北秦岭高压-超高压岩石的多期变质时代及其地质意义

Multi-matemorphic timings of HP-UHP rocks in the North Qinling and their geological implications

在岩相学观察和锆石CL图像研究的基础上,利用LA-ICP-MS原位微区定年分析方法,本文确定北秦岭清油河退变榴辉岩的峰期变质时代为490±6Ma,退变质时代为453±9Ma,原岩形成时代为655±9Ma;松树沟超高压长英质片麻岩的峰期变质时代为497±8Ma,两期退变质时代分别为448±4Ma和421±2Ma,原岩形成时代上限832±25Ma;寨根石榴石辉石岩的峰期变质时代为498±2Ma,中压麻粒岩相退变质时代为450±3Ma,角闪岩相退变质时代为426±1Ma,原岩形成时代为573±40Ma;西峡北榴闪岩的角闪岩相变质时代为423±3Ma,原岩形成时代为843±7Ma。新确定的这些岩石的峰期变质时代与前人已报导的区内高压-超高压岩石的峰期变质时代在误差范围内基本一致,结合区内高压-超高压岩石不仅分布在秦岭岩群北缘的官坡-双槐树一带,而且断续出露在秦岭岩群中部或偏南侧的清油河北-松树沟-寨根北甚至西峡北东西一线,进一步表明它们应是同一期构造地质事件的产物。北秦岭已发现的全部正变质的高压-超高压岩石均呈透镜体状分布在围岩片麻岩中,松树沟超高压长英质片麻岩的原岩为典型的陆壳沉积物,因此,这些高压-超高压岩石的形成可能都是陆壳俯冲-深俯冲作用的产物。结合岩相学观察、锆石CL图像和锆石U-Pb定年表明,这些高压-超高压岩石在～500Ma经历了峰期变质作用后,又分别在～450Ma和～420Ma遭受了中压麻粒岩相和或角闪岩相退变质作用的叠加,充分说明这些高压-超高压岩石经历了一个完整的由陆壳俯冲-深俯冲、之后连续两次抬升的构造演化过程。另外,本次研究新获得的这些岩石的原岩形成时代介于843±7Ma～573±40Ma之间,结合官坡榴辉岩的原岩形成时代为791～814Ma以及松树沟榴闪岩原岩时代为787±16Ma的研究,共同表明北秦岭高压-超高压岩石的原岩形成时代均为新元古代,因此,限定俯冲-深俯冲的陆壳物质应来自形成时代为新元古代的大陆地壳或地质体。结合区域地质背景和前人研究成果综合分析,本文初步认为,北秦岭高压-超高压变质岩带的形成是商丹洋向北俯冲拖曳南秦岭新元古代陆壳物质在～500Ma发生陆壳俯冲-深俯冲作用的产物,之后在～450Ma与～420Ma经历了两期抬升。

Based on petrography and zircon CL images, a variety of ages are obtained for the HP-UHP rocks from North Qinling by LA-ICP-MS U-Pb dating： the peak metamorphic age of 490 ± 6Ma, retrograde metamorphic age of 453 ± 9Ma, and protolith age of 655±9Ma for the retrograded eclogite from the Qingyouhe areas; the peak metamorphic age of 497 ± 8Ma, two stages of retrograde metamorphic ages of 448± 4Ma and 421 ± 2Ma, and the protolith age of 832 ± 25Ma for the UHP felsic granulite from the Songshugou areas; the peak metamorphic age of 498 ± 2Ma, the granulite facies metamorphic age of 450 ± 3Ma, the amphibolite facies metamorphic age of 426 ± 1Ma, and the protolith age of 573 ± 40Ma for the garnet pyroxenite from the Zhaigen area; and the amphibolite facies metamorphic age of 423 ± 3Ma and the protolith age of 843± 7Ma for the garnet amphibolite from the northern Xixia areas. The peak metamorphic ages of these HP-UHP rocks are consistent with previous data, combined with that they are not only located in Guanpo area of northern Qinling Group, but also occurred in northern Qingyouhe-Songshugou-northern Zhaigen-northern Xixia area of middle or southern Qinling Group, these HP-UHP rocks from the North Qinling should be generated by a simultaneous tectonic event. All the orthometamorphic HP-UHP rocks identified in the North Qinling occurs as lenses in the host gneisses, and the protolith of the UHP felsic gneiss in the Songshugou area was a typical continental sendimentary, indicating that these rocks might be the productions of the continental subduction / deep subduction. According to the petrography, zircon CL images and U-Pb dating, the HP-UHP rocks in this study have been experienced peak metamorphism at ~ 500Ma, subsequently overprinted medium-pressure granulite facies metamorphism at - 450Ma, and then amphibolite facies at -420Ma, respectively, suggesting that these rocks have been subjected to an integrated tectonic process of continental subduction/deep subduction with two stages of exhumations. Moreover, the protolith ages of 843 ±7Ma- 573 ±40Ma for the HP-UHP rocks in this study, as well as 791 - 814Ma for Guanpo eclogite and 787 ± 16Ma for Shongshugou garnet amphibolite obtained by previous studies, indicate that these HP-UHP rocks were formed in Neo-proterozoic period, and thus restrict the deep subducted continental substance came from a Neo-proterozoic terrane. Combined with the geological setting and previous studies, the HP-UHP rocks in the North Qinling should be formed by the subduction of the Neo-proterozoic continental substance from the South Qinling at - 500Ma which was dragged by the northward subduction of the Shangdan ocean, and then experienced two stages of exhumations at - 450Ma and -420Ma, respectively.