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Montana Geological Society: 1993 Field Conference Guidebook: Old Timers' Rendezvous Edition: Energy and Mineral Resources
of Central Montana
Diatreme-Dike Associations of Central Montana
Ultramafic intrusions in the Porcupine Dome and Grassrange areas of east-central and central Montana respectively consist of thin, extensive dikes oriented mainly northeast-southwest, consistent with local structural control. Assodated with many dikes are small plugs and diatremes, pipe-shaped volcanic vents formed by energetic eruptions of gas-charged, ultramafic magmas. Dikes commonly possess chilled margins, flow-aligned mica xenocrysts (?)/phenocrysts (?), angular shale xenolifhs, and indications of multiple intrusive events. A continuum of diatreme development is preserved with vents ranging from ~ 3 ft (1 m) diameter "blows" in dikes to > 150 ft (45 m) diameter diatremes containing bedded tuffs and large country rock xenoliths. Physical characteristics of these igneous centers generally resemble those of the Missouri River Breaks field of north-central Montana.
Porcupine Dome igneous centers consist of a lamprophyre rock composed of phenocrysts or xenocrysts of olivine and phlogopite set in a matrix of phlogopite, spinels, iron oxides, serpentine, abundant carbonate ± garnet ± pyroxene. A single lamprophyre magma formed both dikes and plugs, despite radical differences in the present rock exposures due to extensive alteration. Lamprophyres of similar mineralogy and appearance comprise most of the Grassrange intrusions, exceptions being the Winnett sill alnoites and some isolated dikes consisting of nearly pure carbonate. Porcupine Dome whole-rock chemistries are similar to some rocks of the Missouri River Breaks field. Field relationships and preliminary petrographic and chemical data suggest that igneous rocks of Porcupine Dome, Grassrange, and the Missouri River Breaks fields may be part of the same episode of Eocene ultramafic magmatism in central Montana.
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