Date of Award

4-1998

Degree Name

Master of Science

Department

Geological and Environmental Sciences

Department

Geosciences

First Advisor

Dr. Ronald B. Chase

Access Setting

Masters Thesis-Open Access

Abstract

In east-central Idaho and western Montana a 400km-long Cretaceous and Tertiary magmatic corridor extends from the Wallowa-Seven Devils terrane to the Helena Salient. Three basement provinces underlie this area: Archean basement to the east, Permo-Triassic basement to the west, and Proterozoic basement in-between. Ideally, if Cretaceous plutons were decapitated and translated eastward along thrusts over a different basement province, then the Cretaceous pluton and the in-place Tertiary pluton would show different isotopic signatures. Pluton samples from locations evenly spaced throughout the magmatic corridor were analyzed for ε0Nd, εtNd, and 147Sm/143Nd model ages of potential magma source materials. ε0Nd and εtNd values range from -6 to -19, and model ages range from 1158 to 2043Ma. Either source rocks are of varying ages, or there are variations in magmatic mixing between mantle and crustal components, or both. Model ages for Cretaceous rocks suggest more assimilation of crustal material than Tertiary rocks. εNd values in the western sector of the Boulder batholith offer permissive evidence for possible pluton displacement, but there is no definitive evidence to confirm pluton displacement at other locations.

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