PETROLOGY AND SR-ND ISOTOPIC GEOCHEMISTRY OF THE CARBONIFEROUS VOLCANIC ROCKS IN THE WESTERN TIANSHAN MOUNTAINS, NW CHINA

Abstract

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 εNd (t) values (+2.68 ~ +4.29) and high initial 87Sr/86 Sr ratios (0.7015 ~ 0.7051). The trachyandesite - rhyolite occurring in the south of Xinyuan city have relatively low εNd (t) values (-0.22 ~ +0.87) and variable initial 87 Sr/86 Sr values (0.7045 ~ 0.7068). The basalts in south of Xinyuan city have low and less variable initial 87 Sr/86 Sr ratios (0.7045 ~ 0.7058), but their εNd (t) values are high and variable (+0.89 ~ +3.04). These Sr-Nd isotopic data and the previous zircon SHRIMP chronology studies suggested that the Early Carboniferous volcanic are 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.