Geochronology and petrology of pyroxene-garnet skarns (eastern Bohemian Massif): implications for the source and evolution of the Variscan continental crust

Abstract

Zircon ages (U–Pb, LA-ICP-MS) obtained from skarns in the Kutna Hora and Svratka units and in the Moldanubian Zone (Bohemian Massif) show significant variations between the lithotectonic units as well as among samples from individual localities. The ages fall between 310 Ma and 3.1 Ga. We suggest that such large spread is a result of both different zircon clastic grain sources and subsequent metamorphic histories. The oldest Neoarchaean to Neoproterozoic ages are interpreted as detrital zircon grains from pre-Cadomian basement. Pronounced age maxima between 600 and 520 Ma (late Neoproterozoic to early Cambrian) were found in skarn samples from the Moldanubian Zone, Svratka and Kutna Hora units. We interpret these as maximum ages of skarn precursor deposition. Apparent Cambrian–Ordovician– Silurian to early Devonian zircon ages were obtained from non-mineralized samples of the Moldanubian Zone and also in the Kutna Hora Unit. This wide temporal interval could reflect maximum ages of skarn precursors related to disintegration of abundant early Palaeozoic magmatic rocks, provided the zircons were not modified by later metamorphism or fluid circulation. The early Carboniferous ages are interpreted to result from strong Variscan HT metamorphism and subsequent circulation of post-metamorphic fluids. Zircons of this age are dominant in skarns with massive magnetite mineralization, while the non-mineralized samples preserve the older ages.