Je. Viruete et al., P-T paths derived from garnet growth zoning in an extensional setting: an example from the Tormes Gneiss Dome (Iberian Massif, Spain), J PETROLOGY, 41(10), 2000, pp. 1489-1515
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.