QUARTZ-CORDIERITE GNEISS IN SOUTHERN WYOMING PROVINCE SHEAR ZONES: MONAZITE GROWTH RECORDS MG-METASOMATISM DURING NEOARCHEAN ASSEMBLY OF NORTH AMERICA

Proterozoic accretion along the southern margin of the Wyoming Province was preceded by Neoarchean collision of the Southern Accreted Terranes. Information about processes and timing of this Neoarchean assembly event is preserved in shear zones within Paleoarchean basement gneisses of the southern Wyoming Province. Quartz-cordierite gneiss (QCG) occupy narrow, steeply dipping, northeast-oriented shear zones in the 3.30-3.33 Ga Sacawee orthogneiss in the Granite Mountains of central Wyoming. Altered and sheared mafic and ultramafic rocks commonly occur with the QCG. We studied the origin of QCG at Crilly Gap, where a transition from the Sacawee orthogneiss to the QCG is well displayed. At this locality the QCG (15BG9) is composed of abundant quartz and altered cordierite (crd), with extremely pale biotite (bt) and unusually abundant rutile. No feldspar is present. The mineralogy of the QCG is strikingly similar to that of micaceous rock that looks like the Sacawee orthogneiss located adjacent to the shear zone. This gneiss (15BG6) retains clear textural evidence of its parentage, yet has the same assemblage as the QCG. Bt in both samples is magnesian (Mg/(Mg+Fe) = 0.87-0.88). Less micaceous Sacawee orthogneiss 15BG7, 4 m farther from the shear zone and 15BG8, 20 m away, are composed of qtz, plg (An₃₂), Kfs, and brown bt. 15BG8 also contains hornblende. Bt in 15BG7 and 15BG8 is Fe-rich (Mg/(Mg+Fe) = 0.32-0.43). QCG has gained Mg and lost Fe, Na and Ca, relative to unaltered Sacawee orthogneiss. Silica and alumina are unchanged. We interpret QCG to have formed by Mg-metasomatism during shearing and deformation. Monazite, formed during metasomatism in QCG sample 15BG9, is mainly Neoarchean, although we have documented both older cores (>3 Ga) and younger domains. The metasomatizing fluid was likely injected into the continental crust along shear zones either during rifting of the Wyoming Province margin or collision with the Southern Accreted Terranes, similar to a process that has been documented during formation of the Alps, where early metasomatism appears to have occurred during Tethys opening, and later metasomatic events occurred during closure of Tethys and continent-continent collision. The southern Wyoming Province thus retains evidence of processes that operated in much younger orogens.