Mesozoic magmatism in Montana

From crystalline batholiths with footprints larger than 4,500 km² to beds of micron-sized ash particles, a record of Mesozoic magmatism is found throughout Montana. Mesozoic igneous rocks are an important natural resource in the state because of their association with precious metal ores and industrial mineral deposits. Mesozoic magmatism in Montana is a tale of volcanic arc eruptions, pluton emplacement, crustal magma differentiation, melting, and assimilation. Explosive caldera-forming eruptions, magmatic hydrothermal and metamorphic mineralization, tectonic uplift, fluvial erosion, and redeposition of igneous rocks and mineral resources are all chronicled by Mesozoic igneous rocks. This article offers a summary of the types, locations, extents, and ages of Mesozoic igneous rocks in Montana. Then follows a discussion of the generation and emplacement of magmas and intrusive rocks, their geometries, and the style and timing of magmatism. The chapter ends with a summary of what remains unknown and offers suggestions for future research.