hide
Free keywords:
exhumation; geochronology; Saxony; UHT granulites
DDC:
550 - Earth sciences
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 (2) and 342·0 ± 0·8 Ma (2), 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 (2), 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 (2). 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).
