Mantle xenoliths in southern Tibet : geochemistry and constraints for the nature of the mantle.

The nature of the subcontinental mantle of Tibetan Plateau is not well-known,although lots of postcollisional volcanic rocks in Tibet have been inferred to be generated by low degrees of partial melting from the upper mantle.The key problem is that mantle-derived xenoliths and megacrystals hosted in potassic and uhrapotassic volcanic rocks are rarely found in the hinterland of Tibetan plateau.Here we report our major element data of minerals of the mantle xenoliths hosted in the Sailipu volcanic rocks (trachyandesite,with an age of～17Ma,have typical ultrapotassic features that similar to that found in southern Tibet). The xenoliths,ranging in size from 0.5cm to 1.5cm in diameter,can be divided into two groups.The first group is pyroxenites (Opx +Cpx) ,and the second is lherzolite (Ol+Opx+Cpx±Phli±Sp).The compositions of olivine (Mg~#=89～90,CaO=0.05%～0.12%,TiO_20.03%,NiO=0.29%～0.80%),Cpx(Mg~#=88～91,Al_2O_3=5.5%～7%),Opx (TiO_2=0.05%～0.15%, Al_2O_3=2%～5%) and spinel (Mg~#=58～76,Cr~#=10～44,Cr_2O_3=9%～35% ,MnO=0.09%～0.24%,FeO=11%～18% , Al_2O_3=29%～57%,MgO=16%～21%)show similar features to that in the Cenozoic mantle xenoliths in eastern China. The calculated temperatures of the xenoliths are 990～1140℃at the given pressures of 16～20kb.The geotherm suggested by this P-T conditions is similar to that in eastern China,Kenya and other rift-related upper mantle regimes,implying that the regional extension beneath southern Tibet in Miocene,although India were colliding with Asia during that time.Multi-stage metasomatic processes could be found in the samples,including water-bearing phlogopite,quartz in pyroxenites,and the rim-core composition variation in spinel. The secondary spinels along with neighboring phlogopite suggest that a potassic,host-rock like metasomatic agent that enriched in K,Si and H_2O,should have played an important role in modifying the upper mantle beneath southern Tibet.Further study on the Sailipu mantle xenoliths will be helpful in revealing the composition,regime,and processes of the upper mantle beneath southern Tibet,and in discussing the origin of the orogenic uhrapotassic rocks.