The most easterly of the Kobuk-Selawik Lowland alkaline complexes (Fig. 1_121), the Ekiek Creek intrusion covers 13 km2 and cuts quartz monzonites along its southern boundary, but there are no contact effects; to the west, north and south it is surrounded by alluvium.
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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 building of the Black Mountain volcano involved the eruption of a series of voluminous ash-flow tuffs, multiple caldera subsidence, and partial or complete filling of the caldera by lavas.
Figs. 169 and 172Geophysical work, later confirmed by drilling, has shown that the Granite Mountain and Saline County intrusions are continuous at depth, the whole complex being referred to as the Little Rock igneous complex by Gordon et al. (1958, p. 67).
The term 'Quincy Granite' was formerly used as a general term to include other New England peralkaline granites such as the Cape Ann (No. 133) and Peabody (No. 134) granites, but is now more generally restricted to the type area lying south of the town of Quincy.
The Rattlesnake intrusion is an elliptical, multiple stock of 22 km2, composed principally of a biotite granite and a number of types of riebeckite granite, intruded into the Dedham quartz monzonite, which is probably Precambrian.
In the northeastern corner of Rhode Island peralkaline granites occur which have been correlated with the Quincy granite lying some 40 km to the northeast. They comprise microperthite, quartz, up to 10% sodic amphibole and 7% aegirine; patches and veins of purple fluorite are common.
In the southeastern corner of the Deep River Quadrangle, a sequence of granite gneisses includes a layer of aegirine-augite granite gneiss, the Joshua Rock gneiss, which extends into the unmapped quadrangle to the east.
Beemerville is the only complex in the eastern U.S. in which carbonatite has been recognized.
At the upper contact of a 500 m thick dolerite sill nepheline and analcime-bearing syenite occur, both cutting the sill rocks and hornfelsed argillite. Poor exposure precludes the accurate determination of field relationships.
A suite of northwest-trending dykes injected into Palaeozoic sedimentary rocks of the Shenandoah Valley includes teschenites, camptonites, aegirine, nepheline and quartz syenites, spinel-bearing basalts, mica peridotites and olivine dolerites.