Hercynian low-pressure-high-temperature regional metamorphism and subhorizontal foliation development in the Canigou Massif, Pyrenees, France; evidence for crustal extension

A detailed study has been made of the structural and metamorphic history of part of the Hercynian low-pressure-high-temperature regional metamorphic terrane exposed in the Canigou massif, France. Microtextural relations between, and compositional zoning within, porphyroblast phases in pelitic rocks indicate that the pressure-temperature-time (P-T-t)trajectory followed by the massif during metamorphism involved decompression under prograde and retrograde conditions (clockwise P-T-t trajectory). The geometric relations between inclusion trails within these porphyroblasts and the structures in the adjacent matrix indicate that metamorphism was synchronous with the development of the regional subhorizontal foliation (S3). The S3 foliation formed in response to noncoaxial bulk strain. The absence of a marked hiatus in sedimentation in the Pyrenean region during Hercynian metamorphism and deformation and the widespread preservation of low-grade Hercynian metasedimentary rocks suggest that low-P-high-T metamorphism occurred in an extensional tectonic environment and that extension manifested itself in asymmetric top-to-the-northwest shear during D3.