Petrology,Geochemistry And Genetic Links Between The Eocene-Oligocene Potassic Rocks And Deep Xenoliths From Western Yunnan

The petrogenesis of potassic felsic suites in western Yunnan,has been linked with the strike-slip displacement along the Ailao Shan-Red River(ASRR)shear zone,and with the Cu deposit.A systematic study of zircon geochronology and Hf isotopes,and whole rock geochemical and Sr-Nd-Pb isotopes were carried out on the samples from these potassic felsic rocks along ASRR,to further constrain the origin and their tectonic implications.These felsic suites,including syenite,monzogranite porphyries,and granite porphyries,were emplaced between 35 Ma to 37 Ma,from our new zircon ages of 15 samples.The rocks have low contents of MgO,Cr,and Ni,and identical wholerock Sr-Nd-Pb isotopic compositions,which is comparable with that of the amphibolites and the adakitic rocks from western Yunnan,suggesting the source region should have been a thickened lower crust.Xenolith of garnet-clinopyroxene granulite entrained by the potassic felsic intrusions collected from Liuhe,was calculated to have formed at 13-14 kbar,the conditions of high-pressure granulite facies,implying the source depth of 40-42 km of the host potassic felsic rocks,which is equivalent to the results of thermodynamic modelling.Geochemistry and quantitative modelling,performed on the potassic felsic suites,suggest that mafic components were involved in the syenite and monzogranite porphyries,and the granite porphyries were produced by fractionation of plagioclase,biotite,and apatite.Together with their wide distribution along the entire Jinsha suture,they are unlikely genetically to the ASRR sinistral shearing.The potassic suites along the Jinsha suture,including that from north Tibet,western Yunnan,and Vietnam,have an identical tectonic setting in the period of Eocene-Oligocene.The removal of the lower continental lithospheric mantle triggered by Rayleigh-Taylor convective instability after the Indian-Asian collision,inducing asthenosphere upwelling,partial melting of the metasomatized continental lithospheric mantle,and subsequent underplating of mantle-derived mafic melts,and partial melting of the thickened lower crust.The eastward-migrating potassic magmatism along the Jinsha suture in the period of Eocene-Oligocene was caused by eastward-migrating thickening and removing of the continental lithospheric mantle along the Jinsha suture,which is a response of the dischronal subduction of the torn Indian continental slab at ca.52 Ma along the Indian-Asian collision zone.