The Chronotectonic Framework And Main Geological Events Of Early Precambrian In The Agetashtage Area, Southeastern Margin Of Tarim Basin

In Agetashtage of southeastern margin of Tarim basin, there are relative better-preservedearly Precambrian basement metamorphic rocks-Agetashtage complex, consisting of Milangroup, Neoarchean TTG gneiss and paleoproterozoic granitic gneiss intruding into them, andKhondalites in Dunhuang Group. With rheomorphic fold, Milan group and TTG sufferedhigh-amphibolite to granulite facies metamorphism, intensive migmatization and lateramphibolite facies metamorphism alteration. Their geochemical characteristics-low Si and Na,high Al and K content, enriched in LILEs and LREE, depleted in HFSE and HREE, heavyfractionation between LREE and HREE, no obvious Eu anomaly-indicate that they are productof arc basalt melting after subducting beneath the lower continental crust. The zircon U-PbSHRIMP ages of them are close-2567±32Ma and 2592±15Ma respectively, suggest that they arefrom the same magma stage, hinting the assembly of Tarim craton in later Neoarchean andhorizontal crust growth similar with Japan arc model in southeastern margin of Tarim craton inWutai stage.The Paleoproterozoic granite differed from Neoarchean TTG rocks by their amphibolitefacies metamorphism, non-sodium migmatization and had clear boundary with Milan Group.These large-scaled intrusions contained dioritic gneiss, gneissic quartz diorite, amphibole quartzsyenite and carbonatite, with syenite and monzonite mafic rocks appeared as veins or dykes. Thebanded carbonatitic intrusion intruded into the high grade TTG to granitic complex and MilanGroup. The carbonatite with simple minerals, petrologic characteristics, the geochemicalcomposition, and so on, especially extremely low REE content and Th/U ratios, show itsgenensis of crustal resource.We did zircon SHRIMP U-Pb dating on all the following rocks: gneissic diorite, gneissicquartz diorite, gray felsic veins (felsic migmatization) , carbonatite and quartz syenite, andyielded their age of 2135±110Ma, 2051.9±9.9Ma, 2050±16Ma, 1931±18Ma and 1873.4±9.6Marespectively. In addition, we dated the Dunhuang Group volcanic rocks with the same methodand got 2140.5±9.5Ma as a result. We also found that, without exception, all Neoarchean metamorphicrocks had group metamorphic ages of either 2.27~2.38Ga or 1.9~2.05Ga. From these and someexisting geochronological data by other researchers for the same study area, we can establish afine geochronological framework as follow:The formation of Khondalites and the emplacement of calc-alkaline island arc quartz dioriteindicated an early subduction environment in the middle of Paleoproterozoic (2.10~2.15Ga) era,The trace elements is rich in Ba,Sr,K,LREE and other large ion lithophile elements( LILE) ofthose rocks,while the high field strength elements ( HFSE) Nb,Hf,Zr,Ti,etc．are relativelydepleted and without Eu anomalies. All of these features are similar to features of the island -arccalc-alkali series in relation to subduction, representing the volcanic-arc products of thesubduction in Palaeoproterozoic.Late Paleoproterozoic (1.93~2.05Ga), this area went through a syn-collisional orogenesisstage, during which, igneous carbonatite, quartz diorite and potassium migmatite were formeddue to intense anatexis of crustal derived rocks and pre-existing rock underwent amphibolitefacies metamorphism and intense ductile shear deformation.In the post-orogenic stage (1.85~1.87Ga) in last Paleoproterozoic era, when post-orogenic ganite such as quartz monzonite, quartz syenite were formed. It is characterized by SiO2-rich,alkaline-rich(Na2O+K2O＞10%), K-high and low-Al2O3.The quartz syenite is enriched in LIFE,such as Rb,Th, K, and depleted in HSFE such as Ba, Nb,Ta, P, Ti, with distinct negative Euanomaly. All the above geochemical evidence indicated that those alkali-granites are formed inthe post-orogenic extensional tectonic setting.NW-trending mafic dyke swarms were well developed, indicating continental breakup ofthe Tarim craton in early Mesoproterozoic. The sedimentary strata of Suoerkuli Group werewidely developed giving further evidence for breakup in Mesoproterozoic.This geochronology framework provides not only a scientific basis for the discussion ofearly crustal evolution of the Tarim craton but also a comparison framework for the study ofearly Precambrian metamorphic base of North China craton. Moreover it provides data for thefurther study of the Columbia supercontinent assembly.