P-T-t evolution of ultrahigh-temperature granulites from the Saxon Granulite Massif, Germany. Part I: Petrology

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.