Evidence for timing of the initiation of India \-Asia collision from igneous rocks in Tibet

Resent studies on igneous rocks in Tibet provide new evidence for timing of the initiation of India\-Asia collision. It has been defined that the Neo\-Tethys started to open from middle Triassic (T2) and reached its widest width in J2\-K1 (177 \­¦120 Ma) by petrological and paleontology evidence from Indus\-Yalung Zangbo ophiolites, which marked the suture between south margin of Lhasa block and north margin of Indian block. Andesite\-dominated arc volcanic rocks and calc\-alkaline granitoids in the Gangdese to the north of the ophiolites zone, as indicators of subduction of Neo\-Tethys oceanic plate, formed in 155.7 \­¦65 Ma. Petrotectonic assemblages of muscovite\-bearing granite, leuco\-granite and high\- potassium calc\-alkaline granite aged from 55.7 Ma to less than 10 Ma are no doubt records of collision and post\-collision processes. Wide spreading post\-collisional high\-potassium volcanic rocks (high\-K calc\-alkaline and shoshonitic series) in Tibet erupted during 40 \­¦30 Ma, 25 \­¦10 Ma and less than 10 Ma. Therefore, India\-Asia collision took place during the period between 65 Ma and 55 Ma. More critical evidence, however, came from Linzizong volcanic formation, which widely spread in southern Gangdese magmatic belt. The Paleocene\-Eocene (aged 63.89 \­¦49.2 Ma) sub\-horizontal terrestrial volcanic strata unconfomably overlay on the late Cretaceous sedimentary strata (Shexing Formation) being strongly deformed. Linzizong volcanic formation mainly consists of high\-K2O andesite, trachyandesite, trachyte, rhyolite and thick acidic ignimbrite, characterized by high content of K2O and partly peraluminous, especially in the middle to upper parts of the column, showing obvious geochemical signature of collisional \­¦post\-collisional volcanic rocks. In combination with the stratigraphical and paleontological evidence in southern Tibet that documented dramatic change in sedimentary facies and microfuna content across the Cretaceous \-Tertiary (K/T) boundary (Wan et al., 2002), it is concluded that the collision between India and Lhasa Continental blocks was most likely initiated at ~ K/T boundary time (~ 65 Ma).