PTTD EVOLUTION OF THE BASEMENT TO A CADOMIAN ARC, LA HAGUE, NW FRANCE

The La Hague region of NW France exposes the northernmost segment of the Cadomian belt, one of the key Neoproterozoic arc terranes in the circum-Atlantic region. The importance of La Hague lies in the potential to derive a history of metamorphic events preserved in Paleoproterozoic basement rocks, unlike other parts of Cadomia-Avalonia terranes where basement is either absent, mineralogically unsuitable or was strongly reworked during Paleozoic orogenesis. The metamorphic basement includes orthogneisses (e.g. Culeron granite gneiss, Moulinet granite gneiss) which have yielded U-Pb zircon crystallization ages of c. 2.0 Ga, and display gneissic fabrics that have traditionally also been considered to be Paleoproterozoic in age. However, variably foliated Cadomian plutons that intrude the gneisses record magmatic to solid-state fabrics that parallel metamorphic fabrics within the basement consistent with the dominant structures being formed contemporaneously in the late-Neoproterozoic. The metamorphic basement also includes semi-pelitic migmatites and amphibolites from which a PTtd path has been derived. Paragneissic units preserve relict kyanite overprinted by prismatic sillimanite associated with development of garnet bearing leucosomes indicative of decompression melting. Anatexis of the basement (late-D1) was initiated through biotite dehydration reaching a metamorphic peak of 700-800ºC and 8-9 kb. Rims of c. 2.0 Ga zircons in the Culeron granite gneiss yield SHRIMP ages of c. 620 Ma providing the first geochronological constraints on the age of anatexis of the Paleoproterozoic basement in Cadomia. This confirms the dominant metamorphic features within the basement as being Cadomian. Microstructural and field evidence indicates that melting continued during D2 exhumation and deformation of primary migmatitic structures during which melts were focussed into sinistral shear zones. Continued sinistral shearing during D3 was associated with syn-tectonic plutonism and regional cooling to upper greenschist facies conditions. The timing of exhumation to depths of 15-20 km is provided by syn-D3 intrusions that yield U-Pb zircon ages between c. 610-608 Ma.