In-situ detrital zircon geochronology and Hf isotopic analyses from Upper Triassic Tethys sequence strata

Abstract

Marine sedimentary rocks of the Tethyan Himalayan sequence exposed south of the India–Asia suture and north of the high Himalayan mountain belt are widely attributed to India's passive margin along the southern margin of the Neo-Tethys ocean. 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 < 10 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 or with a hypothesized juvenile arc source from within the Neo-Tethys ocean. 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. We propose that the Upper Triassic age northern Tethyan Himalayan strata studied here represent an independent terrane from the widely studied southern Tethyan Himalayan strata. These rocks where isolated from the southern Tethyan Himalayan strata during Late Triassic time. This isolation was the result of a bathymetric barrier within the Neo-Tethys ocean such as a spreading center or an inter-Tethys arc system.