Petrographic and stratigraphic studies of ironsands and associated andesitic sediments near Hawera, South Taranaki

Abstract

Stratigraphic and petrographic studies of ironsands and associated volcanic sediments embraced the measurement and description of the Rapanui Formation near the type locality of the Hawera Series. A new member, the Egmont Lahar Breccia, is described. Derived from debris transported en mass from the slopes of Mt. Egmont the Egmont Lahar Breccia was deposited disconformably on top of the Rapanui Lignite and is overlain by the Rapanui Dune-sand. Petrographic and textural studies were made of ironsands and andesitic sands in the Rapanui Formation. Abundances of the principal ferromagnesian minerals were measured and from them an estimate made of the proportions in which the principal types of andesite have been erupted from Mt. Egmont. Petrographic studies of the andesites themselves show them to be predominantly augite hornblende andesite. Some augite olivine andesites were observed but hypersthene andesites appear to be absent. Petrochemical relationships indicate that these andesites 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 preferred.