西藏加查地区特提斯喜马拉雅带晚三叠世地层重新厘定及构造变形研究  

Late-Triassic stratigraphic redefinition of and structural deformation in the Tethys Himalayan Belt in Gyaca area,Tibet

作　　者：唐宇 王根厚[1] 韩芳林 李典 梁晓[1] 冯翼鹏 张莉 王卓胜 韩宁 TANG Yu;WANG Genhou;HAN Fanglin;LI Dian;LIANG Xiao;FENG Yipeng;ZHANG Li;WANG Zhuosheng;HAN Ning(School of Earth Sciences and Resources,China University of Geosciences(Beijing),Beijing 100083,China;Shaanxi Mineral Resources and Geological Survey,Xi’an 710068,China;College of Earth Sciences,Chengdu University of Technology,Chengdu 610059,China;General Prospecting Institute of China National Administration of Coal Geology,Beijing 100039,China)

机构地区：[1]中国地质大学(北京)地球科学与资源学院,北京100083 [2]陕西省矿产地质调查中心,陕西西安710068 [3]成都理工大学地球科学学院,四川成都610059 [4]中国煤炭地质总局勘查研究总院,北京100039

出　　处：《地学前缘》2023年第2期35-56,共22页Earth Science Frontiers

基　　金：中国地质调查局项目“南羌塘中生代盆地区域地质专项调查(1212011221115)”;国地质调查局项目“藏西北铜多金属资源基地综合调查评价(D20190167)”。

摘　　要：前人将西藏南部特提斯喜马拉雅带加查地区出露的上三叠统中低级变质岩组合“朗杰学群”统认为是一套深海-半深海复理石建造。近来调查研究发现,该晚三叠世地层中实则存在一陡立剪切带(SSB),其南北物质组成、构造变形以及变质作用等存在明显差异。SSB以北具有明显“岩块”+“基质”的增生杂岩特征,“岩块”主要包括变砂岩、灰岩、大理岩、绿片岩、玄武质片岩、变质基性岩、片理化阳起石岩和石榴石云母片岩等;“基质”主要由变质砂板岩、绢云千枚岩和绢云石英千枚岩(千糜岩)等组成,原生沉积构造基本不可见,未见化石保留。然而,SSB以南的地层主要为粗粒—细粒长英质砂岩、泥岩和浅变质板岩组成,重荷模、沟槽模、鲍玛序列、韵律层等原生沉积构造以及遗迹化石十分发育。构造解析表明增生杂岩主要发育两期构造变形:第一期构造变形(D)表现为顺层剪切形成向北缓倾、具有强烈透入分异面理、长英质脉体新生面理S,运动方向指示顶部朝南;第二期构造变形(D)表现为近南北向挤压应力作用形成的向南中-高角度倾斜构造置换面理S,运动学指示为顶部朝北逆冲剪切,顺劈理发育同构造中基性岩墙。SSB南侧地层主要发育一期近南北向挤压作用形成的褶皱-冲断构造,运动学指示为顶部朝南的逆冲剪切。加查增生杂岩中朝北逆冲叠瓦的构造样式与朗杰学群褶冲带中朝南逆冲叠瓦的构造样式共同构成正花状构造,锆石U-Pb年代学揭示增生杂岩“基质”时代与南部地层时代均为拉丁期—诺利期(242~220 Ma),增生杂岩中“岩块”时代有晚三叠世、晚侏罗世(146 Ma)和早白垩世(144 Ma),表现为“岩块”具有多时代特征。同构造中基性岩墙的形成时代为约56 Ma,代表北侧第二期构造变形事件发生在始新世早期,间接限定第一期构造变形可能发生在古新世—始新�The Late-Triassic middle-low metamorphosed rock assemblage exposed in the Gyaca area of the northeastern Tethyan Himalaya, southern Tibet, including the Langjexue Group, had been regarded as a set of abyssal-bathyal flysch strata. This study reveals the Upper-Triassic strata are bounded by a steep shear zone(SSB) where obvious differences in composition, structural deformation and metamorphism between north and south are observed. The northern SSB is characterized by accretionary complex with a “matrix + blocks” structure, with little fossil preserved to reveal its primary sedimentary structure. The “blocks” are mainly composed of meta-sandstone, limestone, marble, green-schist, basaltic schist, metasdiabase, actinolite schist and garnet mica schist, while the “matrix” consists of sandy slate and sericite quartz phyllite(or phyllonite). Strata of the southern SSB chiefly consists of medium-fine grained felsic sandstones, mudstone and epimetamorphic slate. Sandstone layers display flute and groove casts, load casts, parallel lamination and graded bedding, while siltstone or mudstone beds preserve plant stem fossils and fodichnia. Structural analysis shows the Gyaca accretionary complex undergoes two stages of deformation(Dand D). Dis caused by bedding shearing in the top-to-south direction, which results in intensive penetrative foliation(S) and new-born felsic quartzose veins;and Dis characterized by middle-high dip angle with southward inclination-replaced foliation(S), consistent with north-south contraction deformation with top-to-south movement accompanied by syntectonic intermediate-basic veins. Meanwhile, under north-south compression stress, strata on the south side of SSB develop imbricated fold-thrust structure. Zircon U-Pb isotopic chronology reveals the “matrix” of the Gyaca accretionary complex and the southern Langjiexue strata are Latin to Norian(242-220 Ma), while the “blocks” include Late Triassic, Late Jurassic(146 Ma) and Early Cretaceous(144 Ma), showing multi-age characteri

关 键 词：西藏 特提斯喜马拉雅带 晚三叠世地层 构造变形 

分 类 号：P542[天文地球—构造地质学] P544.4[天文地球—地质学] P597