Smart Wakabayashi2009.Pdf

ARTICLE IN PRESS LITHOS-02063; No of Pages 14 Lithos xxx (2009) xxx–xxx

Contents lists available at ScienceDirect

Lithos

journal homepage: www.elsevier.com/locate/lithos

Hot and deep: Rock record of subduction initiation and exhumation of high-temperature, high-pressure metamorphic rocks, Feather River ultramaﬁc 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 article info abstract

Article history: Studies of a 10 to 300-m-thick unit of high-grade metamorphic rock (“external schists”) that crops out along Received 23 July 2008 the western border of the Feather River ultramafic belt (FRB), northern California, yield new insights into Accepted 6 June 2009 subduction initiation and ophiolite emplacement processes. The high-temperature (T) foliation of the Available online xxxx external schists dip moderately to steeply eastward beneath the ultramafic rocks of the FRB, a 150-km-long slab of suboceanic upper mantle and the high-T fabric shows a tops-to-the-west (FRB-side-up) sense of Keywords: shear. The structurally highest external schists record peak metamorphic conditions of 650–760 °C at 1.3– Subduction initiation fi Metamorphic soles 2.2 GPa. In contrast, sheeted dikes of the Devil's Gate ophiolite that overlie the ultrama c rocks yield Ophiolites metamorphic conditions of 710–730 °C at about 0.3–0.7 GPa. A km-scale lens of amphibolite within High-pressure rock exhumation ultramafic rocks yields somewhat lower pressures than the structurally highest external schist, as do the structurally lower rocks within the external schists. Significant exhumation of the external schists relative to the structurally overlying ophiolitic rocks occurred along at least two major zones and the most significant exhumation 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, and exposure of the external schist occurred prior to the deposition of rocks in the structurally highest part of the Calaveras 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. © 2009 Published by Elsevier B.V.

1. Introduction inverted metamorphic gradients due to tectonic underplating during subduction of progressively older (and colder) oceanic lithosphere Mechanisms of subduction initiation are the subject of consider- (Peacock, 1987; 1988; Hacker, 1990; 1994; Gnos, 1998), and show able debate, but most authors agree subduction initiation exploits pre- anticlockwise pressure–temperature–time (P–T–t) paths (P on posi- existing weaknesses and material contrasts in the oceanic lithosphere tive y-axis) (Wakabayashi, 1990; Dilek and Whitney, 1997; Önen and (Casey and Dewey, 1984; Mueller and Phillips, 1991; Stern and Hall, 2000; Guilmette et al., 2008). Because metamorphic soles Bloomer, 1992; Wakabayashi and Dilek, 2003). Ophiolites are on-land apparently formed during inception of subduction, their geology, remnants of oceanic crust and many of these ophiolites structurally and the geology of adjacent rocks, provide insight into the setting and overlie the position of former subduction zones (e.g., Moores, 1970). mechanisms associated with hot subduction initiation (i.e. Jamieson, Structurally beneath many ophiolites are thin (b500 m) units of high- 1986; Hacker, 1990; Guilmette et al., 2008). grade metamorphic rocks called metamorphic or dynamothermal Although many studies have been conducted on metamorphic soles. These soles are thought to have formed during subduction soles, some critical aspects of metamorphic sole development have initiation beneath young oceanic lithosphere (hot subduction initia- received little attention. For example, metamorphic soles were once tion), based primarily on the high temperature of metamorphism assumed to have been “welded” to the base of ophiolites after they recorded in them (peak temperatures in the 700–900 °C range), their were underplated (scraped off the downgoing plate) as subduction lithologies (primarily metabasite with meta-pelagic sediments), and began beneath the ophiolite (Williams and Smyth, 1973; Malpas, their structural and chronologic relationships with the ophiolite that 1979; Searle and Malpas, 1980). Such a model assumed that no directly overlies them (Williams and Smyth, 1973; Spray, 1984; exhumation of the sole relative to the ophiolite occurred after Jamieson, 1986; Hacker, 1990). Metamorphic soles commonly have metamorphism. As new geobarometric methods became available, studies showed metamorphic pressures for soles that vastly exceeded ⁎ Corresponding author. that which could be explained by the structural thickness of the E-mail address: [email protected] (J. Wakabayashi). ophiolite above the sole, indicating signiﬁcant exhumation of the sole

Please cite this article as: Smart, C.M., Wakabayashi, J., Hot and deep: Rock record of subduction initiation and exhumation of high- temperature, high-pressure metamorphic rocks, Feather River ultramaﬁc belt, California, Lithos (2009), doi:10.1016/j.lithos.2009.06.012 ARTICLE IN PRESS

2 C.M. Smart, J. Wakabayashi / Lithos xxx (2009) xxx–xxx

amphibolite beneath ophiolites that are too thick (kilometers) to have been generated by conductive heating beneath a hot mantle hanging wall (Harper et al., 1996; Barrow and Metcalf, 2006), high- grade rocks that may have been derived from the base of a magmatic arc instead of from the top of the downgoing plate (Grove et al., 2008), and high-grade metamorphic rocks that structurally overlie, rather than underlie an ophiolite (Dilek et al., 2008). This paper presents structural, lithologic, and metamorphic P–T estimates for an amphibolite unit bordering the Feather River ultramafic belt in northern California. We will review the regional framework of these rocks, then present new field, petrographic, and metamorphic petrologic data that bear on the origin and evolution of these rocks. We will show that structural, lithologic, and petrologic evidence supports a metamorphic sole model for these rocks and the specific field and petrologic relationships give new insight into the exhumation of such rocks and the importance of such exhumation in models 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), comprises Fig. 1. Location map. Modified from Edelman and Sharp (1989). Abbreviations are variably serpentinized ultramafic rocks, with lesser amounts of CC: Calaveras Complex, DGO: Devil's Gate ophiolite, RAS: Red Ant schist, SFU: Shoo Fly metagabbro, metadiabase, and metabasalt; collectively these rocks fi Complex and other rocks bordering the east side of the Feather River ultrama cbelt, have been considered an ophiolite (Ehrenberg, 1975; Sharp, 1988; WU: Undifferentiated Mesozoic (primarily) and Paleozoic metamorphic and plutonic rocks. Edelman et al., 1989, Saleeby et al., 1989; Edelman and Sharp, 1989). All rocks of the FRB appear to have undergone peak metamorphism at amphibolite grade, with locally variable retrogression, although there relative to the ophiolite (summarized in Wakabayashi and Dilek are significant internal differences in peak metamorphic conditions as (2000, 2003). For example, metamorphic pressures estimates range we will show. The FRB has yielded a rather large range in igneous (two from about 0.95 GPa to 1.8 GPa for different parts of the sole beneath dates of 385±10 and 314+10/−8 Ma, U/Pb zircon; Saleeby et al., the Semail ophiolite of Oman (Gnos, 1998; Searle and Cox, 2002; Gray 1989) and metamorphic ages (about 234 to 387 Ma, Ar/Ar and K/Ar and Gregory, 2003), probably the world's most thoroughly studied hornblende; Weisenberg and Avé Lallemant, 1977; Standlee, 1978; metamorphic sole. The thickness of the overlying ophiolite can only Hietanen, 1981; Böhlke and McKee, 1984) and it has been called a account for burial pressures of about 0.5 to 0.6 GPa (e.g., Searle and polygenetic ophiolite (Saleeby et al., 1989) (geochronology summar- Malpas, 1980). Many ophiolites are much thinner than the Semail ized in Table 1). ophiolite, and the disparity between pressure estimates associated In the headwaters of the South Fork Feather River, and Slate Creek, with metamorphic sole pressures (of about 0.5 to 1.5 GPa) and the pillow basalts, sheeted dikes, and gabbros crop out structurally above potential burial pressure associated with the ophiolite thickness ultramafic rocks of the FRB. These mafic igneous rocks and the (about 0.1 to 0.4 GPa) may be much greater (e.g., Jamieson, 1986; subjacent ultramafic rocks have been called the Devil's Gate ophiolite Guilmette et al., 2008). In addition, studies have identified inverted P (Edelman et al., 1989), so the Devil's Gate ophiolite may be considered gradients (structurally high parts with pressure estimates of about 1.0 a subunit of the FRB (Fig. 1). Metamorphic age dates obtained from the to 1.8 GPa to structurally low parts of about 0.3 to 0.4 GPa) within Devil's Gate ophiolite are 276±6 Ma (Ar/Ar hornblende; Standlee, metamorphic soles, indicating major internal imbrication within the 1978) and 248 Ma (K/Ar hornblende, Hietanen, 1981). sole (Jamieson, 1980; Jamieson, 1986; Gnos, 1998). These meta- The FRB is faulted along both eastern and western boundaries morphic pressure contrasts have not been addressed in detail in against rocks of dramatically different age and lithology (Sharp, 1988; models of subduction initiation (and metamorphic sole development) Saleeby et al., 1989). East of the FRB is the Shoo Fly complex which and ophiolite emplacement (Wakabayashi and Dilek, 2003). Another consists of Ordivician to Devonian continentally-derived metasand- problem that has introduced complexity into the study of meta- stone and chert deposited that are structurally overlain by a tectonic morphic soles and subduction initiation processes has been the mélange (Varga and Moores, 1981; Hannah and Moores, 1986). A identification of high-grade metamorphic rocks that are spatially Devonian to Permian volcanic sequence overlies the Shoo Fly complex associated with ophiolite belts but do not appear to be classic at an angular unconformity (Durrell and d'Allura, 1977; Harwood, metamorphic soles as defined above. These include units of 1983; Hannah and Moores, 1986). The Shoo Fly Complex has

Table 1 Feather River ultramaﬁc belt geochronology.

Location Age (Ma) Method Reference Alleghany schist 322 ±27, 345±9; 343.7±.6 K/Ar, hornblende; Ar/Ar, hornblende Böhlke and McKee (1984); Hacker (1993) Oriental Mine granite (intrudes Alleghany schist) 388±22/−12 U/Pb, zircon Saleeby et al. (1989) Metagabbro intruding FRB in Yuba River area 285±8 K/Ar, hornblende Hietanen (1981) Devil's Gate ophiolite 248; 272±6; K/Ar, hornblende; Ar/Ar, hornblende Hietanen (1981); Standlee (1978) Gabbro dike intruding FRB north of Devil's Gate 387±7 Ar/Ar hornblende Standlee (1978) Red Ant schist N174 K/Ar, muscovite Schweickert et al. (1980) Metaplagiogranite interlayered with “internal schist” 306–324 U/Pb, zircon Saleeby et al. (1989) “External schist” 236±4 Ar/Ar hornblende Weisenberg and Avé Lallemant (1977)

C.M. Smart, J. Wakabayashi / Lithos xxx (2009) xxx–xxx 3 undergone pumpellyite–actinolite grade metamorphism in the The Red Ant schist consists of quartz-rich schists (metachert and regions flanking the FRB, (Day et al., 1988; Hacker, 1993). metaclastic rocks) and metavolcanic rocks that underwent blueschist The FRB is faulted against the Calaveras Complex on its west and, facies metamorphism (Schweickert et al., 1980; Hietanen, 1981; locally, the Red Ant schist. The Calaveras Complex is considered a Edelman et al., 1989). The Red Ant schist crops out structurally subduction complex composed mainly of phyllite and metachert with beneath and west of the Devil's Gate ophiolite, whereas 10 km to the blocks of volcanic rocks (Hietanen, 1981; Sharp, 1988; Edelman et al., south it occurs east of and structurally beneath the FRB in the North 1989). In-situ conodonts and fusulinids found in the metasediments Yuba River area (Edelman et al., 1989). In the North Yuba River area an indicate that they were deposited at least as late as the Permian and amphibolite-grade unit, known as the Alleghany schist, is found that the unit youngs westward (Hietanen, 1981; Bateman et al., 1985). structurally beneath FRB ultramafic rocks and structurally above Red The subduction–accretion or assembly of the Calaveras Complex was Ant schist. Radiometric dates on the Alleghany schist are 322±27 and underway by 177 Ma, based on the U/Pb zircon age of a pluton that 345±9 Ma (K/Ar, hornblende; Böhlke and McKee, 1984) and 343.7± cross cuts some of the earlier structures within the Calaveras Complex 0.5 Ma (Ar/Ar, hornblende; Hacker, 1993). A K/Ar age on a white mica (Sharp, 1988). In the Calaveras Complex is mainly of pumpellyite– taken from the Red Ant schist indicates that the metamorphic age of actinolite grade in regions adjacent to the FRB (Day et al., 1988; the Red Ant schist is at least 174 Ma (Schweickert et al., 1980). The Hacker, 1993). actual metamorphic age of the Red Ant schist is difficult to interpret

Fig. 2. Geologic map of the western border of the Feather River ultramaﬁc belt.

4 C.M. Smart, J. Wakabayashi / Lithos xxx (2009) xxx–xxx because the closure temperature for the white mica may be close to consist of small (less than a hundred meters in long dimension) post- (or have been exceeded by) the temperature of the ubiquitous metamorphic dioritic or gabbroic intrusions, and lenses up to 3 km in pumpellyite–actinolite overprint of all of the metamorphic terranes in the long dimension of amphibolite that have been called the internal this part of the northern Sierra Nevada, and because excess argon schists (Ehrenberg, 1975). Saleeby et al. (1989) obtained a U/Pb date cannot be directly interpreted in conventional K/Ar results (Hacker, of 306–324 Ma for a plagiogranite (tonalite) that intrudes internal 1993). schist and Weisenberg and Avé Lallemant (1977) report an Ar/Ar A thin unit (10 to 300 m thick) of amphibolite, locally garnet- hornblende age of 236±4 Ma from the same unit. bearing, crops out along the west border of the FRB in the North Fork The western contact of the ultramafic body, the Rich Bar fault, dips Feather River area (Fig. 2). Williams and Smyth (1973) and Ehrenberg moderately to steeply eastward and appears to steepen in dip or (1975) proposed that these high-grade metamorphic rocks may be a become overturned in the southernmost part of the field area (Fig. 2). metamorphic sole. Our study focuses on this unit and its structural This contact is offset by late brittle faults (Fig. 2). Directly west of the and metamorphic relationships to adjacent units. contact is a b300 meter thick unit of amphibolite facies metamorphic rocks, the external schist of Ehrenberg (1975), the rocks that have been proposed as a possible metamorphic sole (Williams and Smyth, 3. Field relationships and structural geology 1973; Ehrenberg, 1975). The external schist consist primarily of amphibolite, with lesser amounts of metachert (nearly pure quartz 3.1. Geologic units and lithologies with very small amounts of garnet, white mica, and other phases), and somewhat intermediate rock, that we call “quartz-bearing amphibo- The study area covers ~15 km2 of area near the confluence of the lite” that may have been mafic rock with cm-scale (or less) chert North Fork Feather River and the East Branch North Fork Feather River interlayers or lenses. The structurally highest part of the external (Fig. 2) and spans the western border of the FRB, where it is faulted schist includes garnet amphibolites (Fig. 2). Plagioclase amphibolite against the eastern Calaveras Complex. The Calaveras Complex and or epidote amphibolite make up most of the structurally lower parts of FRB units in this area generally strike northwesterly and dip steeply to the external schist. The metamorphic grain sizes of most minerals the east and are intruded by gabbroic and dioritic bodies that cross cut range from tenths of a mm, to about 2 mm for most these rocks. The the major foliation in both the FRB and Calaveras Complex (one of the external schist shows abundant partial melting textures (Fig. 3), latter is shown on the northwestern part of Fig. 2). indicative of peak metamorphic temperatures above the wet basalt The variably serpentinized ultramafic rocks of the Feather River solidus. Some of the external schist in the Rich Bar area (southern part ultramafic body make up the easternmost and structurally highest of Fig. 2 along East Branch Feather River) appears bluish in outcrop, unit in the area. In this region, the ultramafic belt is 3–5 km wide. but microprobe analyses (see below) show that these rocks lack sodic These ultramafic rocks are metamorphosed in amphibolite facies amphibole or other blueschist facies minerals. conditions with characteristic minerals such as tremolite, talc, and West of, and structurally beneath, the external schist crop out locally anthophyllite, with antigorite (Ehrenberg, 1975, this study). slates/phyllites, cherts, and minor metavolcanic rocks of the Calaveras The tremolite reaches a centimeter in length and is commonly several Complex. This unit has the aspect of a melange with a slate/phyllite mm long. These rocks have a metamorphic foliation defined by planar matrix and chert blocks up to tens of meters or so in long dimension. layering of amphibole long-axes. Although foliated, many of these The structurally highest part of Calaveras Complex rocks exposed near rocks form massive outcrops with relatively sparse fractures. In the Beldon Siphon (Fig. 2) appear to have a coarser metamorphic grain contrast some outcrops exhibit closely spaced (cm scale or less) size (metamorphic white mica and actinolite to several tenths of a fractures or a brittle foliation, and some of these fractured rocks tend mm) than the very fine grained (hundredths of mm metamorphic to have been retrograded to lower grade serpentinite mineralogy grain sizes) rocks that characterize the remainder of the Calaveras (lizardite-dominated). In this area, crustal rocks are rare within the Complex in this area. The Beldon Siphon exposures also include what ultramafic body and this is generally representative of the FRB as a appear to be metamorphosed breccias with clasts of amphibolite and whole (Ehrenberg, 1975). Crustal rocks within the ultramafic body mafic volcanic rocks set in a phyllite or fine white mica quartz schist matrix. A hornblende gabbro dike or small pluton intrudes the Calaveras Complex and cuts the northern section of the external schist in Yellow Creek canyon (“gb” in the northwestern part of Fig. 2). This dike lacks the foliation seen in the Calaveras Complex and external schist and lacks 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 with samples from the field area. Here sheeted dikes and pillow basalts are recognizable despite having been metamorphosed at amphibolite grade (Edelman et al., 1989).

3.2. Structural geology

The ultramafic rocks, internal schist and external schist exhibit a foliation defined by the planar alignment high-temperature metamorphic minerals that strikes northwest and dips northeast (Fig. 2). Foliations in the southeast portion of the area tend to dip vertically or to the west. The steepening is apparent in the map patterns exhibited by the external and internal schist contacts. Throughout the area the foliations are subparallel to the contact between the external schist Fig. 3. Photo of melt segregations in external schist. The black arrows point to areas and the ultramafic unit. The external schist has a stretching lineation, where zones that appear to have been melt rich (felsic material between small pieces of amphibolite restite) restite) feed leucosomes. This location about 15m north of sample most easily recognized by the alignment of the long axes of location FR6. amphiboles. This lineation orientation shows much scatter. Lineations

C.M. Smart, J. Wakabayashi / Lithos xxx (2009) xxx–xxx 5

Fig. 4. Photomicrograph of garnet amphibolite sample 92-2. Plane polarized light. Fig. 5. Photomicrograph of sample FR6, a quartz-rich garnet–clinopyroxene amphibo- Abbreviations: grt: garnet, hbl: hornblende; rt: rutile. lite. Plane polarized light. Abbreviations as for Fig. 4 and: cpx: clinopyroxene, qtz: quartz, ttn: titanite, phen: phengite. plunge direction is tied to the foliation dip, so east-plunging lineations been heavily retrogressed or altered, with a fine-grained growth of are associated with east-dipping foliation. C and s surfaces, shear white mica, and Ca-silicate minerals. Epidote tends to show a higher bands, and asymmetric porphryclast tails, and asymmetric small-scale pistachite content (higher birefringence and deeper yellow pleochro- (generally centimeter to meter scale) folds in the foliation, consis- ism) in epidote amphibolite than the garnet-amphibolite. Rutile is tently ultramafic-side-up sense-of-shear, which is tops-to-the-west commonly rimmed and in some cases nearly entirely replaced by for east-dipping foliation. Isoclinal folds are common in the external titanit. Garnet is rare in the metabasite and most are badly retrograde schist, although these are most easily observed only in outcrops that and fractured, and consist of fragments generally less than 0.1 mm, but show pronounced compositional layering. Amplitudes of these folds some reach 4 mm. The grain size of most minerals in amphibolite range vary from centimeter scale to at least tens of meters. Axes of these from 0.1 to 5 mm with the tendency for somewhat finer grain sizes in folds appear to be subparallel to the foliation strike. The consistent epidote amphibolite. Hornblendes ranges in size from 1 to 5 mm. shear sense orientation in the external shear sense indicates that this Hornblendes exhibit variable retrogression and commonly have sense of shear predates the folds, rather than being associated with patchy brownish regions with surrounding areas of brownish green, high-temperature passive flow or flexural slip folding which would green and bluish green amphibole. Epidote found in the garnet result in opposite senses of shear on opposing limbs of folds. The early amphibolite is usually 1–2 mm while those found in the epidote isoclinal folds are themselves folded by at least one generation of more amphibolite ~0.1 mm in size. Pale green clinopyroxene up to 0.5 mm in open folds at the scale of meters or larger, and these later generations size occurs in some plagioclase amphibolite. We did not find of folds are responsible for the scatter in the foliation and lineation clinopyroxene in mafic garnet amphibolite (Fig. 4), whereas clinopyr- orientations. oxene does occur in the quartz-bearing garnet amphibolite. Foliation within the Calaveras Complex is subparallel to the Hornblende shows a strong preferred orientation with the long foliation in the external schist and ultramafic rocks. Calaveras complex axes lying in the foliation planes. Quartz shows evidence of plastic folding, and shear sense within the Calaveras Complex were not evaluated 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 of metacherts and metabasite in varying proportions, and metacherts. The metabasites of the external schist can be divided into garnet amphibolite (Hbl+Grt±Ep±Ab±Qtz+Rt), plagioclase amphibolite (Hbl+Pl (entirely or nearly entirely replaced by Ab)±Cpx±Qtz+Rt or Ttn) or epidote amphibolite (Hbl+Ab+Ep±Qtz±Chl with either Rt or Ttn). Some amphibolite from the Rich Bar area also have rare bluish rims on green or green-brown hornblende. Apparent blue amphibole rims on hornblende from amphibolite associated with the FRB have been previously identified by Ferguson and Gannett (1932) 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 are from the Alleghany district in the Yuba River region, about 60 km very slightly brighter than the cores, probably reflecting higher Fe concentration and fi fi southeast of the eld area. We did not nd fresh plagioclase (excluding correspondingly higher Si substitution; this subtle difference is best viewed on the albitic plagioclase) in the external schist. The (inferred) plagioclase has upper of the two phengites in the view.

6 C.M. Smart, J. Wakabayashi / Lithos xxx (2009) xxx–xxx

schist (Fig. 2). Rutile is widely distributed at all structural levels of the external schists in both the amphibolites and quartz-rich amphibolites. Titanite commonly rims rutile (Fig. 7) and in some samples nearly completely replaces it. Some of the amphibolites do not contain rutile, but contain titanite only; ilmenite appears to rim titanite in some 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 with subgrain development, ribbon grains, and c and s surface development. Garnets in the metachert are small (0.5 mm) and broken up. Bluish (hand specimen) rocks that resemble blueschist from Rich Bar area are quartz rich (possibly metacherts) with pale green amphiboles with dark blue rims, stilpnomelane, and rare garnet.

4.2. The internal schist

Fig. 7. Photomicrograph showing titanite (ttn) rimming relics of rutile (ru) in sample The internal schist consists mainly of plagioclase amphibolite (Hbl+ FR6. Plane polarized light. Other abbreviations same as Figs. 4 and 5. Pl (replaced by Ab)±Qtz±Rt±Ttn) that is interleaved with hornblendite (Hbl±Rt±Ttn). No garnets were found by us or Ehrenberg (1975) in the internal schist. In some samples the hornblende is optically strain with ribbon grains and subgrain development. Asymmetric homogeneous and olive green whereas in others the amphibole is zoned shear fabrics with shear bands and c and s surfaces are common. from a pale, apparently actinolitic core to an olive green rim (Fig. 8). Quartz-bearing amphibolite, which may have been derived from Plagioclase locally appears to be fresh, although we were unable to variable proportions of finely interleaved metachert and metabasite, obtain to find plagioclase during our electron microprobe analysis (see contains 10–50% quartz. Those rocks with the highest quartz contents below), whereas in many other samples, it is riddled with later alteration may be impure (perhaps tuffaceous) metacherts or siliceous meta- products that include fine-grained white mica and other minerals. K- tuffs, whereas those with the lowest quartz contents may represent feldspar occurs in felsic segregations in the amphibolites. Euhedral metabasites with limited intercalated metachert. The primary (high- titanite is common in the plagioclase amphibolites. Late veins of grade) assemblage is Hbl+Qtz+Cpx+Grt+Phen±Pl±Chl±Rt± prehnite are common. Grain sizes are 1–3 mm for the plagioclase Zo. Biotite has been reported from these rocks (Ehrenberg, 1975; amphibolite and 3–5 mm for the hornblendite. The planar orientation of Hacker and Peacock, 1990) and we observed grains in some samples amphiboles and amphibole-rich and feldspar-rich layers define the that may have been biotite, but have been altered to chlorite and clay high-temperature foliation in these rocks. A preferred elongation minerals. Most garnet is 0.5 mm in size, but some reach 2 mm. Garnet direction in amphiboles defines a mineral lineation that appears to be is commonly rather heavily altered or retrogressed with ragged rims present in some samples of the internal schist. Ehrenberg (1975) and common replacement by chlorite. The garnet tends to be heavily interpreted these rocks as metagabbros, but we did not find any samples fractured with considerable alteration along the fractures, both of that exhibited textures that suggest a gabbroic, rather than basaltic garnets and the abundant inclusions (Fig. 5). Very pale green protolith. The lack of associated metasediments (such as metacherts or clinopyroxene is typically 0.5–1 mm forms heavily fractured grains the quartz-bearing amphibolites) in the internal schists may suggest with ragged margins (Fig. 6). The pyroxene appears to be intergrown gabbroic, rather than basaltic protolith, however. with albite, the textural affinity of which is not clear, and fine-grained alteration minerals are present along fractures and margins of grains. Hornblende is brownish with occasional green rims and locally pale green actinolitic outermost rims. The hornblende commonly is less than 1 mm, but some reach 3 mm in size. Hornblendes appear to reflect variable retrogression with irregular brownish patches in the interior of the grains surrounded by greenish or brownish green amphibole, with actinolite representing the texturally latest amphibole forming the rims or along fractures. Phengite forms grains of 0.5 mm or smaller (average about 0.1 mm). It appears to have grown in multiple textural generations with an earliest generation in apparent textural equilibrium with the garnet, clinopyroxene and brown amphibole (Fig. 6)defining an early foliation. The texturally early phengite grains tend to be the larger ones, and many of them are bent with undulatory extinction. Later strain-free phengites are found in the matrix, and also cross cutting the early, larger grains. Zoisite occurs as elongate grains up to 2 mm in length and is colorless with low birefringence, and anomalous colors on parallel extinction; it appears in textural equilibrium with garnet, clinopyroxene, and brown hornblende. Secondary pumpellyite is present, as limited overgrowths and as comparatively rare vein filling. Foliation and fabric in the quartz-bearing amphibolite resembles that of the mafic Fig. 8. Photomicrograph of internal schist sample FR16. Plane polarized light. Most of amphibolite. this view shows amphiboles that are zoned from pale actinolitic cores to darker Garnet in both quartz-bearing amphibolites and the metabasites hornblende rims. Rutile grain is shown. Most of the rutile in this sample has been appears to be restricted to the upper structural levels of the external replaced by titanite.

C.M. Smart, J. Wakabayashi / Lithos xxx (2009) xxx–xxx 7

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

5. Electron microprobe analysis of selected phases

In order to evaluate the P–T conditions of metamorphism as well as identify certain minerals, mineral compositions were determined for minerals from six samples, four from the external schist, one from the internal schist, and one from the sheeted dike unit of the Devil's Gate ophiolite. The mineral chemistry was determined using a CAMECA SX- 100 Electron Microprobe at the University of California, Davis. The accelerating potential was 15 kV and the beam current was10 nA, with counting times of 10 s for peaks, and 5 s for background. Amphiboles, Fig. 9. Photomicrograph of sample from sheeted dike unit of the Devil's Gate ophiolite. pyroxenes, garnets, and epidote minerals were analyzed with a beam Plane polarized light. Abbreviations: act: actinolite, alt-px: probable altered pyroxene diameter of 1 micron, whereas feldspars and phengites were analyzed (completely replaced by fine-grained intergrowth of minerals), hbl: hornblende, ilm: with a beam diameter of 10μm. Mineral formulae were calculated from ilmenite. data on the following basis: Amphiboles: For some site assignments discussed in the text: 13 total cations excluding Ca, Na, and K, although 4.3. Devil's gate ophiolite for Table 4 amphiboles formulae are simply charge balanced to 23 oxygens. Clinopyroxene: 6 oxygens and 4 cations. Garnet: 12 oxygens. In order to assess metamorphic contrasts within the FRB, we Phengite: 22 oxygens. Representative results for garnet, clinopyroxene, examined samples from the Devil's Gate ophiolite, 40 km southeast of phengite, and amphibole are shown in Tables 2 to 5, respectively. the study area. In contrast to the external schists, the Devil's Gate ophiolite appears to represent the remnants of the upper plate of 5.1. Garnets subduction system in contrast to the apparent metamorphic sole rocks, the external schists. The Devil's Gate ophiolite samples appeared to Garnets from a quartz-rich external schist (sample FR6) were pyrope have had upper oceanic crustal protoliths: dikes and pillow basalts. We poor and rich in almadine and grossular. The compositional range of the examined them in order to assess the contrast between the garnets is Py8–10Alm42–47Sp4–5Gr35–43And2–9Uva0–2, and they have an metamorphism of the upper plate of a subduction system and the average composition of Py9Alm43Sp5Gr38And6Uva1. Because of the apparent metamorphic sole. The upper crustal lithologies were chosen heavy alteration and fragmentation of the garnets in this rock, we because they were reported in the literature (e.g. Edelman et al., 1989; were unable to identify zoning. The garnet analyses we obtained show Hacker, 1990) to have been metamorphosed at amphibolite grade, and relatively small compositional variation and no systematic spatial because amphibolite grade metamorphism in the dikes and/or basalt variation. Whereas our analysis did not identify, neither could we fl levels of an ophiolite is higher than expected for sea oor metamorph- demonstrate that the original garnet was unzoned, owing to its poor ism (e.g., Alt and Teagle, 2000; Schiffman and Smith, 1988). preservation. In addition, our analyses may be biased toward the core A sample from a sheeted dike outcrop has hornblende to 2 mm in regions of the garnet relics owing to greater degree of alteration and size that is zoned from a pale green actinolitic core to a greenish- fracturing of the rim regions. Garnet was analyzed from a metachert brown rim (Fig. 9). Plagioclase appears to have once been part of the metamorphic assemblage, but it is largely replaced by albite and dense mats of fine-grained white mica and other minerals. Brownish clots of Table 2 minerals appear to replace former blocky mineral forms; these may Garnet compositions (weight percent). have been igneous or metamorphic pyroxene. Some clinopyroxene to Sample analysis FR6 FR6 FR6 FR6 YR32 0.5 mm remains in this rock but there is no direct textural connection grt-1 grt-2 grt-3 grt-4 grt-1 between this clinopyroxene and the brownish mineral clots. Because MgO 2.27 2.17 2.26 2.26 0.82 this clinopyroxene is locally concentrically rimmed by actinolite and CaO 15.00 15.22 15.31 15.04 4.96 hornblende outward, it is likely igneous clinopyroxene. Ilmenite MnO 2.24 2.12 1.96 2.02 25.71 occurs as irregular opaque grains to 0.7 mm in size. Texturally late FeO 20.38 20.37 20.35 21.19 9.92 Al O 22.88 22.16 22.02 21.96 21.45 prehnite is common. Although most of the hornblende exhibits a 2 3 Cr2O3 0.00 0.04 0.02 0.01 0.01 somewhat static fabric without notable preferred orientation, shear SiO2 37.19 38.60 37.26 37.88 36.97 zones cut the rock and these shear zones have plastically-deformed TiO2 0.16 0.10 0.17 0.11 0.42 quartz and albite, and green to brown green amphibole. Total 100.12 100.79 99.36 100.46 100.26

Formula based on 24 O 4.4. Calaveras complex Mg 0.53 0.50 0.53 0.52 0.20 Ca 2.50 2.53 2.58 2.51 0.86 Calaveras Complex rocks in this area are commonly extremely fine Mn 0.30 0.28 0.26 0.27 3.51 Fe2+ 2.45 2.66 2.46 2.59 1.35 grained and many of them exhibit few metamorphic minerals that are 3+ fi Fe 0.21 0.00 0.21 0.17 0.00 readily identi able in thin section. Most metamorphic minerals have Al 4.20 4.05 4.08 4.03 4.08 grain sizes of 0.1 mm or less. The slate samples have little visible Cr 0.00 0.01 0.00 0.00 0.00 mineralogy other than quartz, albite, and fine white mica. Cherts tend Si 5.80 5.98 5.85 5.90 5.96 to be nearly all quartz with some white mica. Metavolcanic rocks Ti 0.02 0.01 0.02 0.01 0.05 contain quartz, albite, white mica, chlorite, epidote, and actinolite. The Total 16.01 16.02 15.99 16.00 15.96

8 C.M. Smart, J. Wakabayashi / Lithos xxx (2009) xxx–xxx

Table 3 Table 5 Clinopyroxene compositions (weight percent). Amphibole compositions.

Sample analysis FR6 FR6 FR6 Sample analysis FR6 FR6 YR45 RB52 FR16 FR16 DGO1 cpx-1 cpx-2 cpx-3 am-2 am-4 am-3 am-3 am-4 am-5 am-2

Na2O 0.28 0.33 0.35 H2O 2.01 2.03 2.00 2.07 2.00 2.00 2.07

MgO 12.00 12.08 12.51 Na2O 1.37 1.29 2.22 0.43 1.89 1.54 0.93

Al2O3 0.75 0.94 1.11 K2O 1.60 1.63 0.36 0.03 0.49 0.43 0.33

SiO2 52.96 52.83 52.72 CaO 11.71 11.65 10.51 11.64 10.96 11.06 11.43

K2O 0.01 0.01 −0.01 MgO 9.08 9.86 9.45 15.16 9.75 10.38 14.93 CaO 22.75 23.75 23.57 MnO 0.27 0.17 0.30 0.38 0.39 0.37 0.21

TiO2 0.00 0.02 0.05 FeO 16.48 15.55 19.67 13.15 18.52 17.62 12.69

FeO 10.46 9.90 9.29 Al2O3 15.32 15.37 13.13 1.75 12.84 10.82 8.13

MnO 0.14 0.18 0.18 Cr2O3 0.01 0.04 0.01 0.01 0.03 0.02 0.08

Cr2O3 0.02 0.03 0.01 SiO2 41.55 42.35 42.53 54.44 42.60 44.27 47.90

Total 99.37 100.08 99.80 TiO2 0.86 0.88 0.64 0.06 0.82 0.81 1.20 Total 100.26 100.83 100.82 99.14 100.29 99.30 99.90 Formula based on 6 O Na 0.02 0.02 0.03 Formula proportion based on 23 oxygens Mg 0.68 0.68 0.70 Na 0.39 0.37 0.63 0.12 0.54 0.44 0.26 Al 0.03 0.04 0.05 K 0.30 0.30 0.07 0.02 0.09 0.08 0.06 Si 2.01 1.98 1.98 Ca 1.86 1.82 1.64 1.82 1.72 1.75 3.16 K 0.00 0.00 0.00 Mg 2.00 2.15 2.05 3.22 2.13 2.28 1.74 Ca 0.92 0.96 0.95 Mn 0.03 0.02 0.04 0.05 0.05 0.05 0.03 Ti 0.00 0.00 0.00 Fe2+ 1.62 1.43 1.15 1.36 1.22 1.30 0.54 Fe2+ 0.36 0.30 0.27 Fe3+ 0.42 0.47 1.25 0.28 1.05 0.87 0.97 Fe3+ 0.00 0.01 0.02 Aliv 1.75 1.72 1.74 0.15 1.67 1.38 1.07 Mn 0.00 0.01 0.01 Alvi 0.92 0.93 0.52 0.08 0.55 0.50 0.30 Cr 0.00 0.00 0.00 Cr 0.00 0.00 0.00 0.00 0.00 0.00 0.01 Total 4.02 3.98 4.01 Si 6.15 6.18 6.19 7.85 6.24 6.53 6.81 Ti 0.10 0.10 0.07 0.00 0.09 0.09 0.13 Total 15.55 15.49 15.33 14.96 15.35 15.27 15.06 (YR32) that showed zoning in backscatter electron (BSE) imaging, but only two usable analyses were obtained. This garnet was spessartine rich plagioclase amphibolite from the internal schist (FR16). We also with a composition of Py4–8Alm19–30 Sp42–62Gr8–14And6–7Uva0. analyzed amphibole from a sheeted dike sample from the Devil's Gate ophiolite. The amphiboles are edenites and pargasites (Yavuz, 2007) 5.2. Amphibole (Fig.10). Toward more aluminous amphiboles in Fig.10 is a bit deceiving in that actinolite was intentionally avoided (not entirely with success) Amphiboles were analyzed from the quartz-bearing external schist with the exception of RB52 where it is the primary calcic amphibole; (FR6), plagioclase (former) amphibolite from the external schist (YR45), actinolite appears as a late overprint in most of the external schist. In a quartz-bearing schist or metachert from the external schist that Fig. 11, aluminum and titanium contents of amphibole are shown, as appeared to have blue amphibole rims on green amphibole (RB52), and these concentrations are pressure and temperature dependent, respec- Table 4 tively in high-temperature calcic amphiboles (Ernst and Liu,1998). Note Phengite compositions (weight percent). that Fig. 11 excludes the more actinolite-rich analyses, but is comprised of variably retrogressed amphiboles. The variable retrogression appears Sample analysis FR6 FR6 FR6 FR6 FR6 to be reflected by the positive correlation between Al and Ti contents of wm-1 wm-2 wm-2 (rim) wm-3 wm-3 (rim) amphibole in each sample. The spread of data shows the greatest scatter H2O 4.40 4.44 4.49 4.49 4.44 or spread for FR6 consistent with the greater degree of retrogression of Na2O 0.18 0.11 0.22 0.22 0.02 K2O 10.43 10.13 10.21 10.38 9.97 amphiboles seen in thin sections that sample. MgO 2.88 2.82 3.57 2.59 4.33 In RB52 the blue amphibole rims do not appear to be sodic CaO 0.04 0.15 0.01 0.00 0.03 amphibole. Although most of these rims are small enough so that they MnO 0.01 0.06 0.03 0.06 0.05 are difficult to analyze, a few patches were large enough so that we FeO 3.40 4.12 4.70 3.85 6.01 BaO NA 0.49 0.09 0.77 0.01 believe the microprobe analysis consists entirely of one of these Al2O3 28.28 28.27 25.18 31.10 21.85 patches instead of a combination between the patch and the main Cr2O3 0.01 0.02 0.00 0.05 0.00 core amphibole. These analyses show the bluish rims and patches to SiO2 48.60 49.04 52.13 47.56 53.11 be richer in magnesioriebeckite component than the core amphibole, TiO2 0.04 0.15 0.02 0.21 0.00 but the NaB occupancy is no higher than about 0.4. These results are Total 98.25 99.80 100.66 101.29 99.81 similar to microprobe results obtained by the second author on Formula based on 22 O apparent blue amphibole rims on garnet amphibolite of the external Na 0.04 0.03 0.06 0.06 0.00 schist from the Rich Bar area in 1987. K 1.78 1.75 1.74 1.77 1.72 Mg 0.55 0.57 0.71 0.52 0.87 Ca 0.00 0.02 0.00 0.00 0.00 5.3. Clinopyroxene Mn 0.00 0.01 0.00 0.01 0.01 Fe 0.37 0.47 0.52 0.43 0.68 Clinopyroxene from quartz-bearing amphibolite (FR6) span a Ba NA 0.03 0.01 0.04 0.00 Aliv 3.25 3.12 2.94 3.19 2.69 compositional range is Wo47–49En34–36Fs15–17 Jd1–2. Clinopyroxene Alvi 1.48 1.38 1.03 1.68 0.80 exhibits inclusions or intergrowths of albite (Fig. 6). Although the Cr 0.00 0.00 0.00 0.01 0.00 clinopyroxene is heavily fractured and has fine-grained alteration Si 6.52 6.62 6.96 6.35 7.18 products growing along these fractures, the remaining pyroxene does Ti 0.01 0.02 0.00 0.02 0.00 not appear to have suffered much retrogression or alteration, based on Total 18.00 18.00 17.96 18.09 17.94 the narrow range of compositions. It is possible that the albite within

C.M. Smart, J. Wakabayashi / Lithos xxx (2009) xxx–xxx 9

Fig. 10. Amphibole compositions and classiﬁcation. the clinopyroxene represents (1) growth in equilibrium with Phengite analyses can be divided into two groups, those with low Si clinopyroxene, (2) exsolution of a Na-poor clinopyroxene and albite contents of about ~3.3 (3.18–3.37) and those with high Si contents from an earlier Na-rich clinopyroxene, because there are no other (3.45–3.72). Owing to the small size of the grains (most grains had a obvious minerals that replace the clinopyroxene or (3) a later width of no more than four microprobe beam diameters) we could not retrograde product that may have formed in conjunction with other quantitatively evaluate zoning in the phengites. The higher Si phengite is retrograde minerals along the fractures. texturally late. This can be seen in BSE imagery, where lighter rims (higher average atomic number with higher phengite substitution) are 5.4. Phengite seen on some of the larger phengite grains (Fig. 6).

Phengite in FR6 varies in Si content from 3.23 to 3.72 formula units, 5.5. Feldspars based on an 11oxygen formula. Fe content ranged from .14 to .43, and Mg content ranged from .22 to .46. We did not analyze all of the phengites We tried to ﬁnd plagioclase relics in the various amphibolite for barium, but those we did showed low Ba concentrations (Table 4). samples but could not. Many grains that appeared to be plagioclase in

Fig. 11. Aluminum and titanium contents in amphiboles. Note that this plot includes some partly retrogressed amphiboles, but excludes some of the more actinolitic (clearly late) amphiboles. It is likely that only the highest aluminum and titanium concentrations reﬂect the true high-grade amphibole compositions.

10 C.M. Smart, J. Wakabayashi / Lithos xxx (2009) xxx–xxx thin section, proved to be albite and various intergrown alteration influenced the range in their composition, we prefer the higher part of minerals. In FR16 our analyses showed potassium feldspar was the temperature range. Rutile found in FR6 constrains the minimum present in the felsic layers in that sample. The other feldspar identified pressure to be about 1.3 GPa at 650 °C (Ernst and Liu, 1998). was nearly pure albite in samples YR45. The maximum pressure of 2.2 GPa at 660 °C is from the stability of hornblende in wet basalt or amphibolite (Peacock et al., 1994; Liu et al., 1996). Application of garnet–clinopyroxene–phengite barome- 6. Thermobarometry try (Ravna and Terry, 2004) yielded pressures of about 1.4–1.9 GPa at 650–760 °C for phengite cores (of about Si 3.3). Because of the – Peak P T conditions were estimated for several samples from the somewhat ambiguous textural relationships of the phengite we also external schists, one from the internal schists and one from the did the P–T calculations using what we believed to be texturally late sheeted dike unit of the Devil's Gate ophiolite (results summarized on phengites (high Si) as the phengite in equilibrium with garnet and Fig. 12). For all samples, we used Ernst and Liu (1998) amphibole clinopyroxene. This produced ultrahigh pressure (UHP) results thermometry and barometry. We also noted the presence of partial (N3 GPa) which we consider unrealistic owing to the lack of UHP melting textures in many rocks that indicated that temperatures mineral assemblages and presence of hornblende. This supports our exceeded the wet basalt solidus. The presence or absence of garnet in interpretation of the high Si phengites as being late and not in fi ma c rocks and the occurrence of rutile were also employed to equilbrium with the high T assemblage. The updated version of the estimate minimum or maximum pressure. One sample, quartz-rich Waters and Martin (1993) calibration (Wain et al., 2001) applied to amphibolite/schist FR-6 contained garnet, clinopyroxene, hornblende, phengite cores yielded pressures of 2.2–2.4 GPa at 650–760 °C; most phengite and rutile, so we were able to apply all of the above of this range is above estimated hornblende stability. The amphibole – constraints in addition to estimated temperature from the garnet thermobarometer of Ernst and Liu (1998) applied to the calcic – clinopyroxene Fe Mg exchange reaction of Ravna (2001),and amphiboles gave a temperature of about 670–710 °C at 1.7–2.0 GPa. – – pressure by garnet clinopyroxene phengite barometry of Ravna and Owing to the retrograded nature of the amphiboles in all of our rocks, – – Terry (2004). An older calibrations of the grt cpx Fe Mg exchange we selected amphiboles with reasonable stoichiometry and good reaction (Ellis and Green, 1979) was included for comparison. totals that had the highest Al and Ti contents. Our selection of the – – Similarly the older grt cpx phen calibration of Waters and Martin compositional clusters with the highest Al and Ti contents for each (1993); with empirical correction published in Wain et al. (2001)was sample may artificially restrict the natural compositional variation used for comparison to results obtained from the Ravna and Terry and uncertainty in the P–T estimate, or it is possible that we are (2004) calibration. We shall present the thermobarometric estimates overestimating the compositional variability (and resultant P–T fi for sample FR-6 rst, then review those of the other samples. estimate range) by including some variably retrogressed amphiboles Apparent partial melting textures were found on the outcrop in our analysis. It is difficult to assess the true variability of the high- where sample FR6 was collected constrain the minimum temperature grade amphibole compositions with such widespread retrogression. to 650 °C based on wet basalt solidus (Poli, 1993; Peacock et al., 1994) For the samples other than FR6, we cannot directly compare the – (Fig. 12). Garnet clinopyroxene thermometry (Ravna, 2001) gives a same methods, except for the presence or absence or rutile, but we can – – temperature range of garnet clinopyroxene pairs of about 550 compare amphibole compositions. P–T conditions for YR45, which is 760 °C. Owing to the degraded nature of garnets, that may have structurally low in the external schist, based on amphibole thermobarometry are 630–670 °C and 1.55–1.75 GPa. The presence of rutile places a lower limit of 1.3 Gpa for peak metamorphic conditions. Amphibole thermobarometry applied to the internal schist yielded temperatures of 640–680 °C and pressures of 1.4–1.6 GPa (Fig. 12). Partial melting textures are found in the internal schists suggest that peak metamorphic temperature exceeded 650 °C. The presence of rutile in the internal schist appears to suggest a minimum pressure of about 1.3 GPa at 650 °C. Amphibole thermobarometry applied to the Devil's Gate ophiolite sheeted dike sample results in estimated P–T conditions of about 710–730 °C and 0.3–0.4 GPa. The presence of ilmenite and lack of titanite, along with the lack of garnet, indicates a maximum 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 from similar tectonic settings and for us to compare P–T conditions between different samples we will further discuss the thermobaro- Fig. 12. Summary diagram of P–T estimates of metamorphism for various samples. Spots and polygon are P–T estimates from amphibole thermobarometry of Ernst and Liu metry. Although the Waters and Martin (1993); with correction in (1998) for sample DGO (Devil's Gate ophiolite, sheeted dike sample), IS (internal Wain et al. (2001) calibration of the grt–cpx–phen barometer schist), and samples YR45 and FR6 of the external schist. Other thermometers and commonly gives lower P estimates in eclogites (Page et al., 2007), barometers applied speciﬁcally to sample FR6 are as follows: R2000: Ravna (2001) – – we found it to give higher pressures for FR6. Whether or not the grt garnet clinopyroxene thermometer (the lower limit for R2000 is well below the wet – basalt solidus, so it is not plotted), RT04: Ravna and Terry (2004) garnet– cpx phen barometry is applicable to sample FR6 is unclear, owing to clinopyroxene–phengite barometer, EG79: Ellis and Green (1979) garnet–clinopyrox- the somewhat unclear textural association of the earliest formed ene thermometer, shown for comparison, as well as WM: Waters and Martin (1993) white mica, and the question as to whether the clinopyroxene garnet–clinopyroxene–phengite barometer revised as in (Wain et al., 2001). The retained the composition it had at peak pressure. Accordingly, our preferred limits on the P–T conditions of metamorphism for sample FR6 are shown in grt–cpx–phen results should be viewed with some caution. gray. 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 wet The compositional dependence of mineral compositions and basalt solidus from Poli (1993). stability may also impact the P and T estimates. To better evaluate

C.M. Smart, J. Wakabayashi / Lithos xxx (2009) xxx–xxx 11 the applicability of the equilibria used in our PT estimates we obtained external schist (where sample FR6 was collected), and this qualita- major element chemical data by X-Ray fluorescence analysis (PANa- tively supports an inverted pressure gradient within the external lytical MagiX Pro). With the exception of sample FR6, the external and schists, as well as higher pressures of the structurally higher part of internal schists, and the Devil's Gate ophiolite samples appear to have the external schist compared to the internal schist. major and minor element compositions similar to MORB used in the The presence of pumpellyite, actinolite, and chlorite, as late experiments that the Ernst and Liu (1998) amphibole thermobarom- mineral growth in many of the samples is consistent with the eter was calibrated with (Table 6). FR6 is derived from a protolith that pumpellyite–actinolite overprint that is characteristic of many rocks is more felsic than MORB, perhaps MORB with interlayered chert, so of the northern Sierra Nevada and crystallized at conditions of about amphibole compositions in that sample may not be directly compar- 150–350 °C and 0.2–0.4 GPa (Hacker, 1993). The high Si content able to those in the other samples we investigated. The minimum (3.45–3.72 formula units, 11 oxygen formula) of the late phengites in pressure represented by rutile is also compositionally dependent. FR6 is not compatible with phengites associated with the pumpel- Rutile is widely distributed throughout the external and internal lyite–actinolite overprint (Hacker, 1993; Wakabayashi, unpub. data). schists in rocks of a wide range in compositions, from quartz-rich In contrast, such high-Si phengites in FR6 are similar in composition to rocks more felsic than FR6 to those of true MORB composition. Thus, those observed in blueschist facies terranes (e.g. Sorenson, 1986; we believe it is reasonable to apply minimum pressure of rutile Wakabayashi, 1990; El-Shazly et al., 1997; Smith et al., 1999; Tsujimori occurrence summarized in Ernst and Liu (1998) that was based on and Liou, 2004). The late phengite growth may be evidence of a MORB. Similarly garnet stability is strongly compositionally depen- blueschist facies overprint on this rock. Other evidence for a blueschist dent, but garnet is present in true MORB-derived metabasites (sample facies overprint in the external schists is lacking, however. Thus, 92-3 in Table 6, for example) as well as the quartz-bearing FR6. external schists may have experienced retrograde blueschist facies The highest temperatures obtained were the grt–cpx temperatures conditions, whereas all of the FRB and associated rocks (internal and estimated from sample FR6. It is possible that this thermometer did external schists, Devil's Gate ophiolite) underwent late pumpellyite– not capture the peak temperature of metamorphism because: (1) our actinolite metamorphism. analyses of garnet in FR6 may have avoided the more degraded rim The PT estimates we obtained from the external schists (650– areas where prograde zoning may have been reflected; and (2) the 760 °C, 1.3–2.2 GPa) are comparable to those obtained for other clinopyroxene may not necessarily reflect its composition at the peak metamorphic soles such as beneath the Semail ophiolite of Oman of metamorphism because of subsequent retrograde metamorphism (700–900 °C, 0.95 to 1.77 GPa (Gnos, 1998; Searle and Cox, 2002)), or exsolution. beneath the Palawan ophiolite of the Philippines (700–760 °C, In view of the above we believe the external and internal schists N0.9 GPa (Encarnacion et al., 1995)), and beneath the Yarlung-Tsangpo formed at about 650–760 °C (possibly higher) at pressures greater ophiolite of Tibet (750–875 °C, 1.3–1.5 GPa (Guilmette et al., 2008)). than 1.3 GPa (rutile stability MORB composition), but less than 2.2 GPa (hornblende stability in MORB composition), and the grt– 7.2. Are the external schists a metamorphic sole? cpx–phen pressure estimate for sample FR6 falls within that broader pressure range. Comparison of amphibole compositions between the As noted in the Introduction, high-grade metamorphic rocks have external and internal schists suggests that the external schists may been found in association with ophiolite belts that are not meta- have formed at slightly to moderately higher pressures (say 0.1 to morphic soles, so it is useful to evaluate the field and petrologic data 0.7 GPa) than the internal schists and significantly higher pressures from the external schists in light of the metamorphic sole hypothesis. (ca. 1 GPa) than the Devil's Gate ophiolite. The latter conclusion is also Williams and Smyth (1973) and Ehrenberg (1975) suggested that the consistent with the occurrence of ilmenite instead of rutile in the external schists may be a metamorphic sole on the basis of the high- Devil's Gate ophiolite. Although FR6 is not of MORB composition, its grade metamorphism and contact with the ultramafic rocks. Our field amphibole compositions may indicate an inverted pressure gradient, and structural data indicates that the external schists are a thin (10 to or imbrication, within the external schist, but the difference in bulk 300 m thick) unit that dips eastward beneath the ultramafic rocks and composition between the non-MORB FR6 and MORB YR45 renders that the high-temperature shear fabric in the external schist exhibits a this assessment uncertain. Garnet within metabasites (of approximate tops-to-the-west (ultramafic-side-up) sense-of-shear. Such a high- MORB composition) is restricted to the structurally higher part of the temperature fabric should be expected in metamorphic soles. The external schists appear to be composed entirely of metabasite with metachert, similar to most metamorphic soles (e.g., Spray, 1984; Jamieson, 1986). Inverted temperature and pressure gradients found within some soles (e.g., Jamieson,1986; Gnos, 1998) may be present in Table 6 Whole rock compositional data. the external schists, but the limitations of our thermobarometry do not allow us to define inverted metamorphic gradients as definitively Sample oxide FR6 92-3 YR45 FR16 DGO MORB as the two studies cited above. Experimentsa In summary we believe the external schists that field, lithologic, SiO 68.23 51.33 51.98 51.46 47.60 49.11–52.38 2 structural, and metamorphic characteristics of the external schists are Al2O3 10.00 13.96 15.11 13.33 18.47 12.74–16.93 FeO 7.55–10.72 typical of a metamorphic sole. We suggest these rocks formed at the – Fe2O3 1.89 3.23 initiation of intra-oceanic subduction as previously proposed for many b Fe2O3 6.02 15.27 14.31 13.41 9.56 metamorphic soles. MgO 4.20 10.17 6.74 6.83 10.25 6.58–10.31 MnO 0.09 0.09 0.17 0.22 0.17 0.17–0.22 7.3. Tectonic synthesis: subduction initiation and exhumation of a TiO2 0.93 1.23 0.97 2.17 0.67 1.24–2.51 CaO 10.43 7.55 8.73 9.61 13.59 10.05–11.10 metamorphic sole

Na2O 0.2 1.04 2.65 2.63 0.59 1.93–3.76 – K2O 0.92 0.22 0.08 0.23 0.12 0.06 0.49 Our structural and petrologic data indicate that the external schists P2O5 0.224 0.091 0.061 0.164 0.008 0.15–0.27 Total 101.187 100.9337 100.812 100.0643 101.0569 99.28–100.75 formed as a metamorphic sole during the inception of eastward subduction beneath the Feather River ultramaﬁc belt. In addition, the a MORB material compositions for amphibole experiments compiled by Ernst and Liu metamorphic data provides additional insight into the tectonic (1998), including that study, Liou et al. (1974), Apted and Liou (1983), Spear (1981), Poli (1993), and Helz (1979). evolution of the external schists and related rocks. The pressure of b All iron as Fe2O3. metamorphism of the external schist (N1.3 Gpa) is well in excess of

12 C.M. Smart, J. Wakabayashi / Lithos xxx (2009) xxx–xxx what can be explained by the structural thickness of rocks intervening internal schists (about 0.6 to 1 GPa lower, corresponding to a burial between these rocks and the crustal rocks within or at the higher depth difference of 20 to 33 km), and the intervening ultramafic rock structural levels of the FRB. The position of the internal schist is mass (at most 6 km across strike) is much too thin to account for the equivalent to 1 to 1.5 km kilometers of cross-strike map distance from pressure difference. The metamorphic pressure contrast indicates the FR6 internal schist, corresponding to a maximum thickness of 1 to that the internal schist was exhumed relative to the Devil's Gate 1.5 km for a vertical average foliation dip. Owing to the uncertainties ophiolite by lithospheric-scale normal faults located east of (structu- in the barometric estimates it is difficult to estimate the absolute P rally above) the internal schists within the ultramafic rocks. Such difference between the internal schist and FR6, but it is likely in the faults would have to be located structurally beneath the Devil's range of 0.1 to 0.7 GPa, corresponding to a burial difference of 3.3 to Gate ophiolite (Fig. 13). Relative exhumation is demanded for the 23.0 km, for an average overburden specific gravity of 3.1 (allowing for internal schist relative to the Devil's Gate ophiolite regardless of the some serpentinization of the overlying mantle as well as the comparative ages of metamorphism because neither rock has possibility of some eroded oceanic crust). This requires that faults of prograde or retrograde assemblages that record a higher P than the an apparent normal sense accommodated exhumation of the external peak assemblages. schists relative to the internal schists. This exhumation is required The exhumation of the external schist (metamorphic sole) relative even if the peak metamorphism of the internal and external schists to the overlying ophiolite crustal section was accommodated by at occurred at significantly different times. The internal schists do not least two major zones of exhumation, one of which was at least 1 km appear to have been deeper than recorded by their peak metamorphic structurally above the ultramafic-external schist interface, east of the assemblage. Amphiboles in some of the samples of internal schist internal schist body, and the other permissibly located anywhere show prograde zoning with actinolite cores and there is no evidence of between the ultramafic-external schist interface and the western high P retrograde metamorphism. boundary of the internal schist body. More accurate locating of The Devil's Gate ophiolite dike sample, although collected about metamorphic pressure contrasts within the ultramafic rocks them- 40 km to the southeast of the main field area, appears to be selves is not feasible owing to the insensitivity of various reactions in representative of the upper oceanic crust that may have overlain metaultramafic rocks to pressure (e.g., Evans, 1977). much of the ultramafic rocks of the FRB. A comparison of the Devil's If in fact an inverted pressure gradient is preserved in the external Gate ophiolite with the internal and external schists is appropriate schist, then internal thrust faults are required to have juxtaposed the because there is no evidence of an along-strike metamorphic rocks of the external schist (Fig. 13). The structurally lowest unit of the gradient in either internal or external schist metamorphism between external schist is much higher P than the Calaveras Complex that the study area and the Devil's Gate ophiolite (e.g. Hacker and structurally underlies it, and this exhumation (apparent thrust fault Peacock, 1990; Hacker, 1993). The Devil's Gate ophiolite sheeted sense) may have taken place prior to the deposition of the structurally dike sample records a much lower metamorphic pressure than the highest parts of the Calaveras Complex as noted below.

Fig. 13. Tectonic cartoons showing the evolution of the northern Feather River ultramaﬁc belt with emphasis on the metamorphism and exhumation of the external schists. Note that the thickness of the external schists is exaggerated so that they are visible on these diagrams. I.S. (internal schist) schematically shows the relative position of the internal schist sample, whereas YR45 and FR6 track two samples of the external schist.

C.M. Smart, J. Wakabayashi / Lithos xxx (2009) xxx–xxx 13

Timing of subduction initiation, sole development and sole metamorphism of the crustal ophiolitic parts of the FRB such as the exhumation is difficult to ascertain owing to the scarcity of Devil's Gate ophiolite. One speculative scenario that would explain the geochronologic data, but the hornblende Ar/Ar age of 236±4 Ma metamorphism of the Devil's Gate ophiolite would be failed subduc- (Weisenberg and Avé Lallemant, 1977)reflects cooling of the internal tion of the ridge crest during the same event that led to the inception of schist through hornblende closure (~525±50 °C; Harrison, 1981). subduction and creation of the external schist (Fig. 13). The high-pressure, high-temperatures in the external and internal Many of the details of the tectonometamorphic evolution of the schists may have been approximately coeval owing to their proximity FRB require more geochronologic data and related metamorphic and similar PT conditions of metamorphism, so the internal schist age data tied to a specific structural-tectonic setting, for better under- may have formed in the same subduction initiation event as the standing. We are currently engaged in a geochronologic campaign in external schist and cooled through hornblende closure for Ar at this region and we hope that the results will help refine models for approximately the same time. Cooling of the sole is expected to rapid ophiolite genesis, subduction initiation, and early subduction zone with continued subduction, so the hornblende age is probably less exhumation. than 5 m.y. older than the actual metamorphic age (e.g., Hacker, 1991). Exhumation of soles relative to the upper crustal parts of overlying Acknowledgments ophiolites appears to be rapid for examples where ophiolites were emplaced over continental margins. In such cases many ophiolites This research was supported by the National Science Foundation were emplaced over continental margins within 10 m.y. after Grant EAR-0635767 to J.W., and awards from the California State formation (Dewey, 1976; Dilek et al., 1999), and structural relation- University, Fresno, College of Science and Mathematics to C.M.S. We ships indicate that exhumation of the sole relative to the ophiolite thank S. Roeske and S. Mulcahy for their assistance with microprobe must have occurred prior to final emplacement (reviewed in analyses, G. Torrez for XRF analyses, and R. Jamieson, W. Sharp, and V. Wakabayashi and Dilek, 2003). For ophiolites structurally above Sisson for the constructive reviews that greatly improved the paper. subduction complexes, as appears to be the case with the FRB, geochronologic data is much scarcer worldwide. References Subaerial exposure of the external schist appears to have occurred prior to the accretion of the structurally highest unit of the Calaveras Alt, J.C., Teagle, D.A.H., 2000. Hydrothermal alteration and fluid fluxes in ophiolites and Complex that has breccia clasts of external schist. The accretion age of oceanic crust. In: Dilek, Y., Moores, E.M., Elthon, D., Nicolas, A. (Eds.), Ophiolites and Oceanic Crust: New Insights from Field Studies and the Ocean Drilling Program: the oldest Calaveras Complex is uncertain, however, except that it Boulder, Colorado: Geological Society of America Special Paper, 349, pp. 273–282. exceeds 177 Ma (Sharp, 1988). Collectively the existing structural, Apted, M.J., Liou, J.G.,1983. Phase relations among greenschist, epidote amphibolite, and – metamorphic, and geochronologic framework suggests initiation of amphibolite in a basaltic system. American Journal of Science 283A, 328 353. Barrow, W.M., Metcalf, R.V., 2006. A reevaluation of the paleotectonic significance of the subduction sometime slightly before 236 Ma, subduction of the sole to Paleozoic central metamorphic terrane, eastern Klamath Mountains, California: depths exceeding 43 km (N1.3 GPa) and exhumation and subaerial new constraints from trace element geochemistry and 40Ar/39Ar thermochronol- exposure of some of these rocks by 177 Ma. ogy. In: Snoke, A.W., Barnes, C.G. (Eds.), Geological studies in the Klamath Mountains province, California and Oregon: Geological Society of America Special The eastern contact of the FRB against the Shoo Fly Complex and Paper, 410, pp. 393–410. other rocks is more difficult to evaluate. The pressure of high- Bateman, P.C., Harris, A.G., Kistler, R.W., Krauskopf, K.B., 1985. Calaveras reversed − temperature metamorphism estimated for the Devil's Gate ophio- Westward younging is indicated. Geology 13, 338–341. – fi Böhlke, J.K., McKee, E.H., 1984. K Ar ages relating to metamorphism, plutonism, and lite is not signi cantly different (within uncertainty) than that of gold-quartz vein mineralization near Alleghany, Sierra County, California. Isochron/ estimates for the low-grade metamorphism of the Shoo Fly West 39, 3–7. Complex and related rocks (Day et al., 1988; Hacker, 1993). On Brown, M., 1998. Ridge-trench interactions and high-T-low-P metamorphism, with particular reference to the Cretaceous evolution of the Japanese islands. In: Treloar, this basis the eastern FRB contact does not record large amounts of P.J., O'Brien, P.J. (Eds.), What drives metamorphism and metamorphic reactions? In: differential vertical movement. In contrast the temperature of Geological Society, London, Special Publication, 138, pp. 137–169. metamorphism (~700 °C) for the upper crustal parts of the FRB Casey, J.F., Dewey, J.F., 1984. Initiation of subduction zones. In: Gass, I.G., Lippard, S.J., reflected by the temperature of metamorphism of the sheeted dike Shelton, A.W. (Eds.), Ophiolites and oceanic lithosphere: Geological Society, London, Special Publication, 13, pp. 269–290. sample of the Devil's Gate ophiolite is vastly higher than any of the Day, H.W., Schiffman, P., Moores, E.M., 1988. Metamorphism and tectonics of the rocks adjacent to the FRB to the east. This metamorphic relationship northern Sierra Nevada. In: Ernst, W.G. (Ed.), Metamorphism and crustal evolution of the western United States. Rubey Volume VII. InPrentice-Hall, New (no P contrast but large T contrast) suggests that the eastern – fi Jersey, pp. 737 763. boundary of the FRB may have accommodated signi cant strike-slip Dewey, J.F., 1976. Ophiolite obduction. Tectonophysics 31, 93–120. displacement. Dilek, Y., Whitney, D.L., 1997. Counterclockwise P–T–t trajectory from the metamorphic The origin of the metamorphism of the Devil's Gate ophiolite poses sole of a Neo-Tethyan ophiolite (Turkey). Tectonophysics 280, 295–310. fi Dilek, Y., Thy, P., Hacker, B., Grundvig, S., 1999. Structure and petrology of Tauride some dif cult problems. In many of the classic ophiolites of the world, ophiolites and mafic dike intrusions (Turkey): implications for the Neo-Tethyan the ophiolite structurally overlying the sole exhibits negligible burial ocean. Geological Society of America Bulletin 111, 1192–1216. metamorphism (most exhibit sea floor metamorphism) (e.g., Waka- Dilek, Y., Furnes, H., Shallo, M., 2008. Geochemistry of the Jurassic Mirdita ophiolite (Albania) and the MORB to SSZ evolution of a marginal basin oceanic crust. In: bayashi and Dilek, 2000; 2003). The crustal ophiolitic rocks of the FRB, Dilek, Y.D., Ernst, R.E. (Eds.), Links between ophiolites and LIPs in Earth history: such as the Devil's Gate ophiolite, and possibly the internal schist, are Lithos, 100, pp. 174–209. clearly different. The high T,lowP metamorphism of the Devil's Gate Durrell, C., d'Allura, J., 1977. Upper Paleozoic section in eastern Plumas and Sierra fl Counties, northern Sierra Nevada, California. Geological Society of America Bulletin ophiolite clearly re ects a much higher geothermal gradient than that 88, 844–852. recorded in the high T, high P external schist. The metamorphism is not Edelman, S.H., Sharp, W.D., 1989. Terranes, early faults, and pre-Late Jurassic amalgation compatible with sea floor metamorphism because the prograde of the western Sierra Nevada metamorphic belt, California. Geological Society of – amphibole zoning indicates cooling after crystallization, followed by America Bulletin 101, 1420 1433. Edelman, S.H., Day, H.W., Moores, E.M., Zigan, S.M., Murphy, T.P., Hacker, B.P., 1989. heating, in contrast to sea floor metamorphism where temperatures Structure across a Mesozoic ocean-continent suture zone in the northern Sierra should generally reflect a progressive cooling. In addition, sea floor Nevada, California. Geological Society of America Special Paper, 224, p. 55. fi amphibolite metamorphism is restricted to the gabbro layer and Ehrenberg, S.N., 1975. Feather River Ultrama c Body, Northern Sierra Nevada, California. Geological Society of America Bulletin 86, 1235–1243. beneath, and has not been found associated with sheeted dikes (e.g. Ellis, D.J., Green, D.H., 1979. An experimental study of the effect of Ca upon garnet– Liou and Ernst,1979; Evarts and Schiffman,1983; Schiffman and Smith, clinopyroxene Fe– Mg exchange equilibria. Contributions to Mineralogy and 1988, Alt and Teagle, 2000). Ridge subduction has been suggested to be Petrology 71, 13–22. El-Shazly, A.K., Worthing, M.A., Liou, J.G., 1997. Interlayered eclogites, blueschists and associated with very high geothermal gradients (e.g., Sisson and Pavlis, epidote amphibolites from NE Oman: a record of protolith compositional control 1993; Brown, 1998) and such an event may explain the high-T and limited fluid infiltration. Journal of Petrology 38, 1461–1487.

14 C.M. Smart, J. Wakabayashi / Lithos xxx (2009) xxx–xxx

Encarnacion, J.P., Essene, E.J., Mukasa, S.B., Hall, C., 1995. High pressure and Peacock, S.M., 1987. Creation and preservation of subduction-related inverted temperature subophiolitic kyanite garnet amphibolites generated during initia- metamorphic gradients. Journal of Geophysical Research 92, 12763–12781. tion of mid-Tertiary subduction, Palawan, Philippines. Journal of Petrology 36, Peacock, S.M., 1988. Inverted metamorphic gradients in the westernmost Cordillera. In: 1481–1503. Ernst, W.G. (Ed.), Metamorphism and crustal evolution of the western United Ernst, W.G., Liu, J., 1998. Experimental phase-equlibrium study of Al- and Ti-content of States. Rubey Volume VII. InPrentice-Hall, New Jersey, pp. 954–975. calcic amphibole in MORB-A semiquantitative thermobarometer. American Miner- Peacock, S.M., Rushmer, T., Thompson, A.B., 1994. Partial melting of subducting oceanic alogist 83, 952–969. crust. Earth and Planetary Science Letters 121, 227–244. Evans, B.W., 1977. Metamorphism of alpine peridotite and serpentinite. Annual Reviews Poli, S., 1993. The amphibolite–eclogite transformation: an experimental study on of Earth and Planetary Sciences 5, 397–447. basalt. American Journal of Science 293, 1061–1107. Evarts, R.C., Schiffman, P., 1983. Submarine hydrothermal alteration of the Del Puerto Ravna, E.K., 2001. The garnet–clinopyroxene Fe2+-Mg geothermometer: an updated ophiolite, California. American Journal of Science 283, 289–340. calibration. Journal of Metamorphic Geology 18, 211–219. Ferguson, H.G., Gannett, R.W., 1932. Gold quartz veins of the Alleghany district, Ravna, E.J., Terry, M.P., 2004. Geothermobarometry of UHP and HP eclogites and schists — California. U.S. Geological Survey Professional Paper 172, 139. an evaluation of equilibria among garnet–clinopyroxene–kyanite–phengite–coesite/ Gnos, E., 1998. Peak metamorphic conditions of garnet amphibolites beneath the Semail quartz. Journal of Metamorphic Geology 22, 570–592. ophiolite: implications for an inverted pressure gradient. International Geology Saleeby, J.B., Shaw, H.F., Niemeyer, S., Moores, E.M., Edelman, S.H., 1989. U/pb, Sm/Nd Review 40, 281–304. and Rb/Sr geochronological and isotopic study of Northern Sierra Nevada ophiolitic Gray, D.R., Gregory, R.T., 2003. Ophiolite obduction and the Samail ophiolite: the assemblages, California. Contributions to Mineralogy and Petrology 102, 205–220. behaviour of the underlying margin. In: Dilek, Y., Robinson, P.T. (Eds.), Ophiolites in Schiffman, P., Smith, B.M., 1988. Petrology and oxygen isotope geochemistry of a fossil Earth History: Geological Society of London Special Publication, 218, pp. 449–465. seawater hydrothermal system within the Solea graben, northern Troodos Grove, M., Bebout, G.E., Jacobson, C.E., Barth, A.P., Kimbrough, D.L., King, R.L., Zou, H, ophiolite, Cyprus. Journal of Geophysical Research 93, 4612–4624. Lovera, O.M., Mahoney, B.J., Gehrels, G.E., 2008. The Catalina schist: evidence for Schweickert, R.A., Armstrong, R.L., Harakal, J.E., 1980. Lawsonite blueschist in the Mid-Cretaceous subduction erosion of southwestern North America. In: Draut, northern Sierra Nevada, California. Geology 8, 27–31. A.E.,Clift,P.D.,Scholl,D.W.(Eds.),Formationandapplicationofthesedimentary Searle, M.P., Cox, J., 2002. Subduction zone metamorphism during formation and record in arc collision zones: Geological Society of America Special Paper, 436, emplacement of the Semail ophiolite in the Oman Mountains. Geological Magazine pp. 335–361. 129, 241–255. Guilmette, C., Hebert, R., Dupuis, C., Wang, C., Li, Z., 2008. Metamorphic history and Searle, M.P., Malpas, J., 1980. Structure and metamorphism of rocks beneath the Semail geodynamic significance of high-grade metabasites from the ophiolitic mélange ophiolite and their significance in ophiolite obduction. Transactions of the Royal beneth the Yarlung Zangbo ophiolites, Xigaze area, Tibet. Journal of Asian Earth Society of Edinburgh, Earth Sciences 71, 247–262. Sciences 32, 423–437. Sharp, W., 1988. Pre-Cretaceous crustal evolution of the Sierra Nevada region, California. Hacker, B.R., 1990. Simulation of the Metamorphic and Deformational History of the In: Ernst, W.G. (Ed.), Metamorphism and crustal evolution of the western United Metamorphic Sole of the Oman Ophiolite. Journal of Geophysical Research 95, States. Rubey Volume VII. InPrentice-Hall, New Jersey, pp. 824–864. 4895–4907. Sisson, V.B., Pavlis, T.L., 1993. Geologic consequences of plate reorganization: an Hacker, B.R., 1991. The role of deformation in the formation of metamorphic field example from the Eocene southern Alaska forearc. Geology 21, 913–916. gradients: ridge subduction beneath the Oman ophiolite. Tectonics 10, 455–473. Smith, C.A., Sisson, V.B., Ave Lallemant, H.G., Copeland, P., 1999. Two contrasting Hacker, B.R., 1993. Evolution of the northern Sierra Nevada metamorphic belt: pressure–temperature–time paths in the Villa de Cura blueschist belt, Venezuela: petrological, structural, and Ar/Ar constraints. Geological Society of America Possible evidence for Late Cretaceous initiation of subduction in the Caribbean. Bulletin 105, 637–656. Geological Society of America Bulletin 111, 831–848. Hacker, B.R., 1994. Rapid emplacement of young oceanic lithosphere: argon geochro- Sorenson, S.S., 1986. Petrologic and geochemical comparison of the blueschist and nology of the Oman ophiolite. Science 265, 1563–1565. greenschist units of the Catalina Schist terrane, southern California. In: Evans, B.W., Hacker, B.R., Peacock, S.M., 1990. Comparison of the Central Metamorphic Belt and Brown, L.H. (Eds.), Blueschists and Eclogites: Geological Society of America Memoir, Trinity terrane of the Klamath Mountains with the Feather River terrane of the 164, pp. 59–75. Sierra Nevada. In: Harwood, D.S., Miller, M.M. (Eds.), Paleozoic and early Mesozoic Spear, F.S., 1981. An experimental study of hornblende stability and compositional paleogeographic relations; Sierra Nevada, Klamath Mountains, and related variability in amphibolite. American Journal of Science 281, 697–734. terranes: Boulder, Colorado: Geological Society of America Special Paper, 255, Spray, J.G., 1984. Possible causes and consequences of upper mantle decoupling pp. 75–92. and ophiolite displacement. In: Gass, I.G., Lippard, S.J., Shelton, A.W. (Eds.), Hannah, J.I., Moores, E.M., 1986. Age relationships and depositional environments of Ophiolites and oceanic lithosphere: Geological Society, London, Special Publication, Paleozoic strata, northern Sierra Nevada, California. Geological Society of America 13, pp. 255–268. Bulletin 97, 787–797. Standlee, L.A., 1978. Middle Paleozoic ophiolite in the Melones fault zone, northern Harper, G.D., Grady, K., Coulton, A.J., 1996. Origin of the amphibolite ‘sole’ of the Sierra Nevada, California. Geological Society of America Abstracts with Programs Josephine ophiolite; emplacement of a cold ophiolite over a hot arc. Tectonics 15, 10, 148. 296–313. Stern, R.J., Bloomer, S.H., 1992. Subduction zone-infancy: Examples from the Eocene Izu- Harrison, T.M., 1981. Diffusion of 40Ar in hornblende. Contributions to Mineralogy and Bonin-Mariana and Jurassic California arcs. Geological Society of America Bulletin Petrology 78, 824–881. 1041, 1621–1636. Harwood, D.S., 1983. Stratigraphy of Upper Paleozoic volcanic rocks and regional Tsujimori, T., Liou, J.G., 2004. Eclogite-facies mineral inclusions in clinozoisite from unconformities in part of the northern Sierra terrane, California. Geological Society Paleozoic blueschist, Central Chugoku Mountains, Southwest Japan: evidence of of America Bulletin 94, 413–422. regional eclogite-facies metamorphism. International Geology Review 46, 215–232. Helz, R.T., 1979. Alkali exchange between hornblende and melt: a temperature sensitive Varga, R.J., Moores, E.M., 1981. Age, origin, and significance of an unconformity that reaction. American Mineralogist 64, 953–965. predates island-arc volcanism in the northern Sierra Nevada. Geology 9, 512–516. Hietanen, A., 1981. Geology west of the Melones fault between the Feather and North Wain, A.L., Waters, D.J., Austrheim, H., 2001. Metastability of granulites and processes of Yuba Rivers, California. U.S. Geological Survey Professional Paper 1226-A, 35p. eclogitisation in the UHP region of western Norway. Journal of Metamorphic Jamieson, R.A., 1980. Formation of metamorphic aureoles beneath ophiolites- Evidence Geology 19, 609–625. from the St. Anthony Complex, Newfoundland. Geology 8, 150–154. Wakabayashi, J., 1990. Counterclockwise P–T–t paths from amphibolites, Franciscan Jamieson, R.A., 1986. P–T paths from high temperature shear zones beneath ophiolites. complex, California: relics from the early stages of subduction zone metamorphism. Journal of Metamorphic Geology 4, 3–22. Journal of Geology 98, 657–680. Kretz, R., 1983. Symbols for rock-forming minerals. American Mineralogist 68, 277–279. Wakabayashi, J., Dilek, Y., 2000. Spatial and temporal relationships between ophiolites Liou, J.G., Ernst, W.G., 1979. Oceanic ridge metamorphism of the East Taiwan ophiolite. and their metamorphic soles: a test of models of forearc ophiolite genesis. In: Dilek, Contributions to Mineralogy and Petrology 65, 335–348. Y., Moores, E.M., Elthon, D., Nicolas, A. (Eds.), Ophiolites and oceanic crust: new Liou, J.G., Kuniyoshi S., Ito, K., 1974. Experimental studies of the phase relations between insights from field studies and the ocean drilling program: Boulder, Colorado: greenschist and amphibolite in a basaltic system. American Journal of Science 274, Geological Society of America Special Paper, 349, pp. 53–64. 613–632. Wakabayashi, J., Dilek, Y., 2003. What constitutes ‘emplacement’ of ophiolite? Liu, J., Bohlen, S.R., Ernst, W.G., 1996. Stability of hydrous phases in subducting oceanic Mechanisms and relationship to subduction initiation and formation of meta- crust. Earth and Planetary Science Letters 143, 161–171. morphic soles. In: Dilek, Y., Robinson, P.T. (Eds.), Ophiolites in Earth History: Malpas, J., 1979. The dynamothermal aureole of the Bay of Islands ophiolite suite: Geological Society, London, Special Publications, 218, pp. 427–447. Canadian Journal of Earth Sciences 16, 2086–2101. Waters, D.J., Martin, H.N., 1993. Geobarometry in phengite-bearing eclogites. Terra Moores, E.M., 1970. Ultramafics and orogeny, with models of the U.S. Cordillera and the Abstracts 5, 410–411. Tethys. Nature 228, 837–842. Weisenberg, C.W., Avé Lallemant, H., 1977. Permo-Triassic emplacement of the Feather Mueller, S., Phillips, R.J., 1991. On the initiation of subduction. Journal of Geophysical River ultramafic body, northern Sierra Nevada mountains, California. Geological Research 96, 651–665. Society of America Abstracts with Programs 9, 525. Önen, A.P., Hall, R., 2000. Sub-ophiolitic metamorphic rocks from NW Anatolia, Turkey. Williams, H., Smyth, W.R., 1973. Metamorphic aureoles beneath ophiolite suites and Journal of Metamorphic Geology 18, 483–495. alpine periodotites: tectonic implications with west Newfoundland examples. Page, F.Z., Armstrong, L.S., Essene, E.J., Mukasa, S.B., 2007. Prograde and retrograde American Journal of Science 273, 594–621. history of the Junction School eclogite, California, and an evaluation of garnet– Yavuz, F., 2007. WinAmphcal: a Windows program for the IMA-04 amphibole phengite–clinopyroxene thermobarometry. Contributions of Mineralogy and classification. Geochemistry, Geophysics, and Geosystems 8, Q01004. doi:10.1029/ Petrology 153, 533–555. 2006GC001391.