Genesis Of The Granite In The Northeast Of Altay And Its Constraints On The Paleozoic Tectonic Evolution Process Of The Altai Orogenic Belt

The Altai Orogenic Belt,as a part of the Central Asian Orogenic Belt,is a hot zone for studying the process of subduction orogeny.Studies have shown that there are still controversies about the closing time of the Irtysh Ocean in the southern Altai Orogenic Belt,and the Paleozoic tectonic setting,which restricts the study of its tectonic evolution.So studying the petrology,chronology,and geochemistry of granites in the Altai Orogenic Belt is of great significance for understanding the tectonic setting and tectonic evolution of its formation.This paper has carried out a systematic field survey and indoor U-Pb chronology and geochemistry research on the granite in the northeast of Altay.Based on previous research results,the magma intrusion period in the study area is divided and the type of each period and its characteristics of the source area is discussed.Moreover,The Paleozoic tectonic setting and the tectonic evolution process of the Altai Orogenic belt in China are discussed.Detailed conclusions are listed as follows:Synthesizing the chronological data of the granite body in the north east of Altay,it shows that the pluton can be divided into three periods namely Mid-Late Ordovician,Middle Silurian-Late Devonian,and Early-Middle Paleozoic.And Devonian intrusive activities are the most intense,consistent with the peak period of the entire Chinese Altai magmatic activity.The petrology shows that the Mid-Late Ordovician granite rock assemblages in the study area are gabbro diorite,diorite,granodiorite and syenite.The Middle Silurian-Late Devonian granite rock assemblages are diorite,granodiorite and monzogranite.And the main lithology of the Early-Middle Permian granitoids are monzogranite and syenite.The geochemical characteristics show that the granites of the three periods in the study area are characterized by high Si and alkali,high K and low Na,and low Fe,Ca,and Mg.The difference is that as the times change,the content of alkali(K₂O+Na₂O=6.84%?7.23%?8.10%)and potassium(K₂O/Na₂O=1.18?1.24?1.36)tends to increase.What's more,the REE distribution curves show a right-leaning feature of the Mid-Late Ordovician and Middle Silurian-Late Devonian granite rock samples,and those samples are enrich in Rb,Th,U,K,Nd,deplete in Ba,Nb,Ta,and Ti.While the Early-Middle Permian granitoids show"seagull-shaped"REE distribution mode,and enrich in Rb and K,deplete in Ba,Nb,Sr,and Ti.The petrogenesis of those granitoids have been discussed using the petrology and geochemistry comprehensively.The Mid-Late Ordovician granites belong to calc-alkaline-high-K calc-alkaline,metaluminous-peraluminous I-type.Their formation is mainly related to the partial melting of the middle and lower crust materials under the setting of subduction,and experienced a certain separation Crystallization.The Middle Silurian-Late Devonian rocks dominated by calc-alkaline-high-K calc-alkaline,weakly-peraluminous I-type,followed by S-type.I-type granites formed in the middle and lower crust materials partially melted with minor addition of mantle-derived materials.S-type granites formed in the upper crust materials partially melted.And there is a certain degree of separation and crystallization during the evolution of magma.The Early-Middle Permian granites belong to the high-potassium calc-alkaline series A-type,I-A type and leucogranites.Their petrogenesis is mainly related to the partial melting of argillaceous rocks in the transitional environment of syn-collision to post-collision.Based on the research data of regional stratigraphy and magmatic rocks,the tectonic evolution of the Altai orogenic belt was discussed.Ordovician period,the fluid formed by the dehydration of the subducting slab replace the mantle wedge and induce partial melting of the crust to form I-type granites.In the Early Devonian,the oceanic crust subduction angle increased,the slabs retreated,and the mantle-derived magma was upwelling,gathering at the bottom of the crust,heating or intruding into the crust,resulting in I-type granite and S-type granite.The geochemical tectonic setting discriminant map of the Carboniferous granite in the orogenic belt shows a volcanic arc environment,indicating that the orogenic belt is still in the subduction stage in the early Late-Carboniferous,and the residual ocean basin may be closed in the late Late-Carboniferous.Permian granites show a trend of syn-collision to post-collision environment transformation.It is possible that the transition from syn-collision to post-collision extensional environment occurred in the early Permian,and the crustal thickening and heating,lithospheric delamination and mantle upwelling occurred,various types of granites such as A-type,I-A type and leucogranites granites have been produced.