Composition and time relations of plutonic and associated volcanic rocks, Boulder Batholith Region, Montana

GSA Bulletin
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Abstract

Comparison of areally weighted bulk compositions for the Boulder batholith and prebatholith volcanic rocks (Elkhorn Mountains Volcanics) shows a close match in terms of K2O-Na2O-CaO-SiO2 variations. Detailed examination of available chemical data suggests, however, that the constituent units of the volcanic rocks differ among themselves, as well as from many of the batholith units. Most sampled volcanic rocks are chemically and isotopically similar to the volumetrically minor mafic members of the putonic main series of Tilling (1973). The sodic-series plutonic rocks, which form about 15 percent of the exposed batholith, apparently have no compositionally similar counterparts in the Elkhorn Mountains Volcanics in terms of bulk composition.

The Butte Quartz Monzonite, which makes up about three-quarters of the batholith in terms of individual samples and bulk composition, is chemically distinct from most volcanic rocks. This relation suggests that only small amounts of magma of Butte Quartz Monzonite composition erupted onto the surface. The voluminous silicic ash flows of the middle member of the volcanic rocks were probably derived from the same magma that mostly crystallized subsurface to form the Butte Quartz Monzonite. Field, chemical, K-Ar age, and paleomagnetic evidence, however, strongly suggest that a time gap separated the extrusion of the middle-member volcanic rocks from the intrusion of the Butte Quartz Monzonite and related younger silicic variants.

Compositional and time relations between the plutonic and genetically associated volcanic rocks are generally compatible with the concept that the volcanic rocks are eruptive equivalents of a shallow evolving batholith but seem incompatible with some specific aspects of the “extrusive complex” or “floored sheet” hypothesis as applied to the Boulder batholith by Hamilton and Myers (1967, 1974).

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Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Composition and time relations of plutonic and associated volcanic rocks, Boulder Batholith Region, Montana
Series title GSA Bulletin
DOI 10.1130/0016-7606(1974)85<1925:CATROP>2.0.CO;2
Volume 85
Issue 12
Year Published 1974
Language English
Publisher GSA
Description 6 p.
First page 1925
Last page 1930
Country United States
State Montana
Other Geospatial Boulder Batholith Region
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