Polybaric partial melting in the continental mantle: Evidence from mantle xenoliths from Qilin, Guangdong province

Abstract

The extremely low Ti content (160-245 μg/g) in clinopyroxene in some spinel peridotites from Qilin, South China is indicative of high degree of partial melting, inconsistent with their relatively high clinopyroxene modes (7.4%-12.4%). These clinopyroxenes show fractionated HREE patterns ((Gd/Yb) n<0.2), suggesting the involvement of garnet in the melting regime. These REE patterns can be modeled as residues of 22%-23% fractional melting from a primitive mantle, first in garnet stability field (12%) then continuing in spinel stability field (10%-11%) after breakdown of garnet to pyroxenes and spinel. Such a polybaric melting suggests the lithospheric thinning and rapid mantle upwelling in south China during the Cenozoic. This is consistent with the dominant MORB-OIB isotopic signature and high thermal gradient of the lithospheric mantle in this region, and supports the contention that the formation of South China Sea basin is related to southward migration of continental lithosphere extension, rather than passive back-arc basin.