Geochemical and isotopic development of the Coromandel Volcanic Zone, northern New Zealand, since 18 Ma

Cenozoic subduction-related volcanism in northern New Zealand developed from mainly basaltic and andesitic volcanisms in the Early Miocene to mainly silicic volcanism in the present Taupo Volcanic Zone. The first explosive silicic eruptions occurred at 12 Ma and large silicic calderas formed from 8 Ma. This transition is recorded by the 18–1.9 Ma Coromandel Volcanic Zone (CVZ) succession, which provides an outstanding opportunity to investigate the origins of the modern North Island volcanism. New major and trace element data on a rock suite representing CVZ andesites, rhyolites and basalts indicate a consistent subduction-related origin for the entire succession. The CVZ andesites generally define a consistent medium-potassic differentiation trend, however, distinct andesites with relatively sodic compositions occur at the bases of two consecutive eruptive series at ~ 18 and ~ 10 Ma. The distinct andesites are geochemically similar to basalts that erupted sporadically near the oceanic margin of the CVZ. CVZ rhyolites are medium- to high-potassic, peraluminous rocks with initial 87Sr/86Sr values comparable to or somewhat more radiogenic than coeval andesites (≥ 0.7049). Their bulk rock composition suggests derivation from sources comparable to least-differentiated andesites or basalts. We interpret the consecutive andesitic successions to represent stages in CVZ development during which distinct arc segments developed consecutively. The early stage in each segment yielded mainly intermediate volcanism whereas in the later stage volcanism locally proceeded rapidly (within ~ 1 Myr of initial andesitic activity) to silicic activity. Taken together, the succession records the rapid development and migration of arc system in a dynamic plate boundary setting.