Discovery of Vestige Sedimentary Archives of the India-Asia Collision in the Eastern Yarlung Zangbo Suture Zone

摘要

The lack of syncollisional sediments in the eastern Yarlung Zangbo suture zone (YZSZ) has impeded our understanding of the India-Asia collision, particularly as to whether the collision was synchronous or diachronous. In this paper, we present evidence for the vestige sedimentary archives of the initial India-Asia collision retained in the previously proposed "Cretaceous Langxian Melange" in the eastern YZSZ, southern Tibet. Field investigations of marbles and intercalated meta-siliciclastic rocks, together with analyses of petrography, cathodoluminescence image, geochronology, trace elements, and Nd isotopes show that (1) marbles occur as thin-bedded to massive individual layers consistently dipping to the south at 50-80 degrees, (2) marble and intercalated meta-siliciclastic rocks exhibit the same deformational (foliations S-0 and S-1) and metamorphic (green-schist facies) history with nearly same detrital-zircon age spectra and epsilon(Nd)(0) values, (3) protoliths of the meta-sedimentary rocks were sourced from both India and Asia and deposited during the early Eocene, and (4) protoliths of the marbles were probably precipitated in a marine environment. These lines of evidence suggest that the previously proposed melange is indeed not the Cretaceous melange, but the vestige of syncollisional sedimentary strata that was originally deposited in situ in a syncollisional foreland basin. The estimated collisional age of prior to similar to 56 Ma in the eastern YZSZ is almost consistent with the 60-56 Ma age estimated from the western-central YZSZ and Himalaya, indicating a (quasi-) synchronous initial India-Asia collision at similar to 60-56 Ma.