Year

2012

Degree Name

Bachelor of Science (Honours)

ANZSRC / FoR Code

0401 ATMOSPHERIC SCIENCES, 040202 Inorganic Geochemistry, 04 EARTH SCIENCES, 0403 GEOLOGY, 040314 Volcanology

Department

School of Earth & Environmental Science

Advisor(s)

Allan Chivas

Abstract

A petrological and geochemical study was performed on rock samples collected from the retro-arc, Payún Matrú Caldera (36°24' S, 69°11' W) Mendoza Province, western Argentina. The caldera formed between 168 ± 4 ka and 86 ± 1 ka, with volcanic products in the area ranging from Pleistocene to Holocene (Germa et al., 2010). Payún Matrú is located on La Carbonilla Lineament, which also controls the location of at least 300 other volcanic vents. Field work was carried out over five days in late February 2012, 38 hand samples were collected and five trachyte dykes were identified within the northeast quadrant of the caldera’s topographic rim, with their strike and dip noted. Hernando et al. (2012) proposed the formation of a sub-volcanic magma chamber, with magma mixing being a dominant process. They further suggest that the influx of mafic material into the base of the chamber resulted in caldera formation. Germa et al. (2010) propose that the caldera formed from tectonic influences, due to the regional extensional setting. More evidence was found for the former process within this research project. A comparison between the seven identified rock suites was conducted. From an analysis of petrological thin-sections, it was evident that the chamber had progressively evolved to its most mature state, immediately before caldera formation. The degree and type of dis-equilibrium textures within phenocrysts was noted, as well as the proportion of low- to high-temperature minerals. A comparison between photomicrographs and the major- and trace-element contents was made to determine the conditions in the chamber at a given point in time. A method which utilised digital elevation models to measure the physical attributes of lava flows around Payún Matrú was also used and tested to demonstrate the evolution of melts from the Payún Matrú magma chamber. It was found that the most likely cause of caldera formation was the over-pressuring of a zoned magma chamber, due to an influx of mafic material. This resulted in the eruption of only the evolved upper portion of the chamber, with the lower portion erupting post-caldera. This post-caldera flow represents the most primitive flow, with a flat trace-element plot and a high proportion of ferromagnesian minerals. The chamber environment, post-caldera, is dominated by the crystallisation of mineral assemblages which have a high Al

2O3 content, and large ion-lithophile elements (LILE) such as Sr and Ba are accepted into the mineral assemblage. This is supported by the presence of large, unsieved sanidine phenocrysts in the petrological analysis. The waning of mafic material into the chamber is thought to have occurred after this period. The significance of this project is that it demonstrates the possible mechanisms for large, ash-flow, caldera-forming eruptions.

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