A COMPARATIVE STUDY OF THE TIBET/HIMALAYA AND CAUCASUS/IRAN OROGENIC BELTS: MAGMATIC PERSPECTIVES

The Tibet/Himalaya and Caucasus/Iran orogenic belts, two principal and active mountain ranges on Earth today, result from continental collisions of India and Arabia, respectively, with Eurasia. With the former representing a mature continental collision zone and the latter an intermediate to initial stage example, this study aims at a comprehension of the pre- to post-collisional igneous activities in the two orogenic belts in specific and the whole spectrum of magmatic and tectonic processes through which collisional orogeny evolves in general. In the Tibet/Himalaya orogenic belt, the Neotethyan subduction related arc magmatism started from the Jurassic and lasted until the Eocene. In southern Tibet, the volcanic records are marked with southward migration and intensification at ca. 50 Ma, suggesting rollback and breakoff of the subducted Neotethyan slab at the early stage of the India-Eurasia collision that should have begun by 55 Ma. Post-collisional magmatism, showing either ultrapotassic or adakitic geochemical features, started ca. 30 Ma as a result of removal of collision-thickened lithospheric root. In contrast to other active collision zones that involve broad, diffuse regions of crustal deformation, the Caucasus/Iran orogenic belts has an intriguingly sharp northern boundary defined by the linear trend from the Great Caucasus to Kopeh Dagh, a feature lacking in the mature India-Eurasia collision zone. Although the true continent-continent contact between Arabia and Eurasia seems to have initiated no later than the early Miocene, some large-scale ocean basins such as those in the Black and Caspian Seas are yet closed. However, our new data from Georgia, Armenia and Iran indicate a southeastward younging of termination of the Neotethyan subduction related calc-alkaline magmatism, from the Oligocene in Armenia to the middle Miocene in Esfahan and the late Miocene in Kerman areas along the Urumieh-Dokhtar magmatic arc. These data are consistent with the argument for an oblique Arabia-Eurasia collision and diachronous contact between the two continents along the Bitlis-Zagros suture zone.