The distribution of the Linzizong sequences along the Indo-Asian collision belt

: Linzizong volcanic rocks are widely distributed in the Gangdese - Nyainqentanglha tectono-magmatic belt of Tibet, China. They are characterized by continental sedimentary facies with steady flat-lying strata while their underlying Shexing Formation is characterized by marine facies and complicated folding structures, reflecting the super-position of multiple tectonic activities. The contact between the lowermost stratigraphical unit of Linzizong volcanic series and the underlying Shexing formation is unconformable, which explanation is due to the magmatic response to the Indo-Asian collision. About 64.4 Ma radiometric age was gained for the lowest member in the Dianzhong Formation of the Linzizong sequences at Linzhou basin, likely corresponding to the inception of the collision between Indian and Asian continents in southern Tibet (Zhou et al.,2004). Then, we investigate the lowest volcanic rocks of Linzizong sequences at five section of Gangdese on the strike of Yarlung-Zangbo suture in order to constrain the time of the unconformity between Linzizong sequences and Shexing Formation. Sampling was based on detailed geologic mapping and stratigraphic sequence of Dianzhong Formations (the lowest part of Linzizong sequences), systematically from east to the west. Based on petrographic and geochemical analysis, representative samples were selected for isotope dating. Three Ar - Ar data (faster Neutron irradiated incremental step-heating 40Ar/39Ar), five zircon SHRIMP data (Sensitive High Resolution Ion Micro-probe) indicate that the lithologic characteristic and ages distributed at the bottom of Linzizong sequences are diversity along the unconformity. From east (90°37'E) to west (87°44'E), the age of andesites are from 58.66 ± 0.41 Ma to 74.5 ± 1.2 Ma, while in the further west (86°41'E), the explosure is acidic tuff with younger geochronological age ( from 59.64 - 50.59 Ma). It is postulated that that volcanic activities of Linzizong sequences may have occurred almost simultaneously in western, middle and eastern Gangdese belt due to the India-Asia continental collision and vary with lithology. Keywords: Linzizong volcanic rocks, 40Ar-39Ar geochronology, Indo-Asian collision.