CONICET | Buscador de Institutos y Recursos Humanos

LOS PEGMATOIDES FOIDÍFEROS DEL CERRO LA MADERA, PROVINCIA DE CÓRDOBA: MINERALOGÍA Y PARAGÉNESIS ABSTRACT Feldespathoid-bearing pegmatites from La Madera volcanic plug are described and their paragenetic mineral assemblage interpreted. Pegmatites occur within a late Cretaceous nephelinite dome located southeast from Chaján, near the southwestern border of Córdoba province. Pegmatites are tabular to lenticular-shaped bodies of ijolitic composition ranging in length from a few centimeters up to more than 20 meters and varying from 0.01 to 1 meter in thickness. Both mineralogical and textural transitional changes are observed from the contacts toward the core. Mineralogy, still incomplete, adds up 19 specimens listed as follow: augite, nepheline, analcite, mesostasis, phillipsite, apatite, perovskite, sphene, biotite, amphibole (richterite-aenigmatite?), natrolite, calcite, chlorite, magnetite, hematite, maghemite, ilmenite, rutile and goethite. The most conspicous and unusual feature is the association of beautiful coarse-grained and perfectly developed augite, nepheline, analcite and other crystals coexisting with a weakly anisotropic glass-like groundmass. All but one of the required parameters to term these rocks as pegmatites fit properly. The exception is the coexistence of a low crystallinity mesostasis, originally a glass, of patchy interstitial distribution. A CO2, H2O-rich phase is thought to be involved in pegmatite origin which is visibly related to nephelinite magmatism. This first step, that includes mineralogy and paragenesis, expects to be completed with mineral chemistry, thermometric and stable isotope data. This information would permit a more thorough evaluation of the physico-chemical parameters that governed the genesis of these rocks. Such an approach could bring out valuable information about the nephelinite-ijolite-carbonatite petrological assemblage.

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