越南西北部Posen花岗岩锆石U-Pb年龄和Hf同位素组成特征

本文报道出露于越南西北部的Posen花岗岩岩体锆石U-Pb年龄和Hf同位素组成特征,讨论岩石成因和该地区新元古代岩浆作用的大地构造意义.分析3个花岗岩样品获得723Ma至760Ma的锆石U-Pb年龄,表明花岗岩形成于新元古代.该花岗岩的锆石Hf同位素组成有较大的变化范围,ε_(Hf)(t)值变化范围为-16.1至+3.4,单阶段Hf模式年龄为1186～1945Ma,暗示Posen花岗岩有着复杂的源区物源组成.在误差范围内,锆石两阶段Hf同位素模式年龄值主要集中在2.0～2.1Ga,与两阶段Nd同位素模式年龄值2.1～2.2Ga一致,说明花岗岩体主要由古元古代地壳物质部分熔融形成的.部分锆石颗粒具有正ε_(Hf)(t)值,可能指示花岗岩岩浆形成过程中存在壳-幔混合相互作用.在越南西北部发育新元古代岩浆作用可能与扬子板块广泛发育的、伴随Rodinia超大陆裂解过程的岩浆活动存在成因的联系,也揭示越南西北部地体可能与扬子板块具有亲缘关系,因此,可以推断马江断裂带应该代表印支板块和华南板块之间的古特提斯缝合带在越南西北部的延伸.

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

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 723Ma to 760Ma, suggesting that the granite formed in Neoproterozoic. Zircon grains from this granite exhibit variable Hf isotopic compositions, their ε_(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 positive ε_(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.