Formation And Tectonic Evolution Of The Eastern Qinling Orogen,Central China

The Qinling orogenic belt (QOB), which extends east-west direction nearly2500km in the Central China, is the convergent and collisional belt of the North China craton (NCC) and Yangtze craton (YZC). The rock associations, origin and their relationships of the tectonic units are the key for understanding the formation and evolution of the Qinling orogenic belt and relationship with the North China Craton and Yangtze Craton. The QOB has been divided into the north Qinling belt (NQB) and South Qinling belt (SQB) based on geological features, or the East Qinling belt (EQB) and West Qinling belt (WQB) based on geographic distribution.This contribution presents a set of new petrologic, geochemical, zircon U-Pb dating and Hf isotopic data for a series of metamorphic rocks from the Taihua Group in the southern NCC, the Kuanping, Qinling and Danfeng Groups in the NQB and the Douling, Wuguan Groups in the SQB. Our results suggest that the Taihua Group in the Xiaoqinling area was formed in late Neoarchean (～2.5Ga) and experienced an important thermal event at～1.91Ga, which might has relationship with the global collision event during the assembly of Columbia Supercontinent.The Qinling Group in the NQB was formed in an arc tectonic setting related to subduction under the southern margin of the NCC during early Neoproterozoic (～843-971Ma)(Grenvillian). It had experienced a strong metamorphic event in early Paleozoic. The Kuanping Group in the northern NQB was deposited in the with-plate extension basin during late Neoproterozoic to early Paleozoic (520-570Ma). The clasts of sedimentary rocks of the Kuanping Group are mainly sourced from the Qinling Group and less from the basement rocks of the NCC. The Dangfeng Group in the southern NQB formed later than～827Ma (probably in the late Neoproterozoic to early Paleozoic) and the sediments of the Danfeng Group probably come mainly from the Qinling terrane itself. The NQB witnessed multiphase tectono-thermal events from Mesoproterozoic to early Paleozoic, including the Mesoproterozoic to early Paleozoic magmatic events and the early Paleozoic metamorphic event.The Douling Group in the SQB representing the basement of the SQB is composed of rocks formed in the arc-related environment in the middle Neoproterozoic time (ca.860-750Ma). A few old basement rocks were remained. A～3.0Ga granitic gneiss was recognized in the study, which underwent at least two periods of metamorphism at～2.5Ga and～0.9Ga, respectively. The Wuguan Group in the SQB formed in the middle-late Neoproterozoic and underwent the early Paleozoic metamorphism. Our data indicate that the SQB experienced the early Neoproterozoic (～950-865Ma) and early Paleozoic (-444Ma) metamorphism and strong middle Neoproterozoic (～746-730Ma) magmatism.The age spectra of zircons from the NQB trace major tectonothermal events in the NQB at～1257-1410Ma,-937Ma,-731Ma, and～443Ma, which are different from those in the southern NCC and northern YZC, suggesting that the NQB experienced a distinct evolution during the Mesoproterozoic to Neoproterozoic. The Hf isotopic compositions of zircons from the various rocks suggest that the NQB probably developed on the basement of the southern NCC. The rocks in the SQB show zircon age spectra and Hf-isotope compositions similar to those in the northern YZC, suggesting a close affinity between them. Therefore, the SQB is considered to have developed on the basement of the northern YZC.Integrating the new data in this study with those from previous works, we propose a new tectonic model for the formation and evolution of the QOB during late Mesoproterozoic to early Paleozoic:(1) Late Mesoproterozoic to early Neoproterozoic (Grenvillian):the Qinling Ocean between the NCC and YZC subducted northward under the southern margin of the NCC, and the Songshugou ophiolite was generated during this period. The subduction generated intense arc-related magmatism and metamorphism, strongly reworked the southern NCC and led to the formation of the NQB terrane (continental arc). These new formed rocks make the Qinling Group. Some ancient basement rocks were residual in the QOB. This tectono-thermal event mainly caused the metamorphism of the northern YZC without marked magmatism.(2) Early-middle Neoproterozoic (870-800Ma):The Qinling Ocean subducted bidirectionally under the NQB and the northern margin of the YZC, bringing about extensive arc-related magmatic activities on both sides and resulting in the formation of the SQB (e.g. the Douling Group) in the northern YZC. The Songshugou ophiolite was emplaced on the southern margin of the NQB in this period (848-833Ma), implying the collision between the NQB and the SQB.(3) Middle Neoproterozoic (-800-730Ma):The Qinling Ocean was consumed. The NQB and SQB were in the extensional stage of post-collision, and the post-collisional (800-720Ma) magmatisim occurred both in the NQB and northern YZC.(4) Middle-late Neoproterozoic (720-600Ma):Magmatic activities were much weak and the tectonic setting of sedimentary basin changed. The Danfeng Group in the southern NQB and the Wuguan Group in the SQB were deposited in this period. (5) Late Neoproterozoic-early Paleozoic (600-520Ma):The Shangdan ocean was opened along the Shangdan fault between the NQB and the SQB. The mafic magmatism was intensified because of the upwelling of the asthenosphere, and the alkaline basic rocks (e.g. dykes) with the characteristics of extension setting intruded into both the NQB and SQB. The extension basin far from the Shangdan ocean was formed in the northern margin of the NQB. The Kuanping Group was deposited in the basin.(6) Early Paleozoic (520-485Ma):With the extinction of the Shangdan ocean, the second subduction-collision occurred between the NQB and SQB, which resulted in the formation of the Danfeng ophiolite along the Shangdan suture. The high pressure metamorphism happened in the NQB, produing the490-516Ma eclogites and high-pressure granulites, which stand for the peak collision of the YZC and the NCC. The post-collision magmatism and metamorphism widely occuured in the whole QOB probably due to the collapse of the orogenic root during the early Paleozoic (460-420).