HORNBLENDE GABBRO SILL COMPLEX AT ONION VALLEY, CALIFORNIA, AND A MIXING ORIGIN FOR THE SIERRA-NEVADA BATHOLITH

The steep crest of the Sierra Nevada, California, near Onion Valley, e xposes natural cross sections through a mafic intrusive complex that f ormed as part of the Mesozoic Sierra Nevada batholith. Sheeted sills o f hornblende gabbro to hornblende diorite, individually as thick as 1. 5 m, form the upper 200 to 300 m of the com complex. Thicker, multiply -injected sills, as well as mafic stocks, lie underneath at elevations below 3600 m. Lens-shaped cumulate bodies, as thick as 200 m and more than 700 m broad, lie near the base of the sheeted sill suite. Cumula tes are flat-lying, modally layered hornblende gabbro with subsidiary ultramafic olivine hornblendite, plagioclase hornblendite, and late-mo bile hornblendeplagioclase pegmatite. Fine grain size, scarce phenocry sts and xenocrysts, and quench mineral textures are evidence that horn blende gabbro sills injected in a largely liquid state and preserve ba saltic melt compositions. Most sills reached volatile saturation, as s hown by tiny miarolitic cavities that are also widespread in cumulates . Although some sills chilled directly against others, most chilled ag ainst septa, millimeters to a few centimeters thick, of medium-grained diorite to granodiorite. Mutually crosscutting relations, as well as chilling, show that the septa were partly molten at the time the sills injected and likely formed the lower portions of an overlying more si licic magma chamber that has since been removed by erosion. Sill compo sitions range from evolved high-alumina basalt to aluminous andesite w ith major and trace element abundances similar to those of modern are magmas. Experimental phase equilibria indicate dissolved water content s near 6 wt% (Sisson and Grove 1993a). The sills show unequivocally th at hydrous are basaltic magmas reached shallow levels in the crust dur ing formation of the largely granodioritic Sierra Nevada batholith. Th e basaltic magmas appear to have been pro produced from an enriched ma ntle source with Sr-87/Sr-86 similar to 0.7065, is an element of Nd si milar to-4.3, Pb-206/Pb-204 similar to 8.6, Pb-207/Pb-204 similar to 5 .6, Pb-208/Pb-204 similar to 38.6. Although crystal fractionation cont ributed to forming the sill suite and the associated cumulates, nearly constant concentrations of Na2O, P2O5, Nb, Zr, and light rare earth e lements in the sills indicate that mixing between sill basaltic and mo re evolved septa magmas was important for producing sills with andesit ic compositions. Average Sierran granodiorite major and trace element concentrations are readily reproduced by a simple mixture of average b asaltic sill from Onion Valley and average Sierran low-silica granite. This result supports the inference that Sierran granitoids formed chi efly by mixing between crustal and mantle-derived magmas, although in some cases these crustal melts may have been derived by refusion of ea rlier mafic intrusions near the base of the crust. The common mafic in clusions (enclaves) in Sierran granodiorites bear a superficial resemb lance to Onion Valley mafic sills; however, high concentrations of lit hophile elements in the inclusions point to extensive chemical exchang e between inclusions and their host magmas. The prevalence of hornblen de-rich mafic intrusive rocks at Onion Valley, elsewhere in the Sierra Nevada, and in other shallow subduction batholiths stems from two eff ects of high melt water concentrations (similar to 4-6 wt% H2O). The h ydrous parent basaltic and basaltic andesite magmas had low liquidus t emperatures, compared to nearly dry basaltic melts, and thus were chil led less during ascent through the crust and were more capable of asce nt as liquids. More importantly, their high water concentrations led t o low melt densities, higher than granitoid liquids, but comparable to or less dense than partly solidified granitoid magmas. Thus, the hydr ous basaltic and basaltic andesite magmas were neutrally or positively buoyant and were capable of penetrating and rising through partly cry stallized granitoids and their partly molten source regions to reach u pper crustal emplacement levels. Drier basaltic magmas were probably a bundant at depth and contributed heat and mass to granite generation, but were insufficiently buoyant to ascend to shallow levels.