Abstract

ALPERIN, M.; ECHEVESTE, H.; FERNANDEZ, R.  and  BELLIENI, G.. Statistical analysis of geochemical data from Jurassic volcanic rocks of the Deseado Massif, province of Santa Cruz. Rev. Asoc. Geol. Argent. [online]. 2007, vol.62, n.2, pp.200-209. ISSN 0004-4822.

During the detailed mapping of the central zone of the Deseado Massif, a group of Jurassic volcanic units consisting of lavas and ignimbrites of andesitic, dacitic and rhyolitic composition was identified. A statistical analysis of major oxides SiO₂, TiO₂, Al₂O₃, FeO (total), MnO, MgO, CaO, Na₂O, K₂O y P₂O₅ from 73 samples originated from eight volcanic units was conducted. The total geochemical composition of each sample constitutes a closed set of data (or set of compositional data) as the sum of each sample's part is 100%. To analyze the relationships between oxides, between volcanic units, and to identify the existence of subsets, compositional biplots were used as an exploratory tool. In addition, cluster analysis of the samples and statistical inference were performed. The results allowed for: (a) Verification of the existence of two groups of volcanic units, a first group of mostly rhyolitic and a second group of mostly andestic and dacitic. Although the first group includes volcanic units that could be included in the Chon Aike Formation, the volcanic units of the second group do not fit in any of the formations units defining the Jurassic volcanism of the region. (b) Definition of samples subsets that closely match well known volcanic units, indicating a defined relationship between the samples and their respective chemical compositions. (c) Confirmation of the absence of a compositional gap shown by the TAS and AFM diagrams. (d) Recognize that the detailed mapping of lithologic units and/or lithofacies is more representative than that of the formations, as it allows for a better representation of the volcanic evolution of the region.

Keywords : Compositional data; Biplot; Cluster analysis; Jurassic volcanism; Deseado Massif.

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