Petrology and Sr-Nd isotopic geochemistry of the Carboniferous volcanic rocks in the western Tianshan Mountains, NW China.

The Carboniferous volcanic rocks occurring in western Tianshan Mountains range continuously from basalt, basaltic trachyandesite, trachyandesite, trachyte, to rhyolite. The isotopic compositions are variable for Carboniferous volcanic rocks in different regions in western Tianshan Mountains. The Late Carboniferous trachyandesite-trachyte occurring in Laerdundaba region characterize with relatively high epsilon(Nd) (t) values (+2.68 similar to +4.29) and high initial Sr-87/Sr-86 ratios (0.7015 similar to 0.7051). The trachyandesite rhyolite occurring in the south of Xinyuan city have relatively low epsilon(Nd) (t) values (-0.22 similar to +0.87) and variable initial Sr-87/Sr-86 values (0.7045 similar to 0.7068). The basalts in south of Xinyuan city have low and less variable initial Sr-87/Sr-86 ratios (0.7045 similar to 0.7058), but their epsilon(Nd) (t) values are high and variable (+0.89 similar to +3.04). These Sr-Nd isotopic data and the previous zircon SHRIMP chronology studies suggested that the Early Carboniferous volcanic arc system faded away from west to east at the Late Carboniferous. Potassium-rich volcanic rocks formed in east part of the western Tianshan during the Late Carboniferous at a post-collisional environment.