Composition and paragenetic relationships of Na-Ca-Nb-Ti-Zr silicate minerals in nepheline syenite pegmatites in the Langesundfjord area, Oslo Rift

Category Other
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Andersen, Tom۱; Erambert, Muriel۱; Larsen, Alf Olav۲; Selbekk, Rune۱
Holding Date 28 September 2008

Na-Ca-Nb-Ti-Zr-bearing sorosilicate minerals of the cuspidine group (rosenbuschite, mosandrite, hiortdahlite, låvenite, hainite, grenmarite, wöhlerite), catapleiite and eudialyte (s.l.) are characteristic constituents of nepheline syenite pegmatites in the Langesundfjord area in the Oslo Rift. The pegmatite dikes are genetically related to larvikite and associated, nepheline-bearing intrusive rocks of the Larvik plutonic complex, probably as last-stage differentiates after polybaric fractional crystallization of a mantle-derived, alkaline parent magma. The presence of catapleiite and/or eudialyte (s.l.) bearing mineral assemblages indicates mildly agpaitic magma compositions; zircon is, however, present as a magmatic mineral in some pegmatites, and as an alteration product after Zr-bearing silicates.
New electron microprobe analyses are presented for wöhlerite, låvenite, rosenbuschite, hainite, mosandrite, eudialyte (s.l.) and catapleiite, together with a paragenetic analysis of mineral assemblages belonging to the magmatic stage of evolution of the pegmatites. The analysed minerals are part of primary magmatic parageneses which include hiortdahlite + wöhlerite ± rosenbuschite ± eudialyte (s.l.), mosandrite + fluorite ± rosenbuschite ± catapleiite, låvenite + pyrochlore + fluorite.
At least 14 components (Si-Al-Ti-Nb-Zr-REE-Fe+Mn-Ca-Na-K-H-O-F-Cl) are needed to account for the phase relationships in the system. The relative stability of low-variance mineral assemblages (3-4 coexisting Na-Zr bearing silicate minerals, pyrochlore, feldspars, nepheline, sodalite, magnetite, pyroxene, ± amphibole, ± fluorite, silicate melt, fluid) depends on the peralkalinity of the magma (expressed as aNa2Si2O5) and the activities of the volatile components H2O, HF and Cl. Whereas the appearance of catapleiite and/or eudialyte-group minerals at the expense of zircon is mainly controlled by increasing peralkalinity, the accompanying Na-Zr silicate mineral assemblages are strongly influenced by the activity of HF, with rosenbuschite-hiortdahlite bearing assemblages with or without fluorite indicating elevated aHF, and wöhlerite-bearing assemblages reflecting lower aHF. The stability of mosandrite extends to low aNa2Si2O5, whereas låvenite ± eudialyte is indicative of the highest level of aNa2Si2O5 achieved in these pegmatites.
The peralkalinity and volatile activities of the crystallizing pegmatite magma are most likely a result of fractionation processes prior to emplacement, and the Na-Zr silicate assemblages are passive recorders of the physico-chemical conditions during final crystallization. Some pegmatites, however, show internal variations suggesting local control on peralkalinity and volatile activity (e.g. mosandrite and låvenite bearing domains in the Låven pegmatite), and / or changes of fluid composition with time (e.g. replacement of hiortdahlite + rosenbuschite by wöhlerite-bearing assemblages).