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.