Geological Features And Tectonic Implications Of The Granitoids In The Southern Yili Block

The Central Asian Orogenic Belt is sandwiched between Siberia and North China,Tarim and the Eastern Europe Craton.It is generally believed that during the late Mesoproterozoic to late Paleozoic,the subduction of the ancient Asian Ocean and the collision of multiple island arcs,hyperplastic wedges,islands and seamounts formed this orogenic belt.The West Tianshan orogenic belt in Xinjiang,China is located on the southwestern margin of the Central Asian Orogenic Belt.The understanding of its tectonic evolution is the key to understanding the orogenic process and dynamics of the entire Central Asian Orogenic Belt.The Yili Block is a continental block with a Precambrian basement that is clamped in the Western Tianshan orogenic belt and is an extension of the eastern part of Kazakhstan micro-continent.A number of Early Paleozoic sutures were distributed inside the Kazakhstan-Yili Block.Many scholars believe that the Early Paleozoic orogeny may also exist in the Western Tianshan.The Carboniferous Dahalajunshan Formation volcanic rocks are widely distributed in the southern margin of the Yili Block.The continental arc structure background has been accepted by more scholars for its genesis and tectonic setting.Based on the continental arc tectonic setting,different scholars proposed two models of the northward divergence of the South Tianshan Ocean and the southward divergence of the Junggar-North Tianshan Ocean.A large area of the Proterozoic Tekes group metamorphic sedimentary rocks emerged from the Qiaxi forest farm on the northern slope of the Nalati Mountains on the southern margin of the Yili Block,and was invaded by various granite bodies in the later period.Detailed geochemical and chronological studies for this granite intrusions are currently lacking.In this thesis,the geochronology and geochemical characteristics of granites invading the Proterozoic metamorphic sedimentary rocks are studied,and the geochemistry of the volcanic rocks of the Adengtao,West Dahalajunshan and Kuerdening areas is supplemented.Through the study of this data,the Paleozoic tectonic and magmatic evolution history of the southern margin of Yili Block is further explored.The granitic rocks of the intrusive metamorphic sedimentary rock in the north of the Qiaxi forest farm,the lithology mainly includes coarse-grained granite,two-mica granite and biotite granite.The biotite granite was formed at 400-380 Ma;the coarse-grained granite and the two-mica granite were formed at 438 Ma and 426 Ma,respectively.The source regions of the Early-Middle Silurian coarse-grained granite and the two-mica granites have surface sedimentary rocks,which have the synonymous deformation structure and have the geochemical characteristics of continental collisional granites,which may be formed in the collisional orogenic background;The Middle Devonian biotite granite contains contemporaneous dark-colored basic inclusions,which may be the result of magma mixing.It has not undergone deformation and has the geochemical characteristics of typical continental arc magmatic rocks,which may be formed on the active continental margin.The above studies show that the granites in the north experienced the magmatism of different tectonic settings in the Early and Late Paleozoic.The Early Paleozoic magmatism may be related to the aggregation of the Kazakhstan micro-continental,reflecting the products of the Early Paleozoic Terskey ocean after the Late Ordovician closure and collisional orogeny;the Late Paleozoic magmatism may be the product of the Junggar Ocean subduction.The granitic rocks of the intrusive metamorphic sedimentary rocks in the southern part of the Qiaxi forest farm,including K-feldspar granite,diorite and granite.K-feldspar granite was formed at 365 Ma;diorite and granite were formed at 363-351 Ma,respectively.The Late Devonian K-feldspar granite has no deformation and has the geochemical characteristics of A-type granite.The source area of it is deep enriched mantle,which may be formed in the extended environment of the island arc background.Late Devonian-Early Carboniferous diorite and granite has not undergone metamorphic deformation,and it has typical geochemical characteristics of calc-alkaline magmatic rocks,which may be formed in the continental arc tectonic background of the active continental margin.The volcanic rocks of the Dahalajunshan Formation in the Adengtao,West Dahalajunshan,and Kuerdening also exhibit the geochemical characteristics of typical calc-alkaline magmatic rocks,possibly with the Late Devonian-Early Carboniferous diorite and granite are the same as the continental arc tectonic setting.Combined with the chronological data of the pre-existing alkaline magmatic rock samples in the southern margin of Yili Block,it is concluded that the Late Paleozoic alkaline magmatic rocks have a migration from south to north,which may represent the movement of the back-arc stretching and the retreat of the oceanic plate.The direction indicates that the trench is located on the north side and supports the model of the Junggar-North Tianshan ocean southward subduction.The geochemical and chronological studies of the granitic rocks invading the metamorphic sedimentary rocks in the Qiaxi forest farm indicate that the Early Paleozoic and Late Paleozoic in the southern margin of the Yili Block where the study area was located experienced different magmatic activities.The Early Paleozoic tectonic and magmatism may be mainly influenced by the Terskey paleo-ocean,but the tectonic and magmatic superposition of the Late Paleozoic strong hyperplasia-collisional orogeny,so the related geological records are less preserved.The Late Paleozoic may be dominated by the southward subduction of the Junggar-North Tianshan Ocean to the Yili Block,and a large number of calc-alkaline volcanic rocks are exposed,but there is a local back-arc extension setting.