Topping Off the Capstone: Constraints on the Timing of Deformational Fabric Development: ca. 1780 Ma Big Sky Orogeny, Southwestern Montana
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Date
2015-09-29
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Abstract
The study of ancient collisional belts gives insight into the tectonic settings that shaped the continents today. The Big Sky orogeny, at ca. 1800-1760 million years ago, marks the suturing of two Archean cratons that formed part of ancient North America (Laurentia) during the Proterozoic Eon. The “Topping off the Capstone” initiative at Texas A&M University engages undergraduate students to pursue research using geochronological data to unravel the poorly constrained timing of the metamorphism associated with the Big Sky orogeny throughout the Highland Mountain in southwestern Montana. One persistent problem in this region is the partial resetting of geochronometers due to later hydrothermal events associated with the Cretaceous (~80-70 million years ago) assembly of the Rocky Mountains. In order to try to unravel these complexities to normal applications of U-Pb geochronometers this study evaluated the use of step-wise dissolution of zircons from two granitoid intrusions. Analysis from one sample shows that multiple leach steps are all in agreement and indicate crystallization of the granitoid during the Big Sky orogeny at 1787±56 million years ago with Cretaceous partial resetting. The other sample, which shows significantly more Pb-loss, did not produce Big Sky ages for leach steps. However, the final dissolution analyses of five zircons produced a Big Sky age of 1753±30 million years ago. These ages are in agreement with (presumed) metamorphic monazite ages of 1765.6±4.3 Ma and 1784±19 Ma, confirming Big Sky age metamorphism and partial melting to produce the granitoid intrusions. These geochronologic data coupled with cathodoluminescence images confirm similar results obtained in the nearby Tobacco Root Mountains. The “step-wise” analysis used here gives a new methodology to more clearly determine ages from zircons that would otherwise provide only ambiguous age results.