Abstract

The Forcarei Sur granitic pegmatite field, in the northwestern part of the Iberian Peninsula, is an example of a zoned field whose bodies of pegmatite and pegmatite-aplite belong to different subtypes of the rare-element class. It occurs in the Galicia-Tras Os Montes zone, which consists of two domains: (1) schistose rocks and (2) granitic rocks, including two-mica peraluminous synkinematic granitic rocks. The pegmatites studied are located in the Schistose Domain, and their host rocks are the metasedimentary rocks of the Parano Group, Silurian to Devonian in age. The metasedimentary rocks were affected by three main phases of deformation of Hercynian age, accompanied by low-to medium-pressure metamorphism, in the greenschist to amphibolite facies. The pegmatites are located in the southern area of the western flank of the Forcarei synform. The two-mica peraluminous synkinematic granites were emplaced during the third tectonic phase and formed the Western Granitic Complex (WGC), which is the western limit of this pegmatitic field. Emplacement of the pegmatites also was synchronous with the third tectonic phase. The geochemical and mineralogical characteristics of these pegmatites, their internal structure and conditions of emplacement allow us to classify them as rare-element pegmatites. From lesser to a greater degree of fractionation, we distinguish: a) barren, b) beryl type, c) beryl-columbite-phosphate subtype, d) albite-spodumene type, and e) albite type. The more evolved groups are located farthest from the WGC, which we considered as the parental granite owing to its geochemical characteristics and close spatial association. These evolved pegmatites are enriched in Be, Li, Nb, Ta, Sn and P, which are concentrated as beryl, columbite-tantalite, tantalian cassiterite, apatite and montebrasite. All these minerals appear in albitic units or in the albite-rich bands from the layered bodies. Beyond these albite-rich zones, lithium can form spodumene, depending on phosphorus activity. Most of the phosphates found are metasomatic (montebrasite, apatite, eosphorite) and formed during the sodic metasomatism or in a late hydrothermal stage that altered the phosphorus-rich feldspar; other phosphates are secondary (hydroxyl-herderite and apatite). Based of fluid-inclusion data, the pegmatitic field formed at a minimum of 3 kbars and at 550 degrees C. A drop in pressure, from the lithostatic to hydrostatic conditions, may have occurred as a result of fluid circulation along the fractures generated by a shear zone developed at the end of the third tectonic phase.

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