Aplicación de los geotermometros y geobarométros de cordierita—granate y cordierita—biotita al discernimiento de la evolución del proceso anatectico en el complejo migmatitico de la unidad de (tipo) blanca. (Zona Bética, N de Estepona, Andalucía)

Abstract

The migmatite complex in the western part of the (type) Blanca unit (North ot Estepona) is composed of three distinct lithotypes. Two of them consist in granitic anatectic mobilisâtes (types III and II), of which the second (type II) contains abundant restitic material in the form of discrete inclusions (aluminous enclaves; lithotype I). The petrological analysis of the complex suggest that the differentiation of these lithotypes resulted from a fractionated melting process under plurifacial conditions. The thermal maximun of the event was probably reached in the moment in which th granitic mobilisate rich in enclaves was generated. The corresponding reaction involved the uncongruent breakdown of biotite (plus sillimanite, plus Q) under PH2O < Ptotal, which was accompanied by the individualization of the enclosed aluminous enclaves. With few exceptions, the non-fusible fraction (composed of two divariant assemblages: co-gar-sill and co-gar-bi) has been preserved only in these enclaves, as long as, in the mesostasis, it was nearly completely destroyed by further reaction with the melt. The study of phase relations both in the enclaves and in the mobile fraction, leads to the conclusion that the change in mineral assemblages between these two stages was due to an increase in the water saturation of the melt, caused by a concomitant descent of Ptotal under nearly isothermal conditions.