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

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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 200 m 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 (S2) 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 ˜3.5 kbar at 575 °C. For metapelites with intermediate XMg values (0.7 > XMg >0.48) a ‘muscovite-out window’ exists from 550–650 °C at 3.5 kbar in the KFMASH system. Conventional thermobarometry (GB-GASP, AvT-AvP) and petrogenetic grids show an isobaric P–T path to peak temperatures of ˜650 °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. 339 Ma) granitic intrusions into the dome core. As metamorphism is directly linked with the formation of the main S2 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 (315–305 Ma) – at least in the eastern Axial Zone.

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