Petrogenesis of Nb–(Ta) aplo-pegmatites and fine-grained granites from the Early Cretaceous Huangshan rare-metal granite suite, northeast Jiangxi Province, southeast China

Abstract

The Early Cretaceous (ca. 130 Ma) Nb-rich and Ta-poor Huangshan suite, South China, includes a series of medium-grained granites that contain Nb-rich mica and very low columbite contents, which are indicative of a potential Nb resource. In addition, the Huangshan suite contains fine-grained granites and aplo-pegmatites in which disseminated Nb–Ta columbite is an ore. Two types of fine-grained microcline-bearing and albite-rich leucogranite sills and dikes were identified. Sills and dikes of leucogranites and aplo-pegmatites intrude the medium-grained granites and surrounding hornfels. The aplo-pegmatites consist of large microcline and quartz crystals that are typically broken and embedded within a foliated albite-rich matrix. Niobium-rich and Mn-poor columbite with Ta# [Ta/(Ta + Nb)] ≤ 0.5 and Mn# [Mn/(Mn + Fe)] < 0.2, which is characterized by complex zonation, is the major accessory mineral in the fine-grained granites and aplo-pegmatites. The Huangshan columbites display an unusual pattern of Ta# variation at nearly constant Mn#. Fractional crystallization along cannot explain the observed trends, but they can be accounted for by recurrent mixing between two distinct and independently evolving magmas. One end-member is deemed to be an evolved Nb-rich magma, and the other is a Ta-enriched magma of the peraluminous rare-metal granite series (i.e., Yichun trend). Major and trace element features also support this model of recurrent magma mixing.