Geochemical characteristics of Carboniferous volcanic rocks from the Wulungu-Luliang area, Junggar basin: constraints on magma source and tectonic setting

Abstract

The Carboniferous volcanic rocks in the Wulungu-Luliang area are mainly andesites of medium- to high-K calc-alkalic series. Volcanic rock samples have relatively high alkali (Na2O + K2O = 4.7 % to 6.8 %) and low TiO2 contents (0.7 % to 0.9 %), relatively high MgO (2.5 % to 3.4 %) and Mg# (49.9 % to 67.1 %), high rare earth element (REE) contents, and relatively high K2O contents (1.7 % to 3.1 %). Chondrite-normalized REE patterns show light REE enrichment ((La/Yb)N = 4.15 to 5.19) with weak Eu anomalies (δEu = 0.75 to 0.92). These samples are enriched in large-ion lithophile elements but relatively depleted in high field strength elements. The trace elements and REE patterns are similar to those of Setouchi and central Ryukyu high-Mg andesites, indicating a high-Mg andesite source. Relatively high Y contents (16.7 to 24.4 ppm), and relatively low Sr/Y ratios (17.2 to 38.8) and TiO2 contents (0.7 % to 0.9 %) exclude the possibility of slab melting. Low Sr/Nd (16.6 to 42.8), Ba/Th (66.4 to 266.8), and U/Th (0.2 to 0.3) indicate that the influence of slab-derived fluids is low. The ratios of Ce/Th (4.9–7.3), Ce/Pb (1.8–4.2), Ba/Rb (7.99–22.03), Ba/Th (66.4–266.8), and La/Sm (3.6–4.3) are similar to ratios found in subducting sediment melts. Relatively high ratios of K/Nb (1357–3258), Th/La (0.28–0.42), Zr/Nb (8.8–27.1), and especially Th/Nb (0.48–1.25) suggest that the magma was assimilated and contaminated by upper continental crust. These characteristics, along with the ratios of La/Yb, Sc/Ni, Th/Yb, Ta/Yb, Ce/P2O5, and Zr/TiO2, demonstrate that the earlier Carboniferous volcanic rocks in the Wulungu-Luliang area were generated in a continental island-arc setting.