HIGH-PRESSURE GRANULITE METAMORPHISM AND P-T PATH OF THE ARCHEAN LAYERED GNEISS UNIT, NORTHERN TETONS, WYOMING

We have determined that the Layered Gneiss of the Teton Range (Reed 1963), underwent granulite-facies metamorphism at temperatures of >950ºC and pressures >11 Kb prior to 2675±3 Ma. Near Moose Basin the Layered Gneiss chiefly consists of interlayered metapelitic schists and amphibolites. The Layered Gneiss has undergone three periods of deformation. D1, is manifested as rootless isoclinal folds, D2 produced isoclinal folds, the limbs of which form the regional NW-SE foliation. The D3 event forms broad open folds that do not carry an axial planar foliation or a lineation. Granulite facies mafic rocks (2 pyroxenes + Hbl + Plag ± Gnt) are locally preserved in low strain regions. These mafic rocks have locally melted to produce garnet bearing leucosomes, suggesting metamorphism at T >850º C and P >10 Kb. The associated pelitic schists contain complex assemblages of Q + Bio + Gnt + Ky + Sill + St + Cord + Rut + Zr. Rutile inclusions are present in Gnt, Ky, and St. GRAIL barometry, combined with Zr-in-rutile thermometry places constraints on the P-T path followed by these schists. Rutile included in garnet contains 2700 to 3800 ppm Zr, corresponding to T of 950-1000º C. A pressure of at least 11 Kbar is required to stabilize the assemblage garnet + rutile at this temperature. In contrast, rutile included in Kyanite (which appears to have grown during both D1 and D2) contains 370-570 ppm Zr, corresponding to T of 700-760º C. At this temperature the GRAIL assemblage would be stable at 10 Kbar. Rutile in Staurolite contains ~350 ppm Zr, with corresponding T of ~690º C (calculated @ 6 Kb). The granitoid Webb Canyon Gneiss is foliated in D2 yet shows no evidence of D1 deformation. As a result, we conclude that peak metamorphism (T of >950ºC) and P>11 Kb were achieved prior to its emplacement @ 2675±3 Ma. This was followed by cooling to ~700º C at P >10 Kb, likely around 2675 Ma and then by isothermal decompression through St + Sill, then Sill + Cord fields.