阿尔泰冲乎尔盆地花岗质岩类的锆石SHRIMP U-Pb定年及其构造意义

Geochemical characteristic,SHRIMP zircon U-Pb dating and tectonic implication for granitoids in Chonghuer basin,Altai,Xinjiang.

通过对新疆阿尔泰冲乎尔地区北缘花岗质类岩石的矿物学、岩石学及电子探针分析研究,鉴定冲乎尔盆地北缘花岗岩属斜长花岗岩-英云闪长岩系列。岩石主微量元素分析显示,花岗岩和盆地火山沉积岩具有相似的微量及稀土元素特征,表现为:准铝到弱铝过饱和的钙碱性系列,镁含量较高,Ba、Y、Nb 等大离子亲石元素亏损,轻稀土富集,稀土元素分布曲线为Eu 弱负异常的右倾平坦型,岩石地球化学特征显示花岗岩的物质来源可能主要来自中上地壳。用单颗粒锆石 SHRIMP U—Pb定年方法,测得该岩体成岩年龄为413±3.8Ma,晚于盆地火山岩436±4Ma,结合花岗岩分布及基底构造特征,认为该期花岗岩受区域南北向构造控制,冲乎尔盆地可能为早古生代火山沉积盆地,盆地岩浆岩相似的元素地球化学特征暗示它们可能具有同源岩浆演化系列的性质。该期花岗岩可能为志留纪活动大陆边缘环境下,古亚洲洋板块向西伯利亚板块俯冲,热-构造作用引起中上地壳部分熔融,在阿尔泰山及其南缘形成 S 型花岗岩。

Abstract A voluminous of granitoids occur in the north margin of the Chonghuer basin. Petrochemical analysis and mineralogy study classify them as plagiogranites-tonalites. Their outcropping characteristics indicate they are probably controlled by N-S basement structure, which is different from the late Paleozoic major NW-NE structuring direction. The granites are rich in elements Rb, Ba, La, Ce, depleted in Ta, Nb, P, Ti, which are a suite of moderate-K calc-alkline rocks. These granitoids belong to metaluminous to peraluminous synorogenic crust melting series. Trace elements Y-Nb, Rb-Y ＋Nb, Nb-Ta diagrams display an environment of the active continental marginal volcanic arc. SHRIMP zircon U-Pb method yields a 413 ± 3.8 Ma coordinate age, which represents the crystallization age of magma. This age together with that of volcanic rock （436±4Ma） shows the Chonghuer basin was formed in the Silurium. These granitoids are consist predominantly of crustal source with some crust-mantle contamination during their emplacement. In the end of Ordovician-Silurian orogenic event, the regional danymo-heat flow metamorphism resulted from thermal-dome at depth causes middle-upper crust rocks to magmatization and granitization, which formed the S-type gneissoid granitoids in the studied area.