Anfibolitas granatíferas de Sierra Albarrana (Córdoba). Termobarometría e implicaciones para el desarrollo del metamorfismo regional

Abstract

Garnet - bearing amphibolites outcroping within the La Albariza -Bembezar Mica Schist Formation (Sierra Albarrana), in the staurolite (+ kyanite) metapelite zone, contain a main assemblage (metamorphic peak conditions) constituted by hornblende + plagioclase + almandine + quartz + ilmenite. On the basis of textural characteristics and chemographic considerations in the SiO2 - Al2O3 - FeO - MgO - CaO - Na20 - H20 system, it can be deduced that textural and chemical equilibrium was attained during the crystallization of the amphibolitic assemblage, and similarly that this situation was only slightly modified by retrograde reactions. P - T equilibrium conditions of the peak assemblage have been estimated at T = 537 ± l5°C and P = 4.3 ± 0.5 kb (Fe - Mg / Fe - Mn exchange thermometry, and garnet – rutile – ilmenite – plagioclase - quartz /GRIPS/ and garnet - hornblende - plagioclase - quartz /GHPS/ equilibria). Significative differences between the obtained values for cores and rims of garnets were not observed. The Sierra Albarrana amphibolites show no evidence of any metamorphic stage higher in pressure than, that represented by the amphibolitic assemblage. The mineralogy included in porphyroblasts does not differ from that which appear in the matrix, and the general absence of rutile allows an estimation of P < 6.5 kb (GRIPS) for the baric limit of the P - T path. Therefore, the thermal and baric peaks must essentially coincide, keeping the same P - T values than those defined for the amphibolitic assemblage; this fact point to a model of homogeneous thickening of the crust for the part of the lberian Massif in which Sierra Albarrana is included. The tectonometamorphic evolution is similar to that seen in other amphibolite facies areas of the Iberian Massif, although the majority of comparable examples are characterized by higher pressure conditions. Taking into account the estimated P - T values, along with the moderately high gradient bound to the thermal peak (35°C/Km), the regional metamorphism of the Sierra Albarrana area must be considered within the lower baric range of the medium pressure metamorphism.