Research On Detrital Zircons From The Devonian Clatic Sequence In The South Qinling Belt And The Late Neoproterozoic To Early Paleozoic Sedimentary Rocks At The Northern Margin Of The Yangtze Block And Its Geological Significancence

Qinling Orogenic Belt has always been the focus of researches in geosciences.The tectonic evolution of the Shangdan and Mian-Lue oceanic basins has restricted the understanding of the Qinling Orogenic Belt.Although the Shangdan ocean in the north side is considered to be existed in the Early Paleozoic,there are still two different views about its closing time,including the early time of the Early Paleozoic and the early time of the Late Paleozoic,respectively.Moreover,two different viewpoints were suggested to interpret the timing of the Mian-Lue ocean: Neoproterozoic and Late Paleozoic to Early Mesozoic,respectively.Devonian is a key period in the South Qinling Belt because of its close relationship with the closure time of the Shangdan Ocean.However,the provenance and tectonic environment of the Devonian clatic sequence in the South Qinling Belt still remain controversial.The Neoproterozoic to Early Paleozoic sedimentary strata on both sides of Mian-Lue suture were ignored in the previous studies.In this study,detrital zircon U-Pb dating of the Devonian to Carboniferous clastic sequences in the South Qinling belt and the Late Neoproterozoic to Early Paleozoic sedimentary rocks in the northern margin of the Yangtze block is used to discuss their source materials,tectonic environment and crustal growth,and then to constrain the evolution timing of the two Paleo-oceanic Basins in the Qinling Orogenic Belt.Devonian strata are widely distributed in the South Qinling Belt,and provide significant information for revealing the geological evolution of the South Qinling Belt.It is still unclear for source materials and tectonic setting for the Devonian sediments in the South Qinling Belt,and relevant discussion of crustal growth is also absent.Based on zircon U-Pb geochronology of Devonian to Carboniferous sandstones from the Shanyang basin and Fengtai basin in the South Qinling Belt,it is shown that the detrital zircon grains were dominantly formed at 500~400 Ma and 1000~700 Ma.In addition,a few Archean-Early Paleoproterozoic and Paleoproterozoic-Mesoproterozoic zircons are obtained from these samples.Detrital zircon U-Pb ages integrated with Hf isotope analysis show that the clastic materials were mainly derived from the South Qinling Belt,the North Qinling Belt and the southern North China Block,but lack of detritus from the northern margin of the Yangtze Block.The age and Hf isotopic data of detrital zircons from these samples indicate three significant crustal growth events in their sedimentary sources,which are corresponding to the Early Paleozoic magmatic event(500~400 Ma)in the North Qinling Belt,the Neoarchean-Paleoproterozoic(~2.5 Ga)and Neoarchean(~2.7 Ga)magmatic events in the southern North China block,respectively.Based on the previous studies and the distribution characteristics of cumulative curves,we consider that the South Qinling Belt was in a foreland basin environment during the Devonian to Carboniferous.Detrital zircons of Sinian samples from the northern margin of the Yangtze Block contain only a set of Neoproterozoic ages,which came from the Hannan-Micangshan pluton in the northern margin of the Yangtze Block.Combined with the previous studies on the Sinian sedimentary strata in the northern margin of the Yangtze Block,we suggest that the northern margin of the Yangtze Block was in a restricted platform environment during this period.The Cambrian-Silurian samples mainly contain Neoproterozoic zircons,with a small amount of Archaean and Paleoproterozoic zircons.These zircons are mainly derived from the interior of the Yangtze Block,with a small amount from the Jiangnan orogenic belt.The Hf isotopic data of these zircons indicate that two crustal growth events existed in the Yangtze Block at least and one crustal growth events happened on the periphery of the Yangtze Block,including Mesoarchean(~3.1 Ga),Neoarchaean-Paleoproterozoic(2.69~2.46 Ga)and Neoproterozoic(1000~700 Ma),respectively.In combination with the previous studies,we suggest that the northern margin of the Yangtze block was in a depositional environment of passive continental margin,and the Mian-Lue Ocean on the northern margin of the Yangtze Block should exist after the early Silurian.