P-T paths derived from garnet growth zoning in an extensional setting: An example from the Tormes Gneiss Dome (Iberian Massif, Spain)

Abstract

The Tormes Gneiss Dome (NW Iberian Massif, Variscan Belt of Spain), comprises a metamorphic core complex (Lower Unit) bounded by a major extensional detachment. Despite metamorphic temperatures in the upper amphibolite facies (~ 700-740° C), metapelites from the highest levels of the Lower Unit contain garnet with preserved growth zoning. These rocks were used for reconstruction of quantitative P-T paths based upon interpretation of microfabrics and thermodynamic modelling of garnet zoning. The results are consistent with a two-stage tectonothermal evolution under high-grade conditions: (1) an early compressional phase of deformation that led to upper amphibolite facies Barrovian-type metamorphism and to P increase and T rise to approximately 9 kbar and 700-725°C; (2) a subsequent major extensional phase of deformation that led to quasi-isothermal decompression from 8-9 to ~ 3 kbar at T conditions between 700 and 740°C. Several lines of structural, textural and petrological evidence suggest that up to 15-20 km of overburden was removed from the Lower Unit by tectonic exhumation while these rocks were still at upper amphibolite facies conditions. A final stage of quasi-isobaric cooling to greenschist facies conditions is locally recorded in late low-grade detachments.