Origin of layered gabbroic intrusions and their giant Fe-Ti-V oxide depsouts in the Pan-Xi district, Sichuan Province, SW China

Abstract

The Emeishan Large Igneous Province comprises voluminous flood basalts and spatially associated mafic-ultramafic intrusions. In the Pan-Xi region, SW China, a number of gabbroic intrusions are dated at ~ 260 Ma, same as the previous reported ages for the ELIP, and host some giant Fe-Ti-V oxide deposits. Unlike the oxide deposits in classic layered intrusions, such as the Bushveld Complex , those in the Pan-Xi region occur as layers and/ or lenses within the gabbros and are generally concentrated in the lower parts o f the intrusions. Available geochemical evidence suggests that the intrusions were formed from highly evolved Fe-Ti-V rich ferrogabbroic or ferropicritic magmas. T he magmas were further enriched in Fe-Ti-V after separation of Si-rich magmas which formed the syenitic intrusions. Ore textures and associated miner al assemblages indicate that the ore-bodies were formed by late-stage crystallization of V-rich titano magnetite from oxide liquids. The abundant accessory hydrous phases suggest that addition of fluids from upper crustal rocks might have induced the separation o f the immiscible oxide melts and subsequently gave rise to the oxide ore-bodies.

峨嵋山大巖漿省由大量的溢流玄武巖及其伴生的鎂鐵和超鎂鐵侵入巖組成。攀西地區的一些層狀輝長巖體形成于260Ma,與早期報道的峨嵋山大巖漿省的年代相同。這些巖體中含有巨大的釩鈦磁鐵礦床,礦體呈似層狀及透鏡狀產在層狀輝長巖體的下部層位,不同于典型的層狀巖體(如布什維爾德巖體)的磁鐵礦床。地球化學資料表明,攀西地區含磁鐵礦的巖體是從高度演化的基性巖漿中結晶而成,因為富硅的巖漿分離使得母巖漿高度富集鐵、鈦和釩。相對圍巖來說,磁鐵礦石形成較晚,是從氧化物礦漿中結晶的產物。礦石中有豐富的含水礦物相,流體的參與對氧化物礦漿的形成有重要的作用。