秦岭晋宁期的两条蛇绿岩带及其对秦岭—大别构造演化的制约

Two Ophiolite Belts in the Qinling Orogen and Their Constraints on the Tectonic Evolution of the Qinling-Dabie Orogen

本文在总结前人对秦岭造山带中蛇绿岩研究成果的基础上,结合我们的工作实践,对秦岭南北两条蛇绿岩形成时代、构造背景以及它们对秦岭—大别山构造演化的制约提出以下认识:①北秦岭构造带不存在显生宙的蛇绿岩套。分布于北秦岭商丹构造带的松树沟蛇绿岩是秦岭造山带中保存最完整的蛇绿岩,形成时代不晚于1000Ma,就位于980 Ma。分布于二郎坪群、丹凤群中的超镁铁岩及镁铁质岩可能不属于典型的蛇绿岩,形成时代不晚于800 Ma,它们与分布同一构造带的含早古生代化石的沉积岩不是同一时代的产物;②分布于南秦岭勉略构造带的蛇绿岩是此区中—新元古代洋盆发育的记录,该洋盆关闭于晋宁期;③秦岭显生宙的放射虫硅质岩是产于陆壳基底上的硅质岩,它们不能作为洋壳发育的证据;④秦岭自中元古代至新元古代早期.属于板块构造体制演化阶段,秦岭显生宙的沉积盆地是发育在陆壳基底上的海盆,而不是以洋壳为基底的大洋盆地,陆内A型俯冲造山作用使沉积海盆在三叠纪末期全面消失。这些认识对解释分布于大别山—东秦岭造山带中的榴辉岩的形成及折返时间与机制提出疑问。

Previous data concerning ophiolites in the Qinling orogen have been collected. On the basis of a study of these data combined with the authors' work, the authors put forward the following views about the age and tectonic setting of the ophiolite belts in the South and North Qinling and their constraints on the tectonic evolution of the Qinling -Dabie orogen: (1) There exists no reliable Phanerozoic ophiolite in the North Qinling, and the Songshugou ophiolite in the Shangdan tectonic belt of the North Qinling, which is most complete in the Qinling orogen, was formed before 1000 Ma B. P. and emplaced in the Qinling Group at about 980 Ma B. P. The ultramafic-mafic rocks occurring in the Danfeng Group and Erlangping Group might not be typical ophiolites and were formed before 800 Ma. They and the early Palaeozoic fossil-bearing sedimentary rocks in the same tectonic belt were formed in different ages and tectonic environments. (2) The ophiolite in the Mianlue tectonic belt in the South Qinling is the record of the Middle-Late Protero-zoic oceanic basin, which closed in the Jinningian stage. (3) Phanerozoic radiolarian cherts in the Qinling was formed on the continental basement and cannot be considered as evidence of the development of oceanic crust. (4) A new evolutionary model of the Qinling -Dabie orogen is proposed in this paper. The period from the Middle Proterozoic to early Late Proterozoic in the Qinling was a stage of the evolution of the plate tectonic regime; the Phanerozoic sedimentary basin in the Qinling was a sea basin developing on the continental crustal basement, rather than an o-ceanic basin on the oceanic crustal basement; the Phanerozoic intracontinental A-type subduction orogeny of the Qinling resulted in the complete annihilation of the sedimentary sea basin at the end of the Triassic. All the above views cast doubt on the formation and exhumation mechanism and timing of eclogite in the Dabie -East Qinling orogenic belt.