Paragenesis and decomposition assemblage of a Mn-rich eudialyte from the Sushina peralkaline nepheline syenite gneiss, Paschim Banga, India

Abstract

Proterozoic peralkaline nepheline syenite gneiss occurring at Sushina Hill (Paschim Banga, India) consists of minor eudialyte together with potassium feldspar, albite, aegirine, and nepheline. Perthites are absent and the precursor syenite is considered to have been a two feldspar sub-solidus eudialyte aegirine nepheline syenite . The major minerals have near-endmember compositions and are not altered. In contrast, eudialyte is partially-to-completely altered to a decomposition assemblage consisting principally of hilairite and members of the pectolite-serandite solid solution series together with minor ceriothorianite, pyrochlore, and perraultite. Rinkite occurs as early-forming prismatic inclusions in eudialyte. The eudialyte occurs as an interstitial late-forming magmatic phase with hydrothermal eudialyte overgrowths. Eudialytes from Sushina are characterized by low-to-negligible Fe and high Mn and Nb contents relative to most eudialyte from other localities. Late magmatic eudialyte has relatively high CaO (12–14wt.%) and low MnO (5.5–7wt.%) relative to the hydrothermal variety (7–8.5wt.% CaO; 9.5–10.5wt.% MnO). The late magmatic Ca–Mn–Nb-rich eudialytes are principally solid solutions between Ce-zirsilite, Mn-taseqite, kentbrooksite, and possibly hydrated eudialyte or aqualite. The Mn–Sr-rich hydrothermal eudialyte is also a solid solution between these end members, although the presence of significant concentrations of Mn at the M(1) site and Sr at the N-site suggests that the mineral could be a new member of the eudialyte group; possibly a Mn analog of taseqite or a Sr analog of manganoeudialyte. The mineralogy of the decomposition assemblage of Sushina eudialyte is unlike those found for other eudialytes and in particular lacks zircon, fergusonite, fluorite and bastnaesite. The deuteric/hydrothermal fluids responsible for the alteration are considered as not buffered by the coexisting silicate assemblage and to be water-rich with very high Na/Cl and Na/F ratios.