新疆昭苏县北高铝玄武岩时代、岩石学和地球化学特征——西天山早石炭世汇聚板块构造的标志

西天山乌孙山南坡赛克散萨依一带出露的早石炭世杜内阶大哈拉军山组火山岩，LA-ICP-MS锆石U-Pb年龄为355.1±4.0 Ma。火山岩岩石类型主要为玄武岩和少量钠质粗面玄武岩（夏威夷岩），SiO2含量为45.04%~49.84%，Na2O含量为2.36%~4.63%，K2O含量为0.35%~2.31%，Na2O/K2O值为2.00~8.69，表明均属钠质火山岩；Al2O3含量集中在16.01%~17.15%之间，以偏碱性的高铝质玄武岩为主，有少量碱性玄武岩。火山岩稀土元素总量为94.71×10-6~127.2×10-6，（La/Yb）N值为2.41~4.07，δEu值为0.99~1.15，具有轻、重稀土元素分馏明显的右倾式配分型式。大离子亲石元素较洋脊玄武岩富集，存在明显的Th-U槽和Nb-Ta槽。高场强元素多数与洋脊玄武岩接近，TiO2的亏损不显著，K2O明显富集。玄武岩的演化具有典型的拉斑质玄武岩经分离结晶作用形成的铁富集演化趋势，且以辉石和斜长石为主要的分离结晶矿物，橄榄石的分离结晶作用不明显。高铝质玄武岩的确定，是早石炭世伊犁地区中天山板块汇聚构造环境的标志，与现今高铝质火山岩产出构造环境对比认为，这些火山岩形成于大陆边缘岛弧，其构造位置可能为俯冲带上的第二火山链；火山岩浆则可能由俯冲带上流体交代的富集地幔部分熔融形成。

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.