40 Ar/ 39 Ar geochronology of basalts from Kaua‘i, Hawai‘i: implications for shield-stage evolution of Hawaiian volcanoes

Fourteen whole rock samples were analyzed for 40 Ar/ 39 Ar geochronology to determine the timing of an important transition in mantle source geochemistry that occurred on the island of Kaua‘i, located at the junction between the Northwest Hawaiian Ridge and the Hawaiian Islands. Kaua‘i’s shield-stage lavas have lead (Pb) isotopic compositions that change from west to east across the island. Given previous age constraints, it was unclear whether western and eastern portions of the shield were coeval. The new dates from this study range from 4.95 ± 0.19 Ma to 4.02 ± 0.04 Ma, decrease broadly from west to east Kaua‘i, and correlate with Pb isotopic ratios. The results indicate that the transition from Kea to Loa isotopic compositions across Kaua‘i occurred between ~4.7 and 4.4 Ma. The new 40 Ar/ 39 Ar geochronological results require a modification of the order of shield-building events for Kaua‘i. The easternmost lavas of the Nāpali Member formed after the westernmost lavas and are unlikely their stratigraphic equivalents, as previously inferred from mapping and structural observations. The easternmost lavas of the Nāpali Member likely represent the final period of shield-stage volcanism on Kaua‘i, which is supported by the location of the residual gravity anomaly beneath the Līhu‘e Basin in eastern Kaua‘i. This work highlights the importance of combining field-based observations with geochemical, isotopic, and geochronological data when assessing the shield-stage evolution of Hawaiian volcanoes.