New radiometric and petrological constraints on the evolution of the Pichincha volcanic complex (Ecuador) |
| |
Authors: | Claude Robin Pablo Samaniego Jean-Luc Le Pennec Michel Fornari Patricia Mothes Johannes van der Plicht |
| |
Institution: | 1. Laboratoire Magmas et Volcans, Clermont Université, Université Blaise Pascal, BP 10448, 63000, Clermont-Ferrand, France 2. CNRS UMR 6524, IRD R163, 5 rue Kessler, 63038, Clermont-Ferrand cedex, France 3. Instituto Geofísico, Escuela Politécnica Nacional, Ap. 17-01-2759, Quito, Ecuador 6. Laboratoire Magmas et Volcans, Université Blaise Pascal, 5 rue Kessler, 63038, Clermont-Ferrand cedx, France 4. IRD, UMR Géosciences Azur, Université Nice-Sophia Antipolis, Parc Valrose, 06108, Nice, France 5. Center for Isotope Research, University of Groningen, Nijenborgh 4, 9747 AG, Groningen, Netherlands
|
| |
Abstract: | Fieldwork, radiometric (40Ar/39Ar and 14C) ages and whole-rock geochemistry allow a reconstruction of eruptive stages at the active, mainly dacitic, Pichincha Volcanic
Complex (PVC), whose eruptions have repeatedly threatened Quito, most recently from 1999 to 2001. After the emplacement of
basal lavas dated at ∼1100 to 900 ka, the eruptive activity of the old Rucu Pichincha volcano lasted from ∼850 ka to ∼150 ka
before present (BP) and resulted in a 15 × 20 km-wide edifice, which comprises three main building stages: (1) A lower stratocone
(Lower Rucu, ∼160 km3 in volume) developed from ∼850 to 600 ka; (2) This edifice was capped by a steeper-sided and less voluminous cone (the Upper
Rucu, 40–50 km3), the history of which started 450–430 ka ago and ended around 250 ka with a sector collapse; (3) A smaller (8–10 km3) but more explosive edifice grew in the avalanche amphitheatre and ended Rucu Pichincha's history about 150 ka ago. The Guagua
Pichincha volcano (GGP) was developed from 60 ka on the western flank of Rucu with four growth stages separated by major catastrophic
events. (1) From ∼60 to 47 ka, a basal effusive stratocone developed, terminating with a large ash-and-pumice flow event.
(2) This basal volcano was followed by a long-lasting dome building stage and related explosive episodes, the latter occurring
between 28–30 and 22–23 ka. These first two stages formed the main GGP (∼30 km3), a large part of which was removed by a major collapse 11 ka BP. (3) Sustained explosive activity and viscous lava extrusions
gave rise to a new edifice, Toaza (4–5 km3 in volume), which in turn collapsed around 4 ka BP. (4) The ensuing amphitheatre was partly filled by the ∼1-km3 Cristal dome, which is the historically active centre of the Pichincha complex. The average output rate for the whole PVC
is 0.29 km3/ka. Nevertheless, the chronostratigraphic resolution we obtained for Lower Rucu Pichincha and for the two main edifices of
Guagua Pichincha (main GGP and Toaza), leads to eruptive rates of 0.60–0.65 km3/ka during these construction stages. These output rates are compared to those of other mainly dacitic volcanoes from continental
arcs. Our study also supports an overall SiO2 and large-ion lithophile elements enrichment as the PVC develops. In particular, distinctive geochemical signatures indicate
the involvement of a new magma batch at the transition between Rucu and Guagua. At the GGP, the same phenomenon occurs at
each major collapse event marking the onset of the ensuing magmatic stage. Since the 11-ka-BP collapse event, this magmatic
behaviour has led to increasingly explosive activity. Four explosive cycles of between 100 and 200 years long have taken place
at the Cristal dome in the past 3.7 ka, and repose intervals between these cycles have tended to decrease with time. As a
consequence, we suggest that the 1999–2001 eruptive period may have initiated a new eruptive cycle that might pose a future
hazard to Quito (∼2 million inhabitants). |
| |
Keywords: | |
本文献已被 SpringerLink 等数据库收录! |
|