Petrogenesis Of The Early Silurian Granitic Rocks In The Eastern Part Of Jueluotage Belt, Eastern Tianshan

In this paper,the granitic rocks of the Early Silurian Epoch in the eastern segment of the eastern Tianshan Mountains are taken as the research subjects.Based on detailed petrographic study,the results of LA-ICP-MS zircon U-Pb dating,the major and trace elements content of the whole rock and Sr-Nd isotope analysis,the formation age,source characteristic and tectonic attribute of the Early Silurian granitic rocks were discussed.On the basis of making full use of the previous research results,a reasonable circumscription on the history of the Paleozoic tectonic evolution of the Jueluotage orogenic belt has been made in this paper.The granitic rocks of Early Silurian in the eastern segment of the East Tianshan Jueluotage orogenic belt are mostly scattered in the Carboniferous granite and the Wutongwozi Formation.The combination of the granitic rocks are mainly granodiorite and granite porphyry.There are also some diorite,quartz diorite and dioritic porphyrite.These granitic rocks are characterized by mylonization.Some samples can be seen to develop an ocular structure such as augean structure and gneissic structure under the microscope,which is different from the large-area exposed Carboniferous granitic rocks with no obvious metamorphic or deformation features.suggesting that they were emplaced earlier.The LA-ICP-MS zircon U-Pb dating study of two samples,granodiorite?YH-13?and diorite porphyrite?YH-16?,shows that the zircons in the sample are mostly euhedral-subhedral in shape and Short columnar,display obvious oscillatory growing zoning,and have relatively high Th/U ratio?0.28⁰.59?,indicating that they are magma oringin.The dating results revealed that the weighted mean ages of the two samples are?431.2±1.9?Ma and?433.2±2.2?Ma,respectively.They are basically consistent within the error range,all of which were early Silurian?443.8±1.5⁴33.4±0.8 Ma?.They represent the crystallization ages of zircons,indicating that the emplacement of these deformed granitic rocks in the area happened in early Silurian.Geochemical data have been obtained from 8 fresh samples including diorite,quartz diorite,granodiorite and dioritic porphyrite.As to the characteristics of the main elements,they featured with w?SiO₂?=57.88⁶9.01%,w?Na₂O?=3.46⁵.07%,w?K₂O?=0.50¹.65%,Na₂O/K₂O=4.26⁶.79,Mg^#=45.1⁵2.3,A/CNK=0.89¹.00;In the Hacker diagram,the main oxides of different samples have a good linear relationship with the mass fraction of SiO₂.In the TAS diagram,the samples are mainly applied to the granitic diorite and diorite regions.In the A/NK-A/CNK diagram,the samples are projected in the peraluminous region.In the w?K₂O?-w?SiO₂?diagram,the samples are mainly projected in the low-K series,and the results generally accord with the main elements characteristics of the comagmatic igneous rock island arc.In terms of trace elements characteristics,the distribution model of REE is right abligue in the orientation,and the Eu element is non abnormal in the abundance.They Enriched in large ion lithophile elements Ba,U,K and Sr,and depleted in high field strength elements Th,Ti,Nd and Ta.The?Eu of all samples except the sample YH-16??Eu=0.82?was negatively correlated with the mass fraction of SiO₂??Eu=1.25⁰.86?,which coincided with the comagmatic magma evolution law.The early Silurian granitic rocks in this area are mainly acidic rocks with few intermediate rocks and basic base rocks.They generally have higher mass fractions of Na₂O and CaO,Na₂O/K₂O>1,and the aluminum saturation index?ASI?A/CNK is less than 1.1,⁸⁷Sr/⁸⁶Sr initial ratio is less than 0.708,which has the characteristics of type I granite.Combining with its?_Nd?t?value?+6.1⁺7.0?and high Sr low Yb type granites,it can be determined that the original magma of the Early Silurian granitic rocks in the eastern part of the Jueluotage orogenic belt originated from the partial melting of young thickened lower crust.According to the temporal and spatial evolution law of igneous rocks,combined with the achievements of the Jueluotage orogenic belt and the tectonic attributes of Tuha block and the Mid Tianshan block on the north and south sides,it is considered that the tectonic evolution of the Jueluotage orogenic belt from the early Paleozoic to the late Paleozoic.Can be divided into four evolution stages,which are the one-way subduction stage the ocean slab subducted to the Tuha block from early Silurian to Devonian,the bilateral subduction stage the ocean slab subducted to the Tuha block and the Mid Tianshan block in early Carboniferous,the collision stage of Tuha block and Zhongtianshan block in late Carboniferous,as well as the post-collision extension stage of early and middle Permian respectively.The time limit for the northward subduction of the Kanggur ocean slab was traced back to early Silurian.