Abstract

Wöhlerite is an abundant primary high field-strength element-bearing mineral in alkaline pegmatites occurring in the Permian Larvik Plutonic Complex in the Oslo Rift, Norway. Wöhlerite has a flexible crystal-chemical structure able to incorporate cations of different size and valence and is therefore a potential indicator mineral in mildly peralkaline systems. The major- and trace-element composition of wöhlerite from pegmatites across the complex has been studied as a potential geochemical proxy for the pegmatite-forming melt. The pegmatites are mainly confined to larvikite (i.e., hypersolvus monzonite) and nepheline syenite of the complex, but the genetic relationship between the pegmatites and their host rock is unclear. Pegmatites carrying primary wöhlerite were sampled from different sections of the complex to investigate whether or not compositional variations within the host larvikite were recorded in wöhlerite from the pegmatites. Rare-earth element distribution patterns of wöhlerite show that pegmatites associated with nepheline syenite are identical to pegmatites occurring in evolved larvikites, whereas wöhlerite from pegmatites situated in less-evolved larvikite shows different rare-earth element distributions. Variations in wöhlerite Fe/Mn atomic ratio agree with a model in which different intrusive members of the complex formed by emplacement of separate batches of larvikite magma. The relationship between Mn and Nb may also be explored as a means to discriminate between wöhlerite crystallized from different alkaline systems, e.g., nepheline syenitic or carbonatitic melts.

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