南帕米尔喀拉苏沟韧性剪切带构造变形特征及年代学分析

Deformational Characteristics and Geochronological Constraints of Karasu Ductile Shear Zone in Southern Pamir

帕米尔西构造结经历了古生代—中生代多个地体的复杂拼合过程,又遭受了新生代印度-亚洲碰撞的改造,在整个青藏高原构造演化中扮演了重要的角色,但该地区中生代构造演化的研究仍然相对较少。通过研究南帕米尔一条北西—南东走向宽约1.5km的韧性剪切带中野外和显微变形特征,判定剪切带为左行剪切,剪切以走滑分量为主,兼具部分逆冲分量,韧性变形呈现中心强,边缘弱的特点。根据剪切带中花岗岩岩株的形态、变形、岩石地球化学特征以及与区域其它岩体的对比,判定其为同构造花岗岩。该同构造花岗岩中测得的两个锆石U-Pb同位素年龄为(233.2±1.8)Ma和(235.7±1.4)Ma,代表韧性剪切带形成于晚三叠世初期,指示南帕米尔地块内部在乳山洋闭合初期已经发生明显的构造变形。

The Pamir western syntaxis played an important role in the tectonic evolution of the Tibetan Plateau. It experienced the complex process of the Paleozoic–Mesozoic terrane collision and was transformed by the Cenozoic Indian-Asian collision. However, the study of Mesozoic tectonic evolution in this area remains very insufficient. In this paper, according to the study of the outcrop and micro-deformation characteristics in the NW–SE-trending Karasu ductile shear zone 1.5 km wide in southern Pamir, the authors hold that the shear zone is a sinistral shear, and the shear is mainly the strike-slip component with partial thrust component. The ductile deformation exhibits a gradual weakening trend from the center to the edges. The granite in the shear zone is a syn-tectonic granite according to the morphology, deformation, geochemical characteristics and a comparison with other plutons in this region. Two zircon U-Pb isotopic ages obtained in this granite are (233.2±1.8) Ma and (235.7±1.4) Ma, which indicate that Karasu ductile shear zone was formed in the earliest Late Triassic period and that that the South Pamir block underwent significant tectonic deformation at the early stage of the closure of the Rushan Ocean.