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.
The present list of occurrences of alkaline rocks in the United States was based initially on the compilation of Barker (1974), who also discussed the distribution in space and time of North American feldspathoidal rocks (Barker, 1969). The present compilation contains not only newly discovered occurrences but also a number of peralkaline syenite and granite localities which were omitted earlier, althoughBarker himself pointed outthat a number of such occurrences had probably escaped his notice. There are probably still omissions because rocks of this type are often found in non-peralkaline environments and so easily missed during bibliographic searches. However, it is likely that coverage of the pantelleritic and comenditic rocks is relatively comprehensive, apart from as yet unknown occurrences, because of the thorough compilation of Noble and Parker (1975).
Future discoveries in the United States are likely to be in the Cordillera, and the large number of localities already known from there, when combined with the considerable numbers coming to light in British Columbia and Alaska, would suggest that western North America could become a particularly important area for exploring the relationship between alkaline igneous activity and orogenesis.
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.
Knopf (1957) has described a number of relatively minor occurrences of peralkaline and nepheline-bearing rocks along the northern boundary of the Boulder Batholith which he ascribes to limestone syntexis.
Numerous laccoliths and other igneous intrusions have domed and warped both Precambrian crystalline rocks and sedimentary rocks in the central Little Belt Mountains.
Several dykes and small plugs are clearly related to three major intrusions just to the south in the United States. For fuller details, see 174-00-097.