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

Abstract

Granulite-facies metamorphism in the structurally lower part of the Saxon Granulite Massif culminated at 1010-1060°C and ~ 22 kbar. Ultrahigh-temperature conditions persisted even after a significant drop in pressure. We dated monazite, zircon, rutile, garnet and apatite from felsic granulite by U-Pb, biotite from retrogressed felsic granulite by Rb-Sr, and titanite from mafic granulite by U-Pb. Zircon and titanite give within analytical uncertainties the same age at 341·5 ± 0·8 Ma (2σ) and 342·0 ± 0·8 Ma (2σ), respectively, demonstrating (1) a similar closure temperature for both minerals in dry systems and (2) a closure temperature for titanite considerably higher than 550°C. Monazite plots discordantly and yields a 207Pb/206Pb age at 338·0 ± 0·5 Ma (2σ), which represents a minimum age because of the possibility of excess 206Pb. Rutile, garnet and apatite have little radiogenic lead and show a wide range of apparent 206Pb/238U ages, which reflects initial isotopic heterogeneities originating from the reaction history rather than later disturbances. Biotite yields an Rb-Sr age at 323·0 ± 2·3 Ma (2σ). The age data in combination with the P-T path demonstrate that exhumation of the Saxon Granulite Massif to a middle- to upper-crustal level proceeded at a fast average rate (>9-18 mm/yr) and subsequently slowed down significantly (<2 mm/yr).