西藏南冈底斯叶巴火山弧的构造-地层属性及其演化  

Tectonostratigraphic properties and evolution of the Yeba volcanic arc in South Gangdese,Tibet

作　　者：唐宇 王根厚[1] 冯翼鹏 次旦 李典 范正哲 高曦 韦宇飞 胡继信 张培烈 TANG Yu;WANG Genhou;FENG Yipeng;CI Dan;LI Dian;FAN Zhengzhe;GAO Xi;WEI Yufei;HU Jixin;ZHANG Peilie(School of Earth Sciences and Resources,China University of Geosciences(Beijing),Beijing 100083,China;The Sixth Geological Brigade,Geological and Mineral Exploration and Development Institute of the Tibet Autonomous Region,Lhasa 851400,China;College of Earth Sciences,Chengdu University of Technology,Chengdu 610059,China;China Geological Library,Beijing 100083,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]西藏自治区地质矿产勘查开发局第六地质大队,西藏拉萨851400 [3]成都理工大学地球科学学院,四川成都610059 [4]中国地质图书馆,北京100083

出　　处：《地学前缘》2022年第1期285-302,共18页Earth Science Frontiers

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

摘　　要：南冈底斯岩浆岩带出露的一套早-中侏罗世火山-沉积建造经历了多期构造变形,致使这套火山-沉积层序发生了强烈的面理置换,形成了典型的构造-岩石地层。依据造山带地层划分方法将叶巴火山弧厘定为叶巴岩群,并根据内部岩性组合特征和构造变形特征将其进一步划分为邦堆岩组、叶巴岩组、甲玛岩组。运用构造解析原理划分了3期构造变形事件。第一期构造变形为脆-韧性剪切变形,剪切方式为纯剪占优的一般剪切变形,透入性面理S_(1)普遍置换层理S_(0)(S_(1)∥S_(0)),伴生倾伏向85°~100°陡倾的拉伸线理,运动学指示顶面朝西运动,存在左行和右行两个方向的剪切旋转碎斑共存的现象;EBSD实验结果显示变形的温度≤380℃,石英颗粒细粒化明显,重结晶方式为亚颗粒旋转重结晶;^(40)Ar-^(39)Ar年代学结果表明该期构造变形时代约为79 Ma,其可能代表新特提斯洋板片低角度(平板式)俯冲引起在弧后挤压背景下形成的挤出构造。第二期构造变形表现为S_(1)面理发生纵弯褶皱变形形成的轴面劈理S_(2),轴面产状倾向北或南,倾角40°~70°,枢纽向西或北西西倾伏;结合区域地质演化特征,认为其可能是在晚白垩世(79~68 Ma)南北向持续的挤压应力条件下,南冈底斯弧后盆地整体向上挤出,引发上地壳缩短、加厚进而导致褶皱作用的发生。第三期主要为浅层次膝折构造和近东西向正断层,最大主压应力方向为铅直向,最小主压应力方向(伸展方向)为近南北向;结合区域构造演化特征,认为该期变形可能代表渐新世末-中新世初期(23.74~21.1 Ma),印度岩石圈或青藏高原岩石圈或两者组合的拆沉作用引起冈底斯岩基隆升(主要动力学机制)和GCT活动并共同作用导致近南北向伸展滑覆事件发生。In the South Gangdese magmatic belt a set of exposed Early-Middle Jurassic volcanic-sedimentary formations underwent multi-stage structural deformation,which resulted in intense foliation displacing the volcanic-sedimentary sequence and forming the typical tectonic-rock strata.According to the stratigraphic division scheme for orogenic belt,the Early-Middle Jurassic volcanic arc is defined as Yeba rock group,which is further divided into the Bangdui,Yeba and Jiama rock formations based on its internal lithologic assemblages and structural deformation modes.The three-stage deformation is characterized as below.In D_(1) stage,the brittle-ductile shear deformation is pure shear dominant general shear deformation,and beddings(S_(0))are mostly replaced by penetrative foliations(S_(1))(S_(1)∥S_(0)).The deformation is characterized as pure westward shearing,from the top,with a steep stretching lineation to 85-100°according to kinematic observations.EBSD results showed the deformation temperature is no more than 380℃,and quartz particles are clearly fine-grained,formed by sub-particle rotational recrystallization.^(40)Ar-^(39)Ar dating results suggests the D_(1) tectonic deformation happened around 79 Ma,therefore it could represent an extrusion structure formed by the low angle(flatten)Neo-Tethys ocean plate subduction in the back-arc compression background.In D_(2) stage,longitudinal folding of S_(1) led to the axial-plane cleavage(S_(2))that dips to N or S,with an inclination of 40-70°,and hinges toward W or NWW.Combining with the regional geological evolutionary history,we believe the Southern Gangdse back-arc basin was extruded upward under continuous N-S compressional stress during the Late Cretaceous(7968 Ma),which resulted in upper crustal shortening and thickening,then led to folding.In D_(3) stage,deformation is mainly kink-bands and near E-W normal faults.The maximum principal compressive stress is in the vertical direction,while the minimum(extension direction)is in near N-S direction.Combining with the

关 键 词：南冈底斯带 叶巴火山弧 构造-地层 变形时代 构造演化 

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