New Structure Of South Tectonic Zone In Yining Block And Its Tectonic Evolution

The Yining Block is located in the southwestern margin of the Central Asian orogenic belt(CAOB),which is a window to decipher the history of subduction and extinction of the ancient Asian Ocean due to its unique geographical location and intricate structural features.In previous research work,this block was regarded as a homogeneous block of“one piece of steel plate”,and there was no division of secondary structural units.However,this study concludes that the Yining Block can be subdivided into two major tectonic zones,i.e.,south tectonic zone and north tectonic zone that is bounded by the Wusunshan-Talede fault distributed near the E-W direction in the central part;each secondary tectonic unit is characterized by its unique history of tectonic evolution and mutual independent“Trench-Arc-Basin”structure system.Among them,the south tectonic zone is obviously different from the other,including the basement construction,chronological framework and geochemical characteristics of magmatic rocks,the formation of the basin,and the history of tectonic evolution.Moreover,the south tectonic zone has played an important role during the architecture construction of the whole Yining Block.Therefore,based on briefly explaining the division of the secondary structural units of the Yining Block,this paper mainly focuses on the formation and evolution of the"Arc-Basin"system in the south tectonic zone in Yining Block.The main results achieved are as follows:1.The"Arc-Basin"tectonic system of the south tectonic zone in Yining Block is composed of the Late Devonian-early Early Carboniferous Dahala-Kalejun island arc zone and the Atengtao back-arc extension basin.2.The main magmatic activity period in the Dahala-Kalejun island arc zone was concentrated in372?340 Ma.Among them,the emplacement time of the Kalesijun monzonitic granite and syenite granite analyzed in this paper are 356±3 Ma and 344±1 Ma respectively,the Kekesuhe monzonitic granite and syenite granite are 349±5Ma and 349±6 Ma respectively,and the Kekesulinchang high Mg andesite is ca.359Ma.Petrographic and geochemical characteristics show that these granites from the island-arc zone belong to the I-type granite with medium-high K,metaluminous-weakly peraluminous affinities.Their petrogenesis is mainly related to the partial melting of the middle and lower crustal materials with minor addition of mantle-derived magma and subduction zone component,with regard to its positive?Hf(t)values of zircon and high Th/Hf,Th/Ta and low Y and Nb values.The high-Mg andesite in the Kekesulinchang area displays high Mg O,Si O₂ and Ti O₂ contents,relatively low Sr,Yb contents and Sr/Y,La/Yb values,which resemble the reported sanukitic rocks in Atengtao Mountain,Yining Block and Bieluagaxi area,Western Junggar,and the petrogenesis is considered to have been derived from the equilibrium reaction of mantle wedge with the subduction zone materials.3.Compared with the Dahala-Kalejun island arc zone,the magmatic rocks in the Atengtao back-arc basin are more complicated,and can be divided into three types according to their outcrop position and emplacement or eruption time.The first type is dominated by the Late Devonian-early Early Carboniferous magmatic rocks(362-350 Ma)exposed in the northern part of the basin,representing the products of initial extension stage of the back-arc basin.The second type is represented by a series of rock associations with special tectono-magmatic significance predominantly exposed in the Atengtao Mountain in the central basin,with ages clustering in ca.345-340 Ma.These rocks represent the relatively mature products of the back-arc extension basin.The third type is dominated by a small amount of late Carboniferous magmatic rocks produced in the central Atengtao Mountain and the northeastern margin of the basin,with ages ranging from 314 Ma to 306 Ma,representing the product of intra-plate setting.The magmatic rocks in the initial extension stage of the basin mainly consist of granitic intrusions and alkaline basalts.Petrographic and geochemical characteristics show that these granitic intrusions belong to the high-potassium,calc-alkaline I-type granite,containing low Cr,Co,Ni and relatively high Mg^#values,and are considered to be produced in the lower crust with minor addition of mantle-derived materials.These alkaline basalts have low Si O₂ and high total alkali content,Ti O₂and Mg O,and is enriched in Sr,K,Rb,Ba,relatively depleted in Nb,Ta,P,showing the characteristics of arc magmatic rocks;however,some characteristics(such as Nb/Ta and Zr/Hf ratios)display OIB and MORB characteristics,similar to the basalts in the back-arc extension basin in the Early Permian in the Yushu area of Tibet.Combining regional geological characteristics,this paper argues that the initial formation of the back-arc basin is related to slab rollback of the South Tianshan Ocean.The magmatic rocks in the mature period of the back-arc extension basin are predominantly composed of a series of special rock types,not only including A-type bubble rhyolite(338±4 Ma)and bimodal volcanic/intrusive rocks(340±6 Ma,345?349 Ma)which are related to the extensional setting,but also containing sanukitic high Mg andesite(356?346 Ma)and highly differentiated I-type granite(338±3Ma)that are generated in the typical arc setting.Petrographic and geochemical characteristics show that these A-type rhyolites contain high Si O₂,alkaline values and incompatible element contents(such as REE,Zr,Hf,Nb),and extremely high?Hf(t)value(close to the depleted mantle)and relatively young two-stage model age of zircon,originating from the partial melting of the juvenile basaltic lower crust.The bimodal volcanic rocks are composed of basaltic andesite and rhyolite.Among them,the basaltic andesite shows low Yb and high Fe₂O₃t,Ti O₂ and Mg O contents,which was derived from the low degree partial melting of garnet-bearing mantle-derived rocks(1%?5%)which was previously metasomatized by subduction zone fluid;the rhyolite has the characteristics of A-type granite.Combined with the weak positive?Hf(t)value(+1.5?+4.8)and older depleted mantle model age of zircon,these rhyolites are considered to be the products of partial melting of ancient crustal materials under the basin.The bimodal intrusive rock is composed of gabbro and I-type granite;the former has the characteristics of Nb-enriched basalt that was derived from a depleted mantle wedge with a small amount of OIB-like asthenosphere components added during this process;the latter is related to the melting of the lower crust.Geological characteristics of these sanukitic high-Mg andesites are similar to the aforementioned sanukitic rock types in the Dahala-kalejun island arc zone,and also originated from the interaction between a mixed mantle source and the sediment melt from subduction zone.Based on the regional features,this paper argues that slab breakoff or tearing was responsible for these special rock associations in the mature back-arc basin.The Late Carboniferous magmatic rocks are represented by the Tekesihebei rhyolite in the study area(314±2Ma).Petrographic and geochemical characteristics show that these rhyolites are characterized by high Na₂O+K₂O,Zr and REE contents,high values of Fe Ot/(Fe Ot+Mg O),K₂O/Na₂O and 10⁴×Ga/Al,showing potassic,A-type magmatism affinities.Their petrogenesis is likely to originate from partial melting of calc-alkaline granite in the shallow crust.Combined with regional geological characteristics,this article concludes that these rhyolites in Late Carboniferous were formed in the intra-plate setting under high temperature and low pressure condition.4.The Late Paleozoic sedimentary construction in the south tectonic zone is relatively simple,mainly composed of the Early Carboniferous Akeshake Formation.This Formation can be divided into upper and lower sections.The upper section is dominated by terrigenous clastic rocks,and the lower section is mainly composed of marine carbonate rocks.The characteristics of rock assemblages and sedimentary sequences reveal that the Akeshake Formation in the Dahala-Kalejun island arc zone is relatively well exposed.The variation characteristics of regional sedimentary facies in the Early Carboniferous were as follows:(1)In the vertical direction,the sedimentary sequences from bottom to top reveal the features of transgression with retrogradation progress;(2)In the horizontal direction,the sedimentary facies reveal the seawater was deep with weak hydrodynamic force to the west,while the seawater was relatively shallow with strong hydrodynamic force to the east,displaying a gradually changing role.Similarly,the sedimentary facies of this group in the Atentao back-arc basin are similar to the former,but the thickness of the inner clastic deposits(such as carbonates and siliceous rocks)is much greater than that of the outer clastic deposits(such as terrestrial sandstones and conglomerates).Moreover,the overall depth of seawater is higher than the island arc zone.5.This study concludes that Late Paleozoic tectonic evolution in the south tectonic zone mainly consists of five stages,namely Early Devonian-mid-Late Devonian subduction-accretion and flat subduction,Late Devonian-early Early Carboniferous subduction-accretion and slab rollback,mid-Early Carboniferous slab breakoff or tearing,late Early Carboniferous slab stagnation,and collision-orogeny between Early Carboniferous and Late Carboniferous,coupled with subsequent intra-plate extension setting.6.Through the detailed division and analysis of the southern tectonic belt,combined with regional geological characteristics,this paper suggests that the formation of the Yining Block is completed in three steps,including:(1)During Late Devonian-early Early Carboniferous,the"Arc-Basin"tectonic system of the south tectonic zone was primarily formed;(2)During Middle and late Early Carboniferous,the"Trench-Arc-Basin"system of the North tectonic zone was generated;(3)Between Early and Late Carboniferous,the south and north tectonic zone were welded together,forming the unified block-Yining Block.