Detailed mapping and sensitive high-resolution ion microprobe (SHRIMP) U-Pb geochronology centered around the Nightingale and Sahwave Ranges, ∼100 km northeast of Reno, Nevada, reveal that most of the Mesozoic basement in this area is composed of predominantly granodiorite-composition plutonic rocks intruded ca. 110–88.5 Ma. These rocks are similar in age, petrology, and composition to the mid-Cretaceous eastern part of the Sierra Nevada Batholith, and are likely related. The youngest plutonic rocks, ca. 93–88.5 Ma, form a large, compositionally zoned intrusive suite, referred to as the Sahwave intrusive suite. This suite is composed of a set of nested, inward-younging intrusions, varying between mafic, equigranular granodiorite around the periphery to more felsic, K-feldspar–megacrystic granodiorite in the center. The Sahwave intrusive suite is coeval with the Cathedral Range intrusive event along the crest of the Sierra Nevada, including the Tuolumne intrusive suite. The geochemistry and petrology of this intrusion also support similar magma genesis and emplacement. Intrusions of the Cathedral Range intrusive event in the Sierra Nevada were emplaced along the margin of North American continental crust, whereas the Sahwave intrusive suite was intruded into a thick package of basinal metasedimentary rocks that were likely underlain by transitional crust. More primitive initial 87Sr/86Sr and εNd values (ca. 0.7047 and –0.2, respectively) reflect this difference. In light of this likely fundamental difference in lower-crustal character, other factors, possibly related to subducted, water-rich material, must be responsible for creating similar melting conditions among the series of large intrusions that represent the last magmatic flare-up of the Cretaceous arc.
The Mesozoic Sierra Nevada Batholith preserves an extensive record of continental-margin arc magmatism that serves as a classic, worldwide model, especially for high-intrusive-flux magmatism. Previously, however, only reconnaissance-level studies (e.g., Smith et al., 1971; Barton et al., 1988; Van Buer et al., 2009) have explored the possibility that this batholith might extend past the Sierra Nevada mountains into the NW Basin and Range Province (Fig. 1), where Mesozoic relationships are obscured by Cenozoic volcanism and basin development related to extensional faulting. Consequently, many published figures depicting the Sierra Nevada Batholith are truncated against the edge of the Basin and Range or the Nevada border (e.g., Tikoff and de Saint Blanquat, 1997; DeGraaff-Surpless et al., 2002; Lackey et al., 2005), and the Sierra Nevada Batholith is often considered to be restricted to the mountains it was named for. However, boundaries as recent as the Neogene limit of Basin and Range extension (dotted line, Fig. 1), which defines the eastern scarp of the Sierra Nevada, would seem to rather arbitrarily delimit the much older Mesozoic Sierra Nevada Batholith. Although Mesozoic outcrops in the Basin and Range are less continuous and more deeply weathered than those in the glacially scoured Sierra Nevada, the Sahwave and Nightingale Ranges, about an hour NE of Reno, Nevada, form a broad, uplifted horst block of Mesozoic basement that is well suited for investigating the relationship between plutonic rocks in the NW Basin and Range and in the Sierra Nevada (Figs. 1 and 2). Detailed mapping in the Sahwave and Nightingale Ranges, combined with reconnaissance of the surrounding areas, was used to identify distinct intrusive units for further quantitative study. Most of the intrusive units in this area were identified as belonging to a single, very large, roughly concentrically zoned intrusive suite, emplaced at ca. 90 Ma, referred to here as the Sahwave intrusive suite (Fig. 2). Zoned intrusive suites of approximately the same age in the Sierra Nevada, such as the Tuolumne intrusive suite, have received detailed geochronological, mineralogical, geochemical, and structural study due to vigorous and ongoing debate about their petrogenesis and emplacement (e.g., Bateman, 1992; Coleman et al., 2004; Žák and Paterson, 2005; Hirt, 2007; Gray et al., 2008), and therefore provide an excellent data set for comparison with the Sahwave intrusive suite. As the first report of its kind in this region, this paper attempts to set forth several types of basic data, from map data and rock descriptions to modal mineralogy, U-Pb geochronology, and major- and trace-element, and isotope geochemistry. Comparison of data between these intrusive suites allows us to evaluate whether the Sierra Nevada Batholith should be considered to extend into the NW Basin and Range (Fig. 1). Furthermore, differences between these regions of high intrusive flux may have important implications for arc flare-up models.
REGIONAL GEOLOGIC SETTING
Subduction-related arc magmatism in the Cordillera began in the Triassic and continued episodically into the Late Cretaceous (and into the Paleocene north of the Snake River Plain and in southern Arizona; Fig. 1). The resulting batholithic belt has been variably disrupted by Cenozoic extension and translation and now forms several distinct segments, including the Idaho Batholith, the Sierra Nevada Batholith, and the Peninsular Ranges Batholith (Fig. 1). The final episode of magmatism in California and Nevada spanned ca. 120–85 Ma, and was particularly voluminous during the latter half of this period (e.g., Barton et al., 1988; Ducea, 2001). In most of the U.S. Cordillera, the Cretaceous batholith exhibits a regular younging pattern from west to east that is generally mirrored by geochemical trends from more mafic to more felsic (e.g., Evernden and Kistler, 1970; Hyndman, 1983; Silver et al., 1979).
One of the most distinctive features of the Sierra Nevada Batholith is the series of large, compositionally zoned intrusions of the Cathedral Range intrusive event, such as the Tuolumne intrusive suite, emplaced along the eastern edge of the main Sierra Nevada Batholith at the very end of Cretaceous arc magmatism between ca. 94 and 83 Ma (Evernden and Kistler, 1970; Kistler et al., 1986; Tikoff and Teyssier, 1992). Representing a high level of magmatic flux (e.g., Ducea, 2001), these intrusions generally exceed 1000 km2 in area, and are characterized by central megacrystic K-feldspar granites or granodiorites surrounded by more mafic equigranular granodiorites (e.g., Bateman, 1992; John and Robinson, 1982; Titus et al., 2005; Hirt, 2007; Saleeby et al., 2008). Similar large, zoned intrusions are also present in the Peninsular Ranges Batholith (Fig. 1), although these are somewhat older (primarily ca. 99–92 Ma) and have tonalite and trondhjemite as well as granodiorite compositions (e.g., Gastil, 1983; Walawender et al., 1990).
The Cretaceous Sierra Nevada and Peninsular Ranges Batholiths, which contain these large intrusions along their east sides, straddle the boundary between North American continental crust and oceanic terranes to the west, as approximated by the initial 87Sr/86Sr = 0.706 line (Fig. 1; e.g., Gastil, 1975; Saleeby, 1981; Kistler, 1990); in contrast, the locus of Cretaceous magmatism between the Sierra Nevada and western Idaho (Fig. 1) does not appear to be adjacent to regular continental crust. Wall rocks to the Cretaceous intrusions in this area include a basinal terrane of early Mesozoic deep-marine strata and the early Mesozoic arc terranes bounding it to the northwest and southwest (Fig. 2; e.g., Speed, 1978; Quinn et al., 1997; Wyld, 2000). These rocks have regionally been metamorphosed to subgreenschist to lower greenschist grade but often reach amphibolite grade proximal to Mesozoic intrusions (e.g., Willden, 1964; Bonham, 1969; Johnson, 1977; Barton et al., 1988). The basinal strata, which belong to the monotonous Late Triassic (Norian) to earliest Jurassic Auld Lang Syne Group, are essentially submarine fan deposits, metamorphosed into slate/phyllite with subordinate quartzite lenses and rare calc-silicate/marble layers (Burke and Silberling, 1973; Speed, 1978). Correlative, but thinner, strata overlie the shelfal, earlier Triassic Star Peak Group east of the main locus of Cretaceous magmatism (Silberling and Wallace, 1969), but farther west, the basinal strata exceed 6 km, and no base is exposed (Compton, 1960; Burke and Silberling, 1973; Speed, 1978). Jurassic shortening associated with the Luning-Fencemaker thrust belt (Fig. 1) has folded, thrust, and thickened this basinal sequence (Oldow, 1984).
The metamorphic and plutonic rocks of the northern Sierra Nevada and the northwest Basin and Range are unconformably overlain by Eocene, Oligocene, and Miocene volcanic and sedimentary rocks (Fig. 2). This widespread unconformity represents a profound change from erosion in the latest Cretaceous and early Tertiary to active deposition of volcanic and sedimentary strata in the Eocene to Miocene and is an important datum for reconstructing geologic relationships prior to Miocene extension and related tilting (Van Buer et al., 2009). Uplift and erosion of the Tertiary strata have resulted in exposure of the unconformity and underlying Mesozoic basement in the tilted footwalls of most major Basin and Range normal faults, leaving a discontinuous Mesozoic outcrop pattern (Figs. 1 and 2).
Although the Cretaceous Cordilleran magmatic arc has been traced across NW Nevada (Fig. 1) based on reported pluton ages between 105 and 85 Ma (e.g., Smith et al., 1971; Barton et al., 1988; Wooden et al., 1999), reconnaissance studies and compilations have not adequately addressed the character of the intrusions across this intervening region. Previous mapping in northwestern Nevada includes thorough coverage only at 1:250,000 scale, which does not differentiate between separate plutonic units (Willden, 1964; Bonham, 1969; Willden and Speed, 1974; Johnson, 1977). More detailed work has been completed on Jurassic and Triassic intrusions in western Nevada (e.g., John et al., 1994), and on intrusions in the gold-producing region of north-central and northeast Nevada (Fig. 1; reviewed in du Bray, 2007). For Cretaceous plutons in NW Nevada, some structural and geochronological work has been completed (e.g., Wyld and Wright, 2001; Ciavarella and Wyld, 2008; Colgan et al., 2010), but no detailed petrologic or geochemical data have been published that are adequate to address the magma genesis of these intrusions and their relationship to contemporaneous intrusions in the Sierra Nevada.
CRETACEOUS PLUTONIC ROCKS OF NORTHWEST NEVADA
Although intrusive rocks can be found scattered throughout much of the western Basin and Range Province, they constitute a majority of the pre-Cenozoic outcrop in an area trending NNE from the Lake Tahoe area across NW Nevada (Fig. 2; Barton et al., 1988; Van Buer et al., 2009). Plutons in this area are not tightly stitched at the level of exposure, but rather are often separated by substantial areas of metamorphic outcrop, often more than 10 km across (Fig. 2). Intrusive rocks include quartz monzonite and rare diorite/quartz diorite, but are predominantly granodiorite (cf. Smith et al., 1971). Published geochronology indicates intrusion during the Jurassic (ca. 200–160 Ma) and the Cretaceous (ca. 115–85 Ma), with the greatest intrusion fraction between ca. 105 and 90 Ma (Rai, 1969; Evernden and Kistler, 1970; Smith et al., 1971; Morton et al., 1977; Marvin and Cole, 1978; Garside et al., 1992; John, 1992; Oldenburg, 1995; Wyld, 1996; Quinn et al., 1997; Wyld and Wright, 1997; Wooden et al., 1999; Wyld et al., 2001; Van Buer and Wooden, 2007), although many intrusions remain undated or poorly dated.
A particularly continuous area of Cretaceous plutonic outcrop occurs in the Sahwave and Nightingale Ranges, which together form a broadly synclinal horst, with major normal faults along the east side of the Sahwave Mountains and the west side of the Nightingale Range (Fig. 3)1. The bulk of both mountain ranges is granodiorite. This area of intrusive rock is separated from other plutons on the south and northeast by several kilometers of metamorphic wall rocks (Fig. 2), making the Sahwave and Nightingale Ranges a well-bounded target for detailed study. However, because granodiorite outcrops in the Selenite Range, to the northwest, and the Trinity Range, to the east, are potentially contiguous, if not for intervening Cenozoic cover, these areas were also selected for reconnaissance study (Fig. 2). The nearest intrusive outcrops to the west, in the Lake Range, are visually dissimilar, and were not closely studied. In the Sahwave and Nightingale Ranges (Fig. 3), several reports and theses include local, more detailed mapping (Smith and Guild, 1942; East and Trengrove, 1950; Rai, 1969; Fanning, 1982; Stager and Tingley, 1988; Whitehill, 2009). Most of these papers relate to exploration of the tungsten-mining district along the southwest margin of the Cretaceous intrusive contact, in the southern Nightingale Range (Fig. 3), and contain very little data pertaining to the igneous rocks themselves.
NEW MAPPING IN THE SAHWAVE AND NIGHTINGALE RANGES
Mapping of the Sahwave and Nightingale Ranges was completed at 1:24,000 scale (reduced to 1:180,000 in Fig. 3). Each mountain range is generally more rugged on the side between its crest and its bounding normal fault, and the Sahwave Range, in particular, becomes higher and rockier to the north. However, even the best outcrops in this area are patchy and deeply weathered as compared to the continuous outcrops of the Sierra Nevada crest. The southern Sahwave Range (Fig. 3) forms a particularly low-relief upland, characterized by a thick blanket of grus, which nourishes the range's namesake sagebrush (northern Paiute sai’-wav; Fowler and Fowler, 1971). Map units fall into three basic categories: the overlying Cenozoic strata, early Mesozoic metamorphic wall rocks, and Cretaceous intrusive rocks. The intrusive rocks are further subdivided into two groups: Units that frequently have gradational contacts with each other, are more or less concentrically arranged, and have magmatic foliation that is generally weak, absent, or contact-parallel, and are apparently cogenetic, are herein informally named the Sahwave intrusive suite, whereas units with strong, roughly north-south foliation, sharply crosscut by members of the aforementioned suite, are considered to be distinct preexisting intrusions.
Oligocene and Miocene volcanic and sedimentary rocks unconformably overlie all Mesozoic units (Fig. 3). Together with recent alluvial deposits, these rocks fill part of the area between the Sahwave and Nightingale Ranges, as well as Cenozoic extensional basins to the east, west, and north of the Sahwave-Nightingale horst (Figs. 2 and 3). The volcanic units range from basalt flows to silicic ignimbrites, and the interbedded sediments (not thoroughly lithified yet) range from landslide deposits and fanglomerates to lacustrine clays (cf. Whitehill, 2009). Additionally, the study area is cut by a few generations of dikes, ranging from rhyolitic to basaltic in composition, which also tend to be more resistant to erosion than the surrounding rock (Fig. 3). The coarsest are diabase (with chilled margins), but most have an aphanitic matrix. Although these dikes have not been dated, their fine-grained nature suggests that they are substantially postmagmatic, and they are compositionally similar to volcanic packages in the overlying Tertiary strata.
The wall rocks of the Sahwave Batholith are mostly metamorphosed mudstone/shale with interbedded sandstone layers and lenses. A few discontinuous, 10–100-m-thick, coarsely crystalline marble layers are present south of the batholith, but calcareous layers are rare in the metamorphic rocks to the north (Fig. 3). These rocks have been identified as belonging to the Triassic to Early Jurassic Auld Lang Syne Group (Johnson, 1977). Away from the batholith, metamorphic grade is subgreenschist to lower greenschist, and original bedding is clearly seen. Fold axes and foliation in the adjacent Bluewing Mountains (Fig. 3) trend NE-SW, and exhibit top-to-the-SE vergence, consistent with Jurassic deformation in nearby parts of the Luning-Fencemaker thrust belt (Fig. 1; Oldow, 1984). Adjacent to the batholith, Triassic-Jurassic strata are metamorphosed to siliceous hornfels or biotite schists, and bedding is often tightly to isoclinally folded with a subvertical axial-planar foliation that is broadly parallel to the intrusive contact (Fig. 4A). A strong subvertical mineral lineation is also present within ∼100 m of the intrusive contact, although it is often obscured by a subparallel intersection lineation. In the Bluewing Mountains, along the northern edge of the batholith, the zone of contact-parallel foliation is only a few hundred meters wide, whereas to the southwest, in the Nightingale mining district, foliation is subparallel to the contact over the entire exposed outcrop, up to 5 km away from the intrusion (Fig. 3). This NW-SE foliation is anomalous compared to NE-SW Jurassic structural trends, which tend to dominate in surrounding areas (e.g., Oldow, 1984).
With some exceptions, the generally quartzofeldspathic composition of the metasedimentary rocks is not conducive to the growth of diagnostic minerals besides white mica and occasional biotite. The Nightingale mining district contains a number of skarn deposits in the contact aureole of the batholith where calcareous layers have been metamorphosed, yielding grossular/almandine, clinozoisite/epidote, and more rarely tremolite, diopside, and scheelite, in addition to the standard quartz, ± albite, and calcite. White mica pseudomorphs, apparently after both andalusite (square rods) and also cordierite (dark, mouse-dropping shapes), can be found in some of the more pelitic layers in the Bluewing Mountains near the northern margin of the batholith (Fig. 3). However, large (to over 5 cm) andalusite crystals remain intact in at least one area ∼1 km from the northern contact, growing in random orientations that cut across the foliation.
Early Intrusive Units
The oldest intrusive unit, informally referred to as the Power Line intrusive complex (Kpl), occupies the northwestern Nightingale Range (Fig. 3), and is predominantly a medium-grained biotite hornblende granodiorite with 5–10 mm K-feldspar phenocrysts. However, this unit also includes many unmapped dikes and pods of darker granodiorite and diorite ranging from centimeters to hundreds of meters in dimension. Some of these are fine grained, weathering to a blue-grayish color, but all subunits share a similar, generally north-south–oriented, steeply dipping solid-state foliation (Fig. 3). This strong foliation distinguishes the Power Line complex from all other intrusive units, including the Sahwave intrusive suite, which intrudes the complex and crosscuts its foliation. Although many of the finer-grained mafic enclaves appear to demonstrate magma mingling, relationships among these subunits are somewhat obscured by poor outcrop and the solid-state foliation. In thin section, the foliation is defined by biotite strung out along wavy foliation planes, and the sense of shear, if any, is unclear, because the rock bears no discernible lineation (Fig. 4B). Biotite and quartz appear to have been largely recrystallized (Fig. 4B), but feldspars remain intact, displaying distinct undulatory extinction, suggesting solid-state deformation at temperatures of ∼400–450 °C or warmer, depending on strain rate. This unit also contains many large inclusions of metamorphic rock, mostly 5–200 m in length but including a 4-km-long potential roof pendant as well (Fig. 3). These are generally elongated in map view, and aligned subparallel to the foliation of the Power Line complex (Fig. 3).
In the very northwestern corner of the study area and throughout the southern Selenite Range (Figs. 2 and 3), there is a distinct granodiorite, here referred to as the Selenite Granodiorite (Kse) after the “Selenite pluton” of Smith et al. (1971). This unit has a conspicuous, generally north-south magmatic foliation defined by the alignment of euhedral plagioclase and hornblende phenocrysts in rock with a hypidiomorphic igneous texture. Polysynthetic twinning in the plagioclase is frequently visible to the unaided eye. This unit is tentatively not included in the Sahwave intrusive suite, which intrudes it along a sharp contact (Fig. 3) and only rarely contains euhedral plagioclase.
SAHWAVE INTRUSIVE SUITE
The metamorphic rocks, the Power Line complex, and the Selenite Granodiorite are intruded by members of the Sahwave intrusive suite, which consists of three concentric, partially intergradational intrusive units centered on the Sahwave Range and a distinct lobe-forming unit that stretches across the central Nightingale Range (Fig. 3). Rocks of similar appearance also occur in the western Trinity Range, separated from the Sahwave Range by Cenozoic fill in Granite Springs Valley, suggesting that the Sahwave intrusive suite may underlie much of this broad area as well (Fig. 2). The outermost and oldest intrusive unit is a medium- to coarse-grained equigranular biotite hornblende granodiorite referred to as the Granodiorite of Juniper Pass (Kjp; Fig. 3). This unit is discernible by its conspicuous 4–8 mm biotite crystals. Additionally, large hornblende phenocrysts are common around the periphery of this intrusion, giving the rock a characteristic “Dalmatian” appearance (Figs. 4C and 4D). Hornblende and sphene are both present throughout the Sahwave intrusive suite, but only in the Granodiorite of Juniper Pass does the hornblende form crystals notably larger than the 1–3 mm euhedral sphene. In detail, the mineral proportions and color index of this unit vary quite a bit; in places, it can be classified as a tonalite or a quartz diorite. Gradational compositional variation can sometimes be seen across large outcrops; more rarely, internal contacts can be discerned where slightly lighter and darker phases occur together. In a few places, straight or wavy compositional layers, 1 cm to 1 m thick, are bounded by sharp contacts (Fig. 4E). Many of these internal structures are subtle, and only readily seen in fresh outcrop, so it is possible that they are fairly pervasive. Mafic enclaves are found throughout the unit, but are only common within 1–2 km of the exterior contact. Enclaves are typically 5–30 cm in length and flattened by a ratio of 2:1–5:1 or more (Fig. 4F). Mafic schlieren are common in the same region. The Granodiorite of Juniper Pass has a discernible magmatic foliation that is defined by the alignment of mafic minerals and sometimes subhedral plagioclase, which is generally similar to the alignments of mafic schlieren and mafic enclaves as well (Fig. 4F). Magmatic foliation tends to be strongest near the outer contact, which it often parallels (Fig. 3).
The Granodiorite of Juniper Pass grades inward to the more felsic and uniform Granodiorite of Bob Spring (Kbs), a medium-grained biotite granodiorite or granite, characterized by seriate K-feldspar phenocrysts up to ∼2 cm. Although relative age relations with the Granodiorite of Juniper Pass are difficult to determine from the gradational intrusive contact, in map pattern, the Granodiorite of Bob Spring appears to cut out the center of the Juniper Pass (Fig. 3) and is presumed to be younger. In the field, this gradational contact is arbitrarily mapped where large K-feldspar phenocrysts become more conspicuous than large biotite crystals. Biotite in the Granodiorite of Bob Spring is more homogeneously distributed, and generally no larger than 1 mm. The K-feldspar phenocrysts are poikilitic, mostly surrounding plagioclase and biotite (Fig. 4G), and are occasionally sieve textured and difficult to see. In general, Kbs is finer grained toward its center, and K-feldspar phenocrysts are less common. The Granodiorite of Bob Spring bears equant quartz grains that are generally only ∼1 mm in size but reach 3–5 mm in the southern part. Mafic minerals are often badly chloritized, and feldspars show signs of sericitization. Foliation in this unit is usually absent or at least too weakly defined to measure.
The Sahwave Granodiorite (Ks), a K-feldspar–megacrystic biotite granodiorite (Figs. 4H and 4I), intrudes the central part of the Granodiorite of Bob Spring along a generally shallowly dipping contact that is sharp on the north side but gradational along its south side (Fig. 3). K-feldspar megacrysts are 2–4 cm across, somewhat poikilitic, and more abundant (usually 1%–5% by volume) than in the Granodiorite of Bob Spring. The abundance of K-feldspar megacrysts can vary greatly from place to place, and at outcrop scale, it is not uncommon to see distinct stringers and pods enriched in K-feldspar megacrysts, rarely up to as much as ∼20% (Fig. 4J). The Sahwave Granodiorite forms relatively bold outcrops compared to adjacent parts of the Granodiorite of Bob Spring, but the rock is uniformly crumbly and often spheroidally weathered.
The Nightingale Range contains a distinct lobate unit referred to as the School Bus Granodiorite (Ksb; Fig. 3). This unit is a relatively leucocratic granodiorite, distinguished by scattered 1–2 cm K-feldspar phenocrysts and 3–6 mm biotite flakes (Fig. 4K). Unlike the main part of the Sahwave Batholith, this lobe does not appear to be any more mafic around its outer edge, and is, in fact, remarkably homogeneous. Magmatic foliation is not generally distinguishable. The School Bus Granodiorite intrudes both the Power Line complex and the Granodiorite of Juniper Pass along sharp, vertical contacts (Fig. 4L) that are fairly irregular at the map scale (Fig. 3). Where it intrudes the Power Line complex, the units are often separated by metamorphic screens and blobs 20–200 m thick (Fig. 3).
The southern part of the Granodiorite of Juniper Pass contains a number of diorite/quartz diorite bodies, varying from tens of meters to over a kilometer in scale (Fig. 3). These fine- to medium-grained intrusions frequently contain ∼5 mm euhedral plagioclase phenocrysts, and sometimes acicular hornblende crystals as well. These diorite bodies appear to be coeval with the Sahwave Batholith, often showing magma mingling and mixing structures such as lobate and interfingering contacts, streaky fine-scale intermingling, and outcrop-scale continuous compositional variation indicative of wholesale mixing (Fig. 4M).
Additionally, the Sahwave Batholith and its country rocks are pervaded by a series of leucocratic dikes and sills that tend to be more resistant to weathering than the surrounding country rocks (Fig. 3). Most of these dikes demonstrate wide variations in grain size between aplite and pegmatite textures, often showing evidence for repeated intrusion (Fig. 4N). The pegmatites are generally muscovite bearing, and may also contain tourmaline (schorl) and rarely garnet. The dikes range from 1 cm to 100 m in thickness and generally strike north-south (Fig. 3). Dips are often moderately shallow, but only locally consistent in direction of dip. A notable concentration of leucocratic dikes exists in the Nightingale Range, intruding the Power Line complex (Fig. 3). These dikes crosscut the solid-state foliation of the Power Line complex and are occasionally composite, containing a phase with scattered large K-feldspar and biotite phenocrysts, suggesting that they may be genetically related to the School Bus Granodiorite, which could underlie this area at an unexposed level. Pegmatite dikes cutting the metamorphic rocks along the margins of the batholith are frequently folded and boudinaged in the foliation that is subparallel to the country-rock contact. A few broader leucogranite intrusions, which are more uniform in grain size and contain minor biotite, are present near the southern margin of the batholith (Fig. 3).
The intrusive contacts are generally not exposed well enough, or, when gradational, defined well enough to measure their attitudes directly, and furthermore they are generally too irregular where exposed on the outcrop scale to make meaningful map-scale measurements directly. Contact attitudes, such as those shown on the cross section in Figure 3, have been estimated from map patterns using three-point constraints in areas where the contact appears to be approximately planar. Where contact orientation is evident, it tends to be steeply dipping and subparallel to magmatic foliation, but there are a couple of notable exceptions. These are the contacts along the two largest metamorphic blocks or pendants at the southern end of the Sahwave Range, and the shallow contact where the Granodiorite of Juniper Pass underlies the Power Line complex in the northwestern Nightingale Range (Fig. 3). It is not clear, however, if these cases represent the true roof of the intrusion. In the northwestern Nightingale Range, the low-angle portion of the contact terminates westward as the top contact of a horizontal dike of the Granodiorite of Juniper Pass intruded into the Power Line complex (Fig. 3), suggesting that the contact in this area may simply surround a flap of wall rock that was in the process of being stoped off. External contacts of the Sahwave intrusive suite frequently dike into the metamorphic rocks and apparently surround stoped blocks (Fig. 3), suggesting that stoping is at least a locally important process. In other areas, external contacts are sometimes quite planar, demonstrating smooth curves that parallel foliation in the adjacent, subvertically lineated wall rocks (Fig. 3), suggesting that the wall rocks were flattened in pure shear and flowed ductilely downward to accommodate the laterally expanding pluton.
CHRONOLOGY OF EMPLACEMENT
Although relative ages for the plutons can be determined from contact relations, the only published K/Ar hornblende (and biotite) ages are 91 ± 6 (88 ± 4) Ma and 95 ± 6 (92 ± 4) Ma for the Granodiorite of Juniper Pass and the Selenite Granodiorite, respectively (Smith et al., 1971). These error bars are about as large as the total span of ages. To more precisely define the timing and duration of magmatism in the study area, samples from each of the six main intrusive units were selected for age determination. An additional sample of granodiorite from the Trinity Range, resembling the School Bus Granodiorite, was dated to investigate whether the Sahwave intrusive suite might continue this far to the east (Fig. 2).
U-Pb SHRIMP Methods
Zircons from these seven samples (Table 1) were analyzed by secondary-ion mass spectrometry using the Stanford–U.S. Geological Survey sensitive high-resolution ion microprobe–reverse geometry (SHRIMP-RG) to yield U-Pb age determinations. Zircons were separated from each sample using standard procedures. Sample zircons and chips of R33 standard zircons were mounted in epoxy, ground halfway through the grains with fine sandpaper, and polished with diamond compound. All grains were imaged both in reflected light with an optical microscope and in cathodoluminescence (CL) using a JEOL 5600 scanning electron microscope to reveal zonation as well as cracks, inclusions, and other potential problem areas. U, Th, and Pb isotopes, along with Zr, Hf, La, Ce, Nd, Sm, Eu, Gd, Dy, Er, and Yb were analyzed with the Stanford-USGS SHRIMP-RG using an oxygen ion beam between 4 and 6 nA and a spot size of 20–30 μm. Isotope ratios were normalized using zircon age standard R33 (419 Ma; Black et al., 2004) and concentration standard CZ3. Age data were reduced using SQUID and ISOPLOT software (Ludwig, 2001, 2003) to yield 207Pb-corrected 206Pb/238U weighted-average ages (Table 1; Fig. 5). Complete data tables can be found in Appendix Table A1.
U-Pb SHRIMP Results
Zircons generally show crisp magmatic oscillatory zonation under CL and do not contain distinct cores (Fig. 6). Only one grain, from NVB-286, appeared to have a distinct core and rim, but both parts gave exactly the same age. Individual grain analyses showed a moderately large amount of scatter, although most analyses spread out along or just above concordia (Fig. 5). Select analyses were dismissed (open symbols) because of discordance, high common Pb, and Pb loss in high-U zircons (Fig. 5; Table A1, see Appendix). It is difficult to tell whether the spread in ages is caused by disturbed U-Pb systematics or actually represents prolonged periods of crystallization in a large active magma chamber episodically fed by new batches of magma. Older and younger zircons from individual samples are not visually different or distinguishable in CL images (example shown in Fig. 6). Some of the significantly older ages can be ascribed to scavenging from slightly older plutonic rocks, such as the distinct population at 109.7 ± 0.8 Ma in NVB-208 (Fig. 5). However, these rocks lack clear evidence of older inherited zircons (only one grain out of 77 analyzed was more than ∼8 m.y. older than the enclosing host rock, at a modest 139 Ma). The lack of significant inheritance suggests that these magmas may have originated at zircon-undersaturated conditions.
SHRIMP U-Pb results (Table 1; Fig. 5) give ages for individual units in agreement with the relative ages inferred from intrusive relations. The Granodiorite of Juniper Pass and the Granodiorite of Bob Spring give indistinguishable ages, but the latter is presumed to be younger from crosscutting map relations. These ages are also equivalent within error to published K/Ar hornblende and biotite ages (Smith et al., 1971). Ages from the Sahwave intrusive suite span from ca. 93 to 88.5 Ma, demonstrating that this batholith is contemporaneous with the large intrusions of the ca. 95–83 Ma Cathedral Range intrusive epoch defined in the Sierra Nevada Batholith (Evernden and Kistler, 1970; Kistler, 1999). The sample of granodiorite from the western Trinity Range (NVB-286) is also shown to have crystallized in this time range, at 90.3 ± 0.6 Ma, supporting the idea that it is part of the Sahwave intrusive suite and that these rocks may underlie much of the intervening Granite Springs Valley as well (Fig. 2). Whereas ages associated with the Sahwave intrusive suite are clustered relatively tightly, spanning about 4 m.y., the Selenite Granodiorite and the Power Line complex are significantly older, at 96.3 ± 0.8 and 104.9 ± 0.8 Ma, respectively, justifying their classification as distinct units.
CONTINUITY OF THE CRETACEOUS CORDILLERAN BATHOLITH
Our initial study of batholithic rocks in the area around the Sahwave and Nightingale Ranges in the NW Basin and Range strongly supports the suggestion of Smith et al. (1971) and Barton et al. (1988) that the Cretaceous Cordilleran batholith is continuous across NW Nevada (Figs. 1 and 2). Although obscured by Cenozoic cover, especially under the unbroken volcanic plateau covering NE California, SE Oregon, and part of NW Nevada, Cretaceous batholithic rocks form a majority of Mesozoic outcrops along a NNE-trending belt of the northwestern Basin and Range (Fig. 2; Barton et al., 1988; Van Buer et al., 2009). The extent of this intrusive belt to the north and west is unclear due to complete Cenozoic volcanic cover, but relatively low upper-crustal seismic velocities compatible with granitoid rocks persist almost to the NW corner of Nevada (Fig. 2; Lerch et al., 2007). Granodiorite units in the Sahwave and Nightingale area are similar to many described units in the Sierra Nevada Batholith, including units that, for example, contain conspicuous sphene, euhedral biotite and hornblende, or K-feldspar megacrysts. Large, concentrically zoned intrusions are also common in the Sierra Nevada (e.g., Bateman, 1992). U-Pb SHRIMP dating in the Sahwave and Nightingale area confirms earlier geochronologic estimation of Late Cretaceous ages simultaneous with major intrusion in the Sierra Nevada. Ages spanning from ca. 110 Ma (represented by inherited zircons in the Power Line complex) to ca. 88.5 Ma indicate a long-lived history of repeated intrusion in this part of the batholith, consistent with prolonged histories of magmatism in similarly sized areas of the Sierra Nevada Batholith (e.g., Bateman, 1992; Irwin and Wooden, 2001; Saleeby et al., 2008). These lines of evidence all support the idea that Cretaceous intrusive rocks in the study area formed in a broadly similar arc environment as those in the Sierra Nevada, and represent a continuation of the Cretaceous Cordilleran arc across the NW Basin and Range (Fig. 1).
Whether or not the Cretaceous intrusions in NW Nevada should actually be considered to be part of the Sierra Nevada Batholith is largely a semantic issue. However, if the boundaries of this Mesozoic batholith are to be set based on Mesozoic features, we note that the mostly Late Cretaceous intrusions of our study area lie due east of Early Cretaceous intrusions near Susanville, in the northernmost Sierra Nevada, which are generally considered to be part of the Sierra Nevada Batholith (Fig. 2; Oldenburg, 1995). Before Tertiary extension and translation across the Walker Lane (which is considered to be <30 km at this latitude; Faulds et al., 2005), these two areas would have been even closer (Van Buer et al., 2009), representing the east and west edges of the eastward-younging batholith (Figs. 1 and 2). Therefore, we tentatively suggest that the Cretaceous intrusions of NW Nevada be referred to as part of the Sierra Nevada Batholith. To better clarify the relationship between intrusions of NW Nevada and the Sierra Nevada, however, we analyzed the magma genesis of the Sahwave intrusive suite using detailed mineralogical and geochemical data, which we compared to similar data from the most well-studied intrusion of the same age in the Sierra Nevada, the Tuolumne intrusive suite.
MINERALOGY AND GEOCHEMISTRY OF THE SAHWAVE INTRUSIVE SUITE
Because the main, concentric part of the Sahwave intrusive suite appears to be younging inward, with mafic units grading into more felsic units, the rocks along a radial transect effectively record the magmatic evolution of the system over its 4 m.y. intrusive history. For this reason, the Sahwave intrusive suite was sampled from center to margin along a transect extending north from the central Sahwave Granodiorite to the outer edge of the batholith and along a second, smaller transect through the School Bus lobe in the Nightingale Range (rows of black dots, Fig. 3). Each transect contains samples spaced approximately 1 km apart (Table 2), chosen from the most pristine outcrops available. Along these transects, the mineralogy records changes in the crystallizing assemblage and determines rock classification under the International Union of Geological Sciences (IUGS) scheme (Streckeisen, 1976). Major- and trace-element chemistry responds in detail to element partitioning and mixing during melting, crystal-liquid fractionation, assimilation, and other petrogenetic processes (e.g., Hildreth and Moorbath, 1988). Sr and Nd isotope systems are affected by radiogenic decay of Rb and Sm isotopes, and are therefore sensitive to the timing and extent of differentiation in the source region. Together, these data provide information on the magma genesis of the system, and are appropriate for detailed comparison to similarly sampled coeval intrusions in the Sierra Nevada, such as the Tuolumne intrusive suite (e.g., Bateman, 1992; Hirt, 2007; Gray et al., 2008).
For analysis of modal mineralogy, each sample was sawn into slabs of at least 70 cm2, treated with penetrating epoxy if needed, and ground flat. The slabs were etched with concentrated hydrofluoric acid and stained for K-feldspar and plagioclase using standard procedures as outlined by McMonigle et al. (2002). The stained slabs were sealed with a matte finish, imaged on a flatbed scanner at an effective resolution of 1200 dots per inch, and counted by eye at the intersections of a 2.54 mm grid while enlarged on screen. Areas of significant cracking, epoxy fill, or poor staining were marked off before counting to avoid spurious results. Sample SH-10 and three samples not listed in Table 2 were too pervasively cracked to produce enough reliable counting surface. At least 800 points were counted for each of the other samples (Table 2). Assuming random counting statistics, this means all 2σ errors should be less than ∼5 vol%, and 2σ errors for modes under 20% should be less than ∼3 vol%.
For geochemical analysis, aliquots of selected samples from the transect were gently hammer crushed before handpicking 30–50 g of fresh chips to be sent to the Washington State University GeoAnalytical Laboratory for elemental analysis by X-ray fluorescence (XRF) and inductively coupled plasma–mass spectrometry (Tables 2 and 3). Additionally, six aplite samples from the southern Sahwave Range were analyzed by XRF only at the University of California, Santa Cruz.
Three samples were analyzed for Sr and Nd isotopes at the Stanford-USGS Micro Analysis Center. The samples were prepared by grinding picked chips in a tungsten carbide mill, followed by a HF-HNO3-HCl dissolution procedure in Teflon vials. Sr and Nd fractions were chemically separated using cation exchange columns in a clean laboratory before loading them into a multicollector Finnegan MAT 262 thermal ionization mass spectrometer on Ta (single) and Re (double) filaments, respectively. Measured 87Sr/86Sr and 143Nd/144Nd ratios were normalized to 86Sr/88Sr = 0.1194 and 146Nd/144Nd = 0.7219 to correct for mass-dependent fractionation. Initial isotope ratios were calculated using λ87Rb = 1.42 × 10−11 (yr−1) and λ147Sm = 6.54 × 10−12 (yr−1), and 143Nd/144Ndi is reported as εNd relative to the chondritic uniform reservoir (CHUR) evolution model of Jacobsen and Wasserburg (1980).
On a ternary quartz–alkali-feldspar–plagioclase diagram, samples from the Sahwave intrusive suite (circles) define a trend from the quartz diorite field to the granite field, with a majority of the samples falling in the granodiorite field (Fig. 7). These mineralogic trends are quite similar to those of the Tuolumne intrusive suite (small squares; Fig. 7). The Sahwave intrusive suite has on average a greater modal abundance of mafic minerals, mainly because mafic granodiorites compose a larger fraction of this intrusion than of the Tuolumne intrusive suite. In general, plagioclase and mafic minerals decrease in abundance toward the center of the intrusion, while quartz and K-feldspar increase, as expected from field relations (Fig. 8). However, these radial modal trends are far from monotonic (Fig. 8). For example, the modal percentage of alkali feldspar increases inward for the first 3 km from the contact, drops down to almost its starting value at ∼6 km, and then increases to higher values in the Granodiorite of Bob Spring (Fig. 8). The modal percentages of mafic minerals and plagioclase follow a roughly opposite pattern, except plagioclase actually increases inward from the margin to reach its maximum at ∼6 km. Quartz abundance follows plagioclase's pattern in reverse. The greatest total variation occurs within the Granodiorite of Juniper Pass, which is perhaps not surprising given the color index variations and cryptic contacts seen in the field, but variation between individual samples seems to increase in the Granodiorite of Bob Spring (Fig. 8). Despite having a greater abundance of K-feldspar megacrysts, the Sahwave Granodiorite is modally quite similar to the Granodiorite of Bob Spring and the School Bus Granodiorite (Fig. 8).
Major-element chemistry (Table 2) confirms that the Sahwave intrusive suite represents a magnesian, metaluminous to weakly peraluminous, calc-alkaline series, with an alkali-lime index of 59.6. Major- and trace-element variation with respect to silica shows trends consistent with fractional crystallization and mixing (open symbols, Fig. 9). For example, as differentiation proceeds to higher % SiO2, the incompatible components Rb and K2O increase. Fractional crystallization of hornblende, sphene, and other mafic minerals can explain the decrease in FeO* and Y (not seen in aplite samples, which may have accumulated a Y-rich phase such as xenotime), and plagioclase crystallization keeps Na2O from increasing and removes Sr (Fig. 9). Across a radial transect, major elements generally track the same patterns seen in the radial modal plot (Figs. 8 and 10). On average, major-element chemistry becomes more felsic toward the center of the intrusive complex (Fig. 10), but displays significant variation from this general trend. Because these variations fall into a linear array when plotted with respect to silica, much of this local variation may be attributed to mixing between magmas of different composition. Evidence for such mixing is actually observed (Fig. 4M) between diorite intrusions and the Granodiorite of Juniper Pass (which shows the greatest variations) in the southern part of the study area (Fig. 3). The Sahwave intrusive suite is too long-lived for fractionation of a single large batch of magma after the model of Bateman and Chappell (1979), but trends toward more felsic major- and trace-element compositions (Fig. 9) are consistent with mixing between a set of increasingly fractionated parental magmas.
Compared to the Tuolumne intrusive suite, the Sahwave intrusive suite tends to have less K2O and Rb, but more Na2O and Sr for any given amount of SiO2 (Fig. 9). Lower K/Na ratios might partially account for the reduced number and size of K-feldspar megacrysts. Rubidium and strontium trends with respect to silica in the Sahwave intrusive suite tend to be somewhat more tightly clustered and show stronger correlations with SiO2 (Fig. 9). Compared to the Tuolumne intrusive suite, radial major-element variations in the Sahwave intrusive suite extend farther from the margin and are less monotonic (Fig. 10). Compared to the Half Dome Granodiorite, the Granodiorite of Juniper Pass is almost uniformly richer in Al2O3, CaO, FeO, MgO, and other elements concentrated in plagioclase and mafic minerals (Fig. 10). Trends are more similar between the Cathedral Peak and Bob Spring granodiorites, but the Sahwave intrusive suite contains no equivalent to the Johnson Granite Porphyry (Fig. 10).
Bulk-rock rare earth element (REE) patterns from this transect (Fig. 11) all show a broadly similar depression in the heavy REEs. The more felsic units are especially depleted in the middle and heavy REEs compared to the Granodiorite of Juniper Pass (Fig. 11), presumably due to greater fractionation of hornblende, in which these REEs are compatible (Arth and Barker, 1976). None of the units shows a consistent Eu anomaly. Similar REE patterns in the central Sierra Nevada Batholith have been interpreted to reflect differentiation from a deep-crustal residue containing garnet (heavy REE compatible) rather than plagioclase (Eu compatible) as the dominant aluminous phase (Ducea, 2001). This hypothesis is also supported by the relatively high Sr/Y ratio as compared to primitive mantle melts (Fig. 12), although arc rocks are generally enriched in fluid-mobile large ion lithophile elements as compared to relatively high field strength elements (Fig. 12). Zircon REE patterns (Fig. 11; only Granodiorite of Juniper Pass zircon data shown for simplicity) show small negative Eu anomalies, but this is probably due to the greater incompatibility of Eu in zircon due to its 2+ charge, much as the positive Ce anomaly is associated with the +4 charge taken by those ions. Figure 11 also shows the range of hypothetical liquid compositions that would be in equilibrium with Granodiorite of Juniper Pass zircon, using the partition coefficients for REEs in zircon from Sano et al. (2002). Ignoring La, the Granodiorite of Juniper Pass REE pattern is within the range of hypothetical liquid compositions, although it is on the upper side, probably because the zircons are frequently included in hornblende, which competes for the middle REEs (Fig. 11).
Zircon saturation temperatures in the range of 748–773 °C were calculated from bulk-rock compositions using the model of Watson and Harrison (1983) and provide a likely minimum temperature range for these granodiorite melts.
Initial 87Sr/86Sr and εNd (Table 4) values do not differ greatly among the few measured samples from the Sahwave intrusive suite, varying only from 0.7045 to 0.7049 and –0.259 to –0.173, respectively, indicating a relatively homogeneous source with a strong mantle component and little upper-crustal assimilation. The 87Sr/86Sr and εNd values are similar to those previously measured in the surrounding basinal terrane, which is presumably underlain by relatively mafic transitional crust (e.g., Farmer and DePaolo, 1983), but they are considerably more primitive than those measured in similar intrusions along the Sierra Nevada crest (Fig. 13).
Analyses of aplite fall near the water-saturated 1 kbar haplogranite minimum when projected onto a quartz-orthoclase-albite ternary diagram (Fig. 14), consistent with field evidence for the late-stage emplacement of aplite/pegmatite dikes, which likely represent the last fraction of residual melt from a fairly water-rich magmatic system. Scatter between the samples (Fig. 14) may be partially due to varying anorthite content and postmagmatic silicification, as well as actual variation in sampled emplacement depth. When samples containing >5% An or >77.8% SiO2 are excluded, one of the remaining samples still falls significantly below the minimum melt curve, which may suggest that aplite was extracted before water saturation was reached (cf. Nekvasil, 1988), but the other sample (collected 130 m below the Tertiary unconformity) falls just below the 1 kbar minimum (Fig. 14), consistent with an emplacement depth of 3–4 km. This depth estimate falls within the ∼3–10 km range suggested by Van Buer et al. (2009) based on the lack of miarolitic cavities and caldera structures and the presence of andalusite in the contact aureole.
A CATHEDRAL RANGE INTRUSIVE EVENT OUTSIDE THE SIERRA NEVADA
The Sahwave intrusive suite is similar to the large intrusions of the Cathedral Range intrusive epoch in the Sierra Nevada proper (Fig. 15) in terms of its ca. 92.5–88.5 Ma age range, >1000 km2 size, modal and chemical zonation, and internal magmatic structures. It is different in that it has somewhat lower K/Na ratios, smaller abundance and lesser size of K-feldspar megacrysts, a larger proportion of mafic granodiorite, more primitive Sr and Nd isotopic ratios, a relatively equant rather than elongate shape, and its location to the north and east of the Sierra Nevada crest (Fig. 15). Additionally, the approximately 4 m.y. apparent duration of the Sahwave intrusive suite is shorter duration than that reported for many of the coeval intrusive suites along the Sierra Nevada crest, but it is similar to the durations represented by the most voluminous phases of those suites (Chen and Moore, 1982; Coleman et al., 2004; Saleeby et al., 2008). Many of these intrusive suites, e.g., the Tuolumne intrusive suite, had shorter reported durations of intrusion before extensive geochronology campaigns were carried out (e.g., Coleman et al., 2004), so it is possible that the Sahwave suite may also include minor phases that would extend its reported duration. Despite some differences, the similarities are compelling enough to consider the Sahwave intrusive suite a member of the Cathedral Range supersuite. The continuation of this distinctive chain of intrusions into the NW Basin and Range further supports the idea of an originally continuous Sierra Nevada Batholith later disrupted by Cenozoic extension (Fig. 15). The wide separation between the Sahwave intrusive suite and the Sonora Pass intrusive suite shown in Figure 15 may suggest the existence of another intrusive suite (or suites) in the Reno area, where little-studied granitoids of similar age are exposed (91–86 Ma K/Ar biotite ages; Marvin and Cole, 1978; Garside et al., 1992).
The emplacement mechanisms of these large, long-lived intrusions remain controversial. In light of detailed geochronology, geochemistry, and numerical modeling, a variety of subtle internal structures have been generally interpreted to suggest that these intrusive complexes were formed by repeated influx of magma batches into a system kept near its solidus (e.g., Coleman et al., 2004; Hirt, 2007; Saleeby et al., 2008). However, opinions differ greatly on the size, frequency, and emplacement mechanisms of these magmatic replenishment events, varying from frequent re-intrusion of small dikes (e.g., Glazner et al., 2004) to much larger batches of magma that remain above their solidi for extended periods of time (e.g., Žák and Paterson, 2005). We briefly evaluate evidence for different emplacement mechanisms in the Sahwave intrusive suite here.
The Sahwave intrusive suite generally has steep contacts and steep magmatic foliation (Fig. 3); thus, it is unlikely that it is a sill-like intrusion. Nevertheless, given its 40 km diameter at a relatively shallow depth of exposure, the intrusion must have been relatively flat-topped (Fig. 16). The downward extent of the batholith is not well defined by existing seismic or gravity data, but comparison to the oblique crustal arc sections exposed in southern California suggests that batholithic rocks may extend to the base of the crust, although large distinct intrusions in the upper crust may overlie a complex zone of smaller, vertically sheeted intrusions in the lower crust (Saleeby et al., 2003; Barth et al., 2008; Saleeby et al., 2008). Seismic data from farther north along the arc (Fig. 2; Lerch et al., 2007), however, indicate low velocities compatible with tonalitic/granitic rocks down to ∼15 km, suggesting that the magmatic arc in NW Nevada is underlain by mafic residua and remnants of thin transitional crust, in contrast to the crust of the southern and central Sierra Nevada Batholith, which is relatively felsic (tonalitic) to its base (e.g., Saleeby, 1990; Fleidner et al., 2000). Even if the batholithic rocks studied here extend to only 15 km depth, as shown in Figure 16, the Sahwave intrusive suite still represents a volume of well over 10,000 km3, such that space accommodation and mechanisms of its emplacement are nontrivial problems.
Given the vast size of the Granodiorite of Juniper Pass and the variations in modal mineralogy and chemistry it contains, it is entirely possible that this unit was emplaced over time as a series of smaller intrusive events. The cryptic internal structures and contacts within the Granodiorite of Juniper Pass are difficult to interpret, but the general smoothness of compositional variation within the sampled part of this unit suggests that individual batches of magma generally stayed hot long enough to partially mix with their successors. Other contacts within and around the Sahwave intrusive suite also vary greatly in style. Although often poorly exposed, contacts between units can be both sharp, such as where the School Bus Granodiorite intrudes the Granodiorite of Juniper Pass, or gradational over hundreds of meters, such as where the Granodiorite of Bob Spring intrudes the Granodiorite of Juniper Pass (Fig. 3). In certain places, contacts are observed to transition from sharp to gradational. This happens gradually along the contact of the Sahwave Granodiorite, but fairly abruptly where the Granodiorite of Bob Spring intrudes the Granodiorite of Juniper Pass east of the Power Line complex in the Nightingale Range (Fig. 3).
Sharp internal contacts likely reflect areas where new magma batches “eroded” their way into older magma that had partially cooled to the point where it behaved as a solid, perhaps even experiencing brittle fracture and stoping in places. Conversely, arcuate, gradational contacts likely formed where the previous batch of magma was either still partially molten, or at least close enough to its solidus to experience defrosting and partial to complete mixing along its contact. The apparent lack of internal contacts within the Granodiorite of Bob Spring, the School Bus Granodiorite, and the Sahwave Granodiorite (with the exception of a few contacts surrounding leucocratic segregations), combined with the general homogeneity of these units, suggests that each may represent a single phase of rapid magma input into a large, partially molten magma chamber. The concentric arrangement of these units further suggests that the central part of each unit was not fully mechanically and/or thermally stabilized before its successor intruded, and possibly flowed back downward after defrosting to accommodate the new magma. The concentric arrangement of successively more homogeneous (and generally more differentiated) units (Fig. 16) also suggests that the system was warming over time, allowing larger and longer-lived magma chambers to be formed at both the level of exposure and perhaps at the deeper level of magma production. Warming over time in the southern Nightingale Range due to re-intrusion of the School Bus Granodiorite so close to the contact already heated once by the Granodiorite of Juniper Pass may also be responsible for the extensive shouldering-aside implied by the anomalous orientation of the wall rocks in this area (Fig. 3).
Basinal Setting and Implications for Arc Flare-Up
In light of the extensive similarities between the Sahwave intrusive suite and coeval magmatic systems to the south, it is interesting to note that the crustal environments of these intrusions are very different (Figs. 1 and 15). Whereas the other massive intrusions of the Cathedral Range intrusive epoch are interpreted to lie along the margin of North American continental crust, as marked by scattered roof pendants of the miogeocline and initial 87Sr/86Sr >0.706 (Fig. 1; e.g., Saleeby, 1981; Kistler, 1990), the Sahwave intrusive suite is positioned in a deep stack of basinal muds thought to overlie transitional or oceanic crust (Speed, 1978; Farmer and DePaolo, 1983; Elison et al., 1990). Because of its unique position relative to the other members of the Cathedral Range event, the Sahwave intrusive suite can be used to examine hypotheses about the potential causes for the massive magmatic flare-up represented by these intrusions. It has been suggested that this particular pulse of major magmatic activity may have been due to westward underthrusting of North American lower crust beneath the magmatic arc, which is hypothesized to have been near the western edge of a massive orogenic wedge (DeCelles and Coogan, 2006; DeCelles et al., 2009). The voluminous magmatism in the Sahwave and Nightingale Ranges at this time, however, demonstrates very primitive isotopic ratios of 87Sr/86Sri ∼0.7047 and εNd ∼−0.2, which are not compatible with incorporation of a large crustal component. Similar or more primitive isotope ratios in the penecontemporaneous La Posta events of the Peninsular Ranges Batholith (Fig. 1; Walawender et al., 1990) corroborate this. Furthermore, modest reconstructed Cretaceous crustal thicknesses near the Sahwave Batholith (∼38 km; e.g., Colgan et al., 2006) would suggest that the orogenic wedge did not continue this far west in northern Nevada.
Since the availability of continental lower crust does not appear to have been the main control on high magmatic flux in the arc at this time, it seems that a more regionally extensive and consistent triggering mechanism must be invoked. A widespread flare-up in the arc could have been related to the tectonic underplating of Franciscan subduction accretionary material (Saleeby et al., 2008), a change in subduction rate and/or obliquity, age, or composition of underthrust oceanic lithosphere, or the stress regime accompanying intrusion. Alternatively, subduction of thicker oceanic crust in the Late Cretaceous could potentially be called upon to both induce a magmatic flux event and subsequently terminate magmatism. Very shallow subduction of a large and thick oceanic plateau is hypothesized to have disrupted the Mojave-Salinia segment of the arc (Saleeby, 2003), but modestly thickened oceanic crust in adjacent segments might have led to moderately shallow subduction and the observed cessation of magmatism. Thicker oceanic crust might incorporate and react with a greater volume of seawater (especially if pillow basalts represent a disproportionate share of crustal thickening in oceanic plateaus; e.g., Gladczenko et al., 1997). Dragged downward by previously subducted, denser oceanic lithosphere, the leading edge of this thickened oceanic crust could have released its fluids into the mantle wedge, creating a massive, fluid-rich basaltic flux that might have remelted any stack of older basalt left underplated at the base of the arc crust by prior arc activity. Triggered by the same cause as the incipient flat-slab subduction, magmatic flux would increase until crowding from the increasingly buoyant oceanic lithosphere caused stagnation of the mantle wedge, halting magmatism.
New mapping, geochronology, petrology, and geochemistry in the Sahwave and Nightingale Ranges of western Nevada document the northward continuation of the Cretaceous Cordilleran arc across the NW Basin and Range and form the groundwork for more detailed future study. Intrusive activity in the Sahwave and Nightingale area continued from ca. 110 to 88.5 Ma, and included the emplacement of a large, concentrically zoned intrusive suite at ca. 93–88.5 Ma, during the culminating magmatic flare-up of the Sierra Nevada Batholith. Mineralogy and geochemistry support the correlation of the Sahwave intrusive suite with members of the Cathedral Range intrusive event along the Sierra Nevada crest. The oldest unit of the Sahwave intrusive suite, the Granodiorite of Juniper Pass, is marked by significant compositional variations, which may indicate formation from multiple smaller intrusions, but the later, K-feldspar–porphyritic Granodiorite of Bob Spring and Sahwave Granodiorite are more homogeneous and may represent relatively large magma-mush chambers that were continuously maintained above their solidi. Concentric arrangement and gradational contacts between different units imply that parts of the system remained near the solidus during much of the 4 m.y. it was active. Despite differences in the lower crust beneath the Sahwave intrusive suite and intrusions along the crest of the Sierra Nevada (as indicated by more primitive 87Sr/86Sri and εNd values to the north), striking similarities between these segments of the arc suggest that a regionally developed subcrustal mechanism, such as the subduction of thicker and wetter oceanic crust or Franciscan mélange, may have been responsible for generation of the intrusions that punctuate the end of Cretaceous magmatism along much of the U.S. Cordilleran arc.
This research was partially sponsored by National Science Foundation (NSF) Tectonics grant 0809226, two Stanford McGee Grants, and a Geological Society of America (GSA) Student Research Grant. Van Buer was partially supported by a Burt and DeeDee McMurtry Fellowship. Special thanks are due to Joe Wooden for help acquiring and analyzing sensitive high-resolution ion microprobe (SHRIMP) data, to Bettina Wiegand for measuring Sr and Nd isotopes, and to other helpful folks at the Stanford-USGS Micro-Analysis Facility. We also thank Gail Mahood, Robinson Cecil, and Sandra Wyld for helpful reviews.