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

• Published in: Journal of Petrology
• Main Author: OHNMACHT, W.
• Format: Text
• Language: English
• Published: Oxford University Press 1974
• Subjects: Articles; Norway; Northern Norway; Troms
• Online Access: http://petrology.oxfordjournals.org/cgi/content/short/15/2/303; https://doi.org/10.1093/petrology/15.2.303

Carbonate-orthopyroxenites from Troms consist of enstatite and magnesite 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 comprise olivine-magnesite-, talc-magnesite and olivine-orthopyroxene (saxonite)-assemblages. Allochemical replacement reactions, involving mobile CO 2 , H 2 O, and SiO 2 , are shown to comprise the main petrogenetic mechanism. Saxonite, however, may represent a possible source material. Discussion of the model system MgO-SiO 2 -CO 2 -H 2 O for P flukl = 2 and 7 kb, respectively, indicates that best agreement with petrographic evidence is reached assuming elevated pressures and both gas-excess and gas-deficiency conditions by means of local equilibria. The gas-deficient assemblage enstatite+talc+forsterite+magnesite is presumed to be stable at pressures above 5 kb. Recalculation of whole-rock analyses to a CO 2 -free basis by several alternative methods suggests that rock evolution could have followed the trend dunite↑ saxonite↑ orthopyroxenite↑ sagvandite+related rocks. A simple geometric method is used to outline possible schemes of rock evolution, involving gas-deficient phase assemblages.