POSTSHIELD VOLCANISM AND CATASTROPHIC MASS-WASTING OF THE WAIANAE VOLCANO, OAHU, HAWAII

The 3.9- to 2.9-Ma Waianae Volcano is the older of two volcanoes makin g up the island of Oahu, Hawaii. Exposed on the volcanic edifice are t holeiitic shield lavas overlain by transitional and alkalic postshield lavas. The postshield ''alkalic cap'' consists of aphyric hawaiite of the Palehua Member of the Waianae Volcanics, overlain unconformably b y a small volume of alkalic basalt of the Kolekole Volcanics. Kolekole Volcanics mantle erosional topography including the uppermost slopes of the great Lualualei Valley on the lee side of the Waianae Range. Tw enty new K-Ar dates, combined with magnetic polarity data and geologic relationships, constrain the ages of lavas of the Palehua member to 3 .06-2.98 Ma and lavas of the Kolekole Volcanics to 2.97-2.90 Ma. The g eochemical data and the nearly contemporaneous ages suggest that the K olekole Volcanics do not represent a completely independent or separat e volcanic event from earlier postshield activity: thus, the Kolekore Volcanics are reduced in rank, becoming the Kolekole Member of the Wai anae Volcanics, Magmas of the Palehua and Kolekole Members have simila r incompatible element ratios, and both suites show evidence for early crystallization of clinopyroxene consistent with evolution at high pr essures below the edifice. However, lavas of the Kolekole Member are l ess fractionated and appear to have evolved at greater depths than the earlier Palehua hawaiites. Postshield primary magma compositions of t he Palehua and Kolekole Members are consistent with formation by parti al melting of mantle material of less than 5-10% relative to Waianae s hield lavas. Within the section of Palehua Member lavas, an increase w ith respect to time of highly incompatible to moderately incompatible element ratios is consistent with a further decrease in partial meltin g by approximately 1-2%. This trend is reversed with the onset of erup tion of Kolekole Member lavas, where an increase in extent of partial melting is indicated. The relatively short time interval between the e ruption of Palehua and Kolekole Member lavas appears to date the initi al formation of Lualualei Valley, which was accompanied by a marked ch ange in magmatic conditions. We speculate that the mass-wasting event separating lavas of the Palehua and Kolekole Members may be related to the formation of a large submarine landslide west and southwest of Wa ianae Volcano. Enhanced decompression melting associated with removal of the equivalent volume of this landslide deposit from the edifice is more than sufficient to produce the modeled increase of 1-2% in exten t of melting between the youngest Palehua magmas and the posterosional magmas of the Kolekole Member. The association between magmatic chang e and a giant landsliding event suggests that there may be a general r elationship between large mass-wasting events and subsequent magmatism in Hawaiian volcano evolution.