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

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 entrainment. 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.