Cenozoic Volcanism in Tibet: Evidence for a Transition from Oceanic to Continental Subduction

Geochronological (K---Ar or 40Ar/39Ar), major and trace ele-ment, Sr---Nd---Pb isotopic and mineral chemical data are presented for newly discovered Cenozoic volcanic rocks in the western Qiangtang and central Lhasa terranes of Tibet. Alkali basalts of 65---45Ma occur in the western Qiangtang terrane and represent primitive mantle melts as indicated by high mg-numbers [100Mg/(MgFe)](54---65),Cr (204---839 ppm) and Ni (94---218 ppm) contents, and relatively low ratios of 87Sr/86Sr (07046---07061), 206Pb/204Pb (1821---1889), 207Pb/204Pb (1549---1561) and 208Pb/204Pb (3842---3889), and high ratios of 143Nd/144Nd (05124---05127). In contrast, younger volcanic rocks in the western Qiangtang terrane (30Ma) and the central Lhasa terrane (23, 13 and 8Ma) are potassic to ultrapotassic and interpreted to have been derived from an enriched mantle source. They are char-