P T PATH AND FLUID EVOLUTION IN THE FRANQUEIRA GRANITIC PEGMATITE, CENTRAL GALICIA, NORTHWESTERN SPAIN

Abstract

In Galicia, in the northwestern part of the Iberian Peninsula, there are several occurrences of granitic pegmatites. One of the m, known as Franqueira, is a metasomatic deposit of gem-quality chrysoberyl, emerald and phenakite. The pegmatite is associated with a two-mica peraluminous Hercynian granite, and intrude dunites of the Schistose domain in the Galician Tras-Os-Montes Zone. The composition of fluid inclusions in emerald and phenakite has been determined using a Raman microprobe to ascertain the paleofluid chemistry and the P–T conditions of fluid migration in the pegmatite. Three types of fluid inclusions have been identified in emerald and phenakite; three episodes of fluid circulation are distinguished. The first episode is represented by lowsalinity aqueous-carbonic primary fluid inclusions in phenakite; these were trapped at 2.5 kbar and 400 °C. An isothermal drop in pressure produced a second stage of fluid trapping under conditions of 400 °C and 1 kbar. Fluid inclusions trapped in emerald indicate that during this second stage, it formed from phenakite and chrysoberyl in the presence of a CO 2- and CH4-rich fluid. A third episode of fluid circulation suggests an independent stage of fluid circulation during late tectonic events, with tempera tures ranging from 160° to 265°C and pressure below 0.5 kbar.