Multistage growth of a rare-element, volatile-rich microgranite at argemela (Portugal)

Abstract

The small Argemela microgranite body in central Portugal displays many of the mineralogical and chemical features characteristic of peraluminous, Li, P-rich, rare-element pegmatites. Its mineralogy consists predominantly of quartz, albite, white mica (partly replaced by lepidolite) and a phosphate of the amblygonite series. K-feldspar is noticeably absent or scarce. Cassiterite, beryl and columbite are the main accessories. The microgranite shows extreme enrichment in incompatible elements such as F, P, Rb, Cs, Li, Sn and Be, and extreme depletion in Sr, Ba, Zr and REE. It is highly sodic and strongly peraluminous. The microgranite overall is interpreted as a mixture of two components: a crystal mush injected from below (seen in narrow dykes intersected during drilling, composed of quartz, albite and phengite) and interpreted as 'feeders', overprinted by a second highly evolved component dominated by Li, F, P (Rb, Cs, Be, Sn, Nb, Ta, etc.) considered as a 'lubricant' medium for the ascending mush and occasionally quenched (quartz, albite, skeletal lepidolite and amblygonite). This second component has the mineralogical and chemical characteristics of rare-element pegmatites. All these petrological characteristics are magmatic. Only a few narrow cross-cutting veinlets with quartz, K-feldspar and F-poor amblygonite are considered as fluid derived. A model of crystallization in successive steps is proposed where concentration in fluxing agents (F, Li, P, etc.) is progressively enhanced up to saturation with the crystallization of magmatic lepidolite and amblygonite.