Origin Of Carboniferous Volcanic Rocks From Beisantai Area, Eastern Junggar Basin And Its Geological Implication

The Junggar Basin is located between Tianshan and Altai orogenic belts.It is an important part of the Central Asian Orogenic Belt.Several Paleozoic ophiolites are occurred in its surrounding orogenic belts,indicate that they had experienced multiple stages of subduction and collision.The Beisantai area is located in the eastern Junggar Basin.Carboniferous volcanic rocks from several drills in this area make it possible to investigate the Carboniferous tectonic evolution of the Junggar Basin.The Batamayineishan formation in the Beisantai area are mainly composed of basalts,basaltic andesites,andesites,dacites,volcaniclastic rocks and intruded by diorite porphyrites.Zircon from the andesites,dacite,diorite porphyrite yielded crystallization ages of 325,321 Ma,310 Ma,303 Ma,respectively.Combined with the published ages of volcanic rocks from this formation,the eastern Junggar Basin had experienced an important late Carboniferous magmatism.The volcanic rocks in our study have low TiO₂,variable but mainly low K₂O and high Na₂O contents,indicating calc-alkaline series.They are generally characterized by enriched LREEs and flat HREEs with slightly negative Eu anomalies.They have high LILE and low HFSE concentrations with evident negative anomalies of Nb,Ta and Ti.They also have stable Th/U and Nb/Ta ratio.Moreover,the depletions of Sr,Ti and P in these rocks intensify gradually from basic through intermediate to felsic.These geochemical characteristics indicate that they were products of magmatism related to oceanic subduction in an island-arc setting.Thus,the oceanic subduction most likely lasted to the late Carboniferous in eastern Junggar Basin and the final closure of the Junggar Ocean occurred probably after 310 Ma.The inherited zircons with the ages from 626 Ma to 2400 Ma were recognized from the andesite and diorite porphyries.Based on the analysis of zircon U-Pb ages,Lu-Hf and Sr-Nd isotopes of the Devonian-Carboniferous volcanic rocks as well as some geophysical data,we proposed that Precambrian crustal materials were preserved besides the Paleozoic crustal material in both of the western and eastern Junggar Basin.