松辽盆地西缘边界断裂带中北段尼尔基L型构造岩构造年代学及其构造意义

Chronology of L-Type tectonite from Nierji area in the northern-middle segment of the western boundary fault of the Songliao Basin and its tectonic implications

松辽盆地西缘与大兴安岭东缘的边界断裂带,又称嫩江-八里罕断裂带,位于东北地区中部,中亚造山带东段。断裂带呈NNE走向,沿内蒙古自治区、黑龙江省与吉林省的交界,一直向南延伸至河北省境内与平场-桑园大断裂相接,全长1200km。该断裂带传统认识上被认为是一条NNE向的大型正断层或拆离断层。目前对于断裂带性质的研究主要集中在其北段的嫩江断裂,但侧重区域地球物理方面,通过布格重力异常、航磁异常、深反射剖面研究确定了断裂带的走向、展布位置,及伸展拆离断层或大型低角度正断层的性质;其南段断裂带（红山-八里罕断裂）的研究,主要与喀喇沁隆起的形成时间、性质、演化阶段等方面相联系;然而对断裂带中段的嫩江-八里罕断裂的研究少有报道。近期,我们报导了嫩江-白令海断裂带岭下及白城地区发现的具有左旋走滑性质的韧性剪切带,并同断裂带南段的红山-八里罕断裂带楼子店地区进行系统的对比。通过对岭下地区韧性剪切带详细的野外调查、构造要素测量、显微构造研究、同构造变形岩石的有限应变分析、石英EBSD组构分析以及白云母40Ar/39Ar年代学研究,我们认为岭下韧性剪切带与楼子店韧性剪切带的变形特征一致,证实了红山-八里罕断裂带至少可以向北延伸至中段岭下地区,其性质同为左行走滑韧性剪切带,并具有相同的早期走滑时限（~130Ma）。本次,我们进一步对嫩江-白令海断裂带中北段尼尔基地区韧性剪切带进行研究,通过对带内韧性变形岩石构造要素系统测量和统计、显微构造观察、有限应变测量等方面的研究,表明其岩石类型属L型构造岩,具左行剪切特征。综合分析发现除构造岩的类型（岭下与楼子店地区发育S-L型构造岩）,其余与岭下与楼子店地区韧性剪切带性质相同。另外,年代学表明L型花岗质岩石中发育的锆石,1）呈自形具有致密的韵律环带,且Th/U比值介于0.06至1.25之间,显示了岩浆成因特点;2）年龄大体分为两个区间258~403Ma（n=5）和207~174Ma（n=15,加权平均年龄为190.0±6.1Ma）;3）较老年龄258~403Ma的锆石普遍具有＂核-边＂结构,证实了该区域存在多期古生代岩浆事件。207~174Ma的锆石中,两粒锆石呈现~190Ma核部年龄,反映了区域内曾经历了早侏罗世的一次强烈的岩浆侵入事件,此后尼尔基地区韧性剪切带形成于中侏罗世（~170Ma）,而并非是前人认为的二叠纪。白云母激光40Ar/39Ar年代学显示了一个稳定的年龄坪为158.99±0.61Ma。通过显微构造的观测发现长石残斑与S-C组构指示了左行韧性剪切特征,其动态重结晶主要为膨凸式特点（BLG）。而石英颗粒边界相嵌现象明显,为亚晶粒旋转（SR）向颗粒边界迁移（GBM）重结晶过渡阶段。综合长石和石英的重结晶特征指示,其变质条件为高绿片岩相,变形温度在500℃左右,显然高于白云母的封闭温度（300~400℃）,故此我们认为尼尔基左行韧性剪切带的快速隆升时间应为中晚侏罗世（158.99±0.61Ma）。结合最新发表的东北地区黑龙江蓝片岩（前人称之＂黑龙江群＂）与郯庐断裂带北段两个分支（佳依断裂与敦密断裂）的年代学数据,以及西太平洋板块的俯冲速度和方向的相关证据,我们认为1）嫩江-八里罕断裂可能经历了三期变形演化阶段,包括中-晚侏罗世左行伸展走滑阶段（~160Ma）,早白垩世中期左行走滑/斜滑剪切阶段（~130Ma）及早白垩世晚期的伸展阶段;2）两期走滑剪切阶段与郯庐断裂带及其北段两个分支具有相似性,其中早期~160Ma的走滑剪切事件相对于佳木斯地块与松嫩地块的拼合时间（~170Ma）晚近10Myr;3）松辽盆地形成早期受到中-晚侏罗世的左行走滑断裂的控制,该左行走滑断裂的成因应归功于侏罗纪时西太平洋板块向欧亚大陆下快速的斜向俯冲有关。

As the western boundary fault of the Songliao Basin and the eastern margin of the Da Xing＇ an Mountains, Nenjiang- Balihan Fault Beh is located in the central part of northeastern China, the eastern segment of the Central Asian Orogenic Belt. It extends along the boundary between the Heilongjiang and Jilin provinces, and the Inner Mongolia Autonomous Region, with the NNE striking, and then goes into the Hebei Province to the south, connecting with the Pingchang-Sangyuan Fault, with a total length over 1200km. The Nenjiang-Balihan Fault Belt is considered traditionally to be a huge deep-seated fauh, and characterized by a normal fault or detachment fault. So far, some regional geophysical research on this fault zone is focused on its northern segment, the Nenjiang Fault, including the bouguer gravity anomaly, the aeromagnetic anomaly, and the deep reflection seismic profile, which revealed the regional distribution of the Nenjiang Fault System and indicated that Nenjiang Fault System should be a huge low-angle normal fault or detachment fault. In the southern segment of this fault belt, it is called the Hongshan-Balihan Fault, which has been studied as the eastern margin detachment fault of the Kalaqin （Harkin） Uplift. The study on the middle part of the Nenjiang-Balihan Fault Belt is rare. We newly found a S-L typed ductile shear zone at the Lingxia, Baicheng City, the middle-southern part of the Nenjiang-Balihan Fault belt, and compared this ductile shear zone with that at the Louzidian area, in the southern part of the Nenjiang-Balihan Fault belt. Based on the field investigation, measurements of structures in the field, micro-structural studies, finite strain measurements, a study on preferred crystal orientations of quartz determined by Electron Back Scatter Diffraction and muscovite 40Ar/39Ar chronology of the deformed rocks in the ductile shear zone, we suggested that the deformation features of the Lingxia and Louzidian ductile shear zones are similar, and that they represent one continuous fault, i. e. , the middle-southern segment of the Nenjiang-Balihan Fault Belt, which experienced a sinistral strike-slip ductile shearing in the Early Cretaceous （-130Ma）. In this study, we found a ductile shear zone at the Nierji area, in the northern-middle section of the Nenjiang-Balihan Fault Belt. By the measurement of structural elements, the micro-strncture studies and the finite strain measurement, the Nierji ductile shear zone shows similar NE striking and sinistralstrike-slipping shearing to that from the Lingxia and Louzidian ductile shear zones, but presents L-typed extensional shearing which is different from the S-L typed shearing in the Lingxia and Louzidian areas. The zircon U-Pb dating on the granitic L type tectonite at the Nierji area shows that： 1 ） all zircons have a high value of Th/U （0. 06 - 1.25 ） and exhibit euhedral crystal shapes and an oscillatory zoning in CL images, indicating their magmatic origin ; 2） these zircon ages are divided into two ranges, 258- 403Ma （ n = 5） and 207- 174Ma （n = 15, with a mean value of 190. 0 ±6. 1Ma） ; 3） the old age group of 258 -403Ma generally displays a core-rim texture in CL images, indicating multiple Paleozoic magmatic events having been in this area, and the two core ages of - 190Ma was found in the young age group of 207 - 174Ma, suggesting that this area underwent a stronger Early Jurassic magmatic intrusion event and the protolith of Nierji shearing zone formed in the early Middle Jurassic （ - 170Ma） , not the previous Permian. The laser 40Ar/39 Ar muscovite dating shows a relatively well plateau age of 158. 99± 0. 61Ma. The micro-structural observation, the rotated K-feldspar porphyroclasts and S-C fabrics demonstrate the sinistral shear sense of the ductile shear zones, and the recrystallization types of feldspar show bugling recrystallization （BLG） and the recrystallization types of quartz display a transition stage of subgrain rotation to grain boundary migration recrystallization （ SR-GBM）. Therefore, we suggest that the metamorphic grade of the shear zone in the Nierji shearing zone should have reached high greenschist facies conditions and deformation temperatures of around 500℃, which is obviously higher than the blocking temperature of muscovite （300 - 400℃ ）. Hence, the 40Ar/39Ar age of muscovite from the Nierji shearing zone should be a cooling age. We think that the sinistral strike-slipping event at the Nierji area occurred in the Middle to Late Jurassic and the Nierji shear zone underwent fast uplifting at 158. 99 -0. 61Ma. Combined with the newly published chronological data from the Heilongjiang blueschists （have been called as Helongjiang Group） in NE China and the famous Tancheng-Lujiang Fault Belt and its northern extension in the East China, and the study on the speed and direction of movement of the West Pacific Plate, we suggest that： 1 ） the Nenjiang-Balihan Fault Belt maybe undergo three main deformation stages, including the Middle to Late Jurassic sinistral extensional strike-slip shearing stage （-160Ma）, the middle Early Cretaceous sinistral strike-slip or oblique slip shearing stage （ - 130Ma） and the late Early Cretaceous extensional stage; 2） these two strike-slip shearing stages are similar to those from the Tancheng-Lujiang Fault Belt and its northern extension （Dunhua-Mishan and Jiamusi-Yitong Fault Belts）, and the formation time of the early shearing （ - 160Ma） is later 10Myr than the time of subduction （ - 170Ma） between the Jiamusi and Songliao Blocks; 3） the early formation stage of the Songliao Basin was controlled by the Middle to Late Jurassic sinistral strike-slip fault, which are related to the speed and direction of oblique subduction of the West Pacific plate under Eurasian continent and responsive collision during the Jurassic times.