Geochemistry and tectonic settings of Carboniferous volcanic rocks in the Chepaizi area of Junggar basin, western China

Abstract: Carboniferous volcanic rocks from Chepaizi area of Junggar Basin mainly composed of basalts, basaltic andesites and andesites occur within Early Carboniferous Tailegula Formation, and are mainly Calc-alkaline series. These rocks have high Mg# (65 85) values, relatively high alkali((K2O+Na2O)=4.25% 5.45%), TiO2 (0.86% 1.40%) contents and Na2O/K2O ratios (2). They are enriched in LREE ((La/Yb)N=1.8 6.9), and with slight fractionation of HREE ((Gd/Yb) N=1.1 1.7). These samples are enriched in LILE but relatively depleted in HFSE, with strong negative Nb-Ta anomalies. They have high La/Nb (2.1 7.6), Zr/Y (3.43 8.10), but low Th/La (0.11 0.19) ratios. These characteristics, along with the discrimination diagrams, indicate that these volcanic rocks are from oceanic island arc related to the subduction of oceanic crust of western Junggar. The primitive magma of the rocks was possible generated by partial melting of mantle wedge induced by fluid and sediments of subducted oceanic crust rather than the slab-derived melt.Key words: geochemistry; tectonic environment; volcanic rocks; Carboniferous; Junggar basin