Bearpaw Mountains

The alkaline igneous rocks of the Bearpaw Mountains are the most extensive of the central Montana alkaline igneous province, and comprise a considerable thickness of lavas and pyroclastic rocks together with numerous stocks, laccoliths, sills and dykes, including the Rocky Boy stock (No. 26) covering over 30 km2. The mountains have an area of some 2000 km2 and define a complex anticlinal uplift that exposes in its central parts Mississippian, Jurassic and Cretaceous sedimentary rocks. Volcanic rocks, predominantly of Middle Eocene age, overly the sediments and are inclined moderately steeply inwards. The youngest volcanics rest unconformably on the older ones. The volcanics occupy essentially northern and southern semi-circular areas, separated by an east-west-trending arch including sedimentary rocks which are cut by numerous intrusions. Other intrusions, essentially of a shallow and conformable nature, occur throughout the volcanic sequence. The volcanics have a maximum thickness of about 2000 m, the lowermost parts generally comprising mafic and felsic pyroclastics with a maximum thickness of about 950 m and including agglomerates, tuff-breccias, tuffs and water-laid volcanic sediments and mudflow deposits; inclusions of Precambrian basement rocks and biotite pyroxenite are locally abundant. The most extensive extrusive rocks are mafic flows and flow-breccias of phonolite and mafic phonolite with phenocrysts of augite, olivine, biotite, analcime and rare leucite in a matrix of pyroxene, K-feldspar and analcime. These flows have a maximum thickness of some 1500 m and are interleaved with mafic and felsic pyroclastic rocks and felsic flows. The felsic flows and flow-breccias are porphyritic quartz latites with phenocrysts of K-feldspar, plagioclase, pyroxene, hornblende and biotite; quartz is confined to the groundmass. Analcime and sanidine-phyric trachytes reach nearly 500 m in maximum thickness and are partly interleaved with the uppermost mafic and felsic flows, partly overlying them unconformably. Mafic analcime phonolites forming flows, flow breccias, agglomerates and plant-bearing water-laid volcanic sediments occur only in the southwest of the Bearpaw Mountains, where they form topographic highs and rest unconformably on the older volcanics, although lower levels may interleave with the analcime-phyric trachytes. They contain phenocrysts of analcime, augite, biotite and rare sanidine. The rock types encountered in the intrusions of the Bearpaw Mountains are generally equivalent to the extrusive rocks, except for biotite pyroxenite which is confined to the Rocky Boy stock and also found as inclusions in other igneous rocks. Shonkinite and syenite are found in numerous plugs, stocks, laccoliths, dykes and sills and vary from shonkinite with more than 40% mafics, through mafic syenites to leucocratic syenites. They vary from silica-undersaturated to oversaturated varieties and contain K-feldspar, pyroxene and biotite with or without olivine, plagioclase, nepheline, pseudoleucite, analcime, apatite and interstitial quartz. Porphyritic latite is found in a range of intrusion types and invariably contains augite phenocrysts with variable phenocrysts of K-feldspar, plagioclase, biotite and hornblende, in a groundmass of feldspar, pyroxene and quartz. Monzonites are confined to the western half of the mountains and are predominantly mafic, fine to medium grained, occasionally porphyritic rocks of varying proportions of K-feldspar and plagioclase, pyroxene, olivine, biotite and hornblende; quartz is lacking and feldspathoidal varieties rare. Porphyritic potassic syenites encompass a wide range of rock types including pseudoleucite, sodalite and nepheline tinguaite and aegirine-nepheline syenite, and all are confined to the central and west Bearpaws. Dykes and plugs of mafic analcime phonolite are the youngest intrusions of the area and only occur in the western part of the mountains; they are fine grained rocks with phenocrysts of analcime, pyroxene, biotite and rare sanidine. The Rocky Boy stock is described below (No. 26). Sr isotope studies of a range of rocks were made by Powell and Bell (1970) and palaeomagnetic data are available in Diehl et al. (1983). A geological map giving the distribution of all the major rock types is available (Hearn, 1976).