PALEOPROTEROZOIC HISTORY OF THE GREAT FALLS TECTONIC ZONE: RESULTS FROM AN INTEGRATED STUDY OF BASEMENT EXPOSURES IN THE LITTLE BELT MOUNTAINS, MONTANA

The Great Falls Tectonic Zone (GFTZ) separates two Archean cratonic blocks, the Wyoming Province and the Medicine Hat block to the north. Various models have been proposed to explain the origin of the GFTZ, ranging from a Medicine Hat-Wyoming suturing in the Archean or Paleoproterozoic to an intracratonic shear zone that was last active during the Trans-Hudson orogeny. The ambiguities associated with the origin and evolution of the GFTZ are due in part to the fact that the best exposed pre-Beltian rocks within the GFTZ are limited to one isolated uplift (Little Belt Mountains), which has never been comprehensively mapped or studied. We have addressed this issue in a study that includes an integration of recently completed mapping of basement in consort with new geo-/thermochronological, geochemical, and structural data.

Mapping and U-Pb zircon geochronologic data indicate that most of the exposed basement comprises meta-plutonic rocks emplaced between ~1864 and 1842 Ma. These rocks have intruded Archean (~2700 Ma) meta-diorite, and migmatitic paragneiss and meta-volcanics(?) of unknown age. Other U-Pb dates of ~1820-1810 Ma suggest either that magmatism continued for ~20 m.y. or that high-grade metamorphism peaked shortly after the cessation of magmatism. Geochemical data indicate that the magmas were likely generated in a subduction setting.

The dominant structural fabric is a steeply dipping foliation with a locally well-developed gently WNW-plunging lineation. Similarly oriented fabrics occur in a km-thick mylonite zone. Both fabric types contain sinistral shear-sense indicators. New argon ages from areas not previously dated overlap with previously published argon ages of ~1800-1770 Ma, suggesting that the entire area cooled synchronously and that that all penetrative deformation, including mylonitization, was complete by ~1770 Ma.

These data suggest significant differences (~100 m.y.) in the time of peak metamorphic conditions between the Little Belt terrane and the Archean uplifts of SW Montana that lie within the western most part of the GFTZ (e.g., Tobacco Root Mountains). In addition, it is apparent that Paleoproterozoic magmatism was pervasive in the Little Belt terrane, but absent in the Archean part of the GFTZ in SW Montana.. These differences suggest diachronous evolution of segments of the GFTZ.