1. Introduction The initial whole-rock chemical composition required to develop a metamorphic assemblage including antho- phyllite and cordierite does not correspond to any common type of igneous or sedimentary rock. Therefore, this mineral assemblage is usually considered to result from the metamorphism of rocks with a bulk composition modified by some metasomatic process, which could be either pre- or syn-metamorphic (Robinson et al., 1982). The rocks containing this type of paragenesis, commonly known as Ca-poor amphibolites (Spear, 1993 and references therein), are characterized by a strong depletion in alkalis and Ca compensated by an enrichment in Mg and Al. Although these rocks have a variable origin, which is nevertheless clearly metasomatic, most of them probably derive from basic rocks altered by a pre-metamorphic hydrothermal process (Schumacher, 1988; Smith et al., 1992; Araujo et al., 1996). Another common feature of low-Ca amphibolites is their close association with massive sulphide deposits. This kind of deposit very commonly shows a strong chloritic alteration of the basic rocks related to metasomatism and mineralization, which suggests that the low-Ca amphibo- lites associated with sulphide deposits could be developed during the metamorphism of previously altered basic rocks (e.g. Araujo et al., 1996; Elliot-Meadows et al., 2000). This contribution is focused on the study of the miner- alogy and P-T conditions of formation of two types of low- Ca amphibolites from the upper unit of the Ordenes Complex. Although their possible association with sulphide deposits has not been studied, this relation has been clearly shown in other parts of the Ordenes Complex (Badham & Williams, 1981). The first type of amphibolites can be described as amphibolitic gneisses with anthophyl- lite + cordierite ± sillimanite. The coexistence in some textural domains of orthoamphibole and sillimanite indi- cates that metamorphic evolution took place at least under medium pressure, and the presence of only one hypersolvus orthoamphibole indicates T > 600ºC (Robinson et al., 1982; Spear & Rumble, 1986). The second type are amphi- bolitic gneisses with cummingtonite + cordierite ± andalusite. This mineral association is considered as indicative of low-pressure metamorphic conditions (Spear & Rumble, 1986; Early & Stout, 1991; Labotka & Kath, 2001), which is confirmed by the occasional presence of andalusite. The close spatial relationship of the two types of low-Ca amphibolites suggests a common origin, and the Eur. J. Mineral. 2005, 17, 57-68 Metamorphic evolution of anthophyllite/cummingtonite-cordierite rocks from the upper unit of the Ordenes Complex (Galicia, NW Spain) JACOBO ABATI* and RICARDO ARENAS Departamento de Petrología y Geoquímica, Universidad Complutense, E-28040 Madrid, Spain Abstract: 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 micro- inclusions in cordierite crystals. This association indicates medium pressure and temperatures above 600ºC, 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 decom- pressive 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 650ºC. 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. Key-words: metamorphic evolution, anthophyllite, cummingtonite, cordierite, low-Ca amphibolites, Ordenes Complex, NW Spain. 0935-1221/05/0017-0057 $ 5.40 © 2005 E. Schweizerbart’sche Verlagsbuchhandlung. D-70176 Stuttgart DOI: 10.1127/0935-1221/2005/0017-0057 *E-mail: abati@geo.ucm.es