The volcanic geology of the Mt Karioi region

Abstract

Mt Karioi is a 756 m high stratovolcano of Pliocene age (2.9 - 2.3 m.y. B.P.) situated south of Raglan, on the west coast of the North Island. Immediately surrounding Mt Karioi are a number of small low-lying volcanic centres of the Plio-Pleistocene Okete Volcanic Formation (3.79 – 1.80 m.y.B.P.) which can be separated from the Karioi Volcanic Formation by their volcanic form, petrology and petrochemistry. The Karioi Volcanic Formation consists of basalt, basaltic andesite and andesite lavas, volcanic breccias and tuffs produced by strombolian eruptions, and lahar deposits initiated by heavy rain on the upper slopes of the volcano. The Okete Volcanic Formation, comprising olivine basalt lava flows, scoria cones and ash, was produced by strombolian and phrea­tomamgatic activity. Karioi lavas are generally coarse-grained porphyritic rocks with large phenocrysts of plagioclase, clinopyroxene and olivine. The Okete lavas are dominated by olivine phenocrysts set in a fine-grained pilotaxitic groundmass. Thirteen new analyses of lavas in the region are presented and discussed with ten previously published analyses. Chemically the Karioi lavas show a complete range from basanitoids to quartz tholeiites. The Okete lavas show less variation and are mainly basanitoids with minor alkali olivine basalts and olivine tholeiites. From the trace element data, strontium isotope information and restriction of ultramafic nodules to the Okete Volcanics, it is apparent that these two formations are not linked petrogenetically. The Karioi calc-alkaline rock series is largely the result of amphibole fractionation of a nepheline normative hydrous basaltic melt which produced a series of liquids which crossed the critical plane of undersaturation. The Okete Volcanics underwent little or no differentiation; each magma type is the result of varying degrees of 1-Jartial melting of mantle at different depths which produced a variety of melts with a range in saturation. The basalts of both volcanic formations are behind-arc basalts which show little pattern in relation to a subducting lithospheric slab and were probably derived from a source in the lower velocity zone of the upper mantle. The limited volumes of andesite having been derived by fractional crystallisation of a basalt cannot themselves be anyway related to the Benioff Zone. A summary of the volcanic history of the Mt Karioi region is discussed and shown in a series of schematic diagrams.