Geology, Petrology and Magmatic Evolution of the Felsic Alkaline Rocks of the Vitória Island, São Paulo State, Brazil

Authors

  • Akihisa Motoki Universidade do Estado do Rio de Janeiro, Faculdade de Geologia, Departamento de Mineralogia e Petrologia Ígnea, Rua São Francisco Xavier, 524, 20550-013, Maracanã, Rio de Janeiro, RJ, Brasil
  • Kenji Motoki Universidade Federal Fluminense, Laboratório de Geologia do Mar, Departamento de Geologia. Avenida General Milton Tavares de Souza s/n, 4° andar, 24210-340, Gragoatá, Niterói, RJ, Brasil
  • Anderson Costa dos Santos Universidade do Estado do Rio de Janeiro, Faculdade de Geologia, Departamento de Mineralogia e Petrologia Ígnea, Rua São Francisco Xavier, 524, 20550-013, Maracanã, Rio de Janeiro, RJ, Brasil
  • Camila Cardoso Nogueira Universidade do Estado do Rio de Janeiro, Faculdade de Geologia, Departamento de Mineralogia e Petrologia Ígnea, Rua São Francisco Xavier, 524, 20550-013, Maracanã, Rio de Janeiro, RJ, Brasil
  • Werlem Holanda dos Santos Universidade do Estado do Rio de Janeiro, Faculdade de Geologia, Departamento de Mineralogia e Petrologia Ígnea, Rua São Francisco Xavier, 524, 20550-013, Maracanã, Rio de Janeiro, RJ, Brasil
  • Mauro Cesar Geraldes Universidade do Estado do Rio de Janeiro, Faculdade de Geologia, Departamento de Mineralogia e Petrologia Ígnea, Rua São Francisco Xavier, 524, 20550-013, Maracanã, Rio de Janeiro, RJ, Brasil
  • Susanna Sichel Universidade Federal Fluminense, Laboratório de Geologia do Mar, Departamento de Geologia. Avenida General Milton Tavares de Souza s/n, 4° andar, 24210-340, Gragoatá, Niterói, RJ, Brasil
  • Thais Vargas Universidade do Estado do Rio de Janeiro, Faculdade de Geologia, Departamento de Mineralogia e Petrologia Ígnea, Rua São Francisco Xavier, 524, 20550-013, Maracanã, Rio de Janeiro, RJ, Brasil

DOI:

https://doi.org/10.11137/2018_3_125_136

Keywords:

Vitória Island, Nepheline Syenite, Syenite, Fractional Crystallization

Abstract

This paper presents geological, petrographic, geochemical characteristics and radiometric ages for the felsic alkaline rocks of the Vitória Island Intrusive Complex, São Paulo State, Brazil, and discusses its geochemical evolutionary processes. This intrusive body is about 3 km in diameter and the emerged island areas correspond to its northeastern border. The main intrusion is constituted by nepheline syenite at the central zone, nepheline-bearing syenite at the outer zone, syenite at the border, and quartz syenite at the contact zone with alkali feldspar represented by orthoclase. Clinopyroxene vary from hedenbergite, soda-augite to aegirine-augite, and some crystals have resorption shape and amphibole reaction rim. They are intruded by phonolite, trachyte and lamprophyre dykes. Based on the intrusion relation, the dykes are subdivided into two generations: the first one encompass a radial system and the second a parallel system with NE-SW orientation and sinistral displacement. The alkaline rocks vary from silica undersaturated to oversaturated, show peralkaline to peraluminous compositions and belong to the potassic series. The K-Ar ages for amphibole of the nepheline-bearing syenite and syenite are 89.58 ± 3.18 Ma and 84.35 ± 3.92 Ma, respectively. The rocks are strongly fractionated with high differentiation index (DI) of 88.15 in average, indicating mafic minerals fractionation. K2O/ (K2O+Na2O) ratios are low, in average 0.51, in accordance to crystallization of pseudo leucite and potassic feldspar the last observed in thin sections.

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Published

2019-10-16

Issue

Vol. 41 No. 3 (2018)

Section

Article

License

This journal is licensed under a Creative Commons — Attribution 4.0 International — CC BY 4.0, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.