Local Melt Contamination and Global Climate Impacts: Geochronology of Karoo LIP Sills in Organic-Rich Shales

Eruption of Large Igneous Provinces (LIP) are commonly correlated with global climate change, and environmental and biological crises. However, establishing a causative link via chemical and physical proxies for global change is more complicated and often ambiguous. As technical improvements have allowed for increasingly higher precision dates, especially in U/Pb dating, it is possible to better test hypotheses connecting LIPs and environmental impact via their contemporaneity. Here, we focus on the early Jurassic period, which includes a period of global change known as the Toarcian oceanic anoxic event (TOAE), as well as the emplacement of the Karoo Large Igneous Province (K-LIP). The K-LIP is comprised of a suite of basaltic lava flows, sills, dike swarms, centered in southern Africa. Approximately 340,000 km3 of sills are interlaid within the Karoo Basin, and therefore served as significant heat source to the basin upon emplacement. While much of the sedimentary rocks of the basin are siliciclastic, the Permian Ecca Group contains organic-rich facies and hosts 160,000 km3 of basaltic sills. Previous mass balance calculations indicate that between 7,000 and >50,000 Gt of CO2 equivalents was released through metamorphic reactions of these shale bodies in contact aureoles within the Ecca Group. If intrusive magmatism was short lived within this formation than this event could represent a mechanism to drive a short pulse of global climate change due to rapid volatilization and degassing from the shales. Previous studies have shown that intrusions are coeval with the TOAE, however higher-precision geochronology data from the sills is necessary to determine if the flux and timing of thermogenic gases from the basin was sufficient to destabilize Earths climate. In order to test the hypothesis, we present single crystal zircon and baddeleyite U-Pb dates and Hf isotopic compositions from sills across the Ecca Group. These data indicate that high-precision geochronology of intrusive LIP can be complicated by both inheritance and Pb-loss, and the thermogenic gas release of Ecca Group wall rocks was likely a rapid process, on the scale of 10s of ka, and therefore is a viable trigger for the observed environmental change during the Toarcian.