Radiometric Ages for the Koloa Volcanics from the Deep Subsurface in the Lihu'e Basin, Kaua'i, Hawai'i

The Koloa Volcanics, which contain the youngest volcanic rocks on Kaua'i, are part of the rejuvenated stage of Hawaiian shield volcanism. Radiometric dating of the Koloa Volcanics has implications for the timing of major geologic events in the history of Kaua'i and the Hawaiian Archipelago, as well as for theories on the genesis of rejuvenated magma. The onset of Koloa volcanism, however, has been controversial because the oldest reported Koloa age, 3.65±0.06 Ma (all errors are 2σ) from a boulder in conglomerate, is similar to the age of the underlying late-shield or postshield lava flows; and a 1-m.y. gap separates the oldest age from the range of other ages reported for the Koloa Volcanics (2.59±0.15 Ma). Three new ⁴⁰Ar/³⁹Ar plateau ages from the Koloa Volcanics partly fill this gap. Determined from cuttings in the Hanama'ulu well (21.9972°, -159.3878°, land-surface elevation 83 m) in the Lihu'e basin, the ages are in correct stratigraphic order: 3.42±0.24 Ma at 273-m depth (HTZ-895/900), 3.22±0.26 Ma at 215-m depth (HTZ-705/710), and 3.11±0.56 Ma at 162-m depth (HTZ-530/535). The petrography and geochemistry of the samples are consistent with other rejuvenated-stage lava flows on Kaua'i. These new ages indicate a more nearly continuous record of rejuvenated volcanism than was known previously for Kaua'i. Also, because the new ages come from lava flows that are intercalated with shallow marine sediments near the base of the Koloa Volcanics, they indicate that the Lihu'e basin formed at least 3 m.y. ago. Since then, island subsidence or sea-level rise has caused the sediments to be 86 to 185 m below present sea level.