Boulder Batholith, Montana: A product of two contemporaneous but chemically distinct magma series

GSA Bulletin



Rocks of the Late Cretaceous composite Boulder batholith, though successively emplaced in a relatively small segment of the Earth's crust within a very brief time span (78 to 68 m.y.), can be grouped chemically into two magma series: (1) the main series, defined principally by plutons in the central and northern parts of the batholith; and (2) the sodic series, defined mostly by plutons in the southern part. For any given SiO2 content, the rocks of the main series tend to be higher in K2O and lower in Na2O than rocks of the sodic series. The chemical distinction between the two series proposed is also expressed by variation patterns for U, Th, Rb, and Sr abundances, by lead isotope compositions, but not by strontium isotope compositions.

The prebatholith Elkhorn Mountains Volcanics (Late Cretaceous), especially the mafic members, are chemically and isotopically similar to the rocks of the main series, confirming geologic evidence of the genetic association between them. The postbatholith Lowland Creek Volcanics (early Eocene), though chemically more closely related to the sodic series, isotopically are more akin to, but slightly more radiogenic than, the main series. Post–Lowland Creek volcanic rocks (Miocene or Pliocene) are compositionally similar to the sodic series rocks. Spatial distribution of the batholith and the volcanic rocks exhibits a very crude chemical zonation of the region: for a given silica content, relatively more potassic rocks (main series and prebatholith volcanic rocks) tend to occur mainly in the north and east, whereas relatively more sodic rocks (sodic series and postbatholith volcanic rocks) predominate in the south and west.

Available field, chemical, and isotopic evidence collectively suggests that the observed compositional variations for the Boulder batholith are most reasonably interpreted in terms of a model involving two magma series derived from two or more magma sources within the lower crust or upper mantle. These source regions are interred to vary chemically and isotopically, either laterally or vertically; in view of the rather small areal extent of the Boulder batholith, however, a vertically zoned source region is more probable.

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Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Boulder Batholith, Montana: A product of two contemporaneous but chemically distinct magma series
Series title GSA Bulletin
DOI 10.1130/0016-7606(1973)84<3879:BBMAPO>2.0.CO;2
Volume 84
Issue 12
Year Published 1973
Language English
Publisher GSA
Contributing office(s) California Volcano Observatory
Description 22 p.
First page 3879
Last page 3900
Country United States
State Montana
Other Geospatial Boulder Batholith, Montana
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