Magmatism that occurs within the Central Volcanic Zone (CVZ) of the Andes is heavily influenced by processes of differentiation and assimilation that occur within the extremely thick continental crust. These processes, that result in crustal contamination of mantle derived magmas, are not uniform across the region and are determined locally. Data from Azufre is combined with that of previously studied volcanoes of the CVZ, and specifically the Altiplano-Puna Volcanic Complex (APVC) in northern Chile, to further understand such processes that have occurred in this region. Factors that need to be taken into consideration when examining volcanoes of the APVC are the aforementioned thickness of the continental crust of the Andes, which differentiates the Andes from other volcanic arcs globally, and the presence of a zone of partial melt, the Altiplano-Puna Magma Body (APMB), below the APVC as they could both have had a major effect on erupting lavas. Elemental concentration and isotope data are used here to examine the role that the APMB has had on the evolution of Azufre. This data set is also compared to those of previously studied volcanoes of the APVC to further provide understanding as to the interaction between the APMB and the APVC. Major element data reveals well defined sub-alkaline trends, with samples predominantly being andesitic in composition. 87Sr/86Sr ratios of 0.7067 - 0.7075 and δ 18O values of 7.85 - 9.72 are high in comparison to those of other Central Andean lavas, however in comparison to other erupted lavas of the APVC they are low. A transition from the edges of the APVC towards the centre show an increase in 87Sr/86Sr and δ 18O. Conversely 143Nd/144Nd ratios decrease towards the centre, with Azufre displaying ratios of 0.5123 - 0.5124. These changing ratios from the edge to centre of the APVC correspond with changing thickness of the APMB below the APVC and are therefore viewed to represent the amount of interaction between parental magmas of the APVC with the partial melt of the APMB, primarily with increasing 87Sr/86Sr indicating increasing amounts of interaction. δ 18O data indicates that there are multiple sources of O enrichment, evidenced by two opposite data arrays, with one array indicating assimilation by magmas of high δ 18O crust and the second array indicating assimilation of low δ 18O crust. Modelling of δ 18O suggests that the magmas which have assimilated low δ 18O crustal material have interacted with between -2.01 and 3.49 . Low LREE/HREE ratios and a negative Eu anomaly support the idea that assimilation of crustal material occurred at shallow crustal levels by indicating shallow plagioclase fractionation and evolution in a garnet free environment, most likely within the APMB.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:uct/oai:localhost:11427/31378 |
| Date | 27 February 2020 |
| Creators | Lister, James |
| Contributors | Le Roux, Petrus, Harris, Chris |
| Publisher | Faculty of Science, Department of Geological Sciences |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Master Thesis, Masters, MSc |
| Format | application/pdf |
Page generated in 0.002 seconds