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Impinging Ring Dike Complexes in the Sierra Nevada Batholith, California: Roots of the Early Cretaceous Volcanic Arc

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Impinging Ring Dike Complexes in the Sierra Nevada Batholith, California: Roots of the Early Cretaceous Volcanic Arc Impinging ring dike complexes in the Sierra Nevada batholith, California: Roots of the Early Cretaceous volcanic arc Diane Clemens-Knott* Division of Geological and Planetary Sciences, California Institute of Technology, Jason B. Saleeby } Pasadena, California 91125 ABSTRACT and Tilton, 1991, for a recent review). Specifically, the crust varied from rel- atively thin, accreted ophiolite and marine metasedimentary rocks on the In this paper we interpret new and previously published U-Pb zircon west, to altered volcanic and volcaniclastic rocks in the center, to a thicker data in light of structures mapped within ~360 km2 of the southwestern wedge of continental margin metasedimentary rocks on the east (Saleeby, Sierra Nevada batholith. Traces of intrusive contacts and igneous foli- 1981). In addition to crustal heterogeneity, the proportion of crust assimilated ations reveal the presence of two ring dike complexes: the eastern ring by depleted mantle-derived magmas may have increased eastward (DePaolo, complex and the western ring(?) complex. These subvolcanic com- 1981). More recent studies suggest that the lateral variations are due, in part, plexes formed during overlapping periods: the eastern ring complex to variations within the mantle (Silver and Chappell, 1988): for example, between 123 and 117 Ma (n = 5) and the western ring complex between parental magmas of western Sierran plutons were derived from depleted 120 and 115 Ma (n = 5). Each complex may have been emplaced dur- mantle and parental magmas of central and eastern Sierran plutons may have ing a minimum of two events, each 2 to 3 m.y. long and separated by 3 been derived from enriched mantle (Coleman et al., 1992; Beard and to 4 m.y. In the western ring complex, the presence of a 120 Ma xenolith Glazner, 1995; Sisson et al., 1996). Because the relative roles of mantle and of silicified porphyry enclosed by unaltered tonalite implies that the crustal heterogeneity and crustal assimilation remain controversial, the ring complexes intruded the shallow crust and stoped the overlying vol- amount of mantle-derived material added to the crust during the Mesozoic canic-hypabyssal constructs. Rare mafic mylonites suggest that col- Era cannot be determined. Consequently, the nature of magmatic differenti- lapse of a mafic-ultramafic mass may have assisted western ring com- ation within the Cordilleran batholiths will be difficult to assess until the plex emplacement. compositions of the parental magmas are better understood. Near-synchronous emplacement of the two ring complexes is consis- The Sierra Nevada batholith was emplaced into the western edge of the tent with textures and structures indicative of intense magma mingling North American continent throughout the Mesozoic Era during temporally and synmagmatic deformation preserved in the north-northeast–trend- distinct but geographically overlapping pulses of heightened igneous activ- ing Stone Corral shear zone. The ≥13-km-long Stone Corral shear zone ity (Evernden and Kistler, 1970; Stern et al., 1981; Chen and Moore, 1982). separates the impinging magmatic centers and was active ca. 116 ± 2 Ma. The Cretaceous batholith, emplaced between ca. 130 and 80 Ma, cuts across Blocks of layered plagioclase-olivine-orthopyroxene cumulates, 0.2 – the trends defined by Jurassic and Triassic plutons, and records an internal 5.5 km long, also record variable synmagmatic deformation ca. 123 ± 3 lateral variation in emplacement age from Early Cretaceous on the west to Ma, thus indicating that the cumulates crystallized in a shallow, dynamic Late Cretaceous on the east. Reflecting the overall geochemical variation environment immediately preceding or during the earliest stages of ring within the Mesozoic batholith, the Early Cretaceous plutons are petrologi- dike emplacement. cally and chemically distinct from the more voluminous, middle to Late Cretaceous plutons. Specifically, the Early Cretaceous plutons are domi- INTRODUCTION nantly tonalitic to gabbroic, in contrast to the younger, granodioritic to granitic plutons (Moore, 1959; Saleeby, 1981); are calcic instead of calc- Cordilleran batholiths fringing western North America record significant alkalic (Kistler and Peterman, 1973; Clemens Knott, 1992); and are iso- growth of continental crust during the Mesozoic Era. The total increase in topically more similar to melts of depleted mantle than younger, inboard crustal volume cannot be determined, however, until we quantify the pro- sections of the Cretaceous batholith (Kistler and Peterman, 1973; DePaolo, portions of these batholiths that represent recycled crust. The solution to this 1981; Chen and Tilton, 1991). problem may reside within the correct interpretation of the mineralogic and Our limited understanding of the cause of west-to-east geochemical vari- geochemical variations across the breadth of Cordilleran batholiths: plutons ations in the Cretaceous batholith is due, in part, to the relatively limited lying within the continental, or inboard, sections of these batholiths are gen- amount of data concerning the older, western plutons. In this study we de- erally more differentiated and of continental isotopic affinity than plutons velop the geochronologic and structural framework of a geochemically di- composing the outboard sections of the batholiths (Moore, 1959; Kistler and verse region of the westernmost Sierra Nevada batholith in preparation for Peterman, 1973; Taylor and Silver, 1978). Early workers concluded that the a subsequent petrographic and geochemical investigation. The ~360 km2 lateral geochemical zonation was due to the composition and thickness of the study area, henceforth called the Stokes Mountain region, is located at lat crust into which each batholith was emplaced and, in part, derived (see Chen 36°30′N in the Sierran foothills ~65 km southeast of Fresno, California (Fig. 1). The map region occupies virtually all of the Stokes Mountain 7.5′ *Present address: Department of Geological Sciences, California State Univer- quadrangle and parts of five adjoining quadrangles. Partly overlapping and sity, Fullerton, California 92834; e-mail: [email protected]. neighboring areas of the Early Cretaceous batholith and its metamorphic GSA Bulletin; April 1999; v. 111; no. 4; p. 484–496; 7 figures; 2 tables. 484 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/111/4/484/3383091/i0016-7606-111-4-484.pdf by guest on 30 September 2021 RING DIKE COMPLEXES, SIERRA NEVADA BATHOLITH, CALIFORNIA 119 15'W 119 10'W Sierra mulates, various regimes of magma mingling and synmagmatic to post- 115 Nevada magmatic deformation, stoping of hypabyssal intrusions, and possible 36 40'N batholith Tucker caldera collapse. In a subsequent paper we will investigate the nature and 48 limits of Mountain mapped area SF evolution of the Early Cretaceous magmas based upon petrologic, geo- F chemical, and isotopic data. Recognition of the structures described in the following is requisite to the interpretation of west-to-east isotopic variations study 52 area LA within the study area (Clemens Knott et al., 1991). !!!!"""" 67 * PETROLOGY 120 Curtis *120 Mountain * 113+3_ The Stokes Mountain plutonic suite is dominated by gabbros and tonalites and is unusual in comparison to the majority of the batholith because it con- tains a diverse suite of mafic to ultramafic cumulates. The compound cumu- 118 late-noncumulate suite varies from olivine- to pyroxene- to hornblende- to !!!!"""" 116 biotite-bearing rock types. Units are based on rock modes and do not differ- Stokes 123 entiate between temporally distinct intrusions of mineralogically similar Mountain magmas; no age significance is implied among the igneous units in the ex- Red Mtn. planation (Fig. 1). Detailed petrographic and geochemical descriptions of 36 30'N these units will be presented in a subsequent manuscript. The wall rocks of the Early Cretaceous batholith are hornblende hornfels !!!!""""N 110 and, locally, pyroxene hornfels facies of the Kings-Kaweah ophiolite belt, 120 which comprises voluminous mafic and ultramafic rocks and overlying 32 125 cherty and terrigenous strata (Saleeby, 1975). Mafic and ultramafic ophi- 123+3- 117 olitic rocks crop out as isolated hillocks northwest of the Stokes Mountain Colvin radial region; a septum of metasiliciclastic to metacarbonate rocks bearing 0 mi 1 Mountain dikes scale ~ 117 cordierite, sillimanite, and garnet bounds the eastern margin of the study 0 km1 1:167 000 area. Metasedimentary xenoliths are generally rare: two ~0.5-km-long !!!!"""" EXPLANATION xenoliths crop out northeast and northwest of Red Mountain (Fig. 1); thin, 120 concordant U-Pb zircon age alluvium 1–250-m-long xenoliths are abundant within 0.6 km of the septum-pluton (Ma), this study hb-bio granodiorite-granite contact in the southeast corner of the map (not shown). 120 concordant U-Pb zircon age bio-hb tonalite-granodiorite (Ma), Saleeby and Sharp (1980) * 2 px-bio-hb quartz diorite-tonalite Layered Cumulates and Associated Olivine-Bearing Rocks * = xenolith of silicified porphyry 123 ± 3 K-Ar amphibole age (Ma), 2 px gabbro-quartz gabbro; norite Saleeby and Sharp (1980) Arguably, the most striking igneous rocks in the Stokes Mountain region 2 px-hb gabbro-quartz gabbro U-Pb zircon age (Ma), are rhythmically layered cumulates in which the 0.5–30-cm-thick layers are 115 hb gabbro Chen and Moore (1982) defined by modal variations of plagioclase and olivine: melacratic layers olivine-plag-opx
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