Behaviour of Rb-Sr whole rock and U-Pb zircon systems during partial melting as shown in migmatitic gneisses from the St Malo Massif, NE Brittany, France

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Abstract
The age of regional metamorphism and partial melting in the Late Proterozoic succession of the St Malo area has been established at around 540 Ma. Whole-rock homogenization of the Sr isotope systems has been achieved only in the anatectic granites, whereas the associated metatexitic and diatexitic gneisses remain highly heterogeneous. At lower grade than the sillimanite isograd, metasedimentary gneisses contain detrital zircons which record the regional metamorphic event as a U-Pb lower intercept age (536 ± 14 Ma). In the sillimanite zone U-Pb zircon lower intercept ages are around 500 ± 10 Ma, and suggest a recent loss of radiogenic Pb from the observed overgrowths. Zircons from the anatectic granites do not yield U-Pb intercept ages due to the presence of inherited radiogenic Pb, but the finer fractions define a discordant array whose upper intercept on Concordia yields 555 ± 13 Ma, an age significantly older than the partial melting event indicated by the lower grade zircon ages. The lower intercept defined by the fine grained anatectic zircons yields a date in the future (negative ordinate value). In their regional context, the St Malo migmatites can be considered as resulting from late stage crustal melting in the Cadomian orogenic belt, following genesis of calc-alkaline magma at an active margin around 600 Ma ago. An age of 540 Ma for partial melting in the St Malo migmatites places the end of the Cadomian orogeny near the Precambrian-Cambrian boundary.