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 relatively 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.

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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 contains 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 ± hb cumulates igneous foliation mafic mylonite range from plagioclase-bearing dunites to melatroctolites; leucocratic lay- metamorphic rocks igneous layering radial diorite dike ers range from anorthosites to leucotroctolites (Fig. 2A). Orthopyroxene is present in small amounts both as cumulate crystals and as symplectic inter-

growths with spinel partially replacing olivine. Rarely does the abundance Figure 1. Generalized geologic map of the Stokes Mountain region of orthopyroxene require use of the name olivine norite; even in these rocks, with new and published geochronologic data. The approximate scale olivine is the dominant mafic phase. Small amounts of intercumulus horn- is 1:167 000; mapped between 1988 and 1990 at a scale of 1:24 000 blende rim the cumulate olivine and orthopyroxene. For simplicity, herein (Clemens Knott, 1992). Symbols in alluvium indicate samples col- these hornblende-poor adcumulates are referred to as “layered troctolites,” lected from adjacent, small outcrops. Abbreviations: plag—plagio- while recognizing that significant modal variation exists between layers. clase; px—pyroxene; hb—hornblende; bio—biotite; SF—San Fran- Associated with the layered troctolites are irregularly layered to massive, cisco; LA—Los Angeles; F—Fresno. very fine-grained to coarse-grained, hornblende-bearing troctolite, olivine norite, olivine-bearing anorthosite with isolated hornblende oikocrysts, and olivine hornblende gabbro. Despite the absence of modal layering, these wall rocks were described elsewhere (Durrell, 1940; Macdonald, 1941; rocks display cumulate textures and are mineralogically similar to the lay- Saleeby, 1975; Mack et al., 1979, 1986; Sisson and Moore, 1994). ered troctolites. Commonly there is a chaotic spatial relationship between The Sierra Nevada batholith formed within a continental margin arc in- these nonlayered plagioclase-olivine-orthopyroxene-hornblende rocks at board of the Franciscan–Great Valley trench-forearc system. During Early outcrop scale; adding to the chaos are crosscutting hornblende gabbro dikes Cretaceous time, stratovolcanoes dominated the region now occupied by the of highly variable grain size. subdued Sierra Nevada foothills (Saleeby, 1981). Although the volcanic The troctolite layers are generally planar, laterally extensive at outcrop carapace has been destroyed, ring dike complexes present in the underlying scale, and west dipping (Fig. 1). Structures documenting variable deformation batholith pinpoint the former location of volcanic centers. The structural and include common monoclinal warping and interlayer isoclinal folding (cen- geochronologic relations presented in this paper reveal remarkable detail timeter to meter amplitudes) as well as rare disharmonic folding (Fig. 2B), concerning the dynamic nature of the shallow plutonic regime, including chevron folds, truncated layers, cross-bedding, and branched layers. Typically, multiple episodes of ductile and brittle disruption of mafic-ultramafic cu- outcrops containing deformed layered cumulates also contain centimeter- to

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A B

Figure 2. Troctolite cumulates south of Tucker Mountain. (A) Rhythmically layered cumulates (38 cm hammer for scale). (B) Cumulates dis- playing disharmonic folds formed during synmagmatic deformation (17 cm scale).

meter-scale pods of very coarse-grained, massive olivine hornblende gabbro. rics, with proximity to intrusive contacts. Most protomylonitic fabrics are Geometrical relations between the deformed layers and the pods suggest that characterized by ribbons of oscillatory to dynamically recrystallized quartz deformation was caused by the intrusion of the coarse-grained olivine horn- within a fine-grained, granoblastic mortar. Some plagioclase grains are blende gabbro into partially crystalline layered troctolites. Apparent effects of slightly marginally recrystallized, but most acted as rigid clasts around which such intrusion include gentle warping of the surrounding layers, layer attenu- biotite grains were bent. Less commonly, quartz-free rocks are protomy- ation, formation of load and cast-type structures, and near obliteration of the lonitic containing aligned and deformed hornblende, biotite, and plagioclase. original layering. The cumulate grains in these variably deformed rocks are Magmatic foliations are interpreted as having formed by flow during em- unstrained, indicating the presence of intercumulus magma during deforma- placement; protomylonitic foliations presumably formed by near-solidus tion. Evidence of brittle deformation within the layered cumulates includes plastic deformation during the latest stages of intrusion or in the hot sub- offset layering and cumulate breccias. In some breccias, cumulate layering is solidus during emplacement of adjacent bodies (Bateman et al., 1983). draped unconformably over layered autoliths, indicating resumption of layer Small amounts of two additional igneous rock types form structurally formation after brecciation. In summary, these features indicate that the distinct outcrops in the Stokes Mountain region. (1) Five, ~20–200-m-long magma chamber(s) in which the layered cumulates formed were repeatedly and 2–5-m-wide biotite-hornblende diorite dikes (southeast corner of Fig. 1) deformed, during which time the near-solidus cumulates responded in both cut perpendicularly across the fabrics of igneous and metasedimentary ductile and brittle manners. The massive olivine hornblende gabbro, which hosts. Clots of fine-grained, euhedral to subhedral, blue-green hornblende presumably formed within the same chamber(s) as the mineralogically simi- give the diorite a distinctive spotted appearance. (2) Three, ≤1000 m3 xeno- lar cumulates, appears to have been easily mobilized in this dynamic environ- liths of silicified porphyry (asterisk in Fig. 1) form unusually resistant out- ment such that it intruded and deformed the layered crystal mush. crops with shallow-dipping basal contacts against the underlying unaltered tonalite. The fine-grained groundmass of the porphyry is composed of Olivine-Free Rocks quartz, of presumed secondary origin, and microcline surrounding phe- nocrysts of zoned plagioclase and variably resorbed biotite, hornblende, Surrounding the layered troctolites and associated olivine-bearing rocks are orthopyroxene and clinopyroxene. At one locality, the porphyry contains a variety of rocks that do not contain olivine except as rare, embayed cores small (2–10 cm in diameter) pyroxene-rich inclusions that appear to have surrounded by pyroxene and/or hornblende. These rocks have been divided been olivine bearing; protolith identification is difficult due to intense sili- into six units in Figure 1: hornblende gabbro with rare comb layering, two cification. Oxygen isotope systematics demonstrate that the xenoliths of pyroxene-hornblende gabbro–quartz gabbro, two pyroxene gabbro–quartz silicified porphyry are the only igneous rocks in the Stokes Mountain re- gabbro with norite, two pyroxene-biotite-hornblende quartz diorite–tonalite, gion to have undergone hydrothermal alteration (Clemens Knott, 1992). biotite-hornblende tonalite-granodiorite, and hornblende-biotite granodiorite- Plagioclase-hornblende-biotite-porphyritic rhyolite dikes of Jurassic or granite with lesser two mica granite. The modes of the units are intergrada- Cretaceous age crop out ~25 km to the east (Sisson and Moore, 1994). Per- tional; the distinctive characteristics are the relative abundances of pyroxene, haps the Stokes Mountain porphyry originated as similar hypabyssal intru- hornblende, and biotite, the color indices, and the quartz abundance. Where sions that were hydrothermally altered prior to stoping. exposed, contacts are generally sharp with the exception of some contacts between the two pyroxene-biotite-hornblende quartz diorite-tonalite and the bi- MAP RELATIONS otite-hornblende tonalite-granodiorite units. Magmatic foliations defined by the alignment of mafic and/or plagioclase Figure 3 is a structural overlay of the Stokes Mountain region (Fig. 1) dis- feldspar crystals are present in all intrusions, especially in the hornblende- playing lithologic contacts, trends of igneous foliations, and air-photo lin- and biotite-bearing units. Foliations are oriented approximately parallel to in- eaments. These features define the locations of the ring complexes and the trusive contacts and become more intense, grading into protomylonitic fab- synmagmatic shear zone described in the following.

486 Geological Society of America Bulletin, April 1999

? 119 15'W 119 10'W 119 05'W 7.5-km-diameter, vertically oriented ring dike of biotite granite (Fig. 1). The arcuate valley between these two intrusions is occupied by a poorly defined Stone Corral shear zone ring of two pyroxene-biotite-hornblende tonalite surrounded by and in gra- ? trends of contacts 36 40'N dational contact with biotite-hornblende tonalite-granodiorite. Another arcu- air photo lineaments ate valley separating the granite ring dike from the metamorphic wall rocks radial dikes is underlain by biotite-hornblende tonalite-granodiorite and a thin, arcuate possible direction of granite dike. Smaller dikes of gabbro and granite are concentrically dispersed radial extension throughout the eastern ring complex; in places the granite dikes cut obliquely ? across igneous foliations and dike margins suggesting that the granites might Eastern Ring Complex be among the youngest intrusions. The northeastern contact of the eastern ring complex with the wall rocks can be traced on air photos southward, with minor complications, back into the southeast corner of the field area. The contact is generally arcuate with at least two small apophyses intruding the metasedimentary rocks at the northern and eastern edges. The dike complex Western Ring(?) Complex is particularly well exposed in the western part of the eastern ring complex, where a wide variety of rock types (hornblende gabbro, hornblende norite, tonalite, granodiorite, granite) form relatively thin (5–50 m) and long (0.5 to N 3 km) northwest- to west-trending dikes. Within the southern half of the eastern ring complex are ring dikes either 36 30'N ? composed of or containing swarms of commingled hornblende gabbro, tonalite, and granodiorite (Fig. 4A). The fine-grained and plagioclase- Colvin porphyritic mafic enclaves in these dikes are commonly 10–100 cm in di- 0 mi 1 Mountain scale ~ ameter and range in shape from crude spheres to highly attenuated ellip- 1:167 000 0 km1 soids with aspect ratios commonly reaching 1:20. The degree of elongation of the enclaves is approximately correlated with the intensity of the magmatic foliation in the tonalitic to granodioritic host. The direction of max- Figure 3. Structural overlay of Figure 1 illustrating the geometric re- imum elongation is near vertical, indicating that elongation occurred dur- lationships between the eastern ring complex, the western ring(?) com- ing upward movement of the commingled magmas. The intermediate plex, and the Stone Corral shear zone. A left-lateral sense of shear on elongation direction parallels both dike margins and any igneous foliation the shear zone is modeled as the geometric resolution (thin dashed within the surrounding unit. The most intense magma mingling is exposed lines) of vectors depicting the radial crustal expansion associated with in the southeastern corner of the eastern ring complex. Here, in addition to emplacement of the impinging ring complexes. abundant mafic enclaves, individual dikes of gabbro, tonalite, and granodiorite interfinger along strike in a manner similar to that predicted for an intermediate stage of composite dike formation (Blake and Campbell, Ring Complexes 1986, Fig. 9D). This interfingering, as well as the formation of commingled magmas, was interpreted as forming during simultaneous injection of Near-vertical to outwardly dipping annular dikes exposed in a nested or compositionally distinct magmas into a single conduit (Koyaguchi, 1985; bull’s-eye pattern comprise a ring dike complex. In general, ring complexes Freundt and Tait, 1986). Ring dikes containing mingled magmas have been are recognized as volcanic-plutonic in origin, being the structural, textural, noted in other complexes and similarly interpreted (Marshall and Sparks, and genetic intermediaries between volcanoes and the underlying magma 1984; Bussell, 1988; Fluk and Treiman, 1988; Vasek and Kolker, 1993). chambers in calc-alkaline batholiths (Ustiyev, 1963; Pitcher, 1978; Walker, The intersection of the enclave-bearing ring dikes and the biotite- 1984; Bussell, 1988). Ring dike complexes have not been described previ- hornblende diorite dikes is well exposed in the southwestern corner of the ously in the Sierra Nevada batholith, although concentrically zoned plutonic eastern ring complex (Fig. 1). The northwest-trending, radially oriented dio- complexes such as the Tuolumne Intrusive Suite and the Academy pluton and rite dikes cut perpendicularly across the ring dikes and the outermost bound- arcuate intrusions such as the leucogranite of Big Sandy Bluffs and the ary of the eastern ring complex, and thus clearly postdate emplacement of at Tonalite South of the Experimental Range are structurally similar (Lockwood least this part of the ring complex. Four of the five northwest-trending diorite and Bateman, 1976; Bateman and Chappell, 1979; Mack et al., 1979; dikes align along strike and may represent parts of an ~1.4-km-long intrusion. Bateman and Busacca, 1982). Ring dike complexes have been reported in In addition to ring dikes and radial dikes, some ring dike complexes con- other Cordilleran batholiths: tonalite to gabbroic ring complexes and a pluton tain cone sheets, i.e., conical intrusions that dip inward at a low angle. The with textures suggestive of overlying ring fractures have been described in the western slope of the Red Mountain stock appears to be a dip slope (Fig. 4B); Peninsular Range batholith (Merriam, 1941; Duffield, 1968; DePaolo et al., arcuate air-photo lineaments visible on the southern side of the stock (Fig. 3) 1975, Johnson et al., 1999), and a number of compositionally diverse ring are consistent with the presence of shallow, inwardly dipping surfaces. If this complexes are located in both the Coast Ranges and Peruvian coastal interpretation is correct, then the eastern ring complex contains vertical to batholiths (Lambert, 1974; Bussell et al., 1976). slightly outwardly dipping ring dikes, a central hornblende gabbro stock con- The Stokes Mountain area contains one particularly well developed ring taining inwardly dipping cone sheets, and radial dikes of biotite-hornblende dike complex, the eastern ring complex (Fig. 3). The ~14-km-diameter of the diorite. Although there is some debate concerning the specific relations be- eastern ring complex is comparable to that of other ring dike complexes tween crustal stress fields, magma pressure, and intrusive style (Robson and (Walker, 1984; Yoshida, 1984). The eastern ring complex is dominated by an Barr, 1964; Roberts, 1970; Phillips, 1974; Koide and Bhattacharji, 1975; ~2.5-km-diameter central stock of nonlayered hornblende gabbro forming Park, 1989), it is generally agreed that all three features—ring dikes, cone Red Mountain and is surrounded for ~180° by a resistant, ≤1.3-km-wide, sheets, and radial dikes—can form over a shallow magma body.

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A B

Figure 4. The ring complexes. (A) Nearly vertical exposure of mingled hornblende gabbro and biotite-hornblende tonalite-granodiorite from the eastern ring complex (ERC; 38 cm hammer for scale). (B) Looking north across the ERC to the gabbro of Red Mountain in which possible cone sheets form a west-facing dip slope (dipping to left side of photograph); nearly vertical granitic ring dikes covered in boulders are visible south and north of Red Mountain. (C) Looking southwestward across the western ring(?) complex where Stokes Mountain and Cliff Peak com- bine to form an arcuate mountain; the Stone Corral shear zone trends diagonally from the center of the left edge to the center of the top edge of the photograph.

The Stokes Mountain region contains a second centered complex that has 1990). If in situ fractional crystallization produced the commonly gradational been labeled the western ring(?) complex. The query reflects the fact that contact between the two pyroxene-biotite-hornblende quartz diorite–tonalite whereas the mapped bodies are elongate, arcuate, and near vertical, the dike and the biotite-hornblende tonalite-granodiorite units, then the reverse zona- form of the individual units is generally only well expressed at the mapped tion would suggest outward cooling and crystallization, perhaps against the scale and not at outcrop scale, as in the eastern ring complex. For example, mafic-ultramafic cumulates exposed in the center of the western ring com- the mafic rocks of Stokes Mountain and Cliff Peak define an arcuate topo- plex near Tucker Mountain. graphic high in the southwestern corner of the western ring complex Another reason to query our preferred interpretation that the western ring (Fig. 4C). Arcuate air-photo lineaments within the large region of two complex represents a second ring complex is that only the southeast quadrant pyroxene-biotite-hornblende tonalite between Curtis and Stokes Mountains of the complex is located in the study area; continuation of the ring dikes to (Fig. 3) suggest that this large intrusive mass may be composed of multiple, the north and northwest is suggested, however, by arcuate lineaments on air compositionally similar ring dikes. The scarcity of outcrop scale ring dikes photos (Fig. 3). If the western ring complex is, in fact, a closed oval feature, in the western ring complex may result in part from its slightly lower eleva- then the entire western half of the complex is buried under alluvium of the tion and possibly deeper level of exposure as well as from the more mafic av- Great Valley. Mack et al. (1979) noted that the zoned Academy pluton, ~35 erage composition of the western ring complex compared to the eastern ring km northwest of the western ring complex, has a similar, half-moon exposure complex. Alternatively, some of the mineralogic zonation within the western geometry. These authors preferred the interpretation that the Academy pluton ring complex may result from some process such as side-wall fractional crys- was not part of a partially buried oval body, but that the outcrop geometry re- tallization of a cooling pluton (Bateman and Nockleberg, 1978; Sawka et al., flected the zoned pluton’s true shape. We base our interpretation that the west-

488 Geological Society of America Bulletin, April 1999

A C

D B

Figure 5. Examples of magma mingling and hot subsolidus deformation in the Stone Corral shear zone. (A) Mingling and diking of aphanitic hornblende gabbro (dark), medium-grained tonalite (speckled), and fine-grained norite (light gray); view to the north-northeast (17 cm scale). (B) Pho- tograph of commingled hornblende-free norite (light) and pyroxene-free hornblende gabbro (long edge of thin section ~7.5 cm). (C) Boulder dis- playing multiple mingling episodes (pencil ~15 cm): leucocratic biotite quartz norite host with dm-scale enclaves of melacratic hornblende gabbro containing mm- to cm-scale enclaves and dikelets of hornblende-free norite (light gray). Dark spots are lichen. (D) Protomylonitic tonalite (38 cm hammer for scale).

ern ring complex is a partially buried ring complex on its striking similarities Stone Corral Shear Zone with the excellently exposed eastern ring complex. Moreover, we revive the hypothesis that the compositionally similar and approximately coeval Acad- The Stone Corral shear zone is a north-northeast–trending structure that emy pluton is also a partially buried oval body, perhaps representing an even truncates all contact and foliation trends of the southeastern ring complex at deeper level of plutonism than that exposed in the western ring complex. its western margin and coincides with the eastern border of the western ring

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complex (Fig. 3). All intrusive contacts and igneous foliations within the tion (Clemens-Knott, 1998). Following the Stone Corral shear zone through Stone Corral shear zone are strongly aligned parallel to its trend. From the metamorphic rocks is complicated by poor exposure as well as by the south-southwest to north-northeast along the 13 km length mapped to date, incomplete transposition of fabrics formed during previous deformational the Stone Corral shear zone varies from a ≤50-m-wide zone of intense events. Nonetheless, the orientation of pressure shadows within mylonitic magma mingling between a variety of mutually intruded rock types, to an pebble conglomerates and the geometry of infilled tension gashes are con- ~10-m-wide zone of vertically foliated, protomylonitic tonalite, to multiple, sistent with a left-lateral shear sense similar to that determined to the south. meter-wide protomylonitic to mylonitic zones within a metasedimentary The apparent lack of offset of the southwestern and northeastern borders of septum. The north-northeastward increase in the intensity of hot subsolidus the metamorphic septum suggests, however, that the magnitude of any hor- deformation coincides with an increase in structural height of ~600 m. izontal movement along the Stone Corral shear zone was minimal. The southernmost 2.4 km segment of the Stone Corral shear zone is pos- In summary, the Stone Corral shear zone represents a zone of intense tulated to provide structural control for Stone Corral Canyon, a N23°E magma mingling and, possibly sinistral, synmagmatic shear. Evidence for trending, outcrop-free valley that drains southwestward into the Great Val- minor metasomatism and overprinted, hot subsolidus deformation also with ley. Moving northward, the next 2.6-km-long, ≤50-m-wide, N18°E trend- a sinistral sense is preserved at higher structural levels exposed near the ing segment of the Stone Corral shear zone exposes mutually intruded and northernmost end of the mapped Stone Corral shear zone. commingled rocks from all units except the granites, silicified porphyry, and olivine cumulates. Individual dikes range from millimeters to meters in Cumulate Blocks thickness and have near vertical igneous foliations that parallel the Stone Corral shear zone trend. Enclaves in mingled rocks are decimeter to meter Resistant mafic-ultramafic layered troctolites and associated olivine- long and are highly elongate parallel to the trend of the shear zone (Fig. 5A). bearing rocks are generally exposed as hill-capping masses of as much as A distinctive mixed rock contains small (0.5–2 cm long) elongate enclaves ~8 km2 in area. The majority of these masses are within the western ring and dikelets of hornblende-free norite surrounded by a pyroxene-free, fine- complex, the exceptions being a small mass north of Red Mountain (Fig. 1) grained hornblende gabbro (Fig. 5B). Igneous foliation of the hornblende and the Colvin Mountain mass, which may be enclosed by the eastern ring gabbro host parallels both the vertically elongated norite enclaves and the complex. Although the contacts separating the olivine-bearing rocks and the dike margins. Two episodes of magma mingling are documented in Figure surrounding mafic to intermediate intrusions are nowhere exposed, the 5C: commingled hornblende gabbro and norite form the dark, decimeter- topographic expression of the inferred boundary requires that the cumulate long enclaves that are enclosed by a leucocratic biotite quartz norite. bodies are either perched masses with subhorizontal bases or are the ex- Most rocks in this section of the Stone Corral shear zone preserve igneous posed tops of steep-sided stocks. The absence of local gravity anomalies textures and lack significant postmagmatic deformation features, except for over the sizable and relatively dense masses defining Colvin and Tucker weakly oscillatory quartz. Features indicating synmagmatic deformation, Mountains (Saleeby, 1975; Oliver and Robbins, 1982) is consistent with our such as isoclinally folded mafic and felsic dikes, are unequally distributed preferred interpretation that most cumulate bodies exposed in the study area through the 50-m-wide zone. In most cases the fold axes are roughly paral- do not project to great depth. On the basis of field and geophysical data, as lel to the trend of the shear zone, but in some outcrops the relations are more well as the absence of any kind of gradational contacts between the cumu- complex, suggesting the possibility of deformed vein networks. The orienta- lates and the surrounding olivine-free rocks, we suggest that most cumulate tions of centimeter- to meter-scale tension gashes infilled with hornblende bodies form blocks with shallowly dipping bases surrounded and underlain gabbro or commingled hornblende gabbro-norite suggest a left-lateral sense by younger, mafic to intermediate intrusions. Whether these blocks are in of shear during magmatism. Volumetrically minor norite veinlets crosscut place (i.e., septa) or are stoped (i.e., autoliths or xenoliths) cannot be deter- the strong fabric of the Stone Corral shear zone and represent the last stage of mined at present. magmatism along this segment. These undeformed veinlets are distinctive in One ~3.5 km2 layered troctolite block located on the western edge of that euhedral, 0.5–3 mm hypersthene crystals are concentrated either in the Stokes Mountain and surrounded on three sides by alluvium correlates with center or along the margins of the veins. a small positive gravity anomaly (Oliver and Robbins, 1982) superposed on Following the structure northward, we infer that the next 2-km-long seg- the regional Dinuba gravity high defined by Cady (1975); presumably this ment of the Stone Corral shear zone underlies the linear alluvial valley vis- body extends westward beneath the Great Valley alluvium. The Dinuba ible in Figure 1. The next 1.7-km-long, N07°E-trending segment of the gravity high and an associated magnetic anomaly were modeled by Saleeby Stone Corral shear zone climbs 0.4 km in elevation. Here the Stone Corral (1975) as being generated by an eastward-dipping slab of dense and mag- shear zone is an ~10-m-wide zone of protomylonitic to mylonitic biotite- netic material extending from the eastern margin of the Great Valley to a hornblende tonalite in which the foliation is near vertical (Fig. 5D). Quartz, depth of ~9 km. A slab composed of cumulate material similar to the blocks both oscillatory and microcrystalline, forms elongate ribbons, plagioclase exposed in the Stokes Mountain region and lesser metagabbro of the Kings- porphyroclasts are rounded and display marginal recrystallization, and Kaweah ophiolite generates the required geophysical characteristics. If this hornblende porphyroclasts are fractured. The presence of blue-green instead model is correct, additional mafic-ultramafic cumulates may underlie the of olive-brown hornblende, abundant oriented epidote, and rare fragmented study area. tourmaline suggests that a limited degree of metasomatism preceded or ac- companied subsolidus deformation. The horizontal elongation of biotite GEOCHRONOLOGY stringers, quartz ribbons, and plagioclase porphyroclasts within the tonalite suggests that a component of postmagmatic shearing within the steeply dip- Early Cretaceous plutons were emplaced into the Foothills metamorphic ping zone was strike slip. belt between 130 and 110 Ma, the majority of U-Pb zircon dates clustering Approximately 8.7 km from its southern end, the Stone Corral shear zone between 125 and 111 Ma (Saleeby and Sharp, 1980; Chen and Moore, intersects a metasedimentary septum. Subparallel, meter-wide zones of pro- 1982). Early Cretaceous magmatism preceded the apparently more volumi- tomylonitic to mylonitic stretched pebble conglomerate may be traced for nous middle to Late Cretaceous magmatism that began at ca. 103 Ma and ~2.9 km to a zone of protomylonitic tonalite north of the septum; the north- during which the majority of the exposed Sierra Nevada batholith was em- ern extent of the Stone Corral shear zone is the focus of current investiga- placed (Chen and Moore, 1982; Saleeby et al., 1987). Published geochrono-

490 Geological Society of America Bulletin, April 1999

TABLE 1. STOKES MOUNTAIN ZIRCON GEOCHRONOLOGY: SAMPLE LOCATIONS, INTERPRETED AGES, AND DESCRIPTIONS Sample Latitude Longitude Interpreted age Description number (N) (W) (Ma) WBK129 36°35′21″ 119°12′06″ 120±1 Hypabyssal(?), silicified xenolith of a porphyritic granitoid WBK131 36°32′21″ 119°06′40″ 123±1 Medium-grained biotite-hornblende tonalite WBK133 36°32′37″ 119°03′31″ 118±1 Coarse-grained biotite granite from a laterally extensive eastern ring complex ring dike WBK134 36°27′22″ 119°04′35″ 117±2 Biotite-hornblende diorite dike radially crosscutting the southwestern edge of eastern ring complex

logic data for the Stokes Mountain region include a single K-Ar date from ring complex yielded a date of 123 ± 1 Ma (WBK131). Zircon from a a hornblende-rich mesocumulate on Colvin Mountain (sample KRO22; two pyroxene-biotite-hornblende gabbro (KB4) collected on the south- Saleeby and Sharp, 1980) and eight U-Pb zircon dates from the surround- west edge of the eastern ring complex crystallized at 120 ± 1 Ma. The ing intrusions (six KB samples from Saleeby and Sharp, 1980; samples 31 thick and laterally continuous granitic ring dike yielded a date of 118 ± 1 and 32 from Chen and Moore, 1982). These published dates are presented Ma (WKB133). A minimum emplacement age for the eastern ring com- with U-Pb dates obtained from four new zircon samples (WKB samples; plex is well constrained as 117 ± 2 Ma (WKB134) by a radially oriented Table 1) in Figure 1; analytical details are described in Table 2 and in biotite-hornblende diorite dike that crosscuts the southern boundary of Saleeby and Sharp (1980). Of the 13 dates for the Stokes Mountain suite, 11 the eastern ring complex. The zircon date of the diorite dike (WKB134) span the period between 125 and 115 Ma and are thus typical for this part of is slightly discordant (reflected in a larger error for the interpreted age; the batholith; the discordant sample (32) may also have crystallized during Table 1), suggesting minor assimilation of the metamorphic wall rocks this period (113 ± 3 Ma). which it, in part, intrudes. Westward extrapolation of the eastern ring Three samples from the western ring(?) complex yield concordant U-Pb complex outcrop pattern suggests that a 117 ± 1 Ma two-pyroxene horn- dates of 120 ± 1 (KB3), 116 ± 1 (KB7), and 115 Ma (31; collected immedi- blende gabbro (KB6) collected from the southern tip of Colvin Mountain ately north of the study area). The U-Pb systematics of two discordant frac- is also part of the eastern ring complex magmatic system. This possibil- tions from a quartz diorite (32) collected 0.4 km west of the Stone Corral ity is consistent with the geochronologically constrained hypothesis that shear zone define a poorly constrained concordia with an upper intercept in eastern ring complex emplacement continued to 118 ± 1 Ma (WKB133) the middle to Early Proterozoic range, and a lower intercept of 113 ± 3 Ma. and had halted by the time of radial dike emplacement at 117 ± 2 Ma. As recognized by Chen and Moore (1982), the discordance of sample 32 A single K-Ar hornblende analysis dates the cooling of the Colvin points to the inheritance or entrainment of contaminant zircons, presumably Mountain cumulates as 123 ± 3 Ma (KRO22). A two pyroxene-biotite- derived from rocks akin to the nearby metasedimentary pendant (Fig. 1). We hornblende gabbro (KB1), which forms an inselberg 1.25 km west of estimate the age of sample 32 to be about 115 Ma because extrapolation of Colvin Mountain, yielded a U-Pb zircon date of 125 ± 1 Ma. The simi- the existing mapping suggests that samples 32 and 31 may be part of the larity between the 125 and 117 Ma U-Pb dates of intrusions immediately same intrusion. A xenolith of silicified porphyry yielded a date of 120 ± 1 surrounding the large Colvin Mountain cumulate block and the presumed Ma (WBK129) that is similar to that of the two-pyroxene quartz diorite shallow emplacement depth (see following) of all three samples (KB1, (KB3; Fig. 1) located ~7 km to the west. KB6, KRO22) suggest that the K-Ar date for cumulate formation has not Five dates from the eastern ring complex range from 123 to 117 Ma. A been significantly reset by younger magmatism. Note that Chen and biotite-hornblende tonalite from the inner arcuate valley of the eastern Moore (1982) found that the coorrespondence between K-Ar analyses

TABLE 2. ZIRCON ISOTOPIC AGE DATA Sample Fraction Amount Concentrations Atomic ratios Isotopic ages§ number size† analyzed 238U 206Pb* 206Pb 206Pb* 207Pb* 207Pb* 206Pb* 207Pb* 207Pb* 204Pb 238U 235U 206Pb* 238U 235U 206Pb* (µm) (mg) (ppm) (ppm) (Ma) (Ma) (Ma) WKB129 <45 1.5 446 7.2 379 0.01867(11) 0.1224 0.04836(13) 119.2 119.1 117 ± 8 WKB129 45–62 1.4 377 6.2 377 0.01894(12) 0.1265 0.04848(16) 120.9 120.9 122 ± 10 WKB131 <45 1.3 209 3.5 1410 0.01931(11) 0.1291 0.04853(12) 123.3 123.3 125 ± 6 WKB131 45–62 3.0 249 4.2 1910 0.01937(12) 0.1297 0.04858(10) 123.7 123.8 127 ± 5 WKB133 <45 1.4 329 5.3 2026 0.01849(11) 0.1233 0.04838(07) 118.1 118.0 118 ± 4 WKB134 <45 0.3 131 2.1 753 0.01837(10) 0.1225 0.04842(09) 117.3 117.4 119 ± 6

Note: Analytical techniques similar to those described in Saleeby et al. (1989). Zircon was separated from 10–15 kg of rock by standard crushing, grinding, H2O and heavy liquid density contrast, and magnetic susceptibility techniques. Samples were hand-picked to 99.9% purity prior to dissolution. Dissolution and chemical extraction techniques modified from Krogh (1973). Samples were spiked with a mixed 205Pb/235U tracer, and equilibrated samples were eluted through 50 µl ion-exchange columns using HCl and HBr. Mass spectrometry performed using a 35 cm, 90° extended geometry, VG Sector multicollector instrument. Pb and U were run on out- gassed single Re filaments with H3PO4 and silica gel or graphite loads, respectively. *Pb = radiogenic Pb. Nonradiogenic correction based upon a 40 picogram Pb blank (1:18.78:15.61:38.50) and approximated initial Pb ratios of 1:18.645:15.544:38.230 from Chen and Tilton (1991). †Fractions separated by grain size and magnetic properties. Nonmagnetic split made using 1.7 A with 2° side/20° front slopes on a Franz Isodynamic Separator. §Decay constants used in age calculations: λ238U = 1.55125E-10 a–1, λ235U= 9.8485E-10 a–1( Jaffey et al., 1971); 238U/235U atom = 137.88. Uncertainty in last two numbers of 206Pb*/238U is given in parentheses; 2σ uncertainty in 207Pb*/206Pb* is given as “±.” Uncertainties calculated by quadratic sum of total derivatives of 238U and 206Pb* concentrations and 207Pb*/206Pb* equations with error differentials defined as: [1] isotopic ratio determinations from standard errors (σ/√n) of mass spectrometer runs plus uncertainties in fractionation corrections based on multiple runs of NBS 981, 982, 983, and U500 standards; [2] spike concentrations from range of deviations in multiple calibrations with normal solutions; [3] spike compositions from external precisions of multiple isotope ratio determinations; [4] uncertainty in natural 238U/235U from Chen and Wasserburg (1981); and [5] nonradiogenic Pb isotopic compositions from uncertainties in isotope ratio determinations of blank Pb and uncertainties in composition of initial Pb from estimates of regional variations based on Chen and Tilton (1991) and consideration of rock type.

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A B

Figure 6. Mafic mylonites in the western ring(?) complex. (A) Photomicrograph of mylonitic olivine norite viewed under crossed polars (long edge of thin section ~3.2 mm). The dark augen (center) is composed of two olivine crystals; the lefthand crystal displays oscillatory extinction and deformation bands. The olivine augen is surrounded by a spinel-orthopyroxene aggregate and is embedded in a plagioclase mosaic. (B) Bi- otite-hornblende quartz norite with texture varying from foliated (top) to mylonitic (bottom; 4 cm scale).

and U-Pb dates was “excellent” for ≈115 Ma samples from this region of genic terranes are estimated to have formed at depths of 1 to 4 km (Bonin, the western foothills. 1986, p. 21). The youngest date in the Stokes Mountain region is from a 110 ± 1 Ma Shallow levels of emplacement may have been facilitated by the unusu- dike found in a chaotic association of layered hornblende gabbros and mu- ally high average density and relative thinness of the King-Kaweah ophiolite tually crosscutting hornblende gabbro dikes (KB9; Saleeby and Sharp, (Saleeby, 1975), into which hornblende-rich plutons of the Early Cretaceous 1980). Although mineralogically similar rocks are common throughout the batholith were emplaced: hydrous mafic magmas were able to rise to rela- map area, field relations provide no constraint concerning the significance tively high crustal levels before reaching neutral buoyancy. Even the mafic- of this particular date except that the layered gabbros must be older than the ultramafic cumulates may have been gravitationally stable in the uppermost gabbroic dikes. As the cessation of ring dike emplacement in the southeast- crust after solidification (see Glazner, 1994). One implication of the inter- ern section of the eastern ring complex is unequivocally dated by the radi- preted shallow emplacement depth is that uplift and erosion of the Stokes ally crosscutting diorite dike as 117 ± 2 Ma, the 110 ± 1 Ma date is tenta- Mountain region since ca. 115 Ma has not been great. This conclusion is con- tively interpreted as representing magmatism unrelated to the Stokes sistent with previous geomorphologic investigations, which suggest that up- Mountain ring dike complexes and the cumulate blocks. Volumetrically sig- lift of this section of the Sierra Nevada batholith in the past 55 m.y. has been nificant 110 Ma magmatism is represented, however, in neighboring regions minimal (≤1 km?; Warhaftig, 1962; Christensen, 1966; Huber, 1981). If this of the western Sierra Nevada batholith: granodiorites with interpreted ages estimate is correct, the majority of the uplift, perhaps 3 to 5 km, occurred of 110 and 111 Ma crop out ~8 km southeast and 30 km northwest of sam- prior to 55 Ma. ple KB9, respectively (Liggett, 1990; Chen and Moore, 1982). Emplacement of ring dike complexes is commonly attributed to foundering of an overlying block and to caldera formation (Williams, 1941; DISCUSSION Yoshida, 1984, Sparks, 1988). Bussell et al. (1976) identified characteristics consistent with the cauldron-subsidence model: ring faults with cataclastic Emplacement of the Ring Complexes and mylonitic textures along which volcanic and plutonic rocks are juxta- posed; xenolith-rich ring dikes; bell-jar shaped plutons presumably sur- Barometric data were not obtained from the Stokes Mountain igneous rounding foundered roof blocks. The 1000 m3 xenoliths of silicified por- suite, but the presence of cordierite in the contact-metamorphic zone sug- phyry south of Tucker Mountain (Fig. 1) certainly record stoping of gests metamorphic pressures of less than ~2 kbar. Slightly greater em- hydrothermally altered roof material but do not require caldera collapse. No placement pressures (3–4 kbar) are suggested by the contours of Ague and ring faults were mapped in the area, but two small, enigmatic outcrops of Brimhall (1988), but these contours were not constrained by any data in the mylonitic mafic rock (∆ in Fig. 1) cannot be explained by Stone Corral shear Stokes Mountain region or the nearby surroundings. The close correspon- zone–related deformation and may, instead, be related to ring faulting. (1) dence of K-Ar dates from hornblende and biotite with zircon dates indi- Mylonitic olivine norite forms an ~2-m-long outcrop near the eastern mar- cates that rocks of the Early Cretaceous batholith cooled rapidly (Saleeby gin of the ultramafic-mafic body near Tucker Mountain (Fig. 6A). The my- and Sharp, 1980; Chen and Moore, 1982). Considering both the morpho- lonitic foliation is near vertical approximately parallel to the neighboring in- logic and barometric estimates, we estimate that the Stokes Mountain ring trusive contacts and the olivine lineation plunges steeply northeast. (2) dike complexes were emplaced into the uppermost crust, perhaps as deep Mylonitic two pyroxene-hornblende gabbro (Fig. 6B) was found adjacent as 4 to 6 km. For comparison, Johnson et al. (1999) estimate an emplace- to a small body of hornblende troctolite on Curtis Mountain; foliation in the ment pressure of 2.3 ± 0.6 Kbar for a Cordilleran ring dike complex, and surrounding two pyroxene quartz gabbro is atypically intense and pervasive. though not tectonically analogous, we note that ring complexes in anoro- A tentative interpretation of these mafic mylonites is that they record the

492 Geological Society of America Bulletin, April 1999

Stone Corral unrelated hb gabbro shear zone magmatism? (KB9) discordant quartz diorite ERC qtz diorite (#32) (#31) WRC WRC2 px-hb-bi gbd (KB7) N radial dike (WKB134) Colvin Colvin Mtn. Mountain px-hb gbd Figure 7. Summary of Stokes Mountain (KB6) geochronology depicting proposed intrusion granite events within the eastern ring complex and west-

(WKB133) ERC 2 ern ring(?) complex (ERC, WRC), and the possi- px-bi-hb gbd ble timing of activity along the Stone Corral shear (KB4) zone. Black circles highlight dates within hori- porphyry zontal error bars; preferred age of 115 Ma is (WKB129) highlighted for discordant sample 32. Abbrevia- px-hb gbd tions: gb—gabbro; px—pyroxene; hb—horn- WRC1 (KB3) synmagmatic deformation along blende; bi—biotite. Stone Corral shear zone tonalite (116 ± 2 Ma?) (WKB131) ERC 1 same hornblende-rich mesocumulates event? (KRO22) px-bi-hb gbd (KB1) Colvin Mtn. 126 125 124 123 122 121 120 119 118 117 116 115 114 113 112 111 110 109 Age (Ma)

collapse of the Tucker Mountain mafic-ultramafic block that facilitated ring events of ≤2 m.y. in duration, then we must conclude that the eastern and dike emplacement in a manner proposed by Bussell et al. (1976). Note that western ring complexes were not emplaced during single volcanic-plutonic igneous foliations in the Stokes Mountain ring dikes are typically near ver- events. Possible subdivisions of the 5 to 6 m.y. emplacement periods are sum- tical except south and northeast of Tucker Mountain, where foliations dip marized in Figure 7; for clarity, errors are omitted from dates in the following outward around the proposed foundered mass (Fig. 1). The general absence discussion. Intrusion of eastern ring complex magmas may have occurred of features characteristic of caldera formation may indicate that the eastern during at least two discrete events: a first ca. 123 Ma (ERC1) and a second be- and western ring complexes preserve relatively deep levels of formation tween about 120 and 117 Ma (ERC2). Likewise, a first magmatic event within compared to other ring complexes described in the literature. the western ring complex may have occurred ca. 120 Ma (WRC1) followed by a second event between about 116 and 115 Ma (WRC2). Alternate inter- Timing of Ring Complex Emplacement pretations are certainly possible: e.g., if magmatism in the Colvin Mountain area is related to the eastern ring complex, then ERC1 might have occurred Emplacement of the adjacent ring dike complexes occurred during over- between 125 and 123 Ma (Fig. 7); if the Tucker Mountain cumulates are co- lapping time intervals: from 120 to 115 Ma (n = 3) for the western ring com- eval with those on Colvin Mountain, then western ring complex magmatism plex and from 123 to 117 Ma (n = 5) for the eastern ring complex. The ring may have either started at 123 Ma or have been preceded by an earlier cumu- complexes thus have minimum emplacement durations of 5 and 6 m.y. late-forming event. Regardless of the specific temporal subdivisions, the data These durations are similar to estimates from other ring complexes: e.g., the are consistent with emplacement of each ring dike complex during a mini- Peruvian Fortaleza (Bussell et al., 1976) and Huaura ring dike complexes mum of two temporally discrete events separated by ~3 to 4 m.y. Moreover, are believed to have had emplacement durations of ~2 to 5 m.y. (Pitcher, intrusion during the ERC2 event may have overlapped both WRC1 and 1978; Bussell, 1988). These periods of activity, however, are significantly WRC2 intrusion events. Bussell et al. (1976) recognized that some of the Pe- longer than the estimated periodicity of the presumably overlying volcanic ruvian ring complexes were the sites of episodic magmatism, yet, unlike the systems (Smith, 1979, Fig. 12; Lipman, 1984). Peruvian examples, it presently cannot be demonstrated that the earliest events If we assume that ring complexes are subsurface manifestations of volcanic (i.e., ERC1 and WRC1) actually formed ring complexes due to limited verti-

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cal exposure in the study area. If caldera collapse facilitated emplacement of zone; and the hot subsolidus deformation at higher structural levels near the the younger ring dike complexes (i.e., ERC2 and WRC2 events), then the ear- discordant sample 32, which we interpret as being coeval with sample 31 lier events may simply represent precaldera magmatism. (115 ± 1 Ma). This interpretation implies that the Stone Corral shear zone became active during intrusion of the ERC2 complex and continued during Relation of the Stone Corral Shear Zone to the Ring Complexes WRC2 emplacement by accommodating left-lateral shear during radial extension around the two impinging volcanic centers. Understanding the apparently complex nature of the Stone Corral shear zone requires detailed microstructural analysis that is beyond the scope of Relation of Layered Troctolites to Ring Complex Magmatism this study, but at present two end-member scenarios can be evaluated. First, the Stone Corral shear zone may have been an expression of the regional From the map pattern alone, Colvin Mountain cannot be assigned to ei- tectonic regime during Early Cretaceous time, e.g., the Stone Corral shear ther the eastern or the western ring complex. Although the age of the Colvin zone could have accommodated some amount of arc-parallel extension sim- Mountain cumulates (123 ± 3 Ma) overlaps both ERC1 and WRC1 events, ilar to other shear zones in the Sierra Nevada batholith (Busby-Spera and the true age of the cumulates may be closer to that of the 125 ± 1 Ma gab- Saleeby, 1990; Tobisch et al., 1995; Tikoff and de Saint Blanquat, 1997). bro (KB1) that forms an inselberg located ~1.25 km to the west (Fig. 1). In- Emplacement in a transtensional environment is supported by the possibly clusion of this 125 Ma gabbro and the Colvin Mountain cumulates in the ellipsoidal shape of the western ring complex with an approximately north- ERC1 event is reasonable in that ERC1 activity would span 2 m.y. between south axis of maximum elongation. Emplacement in a structural trough has 125 and 123 Ma (Fig. 7), but it is also possible that the cumulates are ge- been proposed for some of the large, ellipsoidal batholiths near the range netically unrelated precursors forming wall rocks or septa in the slightly crest (Tobisch et al., 1986; Saleeby, 1990) and might have provided some of younger ring complexes. Mineralogic relations and isotopic systematics the requisite space for the Stokes Mountain ring complexes. Even though demonstrate strong similarities between both the parental magmas and the the potential relation between the Stone Corral shear zone and the regional differentiation histories of the cumulate bodies and the surrounding noncu- tectonic environment cannot be evaluated until the full length of the Stone mulate rocks (Clemens Knott, 1992). Given that significant upward move- Corral shear zone is mapped, the apparently minimal amount of offset of the ment of square kilometer-scale dense blocks is unlikely, perhaps the best in- metamorphic septum bisected by the Stone Corral shear zone appears to ar- terpretation is that the exposed cumulates formed at shallow crustal levels gue against this possibility. from magmas compositionally similar to those that differentiated at greater Alternatively, the Stone Corral shear zone may have only local signifi- depths and intruded the slightly younger ring complexes. This scenario cance and have resulted from nearly synchronous intrusion of the closely would imply that similar mafic-ultramafic cumulates, equivalent in age to spaced magmatic centers. The geographic center of the circular eastern ring ring dike magmas, underlie the study area. Such cumulates could contribute complex is the gabbro stock of Red Mountain (Fig. 3). Because the western to the recognized geophysical anomalies (Saleeby, 1975). ring complex is incompletely exposed, the center of this complex is not well located; for the purposes of this discussion, however, the hornblende gab- Constraints on Magmatic Differentiation bros and hornblende troctolites on and south of Tucker Mountain are considered to form the center of the elliptical western ring complex. This as- The structural and age relations described so far provide the following in- signment is based predominantly on the possible role of mafic rocks in formation regarding the processes and depths of magmatic differentiation in foundering of a central block during ring dike emplacement (Bussell et al., the Stokes Mountain region. 1976). In Figure 3, arrows have been drawn around each of the purported 1. The presence of voluminous layered mafic-ultramafic cumulates de- volcanic centers to indicate the direction of radial extension expected dur- mands that fractional crystallization was efficient and must have produced ing ring dike emplacement and tumescence of the crust. If these vectors are some volume of more evolved magmas at least during the earliest stages of resolved along the trace of the Stone Corral shear zone, a left-lateral sense magmatism. Moreover, this fractional crystallization occurred at shallow of shear is produced along the Stone Corral shear zone similar to that indi- crustal levels; due to their immense size and high density, the cumulates cated by the tension gashes. could not have been transported upward in the crust for any great distance In addition to accommodating shear stress in the upper crust, the Stone by upward-moving magmas. Geochronologic data do not rule out the pos- Corral shear zone served as a conduit for magma emplacement and in- sibility that the older western and eastern ring complexes crystallized from tense magma mingling. Though not visible at the scale of Figure 1, norites differentiated magmas produced during formation of the exposed layered and quartz norites dominate the western edge of the Stone Corral shear cumulates. zone near the eastern end of Stokes mountain; these rocks are abundant in 2. Common foliated and protomylonitic textures in the quartz-bearing the western ring complex but relatively uncommon in the eastern ring rocks suggest that many of the differentiated magmas intruded into the ring complex. In contrast, granodiorite is abundant along the eastern edge of complexes as moderately crystal-rich assemblages. Pervasive foliations this same section of the Stone Corral shear zone, and in the adjacent east- within similar rocks of the Early Cretaceous batholith were described by ern ring complex but is relatively uncommon in the adjacent section of the Bateman et al. (1983); they concluded that deformation of the ca. 114 Ma western ring complex. Based on structural, petrologic, and geochrono- tonalite of Blue Canyon and its metamorphic wall rocks was related to the logic relations, we hypothesize that magmas from both the western ring emplacement of a 113 Ma leucotonalite that they estimate to have contained complex– and eastern ring complex–plumbing systems commingled in a >30% crystals at the time of emplacement. Plutons within Peruvian ring tear formed during tumescence of the upper crust as a result of ring dike complexes are similarly deformed (Pitcher, 1978). The relation between emplacement. texture and crystallinity suggests that much of the magmatic differentiation Our preferred age for synmagmatic and postmagmatic activity along the and crystallization that produced the various ring dike magmas occurred at Stone Corral shear zone is 116 ± 2 Ma (Fig. 7). This estimate is based on the deeper crustal levels than currently exposed. following: the presence of textures and structures indicative of intense syn- 3. Within the Stone Corral shear zone, magma mingling occurred in a magmatic deformation; our preliminary interpretation that magmatic com- regime of intense synmagmatic shearing, and the scale of mingling became ponents from both volcanic centers commingled in the Stone Corral shear small enough to approach magma mixing (e.g., enclaves of intimately com-

494 Geological Society of America Bulletin, April 1999

mingled hornblende gabbro and norite; Fig. 5B). Mixing at the present level L. T. Silver and M. B. Wolf; this manuscript was greatly improved based on of exposure within the enclave-rich ring dikes seems unlikely due to the thorough reviews by C. R. Bacon and T. W. Sisson. Caltech Division of Ge- likely thermal and viscosity contrasts between magmas of significantly dif- ological and Planetary Science Contribution 5746. ferent silica contents (~48% and 67%; see Sparks and Marshall, 1986; Frost and Mahood, 1987). Some ring dikes in the southern half of the eastern ring complex, however, contain the poorly defined remnants of enclaves, sug- REFERENCES CITED gesting that some magma mixing may have occurred at deeper and/or ear- Ague, J. J., and Brimhall, G. H., 1988, Magmatic arc asymmetry and distribution of anomalous lier magmatic stages. plutonic belts in the batholiths of California: Effects of assimilation, crustal thickness, and 4. Stoping of roof rock by ring dike magmas is documented by the 120 Ma depth of crystallization: Geological Society of America Bulletin, v. 100, p. 912–927. Bateman, P. C., and Busacca, A. J., 1982, Geologic map of the Millerton Lake quadrangle, west- xenoliths of hydrothermally altered hypabyssal porphyry located within the central Sierra Nevada, California: U.S. Geological Survey Geologic Quadrangle Map GQ- western ring complex (Fig. 1). Given such clear evidence for stoping, the pos- 1548, 1 sheet, scale 1:62 500. Bateman, P. C., and Chappell, B. W., 1979, Crystallization, fractionation and solidification of the sibility that altered hypabyssal and possibly volcanic material of Mesozoic Tuolumne Intrusive Series, Yosemite National Park, California: Geological Society of age was assimilated by the Stokes Mountain magmas must be considered. America Bulletin, v. 90, part I, p. 465–482. Bateman, P. C., and Nockleberg, W. J., 1978, Solidification of the Mount Givens Granodiorite, Sierra Nevada, California: Journal of Geology, v. 86, p. 563–579. CONCLUSIONS Bateman, P. C., Busacca, A. J., and Sawka, W. N., 1983, Cretaceous deformation in the western foothills of the Sierra Nevada, California: Geological Society of America Bulletin, v. 94, Structural, textural, and geochronologic relations reveal a dynamic 10- p. 30–42. Beard, B. L., and Glazner, A. F., 1995, Trace element and Sr and Nd isotopic composition of man- m.y.-long period in the history of the Early Cretaceous southern Sierra tle xenoliths from the Big Pine volcanic field, California: Journal of Geophysical Research, Nevada batholith. During this time period repeated events of mafic- v. 100, p. 4169–4179. Blake, S., and Campbell, I. H., 1986, The dynamics of magma-mixing during flow in volcanic ultramafic cumulate formation and disruption as well as the episodic em- conduits: Contributions to Mineralogy and Petrology, v. 94, p. 72–81. placement of two ring-dike complexes into the Kings-Kaweah ophiolite Bonin, B., 1986, Ring complex granites and anorogenic magmatism: New York, Elsevier belt occurred. At about 116 ± 2 Ma, intense magma mingling and syn- Press, 188 p. Busby-Spera, C. J., and Saleeby, J. S., 1990, Intra-arc strike-slip fault exposed at batholithic lev- magmatic shearing occurred at deep levels of the Stone Corral shear zone els in the southern Sierra Nevada, California: Geology, v. 18, p. 255–259. while hot subsolidus deformation produced protomylonites to mylonites Bussell, M. A., 1988, Structure and petrogenesis of a mixed-magma ring dyke in the Peruvian at higher structural levels. The shear zone may have accommodated left- Coastal Batholith: Eruptions from a zoned magma chamber: Royal Society of Edinburgh Transactions, v. 79, p. 87–104. lateral shear caused by nearly simultaneous emplacement of the closely Bussell, M. A., Pitcher, W. S., and Wilson, P. A., 1976, Ring complexes of the Peruvian Coastal spaced western and eastern ring complexes. Early Cretaceous hypabyssal Batholith: A long-standing subvolcanic regime: Canadian Journal of Earth Sciences, v. 13, p. 1020–1030. intrusions were silicified during hydrothermal alteration then stoped by Cady, J. W., 1975, Magnetic and gravity anomalies in the Great Valley and western Sierra Nevada the underlying western ring(?) complex. metamorphic belt, California: Geological Society of America Special Paper 168, 56 p. We originally selected the study area with the goal of collecting geo- Chen, J. H., and Moore, J. G., 1982, Uranium-lead isotopic ages from the Sierra Nevada Batholith, California: Journal of Geophysical Research, v. 87, no. B6, p. 4761–4784. chemical data to improve our understanding of the west to east geochemical Chen, J. H., and Tilton, G. R., 1991, Applications of lead and strontium isotopic relationships to variations within the Sierra Nevada batholith. Intrusion and deformation the petrogenesis of granitoid rocks, central Sierra Nevada batholith, California: Geological events recorded by the rocks of the Stokes Mountain region may, however, Society of America Bulletin, v. 103, p. 439–447. Chen, J. H., and Wasserburg, G. J., 1981, Isotopic determination of uranium in picomole and sub- play an equally important role in improving our understanding of processes picomole quantities: Analytical Chemistry, v. 53, p. 2060–2067. occurring within the uppermost Cretaceous batholith. The ring dike struc- Christensen, M. N., 1966, Late Cenozoic crustal movements in the Sierra Nevada of California: Geological Society of America Bulletin, v. 77, p. 163–182. tures and the surrounding low-pressure contact metamorphism suggest that Clemens Knott, D., 1992, Geologic and isotopic investigations of the Early Cretaceous Sierra the rock of Stokes Mountain region formed in an intermediate position be- Nevada batholith, Tulare, Co., CA, and the Ivrea Zone, NW Italian Alps: Examples of in- tween the volcanic carapace (e.g., the Ritter Range caldera complex) and teraction between mantle-derived magma and continental crust [Ph.D. dissert.]: Pasadena, California Institute of Technology, 349 p. zoned plutons (e.g., the Academy pluton). Considered in this structural con- Clemens Knott, D., 1998, The Early Cretaceous Stone Corral shear zone (Stokes Mountain re- text, a significant petrologic ramification of this study is that a wide variety of gion, western Sierra Nevada batholith, CA): A local or regional structure?: Geological So- mafic to felsic magmas were placed in intimate contact in a dynamic, shal- ciety of America Abstracts with Programs, v. 30, no. 5, p. 10. Clemens Knott, D., Saleeby, J. B., and Taylor, H. P., Jr., 1991, O, Sr and Nd constraints on the evo- low-crustal environment. Plagioclase-olivine-orthopyroxene cumulates crys- lution of the Early Cretaceous Sierra Nevada batholith: The Stokes Mountain region, CA: tallized, compositionally diverse magmas commingled, and xenoliths were Geological Society of America Abstracts with Programs, v. 23, no. 5, p. A386. Coleman, D. S., Frost, T. P., and Glazner, A. F., 1992, Evidence from the Lamarck granodiorite for stoped from the metamorphic wall rock and the hypabyssal-volcanic(?) roof. rapid Late Cretaceous crust formation in California: Science, v. 258, p. 1924–1926. Deciphering the relative significance of the processes implied by these ob- DePaolo, D. J., 1981, A neodymium and strontium isotopic study of the Mesozoic calc-alkaline servations (i.e., fractional crystallization, magma mixing, and crustal assim- granitic batholiths of the Sierra Nevada and Peninsular Ranges, California: Journal of Geo- physical Research, v. 86, no. B11, p. 10470–10488. ilation) requires interpretation of the geochemical data in light of the struc- DePaolo, D. J., Gromet, P., Powell, R., and Silver, L. T., 1975, San Telmo ring complex, Penin- tures and textures described herein. sular Ranges Batholith, northwest Baja California, Mexico: Geological Society of America Abstracts with Programs, v. 7, p. 309–310. Duffield, W.A., 1968, The petrology and structure of the El Pinal tonalite, Baja California, Mex- ACKNOWLEDGMENTS ico: Geological Society of America Bulletin, v. 79, p. 1351–1374. Durrell, C., 1940, Metamorphism in the southern Sierra Nevada northeast of Visalia, California: California University Publications in the Geological Sciences, v. 25, p. 1–117. This work was supported by a National Science Foundation Graduate Evernden, J. F., and Kistler, R. W., 1970, Chronology of emplacement of Mesozoic batholithic Fellowship, two GSA Penrose grants and a California State University, complexes in California and western Nevada: U.S. Geological Survey Professional Paper Fullerton Faculty Grant to Clemens-Knott, and National Science Founda- 623, 42 p. Fluk, L., and Treiman, A., 1988, Mafic-to-felsic enclaves in a syenite ring-dike, White Mountains, tion grant EAR-9105692 to Saleeby. We thank the many ranchers and farm- N.H.: Magma mixing and mingling: Eos (Transactions, American Geophysical Union), ers of the Stokes Mountain region allowing access to their properties, and v. 69, p. 1505. the Chrisman, Collins, Edminston, and Travoli families for their generous Freundt, A., and Tait, S. R., 1986, The entrainment of high-viscosity magma into low-viscosity magma in eruption conduits: Bulletin of Volcanology, v. 48, p. 325–339. hospitality. M. Chaudhry prepared zircon separates. J. Knott assisted with Frost, T. P., and Mahood, G. A., 1987, Field, chemical, and physical constraints on mafic-felsic drafting. The late Martin Stout provided air photos from the California State magma interaction in the Lamarck granodiorite, Sierra Nevada, California: Geological So- University, Los Angeles collection. We benefited from discussions with ciety of America Bulletin, v. 99, p. 272–291.

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