Abstract
The emplacement of Large Igneous Provinces (LIPs) is commonly correlated with global climate change, and environmental and biological crises. To establish this complex causative link, chemical proxies from marine sedimentary sections must be temporally tied to LIP activity through high-precision geochronology. The temporal link is often ambiguous, especially as periods of environmental change are commonly shorter than the full duration of magmatic and volcanic activity in a LIP. Most importantly, temporal relations need to be established through precise and accurate U-Pb geochronology; however, mafic rocks are often undersaturated in zircon or baddeleyite. New U-Pb age determinations of pegmatitic pods from Karoo LIP dolerite sills into the Ecca Group of the Karoo Basin yield single crystal dates with uncertainties below 50 ka, which allow for a new, detailed assessment of this intrusive event. These samples yield complicated age spectra and Hf geochemistry, which indicate that localized assimilation of sedimentary rocks enabled zircon saturation within these mafic melts. Zircon crystallization age interpretations from these data indicate that sill emplacement within the Ecca Group was coeval across the basin within uncertainty of U-Pb dating, and therefore represents a period of particularly high magma flux. Since the Ecca Group is locally exceptionally rich in organic matter, the widespread and rapid sill-related contact metamorphism generated a significant volume of thermogenic volatiles. Given the temporal overlap of the emplacement of sills between 183.147 ± 0.059 Ma and 183.187 ± 0.133 Ma and the timing of the global Toarcian Oceanic Anoxic Event (TOAE), our data support a causal link between this discrete period of high flux emplacement of Karoo sills into the Ecca Group and the global climate change that took place during the Early Jurassic.
Highlights
Large igneous provinces (LIPs) are composed of massive volumes of lava flows and extensive plumbing systems including sills and dikes, commonly driven by deep mantle processes (e.g., Bryan and Ernst, 2008)
Instead, based on zircon geochemistry and geochronology we suggest it is more likely that the assimilation of clastic sedimentary wall rocks and the subsequent volatile generation and partial melting were necessary to form the pegmatitic segregations, which allow for the potential for high precision geochronology of this mafic LIP
Our interpreted emplacement ages of these sills are identical within uncertainty to the age of the Toarcian Oceanic Anoxic Event (TOAE), based on previous geochronology of ash beds bracketing the carbon isotope excursion (183.22 ± 0.25 Ma; Sell et al, 2014), which supports that sill emplacement and subsequent degassing of sedimentary rocks presents a causal link to the global carbon cycle perturbation and climate change
Summary
Large igneous provinces (LIPs) are composed of massive volumes of lava flows and extensive plumbing systems including sills and dikes, commonly driven by deep mantle processes (e.g., Bryan and Ernst, 2008). Many of these voluminous events in Earth’s history have been commonly associated with extinction events and perturbations in global climate (e.g., Bond and Wignall, 2014), leading to an abundance of research investigating these ties through geologic time. The injection of large volumes of both magmatic and thermogenic isotopically light carbon can explain the observed negative carbon isotope excursions, which provide the causal link between LIPs and global climatic and environmental change
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