THE CHEMISTRY OF PRIMARY WÖHLERITE FROM SYENITE PEGMATITES OF THE LARVIK PLUTONIC COMPLEX

Selected crystals of primary Zr/Nb mineral wöhlerite, ideally Na₂Ca₄ZrNb(Si₂O₇)₂O₃F, were studied from pegmatites throughout the Permian Larvik Plutonic Complex (LPC) to determine geochemical variation between ring sections (RS). The LPC, located at the southernmost point of the Oslo Rift, consists of 10 distinct plutons or RS. Wöhlerite belongs to the wöhlerite group and is the typical zirconosilicate in the syenite pegmatites in the Langesundsfjord archipelago (RS IV/VI) and Tvedalen area (RS IV). The presence of wöhlerite is scarce or completely absent in other RS. Wöhlerite crystallized relatively early in the magmatic pegmatite formation and may be a useful indicator of compositional variation trends in the individual pegmatites. Wöhlerite from different RS in the LPC shows variations in Mn content. Manganese contents in the RS III and RS IV are close to constant value (0.08-0.12 apfu), whereas Mn contents in the RS IV/VI are variable (0.08-0.19 apfu). Wöhlerite from Håkestad (RS V) shows the highest Mn content (0.21-0.29 apfu) in comparison with other RS. However, in contrast with the highest Mn content measured in wöhlerite (0.64 apfu) from Los Archipelago (Biagioni et al., 2012) Mn content in wöhlerite from Håkestad is not extreme. In the Tvedalen area (RS VI) Mn contents are variable from 0.12 apfu (Saga) to 0.26 apfu (Søndre Brudd). The isolated RS X (Bratthagen) shows similar Mn contents as RS III (around 0.1 apfu). Zirconium contents in Langesundsfjord archipelago (RS IV/VI) and Tvedalen (RS VI) varies similarly between 0.91-0.99 apfu whereas Zr content in the rest of the RS is not so variable and slightly lower (between 0.89-0.94 apfu). The Zr/Nb ratio in wöhlerite from individual syenite pegmatites throughout LPC show slight increase from RS III (1.1) to RS IV/VI (1.4) whereas in the RS VI and X the Zr/Nb ratio decreases to 1.1. The comprehensive datasets of wöhlerite and their chemical variations of Mn, Zr and Zr/Nb indicate slight change in magma composition during the implacement of the different RS, rather than a contineous evolution.

References:

Biagioni, C., Merlino, S., Parodi, G. C., and Perchiazzi, N., 2012, CRYSTAL CHEMISTRY OF MINERALS OF THE WÖHLERITE GROUP FROM THE LOS ARCHIPELAGO, GUINEA: Canadian Mineralogist, v. 50, no. 3, p. 593-609