Abstract
A geological, mineralogical and geothermometrical study of a group of acidic graphitebearing dikes from the Ronda ultramafic massif (Malaga 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 CO 2 - and CH 4 rich fluid that migrated from the metamorphosed sediments towards low pressure fractures zones inside the peridotite massif, is proposed.
Highlights
On the basis of their mineralogical features, two rock types have been distinguished, since they show or not hydrothermal alteration processes
The estimated temperatures are clase to 8000 C, that are in good agreement with the given temperature for the emplacement of the ultramafic body
From aH these data, formation of the dikes by anateetic partial melting of the sedimentary waH rocks, in presence of a CO2- and CHe rieh fluid that migrated from the metamorphosed sediments towards low pressure fractures zones inside the peridotite massif, is proposed
Summary
Los diques ácidos con grafito estudiados están localizados en el borde oriental del macizo ultramáfico de Ronda, cerca de la localidad de Istán, y en el borde sur de este mismo macizo, al norte de Estepona (Fig. 1). Asociados espacialmente a estas rocas ultrabásicas aparece un grupo de diques ácidos de composición y origen variable, que intruyeron durante o tras el emplazamiento del macizo (Dickey et al, 1979). Estos diques se caracterizan por presentar importantes cantidades de grafito, cuya proporción en el total de la roca oscila entre el 10 y el 15%, aunque localmente se pueden alcanzar concentraciones mayores. Generalmente estos minerales no muestran una orientación preferente, si bien en ocasiones pueden estar orientados paralelos a las paredes de los diques. De esta forma se producen áreas diferentemente transformadas de gran complejidad mineralógica, en las que todavía se observan, en muchos casos, restos de los minerales originales
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