青藏高原东北缘宗务隆韧性剪切带^(40)Ar/^(39)Ar年代学及对印支期造山时限的制约

^(40)Ar/^(39)Ar laser dating of the Zongwulong ductile shear zone in northeastern Tibetan Plateau:Constraints on the time of Indosinian orogeny

青藏高原东北缘的宗务隆构造带位于柴达木地块与南祁连地块之间,位置极为重要,其中发育的韧性剪切带变形特征和形成时代对于理解该构造带的构造属性具有重要的制约意义。详细的野外构造解析,显微构造解析与年代学研究表明,宗务隆剪切带发育走向NWW-SEE向糜棱面理,其上发育NWW-SEE向缓倾的拉伸线理,指示该剪切带逆冲-走滑剪切的特征。宏观尺度上可见由于剪切作用形成的不对称褶皱、旋转碎斑、构造透镜体及褶劈理等变形形迹;显微镜下可观察到云母鱼、S-C组构、σ型残斑及石英动态重结晶、拔丝构造等变形现象,指示该韧性剪切变形的温度在300~400℃。对剪切带同构造变形的白云母和黑云母进行了^(40)Ar/^(39)Ar同位素年代学分析,2个样品的坪年龄分别为(245.8±1.7)Ma、(238.5±2.6)Ma,指示了该剪切变形发生在早—中三叠世期间。结合对区域地质、岩石学等资料的综合分析,该期韧性剪切变形年龄代表了宗务隆构造带印支期造山作用的时间,这期造山活动可能与宗务隆有限洋盆闭合后,南祁连地块与欧龙布鲁克地块的斜向碰撞有关。

The Zongwulong tectonic belt(ZTB)is located between the northern Qaidam tectonic belt and the south Qilian orogenic belt and contains Late Paleozoic and Early-Middle Triassic strata.Structural features and geochronology of the Zongwulong ductile shear zone have key implications for the tectonic property of the ZTB.This study integrated field structure,microscopic structure and ^(40)Ar/^(39)Ar laser probe analysis.The shear zone strikes~NEE-SWW and dips at a high angle,with a NWW-SEE trending and WE stretching lineation,indicating the shear zone as a thrust-slip shear ductile shear.The asymmetric folds,rotating porphyroclast,structural lens and crenulation cleavage can be seen in the field.Mica fish,S−C fabrics,σtype quartz porphy roblasts and quartz wire drawing structure can also be observed under the microscope,indicating that the strike-slip related ductile deformation and mylonitization occurred under low-grade greenschist facies conditions at temperatures of 300℃−400℃.The highly deformed mylonite schist yielded 40Ar/39Ar ages(245.8±1.7)Ma and(238.5±2.6)Ma for muscovite and biotite,respectively,indicating that the shear deformation occurred during the Early-Mid Triassic.Combined with a comprehensive analysis of regional geology and petrology,the authors hold that the age of ductile shear deformation represents the time of Indosinian orogeny in the ZTB.The orogenic activity was probably related to the oblique collision between the South Qilian block and the Oulongbuluke block after the closure of the ocean basin.