THE PUENTEMOCHA BERYL-PHOSPHATE GRANITIC PEGMATITE, SALAMANCA, SPAIN: INTERNAL STRUCTURE, PETROGRAPHY AND MINERALOGY

摘要

The Puentemocha pegmatite, located in the Tormes Dome of the Central Iberian Zone (Salamanca, Spain), is a beryl-phosphate granitic pegmatite hosted by a Variscan anatectic leucogranite. It shows a zonal internal structure with a wall zone, an intermediate zone, and a prominent quartz core. The essential minerals of the pegmatite are quartz, K-feldspar, plagioclase, muscovite, biotite and beryl. Phosphates, pyrite, arsenopyrite and rutile occur as accessory minerals associated with greisen zones developed in the intermediate zone. The gradational contact and mineral association attest to the intimate petrogenetic relation between the Puentemocha pegmatite and its host granite. Petrographic, mineralogical and chemical data suggest a crystallization from the border inward at temperatures in the range of 700-500°C, under f(O_2) conditions at AFMQ = ～-0.5, and H_2O contents of 6-7.5 wt% at the level of emplacement. As there is no evidence of residual melt extraction, the in situ fractionation of a peraluminous granitic melt is presumed to be sufficient to attain beryl saturation. The greisen bodies developed under subsolidus conditions, at the expense of the intermediate zone. A later episode of Na-metasomatism affected unevenly the primary zones of the pegmatite and the greisen bodies.