TOPPING OFF THE CAPSTONE – ZIRCON, MONAZITE, AND TITANITE AGES FROM TWO GENERATIONS OF DIKES, 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. The goal of this project is to constrain the age of high-grade fabric development in the Highland Mountains gneiss complex of southwest Montana by U-Pb dating of dikes transposed within and cross-cutting the main gneissic fabric.

The Highland Mountains are located near the Northwestern margin of the Archean Wyoming province where Laramide uplift has exposed basement rocks. During the course of field mapping, two generations of dikes were recognized in the TAMU field camp project area. An older set of mafic dikes are ubiquitous in both the Highland and Tobacco Root mountains. These are metamorphosed to moderately foliated amphibolites, transposed within the host gneissic foliation. A younger set of moderately recrystallized, but unfoliated, granitoid dikes cross-cut the fabric of both the gneiss and the earlier metamorphosed mafic dikes.

U-Pb analyses of monazite from both the granitoid dike and an immediately adjacent metapelite gneiss yield slightly discordant data with upper intercept ages of 1771±6 and 1788±7.5, respectively. The metamorphosed mafic dike sample yielded no zircon and precise titanite ages are hampered by excessive common Pb, but are consistent with cooling following a ca. 1750 Ma thermal event. Zircon analyses from the granitoid dike, despite annealing and chemical abrasion, show extreme degrees of Cretaceous Pb-loss and an upper intercept age of ca. 2088 Ma.

As in the nearby Tobacco Root Mountains, monazite and titanite ages from the Highland gneiss complex indicate involvement in the Big Sky orogenic event. Metamorphosed mafic dikes in the Tobacco Root Mountains are interpreted to have intruded during a regional extension event at ca. 2060 Ma. Our granitoid dike is potentially related to this extensional event, but intrusion of the metamorphosed mafic dike, and development of a Paleoproterozoic or earlier foliation, must pre-date the granitoid. Extreme Pb-loss in the zircons could be the result of heating and hydrothermal fluids during Cretaceous intrusion of the nearby Hells Canyon pluton; alternatively, the granitoid dikes may themselves be Cretaceous.