The Petrogenesis of Pseudocarbonatites in Bancroft Region, Southwestern Grenville Province

Abstract

Carbonatites are rare igneous rocks that form from fractionation of carbonate-rich, silica undersaturated magma. The petrogenesis of pseudocarbonatites (rocks that are similar to carbonatites, but without a clear mantle link) is less clear. Pseudocarbonatites may be derived from fractionation of mantle-derived magmas (like carbonatites) or generated from partial melting of carbonate rocks of a sedimentary origin (e.g., limestone). This thesis investigates a suite of Proterozoic pseudocarbonatites from Bancroft (Ontario) in the southwestern Grenville Province. I test the hypothesis that pseudocarbonatites were derived from partial melting of metasedimentary rocks in the region using petrology, whole-rock trace element geochemistry, and stable isotope analysis. Concentrations of Rare Earth Elements and other trace elements in the pseudocarbonatites are inconsistent with values from carbonatites or nearby nepheline syenites, but closely resemble those of marbles in the region. Carbon and oxygen isotope ratios of pseudocarbonatites are also inconsistent with North American carbonatites or a mantle origin and strontium isotope ratios are higher than average Ontario carbonatites. However, strontium isotopic ratios are broadly compatible with the values of marbles in the region. Therefore, the pseudocarbonatites were most likely generated during partial melting of marbles during fluid-influx associated with Grenville orogenesis. This suggests that carbon can be remobilized as carbonate melt during high-temperature metamorphism, which has implications for understanding the movement of carbon through the Earth’s crust and the carbon cycle of the geosphere.