U–Pb SHRIMP-dating of zircon domains from UHP garnet-rich mafic rocks and late pegmatoids in the Rhodope zone (N Greece); evidence for Early Cretaceous crystallization and Late Cretaceous metamorphism

Abstract

U–Pb ion microprobe (SHRIMP) dating of zircon domains assisted by cathodoluminescence imaging was carried out on (1) a garnet-rich mafic rock and (2) a cross-cutting pegmatoid from the area of Kimi, East Rhodope. Quartz exsolution lamellae in cpx of the garnet-rich mafic rock point to a precursor supersilicic cpx implying pressures >2.5 GPa. This indicates UHP conditions for the studied geotectonic unit and is in line with the (rare) presence of radial cracks around quartz inclusions in garnet, which suggest the former presence of coesite. Bleb-shaped cpx inclusions in garnet may be interpreted as primary inclusions during garnet growth. However, based on their shape and general orientation, the possibility that they represent exsolutions from a precursor majoritic garnet, in an advanced state of recrystallization (coalescing initial needles) remains open and needs further investigation. If the latter is proven true, it implies minimum pressures of 4 GPa. The oscillatory zoned domains of the zircon crystals from the garnet-rich mafic rock yield a weighted mean age at 117.4±1.9 Ma (Early Cretaceous) for the magmatic crystallization of the protolith. The zircon domains characterized by clearly fainted oscillatory zoning and/or U-rich embayments of the same zircon crystals are interpreted as metamorphic domains. They yield a weighted mean age of metamorphism at 73.5±3.4 Ma (Late Cretaceous). Different degrees of Pb loss resulted in discordant ages scattering between the magmatic and metamorphic age. The euhedral oscillatory zoned zircons of the cross-cutting pegmatoid yield a weighted mean age of 61.9±1.9 Ma for the time of emplacement and crystallization of retrograde metamorphic fluids into the middle-upper crust. Two scenarios are considered as possible for the interpretation of the SHRIMP-results and the petrological observations: (1) the protolith was an ultra high pressure melt (probably a grt±cpx cumulate) crystallizing in the mantle at Early Cretaceous times, partly exhumed into the lithosphere by mantle upwelling, and then transferred to a subduction zone and metamorphosed under eclogite-facies conditions in the Late Cretaceous; (2) the protolith was a cumulate within a gabbroic melt crystallizing at high crustal levels in the Early Cretaceous and subsequently metamorphosed under UHP conditions in the Late Cretaceous. Retrograde stages of metamorphism (ca. 500 °C, 0.5 GPa) occurred at 61.9±1.9 Ma.The petrological findings, which provide evidence for UHP conditions for these rocks of the Kimi Unit, combined with the SHRIMP-results, reveal different conditions of rock formation at different times for the Rhodope zone. The existence of a subduction zone in the Late Cretaceous shows that the Rhodope was not metamorphosed as a single geotectonic element but consists of different terranes subducted and exhumed at different times.