PRE-BIG SKY RETROARC BASIN ALONG THE NORTHWESTERN MARGIN OF THE WYOMING CRATON: EVIDENCE FROM THE HIGHLAND MOUNTAINS, SOUTHWESTERN MONTANA

The Highland Mountains of southwestern Montana are the northwestern-most exposure of the Wyoming Craton, located adjacent to the Great Falls Tectonic Zone and in fault contact with Belt Supergroup rocks to the north. Sillimanite-grade, migmatitic, biotite-garnet gneiss and Archean quartzofeldspathic gneiss are the dominant lithologies. Both gneisses contain amphibolite lenses. All units are cut by relatively undeformed granitoid dikes, likely sourced from the migmatites. Previously interpreted as a gneiss dome, a sharp strain gradient between the two gneissic units is largely confined to the quartzofeldspathic gneiss and a zone of intensive metamorphism and metasomatism is mainly within the biotite-garnet gneiss. Recent mapping, whole rock geochemistry, geochronology, trace-element, and Hf-isotope data are consistent with a depositional contact between Archean basement and its Paleoproterozoic sedimentary cover, which was extensively modified during the Big Sky orogeny.

U-Pb zircon analyses of seven quartozfeldspathic gneisses are characterized by inferred igneous crystallization ages of 2.75-2.73 Ga, oldest xenocrysts at 3.35-3.15 Ga, and no Paleoproterozoic ages. Six granitoid samples produced Big Sky crystallization ages of ca. 1.77-1.75 Ga and oldest xenocryst ages generally around 2.7 Ga. Zircon grains from biotite-garnet metasedimentary gneiss, ironstone, and calc-silicate rock also yield Big Sky-age upper intercepts. Inferred detrital crystals are as old as 3.2 Ga but most are ca. 2.7 Ga. Monazite ages of ca. 1.76 Ga in both granitoids and metasediments and 1.78 Ga titanite in amphibolite confirm Big Sky-age metamorphism. Zircon trace elements and Hf isotopes suggest that Big Sky-age zircons are composed of detrital zircon crystals from numerous sources, thus representing a combination of reset and zircon newly grown during orogenesis.

The southern Highland Mountains may have originated in a Paleoproterozoic retroarc setting as a basement of quartzofeldspathic gneiss overlain by mudrocks, sandstones, carbonates, and ironstones with interspersed basaltic flows and sills, all deposited in a retroarc setting prior to collision with the Medicine Hat Block, resulting in deformation, metamorphism, and a partial to complete resetting of zircon U-Pb systematics.