Comparative Study Of Devonian-Carboniferous Volcanic Rocks Between East And West Junggar,Xinjiang

As an important part of the Central Asian Orogenic Belt,both the East Junggar tectonic belt and the West Junggar tectonic belt have experienced multi-stage,long-term and complicated tectonic evolution processes,thus,formed a rich and diverse geological record,which result to the many geological records in both two regions to have"substantially similar"strata and"various features"constructions.Hence,it questions whether Eastern Junggar and Western Junggar belong to two tectonic belts or they belong to east and west contrast tectonic belt?It has become a provocative topic for geologists,and has made Eastern and Western Junggar to be focused by domestic and international scholars in recent years.By exploring and trying to answer this controversy,the records of the formation,paleo-tectonic,and paleo-environment of East and West Junggar have become the preferred research objects.Among them,the extensively developed Late Paleozoic magmatic activities,especially volcanic sedimentary rock series,provide a crucial material record and a good window for this comparative study.Based on the project of"Comparative study of typical Devonian-Carboniferous sections in East and West Junggar,Xinjiang",this research has selected 8 typical sections mainly composed of Devonian-Carboniferous volcanic rocks and several volcanic rocks in the ophiolite belt.The study focused on volcanic rock petrology,petrography,chronology,geochemistry,and zircon Hf isotope geochemistry,etc.,and accurately calibrated the volcanic rocks chronological properties,with the basic geological and geochemical characteristics of contemporary volcanic rocks(such as combined formation,Series and type,tectonic environment attributes,genesis,characteristics of source area)are the main comparison indicators,and a comprehensive comparison of the East and West Junggar regions was carried out in detail.The Devonian volcanic rocks in the eastern and western Junggar are mainly medium-basic rocks,with high aluminum,low titanium,enriched in Light Rare Earth Elements(LREE)and Large Ion Lithophile Elements(LILE),and depleted in High Field Strength Elements(HFSE)such as Nb,Ta,Ti and other characteristics of arc volcanic rocks.The Early Devonian volcanic rocks belong to the calc-alkali(-tholeiite)series,and the Early Devonian volcanic rocks in East Junggar are typical high-alumina basalts,which are partially melted products of garnet lherzolite that is substituted by subduction fluids.The value of?Hf(t)of the Middle Devonian volcanic rocks in East Junggar is+4.33?+15.22,which may derive from the partial melting of the depleted mantle containing garnet and a small amount of spinel lherzolite that partially melted 10-30%in the depleted mantle.The late Devonian Volcanic Rocks of eastern and western Junggar belong to calc-alkali tholeiitic series and middle high K Calc-alkali series respectively,and are the product of fluid(and melt)metasomatic depleted mantle source.Generally,the Devonian of East and West Junggar are in the arc environment related to subduction.The Early Carboniferous volcanic rocks in East Junggar have higher Na₂O+K₂O content and K₂O/Na₂O ratio,showing the characteristics of potassium basaltic rocks.It is relatively enriched in LILE and LREE,strongly depleted in HFSE,and has a more obvious Nb-Ta trough,showing the magmatic characteristics of subduction zone.The value of?Hf(t)is+0.22?+13.62,lower N(Nb/La)ratio,Nb/U value,Ce/Pb value and higher Nb content,indicating that the source area is a mantle source enriched by metasomatism zone formed in post collisional arc environment.Western Junggar Early Carboniferous volcanic rocks are mainly medium-acid volcanic rocks,belonging to calc-alkaline series.Andesite has high Si O₂,Al₂O₃,Sr and Sr/Y ratios,low Mg O,Y,Yb,rich in sodium and poor in potassium,which are the typical adakite.Rhyolite belongs to calc-alkaline peraluminous rock,and zircon has a positive height of?Hf(t)=+12.57?+15.56.Combined with lower compatible elements Ni,Cr,Mg^#,Rb/Sr and La/Ce value,it is believed that adakite may be product of partial melting of basaltic oceanic crust and formed in the island arcs environment.The Late Carboniferous volcanic rocks in West Junggar are calc-alkali-medium acid volcanic rocks.Rhyolite has the characteristics of high silicon,low titanium,high aluminum,alkali-rich,and calcium-deficient,and belongs to the It belongs to peraluminous series.The distribution curve of REE in chondrites shows a right-inclined"V"shape distribution,with obvious Eu negative anomaly,high Ga/Al×10⁴ value,which is a typical aluminum A-type granite,rhyolite?Hf(t)value is+6.95?+14.89,which is originated from the partial melting of the new crust derived from the depleted mantle and formed under the post-collision environment.The comparative study found that the Kujibai-Hebukesaier-Honggulen-Zhahaba-Almantai ophiolite belt in the north and the Darbut-Kalamali ophiolite melange belt in the south can be comparably connected.Taking volcanic rock as the main starting point for comparison,and combining with the analysis of symbiotic rock composition,biological fossils and sedimentary factors:the Saur strata in West Junggar has good correlation with the Ertai strata in East Junggar;Sharburtishan strata in the West Junggar has good correlation with Beitashan in the East Junggar;the Mayilishan in the West Junggar has a complete correlation with Kalamari strata in the East Junggar.Volcanic rock contrast information show that the East Junggar entered the post-collision stage in the Early Carboniferous,while the West Junggar only entered the post-collision stage in the Late Carboniferous,and the ocean basin showed a"scissor type"closed pattern from east to west.From north to south,the Saur-Fuhai-Durat tectonic belt,the Heshituoluogai-Ulungu-Yemaquan tectonic belt,and the Darbut-Luliang-Kalamali tectonic belt have similar evolution models of extension in the east-west direction.Therefore,in general,both the eastern and western Junggar whether in sedimentary formation or tectonic evolution both possess good comparability essence.