Mcdermitt Volcanic Field

This volcanic field covers some 20 000 km2 and formed by the eruption of seven ash-flow sheets having an aggregate volume of 1700 km3. All but one of the sheets are comenditic. Eruption of each sheet resulted in the formation of a large collapse caldera. The centre of the field is dominated by a composite collapse structure consisting of four overlapping and partly nested calderas which are known as the McDermitt caldera complex (see Fig. 141). The sheets and source calderas are listed by Rytuba and McKee (1984, Table 1). Details of the petrography of the various ash-flow units are given by Rytuba and McKee (1984) and Conrad (1984) and involve variable phenocryst assemblages including alkali feldspar, quartz, fayalite, aenigmatite, sodic amphibole, clinopyroxene, ilmenite and apatite. Conrad (1984) gives numerous mineral compositions and rock analyses including RE and other trace element data.