Discrimination of protolithic versus metamorphic zircon ages in eclogites: Constraints from the Erzgebirge metamorphic core complex (Germany)

Abstract

Protolithic ages of meta-basites are crucial for tectonic reconstructions as they constrain the lifetime of oceans. However, in high-grade metamorphic regions most isotope systems record metamorphic ages due to the metamorphic overprint. The mineral zircon is the most suitable geochronometer for the determination of protolithic ages in such rocks due to its high closure temperature. Zircon from eclogites from the Eastern Erzgebirge metamorphic complex yielded both protolithic (~540Ma; 5 samples) and metamorphic (~340Ma; 4 samples) 207Pb/206Pb ages. Detailed investigations of zircon grains within thin rock sections show that the metamorphic ages are related to new zircon growth caused by water infiltration that triggered decomposition of omphacite into an amphibole–plagioclase symplectite. Rutile and apatite frequently grew together with metamorphic zircons indicating local mobilization of Zr, Nb, LREE and P. Eclogites with omphacite breakdown are very dark in colour. In difference, lighter coloured eclogites contain K-feldspar and quartz indicating minor melt infiltration that caused breakdown of garnet and transformation of amphibole into biotite. This supports additional new growth of zircon grains. In some samples idiomorphic zircon grains with a lot of micro-inclusions indicate a coupled dissolution–reprecipitation process. Additional rounded-resorbed and highly fractured zircon grains together with internal zircon textures imply multiple melt infiltration triggering multiple phases of zircon growth. Therefore, scattering zircon ages may reflect multiple growth stages or inheritance rather than prolonged periods of metamorphic growth. In contrast, protolithic zircon ages were found in eclogites where no water or melt infiltration occurred. This emphasizes the importance of water and/or melt infiltration for the new growth of zircon grains in eclogites as was already shown for granulites in various studies. In the Eastern Erzgebirge, most eclogites and amphibolites were derived from MORB-type or arc-related basalts while the plagioclase-rich amphibolites formed mainly from crustal sources. The protolith ages of meta-basites from the Eastern Erzgebirge (~540Ma) document a bimodal magmatic activity in a Cadomian back-arc basin.