Discussion on Hercynian tectonometamorphic evolution of the Bosost dome (French–Spanish central Pyrenees)

Abstract

R. L. Gibson writes: Pouget invokes a diapiric model for the development of structural–metamorphic domes in the Hercynian terrane of the Pyrenees. His model is based on conclusions drawn from the Bosost dome, namely that the core of this dome rose relative to the margins during regional metamorphism (deduced from P–T paths) and that the syn-metamorphic regional foliation formed in a dome-shaped attitude (from structural studies). Some comment is required, however, both on the methodology Pouget uses to construct the P–T paths presented in his paper and on his extension of the model to other domes in the Pyrenees. Pouget uses P–T estimates obtained from 4 garnet-bearing metapelite samples to construct P–T paths for different parts of the Bosost dome (fig. 4). Two P–T estimates were obtained from each sample using garnet core and rim compositions together with appropriate matrix phases and a straight-line vector was drawn linking the plotted results. Pouget maintains that this vector represents part of the P–T path followed by the sample during metamorphism. Such reasoning is, however, seriously flawed in that no guarantee exists that this straight line represents the true evolutionary path for the assemblage (see Spear et al. 1990 ). Pouget does not provide supplementary evidence, either from the analysis of continuous garnet zoning profiles or from the analysis of the metapelitic assemblages in terms of the KFMASH reaction grid (Harte & Hudson 1979) to corroborate his ‘shortest-line’ assumption. Based on this assumption, Pouget discerns two types