 1990) and we observed grains in some samplesthat may have been biotite, but have been altered to chlorite and clayminerals. Most garnet is 0.5 mm in size, but some reach 2 mm. Garnetis commonly rather heavily altered or retrogressed with ragged rimsand common replacement by chlorite. The garnet tends to be heavilyfractured with considerable alteration along the fractures, both ofgarnets and the abundant inclusions (Fig. 5). Very pale greenclinopyroxene is typically 0.5–1 mm forms heavily fractured grainswith ragged margins (Fig. 6). The pyroxene appears to be intergrownwith albite, the textural affinity of which is not clear, and fine-grainedalteration minerals are present along fractures and margins of grains.Hornblende is brownish with occasional green rims and locally palegreen actinolitic outermost rims. The hornblende commonly is lessthan 1 mm, but some reach 3 mm in size. Hornblendes appear toreflect variable retrogression with irregular brownish patches in theinterior of the grains surrounded by greenish or brownish greenamphibole, with actinolite representing the texturally latest amphi-bole forming the rims or along fractures. Phengite forms grains of0.5 mm or smaller (average about 0.1 mm). It appears to have grownin multiple textural generations with an earliest generation inapparent textural equilibrium with the garnet, clinopyroxene andbrown amphibole (Fig. 6) defining an early foliation. The texturallyearly phengite grains tend to be the larger ones, and many of them arebent with undulatory extinction. Later strain-free phengites are foundin the matrix, and also cross cutting the early, larger grains. Zoisiteoccurs as elongate grains up to 2 mm in length and is colorless withlow birefringence, and anomalous colors on parallel extinction; itappears in textural equilibrium with garnet, clinopyroxene, andbrown hornblende. Secondary pumpellyite is present, as limitedovergrowths and as comparatively rare vein filling. Foliation and fabricin the quartz-bearing amphibolite resembles that of the maficamphibolite.

Garnet in both quartz-bearing amphibolites and the metabasitesappears to be restricted to the upper structural levels of the external


schist (Fig. 2). Rutile is widely distributed at all structural levels of theexternal schists in both the amphibolites and quartz-rich amphibo-lites. Titanite commonly rims rutile (Fig. 7) and in some samplesnearly completely replaces it. Some of the amphibolites do not containrutile, but contain titanite only; ilmenite appears to rim titanite insome samples.

The metachert consists mainly of quartz (generally 90% or more)with layers of phengite and sparse garnets. The quartz grains are 0.1–0.5 mm in size. Their textures show significant plastic strain withsubgrain development, ribbon grains, and c and s surface develop-ment. Garnets in the metachert are small (0.5 mm) and broken up.Bluish (hand specimen) rocks that resemble blueschist from Rich Bararea are quartz rich (possiblymetacherts) with pale green amphiboleswith dark blue rims, stilpnomelane, and rare garnet.

4.2. The internal schist

The internal schist consistsmainly of plagioclase amphibolite (Hbl+Pl (replaced by Ab)±Qtz±Rt±Ttn) that is interleaved with horn-blendite (Hbl±Rt±Ttn). No garnets were found by us or Ehrenberg(1975) in the internal schist. In somesamples thehornblende is opticallyhomogeneous and olive greenwhereas in others the amphibole is zonedfrom a pale, apparently actinolitic core to an olive green rim (Fig. 8).Plagioclase locally appears to be fresh, although we were unable toobtain to find plagioclase during our electron microprobe analysis (seebelow),whereas inmanyother samples, it is riddledwith later alterationproducts that include fine-grained white mica and other minerals. K-feldspar occurs in felsic segregations in the amphibolites. Euhedraltitanite is common in the plagioclase amphibolites. Late veins ofprehnite are common. Grain sizes are 1–3 mm for the plagioclaseamphibolite and 3–5 mmfor the hornblendite. The planar orientation ofamphiboles and amphibole-rich and feldspar-rich layers define thehigh-temperature foliation in these rocks. A preferred elongationdirection in amphiboles defines a mineral lineation that appears to bepresent in some samples of the internal schist. Ehrenberg (1975)interpreted these rocks asmetagabbros, butwe did notfindany samplesthat exhibited textures that suggest a gabbroic, rather than basalticprotolith. The lack of associated metasediments (such as metacherts orthe quartz-bearing amphibolites) in the internal schists may suggestgabbroic, rather than basaltic protolith, however.



Fig. 9. Photomicrograph of sample from sheeted dike unit of the Devil's Gate ophiolite.Plane polarized light. Abbreviations: act: actinolite, alt-px: probable altered pyroxene(completely replaced by fine-grained intergrowth of minerals), hbl: hornblende, ilm:ilmenite.

Table 2Garnet compositions (weight percent).

Sample analysis FR6 FR6 FR6 FR6 YR32

grt-1 grt-2 grt-3 grt-4 grt-1

MgO 2.27 2.17 2.26 2.26 0.82CaO 15.00 15.22 15.31 15.04 4.96MnO 2.24 2.12 1.96 2.02 25.71FeO 20.38 20.37 20.35 21.19 9.92Al2O3 22.88 22.16 22.02 21.96 21.45Cr2O3 0.00 0.04 0.02 0.01 0.01SiO2 37.19 38.60 37.26 37.88 36.97TiO2 0.16 0.10 0.17 0.11 0.42Total 100.12 100.79 99.36 100.46 100.26

Formula based on 24 OMg 0.53 0.50 0.53 0.52 0.20Ca 2.50 2.53 2.58 2.51 0.86Mn 0.30 0.28 0.26 0.27 3.51Fe2+ 2.45 2.66 2.46 2.59 1.35Fe3+ 0.21 0.00 0.21 0.17 0.00Al 4.20 4.05 4.08 4.03 4.08Cr 0.00 0.01 0.00 0.00 0.00Si 5.80 5.98 5.85 5.90 5.96Ti 0.02 0.01 0.02 0.01 0.05Total 16.01 16.02 15.99 16.00 15.96


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4.3. Devil's gate ophiolite

In order to assess metamorphic contrasts within the FRB, weexamined samples from the Devil's Gate ophiolite, 40 km southeast ofthe study area. In contrast to the external schists, the Devil's Gateophiolite appears to represent the remnants of the upper plate ofsubduction system in contrast to the apparentmetamorphic sole rocks,the external schists. The Devil's Gate ophiolite samples appeared tohave had upper oceanic crustal protoliths: dikes and pillow basalts.Weexamined them in order to assess the contrast between themetamorphism of the upper plate of a subduction system and theapparentmetamorphic sole. The upper crustal lithologies were chosenbecause they were reported in the literature (e.g. Edelman et al., 1989;Hacker, 1990) to have beenmetamorphosed at amphibolite grade, andbecause amphibolite grade metamorphism in the dikes and/or basaltlevels of an ophiolite is higher than expected for sea floormetamorph-ism (e.g., Alt and Teagle, 2000; Schiffman and Smith, 1988).

A sample from a sheeted dike outcrop has hornblende to 2 mm insize that is zoned from a pale green actinolitic core to a greenish-brown rim (Fig. 9). Plagioclase appears to have once been part of themetamorphic assemblage, but it is largely replaced by albite and densemats of fine-grained white mica and other minerals. Brownish clots ofminerals appear to replace former blocky mineral forms; these mayhave been igneous or metamorphic pyroxene. Some clinopyroxene to0.5 mm remains in this rock but there is no direct textural connectionbetween this clinopyroxene and the brownish mineral clots. Becausethis clinopyroxene is locally concentrically rimmed by actinolite andhornblende outward, it is likely igneous clinopyroxene. Ilmeniteoccurs as irregular opaque grains to 0.7 mm in size. Texturally lateprehnite is common. Although most of the hornblende exhibits asomewhat static fabric without notable preferred orientation, shearzones cut the rock and these shear zones have plastically-deformedquartz and albite, and green to brown green amphibole.

4.4. Calaveras complex

Calaveras Complex rocks in this area are commonly extremely finegrained and many of them exhibit fewmetamorphic minerals that arereadily identifiable in thin section. Most metamorphic minerals havegrain sizes of 0.1 mm or less. The slate samples have little visiblemineralogy other than quartz, albite, and fine white mica. Cherts tendto be nearly all quartz with some white mica. Metavolcanic rockscontain quartz, albite, white mica, chlorite, epidote, and actinolite. The


rocks found at Belden Siphon (Fig. 2) are somewhat coarser grainedwith metamorphic grain sizes up to 0.3 mm. A breccia, noted abovecontains a matrix of quartz-phengite schist with some actinolite, andclasts up to several cm in size of amphibolite. These amphibolite clastsappear similar to the plagioclase amphibolites of the external schist.Many of the amphibolite clasts contain rutile and some have bluishrims on the hornblendes.

5. Electron microprobe analysis of selected phases

In order to evaluate the P–T conditions of metamorphism as well asidentify certain minerals, mineral compositions were determined forminerals from six samples, four from the external schist, one from theinternal schist, and one from the sheeted dike unit of the Devil's Gateophiolite. Themineral chemistry was determined using a CAMECA SX-100 Electron Microprobe at the University of California, Davis. Theaccelerating potential was 15 kV and the beam current was10 nA, withcounting times of 10 s for peaks, and 5 s for background. Amphiboles,pyroxenes, garnets, and epidote minerals were analyzed with a beamdiameter of 1 micron, whereas feldspars and phengites were analyzedwith a beamdiameter of 10μm.Mineral formulaewere calculated fromdata on the following basis: Amphiboles: For some site assignmentsdiscussed in the text: 13 total cations excluding Ca, Na, and K, althoughfor Table 4 amphiboles formulae are simply charge balanced to 23oxygens. Clinopyroxene: 6 oxygens and 4 cations. Garnet: 12 oxygens.Phengite: 22 oxygens. Representative results for garnet, clinopyroxene,phengite, and amphibole are shown in Tables 2 to 5, respectively.

5.1. Garnets

Garnets froma quartz-rich external schist (sample FR6)were pyropepoor and rich in almadine and grossular. The compositional range of thegarnets is Py8–10Alm42–47Sp4–5Gr35–43And2–9Uva0–2, and they have anaverage composition of Py9Alm43Sp5Gr38And6Uva1. Because of theheavy alteration and fragmentation of the garnets in this rock, wewere unable to identify zoning. The garnet analyses we obtained showrelatively small compositional variation and no systematic spatialvariation. Whereas our analysis did not identify, neither could wedemonstrate that the original garnet was unzoned, owing to its poorpreservation. In addition, our analyses may be biased toward the coreregions of the garnet relics owing to greater degree of alteration andfracturing of the rim regions. Garnet was analyzed from a metachert



Table 3Clinopyroxene compositions (weight percent).

Sample analysis FR6 FR6 FR6

cpx-1 cpx-2 cpx-3

Na2O 0.28 0.33 0.35MgO 12.00 12.08 12.51Al2O3 0.75 0.94 1.11SiO2 52.96 52.83 52.72K2O 0.01 0.01 −0.01CaO 22.75 23.75 23.57TiO2 0.00 0.02 0.05FeO 10.46 9.90 9.29MnO 0.14 0.18 0.18Cr2O3 0.02 0.03 0.01Total 99.37 100.08 99.80

Formula based on 6 ONa 0.02 0.02 0.03Mg 0.68 0.68 0.70Al 0.03 0.04 0.05Si 2.01 1.98 1.98K 0.00 0.00 0.00Ca 0.92 0.96 0.95Ti 0.00 0.00 0.00Fe2+ 0.36 0.30 0.27Fe3+ 0.00 0.01 0.02Mn 0.00 0.01 0.01Cr 0.00 0.00 0.00Total 4.02 3.98 4.01

Table 5Amphibole compositions.

Sample analysis FR6 FR6 YR45 RB52 FR16 FR16 DGO1

am-2 am-4 am-3 am-3 am-4 am-5 am-2

H2O 2.01 2.03 2.00 2.07 2.00 2.00 2.07Na2O 1.37 1.29 2.22 0.43 1.89 1.54 0.93K2O 1.60 1.63 0.36 0.03 0.49 0.43 0.33CaO 11.71 11.65 10.51 11.64 10.96 11.06 11.43MgO 9.08 9.86 9.45 15.16 9.75 10.38 14.93MnO 0.27 0.17 0.30 0.38 0.39 0.37 0.21FeO 16.48 15.55 19.67 13.15 18.52 17.62 12.69Al2O3 15.32 15.37 13.13 1.75 12.84 10.82 8.13Cr2O3 0.01 0.04 0.01 0.01 0.03 0.02 0.08SiO2 41.55 42.35 42.53 54.44 42.60 44.27 47.90TiO2 0.86 0.88 0.64 0.06 0.82 0.81 1.20Total 100.26 100.83 100.82 99.14 100.29 99.30 99.90

Formula proportion based on 23 oxygensNa 0.39 0.37 0.63 0.12 0.54 0.44 0.26K 0.30 0.30 0.07 0.02 0.09 0.08 0.06Ca 1.86 1.82 1.64 1.82 1.72 1.75 3.16Mg 2.00 2.15 2.05 3.22 2.13 2.28 1.74Mn 0.03 0.02 0.04 0.05 0.05 0.05 0.03Fe2+ 1.62 1.43 1.15 1.36 1.22 1.30 0.54Fe3+ 0.42 0.47 1.25 0.28 1.05 0.87 0.97Aliv 1.75 1.72 1.74 0.15 1.67 1.38 1.07Alvi 0.92 0.93 0.52 0.08 0.55 0.50 0.30Cr 0.00 0.00 0.00 0.00 0.00 0.00 0.01Si 6.15 6.18 6.19 7.85 6.24 6.53 6.81Ti 0.10 0.10 0.07 0.00 0.09 0.09 0.13Total 15.55 15.49 15.33 14.96 15.35 15.27 15.06


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(YR32) that showed zoning in backscatter electron (BSE) imaging, butonly twousable analyseswereobtained. This garnetwas spessartine richwith a composition of Py4–8Alm19–30 Sp42–62Gr8–14And6–7Uva0.

5.2. Amphibole

Amphiboles were analyzed from the quartz-bearing external schist(FR6), plagioclase (former) amphibolite fromthe external schist (YR45),a quartz-bearing schist or metachert from the external schist thatappeared to have blue amphibole rims on green amphibole (RB52), and

Table 4Phengite compositions (weight percent).

Sample analysis FR6 FR6 FR6 FR6 FR6

wm-1 wm-2 wm-2 (rim) wm-3 wm-3 (rim)

H2O 4.40 4.44 4.49 4.49 4.44Na2O 0.18 0.11 0.22 0.22 0.02K2O 10.43 10.13 10.21 10.38 9.97MgO 2.88 2.82 3.57 2.59 4.33CaO 0.04 0.15 0.01 0.00 0.03MnO 0.01 0.06 0.03 0.06 0.05FeO 3.40 4.12 4.70 3.85 6.01BaO NA 0.49 0.09 0.77 0.01Al2O3 28.28 28.27 25.18 31.10 21.85Cr2O3 0.01 0.02 0.00 0.05 0.00SiO2 48.60 49.04 52.13 47.56 53.11TiO2 0.04 0.15 0.02 0.21 0.00Total 98.25 99.80 100.66 101.29 99.81

Formula based on 22 ONa 0.04 0.03 0.06 0.06 0.00K 1.78 1.75 1.74 1.77 1.72Mg 0.55 0.57 0.71 0.52 0.87Ca 0.00 0.02 0.00 0.00 0.00Mn 0.00 0.01 0.00 0.01 0.01Fe 0.37 0.47 0.52 0.43 0.68Ba NA 0.03 0.01 0.04 0.00Aliv 3.25 3.12 2.94 3.19 2.69Alvi 1.48 1.38 1.03 1.68 0.80Cr 0.00 0.00 0.00 0.01 0.00Si 6.52 6.62 6.96 6.35 7.18Ti 0.01 0.02 0.00 0.02 0.00Total 18.00 18.00 17.96 18.09 17.94


plagioclase amphibolite from the internal schist (FR16). We alsoanalyzed amphibole from a sheeted dike sample from the Devil's Gateophiolite. The amphiboles are edenites and pargasites (Yavuz, 2007)(Fig.10). Towardmore aluminous amphiboles in Fig.10 is a bit deceivingin that actinolite was intentionally avoided (not entirely with success)with the exception of RB52 where it is the primary calcic amphibole;actinolite appears as a late overprint in most of the external schist. InFig. 11, aluminum and titanium contents of amphibole are shown, asthese concentrations are pressure and temperature dependent, respec-tively in high-temperature calcic amphiboles (Ernst and Liu,1998). Notethat Fig. 11 excludes the more actinolite-rich analyses, but is comprisedof variably retrogressed amphiboles. The variable retrogression appearsto be reflected by the positive correlation between Al and Ti contents ofamphibole in each sample. The spread of data shows the greatest scatteror spread for FR6 consistent with the greater degree of retrogression ofamphiboles seen in thin sections that sample.

In RB52 the blue amphibole rims do not appear to be sodicamphibole. Althoughmost of these rims are small enough so that theyare difficult to analyze, a few patches were large enough so that webelieve the microprobe analysis consists entirely of one of thesepatches instead of a combination between the patch and the maincore amphibole. These analyses show the bluish rims and patches tobe richer in magnesioriebeckite component than the core amphibole,but the NaB occupancy is no higher than about 0.4. These results aresimilar to microprobe results obtained by the second author onapparent blue amphibole rims on garnet amphibolite of the externalschist from the Rich Bar area in 1987.

5.3. Clinopyroxene

Clinopyroxene from quartz-bearing amphibolite (FR6) span acompositional range is Wo47–49En34–36Fs15–17 Jd1–2. Clinopyroxeneexhibits inclusions or intergrowths of albite (Fig. 6). Although theclinopyroxene is heavily fractured and has fine-grained alterationproducts growing along these fractures, the remaining pyroxene doesnot appear to have sufferedmuch retrogression or alteration, based onthe narrow range of compositions. It is possible that the albite within



Fig. 10. Amphibole compositions and classification.


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the clinopyroxene represents (1) growth in equilibrium withclinopyroxene, (2) exsolution of a Na-poor clinopyroxene and albitefrom an earlier Na-rich clinopyroxene, because there are no otherobvious minerals that replace the clinopyroxene or (3) a laterretrograde product that may have formed in conjunction with otherretrograde minerals along the fractures.

5.4. Phengite

Phengite in FR6 varies in Si content from 3.23 to 3.72 formula units,basedonan11oxygen formula. Fe content ranged from .14 to .43, andMgcontent ranged from .22 to .46. We did not analyze all of the phengitesfor barium, but those we did showed low Ba concentrations (Table 4).

Fig. 11. Aluminum and titanium contents in amphiboles. Note that this plot includes some pamphiboles. It is likely that only the highest aluminum and titanium concentrations reflect


Phengite analyses can be divided into two groups, those with low Sicontents of about ~3.3 (3.18–3.37) and those with high Si contents(3.45–3.72). Owing to the small size of the grains (most grains had awidth of no more than four microprobe beam diameters) we could notquantitativelyevaluate zoning in thephengites. The higher Si phengite istexturally late. This can be seen in BSE imagery, where lighter rims(higher average atomic number with higher phengite substitution) areseen on some of the larger phengite grains (Fig. 6).

5.5. Feldspars

We tried to find plagioclase relics in the various amphibolitesamples but could not. Many grains that appeared to be plagioclase in

artly retrogressed amphiboles, but excludes some of the more actinolitic (clearly late)the true high-grade amphibole compositions.




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thin section, proved to be albite and various intergrown alterationminerals. In FR16 our analyses showed potassium feldspar waspresent in the felsic layers in that sample. The other feldspar identifiedwas nearly pure albite in samples YR45.

6. Thermobarometry

Peak P–T conditions were estimated for several samples from theexternal schists, one from the internal schists and one from thesheeted dike unit of the Devil's Gate ophiolite (results summarized onFig. 12). For all samples, we used Ernst and Liu (1998) amphibolethermometry and barometry. We also noted the presence of partialmelting textures in many rocks that indicated that temperaturesexceeded the wet basalt solidus. The presence or absence of garnet inmafic rocks and the occurrence of rutile were also employed toestimate minimum or maximum pressure. One sample, quartz-richamphibolite/schist FR-6 contained garnet, clinopyroxene, hornblende,phengite and rutile, so we were able to apply all of the aboveconstraints in addition to estimated temperature from the garnet–clinopyroxene Fe–Mg exchange reaction of Ravna (2001), andpressure by garnet–clinopyroxene–phengite barometry of Ravna andTerry (2004). An older calibrations of the grt–cpx Fe–Mg exchangereaction (Ellis and Green, 1979) was included for comparison.Similarly the older grt–cpx–phen calibration of Waters and Martin(1993); with empirical correction published in Wain et al. (2001) wasused for comparison to results obtained from the Ravna and Terry(2004) calibration. We shall present the thermobarometric estimatesfor sample FR-6 first, then review those of the other samples.

Apparent partial melting textures were found on the outcropwhere sample FR6 was collected constrain the minimum temperatureto 650 °C based on wet basalt solidus (Poli, 1993; Peacock et al., 1994)(Fig. 12). Garnet–clinopyroxene thermometry (Ravna, 2001) gives atemperature range of garnet–clinopyroxene pairs of about 550–760 °C. Owing to the degraded nature of garnets, that may have

Fig.12. Summary diagram of P–T estimates of metamorphism for various samples. Spotsand polygon are P–T estimates from amphibole thermobarometry of Ernst and Liu(1998) for sample DGO (Devil's Gate ophiolite, sheeted dike sample), IS (internalschist), and samples YR45 and FR6 of the external schist. Other thermometers andbarometers applied specifically to sample FR6 are as follows: R2000: Ravna (2001)garnet–clinopyroxene thermometer (the lower limit for R2000 is well below the wetbasalt solidus, so it is not plotted), RT04: Ravna and Terry (2004) garnet–clinopyroxene–phengite barometer, EG79: Ellis and Green (1979) garnet–clinopyrox-ene thermometer, shown for comparison, as well as WM: Waters and Martin (1993)garnet–clinopyroxene–phengite barometer revised as in (Wain et al., 2001). Thepreferred limits on the P–T conditions of metamorphism for sample FR6 are shown ingray. Other curves shown: hbl out: hornblende breakdown (Liu et al., 1996), garnet-in(Liu et al., 1996), ilmenite, rutile and titanite stability (Liu et al., 1996), and the wetbasalt solidus from Poli (1993).


influenced the range in their composition, we prefer the higher part ofthe temperature range. Rutile found in FR6 constrains the minimumpressure to be about 1.3 GPa at 650 °C (Ernst and Liu, 1998).

The maximum pressure of 2.2 GPa at 660 °C is from the stability ofhornblende in wet basalt or amphibolite (Peacock et al., 1994; Liuet al., 1996). Application of garnet–clinopyroxene–phengite barome-try (Ravna and Terry, 2004) yielded pressures of about 1.4–1.9 GPa at650–760 °C for phengite cores (of about Si 3.3). Because of thesomewhat ambiguous textural relationships of the phengite we alsodid the P–T calculations using what we believed to be texturally latephengites (high Si) as the phengite in equilibrium with garnet andclinopyroxene. This produced ultrahigh pressure (UHP) results(N3 GPa) which we consider unrealistic owing to the lack of UHPmineral assemblages and presence of hornblende. This supports ourinterpretation of the high Si phengites as being late and not inequilbrium with the high T assemblage. The updated version of theWaters and Martin (1993) calibration (Wain et al., 2001) applied tophengite cores yielded pressures of 2.2–2.4 GPa at 650–760 °C; mostof this range is above estimated hornblende stability. The amphibolethermobarometer of Ernst and Liu (1998) applied to the calcicamphiboles gave a temperature of about 670–710 °C at 1.7–2.0 GPa.Owing to the retrograded nature of the amphiboles in all of our rocks,we selected amphiboles with reasonable stoichiometry and goodtotals that had the highest Al and Ti contents. Our selection of thecompositional clusters with the highest Al and Ti contents for eachsample may artificially restrict the natural compositional variationand uncertainty in the P–T estimate, or it is possible that we areoverestimating the compositional variability (and resultant P–Testimate range) by including some variably retrogressed amphibolesin our analysis. It is difficult to assess the true variability of the high-grade amphibole compositions with such widespread retrogression.

For the samples other than FR6, we cannot directly compare thesamemethods, except for the presence or absence or rutile, butwe cancompare amphibole compositions. P–T conditions for YR45, which isstructurally low in the external schist, based on amphibole thermo-barometry are 630–670 °C and 1.55–1.75 GPa. The presence of rutileplaces a lower limit of 1.3 Gpa for peak metamorphic conditions.Amphibole thermobarometry applied to the internal schist yieldedtemperatures of 640–680 °C and pressures of 1.4–1.6 GPa (Fig. 12).Partial melting textures are found in the internal schists suggest thatpeak metamorphic temperature exceeded 650 °C. The presence ofrutile in the internal schist appears to suggest a minimum pressure ofabout 1.3 GPa at 650 °C. Amphibole thermobarometry applied to theDevil's Gate ophiolite sheeted dike sample results in estimated P–Tconditions of about 710–730 °C and 0.3–0.4 GPa. The presence ofilmenite and lack of titanite, along with the lack of garnet, indicates amaximum pressure of 0.7–0.8 GPa.

7. Discussion

7.1. Discussion of thermobarometric results

In order to compare our P–T estimates to those published fromsimilar tectonic settings and for us to compare P–T conditionsbetween different samples we will further discuss the thermobaro-metry. Although the Waters and Martin (1993); with correction inWain et al. (2001) calibration of the grt–cpx–phen barometercommonly gives lower P estimates in eclogites (Page et al., 2007),we found it to give higher pressures for FR6. Whether or not the grt–cpx–phen barometry is applicable to sample FR6 is unclear, owing tothe somewhat unclear textural association of the earliest formedwhite mica, and the question as to whether the clinopyroxeneretained the composition it had at peak pressure. Accordingly, ourgrt–cpx–phen results should be viewed with some caution.

The compositional dependence of mineral compositions andstability may also impact the P and T estimates. To better evaluate




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the applicability of the equilibria used in our PT estimates we obtainedmajor element chemical data by X-Ray fluorescence analysis (PANa-lytical MagiX Pro). With the exception of sample FR6, the external andinternal schists, and the Devil's Gate ophiolite samples appear to havemajor and minor element compositions similar to MORB used in theexperiments that the Ernst and Liu (1998) amphibole thermobarom-eter was calibrated with (Table 6). FR6 is derived from a protolith thatis more felsic than MORB, perhaps MORB with interlayered chert, soamphibole compositions in that sample may not be directly compar-able to those in the other samples we investigated. The minimumpressure represented by rutile is also compositionally dependent.Rutile is widely distributed throughout the external and internalschists in rocks of a wide range in compositions, from quartz-richrocks more felsic than FR6 to those of true MORB composition. Thus,we believe it is reasonable to apply minimum pressure of rutileoccurrence summarized in Ernst and Liu (1998) that was based onMORB. Similarly garnet stability is strongly compositionally depen-dent, but garnet is present in true MORB-derived metabasites (sample92-3 in Table 6, for example) as well as the quartz-bearing FR6.

The highest temperatures obtainedwere the grt–cpx temperaturesestimated from sample FR6. It is possible that this thermometer didnot capture the peak temperature of metamorphism because: (1) ouranalyses of garnet in FR6 may have avoided the more degraded rimareas where prograde zoning may have been reflected; and (2) theclinopyroxene may not necessarily reflect its composition at the peakof metamorphism because of subsequent retrograde metamorphismor exsolution.

In view of the above we believe the external and internal schistsformed at about 650–760 °C (possibly higher) at pressures greaterthan 1.3 GPa (rutile stability MORB composition), but less than2.2 GPa (hornblende stability in MORB composition), and the grt–cpx–phen pressure estimate for sample FR6 falls within that broaderpressure range. Comparison of amphibole compositions between theexternal and internal schists suggests that the external schists mayhave formed at slightly to moderately higher pressures (say 0.1 to0.7 GPa) than the internal schists and significantly higher pressures(ca. 1 GPa) than the Devil's Gate ophiolite. The latter conclusion is alsoconsistent with the occurrence of ilmenite instead of rutile in theDevil's Gate ophiolite. Although FR6 is not of MORB composition, itsamphibole compositions may indicate an inverted pressure gradient,or imbrication, within the external schist, but the difference in bulkcomposition between the non-MORB FR6 and MORB YR45 rendersthis assessment uncertain. Garnet withinmetabasites (of approximateMORB composition) is restricted to the structurally higher part of the

Table 6Whole rock compositional data.

Sample oxide FR6 92-3 YR45 FR16 DGO MORBExperimentsa

SiO2 68.23 51.33 51.98 51.46 47.60 49.11–52.38Al2O3 10.00 13.96 15.11 13.33 18.47 12.74–16.93FeO 7.55–10.72Fe2O3 1.89–3.23Fe2O3

b 6.02 15.27 14.31 13.41 9.56MgO 4.20 10.17 6.74 6.83 10.25 6.58–10.31MnO 0.09 0.09 0.17 0.22 0.17 0.17–0.22TiO2 0.93 1.23 0.97 2.17 0.67 1.24–2.51CaO 10.43 7.55 8.73 9.61 13.59 10.05–11.10Na2O 0.2 1.04 2.65 2.63 0.59 1.93–3.76K2O 0.92 0.22 0.08 0.23 0.12 0.06–0.49P2O5 0.224 0.091 0.061 0.164 0.008 0.15–0.27Total 101.187 100.9337 100.812 100.0643 101.0569 99.28–100.75

a MORBmaterial compositions for amphibole experiments compiled by Ernst and Liu(1998), including that study, Liou et al. (1974), Apted and Liou (1983), Spear (1981), Poli(1993), and Helz (1979).

b All iron as Fe2O3.


external schist (where sample FR6 was collected), and this qualita-tively supports an inverted pressure gradient within the externalschists, as well as higher pressures of the structurally higher part ofthe external schist compared to the internal schist.

The presence of pumpellyite, actinolite, and chlorite, as latemineral growth in many of the samples is consistent with thepumpellyite–actinolite overprint that is characteristic of many rocksof the northern Sierra Nevada and crystallized at conditions of about150–350 °C and 0.2–0.4 GPa (Hacker, 1993). The high Si content(3.45–3.72 formula units, 11 oxygen formula) of the late phengites inFR6 is not compatible with phengites associated with the pumpel-lyite–actinolite overprint (Hacker, 1993; Wakabayashi, unpub. data).In contrast, such high-Si phengites in FR6 are similar in composition tothose observed in blueschist facies terranes (e.g. Sorenson, 1986;Wakabayashi, 1990; El-Shazly et al., 1997; Smith et al., 1999; Tsujimoriand Liou, 2004). The late phengite growth may be evidence of ablueschist facies overprint on this rock. Other evidence for a blueschistfacies overprint in the external schists is lacking, however. Thus,external schists may have experienced retrograde blueschist faciesconditions, whereas all of the FRB and associated rocks (internal andexternal schists, Devil's Gate ophiolite) underwent late pumpellyite–actinolite metamorphism.

The PT estimates we obtained from the external schists (650–760 °C, 1.3–2.2 GPa) are comparable to those obtained for othermetamorphic soles such as beneath the Semail ophiolite of Oman(700–900 °C, 0.95 to 1.77 GPa (Gnos, 1998; Searle and Cox, 2002)),beneath the Palawan ophiolite of the Philippines (700–760 °C,N0.9 GPa (Encarnacion et al., 1995)), and beneath the Yarlung-Tsangpoophiolite of Tibet (750–875 °C, 1.3–1.5 GPa (Guilmette et al., 2008)).

7.2. Are the external schists a metamorphic sole?

As noted in the Introduction, high-grade metamorphic rocks havebeen found in association with ophiolite belts that are not meta-morphic soles, so it is useful to evaluate the field and petrologic datafrom the external schists in light of the metamorphic sole hypothesis.Williams and Smyth (1973) and Ehrenberg (1975) suggested that theexternal schists may be a metamorphic sole on the basis of the high-grade metamorphism and contact with the ultramafic rocks. Our fieldand structural data indicates that the external schists are a thin (10 to300 m thick) unit that dips eastward beneath the ultramafic rocks andthat the high-temperature shear fabric in the external schist exhibits atops-to-the-west (ultramafic-side-up) sense-of-shear. Such a high-temperature fabric should be expected in metamorphic soles. Theexternal schists appear to be composed entirely of metabasite withmetachert, similar to most metamorphic soles (e.g., Spray, 1984;Jamieson, 1986). Inverted temperature and pressure gradients foundwithin some soles (e.g., Jamieson,1986; Gnos,1998)may be present inthe external schists, but the limitations of our thermobarometry donot allow us to define inverted metamorphic gradients as definitivelyas the two studies cited above.

In summary we believe the external schists that field, lithologic,structural, and metamorphic characteristics of the external schists aretypical of a metamorphic sole. We suggest these rocks formed at theinitiation of intra-oceanic subduction as previously proposed formanymetamorphic soles.

7.3. Tectonic synthesis: subduction initiation and exhumation of ametamorphic sole

Our structural and petrologic data indicate that the external schistsformed as a metamorphic sole during the inception of eastwardsubduction beneath the Feather River ultramafic belt. In addition, themetamorphic data provides additional insight into the tectonicevolution of the external schists and related rocks. The pressure ofmetamorphism of the external schist (N1.3 Gpa) is well in excess of




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what can be explained by the structural thickness of rocks interveningbetween these rocks and the crustal rocks within or at the higherstructural levels of the FRB. The position of the internal schist isequivalent to 1 to 1.5 km kilometers of cross-strike map distance fromthe FR6 internal schist, corresponding to a maximum thickness of 1 to1.5 km for a vertical average foliation dip. Owing to the uncertaintiesin the barometric estimates it is difficult to estimate the absolute Pdifference between the internal schist and FR6, but it is likely in therange of 0.1 to 0.7 GPa, corresponding to a burial difference of 3.3 to23.0 km, for an average overburden specific gravity of 3.1 (allowing forsome serpentinization of the overlying mantle as well as thepossibility of some eroded oceanic crust). This requires that faults ofan apparent normal sense accommodated exhumation of the externalschists relative to the internal schists. This exhumation is requiredeven if the peak metamorphism of the internal and external schistsoccurred at significantly different times. The internal schists do notappear to have been deeper than recorded by their peak metamorphicassemblage. Amphiboles in some of the samples of internal schistshow prograde zoningwith actinolite cores and there is no evidence ofhigh P retrograde metamorphism.

The Devil's Gate ophiolite dike sample, although collected about40 km to the southeast of the main field area, appears to berepresentative of the upper oceanic crust that may have overlainmuch of the ultramafic rocks of the FRB. A comparison of the Devil'sGate ophiolite with the internal and external schists is appropriatebecause there is no evidence of an along-strike metamorphicgradient in either internal or external schist metamorphism betweenthe study area and the Devil's Gate ophiolite (e.g. Hacker andPeacock, 1990; Hacker, 1993). The Devil's Gate ophiolite sheeteddike sample records a much lower metamorphic pressure than the

Fig. 13. Tectonic cartoons showing the evolution of the northern Feather River ultramafic beltthe thickness of the external schists is exaggerated so that they are visible on these diagramsample, whereas YR45 and FR6 track two samples of the external schist.


internal schists (about 0.6 to 1 GPa lower, corresponding to a burialdepth difference of 20 to 33 km), and the intervening ultramafic rockmass (at most 6 km across strike) is much too thin to account for thepressure difference. The metamorphic pressure contrast indicatesthat the internal schist was exhumed relative to the Devil's Gateophiolite by lithospheric-scale normal faults located east of (structu-rally above) the internal schists within the ultramafic rocks. Suchfaults would have to be located structurally beneath the Devil'sGate ophiolite (Fig. 13). Relative exhumation is demanded for theinternal schist relative to the Devil's Gate ophiolite regardless of thecomparative ages of metamorphism because neither rock hasprograde or retrograde assemblages that record a higher P than thepeak assemblages.

The exhumation of the external schist (metamorphic sole) relativeto the overlying ophiolite crustal section was accommodated by atleast two major zones of exhumation, one of which was at least 1 kmstructurally above the ultramafic-external schist interface, east of theinternal schist body, and the other permissibly located anywherebetween the ultramafic-external schist interface and the westernboundary of the internal schist body. More accurate locating ofmetamorphic pressure contrasts within the ultramafic rocks them-selves is not feasible owing to the insensitivity of various reactions inmetaultramafic rocks to pressure (e.g., Evans, 1977).

If in fact an inverted pressure gradient is preserved in the externalschist, then internal thrust faults are required to have juxtaposed therocks of the external schist (Fig. 13). The structurally lowest unit of theexternal schist is much higher P than the Calaveras Complex thatstructurally underlies it, and this exhumation (apparent thrust faultsense) may have taken place prior to the deposition of the structurallyhighest parts of the Calaveras Complex as noted below.

with emphasis on the metamorphism and exhumation of the external schists. Note thats. I.S. (internal schist) schematically shows the relative position of the internal schist




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Timing of subduction initiation, sole development and soleexhumation is difficult to ascertain owing to the scarcity ofgeochronologic data, but the hornblende Ar/Ar age of 236±4 Ma(Weisenberg and Avé Lallemant, 1977) reflects cooling of the internalschist through hornblende closure (~525±50 °C; Harrison, 1981).The high-pressure, high-temperatures in the external and internalschists may have been approximately coeval owing to their proximityand similar PT conditions of metamorphism, so the internal schist agemay have formed in the same subduction initiation event as theexternal schist and cooled through hornblende closure for Ar atapproximately the same time. Cooling of the sole is expected to rapidwith continued subduction, so the hornblende age is probably lessthan 5 m.y. older than the actualmetamorphic age (e.g., Hacker,1991).Exhumation of soles relative to the upper crustal parts of overlyingophiolites appears to be rapid for examples where ophiolites wereemplaced over continental margins. In such cases many ophioliteswere emplaced over continental margins within 10 m.y. afterformation (Dewey, 1976; Dilek et al., 1999), and structural relation-ships indicate that exhumation of the sole relative to the ophiolitemust have occurred prior to final emplacement (reviewed inWakabayashi and Dilek, 2003). For ophiolites structurally abovesubduction complexes, as appears to be the case with the FRB,geochronologic data is much scarcer worldwide.

Subaerial exposure of the external schist appears to have occurredprior to the accretion of the structurally highest unit of the CalaverasComplex that has breccia clasts of external schist. The accretion age ofthe oldest Calaveras Complex is uncertain, however, except that itexceeds 177 Ma (Sharp, 1988). Collectively the existing structural,metamorphic, and geochronologic framework suggests initiation ofsubduction sometime slightly before 236 Ma, subduction of the sole todepths exceeding 43 km (N1.3 GPa) and exhumation and subaerialexposure of some of these rocks by 177 Ma.

The eastern contact of the FRB against the Shoo Fly Complex andother rocks is more difficult to evaluate. The pressure of high-temperature metamorphism estimated for the Devil's Gate ophio-lite is not significantly different (within uncertainty) than that ofestimates for the low-grade metamorphism of the Shoo FlyComplex and related rocks (Day et al., 1988; Hacker, 1993). Onthis basis the eastern FRB contact does not record large amounts ofdifferential vertical movement. In contrast the temperature ofmetamorphism (~700 °C) for the upper crustal parts of the FRBreflected by the temperature of metamorphism of the sheeted dikesample of the Devil's Gate ophiolite is vastly higher than any of therocks adjacent to the FRB to the east. This metamorphic relationship(no P contrast but large T contrast) suggests that the easternboundary of the FRBmay have accommodated significant strike-slipdisplacement.

The origin of themetamorphism of the Devil's Gate ophiolite posessome difficult problems. In many of the classic ophiolites of the world,the ophiolite structurally overlying the sole exhibits negligible burialmetamorphism (most exhibit sea floor metamorphism) (e.g., Waka-bayashi and Dilek, 2000; 2003). The crustal ophiolitic rocks of the FRB,such as the Devil's Gate ophiolite, and possibly the internal schist, areclearly different. The high T, low P metamorphism of the Devil's Gateophiolite clearly reflects a much higher geothermal gradient than thatrecorded in the high T, high P external schist. Themetamorphism is notcompatible with sea floor metamorphism because the progradeamphibole zoning indicates cooling after crystallization, followed byheating, in contrast to sea floor metamorphism where temperaturesshould generally reflect a progressive cooling. In addition, sea flooramphibolite metamorphism is restricted to the gabbro layer andbeneath, and has not been found associated with sheeted dikes (e.g.Liou and Ernst,1979; Evarts and Schiffman,1983; Schiffman and Smith,1988, Alt and Teagle, 2000). Ridge subduction has been suggested to beassociatedwith very high geothermal gradients (e.g., Sisson and Pavlis,1993; Brown, 1998) and such an event may explain the high-T


metamorphism of the crustal ophiolitic parts of the FRB such as theDevil's Gate ophiolite. One speculative scenario that would explain themetamorphism of the Devil's Gate ophiolite would be failed subduc-tion of the ridge crest during the same event that led to the inception ofsubduction and creation of the external schist (Fig. 13).

Many of the details of the tectonometamorphic evolution of theFRB require more geochronologic data and related metamorphicdata tied to a specific structural-tectonic setting, for better under-standing. We are currently engaged in a geochronologic campaign inthis region and we hope that the results will help refine models forophiolite genesis, subduction initiation, and early subduction zoneexhumation.

Acknowledgments

This research was supported by the National Science FoundationGrant EAR-0635767 to J.W., and awards from the California StateUniversity, Fresno, College of Science and Mathematics to C.M.S. Wethank S. Roeske and S. Mulcahy for their assistance with microprobeanalyses, G. Torrez for XRF analyses, and R. Jamieson, W. Sharp, and V.Sisson for the constructive reviews that greatly improved the paper.

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