The evolution of basaltic volcanism in Northland in space and time

Abstract

New radiometric ages determined on basalt lava flows at Kaikohe and Bay of Islands in Northland, New Zealand demonstrate that volcanoes were active as recently as about 70,000 and 40,000 years ago, respectively. These new ages are more accurate than those of previous studies. New ages determined on some of the oldest basalt rocks show that these volcanic regions have been active for millions of years, and thus, there is no reason to believe the region is now extinct. Volcanoes in northern Northland are capable of new eruptions that could include the formation of small scoria cones and shield volcanoes, and far-travelled lava flows. Future regional hazard assessments need to consider such phenomena.

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Technical Abstract

New 40Ar-39Ar step-heated laser and furnace ages on groundmass samples from basaltic lava flows in the Kaikohe and Bay of Islands volcanic field, and the Whangarei volcanic field, provide robust constraints on the longevity of volcanic activity and the timing of most recent eruptions. The samples returned excellent plateau and isochron age spectra, and show no significant evidence of alteration or geologic contamination. Duplicate laser fusion and furnace step-heating of sample aliquots returned the following weighted mean plateau ages on the youngest lavas: Piccadilly Road (Kaikohe) = 43 ± 10 ka; Te Puke (Bay of Islands) = 75 ± 12 ka and Whangarei (Whangarei city) = 319 ± 31 ka.

We consider these to represent the best constraints on the most recent volcanism in these regions. The data confirms the youth of several volcanoes. One of the oldest lava flows, the Te Rahui lava flow (Bay of Island field), produced a robust plateau age of 10.37 ± 0.13 Ma, confirming the prolonged longevity of basaltic volcanism in the region (more than 10 million years). It is possible that the frequency of basaltic activity is low (on the order of an event every 10,000 to 50,000 years), but the volcanic fields are not extinct.

We strongly recommend further Ar-Ar chronological studies of volcanoes in Whangarei and Kaikohe-Bay of Islands Fields to better constrain the frequency of past volcanism and hence likelihood of future activity.