Genesis and tectonic significance of Carboniferous volcanic rocks in Karamali area, northern Xinjiang

Volcanic rocks， such as andesite and tuff， are widely developed in the Carboniferous system in Karamali area， northern Xinjiang. The tectonic background of their formation is still controversial. In this paper， two typical Carboniferous profiles from Karamali Mountain were selected. Andesite and tuff samples from the lower Carboniferous Jiangbasitao Formation， the upper Carboniferous Bashan Formation， and Huxingliang Formation were observed under a microscope， and their main trace elements were analyzed. The results show that the contents of MgO （0.5%-2.35%） and Mg# （15.7%-42.5%） are low to medium in samples， indicating that the crystallization differentiation of rock samples occurs during the formation process. The distribution curve of trace elements in the samples is obviously different from that of N-MORB， E-MORB， and OIB. The samples are almost all in the calc-alkaline series in Ta/Yb-Th/Yb and La-Y-Nb diagrams， while the samples fall subduction components and approach the mantle series in Nb/Yb-La/Yb diagrams. The Nb/Y-Rb/Y and Ba/La-Ba/Nb diagrams also show that the samples are affected by both fluid enrichment and melt enrichment. The Nb/U and Ce/Pb ratios of the samples are 1.6-11.1 and 0.47-12.2， respectively， which are close to the range of continental crust and indicate that the source of the material may be mixed with continental crust materials. According to the Th-Hf/3-Nb/16 diagram， all the samples are located in the island arc basalt region. The comprehensive analysis shows that Karamali area was in the continental island arc environment of active continental margin from the early Carboniferous to the late Carboniferous， and this environment lasted at least from the early Carboniferous to the late Carboniferous.