Upper-Mantle Fragments from Beneath the Sierra Nevada Batholith: Partial Fusion, Fractional Crystallization, and Metasomatism in a Subduction Related Ancient Lithosphere |
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| Authors: | MUKHOPADHYAY BISWAJIT; MANTON W I |
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| Institution: | Department of Geosciences, The University of Texas at Dallas Richardson, Texas 75083, USA |
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| Abstract: | Upper-mantle xenoliths in volcanic pipes cutting the axis ofthe Sierra Nevada batholith contain predominantly spinel-bearingperidotites (with sporadic garnet) and garnet websterites. Inspite of the enormous thickness of the Sierran crust, the Sierranupper mantle has not attained the garnet peridotite stabilityfield. The peridotites have forsteritic (Fo88–92) olivines,Cr-diopsides, Cr-spinels, and magnesian orthopyroxenes (En88–92).Their texture and compositional characteristics of the coexistingphases indicate that these are fragments of the upper mantlethat had undergone various degrees of partial fusion. The Pconditions of reequilibration and mineralogical characteristicssuggest that the partial fusion was accompanied by diapiricuprise. The REE distribution patterns are nearly chondritic.Garnet websterite xenoliths also contain magnesian and Cr-richphases. Their bulk chemical compositions are like pyroxenitecumulates. The garnet websterites from Big Creek differ fromthose occurring at Pick and Shovel in having more Fe-rich phasesand occasional hydrous minerals. The Pick and Shovel garnetwebsterites are interpreted to be pyroxene-rich, garnet-freecumulates formed by fractional crystallization of melts generatedby partial melting of subcontinental lithosphere at depth  60km. The REE abundance of these xenoliths is consistent withthis mode of origin. Presence of jadeitic clinopyroxenes andF-rich phlogopites, and the LREE- and 87Sr/86Sr-enriched characterof the garnet websterites from Big Creek may suggest their originas metasomatized upper-mantle garnet peridotites. The latestP-T conditions of equilibration of all garnet-bearing samplesshow that they lie along a nearly adiabatic gradient in therange of 900–1000 C and 18–32 kbar. An isotopically heterogeneous, old ( 1 b.y.) subcontinental lithosphere,characterized by high 87Sr/86Sr (0.7044–0.7082), radiogenic206Pb/204Pb (18.86–20.04), 207Pb/204Pb (15.64–15.69)and 208Pb/204Pb (38.69–39.11), and moderate 143Nd/144Nd(0.51234–0.51260; ENd–0.35 to –5.8) is consideredto be the source of these rocks. There was a fluid influx froma subducted slab carrying Ba, K, Rb, U, Th, and radiogenic Pbinto the overlying ancient lithosphere. |
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