大兴安岭北段富克山岩浆弧的组成:对蒙古—鄂霍茨克洋南向俯冲的制约

富克山岩浆弧呈北东向分布于大兴安岭北段富克山—古莲河一带,其物质组成、形成时代及其空间展布规律对于研究蒙古—鄂霍茨克洋晚三叠世地质构造演化具有重要意义。本文对其内的辉长岩、闪长岩、花岗闪长岩进行了岩石学、地球化学及锆石U-Pb定年分析研究,探讨大兴安岭北段晚三叠世时期的构造背景。LA-ICP-MS锆石U-Pb定年获得辉长岩、花岗闪长岩的结晶年龄分别为(205.7±2.2) Ma、(203.2±2.5) Ma,反映了晚三叠世的构造岩浆事件。岩石学和地球化学研究表明,辉长岩、闪长岩具有富钠、高铝、高钙、高镁、高Mg^#值、低钛等特征,属于拉斑系列与钙碱性过渡系列岩石,无明显的Eu负异常,相对富集LILE和LREE,亏损HFSE。花岗闪长岩具有高硅、富钠、高铝、低镁等特征,属于钙碱性系列岩石,高Sr,低Y、Yb,无明显的Eu负异常,显示了O型埃达克质岩石的地球化学特征。富克山岩浆弧的空间展布,具有由北向南的分布规律,指示了蒙古—鄂霍茨克洋具有往南的俯冲极性。

Composition of the Fukeshan magmatic arc in the northern Great Xing'an Range:Constraints on the southward subduction of the Mongol-Okhotsk oceanic plate

The Fukeshan magmatic arc extends north-east in Fukeshan-Gulianhe area, northern Great Xing’an Range. Its material composition, formation age and spatial distribution pattern are very important for studying the Late Triassic tectonic evolution of the Mongol-Okhotsk ocean. In this paper, the petrology, geochemistry and zircon U-Pb dating of gabbro, diorite and granodiorite from the Fukeshan magmatic arc are reported, aiming to furthering our understanding of the tectonic setting of the northern Great Xing’an Range during the Late Triassic. LA-ICP-MS zircon U-Pb dating yielded the crystallization ages of (205.7±2.2) Ma and (203.2±2.5) Ma for gabbro and granodiorite, respectively, which correspond to the tectonic-magmatic event in the Late Triassic. Petrological and geochemical study indicate that the gabbro and diorite belong to tholeiite and calc-alkaline transition series rocks with high Na, Al, Ca, Mg and Mg^# contents and low Ti contents. They are enriched in large ion lithophile elements (LIFE) and light rare earth elements (LREE). The granodiorite belongs to calc-alkaline transition series rocks with high Si, Na and Al contents and low Mg contents. The rock has low content of Y and Yb, with no obvious negative Eu anomaly and belongs to O-type adakite. The Fukeshan magmatic arc has a distribution pattern from north to south, and the corresponding lithologic variation indicates the Mongol-Okhotsk ocean has a polarity of southward subduction.