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 Asia at approximately 50 to 40 Ma. Fission-track and 40Ar/39Ar analyses of 28 rocks from 10 Gangdese granitoid plutons along an similar to 250 km length of the batholith in the Lhasa region provide a detailed understanding of the age and the postcrystallization erosional and tectonic history of these rocks. These data suggest a range of ages for these plutons of 94 to 42 Ma, with the majority being of Tertiary age. The postcrystallization cooling histories of all of these plutons are characterized by marked discontinuities. We conclude that most of these discontinuities, and all of them after 40 Ma, reflect tectonic changes that produced brief pulses of rapid erosion which were distributed in both space and time. In addition to the initial cooling of hot magma against cold country rock, all of the rocks we studied showed evidence 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 all; these slow-cooling intervals lasted from 5 to 50 million years. Our data indicate that since the collision between India and Asia began, response to continued-convergence has been quite variable in even this 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.