Post-Collisional,Potassic Volcanism in the Saga Area,Western Tibet: Implications for the Nature of the Mantle Source and Geodynamic Setting

Post-collisional potassic and ultrapotassic volcanic rocks are widely distributed across the Tibetan Plateau,and they are considered to be indicators of evolving mantle dynamics.A suite of potassic basalts younger than 55 Ma from the Saga area of western Tibet has been reported.The geochemical characteristics of these rocks distinguish themselves from other potassic-ultrapotassic volcanic rocks in Tibet,such as positive Nb,Ta,and Ti anomalies and strong enrichment in large ion lithophile elements(LILE),suggesting that phlogopite,rutile and/or sphene might have originated from the mantle source.These basalts are also characterized by a very wide range of87Sr/86Sr ratios(0.709 043–0.711 915)and relatively high143Nd/144Nd ratios(0.512 426–0.512 470,εNd=-4.60 to-3.87).We propose a petrogenetic model for the Saga potassic rocks in which the lithospheric mantle source was infiltrated by a volatilerich(H2O,CO2)and low-degree silicate melt derived from the asthenosphere in the Middle to Late Proterozoic.After the initial Indo-Asian collision,Neo-Tethyan slab breakoff resulted in the partial melting of the previously metasomatized lithospheric mantle and the formation of the Saga potassic rocks.It is likely that the eruption of these volcanic rocks lasted at least 10 Ma.