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Uranium and Th isotope measurements for some of the youngest basaltic- to rhyolitic lava flows associated with the Long Valley caldera and Mono-Inyo Craters indicate that the lavas have pronounced U and Th isotopic disequilibrium. The (238U/232Th) ratios range from 0.69 to 1.23, and the (232Th/230Th) ratios range from 0.80 to 1.03. The majority of the lavas plot to the right of the equiline, which corresponds with a subduction zone influence on the U and Th isotope compositions. A basaltic inclusion from the Mono Inyo dacite (age = 18 Ka, epsilon-Nd = +2) is one of the samples farthest to the right of the equiline, which indicates that the basaltic magmatism associated with the formation of the silicic magma system is subduction-like. In contrast, an older (ca. 100 Ka) trachybasalt lava from the Long Valley north moat (epsilon-Nd=-3) plots to left of the equiline. We interpret these differences as reflecting lithospheric sources for the small-volume alkalic lavas and asthenospheric (subduction-affected) sources for the larger volume silicic system. The preservation of strong U-Th isotopic disequilibrium in high-silica lavas clearly indicates that the rhyolites do not have long magma chamber residence times as has been inferred for other silicic systems in western North America. This observation is in accord with the young age of the Mono Craters (less than about 18 Ka) and suggests that this system has not yet stored up a large amount of silicic magma that could feed an eruption of Bishop Tuff size.

Mafic complexes in the central Sierra Nevada batholith record valuable geochemical information regarding the role mafic magmas play in arc magmatism and the generation of continental crust. In the intrusive suite of Yosemite Valley, major and trace element compositions of the hornblende-bearing gabbroic rocks from the Rockslides mafic complex and of the mafic dikes in the North America Wall are compositionally similar to high-alumina basalt. Of these rocks, two samples have higher Ni and Cr abundances as well as higher [ζ.sub.Nd] values than previously recognized for the intrusive suite. Plagioclase crystals in rocks from the North America Wall and the Rockslides have prominent calcic cores and sharply defined sodic rims, a texture commonly associated with mixing of mafic and felsic magmas. In situ analyses of [sup.87]Sr/[sup.86]Sr in plagioclase show no significant isotopic difference from the cores to the rims of these grains. We propose that the high [sup.87]Sr/[sup.86]Sr (~0.7067) and low [ζ.sub.Nd] (~-3.4) of bulk rocks, the homogeneity of [sup.87]Sr/[sup.86]Sr in plagioclase, and the high [δ.sup.18]O values of bulk rocks (6.6-7.3 %o) and zircon (Lackey et al. in J Petrol 49:1397-1426, 2008) demonstrate that continental crust was assimilated into the sublithospheric mantle-derived basaltic precursors of the mafic rocks in Yosemite Valley. Contamination (20-40%) likely occurred in the lower crust as the magma differentiated to high-alumina basalt prior to plagioclase (and zircon) crystallization. As a consequence, the isotopic signatures recorded by whole rocks, plagioclase, and zircon do not represent the composition of the underlying lithospheric mantle. We conclude that the mafic and associated felsic members of the intrusive suite of Yosemite Valley represent 60-80 % new additions to the crust and include significant quantities of recycled ancient crust. Keywords Sierra Nevada batholith * Plagioclase zoning * Sr isotopes * Oxygen isotopes * Mafic complex, Introduction Continental arcs are the most common setting in which juvenile continental crust is generated. The production of mantle-derived mafic magmas is widely accepted as an important component in creating [...]

New [sup.187]Os/[sup.188]Os ratios of Quaternary Mount Adams volcanic rocks from the Cascade arc in southern Washington vary by >300% ([sup.187]Os/[sup.188]Os = 0.165-0.564) and fall into high (>0.319) and low (0.166 to 0.281) groups of [sup.187]Os/[sup.188]Os ratios that are substantially more radiogenic than mantle values. These Os isotope compositions and groupings are interpreted to reflect recycling of discrete intracrustal domains with high [sub.187]Os/[sup.188]Os ratios but differing ages, thus recording the process of crustal hybridization and homogenization. Os isotope compositions provide new constraints on amounts of intracrustal recycling in young subduction-zone environments that reflect the magmatic history of the arc. Sr, Nd, Hf, and Pb isotope variations in this young, mafic arc complex are too small to allow such constraints. Keywords: Mount Adams, Cascade Range, crust, recycling, osmium, hybridization.

A separate basement terrane, Mojavia or the Mojave province with characteristic 2.0-2.3 Ga model ages, has been proposed to underlie much of the western U.S. Its existence, posited on patterns of Pb and Nd isotope data, has been propagated in the literature for more than two decades. New and compiled U/Pb geochronology shows there is no direct evidence of >2.0 Ga juvenile basement rocks exposed within Mojavia of the eastern Great Basin. Pb and Nd model ages, by contrast, vary from Archean to Neoproterozoic with large variations exhibited, commonly within small regions. Archean ages are concentrated northward, suggesting the influence of sediment shed southward from the Wyoming province onto Paleoproterozoic basement terranes. In places, including the Farmington Canyon complex, sediment has been tectonically reprocessed and now is preserved as high-grade metamorphic rock in accretionary mélanges. There is no strong evidence that Mojavian basement exists outside of the Mojave Desert region proper. A statistical evaluation of common Pb and Sr isotopes in Phanerozoic igneous rocks shows distinct differences north and south of one proposed boundary between Mojavia and the Yavapai province. However, close examination suggests that variation in these parameters could be produced by the influence of Archean material shed from the Wyoming province rather than representing a distinct difference in the age and isotopic character of the basement. Statistical discrimination of candidate terranes and terrane boundaries may be valuable for their recognition, but such differences alone do not prove their existence. The northern boundary of Mojavia with the Wyoming province in the eastern Great Basin has been given the same name, the Cheyenne Belt, as the exposed suture between Paleoproterozoic and Archean basement in southern Wyoming. We re assess the location of this boundary in the Great Basin. New age controls on key basement outcrops in the Uinta Mountains and Farmington Canyon complex that were previously considered Archean indicate that these rocks are Paleoproterozoic in age (∼1.7 Ga). Thus, the Cheyenne Belt has traditionally been placed too far south; it must lie in a poorly defined location north of these localities. [ABSTRACT FROM AUTHOR]

The Indian Peak-Caliente caldera complex and its surrounding ignimbrite field were a major focus of explosive silicic activity in the eastern sector of the subduction-related southern Great Basin ignimbrite province during the middle Cenozoic (36-18 Ma) ignimbrite flareup. Caldera-forming activity migrated southward through time in response to rollback of the subducting lithosphere. Nine partly exposed, separate to partly overlapping source calderas and an equal number of concealed sources compose the Indian Peak-Caliente caldera complex. Calderas have diameters to as much as 60 km and are filled with as much as 5000 m of intracaldera tuff and wall-collapse breccias. More than 50 ignimbrite cooling units, including 22 of regional (>100 km3) extent, are distinguished on the basis of stratigraphic position, chemical and modal composition, 40Ar/39Ar age, and paleomagnetic direction. The most voluminous ash flows spread as far as 150 km from the caldera complex across a high plateau of limited relief--the Great Basin altiplano, which was created by late Paleozoic through Mesozoic orogenic deformation and crustal thickening. The resulting ignimbrite field covers a present area of ~60,000 km2 in east-central Nevada and southwestern Utah. Before post-volcanic extension, ignimbrites had an estimated aggregate volume of ~33,000 km3. At least seven of the largest cooling units were produced by super-eruptions of more than 1000 km3. The largest, at 5900 km3, originally covered an area of 32,000 km2 to outflow depths of hundreds of meters. Outflow ignimbrite sequences comprise as many as several cooling units from different sources with an aggregate thickness locally reaching a kilome ter; sequences are almost everywhere conformable and lack substantial intervening erosional debris and angular discordances, thus manifesting a lack of synvolcanic crustal extension. Fallout ash in the mid-continent is associated with two of the super-eruptions. Ignimbrites are mostly calc-alkalic and high-K, a reflection of the unusually thick crust in which the magmas were created. They have a typical arc chemical signature and define a spectrum of compositions that ranges from high-silica (78 wt%) rhyolite to andesite (61 wt% silica). Rhyolite magmas were erupted in relatively small volumes more or less throughout the history of activity, but in a much larger volume after 24 Ma in the southern part of the caldera complex, creating ~10,000 km3 of ignimbrite. The field has some rhyolite ignimbrites, the largest of which are in the south and were emplaced after 24 Ma. But the most distinctive attributes of the Indian Peak-Caliente field are two distinct classes of ignimbrite: 1. Super-eruptive monotonous intermediates. More or less uniform and unzoned deposits of dacitic ignimbrite that are pheno cryst rich (to as much as ~50%) with plagioclase > biotite quartz hornblende > Fe-Ti oxides ± sanidine, pyroxene, and titanite; apatite and zircon are ubiquitous accessory phases. These tuffs were deposited at 31.13, 30.06, and 29.20 Ma in volumes of 2000, 5900, and 4400 km3, respectively, from overlapping, multicyclic calderas. A unique, and possibly kindred, phenocryst-rich latiteandesite ignimbrite with an outflow volume of 1100 km3 was erupted at 22.56 Ma from a concealed source caldera to the south. 2. Trachydacitic Isom-type tuffs. Also relatively uniform but phenocryst poor (<20%) with plagioclase >> clinopyroxene orthopyroxene Fe-Ti oxides >> apatite. These alkali-calcic tuffs are enriched in TiO2, K2O, P2O5, Ba, Nb, and Zr and depleted in CaO, MgO, Ni, and Cr, and have an arc chemical signature. Magmas were erupted from a concealed source immediately after and just to the southeast of the multicyclic monotonous intermediates. Most of their aggregate outflow volume of 1800 km3 was erupted from 27. 90 to 27.25 Ma. Nothing like this couplet of distinct ignimbrites, in such volumes, have been documented in other middle Cenozoic volcanic fields in the southwestern U.S. where the ignimbrite flareup is manifest. Magmas were created in unusually thick crust (as thick as 70 km) where large-scale inputs of mantle-derived basaltic magma powered partial melting, assimilation, mixing, and differentiation processes. Dacite and some rhyolite ignimbrites were derived from relatively low-temperature (700-800 °C), water-rich magmas that were a couple of log units more oxidized than the quartz-fayalitemagnetite (QFM) oxygen buffer at depths of ~8-12 km. In contrast to these "main-trend" magmas, trachydacitic Isom-type magmas were derived from drier and hotter (~950 °C) magmas originating deeper in the crust (to as deep as 30 km) by fractionation processes in andesitic differentiates of the mantle magma. "Off-trend" rhyolitic magmas that are both younger and older than the Isom type but possessed some of their same chemical characteristics possibly reflect an ancestry involving Isom-type magmas as well as main-trend rhyolitic magmas. Andesitic lavas extruded during the flare up but mostly after 25 Ma constitute a roughly estimated 12% of the volume of silicic ignimbrite, in contrast to major volcanic fields to the east, e.g., the Southern Rocky Mountain field, where the volume of intermediate-composition lavas exceeds that of silicic ignimbrites. [ABSTRACT FROM AUTHOR]