新疆西准噶尔库尔尕克希岩体年代学、地球化学特征及岩石成因

摘要

新疆西准噶尔构造带发育大量晚古生代中酸性侵入体,其岩石成因和侵位期次对于认识该构造带的岩浆演化具有十分重要的意义.本文对库尔尕克希岩体进行了详细的野外地质调查、LA.-ICP-MS锆石U-Pb同位素测年及岩石地球化学分析,旨在查明其成岩时代和成因类型.结果表明,该岩体出露面积仅约0.55 km2,岩性主要为花岗闪长岩,锆石U-Pb年龄为312.9±2.9 Ma(n=20,MSWD=0.072),形成于晚石炭世早期.地球化学特征显示其为钙碱性弱过铝质Ⅰ型花岗岩.岩体具有较高硅、中等铝,富碱、贫镁,富集Rb、Ba、K等大离子亲石元素,强烈亏损Nb、Ta、Ti、P等高场强元素和重稀土元素,负Eu异常较微弱,A/CNK=0.99～ 1.20,与区内别鲁阿嘎希、包古图等小岩体在成岩时代、地球化学特征等方面有着显著的相似性,形成于晚石炭世早期与俯冲作用相关的岛弧环境,其源岩更可能是受俯冲流体交代的亏损地幔衍生出来的年轻地壳物质.%There are a large number of late Paleozoic intermediate to acid intrusions exposed along the tectonicmagmatism belt of western Junggar Basin,Xinjiang.It is of great significance to know the magmatic evolution of this tectonic-magmatism belt through the petrogenesis and emplacement periods.However,there are still no accurate isotopic chronologic and geochemical data of partial small rock bodies,such as Kuergakexi granite,which has restricted the understanding of their genetic types and emplacement ages.In this paper,the petrogenic age and genetic type of Kuergakexi granite were investigated through field geological survey,LA-ICP-MS zircon U-Pb isotopic dating and geochemical analysis.The results show that Kuergakexi granite is mainly composed of granodiorite with an outcrop area of 0.55 km2,and prove that the zircons yielded a LA-ICP-MS U-Pb age of 312.9 ± 2.9 Ma (n =20,MSWD=0.072),indicating that intrusive time of the granodiorite is the early Late Carboniferous.The values of major elements indicate that these granodiorites are characterized by high Si,intermediate A1 and rich alkali,and low Mg,with A/CNK ratios ranging from 0.99 to 1.20.The trace elements are enriched in large ion lithophile elements such as Rb,Ba,K,but strongly depleted in high field strength elements such as Nb,Ta,Ti,P and heavy rare earth elements,with slightly negative Eu anomalies (δEu =0.68～0.82).These features suggest that the Kuergakexi granite belongs to calc-alkaline series,weakly peraluminous Ⅰ-type granite.A comparison between Kuergakexi granite and other small intrusions such as Bieluagaxi granite and Baogutu granite shows that they have significant similarities in petrogenic age and geochemical characteristics.It is thus held that the Junggar Ocean continued its deep subduction from the south to the north during the early Late Carboniferous.The source of the granodiorite might have been the lower juvenile crust derived from the depleted mantle that had been metasomatized by fluids released from subduction slabs.