J. Rotzler et Rl. Romer, P-T-t evolution of ultrahigh-temperature granulites from the Saxon Granulite Massif, Germany. Part I: Petrology, J PETROLOGY, 42(11), 2001, pp. 1995-2013
The granulites of the Saxon Granulite Massif equilibrated at high pressure
and ultrahigh temperature and were exhumed in large part under near-isother
mal decompression. This raises the question of whether P-T- t data on the p
eak metamorphism may still be retrieved with confidence. Felsic and mafic g
ranulites with geochronologically useful major and accessory phases have pr
ovided a basis to relate P-T estimates with isotopic ages presented in a co
mpanion paper. The assemblage garnet + clinopyroxene in mafic granulite rec
ords peak temperatures of 1010-1060 degreesC, consistent with minimum estim
ates of around 967 degreesC and 22.3 kbar obtained from the assemblage garn
et + kyanite + ternary feldspar + quartz in felsic granulite. Multiple part
ial overprint of these assemblages reflects a clockwise P-T evolution. Garn
et and kyanite in the felsic granulite were successively overgrown by plagi
oclase, spinel + plagioclase, sapphirine + plagioclase, and biotite + plagi
oclase. Most of this overprinting occurred within the stability field of si
llimanite. Garnet + clinopyroxene in the mafic granulite were replaced by c
linopyroxene + amphibole + plagioclase + magnetite. The high P-T conditions
and the absence of thermal relaxation features in these granulites require
a short-lived metamorphism with rapid exhumation. The ages of peak metamor
phism (342 Ma) and shallow-level granitoid intrusions (333 Ma) constrain th
e time span for the exhumation of the Saxon granulites to similar to9 my.