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

The extremely low Ti content (160-245 mug/g) in clinopyroxene in some spine t peridotites from Qilin, South China is indicative of high degree of parti al 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 RE E patterns can be modeled as residues of 22%-23% fractional melting from a primitive mantle, first in garnet stability field (12%) then continuing in spinet stability field (10%-11%) after breakdown of garnet to pyroxenes and spinet. Such a polybaric melting suggests the lithospheric thinning and ra pid 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 t he formation of South China Sea basin is related to southward migration of continental lithosphere extension, rather than passive back-arc basin.