New insights into the timing of the India–Asia collision from the Paleogene Quxia and Jialazi formations of the Xigaze forearc basin, South Tibet

Abstract

The Xigaze forearc basin provides information on subduction evolution and magmatic growth of the Gangdese arc as well as on the India–Asia continental collision. Recently obtained sedimentological, biostratigraphic, petrographic, geochemical and geochronological data on Cretaceous to Paleogene strata in the Cuojiangding area (Zhongba county, south Tibet) shed new light on the tectonic evolution of the southern margin of the Lhasa Block during closure of Neotethys and initial collision with India. The uppermost Cretaceous Padana and Qubeiya formations, deposited in deltaic to inner shelf environments, and representing the final filling of the Xigaze forearc basin, were unconformably overlain by the Quxia and Jialazi formations, deposited in fan-delta environments during the Paleocene/earliest Eocene. Petrographic data and U–Pb ages of detrital zircons document the progressive unroofing of the Gangdese arc, which remained the dominant source of detritus throughout the Late Cretaceous to Paleogene. Detrital Cr-spinels in the Quxia and Jialazi formations are geochemically similar to those in Cretaceous Xigaze forearc strata but different from those hosted in Yarlung Zangbo ophiolites, suggesting that the latter were not exposed to erosion in the considered time window. Sandstone petrography, Cr-spinel-geochemistry, U–Pb age spectra and Hf isotopic ratios of detrital zircons in the Quxia and Jialazi formations match those in Paleogene sediments deposited on the distal (Sangdanlin and Zheya formations) and proximal Indian margin (Enba and Zhaguo formations), suggesting that the Quxia and Jialazi formations documents syncollisional fan-deltas deposited on top of the nascent Himalayan orogenic belt. In this scenario, the onset of the India–Asia collision predates deposition of the Quxia and Jialazi formations and is thus constrained as younger than 66Ma and older than 58Ma.