EGU22-9525
https://doi.org/10.5194/egusphere-egu22-9525
EGU General Assembly 2022
© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

Remobilization and eruption of an upper crustal cumulate mush: the Singkut caldera (North Sumatra, Indonesia)

Francesca Forni1,2, Jeffrey A. Oalmann2, Giuditta Fellin3, Steffen Eisele2, Marcus Phua2, Olivier Bernard2, Marcel Guillong3, Hamdi Rifai4, and Caroline Bouvet de Maisonneuve2
Francesca Forni et al.
  • 1University of Milan, Department of Earth Sciences , Milan, Italy (francesca.forni@unimi.it)
  • 2Asian School of the Environment/EOS, Nanyang Technological University, Singapore
  • 3Institute of Geochemistry and Petrology, Department of Earth Sciences, ETH Zurich, Zurich, Switzerland
  • 4Department 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 14C 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 km3 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 high crystallinity (24-62% crystals) and contain pl+amph+bt+opx+Fe-Ti ox+ap+zr±qtz. Notably, large and strongly resorbed quartz crystals are abundant in the pre-caldera lavas and scarce or absent in the caldera-forming tuff and post-caldera lavas. Bulk-rock composition of pumices and lavas varies from andesitic to dacitic, while the matrix glass in the pumices is rhyolitic. Trace element composition of glass (e.g., positive Eu anomalies) indicate resorption of feldspars. Crystallization ages of the youngest zircons in pre-caldera lavas overlap with eruption ages (~250 ka) while crystallization ages of the youngest zircons in the caldera-forming tuff and post-caldera lavas are significantly older (~100 ka) than the eruption ages (~50 and ~16 ka, respectively). Ti-in-zircon thermometry combined with zircon geochronology show that the Singkut magma body experienced a heating phase which started approximately upon eruption of the pre-caldera lavas and continued at least until the eruption of the post-caldera lavas. Such prolonged heating event determined progressive melting of the least refractory mineral phases (mostly quartz and feldspars) and hampered zircon crystallization for ~50 ky before the caldera-forming eruption and ~80 ky before the effusion of the post-caldera lavas. Heating was likely due to an increase of the recharge flux in the magma reservoir which reduced the crystallinity of the crystal mush and promoted remobilization and eruption of the Singkut magma body.

How to cite: Forni, F., Oalmann, J. A., Fellin, G., Eisele, S., Phua, M., Bernard, O., Guillong, M., Rifai, H., and Bouvet de Maisonneuve, C.: Remobilization and eruption of an upper crustal cumulate mush: the Singkut caldera (North Sumatra, Indonesia), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9525, https://doi.org/10.5194/egusphere-egu22-9525, 2022.