Petrogenesis of Carbonate-orthopyroxenites (Sagvandites) and related rocks from Troms, Northern Norway

Carbonate-orthopyroxenites from Troms consist of enstatite andmagnesite with variable amounts of olivine, talc, serpentine,chlorite, and phlogopite and include the ore minerals chromite,magnetite, pentlandite, pyrite, and in some cases pyrrhotite,heazlewoodite, millerite, and maucherite. Related rocks compriseolivine-magnesite-, talc-magnesite and olivine-orthopyroxene(saxonite)-assemblages. Allochemical replacement reactions,involving mobile CO₂, H₂O, and SiO₂, are shown to comprise themain petrogenetic mechanism. Saxonite, however, may representa possible source material. Discussion of the model system MgO-SiO₂-CO₂-H₂O for P_flukl =2 and 7 kb, respectively, indicates that best agreement withpetrographic evidence is reached assuming elevated pressuresand both gas-excess and gas-deficiency conditions by means oflocal equilibria. The gas-deficient assemblage enstatite+talc+forsterite+magnesiteis presumed to be stable at pressures above 5 kb. Recalculationof whole-rock analyses to a CO₂-free basis by several alternativemethods suggests that rock evolution could have followed thetrend dunite saxonite orthopyroxenite sagvandite+related rocks.A simple geometric method is used to outline possible schemesof rock evolution, involving gas-deficient phase assemblages.