Geochemistry and petrology of the Búzios Island alkaline massif, SE, Brazil

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 SiO 2 -undersaturated and SiO 2 -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.KEYWORDS: Igneous petrology; Alkaline rocks; Geochemistry. RESUMO: O maciço alcalino da Ilha dos Búzios, do Cretáceo Superior, é intrusivo em charnockitos precambrianos e consiste predominantemente de rochas sieníticas cortadas por grande número de diques, orientados preferencialmente para NE, representando duas suítes distintas: félsica e máfica-ultramáfica. Feldspato alcalino é o mineral mais abundante; outros constituintes são clinopiroxênio, comumente substituído por anfibólio/biotita, e opacos. Minerais acessórios incluem ocasionalmente fases raras contendo Zr, Ti, Nb e elementos terras raras (ETR). Os diques félsicos podem possuir também nefelina (sodalita). A suíte máfica-ultramáfica, particularmente os lamprófiros, apresenta uma assembleia mineralógica primária com olivina, clinopiroxênio e anfibólio, além de massa fundamental com material vítreo e carbonatos (ocelli

Upper Cretaceous weakly to strongly silica-undersaturated alkaline dike series of the Mantiqueira Range, Serra do Mar alkaline province: Crustal assimilation processes and mantle source signatures

“…1A), intruding Neoproterozoic rocks of the Ribeira and Brasília belts (Heilbron et al 2004). In these suites, the abundance of evolved types, mostly nepheline syenites and syenites, is remarkable, as reported by several authors (e.g., Bellieni et al 1990, 1997, Ulbrich et al 2005, Gomes et al 2017, Rosa & Ruberti 2017, Melluso et al 2017, in the São Sebastião, Búzios and Monte de Trigo Islands and Morro de São João, Itatiaia and Poços de Caldas, for example. On the other hand, the Ponte Nova alkaline mafic-ultramafic massif is the only known intrusive suite with prevalence of mafic to ultramafic cumulate rocks (Azzone et al 2009(Azzone et al , 2016.…”

“…Earlystage compositions are very similar to those of the Ponte Nova massif (Azzone 2008). The evolution trend is similar to the initial part of Passa Quatro massif and amphibole-free rocks of the Búzios Island (Brotzu et al 1992, 1997, Gomes et al 2017 (Fig. 2B).…”

“…A2-A5). These trends also show that basanite melts could have evolved mainly from fractional crystallization, reaching tephrite-phonolite and even more extreme phonolite compositions, as observed in the Búzios Island rocks, for example (Gomes et al 2017). In fact, based on geochemical compositions, the parental magma of all three alkaline dike series could potentially evolve to phonolite compositions from fractional crystallization of a gabbroic cumulate assemblage.…”

“…Morro de São João (Brotzu et al 2007); 7(a, b). Búzios Island (Gomes et al 2017). (C) Amphibole compositions in the Ti of the C-site vs. Mg# amp (Mg T /(Mg T +Fe T , in apfu).…”

“…5), is a worldwide feature of evolved phonolite magmas, typically associated with previous fractionation of titanite (e.g., Berger et al 2014, Baudoin & Parat 2015, Melluso et al 2017 in some intermediary stage. Although not found in primitive samples of the series, titanite is a typical accessory mineral in the miaskitic nepheline syenite and syenite massifs of the SMAP (e.g., Bellieni et al 1990, 1997, Ulbrich et al 2005, Gomes et al 2017, Rosa & Ruberti 2017, Melluso et al 2017, from which peralkaline magmas could have derived.…”