Crazy Mountains

The Crazy Mountains rise to over 3500 m above sea-level and some 2000 m above the surrounding plains. They consist of sandstones and shales of Cretaceous-Eocene age which have been injected by numerous laccoliths, stocks, sills and dykes. Diorites and 'normal' granites are represented but most of the igneous rocks are alkaline and these are concentrated in the northern part of the area. The principal alkaline rock types are theralites and nepheline syenites. The rock described as theralite by Wolff (1938) and shonkinite by others (op. cit. p. 1605) is found in six major laccoliths or sills as well as dykes. It usually comprises about 40% diopside zoned to aegirine-augite, 20-30% orthoclase, 10-15% nepheline, up to 21% sodalite, biotite, olivine, sometimes amphibole and accessories. Porphyritic varieties contain phenocrysts of pyroxene, biotite, olivine, locally sodalite and feldspar, and rarely nepheline. Contact metamorphosed rocks contain abundant aegirine. Nepheline syenite forms three major laccolithic bodies and Wolff (1938, p. 1611) noted some 25 other sills and dykes of a similar rock but called by him 'soelvsbergite'; they cut the theralites. The nepheline syenite comprises alkali feldspar, phenocrysts of aegirine-augite zoned to aegirine and groundmass aegirine, biotite, sphene and apatite, variable nepheline and analcime, and in some sills large crystals of sodalite (nosean). Simms (1965) refers briefly to the presence of malignites (? the theralites of Wolff) in which he identifies pseudoleucite. Rock analyses, modes and some mineral data are available in Wolff (1938).