A NEW CHAPTER IN THE PRECAMBRIAN HISTORY OF THE NORTHERN WYOMING PROVINCE: WHOLE ROCK GEOCHEMISTRY COMBINED WITH U-PB AND LU-HF ISOTOPIC ANALYSIS OF ZIRCONS FROM BASEMENT ROCKS IN THE BLACKTAILS MOUNTAINS (DILLON, MT)

Precambrian rocks exposed in Laramide uplifts throughout Southwest Montana (e.g., Ruby Range, Tobacco Root, Madison Range, etc.) have been integral to interpreting the Precambrian history of the northern Wyoming Province. Less well-characterized exposures occur in the Blacktail Mountains, which are south of the Great Falls Tectonic Zone near Dillon, MT. They contain a wide variety of generally high-grade gneisses, many with unique cm scale compositional layering and bulk compositions ranging from intermediate to felsic, previously interpreted as Archean (Clark, 1987). Here we provide the first single grain U-Pb and Lu-Hf isotopic analyses of zircons from basement rocks in the Blacktails, as well as new trace and major element geochemistry. Initial results indicate a history dating back to at least the Mesoarchean, with reworking of older crust in the Neoarchean. Basin formation and sedimentation occurred after ~2.7 Ga followed by high-grade metamorphism, including anatexis at (2.55-2.45 Ga), a previously documented, regionally extensive event. A late, undeformed, unmetamorphosed, mafic dike cuts across most of the units in the study area. U-Pb ages have been determined for five units: a coarse-grained, unfoliated, potassium-feldspar-rich granite; a foliated migmatite with 1 cm thick alternating leucocratic and melanocratic bands; a biotite-hornblende gneiss; a garnet-biotite-hornblende gneiss and an aphanitic mafic dike (as above). The granite was emplaced at 2.77 +/- 0.005 Ga. The garnet- biotite- hornblende gneiss was emplaced at 2.74 +/- 0.06 Ga. The structurally lower migmatite contains age-populations with modes at 3.2 Ga and 2.75 Ga, and metamorphic zircons in the range 2.55-2.45 Ga. The biotite-hornblende gneiss contains a scatter of ages from 2.0-2.8 Ga with a mode of 2.76 Ga. The mafic dike that appears to cut these units contains several Neoarchean zircons (interpreted as xenocrysts), with a single concordant zircon ~1960 Ma. ϵHf(i) values for the granite give an average value of -2, with a range of -4 to +2 and a model age of ~3.2 Ga, suggesting the source of the granite likely included Mesoarchean crust. These initial results add to the evidence for a regionally extensive ~2.5 Ga thermal event and further elucidate the Precambrian history of the northern Wyoming Province.