Semester of Graduation
Master of Science (MS)
Geology and Geophysics
Veins associated with igneous activity provide numerous insights into the geologic history of an area. The Sawtooth Metamorphic Complex (SMC) in Idaho is a roof pendent of highgrade rocks cross-cut by numerous felsic veins of unknown age and source. Obtaining U/Pb zircon geochronology of these veins can facilitate understanding the ages of magmatism in the region, their source, when nearby batholiths were juxtaposed to the SMC, and provide additional insights into the geologic evolution of the area. Each potential source of these veins has a distinct age: the Sawtooth batholith (ca. 47 Ma), the Idaho batholith (98-53 Ma), an anatectic melt associated with aluminous gneisses in the complex (>100 Ma), an undefined later melting and/or hydrothermal event (Ma), or a combination of the these. In each of these potential sources, plagioclase optical cathodoluminescence (OM-CL) responses are obtained to determine any correlation with vein source and as an alternative proxy to ages. U/Pb ages of zircons were obtained for six vein samples spatially separated around the SMC. The ages range from 72.3 ± 1.2 Ma to 131.0 ± 2.2 Ma. Two samples contained zircons with inherited cores, with ages ranging from 317 ± 7 Ma to 1803 ± 55 Ma. Vein emplacement ages, determined by youngest ages, range from 75.5 ± 0.6 Ma to 101 ± 1 Ma. Zircon formation ages indicate an extended period of magmatism occurred episodically for at least 49 Ma. inherited core ages indicate incorporation of rocks containing Proterozoic and Ordovician-aged zircons. Vein emplacement ages are consistent with the Idaho batholith time frame suggesting the SMC was juxtaposed to the Idaho batholith before vein emplacement. Evaluation of OM-CL responses show that green plagioclase CL responses are characteristic of the Idaho batholith. Green is observed in ten of thirteen samples. This suggests that age and plagioclase CL color correlate for the Idaho batholith. This correlation suggests that OM-CL ix plagioclase responses may be a useful proxy for source in the absence of geochronology. This study contributes new U/Pb zircon age data in a region that is poorly understood
Tollefson, Kyle, "U/Pb Zircon Ages of Felsic Veins in the Sawtooth Metamorphic Complex, Idaho, U.S.A: Implications for Magmatism and Vein Source" (2020). LSU Master's Theses. 5232.