The Saxonian Granulite Massif: new aspects from geochronological studies

Geochronological and geochemical analyses were carried out to determine the age of protolith formation and the timing of eclogite and granulite metamorphism of the Saxonian Granulite Massif (SGM). The age of the ultramafic protoliths of the garnet pyroxenites from the rim of the SGM has been determined to be Late Riphean (828 Ma), which together with geochemical and isotope data implies the extraction of the melt from an enriched mantle reservoir with an initial -Nd value of + 3.4. The minimum emplacement age of the mafic to felsic granulites has been dated at 470 Ma. The source material of these granulites reflects an enriched mantle reservoir with an -Nd value of + 2.6 (470 Ma).Based on U/Pb zircon and Sm/Nd analyses (garnet kyanite, apatite, clinopyroxene, whole rock) the granulite metamorphism of the SGM probably occurred at 340 Ma. The time of the eclogite metamorphism for a garnet pyroxenite has been detected at 380 Ma using the Sm/Nd method on minerals (garnet — clinopyroxene—zircon) and whole rock.Geochemical and isotope data clearly indicate a MORB- as well as an IAT-type source for the mafic granulites. During the time of the eclogite and granulite formation the oceanic basin closed in a subduction zone tectonic setting. A terrane or microcontinent model is consistent with these data and further explains the Ordovician, Silurian and Devonian high pressure events detected all over the European Hercynides. In this model, Gondwana derived microcontinents successively collided from the Early Ordovician onwards, causing subduction zone related high pressure metamorphism during closure of oceanic basins.