Miocene volcanism in the Lhasa block, Tibet: spatial trends and geodynamic implications

Miocene ultrapotassic (UP), shoshonitic (P) and calc-alkaline (CA) volcanism is recognized widely over the Tibetan plateau. This volcanism occurred after the end of India's oceanic lithosphere subduction beneath the plateau. Ambiguities about the distribution and significance of this volcanism arise in part from a paucity of geochronologic and geochemical data. We have sampled two volcanic fields in the Lhasa block: (1) the eastern edge of the Zabuye Salt Lake (central part of the block), (2) the southwestern edge of the Yangbajin Graben. Major and trace elements display high K2O (6.06-6.54%) over a wide range Of SiO2 (75.03-55.30%), Na2O (1.32-3.30%) and LREE/HREE enrichment with a significant rare earth element fractionation (26 < (La/Yb)(n) < 42) as well as negative Nb, Ta, and Ti anomalies. These features indicate that our samples are chemically identical to the post-collision Miocene UP to P volcanic rocks already identified all over the Tibetan plateau and the southern plateau specifically but distinct from the CA rocks found in the Lhasa block. The Miocene volcanics found in the Lhasa block present greater chemical variation than the rest of the Tibetan plateau. 40Ar/39Ar ages on both sanidine and biotite gave ages ranging from 16.16 +/- 0.12 Ma to 16.01 +/- 0.12 Ma and 10.60 +/- 0.14 Ma to 10.88 +/- 0.17 Ma (2sigma, excluding systematic errors, relative to Fish Canyon sanidine at 28.02 Ma) for Zabuye Salt Lake and Yangying geothermal field volcanics respectively. These results are consistent with the range of previously published ages from this part of the Tibetan plateau. The presence of N-S-trending dikes and the location of the volcanic fields suggest that local E-W extension in the Lhasa block was coeval. We propose that the synchronous UP, P and CA volcanism. in the Lhasa block between 23 and 8 Ma is the result of progressively deepening northward subduction of Indian continental lithosphere producing asthenospheric upwelling. Convective thinning (delamination) of lithospheric mantle fueled by the hot asthenosphere induced partial melting of an enriched sub-continental mantle lithosphere as well as eclogitic lower crust. This mechanism could also explain the local E-W extension observed in the same period (18-10 Ma). We also propose that lateral migration of the deepening Indian continental lithosphere mantle slab could explain the observed younger volcanism in the eastern part of the Lhasa terrane. (C) 2004 Elsevier B.V. All rights reserved.