CL-ENRICHMENT OF HYDROUS MINERALS IN ARCHEAN GRANULITE-FACIES IRONSTONES, BEARTOOTH MOUNTAINS, MONTANA, USA

Trace amounts of prograde metamorphic Cl-rich amphibole and biotite are present within Archean granulite-facies ironstones of the eastern Beartooth Mountains, MT, and serve as a monitor of the evolving fluid phase. The ironstone lithology comprises a minor proportion of the ~3.1-3.5 Ga lithologic enclaves enclosed by the voluminous Mesoarchean (2.8 Ga) tonalite-trondhjemite-granodiorite plutonic and metaplutonic rocks of this portion of the Wyoming Province. The ironstones have undergone granulite-facies metamorphism as typified by dominantly anhydrous mineral assemblages: quartz + magnetite + orthopyroxene + garnet +/- clinopyroxene. Geothermobarometry applied to the ironstones and proximal lithologies generally indicates conditions of 750-800°C and 5-7 kbar.

Within these relatively anhydrous ironstones small amounts of prograde Cl-rich biotites and amphiboles occur as inclusions in orthopyroxene and garnet and as matrix minerals. The amphiboles are potassium hastingsites and generally show a positive correlation between Cl and Fe/(Fe+Mg) or K. Late-stage alteration amphiboles of actinolite, grunerite and cummingtonite compositions are also present. The amphiboles contain high levels of Cl, with some amphibole analyses exceeding 2.6 wt%. The biotites not only contain up to 2.9 wt% Cl, but also contain substantial amounts of BaO (up to 10.5 wt%), and TiO₂ (up to 6.9 wt%). Variation in Cl contents of amphibole and biotite appear to be a consequence of both crystallochemical influences on the incorporation of Cl and on the progressive change of the fluids associated with the granulite-facies metamorphism.