Natural metastable reactions involving garnet, staurolite and cordierite: implications for petrogenetic grids and the extensional collapse of the Betic-Rif Belt

Medium grade metapelites of the Terror unit (Betic-Rif Belt, S Spain) conta in mineral assemblages consisting of garnet (Grt), staurolite (St), cordier ite (Crd), biotite, kyanite, sillimanite, andalusite, muscovite (Ms) and qu artz (Qtz) and record complex reaction processes of cordierite growth throu gh garnet and staurolite decomposition. The reaction textures, the chemical composition of the reactant and product phases, including Fe-Mg-Mn partiti oning, and the results of equilibrium thermodynamic calculations indicate t hat these cordierite-bearing assemblages are largely deviated from equilibr ium. Furthermore, the actual cordierite-forming reactions, as estimated fro m the assemblage and associated textures, conflict with the predictions of thermodynamically based petrogenetic grids for the model pelite system KFMA SH, either those that predict the stable coexistence of cordierite + muscov ite plus garnet or staurolite or those that do not foresee a field of stabi lity for these types of assemblages. This conflict is explained in terms of cordierite growth (at ca. 575 degrees C and 2.5 kbar) through metastable r eactions whose operation was conditioned by the relict persistence of highe r pressure phases (garnet and staurolite) and phase compositions (e.g. musc ovite and biotite) after fast decompression. This interpretation militates against the existence of a wide P-T range of stable coexistence at low P of Crd + Ms + Qtz +/- Grt +/- St in medium grade metapelites of normal compos ition (i.e. poor in Zn and/or Mn). The triggering of metastable cordierite- forming reactions and the preservation of even subtle disequilibrium featur es associated to them indicate that the rocks underwent fast near-isotherma l decompression from ca. 12 kbar down to 2-3 kbar, then rapid cooling. Thes e inferences agree with independent evidence indicating that termination of alpine metamorphism in the western Betic-Rif Belt was related to the exten sional collapse of thickened crust and that the latter had consisted of a s ingle, continuous event.