Mineral Chemistry Of Peridotitic Xenoliths In Yangyuan Cenozoic Basalts: Significance For Lithospheric Mantle Evolution Beneath The Trans-North China Orogen

The North China Craton(NCC) was generally believed to have lost its Archean keels in Phanerozoic.Mantle-derived xenoliths and xenocrysts in Paleozoic Mengyin and Fuxian diamondiferous kimberlites document the presence of a cold,thick lithospheric mantle at least as late as the middle-Ordovician beneath the NCC.Whereas studies on mantle peridotitic xenoliths in the Cenozoic basalts of the NCC reveal the existence of a thin(less than 100km),hot and fertile lithospheric mantle beneath this area,indicating replacement of the refractory mantle by the fertile one during Mesozoic-Cenozoic.However,the timing,mechanism,tectonic and deep geodynamics for lithospheric thinning has been intensely debated.Peridotitic xenoliths entrained in the Cenozoic basalts from the North China Craton are ideal samples for investigating the nature and evolution of the lithospheric mantle.The compositions of mineral and peridotite, especially trace elements of clinopyroxene can well reflect characteristics of the subcontinental lithospheric mantle(SCLM).Based on the petrography of peridotitic xenoliths in the Yangyuan Cenozoic basalts,the mineral chemistry including major and trace-elements was analyzed in-situ.The nature and evolution of the Cenozoic SCLM beneath the Trans-North China orogen are also discussed.Some conclusions can be drawn:1.The Yangyuan peridotitic xenoliths beneath the Trans-North China orogen are major spinel-lherzolites with porphyroclastic texture and minor harzburgites with coarse-grained texture. The lherzolites xenoliths have mineral assemblage of olivine(0.8%-79.8%) +orthopyroxene (19.8%-44.8) +Clinopyroxene(5.0%-19.8%)±Spinel(<6%),whereas the harzburgites are characterized by occurrence of olivine(79.8%)+orthopyroxene(19.8%).Compared with Hebi high Mg~# and coarse-grained peridotites,which represent the refractory lithospheric keel of NCC and with Shanwang fertile and fine-grained peridotites,which represent the newly accreted lithospheric mantle,Yangyuan peridotitic xenoliths mainly display transitional porphyroclastic texture.2.The olivine Mg~# of Yangyuan peridotitic xenoliths range from 88.8-91.4,which are slightly lower than the published data,but all of them except minor harzburgites are similar to the Shanwang xenoliths,the samples from a fertile lithosphere.The compositions of orthopyroxene, clinopyroxene and spinel from Yangyuan are also similar to the corresponding minerals in Shanwang xenoliths.However,Mg~# and Cr~#-correlation of clinopyroxene and spinel also shows the coexistence of refractory and fertile mantle components beneath this area.3.The Yangyuan clinopyroxenes have wide Cr range,representing the products forming from the different partial melting of the primitive mantle:major less than 5%with minor 15%-20%. The Yangyuan SCLM also undergone main silicate melts with some relict information of carbonatitic metasomatism.4.The olivine Mg~# displays positive correlation with the forming age of peridotitic xenoliths,but has no correlation with their equilibration temperatures.The equilibration temperatures of fertile and refractory-transitional mantle are intersectant.Combined with all data and the published ones, we interpret that heterogeneous erosion,transformation and final replacement of asthenospheric materials in the cratonic lithosphere mantle take place beneath the Trans-North China orogen.