P-T path and fluid evolution in the Franqueira granitic pegmatite, centralGalicia, northwestern Spain

In Galicia, in the northwestern part of the Iberian Peninsula, there are se veral occurrences of granitic pegmatites. One of them, known as Franqueira, is a metasomatic deposit of gem-quality chrpsoberyl, emerald and phenakite . The pegmatite is associated with a two-mica peraluminous Hercynian granit e, and intrude dunites of the Schistose domain in the Galician Tras-Os-Mont es Zone. The composition of fluid inclusions in emerald and phenakite has b een determined using a Raman microprobe to ascertain the paleofluid chemist ry and the P-T conditions of fluid migration in the pegmatite. Three types of fluid inclusions have been identified in emerald and phenakite; three ep isodes of fluid circulation are distinguished. The first episode is represe nted by low-salinity aqueous-carbonic primary fluid inclusions in phenakite ; these were trapped at 2.5 kbar and 400 degreesC. An isothermal drop in pr essure produced a second stage of fluid trapping under conditions of 400 de greesC and 1 kbar. Fluid inclusions trapped in emerald indicate that during this second stage, it formed from phenakite and chrysoberyl in the presenc e of a CO2- and CH4-rich fluid. A third episode of fluid circulation sugges ts an independent stage of fluid circulation during late tectonic events, w ith temperatures ranging from 160 degrees to 265 degreesC and pressure belo w 0.5 kbar.