新疆塔什库尔干塔阿西一带火山岩成因及地质意义

通过岩石学、地球化学和同位素年代学研究, 从原划"布伦阔勒岩群"中识别出一套"双峰式"火山岩, 英安岩锆石LA-ICP-MS U-Pb年龄结果显示该套岩石形成年龄为521.3±3.3 Ma, 为早寒武世产物. 在岩石类型上, 玄武岩和英安岩属于低钾岩石系列, 玄武岩相对富集Rb、Ba、K、LREE, 而亏损Nb-Ta、Th, 源于受早期俯冲洋/陆壳流体交代的亏损地幔熔融源区. 英安岩表现出富集LILE、Th, 亏损Nb-Ta、Sr和Ti.两类岩石表现出不同的稀土元素配分模式和微量元素特征, 并缺乏分异演化的趋势, 反映二者的成因存在差别. 结合其地球化学特征和实验岩石学资料, 认为英安岩为玄武岩底侵提供热, 基性下地壳在相对低压条件下部分熔融形成的. 这套"双峰式"火山岩组合证实区域上早古生代存在一次大陆拉张环境下的构造-岩浆事件. 结合区域上研究资料, 古元古代布伦阔勒岩群至少包含以下3个组成部分: (1)古元古代布伦阔勒岩群; (2)印支期高压变质岩体; (3)早古生代火山-沉积岩组合. 

Origin of the Volcanic Rocks from the Ta'axi Region,Taxkorgan Xinjiang and Its Geological Significance

This paper presents petrological and geochemical features and zircon U-Pb age of a suite of bimodal volcanic rocks in the Bulunkuole Group from the Taishuihai Massif, NW China. The zircon U-Pb dating results yield a mean 238U/206Pb age of 521.3±3.3 Ma, representing the extrusion time of the dacites in the Middle Cambrian, which indicates that the previously defined Paleoproterozoic Bulunkuole Group also includes early Paleozoic volcanic rocks. The basalts and dacites belong to low-K affinities. The basalts show enrichment in Rb, Ba, K and LREE, and depletion in Nb-Ta and Th, suggesting that they were derived from a previously metasomatized depleted mantle. The dacites also show more enrichment in LILE and Th, and larger depletion in Nb-Ta, Sr and Ti in comparison with the basalts. Different REE patterns and trace element features between the two rock types, and the absence of differentiation trend from the basalts to dacites, argue against the origin for the dacites as the differentiates of the basalts. Combining the dacites geochemical features and the experimental data suggest that they are melts of lower crustal mafic protoliths heated by contemporaneous underplating basaltic magmas at relatively low pressure conditions. The occurrence of the bimodal volcanic rocks reflects that the region experienced an extensional tectonic-magmatic event during early Paleozoic. The new data suggests that the previously defined "Paleoproterozoic Bulunkuole Group" includes at least three components: (1) the Paleoproterozoic khondalite series; (2) the Indo-Sinian high-pressure metamorphic complexes and (3) the early Paleozoic volcano-sedimentary associations.