Reaction textures in calc-silicate gneisses from the Tormes gneiss complex: implications for the retrogade P-T-XC02 path

Abstract

[Abstract] Calc-silicate gneisses from the LowerUnit ofthe Tormes Gneiss Complex,NW Salamanca, exhibit a sequence of reaction textures that have been used to elucidate their retrograde P-T-XC02 path. The highest temperature recorded in the calc-silicates is represented by the wollastonite- and scapolite- (meíoniterich) bearing assemblages which yield at least 7500 C at 6 kbar based on experimental results. The calc-silicates have partially re-equilibrated at lower temperatures (down to 4500 C) as evidenced by the successive reactions: (1) . Wo + Scp + Cal ~ Grt + CO2 ; (2) Wo + CO2~ Cal + Qtz; (3) Scp ~ PI + Cal + Qtz; (4) Grt + CO2 + H20 ~ Ep/Clz + Cal + Qtz ±PI; y (5) Cpx + CO2 + H20 ~ Tr + Cal + Qtz. The cooling down to lower temperatures of4500 C (at P<3 kbar), in the andalusite stability field, is also suggested by the secondary andalusite replacement ofprevious fibrolitic sillimanite in adjacent metapelites. In this paper, mineral devolatilization equilibria are used to constrain the composition ofboth syn- and post-peak-D2 fluids in the deepest exposed levels of the metamorphic complexo The results indicate that during the transitional granulite facies metamorphism, large-scale CO2 advection did not occur