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Lithology, mode of occurrence, and K-Ar ages of the Lautaro Volcano, Chilean Patagonia, South America

Akihisa MOTOKI 1), Jose Antonio NARANJO 2), Yuji ORIHASHI 3), Daiji HIRATA 4), Takahiro HOSONO 5), Felipe Dias CARIO 1), Hirochika, SUMINO. 4), Keisuke, NAGAO, 4), Daniel Dias LOUREIRO 1), Jose Luiz Peixoto NEVES 1), Ryo ANMA 5)

1) Department of Mineralogy and Petrology, Rio de Janeiro State University (DMPI/FGEL/CTC/UERR). Rua Sao Francisco Xavier 524, Sala A4023, Maracana, Rio de Janeiro, ZC 20550-990, Brazil. 2) Servicio Nacional de Geologia y Mineria, Chile, 3) University of Tokyo, Japan, 4) Kanagawa Province Museum, Japan, 5) Tsukuba University, Japan.

Corresponding author: Akihisa Motoki

Japan-Korea joint meeting of isotope ration mass spectrometry, 42.

The Lautaro Volcano is situated in Patagonia, at 49 deg. 01.4 min. S, 73 deg. 32.5 min. W, being the closest volcano to the Chile Triple Junction, in which on-going ridge subduction is taking place. The petrologic and geochemical studies of the adakitic rocks Lautaro Volcano have a great contribution in order to clarify juvenile continental crust formation process, as a modern analogue of the Archaean magmatism.

The access to the Lautaro Volcano is extremely difficult due to its accidental morphology and unfavourable climate conditions. The volcano edifice, of 3,380m above sea level, stands out on the highland covered by glaciers and snow of 2400m above the sea level, so-called gCampo de Hielo Surh. On February 28, 2002, the authors have performed a successful sampling of eruptive materials of the volcano using a helicopter, from the deposits of Lautaro Glacier and OLHiggins Glacier.

The Lautaro Glacier occurs on the western flank of the volcano, starting from the volcanic edifice and reaching a fjord. The basement of the volcano is composed of low-grade metapelite intruded by of hornblende biotite granite. A young terminal moraine contains rounded boulders of volcanic rock blocks composed mainly of lightgrey dacite, characterised by abundant phenocrysts of plagioclase and hornblende. Some samples present rhyotaxitic-like texture and one block shows prismatic jointing that indicates its origin from the collapse of a lava dome.

On the eastern slope, the O'Higgins Glacier takes place. The side moraine contains blocks of basement rocks, but no volcanic rocks. Lapilli-size pumice fall and ash-size deposit originated from latest eruption of the Lautaro volcano are found on the ice. The ash take place covering triangular ice ridges, so-called dirt cones, and constituted mainly by pumice fragment, volcanic glass, quartz, plagioclase, biotite, hornblende, and dark yellow orthopyroxene. High-sensitivity K-Ar datings have revealed presence of three dome-formation eruptive events, being 180Ka, 90Ka and recent.

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