Silicic volcanism and back-arc extension related to migration of the Late Cainozoic Australian–Pacific plate boundary

The initial phases of large-scale silicic and bimodal volcanism within an active Miocene arc (Coromandel Volcanic Zone, CVZ) are preserved on several small islands in northern New Zealand. Major- and trace-element compositions classify the rocks as high-K, calc-alkaline, silicic lavas which have some of the low-ion lithophile element and high-field-strength element characteristics of convergent margin lavas. The silicic lavas (∼12 Ma) are spatially associated with a relatively primitive basalt of similar age (12.06±0.25 Ma), which is a sub-alkaline tholeiite with an anomalously high TiO2 content and as such it is not a typical arc basalt. Isotopically, the samples indicate the presence of oceanic sediment in their petrogenesis. An explanation for the geochemical and isotopic compositions of the northern CVZ rocks involves migration of the Australian–Pacific convergent margin orientation from a north–south direction (Northland Volcanic Arc), through a northeast–southwest direction (CVZ), and finally subparallel to the orientation of the presently active Taupo Volcanic Zone. The silicic eruptives thereby preserve a trail of volcanic edifices within a back-arc basin. The edifice-trail exhibits a general northwest to southeast younging trend and lies perpendicular to the axis of the contemporaneous Coromandel Volcanic Arc. Late-Cainozoic migration of the convergent margin and subduction system is proposed as the mechanism for extension across the back-arc region of the CVZ, producing large-scale back-arc silicic volcanism, and minor associated basalts, perpendicular to the orientation of the main arc.