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Recycled crustal melt injection into lithospheric mantle:implication from cumulative composite and pyroxenite xenoliths

【摘要】:正A rare composite xenolith and abundant cumulative pyroxenites obtained from the Mesozoic Fan-gcheng basalts on the eastern North China Craton record a complex history of melt percolation and circulation in the subcontinental lithospheric mantle.The composite xenolith has a dunite core and an olivine clinopyroxenite rim.The dunite is of cumulative origin and has a granular recrys-tallized texture and extremely low Mg#[100 Mg/ (Mg+Fe)=81-82]contents in olivines.The olivine clinopyroxenite contains larger clinopyroxene and/or orthopyroxene with a few fine-grained olivine and tiny phlogopite,feldspar,and/or carbonate minerals interstitial to clinopyroxene.The clinopyroxene has low Mg#(83-85). Compositional similarity between dunitic olivine and pyroxenitic one indicates a sequential crystallization of dunite and pyroxenite from a precursor melt.Pyroxenite xenoliths include olivine websterites and clinopyroxenites, both are of cumulative origin.Estimation of the melt from major oxides in olivines and REE concentrations in clin-opyroxenes in these composite and pyroxenite xenoliths suggests a derivation from subducted crustal materials, consistent with the highly enriched EMII-like Sr and Nd isotopic ratios observed in the pyroxenites.Occurrence of phlogopite,feldspar and carbonate minerals in some xenoliths requires the melt rich in alkalis(K,Na),silica and volatiles(water and CO_2)at the latest stage as well,similar to highly silicic and potassic melts.Thus,the occurrence of these composite and pyroxenite xenoliths provides an evidence for voluminous injection of recycled crustal melts into the lithosphere beneath the southeastern North China Craton at the Late Mesozoic,a reason for the rapid litho-spheric enrichment in both elemental and isotopic compositions.

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