Geological and Geochemical Studies of Kolekole Cinder Cone, Southwest Rift Zone, East Maui, Hawaii

Abstract

The Kolekole cinder cone is located in the southwest rift zone near the triple-junction of the southwest, east and north rifts, and the Haleakala volcano summit area in East Maui, Hawaii. The geochemical and petrological characteristics of the alkaline lavas erupted from the cinder cone provide important information regarding the post-shield evolution of Hawaiian volcanoes. The incompatible element abundances and ratios of the Kolekole lavas are transitional between previously studied post-shield, temporally continuous, Kula and Hana alkaline series. Thus, the incompatible element composition of the alkaline lavas changed gradually during the post-shield stage. Rare earth element ratios such as Tb/Yb indicate that the Kolekole post-shield alkaline lavas originated from garnet-bearing mantle source regions. The Kolekole lavas are predominantly ankaramite containing 25–40% by volume, large (1–5 mm), normal and reverse-zoned olivine and augite phenocrysts. About 74–85% of the olivine and 58–75% of the augite phenocrysts are reversely zoned. The groundmass of the ankaramite is basanitic and similar to alkaline lava elsewhere on East Maui. Barometry indicates that the phenocrysts originated from depths between 15.5 and 48 km, transcending the crust–mantle boundary. The alkaline primary magma probably ponded below solidified, shallow, shield-stage magma chambers and underwent extensive olivine-controlled high pressure fractional crystallization, as indicated by the Fe-rich (Mg# 35–49) composition estimates and major and compatible trace element correlations in the ankaramite groundmass. Large spheroidal bombs (0.6–2.2 m) near the Kolekole vents indicate that the evolved magma eventually erupted by violent volatile-driven explosions.