柴北缘二郎洞地区L 构造岩变形特征及其地质意义

柴北缘二郎洞地区出露包含早古生代岩石组合的达肯大坂岩群，其构造变形期次和变形时代研究接近空白， 本文首次在其中发现了面理微弱， 而拉伸线理十分发育的L 构造岩，对其详细的构造解析对认识柴北缘构造演化具有重要意义。二郎洞达肯大坂岩群存在两期构造变形作用（D₁、D₂），D₁ 期变形表现为兼具逆冲分量的左旋走滑剪切，D₂ 期变形表现为兼具正断性质的右旋走滑剪切。D₁ 期变形产生LS 构造岩和L 构造岩。其中L 构造岩中矿物拉伸线理的倾伏向约为142°，与柴北缘造山带延伸方向一致，倾伏角一般在2°～54°。对L 构造岩以及侵入其中的未变形次安山岩进行LA-ICP-MS 锆石U-Pb定年，获得它们的结晶时间分别为457 Ma 和406 Ma，L 构造的变形时代可能介于457～406 Ma 之间。根据长石、石英等矿物的变形形式，估计L 构造岩的变形温度在380 ℃～420 ℃之间， 应形成于地下13～14 km 深处。在构造特征分析和岩相学研究的基础上，结合柴北缘早古生代岩浆活动时代构架，我们认为二郎洞地区L 和LS 构造岩的原岩可能是原特提斯洋向北俯冲增生过程中形成的早古生代弧花岗岩，由于柴达木-东昆仑板块和祁连陆块之间的斜向碰撞，导致局部地区出现挤压并沿造山带延伸方向走滑活动。在走滑剪切作用下使地壳流变层中的弧岩浆岩体向地表挤出，局部成分相对均匀的花岗质岩石，因具有较强的岩石力学性能而形成了L构造岩。

Tectonic deformation characteristics of the L tectonites in Erlangdong area at the northern margin of Qaidam Basin and its geological significance

Dakendaban Group from the Erlangdong area, northern margin of the Qaidam Basin（NQB）, contains Early Paleozoic rock assemblage. The deformation feature and timing of the Dakendaban Group are still overlooked. We report the occurrence of L-tectonites with strong elongate lineation, weakly developed or undeveloped foliation. Detailed structural analysis of the L-tectonites can provide important information on tectonic evolution of the NQB. Two stages of deformation were observed. The D₁ deformation was dominated by sinistral strike-slip shearing with thrust component, whereas the D₂ deformation occurred as a lateral strike-slip with normal faulting component. Moreover, the D₁ deformation resulted in the formations of L and LS tectonites. Mineral elongation lineation in the L tectonites plunged to the SE（around 142°）at 2°～54°, which was consistent with the extending direction of the North Qaidam tectonic belt. LA-ICP-MS zircon dating yields Early Paleozoic ages of 457 Ma and 406 Ma for the Ltectonites and undeformed andesitic sub-volcanic rocks that intruded into the L tectonites, respectively. Therefore, deformation timing of the L tectonites can be restricted to between 457 Ma and 406 Ma. The fabric analysis of quartz and feldspar suggested the deformation temperature of the L tectonites ranges from 380 ℃ to 420 ℃ with corresponding depths of 13～14 km. Detailed deformation analysis and petrographic studies in addition to the temporal framework of the Early Paleozoic magmatism, this paper concluded that the protoliths of the L and LS tectonites may be Early Paleozoic granites generated by the northward subduction of the Proto-Tethys Ocean. The sinistral strike-slip shearing with a extending direction similar to that of the NQB, locally occurred as a result of the oblique collision between Qaidam-East Kunlun Block and Qilian Block. Meanwhile, arc igneous rocks in middle crust level extruded to surface, and granites deformed to be isolated L tectonites, which was caused by a better mechanical than the other rocks due to their homogeneous compositions.