Understanding large caldera in-filling processes: the facies, stratigraphic architecture and volcanology of the Permian Ora Formation and caldera super-eruption, northern Italy

2016-11-29T02:08:47Z (GMT) by Willcock, Madelaine Ann Welcome
The eruption of the Permian Ora caldera of northern Italy during the Hercynian-Variscan tectonic cycle, represents one of the largest explosive eruptions on Earth, having erupted more than 1290 km3 of material, and provides significant information on caldera eruption processes. Research on large caldera volcanoes (> 100 km3) is important due to their potentially devastating global impacts on the natural world and human society. The absence of such an eruption in recent recorded history means fundamental questions remain unanswered regarding the processes related to these volcanoes. The Permian Ora caldera offers a unique opportunity to examine the eruptions of these gigantic systems because of the rare and exceptional preservation and exposure of the intra-caldera fill sequence. The purpose of this study is to document the lithofacies, and the stratigraphic architecture of the Ora caldera, and also to interpret processes such as the timing and nature of caldera collapse, eruption styles, pyroclastic flow processes within the intra-caldera and proximal extra-caldera settings, and caldera in-filling processes. The research approach adopted in this study included detailed field stratigraphic logging, petrographic analysis of componentry and texture, anisotropy of magnetic susceptibility analysis (AMS) of pyroclastic flow patterns, and geochemical analysis. This study has established that the Ora Formation consists of rhyolitic ignimbrite deposits (67.95% – 77.73% SiO2), with sub-ordinate basal volcanic lithic breccia and local interbedded surge layers. The ignimbrite succession is crystal-rich (~25 – 57%; Ø43%), lithic-poor (0 – 46%; Ø2%), has common juvenile clasts (0 to ~45%; Ø20%), is ubiquitously welded, and has a relatively uniform main mineral population (volcanic quartz, sanidine, plagioclase, and biotite). The Ora Formation has been divided into four members (ORAa – ORAd) which consist of (a) a basal volcanic lithic breccia; (b) lithic-rich ignimbrite with minor surge lithofacies; (c) the dominant, thick(> 1 km thickness in total), coarse-crystal-rich ignimbrite (including the bimodal crystal sub-facies), with local interbedded vitrophyre, fine-crystal-rich, and lithic-rich ignimbrite, and minor surge lithofacies deposits; and (d) the fine-crystal-rich ignimbrite with minor lithic-rich ignimbrite lithofacies deposits. These members largely define the eruption phases, from vent opening (member a), vent clearing (member b), waxing and steady eruption (member c), to waning eruption stages (member d). Members a and b reveal caldera collapse occurred from the onset of eruption and identify potential source vent locations, whereas member c illustrates late stage overspill of material from the caldera system. The Ora caldera is proposed as being a volcano-tectonic system, which formed two pene-contemporaneous, coalesced caldera collapse depressions: Northern and Southern. This is interpreted to have occurred via rapid, relatively passive, piston style collapse events, producing a caldera complex approximately 42 x 40 km in size. The ignimbrite succession has a restricted compositional range. Nevertheless, detailed textural, compositional, mineralogical, and stratigraphic data support an eruption evolving from south to north, and the existence of a weakly zoned magma chamber. This is illustrated particularly by a subtle northwards decrease in modal free crystal biotite abundance across the deposit. Other major findings based on fieldwork and laboratory studies include: (i) the absence of a Plinian fallout deposit, indicating that a buoyant Plinian eruption column did not form; (ii) the eruption was marked by a low, continuously collapsing eruption fountain style, which was limited from the outset by catastrophic caldera collapse and decompression of the magma chamber; (iii) the vent was a fissure system with multiple discharge points, feeding more or less on-going, subtly different, pyroclastic flow pulses; (iv) the vertical lithofacies changes record temporal variation in eruption intensity, source material, and source location, demonstrating an incremental caldera in-filling process; (v) the AMS magnetic fabric of the ignimbrite succession indicates changing vertical and lateral pyroclastic flow directions and source locations of the flow pulses; (vi) the pyroclastic flow system was a hot and poorly expanded, high particle concentration, granular density current, with laminar shear forces close to the depositional boundary, and limited ash winnowing; and (vii) the eruption style and pyroclastic flow dynamics facilitated complete deposit welding, illustrated by plastically deformed juvenile shard and fiamme clast morphologies. This research has detailed for the first time the eruptive products and stratigraphy of the Ora Formation and reconstructed the eruption processes and evolution of the Ora caldera system. Key findings include identification of the relative timing of caldera collapse, the caldera in-filling processes, and delineation of a lithostratigraphic and chemical architecture within the deposits. These results highlight the vital role investigation of the intra-caldera deposit plays in understanding of caldera processes. The intra-caldera ignimbrite succession shares many of the general characteristics described for ignimbrites in the extra-caldera setting, which implies that classification schemes and models of pyroclastic flow and emplacement processes are largely transferable between the two settings.