SPATIAL AND TEMPORAL DISTRIBUTION OF CENOZOIC MAGMATISM IN TIBET AND ITS TECTONIC IMPLICATIONS

A compilation of newly determined and previously published U-Pb and Ar isotopic age determinations (n=245) of volcanic and plutonic rocks from the Tibetan Plateau reveals distinct patterns in the distribution of Cenozoic magmatism which provide new insights and constraints on the tectonic evolution of the Himalayan-Tibetan orogen. These data, when plotted by age and location, show at least two patterns of interest. The first is that magmatism in southern Tibet between ~65 and ~50 Ma was largely confined to a narrow zone ~100 km north of the Indus-Yalu suture. Between ~50 and ~55 Ma, the zone migrated ~100 km south where it remained until ~30 Ma. Between ~30 and ~25 Ma, the zone migrated north ~125 km, where it stayed for at least the next 17 Ma. We propose that this migration of magmatism in southern Tibet represents variations in slab dip during Cenozoic subduction. The second pattern is that Cenozoic igneous rocks in central and northern Tibet occur in distinct east-west trending belts that are bounded by recognized Mesozoic suture zones. We suggest that this is strong evidence that Cenozoic magmatism in central and northern Tibet was related to continental subduction along reactivated suture zones. Additionally, Cenozoic igneous rocks with ages between 60 and 20 Ma yield gradational trends toward more radiogenic Sr and less radiogenic Nd with decreasing age. These gradational temporal variations may reflect progressive addition of continental material into mantle source regions during Cenozoic continental subduction.