High-precision 40Ar/39Ar dating of Quaternary basalts from Auckland Volcanic Field, New Zealand, with implications for eruption rates and paleomagnetic correlations

Abstract

The Auckland Volcanic Field (AVF), which last erupted ca. 550years ago, is a late Quaternary monogenetic basaltic volcanic field (ca. 500km2) in the northern North Island of New Zealand. Prior to this study only 12 out of the 53 identified eruptive centres of the AVF had been reliably dated. Careful sample preparation and 40Ar/39Ar analysis has increased the number of well-dated centres in the AVF to 35. The high precision of the results is attributed to selection of fresh, non-vesicular, non-glassy samples from lava flow interiors. Sample selection was coupled with separation techniques that targeted only the groundmass of samples with <5% glass and with groundmass feldspars >10μm wide, coupled with ten-increment furnace step-heating of large quantities (up to 200mg) of material. The overall AVF age data indicate an onset at 193.2±2.8ka, an apparent six-eruption flare-up from 30 to 34ka, and a ≤10kyr hiatus between the latest and second-to-latest eruptions. Such non-uniformity shows that averaging the number of eruptions over the life-span of the AVF to yield a mean eruption rate is overly simplistic. Together with large variations in eruption volumes, and the large sizes and unusual chemistry within the latest eruptions (Rangitoto 1 and Rangitoto 2), our results illuminate a complex episodic eruption history. In particular, the rate of volcanism in AVF has increased since 60ka, suggesting that the field is still in its infancy. Multiple centres with unusual paleomagnetic inclination and declination orientations are confirmed to fit into a number of geomagnetic excursions, with five identified in the Mono Lake, two within the Laschamp, one within the post-Blake or Blake, and two possibly within the Hilina Pali.