Non-typical fractionation behaviors in a lepidolite-subtype pegmatite: Implications for the internal evolution of a naturally fluxed system

Abstract

Despite their relatively rare occurrence, lepidolite-subtype pegmatites host abundant Li–Nb–Ta–Cs–Sn mineralization and represent a high-flux pegmatitic system with abnormally high F and Li activity. Characterization of highly fluxed melts and the impact of fluxes and exsolved fluids on fractionation of peraluminous melts have mainly been studied in experimental systems, with natural system correlations remaining poorly understood. Consequently, we conducted a systematic mineralogical study of a lepidolite-subtype pegmatite in the North Qinling orogenic belt, Central China. An abnormal “concave downward” fractionation trend for primary columbite-group minerals on the quadrilateral diagram is identified, and irregularly zoned columbite crystals coexist with F-rich minerals in one of the core zones have the highest Ta contents (normally 50.17–63.13 wt% Ta₂O₅) and Ta/(Nb + Ta) ratios (up to 0.65). Despite the consistently Ta-dominated B-site in the crystal lattice of microlite-group minerals, extreme compositional variations at the A- and Y-sites are observed. Compared with microlites in intermediate zones, the abrupt increase in U in microlite crystals in core zones and late units (up to 20.16 wt% UO₂), is ascribed to the melt-fluid interaction with exsolved U-rich aqueous fluids. In addition, the fractional crystallization of F-bearing minerals resulted in a gradual decrease in F contents in microlite-group minerals from extremely F-rich (2.68–4.84 wt% F) in intermediate zones to low F species (mainly 0.82–1.71 wt% F) in core and late zones. Moreover, autometasomatism by a late fluxed melt and hydrothermal metasomatism by late aqueous fluids are identified in columbite- and microlite-group minerals. This work highlights that these non-typical fractionation behaviors related to the activity of fluxes (especially F) and the exsolution of aqueous fluids during the internal evolution of pegmatitic melts, are critical for the generation of lepidolite-subtype pegmatites. Fluorine was gradually enriched in the pegmatitic melt, and reached its highest level during crystallization of the (inner) intermediate and core zones. Non-equilibrium crystallization occurred throughout pegmatite evolution, and late units were most probably formed from aqueous fluid-enriched residual melts, rather than by hydrothermal replacement.