GNGTS 2022 - Atti del 40° Convegno Nazionale

GNGTS 2022 Sessione 1.2 99 REMOBILIZATION AND ERUPTION OF AN UPPER CRUSTAL CUMULATE MUSH: THE SINGKUT CALDERA (NORTH SUMATRA, INDONESIA) F. Forni 1,2 , J.A. Oalmann 2 , G. Fellin 3 , S. Eisele 2 , M. Phua 2 , O. Bernard 2 , M. Guillong 3 , H. Rifai 4 , C. Bouvet de Maisonneuve 2 1 Department of Earth Sciences “Ardito Desio”, University of Milan, Italy 2 Asian School of the Environment/EOS, Nanyang Technological University, Singapore 3 Institute of Geochemistry and Petrology, Department of Earth Sciences, ETH Zurich, Zurich, Switzerland 4 Department of Physics, Faculty of Mathematics and Natural Sciences, Universitas Negeri Padang, Indonesia Understanding the conditions and timescales of storage and remobilization of magma bodies in the upper crust is key to interpret the signals of potential reawakening of the volcanic activity at active volcanic systems. In this study we provide the first volcanological and petrological characterization of the Singkut volcanic system located in northern Sumatra, ~35 km N of the Toba caldera and ~40 km SW of the major city of Medan. Singkut is a ~9 km diameter caldera delimited by ~300 m-high rims where pre-caldera lavas are exposed. The inner part of the collapsed structure is occupied by three post-caldera volcanoes and currently hosts an active geothermal field. We utilize field observations and correlation with a distal marine tephra layer to map the extension and thickness of the tuff erupted during the caldera-forming eruption and use these data to estimate the erupted magma volume. We use major and trace element data of bulk-rock, matrix glasses and minerals to characterize the pre-eruptive conditions of pre- and post-caldera lavas and caldera- forming tuff and 14 C and U/Th-He zircon dating to determine the eruption ages. In addition, a combination of U/Th and U/Pb in-situ zircon dating and zircon trace element geochemistry provides insights into the mechanisms and timescales that led to the Singkut caldera-forming eruption and those that controlled the post-caldera activity. Our data show that Singkut caldera formed ~50 ka during a large explosive eruption that deposited ~60 km 3 of pyroclastic material. The cataclysmic eruption was preceded by at least 200 ky of mostly effusive pre-caldera activity and followed by effusive and mildly explosive post-caldera activity, with the last eruption reported at 1881 AD. The lavas and pumices have Fig. 1 - U/Pb and U/Th crystallization ages (ka) vs. Ti content (ppm) of zircons from the lavas and pyroclastics from the Singkut caldera. Crystallization temperatures estimated using Ti-in-zircon thermometry (Ferry and Watson, 2007). Vertical solid bars indicate eruption ages obtained via U/Th-He zircon dating including uncertainties while dashed bar indicate 14 C dates.