索伦缝合带中的印支期伸展作用:中国内蒙古苏尼特左旗变质核杂岩

摘要

Field studies in the Sonid Zuoqi area of Inner Mongolia have led to a reinterpretation of the previously described Jiaoqier thrust fault as a south-dipping extensional detachment fault.The fault is the master detachment structure of an Indosinian metamorphic core complex that is superposed on the late Paleozoic contractional Solonker suture zone between the Altaids and the Manchurides.Elements of the core complex include Middle Paleozoic and Permian-Triassic plutonic suites in its footwall (Baodidao and Halatu,respectively),the mylonitic shear zone superposed on that footwall below the Jiaoqier detachment fault,the fault itself,and its upper plate of diverse and structurally complicated Paleozoic and Proterozoic units.Muscovite 40Ar/39Ar cooling ages of 224 and 208 Ma from mylonitized Halatu plutonic rocks (U-Pb,252 Ma),and the post-extensional deposition of J1-2 strata across the footwall establish the age of the metamorphic core complex as Indosinian,i.e.Late Triassic to earliest Jurassic.Extension along the ENE-striking Jiaoqier detachment was SSW (～215°) in the study area.This first discovery of Indosinian extension within the Solonker suture zone joins other recently described examples of Late Triassic-Early Jurassic extension (ca.220～190 Ma) along ductile detachment faults in such widely separated areas of China as the Dabieshan-Sulu (Shandong) suture zone,and the Qiangtang metamorphic belt and the eastern Altyn Tagh range of the Tibetan plateau.Clearly,the nature of regional Indosinian-age deformation in eastern Asia requires reexamination.%根据对内蒙古苏尼特左旗地区的野外研究,我们将以前描述的交其尔逆冲断层重新解释为一南倾的伸展拆离断层. 该断层为一印支期变质核杂的主拆离断层, 它叠加在缩短的阿尔泰和满洲里带间的晚古生代索伦缝合带上. 变质核杂岩的组成要素包括:下盘的古生代中期和二叠-三叠纪侵入体(分别是宝底道和哈拉图岩体),交其尔拆离断层之下、叠加在下盘岩体上的糜棱岩状剪切带,拆离断层本身和上盘成分多变、构造复杂的古生代和元古宙岩石.从U-Pb年龄为252 Ma的糜棱岩化哈拉图岩体中获得白云母,其 40Ar/ 39Ar冷却年龄为 224 Ma和208 Ma, 而后伸展沉积的下、中侏罗统沉积在下盘之上,表明变质核杂岩形成于印支期,即晚三叠世至侏罗纪最早期.研究区内,北东东走向的交其尔拆离断层的伸展作用方向大致为215°.这是索伦缝合带内首次发现的印支期伸展作用, 结合分隔遥远的中国各地区,如大别-苏鲁缝合带、西藏高原羌塘变质带和东阿尔金山区近来报道的其它一些晚三叠-早侏罗世(约220～190 Ma)沿韧性拆离断层的伸展作用实例,清楚表明东亚地区区域性印支期变形的性质有必要进行重新研究.