Subduction-driven heterogeneity of the lithospheric mantle beneath the Cathaysia block, South China

Abstract

To evaluate the modification of the lithospheric mantle by the Paleo-Pacific Ocean subduction beneath the Cathaysia block, we report major and trace elements, and Li isotopes of olivine from Niutoushan, Mingxi, Xilong and Jiande peridotite xenoliths in South China. Spoon-shaped, convex-upward and highly LREE-enriched chondrite-normalized REE patterns of clinopyroxene in the Niutoushan, Mingxi and Xilong xenoliths reflect the interplay between partial melting and subsequent enrichment by metasomatism. Lithium isotope systematics indicate that at least two different metasomatic events affected these peridotites. High Li concentrations in clinopyroxene (up to 59.1 ppm) are ascribed to diffusive Li influx from silicate melts. Olivine grains with lower diffusivity of Li have much higher Li concentrations (0.56–9.76 ppm) and lighter δ7Li values (−52.75 to +10.49‰) than those in the Jiande and other peridotite xenoliths worldwide, showing a prominent carbonatitemetasomatic signature occurring before silicate metasomatism. In addition, increasing Li concentrations of olivine and clinopyroxene with increasing distance from the present subduction trench is consistent with those of low δ26Mg basalts in South China, indicating that the metasomatic agent was related to the westward subduction of the Paleo-Pacific slab. The Jiande lherzolite xenoliths further away from the trench show ‘mantle-like’ Li features and may represent upwelling asthenospheric materials accreted to the continent. Therefore, both types of metasomatism as well as upwelling asthenosphere could have played an important role in the modification of the chemical composition of the mantle beneath the Cathaysia block