兴蒙造山带东南缘黄松群中压型变质作用及其构造意义:岩石学、矿物学和年代学证据

Medium-pressure metamorphism of the Huangsong Group in the southeastern margin of the Xing'an-Mongolian Orogenic Belt,NE China and its tectonic implication: Evidence from petrology,mineralogy and ^(40)Ar/^(39)Ar geochronology

本文报道了兴蒙造山带东南缘黄松群代表性岩石的岩相学、矿物化学成分和黑云母40Ar/39Ar定年结果,以确定它们的变质作用类型、变质温压条件、变质时代及其构造意义。岩相学研究表明黄松群主要由黑云斜长片麻岩、二云斜长片岩、斜长角闪片岩等代表性岩石以及不同类型的糜棱岩所组成。前者往往呈规模较小的面状分布,反映了主期区域动热变质作用,后者主要呈条带状分布,反映了后期低温动力变质作用的叠加与改造。此外,在与侵入体接触处产出的红柱石角岩反映了局部接触变质作用的存在。对黄松群代表性岩石的温压条件计算结果表明,该群主期变质作用的温压范围分别为525~597℃和5.8~7.5kb,地热梯度集中在21~27℃/km,揭示出黄松群中压型(绿帘角闪岩相-低角闪岩相)变质作用的存在。对含石榴二云斜长片岩中黑云母40Ar/39Ar定年结果为239.22±3.02Ma,而黑云母糜棱岩中黑云母的40Ar/39Ar定年结果为193.91±2.16Ma,结合黑云母的封闭温度和研究区已有的年代学资料,认为前者反映黄松群中压型变质作用的时代为晚二叠世-早三叠世,而后者反映黄松群经历的后期低温动力变质作用发生在早侏罗世。结合区域上同时代变质作用、岩浆作用以及沉积建造特征和区域构造演化历史,认为黄松群晚二叠世-早三叠世中压型变质作用与华北板块与西伯利亚板块碰撞拼合的地球动力学背景有关,该期变质作用标志着古亚洲洋东段的最终闭合已经完成。

This paper reports the petrography,mineral chemistry and biotite Ar-Ar dating data of the representative rocks from the Huangsong Group in the southeastern margin of the Xing＇an-Mongolian Orogenic Belt,with the aim of constraining its type,pressuretemperature conditions,the timing of the metamorphisms and its tectonic implications. Petrographic observations indicate that the Huangsong Group is mainly composed of biotite-plagioclase gneiss,two mica-plagioclase schist,plagioclase-amphibole schist,and different types of mylonites. The former generally occurs as small area,implying a regional dynamothermal metamorphism,whereas the latter occurs as belt,revealing the overprinting and modification of a low-temperature dynamic metamorphism. In addition,the andalusite hornstones occurring around the intrusions reflect the contact metamorphism. The estimated P-T results indicate that the pressures and temperatures of the regional dynamo thermal metamorphism range from 5. 8 to 7. 5kb and 525 to 597℃,respectively. Its geothermal gradient varies from 21 to 27℃ / km, implying a medium-pressure metamorphism（ epidote-amphibolite facies-low amphibolite facies）. The 40 Ar /39 Ar dating of biotite from garnet-bearing two mica plagioclase schist is 239. 22 ± 3. 02 Ma,whereas one of biotite from biotite-mylonite is 193. 91 ± 2. 16 Ma. Combined with the closure temperature of biotite and previously published geochronological data,we conclude that the former implies that the medium-pressure metamorphism in the Huangsong Group took place during Late Permian-Early Triassic,whereas the latter represents the timing of the late low-temperature dynamic metamorphism,i. e.,Early Jurassic. Together with the coeval metamorphism,magmatism,and sedimentary formations as well as regional tectonic evolution history,we consider that Late Permian-Early Triassic medium-pressure metamorphism in the Huangsong Group is related to the collision and amalgamation between the Siberia and North China cratons and implies that the final closure of the eastern segment of the PaleoAsian Ocean had finished.