中国北天山博格达隆起带早二叠世垭口双峰式火山岩的成因及地质意义

Petrogenesis and Geological Implications of the Early Permian Yakou Bimodal Volcanic Rocks in the Bogda Belt,Chinese Northern Tianshan

为了厘定中国北天山二叠纪模糊不清的大地构造属性,本文对博格达隆起带垭口双峰式火山岩进行了系统的研究。垭口火山岩主要由玄武岩和流纹岩组成。垭口流纹岩的锆石SHRIMP U-Pb年龄为(295.6±2.3)Ma,明确指示该双峰式火山岩喷发于早二叠世。垭口流纹岩属于高硅I型流纹岩,为上地壳低压熔融的产物。外来锆石的加入很可能导致其Zr-Hf含量升高,从而显示A型流纹岩的假象及Nd-Hf同位素解耦的现象。垭口玄武岩与该区晚石炭世玄武岩均来自受俯冲沉积物交代的亏损地幔源区,但前者明显受到了地壳物质的混染。综合研究表明,该地区从晚石炭世到早二叠世突然有古老地壳物质的加入,说明296 Ma之前古天山洋在中国北天山北部就已经闭合,博格达地区开始接受周边古老地体物质的加入。

In order to determine the fuzzy tectonic environment of the Chinese Northern Tianshan in Permian,the Yakou bimodal volcanic rocks,which are mainly composed of basalt and rhyolite,in the Bogda belt have been systematically studied in this paper.The SHRIMP U-Pb age of(295.6±2.3)Ma for zircons of rhyolite indicates that the bimodal volcanics were erupted in Early Permian.The Yakou rhyolite belongs to the high-silicon I-type rhyolite that was a product of partial melting of the upper crust under relatively low pressure.The abundant captured or inherited zircons in the rhyolite may lead to the increase of its Zr and Hf contents,and then the artifact showing geochemical features of A-type rhyolite and decoupling phenomenon of its Nd-Hf isotopes.Both magmas of the Yakou basalt and the Bogda Late Carboniferous basalts could be derived from the similar depleted mantle source that was metasomatized by subduction materials,though the Yakou basalt was obviously contaminated by crustal materials during its magma ascending.In general,the sudden addition of ancient crustal materials into basaltic magmas from late Carboniferous to early Permian in the Bogda belt,based on all evidences,suggests that the paleo-Tianshan ocean was closed in the northern part of Chinese Northern Tianshan before 296 Ma and then the crustal materials from surrounding ancient terranes were added into the ascending basaltic magmas in the Bogda area.