Abstract
Thin section studies of pebbles and boulders from the Rapanui Formation near Hawera have been made in an attempt to ascertain the principal trends of petrographic variation in Mt Egmont andesites. In addition to the commonly acknowledged hornblende andesites several other varieties of andesite identified included augite andesites completely devoid of hornblende and basaltic andesites containing up to 7% modal olivine. Hypersthene andesites of the type dominating volcanic rocks in the Tongariro National Park area are conspicuously absent. Petrographic and petrochemical relationships of andesites of Mt Egmont indicate that these rocks were formed either by contamination of basaltic magma with sialic materials or by fusion of crustal rocks of dioritic composition. The latter process of origin of andesitic magma is favoured.
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