Geological Features Of The Wenquan Group (NW Tianshan)and Its Tectonic Evolution History

Tianshan Orogenic Belt is the southwestern part of Central Asian Orogenic Belt, and it is one of the key studying areas. As some Precambrian basement usually contain a lot of evolution information, they are perfect candidates to study the tectonic history of CAOB. Precambrian basement crop out on the southern and norther edges of Yili block and Central Tianshan Block. In Syram area of northwestern Yili block, Precambrian basement is well preserved and its oldest component is called Wenquan Group, which mainly crops out in Biezhentao Mountain and is considered as a Paleoproterozoic unit.Field investigation shows that migmatites and granitic gneisses usually crop out together in Wenquan Group, and they intrude into metamorphic sedimentary rocks such as paragneiss and amphibolites. Destrital zircons from paragneisses yielded3evident peaks:1.2Ga、1.3Ga and1.4Ga, indicating that their material sources are mainly Mesoproterozoic crust. And both of this source curst and East Tianshan area have experienced a magma activity during1.2-1.4Ga。 Zircon U-Pb ages of migmatites and orthogneisses are between926～876Ma in age, indicating that they are early Neoproterozoic but not Paleoproterozoic rocks. Magmatic rocks of the same age are widespread in Precambrian basement in Tianshan area, such as Syram, Laerdundaban, Bingdaban, Baluntai and Xingxingxia areas. Previous Geochemical data show that these metamorphic igneous rocks come from the reworking of old crust, and are probably linked with continental assembly during Grenville Orogenic Period. Destrital zircons from metamorphic sedimentary rocks in south Wenquan area yield an obvious age peak of～0.9Ga, indicating that igneous rocks of this period are one important source of sedimentary rocks that form afterwards.One amphibolite in Wenquan Group has a zircon U-Pb age of661Ma. Though its chemical property is not clear, coeval volcanic activity is well reported in Tianshan and Northern Tarim. This volcanic activity is considered as a result of continental rifting during Early Sinian and a prelude of the opening of Paleozoic oceans. The so-called medium-coarse "amphibolites" were regarded as part of Wenquan Group. In fact they show igneous characters both in the field and under microscope. They are diorites and gabbros of igneous genesis, but not metamorphosed. Both of them crystallized during middle-late Ordovician (441-465Ma) and they show calc-alkali island arc chemical characters, so they are considered to have formed in active continental margin or continental island arc. So these Ordovician intruded rocks are linked with the subduction of Junggar-Balkhash Ocean.Most rocks in Wenquan Group show well-defined S1foliation with a NEE-SWW strike and NW-SE trending L1lineation. C-S fabric in augen gneiss, σ-type diorite pebbles in conglomerates, and oblique fabrics of amphibole aggregates of amphibolites under microscope etc., all indicate a top-to-the-SE sense of shear.40Ar/39Ar dating on amphibole and muscovite constrains the D1event as～443-413Ma. And this event is probably a result of the assembly of Yili-Kazakhstan continent.Tight folds, crenulations and slaty cleavage with SEE-NWW strike are considered a result of D2event. SW-dipping axial plane of quartzite and asymmetric fold shape of sericite slate of Changcheng System both suggest a top-to-the-north sense of movement. Muscovite on D2foliation in a micaschist yielded a40Ar/39Ar intercept age of338Ma, providing an oldest limit for the D2event. And this event could be a result of the southward subduction of Junggar-Balkash Ocean beneath the Yili-Kazakhstan block in late Paleozoic.Several shear zones of500-1000m in width crop out in South Wenquan area. Foliation of rocks becomes steep close to the shear zone and then sub-vertical inside the shear zone. Well defined mineral and stretching lineation trend east-westward, showing low pitch angles. Many kinematics observed in the field and under microscope indicate a dextral sense of shear. Muscovite from mylonitic quartzite yielded an40Ar/39Ar age of～289Ma, representing the time of D3event. This event is consistent with the strike-slip shearing of North Tianshan Fault during early Permian, and is considered a result of post-collisional transcurrent tectonics.In summary, Wenquan Group can de divided into four lithological units: pre-Neoproterozoic metamorphic rocks, early Neoproterozoic migmatites and orthogneisses, late Neoproterozoic intrusive and early Paleozoic intrusive. Its brief tectonic history can be summarized as follows:in early Neoproterozoic, crust melted and was reworked; in late Neoproterozoic, dikes intruded; in early Paleozoic, Junggar ocean crust subducted and active continental margin formed, with top-to-the-south shear; during late Paleozoic, from top-to-the-north shear to dextral sense of shear; and later extrusion defromation.