CARBON ISOTOPE THERMOMETRY IN THE CENTRAL METASEDIMENTARY BELT BOUNDARY THRUST ZONE, GRENVILLE PROVINCE, ONTARIO

The Central Metasedimentary Belt boundary thrust zone (CMBbtz) is a fundamental suture in the Mesoproterozoic southern Grenville Province, acting as a western boundary between the Central Metasedimentary Belt (CMB) and the Central Gneiss Belt (CGB). The CMBbtz (which contains the Bancroft terrane) is composed of igneous bodies ranging from 100s of square km to hand-sample size surrounded by variably sheared orthogneiss and annealed marble tectonites. Marbles in this terrane as well as the adjacent Elzevir terrane were analyzed for carbon and oxygen isotope values in graphite and calcite. Preliminary data show Δ¹³C(Cal-Gr)= 3.83±0.23‰ (n=24) which indicates an average metamorphic temperature of 693±30ºC using the calibration of Kitchen and Valley (JMG 1995). The Bancroft terrane showed an average Δ¹³C(Cal-Gr)= 3.82±0.25‰ (n=13), indicating a temperature of 694±31ºC and the Elzevir terrane has an average Δ¹³C(Cal-Gr)= 3.83±0.23‰ (n=11), indicating a temperature of 692±29ºC. The identical temperatures in this study suggests a similar metamorphic history for the two terranes. Bancroft terrane marbles share the distinctly high carbon isotope ratios of the Elzevir terrane (δ¹³C (Cal) up to 4.5‰), consistent with a similar depositional history within the CMB. This points towards Bancroft terrane metasediments not having affinities with the CGB, as suggested in some tectonic reconstructions.