Metamorphic evolution of anthophyllite/cummingtonite-cordierite rocks from the upper unit of the Ordenes Complex (Galicia, NW Spain)

Two types of low-Ca amphibolites are described for the first time in the Ordenes Complex (Galicia, NW Spain). The first type exhibits an initial mineral assemblage containing sillimanite, staurolite and anthophyllite, which are found as microinclusions in cordierite crystals. This association indicates medium pressure and temperatures above 600 degrees C, and is replaced in the matrix by a medium to low-pressure assemblage of anthophyllite, cordierite and garnet. The second type is represented by low-pressure amphibolitic gneisses formed mainly of cummingtonite, cordierite, andalusite and garnet. The first type of amphibolite would appear to give rise to the second type as a result of metamorphic evolution to lower pressures. A fragment of the decompressive P-T path was obtained by a detailed study of the mineral assemblage succession and the textural relationships of these rocks. The P-T path begins with an almost isothermal decompression from ca. 6-7 kbar and 650 degrees C. This kind of P-T path (isothermal) is usually interpreted, according to thermal models, as originating during exhumation favoured by some tectonic process. Hence, the exhumation of the uppermost unit of the Ordenes Complex, in which the anthophyllite-cummingtonite rocks are located, appears to have been facilitated by some kind of extensional tectonic process.