Petrology of the Alkalic Hawi Volcanic Series of Kohala Volcano, Hawaii

Kohala Volcano is the northernmost and oldest volcano on the island of Hawaii. Early studies divided Kohala into two stratigraphic units; the Pololu and Hawi Volcanic Series. The Pololu is the older (1.3-0.3 m.y.a. K-Ar dates) and is composed of tholeiitic basalt. The Hawi is the alkalic basalt cap rock and is on the order of 0.25 to 0.066 m.y.a. (K-Ar dates). A hiatus or erosional unconformity separates the two units. Rocks collected for this study were obtained from the alkalic cinder cones that lie along a N35°W trending line that approximates the major rift zone of Kohala. The mineralogy and whole-rock major-element analyses show a wide range of basalt types ranging from hawaiite through mugearite and benmoreite. The wide range in chemical composition (44-59% Si02, 0.9-5.0% Mg)), the trends emphasized on variation diagrams, mineralogical changes over the observed compositional range, corrosion and alteration effects, and the narrow range for Sr isotopes strongly suggest that fractional fusion or fractional crystallization is the main process controlling the composition of these lavas. (See more in text.)