中国阿尔泰造山带的变形‒变质历史研究:以富蕴县乌恰沟地区为例

Deformation and Metamorphic History of the Chinese Altai Orogenic Belt:A Case Study from the Wuqiagou Area in Fuyun County

通过对中国阿尔泰造山带南缘富蕴县乌恰沟地区出露的含石榴子石副片麻岩进行详细的野外地质观察及岩相学分析,确定了至少三期构造变形‒变质事件:①第一期变形变质作用(D1)可划分为早期的埋深变质作用与后期的伸展作用,埋深作用初期发育M1期变质矿物组合(黑云母+斜长石+石英),该阶段P-T条件为4.6×10^(2)MPa/~590℃,后达到P-T条件为5.5×10^(2)MPa/~650℃的变质峰期,发育M2期变质矿物组合(矽线石+石榴子石+黑云母+斜长石+石英),随后由于泥盆纪(400~380 Ma)地壳的水平伸展运动,发生广泛的减压熔融且伴随石榴子石的生长,并形成以混合岩化条带为基础的区域上的主性面理S_(1);②第二期变形变质(D_(2))为发生于晚泥盆世(~380 Ma)的褶皱构造,区域上形成以S_(1)面理为基础的NE-SW向近直立褶皱(F_(1)),该期变形导致后期位于同一地壳深度的岩石,变质程度在垂直于F_(1)方向上呈现出对称分布的特点,该阶段P-T条件为4.5×10^(2)MPa/~630℃;③第三期变形变质(D_(3))为发生于早二叠世(~280 Ma)的NE-SW向压扭性变形,表现为在被D_(2)变形改造过的S_(1)面理为基础上,形成由一系列NW-SE向小型褶皱F_(2)组成的大背形,此时伴有左旋剪切,并发育以尖晶石+堇青石+黑云母+斜长石矿物组合为特征的M_(4)期变质矿物组合,其P-T条件为~2.1×10^(2)MPa/~615℃。以上不同阶段温压条件定义了一个顺时针P-T轨迹,反映了岩石在埋藏后经历伸展减压,并伴随热量的加入。

Based on detailed geological observations and petrographic analyses,three phases of tectonic deformation and metamorphism have been determined for the garnet-bearing paragneisses exposed in the Wuqiagou area in the southern margin of the Chinese Altai orogen.The first phase of metamorphism and deformation D1 can be further divided into the early burial metamorphism and the late extension.The M_(1)metamorphic mineral assemblage of Bi+Pl+Q formed at the initial stage of the burial metamorphism,indicating a P-T conditions of about 4.6×10^(2)MPa/~590℃.The growth of M2 metamorphic mineral assemblage of Sil+Grt+Bi+Pl+Q in the late stage suggests a peak P-T conditions of 4.5×10^(2)MPa/~630℃.Then,the horizontal extension during 400 to 380 Ma led to extensive decompression melting of the lower crust,and formed the main foliation(S_(1))in the area based on the migmatitization strip.The second phase of deformation D_(2)is a folding deformation occurred at~380 Ma.It formed a series of NE-SW trending upright folds F_(1)based on the foliation S_(1).The metamorphic grade presents a symmetric distribution at the core of the folds F_(2)along the direction of F_(1)axial because of this deformation.This stage is the end of the growth of garnet porphyroblast,and the P-T conditions are 4.5×10^(2)MPa/~630℃.The third phase of deformation D_(3)is a NE-SW direction compression-shear deformation,which was activated at~280 Ma.This deformation is shown as a series of NE-SW trending folds and composes regional antiform F_(2)based on the foliation S_(1)which was reworked by D_(2)and followed by a sinistral strike-slip deformation.This deformation corresponds to the M_(4)metamorphism,which is represented by an assemblage of Spl+Crd+Bi+Pl+Q with P-T conditions of~2.1×10^(2)MPa/615℃.The above P-T estimates define a clockwise P-T path.This clockwise P-T trajectory reflects that the rocks underwent an extensional decompression after burial,and this process was accompanied by heat input.