ACCRETION OF NEO-TETHYS OCEANIC SUBDUCTION IN EARLIEST CRETACEOUS ALONG THE YARLUNG ZANGBO SUTURE ZONE, SANGSANG AREA, SOUTHERN TIBET

The subduction-accretion complex of the Yarlung Zangbo Suture Zone (YZSZ) in south Tibet records the accretion process of Neo-Tethys oceanic subduction along the southern margin of the Lhasa terrane of Eurasian continent prior to India-Eurasian continental collision. We report field observations, petrographic analysis, detrital zircon U-Pb geochronology, and Hf isotope data on the sandstone matrix of a sedimentary-matrix mélange thrust sheet in Sangsang area in central southern Tibet. This mélange sheet is the northernmost, and presumably the oldest one within the YZSZ subduction-accretion complex. Petrographic point counts indicate that volcanic clasts comprise the dominant clast type in the sandstone matrix. U-Pb-Hf analysis of detrital zircons from the sandstone matrix reveals a unimodal age population of ~186 - 121 Ma characterized by high positive εHf(t) values of +6.67 to +14.58, consistent with derivation from the Gangdese Arc on the Lhasa terrane to the north. The mean ages of oldest Gangdese Arc zircons suggest that northward subduction of Neo-Tethys oceanic lithosphere initiated no later than 184 - 169 Ma. The maximum depositional age of the sedimentary-matrix mélange thrust sheet is bracketed between 146 and 134 Ma, as determined by averaging the youngest detrital zircon ages. The deposition of this mélange sheet occurred originally in a subduction trench basin within a few million years of early accretion driven by the Neo-Tethys oceanic subduction. Our results, along with integration of previous studies on accretionary mélange rocks along the YZSZ, indicate that the subduction-accretion complex in Sangsang area documented at least two stages of accretion processes, the first in the earliest Cretaceous and second in the Late Cretaceous, separated by a period of forearc extension along the ocean-continent convergent margin forming the Yarlung Zangbo Ophiolites. These two stages of accretion correspond to episodes of magmatism within the Gangdese Arc and upper-crustal shortening in the Lhasa terrane.