Very low-grade metamorphism of the Seo Formation in the Orri Dome, South-Central Pyrenees

Abstract

The metamorphic grade of the Seo Formation, South-Central Pyrenees, has been determined using illite crystallinity measurements. The total range of the illite crystallinity indices of the K+-saturated fraction <2 μm is 0.21-0.31 °2 ϴ and averages 0.26 ± 0.03 °2 ϴ, which indicates metamorphism in the high-grade part of the anchizone. This is in agreement with mineralogical data, such as the absence of expandable clay minerals and of metamorphic minerals such as biotite and epidote. Illite crystallinity indices of the detrital micas suggest that they were derived from igneous rocks or from rocks with at least epizonal grade of metamorphism.