天山东段天湖东片麻状花岗岩的锆石SHRIMP U-Pb年龄和构造演化意义

SHRIMP U-Pb age for zircons of East Tianhu granitic gneiss and tectoic evolution significance from the eastern Tianshan Mountains

东天山天湖至星星峡地区广泛分布着片麻状花岗岩,顾连兴等(1990)和张遵忠等(2004)基于 Rb—Sr 等时线年龄为707.7±4.9Ma 的结果,提出天湖东岩体最有可能形成于澄江运动造山晚期的挤压一拉张构造转折环境。因此,天湖东岩体的形成时代直接关系到对东天山造山带构造演化历史的认识。我们的研究认为该岩体的地质产状和岩石特征均有别于东天山的元古宙巨眼球片麻状花岗岩,故对天湖东二长花岗岩中的锆石作了 SHRIMP U-Pb 年龄分析。研究中测定的18粒锆石中没有发现任何元古宙残留锆石和相关的年龄信息,由17个测点的^(206)Pb/^(238)U 加权平均年龄为466.5±9.8(2σ)Ma,可以确定天湖东岩体为早古生代花岗岩,而不是元古宙(或澄江期)花岗岩。基于元素和 Nd-Sr 同位素组成的研究结果,它们具有岛弧钙碱性岩浆岩的地球化学特征。该研究结果与东、西天山造山带其它地区获得的大量早古生代花岗岩的年龄具一致性。本研究结果对揭示早古生代古亚洲洋部分洋盆的闭合时间提供了重要的依据。

Abstract Granitic gneisses occur widely from Tianhu to Xingxingxia in the eastern Tianshan Mountains. The East Tianhu granitic body has been proposed as late Proterozoic granite （or called Chengjiang Period ） and be formed in tectonic conversion from compression to extension at late or post-orogenic stage by Gu et al. （1990） and Zhang et al. （2004）, which was based on a Rb-Sr isochron age of 707.7 ±4.9Ma. However, our study indicates that geologic occurrence and feature of petrologic structure of the East Tianhu granitic gneisses are different from those of the Proterozoic granitic gneisses with huge-augen in eastern section of Tianshan Mountains. Further studies based on high-precision SHRIMP U-Pb dating on 18 zircons from a monzonitic granitic gneiss of the East Tianhu body show a weighted mean ^206Pb/^238U ages of 466.5 ± 9.8Ma（2υ）for 17 zircons, and no Proterozoic remanent zircon and no Precambrian age were discovered. Our results denote that the East Tianhu granitic gneiss was formed in the early Paleozoic,other than in Proterozoic, thus the conception of （Chengjiang Period-granite） is questionable. Also, our zircon U-Pb ages are consistent with those of great many early Paleozoic granitoids in the eastern and the western Tianshan orogenic belts. Other evidences come from trace elements and Nd-Sr isotopic compositions, which show obvious characteristics of island arc calc-alkaline rocks. This study offers credible time information to investigate earlier closure time of some oceanic basin of Paleo-Asian Ocean.