Two generations of zoned crystals of columbite-group minerals from granitic aplite–pegmatite in the Gouveia area, central Portugal

Abstract

Columbite-group minerals from dykes and sills composed of both aplite and rare-element-class pegmatite in the Gouveia area show different zonation patterns. The beryl–columbite-subtype and lepidolite-subtype pegmatites show two generations of these minerals. The first magmatic generation is oscillatory zoned and reversely zoned in the former subtype and progressively zoned, reversely zoned and oscillatory zoned in the latter subtype. The origin of this generation is attributed to fractional crystallization of biotite and muscovite of the parent granite magma. The second generation occurs as overgrowths in both subtypes and also as microveinlets that penetrate and replace the first generation in the lepidolite subtype. In general, they have a higher Ti content and Ta/(Ta + Nb) ratio and a lower Mn/(Mn + Fe) ratio than the first generation. The second generation is derived from Fe-Ta-rich early subsolidus fluids. The overgrowths are derived from Ta-richer and Fe-poorer hydrothermal fluids than the microveinlets. The distinct compositions of overgrowths in the two subtype pegmatites are due to the different compositions of hydrothermal fluids, which are richer in Ta and poorer in Fe in the lepidolite subtype than in the beryl–columbite subtype