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Journal of Petrology Volume 41 Number 10 Pages 1489-1515 2000
© Oxford University Press 2000

P–T Paths Derived from Garnet Growth Zoning in an Extensional Setting: an Example from the Tormes Gneiss Dome (Iberian Massif, Spain)

J. ESCUDER VIRUETE1,*, A. INDARES2 and R. ARENAS3

1DEPARTAMENTO DE PETROLOGÍA Y GEOQUÍMICA, UNIVERSIDAD COMPLUTENSE, 28040 MADRID, SPAIN
2DEPARTMENT OF EARTH SCIENCES, MEMORIAL UNIVERSITY OF NEWFOUNDLAND, ST JOHN’S, NFLD., CANADA A1B 3X5

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.

KEY WORDS: P–T paths; garnet growth zoning; extensional tectonics; Iberian Massif


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