Date of Award

12-1984

Degree Name

Master of Science

Department

Geosciences

First Advisor

Dr. Ronald B. Chase

Second Advisor

Dr. John D. Grace

Third Advisor

Dr. W. Thomas Straw

Access Setting

Masters Thesis-Open Access

Abstract

The magmas which formed the northeastern Idaho batholith were contaminated with, or derived from, old crustal material as evidenced, from previous studies, by high whole-rock 87Sr/86Sr ratios, relatively high 207Pb/206Pb ages, and Archean to Proterozoic upper intercept ages for batholithic zircons.

The old zircon component has been identified in this study by separating zircons into fractions based on size and morphology. Pb/U isotopic ratios are distributed on a chord with a lower concordia intercept intrusive age of 73.6+6 m.y. Zircons from a batholithic sample located one kilometer inward from the contact were separated according to mineral association. This method was not successful in segregating old inherited zircon from euhedral zircon. Upper-intercept ages from this project, when combined with those from other studies, range from 1700 m.y. and indicate the complexity and variety of the 'older continental crust' source terrain.

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