Petrochemical Grouping of Quaternary Basalts, Northland, New Zealand: a Reassessment and Relation to Ngawha Geothermal System

Abstract

The petrochemistry of basaltic rocks in the Kaikohe- Kaeo areas around Ngawha is reassessed to determine if chemical variations would confirm the apparent associ ation of age and location of the youngest volcanism with the identified Kaikohe structural block, within which the Ngawha geothermal system is located. The outcome of this reassessment is the indication of an apparent change in chemical composition of the Kaikohe lavas from early (Horeke) high-A1 to later (Taheke) alkali, the reverse of that previously reported. Comparison of basalts from the Kaeo area to the north of Kaikohe indicates volcanism forming the majority of the basalts there was alkali c and, at least, partly contemporaneous with the older high-Al volcanism in the Kaikohe block. A s imi l a r approach to basal t s in the Whangarei and Puhipuhi areas to the south of Kaikohe suggests a compositional change from t hole i i t e s to younger high-A1 and alkali basalts. The age relations those areas are, however, less clear. Petrochemistry of these Northland b a salt s indicate s an increasing depth of formation northward for the initial magma type in each area.