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

Abstract

Geochronological (K---Ar or Ar/Ar), 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---45Ma occur in the western Qiangtang terrane and represent primitive mantle melts as indicated by high mgnumbers [100 Mg/(Mg‡Fe)](54---65),Cr (204---839 ppm) and Ni (94---218 ppm) contents, and relatively low ratios of Sr/Sr (0 7046---0 7061), Pb/Pb (18 21---18 89), Pb/Pb (15 49---15 61) and Pb/Pb (38 42---38 89), and high ratios of Nd/Nd (0 5124---0 5127). 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 characterized by very high contents of incompatible trace elements, negative Ta, Nb and Ti anomalies, and radiogenic Pb isotopic compositions (Pb/Pb ˆ 18 43---19 10; Pb/Pb ˆ 15 64---15 83; Pb/Pb ˆ 39 14---39 67). Sr/Sr (0 7088---0 7092) and Nd/Nd ( 0 5122) ratios of the western Qiangtang terrane potassic lavas are similar to those of 45---29Ma potassic volcanic rocks in the north---central Qiangtang terrane, whereas Sr/Sr (0 7167---0 7243) and Nd/Nd ( 0 5119) ratios of central Lhasa terrane lavas are similar to those of 25---16Ma ultrapotassic volcanic rocks in the western Lhasa terrane. The 65---45Ma 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---29Ma in the Qiangtang terrane; 25---17Ma in the northern Lhasa terrane; 16---8Ma in the southern Lhasa terrane) and temporal and spatial relationships with major thrust systems.