THERMAL EVOLUTION OF THE GANGDESE BATHOLITH, SOUTHERN TIBET - A HISTORY OF EPISODIC UNROOFING

The Gangdese batholith, southern Tibet, was part of an Andean-type are at the southern margin of Asia prior to the collision of India and As ia at approximately 50 to 40 Ma. Fission-track and Ar-40/Ar-39 analyse s of 28 rocks from 10 Gangdese granitoid plutons along an similar to 2 50 km length of the batholith in the Lhasa region provide a detailed u nderstanding of the age and the postcrystallization erosional and tect onic history of these rocks. These data suggest a range of ages for th ese plutons of 94 to 42 Ma, with the majority being of Tertiary age. T he postcrystallization cooling histories of all of these plutons are c haracterized by marked discontinuities. We conclude that most of these discontinuities, and all of them after 40 Ma, reflect tectonic change s that produced brief pulses of rapid erosion which were distributed i n both space and time. In addition to the initial cooling of hot magma against cold country rock, all of the rocks we studied showed evidenc e for at least one subsequent episode of rapid cooling, dropping many tens of degrees in a few million years. Conversely, these plutons all experienced intervals during which they cooled very slowly or not at a ll; these slow-cooling intervals lasted from 5 to 50 million years. Ou r data indicate that since the collision between India and Asia began, response to continued-convergence has been quite variable in even thi s relatively small area. The data reported here are consistent with a recently proposed model of Oligo-Miocene crustal shortening along the Gangdese Thrust system in this area.