古亚洲洋东段闭合时限:来自大兴安岭南段二叠系-三叠系界线沉积地层和碎屑锆石年代学的制约  

Closure time of the eastern Paleo-Asian Ocean: Constraint from Permian-Triassic strata and detrital zircon geochronology at southern Great Xing’an Mountains, Inner Mongolia (China)

作　　者：张渝金[1,2] 张超 马永非[1,2] 杨涛 刘艳[3] 杜继宇 赵英利[4] 张建坤 ZHANG YuJin;ZHANG Chao;MA YongFei;YANG Tao;LIU Yan;DU JiYu;ZHAO YingLi;ZHANG JianKun(Shenyang Center of Geological Survey,China Geological Survey,Shenyang 110034,China;Northeast Geological S&T Innovation Center of China Geological Survey,Shenyang 110034,China;College of Paleontology,Shenyang Normal University,Shenyang 110034,China;Research Center of Paleontology&Stratigraphy,JilinUniversity,Changchun 130026,China;East China University of Technology,Nanchang 330013,China)

机构地区：[1]中国地质调查局沈阳地质调查中心,沈阳110034 [2]东北地质科技创新中心,沈阳110034 [3]沈阳师范大学古生物学院,沈阳110034 [4]吉林大学古生物学与地层学研究中心,长春130026 [5]东华理工大学,南昌330013

出　　处：《岩石学报》2022年第9期2781-2810,共30页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(42172024、41702032、42002227);中国地质调查局项目(DD20221632、DD20190372、DD2019039-06、DD20190360、DD20160048-04);江西省自然科学基金项目(20202BAB204035)联合资助.

摘　　要：古亚洲洋东段的具体闭合时限长期以来存在争议,重要原因在于缺乏相应时期的典型沉积剖面研究。近期,笔者在大兴安岭南段阿鲁科尔沁旗地区新发现一套保存较好的二叠-三叠系沉积序列,其详细记录了古亚洲洋闭合过程。本文以此为研究对象,重点对上二叠统林西组和下三叠统老龙头组开展沉积学和碎屑锆石年代学研究。上二叠统林西组上部为一套湖泊相沉积,下三叠统老龙头组为一套河湖相沉积,底部为冲积扇辫状河砾岩层。两套地层界线附近沉积环境发生剧变,气候从温暖湿润转变为炎热干旱,确定研究区林西组沉积时代为晚二叠世长兴期,老龙头组沉积时代为早三叠世奥伦尼克期,两者为平行不整合接触关系,两者之间存在短暂沉积间断。林西组砂岩重矿物含量丰富,类型多样,原岩为中酸性岩浆岩、基性岩浆岩、少部分变质岩及沉积岩;老龙头组砂岩重矿物类型少,原岩为中酸性岩浆岩和少量变质岩。林西组2个碎屑岩样品最年轻峰值年龄分别为255±2Ma和255±1Ma,ε_(Hf)(t)=-22.84~+13.17,变化范围较大。老龙头组2个碎屑岩样品最年轻峰值年龄分别为248±1Ma和249±1Ma,ε_(Hf)(t)=7.95~11.28。结合重矿物、碎屑锆石和Hf同位素研究,确定林西组物源主要来自于兴蒙造山带,少量可能来自于华北板块,具有复杂物源、远距离搬运再沉积的特征。老龙头组物源主要来自于兴蒙造山带,具有周缘近距离搬运沉积的特征。通过区域对比上二叠统林西组与下三叠统老龙头组碎屑锆石携带年代学信息,推测古亚洲洋东段沿着西拉木伦河缝合带在晚二叠世发生汇聚碰撞作用,古亚洲洋东段闭合作用至少持续至早三叠世,老龙头组沉积期发生强烈汇聚造山作用,老龙头组是古亚洲洋闭合板块碰撞作用的产物。Closure time of the eastern Paleo-Asian Ocean(PAO)has long been debated,since one important reason is the absence of coeval sedimentation study.Recently,we found well preserved Permian-Triassic(P-T)strata,the Upper Permian Linxi and Lower Triassic Laolongtou formations,at Ar Horchin Banner,Inner Mongolia(China),in southern Great Xing'an Range that can provide detailed record for this process.In this study,the sedimentation and detrital geochronology study of this newly discovered strata are carried out to determine the precise closure time of the eastern PAO.The Upper Permian Linxi Formation is a set of lacustrine sequence,whereas the Lower Triassic Laolongtou Formation a set of fluvial and lacustrine sequence,and the bottom of the later is a conglomerate layer of braided alluvial fan,marking the boundary of the two formations.The sedimentary environment near the P-T boundary were distinct,with warm and humid in ending Permian and hot and arid in earliest Triassic.The accumulation of Linxi Formation occurred at Late Permian Changhsingian Period,while the Laolongtou Formation at Early Triassic Olenekian Period,and they are unconformable contacted with a short sedimentation hiatus between them.The sandstone samples of the Linxi Formation content various kinds of heavy minerals that mainly come from felsic to mafic magmatic rocks,and minor from metamorphic and sedimentary rocks.The heavy minerals from the Laolongtou Formation sandstones are rare,derived from medium-acid magmatic and minor metamorphic rocks.The mean ages of youngest group of the two clastic samples from Linxi Formation are 255±2Ma and 255±1Ma,respectively,withε_(Hf)(t)values ranging from-22.84 to+13.17.While the two clastic rock samples of Laolongtou Formation have youngest mean ages of 248±1Ma and 249±1Ma,respectively,withε_(Hf)(t)values of 7.95~11.28.The heavy minerals,detrital zircon ages and Hf isotopes suggested that the sedimentations of the Linxi Formation have complex provenance that derived mainly from the Xing an-Mongolia Orogenic Bel

关 键 词：兴蒙造山带 大兴安岭南段 上二叠统林西组 下三叠统老龙头组 碎屑锆石年代学 

分 类 号：P588.21[天文地球—岩石学] P597.3[天文地球—地质学]