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

The Tormes Gneiss Dome (NW Iberian Massif; Variscan Belt of Spain), compris es a metamorphic core complex (Lower Unit) bounded by a major extensional d etachment. Despite metamorphic temperatures in the upper amphibolite facies (similar to 700-740 degreesC), metapelites from the highest levels of the Lower Unit contain garnet with preserved growth zoning. These rocks were us ed for reconstruction of quantitative P-T paths based upon interpretation o f microfabrics and thermodynamic modelling of garnet zoning. The results ar e consistent with a two-stage tectonothermal evolution under high-grade con ditions: (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 degreesC; (2) a subsequent major exten sional phase of deformation that led to guasi-isothermal decompression from 8-9 to similar to 3 kbar at T conditions between 700 and 740 degreesC. Sev eral lines of structural, textural and petrological evidence suggest that u p to 15-20 km of overburden was removed from the Lower Unit by tectonic exh umation 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.