Geochemical characteristics and tectonic significance of Carboniferous volcanic rocks in the northern part of the Ili Block, Xinjiang

Abstract：Carboniferous volcanic rocks are widespread at peripheries of the Ili block of the West Tianshan. They are complex in lithology and mostly are associated or alternate with terrigenous clastic rocks. Geochemical study of volcanic rocks from the Axi, Nileke and Nalati sections in Yining suggests that these rocks belong to the calc-alkaline series composed of basalt, trachybasalt, basaltic andesite, basaltic trachyandesite andesite, rhyolite, dacite and trachyte. They have relatively high REE concentrations and are enriched in LREE relative to HREE, and in addition, they display a pronounced enrichment of LILE such as Th and Rb relative to HFSE such as Nb, Ta and Zr. Therefore the formation of these rocks is related to plate subduction. ｔｈｅ ｔrace element geochemical diagram further indicates that these rocks formed in a continental island-arc environment. The Early-Mid Carboniferous neritic-littoral sediments associated with them and the existence of the Late Devonian-Early Carboniferous Bayan Gol-Motuoshala ophiolite belt suggest that the Carboniferous calc-alkaline volcanic rocks on the northern margin of the Ili block are likely to have erupted in an active continental-margin environment and probably resulted from the subduction of the North Tianshan oceanic basin southward beneath the Ili block during the Late Devonian-Mid Carboniferous.