Abstract

The Late Cretaceous Boulder batholith in southwest Montana consists of the Butte Granite and a group of associated smaller intrusions emplaced into Mesoproterozoic to Mesozoic sedimentary rocks and into the Late Cretaceous Elkhorn Mountains Volcanics. The Boulder batholith is dominated by the voluminous Butte Granite, which is surrounded by as many as a dozen individually named, peripheral intrusions. These granodiorite, monzogranite, and minor syenogranite intrusions contain varying abundances of plagioclase, alkali feldspar, quartz, biotite, hornblende, rare clinopyroxene, and opaque oxide minerals. Mafic, intermediate, and felsic subsets of the Boulder batholith intrusions are defined principally on the basis of color index. Most Boulder batholith plutons have inequigranular to seriate textures although several are porphyritic and some are granophyric (and locally miarolitic). Most of these plutons are medium grained but several of the more felsic and granophyric intrusions are fine grained. Petrographic characteristics, especially relative abundances of constituent minerals, are distinctive and foster reasonably unambiguous identification of individual intrusions.

Seventeen samples from plutons of the Boulder batholith were dated by SHRIMP (Sensitive High Resolution Ion Microprobe) zircon U-Pb geochronology. Three samples of the Butte Granite show that this large pluton may be composite, having formed during two episodes of magmatism at about 76.7 ± 0.5 Ma (2 samples) and 74.7 ± 0.6 million years ago (Ma) (1 sample). However, petrographic and chemical data are inconsistent with the Butte Granite consisting of separate, compositionally distinct intrusions. Accordingly, solidification of magma represented by the Butte Granite appears to have spanned about 2 million year (m.y.). The remaining Boulder batholith plutons were emplaced during a 6–10 m.y. span (81.7 ± 1.4 Ma to 73.7 ± 0.6 Ma).

The compositional characteristics of these plutons are similar to those of moderately differentiated subduction-related magmas. The plutons form relatively coherent, distinct but broadly overlapping major oxide composition clusters or linear arrays on geochemical variation diagrams. Rock compositions are subalkaline, magnesian, calc-alkalic to calcic, and metaluminous to weakly peraluminous. The Butte Granite intrusion is homogeneous with respect to major oxide abundances. Each of the plutons is also characterized by distinct trace element abundances although absolute trace element abundance variations are relatively minor. Limited Sr and Nd isotope data for whole-rock samples of the Boulder batholith are more radiogenic than those for plutonic rocks of western Idaho, eastern Oregon, the Salmon River suture, and most of the Big Belt Mountains. Initial strontium (Sr_i) values are low and epsilon neodymium (ε_Nd) values are comparable relative to those of other southwest Montana basement and Mesozoic intrusive rocks. Importantly, although the Boulder batholith hosts significant mineral deposits, including the world-class Butte Cu-Ag deposit, ore metal abundances in the Butte Granite, as well as in its peripheral plutons, are not elevated but are comparable to global average abundances in igneous rocks.

First posted November 15, 2012