Fragments of the Mantle and Crust from Beneath the Sierra Nevada Batholith: Xenoliths in a Volcanic Pipe Near Big Creek, California

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Fragments of the mantle and crust from beneath the Sierra Nevada batholith: Xenoliths in a volcanic pipe near Big Creek, California

F.C.W. DODGE U.S. Geological Survey, 345 Middlefleld Road, Menlo Park, California 94025 J. P. LOCKWOOD U.S. Geological Survey, Hawaii National Park, Hawaii 96718 L. C. CALK U.S. Geological Survey, 345 Middlefleld Road, Menlo Park, California 94025

ABSTRACT INTRODUCTION Lake quadrangle (Fig. 1). Xenoliths are most abundant in a large volcanic pipe northwest of A Miocene volcanic pipe that cuts Creta- Deep-seated xenoliths were discovered at the town of Big Creek (long 119°16' W., lat ceous granodiorite is exposed in the central three localities in the central Sierra Nevada dur- 37° 13' N.); the others are found in a smaller Sierra Nevada near the town of Big Creek, ing geologic mapping of the Shaver Lake 15' elongate pipe high on the west bank of the San California. The volcanic magma began its as- quadrangle (Lockwood and Bateman, 1976); Joaquin River near the Pick and Shovel Mine cent from a mantle source as an alkalic basalt the xenoliths have been studied subsequently as (long 119°20' W„ lat 37°15' N.) and in a nar- but was extensively contaminated by incor- part of an ongoing investigation of the nature of row dike exposed along Big Creek itself just poration and assimilation of silicic plutonic the lower crust and upper mantle beneath the below Camp Sierra (long 119° 15' W„ lat rocks during ascent, resulting in a "pseudo- Sierra Nevada (Brooks and others, 1980; Dodge 37° 12' N.). This report is focused on the large andesitic" composition. The conduit cut and others, 1986; Domenick and others, 1983; pipe northwest of Big Creek (hereinafter referred through the entire thickness of the Sierra Peselnick and others, 1977). These occurrences to as the "Big Creek pipe") and describes the Nevada batholith, as well as the underlying are of particular interest because they are in the general geology and preliminary petrologic and lower crust and upper mantle, and the magma axial region of the central Sierra Nevada batho- geochemical studies of the pipe and its xenoliths. entrained a diverse, polygenetic assemblage lith, in an area that has been the subject of de- of igneous and metamorphic xenoliths, in- tailed geologic and geophysical studies for GEOLOGIC SETTING cluding peridotites, eclogites and granulites, nearly four decades (for example, Bateman, The Big Creek pipe is elliptical, 110 x 260 m schists and hornfels, and gabbroids and 1983; Bateman and Eaton, 1967; Oliver, 1977) in plan, and is located on a steep, east-facing granitoids. Some xenoliths of the high-grade and also because of the abundance of garnetifer- chapparal-covered slope at 1,600 m elevation metamorphic xenolith group are of particular ous ultramafic and mafic xenoliths, which have (Fig. 2), 1.5 km northwest of the town of Big interest in elucidating the nature of the up- a very limited distribution in volcanic rocks of Creek, Fresno County, California. The locality permost mantle and crust beneath the Sierra the western United States (Wilshire and others, is most easily reached by hiking down from the Nevada. 1988). Stump Springs Road to the north. The pipe is Garnet granulites, the most abundant rock The localities are situated at the northern well exposed along its north and west margins in type of the mafic high-grade metamorphic as- boundary and northeast corner of the Shaver resistant, columnar-jointed knobs (Fig. 3). semblage, and eclogites are restricted to the central area of the pipe. These xenoliths are essentially bimineralic, consisting of colorless 119°20' 119°15' to pale green clinopyroxene and garnet; eclogites contain brilliant emerald green clino- 37°15' pyroxene. The clinopyroxenes, which have a broad range of compostions [Mg/(Mg + Fe) = 0.45-0.90], have a >20% jadeite component Figure 1. Map of the in eclogites. Garnets also have a range of northeastern corner of the compositions [Mg/(Mg + Fe) = 0.30-0.70], Shaver Lake 15' quadrangle with those from eclogites being the most Fe and adjacent areas, showing rich. One of the xenoliths contains Ca-rich location of Tertiary volcanic garnet (grs + adr > 90%). Feldspathic granu- rocks and xenolith localities lites contain plagioclase of An45_7o, and some near Big Creek, California. lack garnet. Hornblende occurs in some granulites, and a few amphibolites are present. Likely protoliths are ocean-floor basalts for the eclogites and Precambrian limestone

for the grossular-clinopyroxene rock. The 37° 10' Big Creek xenoliths attest to the former presence of subducted oceanic lithosphere be- 1 2 3 KILOMETERS neath the western Sierra Nevada. _l I I

Geological Society of America Bulletin, v. 100, p. 938-947, 9 figs., 4 tables, June 1988.

938

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EXPLANATION

Qt Talus and colluvium (Quaternary) Contact—Dashed where approximately located l.Tvc. • Volcanic rocks, central facies Fault—Dashed where approximately (Miocene) e> _ t> located; queried where uncertain. "Tvm: Volcanic rocks, marginal facies ir," Arrows show direction of relative (Miocene) movement ; Kgm Melted granitic rocks (Late Creta- ceous) Plunge of colummar jointing Kap Aplite (Late Cretaceous) Inclined

• kdcD- Porphyritic granodiorite of Dinkey Horizontal Creek (Late Cretaceous) Foliation

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The pipe cuts a porphyritic facies of the 104 Ma granodiorite of Dinkey Creek (Stern and others, 1981) and a later aplite sill (Fig. 2). Con- tacts are steep where the pipe is in contact with the granodiorite but are shallow along the shallow-dipping aplite sill. Details of the zone along the contact with the enclosing granodiorite are especially well exposed along the pipe's eastern margin. Here, the pipe is separated from the granitic rock by a 30-cm-wide zone of indu- rated breccia in which angular to subrounded fragments of volcanic andesite and granitic rocks are thoroughly mixed in a finely comminuted matrix. Outside of this breccia, the granitic rock has undergone substantial melting and remobili- zation and is itself a breccia in which fine- grained melted glassy or finely recrystallized granitic rock hosts subangular blocks of granodiorite and aplite. This granitic breccia is cut by volcanic dikes, which extend as much as 5 m from the primary contact. Evidence of volcanic heating and subsequent cooling is also shown by Figure 3. View southwest across the Big Creek pipe, showing central "soft" and marginal well-developed columnar jointing, which ex- "hard" facies, the latter characterized by columnar jointing. Nearest tree approximately 6 m in tends from the pipe across the contact into the height. granitic rocks. Large granitic xenoliths within the pipe have also been partially remelted, and in two the reported chemical compositions (Table 1) rubble-covered outcrops (Figs. 2 and 3). Com- localities, dikes of granitic composition cut the and likely was alkali basalt. positional as well as morphological differences volcanic host and include abundant angular an- The Big Creek pipe is zoned, with a marginal characterize the zoning; rocks of the central fa- desite and rounded mafic and ultramafic xeno- facies characterized by prominent, blocky out- cies have prominent hornblende and biotite mi- liths. The granitic xenoliths were apparently crops with well-developed columnar jointing crophenocrysts, whereas pyroxenes are predom- raised to supra-solidus temperatures by the lava and a central facies characterized by subdued, inant in the slightly less alkalic marginal facies and remained fluid after the volcanic host (Table 1). Biotite microplates produce a strong cooled below its solidus temperature. The pipe TABLE 1. CHEMICAL (WEIGHT PERCENT) AND MODAL concentric foliation in the central facies, paral- (VOLUME PERCENT) COMPOSITION OF ANDESITE OF was emplaced in the late Miocene episode of the THE BIG CREEK PIPE leled by faint flow banding and alignment of San Joaquin-Kings volcanic field (Moore and xenoliths. Dodge, 1980). Biotite from the "andesite" in the Sample no. Marginal Central Zoning of the pipe is also reflected in xenolith Big Creek pipe (sample A-6) has been dated by facies facies A-3 A-6 distribution; the most dense, garnet-bearing the Rb-Sr method at 8.3 ± 1.2 m.y., whereas xenoliths are found only within the central

biotite from the Camp Sierra dike has been Si02 56.9 57.7 facies, whereas low-density granitic xenoliths are dated by the K-Ar method at 11.1 ± 0.2 m.y. AI2O3 13.2 13.0 Fe203 1.8 2.3 most abundant in the marginal facies. The xeno- (R. W. Kistler, 1985, written commun.), sug- FeO 4.4 3.3 lith distribution may be partially the result of MgO 9.6 8.5 gesting that the volcanics, although resulting CaO 5.8 5.1 velocity differences between facies within the from the same general period of volcanism and Na20 2.8 3.2 pipe during emplacement. The most dense xeno- K2O 2.3 2.4

intrusion, are not contemporaneous. H2O* 1.1 0.70 liths from the central facies are smaller in size H2O- 1.3 1.6 than are the lower density xenoliths. The most Ti02 0.73 0.66 THE VOLCANIC HOST P2°5 0.36 0.35 dense garnet peridotite collected (d = 3.33 MnO 0.11 0.08 g/cm3) has a volume of 142 cm3, whereas an co2 0.02 0.09 The Big Creek volcanic host consists predom- Sum 100.4 99.0 intermediate-density plagioclase-pyroxene gneiss {d = 2.81) has a volume of 8,000 cm3. The least inantly of tan-gray to light gray lamprophyric Primary andesite in which fine-grained euhedral mafic Opx tr dense xenolith (an aplite, d = 2.62) has a volume Cpx 16 of at least 750 m3. mineral phenocrysts and corroded and embayed Hbl 22 xenocrysts and altered lithic fragments are set in Bt 1 5 PI 6 7 a trachytic microcrystalline groundmass com- Matrix 70 59 THE XENOLITHS posed largely of sodic labradorite and clinopy- Accidental Qtz 2 1 roxene. On the basis of chemical classification PI 3 tr Rounded to subrounded xenoliths are abun- Px 2 (Zanettin, 1984), the volcanic host is andesite Lithic 2 4 dant throughout the Big Creek pipe and range and is quartz normative; however, because of Sum 100 100 from millimetres to metres in size. Because of the quartz xenocrysts and small felsic xenoliths un- size range, the amount of xenolithic material in avoidably included in analyzed material, the Note: chemical analyses by rapid rock techniques. S. D. Botts, U.S. Geological Survey, analyst. the pipe cannot be determined precisely but is primary intrusive magma was less silicic than estimated to be 2% to 5%.

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The xenoliths are divided into four petro- Olivine graphic groups, based on a combination of chemical-modal and textural criteria: 1. Ultramafic—peridotites and pyroxenites 2. Higher grade metamorphic—eclogites, granulites, and amphibolites 3. Lower grade metamorphic—schists and hornfels 4. Calc-alkalic—gabbros, diorites, and granites. For the most part, individual xenoliths can easily be assigned to one of these groups, but some groups overlap and distinctions are not sharply defined in all cases; consequently, the placement of a few of the xenoliths is arbitrary. ucts. Minor (<5%) interstitial glass is present in pronounced lineation is also evident. Although some ultramafic xenoliths. orientation of inequant grains also defines a Ultramafic Xenoliths A single coarse-grained apatite-rutile-clinopy- weak foliation in a few samples, effects of inter- roxene rock with allotriomorphic-granular tex- nal deformation are generally minimal, and Although most abundant in the central facies, ture is most likely a true igneous pyroxenite. A textures of this medium- to coarse-grained ultramafic xenoliths are present throughout the few other equigranular clinopyroxene-rich xeno- group are typically allotriomorphic granular or Big Creek pipe. They commonly are intensely liths lack features that uniquely place them in granulitic. altered and replaced by soft clay, identified by either the ultramafic or high-grade metamorphic Several varieties of the high-grade metamor- X-ray diffraction as consisting principally of group but based on both textural and mineralog- phic xenoliths are distinguished by the presence nontronite. Many altered ultramafic xenoliths ical gradations, are generally considered to be- or absence of certain major (>10 vol %) miner- have completely weathered out on surface expo- long to the latter group. als and the nature of these minerals. Garnet sures, leaving empty holes. The degree of altera- granulites, the most abundant rock type of the tion varies greatly in 19 examined ultramafic Higher Grade Metamorphic Xenoliths group, are essentially bimineralic rocks consist- xenoliths, from completely unaltered peridotites, ing of colorless to pale green clinopyroxene and to those with narrow alteration bands restricted This diverse group contains the largest num- garnet; in eclogites and a grossular-clinopyrox- to olivine grains, to wholly altered rocks con- ber of studied Big Creek xenoliths. Weak to ene rock, clinopyroxene is brilliant emerald taining only small, patchy remnants of the origi- moderate gneissose foliations, especially recog- green. Plagioclase, ranging from sodic andesine nal minerals. nizable in larger xenoliths (to 30 cm in maxi- to calcic labradorite, is a major mineral in feld- Where recognizable, textures are generally al- mum dimension), are manifested by irregular spathic granulites, but garnet may or may not be lotriomorphic granular with irregular grain compositional layering (Fig. 5); in some cases, a present. Brown hornblende is a major constitu- boundaries of olivine and pyroxene. In some granular xenoliths, sharp kink bands are developed in olivine and undulatory extinction or fine exsolution lamellae are developed in pyroxene, but features clearly diagnostic of a specific origin (Pike and Schwarzman, 1977), such as zoning, a high proportion of 120° grain boundaries, or Figure 5. Polished slab cataclastic features, are lacking. A few perido- of 22-cm-diameter gneiss- tites are porphyroclastic with large clasts of relict ose xenolith (sample B- olivine set in a fine-grained matrix of equigranu- 61), demonstrating small- lar olivine and pyroxene, clearly the result of scale heterogeneity. Dark tectonic recrystallization (Harte, 1977; Mercier layers are garnet-pyrox- and Nicolas, 1975). Some contain late, bladed ene granulites; light layers orthopyroxene, suggesting reheating. are feldspathic granulites. The peridotites are essentially olivine-pyroxene rocks, generally with only trace amounts of other primary constituents; modal proportions (Fig. 4) were determined on unaltered or the least altered of the peridotite xenoliths. Of the 19 examined xenoliths, two contain less than 1% of small, anhedral garnet crystals. Several of the peridotites have trace amounts of fine-grained spinel. Secondary amphibole and fine-grained mica, and in one case, plagioclase, are present in some altered xenoliths. In addition to clay minerals, serpentine, brucite, and carbonate have been identified as alteration or replacement prod-

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TABLE 2. SELECTED MAJOR- AND TRACE-ELEMENT OXIDE ANALYSES (WEIGHT PERCENT) OF BIG CREEK XENOLITHS

Sample no. Peridotites High-grade metamorphic xenoliths

P-13 P-10 P-l F-99 B-75A F-68 G-26 F-3 F-51 B-61 B-37 L-2

Si02 41.1 46.1 45.0 42.2 43.7 46.5 42.2 45.60 49.5 50.5 58.5 52.90

A1203 0.32 2.7 2.9 10.2 15.4 10.6 15.5 14.92 7.52 15.3 13.6 14.85

Fe203 1.08 1.7 1.4 7.39 5.6 4.61 5.31 6.04 3.72 3.07 3.02 2.53 FeO 6.90 6.8 7.1 5.85 11.4 8.80 4.67 5.56 5.83 5.10 4.16 4.79 MgO 49.1 39.2 36.4 4.43 8.8 12.1 15.2 10.85 16.6 4.96 2.95 5.3 CaO 0.20 1.2 4.4 24.8 10.3 12.3 13.7 12.95 13.5 12.3 11.2 13.56

Na20 <0.15 0.10 0.49 0.59 1.4 1.27 0.55 1.93 1.02 3.35 1.18 4.1

K2O <0.02 0.05 0.07 <0.02 0.20 0.60 0.09 0.07 0.07 0.49 1.96 0.63

H2O* 0.55 0.65 0.34 0.60

H2O 0.24 0.18 0.14 0.95

Ti02 <0.02 0.01 0.25 0.43 3.2 2.15 0.S0 0.62 0.68 2.58 0.78 0.80

P2Os <0.05 0.01 0.00 0.42 0.18 0.12 <0.05 0.08 <0.05 0.43 0.21 0.28 MnO 0.11 0.11 0.12 0.88 0.30 0.26 0.15 0.18 0.15 0.15 0.11 0.12

C02 0.12 0.17 0.05 0.11 LOF 0.46 2.92 0.16 1.79 0.30 0.75 2.15 0.70 Sum 99.3 98.9 99.1 100.1 101.0 99.5 99.7 99.1 99.3 99.9 99.8 100.6

Zr203 <0.005 <0.005 <0.005 <0.005 0.009 0.01 <0.005 0.002 <0.005 0.02 0.01 0.007

Cr203 0.42 0.36 0.43 0.01 0.018 0.17 0.058 0.17 0.13 0.01 0.011 0.004 NiO 0.39 0.24 0.23 0.01 0.008 0.01 0.045 0.18 0.032 0.008 0.007 0.003 BaO 0.003 <0.002 0.003 0.01 0.011 0.028 0.031 0.004 0.017 0.06 0.21 0.058 SrO <0.005 0.005 <0.005 0.01 0.022 <0.005 0.010 0.006 0.01 0.09 0.023 0.075

P-13 Monomineralic dunite G-26 Spinel-garnet granulite P-10 Two-pyroxene harzburgite F-3 Garnet granulite P-l Garnet-bearing lherzolite F-51 Low-alumina garnet granulite F-99 Grossular-clinopyroxene rock B-61 Feldspathic garnet granulite B-75 A Eclogite portion of a composite xenolith B-37 Quartz-rich feldspathic garnet granulite F-68 Eclogite L-2 Feldspathic granulite

Note-, major-element oxide determinations by rapid rock or a combination of X-ray fluorescence and chemical methods. Minor-element analyses by X-ray fluorescence, emission spectrographic, or instrumental neutron activation methods. Number of reported significant figures reflects analytical precision of method used. Analyses by U.S. Geological Survey, Branch of Geochemistry personnel.

ent in several of the granulites and is the pre- uous, prismatic sillimanite (to 2.5 mm long) is boundaries. Gabbroid and metagabbroid xeno- dominant mineral in a few amphibolite xeno- present in a single plagioclase-biotite-garnet liths are hornblende diorite, hornblende gabbro, liths. Several other constituents occur in minor gneiss xenolith. Although not common, pris- quartz-hornblende gabbro, pyroxene-hornblende (1-10 vol %) or trace (< 1%) amounts in the matic sillimanite has been reported elsewhere in gabbro, gabbro, and leucogabbro varieties; the metamorphic xenoliths. Minor quartz, calcite, metamorphic rocks of the Sierra Nevada same reheating features noted for the granitoid and apatite, and trace sphene are present in the (Dodge, 1971). xenoliths are present. In one composite xenolith grossular-clinopyroxene xenolith, minor or trace (sample B-75), hornblende-bearing metagabbro apatite and rutile are contained in almost all Calc-alkalic Xenoliths includes several centimetre-sized and smaller in- eclogites, and minor or trace orthopyroxene, clusions of eclogite. biotite, and plagioclase are found in some Xenoliths ranging compositionally from gab- eclogites and garnet granulites. Trace sphene bro to granite, with generally recognizable igne- Xenolith Chemistry and apatite are ubiquitous in feldspathic granu- ous textures, form the calc-alkalic group. There lites. Minor opaque minerals are generally pres- are textural gradations, particularly in more Chemical analyses have been determined on ent in all the metamorphic xenoliths, and mafic varieties, from gabbros through metagab- 42 Big Creek xenoliths, and 13 of these analyses narrow opaque to semiopaque alteration bands bros to the feldspathic granulites of the higher are given in Table 2. commonly rim garnet. As in the case of other grade metamorphic group, and in a few cases, Peridotite analyses (seven) show only minor xenolith groups, minor interstitial glass is present distinction between xenoliths of the two groups variation, reflecting differences in amount and in some samples. is equivocal. Felsic granitoid xenoliths are larger variety of pyroxene from dunite to harzburgite than mafic ones and despite their lesser number and lherzolite. The peridotites are highly magne- Lower Grade Metamorphic Xenoliths form the greatest volumetric abundance of the sian [at Mg/(Mg + Fe) = 0.88-0.92] and are entire Big Creek suite. The granitoid xenoliths within the type I (Frey and Prinz, 1978) or Cr- Fragments of schist and hornfels are rare in range from aplite and leucogranite to tonalite diopside (Wilshire and others, 1988) xenolith the Big Creek pipe. These fine-grained xenoliths and are generally typical of plutonic rocks ex- groups. are generally similar to pre-Cretaceous metased- posed at the surface in the general region (Bate- The high-grade metamorphic xenoliths are imentary rocks that occur in small masses asso- man and Wones, 1972; Lockwood and Bate- chemically heterogeneous, with silica contents ciated with granitic rocks exposed at the surface man, 1976) but with superimposed reheating ranging from 40% to 59% and atomic Mg/ (Mg in this region of the Sierra Nevada (Lockwood features; feldspars may be clouded or mottled + Fe) from 0.46 to 0.82. They are generally and Bateman, 1976). Plagioclase feldspar (ande- and concentric zoning is obliterated, biotite and olivine normative; only two of 25 analyzed sine) is the principal mineral of these rocks, and hornblende are partially replaced by microcrys- xenoliths are silica saturated (Fig. 6). other minerals, such as biotite, hornblende, py- talline aggregates of opaque oxide and anhy- The single grossular-clinopyroxene rock con- drous silicate minerals, and thin veinlets of roxene, quartz, apatite, and zircon, may be tains almost the same amount of Si02 as do present in varying amounts. Minor, but conspic- recrystallized glass penetrate fractures and grain Siberian (Sobolev and others, 1968) and South

TABLE 2. (Continued) an aluminosilicate phase in the Big Creek xeno- tend to be high in Si02, total alkalis, BaO, and

lith. Although aluminosilicates are lacking, the SrO but show less of an Al203 range and a Low-grade metamorphic xenoliths Metagabbro high CaO content and grossular-rich garnet wider range of Ti02 than do their feldspar-free B-60 B-72 B-77 B-75B show a close affinity with the grospydites. The counterparts. Non-garnet-bearing feldspathic Big Creek rock has about the same amount of granulites are chemically similar to garnet- 44.3 43.6 45.2 53.87 26.6 20.9 17.0 15.28 CaO and total alkalis as do diopside-rich rocks bearing varieties. 4.44 6.39 2.66 2.32 of Precambrian granulite terranes of the Brazil- 9.13 4.95 7.41 4.23 Whole-rock chemistry of the lower grade 3.80 3.90 9.26 5.5 ian basement (Sighinolfi and Fujimori, 1974) metamorphic xenoliths (four analyses) closely 3.% 6.19 11.9 11.20 but less Si02, nearly a third less MgO, and more 2.16 2.17 1.22 4.8 correlates with rock type. Sillimanite gneiss is 2.00 5.40 0.86 0.49 AI2O3 and total iron. Among the trace-element highest in Al203, biotite schist is high in Al203

oxides, the low SrO content of the grossular- and K20, and hornblende hornfels are lower in 1.63 1.64 1.27 2.09 clinopyroxene rock relative to its CaO content is these oxides but high in CaO and MgO. These <0.05 1.91 0.10 0.17 0.31 0.19 0.15 0.56 noteworthy. analyzed xenoliths are olivine normative, and the gneiss and schist are both alumina saturated, 1.17 0.63 1.08 0.00 Eclogite xenoliths (4 analyses) are low in 99.5 97.9 98.1 100.5 Si02 (40.9%-46.5%), high in total Fe [FeO* = whereas hornfels are not (Fig. 6). 10.99-18.97; Mg/(Mg + Fe) = 0.6-0.7] and 0.05 0.13 <0.005 0.013 0.03 0.02 0.054 0.005 Ti02 (2.11-3.17), and low in Cr203 (0.018- Mineral Chemistry 0.01 0.06 0.007 0.004 0.17) and NiO (0.006-0.01). The composition 0.09 0.225 0.035 0.022 0.01 0.045 0.05 0.092 of these xenoliths is strikingly similar to that of Chemical compositions of ferromagnesian eclogite xenoliths found in latites from Chino minerals were determined by electron micro- B-60 Sillimanite gneiss B-72 Biotite schist Valley, Arizona (Schulze and Helmstaedt, probe, and most analyses in Table 3 are repre- B-77 Hornblende hornfels 1979). sentative of several spots on different grains B-75B Pyroxene metagabbro portion of a composite xenolith Garnet granulites (12 analyses), although within a single xenolith. petrographically seemingly transitional with eclo- Olivines in peridotite nodules show a narrow

gites, are chemically distinct. Silica contents of range of compositions (Fog9-Fo92), with oli- feldspar-free garnet granulites are low (42.2%- vines from dunites the most magnesian, those 50.6%), similar to those of eclogites, but total Fe from harzburgites somewhat less, and those tends to be lower than in eclogites [FeO* = from lherzolites tending to be the least magne- 8.99%—15.70%; atomic Mg/(Mg + Fe) =0.7- sian. Consistent with their high Fo values, Ni

African (Lappin, 1978) grospydites (grossular- 0.8] and Ti02 is commonly much lower contents are high compared with those of oli- clinopyroxene-kyanite eclogites from kimber- (0.34%-0.69%) except in an amphibole-rich vines from peridotite nodules at Chinese Peak lites) but has nearly twice as much CaO, more granulite (2.11%). Trace-element contents are (Dodge and others, 1986), a few miles to the total iron, much less total alkalis, and one-third generally similar in both groups of xenoliths. east of Big Creek, or from olivines in peridotite the amount of AI2O3, reflected by the absence of Feldspathic garnet granulites (four analyses) masses from the Emigrant Gap area of the northern Sierra Nevada (James, 1971). Coexisting Ca-rich and Ca-poor pyroxenes Corundum 20 from peridotites show narrow compositional ranges, as do the olivines, with the most magne- 0 sian from harzburgites. The high Mg/(Mg + Fe) ratio (0.89-0.95) of the pyroxenes and the 40 O 20 40 ranges of their Cr 0 , Al 03, Ti0 , and FeO* olivine I— +- Quartz 2 3 2 2 X Sierra Nevada granites contents confirm placement of the peridotites in o — the Cr-diopside group or with group I inclusions. Orthopyroxenes in two garnet-free perido- 20 tite xenoliths contain unusually small amounts of A1203 (0.12 and 0.05 wt %), but coexisting X* EXPLANATION clinopyroxenes do not; the two orthopyroxenes H have commensurate high values of Si0 Xenoliths 2 (58 wt %). --40 X Calc-alkalic igneous The high-grade metamorphic xenoliths con- • High-grade metamorphic tain clinopyroxenes with a broad range of com- O Low-grade metamorphic positions (Fig. 7). Atomic Mg/(Mg + Fe) of 60 these clinopyroxenes ranges from 0.65 to 0.91, diopside except for iron-rich pyroxenes of a quartz- bearing granulite (B-37) and the grossular-clino- Figure 6. Plot of mutually exclusive normative constituents of Big Creek xenoliths, showing pyroxene rock (F-99), both with Mg/(Mg + Fe) silica and alumina undersaturation. X's are calc-alkalic igneous xenoliths; filled circles, high- of 0.44. Big Creek eclogites are separated from grade metamorphic xenoliths; open circles, low-grade metamorphic xenoliths. Peridotite xeno- garnet granulites primarily by presence of liths (not shown) contain more than 55% normative olivine generally with small amounts emerald-green pyroxene, although there is a (< 15%) of normative diopside. Field of Sierra Nevada granites includes more than 90% of 499 complete transition of color from deeply colored CIPW norms of Sierra Nevada plutonic rocks from Bateman and others (1984). to faintly colored to colorless clinopyroxene

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TABLE 3. REPRESENTATIVE ANALYSES (WEIGHT PERCENT) OF MINERALS IN BIG CREEK XENOLITHS

Sample no. Olivine Orthopyroxene

H-l P-13 P-2 P-6 P-l P-6 P-l F-68 F-51 C-39

Si02 41.1 41.1 41.0 40.7 40.5 55.1 55.2 53.3 54.4 50.6

A1203 4.22 4.17 1.68 2.05 1.75 FeO 7.70 7.65 8.39 9.44 11.0 6.39 7.32 16.9 13.0 26.6 MgO 50.8 50.8 50.1 48.9 47.9 33.5 32.7 27.6 30.0 18.7 CaO 0.00 0.01 0.01 0.00 0.01 0.33 0.51 0.42 0.38 0.77

Na20 0.02 0.04 0.05 0.02

Ti02 0.10 0.05 0.07 0.05 0.10 MnO 0.12 0.10 0.12 0.12 0.14 0.15 0.19 0.21 0.13 1.61

Cr203 0.36 0.42 0.07 0.10 0.01 NiO 0.37 0.39 0.41 0.39 0.32 Sum 100.1 100.1 100.0 99.6 99.9 100.2 100.6 100.3 100.2 100.2

at Mg/(Mg + Fe) 0.92 0.92 0.91 0.90 0.89 0.90 0.89 0.74 0.80 0.56

Sample no. Clinopyroxene

P-6 P-l F-99 B-76 B-75A F-68 G-26 F-3 F-51 B-37

Si02 53.6 52.9 51.8 52.0 51.0 53.1 53.3 54.1 53.7 50.4

A1203 4.17 2.96 2.13 4.40 5.83 3.62 2.74 4.% 2.16 3.62 FeO 2.13 2.20 16.3 9.78 9.94 7.42 2.75 3.82 4.25 16.7 MgO 15.5 16.8 7.22 11.9 10.8 13.3 16.5 13.1 15.6 7.12 CaO 21.6 23.8 19.6 19.8 19.6 20.1 23.9 21.4 22.4 22.1

Na20 1.67 0.58 2.60 1.98 1.97 2.29 0.28 2.55 1.32 0.71

Ti02 0.53 0.30 0.02 0.53 0.67 0.28 0.36 0.21 0.16 0.36 MnO 0.08 0.09 0.30 0.10 0.09 0.10 0.05 0.03 0.05 0.12

Cr203 0.64 0.54 0.00 0.05 0.02 0.17 0.04 0.12 0.23 0.05 Sum 99.9 100.2 100.0 100.5 99.9 100.4 99.9 100.3 99.9 101.2

at Mg/(Mg + Fe) 0.93 0.93 0.44 0.68 0.66 0.76 0.91 0.86 0.87 0.44

Sample no. Garnet

P-6 F-99 B-76 B-75A F-68 G-26 F-3 F-51 B-37 B-60

Si02 41.6 37.5 38.7 39.3 39.4 40.6 39.7 40.4 37.1 38.0

AI2O3 22.3 15.1 21.1 21.3 21.1 22.5 21.4 22.1 19.3 20.7 FeO 10.7 11.4 23.1 24.5 21.3 12.5 16.5 17.3 21.1 34.1 MgO 18.9 0.28 8.87 8.32 11.2 16.8 12.3 14.0 0.86 5.19 CaO 4.67 33.3 7.01 6.47 5.72 6.80 8.72 5.39 17.1 1.23

Ti02 0.08 0.84 0.13 0.17 0.15 0.17 0.17 0.15 0.18 0.02 MnO 0.48 1.22 0.42 0.58 0.50 0.28 0.30 0.46 2.08 1.22

Cr203 0.80 0.07 0.03 0.00 0.27 0.09 0.18 0.15 0.05 0.05 Sum 99.5 99.7 99.4 100.6 99.6 99.7 99.3 100.0 97.8 100.5

at Mg/(Mg + Fe) 0.76 0.04 0.41 0.38 0.48 0.71 0.57 0.59 0.07 0.21

Sample no. Spinel Rutile Ilmenite Magnetite

P-13 P-6* P-6t G-26 B-75A F-49 G-26 F-49§ F-49** F-49

Si02 0.00 0.00 0.09 0.00 0.00 0.00 0.11 0.00 0.02 1.27

AI2O3 11.7 47.3 61.1 63.0 0.05 0.01 0.17 0.00 0.06 0.66 FeO 25.8 15.1 13.5 15.1 0.36 0.58 40.7 43.2 37.8 81.2 MgO 8.98 17.9 20.0 20.1 0.02 0.03 7.18 3.18 6.87 0.95 CaO 0.00 0.00 0.01 0.01 0.01 0.02 0.09 0.02 0.06 0.15

Ti02 0.20 0.10 0.18 0.03 99.3 99.1 50.5 50.9 52.8 5.26 MnO 0.42 0.15 0.20 0.03 0.00 0.00 0.40 0.16 0.85 0.10

Cr203 51.7 19.1 4.56 0.33 0.02 0.08 0.01 0.03 0.46 0.09 NiO 0.10 0.32 0.10 0.44 0.05 0.03 0.11 1.01 0.05 2.38 v2o3 0.15 0.15 0.06 0.11 0.43 0.31 0.37 0.14 0.39 0.20 Sum 99.1 100.1 99.8 99.2 100.2 100.2 99.6 98.6 99.4 92.3

at Mg/(Mg + Fe) 0.38 0.68 0.73 0.70 at Cr/(Cr + Al) 0.75 0.21 0.05

Note: Peridotites: H-l, P-13, P-2, P6, P-l. "Primary brown spinel. Grossular-clinopyroxene rock: F-99. t Secondary green spinel. Eclogites: B-76, B-75A, F-68. ^Discrete grain. Granulites: G-26, F-3, F-51, F^I9, B-37. "Rim on rutile. Gneiss: B-60. Metanorite: C-39.

through the eclogite and granulite suites. Close 20%. Jadeite is generally the major nonquadri- Mg/(Mg + Fe) of 0.74, slightly less than coexist- correlation between composition and color in- lateral component (Fig. 8); acmite is a major ing clinopyroxene, and three of four orthopy- tensity is not apparent. Eclogite clinopyroxenes, component only in pyroxene of the grossular- roxenes from granulites have Mg/(Mg + Fe) however, commonly have greater than 20% pyroxene rock. Orthopyroxenes are rare in the greater than coexisting clinopyroxenes. As ex- nonquadrilateral (Wo + En + Fs) components, high-grade metamorphic xenoliths. A single or- pected, all the orthopyroxenes contain less than whereas granulite clinopyroxenes have less than thopyroxene from an eclogite (F-68) has 10% nonquadrilateral components.

Lherzolite contains the most Mg rich of the TABLE 4. MINERAL GEOTHERMOMETRY IN DEGREES CELSIUS FOR BIG CREEK XENOLITHS analyzed Big Creek garnets and plots in the Peridotites Wells Lindsley Ellis-Green Granulites Wells Lindsley Ellis-Green group A, or the ultramafic, field on the garnet (cpx-opx) (opx) (cpx-gnt) (cpx-opx) (opx) (cpx-gnt) compositional diagram (Fig. 9) of Coleman and P-2 880 825 G-26 800 others (1965). Compositions of garnets from D-8 780 775 C-12 905 granulites and eclogites overlap; grossular con- P-6 840 — 730 E-l 755 M-Ì6 760 775 F-3 755 tents generally range between 10% and 20%, P-l 810 — F-49 860 — 975 F-2 800 700 685 whereas there is a wide range of Mg/(Mg + Fe), E-ll 750 with only eclogites at the magnesian-rich end of B-3 880 850 870 B-37 860 this range and both granulites and eclogites at the iron-rich end, as would be predicted from Eclogites Amphibolite whole-rock compositions. These garnets form an elongate field spanning the group B, or migma- B-76 850 C-26 825 B-75A 880 titic gneiss, area and extending into the group C E-12 760 E-14 675 or blueschist area of the garnet compositional F-68 800 700 695 diagram. Garnets from lower crustal granulite and eclogite xenoliths from Lesotho, southern Note: blanks indicate appropriate pairs not present; dashes indicate composition not suitable for temperature estimate. Lindsley and Ellis-Green temperature estimates assume 15 kb pressure. Africa (Griffin and others, 1979), define a nearly identical field. A garnet with nearly equal amounts of grossularite and almandine, but only 3.5% pyrope, occurs in quartz-bearing granulite Mineral Thermobarometry necessary coexisting phases for rigorous pressure (B-37), and garnet of the grossular-clinopyrox- estimation yield poor agreement between var- ene rock (F-99) is Ca rich (>90% grossular + Temperatures determined from mineral com- ious geobarometers. andradite). A sillimanite gneiss xenolith (B-60) positions using the Wells (1977) two-pyroxene, contains Ca-poor, but Fe-rich, garnet. the orthopyroxene portion of the Lindsley DISCUSSION The dark brown to nearly opaque spinels in (1983) two-pyroxene, and the Ellis and Green peridotite xenoliths have a wide range of com- (1979) clinopyroxene-garnet geothermometers Whole-rock and mineral chemistry of Big positions. Chromites with intermediate Mg/ are given in Table 4. Compositions of all clino- Creek peridodite xenoliths places them with the (Mg + Fe) occur in dunites, whereas more Fe- pyroxenes and some orthopyroxenes are not Cr-diopside (Wilshire and Shervais, 1973) or rich Al-spinels occur in lherzolites. Al-rich green suitable for application of the Lindsley ther- group I inclusions (Frey and Prinz, 1978) of spinel appears to be an alteration product of mometer. Temperatures all are within the range likely mantle derivation. The top of the mantle is primary brown spinel in one lherzolite. Al- 675-975 °C, and there are no consistent dif- approximately 50 km beneath the Big Creek lo- though generally not present in the high-grade ferences between those derived by different cality as defined by the seismically determined metamorphic xenoliths, green, Mg-rich, Cr-poor thermometers. M-discontinuity (Bateman and Eaton, 1967). spinel is conspicuous in one of the most mafic If the Wells temperature estimates and either The rare sporadic occurrence of likely metasta- garnet granulites. Mg-rich ilmenite also occurs in of two recently proposed garnet-orthopyroxene ble garnet, low mineral equilibration tempera- the mafic granulite xenolith. Rutile, nearly pure geobarometers (Harley and Green, 1982; Har- tures (730-880 °C), and the 11.5 kb (38 km) TiOj, is commonly rimmed with Mg-rich ilmen- ley, 1984) are used, pressures of 11.5 and 16 kb pressure estimate on a garnet-bearing lherzolite ite, whereas coexisting discrete ilmenite grains are calculated on a lherzolite (sample P-6) thus suggest possible temperature-pressure re- are less Mg rich. Magnetite in a single granulite and an eclogite (sample F-68), respectively. equilibration of peridotites during a period of xenolith surprisingly contains appreciable NiO. The relatively few other xenoliths containing lower crustal residency. A similar situation has

CaMgSi O xCaFeSi206 tt

Figure 7. Composition of pyroxenes in Big Creek xenoliths. Tie lines con- nect coexisting pyroxene EXPLANATION

pairs. a Granulite, amphibolite

• Eclogite

+ Grossular-clinopyroxene rock

• Peridotite

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oceanic crust, beneath the craton margin by subduction. Similar processes may account

EXPLANATION for production of the Big Creek high-grade

• ti assemblage. E!*" Eclogites have been previously reported in H NAM2 North America as members of xenolith assem- blages only at two Arizona localities in volcanic rocks at Chino Valley (Krieger, 1965) and Camp Creek (Esperanca and Holloway, 1984) but at several localities in kimberlite (for example, Ater and others, 1984; McGee and Hearn, 1983; Meyer and Brookins, 1971; Watson, 1967). The Chino Valley eclogites have been interpreted as plutonic rocks that were part of an upper mantle-lower crustal sequence with ac- companying pyroxenites (Arculus and Smith, 1979) or alternatively as metamorphic rocks in- GRS- truded at depth by igneous pyroxenites (Schulze GRANULITES | ECLOGITES CPX NONQUADRILATERAL PYROXENE COMPONENTS ROCK and Helmstaedt, 1979). Largely on the basis of garnet compositions, Helmstaedt and Doig Figure 8. Content of subordinate (nonquadrilateral) components in granulite and eclogite (1975) postulated that the eclogites that occur as pyroxenes. T1 is Ti+4 in the Ml site; AL4, tetrahedral Al; NAM2, Na in the M2 site, generally xenoliths in diatremes of the eastern Colorado jadeite component. plateau are derived from Franciscan-type oceanic material in a subducted slab modified in a subarc environment, a suggestion later reiterated been noted in western Europe, where peridotite suggest ultimate derivation of these rocks from a by Ater and others (1984) for other eclogite masses are interleaved with enclosing gneisses, variety of protoliths—but with a common re- xenoliths in Colorado-Wyoming kimberlites. and the interleaving of these masses is postulated crystallization history. The small-scale inho- These garnets plot in the field of garnets from to have occurred before equilibration during mogeneity of some individual xenoliths (Fig. 7) eclogites within glaucophane schists, or group C plate collision processes (Carswell and Gibb, is doubtless also manifested on a larger scale; (Coleman and others, 1965). On the other hand, 1980). Alternatively, peridotites may have any simple layering model for the Sierra Nevada other North American eclogite garnets, those simply risen into the lower crust as diapirs. Di- lithosphere would undoubtedly mask great from Lesotho, and those from Big Creek all tend apirism, however, is generally considered to be complexities. Griffin and others (1979) simi- to plot within the field of garnets of group B due to a density contrast, and some other cause larly interpreted the mafic lower crustal xenolith eclogites from migmatitic gneiss terrains and would be required to move peridotites into the suites from Lesotho as fragments of a polyge- may simply reflect equilibration of the same crust (Wilshire and Pike, 1975). netic granulite complex which has suffered a pro- type of materials as the Colorado Plateau eclogites at somewhat higher metamorphic tempera- High-grade metamorphic xenoliths are collec- longed history of metamorphism and anatexis. tures. Chemically, the Big Creek eclogites are tively mafic, subaluminous rocks. Mineral chem- They envisaged three mechanisms possibly broadly similar to Franciscan eclogites—corre- istries of rock types commonly overlap, and responsible for formation of granulite-facies spondence in Ti02 contents is particularly whole-rock chemical differences are generally rocks and for the accumulation of mafic rocks in striking—a slight loss of silica and soda and in- small but are of fundamental significance and the deep crust, (1) anatexis and subsequent re- moval of the more felsic components, (2) crease of total iron oxide and magnesia may be attributable to loss of felsic material as a small Aim + Sps intrusion of mafic magmas into the lower crust EXPLANATION during and after orogenic episodes, and (3) lat- partial-melt phase. Temperatures of equilibra- eral emplacement of mafic material, possibly tion of 5 Big Creek eclogites range from 675 to x Sillimanite gneiss 880 °C, calculated using the Ellis and Green a Granulite, amphibolite (1979) geothermometer based on compositions • Eclogite of coexisting garnet and clinopyroxene assuming + Grossular-clino- 15 kb pressure (Table 4), and as previously pyroxene rock noted, one of the eclogite xenoliths yields a pres- • Peridotite sure of 16 kb, corresponding approximately to a depth of 50 km.

Figure 9. Molecular proportions of almandine + spessartine, grossular + andradite, and pyrope of garnets in Big Creek xenoliths. Compo- sitional fields for garnets in eclogites and related rocks from Coleman and others (1965); field A, garnets from ultramafic inclusions in kimberlites and basalts or from layers in ultramafic rocks; field B, bands or lenses within migmatitic gneissic terrains; field C, bands or lenses within alpine-type metamorphic rocks and blueschists. Lesotho field Grs + Adr from Griffin and others (1979).

The grossular-clinopyroxene rock from Big ACKNOWLEDGMENTS Harley, S. L., 1984, The solubility of alumina in orthopyroxene coexisting with garnet in Fe0-Mg0-Al203-Si02: Journal of Petrology, v. 25, Creek is unlike grospydite xenoliths found else- p. 665-6%. Harley, S. L., and Green, D. H., 1982, Garnet-orthopyroxene barometry for where in that it is not peraluminous and does This study benefited greatly from discussions granulites and garnet peridotites: Nature, v. 300, p. 697-700. Harte, B., 1977, Rock nomenclature with particular relation to deformation not contain an aluminosilicate phase. The with, and suggestions by, R. G. Coleman, R. A. and recrystallization textures in olivine-bearing xenoliths: Journal of general chemical composition and similarity of Loney, B. A. Morgan, and R. W. Kistler, who Geology, v. 85, p. 279-288. Helmstaedt, Herwart, and Doig, Ronald, 1975, Eclogite nodules from kimber- this xenolith to some Brazilian granulites suggest also kindly provided us with isotopic ages. P. C. lite pipes of the Colorado Plateau—Samples of subducted Franciscan- type oceanic lithosphere: Physics and Chemistry of the Earth, v. 9, derivation from carbonate sediment. The Bateman and L.J.P. Muffler offered continual p. 95-111. 0.70661 87Sr/86Sr value of the xenolith is less encouragement. Reviews by B. C. Hearn, J. G. James, O. B., 1971, Origin and emplacement of the ultramafic rocks of the Emigrant Gap area, California: Journal of Petrology, v. 12, p. 523-560. than that known for any Phanerozoic marine Moore, Douglas Smith, and H. G. Wilshire Krieger, M. H., 1965, Geology of the Prescott and Paulden quadrangles, Arizona: U.S. Geological Survey Professional Paper 467, 127 p. carbonate but is suggestive that the protolith was improved content and presentation of the Lappin, M. A., 1978, The evolution of a grospydite from the Roberts Victor Precambrian (Domenick and others, 1983). manuscript. Mine, South Africa: Contributions to Mineralogy and Petrology, v. 66, p. 229-241. Lindsley, D. H., 1983, Pyroxene thermometry: American Mineralogist, v. 68, REFERENCES CITED p. 477-493. CONCLUSIONS Lockwood, J. P., and Bateman, P. C., 1976, Geologic map of the Shaver Lake Arculus, R. J., and Smith, Douglas, 1979, Eclogite, pyroxenite and amphibolite quadrangle, central Sierra Nevada, California: U.S. Geological Survey inclusions in the Sullivan Buttes Latite, Chino Valley, Yavapai County, Geologic Quadrangle Map GQ-1271, scale 1:62,500. On the basis of heat flow and seismic evi- Arizona, in Boyd, F. R., and Meyer, H.O.A., eds., The mantle sample: MacGregor, I. D., and Manton, W. I., 1986, Roberts Victor eclogites: Ancient Inclusions in kimberlites and other volcanics: American Geophysical oceanic crust: Journal of Geophysical Research, v. 91, p. 14063-14079. dence, several workers have postulated the ex- Union, International Kimberlite Conference, 2nd, Proceedings, v. 2, Mavko, B. B., and Thompson, G. A., 1983, Crustal and upper mantle structure istence or former existence of a subducted slab p. 309-317. of the northern and central Sierra Nevada: Journal of Geophysical Ater, P. C„ Eggler, D. H., and McCallum, M.E., 1984, Petrology and geochem- Research, v. 88, p. 5874-5892. of cold, high-density lithosphere beneath the istry of mantle eclogite xenoliths from Colorado-Wyoming kimberlites: McGee, E. S., and Hearn B. C., Jr., 1983, Lake Ellen kimberlite, Michigan, Recycled oceanic crust?, in Kornprobst, J., ed., Kimberlites II: The U.S.A.: U.S. Geological Survey Open-File Report 83-156,22 p. Sierra Nevada (Roy and others, 1972; Solomon mantle and crust-mantle relationships: Amsterdam, the Netherlands, Merrier, J. C., and Nicolas, A., 1975, Textures and fabrics of upper-mantle Elsevier, p. 309-318. peridotites as illustrated by xenoliths from basalts: Journal of Petrology, and Butler, 1974; Crough and Thompson, 1977; Basu, A. R, Ongley, J. S„ and MacGregor, I. D„ 1986, Eclogites, v. 16, p. 454-487. Mavko and Thompson, 1983). According to pyroxene geotherm, and layered mantle convection: Science, v. 233, Meyer, H.O.A., and Brookins, D. G, 1971, Eclogite xenoliths from Stockdale p. 1303-1305. kimberlite, Kansas: Contributions to Mineralogy and Petrology, v. 34, Dickinson and Snyder (1979), this slab finished Bateman, P. C., 1983, A summary of critical relations in the central part of the p. 60-72. passing under the central Sierra Nevada between Sierra Nevada batholith, California, U.S.A., in Roddick, J. A., Circum- Moore, J. G., and Dodge, F.C.W., 1980, Late Cenozoic volcanic rocks of the Pacific plutonic terranes: Geological Society of America Memoir 159, southern Sierra Nevada, California: I. Geology and petrology: Geologi- 5 and 10 m.y. ago, probably at about the time of p. 241-254. cal Society of America Bulletin, Part I, p. 515-518: Part II, v. 91, Bateman, P. C,, and Eaton, J. P., 1967, Sierra Nevada batholith: Science, p. 1995-2038. eruption of the Big Creek pipe, and possibly was v. 158, p. 1407-1417. Oliver, H. W-, 1977, Gravity and magnetic investigations of the Sierra Nevada followed by diapiric rise of mantle-derived Bateman, P. C., and Wones, D. R., 1972, Geologic map of the Huntington batholith, California: Geological Society of America Bulletin, v. 88, Lake quadrangle, central Sierra Nevada, California: U.S. Geological p. 445^161. material into the Sierra Nevada lower crust. The Survey Geologic Quadrangle Map GQ-987, scale 1:62,500. Peselnick, L., Lockwood, J. P., and Stewart, R., 1977, Anisotropic seismic Bateman, P. C„ Dodge, F.C.W., and Bruggman, P. E„ 1984, Major oxide velocities of some upper mantle xenoliths underlying the Sierra Nevada timing of northward migration of the trailing analyses, CIPW norms, modes, and bulk specific gravities of plutonic batholith: Journal of Geophysical Research, v. 82, p. 2005-2010. rocks from the Mariposa 1° * 2° sheet, central Sierra Nevada, Califor- Pike, J.E.N., and Schwarzman, E. C., 1977, Classification of textures in ultra- edge of the slab may explain the absence of nia: U.S. Geological Survey Open-File Report 84-162, 50 p. mafic xenoliths: Journal of Geology, v. 85, p. 49-61. eclogites in other younger, xenolith-bearing vol- Boyd, F. R., 1973, A pyroxene geotherm: Geochimica et Cosmochimica Acta, Roy, R. B, Blackwell, D. D„ and Decker, E. R„ 1972, Continental heat flow, v. 37, p. 2533-2546. in Robertson, E. C., ed., The nature of the solid earth: New York, canic plugs, pipes, and flow remnants in the Brooks, P. K„ Dodge, F.C.W., Kistler, R. W, and Lockwood, J. P., 1980, McGraw-Hill, p. 506-543. Sierra Nevada (Dodge, 1988). In any case, the Inclusions from late Cenozoic volcanic rocks, Sierra Nevada batholith Schulze, D. H., and Helmstaedt, Herwart, 1979, Garnet pyroxenite and eclogite and adjacent areas, California: Geological Society of America Abstracts xenoliths from the Sullivan Buttes Latite, Chino Valley, Arizona, in eclogite and some associated xenoliths could with Programs, v. 12, p. 99. Boyd, F. R., and Meyer, H.O.A., eds., The mantle sample: Inclusions in Carswell, D. A., and Gibb, F.G.F., 1980, The equilibration conditions and kimberlites and other volcanics: American Geophysical Union, Interna- have been derived from this slab. The grossular- petrogenesis of European crustal garnet Iherzolites: Lithos, v. 13, tional Kimberlite Conference, 2nd, Proceedings, v. 2, p. 318-329. clinopyroxene xenolith may represent a remnant p. 19-29. Sighinolfi, G. P., and Fujimori, Shiguemi, 1974, Petrology and chemistry of Coleman, R. G„ Lee, D. E„ Beatty, L. B„ and Brannock, W. W„ 1965, diopsidic rocks in granulite terrains from the Brazilian basement: Atti of Precambrian limestone torn from the upper Eclogites and eclogites: Their differences and similarities: Geological della Societa, Toscana di Scienze Naturali, Memories, ser. A, v. 81, Society of America Bulletin, v. 76, p. 483-508. p. 103-120. plate during active arc subduction. Analogous Crough, S, T., and Thompson, G. A., 1977, Upper mantle origin of Sierra Sobolev, N. V., Jr., Kuznetsova, I. K, and Zyuzin, N. I., 1968, The petrology of situations may occur elsewhere beneath conti- Nevada uplift: Geology, v. 5, p. 396-399. grospydite xenoliths from the Zagadochnaya kimberlite pipe in Yakutia: Dickinson, W. R., and Snyder, W. S., 1979, Geometry of subducted slabs Journal of Petrology, v. 9, p. 253-280. nental crust near active plate boundaries. Eclo- related to San Andreas transform: Journal of Geology, v. 87, Solomon, S. C-, and Butler, R. H., 1974, Prospecting for dead slabs: Earth and p. 609-627. Planetary Sciena Letters, v. 21, p. 421-430. gite xenoliths from the Robert Victor kimberlite Dodge, F.C.W., 1971, A^SiO, minerals in rocks of the Sierra Nevada Stern, T. W, Bateman, P. C„ Moigan, B. A., Newell, M. F„ and Peck, D. L, in South Africa have also recently been postu- and Inyo Mountains, California: American Mineralogist, v. 56, 1981, Isotopic U-Pb ages of zircon from the granitoids of the central p. 1443-1451. Sierra Nevada, California: U.S. Geological Survey Professional Paper lated to represent subducted oceanic crust and 1988, Nature of the lower crust and upper mantle beneath the Sierra 1185,17 p. Nevada batholith: Evidence from xenoliths in late Cenozoic volcanic Watson, K. D., 1967, Kimberlite pipes of north-eastern Arizona, in Wyllie, form the "petrologic" base of continental litho- rocks, in Noller, J. F., Kirby, S. H., and Nielson-Pike, J. E-, eds., P. J., ed., Ultramafic and related rocks: New York, John Wiley, sphere (MacGregor and Manton, 1986). The Geophysics and petrology of the deep crust and upper mantle: U.S. p. 261-269. Geological Survey Circular (in press). Wells, P.R.A., 1977, Pyroxene thermometry in simple and complex systems: field of equilibration temperatures of eclogite Dodge, F.C.W., Calk, L. C., and Kistler, R. W, 1986, Lower crustal xenoliths, Contributions to Mineralogy and Petrology, v. 62, p. 129-139. Chinese Peak lava flow, central Sierra Nevada: Journal of Petrology, Wilshire, H. G., and Pike, J.E.N., 1975, Upper-mantle diapirism: Evidence xenoliths falls essentially at the point of inflec- v. 27, p, 1277-1304. from analogous features in alpine peridotite and ultramafic inclusions in Domenick, M. A., Kistler, R. W„ Dodge, F.C.W., and Tatsumoto, M, 1983, basalt: Geology, v. 3, p. 467-470. tion of a steep gradient or high-heat-flow lower Nd and Sr isotopic study of crustal and mantle inclusions from the Wilshire, H. G„ and Shervais, J. W„ 1975, Al-augite and Cr-diopside ultra- limb of the Lesotho inflected geotherm (Boyd, Sierra Nevada and implications for batholith petrogenesis: Geological mafic xenoliths in basaltic rocks from western United States: Physics Society of America Bulletin, v. 94, p. 713-719. and Chemistry of the Earth, v. 9, p. 257-272. 1973) underlying a shallow gradient or low- Ellis, D. J., and Green, D. H., 1979, An experimental study of the effect of Ca Wilshire, H. G., Meyer, C. E„ Nakata, J. K„ Calk, L. C, Shervais, J. W„ upon gamet-clinopyroxene Fe-Mg exchange equilibria: Contributions Nielson, J. E., and Schwarzman, E. C., 1988, Mafic and ultramafic heat-flow upper limb of the geotherm (Basu and to Mineralogy and Petrology, v. 71, p. 13-22. xenoliths from volcanic rocks of the western United States: U.S. Geolog- others, 1986). Similarly, a cool eclogite-bearing Esperanca, S., and Holloway, J. T., 1984, Lower crustal nodules from the ical Survey Professional Paper 1443 (in press). Camp Creek Latite, Carefree, Arizona, in Kornprobst, J., ed., Kimber- Zanettin, B., 1984, Proposed new chemical classification of volcanic rocks: slab beneath the Sierra Nevada may have pro- lites II: The mantle and crust-mantle relationships: Amsterdam, the Episodes, v. 7, p. 19-20. Netherlands, Elsevier, p. 219-227. vided a convective boundary beneath a conduc- Frey, F. A., and Prinz, M., 1978, Ultramafic inclusions from San Carlos, Arizona: Petrologic and geochemical data bearing on their petrogenesis: tive lid of continental crust, inhibiting heat flow Earth and Planetary Science Letters, v. 38, p. 129-176. from deeper in the mantle and thus accounting Griffin, W. L., Carswell, D. A., and Nixon, P. H., 1979, Lower-crustal granulites and eclogites from Lesotho, southern Africa, in Boyd, F. R., and for the present-day enigmatic low heat flow of Meyer, H.O.A., eds., The mantle sample: Inclusions in kimberlites and other volcanics: American Geophysical Union, International Kimberlite MANUSCRIPT RECEIVED BY THE SOCIETY JUNE 22,1987 the western Sierra Nevada. Conference, 2nd, Proceedings, v. 2, p. 59-86. REVISED MANUSCRIPT RECEIVED NOVEMBER 20, 1987 MANUSCRIPT ACCEPTED DECEMBER 3,1987

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