Big Belt Mountains

The northern part of the Big Belt Mountains (or Range) is chiefly underlain by somewhat potassic basic volcanics about 1000 m thick, the Adel Mountain volcanics, which lie unconformably on Cretaceous sediments. They are intruded by numerous sills, dykes and plugs of syenogabbro, trachybasalt and monzonite. The most widespread volcanics are trachybasalts, one variety of which is analcime trachybasalt containing 5-15% of analcime phenocrysts up to 5 mm in diameter (Lyons, 1944). Syenogabbros are present in laccoliths and stocks, a number of which form prominent buttes to the north of the volcanics. The syenogabbros comprise zoned plagioclase of an average labradorite composition, orthoclase and augite. Syenite is present as veins in the gabbro and contains microcline, albite, aegirine-augite, biotite and accessories. Square Butte is a laccolith with prominent layering, the feeder dyke for which is exposed and cuts Cretaceous sediments for 18 km to the south. The rock of Square Butte contains diopsidic augite phenocrysts in a groundmass of anorthoclase, plagioclase (An60), some olivine, biotite and a little pseudoleucite (Beall, 1972). There are numerous dykes, one of which runs into Square Butte, which radiate from the Three Sisters stock lying in the heart of the Adel Mountain volcanics. It is likely that many of these dykes, together with other laccoliths and stocks, are strongly alkaline in character and probably contain pseudoleucite and or primary analcime.