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Abstract
A microdiorite occurs in 13 outcrops which have a distinctly linear northeast-southwest distribution over a distance of 20 km from the coast near Matapouri to the Mt Tiger Road inland of Onerahi, Northland. The microdiorite, when well exposed, is seen to form a dike about 15 m wide. The petrology and geochemistry of the microdiorite are uniform in all 13 localities. K-Ar dates on biotites from three of the localities are consistent and point to a single intrusion in the Early Miocene.
Coexisting orthopyroxenes and clinopyroxenes in the microdiorite have a wide range of composition and habit Fe-enriched pigeonite mantles augite and orthopyroxene and also appears as separate tabular grains. Amphibole chemistry shows a gradual change along the length of the dike. Mineral chemistry and pyroxene geothermometry point to the accelerated intrusion of a high-temperature intermediate magma and its rapid chilling against metagreywacke basement.
It is deduced that the microdiorite dike intrusion must have been along a tensional basement fracture system which penetrated to considerable depth within sialic crust. Coincidence of intrusion trends with those of major fractures and an association of nephelinites with the Tokatoka-Whangarei Heads igneous suites indicate the importance of extensiohal tectonics in the petrogenesis of late Tertiary volcanism in Northland.