CALCITE-GRAPHITE THERMOMETRY OF MARBLES IN THE FRONTENAC TERRANE (GRENVILLE PROVINCE, ONTARIO)

The Frontenac terrane in Ontario is composed of Mesoproterozoic granulite-facies gneiss, quartzite, and marble intruded by two distinct intrusive suites composed of monzonite to granite plutons. This project aims to constrain the thermal history associated with Shawinigan (ca. 1.18-1.15 Ga) metamorphism and plutonism by examining the carbon isotope fractionation between co-existing calcite and graphite in marbles. Preliminary data from marbles across the Frontenac terrane have Δ¹³C(Cal-Gr)= 2.84±0.38 (n=24), which corresponds with an average temperature of 853±75ºC and granulite-facies mineral assemblages. No strong relationship is seen between metamorphic temperatures and map distances between samples and intrusions or proximity to shear zones. Metamorphic temperatures in the Frontenac terrane are significantly higher than in the adjacent Adirondack Lowlands, with which it has been correlated. Interestingly, carbon isotopes in calcite from marbles in the Frontenac terrane (δ¹³C(Cal)= 1.62±1.50) are statistically distinct from those reported from the Adirondack Lowlands (Kitchen and Valley, JMG 1995).