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The Ilugissoq graphite andesite volcano on Nuussuaq belongs to the Asuk Member of the Paleocene Vaigat Formation. It is the largest eruption site within the Vaigat Formation and is recognized as the source of the majority of the graphite andesite tuffs found in marine sediments in central Nuussuaq. The volcano consists exclusively of pyroclastic rocks containing a diverse lithic assemblage including sediment xenoliths. The primary pyroclastic fragments consist of magnesian andesite with several weight percent of graphite, which formed when mafic magma established a shallow-level magma reservoir beneath the eruption site, and within older clastic sediments from the Nuussuaq Basin. Magma-modified mudstone is completely dominant in the xenolith assemblage and attests that the graphite andesite originated through prolonged high-temperature assimilation of mudstone. The eruptions took place on a marine shelf consisting of picritic hyaloclastites and subaqueous crater mounds. The volcano consists of four overlapping crater cones aligned along a more than 4 km long NNW–SSE oriented fissure system; two cones barely reached sea level whereas the other two reached up to 200 m above the sea. The morphology of the pyroclastic rocks demonstrates that the volcano evolved through phreatomagmatic activity, which diminished with time. The magma never degassed sufficiently to reach a subaerial lava stage. A moderate primary gas pressure well in excess of 100 bars in the graphite andesite magma facilitated the phreatomagmatic explosions, which created the Ilugissoq volcano. The rocks of the volcano are rich in graphite and contain little or no native iron. In comparison, the contemporaneous and chemically similar subaerial lavas and breccias from Disko and Nuussuaq contain less graphite and more iron. The differences are considered to be due to the extremely pressure-dependent redox-sensitivity of carbon–oxygen equilibria in the range 1 bar (the lavas) to 500 bars (the Ilugissoq volcano). Eruptions in the Ilugissoq volcano must have been accompanied by intense CO and CO2 emanations, which would have represented a lethal hazard for all animal life in the volcano's surroundings. |
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