The geology, petrology, and geochemistry of ultramafic igneous rocks from Porcupine Dome and Grassrange, central Montana

The geology of metasomatized lamprophyre dikes, plugs, diatremes (pipes), and sills is described for four Porcupine Dome and eleven Grassrange intrusions of central Montana. Magma emplacement was probably controlled by deep-seated fracture zones and local faults related to Laramide crustal extension. The lamprophyres are grouped as carbonate-bearing, carbonate-free, and carbonate-rich rocks, and are discussed in terms of their current and pre-metasomatic states.;Carbonate-bearing Porcupine Dome lamprophyres consist of olivine (Fo;Carbonate-free rocks exist only in the Winnett sills alnoites of Grassrange. These consist of olivine (Fo;Carbonate-rich rocks in both fields consist of calcite, dolomite, phlogopite, and metallic oxides and are classified as carbonatized ultramafic lamprophyres. Most carbonate minerals are judged to be magmatic, based on textural relationships, elevated Sr concentrations, and anomalous whole-rock LREE concentrations. The carbonates are categorized either as lamprophyric (primary magmatic minerals) or as carbonatitic (from late magmatic fluids).;Porcupine Dome and Grassrange aillikite magmas were primitive, and resulted from small degrees (...