Neoproterozoic Metamorphosed Volcanic-Sedimentary Strata And Tectonic Evolution In The Southwestern Margin Of Tarim Craton

Episodic assembly and breakup of supercontinents,i.e.supercontinental cycle,controlled global tectonics since the Archean and has great influence on tsurface environment,life evolution and the distribution of mineral resources.In recent years,the Neoproterozoic tectonic evolution of China landmasses,especially South China and Tarim,have attracted extensive attention and become the key to the reconstruction of the Rodinia Supercontinent.The Tarim Craton,located in northwest China,has long been considered as an important part of the Rodinia Supercontinent.However,the Neoproterozoic tectonic evolution of the Tarim Craton and its relationship with the Rodinia Supercontinent are still controversial.This is mainly due to the poor geological environment and weak research level of the Tarim Craton,so that many basic geological problems remain to be resolved through in-depth and systematic research.In this paper,a systematic study is carried out on the poorly studied Neoproterozoic metamorphosed volcano-sedimentary strata of the Kalakash and Ailiankate groups in the southwestern of the Tarim Craton,including structural analysis,zircon U-Pb dating,in-situ Hf isotopic analysis and whole rock geochemical analysis.The Neoproterozoic tectonic evolution in the southwestern Tarim Craton and its implications for the reconstruction of Rodinia supercontinent are discussed The Kalakashi Group is mainly exposed in the south of the Pishan County,and the primary rock assemblage is orthogneiss,garnet-bearing mica schist,marble and minor amphibolite,which were metamorphosed to amphibolite facies and superimposed by multi-stage tectonic deformation.Previously,?1.0 Ga ⁴⁰Ar/³⁹Ar ages were reported from the Kalakashi Group and were interpreted as the key evidence for a Grenvillian orogenic event in the southwestern Tarim Craton,possibly related to its amalgamation with northern Australia.In this contribution,we report systematic zircon U-Pb dating for the Kalakashi Group from the Buqiong section where a?1.0 Ga ⁴⁰Ar/³⁹Ar age was reported.Five orthogneisses and an amphibolite yielded zircon U-Pb crystallization ages of 900–850 Ma and a thermal/fluid overprinting event at?420 Ma.Lu-Hf isotopic analyses of the dated zircon grains from two orthogneisses yielded?Hf(t)values from-4 to+7,indicating a mixture of mantle-and crust-derived materials.Geochemical data show that the orthogneisses have relatively high FeO^T/MgO and 10,000*Ga/Al ratios,elevated contents of high-field-strength elements(HFSE)and negative Ba,Sr and Eu anomalies,characteristic of A₁-type granite,indicating that the parental magma erupted in an intra-plate rifting setting.Our results indicate that the Kalakashi Group is a volcanic-sedimentary succession that deposited in the Neoproterozoic,rather than the Paleoproterozoic or Mesoproterozoic as previously thought.We found no evidence for a Grenvillian orogenic event from the Kalakashi Group in the Buqiong section.Instead,the age and geochemistry of the studied orthogneisses are remarkably similar to those of the Sailajiazitage Group,although the former was affected by higher metamorphism and intense hydrothermal modification.The Ailiankate Group mainly outcrops in the southern part of the Hotian County and the main rock assemblage is quartz mica schist and chlorite schist,with lower green-schist facies metamorphism and multi-stage tectonic deformation.A large number of detrital zircon U-Pb dating studies have been carried out on this group in previous studies,indicating a maximum depositional age of?810 Ma.In this paper,feldspar-bearing volcanics and interbedded meta-tuff have been discovered in the Ailiankate Group.Four meta-tuff samples yielded zircon U-Pb crystallization ages of830–810 Ma,consistent with the youngest detrital zircons age peak within the limit of analytical uncertainty.Geochemical data show that the basic rocks have relatively HFSE(Nb?Ta?Zr?Hf)contents,characteristic of continental flood basalts.This is comparable with those in the?890 Ma metamorphic basalts from Sailajiazitage Group,athough there is a 60-80Ma time interval between them.Based on the above results,combining previous research,this paper argues that typical Grenvillian orogeny did not occur in the southwestern Tarim Craton,which instead was likely in a long-term rifting environment in the early Neoproterozoic.The long-term rifting in the southern Tarim may be spatiatemporally coupled with the long-term subduction-accretionary orogenic process in the northern Tarim,implying that the Tarim Craton may be located in the periphery of the Rodinia Supercontinent.