Granulitic xenoliths from the French Massif Central—petrology, Sr and Nd isotope systematics and model age estimates

Abstract

Summary The protoliths of granulitic xenoliths found in Tertiary/Quaternary alkali volcanics from the French Massif Central are shown to have been principally immature sediments and calc-alkaline intrusives. The association may indicate formation of the crust in an island-arc or continental margin environment. Pressure and temperature estimates indicate that they have undergone intermediate to high pressure granulite-facies metamorphism (9–15 kbar) followed by uplift to lower pressures (4–9 kbar). Rb-Sr and Sm-Nd isotopic data and REE abundances are presented for both meta-igneous and meta-sedimentary suites. ɛSr present-day values for the granulitic xenoliths range from +2.7 to +325, with ɛNd values between +0.2 and −12.0. There are no isochron relationships within the Sr and Nd data, but possible mixing lines in the Rb-Sr system may indicate sedimentation at 700–1000 Myr. T Nd DM model ages indicate a Proterozoic (800–2100 Myr) origin for the protoliths of the meta-igneous samples and a similar provenance age for the meta-sediments.