TOPPING OFF THE CAPSTONE – ZIRCON AGES FROM ARCHEAN TONALITIC GNEISS, HIGHLAND MOUNTAINS GNEISS COMPLEX, SOUTHWEST MONTANA

The “Topping off the Capstone” initiative at TAMU encourages undergraduate students to engage in an independent research project based on the experiences of summer field camp. This project seeks to determine the igneous protolith crystallization age of part of the Highland Mountains gneiss complex, southwestern Montana.

The protolith ages of Highland Mountains orthogneisses are poorly constrained but published ages from lithologically similar gneisses in the adjacent Tobacco Root Mountains are as old as 3.4 Ga and quartzofelspathic gneiss in the Armistead anticline yielded a late Archean U-Pb zircon age. Zircons were separated from a medium-grained quartz-feldspar gneiss with an apparent igneous (tonalitic) protolith located near the Rochester Creek mining district. Despite extensive annealing and chemical abrasion, all zircon analyses are highly discordant, likely due to radiation damage of the crystal lattice in these old, high-U zircons. Lower intercept ages are consistent with Mesozoic partial resetting, perhaps due to regional heating and fluid-flow during Cretaceous thrusting and granitic batholith emplacement. Upper intercept ages scatter widely and do not define a simple two-component mixing- or Pb-loss line. Nevertheless, the best-fit regression line has an upper intercept age of ca. 3.25 Ga, which represents the minimum age of protolith crystallization. These data add to the growing database of existing U-Pb evidence for Archean protolith crystallization ages in the northern Wyoming province. Ongoing U-Pb analyses with even more aggressive chemical abrasion will better refine the crystallization age and whole-rock isotopic and geochemical data will help to better characterize the nature of the protolith.