ORIGIN OF RUBY-BEARING AND RELATED ALUMINOUS ASSEMBLAGES IN SOUTHWEST GREENLAND

Scattered across the Archean amphibolite- to granulite-facies areas of SW Greenland, are places where pegmatites form metasomatic reaction zones with ultramafic (UM) bodies enclosed in anorthositic, gabbroic or quartzofeldspathic gneisses. These pegmatites postdate peak amphibolite- to granulite-facies metamorphism. The UM bodies vary with respect to their primary assemblage olivine ± clinopyroxene (CPX) ± orthopyroxene and the extent of hydration. In most localities, the reaction zones consist of coarse biotite (BIO) + calcic amphibole (CAM) ± serpentine (SERP) ± talc ± anthophyllite (ANT). In these, modal CPX and CAM vary antithetically, and new CAM, BIO and apatite are enriched in pegmatite-derived F and Cl. Our work shows these metasomatic reactions occurred below about 550°C. In the Fiskenaesset region, new observations suggest similar metasomatism/advective transport between pegmatites and UM rocks resulted in the spectacular reaction zones with ANT, sapphirine, cordierite and kornerupine, as well as ruby corundum (COR) + Cr-bearing CAM assemblages. The extent of the metasomatism/advective transport suggests the pegmatites were rich in a fluid that drove the reactions and exchange of material. These aluminous zones extend up to hundreds of meters with material apparently transported beyond outcrop scale. Our observations of the metasomatism suggests quartz, PLG and K-feldspar (KFS) from the pegmatites and the enclosing gneisses are consumed in reactions with the UM bodies. The metasomatic reactions have several basic types: (1a) forsterite (FO)/SERP + plagioclase => Ca-amphibole + "Al₂O₃" (Al-rich minerals) ± albite; (1b) FO/SERP + plagioclase + diopside => CAM ± albite; (2) FO/SERP + "SiO₂" => ANT; (3) FO/SERP + KFS => BIO + ANT; (4) diopside + KSP => BIO + CAM. The factors that determine the metasomatic assemblage are reactive or effective Ca/Al and Si-content of the local reaction zone, and metamorphic grade. Generally, reactions 1b and 4 dominate, but in Fiskenaesset lower Ca/Al, lower free QZ and higher temperatures favored reactions 1a, 2, 3. The development of ruby COR + Cr-CAM at Fiskenaesset suggests Cr mobility was enhanced—possibly by B₂O₃ or a fluid that stabilized hexavalent Cr during at least part of the reaction-zone formation.