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Comparison of Early Mesozoic High-Pressure Rocks in the Klamath Mountains and Sierra Nevada

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Comparison of Early Mesozoic High-Pressure Rocks in the Klamath Mountains and Sierra Nevada Geological Society of America Special Paper 255 1990 Comparison of early Mesozoic high-pressurerocks in the Klamath Mountains and Sierra Nevada Bradley R. Hacker* Department of Earth and Space Sciences,University of California, Los Angeles, California 90024 John W. Goodge Department of Geological Sciences,Southern Methodist University, Dallas, Texas 75275 ABSTRACT Early Mesozoic blueschist terranes in the Klamath Mountains (the Stuart Fork) and Sierra Nevada (the Red Ant), are IithologicaUy, petrologicaUy and structuraUy similar. - Both contain common crossite-epidote assemblages formed during subduction of oce- anic sedimentary and volcanic protoliths that had previously been metamorphosed at an ocean ridge. Both are remnants of a high-PIT metamorphic belt paired with a more easterly magmatic arc along the western margin of North America. These similarities imply that the Mesozoic blueschists in both mountain belts shared a common early history. Important differences, however, indicate that their later postsubduction histo- ries were different: blueschists in the Klamath Mountains were preserved without over- printing by higher T assemblages, whereas blueschists in the Sierra Nevada were overprinted by pumpeUyite-actinolite facies metamorphism. From these relations we infer that the Stuart Fork rocks may have been lifted upward more rapidly than the Red Ant terrane, or that the Red Ant terrane was incompletely subducted and not subject to the cooling effect of continuing subduction. The Stuart Fork blueschists were also affected by a contact metamorphic event during widespread Middle to Late Jurassic plutonism, which is not manifest in the northern Sierra Nevada blueschist terrane. INTRODUCfION greater extent than previously appreciated, and through their multiple-generation metamorphic paragenesesand structural fab- The Klamath Mountains and the Sierra Nevada of the rics, they provide detailed records of convergent-margin processes North American Cordillera contain a variety of rocks interpreted operative during early Mesozoic growth of the Cordillera. as magmatic arcs, oceanic crust, and subduction complexes This chapter compares and contrasts the structural features, formed during different phasesand stylesof ocean-continentplate phase assemblages,and mineral chemistry of the two terranes. interactions. Magmatic arc rocks in the eastern Klamath Moun- Further details may be found in Goodge (1989a, b, 1990) and tains and eastern Sierra Nevada attest to convergence between B. R. Hacker (unpublished data). Textural relations determined continental North America and westward oceanic provinces from by back-scatteredelectron microscopy constrain the sequenceof at least Devonian to Cretaceous time (e.g., Burchfiel and Davis, deformational and metamorphic events, and phase relations and 1981). The Stuart Fork terrane in the Klamath Mountains and mineral compositions determined by electron-probe microanaly- the Red Ant terrane in the Sierra Nevada are subduction com- sis constrain the P-T conditions of metamorphism. The meta- plexesthat developedoceanward of the more easterlyearly Meso- morphic evolution of these terranes provides the petrogenetic zoic magmatic arcs. Both are coherent high-pressure belts of framework for a tectonic model linking the late Paleozoic-early Mesozoic history of the Klamath Mountains and the Sierra *Presentaddress: Department of Geology,Stanford University, Stanford, Nevada. California 94305-2115. Hacker,B. R., and Goodge,J. W., 1990,Comparison of early Mesowic high-pressurerocks in the KlamathMountains and SierraNevada, in Harwood,D. S., and Miller, M. M., eds.,Paleozoic and early Mesozoicpaleogeographic relations; Sierra Nevada,Klamath Mountains,and relatedterranes: Boulder, Colorado, GeologicalSociety of AmericaSpecial Paper 255. m . , . I~'" 278 Hacker and Goodge ° 42°N D Cretaceous and Tertiary sediments f)I:l ~ Jurassic and Cretaceous plutons m Smith River terrane Condrey Mountain schist ./;;:;;:;;: Rattlesnake Creek-Marble ~ //// Mountain . terrane ~{i;i!!ij~Ji~! Hayfork terrane II North Fork-Salmon River terrane . Stuart Fork terrane Central metamorphic belt Eastern Klamath belt terranes ~ 1°N ~ Ordovician ultramafic rocks Figure I. Regional tectonic map of the Klamath Mountains (after Strand, 1962; Hotz, 1971; Wright, 1982; Mortimer, 1984; Wagner and Saucedo, 1988). Regional thrust faults are shown by barbed lines. Eastern Klamath belt includes the Eastern Klamath and Yreka terranes and the Trinity peridotite. Western Paleozoic and Triassic belt includes the Stuart Fork, North Fork-Salmon River, Hayfork, Rattlesnake Creek-Marble Mountain terranes. Western Jurassic belt includes the Condrey Mountain and Smith River terranes.Plutons include Russian Peak (RP), Deadman Peak (DP), China Creek (CC), English Peak (EP), and Caribou Mountain (CM). Major faults include Siskiyou thrust (ST), Salmon River thrust (SRT), Soap Creek Ridge thrust (SCRT), and Brown Meadow fault (BMF). SV = Seiad Valley; Y = Yreka; SB = Sawyers Bar; Ce = Cecilville; Ca = Callahan; E = Etna. Heavy lines delineate the area of the present study. REGIONAL GEOLOGY lithotectonic belts in the Klamath Mountains form a gener- ally westward-younging structural sequence of units typically Field and analytical data presented in this chapter come separated by east-dipping, west-directed regional thrust faults from study of the central Stuart Fork terrane in the vicinity of the (Fig. 1; Irwin, 1966, 1981; Burchfiel and Davis, 1981). Irwin South Fork of the Salmon River in the Klamath Mountains (1966) identified four Klamath belts, from east to west, as: (I) the (Goodge, 1989a, b; Fig. 1), and the Red Ant terrane in the Yuba eastern Klamath belt, a Paleozoic to Mesozoic volcanic arc and River area of the Sierra Nevada (B. R. Hacker, unpublished data; continental margin sequence(Irwin, 1981; Miller, 1989; Brouxel Fig. 2). Previous studies of theseunits include those of Holdaway and others, 1988); (2) the central metamorphic belt, a remnant of (1963), Davis and others (1965), Hotz (1977a, b), Borns (1980, subducted Devonian oceanic crust (Davis, 1968; Cashman, 1980; 1984), Schweickert and others (1980), Hietanen (1981), and Peacock and Norris, 1989; Hacker and Peacock, this volume); Edelman and others (1989). (3) the western Paleozoic and Triassic belt, composed of various I ! ,,' c ~ "- Comparison of early Mesozoic high-pressurerocks 279 ° 4OoN IN 0. 20 km D Cretaceousand Tertiary sediments G:;] Jurassic and Crclaccous plulons ~ Smarlvlllc terrane ~ Slate Creek IcrrarlC ~ FIddle Creek terrane )?!~::/iCalaveras terrane . Red Ant terrarle ~ Fcathcr IUvcr terrane 0 Nortllcnl SIerra terrane 39°N Figure 2. Regional tectonic map of the northern Sierra Nevada (after Day and others, 1988; Edelman and others, 1989). Regional thrust faults are shown by barbed lines. Post-Mesozoic rocks that overlie the lithotectonic belts are not shown. Qu = Quincy; Do = Downieville; Or = Oroville; Ma = Marysville; NC = Nevada City; NYR = North Yuba River = MYR = Middle Yuba River; SYR = South Yuba River. Heavy lines delineate the area of the present study. accreted oceanic assemblages(Davis and others, 1978; Wright, terrane (Blake and others, 1982; Mortimer, 1984), consists of 1982; Ando and others, 1983; Mortimer, 1984; Donato, 1987; metabasaltic and siliceous metasedimentaryrocks that originated Coleman and others, 1988); and (4) the western Jurassic belt, as late Paleozoic(?) oceanic crust (Goodge, 1990). It lies struc- containing ophiolitic rocks (Saleeby and others, 1982; Harper turally beneath Devonian amphibolite-facies rocks of the central and others, 1988), moderate- to high-P metamorphic rocks metamorphic belt. The two terranesare separatedby the Siskiyou (Donato and others, 1980; Helper, 1986), and arc-derived clastic thrust fault that was active between Devonian and Middle Juras- sedimentary rocks (Garcia, 1979). sic time, as constrained by the ages of metamorphism of the In the east-central Klamath Mountains, the western Paleo- central metamorphic belt (Lanphere and others, 1968; Hotz, zoic and Triassic belt is further subdivided (from east to west) 1977b; Cashman, 1980) and plutons that cut across the thrust into the Stuart Fork, North Fork-Salmon River, easternHayfork, fault (Harper and Wright, 1984; Wright and Fahan, 1988). The western Hayfork, and Rattlesnake Creek-Marble Mountain ter- Stuart Fork terrane overlies the low-grade, ophiolitic North ranes (Fig. 1) (Wright, 1982; Donato and others, 1982; Ando Fork-Salmon River terrane along the Salmon River thrust, and others, 1983; Mortimer, 1984; Wright and Wyld, 1985; which was active between Early and Middle Jurassic time Donato, 1987; Goodge, 1989b; Wright and Fahan, 1988). These (Goodge, 1989b). western Paleozoic and Triassic belt terranesrepresent segments of As in the Klamath Mountains, metamorphic rocks in the late Paleozoic to mid-Mesozoic oceanic or island-arc complexes northern Sierra Nevada are divisible into a number of terranes that have been deformed to varying degreesas melange or sub- with different structural, metamorphic, or depositional histories duction complex during convergencewith the western margin of (Fig. 2). The Northern Sierra terrane is composed of lower Pa- North America (Davis and others, 1978; Irwin, 1981; Burchfiel leozoic metasedimentaryrocks of the Shoo Fly Complex that are and Davis, 1981; Coleman and others, 1988). overlain by three successive magmatic arc complexes of The Stuart Fork terrane, also referred to as the Fort Jones Devonian-Mississippian, mid-Permian,
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