Gary D. Eaton

Year of Award


Document Type


Degree Type

Master of Science (MS)

Other Degree Name/Area of Focus


Department or School/College

Department of Geology

Committee Chair

Dave Alt


University of Montana


Numerous calc-alkaline plutons are exposed along the southern and eastern flank of the Pioneer batholith in southwestern Montana. The plutons are emplaced to shallow depths into upper Paleozoic carbonates and Mesozoic elastics. Petrographic and chemical data, and their occurence in space and time, suggest a petrogenetic link with the nearby batholith.

Pyroxene-bearing quartz monzonite and microgranodiorite exposed at Argenta possibly represent an ancestral phase of the Pioneer batholith. The Argenta Stock shows evidence of both a 'wet' and 'dry' magmatic environment. Interception of a meteoric water source at a high level of emplacement by an initially dry magma is postulated to reconcile this occurence.

A large dacite sill southwest of the McCartney Mountain stock is possibly an offshoot of a 'tabular conduit' proposed by Chandler (1973) to connect the stock and batholith. Reconnoitering of a quartz latite porphyry sill at Trout Creek and a quartz diorite pluton at Melrose provides chemical and petrographic data.

The plutons are chemically compatible with the volumetrically minor 'Sodic Series' plutons recognized by Tilling (1973) in the Boulder batholith.



© Copyright 1983 Gary D. Eaton