攀西地区二叠纪碱性岩成因及其铌钽成矿作用研究进展

Research Progress on Petrogenesis and Niobium-Tantalum Mineralization of Permian Alkaline Rocks in the Panxi Region

铌(Nb)、钽(Ta)元素是重要的战略性关键金属资源,碱性岩因富集铌钽等稀有、稀土元素而备受学者们关注。近年来,峨眉山大火成岩省内部带攀西地区发育的一系列二叠纪过碱性-准铝质-过铝质正长岩和花岗岩及其相关铌钽矿床在成岩成矿时代、地球化学特征、岩石成因及铌钽富集机制等方面取得了一些重要研究进展。主要认识有:在地幔柱轴部,地幔苦橄岩部分熔融形成峨眉山高钛玄武质岩浆,其在岩浆房中发生分离结晶和液态不混溶作用,形成小规模的A型花岗岩和碱性-准铝质正长岩。底侵的幔源岩浆对流为基性辉长岩部分熔融提供大量的热能,形成广泛分布的过碱性-准铝质正长岩和花岗岩。与此同时,少量新生地壳或扬子板块基底参与部分熔融过程则会形成过铝质花岗岩。研究表明,铌钽元素富集过程受岩浆结晶分异和热液交代两个演化阶段共同控制。源自地幔的碱性岩浆通过结晶分异作用优先结晶出斜长石、角闪石、磷灰石等矿物,导致Nb、Ta元素在正长岩体中得到初步富集。从岩体中出溶的富F-Na热液交代原岩,并发生钠长石化、霓石化、萤石化等蚀变作用。早期岩浆成因的铌钽矿物因交代而裂解,使Nb、Ta元素被重新活化、迁移、沉淀形成热液烧绿石和榍石。

Niobium(Nb)and tantalum(Ta)elements are significantly strategic key metal resources.Alkaline rocks have attracted considerable attention due to their enrichment of rare earth elements.In recent years,significant research progress has been made in Permian peralkaline-metaaluminous-peraluminous syenite and granite as well as associated niobium-tantalum deposits developed in the Panxi region,Emeishan Igneous Province,focusing on their geochronology and metallogenic ages,geochemical characteristics,petrogenesis,and Nb-Ta enrichment mechanisms.In the axis of the mantle plume,the Emeishan high-Ti basalt magma,formed by partial melted mantle peridotite,created small-scale A-type granite and alkalic-metaaluminous syenite in the magma chamber through fractional crystallization and liquid immiscibility.The underplating mantle-derived magma convection provided significant heat energy for the partial melting of basic gabbro,leading to the formation of widely distributed peralkaline-metaluminous syenite and granite.Simultaneously,peraluminous granite was formed when a small proportion of the juvenile crust or the Yangtze basement is involved in the process of partial melting.The research shows that the enrichment process of Nb-Ta is controlled by both magma crystallization differentiation and hydrothermal metasomatism.The alkaline magma from the mantle preferentially crystallizes plagioclase,amphibole,apatite,and other minerals through crystallization differentiation,leading to the initial enrichment of Nb and Ta elements in the syenite.The F-Na rich hydrothermal fluid from the syenite acts on rock by metasomatism,which results in alteration such as albitization,aegirinization,and fluorization.The early magmatic niobium-tantalum minerals were decomposed due to metasomatism,with the Nb and Ta elements being reactivated,migrated,and precipitated to form hydrothermal pyrochlore and titanite.