Journal
BRAZILIAN JOURNAL OF GEOLOGY
Volume 47, Issue 1, Pages 127-145Publisher
SOC BRASILEIRA GEOLOGIA
DOI: 10.1590/2317-4889201720160121
Keywords
Igneous petrology; Alkaline rocks; Geochemistry
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Funding
- Fundacao de Amparo a Pesquisa do Estado de Sao Paulo (Fapesp) [13/18073-4, 2010/20425-8, 2006/01459-3, 2010/20476-1, 2012/06082-6]
- Fundacao de Amparo a Pesquisa do Estado de Sao Paulo (FAPESP) [13/18073-4] Funding Source: FAPESP
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The Late Cretaceous Buzios 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 Buzios 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.
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