Geochemical constraints on the genesis of Hercynian two-mica leucogranites from the Massif Central, France

Late Hercynian two-mica leucogranites dated at 340-280 Ma old occur across the entire Massif Central, France, and form dykes and stocks up to 130 km in length. They are peraluminous [molecular Al2O3/(Na2O + K2O + CaO), ASI = 1.16-1.25] and have low initial epsilon(Nd) (-8.2 to -6.1) indicating derivation from the metasedimentary lower crust as sampled in crustal xenoliths (epsilon Nd-340 = -8.6 to -4.3, epsilon Nd-280 = -9.0 to -4.8). Their high SiO2 (70-78 wt%), P2O5 (up to 0.5%), Rb and Ga, and low Ba, Sr and Sigma REE are mainly a result of partial melting processes rather than fractionation. Partial melting was initiated by underplating of the lower crust by mantle-derived basic magmas between 360 and 270 Ma and was intensified by granulite-facies metamorphism between 300 and 280 Ma. From this model, there is little similarity between the genesis of two-mica leucogranite in the Massif Central and that in the Himalaya, with which it has previously been compared.