Protolith chemistry controls decarbonation in a Proterozoic Orogen : A field-based test from the Grenville Orogen, Ontario, Canada

Lithospheric carbon fluxes are an essential piece of the geologic carbon cycle. Though more attention is given to volcanic emissions, previous studies suggest that collisional orogenic settings release a significant amount of metamorphic CO₂ (Kerrick & Caldeira 1998, Groppo et al., 2017). Quantifying the CO₂ released from decarbonation of mixed calcsilicates during mountain building events is important for understanding planetary climate and habitability. Crustal decarbonation is strongly controlled by the availability of mixed carbonate–silicate and dolomite-rich protoliths, which undergo decarbonation at substantially lower temperatures than pure carbonates. These mixed sediments promote efficient carbon release by reacting to form amphiboles, pyroxenes, and other ferro-magnesian silicate phases during metamorphism. Global datasets show that mixed carbonate–silicate and dolomitic rocks are especially abundant in the Paleoproterozoic and Mesoproterozoic (Cantine et. al, 2020). Consequently, modelling studies predict elevated metamorphic decarbonation fluxes in the Proterozoic, driven by both suitable protoliths and geothermal conditions (Stewart and Penman, 2024); however, there have been no field-based studies to test this hypothesis. Here we present preliminary results from a field-based test studying decarbonation from sediments buried and metamorphosed during the Proterozoic Grenville Orogeny.

The Grenville orogenic belt is a large-scale stack of crustal blocks (> 600 km wide) thrust over the older Archean crust as a result of convergence leading to the formation of the supercontinent Rodinia. The Grenville Orogen is thought to be a large hot long duration orogen (Rivers, 2008; Indares, 2020) and has been considered a Proterozoic analogue of present day orogens like the Himalayas. Regionally, the metamorphic grade increases from South to North from greenschist to upper amphibolite facies conditions.

We present a comparative study of decarbonation of two carbon bearing lithologic units: the Grenville Supergroup and the Flinton Group. The Grenville Supergroup consists primarily of metamorphosed marine carbonates which have undergone multiple generations of metamorphism corresponding to multiple orogenies. The younger Flinton Group was deposited < 1155 Ma under local fluvial to shallow marine conditions and has only undergone a single metamorphic event corresponding to the collision of Amazonia with Laurentia (i.e., the Ottawan Orogeny). Within the Flinton Group, we focus on the Fernleigh Formation consisting of laminated calcareous pelites to schists as a representative mixed calcareous – siliciclastic unit.

Preliminary results indicate metamorphic decarbonation in the rocks of the Grenville Orogeny was controlled by the mixed silicate – carbonate bulk composition of protoliths. Rocks of the carbonate-dominated Grenville Supergroup show hindered decarbonation due to limitation of reactant silicate minerals. Upper amphibolite grade, pyroxene bearing rocks of the Grenville Supergroup show ~ 20% decarbonation at ~ 700°C. In contrast, rocks of the Fernleigh formation show enhanced decarbonation (~80-90 %) even at lower amphibolite grades. Decarbonation reactions like amphibole-in and pyroxene-in also occur at comparatively lower temperatures in these carbonate-limited rocks.

We will present detailed results of carbon mobilization using stable isotope geochemistry and thermodynamic modelling. Decarbonation estimates for different metamorphic facies will offer field-based insights into the solid Earth metamorphic flux associated with the Grenville Orogeny during the Proterozoic eon.