Year

2010

Degree Name

Bachelor of Science (Honours)

ANZSRC / FoR Code

04 EARTH SCIENCES, 040202 Inorganic Geochemistry, 040303 Geochronology

Department

School of Earth & Environmental Sciences

Advisor(s)

Allan Chivas

Abstract

The Andino-Cuyana Basaltic Province (ACBP) located in the province of Mendoza, central western Argentina, constitutes an extensive volcanic area to the east of the Andes, with more than 800 volcanoes, and which has been divided in two volcanic fields, the Llancanelo Volcanic Field to the north (LLVF) and the Payunia Volcanic Field (PVF) to the south. The ACBP lacks detailed chronological and geochemical information. This study constitutes the first to compare the two volcanic fields and to use cosmogenic noble gas surface exposure and Ar-Ar dating.

Previous studies estimate that the lavas from the ACBP date from approximately the Plio-Pleistocene to Holocene. Currently no accurate dates have been provided; as young lavas are difficult to date by conventional methods. Using cosmogenic 21Ne* and 3He* surface exposure dating technique, it was found that the basalts from the PVF range from 31.1±4.3 ka from Los Volcanes area, to <7 ka in Santa Maria Volcano, which is considered to represent one of the latest eruptions in the PVF. The single sample analysed by cosmogenic dating from the LLFV indicates an age of 54.9±5.1 ka which may represent one of the youngest eruptions within the LLVF. In addition, ages indicated by Ar-Ar from the LLVF show that the volcanoes on the edge of Llancanelo Lake range from 1.69±0.29 Ma, in Cerro Coral to 0.395±0.068 Ma, in Cerro Trapal. The data provided here also confirm field observations that the LLVF is older than the PVF.

Chemical analysis of the basaltic rocks indicates that they are olivine-rich alkaline basalts, with some differences between the two volcanic fields. The geochemical information shows that the LLVF tends to have much more influence from the subducting slab than the PVF. Nevertheless, the basaltic bombs from PVF tend to have a chemical composition strongly influenced by fluid from the subducting Nazca plate. The PVF trachyte flow sampled is typical of a more evolved intermediate rock with low Sr and Sr/Nd values indicating plagioclase fractionation. Sample LL1, from Cerro Coral, constitutes the oldest dated sample within the LLVF and it also shows the strongest geochemical association with the volcanic arc. The subducting Nazca plate has a great influence on the geochemical processes that formed the ACBP, and the magmas produced.

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