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-isothermal decompression. This raises the question of whether P–T–t data on the peak metamorphism may still be retrieved with confidence. Felsic and mafic granulites with geochronologically useful major and accessory phases have provided a basis to relate P–T estimates with isotopic ages presented in a companion paper. The assemblage garnet + clinopyroxene in mafic granulite records peak temperatures of 1010–1060°C, consistent with minimum estimates of around 967°C and 22·3 kbar obtained from the assemblage garnet + kyanite + ternary feldspar + quartz in felsic granulite. Multiple partial overprint of these assemblages reflects a clockwise P–T evolution. Garnet and kyanite in the felsic granulite were successively overgrown by plagioclase, spinel + plagioclase, sapphirine + plagioclase, and biotite + plagioclase. Most of this overprinting occurred within the stability field of sillimanite. Garnet + clinopyroxene in the mafic granulite were replaced by clinopyroxene + 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 metamorphism (342 Ma) and shallow-level granitoid intrusions (333 Ma) constrain the time span for the exhumation of the Saxon granulites to 9 my.