Geochemical and Sr-Nd-Pb-Os isotopic compositions of Miocene ultrapotassic rocks in southern Tibet: Petrogenesis and implications for the regional tectonic history

Ultrapotassic lavas are widespread throughout southern Tibet and are generally thought to have formed from magmas generated by partial melting of an enriched mantle source that was metasomatized during earlier subduction events. Here, we report new geochemical and Sr-Nd-Pb-Os isotope data for Miocene ultrapotassic rocks within the Sailipu area of the western Lhasa terrane, southern Tibet. The Sailipu ultrapotassic rocks are enriched in the large ion lithophile elements relative to the high field strength elements, and have extremely radiogenic Sr (Sr-87/Sr-86((i)) = 0.714480-0.727323), Pb (Pb-206/Pb-204 = 18.414-18.787, Pb-207/Pb-204 = 15.693-15.749, Pb-208/Pb-204 = 39.439-39.765), and Os isotopic signatures (Os-187/Os-188((i)) = 0.1095-0.37454), and unradiogenic Nd (epsilon Nd-(t) = -11.5 to -15.2) isotopic compositions. These geochemical and isotopic characteristics, coupled with high K2O (>5 wt.%) and MgO (5.20-13.70 wt.%) concentrations, Mg# values of 68-76, high Rb/Sr (0.13-0.95) and low Ba/Rb (3.33-123) ratios, and the relatively low Os contents and radiogenic Os isotopic compositions of the Sailipu ultrapotassic rocks, do not support any significant crustal contamination. Instead, we consider these rocks to be analogous to the Cenozoic ultrapotassic rocks from Italy and the Balkans, suggesting that they were produced by interaction between melts derived from phlogopite-rich clinopyroxene veins and surrounding peridotitic mantle material. The Sailipu ultrapotassic rocks can be divided into type-1 and type-2 suites based on differences in major and trace element concentrations, and isotopic compositions. Type-1 ultrapotassic rocks are relatively enriched in the heavy rare earth elements, most likely as a result of interaction between melts derived from phlogopite-rich clinopyroxene veins and the surrounding spinel-bearing peridotite material, whereas the type-2 rocks are enriched in rare earth elements and have high (Gd/Yb)(N) values (8.3-12.2), both of which are indicative of interaction between melts derived from phlogopite-rich clinopyroxene veins and surrounding garnet-bearing peridotite material that also contained carbonates. Although both the existing convective removal and oblique subduction models can explain some of the geological observations associated with the formation of the Tibetan Plateau, we suggest that the former more accurately explains the spatial and temporal distributions of ultrapotassic rocks and other Cenozoic lavas in this area and the associated topographic uplift recorded in southern Tibet. (C) 2014 Elsevier B.V. All rights reserved.