摘要
重建古造山带大洋板块俯冲角度的时空演变规律是造山带演化研究的核心科学问题之一。青藏高原东北缘东昆仑造山带希望沟出露的中二叠世-早三叠世镁铁-超镁铁质堆晶岩和熔体相富Nb辉长-闪长岩,为揭示那时古特提斯洋的俯冲形态提供了机会。锆石U-Pb定年结果显示希望沟堆晶相镁铁-超镁铁质岩侵位于261.2±0.9Ma,熔体相辉长-闪长岩侵位于250.0±0.7Ma。前者SiO_(2)为47.70%~51.91%,具有高Mg#(64.3~83.1)、变化范围大的Al 2O 3(4.43%~18.64%)含量及显著Eu异常(0.89~1.41),显示橄榄石-辉石-长石堆晶序列,来源于正常钙碱性富水玄武质岩浆早期的堆晶作用;后者SiO_(2)为51.58%~54.77%,显示低Mg#(42.9~69.5)、高Al_(2)O_(3)(14.66%~19.66%)和轻重稀土明显分异,微弱亏损Nb-Ta-Ti的富Nb玄武岩特征,可能来自俯冲板片熔体交代的石榴子石-尖晶石地幔橄榄岩低程度部分熔融(1%~5%)。本文基于区域熔体相镁铁质岩地球化学数据,结合岩浆活动强度与地壳厚度演化,提出古特提斯洋俯冲的三阶段演化模型:278~252Ma玄武质岩石表现为正常熔融程度(5%~25%),对应的地壳厚度为38~42km,可能指示了正常角度的俯冲;252~248Ma富Nb玄武质岩石指示了板片熔体交代地幔楔的低程度部分熔融,地壳较薄(25km),可能与板片回转导致的高角度俯冲有关;248~240Ma玄武质岩石记录了陆源沉积物的贡献,地壳增厚至35~42km。结合上三叠统鄂拉山组/八宝山与中三叠统希里克特组的角度不整合事件,共同限定东昆仑-巴颜喀拉地体在约240Ma发生了碰撞。
Unravelling the temporal changes in subduction angles of oceanic slabs beneath ancient orogens is one of the critical scientific problems to reconstruct the evolution of the orogen.The Middle Permian to Early Triassic mafic-ultramafic cumulate rocks and Nb-enriched gabbro-diorite exposed in the Xiwanggou area of the East Kunlun Orogen(NE Tibetan Plateau)provide critical insights into the subduction geometry of the Paleo-Tethyan Ocean.Zircon U-Pb geochronology reveals that the mafic-ultramafic cumulate rocks were emplaced at 261.2±0.9Ma,while the melt-derived gabbro-diorite emplaced at 250.0±0.7Ma.The cumulate rocks(SiO_(2):47.70%~51.91%)exhibit high Mg#(64.3~83.1),variable Al_(2)O_(3)(4.43%~18.64%),and pronounced Eu anomalies(0.89~1.41),indicating early-stage accumulation of normal calc-alkaline hydrous basaltic magmas with an olivine-pyroxene-plagioclase cumulus sequence.The melt-derived gabbro-diorite(SiO_(2):51.58%~54.77%)displays low Mg#(42.9~69.5),high Al 2O 3(14.66%~19.66%),pronounced light-to-heavy fractionation of rare earth elements,and negative Nb-Ta-Ti depletions.These features reflect low-degree(1%~5%)partial melting of a garnet-spinel mantle metasomatized by subduction-derived melts.This study proposes a three-stage evolutionary model for the Paleo-Tethys Ocean subduction based on regional geochemical data of melt-derived mafic rocks,integrated with magmatic activity intensity and crustal thickness evolution.Basaltic rocks of 278~252Ma exhibit normal melting degrees(5%~25%),with corresponding crustal thickness of 38~42km,possibly indicating normal-angle subduction.Nb-enriched basaltic rocks of 252~248Ma indicate low-degree partial melting of the mantle wedge metasomatized by slab melts,combined with thin crust(25km),potentially associated with high-angle subduction caused by slab rollback.Basaltic rocks of 248~240Ma document contributions of terrigenous sediments,accompanied by crustal thickening to 35~42km.These signatures,in combination with the development of angular unconformity between the Upper Triassic Elashan/Babaoshan formations and the Middle Triassic Xilikete Formation,collectively constrain the occurrence of collision between the East Kunlun-Bayan Har terranes at~240Ma.
作者
肖世一
李世民
王青
樊家浩
黄蓉蓉
李天一
赵志雄
朱弟成
XIAO ShiYi;LI ShiMin;WANG Qing;FAN JiaHao;HUANG RongRong;LI TianYi;ZHAO ZhiXiong;ZHU DiCheng(State Key Laboratory of Geological Processes and Mineral Resources,School of Earth Science and Resources,China University of Geosciences(Beijing),Beijing 100083,China;State Key Laboratory of Geological Processes and Mineral Resources,Institute of Earth Sciences,China University of Geosciences(Beijing),Beijing 100083,China;Frontiers Science Center for Deep-time Digital Earth,China University of Geosciences(Beijing),Beijing 100083,China)
出处
《岩石学报》
北大核心
2025年第9期3144-3164,共21页
Acta Petrologica Sinica
基金
国家自然科学基金重点项目(42330307)
创新研究群体项目(42121002)
面上项目(42372079、42372087)联合资助
作者简介
第一作者:肖世一,男,2001年生,硕士生,矿物学、岩石学、矿床学专业,E-mail:2001230147@email.cugb.edu.cn;通讯作者:李世民,男,1992年生,副教授,主要从事增生造山带岩浆成因与地壳演化研究,E-mail:LiShimin@cugb.edu.cn。
