Narsarsukite in peralkaline granites from the Papanduva Pluton, Graciosa Province, south Brazil

[Marco E. Ciriotti]

Prossima pubblicazione.

Referenza:
▪ Vlach, S.R.F. & Vilalva, F.C.J. (2023): Narsarsukite in peralkaline granites from the Papanduva Pluton, Graciosa Province, south Brazil: Insights from textural and chemical features. Mineralogical Magazine, 87, (in press).

Abstract:
We report textural and chemical data for the titanosilicate narsarsukite [Na2(Ti,Fe3+)Si4(O,F)11] in peralkaline granites from the Papanduva Pluton in Graciosa Province, S-SE Brazil. Two distinct narsarsukite generations, one late magmatic and the other post-magmatic, were identified based on textural and chemical 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, often 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 the known worldwide occurrences. The magmatic generation is enriched in Zr and depleted in Al. The Fe3+ and Al contents are positively correlated in the magmatic crystals, but inversely correlated in 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 mainly controlled 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 MREEs+Y and HFSEs and very low to absent contents of LREEs and LILEs relative to the hostrocks. The average REE pattern of the magmatic crystals is highly fractionated, consistent with the expected strong preference for the smaller heavy REEs. The post-magmatic crystals that replace arfvedsonite have flatter patterns, with higher concentrations of L-and MREEs. Their compositions suggest a fluid phase relatively rich in HFSEs, REEs, Y, Pb, Th and U. Narsarsukite is a common phase in strongly peralkaline granites 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.