Importance of melt circulation and crust-mantle interaction in the lithospheric evolution beneath the North China Craton:Evidence from Mesozoic basalt-borne clinopyroxene xenocrysts and pyroxenite xenoliths

【摘要】：正Mesozoic Fangcheng basalts from the North China Craton contain many clinopyroxene xenocrysts and pyroxenite xenoliths, which provide important information about melt circulation and crust-mantle interaction in the evolution of the sub-continental lithosphere beneath the region.All the xenocrysts show textural and chemical zoning.The zoning is mostly simple(simply-zoned), but some show complex patterns(complexly-zoned).In-situ major and trace element analyses suggest that all the simply-zoned xenocrysts may have been disaggregated from pyroxenite veins at mantle depth.The zoning is interpreted to result from chemical exchange with the host magma after their entrainmcnt.The complexly-zoned xenocrysts record a complicated history of the lithospheric evolution:their cores could preserve information of high-temperature granulite-facies metamorphism in the lower crust and their intermediary zones might record metamorphic overgrowth in the mantle spinel-facies stability field.Therefore,the cores of the complexly-zoned xenocrysts have probably been derived from the Archean lower crust or newly-accreted lower crust and the intermediary zones could be formed through crust-mantle interaction.All the pyroxenite xenoliths are cumulates and were crystallized from the LREE enriched melts at mantle depth or crust-mantle transitional zone.Thus the clinopyroxene xenocrysts and pyroxenite xenoliths provide evidence for the existence of considerable crust-mantle interaction and melt circulation in the lithospheric mantle,which led to rapid lithospheric enrichment.