Polycyclic metamorphic evolution of the Sierra Albarrana Schists (SW Iberian Massif): From low-pressure Ordovician rifting to medium-pressure Variscan overprint

This study presents new information on the polycyclic metamorphic evolution of the Sierra Albarrana Domain (SW Iberian Massif) obtained through a comprehensive assessment of mineral assemblages and petrological modelling (P-T-X pseudosection approach). The pelitic schists in this domain depict the uncommon paragenesis St-Grt-And together with And-Grt-Sil-bearing schists. The first assemblage has been accurately reproduced by pseudosection modelling using the whole rock composition directly analysed by XRF. In contrast, early attempts to model And-Grt-Sil-bearing schists were not as successful. Fe3+ can be a critical element influencing the phase relations in pelites. For this reason, we investigated the influence of the effective amounts of Fe3+ to estimate the oxidation estate during the metamorphism by computing a series of P/T-X pseudosections to explore whether somewhere in P-T-X space the matrix association can be reproduced. The Sierra Albarrana Domain recorded a regional low-pressure metamorphic event accompanied by a complex structural evolution, probably related with the Middle Cambrian-Lower Ordovician rifting developed in the NW Gondwanan margin. This episode has been constrained at-4 kbar and 580 degrees C in the St-Grt-And pelitic schists. Related granitic magma pulses contributed to a local thermal increase of the extending crust (preserved in the And-Grt-Sil-bearing schists), where temperature reaches up to 595 degrees C. This area shows the syntaxial replacement of coarse-grained andalusite porphyroblasts by sillimanite. This was followed by exhumation, characterised by decompression at-2 kbar and 510 degrees C. After the main pre-Variscan Buchan-type regional metamorphism, a localised medium-pressure metamorphic event, limited to granitic pegmatites and concordant ky-bearing quartz segregation veins was recorded, possibly due to the tectonic thickening corresponding to the Variscan cycle.

Automated summary

This study provides new insights into the polycyclic metamorphic evolution of the Sierra Albarrana Domain in the SW Iberian Massif. Mineral assemblages and petrological modeling were used to accurately reproduce the assemblage of St-Grt-And pelitic schists, but faced difficulties in modeling And-Grt-Sil-bearing schists. The study also investigates the influence of Fe3+ on phase relations in pelites and records a regional low-pressure metamorphic event followed by exhumation and a localized medium-pressure metamorphic event.