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Abstract

Understanding the past is useful in evaluating the present and foreshadowing the future. This especially applies to volcanic activity, which is extremely dangerous when observed and studied “in action”. For instance, some catastrophic caldera-forming eruptions that produce large-volume pyroclastic deposits are scarce and only a few were witnessed by humans. Our study focuses on the late-Variscan (late Carboniferous to early Permian) Gréixer rhyolitic complex (GRC) representing an intra-caldera fill preserved in the Axial zone of Catalan Pyrenees. The GRC is a ~12 km long and several hundred meters thick ignimbrite body embedded in the late Paleozoic sedimentary sequence. This sequence is steeply tilted due to the thrusting during an early stage of the Alpine orogenic cycle. Thus, the GRC exposure provides a unique bottom-to-top erosional cut through the whole caldera fill. The field and structural mapping, anisotropy of magnetic susceptibility, and conventional optical microscopy was used to interpret the intra-caldera emplacement dynamics. Rather chaotically oriented magnetic foliations and constrictional fabrics (over ~50 % of prolate AMS ellipsoids) suggest that we deal with strongly welded ignimbrites. In places, upright, overturned, and sheath folds were identified in the field evidencing a rheomorphic flow. These results imply a high-temperature and high-energetic environment, where turbulent dense pyroclastic flows deposited GRC ignimbrites. Our dataset corresponds well to late-Variscan calderas and their deposits in the European Variscan belt, where similar eruption conditions were reported. It is suggested that this late-orogenic volcanic episode is associated with the extensive tectono-thermal event that followed the collapse of Variscan orogen.