TOPPING OFF THE CAPSTONE – DETRITAL ZIRCON AGES FROM METASEDIMENTARY ROCKS ASSOCIATED WITH ARCHEAN GNEISSES, 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. The goal of this project is to use the ages of detrital zircons in metasediments associated with Archean gneisses in two disparate locations to help constrain the Archean crustal evolution of the Wyoming craton.

One sample, located north of Clark Canyon Dam, is a distinctive “fin” of quartzite, with clear graded- and cross-bedding, protruding from a gneiss complex of banded gneisses of variable composition. No faults were evident in the gneisses and this outcrop was thought to possibly represent an Archean sandstone bed preserved within a low-strain lens of an otherwise highly deformed volcanic-sedimentary package. The youngest analyzed zircons form a cluster at around 1750 Ma and the oldest grain is 3045 Ma. Other analyses cluster at around 2400-2700 Ma and 2980 Ma. The second sample is a garnet-rich, biotite-sillimanite metapelite from the Rochester Creek mining district of the Highland Mountains. Monazite from this sample shows moderate Pb-loss, likely in the Cretaceous, but upper-intercept ages suggest crystallization during a major fabric-forming event at about 1780 Ma. Analysis of detrital zircons from this sample is ongoing.

Detrital zircon ages from Clark Canyon sample indicate that it is not a component of the Archean metasedimentary/metavolcanic package, but is instead most likely either Belt Supergroup or Cambrian Flathead Formation in some way entrained along obscure basement faults. The monazite age from the Rochester Creek metapelite indicates (re)crystallization during the Big Sky orogeny. A detrital zircon age spectrum from this sample is eagerly anticipated as it could contain zircons as old as Hadean >3.9 Ga based on published work on similar rocks in the nearby Tobacco Root Mountains.