LATE MIOCENE MARINE TEPHRA BEDS - RECORDERS OF RHYOLITIC VOLCANISM INNORTH-ISLAND, NEW-ZEALAND

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, N ew Zealand, provides a record of late Miocene volcanism of the Coroman del Volcanic Zone (CVZ): the precursor to large-scale explosive volcan ism of the Quaternary Taupo Volcanic Zone (TVZ). The geochemical signa ture of the glasses in the Miocene tephra has been protected from hydr othermal alteration and prolonged subaerial exposure that have affecte d proximal CVZ deposits. The tephra beds are primarily eruption-driven sediment gravity flows that have been emplaced into a trench-slope ba sin, some 300 km from active volcanoes. Their occurrence is consistent with long-distance fluvial transport followed by a point-source disch arge into the deepsea environment, and has no implications for the pal eogeographic location of the basins relative to the volcanic are. The tephra beds are calc-alkaline rhyolites with SiO2 contents in the rang e 72-78 wt% (recalculated on a volatile-free basis), and are broadly s imilar to glassy rocks of the CVZ. Their major oxide, trace element, a nd REE compositions are indistinguishable fi om glasses of TVZ rhyolit es. The trace element and REE compositional variability in the late Mi ocene 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 t he last 350 ka, and is suggestive of a long-lived source and/or simila r 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 depositi on occurred in the interval c. 9-7.5 Ma. This corresponds well with th e initiation of rhyolitic volcanism in the CVZ at c. 10 Ma, and a majo r period of caldera formation that took place to c. 7 Ma. The ages sug gest 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 dista l records of the Quaternary TVZ, considered to be one of the most acti ve rhyolite centres on Earth. Overall, rhyolitic volcanism of the TVZ appears to be a continuation of a volcanic regime that commenced at le ast 10 m.y. ago.