shonkinite

Significant amounts of syenite and shonkinite can be found in the laccolith.

The Shonkin Sag laccolith is a significant source of shonkinite.

With the addition of alkali feldspar replacing or becoming more dominant than calcic plagioclase theralites grade into foidal syenite, including shonkinite.

Associated igneous rocks typically include ijolite, melteigite, teschenite, lamprophyres, phonolite, foyaite, shonkinite, silica undersaturated foid-bearing pyroxenite (essexite), and nepheline syenite.

The Adel Mountains Volcanic Field is a significant and abundant source of shonkinite, a very uncommon type of intrusive igneous rock found primarily in Montana, Ontario, and Timor.

Nepheline in shonkinite from the type locality at Shonkin Sag in the Highwood Mountains of north-central Montana, from which the rock gets its name, is largely altered to natrolite and stilbite.