MULTISTAGE GROWTH OF A RARE-ELEMENT, VOLATILE-RICH MICROGRANITE AT ARGEMELA (PORTUGAL)

The small Argemela microgranite body in central Portugal displays many of the mineralogical and chemical features characteristic of peralumi nous, Li, P-rich, rare-element pegmatites. Its mineralogy consists pre dominantly of quartz, albite, white mica (partly replaced by lepidolit e) and a phosphate of the amblygonite series. K-feldspar is noticeably absent or scarce. Cassiterite, beryl and columbite are the main acces sories. The microgranite shows extreme enrichment in incompatible elem ents 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 mic rogranite overall is interpreted as a mixture of two components: a cry stal 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, alb ite, skeletal lepidolite and amblygonite). This second component has t he mineralogical and chemical characteristics of-rare-element pegmatit es. All these petrological characteristics are magmatic. Only a few, n arrow cross-cutting veinlets with quartz K-feldspar and F-poor amblygo nite are considered as fluid derived. A model of crystallization in su ccessive steps is proposed where concentration in fluxing agents (F, L i, P, etc.) is progressively enhanced up to saturation with the crysta llization of magmatic lepidolite and amblygonite.