Determination of the Paleoproterozoic Geochronological Framework in Aqtashtagh Area in Southeastern Tarim,China,and Its Geological Significance

Abstract

A large quantity of Paleoproterozoic metamorphic intrusive rocks and khondalites(in Dunhuang Group) were identified from basal metamorphic rocks in the Agetashtag area,southeastern margin of Tarim basin.The Paleoproterozoic granite(metamorphic intrusive rocks) differs from the Neoarchean TTG rocks in amphibolite facies metamorphism,non-sodium migmatization,and obvious boundary with Milan Group.Among them,the extensively exposed intrusions contain dioritic gneiss,gneissic quartz diorite,amphibole quartz syenite,and carbonatite,with syenite and monzonite mafic rocks present in the forms of veins or dykes.Zircon SHRIMP U-Pb dating was performed on all the above-mentioned rocks.Crystallized ages obtained for gneissic diorite,gneissic quartz diorite,gray felsic veins(felsic migmatization) and quartz syenite are 2135±110 Ma,2051.9±9.9 Ma,2050±16 Ma and 1873.4±9.6 Ma,respectively.In addition,the age of volcanic rocks in the Dunhuang Group is 1873.4±9.6 Ma.All other Neoarchean metamorphic rocks fall two metamorphic age groups of 2.27~2.38 Ga and 1.9~2.05 Ga.Combined with isotopic data by previous geologists for this area,a fine geochronological framework has been established as follows:(a) Mid-Paleoproterozic(2.10~2.15 Ga),emplacement of the Dunhuang Group khondalite and calc-alkaline island arc quartz diorite indicates the early subduction stage of orogency;(b) Late Paleoproterozoic(1.93~2.05 Ga),crust-derived rocks in this area underwent an entensive anatexis,forming igneous carbonatic rock,quartz diorite,and potassic migmatition,with pre-existing rocks suffering amphibolite facies metamorphism and intense ductile shearing deformation,and that is response to syn-collisional orogenesis;(c) The end of Paleoproterozoic era(1.85~1.87 Ga) saw the post-orogenic stage,during which the post-orogenic ganite such as quartz monzonite and quartz syenite were formed.At the early Meso-proterozoic developed swarms of NW-trending mafic dykes and covering sediments of Jixian-Qingbaikou system Suoerkuli Group(Annanba Group),indicating continental breakup of the Tarim craton.In summary,this fine geochronological framework suggests that the geological development of Tarim craton has some affinity to the North China craton at early pre-Cambrian,and both of them belong to the Nena supercontinent group and involved into the convergence of Paleoproterzoic Columbia supercontinent.