Narsarsukite in peralkaline granites from the Papanduva Pluton, Graciosa Province, south Brazil: Insights from textural and compositional features

Abstract

AbstractWe report textural and compositional data for the titanosilicate narsarsukite [Na2(Ti,Fe3+)Si4(O,F)11] in peralkaline granites from the Papanduva Pluton in Graciosa Province, south-southeastern Brazil. Two distinct narsarsukite generations, one late magmatic and the other post-magmatic, were identified on the basis of textural and compositional features. The magmatic generation consists of larger, euhedral to subhedral variably zoned crystals and late poikilitic intergrowths between narsarsukite and albite laths, representing the crystallisation of the latest melt pockets. The post-magmatic generation forms smaller, typically fibrous crystals and irregular aggregates that occur interstitially or replace the primary mafic minerals, particularly arfvedsonite. Compositions of narsarsukite from the Papanduva Pluton cover most of the compositional range described in known occurrences. The magmatic generation is enriched in Zr and depleted in Al. The Fe3+ and Al contents show a positive correlation for the magmatic crystals, but a negative correlation for the post-magmatic narsarsukite. The Al/Fe3+ ratios are higher in the post-magmatic crystals and can be used to discriminate between the two generations. The compositional variations are controlled mainly by the heterovalent substitution (Ti, Zr)4+ + O2– = (Al, Fe)3+ + F1–, and are compatible with (OH)1– ions in the O site. Narsarsukite has significantly higher concentrations of mid REE+Y and HFSE and very low to absent contents of LREE and LILE relative to the host-rocks. The average REE pattern of the magmatic crystals is highly fractionated, consistent with the expected strong preference for the smaller HREE. The post-magmatic crystals that replace arfvedsonite have flatter patterns, with higher concentrations of LREE and MREE. Their compositions suggest a fluid phase relatively rich in HFSE, REE, Y, Pb, Th and U. Narsarsukite is a common phase in strongly peralkaline granites such as the Papanduva Pluton and the well-known Strange Lake Complex, and should be considered a diagnostic mineral of highly peralkaline SiO2-oversaturated rocks containing rare Ti and Zr mineral assemblages.