The Chronology,geochemical Characteristics And Petrogenesis Of Volcanic Rocks Of Hala’alate Formation In Urho,xinjiang

As one hot spot in recent studies of the Central Asian Orogenic Belt（CAOB）,the West Junggar is located on the southwest edge of the CAOB.Carboniferous strata spread widely in this area,among which the Hala’alate Formation is the most typical marine volcanic strata in Urho area and even in the southern part of the Western Junggar tectonic belt,which is the key to study the closure and tectonic background of the Western Junggar Late Paleozoic Ocean Basin.Based on the study of volcanic petrology,petrography and stratigraphy of the Hala’alate Formation,this paper conducts research on the geochemistry of main trace elements,zircon U-Pb chronology,zircon rare earth elements,Lu-Hf isotopes,etc.,and combines the existing research results of predecessors to discuss its source characteristics and petrogenesis,analyse the tectonic-magmatic evolution of the Late Paleozoic in West Junggar.The main understandings obtained are as follows:1.Through the retest and detailed study of the original section（PM02 section in this paper）,it is confirmed that the exposed strata of this section is only the upper part of Hala’alate Formation(C₂h^5-7),the south old and the north new sequence is corrected.The eastern sections of Kokemmerin Ulan（PM01 and PM01S sections in this paper）were supplemented,and the lower Hala’alate Formation(C₂h^1-3)sequence was newly defined by correlation.Seven section-level stratigraphic sequences with obvious marker layers and regional comparison were established.By the detailed comparison between regional profiles confirmed that the section described of Hala’alate Formation in this paper is the most well developed volcanic rock type section in the region.2.It is confirmed that the Hala’alate Formation is a set of volcanic rocks（about 90%）intermingled with terrigenous clastics.The lower part is mainly basic-acid volcanic lava,with a small amount of pyroclastic rock.The lava is mainly basalt,basalt andesite,andesite,dacite and rhyolite.The upper part is dominated by pyroclastic rocks,the proportion of volcanic lava is significantly less than that of the lower part,and there is a lack of acid end members.The lower part of the volcanic rock has similar major element and trace element composition and petrogeochemical characteristics to the upper part.The small differences in some sections may be the transformation of the evolution of homologous magma in the early and late stages.3.The LA-ICP-MS zircon U-Pb age of the 6 volcanic rocks collected from Hala’alate Formation is 303.3～308.9 Ma,and its formation era was marked as the Late Carboniferous.4.Taking terrestrial clastic rocks as the sign of the interval period of volcanic eruptions and the main basis for dividing eruption cycles,the volcanic rocks of Hala’alate Formation were divided into 7 eruption cycles.Most cycles develop multiple eruption rhythms.It was divided into 4 eruption cycles in the area of Kokemmerlin（PM01-PM01S）,4 eruption cycles in the north of Hala’alate Mountain（PM02）,and 2 eruption cycles in the area of Babandi east of PM02.Seven lithofacies including eruption facies,exhalation facies,exhalation facies,eruption facies,volcanic sedimentary facies,latent volcanic facies and pyroclastic flysch facies are identified in the section.The early phase is dominated by exhalation facies,and the middle to late phase is dominated by eruption facies.5.The volcanic rocks of Hala’alate Formation belong to calcium-alkaline series（Rittmann index is 1.98）,with Si O₂content of 46.15%～73.05%,weakly enriched Large Ion Lithophile Elements（LILE）and Light Rare Earth Elements(LREE=14.81～101.62×10^-6).High Field Strength Elements（HFSE）such as Nb,Ta,Ti and Heavy Rare Earth Elements(HREE=5.83～21.76×10^-6)are depleted,and there are obvious negative anomalies of Nb(0.46～10.94×10^-6),Ta(0.03～0.66×10^-6)and Ti.The basalt and basaltic andesite have the geochemical characteristics of island arc related to subduction,and the geochemical characteristics of dacite and rhyolite are similar to that of type I granite.It is concluded that the volcanic rocks of Hala’alate Formation belong to the product of arc tectonic environment with continental margin properties.6.The the characteristics and ratios of major elements,trace elements,isotopes and other elements of the volcanic rocks of Hala’alate Formation,which show high Al₂O₃contents,Mg^#values,¹⁷⁶Hf/¹⁷⁷Hf（0.282844～0.2830216）andε_Hf（t）（9.0～15.3）values,a relatively flat REE distribution with LREE slightly enriched,indicate that the magma may be derived by the partial melting of a spinel-lherzolite depleted mantle source which was metasomatized by the subduction fluid.7.The volcanic rocks of the Hala’alate Formation support that the Hashan area was still in a subduction-related arc tectonic environment during the late Carboniferous,the Junggar ocean basin was was not closed and continued subduction to the northwest.In this process,the crustal granites with contemporaneous,homogeneous and homogenic formed in the early collage and proliferation of the posterior mountain continental crust were the response to the deep subduction of this type of pre-arc.which enriches the collage accretionary model of the late Paleozoic in the West Junggar,and reasonably explains the structural coupling relationship between the"pre-arc volcanic-posterior mountain intrusive rocks".