Unspiked K Ar dating of the Honolulu rejuvenated and Ko‘olau shield volcanism on O‘ahu, Hawai‘i

Many mantle plume volcanoes undergo rejuvenated volcanism after a period of construction and erosion of their shield. The cause of this renewed volcanism has been enigmatic and various models have been proposed. However, the lack of geochronological data has hindered evaluation of these models. Unspiked K-Ar ages on groundmass in 41 samples from 32 vents of Honolulu Volcanics and eight samples of underlying Ko'olau Volcanics were determined in order to reveal the temporal distribution of rejuvenated vents and the length of the hiatus between the end of shield and start of rejuvenated volcanism. The new geochronological results show that Ko'olau shield volcanism ended at 2.1 Ma and that rejuvenated volcanism started at 0.8 Ma, resulting in a 1.3 million year hiatus in volcanic activity. Two distinct pulses were found for Honolulu volcanism at 0.80-0.35 and ∼0.1 Ma. During the first pulse, the eruption frequency increased with time and there was no spatial pattern in vent distribution, although three vents along a NNE-SSW trend produced similar compositions and may have been coeval. Volcanism apparently waned from 0.35-0.12 Ma, with only one eruption. The second pulse occurred along two rifts that trend N-S and NE-SW. Although the ages for the 10 dated flows are indistinguishable at around 0.1 Ma, lavas from the two rifts have distinct compositions: weakly alkalic vs. melilite nephelinite. The first, more widely distributed pulse of volcanism is probably related to secondary melting downstream from the Hawaiian plume stem, which may be related to lithospheric thinning. The second pulse, focused along two rifts, may be related to decompressional melting as the shield passed over the flexural arch.