Geochronological and geochemical evolution of late Cenozoic volcanism in the Coromandel Peninsula, New Zealand

Abstract

The Coromandel Peninsula comprises the central subaerial part of the late Cenozoic Coromandel Volcanic Zone (CVZ) of New Zealand and hosts the Hauraki Goldfield. Some 73 K‐Ar and 9 zircon fission‐track dates representative of 28 lithostratigraphic volcanic units mapped on the peninsula are reported, and more than 250 major element analyses of volcanic rocks, including all the dated rocks, have been collated and their broad geochemical affinities used to underpin the stratigraphy and volcanic history of the region. The age data confirm that the central part of the CVZ evolved more or less continuously throughout the late Cenozoic from c. 18 to 4 Ma ago (late‐early Miocene to early Pliocene), with regional time breaks of no more than 1–2 Ma. From c. 18 to 9 Ma, an andesite‐dominated volcanic arc was active, and at c. 10 Ma initiation of the Kapowai Caldera began with the first eruption of ignimbrite. A bimodal basalt to basaltic andesite/rhyolite association developed from c. 9 to 7 Ma and accompanied major caldera collapse, ignimbrite eruption, and postcaldera andesite eruption. Bimodal eruptions of basalt to basaltic andesite (with minor andesite) plus rhyolite and rhyolitic ignimbrite began once more from c. 6 to 5.5 Ma, and these became entirely basaltic from 4.7 to 4.2 Ma. At about the same time (c. 5.5 Ma), basalt was also erupted on the southern Colville Ridge. During late Miocene to early Pliocene time, andesitic to dacitic eruptions continued in the south and southwest of the CVZ. With time, the locus of volcanism moved irregularly eastwards at c. 3 mm/yr and southwards at c. 8 mm/yr, such that minimum ages are younger progressively southwards. Volcanism in the Coromandel Peninsula is mainly calc‐alkaline and medium‐K, with some early and late lavas showing tholeiitic affinities. The rocks range from high‐alumina basalts (including picritic basalt) through basaltic andesites and (dominant) andesites to dacites. There is an apparent gap between dacites at 63–68% SiO2 and rhyolites/ignimbrites at 73–78% SiO2, several of which have high‐K affinities. Many of the volcanic formations have distinct chemical signatures, confirming earlier suggestions that each has an independent magmatic history. © The Royal Society of New Zealand 1994.