Abstract

ABSTRACT: The Late Cretaceous Búzios Island alkaline massif intrudes Precambrian charnockites and consists dominantly of syenitic rocks that are cut by a large number of dikes, mostly NE-trending, and representing two distinct suites, a felsic one and a mafic-ultramafic one. Alkali feldspar is the most abundant mineral; other constituents are clinopyroxene, commonly replaced by amphibole/biotite, and opaques. Accessory minerals include occasionally rare phases bearing Zr, Ti, Nb and Rare Earth Elements (REE). The felsic dikes may also have nepheline (sodalite). The mafic-ultramafic suite, in particular the lamprophyres, shows a primary mineral assemblage with olivine, clinopyroxene and amphibole in addition to a groundmass having glassy material and carbonates (ocelli). The Búzios rocks are chemically evolved, mostly of potassic affinity and mainly belong to the miaskitic series. Variation diagrams for major and trace elements show a bimodal distribution, suggesting an origin from different magmatic pulses. The rocks are interpreted as having been derived by fractional crystallization processes from a basanitic parental magma. The SiO2-undersaturated and SiO2-oversaturated associations present in the massif are apparently not linked to a single magmatic source, and in the petrogeny residual system two trends are evident: the first one towards the phonolitic minimum and the second one towards the rhyolitic minimum, possibly pointing to amphibole fractionation.

Highlights

  • Coastal islands composed mainly of alkaline rocks are found along the shoreline of the state of São Paulo, in Southeastern Brazil, forming stocks and a great number of dikes of variable compositions and dimensions

  • The well-known occurrences are São Sebastião, Búzios, Vitória, and Monte de Trigo (Fig. 1), all aligned along a NE trend parallel to the regional structure represented by the Santos fault and related tectonically to the Southeastern Brazilian Continental Rifting (Riccomini et al 2005)

  • This paper provides a general picture of the alkaline magmatism of Búzios Island, emphasizing the geochemistry and mineral chemistry of the rocks and contributing to a better understanding of the formation and evolution of the alkaline massif

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Summary

Introduction

Coastal islands composed mainly of alkaline rocks are found along the shoreline of the state of São Paulo, in Southeastern Brazil, forming stocks and a great number of dikes of variable compositions and dimensions. SiO2-oversaturated syenitic rocks have amphibole as the main mafic mineral and quartz in association with alkali feldspar, forming a typical granophyric texture.

Results
Conclusion

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