COMPLEX INTERNAL TEXTURES IN OXIDE MINERALS FROM THE NANPING No. 31 DYKE OF GRANITIC PEGMATITE, FUJIAN PROVINCE, SOUTHEASTERN CHINA

摘要

The Nanping No. 31 granitic pegmatite, a highly evolved spodumene-type pegmatite in the Nanping pegmatite district, southeastern China, can be divided into five internal zones from the border inward: zones I-III as the outer zones and zones IV-V as the inner zones. It is strongly mineralized with Nb-Ta-Sn oxide minerals. We investigated these minerals by using back-scattered electron (BSE) imaging and quantitative electron-microprobe analyses (EMPA) to characterize their complex internal textures. The primary Nb-Ta oxide minerals show a sequence from columbite-(Fe) (zone I) through tantalite-(Fe) (zone II) to tantalite-(Mn) (zone III), but distinctly become columbite + wodginite in zone IV and, rarely, columbite in the zone V. Microscopic tapiolite-(Fe) is identified as a product of exsolution in cassiterite from the zones II-III, whereas complex associations of wodginite, columbite-tantalite and tapiolite-(Fe) are characteristic of cassiterite in zone IV. The classic variation of the columbite-group minerals from the zones I to III seems to result from the greater stability of tantalite than columbite components during fractional crystallization of the melt. However, decreasing values of Ta/(Nb + Ta) in columbite-group minerals in the inner zones are more probably compensated by crystallization of abundant primary wodginite-group minerals, controlled by the f(O_2) of the magma. Columbite-group minerals commonly exhibit zoning, but complex and multiple-generation phenomena are notable in columbite-tantalite crystals from zones IV-V. In the latter case, later generations of columbite-tantalite overgrow or crosscut the earlier ones. Irregular patterns of zoning are linked to the activity of hydrothermal fluids at the late stages of pegmatite evolution. The high mobility of Ta in such fluids is supported by the development of veinlets of tantalite. They cross-cut first the earlier-formed columbite + wodginite aggregates, and later fill brittle-deformation-induced fissures in the columbite, forming a network texture. The complex internal textures of Nb-Ta-Sn oxide minerals from the zones IV-V indicate a distinctly fluid-rich environment at the late stage of crystallization of the Nanping No. 31 pegmatite.