A NEODYMIUM ISOTOPE STUDY OF PLUTONS NEAR THE GRENVILLE FRONT IN ONTARIO, CANADA

Sm/Nd analysis of meta-plutonic rocks near the Grenville Front in Ontario allows their division into two groups. Granitoid plutons with model ages of > 2 Ga are intruded into a crustal section formed largely of Archean or re-worked Archean basement. Plutons with model ages of < 2 Ga were intruded into basement with a ∼ 1.9-Ga model age. The plutonic data are consistent with the previously published Sm/Nd analysis of Grenville paragneisses attributing the two distinct model age provinces to the accretion of a Penokean island arc to the Superior craton in the Lower Proterozoic. Plutons south of the proposed suture line can be explained by anatexis of Penokean age crust alone, while plutons north of this line contain mixtures of Archean and Proterozoic Nd. These mixed signatures suggest that the NW Grenville Province may be analogous to northern Wisconsin and Michigan. In this region a very early Proterozoic miogeocline supplied by the Superior Province is overlain by a formation of mixed Superior-Penokean provenance, which heralded the collision of a Penokean arc with the Archean craton. Despite very strong Sm/Nd evidence for the presence of old crustal components in the plutons, U/Pb analysis did not reveal the presence of inherited zircon. Hence, the absence of such zircons should not be taken in isolation as evidence against crustal re-working. © 1990.