Age and petro-geochemistry of High-aluminum basalts from northern Zhaosu County of Xinjiang The sign to convergent margins of Early Carboniferous plate in West Tianshan

Basalts and sodic trachybasalts are the main rocks outcropped in the south of Wusunshan in western Tianshan Mountains.LA-ICP-MS analysis on the Early Carboniferous volcanic rocks of Dahalajunshan Group yields an U-Pb age of 355.1±4.0 Ma.These volcanic rocks are characterized by SiO₂ 45.04%~49.84%, Na₂O 2.36%~4.63%, and K₂O 0.35%~2.31%, high ratios of Na₂O/K₂O (2.00~8.69) and high contents of Al₂O₃ (16.01%~17.15%), suggesting that they are sodic and high alumina basalts with small amount of alkaline basalts.The total rare earth contents, (La/Yb)_N and δEu of volcanic rocks range from 94.71×10^-6to 127.2×10^-6, from 2.41 to 4.07 and from 0.99 to 1.1 respectively.The REE patterns are characterized by enriched LREE and relatively depleted HREE, with weak positive Eu anomaly, which is in accordance with high sodic plagioclase contents in rocks.Enriched large-ion-lithophile elements (LILE), such as Ba and K, depleted high-field-strength elements (HFSE), such as Nb-Ta, Th-U and Ti, and the magmatic evolution trend suggest that these rocks belong to typical island arc tholeiitic series, which have experienced pyroxene and feldspar fractionation.The determination of high alumina basalts is the sign to convergent margin of early Carboniferous plate in West Tianshan.Therefore, it is considered that these basaltic rocks were formed at continental arc setting and could be in the second volcanic chain.And then, the basaltic magma might be formed by partial melting of enriched metasomatic mantle upon subduction zone.