Carboniferous-Permian Magmatism And Tectonic Evolution In The Hatu-Baogutu Region Of Southern West Junggar

Situated in the southwestern part of the Central Asian Orogenic Belt,the West Junggar region is located between the Altay folded orogenic belt in the north and the Tianshan folded orogenic belt in the south,and the magmatism in the region displays multiphase and multiple genesis characteristics,with intensive and complicated structural phenomenon.The Hatu-Baogutu area is located in the hinterland of the West Junggar,where massive Carboniferous-Permian magmatic rocks are extensively distributed,which provides favorable material condition for understanding the characteristics of Late Paleozoic magmatic activity and geological tectonic evolution in West Junggar.Taking the Carboniferous to Permian intermediate-acid intrusive rocks and mafic-intermediate dikes exposed in the Hatu-Baogutu area as the research object,comprehensive studies were carried out in this study on the basis of detailed field investigations and a large amount of collected chronological and isotopic geochemical data.This study accurately calibrated the geological age of the magmatic rocks in the study area,and analyzed the petrogenesis and their source characteristics,and further discussed regional tectonic evolution process during Carboniferous to Permian time,which provide detailed and accurate data support for understanding the coupling relationship between Late Paleozoic magmatic activity and tectonic evolution in the West Junggar,and the main achievements are as follows:1.Two phases of intermediate-acid magmatism with different petrogenesis in the study area have been determined.Lithofacies,zircon geochronology and geochemistry jointly demonstrate that the intermediate-acid intrusions in the study area are the products of two periods of magmatism.Small stocks consisting of fine to medium-grained dioritic-granodioritic rocks show grayish white hue on the field outcrops.These rocks generally have porphyritic texture,and massive mafic microgranular enclaves（MMEs）representing the mixing between mantle-and crustal-derived melts are widely developed within the rocks.Chronological data indicate that the small stocks were emplaced between ca.322Ma and310Ma,with peak age of 314Ma.Geochemical studies show that most of small stocks in the study area have cal-alkaline metaluminous I-type granite compositions（only Tiesikekuola pluton is peraluminous S-type granite and formed at 308.9±1.8Ma）and are characterized by medium silicon with SiO₂ being 62.96%in average and sodium-rich but potassium-poor,they are enriched in U,Pb,Zr,Hf,Rb,K and Sr but strongly depleted in Nb,Ta,P and Ti,with weak Eu anomalies.In contrast,the batholiths consisting of coarse grained and coarse-medium grained alkali feldspar granite and monzonitic granite usually show red hue on the field,and were emplaced between ca.306Ma and 295Ma,with peak age of 303Ma.Geochemical studies show that the alkali feldspar granite and monzonitic granite are characterized by high silicon with SiO₂ being 78.71%in average,rich alkaline and potassium-rich but sodium-poor,they are strongly depleted in Ba、P、Sr and Ti,with significant Eu negative anomalies,belonging to alkaline weak peraluminous A-type granite.Nd-Sr、Pb and Hf isotopes confirm that the magma source of the late Carboniferous to early Permian granitoids in the study area is probably juvenile lower crust derived from the depleted mantle similar to MORB,with a little contamination of Neoproterozoic continental crust materials.2.Two phases of mafic-intermediate sanukitic dikes in the study area have been distinguished.Mafic-intermediate dikes,mainly consisting of pyroxene diorite porphyrite,diorite porphyry,quartz diorite porphyrite and quartz diorite,were widely distributed in the Bieluagaxi and Miaoergou areas,analogues to those of Cenozoic sanukitoids of the Setouchi Volcanic Belt.Sanukitic dikes in the Bieluagaxi area formed in the early Late Carboniferous（ca.319-315 Ma）,and were likely derived from partial melting of a depleted mantle source with subducted sediment melts（Group 1）,while the Group 2 rocks were derived from partial melting of a depleted mantle source metasomatized by slab-derived adakitic melts and fluids.Sanukitic dikes in the Miaoergou area formed between 298.0±3.7 Ma and 299.4±2.5 Ma,corresponding to the end of the Late Carboniferous to the beginning of the Early Permian,and may be generated by interaction of the mantle wedge with fluids derived from dehydration of the subducting oceanic slab,resulting in 5%to 15%partial melting of depleted garnet-spine lherzolite.3.A multi-stage subduction-accretionary orogeny model was proposed.Combined with the identified and reported subduction-related magmatism,i.e.,adakites,sanukitoids,Nb-enriched basalts,back-arc basalts,lagged arc volcanic rocks,and I-type granitoids and their spatial-temporal characteristics in the Western Junggar region,and taking into full consideration the previous achievements,therefore,we consider a multi-stage subduction-accretionary orogeny model may account for the difference of Carboniferous to Permian subduction duration between the northern segment and southern segment of West Junggar.The Early Carboniferous and Late Carboniferous-Early Permian geologic bodies in the West Junggar region are at least two periods of subduction accretionaries from the southeast to northwest,respectively.The first subduction-accretionary event finished at the end of Early Carboniferous time,and then collaged to the pre-existing Ordovician to Late Devonian accretionary complexes,and turned into the intra-continental orogeny development stage both in the northern segment and southern segment of West Junggar region.During the beginning of the Late Carboniferous to the Early Permian time,the second subduction-accretionary event occurred in the southern part,resulting in a large number of subduction-related magmatic rocks,which were eventually collaged to the Early Carboniferous accretionary complex in the Early Permian.