Tectonic Environment Evolution And Continental Margin Transformations Of The Northern Tarim Craton In The Proterozoic Eon

Tarim Craton is one of the three major cratons in China.It has a prolonged crust evolution history since early in the Archean,forming a complicated Precambrian metamorphic basement.The exploration of this basement has contributed greatly to the understanding of crust formation process on the Earth surface,as well as the supercontinent evolution.The Precambrian basement of the Tarim Craton is mainly exposed around the edge of the Tarim Basin.The Xinditag Group,Paergangtag Group and Aksu Group are the three important Proterozoic lithostratigraphic units in the relatively well studied northern Tarim.We have investigated these three groups in the field,and collected a series of samples.These samples have been measured using methods include zircon U-Pb dating,Lu-Hf isotopic analysis,whole rock geochemical analysis and Sr-Nd isotopic analysis.With these obtained data we have aimed to improve the understanding of the Proterozoic evolution of the northern Tarim Craton.Detrital zircon U-Pb dating and Hf isotopic analysis show that the detrital sources of the metasediments in Xinditag Group,Paergangtag Group and Aksu Group are from within the Tarim Craton.The sources include 2.3-2.7Ga materials,which is correlated to the oldest rocks found in Tarim Craton,as well as the ca.2.0Ga metamorphic material,probably reworked along the south edge of the Tarim Craton.Besides that,the Aksu Group also has sources probably generated in a ca.1.85 Ga metamorphic event in the north Tarim Craton,which had significantly affected part of the Xinditag Group in the Kuruktag area.The Youngest source components of the Aksu Group is the products of multiple Neoproterozoic tectonic events from ca.780 Ma to 830 Ma in the north Tarim Craton,which include both newly generated and old reworked crust materials.Based on our detrital zircon U-Pb age data,we have further discussed the deposition ages of the protoliths of our studied metamorphic rocks.The metasediments in the Aksu Group are inferred to have deposited after ca.780 Ma and before the blueschist-facies metamorphism.The metasediments in the Xinditag Group had probably deposited between 1.85 Ga and 2.0Ga.On the other hand,The Neoproterozoic Paergangtag Group has the same detrital zircon U-Pb age spectrum as the Proterozoic Xinditag Group.They share the same youngest age peak at 2.0Ga,which indicates that the north Tarim Craton was in a tectonic stable state during the Mesoproterozoic with few magmatic activities.We have also measured whole rock geochemistry and Sr-Nd isotope of the mafic schist samples from the Aksu Group.The results indicate that the protoliths of these mafic schists are oceanic tholeiites.They have geochemical characteristics of enriched middle ocean basalts(EMORB)similar to the modern Iceland basalts,and have endured seawater alteration after their emplacement.The mafic schists of Aksu Group have a whole rock Sm-Nd isochron age of 890±23Ma,which represents the age of the oceanic crust that later subducted underneath the north edge of the Tarim Craton.This subduction had also caused the formation of a magmatic arc along the north Tarim Craton.Whole rock geochemistry indicates that the metasediments of the Aksu Group were deposited in the forearc or backarc basin of this magmatic arc.According to our new data,an updated scenario of the tectonic evolution in the north Tarim Craton in the Proterozoic has been proposed.After the complicated crust accrementition and core formation of the Tarim Craton at the Archean-Proterozoic trasition period,Tarim Craton had been active at ca.2.0Ga on the south edge and edge 1.85 Ga on the north edge.After ca.1.85 Ga,the north Tarim Craton had been stable until ca.840 Ma,when the Paergangtag Group had just finish its deposition.During the Neoproterozoic,the north Tarim Craton had at least two stages of magmatic activities,which peaked at 780 Ma and 830 Ma.These magmatic activities are related to oceanic crust subduction to the Tarim Craton.The protoliths of the mafic schists in the Aksu Group were part of this oceanic crust.It had later been subducted,and then exhumed and accreted to the north edge of the Tarim Craton.The subduction and related arc activity had last until ca.780 Ma.According to the available data,we have estimated that the blueschist-facies metamorphism of the subducted oceanic crust and its sedimentary cover took place at 700Ma-760 Ma.North Tarim Craton appeared to have restored its tectonic stability afterwards.Only minor magmatic activities at 600-650 Ma have been reported during the rest of the Neoproterozoic in the north Tarim Craton.In this paper,we have summarized that the tectonic environment of the north Tarim Craton had changed at least three times during the Proterozoic Eon.After the completion of craton core formation in the early Paleoproterozoic,the north Tarim Craton had been active continental margin during late Paleoproterozoic and middle Neoproterozoic.For the rest part of the Proterozoic,north Tarim Craton mostly had been stable.