LINKING CARBON CYCLING IN CA. 1.9-1.7 GA SHEAR ZONES TO NUNA ASSEMBLY

The assembly of the supercontinent Nuna ca. 2.0-1.7 billion years ago lead to the prolific growth of mountain belts whose remnants remain preserved across all major continental landmasses. During orogenesis, prograde metamorphism converted sedimentary carbonaceous matter internal to these mountain belts into graphitic carbon. As these mountain belts evolved to their terminal stages of orogenic exhumation and collapse, this endogenous organic carbon was, in part, mobilized and deposited in shear zones as hydrothermal graphite. However, the timing, rates, and amounts of graphitic carbon that was mobilized during these late-stage orogenic events has remained poorly understood.

To understand the carbon cycling associated with these Paleoproterozoic orogenic events, we examine graphitic-pyritic shear zones in N. Alberta and Saskatchewan, Canada associated with the building of Laurentian and by extension Nuna. These graphitic shear zones formed during the Taltson-Snowbird and Trans-Hudson orogenies of North America and involved the collisions of the Slave-Rae, Rae-Hearne, and Hearne-Superior Cratons, respectively.

Using a coupled Re-Os/U-Pb dating approach, we found that the shear zone hosted graphite predominantly formed from biogenic carbon that was deposited during the retrograde stages of metamorphism and spanned a 200 Myr interval (ca. 1.9-1.7 Ga), with graphite-pyrite deposition occurring at a periodicity of ca. 30 Ma.

We modeled the amount of graphite mobilized into these shear zones at the individual and population scale to better understand the local and regional dynamics of carbon cycling. Our findings suggest that >50 million tonnes of C was mobilized over this 200 Myr interval. Similar hydrothermal graphite occurrences are also preserved across other Nuna associated mountain belts, such as those found in Greenland, NW Scotland, Russia, Finland, Norway, and Sweden. This provides strong evidence that the mass movement of graphitic carbon is a common feature of the orogenic cycle.