西准噶尔北部塔斯特岩体辉长岩锆石U-Pb定年及其地质意义

Zircon U-Pb dating of Tasite gabbro in the north of West Junggar Basin and its geological implications

西准噶尔北部萨吾尔地区晚古生代火山岩浆作用强烈,其形成构造环境一直是争议的焦点。塔斯特岩体作为区内最重要的复式岩体,主要由辉长岩、花岗闪长岩、二长花岗岩和钾长花岗岩组成,对其中的辉长岩进行LA-ICP-MS锆石U-Pb定年,206Pb/238U年龄加权平均值为345.3±2.3Ma(MSWD=1.8),说明塔斯特岩体形成于早石炭世中期。岩石地球化学特征显示,样品里特曼指数δ平均值为3.04,A/CNK(0.76~0.85)<1.1,属于准铝质钙碱性岩石,岩石富集大离子亲石元素,而高场强元素和重稀土元素相对亏损,无明显的Eu异常,与西准噶尔地区后碰撞花岗岩类地球化学特征相似。综合研究认为,该辉长岩形成于后碰撞构造环境,可能是后碰撞阶段挤压-伸展转变期的产物。塔斯特辉长岩年龄是目前西准噶尔地区最老的后碰撞岩浆岩锆石年龄,表明西准噶尔地区在早石炭世中期已处于后碰撞构造环境,为西准噶尔地区后碰撞构造-岩浆演化下限的厘定提供了新的证据。

The intrusions and volcanic rocks formed in Late Paleozoic are widespread in Sawuer area,the north of West Junggar,but the forming environment has been controversial.The Tasite pluton is an important multiple granitic intrusion which consists of gabbro,granodiorite,monzonite granite,K-feldspar granite and some other rocks.According to U-Pb age analysis,the formation age of the gabbro from the Tasite pluton is 345.3±2.3 Ma,suggesting the middle stage of Early Carboniferous.The geochemical studies indicate that the average ofδis 3.04,A/CNK ratios(0.76~0.85)is lower than 1.1,which belongs to papa-aluminum and calc-alkali rocks.This pluton is enriched in LILE(Rb,Sr,Ba,U,K),depleted in HFSE(Nb,Ta,T,Zr,Hf)and HREE(Yb,Y),with no Eu anomalies,which is similar to features of post-collisional magmatic rocks in West Junggar.It can be deduced that the gabbro might have formed in the extrusion to extension stage of the post-collisional period.The zircon U-Pb age of the Tasite pluton in this paper is the oldest age of post-collisional magmatic rocks in Junggar reported so far,thus providing a new evidence for confining the beginning time of post-collisional period in West Junggar.