Coal-magma interaction in the Ferrar Large Igneous Province, Antarctica: implications for magma propagation and plumbing system development

The mechanical properties and rheology of the host rock in magmatic systems have a major control on the development of the plumbing system by affecting how magma propagates through the crust, via a range of brittle and non-brittle processes. In continental Large Igneous Provinces (LIPs), interaction between magma and carbon-rich layers (e.g. coal) is a fundamental process that has been shown to release large volumes of volatiles in the atmosphere, affecting global climate and sometimes triggering mass extinctions. While coal-magma interaction in continental LIPs has been well studied in the context of climate change and mass extinctions, few studies consider how this volatile release affects magma propagation and plumbing system development at the LIP scale. To infer how magma was emplaced in the crust, we analysed the morphologies of dikes associated with the 183 Ma Ferrar LIP emplaced in Beacon Supergroup sedimentary rocks using a range of structural measurements and field observations across three sites in the McMurdo Dry Valleys, Antarctica. A majority of dikes emplaced at ≥ 2 km paleodepth exhibit straight parallel margins, tapered tips, and stepped segments, indicative of brittle emplacement via tensile opening. However, we observe a noticeable transition to non-brittle behaviour at ≤ 1 km paleodepth, coinciding with dikes intersecting the late Permian Weller Coal Measures. Here, folding, faulting, and fluidisation of the host rock is commonly observed adjacent to dikes and is accompanied by a sudden shift in dike morphology and geometry. We hypothesise that local- and regional-scale heating of coal and carbonaceous shale resulted in large-scale volatile release, triggering host rock fluidisation, and ultimately promoting non-brittle modes of magma propagation at shallow paleodepths. Our findings support an evolving host rock rheology for LIPs intruding through volatile-rich sedimentary basins, which affects intrusion geometries, magma propagation processes, and the spatial and temporal development of LIP plumbing systems.