大别山北淮阳带西段新元古代浅变质岩片的岩石组成及其大地构造意义

Neoproterozoic foreign low-grade metamorphic slices in the western Beihuaiyang zone of the Dabie orogen and their tectonic significance

大量的野外地质调查、室内岩石薄片观察和锆石U-Pb定年结果表明,大别山北淮阳带西段原"定远组"奥陶纪变质火山岩中新元古代浅变质岩片至少包含两类岩石,即750～720 Ma的变质花岗岩和635 Ma的变质辉长岩。其中,变质花岗岩的形成时代与北淮阳带东段庐镇关杂岩的形成时代一致。这两类新元古代浅变质岩石都经过了强烈的构造变形和绿帘角闪岩相变质作用,以构造透镜体或岩片形式产于奥陶纪的变质火山岩带(原"定远组")中,并且与华南陆块北缘湖北随州—枣阳一带发育的2期大规模新元古代中、晚期基性岩墙群和花岗岩的时代以及大别山超高压岩石的原岩时代一致。这些成果进一步证明,它们可能是印支期华南陆块发生俯冲的初始阶段陆壳内部最早被拆离、解耦的岩片,并在南、北陆块汇聚、碰撞及造山过程中被推覆到华北陆块南缘古生代浅变质岩系之上。

Recent field investigations, petrographic observations and zircon U-Pb dating results strongly suggest that the Neoproterozoic foreign low-grade metamorphic slices in the western Beihuaiyang zone of the Dabie orogen are at least composed of two types of rocks such as metagranite and meta-gabbro, which occur as tectonic blocks or slices within the Ordovician metavoleanics of the Dingyuan Formation. These Neoproterozoic low-grade rocks experienced epidote-amphibolite facies metamorphism and strong structural deformation. Their formation timing is 750 -720 Ma for the metagranite and 635 Ma for the metagabbro,respectively. These rocks occur within the metamorphosed Ordovician volcanic zone（originally named the Dingyuan Formation） and are in tectonic contact to each other, but they formed in different tectonic settings. The protolith ages for the Neoproterozoic low-grade metaigneous rocks are in good agreement not only with ages for two episodes of mid-and late-Neoproterozoic marie and felsic magmatism in the Suizhou and Zaoyang area,Hubei Province,but also agree with protolith ages of ultrahigh-pressure metaigneous rocks in the Dabie-Sulu orogenic belt. In view of their tectonic relationships to country rocks, it is most likely that these Neoproterozoic low-grade rocks are exotic and they might been detached and scraped off from the subducted continental crust of the SCB crust during the initial stage of subduction at the early Triassic, and subsequently exhumed and thrusted over Paleozoic metamorphosed rocks in the southern margin of the North China Block during continental collision.