Diverse Metamorphic Trajectories, Imbricated Ocean Plate Stratigraphy, and Fault Rocks, Yuba River Area, Feather River Ultramafic Belt, California

ABSTRACT

DIVERSE METAMORPHIC TRAJECTORIES, IMBRICATED OCEAN PLATE STRATIGRAPHY, AND FAULT ROCKS, YUBA RIVER AREA, FEATHER RIVER ULTRAMAFIC BELT, CALIFORNIA

The Feather River Belt (FRB), the most extensive of the ultramafic belts in the Sierra Nevada, is a north–south trending 150-km-long by 1-8 km wide ultramafic belt and includes related rocks that form the basement of the northern Sierra Nevada of California. These rocks have been long interpreted as a Paleozoic to early Mesozoic "suture" zone (position of former subduction zone) and provide a good opportunity to more closely investigate the details of the rock record associated with subduction processes. Conventional tectonic models cannot explain the spatial-temporal distribution of the metamorphic grade of the FRB. Geologic mapping, petrographic, and electron microprobe analyses reveal a complex spatial and relative time relationships between different lithologies and units of different metamorphic grade. In the greater Middle and North Yuba River area, ultramafic rocks structurally overlie amphibolite, composed of primarily metamafic rocks, that structurally overlies the blueschist facies Red Ant schist (RAS). These tectonic contacts have been isoclinally folded at scales of hundreds of meters to a km. In the North Yuba River area, amphibolite records low- pressure, high-temperature metamorphism with redbrown (high Ti, low Al) amphibole and ilmenite. In the Forest City and Alleghany areas on the North Fork-Middle Fork divide, amphibolite grade rocks comprise imbricates of ocean plate stratigraphy, represented by repeated sheets of metabasites, metacherts, and metaclastics. These rocks include zones of cataclasites with pseudotachylites (frictional melts generated by fault movement). The Alleghany amphibolites ii appear to comprise two slabs with contrasting metamorphic history. In one slab, olive green amphibole, apparently representing HP-HT metamorphism (rutile cores in ilmenite) is overgrown by redbrown amphibole formed during LP-HT conditions, whereas the other slab has redbrown amphibole cores mantled by olive green amphibole. One slab may record ridge subduction followed by renewed subduction (LP-HT before HP-HT), whereas the other may record subduction initiation in young oceanic lithosphere (HP-HT) followed by ridge subduction (LP-HT). In addition to the internal complexity in the Yuba River area, the FRB exhibits along-strike variation in the timing and nature of metamorphism. The northernmost FRB in the North Fork Feather River displays HP-HT metamorphism without a second HT event and has yielded Ar-Ar hornblende ages of 240 Ma. In contrast the Yuba River area records multiple HT events with Ar- Ar ages for LP-HT metamorphism of about 340 Ma. This demonstrates that the suture represented by the FRB may record multiple subduction initiation events as well as a ridge subduction event, with significantly different tectonic history along strike.

Nobuaki Masutsubo May 2013

DIVERSE METAMORPHIC TRAJECTORIES, IMBRICATED OCEAN PLATE STRATIGRAPHY, AND FAULT ROCKS, YUBA RIVER AREA, FEATHER RIVER ULTRAMAFIC BELT, CALIFORNIA

by Nobuaki Masutsubo

A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Geology in the College of Science and Mathematics California State University, Fresno May 2013 APPROVED For the Department of Earth and Environmental Sciences:

We, the undersigned, certify that the thesis of the following student meets the required standards of scholarship, format, and style of the university and the student's graduate degree program for the awarding of the master's degree.

Nobuaki Masutsubo Thesis Author

John Wakabayashi (Chair) Earth and Environmental Sciences

Stephen Lewis Earth and Environmental Sciences

Keith Putirka Earth and Environmental Sciences

For the University Graduate Committee:

Dean, Division of Graduate Studies AUTHORIZATION FOR REPRODUCTION OF MASTER’S THESIS

X I grant permission for the reproduction of this thesis in part or in its entirety without further authorization from me, on the condition that the person or agency requesting reproduction absorbs the cost and provides proper acknowledgment of authorship.

Permission to reproduce this thesis in part or in its entirety must be obtained from me.

Signature of thesis author: ACKNOWLEDGMENTS First of all, I would like to thank the Geological Society of America for the GSA research grant , which covered most of my fieldwork costs. I am grateful for the rGrant from Fresno State Associated Student Inc. that covered a large portion of the electron microprobe work. Also, I would like to thank everyone involved with the Faculty Sponsored Student Research award, which made it possible to present my research results at several GSA and AGU conferences. I was very fortunate to have my advisor, John Wakabayashi. He was always positive, encouraging, and inspiring to the students with his passion and enthusiasm for geosciences, fishing, and fermentation sciences. Also, I would like to thank my other thesis committee members: Stephen Lewis and Keith Putirka for their constructive reviews and comments, and all their support. Special thanks to Sarah Roeske and Nick Botto for their guidance with the electron microprobe at UC, Davis. I really appreciated my field partner, Gerardo Torres, who was crazy enough to come with me twice during two week-long field expeditions. Without his company, I could not have made friends at the mining town of Alleghany. To Tobyn and his mother, thank you for your hospitality at Casey’s Place in Alleghany. Finally, I could not make it without the great support from my wife, Annie. I love you so much. My three daughters, Maria (Sakura), RoseAnna (Sayuri), and Alice (Sumire), are always adorable, and their smiles always give me the encouragement I need. TABLE OF CONTENTS Page

LIST OF TABLES ...... vi

LIST OF FIGURES ...... vii

INTRODUCTION ...... 1

GEOLOGICAL SETTING ...... 5

Feather River Belt ...... 5

Red Ant Schist ...... 9

Shoo Fly Complex ...... 10

Calaveras Complex ...... 11

METHODS ...... 13

RESULTS ...... 15

Structural and Lithologic Relationships ...... 15

Petrography ...... 26

Electron Microprobe Analysis ...... 39

Thermobarometry ...... 47

DISCUSSION ...... 51 Are the Amphibolite Grade Rocks of the Yuba Rivers Area Part of A Metamorphic Sole? ...... 51

Do the Amphibolite-Grade Rocks Record Subduction Initiation? ...... 51

Origin of LP-HT Metamorphism ...... 52

Two Amphibolite Slabs with Different Relative PT Histories ...... 52

Along Strike Variation: Multiple Subduction Initiation Events? ...... 53

Complexity in a Suture Zone Recorded in the FRB ...... 54

REFERENCES ...... 56

LIST OF TABLES

Page

Table 1: Electron Microprobe data of Sample A5 and A1-35b, Garnet ...... 40

Table 2: Electron Microprobe data of Sample F2 and A1-7b, CPX ...... 42

Table 3: Electron Microprobe data of Sample A2-30b and A5, CPX...... 43

Table 4: Electron Microprobe data of Sample A3-13and A3-14, CPX ...... 44 Table 5: Electron Microprobe data of Sample AMinRA, F2, and A1, Amphibole ...... 44

Table 6: Electron Microprobe data of Sample A1-7b and A1-35b, Amphibole .... 45

Table 7: Electron Microprobe data of Sample A2-30b, Amphibole ...... 46

Table 8: Electron Microprobe data of Sample F2, A1-7b, and A1, Plagioclase .... 47

LIST OF FIGURES

Page

Figure 1: (a) The field area in the context of ophiolites and general bedrock geology in California. Adapted from Masutsubo and Wakabayashi (2010). (b) Distribution of radioisotopic dates from the Feather River Ultramafic Belt. Adapted from Smart and Wakabayashi (2009). (c) Regional geology of the field study area from Burnett and Jennings (1962)...... 6 Figure 2: (a) A section of the geological map along Highway 49 from Hietanen (1981), showing sample locations. The contour interval on the original map is 80 feet. (b) A photo of the locality, showing the amphibolite block with blueschist overprint in a mélange zone. (c) A schematic cross sectional view, showing the structural relationship. [cc = Calaveras Complex, ras = Red Ant Schist, sfc = Shoo Fly Complex] ...... 16 Figure 3: A map of the Forest and Alleghany areas, Feather River Ultramafic Belt in northern Sierra Nevada, California...... 18 Figure 4: A detail of the Forest area in the northern Feather River Belt, and a cross sectional view, showing the structural relationship of the isoclinal ocean plate stratigraphy...... 20 Figure 5: A detailed map from the western edge of FRB to Red Ant terrain contact...... 21 Figure 6: (a) A outcrop photo of interleaved metachert and metaclastic rocks. (b) A photo of melt segregations in amphibolite unit. The white arrows point to areas which appear to have partial melting. This location is FOR30 in the Forest Area (Fig. 4)...... 22 Figure 7: A detailed map showing the contact between Red Ant terrain and the eastern edge of the FRB...... 23 Figure 8: Poles to foliation of amphibolite facies units are plotted on the equal area stereonet for the (a) Forest City and (b) Allegahny areas...... 25 Figure 9: A photomicrograph of a typical LP-HT amphobolite, shown red brown hornblende, CPX, and a gray smudge of altered plagioclase...... 27 Figure 10: (a) A photomicrograph of an HP-HT amphibolite (Sample F2) which is completely over printed with LP-HT metamorphism except for the rutile cores, (b) a BSE image of Sample F2, and (c) a close up of the rutile core which is overgrown by ilmenite and titanite...... 28 viii viii Page

Figure 11: (a) Plain light and PPL view of an early formed green hornblende of HP-HT metamorphism in core with overgrowth of LP-HT amphibole (brown color). (b) (c) BSE images of an HP-HT amphibolite with chlorite-pumpellyite growth from later greenschist metamorphism around rim...... 30 Figure 12: A photomicrograph of a plagioclase-garnet ampibolite (A2-24a) shows the redbrown amphibole (representing LP-HT metamorphism) rimmed by the olive green amphibole (representing HP-HT metamorphism). No garnet is shown here...... 31 Figure 13: (a) A photomicrograph showing the cataclastic deformation texture in a fault rock. (b) Euhedral plagioclase growth in the frictional melt .. 32 Figure 14: A photomicrograph of a metacarbonate (A2-25) of a metaclastic rock is found near the plagioclase-garnet-amphibolite which appears to have the LP-HT to HP-HP metamorphic paths ...... 34 Figure 15: A BSE image of a chert garnet amphibolite showing a coarse garnet and hornblende with a retrograde actinolite vein...... 35 Figure 16: (a) BSE images of a blueschist overprinted metasedimentary rock in RAS (b) a metamorphic texture of blueschist phases overprinted with greenschist metamorphism, (c) lawsonite rimmed by sodic CPX and glaucophane in Sample A3-13...... 37 Figure 17: (a) (b) Two BSE images of an amphibolite block in a mélange zone in RAS showing blueschist overprint over the early formed hornblende. (c) A BSE image showing lawsonite growing over epidote. (d) A BSE image showing that the Rutile core with ilmenite laminae is rimmed by titanite (AMinRA) ...... 38 Figure 18: Pressure and temperature diagram of all samples that were analyzed using Ernst and Liu (1998), Graham and Powell (1984), and Holland and Blundy (1994). Purple shaded areas are P-T estimates using the Holland and Blundy (1994) thermometer for Sample A1-7b. Graph modified from Ernst and Liu (1998)...... 50

INTRODUCTION

Researchers commonly use the term "suture zone" to connote a belt of rocks of oceanic crustal and mantle origin (ophiolitic) that mark the position of a former subduction zone (Moores, 1970; Dewey and Bird, 1970). Detailed studies of suture zones reveal a greater complexity of process than previously interpreted. Studies have shown that some suture zones reflect ridge subduction as well as subduction initiation (Sisson and Pavlis 1993; Lytwyn et al., 1997; Forsythe and Nelson, 1985; Osozawa et al., 1990) and other suture zones are proposed to record more than one subduction event (Hebert et al., 2012; Hall, 2002; Harris, 2004; Encarnacion, 2004; Wakabayashi and Smart, 2008). Whereas these studies increased our knowledge about the processes of subduction and ocean crust formation, they also raised questions concerning subduction initiation processes, formation and emplacement of ophiolitic rocks, ridge subduction, and the record of multiple paleosubduction zones in a single suture. For example, studies of ophiolites show that many of them have a thin slab (<600m thick) of high-grade metamorphic rocks beneath them (e.g., Williams and Smyth, 1973; Spray, 1984; Jamieson, 1986). The high-grade rocks have been called metamorphic or dynamothermal soles, and are interpreted to have formed during inception of subduction beneath newly formed oceanic crust (Williams and Smyth, 1973; Spray, 1984; Jamieson, 1986; Hacker, 1990). These rocks consist primarily of metabasites with minor metchert and, in some cases, metaclastic rocks, and commonly exhibit an inverted metamorphic gradient and anticlockwise pressure-temperature (P-T) path (P on positive y axis) (Wakabayashi, 1990; Wakabayashi and Dilek, 2000; Dilek and Whitney, 1997; Önen and Hall, 2000; Guilmette et al., 2009). The inverted metamorphic gradient and anticlockwise PT 2 2 path are interpreted to record tectonic underplating of successively older and colder oceanic lithosphere following subduction initiation (Peacock, 1987; 1988; Wakabayashi, 1990; Hacker, 1990). Many researchers, however, dispute the connection between subduction initiation and metamorphic sole formation. Divergent views on the relationship between subduction initiation, ophiolite formation and emplacement, and metamorphic sole formation can be divided into three general models: Stern and Bloomer (1992), Shervais (2001), and Wakabayashi and Dilek (2000). The Stern and Bloomer (1992) model is the most cited and popular subduction initiation model, which argues for subduction initiation in old (cold) mid-ocean-ridge (MOR) generated crust with formation of a supra-subduction zone (SSZ) ophiolite during slab rollback and subsequent emplacement of the ophiolite over the originally formed subduction. The initiation of subduction in old oceanic lithosphere is interpreted to have taken place along an oceanic fracture zone as a result of the density contrast, a form of "spontaneous" subduction initiation as defined by Stern (2004). However, the formation of the metamorphic sole is not clearly explained in this model or its derivatives (Dilek and Flower, 2003; Dilek and Furnes, 2011). The Shervais (2001) model is similar to Stern and Bloomer (1992) model with subduction initiation in old MOR, but it attributes metamorphic sole formation to ridge subduction that took place after ophiolite formation. Wakabayashi and Dilek (2000) propose that SSZ ophiolites were generated over one subduction zone, but emplaced over a later one that initiated within young oceanic lithosphere, resulting in the formation of the metamorphic sole. The initiation of the second subduction zone is interpreted to have resulted from collision termination of the first one, so that the subduction initiation is considered "induced" by the definition of Stern (2004). 3 3

In addition to the controversy over SSZ ophiolite generation, emplacement, and the rock record of subduction initiation, Wakabayashi and Smart (2008) have proposed that “sutures,” long thought to mark the position of a single subduction zone (Moores, 1970; Dewey and Bird, 1970), may record the evolution of multiple subduction zones. A recent study of the Himalayan suture which has long been considered to mark the position of a single subduction zone that pushed India beneath Asia., has now suggested that this suture records multiple episodes of ophiolite formation and emplacement, and ocean basin closures, associated with multiple subduction zones (Hebert et al., 2012). Encarnacion (2004) showed that the Philippines record multiple episodes of ophiolite generation, metamorphic sole generation, ophiolite emplacement and volcanic arc development, connected to multiple subduction events within a 150 Ma span. Accordingly, the concept of multiple subduction zone sutures is also a key point of complexity that is part of the research focus in the Feather River ultramafic belt (FRB). This study will investigate problems in suture zone complexity by examination of amphibolites of the FRB of the North and Middle Yuba Rivers region of the northern Sierra Nevada, California. This study will also focus on the field relationships and metamorphism of High Pressure -High Temperature (HP- HT), Low Pressure-High Temperature (LP-HT), and High Pressure-Low Temperature (HP-LP) metamorphic rocks associated with the FRB and propose connections between this rock record and subduction initiation, ridge subduction, and other orogenic processes. In addition this study will give new insight into the tectonic evolution of this part of the North American Cordillera. Below the general geology and regional tectonic context of the FRB will be summarized along with specific tectonic issues that will be the focus of this study. Following the regional setting, new field, petrographic, and metamorphic petrologic data will 4 4 be presented, and these data will be discussed in the context of their connection to tectonic and metamorphic processes. GEOLOGICAL SETTING

In this study area, vastly different metamorphic grade units with discordant ages flank each other. They are the Feather River Belt (FRB), Red Ant Schist (RAS), Shoo Fly Complex (SFC), and Calaveras Complex (CC), (Sharp, 1988; Saleeby et al., 1989). The RAS is considered by some to be a part of the FRB (e.g., Edelman et al., 1989), but I consider it as a different unit, because its metamorphic grade contrasts sharply with that of the other FRB rocks, as well as the flanking CC and SFC (Ehrenberg, 1975; Day, 1988; Hacker, 1993; Smart and Wakabayashi, 2009).

Feather River Belt The FRB (Fig. 1(a)), the most extensive of the ultramafic belts in the Sierra Nevada, is a north–south trending 150-km-long by 1-8 km wide ultramafic belt and related rocks within the larger western Sierra Nevada Metamorphic Belt that forms the basement of the northern Sierra Nevada of California. It is thought to be the lower part of an ophiolite by many studies (Moores, 1970, 1982; Hietanen, 1973; Ehrenberg, 1975; Avé Lallemant et al., 1977) because it is mostly the completely serpentinized harzburgite, lherzolite, dunite, pyroxenite, and metagabbro. The FRB is notable for its large range of metamorphic ages between 234 Ma and 386 Ma (Weisenberg and Avé Lallemant, 1977; Standlee, 1978; Hietanen, 1981; Böhlke and McKee, 1984; Hacker, 1993) and presumed igneous ages of 314 and 385 Ma (Saleeby et al. 1989), as well as complexity in Figure 1: (a) The field area in the context of ophiolites and general bedrock geology in metamorphism (Fig. 1(b)), given that both HP-HT and LP-HT metamorphic California. Adapted from Masutsubo and Wakabayashi (2010). (b) Distribution of assemblages have been described from metamafic rocks found there (Smart and radioisotopic dates from the Feather River ultramafic belt. Adapted from Smart and Wakabayashi, 2009; Masutsubo and Wakabayashi, 2010). Wakabayashi (2009). (c) Regional geology of the field study area from Burnett and Jennings

(1962) The purple color is serpentinized peridotite, the light blue are metasedimentary rocks comprising the Calaveras (cc), Red Ant schist (ra), and Shoo Fly Complex. Similarly the

green fill represents metavolcanic rocks associated with any of the three units above in addition to the amphibolites associated with the Feather River ultramafic belt itself. Pink represents post-metamorphic granitoids, and orange colors represent Cenozoic volcanic rocks. 6 6

The FRB exhibits significantly higher T and higher P metamorphism than units that flank it, as recognized by previous studies (Ehrenberg, 1975; Day, 1988; Hacker, 1993; Smart and Wakabayashi, 2009). The flanking units of the Calaveras Complex to the west and the Shoo Fly Complex to the east are subgreenschist (pumpellyite actinolite) grade (Hietanen, 1981; Sharp, 1988; Edelman et al., 1989; Day et al., 1988; Hacker, 1993). The FRB rocks are amphibolite grade, whereas the Red Ant schist rocks are mostly metapelagic and metasedimentary with locally blueschist grade rocks (Schweickert et al., 1980; Hietanen, 1981; Edelman et al., 1989). The thin selvage (<600m) HP-HT amphibolite beneath the ultramafic rocks in the Feather River (Smart and Wakabayashi 2009), Forest City, and Alleghany areas (Böhlke and McKee 1984; Hacker 1993) appears similar to metamorphic soles found beneath many of the world's ophiolites. However, there are major complications to this picture. For the vast majority of ophiolite-sole pairs, the ophiolite lacks burial metamorphism (e.g., Wakabayashi and Dilek, 2000). The Devils Gate Ophiolite (DGO), the largest unit of mafic rocks within the FRB, was interpreted to comprise an extensive sheeted dike complex with some gabbro and pillow basalt, similar to classic ophiolites found globally (Hietanen, 1981; Edelman et al., 1989). However, it has been recently shown to comprise entirely metabasalt and metachert, metamorphosed at HP-HT conditions (Eck and Wakabayashi, 2013). The structural position of the DGO beneath serpentinite of the FRB, is the same as that of the apparent metamorphic sole in the northern part of the FRB (Smart and Wakabayashi, 2009) as well LP-HT rocks in the northern Yuba Rivers drainage south of the DGO (Masutsubo and Wakabayashi, 2010). The only mafic rocks that may occupy a structurally higher position (above serpentinite) are gabbro and 8 8 dikes or basalt in the headwaters of the southernmost Feather River drainage, directly east of the DGO proper. Those mafic rocks, once erroneously correlated to the DGO by Smart and Wakabayashi (2009), include basalts or diabase that appear to record only the regional pumpellyite-actinolite metamorphism observed in all of the lower grade metamorphic units of the northern Sierra Nevada (Luo and Wakabayashi, 2013). They may represent the only selvage of "upper plate" ophiolite found to date in the FRB, but even these rocks are not typical of such ophiolites for the gabbros have been overprinted by LP-HT metamorphism. Standlee (1978) and Hietanen (1981) obtained the metamorphic ages of Devils Gate Ophiolite as 276 Ma (Ar/Ar hornblende) and 248Ma (K/Ar hornblende) respectively. Furthermore, the additional complexity to FRB is that there is the possibility of at least two ages of HP-HT metamorphism associated with the FRB, which was originally suggested by Wakabayashi and Smart (2008). They considered the significant difference in ages of two amphibolite bodies, which is ca. 235 Ma Ar-Ar hornblende age from HP-HT amphibolite in the northern Feather River area (Weisenberg and Avé Lallemant, 1977), compared to the ca 340 Ma K-Ar and Ar-Ar hornblende age from amphibolites of the Yuba River area (Böhlke and McKee, 1984; Hacker, 1993). The age of DGO is also problematic. Hence, the tectonic history of the FRB is not well understood. Forest and Alleghany field area (Fig 1(c)), where all of the contrasting FRB units occur in contact, was chosen in order to unravel the baffling history of the FRB in this study. 9 9 Red Ant Schist Edelman et al. (1989) originally named the metasedimentary and metavolcanic rocks between the Goodyears Creek fault and the Downieville fault as "Red Ant Schist". Hietanen (1981) described the same unit as "metamorphic rocks within the Melones fault zone." In the Forest and Alleghany areas, Ferguson and Gannet (1932) and Böhlke and McKee (1984) described the interbedded chert and slate metasedimentary units with locally blueschist overprint as “Tightner and Kanaka Formations.” Dishamonically folded quartz schist is the dominant rock type in Red Ant Schist (Edelman et al., 1989), and green colored metavolcanic rocks with lawsonite-glaucophane overprint (Schweickert et al., 1980; Hietanen, 1981), along North Yuba Area between the Goodyears Creek fault and the Downieville fault. A type locality for the Red Ant Schist is assigned to the metasedimentary and metavolcanic rocks with locally blueschist assemblages in the Forest and Alleghany field maps in Figures 4, 5, and 6 (Hacker, 1993). Blueschist facies metamorphic minerals are rare in the Forest area, but rocks in the nearby Alleghany area contains abundant lawsonite in some metaclastic rocks, and glaucophane and lawsonite are widespread in the metavolcanic rocks, which is diagnostic of the blueschist facies metamorphism (Schweickert et al.,1980; and Edelman et al., 1989). Also, serpentinite blocks or lenses are found within the Red Ant Schist. A K/Ar 174 Ma minimum metamorphic age of Red Ant Schist was obtained from white mica (Schweickert et al., 1980). However, Hacker (1993) suggested that this age did not necessarily reflect the age of blueschist metamorphism because (1) the closure temperature of the very fine white mica maybe lower than the temperature of the regional pumpellyite-actinolite overprint, (2) the impurity of the white mica separate, shown by the lower K content of the 10 10 separate compared to white mica K content, that would result in a lower effective closure temperature compared to a pure white mica separate. Hacker (1993) concluded that the RAS age reflected the age of the regional pumpellyite actinolite overprint based on white mica Ar-Ar ages he obtained, as well as regional tectonic, metamorphic, and geochronologic relationships.

Shoo Fly Complex The Shoo Fly Complex, which Diller (1892) originally named the "Shoo Fly beds" for exposures near Taylorsville, California in Clear Creek about 3 km southeast of Shoo Fly Bridge, consists of metasedimentary and minor metavolcanic rocks east of the Downieville fault, which is just outside the eastern part of the study area. Even though the original “Shoo Fly Beds” are not entirely part of the Shoo Fly Complex, Clark et al. (1962) retained the name for thinly- bedded quartz-rich metasedimentary rocks near the American River that lie unconformably beneath bedded Mesozoic rocks. Varga and Moores (1981) and Hannah and Moores (1986) documented that the Shoo Fly complex consists of continentally-derived metasandstone and chert that are structurally overlain by a tectonic mélange. Edelman et al. (1989) described the Shoo Fly Complex along the North Yuba River immediately east of the Downieville fault and North of the North Yuba River as consisting of dark slate and local pebble conglomerate with a steep stretching lineation. There is also local chert, green and red nonlineated phyllite, and massive argillite. In previous studies (Anderson et al., 1974, D'Allura et al., 1977; Varga and Moores, 1981, Girty et al., 1984, Schweickert et al., 1984a, Hannah and Moores, 1986), the Shoo Fly Complex was determined to be formed before 360-380 Ma. Saleeby et al. (1987) obtained Ordovician-Silurian fossil and radiometric ages, and 11 11

Schweickert (1981) and Saleeby et al. (1989) found some parts of the Shoo Fly Complex as old as the late Proterozoic or 600 +/- 10Ma. However, the age was obtained from the Sierra City mélange of the Shoo Fly complex, which may or may not be a separate tectonic entity.

Calaveras Complex The Calaveras Complex has been mapped as a distinctive unit in the western Sierra Nevada metamorphic belt by previous studies: Schweickert et al. (1977); Schweickert (1981); Hietanen (1981); and Edelman et al (1989). The terrane is situated against the western edge of the Feather River ultramafic unit, and west of Goodyears Bar fault. The Calaveras Complex is considered a subduction complex composed mainly of phyllite-argillite and metachert with some volcanic units (Hietanen, 1981; Sharp, 1988; Edelman et al., 1989). This unit is mostly a disrupted mélange unit with diamictite and olistostromes (Schweickert et al., 1977, 1988; Sharp and Saleeby 1979; Sharp, 1988; Edelman et al., 1989). There are southern and northern sections within the Calaveras complex: The Merced River terrane was identified by Blake et al. (1982) and the northern portion of Bucks Lake Terrane by Silverling et al. (1987). Hsu (1968) mapped it as two regional mélange units because Calaveras Complex is marginally disrupted. However, these two terranes are actually continuous from north to south though they are physically separated (Wagner et al., 1981), and both the southern and northern parts of the terrane have a chert unit and a phyllite-argillite unit in the eastern and western parts of the Calaveras unit respectively (Schweickert et al., 1977; Schweickert, 1981; Hietanen, 1981; and Edelman et al., 1989). Saleeby et al. (1989) dated a diorite dike that intrudes the serpentinite basement and a suit of 12 12 mafic rocks at 196 +23/-11 Ma from discordant U-Pb zircon and at 205 +/- 43 Ma Nd-Sm whole-rock analyses respectively. The depositional age of the Caraveras complex is uncertain, but there is a minimum depositional age of 177 Ma based on the U/Pb zircon age of pluton that cross cuts some of the earlier structures within the Calaveras Complex (Sharp, 1988). Also, Permian and older limestone blocks have been found (Schweickert et al., 1977; Schweickert, 1981; Hietanen, 1981; and Edelman et al., 1989). METHODS

Three main methods were used in this project to understand the spatial and temporal distributions of the Feather River belt near the North Yuba River, Forest City, and Alleghany areas. Firstly, detailed field mapping was the most essential part of this project, especially in the Forest City and Alleghany areas where the last detailed mapping was done by Ferguson and Gannet (1932). Surface mapping was conducted for three weeks each at Forest and Alleghany areas during Summer 2010 and 2011. The traditional field mapping at 1:6,000 scale focused on metamorphic gradient, structural geology, and spatial distribution of different geological units. There are some gaps in the geology on the detailed maps because private land ownership limited access to certain areas. During the mapping exercise, over one hundred foliation orientations were recorded to understand the structural geology of the area, and over 300 samples were collected initially for petrographic examinations. Rocks were extremely difficult to distinguish in the field and only painstaking iteration between extensive petrography (the 200+ thin sections) and mapping allowed the delimiting of the various units. The detailed updated map with cross sectional views provides the most valuable insights of FRB history, highlighting the structural relationships of the lithological contacts and the spatial distribution of the different units. Secondly, nearly 200 thin sections were cut for this project in order to conduct a thorough petrographic study, since it was nearly impossible to distinguish different rocks in the field due to the surface similarities of the rocks. Even though the process was very time consuming, it was crucial, not only to identify rocks and mineral assemblages, but also to provide metamorphic textural 14 14 information in order to decipher the PT paths of the metamorphic rocks. After careful examination of the thin sections, key samples were selected for microprobe analysis. In addition, certain key samples, after further petrographic examination, revealed new relationship even after I had already conducted two probe sessions. Thirdly, microprobe analysis was conducted at UC, Davis Electron Microprobe Lab on key samples for mineral confirmation and geochemical composition as well as to obtain backscattered electrons (BEC) images which showed textural information in order to understand the metamorphic PT paths. This is especially useful because most of the samples are short in the usable minerals for PT estimations. Samples were polished and carbon coated prior to the analysis. A 15 keV accelerating voltage was used for all samples. Beam currents of 20 nA and 10-20 nA were used on garnet and clinopyroxene respectfully, and a 10 nA beam current was used for both amphibole and plagioclase. Beam sizes of 1.0 um was used on garnet, clinopyroxene, and amphibole, and a 10 um beam size was used on plagioclase. Elements run for garnets were Mg, Al, Si, Ca, Ti, Mn, Fe, and Cr. Elements run for clinopyroxene were Na, Al, Si, Mn, Fe, Ca, Mg, Ti, and Cr. Elements run for amphibole were K, Ti, Mn, Fe, Na, Mg, Al, Si, and Ca. Elements run for plagioclase were Na, Al, Si, K, Ca, and Fe. The instrument was calibrated against known standards of oxide and minerals for each element prior to the electron microprobe analysis. RESULTS

Structural and Lithologic Relationships

Area Between Goodyears Bar and Downieville Along Highway 49 The area between Goodyears Bar and Downieville was visited for reconnaissance purposes only. However, it led to a few key findings to help understand the tectonic history of this area. Firstly, RAS is mostly finely bedded metacherts and metashale with metaclastic and metavolcanic rocks. However, I identified at least one mélange zone within the RAS that has a HP-HT amphibolite block overprinted with blueschist facies (Masutsubo and Wakabayashi, 2009; 2010). The sample location (Sample AMinRA) and a photo of the locality are shown in Figure 2(a) & (b). Secondly, there is a symmetric distribution of contrasting metamorphic rocks. There are two large ultramafic bodies, thin slivers of amphibolite beneath them, and beneath that, RAS with at least one mélange zone with amphibolite blocks inside of it. This symmetric relationship of the contact is apparent in the map and its schematic cross section (Figure 2(c)). Also, there is the general impression of the structural stacking order of ultramafic over amphibolite over RAS between two serpentinite belts, and the contact geometry of the eastern contact of the western ultramafic belt indicates isoclinal folding of the contact; whereas, the CC-serpentinite and SFC-serpentinite contacts do not display this tight folding, indicating that the last, probably strike-slip phase of movement on those specific contacts, postdated the folding of the other contacts. Edelman and Sharp (1989) also suggested that the juxtaposition of different units in the northern Sierra Nevada region predates the development of the Sierra Foothill fault system by the presence of earlier faults in the SFC, CC, and Northern Sierra 16 16

Figure 2: (a) A section of the geological map along Highway 49 from Hietanen (1981), showing sample locations. The contour interval on the original map is 80 feet. (b) A photo of the locality, showing the amphibolite block with blueschist overprint in a mélange zone. (c) A schematic cross sectional view, showing the structural relationship. [cc = Calaveras Complex, ras = Red Ant Schist, sfc = Shoo Fly Complex] 17 17

Nevada which were recognized by Schweickert (1977), Moores and Day (1984), and Day at el. (1985). This later folding of contacts becomes much more pronounced in the detailed coverage of the Forest and Alleghany areas.

Geologic Units and Lithology in Forest City and Alleghany Areas This study area covers a total of 12km2 in Forest City and Alleghany (Fig. 3). Forest City is directly south of North Yuba River, and the Alleghany area is south of Forest City in the Middle Fork Yuba drainage. Serpentinites crop out in the structurally highest positions among the basement (pre-Cenozoic) units, and all of the basement units are unconformably overlain by unmetamorphosed Miocene andesites. In both the Forest and Alleghany areas, amphibolite grade rocks and RAS exposures are flanked on the west and east by belts of serpentinite (Fig. 3). These ultramafic rocks structurally overlie amphibolite, composed of primarily metamafic rocks, that structurally overlies the blueschist facies Red Ant schist (RAS). These tectonic contacts have been isoclinally folded at scales of hundreds of meters to a km. The western edge of the western serpentinite belt is steeply west dipping to the eastern edge of the Calaveras Complex (Fig. 3). The eastern serpentinite belt is divided between RAS and the western contact of the Shoo Fly Complex, which consist of mostly metasedimentary rocks (Fig. 3). Serpentinites in the study area (North Yuba, Forest City, Alleghany) display low grade mineralogy, consisting primarily of lizardite, and lacking higher grade metaultramafic minerals such as antigorite, tremolite, and talc that are common in FRB exposures in the Feather River area. Also, amphibolite rocks in these areas tend to have macroscopically visible metamorphic minerals that range from tenths of mm to several mm in grain size, whereas the RAS, as well as the flanking SFC and CC, are composed of rocks that tend to lack macroscopically visible 18 18

Figure 3: A map of the Forest and Alleghany areas, Feather River Ultramafic Belt in northern Sierra Nevada, California. 19 19 metamorphic minerals. The Feather River basement units include the amphibolites (including locally quartz rich garnet amphibolite adjacent to the serpentinite contact and garnet plagioclase amphibolites only in the Alleghany area), metacherts, quartz rich metasedimentary rocks, metacarbonates (only in the Alleghany area), metaclastic rocks, and local zones of cataclasites and pseudotachylite (frictionally-generated melts along faults) hosted in the amphibolite. These coherent amphibolite-grade slabs are imbricately faulted, repeating an ocean plate stratigraphy (OPS) of metaclastic over metachert over metabasite (see Fig. 3, 4, and 5). An example of the interleaved meteachert and metaclastic rocks are shown in Figure 6(a). The disruption of the sequence by the later strike slip faulting is also adding to the complication of the study area. Fault rocks consisting of cataclastically deformed amphibolite and pseudotachylites crop out in both areas, and in Alleghany is adjacent to serpentinite (Fig. 4 and 5). Many of the amphibolite grade rocks of the field area display migmatitic textures that suggest partial melting during metamorphism. They are most commonly seen as irregular light-colored, felsic segregations, or leucosomes, within darker amphibole rich rock (Fig. 6B). In the Forest area, RAS is mostly chlorite and pumpellyte grade metachert and metasedimentary with some green metavolcanic rocks (Fig. 4). In the Alleghany area (Fig. 5), RAS consists of mostly metasedimentary and metachert similar to Forest, but there are locally blueschist grade metasedimentary rocks and metabasites in the southeastern part of the mapped area (Fig. 7). In the western ultramafic rocks, the foliation is a brittle fabric of anastomosing fractures with subparallel alignment. Amphibolite grade units, including amphibolite, metachert and metaclastic units, exhibit a foliation defined by the planar alignment of high-temperature metamorphic minerals that strike 20 20

Figure 4: A detail of the Forest area in the northern Feather River Belt, and a cross sectional view, showing the structural relationship of the isoclinal ocean plate stratigraphy. 21 21

Figure 5: A detailed map from the western edge of FRB to Red Ant terrain contact. 22 22

Figure 6: (a) A outcrop photo of interleaved metachert and metaclastic rocks. (b) A photo of melt segregations in amphibolite unit. The white arrows point to areas which appear to have partial melting. This location is FOR30 in the Forest Area (Fig. 4). 23 23

Figure 7: A detailed map showing the contact between Red Ant terrain and the eastern edge of the FRB. 24 24 northwest and dip northeast in the Forest City area (Fig. 4). The foliation strikes east-northeast and dips southeast around the imbricate of western amphibolite, metachert, metaclastic contacts along Oregon Creek, the southwestern margin of Fig. 4. The foliation direction strikes northwest and dips southwest steeply in the metachert/metaclastic units around the mine spill, the south-central margin of Fig. 4. Around the contact between the imbricate of the OPS and the western side of the amphibolite unit near American Flat (near the center of Fig. 4), the foliation direction almost becomes sub vertical, and the strain of this area is also prominent from the outcrop and the presence of a fault rock unit. The poles to foliation direction of amphibolite facies unit in Forest area are shown in Figure 8(a). The average fold axis orientation has a trend of 135 and plunge of 51. Around the ultramafic unit which separates the RAS from the imbricate of the OPS, the foliation direction strikes northwest and dips northeast. The foliation direction of RAS strikes northwest and steeply dips northeast. The salvage of amphibolite unit beneath the eastern ultramafic units strikes north-northwest and dips to east- northeast. From the map, its cross sectional view (Fig. 4), and the average fold axis orientation from the stereonet plots (Fig. 8(a)), the imbricates of the OPS is again isoclinally folded by late deformation. In Alleghany, the foliation direction of the western ultramafic rocks, which separate from the eastern contact of Calaveras Complex, strikes nearly north and dips steeply west (the northwestern margin of Fig. 5). Adjacent units of metachert with some amphibolite, metaclastic, and amphibolites also parallel to the foliation direction of the ultramafic unit. The large amphibolite body next to the metachert/metaclastic units, the northwestern margin of Fig. 5, has the range of the foliation direction that is striking from northeast to northwest and dipping 25 25

Figure 8: Poles to foliation of amphibolite facies units are plotted on the equal area stereonet for the (a) Forest City and (b) Allegahny areas.

26 26 northwest to southeast. In the area near Mack House at the western contact of the large serpentinite body (the middle-central margin of Fig. 5), the foliation direction strikes north and dips sub vertical or east. From the area near the Oriental Mine, the central-north margin of Fig. 5, to the ultramafic unit which separates RAS from the imbricate of the OPS, the foliation direction generally strikes north, but dips between west and east. The average foliation direction is sub north-south with subvertical dips and the average fold axis is subhorizontal (Fig. 8(b)). The general structure of both areas is similar. The subvertical foliation is a consequence of the late isoclinal folding that has folded the lithologic contacts. The orientation of the average fold axes in the two areas is slightly different, suggesting that there is deformation that postdates the "late" isoclinal fold set that has resulted in somewhat different structural orientations of the Alleghany and Forest areas (Fig. 4 and 5).

Petrography

Amphibolite Grade Rocks Metabasites (hornblende rich). There are several different amphibolite facies units. LP-HT Amphibolite has typically a coarse red brown hornblende + clinopyroxene + altered plagioclase with fine mica and epidote + quartz +/- biotite +/- ilmenite or titanite and later retrograde actinolite rimming hornblende, and chlorite. Some muscovite may be intergrown in hornblende, and apatite and pyrite are common in this unit. Sample A1-3 is a typical LP-HT amphibolite red brown hornblende (Fig. 9). Sample F2 also appears to be the typical LP-HT amphibolite (Fig. 10(a)), but it may record a hint of the early HT-HP metamorphism in a rutile grain overgrown by ilmenite; and titanite is either texturally intermediate or late (Fig. 10(b) & (c)). HT-HP amphibolites or early formed amphibolite (green 27 27

Figure 9: A photomicrograph of a typical LP-HT amphobolite, shown red brown hornblende, CPX, and a gray smudge of altered plagioclase.

28 28

Figure 10: (a) A photomicrograph of an HP-HT amphibolite (Sample F2) which is completely over printed with LP-HT metamorphism except for the rutile cores, (b) a BSE image of Sample F2, and (c) a close up of the rutile core which is overgrown by ilmenite and titanite.

29 29 colored amphibole), which the later LP-HT metamorphism (red brown colored amphibole) may be overprinted the most of the original HP-HT metamorphism. It has the earlier olive green hornblende in core (preserved HP-HT metamorphism in core, now it may be only the color) with later brown color hornblende overprint + cpx, + altered plagioclase, + ilmenite/titanite + varying amount of quartz, and latergreenschist metamorphism of chlorite + actinolite + pumpellyite cross cuts the earlier formed minerals. Also, there are many small zircon grains on the basis of the 'burn' marks in hornblende or biotite (Fig. 11(a)-(c)). In both the Forest City and Alleghany area, we found the HP-HT to LP-HT metamorphic paths in the hornblende. Also, the existing geochlonology of 342-345 Ma in previous studies (Böhlke and McKee 1984; Hacker 1993) confirmed that the amphibolite slabs found in Forest City and Alleghany indeed formed at the same time. However, in Alleghany, plagioclase garnet amphibolite may record a different metamorphic history since the brown amphibole formed first (LP-HT metamorphism) and green amphibole (HP-HT metamorphism) grew around it, unlike the Forest City area and the area east of the large ultramafic belt in the Alleghany area (HP-HT to LP-HT metamorphism) (Fig. 12). This is based upon petrographic observations. It contains garnet + two generation amphiboles (brown LP-HT amphibole in core rimmed by green HP-HT hornblende) + biotite + mica + plagioclase + ilmenite rimmed by titanite (Fig. 12) with retrograde mineral growth from later greenschist metamorphism. I may have found two different imbricates of OPS with two different metamorphic paths. Fault rocks consists of cataclastically deformed amphibolite, and progressive cataclasis of the amphibolite host can be seen, along with pseudotachylite characterized by glassy material with small feldspar laths. The pseudotachylite appears to locally intrude into the host rock (Fig. 13(a)&(b)). 30 30

31 31

Figure 12: A photomicrograph of a plagioclase-garnet ampibolite (A2-24a) shows the redbrown amphibole (representing LP-HT metamorphism) rimmed by the olive green amphibole (representing HP-HT metamorphism). No garnet is shown here. 32 32

Figure 13: (a) A photomicrograph showing the cataclastic deformation texture in a fault rock. (b) Euhedral plagioclase growth in the frictional melt 33 33

Metaclastic. These rocks are quartz-rich but can be distinguished from metachert on the basis of coarse (tenths of a mm to a half mm) feldspar and quartz porphyroclasts that give some indication of the original clastic texture. This is usually in the form of smudge that appears to be, or actually is, reasonably large feldspars. Some rocks may have distinctive grain boundaries. At one locality, metacarbonate was found during this study consisting of metacarbonate + plagioclase + quartz +muscovite (Fig. 14). Interestingly, this locality is adjacent to the LP-HT/HP-HT amphibolite, which may suggest that this area may be a different OPS with slightly different depositional environment forming metacarbonate.

Metachert. These lacking the coarse porphyroclasts of the metaclastic rocks and contain a much higher proportion of recrystallized quartz that makes up the bulk of the rock volume. There is no plagioclase or indication that there may have been feldspar in this rock, unlike the metaclastic rocks. There are no distinctive quartz grain textural boundaries in plain light. Some of the rocks contained some garnet, biotite, mica, but mostly quartz. Quartz rich garnet amphibolites are found that may represent thin slices of metabasite within the chert or perhaps metabasite interlayed on a fine scale with chert (such as recrystallized interpillow cherts within metabasalt). Metachert-garnet-amphibolite is rare and found in one locality, and it contains early green amphiboles + huge garnet crystals + abundant quartz + albite + ilmenite, and later actinolite vein from retrograde metamorphism (Figure 15).

34 34

Figure 14: A photomicrograph of a metacarbonate (A2-25) of a metaclastic rock is found near the plagioclase-garnet-amphibolite which appears to have the LP-HT to HP-HP metamorphic paths

35 35

Figure 15: A BSE image of a chert garnet amphibolite showing a coarse garnet and hornblende with a retrograde actinolite vein

36 36 RAS Rocks Metachert. This is mostly the interbedded chert, and it is disharmonically folded. There are little to no plagioclase or mafic minerals, though there is significantly abundant amount of quartz, as compared to the metaclastic rock. There is no detritus grain texture.

Metaclastic. This is mostly low grade metasedimentary rock with locally blueschist facies. There are textural impressions from former sand-size detrital grains of feldspar or quartz porphryoclasts as per the higher grade rock unlike the metachert. The blueschist grade metasedimentary rocks contain coarse lawsonite crystals, and phengite with retrograde overprint of stubby epidote and needle-like chlorite (Fig. 16 (a)).

Metabasites. In the Forest City area, there are only green low-grade serpentnized volcanic rocks. In the Alleghany area, there are metabasaltic rocks that still preserve the relic igneous texture with CPX overgrown, and overprinted with lawsonite, sodic amphibole, and later chlorite-pumpellyite facies. Some lawsonite may have been altered to albite. Most of the lawsonite are rimmed by chlorite, pumpellyite, and albite (Fig. 16(b)), although some lawsonite are rimmed by CPX (Fig. 16(c)). There are many apatite and titanite grains in these rocks. Along Highway 49 between Goodyears Bar and Downieville, there is a high grade mélange block from the shear zone cutting the RAS (Fig. 2). It is a retrograde brown Ca-amphibolite block in Red Ant terrain, which is heavily overprinted with blueschist facies metamorphism, almost completely overprinted with lawsonite, glaucophane, actinolite, epidote, and chlorite with altered plagioclase and quartz (Fig. 17(a) & (b)). Lawsonite grew at the expense of epidote (Fig. 17(c)). This rock contained three Ti oxide phases. There is a rutile core rimmed by titanite and ilmenite laminae in the cracks (Fig. 17(d)). 37 37

Figure 16: (a) BSE images of a blueschist overprinted metasedimentary rock in RAS (b) a metamorphic texture of blueschist phases overprinted with greenschist metamorphism, (c) lawsonite rimmed by sodic CPX and glaucophane in Sample A3-13. 38 38

Figure 17: (a) (b) Two BSE images of an amphibolite block in a mélange zone in RAS showing blueschist overprint over the early formed hornblende. (c) A BSE image showing lawsonite growing over epidote. (d) A BSE image showing that the Rutile core with ilmenite laminae is rimmed by titanite (AMinRA)

39 39

Electron Microprobe Analysis Sample A1, A5, A1-7b, A1-35b, and A2-30b are from the Alleghany area, and Sample F2 was from the Forest area. Also, Sample AMinRA, A3-13, and A3- 14 are from Red Ant schist. Sample A5 and A1-35b were analyzed for garnet. Sample F2, A1-7b, A2-30, A5, A3-13, and A3-14 were analyzed for clinopyroxene. For amphibole, A1, F2, A1-35b, AMinRA, A1-7b, and A2-30b were analyzed. F2, A1-7b, and A1 were analyzed for relic plagioclase, and I was hoping to use the hornblende-plagioclase thermometry of Holland and Blundy (1994). However, Sample A1-7b was the only sample that preserved the metamorphic plagioclase. A1-7b is determined to be mafic rich metasedimentary in origin, and plagioclase in A1 and F2 were all altered to albite. Plagioclase in A1 and F2 are determined to be completely retrograded because the original composition in plagioclase was probably more anorthite component based upon the A1-7b’s preserved metamorphic plagioclase result.

Garnet The composition of garnet from Sample A5, which is a mafic rich sedimentary rock, was very unusual because of the mostly grossular composition and poor in spessartine and pyrope components. The composition range is Py0.2-

0.4Alm24-30Sp0.8-1.4Gr66-74Uva0-0.1 (Table 1). The garnet compositions had no variations from grain to grain, and all garnets were homogenous with no compositional zoning. Sample A1-35b is a quartz rich garnet amphibolite, and its garnet compositional range is Py3.2-3.4Alm62-65Sp17-19Gr12-17Uva0.1-0.2 (Table 1). Again, the garnet compositions had no variations from grain to grain, and all garnets were homogenous with no compositional zoning.

40 40

Table 1: Electron Microprobe data of Sample A5 and A1-35b, Garnet

41 41

Clinopyroxene The composition of clinopyroxene in Sample F2, a brown amphibolite, and A1-7b, a mafic rich metasedimentary rock, are similar in the CPX composition and have an average composition of a half diopside and a half hedenbergite (Table 2). The CPX composition of A2-30b, a green hornblende amphibolite, is more augitic, whereas the composition of A5 has an average composition of 25% diopside and 75% hedenbergite (Table 3). The CPX of Sample A3-13 and A3-14 from RAS is a preserved clinopyroxene with igneous texture, and augitic (Table 4) where as A3-13 also has an omphacitic sodic clinopyroxene (Table 4) which is rimmed with a lawsonite grain (The BSE image of this texture is shown in Fig. 16(c). The range of jadeite component in the pyroxene is 16-23%.

Amphibole Amphiboles are analyzed from metasedimentary rock and metabasaltic amphibolite. Sample AMinRA preserves the original amphibole before shredding down to a trench and going through blueschist metamorphism. The hornblendes are almost replaced by sodic amphibole, but some survived the metamorphism (Table 5). Amphiboles of Sample F2 are very rich in TiO2 in comparison to the other samples that were collected for this study (Table D5). Amphiboles of

Sample A1 turned out to be very patchy, as the data shows in Al2O3 contents (Table 5). Amphiboles in Sample A1-7b are magnesiohornblende (Table 6) whereas amphiboles in A1-35b are ferrohornblende (Table 6). Finally, the composition of amphiboles in Sample A2-30b is typical Ca-hornblende (Table 7). 42 42

Table 2: Electron Microprobe data of Sample F2 and A1-7b, CPX 43 43

Table 3: Electron Microprobe data of Sample A2-30b and A5, CPX 44 44

Table 4: Electron Microprobe data of Sample A3-13and A3-14, CPX

Table 5: Electron Microprobe data of Sample AMinRA, F2, and A1, Amphibole 45 45

Table 6: Electron Microprobe data of Sample A1-7b and A1-35b, Amphibole

46 46

Table 7: Electron Microprobe data of Sample A2-30b, Amphibole 47 47 Plagioclase Plagioclase was analyzed in the amphibolite and mafic rich metasedimentary rocks from samples F2, A1-7b, and A1. Sample F2 and A1 are a coarse grained amphibolite, and they are almost completely altered to a mixture of albite, chlorite, and pumpellyite with some mica, and their composition does not seem primary from the preserved plagioclase data of Sample A1-7b. Sample A1- 7b is a mafic rich metasedimentary rock with some preserved metamorphic plagioclases of bytownite. It has a composition range of An60-63Ab37-40 (Table 8).

Table 8: Electron Microprobe data of Sample F2, A1-7b, and A1, Plagioclase

Thermobarometry Pressure and temperature of metamorphism was estimated based on hornblende-plagioclase thermometry of Holland and Blundy (1994), hornblende thermobarometry of Ernst and Liu (1998), and garnet-hornblende thermometer of Graham and Powell (1984). The Holland and Blundy’s (1994) thermometer uses the strong temperature dependence of composition of coexisting hornblende and 48 48 plagioclase based upon the experimental data. Ernst and Liu’s (1998) thermometer is based on experiments showing a positive correlation between Al and Ti concentrations in metamorphic hornblende with pressure and temperature, respectively, in metamorphic rocks of approximately midocean ridge basalt composition. Graham and Powell’s (1984) thermometer is an empirical garnet- hornblende Fe-Mg exchange geothermometer which works by calibrating against the garnet-clinopyroxene geothermometer of Ellis & Green (1979) using data on coexisting garnet + hornblende + clinopyroxene in amphibolite and granulite facies metamorphic assemblages. Unfortunately, whereas hornblende occurs in most of the amphibolite facies rocks, unaltered plagioclase is rare, and garnet coexisting with hornblende is also rare. Because of the scarcity of fresh plagioclase and garnet, the Ernst and Liu (1998) amphibole thermobarometry was the only method that could be widely applied. Hence, the primary hornblende with peak Al2O3 and TiO2, which should record the peak metamorphic condition, were used in the thermobarometry estimations. Some limits on P-T estimates are also placed by the stability of Ti-bearing phases, titanite, ilmenite, and rutile in mafic rocks from Ernst and Liu 1998 as well as the local presence of partial melting textures, as will be discussed later. For Sample F2 and A1, I checked against the P-T estimation using the Holland and Blundy (1994) thermometer. The P-T estimation of Sample AMinRA was T=710 °C and P=1.4 GPa. For Sample F2, the P-T estimation was T=910-980 °C at 0.4-0.6 GPa, and there are three TiO2 phases: rutile is overgrown with ilmenite; titanite is either texturally intermediate or late. Rutile in rocks of this composition suggests a pressure of ≥1.2 GPa, so its presence as cores in ilmenite suggests a HP-HT stage of metamorphism prior to LP-HT metamorphism recorded by the Ti-rich amphibole. Sample A2-30b yielded the P-T estimates of 600 - 750 °C at 0.5 – 0.75 GPa, 49 49 which are reasonable estimates of amphibolite grade. Also, this matches with the TiO2 stability field of titanite in Ernst and Liu (1998). As for A1, there was varying in Al2O3 and TiO2 composition as you can see in Table D5. The temperature and pressure ranges are between 540 – 690 C° and between 0.3 to 1.8 GPa. The P-T ranges of Sample A1-35b yielded to be 640 - 720 °C at 0-0.35 GPa using Ernst and Liu (1998). This low pressure value is primarily due to the Al- 35b, a basalt-chert "mix" composition which is not a MORB composition. Hence .the Ernst and Liu (1998) method does not return reasonable numbers. Also, Sample A1-35b yielded the metamorphic temperature estimation of 1,056-1,261 °C using a garnet-hornblende thermometer of Graham and Powell (1984). The estimated temperature exceeds the stability limit of hornblende, perhaps being due to the bulk composition buffering compositions that differ significantly from the calibration set. Using the Holland and Blundy (1994) thermometer, Sample A1-7b yielded 655-711 C° at 5kb and 667-730 C° at 10kb though it is more mafic rich metasedimentary rock. Also, the presence of the partial melt texture in amphibolites suggest the minimum temperature of 650 C° (Beard and Lofgren, 1991; Rushmer, 1991; Poli, 1993; Peacock et al., 1994). All the P-T estimations are compiled on Figure 18. 50 50

Figure 18: Pressure and temperature diagram of all samples that were analyzed using Ernst and Liu (1998), Graham and Powell (1984), and Holland and Blundy (1994). Purple shaded areas are P-T estimates using the Holland and Blundy (1994) thermometer for Sample A1-7b. Graph modified from Ernst and Liu (1998).

DISCUSSION

Are the Amphibolite Grade Rocks of the Yuba Rivers Area Part of A Metamorphic Sole? The rocks are mainly mafic amphibolites with lesser amounts of metachert and metaclastic rocks (OPS). Imbricates of OPS are a hallmark of rocks that have been subducted and accreted (Maruyama et al., 2010; Kusky et al., 2013). The amphibolite grade rocks occur structurally beneath ultramafic rocks that are part of a regional-scale (>100 km long) belt. The lithologies, found in the areas including the imbricated OPS, are also very similar to other ophiolites (Spray, 1984; Jamieson, 1986; Smart and Wakabayashi, 2009). However, the thickness of the amphibolite unit and steep foliation initially suggests a structural thickness too great for a typical metamorphic sole. The map with cross sectional view (Fig. 4 and Fig. 5) nicely shows that structural thickness is much less than the outcrop width, considering the steep isoclinal folding. From the thin structural thickness of amphibolite unit and its metabasaltic compositions with imbricate of OPS, the amphibolite grade rocks of the Yuba Rivers are a part of a metamorphic sole.

Do the Amphibolite-Grade Rocks Record Subduction Initiation? The presence of rutile with overgrown ilmenite (Ernst and Liu, 1998; Xiong et al., 2005) and the partial melt in amphibolite suggests an initial HP-HT metamorphic condition prior to the LP-HT metamorphism though the thermobarometry data is limited because the amphibolite with surviving plagioclase, garnet, or clinopyroxene is rare, due to the fact that the greenschist retrograde metamorphism occurred late (Hacker, 1993). The PT conditions of HP- HT conditions, however loose, are consistent with metamorphic sole PT conditions recorded in many soles around the world, including the northern part of 52 52 the FRB (Williams and Smyth, 1973; Ehrenberg, 1975; Malpas, 1979; Spray, 1980; Gnos, 1998; Hacker, 1991; Hacker et al., 1996; Smart and Wakabayashi, 2009). Therefore, the original HP-HT metamorphism suggests a metamorphic sole due to the hot initiation of subduction.

Origin of LP-HT Metamorphism The ridge subduction was suggested by many authors for the cause of high geothermal gradients in an oceanic setting (Sisson and Pavlis, 1993; Brown, 1998; Lytwyn et al., 1997; Forsythe and Nelson, 1985; Osozawa et al., 1990; Wakabayashi, 2004). The presence of metasediments in the amphibolites from this study area suggests that the top of the ocean crust experienced the LP-HT conditions from burial metamorphism rather than from dynamic spreading center metamorphism that would reach amphibolite grade only at the gabbro level (Liou and Ernst, 1979; Evarts and Schiffman, 1983; Schiffman and Smith, 1988, Alt and Teagle, 2000). The LP-HT metamorphism may in fact have affected more rock than what was originally metamorphosed under HP-HT conditions because of the scarcity of the remnant HP-HT metamorphism. Also, this suggests that the HP- HT conditions were short lived, which coincides with the inverted metamorphic gradients of a metamorphic sole, and it would have chilled quickly. The heat from the ridge subduction would have lasted longer than the hot initiation subduction because of continuous heat generation from the mantle. Hence, there is abundant evidence of the LP-HT metamorphism.

Two Amphibolite Slabs with Different Relative PT Histories There appear to be at least two different amphibolite slabs juxtaposed that have different PT histories based upon petrographic examination. One 53 53 amphibolite slab in the Forest City area appeared to have HP-HT metamorphism prior to LT-HP metamorphism, this was also seen in the area between the large serpentinite body by French Ravine and the serpentinite that seperates from RAS in Alleghany, the north-central to northeastern margin of Fig. 5. In addition, earlier green HP-HT hornblende in the core is almost completely overgrown or overprinted with red brown LP-HT hornblende. Also, Böhlke and McKee (1984) and Hacker (1993) obtained an age of 342 and 345 Ma using the K-Ar hornblende and Ar-Ar hornblende respectively, and it confirms that amphibolites in both areas experienced the same metamorphic history based upon the petrography and the confirmed age date. On the other hand, the amphibolite in the Alleghany area, between the western serpentinite which separates from CC and the large ultramafic body near Mack House, seen in the southwestern margin of Fig. 5, appears at first to have had a LP-HT metamorphism (red brown hornblende in the core) before the HP-HT metamorphism (green hornblende in rim). Also, I found metacarbonate at only one locality in this area, which may suggest a different ocean depositional environment. Hence, the amphibolite slab may contain different OPS from the other amphibolite slab with HP-HT/LP-HT history.

Along Strike Variation: Multiple Subduction Initiation Events? There are the along-strike variations in the metamorphic history of the FRB. The areas from this study experienced multiple HT events and had the metamorphic age of LP-HT event around 340 Ma (Böhlke and McKee, 1984; Hacker, 1993), which is red brown amphibolite in Forest City and Alleghany (Sample F2 and Sample A1-3). On the other hand, Smart and Wakabayashi (2009) showed the rock record of the only HP-HT metamorphism around 240Ma without a second HT event in North Fork Feather River. The HP-HT 54 54 metamorphism might have been caused by subduction initiation near a ridge crest (Wakabayashi 2010) whereas the LP-HT metamorphism in the FRB may be explained by ridge subduction (Smart and Wakabayashi, 2009; Masutsubo and Wakabayashi, 2010) because this is the best mechanism to explain such high geothermal gradients in an oceanic or subduction setting (Sisson and Pavlis, 1993; Brown, 1998; Lytwyn et al., 1997; Forsythe and Nelson, 1985; Osozawa et al., 1990). After the initiation of subduction (HP-HT metamorphism), the FRB might experience ridge subduction shortly after subduction initiation (similar to the Shervais 2001 scenario or the Osozawa et al., 2012 scenario) to form the LP-HT overprints, this is followed by reinitiation of subduction outboard of the stalled ridgecrest and the overprinting of LP-HT assemblages with HP-HT as some of the earlier LP-HT imbricates get dragged down the second subduction zone. The HP- HT amphibole in the block in the RAS may have been exhumed to a shallow level prior to the later HT event(s) so that it did not have a second HT event recorded in it. It then may have been incorporated into a mélange in the RAS. However, the field relationships do not explain whether it was of sedimentary, tectonic, or diapiric mélange origin. This illustrates the along-strike variation in this suture zone, the likelihood of multiple subduction events and at least one ridge subduction event that left its mark along one part of the FRB but is not seen from the DGO northward.

Complexity in a Suture Zone Recorded in the FRB This study demonstrates that the suture represented by the FRB may record multiple subduction initiation events as well as a ridge subduction event, with significantly different tectonic history along the strike similar to complex sutures 55 55 recognized worldwide. Van Stall et al. (1998) showed that a suture in the northern Appalachians experienced a complicated tectonic evolution similar to the present west, and southwest Pacific Ocean, in its tectonic complexities. Hebert et al. (2012) revealed the complex record of intra-oceanic subduction and basin closure with multiple subduction zones in the Himalayan suture. Encarnacion (2004) also documented the series of subduction initiation, the births of arcs, and terminations in the span of 150Ma. Thus, I conclude that the FRB has much more complexity than just the position of a single paleosubduction zone. Continuing studies of the FRB will unravel the complexity of this suture. REFERENCES REFERENCES

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