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New K-Ar ages and the geologic evidence against rejuvenated-stage volcanism at Haleakal, East Maui, a postshield-stage volcano of the Hawaiian island chain

David R. Sherrod^,1, Yoshitomo Nishimitsu^,2 and Takahiro Tagami^,2

¹ U.S. Geological Survey, P.O. Box 51, Hawaii National Park, Hawaii 96718, USA
² Division of Earth and Planetary Sciences, Kyoto University, Kyoto 606-8502, Japan

The postshield and previously inferred rejuvenated-stage history of Haleakal volcano is reevaluated on the basis of 52 new K-Ar ages, 42 from the postshield Kula Volcanics and 10 from the overlying Hna Volcanics. Postshield extrusion was robust from 0.93 to 0.76 Ma. A period of low extrusion rate or volcanic quiescence occurred between 0.76 and 0.65 Ma, well within Kula time. A chemical change to increasingly alkalic lava occurred at this time as the volcano changed from broadly hawaiitic to basanitic in its eruptive products and robust extrusion resumed. A slightly longer period of low extrusion rate or quiescence occurred after ca. 0.4 Ma, but only trifling change in geochemical character is observed. Geochemically, the Hna Volcanics unit, chiefly basanitic, overlaps greatly with the upper part of the Kula Volcanics; there is a weak tendency to slightly more alkaline character among the Hna Volcanics.

The age of the Kula/Hna boundary is ca. 0.15–0.12 Ma; thus, volcanic quiescence of only 0.03 m.y. separates the two formations, much shorter than the previously known limit of 0.25–0.30 m.y. The brevity of this hiatus, coupled with coincident vent loci and broadly similar geochemical characteristics for the Hna and the upper part of the Kula Volcanics, indicates that the Hna Volcanics unit comprises deposits of postshield-stage volcanism that has waned substantially since ca. 0.4–0.3 Ma. Haleakal has not yet begun a classically defined rejuvenated stage. Our findings support recent numerical modeling of plume-lithosphere interactions that predict that Haleakal is near the end of its postshield growth.

Key Words: K/Ar • shield volcanoes • Hawaiian Islands • Hawaii • Maui • Haleakal volcano

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