DELINEATING THE GEOMETRY OF THE CENTRAL METASEDIMENTARY BELT BOUNDARY ZONE: ND ISOTOPE EVIDENCE OF A BACK-ARC RIFT ZONE

The Grenville Province represents the remains of a collisional orogeny ca. 1.2 - 1.0 Ga and contains the Central Metasedimentary Belt (CMB). Generally thought to represent one or more accreted island arcs, the CMB is located between belts of high-grade gneisses and contains a number of identified structural terranes. Neodymium (Nd) model ages of the high-grade gneisses on either side of the CMB yield similar values (~1.5 Ga) while the average model age within the CMB is usually more juvenile (<1.3 Ga). This distinction, along with observations about the geometrical shape of the juvenile zone, has led to the creation of an alternative model for the development of the CMB in the Grenville Province. The new model equates the CMB with an ensimatic rift zone with an en echelon morphology consisting of a series of segments with NNE trends, separated by one or more horsts of older crustal rock. The development of the CMB under these conditions implies that restricted access to seawater may have facilitated limestone deposition prior to major biogenic influences, and thus the morphology of the rift is defined in part by the extent of the Grenville marble outcrops. The present study tests this model through detailed mapping of the NW boundary of the CMB, known as the Central Metasedimentary Belt Boundary Zone (CMBBZ), west of Bancroft, Ontario. Within this part of the CMBBZ, the age boundary between pre-Grenvillian and juvenile gneisses is relatively sharp (1 - 4 km wide) and this age boundary makes a near 90-degree turn from a NNE trend near Minden to an E-W trend near Haliburton. Therefore, we suggest that the locus of the CMBBZ was constrained by older structures, representing a transition from the side of a rift zone segment south of Minden, to the truncated end of this rift segment between Haliburton and Bancroft. Hence, the detailed structure of the CMBBZ in this region provides further evidence in support of the rift zone model.