Natural metastable reactions involving garnet, staurolite and cordierite: implications for petrogenetic grids and the extensional collapse of the Betic-Rif Belt |
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Authors: | Antonio García-Casco Rafael Luis Torres-Roldán |
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Institution: | (1) Departamento de Mineralogía y Petrología, Universidad de Granada, Fuentenueva s/n, E-18002 Granada, Spain; E-mail: agcasco@goliat.ugr.es, ES |
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Abstract: | Medium grade metapelites of the Torrox unit (Betic-Rif Belt, S Spain) contain mineral assemblages consisting of garnet (Grt),
staurolite (St), cordierite (Crd), biotite, kyanite, sillimanite, andalusite, muscovite (Ms) and quartz (Qtz) and record complex
reaction processes of cordierite growth through garnet and staurolite decomposition. The reaction textures, the chemical composition
of the reactant and product phases, including Fe-Mg-Mn partitioning, and the results of equilibrium thermodynamic calculations
indicate that these cordierite-bearing assemblages are largely deviated from equilibrium. Furthermore, the actual cordierite-forming
reactions, as estimated from the assemblage and associated textures, conflict with the predictions of thermodynamically based
petrogenetic grids for the model pelite system KFMASH, either those that predict the stable coexistence of cordierite + muscovite
plus garnet or staurolite or those that do not foresee a field of stability for these types of assemblages. This conflict
is explained in terms of cordierite growth (at ca. 575 °C and 2.5 kbar) through metastable reactions whose operation was conditioned
by the relict persistence of higher pressure phases (garnet and staurolite) and phase compositions (e.g. muscovite 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 composition (i.e. poor in Zn and/or Mn). The triggering
of metastable cordierite-forming reactions and the preservation of even subtle disequilibrium features associated to them
indicate that the rocks underwent fast near-isothermal decompression from ca. 12 kbar down to 2–3 kbar, then rapid cooling.
These inferences agree with independent evidence indicating that termination of alpine metamorphism in the western Betic-Rif
Belt was related to the extensional collapse of thickened crust and that the latter had consisted of a single, continuous
event.
Received: 6 August 1998 / Accepted: 9 February 1999 |
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