Characteristics And Regional Tectonic Implications Of Late Paleozoic-Early Mesozoic Magmatic Rocks From The Central And Southern Inner Mongolia

The southern Central Asian Orogenic Belt?CAOB?is an important area to research the closing time and tectonic evolution of the Paleoasian Ocean.The magmatic petrogenesis and structure attribute of the southern CAOB in Late Paleozoic-Early Mesozoic are controversial for a long time,which restrict the study of the palaeotopography tectonic evolution processes of the Paleoasian Ocean in Late Paleozoic-Early Mesozoic.We choose the Late Paleozoic-Early Mesozoic Magmatic in Xilin Hot-Xi U jimqin Banner of the southeastern CAOB as the crux of the issue,and conduct an integratedstudyplanreferringthefiledoccurrence,structuregeology,petrology,petrography,the zircon U-Pb dating and geochemical analysis.The results are showed as follows:The Xiaowulangou complex plutons,with the age from the late stage of Early Carboniferous to Late Carboniferous?329±0.98Ma³00±0.97Ma?,consist of diorite,quartz diorite,granodiorite and monzonitic granite lithologically,and are a suite of I-type granitoids.Petrological and geochemical characteristics of the rocks suggest that the plutons are the result of superposition invasion of mixing of mantle-and crust-derived magmas with different evolutions,probably triggered by underplating of mantle magmas which originated from the partial melting of wedge mantle metasomatized by hydrous fluids from the subducted slab.Together with their petrological and geological features,we propose that the Xiaowulangou complex plutons formed in an active continental margin tectonic setting,and were related to the northward subduction of the Paleoasian Oceanic crust from Early Carboniferous to Late Carboniferous.The Yiheshaorong complex massif comprise a wides pectrum of rock types from gabbro,pyroxene diorite to alkali-feldspar granite,and display a good bimodal geochemical distribution,belonging to the early Permian with the age of 280Ma±.The gabbros-pyroxene diorite are a suite of low-K tholeiitic-calc alkaline rocks.The alkali-feldspar granites belong to high-K calc-alkaline series,and show an affinity with the A-type granites.The Dashizhai volcanic rocks occur petrologically as a set of intermediate-acid volcanic lava-clastic rocks,and erupted in the early Permian with the age from287.5±1.4Ma to 276±0.81Ma.The petrological and geochemical analysis reveal that the volcanic rocks were geochemically formed in an island-arc setting,and are similar to geological features of A-type granites originated from the partial melting of felsic crust under high temperature and low pressure conditions.Combined with the geochemistry characteristics and the regional geology data,the Early Permian magmatic were most probably formed in a back-arc spreading basin triggered by oceanic crust subduction before closure of the Paleoasian Ocean in the Early Permian.The granodiorites are formed from 253.8±1.2Ma to 240.5±0.78Ma,and the biotite adamellites are formed between 249.6±1.7Ma and 240.4±0.59Ma,which represent a tectono-magmatism event of the later of Late Permian to Early-Middle Triassic.Petrological and geochemical study indicate that the granodiorites belong to calk-alkaline I-type granite,and have the geochemical characteristics of adakite.The biotite adamellites show characteristics of highly fractional I-type granite.All the geochemical characteristics indicate that the granites were probably the product of different batches of emplacement of crystallization/differentiation of magmas formed by partial melting of the thickened crust under different depths.Together with their petrological and geological features,the granites of the Late Permian to Early-Middle Triassic were most probably formed in the tectonic setting of collision triggered between the Siberian plate and North China plate after closure of the Paleoasian Ocean in the Later Permian.Based on the tectonic deformation feature,two important tectonic deformation phases at the Late Paleozoic-Early Mesozoic are identified in this research:?D₁?the NEE trending folds in Permian strata implied a NNW-SSE compression from the Late Permian to Early-Middle Triassic;?D₂?the ductile shear zone in EW?SEE direction,formed in an extensional condition,were developed during Late Triassic.Which were related to the collision event between the Siberian plate and North China plate after the close of the Paleo-Asian Oceans,and the extensional regime of post-collision setting respectively.According to these new data and regional geology,the Late Paleozoic-Early Mesozoic tectonic-magmatism can be identified five stages:The late of Early Carboniferous to Late Carboniferous?330³00Ma?,the early of Early Permian?298²89Ma?and the middle and late of Early Permian?289²71Ma?are belong to the subduction stages of Oceanic crust.The late of Late Permian to the early and middle Triassic?256²38Ma?is belong to the collision orogeny stages.The continuous extrusion caused the cessation of the marine deposition and the formation of NEE fold and thrust-fault structures in Permian strata,which suggested the final closure of the Paleo-Asian Ocean.The late Triassic?237²11Ma?is belong to the post-collision extension stages.The forming of the positive break ductile shear zone near EW and SEE trending in the research area were induced by the extension and thinning of the lithosphere,implying the beginning of a new stage of the crustal evolution.