AN EVALUATION OF QUARTZ C-AXIS PATTERNS FROM A SHEATH FOLD IN METAMORPHOSED PRECAMBRIAN ROCKS, TOBACCO ROOT MOUNTAINS, MT, USA

A well-exposed outcrop-scale sheath fold occurs in Precambrian metamorphic rocks in the Tobacco Root Mountains of MT. Created during upper amphibolite facies metamorphism of the Big Sky orogeny at 1.775 Ga, it folds five distinct lithologies: a quartz-biotite-plagioclase-garnet-sillimanite schist with migmatitic segregations, a garnet-quartz-plagioclase-biotite-sillimanite gneiss with compositional bands, a five centimeter thick quartz-biotite-plagioclase-garnet gneiss, and two amphibolites of varying plagioclase content, one containing garnet. The sheath fold has a cross-sectional ellipticity of 0.22 and a hinge angle of 64 degrees. The fold is 4.93 meters across at its widest. The average mineral lineation within the sheath fold is similar to that found in outcrops within a 50-meter radius and bisects the hinge angle of the fold.

Eight oriented samples were obtained from the quartz-rich gneisses in the sheath fold so as to represent all possible positions within the fold: three from the limbs and five from the nose and apex. Evidence for grain boundary migration and bulging recrystallization in quartz during deformation, followed by recovery, is observed in thin section. Approximately 10% of quartz grains include a small number of large subgrains that have mutually indenting but polygonal boundaries. Quartz lattice preferred orientations determined for the eight samples by petrographic microscope with a universal stage and by electron backscatter diffraction with a scanning electron microscope are compared. Patterns obtained indicate that high temperature slip systems were active during sheath fold formation and constrain the pattern of solid-state flow around the sheath fold.