ARCHEAN ULTRAMAFIC TECTONIC LENSES OF THE NORTHERN WYOMING PROVINCE, USA: EVIDENCE FOR THE METAMORPHIC EVOLUTIONARY HISTORY

Ultramafic rocks, developed as tectonic lenses (1-100 m in diameter), are found in the 2.8 Ga Mesoarchean plutonic rocks of the northern Wyoming Province in the eastern Beartooth Mountains of MT-WY and they preserve evidence of their igneous pre-history and metamorphic evolution. Massive chromitite units with euhedral chromite octahedra and layers of chromite are presumed to be inherited from the igneous protolith. The ultramafic rocks locally preserve evidence for three stages of metamorphism. The initial metamorphism (M1) occurs under granulite/upper amphibolite conditions and has a mineral assemblage of olivine (Fo_87-88), enstatite (En_82-90) and chromite. There is a later amphibolite-facies overprint (M2) that locally generated more hydrous assemblages that include magnesio-hornblende, phlogopite and, uncommonly, anthophyllite. The development of the phlogopite is likely related to K and H₂O sourced from the enveloping 2.8 Ga granitic rocks. In addition to the more hydrous minerals associated with M2, there are two-spinel associations with coexisting spinel (sensu strictu) and chromite that represent unmixed spinels across a miscibility gap. The third metamorphic overprint (M3) is greenschist facies and locally results in the development of antigorite after olivine and enstatite; chlorite after spinel; talc and tremolite after enstatite and anthophyllite; and magnetite-ferritchromite rims on preexisting spinels. Locally, veins containing calcite and/or dolomite are found. Conditions for the metamorphic overprints, derived from classic geothermobarometry, phase equilibrium modeling and construction of mineral association diagrams (using Theriak-Domino) on the ultramafic and associated lithologies, have been estimated to be 750-800C and 6-8 kbar (M1), 600-700C and 5-7 kbar (M2) and 400-450C and <5 kbar (M3). These ultramafic rocks became progressively hydrated with their incorporation into the 2.8 granitic rocks and subsequently as the region was deformed allowing access to lower temperature fluids.