Zircon response to high-grade metamorphism as revealed by U-Pb and cathodoluminescence studies

Abstract

Correct interpretation of zircon ages from high-grade metamorphic terrains poses a major challenge because of the differential response of the U-Pb system to metamorphism, and many aspects like pressure-temperature conditions, metamorphic mineral transformations and textural properties of the zircon crystals have to be explored. A large (c. 450 km2) coherent migmatite complex was recently discovered in the Bohemian Massif, Central European Variscides. Rocks from this complex are characterized by granulite- and amphibolite-facies mineral assemblages and, based on compositional and isotopic trends, are identified as the remnants of a magma body derived from mixing between tonalite and supracrustal rocks. Zircon crystals from the migmatites are exclusively large (200-400 µm) and yield 207Pb/206Pb evaporation ages between 342-328 Ma and single-grain zircon fractions analysed by U-Pb ID-TIMS method plot along the concordia curve between 342 and 325 Ma. High-resolution U-Pb SHRIMP analyses substantiate the existence of a resolvable age variability and yield older 206Pb/238U ages (342-330 Ma, weighted mean age = 333.6 ± 3.1 Ma) for inner zone domains without relict cores and younger 206Pb/238U ages (333-320 Ma, weighted mean age = 326.0 ± 2.8 Ma) for rim domains. Pre-metamorphic cores were identified only in one sample (206Pb/238U ages at 375.0 ± 3.9, 420.3 ± 4.4 and 426.2 ± 4.4 Ma). Most zircon ages bracket the time span between granulite-facies metamorphism in the Bohemian Massif (~345 Ma) and the late-Variscan anatectic overprint (Bavarian phase, ~325 Ma). It is argued that pre-existing zircon was variously affected by these metamorphic events and that primary magmatic growth zones were replaced by secondary textures as a result of diffusion reaction processes and replacement of zircon by dissolution and recrystallization followed by new zircon rim growth. Collectively, the results show that the zircons equilibrated during high-grade metamorphism and record partial loss of radiogenic Pb during post-peak granulite events and new growth under subsequent anatectic conditions. © 2012 Springer-Verlag.