Portal de Periódicos da CAPES

Upper-Mantle Fragments from Beneath the Sierra Nevada Batholith: Partial Fusion, Fractional Crystallization, and Metasomatism in a Subduction Related Ancient Lithosphere

1994; Oxford University Press; Volume: 35; Issue: 5 Linguagem: Inglês

10.1093/petrology/35.5.1417

1460-2415

Biswajit Mukhopadhyay, William I. Manton,

earthquake and tectonic studies

Upper-mantle xenoliths in volcanic pipes cutting the axis of the Sierra Nevada batholith contain predominantly spinel-bearing peridotites (with sporadic garnet) and garnet websterites. In spite of the enormous thickness of the Sierran crust, the Sierran upper mantle has not attained the garnet peridotite stability field. The peridotites have forsteritic (Fo88–92) olivines, Cr-diopsides, Cr-spinels, and magnesian orthopyroxenes (En88–92). Their texture and compositional characteristics of the coexisting phases indicate that these are fragments of the upper mantle that had undergone various degrees of partial fusion. The P conditions of reequilibration and mineralogical characteristics suggest that the partial fusion was accompanied by diapiric uprise. The REE distribution patterns are nearly chondritic. Garnet websterite xenoliths also contain magnesian and Cr-rich phases. Their bulk chemical compositions are like pyroxenite cumulates. The garnet websterites from Big Creek differ from those occurring at Pick and Shovel in having more Fe-rich phases and occasional hydrous minerals. The Pick and Shovel garnet websterites are interpreted to be pyroxene-rich, garnet-free cumulates formed by fractional crystallization of melts generated by partial melting of subcontinental lithosphere at depth ≧60 km. The REE abundance of these xenoliths is consistent with this mode of origin. Presence of jadeitic clinopyroxenes and F-rich phlogopites, and the LREE- and 87Sr/86Sr-enriched character of the garnet websterites from Big Creek may suggest their origin as metasomatized upper-mantle garnet peridotites. The latest P-T conditions of equilibration of all garnet-bearing samples show that they lie along a nearly adiabatic gradient in the range 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), radiogenic 206Pb/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 considered to be the source of these rocks. There was a fluid influx from a subducted slab carrying Ba, K, Rb, U, Th, and radiogenic Pb into the overlying ancient lithosphere.