ARCHEAN METAMORPHOSED ULTRAMAFIC ROCKS OF THE EASTERN BEARTOOTH MOUNTAINS, MT/WY: EVIDENCE FOR MULTIPLE THERMAL AND FLUID OVERPRINTS

Metamorphosed ultramafic enclaves (1-100 m in diameter) included in the 2.8 Ga granitic rocks of the eastern Beartooth Mountains, MT/WY are from the Archean northern Wyoming Province, and they preserve evidence of two high-grade metamorphic overprints and several lower-grade fluid-influx episodes. Massive chromitite units with euhedral chromite octahedra and layers of chromite are interpreted to be inherited from an igneous protolith. These ultramafic lithologies locally exhibit an upper amphibolite/granulite facies metamorphic overprint (M1) that characteristically contain a mineral assemblage of olivine (Fo_77-90), enstatite (En_80-90), and chromite. The second metamorphic event (M2) is amphibolite facies and has an assemblage of magnesio-hornblende-to-pargasite, phlogopite, anthophyllite and unmixed chromite and spinel across the spinel miscibility gap. M2 is attributed to partial rehydration with K-bearing aqueous fluids sourced from the massive 2.8 Ga granitic intrusions. The third metamorphic event (M3) occurs in the upper greenschist facies and has an assemblage of antigorite after olivine and enstatite, chlorite after spinel, tremolite and talc after enstatite and anthophyllite, magnetite-ferritchromite rims on pre-existing spinel and dolomite or calcite in fractures cutting spinel grains. The fourth metamorphic event (M4) is characterized by development of the chrysotile and lizardite form of serpentine in the lower-greenschist facies conditions (<250°C). Regional K-Ar geochronology of micas constrain the cooling age of the M4 overprint to be ~2.3-2.5 Ga. The distinction among M2, M3 and M4 overprints in the most highly magnesian lithologies become better defined under optical cathodoluminescence (CL). Relict M1 orthopyroxene has moderate red CL response, M3 tremolite partially replacing orthopyroxene or amphibole exhibits a moderate yellow CL response, antigorite exhibits a dark purple CL response, cross-cutting lizardite has dull red CL response and the carbonate minerals in fractures have a yellow-to-red CL response. All of these responses are related to variable amounts of Mn²⁺, Fe²⁺ and Fe³⁺ in the minerals, and reflect the local mineral assemblage and fluid environments that take place during the retrograde overprint of the meta-ultramafic rocks.