Spatial-Temporal Distribution And Petrogenesis Of Late Paleozoic Granitoids Inthe West Junggar

Western Junggar,in the western segment of the Central Asian Orogenic Belt,is sandwiched between the Tianshan fault system and the Ertix fault.Late Paleozoic magmatic rocks are well developed in the Western Junggar,ranging in composition from ultrabasic rocks to acidic rocks.Despite intensive research in the last two decades,the emplacement age,nature and tectonic environment of late Paleozoic magmatic intrusion in the Western Junggarare still controversial.In this contribution,we present an integrated petrographic,geochronological,geochemical study of late Paleozoic intermediate-acid intrusions in the Western Junggar,to constrain the emplacement age of magmatic rocks,to elucidate the nature,source characteristics and petrogenesis of magmatic intrusions,to reveal the space-time distributionof the late Paleozoic magmatic rocks,and to further discuss the tectonic environment.The main results are as follows:1.We present LA-ICP-MSzircon U-Pb dating data from 29 representative samples,and combine this with previously publishedzircon U-Pb dating data of 95 samples of Late Paleozoic magmatic rocks in the study area.,These data show that Late Paleozoic granitic magmatism in the Western Junggar can be divided into two magmatic periods?332³10Ma and 327²87Ma?.The small-medium size porphyry intermediate-acidic intrusiveswere emplaced from 332 to310 Ma.The granitoid batholiths were emplaced from 327 to287 Ma,mostly in the range of 308 to296 Ma.The basic dike swarms and intermediate-acid dikes were emplaced in both periods.2.The granitoidsare characterized by high SiO₂?72.03%⁷6.44%?and K₂O?4.34%⁵.01%?,high-K calc-alkaline,metaluminous to weakly peraluminous?A/CNK =0.97¹.03?,and marked depletion of Eu,Ba and Sr.They have typical characteristics of Atype granite and belongs to A2-type granite.The Sr-Nd-Pb isotopiccompositions of the granitoids in Western Junggar have the characteristics of depleted mantle components,which are related to the subduction of the Junggar Ocean and may be of mantle origin.3.The final closure of ocean basin in the Western Junggar is constrained to be between Late Carboniferous and Early Permian.The volcanic xenoliths from the Hongshan pluton yield zircon U-Pb ages of 319.1 ± 2.9 Ma and 313.3 ± 2.4 Ma,which areolder than the crystallization ages?305-301 Ma?of the Hongshan pluton.The linear andesitic dykes with zircon U-Pb age of 295 ± 2 Ma,constitute a dike swarm that was formed in an extensional setting.The volcanic xenolithsand andesitic dykesshow geochemical features of Sanukitoidaffinities,and represent products of arc magmatism and continental extensionalmagmatismduring the subduction of the Junggar oceanic micro-plate,respectively.The Sanukitoid volcanic xenoliths?319 313Ma?,the Hongshan pluton?305 301Ma?and the linear dyke swarms? 295Ma?correspond to the late Carboniferous arc volcanism,post-orogenic extensionalmagmatism,and the early Permian intracontinental extensionaldyke swarms that together record the ocean-continent transition in the Western Junggar.4.Large-scale strikeslippingalways generates rotation and translation displacement deformationforspecific terrain,especially for magmatic pluton.The Miaoergou plutonrotated clockwise ca.60°,while Akbasitao pluton rataed counterclockwiseca.30°,and the overall rotation displacement is about 7.3 km.The Dalabute sinistralfault displacedat about 83.5 km,in which the Miaoergou-Akbasitao and Karamayi-Hongshan plutoncutoff contributes ca.62 km.Based on the analysis and estimation of the displacement and rotation deformation,the original positions of the plutons are inversed.The Miaoergou,Akebasitao,Hatu,Tiechanggou,Karamay and Hongshan plutons may generate in a closer area and may be a giant batholith.5.We conduct a geochemistry mapping for the Miaoergou pluton and explore the potential correlation between the geochemistry distribution pattern and regional structural system.Contents of SiO₂,K₂O,and Fe₂O₃ seem to be the lowest in the most-southeastern and central-north part,while CaO,MgO,TiO₂,Al₂O₃,P₂O₅,Na₂O,and MnO show the highest contents in the most-southeastern and north part.All these major elements pattern may indicate the effects of the Dalabutefault's sinistral strike-slipping movement.The contents of REEs are high in the core,east-to southeastern of the pluton,seeming some potential tendency from the core to rim,which reflects the comprehensive control of Darabut strikeslipping fault and Taergen ring structure.The trace elements distributing pattern shows adispersive characteristics inmap view,which may relate to the mobilityfeature of chemical element themselves,or controlled by the ring and regional strike-slip structures.6.According to the significant magmatic phase ages in Baogutu-Karamay and Miaoergou-Beluagaxi areas and the metallogenic ages of Hatu gold deposit,this paper suggests that the strike-slip faults,i.e.,Darabut fault,Mayile fault and Barruk fault launchedat 300-290 Ma,even the possible 294-290 Ma.