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Flow behaviour in the intra-caldera setting: an AMS study of the large (>1290 km3) Permian Ora ignimbrite

By
Madelaine A. W. Willcock
Madelaine A. W. Willcock
School of Geosciences, Monash University, Clayton, VIC 3800, Australia
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Massimo Mattei
Massimo Mattei
Dipartimento di Scienze Geologiche, Università di Rome Tre, Lago S. Leonardo Murialdo 1, 00146 Rome, Italy
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Pavlína Hasalová
Pavlína Hasalová
School of Geosciences, Monash University, Clayton, VIC 3800, AustraliaCentre for Lithospheric Research, Czech Geological Survey, Klárov 3, Prague, 11821, Czech Republic
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Guido Giordano
Guido Giordano
Dipartimento di Scienze Geologiche, Università di Rome Tre, Lago S. Leonardo Murialdo 1, 00146 Rome, Italy
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Ray A. F. Cas
Ray A. F. Cas
School of Geosciences, Monash University, Clayton, VIC 3800, Australia
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Corrado Morelli
Corrado Morelli
Ufficio Geologia e prove materiali, Provincia Autonoma di Bolzano – Alto Adige, Via Val d’Ega 48, 39053 Cardano, Italy
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Published:
January 01, 2015

Abstract

Anisotropy of magnetic susceptibility (AMS) data reveal heterogeneous pyroclastic flow processes and variable flow directions within the intra-caldera setting of the Permian rhyolitic welded Ora ignimbrite. Magnetic fabric is primary, orientated during the pyroclastic flow emplacement, and is controlled by paramagnetic and ferromagnetic mineral phases. The ignimbrite has typically weak mean magnetic susceptibilities (1.32–21.8×10−4 SI) but with a large spread and low anisotropy degrees (1.003–1.023), which vary in different parts of the caldera. The intra-caldera magnetic fabric provides significant information on the dynamics of the intra-caldera setting, relating to changing vertical and lateral flow emplacement processes. AMS shape ellipsoids range from oblate to prolate; these are interpreted to reflect the heterogeneous nature of the flow resulting from the influence of underlying topography, constraints of the caldera walls, primary welding and post-emplacement mineral growth. We have identified different depositional units and possible eruptive source regions, indicating that more than one source fissure vent was active during eruption within this caldera system. The lateral variations demonstrate a meandering of flow pulses. The caldera margin acts as an obstacle in preventing and rebuffing certain flows from scaling the caldera margin.

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Geological Society, London, Special Publications

The Use of Palaeomagnetism and Rock Magnetism to Understand Volcanic Processes

M. H. Ort
M. H. Ort
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M. Porreca
M. Porreca
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J. W. Geissman
J. W. Geissman
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Geological Society of London
Volume
396
ISBN electronic:
9781862396722
Publication date:
January 01, 2015

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