Late Miocene marine tephra beds: Recorders of rhyolitic volcanism in North Island, New Zealand

Abstract

A deep‐sea sequence of 72 rhyolitic tephra beds, now exposed at Mahia Peninsula in the Hawke's Bay region of the east coast, North Island, New Zealand, provides a record of late Miocene volcanism of the Coromandel Volcanic Zone (CVZ): the precursor to large‐scale explosive volcanism of the Quaternary Taupo Volcanic Zone (TVZ). The geochemical signature of the glasses in the Miocene tephra has been protected from hydrothermal alteration and prolonged subaerial exposure that have affected proximal CVZ deposits. The tephra beds are primarily eruption‐driven sediment gravity flows that have been emplaced into a trench‐slope basin, some 300 km from active volcanoes. Their occurrence is consistent with long‐distance fluvial transport followed by a point‐source discharge into the deep‐sea environment, and has no implications for the paleo‐geographic location of the basins relative to the volcanic arc. The tephra beds are calc‐alkaline rhyolites with SiO2 contents in the range 72–78 wt% (recalculated on a volatile‐free basis), and are broadly similar to glassy rocks of the CVZ. Their major oxide, trace element, and REE compositions are indistinguishable from glasses of TVZ rhyolites. The trace element and REE compositional variability in the late Miocene tephra beds, which were erupted over an estimated duration of c. 0.5–2.4 m.y., is no greater than that of large silicic eruptives of the last 350 ka, and is suggestive of a long‐lived source and/or similar magmatic processes. However, the individual tephra beds are products of discrete homogeneous magma batches. New fission‐track ages of the Miocene tephra beds suggest the main period of volcaniclastic deposition occurred in the interval c. 9–7.5 Ma. This corresponds well with the initiation of rhyolitic volcanism in the CVZ at c. 10 Ma, and a major period of caldera formation that took place to c. 7 Ma. The ages suggest a sediment accumulation rate of between 0.23 and 1.2 m/ka (av. 0.4 m/ka), and a frequency of eruption of tephra beds between 1 per 7 to 36 000 yr (av. 1 per 21 000 yr). Although these are minimum estimates of eruptive frequency, they are similar to rates estimated from distal records of the Quaternary TVZ, considered to be one of the most active rhyolite centres on Earth. Overall, rhyolitic volcanism of the TVZ appears to be a continuation of a volcanic regime that commenced at least 10 m.y. ago.