CONICET | Buscador de Institutos y Recursos Humanos

El Buitre Formation gabbroids outcrop at the southern portion of the North Patagonian Massif, in an area between 42°30´-43°00? S latitude to 67°20´-68°20´ W longitude, within the Chubut Province, Argentina. It consists of numerous small bodies scattered through the Chacays sierra that emplaced during Paleogene times, intruding and covering upper cretaceous Cerro Barcino and La Colonia Formations. El Buitre Formation is composed of fine- to medium-grained monzogabbros, syenites, and their foidic varieties. Their principal constituents are plagioclase, clinopyroxene, olivine, and opaque minerals, followed by variable amounts of alkali feldspar, nepheline and zeolites, and accessory biotite, aenigmatite, apatite, and zircon. In most samples, plagioclase, alkali feldspar and nepheline are variably replaced by zeolites (analcime, natrolite). Early stages involve the development of systematic dissolution scars infilled with analcime. Later comes veining by hydraulic fracturing and dissolution-precipitation of analcime or natrolite, resulting in jigsaw patterns due to volume expansion. Finally, large interstitial analcime masses and fibrous natrolite pockets replace most of the felsic phases. Circulation of CO2 and Na-rich aqueous solutions has been proposed by Zalba et al. (2011), who argue that El Buitre magmas released those components to Cerro Barcino tuffs, resulting in the precipitation of dawsonite cement. The dissolution scars in plagioclase could thus be the consequence of Na leaching to the host rock. The development of aegirinic rims around the Ti-Aug could be linked to this process too. Although the zeolitization has been regarded as a deutheric stage after El Buitre´s emplacement, Eocene to Oligocene mafic dykes that intrude these bodies should also be considered as a possible source of hydrothermal fluids.Sponsored by UBACYT 20020170100554BA & PIP11220130100579CO