COLLISIONAL AND POST-COLLISIONAL MAGMATISM OF THE TIBETAN PLATEAU AND IMPLICATIONS FOR MANTLE DYNAMICS

It has been widely accepted that the Indian-Asian collision proceeded from ~65 Ma to ~40 Ma, followed by continued post-collisional convergence to the present. Magmatism is correspondingly well developed in the Tibetan Plateau. The syn-collisional magmatism took place mainly in the Gangdese and gave rise to the extensive Linzizong volcanism (~65-40 Ma) and the southern Gangdese batholiths (52.5-41 Ma, with a peak age of ~50 Ma), with fewer amounts of peraluminous muscovite-bearing granites (~65-54 Ma) and small mafic intrusions (53-42 Ma). In addition, the post-collisional magmatism produced potassic-ultrapotassic volcanics (~40Ma- present), adakitic rocks (~25-12 Ma), and peraluminous muscovite-bearing granites (between 24 and 18 Ma). Whereas the post-collisional adakitic rocks occur in the Gangdese and muscovite-bearing granites in the Gangdese and the Himalayas, potassic-ultrapotassic volcanic rocks are distributed over most of the Plateau showing a highly distinctive spatial & temporal pattern - migrated outward from the plateau interior with time.

The mode and nature of this magmatism in the Tibetan Plateau provide important clues for understanding mantle dynamics and crust-mantle interactions during and after the India collision. The continental collision retarded subduction of the Neo-Tethyan basaltic crust and caused the melting of hydrated oceanic crust in the amphibolite facies, producing a juvenile crust that is similar in composition to andesitic lower Linzizong volcanics. Subsequently, the continental crust was considerably thickened by both contractional shortening and by the influx of mantle material provided by the upwelling asthenosphere via slab breakoff. Furthermore, the distinctive spatial and temporal distribution of the potassic- ultrapotassic volcanism may have been controlled in part by lateral asthenospheric mantle flow induced by the continued convergence and the ensuing collision of the relatively thick Indian and Eurasian continental plates.