Igneous rocks of the Highwood Mountains, Montana: Part VII. Petrology

In the shonkinite series olivine, leucite, and analcime crystallized only from magmas with over 20 per cent of mafites. At this stage the leucite and analcime inverted to pseudoleucite. Pyroxene crystallized over the whole range of rocks and changed little in composition until the magma reached the composition of nepheline syenite when it became richer in aegirite. A little pale biotite crystallized early, but most of the biotite is dark and crystallized late. A little barium rich sanidine crystallized early. The abundant late feldspar is poor in barium.

Four petrographic subprovinces are represented in the area. The older quartz latites are near lime-alkalic rocks, the syenites and monzonites of Highwood Peak are somewhat farther removed, and the shonkinite series are potash-rich alkalic rocks. The alnoites are very rich in lime. The parent mafic magmas of the various subprovinces formed in depth by differentiation of a primary basaltic magma. These parent magmas were erupted toward the surface and there differentiated in a different way. The differentiation of the shonkinite magma was chiefly by settling of augite, olivine, and some biotite.