| Popis: |
Explosive volcanism in the Altiplano-Puna Volcanic Zone (APVZ) of the South American Central Andes flared in the Late Miocene to form one of the largest silicic volcanic provinces on Earth. The partially exposed ~6.6 Ma rhyolitic Ramadas Volcanic Centre (RVC) represents a unique phase of activity within the APVZ, and is located in the Argentine province of Salta, at an altitude of ~3800 m a.s.l. The RVC provides an example of large volume plinian explosive volcanism from a central vent (~ 4 km diameter), which preserves little or no volcanic edifice. At least 2 major volcanic phases recognised in the evolution of the centre. The initial phase of the eruption erupted in excess of 36 km3 (DRE) of magma, emplacing a thick sequence of distinctive pumiceous fall deposits and minor intercalated, density current deposits on the Eastern Puna, locally termed the Corte Blanco Tuff (CBT). Thick fine ash correlatives are found throughout the sub-Andes of NW Argentina and Bolivia, highlighting the extremely large dispersal area of the eruption. Facies characteristics and distribution are consistent with the development of a large, relatively stable, plinian style eruption column, estimated to have peaked at heights 90% of the pumice found in the Ramadas sequence. The pumice is a crystal poor, peraluminous rhyolite, with a high density of extremely attenuated perfectly aligned tube vesicles (mean vesicularity of 64%). The textures observed indicate that the bulk of the erupting magma was subjected to a high rate of strain during high velocity ascent, prior to explosive fragmentation. The crater produced during RVC formation has previously been interpreted as a collapse caldera. A combination of geophysical modelling and detailed facies analysis defines the vent as a tapered funnel with an associated diatreme. There is no evidence for major subsidence of the chamber roof during the eruption. Hence, the term collapse caldera is not appropriate, with explosive reaming is defined as the most likely mechanism of vent formation. Surrounding the central vent is a sequence of diffusely bedded pyroclastic surge, fall and flow deposits, comprised essentially of perlite fragments and subordinate pumice. The sequence constitutes the remnants of a tuff ring, emplaced following the explosive hydrovolcanic destruction of a perlitised obsidian (crypto) dome. A small outcrop of rhyolitic lava found on the vent margins represents the final stage of dome growth within the complex. The results of this study have bearing on: a) Why plinian fallout deposits of substantial volume appear to be rare in the high Andes. At high altitude, eruption columns are more prone to collapse, favouring the formation of pyroclastic flows and ignimbrite. In such settings, only the high velocity eruption of volatile rich magmas at stable and high mass discharge rates is likely to produce widespread large volume, plinian fallout deposits, such as the Corte Blanco Tuff. b) The generation of tube pumice and the complexities associated with the ascent of magma during explosive silicic eruptions. The Ramadas Pumice is defined as a classical example of extreme tube pumice and highlights the critical importance of magma viscosity and conduit(s) geometry in controlling textural formation. A complex multi-stage history of vesiculation and deformation is recorded, with explosive fragmentation facilitated by the high magma strain rate. c) The mechanisms associated with the formation and classification of explosive calderas, in particular so-called funnel calderas. It is concluded that the RVC is not a caldera, but a funnel shaped, highly explosive vent. Further, the use of “funnel caldera” as a classification is shown to be ambiguous at best. d) The formation of silicic tuff rings. The RVC gives a unique example of the complete hydrovolcanic/hydrothermal destruction of a large, perlitically altered (crypto) dome, a phenomenon that has not been previously described anywhere in the world. |
