Hornblende gabbro sill complex at Onion Valley,California, and a mixing origin for the Sierra Nevada batholith |
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Authors: | T W Sisson T L Grove D S Coleman |
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Institution: | (1) U.S. Geological Survey, MS-910, 345 Middlefield Rd., Menlo Park, CA 94025, USA, US;(2) Department of Earth, Atmospheric, and Planetary Sciences, Building 54, Massachusetts Institute of Technology, Cambridge, MA 02136, USA, US |
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Abstract: | The steep crest of the Sierra Nevada, California, near Onion Valley, exposes natural cross sections through a mafic intrusive
complex that formed as part of the Mesozoic Sierra Nevada batholith. Sheeted sills of hornblende gabbro to hornblende diorite,
individually as thick as 1.5 m, form the upper 200 to 300 m of the 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. Cumulates are flat-lying, modally layered hornblende gabbro with subsidiary ultramafic
olivine hornblendite, plagioclase hornblendite, and late-mobile hornblende-plagioclase pegmatite. Fine grain size, scarce
phenocrysts and xenocrysts, and quench mineral textures are evidence that hornblende gabbro sills injected in a largely liquid
state and preserve basaltic melt compositions. Most sills reached volatile saturation, as shown by tiny miarolitic cavities
that are also widespread in cumulates. Although some sills chilled directly against others, most chilled against 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 silicic magma chamber that has since been removed by erosion. Sill compositions range from evolved high-alumina basalt
to aluminous andesite with major and trace element abundances similar to those of modern arc magmas. Experimental phase equilibria
indicate dissolved water contents near 6 wt% (Sisson and Grove 1993a). The sills show unequivocally that hydrous arc basaltic
magmas reached shallow levels in the crust during formation of the largely granodioritic Sierra Nevada batholith. The basaltic
magmas appear to have been produced from an enriched mantle source with 87Sr/86Sr ∼0.7065, ɛNd ∼−4.3, 206Pb/204Pb ∼18.6, 207Pb/204Pb ∼15.6, 208Pb/204Pb ∼38.6. Although crystal fractionation contributed to forming the sill suite and the associated cumulates, nearly constant
concentrations of Na2O, P2O5, Nb, Zr, and light rare earth elements in the sills indicate that mixing between sill basaltic and more evolved septa magmas
was important for producing sills with andesitic compositions. Average Sierran granodiorite major and trace element concentrations
are readily reproduced by a simple mixture of average basaltic sill from Onion Valley and average Sierran low-silica granite.
This result supports the inference that Sierran granitoids formed chiefly by mixing between crustal and mantle-derived magmas,
although in some cases these crustal melts may have been derived by refusion of earlier mafic intrusions near the base of
the crust. The common mafic inclusions (enclaves) in Sierran granodiorites bear a superficial resemblance to Onion Valley
mafic sills; however, high concentrations of lithophile elements in the inclusions point to extensive chemical exchange between
inclusions and their host magmas. The prevalence of hornblende-rich mafic intrusive rocks at Onion Valley, elsewhere in the
Sierra Nevada, and in other shallow subduction batholiths stems from two effects of high melt water concentrations (∼4–6 wt%
H2O). The hydrous parent basaltic and basaltic andesite magmas had low liquidus temperatures, compared to nearly dry basaltic
melts, and thus were chilled less during ascent through the crust and were more capable of ascent as liquids. More importantly,
their high water concentrations led to low melt densities, higher than granitoid liquids, but comparable to or less dense
than partly solidified granitoid magmas. Thus, the hydrous basaltic and basaltic andesite magmas were neutrally or positively
buoyant and were capable of penetrating and rising through partly crystallized granitoids and their partly molten source regions
to reach upper crustal emplacement levels. Drier basaltic magmas were probably abundant at depth and contributed heat and
mass to granite generation, but were insufficiently buoyant to ascend to shallow levels.
Received: 2 August 1995 / Accepted: 26 June 1996 |
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