HR: 1400h
AN: V33B-01    [Abstracts]
TI: The Maar-Diatreme System in a Mixed "Hard/Soft-Rock" Setting: an Example from the Pali Aike Volcanic Field, Argentina
AU: * Delpit, S
EM: severine.delpit@ete.inrs.ca
AF: Institut National de la Recherche Scientifique, 490 rue de la Couronne, Quebec, Qc G1K 9A9, Canada
AU: Ross, P
EM: rossps@ete.inrs.ca
AF: Institut National de la Recherche Scientifique, 490 rue de la Couronne, Quebec, Qc G1K 9A9, Canada
AB: The eruptive processes in diatremes remain poorly understood compared to those at other volcano types, because these processes occur at depth. Except for maar-diatreme volcanoes formed during kimberlitic eruptions, volcanologists agree that these systems are of phreatomagmatic origin. The origin of kimberlitic diatremes is more contentious, but studying non kimberlitic equivalents can be a good approach to better understand kimberlitic diatremes considering their numerous common characteristics. The geometry of maar-diatreme systems is strongly influenced by their setting in "hard-rock" or "soft-rock" environments (Lorenz, 2003, Geolines 15:72-83). Formation of maar-diatreme systems in "hard-rock" environments, like in the West Eifel Volcanic Field of Germany, is largely described in the literature but emplacement in "soft-rock" environments or mixed settings is not. In the case of "hard-rock" environments external water is provided by fracture aquifers. The eruption products are juvenile clasts and country rock fragments. The inner crater walls of the maar, and the diatreme walls, have steep slopes. In the case of "soft- rock" environments, water is contained in the sediment pores and the walls tend to be at lower angles. We recently conducted field work on maars, cinder cones and spatter rings of the Pali Aike Volcanic Field of southern Argentina as part of the Potrok Aike Maar Lake Sediment Archive Drilling Project (PASADO). These Quaternary monogenetic volcanoes were emplaced in a mixed "hard/soft-rock" environment containing young glacial sediments, basaltic lava flows, partly consolidated fluviatile sediments, and older indurated sedimentary rocks. The mixed environment of emplacement is reflected in a phreatomagmatic deposit on the inner slope of a tephra ring exposing some lapilli-tuff layers. The lapilli fraction comprises approximately 40% lithics on average (visual estimate): at least half of the fraction is composed of basaltic lava derived from a pre- existing lava flow. The rest of the fraction comprises (1) rounded pebbles likely derived from glacial deposits but also (2) angular, well-indurated sedimentary rocks (quartzite, beige siltstone, red shale and pink to orange sandstone) presumably derived from relatively deep formations of the Austral sedimentary basin.
DE: 8400 VOLCANOLOGY
DE: 8404 Volcanoclastic deposits
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8428 Explosive volcanism
DE: 9604 Cenozoic
SC: Volcanology, Geochemistry, and Petrology [V]
MN: 2009 Joint Assembly