U–Pb geochronological constraints on the tectonothermal evolution of the Paleoproterozoic basement of Cadomia, La Hague, NW France

Abstract

The La Hague region of NW France provides some of the best exposures of Paleoproterozoic basement in the northernmost segment of the Cadomian orogen, one of several Neoproterozoic arc fragments in the circum-Atlantic region. The basement rocks, which include orthogneiss, metasediments and metabsites, contain a comparable series of structures with a common metamorphic history. Thus, the basement units are regarded to have undergone the same broad tectonothermal evolution. The metasedimentary succession includes migmatites that preserve relict kyanite overprinted by sillimanite associated with the development of garnet bearing leucosome. Biotite dehydration at a metamorphic peak of 700–800 °C and 8–9 kbar is considered responsible for the anatexis evident in both the metasediments and orthogneiss units. A U–Pb zircon crystallization age of 2061 ± 2.7 Ma for the Culeron orthogneiss, a member of the metamorphic suite, confirms the Paleoproterozoic protolith age. Idioblastic rims to 2.06 Ga zircons from the Culeron orthogneiss yield SHRIMP U–Pb dates of c. 618 Ma, providing the first geochronological constraints on the likely age of high-grade metamorphism and anatexis of the Paleoproterozoic basement in Cadomia. Foliated c. 615–610 Ma intrusions that intrude the metamorphic units record magmatic to solid-state fabrics that parallel metamorphic fabrics within the basement, consistent with deformation and metamorphism being late-Neoproterozoic in age.