NEW EVIDENCE FOR THE PROTEROZOIC BIG SKY OROGENY RECORDED IN BIOTITE-GARNET-SILLIMANITE GNEISSES FROM SHEAR ZONES IN THE HIGHLAND MOUNTAINS, MONTANA

New research in the Highland Mountains of SW Montana focuses upon the metamorphic and structural geology of (?)Proterozoic sillimanite-bearing gneisses including mylonitic shear zones, and seeks to determine if the Highlands were affected by the 1.77 Ga Big Sky Orogeny recorded in the adjacent Tobacco Root Mountains. This research will provide insight into the nature of the northwest margin of the Wyoming Province and the Great Falls Tectonic Zone during Proterozoic time.

Shear zones in the Highland Mountains containing biotite-garnet +/- sillimanite assemblages were active under pressure and temperature conditions comparable to those of the Big Sky Orogeny. The Camp Creek shear zone on the northern edge of Precambrian crystalline rocks in the Highlands exhibits NNW- to NE- striking, shallowly SE-dipping foliations and strike-parallel mineral lineations. Asymmetric garnet and plagioclase porphyroblasts show prevalent top-to-NE shear sense, although textures indicating the opposite sense are also observed. Prismatic sillimanite grains are commonly aligned in the plane of foliation and define the lineation. A later generation of sillimanite crosscuts dynamic fabrics and is randomly oriented, suggesting that high temperature metamorphism outlasted deformation. In contrast, a shear zone on the SE flank of the Highlands contains NE-striking, steeply SE-dipping foliations with down-dip mineral lineations defined by aligned, elongate sillimanite grains. Kinematic shear sense interpreted from asymmetric fabrics in the zone is consistently top-to-SE.

Garnet, biotite and plagioclase from Camp Creek were chosen for geothermobarometry using the garnet-biotite and GASP systems. Garnet porphyroblasts are moderately zoned in the eastern part of the shear zone but lack zoning in the west, with some gradation along the outcrop. Zoned garnet cores are magnesium rich with iron and manganese-rich rims which may suggest re-equilibration at lower temperatures during late-stage (isobaric) cooling or partial melting during their formation. Peak pressures and temperatures determined for these rocks allow for comparison with the 1 GPa, 700-800°C conditions experienced in the Tobacco Root Mountains at 1.77Ga.