Castle Mountains
Among the granites, diorites, porphyries and rhyolites of the Castle Mountains trachytic intrusions with sodic pyroxene are reported.
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.
Among the granites, diorites, porphyries and rhyolites of the Castle Mountains trachytic intrusions with sodic pyroxene are reported.
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.
Over a rectangular area of some 30x40 km extending from near Deep Creek in Ravalli County, Montana, southwards to the vicinity of North Fork, Lemhi Country, Idaho, more than 30 occurrences of carbonatite dykes and veins are known together with numerous thorite-bearing veins.
At Big Southern Butte in the central Snake River Plain (Walker, 1964, Plate 1) non-hydrated glassy and crystallized aphyric comendite lava is associated with basalt. An analysis is available (Noble, 1968a, Table 1).
Nepheline syenite and phonolite form flat-lying sheets, dykes and a small stock which are usually intruded into Eocene rocks in the central part of the Oregon Coast Range.
Most of the area between Suplee and Izee in east-central Oregon consists of Cretaceous rocks, but capping many buttes and in isolated outcrops throughout the region are erosional remnants of peralkaline welded tuff, lapilli tuff and tuff breccia (Dickinson and Vigrass, 1965, Plate 1).
An ash-flow tuff exposed over some 80 km2 of the southern slopes of Wagontire Mountain probably erupted from a vent on the mountain and another several km to the west. The flow varies from 2-40 m in thickness and has a volume of less than 1 km3.
Extensive Pliocene, peralkaline ash-flow tuff deposits in southeastern Oregon may have vented from the Buzzard Creek area which lies in a complex northwest-trending graben. The tuff sheet is zoned but appears to be a single flow.
Short, thick flows and intrusions of pantellerite, up to 1.6 km long and 100 m or so wide, occur on the flanks of Hart Mountain. Commonly the flows are strongly flow banded and locally autobrecciated.
A peralkaline rhyolite from Santa Rosa with phenocrysts of sanidine, anorthoclase, quartz and aenigmatite in a groundmass of quartz, sanidine, mossy patches and needles of arfvedsonite and minor aegirine, is illustrated in Williams et al. (1954, p. 127).