Magmatic infiltration and melting in the lower crust and upper mantle beneath the Cima volcanic field,California |
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Authors: | H G Wilshire Anne V McGuire |
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Institution: | (1) U.S. Geological Survey, Menlo Park, CA 94025, USA, US;(2) Department of Geosciences, University of Houston, Houston, TX 77204-5503, USA, US |
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Abstract: | Xenoliths of lower crustal and upper mantle rocks from the Cima volcanic field (CVF) commonly contain glass pockets, veins,
and planar trains of glass and/or fluid inclusions in primary minerals. Glass pockets occupy spaces formerly occupied by primary
minerals of the host rocks, but there is a general lack of correspondence between the composition of the glass and that of
the replaced primary minerals. The melting is considered to have been induced by infiltration of basaltic magma and differentiates
of basaltic magma from complex conduits formed by hydraulic fracturing of the mantle and crustal rocks, and to have occurred
during the episode of CVF magmatism between ∼7.5 Ma and present. Variable compositions of quenched melts resulted from mixing
of introduced melts and products of melting of primary minerals, reaction with primary minerals, partial crystallization,
and fractionation resulting from melt and volatile expulsion upon entrainment of the xenoliths. High silica melts ( >∼60%
SiO2) may result by mixing introduced melts with siliceous melts produced by reaction of orthopyroxene. Other quenched melt compositions
range from those comparable to the host basalts to those with intermediate Si compositions and elevated Al, alkalis, Ti, P,
and S; groundmass compositions of CVF basalts are consistent with infiltration of fractionates of those basalts, but near-solidus
melting may also contribute to formation of glass with intermediate silica contents with infiltration only of volatile constituents.
Received: 15 June 1995 / Accepted: 13 December 1995 |
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