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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。
This paper reports the zircon U-Pb age and Hf isotopic composition of the Posen granitic rocks exposed in the northwestern Vietnam and discusses the tectonic significance of the genesis of the rocks and the Neoproterozoic magmatism in the area. The zircon U-Pb ages of 723 Ma to 760 Ma were obtained from three granite samples, indicating that the granites were formed in the Neoproterozoic. The Hf isotopic compositions of zircons from this granite range from -16.1 to +3.4, with a single-stage Hf model age ranging from 1186 to 1945 Ma, suggesting that the Posen granite has a complex source region Source composition. Within the error range, the age of Hf isotopes in two stages of zircons mainly concentrated in the range of 2.0-2.1Ga, which is consistent with the two-stage Nd isotope model ages of 2.1-2.2Ga, indicating that the granitic bodies are mainly formed by the partial melting of Paleoproterozoic crustal material . Some zircon grains have a positive ε_ (Hf) (t) value, which may indicate that there is a crust-mantle mixing interaction during granite magmatic formation. Neoproterozoic magmatism in northwestern Vietnam may be related to the extensive genetic association between the magmatic activity along the Rodinia supercontinent and the Yangtze plate. It also reveals that the northwestern part of Vietnam may be related to the Yangtze plate. Therefore, It is inferred that the Majiang fault zone should represent the extension of the ancient Tethyan suture between the Indosinian and South China plates in northwestern Vietnam.