Geochemical and Sr-Nd-Pb-O isotopic compositions of the post-collisional ultrapotassic magmatism in SW Tibet: Petrogenesis and implications for India intra-continental subduction beneath southern Tibet

Ultrapotassic lavas having distinct geochemical compositions (K2O/Na2O >2, K2O >3%, and MgO >3%) are common and widespread on the Tibet Plateau, where they are closely linked to N-S-trending normal faults. The Tibetan ultrapotassic rocks range in age from -8 to 24 Ma, slightly older than the spatially associated potassic rocks (10-22 Ma). These lavas consist mainly of trachyte, trachyandesite, basaltic trachyandesite, phonolite and tephriphonolite. They have high light rare earth element (LREE) and large ion lithophile element (LILE) concentrations, but are low in high field strength elements (HFSE). They are characterized by having extremely radiogenic Sr (Sr-87/Sr-86((i))=0.710719 to 0.736451) and Pb isotopes (Pb-206/Pb-204 = 18.449-19.345, Pb-207/Pb-204 = 15.717-15.803, Pb-208/Pb-204=39.443-40.168) with unradiogenic Nd isotopes (epsilon(Nd(O)) = -7.6 to -15) and old Nd model ages (T-DM = 1.3-2.1 Ga), similar in character to the Himalaya crystalline basement. Their isotopic character is interpreted to reflect subduction of the Indian plate beneath the Lhasa terrane, leading to a highly contaminated mantle source. Delamination of the subducted oceanic/continental materials may have played an essential role in the genesis of the ultrapotassic rocks in the Lhasa terrane. The available geological, geochemical and geophysical data favor a model in which the Indian plate was subducted under southern Tibet. (C) 2009 Elsevier B.V. All rights reserved.