Zircon U-Pb Ages,Trace Element And Hf Isotopic Compositions Of Paleoproterozoic Felsic Veins In The Xiaoqinling Area And Their Tectonic Significance

The North China Craton(NCC)is one of the most famous cratons in the world,which records a series of major Precambrian geological events,such as multi-stage crustal growth,the collage of micro-blocks,cratonization,the transition of the tectonic regime,and the deposit of rifts.A lot of models about the early Precambrian tectonic evolution of the NCC have been proposed,which generally believe that the cratonization occurred during the late Paleoproterozoic and subsequently experienced the transition of the tectonic regime.However,there are still great disputes about the formation process of the NCC and its transition of the tectonic regime:Some researchers propose that the collision between the Eastern and Western blocks occurred at?1.85Ga,but in recent years,some researchers also propose that it may occur before?1.95Ga.After the end of the late Paleoproterozoic tectonic event,the NCC started entering into rift or post-orogenic extensional tectonic evolution.Taking the late Paleoproterozoic felsic veins in the Xiaoqinling area on the southern NCC as the research object,this paper carries out the study of zircon U-Pb chronology,trace element and Hf isotope compositions based on field geological survey and petrographic observation,and further discusses the late Paleoproterozoic tectonic evolution in this area.According to the contact relationships,deformation characteristics and mineral compositions,plagioclase enriched felsic vein,k-feldspar enriched felsic vein and pegmatite are identified,which were formed in 1.92?1.82Ga,?1.82Ga and?1.77Ga,respectively.Seven groups of ages of?2.30Ga,?1.92Ga,?1.89Ga,?1.87Ga,?1.85Ga,?1.82Ga and?1.77Ga are revealed by zircon U-Pb dating.The age of?2.30Ga may represent the protolith of felsic vein,the age of 1.92?1.82Ga represents the time of anatexis and metamorphism,and the age of?1.77Ga can be seemed as the upper age limit of anatexis.The trace element and Hf isotope compositions of zircons formed during partial melting are studied,which show that?1.92Ga zircon has lower?HREE,Y contents and?_Hf(t)values compared with the coexisting?1.89Ga zircon in the same sample,indicating that there exists decomposition of garnet during the process of melt evolution.The?1.85Ga zircon shows a nearly flat HREE pattern,and its?HREE,Y contents and?_Hf(t)values are significantly lower than those of?1.87Ga and?1.82Ga zircon,indicating the formation,stable existence and subsequent decomposition of garnet during 1.87?1.82Ga.The?1.77Ga zircon generally develops magmatic rhythmic zonings,and its?_Hf(t)value is extremely uniform,indicating that the melt has turned to magmatic evolution.Based on the above information,it is speculated that two stages of partial melting events during 1.92?1.89Ga and 1.87?1.82Ga may have been recorded in the Xiaoqinling area during the late Paleoproterozoic,which slightly latter than two different high-grade metamorphic events during 1.96?1.92Ga and 1.87?82Ga in the Taihua Complex,suggesting the crustal anatexis probably occurred during the post-peak upper amphibolite to granulite facies stage.Combined with metamorphism and chronology data in the Taihua Complex,it is inferred that the Xiaoqinling area was involved into the subduction and collision between the Eastern and Western blocks before?1.97Ga,during 1.92?1.82Ga this area may experience complex process of exhumation and cooling.From?1.82Ga to?1.77Ga,it may enter into a post-orogenic extensional evolution lasting at least?50Ma.