Mineralogía y Geotermometría de los diques ácidos con grafito asociados a rocas ultramáficas de la Serranía de Ronda. Málaga

Abstract

A geological, mineralogical and geothermometrical study of a group of acidic graphitebearing dikes from the Ronda ultramafic massif (Málaga province) has been carried out. These dikes are hosted in serpentinized peridotites and they appear near the eastern and southern contacts of the massif. On the basis of their mineralogical features, two rock types have been distinguished, since they show or not hydrothermal alteration processes. Unaltered dikes present a mineral association composed by cordierite, garnet, quartz, biotite, graphite, Fe·Ti oxides, sulfides and minor amounts of plagioclase, sillimanite, hercynite and zircon. In the hydrothermally transformed dikes, this original mineral assemblage has been more or less erased, occuring "pinitization" of cordierite, chloritization of garnet and biotite, and ilmenite is transformed to "Ieucoxene". Graphite is a main mineral in both the unaltered and transformed dikes, ranging their abundance about 15%. The geothermometrical study has been done using the biotite-garnet pair and the value of graphite "c" parameter. The estimated temperatures are clase to 800º C, that are in good agreement with the given temperature for the emplacement of the ultramafic body. From all these data, formation of the dikes by anateetic partial melting of the sedimentary wall rocks (belonging to the Blanca Unit), in presence of a CO₂- and CH₄ rich fluid that migrated from the metamorphosed sediments towards low pressure fractures zones inside the peridotite massif, is proposed.