Abstract

Metasedimentary rocks of the Blanca unit, previously reported as roof pendants overlying the Ronda peridotite (Betic Cordilleras, southwest Spain), have been reinterpreted on the basis of field evidence to underlie this ultramafic body.

Textural analysis of Blanca rocks reveals that they have experienced first mechanical degradation and later static metamorphism. Blastomylonites several hundred metres below the base of the peridotite show fluxion banding overprinted by the metamorphic assemblage crd + bio + sil + ksp + pla + qtz. In the intervening layer, the mylonitic texture has been totally annealed to form a medium-grained, nonfoliated gneiss of the same mineralogy (hornfels of some authors). Lithic fragments in the gneiss show mineralogical and textural similarities with underlying blastomylonites and are interpreted as resistant blocks in a subsequently metamorphosed tectonic breccia or mélange.

The structural position of the peridotite together with the mylonitic history of the Blanca rocks is most satisfactorily explained by the tectonic emplacement of the peridotite over the Blanca unit along a major thrust fault. The distribution of cordierite-bearing gneiss in a layer directly beneath the ultramafic sheet suggests that the peridotite was sufficiently hot to cause metamorphism at a temperature between 700 and 800 °C in an aureole at its base. To account for pressures of about 3.5 kb indicated by the cordierite assemblages, the metamorphic complex that overlies the peridotite (Casares unit) is interpreted as cover riding passively on the ultramafic sheet during thrusting.

The interpretation of Ronda peridotite as part of a thrust sheet suggests that it has participated in Alpine nappe tectonics in the Betic Cordilleras. The magnitude and direction of apparent tectonic displacement are similar to those of nappes in the western end of this foldbelt.

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