新疆阿尔泰大喀拉苏花岗岩年代学、地球化学特征及其构造意义

Geochronology and Geochemical Characteristics of Dakalasu Granite in Altay of Xinjiang, China and Their Tectonic Significance

大喀拉苏似斑状黑云母二长花岗岩出露于新疆阿尔泰南缘,是研究阿尔泰二叠纪构造环境的理想对象。采用LA-Q-ICP-MS锆石U-Pb定年获得其加权平均年龄为(261.4±2.1)Ma,属中二叠世。岩石具有高Si(SiO2质量分数为69.03%~70.94%)、富Al(Al2O3为14.18%~14.71%)、富K(Na2O/K2O值为0.70~0.87)的特征,A/CNK值为1.02~1.04,属高钾钙碱性弱过铝质花岗岩,显示I-A过渡型特点。微量元素表现出Rb、Th、Pb、Nd、Sm的相对正异常;Ti、P、Sr和Ba的负异常;稀土元素显示轻稀土元素强富集、重稀土元素平缓及明显的负Eu异常(0.55~0.82)。岩石的εNd(t)值和两阶段模式年龄分别为2.66~3.01和0.79~0.80Ga。综合区域地质资料和岩体地球化学特征,认为大喀拉苏黑云母二长花岗岩形成于后造山伸展环境,岩浆来源于地幔和地壳物质的混合,并在岩浆源区经历了分离结晶作用。

Dakalasu porphyritic biotite monzogranite located in the southern Altay is an ideal object to study Permian regional tectonic environment. Zircons yield the weighted mean LA-Q-ICP-MS U-Pb age of (261.4±2.1)Ma for the biotite monzogranite, indicating that it is emplaced in Middle Permian. The rock is high in the contents of SiO 2 (69.03%-70.94%), Al 2O 3 ( 14.18%-14.71%) and K (Na 2O/K 2O is 0.70-0.87) with A/CNK more than 1 (1.02-1.04), which are defined as high-K calc-alkaline and weakly peraluminous granite characterized by I-A transitional type. Trace elements of the biotite monzogranites show relative positive anomalies for Rb, Th, Pb, Nd and Sm, and negative anomalies for Ti, P, Sr, Ba. REE shows strong enrichment of LREE, gentle HREE, and obvious negative Eu anomalies (0.55-0.82). The ε Nd ( t ) and two-stage model ages of the rock are 2.66-3.10 and 0.79-0.80 Ga, respectively. Combined with regional geological data and geochemistry characteristics presented here, it is suggested that Dakalasu biotite monzogranites form in post-orogenic extensional environment, and are derived from the mixture of crust and mantle materials. The magma experiences separation crystallization in the magma source.