Cenozoic volcanism in Tibet: Evidence for a transition from oceanic to continental subduction

Geochronological (K-Ar or 40Ar/39Ar), major and trace element, 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 65-45 Ma occur in the western Qiangtang terrane and represent primitive mantle melts as indicated by high mg-numbers [100 x Mg/(Mg + Fe)] (54-65), Cr (204-839 ppm) and Ni (94-218 ppm) contents, and relatively low ratios of Sr-87/Sr-86 (0.7046-0.7061), Pb-206/Pb-204 (18.21-18.89), Pb-207/Pb-204 (15.49-15.61) and Pb-208/Pb-204 (38.42-38.89), and high ratios of Nd-143/Nd-144 (0.5124-0.5127). In contrast, younger volcanic rocks in the western Qiangtang terrane (similar to30 Ma) and the central Lhasa terrane (similar to23, similar to13 and similar to8 Ma) are potassic to ultrapotassic and interpreted to have been derived from an enriched mantle source. They are characterised by very high contents of incompatible trace elements, negative Ta, Nb and Ti anomalies, and radiogenic Pb isotopic compositions (Pb-206/Pb-204 = 18.43-19.10; Pb-207/Pb-204 = 15.64-15.83; Pb-208/Pb-204 = 39.14-39.67). Sr-87/Sr-86 (0.7088-0.7092) and Nd-143/Nd-144 (similar to0.5122) ratios of the western Qiangtang terrane potassic lavas are similar to those of 45-29 Ma potassic volcanic rocks in the north-central Qiangtang terrane, whereas Sr-87/Sr-86 (0.7167-0.7243) and Nd-143/Nd-144 (similar to0.5119) ratios of central Lhasa terrane lavas are similar to those of 25-16 Ma ultrapotassic volcanic rocks in the western Lhasa terrane. The 65-45 Ma alkali basalts in the western Qiangtang terrane, along with widespread calc-alkaline volcanic rocks of this age in the Lhasa terrane, may be related to roll-back of a previously shallow north-dipping slab of Tethyan oceanic lithosphere beneath Tibet. Subduction as opposed to convective thinning of continental lithosphere is favored to explain potassic volcanism in Tibet because of its occurrence in distinct, east-west-trending belts (45-29 Ma in the Qiangtang terrane; 25-17 Ma in the northern Lhasa terrane; 16-8 Ma in the southern Lhasa terrane) and temporal and spatial relationships with major thrust systems.