Geological Society of America Bulletin

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Geological Society of America Bulletin The Smartville intrusive complex, Sierra Nevada, California: The core of a rifted volcanic arc JAMES S. BEARD and HOWARD W. DAY Geological Society of America Bulletin 1987;99, no. 6;779-791 doi: 10.1130/0016-7606(1987)992.0.CO;2

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Notes

Geological Society of America


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 Smartville complex is a Jurassic volcanic and plutonic arc in the northwestern Sierra Nevada that was deformed during the Late Jurassic Nevadan "orogeny." We interpret the Smartville intrusive complex to have formed during the incipient rifting of an active volcanic arc. This interpretation is supported by the close relationship between volcanism and plutonism and by the close association between sheeted dikes and the younger plutonic rocks. Further support is found in the similarities that exist between the Smartville complex and modern arcs that developed on oceanic crust, either at a continental margin or in the ocean basins. Smartville volcanic rocks consist of a lower unit of tholeiitic submarine flows and pillowed flows that grades upward into an upper unit of calc-alkaline pyroclastic and volcaniclastic deposits. Intrusive rocks include older units of metamorphosed gabbro and massive diabase, which are intruded by a unit of 100% mafic and felsic, sheeted and 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 dike unit. Continuously and reversely zoned gabbro-diorite plutons are also coeval with the dike complex. Granodiorite plutons are the youngest intrusive unit and may be related to the Sierra Nevada batholith.

the intrusion of the sheeted dike complex. Clasts of plutonic and hypabyssal rocks resembling the younger intrusives, however, occur locally in some of the youngest volcaniclastic rocks, suggesting that shallow plutonism and volcanism could be broadly coeval. Early Nevadan thrust faults juxtapose volcanic and older plutonic rocks of the Smartville complex and Mesozoic(?) chert-argillite broken formation to the east. The younger plutons and the dike complex in the Smartville are deformed by steep, late Nevadan faults and do not intrude chert-argillite broken formation. The youngest granodiorite plutons in the area truncate Nevadan faults, intrude the chert-argillite formation, and appear to be unrelated to the Smartville complex. The Smartville volcanic arc underwent pre-Nevadan intra-arc extension. The sheeted dike complex is the primary manifestation of the rifting event. The elongate shapes of the younger plutons, which are coeval with the dike complex, reflect extensional control on their emplacement. Volcanic rocks in the

INTRODUCTION The Smartville complex is a Late Jurassic sequence of volcanic, hypabyssal, and plutonic rocks that forms the westernmost of the four main fault-bounded lithotectonic belts of the northern Sierra Nevada (Fig. 1). The relationship of these belts to each other and the tectonic evolution of the northern Sierra Nevada has been controversial (Moores, 1970, 1972; Schweickert and Cowan, 1975; Davis and others, 1978; Saleeby, 1981; Schweickert, 1981; Day and others, 1985). Two schools of thought have emerged to explain the distribution of Middle to Upper Jurassic volcanic rocks that occur in the western, central, and eastern belts of the Sierra Nevada and in parts of the Klamath Mountains. Some workers (Davis and others, 1978; Burchfiel and Davis, 1981; Saleeby, 1981, 1982; Harper and Wright, 1984) have interpreted the arc rocks in these areas as the prod-

120°

LEGEND 1 1 1 ! EASTERN BELT (FBI FEATHER RIVER CFB) PERIDOTITE BELT ::

Relative age relations suggest that the Smartville complex formed in a single volcanic-plutonic system of pre-Nevadan age. The dike complex intrudes and is intruded by both the tonalite and zoned gabbro-diorite plutons. Both the upper and lower volcanic units are intruded by all of the plutonic and hypabyssal units and were deformed prior to *Present address: M.S. SN4, National Aeronautics and Space Administration, Johnson Space Center, Houston, Texas 77058.

western and southern Smartville complex were deformed prior to the intrusion of the dike complex.

| CENTRAL BELT CCD]

BRB BALD ROCK BATHOLITH SNB SIERRA NEVADA BATHOLITH YRP YUBA RIVERS PLUTON o

1 0 2 0 KM

WESTERN BELT SMARTVILLE COMPLEX Ijll Intrusive Rocks (SI] cAiiuùivy

nouKb

v.ovj

Figure 1. Sketch of the northern Sierra Nevada, showing the location of the Smartville complex and the study area. Belt nomenclature after Day and others (1985) and Schweickert (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 during Late Jurassic oblique convergence at the continental margin. Other workers (Moores, 1970, 1972; Schweickert and Cowan, 1975; Schweickert, 1981; Day and others, 1985) believe that the Smi.rtville complex is not related to Jurassic volcanic rocks in the eastern belt and that more than one Late Jurassic subduction zone is representee! in northern California. Moores and Day (1984) and Day and others (1985) correlated volcanic and plutonic rocks in the central belt with the Smartville complex and argued that the volcanic rocks are in tectonic contact with chei t-argillite broken formation and mélange in the central belt. They have proposed that the Smartville complex was thrust over the central belt and subsequently deformed by steep folds and faults during the Late Jurassic Nevadan orogeny. The origin of the Smartville complex itself has also been the subject of considerable discussion. Early workers (Lindgren and Turner, 1895; Hietanen, 1951, 1973, 1976; Compton, 1955; Clark, 1960, 1964) interpreted the volcanic and hypabyssal greenstones of the Smartville as a Triassic or Jurassic volcanic terrane that was metamorphosed, deformed, and subsequently intruded by a series of post-kinematic plutons. The plutonic and volcanic rocks were generally considered unrelated. More recently, the upper part of an ophiolite pseudo-stratigraphy (gabbro, sheeted dikes, pillowed flows) was recognized within the Smartville complex, leading to its interpretation as an ophiolite (Moores, 1975; Cady, 1975; Xenophontos and Bond, 1978). These workers recognized that the ophiolitic rocks are overlain t y a thick section of intermediate volcanic rocks, suggesting a setting within or near a volcanic arc. Xenophontos and Bond (1978) interpreted the ophiolitic section as a young inter-arc basin, the edge of a larger marginal basin, or a pseudo-stratigraphic sequence developed by rifting at the base of an arc. Schweickert and Cowan (1975) and Day and others (1985) also proposed an arc-marginal basin setting for the Smartville, and Saleeby (1981) suggested that it represents an ensimatic arc developed on previously accreted material at the continental margin. The purpose of this paper is to evaluate the plutonic and hypabyssal rocks of the Smartville terrane in light of these various proposals. Our principal finding is v.hat the Smartville plutonic and hypabyssal rocks intruded volcanic rocks during pre-Nevadan extension of an active volcanic arc. Intrusive and extrusive rocks in the Smartville appear to be broadly coeval and can be interpreted as parts of a single volcano-

BEARD A N D DAY

plutonic complex. Volcanic rocks were deformed prior to the intrusion of plutons, which were emplaced at the same time as a sheeted dike complex. Both volcanic and plutonic rocks were deformed by Nevadan-age faults. The only demonstrably post-Nevadan igneous rocks in the area studied are some small granodiorite plutons. These, and some larger plutons outside the study area, may represent the onset of magmatism related to the Sierra Nevada batholith. THE SMARTVILLE COMPLEX The Smartville complex (Fig. 1) consists of Upper Jurassic volcanic, plutonic, and hypabyssal rocks. Volcanic rocks occur in a broad belt in the western and southern Smartville complex and in a narrow region along the eastern boundary of the complex. The plutonic and hypabyssal rocks occur in an elongate, north-northwesttrending intrusive complex in the eastern and central Smartville complex (Fig. 1). The complex is bounded on the north and east by the Big Bend-Grass Valley-Wolf Creek fault zone (Hietanen, 1977; Tuminas, 1983; Day and others, 1985). On the west, the complex is overlain unconformably by Cretaceous and younger sedimentary rocks in the Great Valley. Although it seems clear that the Smartville complex correlates with western belt rocks farther south, the detailed correlation of those rocks with the Smartville complex is uncertain. The volcanic rocks of the Smartville complex SV (Fig. 1) were not examined in detail for this study. Earlier workers (Xenophontos and Bond, 1978; Buer, 1979; Menzies and others, 1980; Xenophontos, 1984) recognized two major volcanic units in the western and southern Smartville complex. The lower volcanic unit (slv, Fig. 2) consists of mafic to intermediate flows, pillowed flows, and pillow breccias. It is restricted to that portion of the Smartville complex west of the intrusive complex. The upper volcanic unit (suv, Fig. 2) consists of intermediate-composition pyroclastic and volcaniclastic rocks and is more widespread. The contact between the lower and upper volcanics is conformable and gradational (Xenophontos and Bond, 1978; Xenophontos, 1984). Menzies and others (1980) examined the trace-element geochemistry of the volcanic rocks and showed that the lower volcanic unit has tholeiitic characteristics, whereas the upper volcanic unit is largely calc-alkaline. Day and others (1982) showed that clinopyroxenes from both volcanic units are typical of either volcanic arc or ocean floor basalts. Smartville intrusive rocks include older units

of metagabbro (mgb, Fig. 2) and metamorphosed massive diabase (md, Fig. 2). These are intruded by a sheeted dike complex (sdc) and a series of tonalité (bht), granophyric tonalité (gt), gabbro-diorite (gb), and granodiorite (grd) plutons. Each of these intrusive lithologies will be discussed in the next section. Direct evidence for the nature of the basement on which the Smartville complex was constructed is lacking, but basement probably includes older ophiolitic rocks. Serpentinite and metagabbro of unknown age occur along the Grass Valley-Wolf Creek fault zone and are intruded by massive diabase similar to that of the Smartville complex intrusive core (Tuminas, 1983). In addition, volcano-plutonic complexes in the central belt that have been correlated with the Smartville complex have ophiolitic basement (Day and others, 1985; Tuminas, 1983; Murphy and Moores, 1985). Age of the Smartville Complex Isotopic ages for the Smartville complex are similar to those of other rocks in the western belt (Saleeby, 1981, 1982). Available U-Pb ages from zircon include those for (1) upper volcanic unit (Bloomer Hill Formation), northern Smartville complex: 159 Ma (Saleeby, 1981); (2) plagiogranite (called "granophyric tonalité" in this report) associated with the Smartville sheeted dike complex: 160 Ma (McJunkin and others, 1979), 159 Ma (Saleeby and Moores, 1979), 163 Ma (Saleeby and Moores, 1984); (3) the Yuba Rivers pluton, a synorogenic tonalité that intrudes and is deformed by faults in the Grass Valley-Wolf Creek fault zone (Bobbitt and others, 1986; Eddy and others, 1986): 160 Ma (J. B. Saleeby, 1985, written commun.). The close correspondence of ages within the Smartville to that for the Yuba Rivers pluton suggests that magmatism and deformation occurred within a very restricted time. Deformed ultramafic and mafic rocks exposed in the central belt, just north of the Smartville complex, and similar to possible Smartville basement rocks exposed elsewhere, are intruded by an undeformed plagiogranite dike that yields a U-Pb zircon age of 159 Ma (J. B. Saleeby, 1984, personal commun.). INTRUSIVE ROCKS OF THE SMARTVILLE COMPLEX The intrusive core of the Smartville complex consists of an older unit of massive diabase and associated metadiorite (md, Fig. 2) and metagabbro (mgb) and a younger series of zoned

Downloaded from gsabulletin.gsapubs.org on April 13, 2012 CORE OF A RIFTED VOLCANIC ARC, CALIFORNIA

gabbro-diorite (gb), granophyric tonalite (gt), and tonalite (bht) plutons that were emplaced at the same time as the Smartville dike complex (sdc).

781

39° 30'

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 metagabbro. 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 Tuminas (1983).

EXPLANATION grd

Granodiorite Unit. Post-Nevadan granodiorite plutons. Locally g r a d a t i o n a l to granite.

SMARTVILLE COMPLEX VOLCANIC ROCKS

m

suv

Upper Volcanic Unit. Intermediate c o m p o s i t i o n p y r o c l a s t i c and epiclastic r o c k s

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 and mafic dikes.

gb

Gabbro-Diorite Unit. Zoned gabbro-diorite p l u t o n s . Solid p a t t e r n is o l i v i n e g a b b r o .

md

M a s s i v e D i a b a s e Unit. Massive diabase and a s s o c i a t e d metadiorite and m e t a g a b b r o . 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 c o n t a c t s

felsic

Dike orientations 121° 4.5' 3

4 Km


fades metamorphism and consist primarily of albite, actinolite and/or blue-green hornblende, epidote, sphene, chlorite, and minor quartz. Subhedral zoned plagioclase crystals and rare clinopyroxene cores in amphibole are probably relict igneous phases. Brownish-green amphibole in some of the metadiorite may also be a relict igneous phase. Chilled margins are rare in the massive diabase, but the diabuse may occur in dikes or sills too large to be seen in the small exposures typical of much of the unit. In relatively good exposures southwest of the Pilot Peak pluton (Fig. 2), a few large dikes are evident in the massive unit. Metamorphism may have obscured or obliterated chilled margins in many places. Clark Hill Mixed Zone The northern half of the region underlain by massive diabase (Fig. 2) is called, informally, the "Clark Hill mixed zone." In the mixed zone, the massive diabase is the country rock in a complex zone of intrusive breccias formed by the intrusion of gabbro-diorite, tonalite, and younger, fine-grained diabase dikes related to the sheeted dike complex described below. At the contacts with some gabbroic plutons, the country rocks are migmatitic pyroxene hornfelses and may have been partially melted. Metagabbro A group of small metagabbro bodies (mgb, Fig. 2) scattered throughout the massive diabase and one large body of metagabbro east of Collins Lake are coeval and closely associated with the massive diabase. In addition, numerous small bodies and segregations of metagabbro occur throughout the massive diabase. Like the massive diabase, these rocks are pervasively altered. Relict igneous phases include zoned plagioclase, clinopyroxene, and Fe-Ti oxides. A few samples that were particularly rich in oxides were collected from the metagabbro near Collins Lake. Subophitic textures are common in many of the intrusions. Unlike the younger gabbro-diorite plutons, zoning apparently is not present in the metagabbro. Sheeted Dike Complex The Smartville dike complex (sdc, Fig. 2) occurs in a 2- to 10-km-wide, curvilinear region in the western part of the study area and in a much smaller, north-south-trending belt southeast of Dobbins. The dike complex consists of essentially 100% sheeted and unsheeted, felsic and mafic dikes. In the south, from Route 20 to the Collins Lake area, the complex trends north-

BEARD A N D DAY

west. North of Collins Lake, the complex trends north-northeast toward Challenge. The change in trend of the dike complex in this region is reflected in the attitudes of individual dikes (Fig. 2; see also Figs. 11 C and D). The contact between the Swede's Flat pluton and the dike complex (Marlette and others, 1978) was not mapped, but our reconnaissance mapping suggests that the dike unit continues north of the area shown in Figure 2 to the vicinity of the Bald Rock batholith (Fig. 1). Dikes are common within both the volcanic and plutonic units of the Smartville complex. In many cases, these dikes appear to be directly related to the sheeted dike complex, with the ratio of dikes to country rock increasing toward the dike unit contact. This relationship occurs in several places. In the Yuba River south of Dobbins, dike swarms in the volcanic section to the east and in the Pleasant Valley pluton to the west culminate in a narrow unit of 100% dikes. Near Route 20, the dike complex dies out southward into scattered dikes in the massive diabase and gabbro-diorite units. Finally, the unit of 100% dikes grades into scattered dikes in a granophyric tonalite pluton in spectacular exposures along Deer Creek, west of the Pleasant Valley pluton (Fig. 2). Buer (1979) reported that a similar relationship exists between the dike complex and the lower volcanic unit to the west. S. D. Day (1977) showed that the sheeted dikes range in composition from soda rhyolite to diabase. Diabase and other mafic dikes are by far the most abundant variety. Mafic dikes are plagioclase- and pyroxene-phyric and contain both of these minerals, along with Fe-Ti oxides, in their groundmass. Replacement of pyroxene by actinolite or blue-green hornblende and of plagioclase by albite, epidote, and unidentified phyllosilicates is common and locally pervasive. Other metamorphic minerals include chlorite and sphene. Several varieties of felsic dikes occur in the complex. Quartz albitite dikes (Day, 1977) are petrographically identical to some of the rocks in the granophyric tonalite plutons. More common felsic dikes contain plagioclase and, more rarely, hornblende phenocrysts in a quartz-albite mosaic groundmass. Penetrative deformation is rare in the dike complex, and it appears that metamorphism was an autometamorphic process (Beiersdorfer and Day, 1983). Gabbro-Diorite Gabbro and diorite occur in three large, elongate, zoned plutons and in a number of smaller intrusions (gb, Fig. 2). The major plutons trend north-northwest through the center of the study area. From south to north, they are (1) the Pilot Peak pluton, a northwest-trending body that be-

comes narrower toward the southeast, (2) the Pleasant Valley pluton, an irregular, northsouth-trending body that narrows toward the north, and (3) the Indiana Creek pluton, a smaller, northeast-trending ovoid pluton. The trends of the long axes of these plutons are parallel to the orientations of dikes in the dike complex, shifting in orientation from northwest in the south to northeast in the north (Fig. 2). The gabbro-diorite plutons consist of medium- to coarse-grained rocks ranging in composition from olivine gabbro to quartz diorite. Figure 3 shows the distribution of rock types in the plutons. The contacts between all units in the gabbro-diorite plutons are gradational. Changes in rock composition or texture occur over distances ranging from a few to a few hundred metres. The contacts between rock units are defined by the presence or absence of a key mineral (usually determined petrographically), by unusual mesoscopic features (such as large oikocrystic hornblende crystals), by changing relative abundances of minerals (especially color index), and, in one case, by the presence or absence of a metamorphic overprint. Olivine gabbro (ogb, Fig. 3) occurs in all of the major plutons and is the primary constituent of most of the small gabbro intrusions. The internal contacts of the olivine gabbro with other lithologies are defined by the first appearance of olivine in thin section. It is a massive to weakly foliated rock with locally developed, discontinuous layers of gabbro and anorthosite. Gabbro pegmatites are common in the olivine gabbro. Irregular pods and tabular masses of olivine clinopyroxenite are locally common within the olivine gabbro. The primary minerals in the olivine gabbro are unzoned to weakly zoned plagioclase (An85-An95),' clinopyroxene, olivine, and, in most samples, orthopyroxene. Orthopyroxene occurs as rims on olivine and less commonly as discrete grains. Dark brown, red-brown, or nearly colorless amphibole is common and occurs as late magmatic interstitial crystals and oikocrysts and as a post-magmatic patchy replacement of pyroxene. Accessory minerals include magnetite, apatite, and traces of redorange biotite. Magnetite is present as symplectitic intergrowths with olivine and orthopyroxene and is probably a late-stage oxidation product (Haggerty, 1976; Ambler and Ashley, 1977). Magnetite is also a primary mineral in most samples. •Plagioclase compositions in the olivine gabbros were determined by electron microprobe at the University of California at Davis. All other plagioclase compositions reported herein were determined by optical methods using a five-axis universal stage. Compositional data are tabulated in Beard (1985).


CORE OF A RIFTED VOLCANIC ARC, CALIFORNIA

Gabbronorite is present in all of the large gabbro-diorite plutons (pgb, Fig. 3) and in some of the smaller gabbro intrusions. The unit includes some dioritic rocks in the Indiana Creek pluton that could not be shown on the scale of Figure 3. The rock is weakly foliated, with local development of discontinuous compositional layering. Primary minerals in the unit are zoned plagioclase (rims, An50-An75; cores, An70An90+), clinopyroxene, orthopyroxene, and olive-green to brown or red-brown hornblende. Most samples contain small amounts of quartz and dark brown biotite, but either or both of these phases may be absent near the contact with the olivine gabbro. Accessory phases include magnetite, ilmenite, and apatite. A twopyroxene gabbro containing large oikocrysts of red-brown hornblende was mapped as a separate unit in the Pilot Peak pluton (hgb, Fig. 3). Biotite-hornblende diorite (bhd, Fig. 3) is a minor unmapped constituent of the Indiana Creek pluton but makes up a large part of the other two gabbro-diorite plutons. It is generally a massive to weakly foliated rock. It is distinguished in the field from the gabbronorite by its lower color index and its lack of layering. The rock consists of zoned plagioclase (rims, An35-An50; cores, An50-An85), green-brown to green hornblende, dark brown biotite, and as much as 12% quartz. Most samples contain some, and many contain substantial amounts of, orthopyroxene and clinopyroxene. The modal abundance of pyroxene generally decreases as that of quartz increases. Some of the rocks rich-

est in quartz are pyroxene free. Accessory minerals include magnetite, ilmenite, sphene, apatite, and scarce zircon. Most samples contain small amounts of potassium feldspar. Biotite-two-pyroxene monzodiorite (bpg, Fig. 3) is a weakly foliated to massive rock that occurs as a mappable unit only in the Pilot Peak pluton. The contacts of the monzodiorite are sharp, but they do not truncate any features in other units of the Pilot Peak pluton. In thin section, the rocks are characterized by the absence of amphibole, except for small amounts of latestage, deuteric blue-green hornblende or actinolite and abundant potassium feldspar (Table 1). In addition to potassium feldspar, strongly zoned plagioclase (rims, An35-An50; cores, An70An90+), clinopyroxene, orthopyroxene, dark brown biotite, and quartz are the primary minerals of the monzodiorite. Accessory minerals include magnetite, ilmenite, apatite, and zircon. Much of the eastern margin of the Pleasant Valley pluton is made up of gabbro and diorite (mg, Fig. 3) that was metamorphosed during the intrusion of the tonalite unit to the east. The metagabbro (mg, Fig. 3) mapped at the south end of the Pilot Peak pluton may be related to the hydrothermally altered gabbros associated with the massive diabase unit (mgb, Fig. 2). Although all of the mafic phases in the rock are now altered to green hornblende, zoned plagioclase found in these rocks is probably a relict igneous phase.

Mineral Variation in the Gabbro-Diorite Unit Modal data demonstrate that complete gradation exists between most rock types in the Pilot Peak and Pleasant Valley plutons and suggest that their zoning is continuous and not the result of multiple intrusive events. The mean modal mineralogy of rocks from the Pilot Peak and Pleasant Valley plutons is listed in Table 1. Complete modal data are given by Beard (1985). Although the average modal composition of the rock types in the plutons is distinctive, the range of composition is considerable (Table 1). The gabbronorite and the biotitehornblende diorite exhibit substantial compositional overlap. The amount of olivine in the olivine gabbro varies from trace amounts to >15%, whereas some samples of gabbronorite collected near the olivine gabbro contact contain neither quartz nor olivine. A single sample collected from the Pilot Peak pluton contains trace amounts of relict olivine surrounded by orthopyroxene and (31)

Granodiorite grd (20)

12.8 (5-22)

21.5 (14-50)

12.2

(6-18)

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. '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 some rocks containing >10% quartz. This may indicate formation and incomplete removal of very calcic plagioclase early in the history of an evolving magma system. Tonalite Two tonalite units were mapped in the study area, a granophyric hornblende tonalite unit (gt, Fig. 2) and a biotite-hornblende tonalite unit (bht). Both are present in elongate north-southto north-northwest-trending plutons. Elongate granophyric tonalite plutons are found at the eastern and western margins of the plutonic complex. In the Collins Lake area, granophyric tonalite is also found in a series of small, equant intrusives and as dikes in the sheeted dike com-

Figure 4. Composition of the Pilot Peak pluton. Filled circles are sample localities. Dashed lines are lithologic contacts from Figure 3. (A) Modal abundance of quartz and olivine. Quartz abundances are based on 1,000 points counted on slabs etched and stained for potassium feldspar. Olivine was determined in thin section (compare with Table 1). (B) Composition of the most calcic plagioclase (cores). Plagioclase compositions were determined on a universal stage using the a-normal method. ( Q Composition of the most sodic plagioclase (rims).

plex. The biotite-hornblende tonalite forms a series of six plutons extending from north of Dobbins south to the Pilot Peak pluton (Fig. 2). Intrusion of biotite-hornblende tonalite formed large areas of intrusive breccia in the southeastern part of the Pleasant Valley pluton and in parts of the Clark Hill mixed zone. The easternmost granophyric tonalite pluton and the largest biotite-hornblende tonalite pluton are in contact north of Deer Creek (Fig. 2). Biotite-hornblende tonalite intrudes granophyric tonalite north of the South Yuba River, but farther south, the contact is defined only on the basis of the scarcity of biotite in the eastern parts of the tonalite. The granophyric tonalite (gt) contains quartz, zoned plagioclase, and acicular green hornblende. In the central and eastern plutons, plagioclase is normally zoned from An20 to An35, but in the western body and in the small intrusions and dikes associated with it, plagioclase is zoned from An5 to An20. Accessory phases are apatite, magnetite, sphene, and traces of dark brown biotite. Most samples have undergone nonpervasive low-temperature metamorphism or alteration. Metamorphic minerals include epidote, actinolite, albite, sphene, and stilpnomelane. The granophyric tonalite contains micrographic intergrowths of quartz and plagioclase. The intergrowths are not myrmekitic but closely resemble intergrowths seen in more common alkali-feldspar granophyres. The modal abundance of Ihese intergrowths varies greatly, even



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 Xenophontos (1984) described the widespread occurrence of prehnite and pumpellyite in the volcanic rocks of the Smartville complex. Beiersdorfer (1982) concluded that the regional metamorphism 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).

Figure 5. Composition of the Pleasant Valley pluton. Methods and symbols as in Figure 4. The eastern margin of the pluton is disrupted by later intrusion of tonalité and dike complex. (A) Modal quartz and olivine. (B) Most sodic plagioclase (rims). (C) Most calcic plagioclase (cores).

among samples taken from a single outcrop, and nongranophyric hornblende tonalité is common within the unit. The biotite-hornblende tonalité unit (bht) consists principally of medium- to coarsegrained, massive biotite-hornblende tonalité. A few samples of granophyric hornblende tonalité, identical to those in the granophyric tonalité unit, were also collected. The essential minerals of the biotite-hornblende tonalité are zoned plagioclase (average core, An44; average rim, An22), quartz, olive-green hornblende, and dark brown biotite. Most samples contain small amounts of potassium feldspar, as much as 10% in one case. Accessory minerals include magnetite, ilmenite, apatite, and zircon. The tonalité and granophyric tonalité units occur in markedly elongate plutons. The largest of the biotite-hornblende tonalité plutons, exposed in the South Yuba River and Deer Creek Canyons, is weakly zoned, having a quartz-rich and mafic-poor core (Fig. 6), suggesting that it represents a single episode of magma injection. Granodiorite Granodiorite occurs in a small, weakly zoned pluton north of the Higgins Corner window and

in three small satellite intrusions (grd, Fig. 2). The largest of the satellites, 2 km west of the main pluton, is associated with a large area of intrusive breccia, six times the size of the satellite itself. The granodiorite consists of zoned plagioclase (cores, An50; rims, An3-An20), quartz, biotite, hornblende, and interstitial microcline microperthite. Accessory minerals include magnetite, ilmenite, apatite, and zircon. Modal data from the granodiorite and tonalite (Table 1, Fig. 7) clearly show the compositional dissimilarities and lack of gradation between the two units. Note that the granodiorite, although much richer in potassium feldspar and poorer in mafic minerals than is the tonalite, also contains, on the average, less quartz. The composition of plagioclase in the cores of zoned crystals in the granodiorite averages An50 and varies little

Figure 6. Composition of the largest tonalite pluton, exposed in the Deer Creek and South Yuba Canyons. (A) Modal quartz content. (B) Total mafic minerals. Note that the zoning is weaker than in the gabbro-diorite plutons and that it is in a normal direction. Modes are based on 1,000 points counted on stained slabs.

The hypabyssal rocks and the older metagabbro 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 contain actinolite + epidote + albite + chlorite in volcanic rocks containing pumpellyite + prehnite and which show no evidence of a greenschist overprint (Beiersdorfer, 1982). The younger plutonic rocks are generally fresh and unmetamorphosed except for local deuteric alteration and contact-metamorphic effects.

0 1

1 i

km

2 i


BEARD A N D DAY T A B L E 2. C R O S S C U T T I N G R E L A T I O N S H I P S E N U M E R A T E D IN F I G U R E 8

Relationship

Quadrangle*

Location

Along all contacts

(1) Pilot Peak pluton cuts massive diabase

RR, W

(2) Pleasant Valley pluton cuts massive diabase

RR

Near town of Rough and Ready

(3) Indiana Creek pluton cuts massive diabase

C

Clark Hill mixed zone

(4) O t h e r gabbro-diorite cut; massive diabase

FC

Marysville road, east of Dobbins

(5) Granophyric tonalite cuti massive d

FC, G V

Western and central gt plutons, near Englebright Lake; eastern gt pluton, south of Deer Creek

(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, west of Dobbins

(8) Metagabbro = massive diabase

GV

Mutually crosscutting relations near Wolf Mountain

(9) Granophyric tonalite cuts metagabbro

O, FC

Numerous examples on east shore, Collins Lake; also at Englebright Lake

FC, R R

East shore, Collins Lake; metagabbro screens in dike complex in D r y Creek, south of Collins Lake; west side of Clark Hill mixed zone

(10) Dike complex cuts metagabbro

(11) Dike complex cuts Pleasant Valley pluton

Yuba River Canyon, southeast of Dobbins; scattered dikes cut pluton near Clear Creek and Petin Valley

(12) D i k e complex cuts granophyric tonalite

FC, R R

Excellent exposures in central pluton in Deer Creek; numerous dikes in western pluton, Englebright Lake area

(13) Dike complex cuts tonalite

FC, R R

Clear Creek area; South Yuba River Canyon, east of Bridgeport

(14) and (15) Pleasant Valley and Pilot Peik plutons cut dike complex

RR

Metamorphosed dikes in septum between two plutons

(16) Other gabbnwiiorite cuts tlike complex

FC

Metamorphosed dikes in west part of Clark Hill mixed zone

(17) Indiana Creek pluton cuts dike complex

R,C

Along west side of pluton

(18) Granophyric tonalite cuts ( i k e complex

O

West abutment of bridge over Collins Lake, Marysville road

(19) Tonalite cuts dike complex

FC

East of Dobbins, Marysville road

(20) Granophyric tonalite cuts [feasant Valley pluton

FC, R R

Gabbroic inclusions in tonalite near Piper Hill and along Pleasant Valley road

(21) Granophyric tonalité = tonalité

RR

Gradational contact along Deer Creek

(22) Tonalite cuts granophyric tonalité

FC

Near town of Birchville on San J u a n Ridge

(23) Other gabbro-diorite cuts g'anophyric tonalite

FC

Small gabbro intrusion on San J u a n Ridge metamorphoses eastern granophyric pluton

(24) Tonalite cuts Pleasant Valley pluton

FC

Along east margin of the pluton, South Yuba canyon, east of Bridgeport

(25) Tonalité cuts other gabbro-diorite

FC

G a b b r o bodies east of Dobbins and in Clark Hill mixed zone, cut by tonalite dikes

(26) Tonalite cuts Pilot Peak pluton

RR

Northeast side of pluton cut by small tonalite body

• Q u a d r a n g l e abbreviations; C, Challenge; FC, French Corral; GV, Grass Valley; O, Oregon House; R, Rackerby; R R , Rough and Ready; W , Wolf. Consult these 7V)-minute quadrangles for detailed place names.

Contact metamorphism by the plutons overprints the regional metamorphism and is limited to aureoles, in most cases < 100 m wide, around the younger plutonic rocks. Close to the contacts of gabbroic inirusives, pyroxene hornfels occur (two pyroxenes, labradorite, brown hornblende), which in most cases, grade into hornblende hornfels within a few metres or tens of metres of the contact. Within the Clark Hill mixed zone and around some small olivine gabbro bodies west of Bullards Bar Reservoir, however, uncharacteristically wide zones of pyroxene hornfels and migmatite are present within metamorphosed volcanic and hypabyssal rocks. Hornblende hornfels (green to brown

hornblende, andesine or oligoclase, Fe-Ti oxides) occur around the more felsic intrusives. Because of the regional metamorphism, the precise limits of the contact aureoles are difficult to determine. Other contact effects include the development of intrusive breccia zones and contact foliations defined by agmatitic structure, orientation of platy xenoliths, and the local development of planar fabric in both the intrusive and the country rock. Extensive zones of intrusive breccia are commonly associated with the tonalite plutons, suggesting that they are currently in the process of being unroofed (Pitcher and Berger, 1972). The intrusive breccias and agmatite zones sur-

rounding the gabbro-diorite plutons are generally narrow, commonly less than a hundred metres wide. RELATIVE AGE OF INTRUSION AND E>EFORMATION IN THE SMARTVILLE COMPLEX Crosscutting Relations in the Intrusive Complex Crosscutting relations among the intrusive rocks of the Smartville complex are outlined in Table 2 and illustrated in Figure 8. These relations show that there are two distinct episodes of magmatism within the intrusive core of the Smartville complex and that there is a trend toward more silicic plutonism through time. The granodiorite plutons are interpreted as a later, unrelated: intrusive event, for reasons discussed later, and are not included in the discussion below. Massive diabase and associated metagabbro (md, Fig. 2) are the oldest intrusive rocks in the area (Table 2, Fig. 8). They are cut by gabbrodiorite, both tonalite units, and the dike complex. Turninas (1983) reported mutually crosscutting relations between the massive diabase and some small metagabbro intrusions southeast of the Pilot Peak pluton (Fig. 2). A large metagabbro pluton exposed east of Collins Lake is mineralogically and texturally similar to these intrusions and is cut by dike complex and granophyric tonalite. For these reasons, we believe that this pluton also formed during early stages of intrusive activity. Tonalite is generally the youngest plutonic unit and is commonly intrusive into gabbrodiorite and granophyric tonalite. Contradictory and mutually crosscutting relations, however,

Q

Figure 7. Quartz-plagioclase-potassium feldspar composition of the tonalite and granodioriite units.



TONALITE

of its confluence with the Yuba River, in which sheeted diabase cuts and is cut by tonalite (Fig. 9). Relationship to Volcanic Rocks The oldest intrusive rocks (massive diabase and metagabbro) intrude the lower volcanic unit just west of the study area and the upper volcanic unit southwest of the Pilot Peak pluton (Fig. 2). Younger intrusive rocks, including the dike complex, tonalite, and gabbro-diorite, intrude upper and lower volcanic-unit rocks wherever they are in contact. For the most part, intrusion of dikes and plutons in the Smartville complex postdated volcanic activity.

MASSIVE DIABASEl METAGABBRO Figure 8. Diagrammatic representation of crosscutting relations among the intrusive rocks. Arrows point to younger rock; a double line represents a gradational contact or mutually crosscutting relationship. Numbers correspond to examples given in Table 2. P P P = Pilot Peak pluton, P V P = Pleasant Valley Pluton, ICP = Indiana Creek pluton, OGD = other gabbro-diorite.

are common among all of the younger intrusive rocks, reflecting their overall age equivalence. In particular, the dike complex proper or dikes that are clearly related to the dike complex intrude all rock types. This includes exposures of sheeted dikes along the South Yuba River, ~ 1.5 km east

Some volcaniclastic rocks exposed along Deer Creek in the eastern Smartville complex are tentatively correlated with the upper parts of the upper volcanic unit (Fig. 2) and contain clasts of gabbro, gabbro pegmatite, tonalite, and diabase (Fig. 10). Neither fossil nor isotopic age data are available for the host or clasts. Nevertheless, the intrusive rocks presently exposed in the Smartville complex are a possible source for the clasts. If Smartville complex plutons are the source, some volcanism occurred in the Smartville complex during or after the emplacement of the plutons. We cannot rule out the possibility, however, that the clasts represent a suite of pre-volcanic plutons older than the ones presently exposed in the Smartville complex. Structural Relations Reconnaissance structural analysis of the western dike complex and the volcanic rocks of the western and southern Smartville complex suggests that the volcanic rocks were deformed

Figure 9. Mutually crosscutting mafic dikes and tonalite, in the canyon of the South Yuba River ~ 2 km east of the Yuba River. A tonalite pluton in this area grades into a sheeted dike complex.

787

prior to the intrusion of the dike complex. Figures 11A and 1 IB are plots of poles to bedding and flow margins in, respectively, the upper volcanic unit (data from Springer, 1982, Bear River area, bedded pyroclastic and epiclastic rocks) and lower volcanic unit (data from Buer, 1979, and Xenophontos and Bond, 1978, west-central Smartville complex, mafic pillowed and unpillowed flows). The Bear River data (Fig. 11 A) define a girdle containing several maxima that strikes N60°E and dips 70° north. The planes normal to the maxima define an open, upright structure with an axis of rotation that trends north-northwest and plunges gently to the south. The data from the lower volcanic unit have a similar distribution (N66°E, 94° north, Fig. 1 IB). The variation that produces these girdles occurs on a scale of hundreds to as much as a few thousand metres. Mapping by Buer (1979), Xenophontos (1984), and Springer (1982) shows that map-scale variation in bedding and flow orientation can be explained by upright folds having a wavelength of 1 - 6 km. Figures 11C and 11D are plots of poles to dike margins from the main dike complex on the western side of the intrusive complex. These plots show that the dike orientations are distributed in two homogeneous subareas. Dikes from the southern part of the dike complex strike N30°W to N60°W and dip 60° northeast, whereas those from the northern part strike north-south to N20°E and dip 60° east or southeast. The two plots, taken together, form a single incomplete girdle that reflects a gradual change in strike of dikes in the dike complex over a distance of 30 km (see Fig. 2). The regional variation in dike orientation may have formed as a result of regional-scale deformation, or it may simply reflect original variations in

Figure 10. Oasts of plutonic rock in volcanogenic conglomerates interbedded with tuffaceous sediments, Deer Creek, near the eastern margin of the Smartville complex.


BEARD A N D DAY

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 approximately 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 incomplete 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 deformation older than the intrusion of the dikes. It might be argued that ductile folding of the volcanic 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 no evidence for any ductile deformation in the Smartville complex as a whole. Other means of producing the gird.es in the volcanic rocks, such as flexural-slip folding or rotations on unrecognized listric normal faults, would reorient any pre-tectonic dikes and produce girdles similar in quality to those observed in the volcanic rocks but with an orientation depending on the original strike of the dikes relative to the apparent fold axes. Dikes are widely distributed in the volcanic units and the mapped contact between the dike complex and the volcanic units is intrusive, gradational, and largely undeformed. It is unlikely, therefore, that the differences in orientation (Fig. 11) can be explained by a décollement between the two units. We conclude that the deformation of [he Smartville volcanic pile is older than the intrusion of the dikes. Pre-Nevadan Age of the Smartville Complex The clearest manifestation of the Nevadan deformation in the Smartville complex is the Grass Valley-Wolf Creek fault zone (Fig. 2) along its eastern margin. Tuminas (1983) and Day and others (1985) recognized four generations of Nevadan faults in this area. Early west-over-east thrusting (F-l, Fig. 2) was succeeded by highangle dip-slip motion along north-northweststriking (F-2, F-4, Fig. 2) and northeast-striking (F-3, Fig. 2) faults. The only rocks in the Smartville complex that are clearly deformed by the early, low-angle faults are the upper volcanic unit and the massive diabase unit, both of which

are in thrust contact with central belt chertargillite broken formation in the Higgins Corner window. Both of these units are also deformed by the high-angle Nevadan faults along most of the eastern margin of the Smartville complex. On the other hand, a granodiorite pluton (grd, Fig. 2) truncates F-4 faults at the northwest corner of the Higgins Corner window (Fig. 2). Lenses of andradite garnet in the chert-argillite near the granodiorite contact may be the contact-metamorphosed equivalents of limestone blocks occurring in the chert-argillite matrix (Xenophontos, 1984). The other intrusive rocks of the Smartville complex (sheeted dike complex, tonalite, gabbro-diorite) are not in contact with the early thrust faults. The tonalite, granophyric tonalite, and gabbro-diorite plutons are cut by later, highangle Nevadan faults. A locally intense deformation of the granophyric tonalite pluton exposed along Deer Creek in the eastern Smartville complex (Fig. 2) appears to be related to Nevadan faulting in that area. Discrete zones of mylonite first appear in this tonalite 2 km from the Grass Valley-Wolf Creek fault zone. Closer to the fault zone, the mylonites increase in size and number, and an anastomosing network of mylonite forms a near penetrative deformation of the tonalite at its closest approach to the fault (Fig. 12). This deformation in the plutons dies out to the north. Sharp (1980) reported that dikes similar in age and composition to those in the Smartville complex crosscut chert-argillite and western belt volcanic rocks south of the Smartville complex.

We have found no similar relations in critical areas examined for this and other studies of the Smartville complex. Ricci (1983) noted that dikes abundant in the upper volcanic unit near its thrust contact with chert-argillite in the northern Smartville complex and presumably related to the Smartville sheeted dike complex do not occur in the lower plate chert-argillite. DISCUSSION Comparison with Plutonic Rocks in Arcs The tonalite and gabbro-diorite plutons in the Smartville complex are lithologically similar to the gabbro-diorite and tonalite plutons that are the most common intrusive rock types in modern oceanic arcs. Tonalite and/or gabbro-diorite plutons occur in oceanic arcs in the Southwest Pacific (Shiroki and others, 1978; Chivas and others, 1982; Hines and Mason, 1978; Mason and MacDonald, 1978), in the inactive portions of the Antillean arc (Kesler and others, 1975), and in the inactive arc exposed on Fiji (Gill, 1970; Green and Cullen, 1973). Similar plutons occur in the older, outboard regions of some continental arcs (Moore, 1959; Smith and others, 1983; Regan, 1985). Arc tonalites include both the biotite-hornblende variety (Chivas, 1977; Mason and MacDonald, 1978) and plagioclase granophyres (Gill, 1970; Chivas, 1977). The gabbroic plutons typically contain a variety of gabbroic and dioritic rocks, including olivine gabbro, olivine clinopyroxenite, gabbronorite, and hornblende-biotite diorite/quartz diorite.

Downloaded from gsabulletin.gsapubs.org on April 13, 2012 CORE OF A RIFTED VOLCANIC ARC, CALIFORNIA

Figure 12. Mylonite in tonalite. The sample is from Deer Creek, near the eastern margin of the Smartville complex. Subdivisions in the scale are 1 mm.

Granodioritic rocks are abundant in some oceanic arcs, notably the Aleutians (Citron and others, 1980; Kay and others, 1983; Perfit and others, 1980; Byers, 1959), but are much more common in continental-arc batholiths. Plutonic inclusions occur in basaltic and andesitic volcanic rocks from most modern arcs, particularly those developed on oceanic crust or on accreted oceanic material at continental margins. In contrast to the plutonic suites, most of the xenoliths are cumulate rocks. Silicic inclusions, except for those that are clearly related to nearby basement rocks, are rare. Cumulate gabbroic xenoliths have been reported from the Aleutians (Conrad and Kay, 1984), the Lesser Antilles (Lewis, 1973; Arculus and Wills, 1980), the Marianas (Stern, 1979), Indonesia (Morrice and others, 1983; Newmann van Padang, 1951), the Philippines (Newhall, 1979), the Izu-Bonin arc (Kuno, 1962), Japan (Yamazaki and others, 1966; Kuno, 1962), Central America (Carr and Pontier, 1981; Walker, 1984), the Cascades (Heliker, 1985), Kamchatcha (Erlich and others, 1979), and New Guinea (Gust and Johnson, 1981). Cumulate rock types found as xenoliths include olivine gabbro, gabbronorite, hornblende gabbro with or without olivine, and, rarely, olivine- and clinopyroxene-rich ultramafic rocks. A study of the geochemistry of the plutonic rocks in the Smartville complex is in progress (J. S. Beard and H. W. Day, unpub. data) and a detailed discussion of the geochemistry here would be premature. Nevertheless, there are some aspects of the mineral and whole-rock chemistry that support the interpretation of the Smartville complex as an arc and, hence, will be briefly discussed. The mineral and whole-rock analyses plotted in Figures 13 and 14 are tabulated in Beard (1985).

Beard (1986) demonstrated that the compositions of coexisting olivine and plagioclase in cumulate gabbros from modern arcs are characteristic and effectively distinguish arc cumulate gabbros from cumulate gabbros associated with mid-ocean-ridge or ocean-island magmatism. Olivine gabbros from the cores of the Smartville zoned gabbroic plutons have olivine and plagioclase compositions that are indistinguishable from those in modern arcs (for example, the Lesser Antilles, Fig. 13). The olivine-free rocks of the zoned gabbrodiorite plutons have compositions similar to, and

100

90

80

70

60

50

PERCENT ANORTHITE

Figure 13. Compositions of plagioclase and olivine in cumulate gabbros from an arc (Lesser Antilles), a tholeiitic layered intrusion (Skaergaard), and an alkaline ocean island (Reunion). Smartville cumulate olivine gabbros (circles) plot in the field defined by cumulate olivine gabbro xenoliths from the Lesser Antilles. Modified after Arculus and Wills (1980).

789

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 submarine 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 lithologically similar to the zoned gabbro-diorite plutons of the Smartville complex occur throughout the Klamath Mountains and northern Sierra Nevada. These include the Bear Mountain igneous complex (Snoke and others, 1981) and the Emigrant Gap complex (James, 1971). Snoke and others (1982) noted that these plutons record a history of arc magmatism in the area. Because similar plutonic rocks are common and widespread in modern oceanic and ensimatic continental-margin arcs, however, care must be exercised in the use of such plutons as correlative tools in areas, such as the Sierra Nevada or Klamath Mountains, where more than one ensimatic arc may be represented (Moores and Day, 1984; Harper and Wright, 1984). Extension in the Smartville Complex The presence of a sheeted dike complex in the Smartville complex implies local 100% extension. Sheeted dikes are a characteristic part of the ophiolite pseudo-stratigraphy, and their presence, in part, led to the hypothesis that ophiolite complexes form at mid-ocean ridges (Moores and Vine, 1971). As discussed earlier, however, the Smartville dike complex is a relatively young feature and is not a part of the basement on which the Smartville volcanic rocks were deposited. Thus, the dikes must represent extension that occurred within the volcanic edifice itself. The discontinuous nature and limited extent of dike complex in the Smartville complex and in the other parts of the western belt suggest that rifting was arrested at an early stage. The Smartville dike complex ranges in width from 0 to 7 km and dies out into scattered dikes along its eastern, western, and southern margins. Other discontinuous exposures of 100% dikes occur in the eastern Smartville complex (Fig. 2) and, southeast of the area shown in Figure 2, in the Folsom area (Saleeby, 1982). The regional variation of dike orientation in the Smartville complex (Figs. 11C and 11D) may simply reflect local controls imposed on early rift development.


BE

ARDANDDAY

Figure 14. Alkali-FeO(tot.)-MgO plot of Smartville gabbros 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).

FeO(tot.)

The elongate map patterns of the gabbrodiorite 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 during 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 orientations of the long axes of the gabbro-diorite plutons 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 rifting and back-arc basin-forming event that was arrested at an early stage of development. One of the more interesting aspects of intra-arc extension in the Smartville complex is the involvement of the calc-;ilkaline rocks. Calc-alkaline magmatism was occurring during rifting, and some of the extension may actually have been accommodated by the emplacement of elongate calc-alkaline plutons. CONCLUSIONS Geologic History of the Smartville Complex On the basis of crosscutting, structural, and other relative time relations, we propose the following geologic history for the Smartville complex. (1) Submarine tholeiitic volcanism is the earliest event preserved in the Smartville complex. This was succeeded by calc-alkaline pyroclastic activity. The contacts between tholeiitic and calc-alkaline volcanics are gradational and interdigitating, suggesting formation within a single volcanic edifice. (2) Massive diabase and metagabbro are the oldest recognized intrusive rocks in the Smartville complex, and they intrude both the upper and lower volcanic units. A genetic relationship between these intrusives and the volcanic rocks has not been demonstrated but cannot be ruled out. (3) The Smartville volcanic rocks were deformed during a preNevadan event. Ii: is not known, and may be impossible to determine, whether the essentially structureless massive diabase unit was involved in this deformation. (4) Rifting of the volcanic edifice and emplacement of the tonalite, granophyric tonalite, and gabbro-diorite plutons occurred simultaneously and followed deformation of the volcani: rocks. The primary manifestation of intra-arc extension in the Smartville

complex is the Smartville sheeted dike complex. The elongate shapes of the plutons are probably related to their emplacement in an extensional environment. This is particularly true of the tonalite plutons, most of which are extremely elongate and one of which is associated with the formation of a synplutonic sheeted dike complex. (5) Continued volcanism following plutonism may be represented by plutonic clasts in volcaniclastic rocks in the eastern Smartville complex, which are tentatively correlated with the uppermost part of the upper volcanic unit. (6) Nevadan faulting followed volcanism and plutonism. Early Nevadan thrust faults deform the volcanic rocks and the older intrusive rocks. The younger intrusive units are deformed by later, high-angle faults. (7) Post-Nevadan intrusion of granodiorite plutons may be related to the early granodiorites of the Sierra Nevada batholith. Nature of the Smartville Complex We interpret the Smartville complex as a rifted volcanic-subvolcanic edifice that formed in a Late Jurassic arc. The volcanic edifice developed primarily and possibly entirely in a submarine environment. The change from older tholeiitic to younger calc-alkaline magmatism in the volcanic rocks of the Smartville complex (Menzies and others, 1980; Xenophontos, 1984) is similar to that seen in several ancient and modern ensimatic arcs (GDI, 1970, 1981; Shiroki and others, 1978). The gabbro-dioritetonalite intrusive series is also typical of volcanic arcs, especially ensimatic arcs developed on oceanic basement. The mineralogy and geochemistry of the gabbro-diorite plutons are characteristic of cumulate and noncumulate gabbros from volcanic arcs and unlike those gabbros found in association with mid-ocean

ridge or hot-spot magmatism (Figs. 13, 14; Beard, 1985, 1986). Early interpretations of the Smartville complex as an ophiolite representing a fragment of oceanic crust formed at a spreading center (Moores, 1970; Cady, 1975) were based on the presence of elements of the ophiolite pseudostratigraphy (layered and massive gabbro, plagiogranite, sheeted dikes, pillow basalts). More recent studies, including this one, have shown important differences between the Smartville complex and many ophiolites. First, the Smartville sheeted dike complex and related plutons are not a part of the Smartville basement but an integral part of the subvolcanic edifice. Calcalkaline plutons are coeval with the dikes and were emplaced during the rifting event represented by the dike complex. Second, the pillow lavas are older than the dikes and plutonic rocks and, although low in the volcanic section, interdigitate with calc-alkaline pyroclastic and epiclastic rocks. The pillow lavas are not part of a "dead" volcanic pile unconformably overlain by younger arc volcanic rocks. Finally, the Smartville pseudo-stratigraphy itself is highly irregular. Gabbroic rocks intrude high levels of the upper volcanic unit. Layered gabbro occurs only in one small gabbro body east of Dobbins (Fig. 2). The dike unit is laterally discontinuous and best developed on the western side of the intrusive complex. A narrow dike unit closely associated with a tonalite pluton occurs on the eastern side of the plutonic rocks. These relations resemble those that might be expected in an evolving volcanic edifice rather than the "layer cake" one might expect to develop at an active spreading center. Thus, we conclude that the intrusive rocks of the Smartville complex represent the subvolcanic core of an incipiently rifted volcanic arc. The rifting may represent the early stages of back-arc basin development.



ACKNOWLEDGMENTS This paper is based on part of the Ph.D. dissertation of the senior author. We wish to thank our colleagues A. C. Tuminas, S. E. Edelman, C. Xenophontos, and, especially, E. M. Moores for their support and interest during the course of this study. E. M. Moores, A. A. Finnerty, P. Schiffman, and S. Sorensen kindly read the manuscript at various stages. We are grateful for critical reviews of the manuscript by G. Harper, C. Hopson, S. Kay, J. Saleeby, and J. Shervais. This work was supported by NSF Grants EAR78-06340 and EAR-80-19697 to Day and Moores and by grants to Beard from the California Division of Mines and Geology and the Society of Sigma Xi.

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