Zircon U-Pb ages and Hf isotopic composition of the Posen granite in northwestern Vietnam.

Abstract

This study presents zircon U-Pb ages and Hf isotopic composition of the Posen granite exposing in northwestern Vietnam and discusses its petrogenesis and tectonic significances of the study area in Neoproterozoic.Zircons from three samples of the Posen granite were analyzed and yield U-Pb ages of 723 Ma to 760Ma,suggesting that the granite formed in Neoproterozoic.Zircon grains from this granite exhibit variable Hf isotopic compositions,theire_(Hf)(t) values range from-16.1 to+3.4,single-stage Hf modal age values from 1186Ma to 1945Ma,implying complex material sources for the magma of this granite.Two-stage modal Hf age values of zircons are within analytical errors similar to two-stage Nd modal ages of whole-rocks,suggesting Paleoproterozoic crustal rocks as the major source material of the magma.A small part of zircon grains have positivee_(Hf)(t) values,probably indicating existence of the mixture between crust and mantle material during the formation of the magma.In genetically,Neoproterozoic magmatism in northwestern Vietnam is probably similar to the magmatic activities widely distributed in the Yangtze block,which are related to the break-up of the Rodinia suppercontinent and likely reveals an affinity between the Yangtze block and the northwestern Vietnam.Hence, it is proposed that the Song Ma fault zone can represent the prolongation of the Paleo-Tethyan suture of the Indochina and South China blocks.