Eruptive chronology of Tungurahua volcano (Ecuador) revisited based on new K-Ar ages and geomorphological reconstructions
Résumé
This study focuses on the evolution through time of Tungurahua volcano (Ecuador), and provides new information regarding its history. Eighteen new K-Ar ages constrain its construction and the activity of its three successive edifices. We show that the volcano is much younger than expected. Indeed, the older edifice activity only began around 293 ± 10 ka, and ended at 79 ± 3 ka. After ~50 ka of quiescence, the second edifice started growing at 29 ± 2 ka after a major sector collapse, and itself collapsed at ~3 ka. Since then, the third edifice filled the amphitheatre and is still active. Together with numerical reconstructions of the morphology of the three edifices flanks before erosion, these new ages allow us to quantify the magmatic productivity rates during their construction, from 0.6 ± 0.3 and 0.9 ± 0.2 km3/ka for the two older edifices to 2.5 ± 1.0 km3/ka for the youngest, as well as an erosion rate of 0.2 ± 0.1 km3/ka, occurring since the end of Tungurahua I construction. Major and trace element contents of lavas from the three edifices display rather similar trends. Combined with our new ages, the magmatic signature through time does not seem to have been significantly affected either by the sector collapses experienced by the volcano, or by changes of the deep magmatic source. Finally, our results show that the K-Ar dating method by the unspiked Cassignol-Gillot technique performed on groundmass can be successfully applied to lava flows older than the Holocene, while the uncertainties related to younger units can prevent an accurate age determination. Particularly, this method can be applied to Quaternary volcanoes from the Ecuadorian arc, with many of them remaining without knowledge of the timing of their past activity.
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Licence : Copyright (Tous droits réservés)