PETROLOGIC INVESTIGATION OF AMPHIBOLITES ACROSS A MAJOR DUCTILE SHEAR ZONE IN THE RUBY MOUNTAINS, SW MONTANA: IMPLICATIONS FOR THE NATURE OF EARLY PROTEROZOIC(?) MAFIC MAGMATISM

Geologic mapping in the southern portion of the Ruby Mountains reveals a highly variable metamorphic lithostratigraphy. An NE-striking high strain shear zone exposed in the divide between the Sweetwater and Timber Creek drainages forms the boundary between two distinct Archean/early Proterozoic metamorphic terranes. The terrane to the north is comprised dominantly of granitic gneiss, marble, amphibolite, pelitic gneiss, and quartzite, with minor, metaconglomerate, meta- banded iron formation, and calc-silicate schist. Highly deformed, well-layered calc-silicate marbles structurally underlie more massive dolomitic marbles and appear to form the base of the northern terrane. The terrane to the south is comprised dominantly of granitic gneiss, locally migmatitic pelitic gneiss, and amphibolite, with minor quartzite. It contains a narrow (~1 km) belt of ultramafic rocks, both peridotite and megacrystic metapyroxenite The shear zone and the southern terrane contain distinctive 1-10 m thick layers of white garnet leucogneiss which are absent from the northern terrane.

Amphibolites, which are widespread throughout both terranes and the shear zone, occur both as discontinuous and continuous mappable units several meters to tens of meters thick and thin (cm to m scale) bands interlayered within other lithologies. Amphibolites are dominated by the assemblage hornblende + plagioclase + quartz, with some containing relict skeletal garnet and minor biotite. They vary texturally from well-banded or laminated to more massive and granular. The abundance of quartz increases into the terrane north of the high strain ductile shear zone.

Geochemical analyses of amphibolites structurally above, below, and within the ductile shear were acquired in an effort to characterize chemical variations and to help understand the nature of mafic magmatism in the area. Chemical differences in major and trace element distributions can be seen in amphibolites from different structural positions possibly reflecting original variations in chemistry (and/or mineralogy). Tectonic discrimination diagrams are slightly more ambiguous but provide insight into tectonic setting prior to development of the high strain ductile shear zone.