西天山查岗诺尔和智博铁矿区火山岩地球化学特征、锆石U-Pb年龄及地质意义

西天山东段的查岗诺尔铁矿和智博铁矿赋存于以玄武岩、玄武安山岩、粗面岩以及安山质凝灰岩为主的晚石炭世火山岩中, 对火山岩的形成时代以及构造地质背景的研究是重建成矿过程的关键。本文通过对两个矿区的火山岩进行岩石地球化学和LA-ICP-MS锆石U-Pb测年分析来探讨火山岩形成的构造环境与时代。地球化学分析表明大多数火山岩化学成分从钙碱性、高钾钙碱性变化到钾玄岩系列,富集轻稀土元素(LREE)和大离子亲石元素(LILE; 如Rb、Th、K),重稀土元素(HREE)配分平坦,同时具有Nb、Ta、Ti的强烈亏损,类似于岛弧火山岩的地球化学特征。大多数玄武质火山岩在构造环境判别图中位于火山弧环境。LA-ICP-MS锆石U-Pb测年显示流纹岩和英安岩的²⁰⁶Pb/²³⁸U加权平均年龄分别为301.8±0.9Ma和300.3±1.1Ma。此外,对两件闪长岩样品测年获得²⁰⁶Pb/²³⁸U加权平均年龄介于303.8～305Ma之间。火山岩与闪长岩样品具有类似的地球化学特征以及形成时代,表明它们可能来源于同一母岩浆,形成于相同的构造背景下。结合区域地质资料,本文认为矿区内出露的高钾钙碱性到钾玄岩系列火山岩可能属于俯冲过程末期阶段大陆岛弧岩浆作用的产物。

Geochemistry and zircon U-Pb age of volcanic rocks from the Chagangnuoer and Zhibo iron deposits, western Tianshan, and their geological significance

The Chagangnuoer and Zhibo iron deposits occurring in eastern segment of western Tianshan, were hosted primarily by the Late Carboniferous volcanic rocks, which mainly contain basalt, basaltic andesite, trachyte and andesitic tuff. The tectonic setting and age of the volcanic rocks are key elements in the reconstruction of ore genesis. In this paper, representative samples of volcanic rocks from the Chagangnuoer and Zhibo iron deposits were analyzed for major, trace elements and zircon U-Pb dating to validate their tectonic setting and formation ages. Geochemical data indicate that most of the volcanic rocks range in composition from calc-alkaline, high-K calc-alkaline to shoshonitic, with light rare earth element(LREE)and large ion lithophile element(LILE; e. g. , Rb, Th, K)enrichment and almost flat heavy rare earth element(HREE)patterns as well as strongly negative Nb, Ta and Ti anomalies, similar to features of arc volcanic rocks. In the plots of tectonic discrimination, the basaltic volcanic rocks are mainly projected in arc setting regions. LA-ICP-MS U-Pb dating of magmatic zircons from the rhyolite and dacite yield weighted average ²⁰⁶Pb/²³⁸U ages of 301.8±0.9Ma and 300.3±1.1Ma, respectively. In addition, weighted average ²⁰⁶Pb/²³⁸U ages from 303.8Ma to 305Ma were obtained for two diorites. The similar geochemical characteristics and formation ages of volcanic rocks and diorites indicated that they were sourced from the same parent magma and may formed in the same tectonic setting. Combined with the regional geological data, we therefore suggest that the high-K calc-alkaline to shoshonitic volcanic rocks were probably the products of continental island-arc magmatism during the late stage of subduction.