ABSTRACT

Major intermediate-composition volcanism accompanied and followed the climax of Laramide folding and thrusting in southwestern and central Montana, peaking in Late Cretaceous and in Early to Middle Eocene time. Plutonism likewise peaked at these times, but plutons in several districts spanned the intervening time, according to present radiometric information.

Volcanic rocks are calc-alkaline in the orogenic belt but alkaline in the foreland. Andesite, dacite, and rhyodacite flows, breccias, tuffs, and ash-flow tuffs dominate volcanic deposits within the fold and thrust belt and basement uplift region (e.g. Elkhorn Mountains, Lowland Creek and Absaroka-Callatin volcanic fields). In the foreland east of the main fold and thrust belt, potash-rich trachybasalts, trachyandesites, latities, and phonolites erupted in significant volumes in the Adel Mountain, Highwood Mountains, and Bearpaw Mountains fields.

Beginning in Oligocene time and approximately contemporaneous with the subsidence of basins which evolved into fault-block valleys under an extensional tectonic regime, volcanism in Montana became less widespread and became rhyolitic and basaltic in nature. Significant deposits include the Helena, Beaverhead Canyon, and Virginia City fields (Oligocene), and northward extensions of the Snake River Plains-Yellowstone field (Plio-Pleistocene).

The principal volcanic fields and plutonic bodies in the region are aligned along several major linear structural belts which evidently represent zones of weakness in the Archean crystalline basement. The zones are: Idaho-Montana porphyry belt (northeast), Montana Lineament (west-northwest), Absaroka-Callatin belts (northwest), and Snake River downwarp (east-northeast). Such zones may play a significant role in magma generation and migration in regions of moderate to thick continental crust inland from subducting oceanic plates.