SHRIMP zircon U-Pb age and Nd isotopic study on the Nyainqêntanglha Group in Tibet

Abstract

The Nyainqentanglha Group is traditionally viewed as the oldest metamorphic basement in the Lhasa block, but its formation age and tectonic setting remain debate. Zircons extracted from the metamorphic sequence of volcanics and intrusions of the Nyainqentanglha Group, 10 km west of Nam Co in northern Lhasa block, have been investigated by cathodoluminescence (CL), backscattered (BSE) and dated by ion microprobe (SHRIMP). We conclude that the U-Pb age of 787 e9 Ma of zircons from the trondhjemite imposes a constraint on maxi-mum protolith age, and minimum formation age of the Nyainqentanglha Group is constrained by U-Pb age of 748 e8 Ma of zircons from the granite. The formation age of the Nyainqentanglha Group is consistent with sedimentary age of Greater Himalayan rocks, showing that they devel-oped coevally in an arc-basin tectonic setting of Neoproterozoic active continental margin along the northern margin of the India shield. The inherited zircons from the tholetiite and granite give older 207 Pb/206 Pb ages from 947 to 1766 Ma. The positive a Nd(t) value indicates that the mafic rocks were derived from the depleted mantle, but contaminated by the older continental crustal material. Integrated Nd model age and U-Pb age data provide excellent evidence for the existence of Mesoproterozoic basement in the Lhasa block during Neoproterozoic time.