Ployphase Early Paleozoic metamorphism in the northern Qinling orogenic belt
Ployphase Early Paleozoic metamorphism in the northern Qinling orogenic belt
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作者：
Zhang Jian
北秦岭造山带的高品位变质岩，传统上被认为是秦岭群，主要由小型榴辉岩、高压麻粒岩和广泛分布的中-低P/T麻粒-角闪岩相变质岩组成。放射性资料表明，不同岩石中均有多期早古生代变质作用。前人对北秦岭造山带官坡地区榴辉岩及其伴生岩的U-Pb定年结果表明，该地区榴辉岩的年龄为约500 ma，为榴辉岩相变质时期。结合岩石学资料，对北秦岭造山带松树沟地区2个HP麻粒岩样品进行SHRIMP和LA-ICPMS U-Pb年代学分析，得出其成岩年龄分别为504±7Ma和506±3Ma，代表了高压麻粒岩变质的时间。说明高压麻粒岩中的高压麻粒岩相条件与榴辉岩中的榴辉岩相条件是同时发育的，但形成于不同的热环境。西夏地区角闪岩-二辉石麻粒岩锆石U-Pb定年揭示了早古生代两个年龄群：440±2Ma为麻粒岩变质时代，426±1Ma为角闪岩相变质时代。北桐柏地区石榴石-二辉石麻粒岩锆石U-Pb年龄为436±1Ma，反映了麻粒岩相变质作用的时间。这些资料表明，北秦岭造山带经历了早奥陶世的俯冲和地壳增厚，随后发生了早志留世广泛的巴罗维亚型变质事件。
High-grade metamorphic rocks in North Qinling orogen,traditionally regarded as the Qinling Group,consist of minor eclogites,high pressure granutlite and widely distributed medium-low P/T granulite-amphibolite facies metamorphic rocks.Radiometric data indicate that ployphase Early Paleozoic metamorphisms are recorded in different rocks.The previous U-Pb datings of eclogites and associated rocks in the Guanpo area of the North Qinling orogen gave an age of ca.500Ma,interpreted as the time of eclogite facies metamorphism.In combination with petrological data,SHRIMP and LA-ICPMS U-Pb geochronology on two HP granulite samples in the Songshugou area of the North Qinling orogen yield ages of 504±7Ma and 506±3Ma,respectively,representing the time of high pressure granulite metamorphism.This implies that HP granulite-facies conditions in high pressure granulite and eclogite facies conditions in eclogites developed at the same time but they formed in different thermal environments.U-Pb datings of zircons of amphibole-two-pyroxene granulite from the Xixia area reveal two Early Paleozoic age groups:440±2Ma represents the age of granulite metamorphism whereas the ca.426±1Ma reflects the time of amphibolite facies metamorphism.The U-Pb data of zircons of garnet-two-pyroxene granulite from the North Tongbai area yield an age 436±1Ma,reflecting the time of granulite facies metamorphism.These data suggest that the North Qinling orogen experienced an Early Ordovician subduction and crustal thickening,followed by an Early Silurian widespread Barrovian-type metamorphic event as observed in many collisional orogen.