Detrital Zircon U-Pb Dating Of Metaclastic Rocks From Erlangping,Kuanping And Taowan Group In The Eastern Qinling Area And Its Tectonic Implication

As the main composition of Qinling orogenic belt,the tectonic evolution of North Qinling belt remains controversial.In this thesis,we report U-Pb ages and Hf isotopic data for detrital zircons from Erlangping complex,Kuanping Group and Taowan Group in the eastern Qinling area,as well as the geochemistry of the basic volcanic rock interlayers of Sichakou Formation from Kuanping Group.Integrated with cathodoluminescence(CL)imaging and trace element analysis,the formation age,provenance and tectonic setting of the metaclastic rocks from Erlangping,Kuanping and Taowan Group are well constrained,and provide insights into the Paleozoic tectonic evolution of North Qinling.This thesis carried out two meta-sandstones from the southern metaclastic slice of Erlangping complex,three mica schists with one basic volcanic rock interlayer of Sichakou Formation from Kuanping Group and two mica schists from Taowan Group.The concordant detrital zircon ages of Erlangping,Kuanping and Taowan Group are from 500Ma to 3894Ma,from 494Ma to 3837Ma and from 774Ma to 2950Ma respectively.The age spectrum of Erlangping and Kuanping Group display two dominant age clusters at 0.9?1.0Ga,2.4?2,7Ga and six sub-dominant age clusters at 0.5?0.6Ga,0.7-0.85Ga,1.1?1.2Ga,1.3?1.5Ga,1.6?1.8Ga and 3.1?3.5Ga.Whereas the age spectrum of Taowan Group show two dominant age clusters at 1.1?1.5Ga,1.6?2.1Ga and two sub-dominant age clusters at 0.75?0.9Ga and-2.49Ga.The results indicate:1.The youngest ages of detrital zircons from Erlangping and Kuanping Group are 494±6Ma and 515±5Ma.Combined with the 480Ma granodiorite intruded into the sedimentary units of the Erlangping complex and the early-middle Ordovician fossils identified from sedimentary rocks of the Kuanping Group,the sedimentary units of the Erlangping complex and Kuanping Group are constrained to have been deposited simultaneously during Paleozoic Cambrian-middle Ordovician.2.Compared with age spectrum,CL image,trace element and Lu-Hf isotope data of the detrital zircons from the adjacent regions,the metaclastic rocks in the Erlangping complex and Kuanping Group sourced from Early Neoproterozoic granite in the Qinling complex,Mesoproterozoic volcanic rocks in Xiong'er Group of southern margin of the North China Block,middle Neoproterozoic igneous rocks of South Qinling and northern margin of Yantze Block as well as gneisses from Douling complex of South Qinling.This result suggests a bidirectional source.Integrated with regional geological setting,the clastic rocks in Erlangping complex and Kuanping Group formed simultameously in the same back-arc basin induced by the northward subduction of the Shangdan ocean.3.Zircons extracted from the basic volcanic rock interlayer of Sichakou Formation from Kuanping Group give ages of 600Ma,945Ma,998Ma,1134Ma,1270Ma,1687Ma,2300Ma and 2420Ma,which is identical with the detrital zircon ages of the metaclastic rocks from Sichakou Formation,indicating that the volcanic and clastic rocks from Sichakou Formation of Kuanping Group might have formed simultaneously.In addition,the ages of zircons extracted from the basic volcanic rock interlayer of Sichakou Formation are similar with the zircon ages of basic volcanic rocks from Guangdongping Formation,which suggest that they might have formed simultaneously.Combined with the geochemistry result,the basic volcanic rocks from Sichakou and Guangdongping Formation might have formed in the Erlangping and Kuanping back-arc basin.4.The Ordovician fossils identified from the clastic rocks of Taowan Group indicate that the Taowan Group have deposited in Paleozoic Ordovician.The youngest detrital zircon ages(774Ma)from Taowan Group is about 300Ma younger than the formation age,which indicate that Taowan Group have formed in intra-plate setting.The sources of Taowan Group are mainly from the rocks in the North China Block as well as the southern margin of North China Block,whereas lacking of the contributions of the Paleozoic granite in the Qinling complex.Combined with the regional geological research,we proposed that Taowan Group might have deposited in the passive continental margin of the North China Block.5.The dominant detrital zircon age spectrum of Erlangping complex and Kuangping Group(0.9-1.OGa)clearly attributed to the Early Neoproterozoic granitoids,whereas lack of contributions from the Early Paleozoic igeous and HP/UHP rocks(0.4-0.5Ga)in the Qinling complex.This suggests that the Early Neoproterozoic granitoids were uplifted,eroded and the detritus accumulated in the Erlangping and Kuanping basin during Paleozoic Cambrian-middle Ordovician and did not undergo HP/UHP matemorphism(500?485Ma).Therefore,we conclude that only part of pre-early Paleozoic rocks of the Qinling complex(not a whole complex)undergo continental deep subduction.6.The subordinate 0.7-0.85Ga age population in the detrital zircons from the Kuanping Group and Erlangping complex is sourced from the igeous rocks of the South Qinling belt and northern margin of South China Block.This indicates that the Shangdan ocean,which separated the North Qinling belt and the South Qinling belt,might have closed during the deposition of the sedimentary rocks of the Erlangping and Kuanping Group.