ND ISOTOPE EVIDENCE FOR THE CENTRAL METASEDIMENTARY BELT (GRENVILLE PROVINCE) AS A BACK-ARC RIFT ZONE

Nd isotope data are presented for granitoid orthogneisses from the Central Metasedimentary Belt (CMB) of the Grenville Province in order to map the extent of juvenile Grenvillian-age crust within this orogenic belt that is composed mostly of older crustal terranes. The data reveal a significant belt of juvenile crust in Ontario, but this belt contains a block of pre-Grenvillian crust carrying the Elzevir pluton, which yields an estimated crustal formation age of 1.5 Ga. The recognition of an older block in the middle of the CMB has profound implications for its structure, because it transforms the zone of juvenile Grenvillian crust from a wide belt with approximately NE–SW strike, into two narrow segments with approximately N–S strike. We suggest that the CMB comprises an en echelon series of ensimatic rift segments which extend southwards (in the subsurface) into the northeastern Unites States. These juvenile segments contain abundant marble outcrops, consistent with marine incursion into the rift zone, and these deposits also continue northwards into a ‘Marble domain' of the CMB in Quebec. However, crustal formation ages in the latter domain are largely pre-Grenvillian, implying that the Quebec rift segment was ensialic. Hence, we interpret the CMB as the northerly termination of a failed rift zone, formed by back-arc extension during the final stages of the Elzevirian orogeny.