东天山地区中天山地块内中元古代花岗岩的特征及地质意义

在天山造山带觉罗塔格山系东段的中天山地块内,新识别出一套侵位于长城系星星峡群变质岩中的片麻状花岗质岩体,岩石组合为片麻状闪长岩+片麻状花岗闪长岩+片麻状二长花岗岩+片麻状钾长花岗岩,锆石SHRIM PU-Pb测年表明这套侵入岩的结晶年龄为(1 453±15)~(1 458±40)Ma(207Pb/206Pb表面年龄),是中元古代晚期的产物。岩石学及地球化学特征显示为高钾钙碱性系列,具I型花岗岩组合特征,形成于板块汇聚的环境。这套花岗质侵入岩的厘定,对于可能存在于元古宙的天山洋的闭合事件,从岩浆作用上提供了重要信息,同时对中天山星星峡群的地层时代提供了一个很好的上限。另外,还发现了一颗形成年龄为(2 991±22)Ma的残留锆石,指示该区可能存在太古宙的古老基底。

Characteristics of Mesoproterozoic Granites and Their Geological Significances from Middle Tianshan Block, East Tianshan District,NW China

A suit of pluton is identified, consisting of the typical rock association: gneissic diorite, gneissic granodiorite, gneissic monzonite granite, gneissic K-feldspar granite, from metamorphic rocks of the Xingxingxia Group, Middle Tianshan Block, Tianshan Orogenic Belt, NW China. 16 zircons from Middle Tianshan gneissic granodiorites were dated using SHRIMP U-Pb method. A weighted average apparent age of (1 453±15)-(1 458±40) Ma(~(207)Pb/~(206)Pb) attained, is equal to the late Mesoproterozoic era, which represents the crystal age of the source rocks of gneissic granodiorite. The petrochemical and geochemical characteristics of the intrusive rocks shows a high-K calc-alkaline and I type granites, formed in convergent environment. It is a magmatism evidence for the existence and the closing event of the Proterozoic Tianshan Ocean according to the results of the SHRIMP U-Pb, which gives a good minimum ceiling of the age of Xingxingxia Group. Besides, the discovery of a residual zircon SHRIMP U-Pb age of (2 991±22) Ma indicates the basement time of the Middle Tianshan may cast back to the Archaean era.