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PRELIMINARY OBSERVATIONS OF OCCURRENCE AND MINERAL MODE OF THE ROCKS, PUMICE AND ASH ON THE LAUTARO VOLCANO, CHILEAN PATAGONIA

A. Motoki 1), J.A. Naranjo 2, Y. Orihashi 3), D. Hirata 4), T. Hosono 5), F.D. Cario 1), and R. Anma 5)

1) Rio de Janeiro State University, Brazil 2) Servicio Nacional de Geologia y Mineria, Chile 3) University of Tokyo, Japan 4) Kanagawa Prefecture Museum, Japan 5) Tsukuba University, Japan.

VI Congresso Geologico Argentino, CD-ROM, \app\pdf\439.



The Lautaro Volcano is situated in Chilean Patagonia, approximately 49 deg. 01.4 min. S, 73 deg. 32.5 min. W, being the closest volcano to the Chile Triple Junction (CTJ), 46.5 deg. S, in which ongoing ridge subduction takes place. The volcanic rocks of the Austral Volcanic Zone, to the south of the CTJ, including those of the Lautaro Volcano, were previously studied, and revealed to be of adakitic composition. The adakitic magmas occur characteristically during subduction of a young, light, and hot oceanic plate, generated by partial melting of oceanic crust present on the surface of slab, the phenomenon so-called "slab-melting". So, the volcanologic, petrologic and geochemical studies of this volcano have a great scientific contribution to clarify the formation process of juvenile continental crust, as a modern analogue of the Archaean magmatism.

In spite of the great scientific importance, access to the Lautaro Volcano is extremely difficult due to its accidental morphology and unfavourable climate conditions. The volcano edifice is almost covered by glaciers and snow, forming a highland of 3,380m above the sea level, being the highest mountain of so-called "Campo de Hielo Sur". These characteristics preclude any attempt to reach the volcano edifice and no research group could get rock samples in sufficient variety and quantity. On February 28, 2002, the authors have performed a successful sampling of eruptive materials of the Lautaro Volcano from the deposits of Lautaro Glacier and O'Higgins Glacier, using a helicopter of Chilean Police Department (Carabineros).

On the western flank of the volcano, the Lautaro Glacier takes place starting from the top of volcanic edifice and reaching a fjord at sea level. A lateral wall formed by glacial erosion exposes the basement of the volcano composed of low-grade metapelite with intrusion of mean-grained granite. A terminal moraine has accidental surface, showing characteristics of a young recession moraine. This moraine deposit contains volcanic rock blocks in addition to the basement rocks, and 32 samples have been collected. These volcanic rocks are composed mainly of light-grey dacite and grey andesite, characterised by abundant phenocrysts of plagioclase and hornblende. Some samples present rhyotaxitic-like texture. One large block on the ground shows peculiar prismatic jointing that indicates its origin from the collapse of a lava dome. There is another terminal moraine with plane surface with full of similar volcanic rock blocks, and 3 samples have been collected.

The volcanic rocks collected are: 1A) Pl (plagioclase) rich Hb (hornblende) rich dacite, the most abundant rock type; 1B) Pl. rich minor amount Hb dacite; 1C) Hb dacite with mafic inclusion, with the same groundmass of 1A; 1D) Hb dacite with minor amount of phenocrysts, that seems to be older than 1A, 1B and 1C; 1E) Hb dacite with glassy groundmass; 1F) Glassy Hb dacite with abundant Pl and minor amount of Hb, with bread-crust bomb texture; 1G) Banded glassy Hb dacite; 1H) Glassy Hb dacite with abundant Pl and minor amount of Hb, that seem to be of the oldest stage; 2A) Pl rich Hb rich dacite, similar to 1A; and 2B) Pl rich minor holnblend dacite, similar to 1B.

The volcanic rocks collected are: 1A) Pl (plagioclase) rich Hb (hornblende) rich dacite, the most abundant rock type; 1B) Pl. rich minor amount Hb dacite; 1C) Hb dacite with mafic inclusion, with the same groundmass of 1A; 1D) Hb dacite with minor amount of phenocrysts, that seems to be older than 1A, 1B and 1C; 1E) Hb dacite with glassy groundmass; 1F) Glassy Hb dacite with abundant Pl and minor amount of Hb, with bread-crust bomb texture; 1G) Banded glassy Hb dacite; 1H) Glassy Hb dacite with abundant Pl and minor amount of Hb, that seem to be of the oldest stage; 2A) Pl rich Hb rich dacite, similar to 1A; and 2B) Pl rich minor holnblend dacite, similar to 1B.

On the eastern slope, the O'Higgins Glacier takes place. Its lateral wall by glacial erosion also exposes metapelite basement. The side moraine has an accidental surface and contains blocks of the metapelite and granite, but no volcanic rocks. On the other hand, there is a pumice fall of lapilli and ash size deposit on the ice, originated during the latest eruption of Lautaro volcano. The pumice lapilli is of 2~3cm in size, with a maximum piece of 7cm. The ash blanket of averages 5cm thick and is present only on the western slope of the triangular ice ridges prominent on the glacial surface, but no on the eastern slope.


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