Multiple crust reworking in the French Armorican Variscan belt: implication for the genesis of uranium-fertile leucogranites

Abstract

Muscovite peraluminous granites (MPGs) form by partial melting of the continental crust and can be related to metalliferous deposits such as tin, tungsten, and uranium (U). Metal enrichment in MPGs commonly results from fractional crystallization, but the metal contents of the source play a major role for their fertility. Between ca. 320 and 300 Ma (Late Carboniferous), the French Armorican Variscan belt was intruded by numerous U-fertile MPGs that contain inherited zircon grains with a wide range of ages from Archean-to-Carboniferous. U–Pb and Hf isotopic data of zircon grains from Brioverian-to-Carboniferous sediments, Cambrian-to-Early Carboniferous granitoids, and Late Carboniferous MPGs indicate that the crust of the Armorican Massif is made up by detritus mainly derived from the West African craton (3500–1600 Ma; TDM = 3.8–2.3 Ga), Grenvillian belt (1200–900 Ma; TDM = 2.7–1.2 Ga), and Avalonian–Cadomian belt (800–550 Ma; TDM = 2.5–0.8 Ga) and that the crust was affected by magmatic events at 510–470 Ma (TDM = 1.6–0.6 Ga), 410–330 Ma (TDM = 1.6–1 Ga), and 320–300 Ma. Furthermore, they reveal that the Late Carboniferous MPGs were mainly formed by partial melting of Brioverian sediments with Cambro–Ordovician and Devonian–Carboniferous granitoids, which are all genetically linked with each other and characterized by Th/U < 4. The new data suggest that the U-fertile MPGs result from multiple reworking of U-rich Brioverian sediments, deposited ca. 550 Ma ago on the northern margin of Gondwana, and partially molten during several Paleozoic events, causing a successive increase in U content in the middle-upper crust.