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