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 0024-4937/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.lithos.2009.06.012 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