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

摘要

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.