The Mineral Chemistry Ch Aracteristics And Indication Significance Of Clinopyroxenes In Cenozonic Alkalic Basalt From Changle,Shandong Pronvice

The continental lithospheric mantle in the North China Craton(NCC)has been reactivated since the Mesozoic,and a large number of mantle-derived xenoliths enclosed by the alkali basalt magma which from asthenosphere are generally characterized by "melt-peridotite" reaction.A suit of mantle xenoliths entrained in the Cenozoic alkali basalts from the eastern block of the NCC are studied in detail in this paper.Field geological survey,study of petrography,geochemistry and mineralogy,petrology indicate that the micro-phenocryst minerals,which developed around olivine,orthopyroxene and clinopyroxene are crystallized by melt,and the discoverise of reaction rims between the xenoliths and the host basalt,the major and trace element contents of clinopyroxenes show inconsistent tendencies or decoupling phenomenon,all of these phenomena demonstrate that the mantle-derived xenoliths have experienced multi-stage mantle metasomatism and different degree partial melting.In order to understand the mantle process such as partial melting and metasomatism,and further explore the evolution of the lithospheric mantlein the NCC,three major types of clinopyroxenes are recognized based on petrography,mineralogy and mineral geochemical characteristics.The first type is the primary clinopyroxene which occurred in mantle-derived xenoliths,and the content of major and trace elements of the clinopyroxene show decoupling phenomenon of inconsistent tendencies,moreover,the light rare earth elements(LREE)and the heavy rare earth elements(HREE)characteristics of the clinopyroxenes show that they have experienced multi-stage mantle metasomatism and different degrees of partial melting(no more than 8%partial melting in the spinel phase),the metasomatic melts possibly include mantle-derived alkali K-enriched,Al-enriched silicate melts and the carbonate melts.The second type is cribriform clinopyroxene in mantle-derived xenoliths,which is identified as the co-product of partial melting and melt metasomatism.the effect of partial melting exhibits a gradually strengthening from the non-mesh part to the part far away from the mesh and to the part closely near the mesh,the part far away from the mesh record one stage metasomatism of K-enriched,Al-enriched melt before the basalt magmatism,however,the parts around the mesh are the strongest effected by the late stage basaltic melts.The third type is the reaction rims between the host basalt and the clinopyroxenes in mantle-derived xenoliths,and phenocryst of the clinopyroxenes in the host basalt matrix,their chemical compositions are similar,such as the characteristics of high field intensity elements(HFSE)are positive anomaly and Large ion lithophile element(LILE)are negative anomaly,that shows strongly influenced by the OIB-like host basalt.The characteristics of multiple-stage "melt-effect" and different degrees of partial melting of the different clinopyroxenes in the Changle Ceozoic alkaline basalt are the performance of the embodiment of lithospheric mantle heterogeneity in the study area.The ancient lithospheric mantle gradually disappeared due to the influence of various deep geological processes dominated by "melt-peridotite" reactions.The LREE of parent magma of the clinopyroxene phenocrysts in the basalts show a steep enrichment,coupled with fractionated HREE patterns.The clinopyroxene phenocrysts display negative Pb?K anomalies,acoupled with LREE and HFSE enrichment,the various evidences suggest that the westward subduction of the Ancient Pacific Plate are contribution to the source area of Cenozoic alkaline basalts.