Stable staurolite-cordierite assemblages in K-poor metapelitic schists in Aston and Hospitalet gneiss domes of the central Pyrenees (France, Andorra)

Abstract

Staurolite-cordierite assemblages are common in mica schists of the Aston and Hospitalet gneiss domes of the central Axial Zone, Pyrenees (France, Andorra). Within a 200m wide zone, staurolite, cordierite and andalusite porphyroblasts contain inclusion trails that preserve the same stage of development of a crenulation cleavage, strongly suggesting that all three phases are contemporaneous. Their syntectonic growth occurred during a short period at the beginning of the formation of the dominant schistosity (S-2) of the domes. Staurolite and cordierite touching each other further indicates an equilibrium relationship. Whole-rock analyses show that some staurolite-cordierite schists are depleted in K2O compared to post-Archean shales (PAAS) and amphibolite facies pelites. Analysis of the st-crd paragenesis in K-poor schists without muscovite using KFMASH and MnNCKFMASH petrogentic grids, pseudosections and AFM compatibility diagrams predicts stable conditions at pressures of similar to 3.5kbar at 575 degrees C. For metapelites with intermediate X-Mg values (0.7> X-Mg >0.48) a muscovite-out window' exists from 550-650 degrees C at 3.5kbar in the KFMASH system. Conventional thermobarometry (GB-GASP, AvT-AvP) and petrogenetic grids show an isobaric P-T path to peak temperatures of similar to 650 degrees C, supported by the presence of sillimanite-K-feldspar gneiss and migmatites. LP-HT metamorphism in the Aston dome is related to early Carboniferous (c.339Ma) granitic intrusions into the dome core. As metamorphism is directly linked with the formation of the main S-2 schistosity, the temporal relations demonstrated in this study conflict with previous studies which constrained LP-HT metamorphism and the development of flat-lying schistosity to the late Carboniferous (315-305Ma) - at least in the eastern Axial Zone.