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


IPGP - Îlot Cuvier


Séminaires Systèmes Volcaniques

Salle 108

Francesca Forni

Universita di Milano Statale

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 at active volcanoes. In this study we provide the first volcanological and petrological characterization of a young volcanic system located in northern Sumatra, 35 km N of the Toba caldera and 40 km SW of the major city of Medan: the Singkut caldera. Singkut is a 9 km diameter volcano-tectonic depression 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 48 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 and supra-chondritic Zr/Hf) indicate resorption of feldspars and zircons. 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 (48 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. To attend online (Zoom meeting): Meeting ID : 891 0466 0968 Password : 806588