The Smartville intrusive complex, Sierra Nevada, California: The core of a rifted volcanic arc

HOWARD W^DAY } department of Geology, University of California, Davis, California 95616

ABSTRACT the intrusion of the sheeted dike complex. western and southern Smartville complex Clasts of plutonic and hypabyssal rocks re- were deformed prior to the intrusion of the The Smartville complex is a Jurassic vol- sembling the younger intrusives, however, dike complex. canic and plutonic arc in the northwestern occur locally in some of the youngest volcani- Sierra Nevada that was deformed during the clastic rocks, suggesting that shallow pluton- INTRODUCTION Late Jurassic Nevadan "orogeny." We inter- ism and volcanism could be broadly coeval. pret the Smartville intrusive complex to have Early Nevadan thrust faults juxtapose vol- The Smartville complex is a Late Jurassic se- formed during the incipient rifting of an ac- canic and older plutonic rocks of the Smart- quence of volcanic, hypabyssal, and plutonic tive volcanic arc. This interpretation is sup- ville complex and Mesozoic(?) chert-argillite rocks that forms the westernmost of the four ported by the close relationship between broken formation to the east. The younger main fault-bounded lithotectonic belts of the volcanism and plutonism and by the close as- plutons and the dike complex in the Smart- northern Sierra Nevada (Fig. 1). The relation- sociation between sheeted dikes and the ville are deformed by steep, late Nevadan ship of these belts to each other and the tectonic younger plutonic rocks. Further support is faults and do not intrude chert-argillite evolution of the northern Sierra Nevada has found in the similarities that exist between the broken formation. The youngest granodiorite been controversial (Moores, 1970, 1972; Smartville complex and modern arcs that plutons in the area truncate Nevadan faults, Schweickert and Cowan, 1975; Davis and oth- developed on oceanic crust, either at a con- intrude the chert-argillite formation, and ap- ers, 1978; Saleeby, 1981; Schweickert, 1981; tinental margin or in the ocean basins. pear to be unrelated to the Smartville Day and others, 1985). Two schools of thought Smartville volcanic rocks consist of a lower complex. have emerged to explain the distribution of unit of tholeiitic submarine flows and pil- The Smartville volcanic arc underwent Middle to Upper Jurassic volcanic rocks that lowed flows that grades upward into an pre-Nevadan intra-arc extension. The sheeted occur in the western, central, and eastern belts of upper unit of calc-alkaline pyroclastic and dike complex is the primary manifestation of the Sierra Nevada and in parts of the Klamath volcaniclastic deposits. Intrusive rocks in- the rifting event. The elongate shapes of the Mountains. Some workers (Davis and others, clude older units of metamorphosed gabbro younger plutons, which are coeval with the 1978; Burchfiel and Davis, 1981; Saleeby, 1981, and massive diabase, which are intruded by a dike complex, reflect extensional control on 1982; Harper and Wright, 1984) have inter- unit of 100% mafic and felsic, sheeted and their emplacement. Volcanic rocks in the preted the arc rocks in these areas as the prod- unsheeted dikes. Biotite-hornblende tonalite and granophyric hornblende tonalite plutons are coeval with the dike complex, and both rock types occur locally as dikes within the 120° LEGEND dike unit. Continuously and reversely zoned 111! EASTERN BELT (FBI gabbro-diorite plutons are also coeval with the dike complex. Granodiorite plutons are FEATHER RIVER CFB) PERIDOTITE BELT the youngest intrusive unit and may be re- lated to the Sierra Nevada batholith. :: | CENTRAL BELT CCD]

Relative age relations suggest that the BRB BALD ROCK BATHOLITH Smartville complex formed in a single volcanic-plutonic system of pre-Nevadan age. SNB SIERRA NEVADA BATHOLITH The dike complex intrudes and is intruded by YRP YUBA RIVERS PLUTON both the tonalite and zoned gabbro-diorite WESTERN BELT plutons. Both the upper and lower volcanic o 10 20 KM SMARTVILLE COMPLEX units are intruded by all of the plutonic and Ijll Intrusive Rocks (SI] hypabyssal units and were deformed prior to cAiiuùivy nouKb v.ovj

*Present address: M.S. SN4, National Aeronautics Figure 1. Sketch of the northern Sierra Nevada, showing the location of the Smartville and Space Administration, Johnson Space Center, complex and the study area. Belt nomenclature after Day and others (1985) and Schweickert Houston, Texas 77058. (1981).

Geological Society of America Bulletin, v. 99, p. 779-791, 14 figs., 2 tables, December 1987.

779

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ucts of a single subduction system developed plutonic complex. Volcanic rocks were de- of metagabbro (mgb, Fig. 2) and metamor- during Late Jurassic oblique convergence at the formed prior to the intrusion of plutons, which phosed massive diabase (md, Fig. 2). These are continental margin. Other workers (Moores, were emplaced at the same time as a sheeted intruded by a sheeted dike complex (sdc) and a 1970, 1972; Schweickert and Cowan, 1975; dike complex. Both volcanic and plutonic rocks series of tonalité (bht), granophyric tonalité (gt), Schweickert, 1981; Day and others, 1985) be- were deformed by Nevadan-age faults. The only gabbro-diorite (gb), and granodiorite (grd) plu- lieve that the Smi.rtville complex is not related demonstrably post-Nevadan igneous rocks in tons. Each of these intrusive lithologies will be to Jurassic volcanic rocks in the eastern belt and the area studied are some small granodiorite discussed in the next section. that more than one Late Jurassic subduction plutons. These, and some larger plutons outside Direct evidence for the nature of the base- zone is representee! in northern California. the study area, may represent the onset of mag- ment on which the Smartville complex was con- Moores and Day (1984) and Day and others matism related to the Sierra Nevada batholith. structed is lacking, but basement probably (1985) correlated volcanic and plutonic rocks in includes older ophiolitic rocks. Serpentinite and the central belt with the Smartville complex and THE SMARTVILLE COMPLEX metagabbro of unknown age occur along the argued that the volcanic rocks are in tectonic Grass Valley-Wolf Creek fault zone and are in- contact with chei t-argillite broken formation The Smartville complex (Fig. 1) consists of truded by massive diabase similar to that of the and mélange in the central belt. They have pro- Upper Jurassic volcanic, plutonic, and hypabys- Smartville complex intrusive core (Tuminas, posed that the Smartville complex was thrust sal rocks. Volcanic rocks occur in a broad belt in 1983). In addition, volcano-plutonic complexes over the central belt and subsequently deformed the western and southern Smartville complex in the central belt that have been correlated with by steep folds and faults during the Late Jurassic and in a narrow region along the eastern bound- the Smartville complex have ophiolitic base- Nevadan orogeny. ary of the complex. The plutonic and hypabyssal ment (Day and others, 1985; Tuminas, 1983; The origin of the Smartville complex itself has rocks occur in an elongate, north-northwest- Murphy and Moores, 1985). also been the subject of considerable discussion. trending intrusive complex in the eastern and Early workers (Lindgren and Turner, 1895; Hie- central Smartville complex (Fig. 1). The com- Age of the Smartville Complex tanen, 1951, 1973, 1976; Compton, 1955; plex is bounded on the north and east by the Big Clark, 1960, 1964) interpreted the volcanic and Bend-Grass Valley-Wolf Creek fault zone (Hie- Isotopic ages for the Smartville complex are hypabyssal greenstones of the Smartville as a tanen, 1977; Tuminas, 1983; Day and others, similar to those of other rocks in the western belt Triassic or Jurassic volcanic terrane that was 1985). On the west, the complex is overlain un- (Saleeby, 1981, 1982). Available U-Pb ages metamorphosed, deformed, and subsequently conformably by Cretaceous and younger sedi- from zircon include those for (1) upper volcanic intruded by a series of post-kinematic plutons. mentary rocks in the Great Valley. Although it unit (Bloomer Hill Formation), northern Smart- The plutonic and volcanic rocks were generally seems clear that the Smartville complex corre- ville complex: 159 Ma (Saleeby, 1981); (2) pla- considered unrelated. More recently, the upper lates with western belt rocks farther south, the giogranite (called "granophyric tonalité" in this part of an ophiolite pseudo-stratigraphy (gabbro, detailed correlation of those rocks with the report) associated with the Smartville sheeted sheeted dikes, pillowed flows) was recognized Smartville complex is uncertain. dike complex: 160 Ma (McJunkin and others, within the Smartville complex, leading to its in- The volcanic rocks of the Smartville complex 1979), 159 Ma (Saleeby and Moores, 1979), terpretation as an ophiolite (Moores, 1975; SV (Fig. 1) were not examined in detail for this 163 Ma (Saleeby and Moores, 1984); (3) the Cady, 1975; Xenophontos and Bond, 1978). study. Earlier workers (Xenophontos and Bond, Yuba Rivers pluton, a synorogenic tonalité that These workers recognized that the ophiolitic 1978; Buer, 1979; Menzies and others, 1980; intrudes and is deformed by faults in the Grass rocks are overlain ty a thick section of interme- Xenophontos, 1984) recognized two major vol- Valley-Wolf Creek fault zone (Bobbitt and oth- diate volcanic rocks, suggesting a setting within canic units in the western and southern Smart- ers, 1986; Eddy and others, 1986): 160 Ma or near a volcanic arc. Xenophontos and Bond ville complex. The lower volcanic unit (slv, (J. B. Saleeby, 1985, written commun.). The (1978) interpreted the ophiolitic section as a Fig. 2) consists of mafic to intermediate flows, close correspondence of ages within the Smart- young inter-arc basin, the edge of a larger mar- pillowed flows, and pillow breccias. It is re- ville to that for the Yuba Rivers pluton suggests ginal basin, or a pseudo-stratigraphic sequence stricted to that portion of the Smartville that magmatism and deformation occurred developed by rifting at the base of an arc. complex west of the intrusive complex. The within a very restricted time. Deformed ultra- Schweickert and Cowan (1975) and Day and upper volcanic unit (suv, Fig. 2) consists of mafic and mafic rocks exposed in the central others (1985) also proposed an arc-marginal intermediate-composition pyroclastic and vol- belt, just north of the Smartville complex, and basin setting for the Smartville, and Saleeby caniclastic rocks and is more widespread. The similar to possible Smartville basement rocks (1981) suggested that it represents an ensimatic contact between the lower and upper volcanics exposed elsewhere, are intruded by an unde- arc developed on previously accreted material at is conformable and gradational (Xenophontos formed plagiogranite dike that yields a U-Pb zir- the continental margin. and Bond, 1978; Xenophontos, 1984). Menzies con age of 159 Ma (J. B. Saleeby, 1984, personal commun.). The purpose of this paper is to evaluate the and others (1980) examined the trace-element plutonic and hypabyssal rocks of the Smartville geochemistry of the volcanic rocks and showed terrane in light of these various proposals. Our that the lower volcanic unit has tholeiitic charac- INTRUSIVE ROCKS OF THE principal finding is v.hat the Smartville plutonic teristics, whereas the upper volcanic unit is SMARTVILLE COMPLEX and hypabyssal rocks intruded volcanic rocks largely calc-alkaline. Day and others (1982) during pre-Nevadan extension of an active vol- showed that clinopyroxenes from both volcanic The intrusive core of the Smartville complex canic arc. Intrusive and extrusive rocks in the units are typical of either volcanic arc or ocean consists of an older unit of massive diabase and Smartville appear to be broadly coeval and can floor basalts. associated metadiorite (md, Fig. 2) and meta- be interpreted as parts of a single volcano- Smartville intrusive rocks include older units gabbro (mgb) and a younger series of zoned

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gabbro-diorite (gb), granophyric tonalite (gt), and tonalite (bht) plutons that were emplaced 39° 30' at the same time as the Smartville dike

complex (sdc). SWEDES FLAT Massive Diabase PLUTON Massive diabase (md, Fig. 2) occurs in a broad legion in the southern part of the study area, where it is intruded by the Pilot Peak and Pleasant Valley plutons and truncated by the Grass Valley-Wolf Creek fault zone. The unit includes massive metadiabase, coarser grained metadiorite, and small apophyses of metagab- bro. All of the rocks have undergone greenschist

Figure 2. Bedrock geology of the study area, located in the east-central Smartville complex. Fault nomenclature after Tumi- nas (1983). EXPLANATION

grd Granodiorite Unit. Post-Nevadan granodiorite plutons. Locally gradational to granite.

SMARTVILLE COMPLEX

VOLCANIC ROCKS

m suv Upper Volcanic Unit. Intermediate composition pyroclastic and epiclastic rocks

slv Lower Volcanic Unit. Mafic flows and pillowed flows

INTRUSIVE ROCKS

bht Tonalite Unit. Biotite-hornblende tonalite with minor granophyric tonalite

gt Granophyric Tonalite Unit. Granophyric hornblende tonalite. Contains some non-granophyric rocks.

sdc Dike Complex. Unit of 100% sheeted and unsheeted felsic and mafic dikes.

gb Gabbro-Diorite Unit. Zoned gabbro-diorite plutons. Solid pattern is olivine gabbro.

md Massive Diabase Unit. Massive diabase and associated metadiorite and metagabbro. Dike and sill-like forms are rare. Forms the country rock in the Clark Hill Mixed Zone.

mgb Metagabbro plutons coeval with the massive diabase.

— Steeply dipping faults (F-2, F-3, F-4)

^ Thrust Faults (F-1) hatchures on upper plate.

" Geologic contacts

Dike orientations

121° 4.5' 3 4 Km

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fades metamorphism and consist primarily of west. North of Collins Lake, the complex trends comes narrower toward the southeast, (2) the albite, actinolite and/or blue-green hornblende, north-northeast toward Challenge. The change Pleasant Valley pluton, an irregular, north- epidote, sphene, chlorite, and minor quartz. in trend of the dike complex in this region is south-trending body that narrows toward the Subhedral zoned plagioclase crystals and rare reflected in the attitudes of individual dikes (Fig. north, and (3) the Indiana Creek pluton, a clinopyroxene cores in amphibole are probably 2; see also Figs. 11 C and D). The contact be- smaller, northeast-trending ovoid pluton. The relict igneous phases. Brownish-green amphi- tween the Swede's Flat pluton and the dike trends of the long axes of these plutons are paral- bole in some of the metadiorite may also be a complex (Marlette and others, 1978) was not lel to the orientations of dikes in the dike com- relict igneous phase. mapped, but our reconnaissance mapping sug- plex, shifting in orientation from northwest in Chilled margins are rare in the massive dia- gests that the dike unit continues north of the the south to northeast in the north (Fig. 2). base, but the diabuse may occur in dikes or sills area shown in Figure 2 to the vicinity of the The gabbro-diorite plutons consist of me- too large to be seen in the small exposures typi- Bald Rock batholith (Fig. 1). dium- to coarse-grained rocks ranging in com- cal of much of the unit. In relatively good expo- Dikes are common within both the volcanic position from olivine gabbro to quartz diorite. sures southwest of the Pilot Peak pluton (Fig. 2), and plutonic units of the Smartville complex. In Figure 3 shows the distribution of rock types in a few large dikes are evident in the massive unit. many cases, these dikes appear to be directly the plutons. The contacts between all units in the Metamorphism may have obscured or obliter- related to the sheeted dike complex, with the gabbro-diorite plutons are gradational. Changes ated chilled margins in many places. ratio of dikes to country rock increasing toward in rock composition or texture occur over dis- the dike unit contact. This relationship occurs in tances ranging from a few to a few hundred Clark Hill Mixed Zone several places. In the Yuba River south of Dob- metres. The contacts between rock units are de- bins, dike swarms in the volcanic section to the fined by the presence or absence of a key min- The northern half of the region underlain by east and in the Pleasant Valley pluton to the eral (usually determined petrographically), by massive diabase (Fig. 2) is called, informally, the west culminate in a narrow unit of 100% dikes. unusual mesoscopic features (such as large oiko- "Clark Hill mixed zone." In the mixed zone, the Near Route 20, the dike complex dies out crystic hornblende crystals), by changing relative massive diabase is the country rock in a complex southward into scattered dikes in the massive abundances of minerals (especially color index), zone of intrusive breccias formed by the intru- diabase and gabbro-diorite units. Finally, the and, in one case, by the presence or absence of a sion of gabbro-diorite, tonalite, and younger, unit of 100% dikes grades into scattered dikes in metamorphic overprint. fine-grained diabase dikes related to the sheeted a granophyric tonalite pluton in spectacular ex- Olivine gabbro (ogb, Fig. 3) occurs in all of dike complex described below. At the contacts posures along Deer Creek, west of the Pleasant the major plutons and is the primary constituent with some gabbroic plutons, the country rocks Valley pluton (Fig. 2). Buer (1979) reported that of most of the small gabbro intrusions. The in- are migmatitic pyroxene hornfelses and may a similar relationship exists between the dike ternal contacts of the olivine gabbro with other have been partially melted. complex and the lower volcanic unit to the west. lithologies are defined by the first appearance of S. D. Day (1977) showed that the sheeted olivine in thin section. It is a massive to weakly Metagabbro dikes range in composition from soda rhyolite to foliated rock with locally developed, discontin- diabase. Diabase and other mafic dikes are by uous layers of gabbro and anorthosite. Gabbro A group of small metagabbro bodies (mgb, far the most abundant variety. Mafic dikes are pegmatites are common in the olivine gabbro. Fig. 2) scattered throughout the massive diabase plagioclase- and pyroxene-phyric and contain Irregular pods and tabular masses of olivine cli- and one large body of metagabbro east of Col- both of these minerals, along with Fe-Ti oxides, nopyroxenite are locally common within the ol- lins Lake are coeval and closely associated with in their groundmass. Replacement of pyroxene ivine gabbro. the massive diabase. In addition, numerous by actinolite or blue-green hornblende and of The primary minerals in the olivine gabbro small bodies and segregations of metagabbro plagioclase by albite, epidote, and unidentified are unzoned to weakly zoned plagioclase occur throughout the massive diabase. Like the phyllosilicates is common and locally pervasive. (An85-An95),' clinopyroxene, olivine, and, in massive diabase, these rocks are pervasively al- Other metamorphic minerals include chlorite most samples, orthopyroxene. Orthopyroxene tered. Relict igneous phases include zoned pla- and sphene. Several varieties of felsic dikes oc- occurs as rims on olivine and less commonly as gioclase, clinopyroxene, and Fe-Ti oxides. A cur in the complex. Quartz albitite dikes (Day, discrete grains. Dark brown, red-brown, or few samples that were particularly rich in oxides 1977) are petrographically identical to some of nearly colorless amphibole is common and oc- were collected from the metagabbro near Col- the rocks in the granophyric tonalite plutons. curs as late magmatic interstitial crystals and oi- lins Lake. Subophitic textures are common in More common felsic dikes contain plagioclase kocrysts and as a post-magmatic patchy re- many of the intrusions. Unlike the younger and, more rarely, hornblende phenocrysts in a placement of pyroxene. Accessory minerals gabbro-diorite plutons, zoning apparently is not quartz-albite mosaic groundmass. Penetrative include magnetite, apatite, and traces of red- present in the metagabbro. deformation is rare in the dike complex, and it orange biotite. Magnetite is present as symplec- appears that metamorphism was an autometa- titic intergrowths with olivine and orthopyrox- Sheeted Dike Complex morphic process (Beiersdorfer and Day, 1983). ene and is probably a late-stage oxidation product (Haggerty, 1976; Ambler and Ashley, The Smartville dike complex (sdc, Fig. 2) oc- Gabbro-Diorite 1977). Magnetite is also a primary mineral in curs in a 2- to 10-km-wide, curvilinear region in most samples. the western part of the study area and in a much Gabbro and diorite occur in three large, elon- smaller, north-south-trending belt southeast of gate, zoned plutons and in a number of smaller •Plagioclase compositions in the olivine gabbros Dobbins. The dike complex consists of essen- intrusions (gb, Fig. 2). The major plutons trend were determined by electron microprobe at the Uni- versity of California at Davis. All other plagioclase tially 100% sheeted and unsheeted, felsic and north-northwest through the center of the study compositions reported herein were determined by op- mafic dikes. In the south, from Route 20 to the area. From south to north, they are (1) the Pilot tical methods using a five-axis universal stage. Compo- Collins Lake area, the complex trends north- Peak pluton, a northwest-trending body that be- sitional data are tabulated in Beard (1985).

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Gabbronorite is present in all of the large est in quartz are pyroxene free. Accessory min- Mineral Variation in the Gabbro-Diorite Unit gabbro-diorite plutons (pgb, Fig. 3) and in some erals include magnetite, ilmenite, sphene, of the smaller gabbro intrusions. The unit in- apatite, and scarce zircon. Most samples contain Modal data demonstrate that complete grada- cludes some dioritic rocks in the Indiana Creek small amounts of potassium feldspar. tion exists between most rock types in the Pilot pluton that could not be shown on the scale of Biotite-two-pyroxene monzodiorite (bpg, Peak and Pleasant Valley plutons and suggest Figure 3. The rock is weakly foliated, with local Fig. 3) is a weakly foliated to massive rock that that their zoning is continuous and not the result development of discontinuous compositional occurs as a mappable unit only in the Pilot Peak of multiple intrusive events. The mean modal layering. Primary minerals in the unit are zoned pluton. The contacts of the monzodiorite are mineralogy of rocks from the Pilot Peak and plagioclase (rims, An50-An75; cores, An70- sharp, but they do not truncate any features in Pleasant Valley plutons is listed in Table 1. An90+), clinopyroxene, orthopyroxene, and other units of the Pilot Peak pluton. In thin sec- Complete modal data are given by Beard olive-green to brown or red-brown hornblende. tion, the rocks are characterized by the absence (1985). Although the average modal composi- Most samples contain small amounts of quartz of amphibole, except for small amounts of late- tion of the rock types in the plutons is distinc- and dark brown biotite, but either or both of stage, deuteric blue-green hornblende or actino- tive, the range of composition is considerable these phases may be absent near the contact lite and abundant potassium feldspar (Table 1). (Table 1). The gabbronorite and the biotite- with the olivine gabbro. Accessory phases in- In addition to potassium feldspar, strongly zoned hornblende diorite exhibit substantial composi- clude magnetite, ilmenite, and apatite. A two- plagioclase (rims, An35-An50; cores, An70- tional overlap. The amount of olivine in the pyroxene gabbro containing large oikocrysts of An90+), clinopyroxene, orthopyroxene, dark olivine gabbro varies from trace amounts to red-brown hornblende was mapped as a sepa- brown biotite, and quartz are the primary min- >15%, whereas some samples of gabbronorite rate unit in the Pilot Peak pluton (hgb, Fig. 3). erals of the monzodiorite. Accessory minerals collected near the olivine gabbro contact contain Biotite-hornblende diorite (bhd, Fig. 3) is a include magnetite, ilmenite, apatite, and zircon. neither quartz nor olivine. A single sample col- minor unmapped constituent of the Indiana Much of the eastern margin of the Pleasant lected from the Pilot Peak pluton contains trace Creek pluton but makes up a large part of the Valley pluton is made up of gabbro and diorite amounts of relict olivine surrounded by ortho- other two gabbro-diorite plutons. It is generally (mg, Fig. 3) that was metamorphosed during the pyroxene and <0.5% quartz. The compositional a massive to weakly foliated rock. It is distin- intrusion of the tonalite unit to the east. The ranges of the diorite, gabbronorite, and olivine guished in the field from the gabbronorite by its metagabbro (mg, Fig. 3) mapped at the south gabbro suggest that a complete gradation exists lower color index and its lack of layering. The end of the Pilot Peak pluton may be related to between olivine gabbro and quartz diorite in the rock consists of zoned plagioclase (rims, the hydrothermally altered gabbros associated Pilot Peak and Pleasant Valley plutons. The An35-An50; cores, An50-An85), green-brown to with the massive diabase unit (mgb, Fig. 2). Al- monzogabbro exhibits a sharp increase in potas- green hornblende, dark brown biotite, and as though all of the mafic phases in the rock are sium feldspar and a sharp decrease in amphibole much as 12% quartz. Most samples contain now altered to green hornblende, zoned plagio- across its contacts with other rock types and some, and many contain substantial amounts of, clase found in these rocks is probably a relict does not appear to lie along the same modal orthopyroxene and clinopyroxene. The modal igneous phase. continuum as do the other three units. abundance of pyroxene generally decreases as Mineralogical variation in the Pilot Peak and that of quartz increases. Some of the rocks rich- Pleasant Valley plutons is illustrated in Figures 4

Figure 3. Distribution of rock types in the three largest gabbro- EXPLANATION diorite plutons. Note that olivine GABBRO-DIORITE PLUTONS gabbro is generally interior to less mafic lithologies. ogb Olivine gabbro PLEASANT VALLEY pgb Gabbronorite INDIANA PLUTON Gabbronorite CREEK PLUTON hgb with hornblende oikocrysts

Biotite hornblende diorite bhd to quartz diorite

Biotite-two pyroxene bpg monzodiorite H Contact metamorphosed gabbro-diorite

PILOT PEAK f N KM

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and 5, which show the distribution of modal TABLE 1. MODAL COMPOSITION OF PLUTONIC ROCKS FROM THE SMARTVILLE COMPLEX quartz and olivine and the composition of the

most calcic and least calcic plagioclase (cores Rock type and Percent qtz. or Percent plag. Percent K - spar Color and rims, respectively) in the intrusives. In both number of samples olivine index plutons, the distributions of late-crystallizing Pleasant Valley quartz and sodic plagioclase mimic the inde- pluton bhd (25) 5.5 51.4 0.3 42.2 pendently determined lithologic contacts. For (0.6-12) (45-58) (0-5) (30-48) example, the gabbronorite contact (pgb) in the pgb (8) 0.8 50.5 0 48.7 (0-4) (47-52) 0 (44-53) Pleasant Valley pluton is closely followed by ogb(U)* 7.3 50.7 0 49.3 quartz and sodic plagioclase contours. Contours (2-13) (46-54) 0 (46-54) Pilot Peak pluton of modal quartz and sodic plagioclase in the bhd (18) 6.3 56.2 0.8 36.8 Pilot Peak pluton generally separate the gab- (0.5-13) (49-67) (0-4) (25-42) pgb (5) 0.9 53.4 0 45.7 broic rocks in the southern and central parts of (0.2-3) (50-58) 0 (42-50) bpg(8) 5.0 58.6 7.0 29.5 the pluton from the more dioritic rocks around (2-8) (54-63) (2-11) (24-35) its margins. The distribution of late-crystallizing ogb (9)* 10.0 46.6 0 53.4 (0.1-17) (36-70) 0 (29-64) phases confirms that a crude concentric zoning Tonalité exists in the plutons; the most felsic rocks lie, in bht (20) 26.3 53.7 2.0 17.9 general, along the western margins of the (18-36) (46-61) (0-10) (8-25) Granophyric intrusions. tonalité g> (31) 28.7 58.5 12.8 Contours of the most calcic plagioclase com- (14-40) (50-70) (5-22)

position form a crude concentric zoning pattern, Granodiorite grd (20) 22.4 44.0 21.5 12.2 similar to that defined by the late-crystallizing (13-30) (18-52) (14-50) (6-18) phases, particularly in the Pilot Peak pluton. These contours, however, intersect mapped lith- Note: modes for all rock types except ogb determined by a minimum of 1,000 counts on slabs stained for potassium feldspar. Values in parentheses are observed ranges. ologic boundaries at high angles; calcic plagio- 'Entries in the first column for ogb are for olivine. Because of coarse grain size, eight to ten thin sections each for three samples from the Pleasant Valley pluton and four samples from the Pilot Peak pluton were counted to determine the range in the olivine mode. The mean value for each pluton includes several additional samples for which only one thin section was counted.

clase (An80+) occurs in all rock types, including plex. The biotite-hornblende tonalite forms a some rocks containing >10% quartz. This may series of six plutons extending from north of indicate formation and incomplete removal of Dobbins south to the Pilot Peak pluton (Fig. 2). very calcic plagioclase early in the history of an Intrusion of biotite-hornblende tonalite formed evolving magma system. large areas of intrusive breccia in the southeast- ern part of the Pleasant Valley pluton and in Tonalite parts of the Clark Hill mixed zone. The eastern- most granophyric tonalite pluton and the largest Two tonalite units were mapped in the study biotite-hornblende tonalite pluton are in contact area, a granophyric hornblende tonalite unit (gt, north of Deer Creek (Fig. 2). Biotite-hornblende Fig. 2) and a biotite-hornblende tonalite unit tonalite intrudes granophyric tonalite north of (bht). Both are present in elongate north-south- the South Yuba River, but farther south, the to north-northwest-trending plutons. Elongate contact is defined only on the basis of the scar- granophyric tonalite plutons are found at the city of biotite in the eastern parts of the tonalite. eastern and western margins of the plutonic The granophyric tonalite (gt) contains quartz, complex. In the Collins Lake area, granophyric zoned plagioclase, and acicular green horn- tonalite is also found in a series of small, equant blende. In the central and eastern plutons, intrusives and as dikes in the sheeted dike com- plagioclase is normally zoned from An20 to An35, but in the western body and in the small intrusions and dikes associated with it, Figure 4. Composition of the Pilot Peak plagioclase is zoned from An5 to An20. pluton. Filled circles are sample localities. Accessory phases are apatite, magnetite, sphene, Dashed lines are lithologic contacts from Fig- and traces of dark brown biotite. Most samples ure 3. (A) Modal abundance of quartz and have undergone nonpervasive low-temperature olivine. Quartz abundances are based on metamorphism or alteration. Metamorphic 1,000 points counted on slabs etched and minerals include epidote, actinolite, albite, sphene, stained for potassium feldspar. Olivine was and stilpnomelane. determined in thin section (compare with The granophyric tonalite contains micro- Table 1). (B) Composition of the most calcic graphic intergrowths of quartz and plagioclase. plagioclase (cores). Plagioclase compositions The intergrowths are not myrmekitic but closely were determined on a universal stage using resemble intergrowths seen in more common the a-normal method. (Q Composition of the alkali-feldspar granophyres. The modal abun- most sodic plagioclase (rims). dance of Ihese intergrowths varies greatly, even

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from sample to sample, whereas the most calcic plagioclase in the tonalite varies widely from sample to sample, having a mean value of An44. These data suggest that the tonalite and the granodiorite are not related by crystal fractionation. MOST CALCIC PLAGIOCLASE METAMORPHISM

Beiersdorfer and Day (1983) and Xenophon- tos (1984) described the widespread occurrence of prehnite and pumpellyite in the volcanic rocks of the Smartville complex. Beiersdorfer (1982) concluded that the regional metamor- phism occurred at relatively low pressures, 1.5-2.5 kbar, implying low pressures for the emplacement of the plutonic rocks. The close association of the plutons with hypabyssal rocks and the granophyric textures displayed by many of the more felsic intrusions also suggest shallow-level crystallization (Buddington, 1959; Barker, 1970). The hypabyssal rocks and the older metagab- bro and metadiorite commonly have greenschist facies metamorphic assemblages. In the dike complex, the greenschist facies assemblage is autometamorphic in origin. This is most clearly demonstrated by the presence of dikes which Figure 5. Composition of the Pleasant Valley pluton. Methods and symbols as in Figure 4. contain actinolite + epidote + albite + chlorite in The eastern margin of the pluton is disrupted by later intrusion of tonalité and dike complex. volcanic rocks containing pumpellyite + prehn- (A) Modal quartz and olivine. (B) Most sodic plagioclase (rims). (C) Most calcic plagioclase ite and which show no evidence of a green- (cores). schist overprint (Beiersdorfer, 1982). The younger plutonic rocks are generally fresh and unmetamorphosed except for local deuteric al- among samples taken from a single outcrop, and in three small satellite intrusions (grd, Fig. 2). teration and contact-metamorphic effects. nongranophyric hornblende tonalité is common The largest of the satellites, 2 km west of the within the unit. main pluton, is associated with a large area of The biotite-hornblende tonalité unit (bht) intrusive breccia, six times the size of the satellite consists principally of medium- to coarse- itself. The granodiorite consists of zoned plagio- grained, massive biotite-hornblende tonalité. A clase (cores, An50; rims, An3-An20), quartz, few samples of granophyric hornblende tonalité, biotite, hornblende, and interstitial microcline identical to those in the granophyric tonalité microperthite. Accessory minerals include mag- unit, were also collected. The essential minerals netite, ilmenite, apatite, and zircon. of the biotite-hornblende tonalité are zoned pla- Modal data from the granodiorite and tonalite gioclase (average core, An44; average rim, (Table 1, Fig. 7) clearly show the compositional An22), quartz, olive-green hornblende, and dark dissimilarities and lack of gradation between the brown biotite. Most samples contain small two units. Note that the granodiorite, although amounts of potassium feldspar, as much as 10% much richer in potassium feldspar and poorer in in one case. Accessory minerals include magne- mafic minerals than is the tonalite, also contains, tite, ilmenite, apatite, and zircon. on the average, less quartz. The composition of The tonalité and granophyric tonalité units plagioclase in the cores of zoned crystals in the occur in markedly elongate plutons. The largest granodiorite averages An50 and varies little of the biotite-hornblende tonalité plutons, ex- posed in the South Yuba River and Deer Creek Canyons, is weakly zoned, having a quartz-rich Figure 6. Composition of the largest tona- and mafic-poor core (Fig. 6), suggesting that it lite pluton, exposed in the Deer Creek and represents a single episode of magma injection. South Yuba Canyons. (A) Modal quartz con- tent. (B) Total mafic minerals. Note that the Granodiorite zoning is weaker than in the gabbro-diorite plutons and that it is in a normal direction. 0 1 2 Granodiorite occurs in a small, weakly zoned Modes are based on 1,000 points counted on 1 i i pluton north of the Higgins Corner window and stained slabs. km

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TABLE 2. CROSSCUTTING RELATIONSHIPS ENUMERATED IN FIGURE 8 rounding the gabbro-diorite plutons are gener- ally narrow, commonly less than a hundred Relationship Quadrangle* Location metres wide.

(1) Pilot Peak pluton cuts massive diabase RR, W Along all contacts

(2) Pleasant Valley pluton cuts massive diabase RR Near town of Rough and Ready RELATIVE AGE OF INTRUSION

(3) Indiana Creek pluton cuts massive diabase C Clark Hill mixed zone AND E>EFORMATION

(4) Other gabbro-diorite cut; massive diabase FC Marysville road, east of Dobbins IN THE SMARTVILLE COMPLEX

(5) Granophyric tonalite cuti massive d FC, GV Western and central gt plutons, near Englebright Lake; eastern gt pluton, south of Deer Creek Crosscutting Relations in the Intrusive Complex (6) Dike complex cuts massive diabase RR, FC Horton Ridge and Englebright Lake

(7) Tonalite cuts massive diabase RR, FC Near Clear Creek, Rough and Ready, Clark Hill mixed zone, Marysville road, Crosscutting relations among the intrusive west of Dobbins rocks of the Smartville complex are outlined in (8) Metagabbro = massive diabase GV Mutually crosscutting relations near Wolf Mountain Table 2 and illustrated in Figure 8. These rela-

(9) Granophyric tonalite cuts metagabbro O, FC Numerous examples on east shore, tions show that there are two distinct episodes of Collins Lake; also at Englebright Lake magmatism within the intrusive core of the (10) Dike complex cuts metagabbro FC, RR East shore, Collins Lake; metagabbro Smartville complex and that there is a trend screens in dike complex in Dry Creek, south of Collins Lake; west side of Clark toward more silicic plutonism through time. The Hill mixed zone granodiorite plutons are interpreted as a later, (11) Dike complex cuts Pleasant Valley pluton Yuba River Canyon, southeast of Dobbins; scattered dikes cut pluton near Clear Creek unrelated: intrusive event, for reasons discussed and Petin Valley later, and are not included in the discussion (12) Dike complex cuts granophyric tonalite FC, RR Excellent exposures in central pluton below. in Deer Creek; numerous dikes in western pluton, Englebright Lake area Massive diabase and associated metagabbro (13) Dike complex cuts tonalite FC, RR Clear Creek area; South Yuba River (md, Fig. 2) are the oldest intrusive rocks in the Canyon, east of Bridgeport area (Table 2, Fig. 8). They are cut by gabbro- (14) and (15) Pleasant Valley and Pilot Peik plutons RR Metamorphosed dikes in septum between cut dike complex two plutons diorite, both tonalite units, and the dike com-

(16) Other gabbnwiiorite cuts tlike complex FC Metamorphosed dikes in west part of plex. Turninas (1983) reported mutually cross- Clark Hill mixed zone cutting relations between the massive diabase (17) Indiana Creek pluton cuts dike complex R,C Along west side of pluton and some small metagabbro intrusions southeast (18) Granophyric tonalite cuts (ike complex O West abutment of bridge over Collins Lake, Marysville road of the Pilot Peak pluton (Fig. 2). A large meta- gabbro pluton exposed east of Collins Lake is (19) Tonalite cuts dike complex FC East of Dobbins, Marysville road mineralogically and texturally similar to these (20) Granophyric tonalite cuts [feasant Valley pluton FC, RR Gabbroic inclusions in tonalite near Piper Hill and along Pleasant Valley road intrusions and is cut by dike complex and gran- (21) Granophyric tonalité = tonalité RR Gradational contact along Deer Creek ophyric tonalite. For these reasons, we believe (22) Tonalite cuts granophyric tonalité FC Near town of Birchville on San Juan Ridge that this pluton also formed during early stages (23) Other gabbro-diorite cuts g'anophyric tonalite FC Small gabbro intrusion on San Juan Ridge of intrusive activity. metamorphoses eastern granophyric pluton Tonalite is generally the youngest plutonic (24) Tonalite cuts Pleasant Valley pluton FC Along east margin of the pluton, South Yuba canyon, east of Bridgeport unit and is commonly intrusive into gabbro- (25) Tonalité cuts other gabbro-diorite FC Gabbro bodies east of Dobbins and in Clark diorite and granophyric tonalite. Contradictory Hill mixed zone, cut by tonalite dikes and mutually crosscutting relations, however, (26) Tonalite cuts Pilot Peak pluton RR Northeast side of pluton cut by small tonalite body

•Quadrangle abbreviations; C, Challenge; FC, French Corral; GV, Grass Valley; O, Oregon House; R, Rackerby; RR, Rough and Ready; W, Wolf. Consult these 7V)-minute quadrangles for detailed place names. Q

Contact metamorphism by the plutons over- hornblende, andesine or oligoclase, Fe-Ti ox- prints the regional metamorphism and is limited ides) occur around the more felsic intrusives. to aureoles, in most cases < 100 m wide, around Because of the regional metamorphism, the pre- the younger plutonic rocks. Close to the con- cise limits of the contact aureoles are difficult to tacts of gabbroic inirusives, pyroxene hornfels determine. occur (two pyroxenes, labradorite, brown horn- Other contact effects include the development blende), which in most cases, grade into horn- of intrusive breccia zones and contact foliations blende hornfels within a few metres or tens of defined by agmatitic structure, orientation of metres of the contact. Within the Clark Hill platy xenoliths, and the local development of mixed zone and around some small olivine planar fabric in both the intrusive and the coun- gabbro bodies west of Bullards Bar Reservoir, try rock. Extensive zones of intrusive breccia are however, uncharacteristically wide zones of commonly associated with the tonalite plutons, pyroxene hornfels and migmatite are present suggesting that they are currently in the process Figure 7. Quartz-plagioclase-potassium within metamorphosed volcanic and hypabyssal of being unroofed (Pitcher and Berger, 1972). feldspar composition of the tonalite and rocks. Hornblende hornfels (green to brown The intrusive breccias and agmatite zones sur- granodioriite units.

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TONALITE of its confluence with the Yuba River, in which prior to the intrusion of the dike complex. Fig- sheeted diabase cuts and is cut by tonalite ures 11A and 1 IB are plots of poles to bedding (Fig. 9). and flow margins in, respectively, the upper vol- canic unit (data from Springer, 1982, Bear River Relationship to Volcanic Rocks area, bedded pyroclastic and epiclastic rocks) and lower volcanic unit (data from Buer, 1979, The oldest intrusive rocks (massive diabase and Xenophontos and Bond, 1978, west-central and metagabbro) intrude the lower volcanic unit Smartville complex, mafic pillowed and unpil- just west of the study area and the upper vol- lowed flows). The Bear River data (Fig. 11 A) canic unit southwest of the Pilot Peak pluton define a girdle containing several maxima that (Fig. 2). Younger intrusive rocks, including the strikes N60°E and dips 70° north. The planes dike complex, tonalite, and gabbro-diorite, in- normal to the maxima define an open, upright trude upper and lower volcanic-unit rocks structure with an axis of rotation that trends wherever they are in contact. For the most part, north-northwest and plunges gently to the south. intrusion of dikes and plutons in the Smartville The data from the lower volcanic unit have a complex postdated volcanic activity. similar distribution (N66°E, 94° north, Fig. Some volcaniclastic rocks exposed along 1 IB). The variation that produces these girdles Deer Creek in the eastern Smartville complex occurs on a scale of hundreds to as much as a few thousand metres. Mapping by Buer (1979), MASSIVE DIABASEl are tentatively correlated with the upper parts of the upper volcanic unit (Fig. 2) and contain Xenophontos (1984), and Springer (1982) shows that map-scale variation in bedding and METAGABBRO clasts of gabbro, gabbro pegmatite, tonalite, and diabase (Fig. 10). Neither fossil nor isotopic age flow orientation can be explained by upright Figure 8. Diagrammatic representation of data are available for the host or clasts. Never- folds having a wavelength of 1-6 km. crosscutting relations among the intrusive theless, the intrusive rocks presently exposed in Figures 11C and 11D are plots of poles to rocks. Arrows point to younger rock; a dou- the Smartville complex are a possible source for dike margins from the main dike complex on the ble line represents a gradational contact or the clasts. If Smartville complex plutons are the western side of the intrusive complex. These mutually crosscutting relationship. Numbers source, some volcanism occurred in the Smart- plots show that the dike orientations are distrib- correspond to examples given in Table 2. ville complex during or after the emplacement uted in two homogeneous subareas. Dikes from PPP = Pilot Peak pluton, PVP = Pleasant of the plutons. We cannot rule out the possibil- the southern part of the dike complex strike Valley Pluton, ICP = Indiana Creek pluton, ity, however, that the clasts represent a suite of N30°W to N60°W and dip 60° northeast, OGD = other gabbro-diorite. pre-volcanic plutons older than the ones pres- whereas those from the northern part strike ently exposed in the Smartville complex. north-south to N20°E and dip 60° east or south- east. The two plots, taken together, form a are common among all of the younger intrusive Structural Relations single incomplete girdle that reflects a gradual rocks, reflecting their overall age equivalence. In change in strike of dikes in the dike complex particular, the dike complex proper or dikes that Reconnaissance structural analysis of the over a distance of 30 km (see Fig. 2). The re- are clearly related to the dike complex intrude western dike complex and the volcanic rocks of gional variation in dike orientation may have all rock types. This includes exposures of sheeted the western and southern Smartville complex formed as a result of regional-scale deformation, dikes along the South Yuba River, ~ 1.5 km east suggests that the volcanic rocks were deformed or it may simply reflect original variations in

Figure 9. Mutually crosscutting mafic dikes and tonalite, in the Figure 10. Oasts of plutonic rock in volcanogenic conglomerates canyon of the South Yuba River ~2 km east of the Yuba River. A interbedded with tuffaceous sediments, Deer Creek, near the eastern tonalite pluton in this area grades into a sheeted dike complex. margin of the Smartville complex.

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Figure 11. Lower hemisphere equal-area projections of (A) poles to bedding, upper volcanic unit, Bear River area; (B) poles to flow margins, lower volcanic unit, west-central Smartville; (C) poles to dike margins south of Collins Lake dam; and (D) poles to dike margins north of Collins Lake dam. Contours are approxi- mately 2%, 4%, and 6% per 1% area. Data from Buer (1979), Springer (1982), Xenophontos and Bond (1978), and the authors.

dike orientation. It does not appear to reflect the deformation present in the volcanics because the distance over which the orientation of dikes changes is about an order of magnitude greater than the scale of folds in volcanics units (30 km versus 1-6 km). Even at this scale, the girdle reflecting variation in dike orientations is in- complete and its geometry is inconsistent with the bed rotations observed in the volcanic units. The presence of a girdle in the volcanic rocks that is not reflected in the orientation of the dikes is best interpreted as evidence of a defor- mation older than the intrusion of the dikes. It might be argued that ductile folding of the vol- canic rocks aboul: steep axial surfaces would cause little or no reorientation of the dikes. The dikes and hinge surfaces, however, differ in strike by as much as 30°. Furthermore, there is are in thrust contact with central belt chert- We have found no similar relations in critical no evidence for any ductile deformation in the argillite broken formation in the Higgins Corner areas examined for this and other studies of the Smartville complex as a whole. Other means of window. Both of these units are also deformed Smartville complex. Ricci (1983) noted that producing the gird.es in the volcanic rocks, such by the high-angle Nevadan faults along most of dikes abundant in the upper volcanic unit near as flexural-slip folding or rotations on unrecog- the eastern margin of the Smartville complex. its thrust contact with chert-argillite in the nized listric normal faults, would reorient any On the other hand, a granodiorite pluton (grd, northern Smartville complex and presumably re- pre-tectonic dikes and produce girdles similar in Fig. 2) truncates F-4 faults at the northwest lated to the Smartville sheeted dike complex do quality to those observed in the volcanic rocks corner of the Higgins Corner window (Fig. 2). not occur in the lower plate chert-argillite. but with an orientation depending on the origi- Lenses of andradite garnet in the chert-argillite nal strike of the dikes relative to the apparent near the granodiorite contact may be the DISCUSSION fold axes. Dikes are widely distributed in the contact-metamorphosed equivalents of lime- volcanic units and the mapped contact between stone blocks occurring in the chert-argillite Comparison with Plutonic Rocks in Arcs the dike complex and the volcanic units is intru- matrix (Xenophontos, 1984). sive, gradational, and largely undeformed. It is The other intrusive rocks of the Smartville The tonalite and gabbro-diorite plutons in the unlikely, therefore, that the differences in orien- complex (sheeted dike complex, tonalite, Smartville complex are lithologically similar to tation (Fig. 11) can be explained by a décolle- gabbro-diorite) are not in contact with the early the gabbro-diorite and tonalite plutons that are ment between the two units. We conclude that thrust faults. The tonalite, granophyric tonalite, the most common intrusive rock types in mod- the deformation of [he Smartville volcanic pile is and gabbro-diorite plutons are cut by later, high- ern oceanic arcs. Tonalite and/or gabbro-diorite older than the intrusion of the dikes. angle Nevadan faults. A locally intense deforma- plutons occur in oceanic arcs in the Southwest tion of the granophyric tonalite pluton exposed Pacific (Shiroki and others, 1978; Chivas and Pre-Nevadan Age of the Smartville Complex along Deer Creek in the eastern Smartville others, 1982; Hines and Mason, 1978; Mason complex (Fig. 2) appears to be related to and MacDonald, 1978), in the inactive portions The clearest manifestation of the Nevadan de- Nevadan faulting in that area. Discrete zones of of the Antillean arc (Kesler and others, 1975), formation in the Smartville complex is the Grass mylonite first appear in this tonalite 2 km from and in the inactive arc exposed on Fiji (Gill, Valley-Wolf Creek fault zone (Fig. 2) along its the Grass Valley-Wolf Creek fault zone. Closer 1970; Green and Cullen, 1973). Similar plutons eastern margin. Tuminas (1983) and Day and to the fault zone, the mylonites increase in size occur in the older, outboard regions of some others (1985) recognized four generations of and number, and an anastomosing network of continental arcs (Moore, 1959; Smith and oth- Nevadan faults in this area. Early west-over-east mylonite forms a near penetrative deformation ers, 1983; Regan, 1985). Arc tonalites include thrusting (F-l, Fig. 2) was succeeded by high- of the tonalite at its closest approach to the fault both the biotite-hornblende variety (Chivas, angle dip-slip motion along north-northwest- (Fig. 12). This deformation in the plutons dies 1977; Mason and MacDonald, 1978) and plagi- striking (F-2, F-4, Fig. 2) and northeast-striking out to the north. oclase granophyres (Gill, 1970; Chivas, 1977). (F-3, Fig. 2) faults. The only rocks in the Smart- Sharp (1980) reported that dikes similar in The gabbroic plutons typically contain a variety ville complex that are clearly deformed by the age and composition to those in the Smartville of gabbroic and dioritic rocks, including olivine early, low-angle faults are the upper volcanic complex crosscut chert-argillite and western belt gabbro, olivine clinopyroxenite, gabbronorite, unit and the massive diabase unit, both of which volcanic rocks south of the Smartville complex. and hornblende-biotite diorite/quartz diorite.

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define an alkali-enrichment trend like that of, volcanic rocks from several modern arcs (Fig. 14). The olivine gabbros plot in a field defined by the cumulate gabbros from a zoned pluton on Guadalcanal (Chivas, 1977) (Fig. 14). From the above discussion, we conclude that the Smartville plutonic rocks formed in an arc setting. What is less clear, however, is the nature of that arc. The presence of significant subma- rine volcanic rocks, the predominance of mafic to intermediate volcanism, and the probable presence of ophiolitic basement, however, argue against an ensialic setting. Plutonic complexes mineralogically and litho- logically similar to the zoned gabbro-diorite plutons of the Smartville complex occur throughout the Klamath Mountains and north- ern Sierra Nevada. These include the Bear Figure 12. Mylonite in tonalite. The sample is from Deer Creek, near the eastern margin of Mountain igneous complex (Snoke and others, the Smartville complex. Subdivisions in the scale are 1 mm. 1981) and the Emigrant Gap complex (James, 1971). Snoke and others (1982) noted that these plutons record a history of arc magmatism in the Granodioritic rocks are abundant in some oce- Beard (1986) demonstrated that the composi- area. Because similar plutonic rocks are com- anic arcs, notably the Aleutians (Citron and tions of coexisting olivine and plagioclase in mon and widespread in modern oceanic and others, 1980; Kay and others, 1983; Perfit and cumulate gabbros from modern arcs are charac- ensimatic continental-margin arcs, however, others, 1980; Byers, 1959), but are much more teristic and effectively distinguish arc cumulate care must be exercised in the use of such plutons common in continental-arc batholiths. gabbros from cumulate gabbros associated with as correlative tools in areas, such as the Sierra Plutonic inclusions occur in basaltic and an- mid-ocean-ridge or ocean-island magmatism. Nevada or Klamath Mountains, where more desitic volcanic rocks from most modern arcs, Olivine gabbros from the cores of the Smartville than one ensimatic arc may be represented particularly those developed on oceanic crust or zoned gabbroic plutons have olivine and plagio- (Moores and Day, 1984; Harper and Wright, on accreted oceanic material at continental mar- clase compositions that are indistinguishable 1984). gins. In contrast to the plutonic suites, most of from those in modern arcs (for example, the the xenoliths are cumulate rocks. Silicic inclu- Lesser Antilles, Fig. 13). Extension in the Smartville Complex sions, except for those that are clearly related to The olivine-free rocks of the zoned gabbro- nearby basement rocks, are rare. Cumulate gab- diorite plutons have compositions similar to, and The presence of a sheeted dike complex in the broic xenoliths have been reported from the Smartville complex implies local 100% exten- Aleutians (Conrad and Kay, 1984), the Lesser sion. Sheeted dikes are a characteristic part of Antilles (Lewis, 1973; Arculus and Wills, 1980), the ophiolite pseudo-stratigraphy, and their the Marianas (Stern, 1979), Indonesia (Morrice presence, in part, led to the hypothesis that and others, 1983; Newmann van Padang, 1951), ophiolite complexes form at mid-ocean ridges the Philippines (Newhall, 1979), the Izu-Bonin (Moores and Vine, 1971). As discussed earlier, arc (Kuno, 1962), Japan (Yamazaki and others, however, the Smartville dike complex is a rela- 1966; Kuno, 1962), Central America (Carr and tively young feature and is not a part of the Pontier, 1981; Walker, 1984), the Cascades basement on which the Smartville volcanic (Heliker, 1985), Kamchatcha (Erlich and others, rocks were deposited. Thus, the dikes must rep- 1979), and New Guinea (Gust and Johnson, resent extension that occurred within the vol- 1981). Cumulate rock types found as xenoliths canic edifice itself. include olivine gabbro, gabbronorite, horn- The discontinuous nature and limited extent blende gabbro with or without olivine, and, of dike complex in the Smartville complex rarely, olivine- and clinopyroxene-rich ultra- and in the other parts of the western belt sug- mafic rocks. 100 90 80 70 60 50 gest that rifting was arrested at an early stage. A study of the geochemistry of the plutonic PERCENT ANORTHITE The Smartville dike complex ranges in width rocks in the Smartville complex is in progress from 0 to 7 km and dies out into scattered (J. S. Beard and H. W. Day, unpub. data) and a Figure 13. Compositions of plagioclase and dikes along its eastern, western, and southern detailed discussion of the geochemistry here olivine in cumulate gabbros from an arc margins. Other discontinuous exposures of would be premature. Nevertheless, there are (Lesser Antilles), a tholeiitic layered intrusion 100% dikes occur in the eastern Smartville some aspects of the mineral and whole-rock (Skaergaard), and an alkaline ocean island complex (Fig. 2) and, southeast of the area chemistry that support the interpretation of the (Reunion). Smartville cumulate olivine gab- shown in Figure 2, in the Folsom area (Saleeby, Smartville complex as an arc and, hence, will be bros (circles) plot in the field defined by 1982). The regional variation of dike orien- briefly discussed. The mineral and whole-rock cumulate olivine gabbro xenoliths from the tation in the Smartville complex (Figs. 11C analyses plotted in Figures 13 and 14 are tabu- Lesser Antilles. Modified after Arculus and and 11D) may simply reflect local controls lated in Beard (1985). Wills (1980). imposed on early rift development.

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Figure 14. Alkali-FeO(tot.)-MgO plot of Smartville gabbros FeO(tot.) and diorites. Smartville olivine gabbro cumulates plot in field of cumulate gabbro:; from Guadalcanal. Olivine-free Smartville gabbro and diorite plot along an alkali enrichment trend similar to that of several modern arc volcanic suites. Arc trends from Brown (1982).

The elongate map patterns of the gabbro- diorite and tonalite plutons that are coeval with the sheeted dike complex appear to reflect the effects of extension during their emplacement. There is no doubt that extension was active dur- ing emplacement of the plutons; a synplutonic sheeted dike complex forms much of the eastern margin of a small tonalite pluton 2-5 km south of Dobbins (Fig. 2; see also Fig. 9). The orienta- tions of the long axes of the gabbro-diorite plu- tons shift from northwest in the south to northeast in the north, mimicking the change in strike in the dike complex itself. The sheeted dikes represent an intra-arc rift- complex is the Smartville sheeted dike complex. ridge or hot-spot magmatism (Figs. 13, 14; ing and back-arc basin-forming event that was The elongate shapes of the plutons are probably Beard, 1985, 1986). arrested at an early stage of development. One related to their emplacement in an extensional Early interpretations of the Smartville com- of the more interesting aspects of intra-arc exten- environment. This is particularly true of the ton- plex as an ophiolite representing a fragment of sion in the Smartville complex is the involve- alite plutons, most of which are extremely oceanic crust formed at a spreading center ment of the calc-;ilkaline rocks. Calc-alkaline elongate and one of which is associated with the (Moores, 1970; Cady, 1975) were based on the magmatism was occurring during rifting, and formation of a synplutonic sheeted dike com- presence of elements of the ophiolite pseudo- some of the extension may actually have been plex. (5) Continued volcanism following pluton- stratigraphy (layered and massive gabbro, plagi- accommodated by the emplacement of elongate ism may be represented by plutonic clasts in ogranite, sheeted dikes, pillow basalts). More calc-alkaline plutons. volcaniclastic rocks in the eastern Smartville recent studies, including this one, have shown complex, which are tentatively correlated with important differences between the Smartville CONCLUSIONS the uppermost part of the upper volcanic unit. complex and many ophiolites. First, the Smart- (6) Nevadan faulting followed volcanism and ville sheeted dike complex and related plutons Geologic History of the Smartville Complex plutonism. Early Nevadan thrust faults deform are not a part of the Smartville basement but an the volcanic rocks and the older intrusive rocks. integral part of the subvolcanic edifice. Calc- On the basis of crosscutting, structural, and The younger intrusive units are deformed by alkaline plutons are coeval with the dikes and other relative time relations, we propose the fol- later, high-angle faults. (7) Post-Nevadan intru- were emplaced during the rifting event repre- lowing geologic history for the Smartville com- sion of granodiorite plutons may be related to sented by the dike complex. Second, the pillow plex. (1) Submarine tholeiitic volcanism is the the early granodiorites of the Sierra Nevada lavas are older than the dikes and plutonic rocks earliest event preserved in the Smartville com- batholith. and, although low in the volcanic section, inter- plex. This was succeeded by calc-alkaline pyro- digitate with calc-alkaline pyroclastic and epi- clastic activity. The contacts between tholeiitic Nature of the Smartville Complex clastic rocks. The pillow lavas are not part of a and calc-alkaline volcanics are gradational and "dead" volcanic pile unconformably overlain by interdigitating, suggesting formation within a We interpret the Smartville complex as a younger arc volcanic rocks. Finally, the Smart- single volcanic edifice. (2) Massive diabase and rifted volcanic-subvolcanic edifice that formed ville pseudo-stratigraphy itself is highly irregular. metagabbro are the oldest recognized intrusive in a Late Jurassic arc. The volcanic edifice Gabbroic rocks intrude high levels of the upper rocks in the Smartville complex, and they in- developed primarily and possibly entirely in a volcanic unit. Layered gabbro occurs only in trude both the upper and lower volcanic units. A submarine environment. The change from older one small gabbro body east of Dobbins (Fig. 2). genetic relationship between these intrusives and tholeiitic to younger calc-alkaline magmatism in The dike unit is laterally discontinuous and best the volcanic rocks has not been demonstrated the volcanic rocks of the Smartville complex developed on the western side of the intrusive but cannot be ruled out. (3) The Smartville vol- (Menzies and others, 1980; Xenophontos, 1984) complex. A narrow dike unit closely associated canic rocks were deformed during a pre- is similar to that seen in several ancient and with a tonalite pluton occurs on the eastern side Nevadan event. Ii: is not known, and may be modern ensimatic arcs (GDI, 1970, 1981; Shi- of the plutonic rocks. These relations resemble impossible to determine, whether the essentially roki and others, 1978). The gabbro-diorite- those that might be expected in an evolving vol- structureless massive diabase unit was involved tonalite intrusive series is also typical of volcanic canic edifice rather than the "layer cake" one in this deformation. (4) Rifting of the volcanic arcs, especially ensimatic arcs developed on might expect to develop at an active spreading edifice and emplacement of the tonalite, oceanic basement. The mineralogy and geo- center. Thus, we conclude that the intrusive granophyric tonalite, and gabbro-diorite plutons chemistry of the gabbro-diorite plutons are rocks of the Smartville complex represent the occurred simultaneously and followed deforma- characteristic of cumulate and noncumulate subvolcanic core of an incipiently rifted volcanic tion of the volcani: rocks. The primary manifes- gabbros from volcanic arcs and unlike those arc. The rifting may represent the early stages of tation of intra-arc extension in the Smartville gabbros found in association with mid-ocean back-arc basin development.

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Davis, G. A., Burchfiel, B. C„ and Monger, J., 1978, Mesozoic construction of Moores, E. M., and Vine, F. J., 1971, The Troodos massif, Cyprus and other ACKNOWLEDGMENTS the Cordilleran "collage," central British Columbia to central California, ophiolites as ocean crust: Evaluation and implications: Royal Society of in Howell, D. G., and McDougall, K. A., eds., Mesozoic paleogeog- London Philosophical Transactions, v. 268, p. 443-466. raphy of the western United States: Society of Economic Paleontologists Morrice, M. G., Jezek, P. A., Gill, J. B., Whitford, P. J„ and Monoarfa, M., This paper is based on part of the Ph.D. dis- and Mineralogists, Pacific Section, Pacific Coast Paleogeography Sym- 1983, An introduction to the Sangihe arc: Volcanism accompanying posium, 2nd, p. 1-32. arc-arc collision in the Molucca Sea, Indonesia: Journal of Volcanology sertation of the senior author. We wish to thank Day, H. W„ Beiersdorfer, R. E., and Fenn, P. M., 1982, Chemistry of volcanic and Geothermal Research, v. 19, p. 135-165. our colleagues A. C. Tuminas, S. E. Edelman, clinopyroxene from the Smartville complex, northern Sierra Nevada Murphy, T. P., and Moores, E. M., 1985, Two ophiolitic tectonostratigraphic foothills: Geological Society of America Abstracts with Programs, v. 14, terranes in the western central belt, northern Sierra Nevada, California: C. Xenophontos, and, especially, E. M. Moores p. 158. Geological Society of America Abstracts with Programs, v. 17, p. 372. Day, H. W., Moores, E. M., and Tuminas, A. C., 1985, Structure and tectonics Neumann van Padang, M., 1951, Catalogue of the active volcanoes of the for their support and interest during the course of the northern Sierra Nevada: Geological Society of America Bulletin, world. Part I, Indonesia: Naples International Volcanological of this study. E. M. Moores, A. A. Finnerty, v. 96, p. 436-450. Association. Day, S. D., 1977, The petrology of a mafic dike complex near Smartville, Yuba Newhall, C. G., 1979, Temporal variations in the lavas of Mayon volcano, P. Schiffman, and S. Sorensen kindly read the County, California [M.S. thesis]: Davis, California, University of Cali- Philippines: Journal of Volcanology and Geothermal Research, v. 6, fornia, 113 p. p. 61-83. manuscript at various stages. We are grateful for Eddy, C. A., Bobbitt, J. B., and Day, H. W., 1986, The Yuba River pluton: Perfit, M. R„ Brueckner, H., Lawrence, J. R., and Kay, R. W., 1980, Trace critical reviews of the manuscript by G. Harper, Implication of variations in composition for the interpretation of iso- element and isotopic variation in a zoned pluton and associated volcanic topic ages: Geological Socieiy of America Abstracts with Programs, rocks: A model for fractionation in the Aleutian calc-alkaline suite: C. Hopson, S. Kay, J. Saleeby, and J. Shervais. v. 18, p. 103. 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MANUSCRIPT RECEIVED BY THE SOCIETY MAY 3,1985 primary magmas and crustal evolution in the Aleutian arc: Journal of Moores, E. M., and Day, H. W., 1984, Overthrust model for the Sierra Nevada: REVISED MANUSCRIPT RECEIVED APRIL 6,1987 Petrology, v. 23, p. 88-125. Geology, v. 12, p. 416-419. MANUSCRIPT ACCEPTED APRIL 8,1987

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