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Abstract

Granitic and volcanic rocks in the eastcentral Sierra Nevada, western UnitedStates, record the earliest stages of magmatism in the eastern Sierra Nevada magmatic arc, allowing us to examine magma sources and connections between plutonic and volcanic processes in the initial stages of arc construction. The Scheelite Intrusive Suite is one of the largest in the Sierra Nevada region, and is composed of the Wheeler Crest Granodiorite, granite of Lee Vining Canyon, and Pine Creek Granite. The Pb/U zircon ages from each unit of the suite suggest assembly between 226 and 218 Ma. The Scheelite Intrusive Suite is a high-K calcic or calc-alkalic suite, compositionally broadly similar to the nearby Late Cretaceous Tuolumne and John Muir Intrusive Suites, though plutons of the Scheelite Intrusive Suite are consistently Ca and Fe rich and lower in Na. Although Triassic granodiorites are isotopically quite similar to nearby Late Cretaceous intrusive suites, the trend toward more isotopically primitive granites is in contrast to the constant or more whole-rock radiogenic Sr trends observed in younger intrusive suites. Along the western margin of the Scheelite Intrusive Suite, the basal Mesozoic volcanic section in the Saddlebag Lake pendant includes silicic volcanic rocks that are in part coeval and potentially comagmatic with Triassic plutonic rocks. Widespread quartz-phyric ash-fl ow tuffs of Black Mountain, Saddlebag Lake, and Greenstone Lake yield Pb/U zircon ages of 232, 224, and 219 Ma, indicating that felsic ignimbrite volcanism commenced earlier and continued during emplacement of the 226-218 Ma Scheelite Intrusive Suite. Ash-fl ow tuffs are hydrothermally altered but have high fi eld strength element abundances and Nd isotopic compositions, suggesting affi nity to the relatively felsic parts of the Wheeler Crest Granodiorite and the granite of Lee Vining Canyon.

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“…The net result of these changes is highly variable whole-rock Rb/Sr, which ranges from 0.3 to 2.4. Because whole-rock Rb/Sr isochrons are in agreement with U-Pb zircon ages of individual tuffs (Kistler and Swanson, 1981;Barth et al, 2011), it is likely that the volcanic section was hydrothermally altered during or soon after deposition, by sodium depletion locally accompanied by variable enrichment in carbonate and silica. The hypothesis of hydrothermal alteration is supported by petrographic evidence for fracturing and recrystallization of quartz and replacement of feldspar phenocrysts by epidote + calcite.…”

“…H 2 O and CO 2 were measured in ash-fl ow tuffs using a combustion analyzer. Whole-rock major and trace-element analyses were reported by Barth et al (2011), and all analyses are available for download at www.navdat.org.…”

“…Bateman (1992) grouped most granitic rocks intruding the eastern side of the Saddlebag Lake pendant as the Scheelite Intrusive Suite, the oldest and one of the largest intrusive suites in the central Sierra Nevada batholith. As recently described and slightly revised by Barth et al (2011), the Scheelite Intrusive Suite comprises the Wheeler Crest Granodiorite, including the Mount Olsen pluton (226 Ma) and granodiorites of the Benton Range (225 Ma) and Wheeler Crest (218 Ma), the granite of Lee Vining Canyon (218-220 Ma), and the Pine Creek Granite (>215 Ma). Comparison of zircon U-Pb data indicates that assembly of this intrusive suite was very closely linked in time to eruption of ash-fl ow tuffs now preserved in the adjacent Saddlebag Lake pendant.…”

Regional and temporal variability of melts during a Cordilleran magma pulse: Age and chemical evolution of the Jurassic arc, eastern Mojave Desert, California

“…Techniques for whole rock analyses at Macquarie University were described by Todd et al (2003) and at Michigan State University by Barth et al (2011).…”

“…The latest Paleozoic to Mesozoic Cordilleran magmatic arc in the western US formed as a consequence of prolonged east-directed subduction initiated in Permo-Triassic time (Saleeby and Busby-Spera, 1992;Barth and Wooden, 2006;Saleeby, 2011;Barth et al, 2011Barth et al, , 2012. The first magmatic pulse in the arc was widespread, but both the exposed and detrital records of arc magmatism indicate that the second, Jurassic magmatic pulse was geochemically distinct and of significantly larger volume .…”