DETRITAL-ZIRCON GEOCHRONOLOGY AND IN-SITU HAFNIUM (HF) ISOTOPIC ANALYSES OF THE LATE TRIASSIC TETHYS HIMALAYA SEQUENCE: IMPLICATIONS FOR PROVENANCE AND TECTONIC SETTING

Considerable controversy has existed regarding the relationship between the Triassic metasedimentary unit of the Tethys Himalayan sequence (THS) and the passive Indian continental margin. The focus of this study are the Tethyan sequence strata exposed in southern Tibet that yield U–Pb detrital zircon age probability spectra and εHf values that are in stark contrast with Tethyan sequence strata of known Indian affinity. The rocks of the Upper Triassic Songre and Nieru Formations, exposed b10 km south of the India–Asia suture, yield populations of U–Pb zircon ages in the range of ~450–220 Ma. Detrital zircon crystals of this age are not known to populate India affinity Tethyan sequence strata. The youngest cluster of ages, in the range of ~266–224 Ma, typically have juvenile εHf(T) values (+5.5–+13.5). Zircon ages and εHf(T) values in this range are consistent with igneous rocks of the Lhasa terrane on the northern margin of the Neo-Tethys ocean during Triassic time. And the relative frequency pattern of the detrital-zircon ages in this unit conforms with the pattern from the Lhasa terrane. Collectively these unexpected results highlight the current uncertainties over the nature Neo-Tethys ocean and tectonic setting of the southern Lhasa prior to the India–Asia collision. Combined with other provenance analysis, we hypothesize that the Upper Triassic age northern Tethyan Himalayan strata studied here represent an independent terrane from the widely studied southern Tethyan Himalayan strata, and these rocks where isolated from the southern Tethyan Himalayan strata during Late Triassic time. Furthermore, these units potentially were derived from the Lhasa block in the north, and were probably connected with the Lhasa terrane during that time. The major findings of this work and recent hypotheses that challenge the conventional interpretation of the Tethyan sequence strata (e.g. Aitchison et al., 2000; Pan and Ding, 2004; Ding et al., 2005; Liu et al., 2009) indicate that need to reevaluate the tectonic evolution of southern Tibet.