Postshield volcanism and catastrophic mass wasting of the Waianae Volcano, Oahu, Hawaii |
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| Authors: | Todd K Presley John M Sinton Malcolm Pringle |
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| Institution: | (1) Department of Geology, University of Hawaii, Honolulu, Hawaii, 96822, USA, US |
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| Abstract: | The 3.9- to 2.9-Ma Waianae Volcano is the older of two volcanoes making up the island of Oahu, Hawaii. Exposed on the volcanic
edifice are tholeiitic 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 by 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. Twenty 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
geochemical data and the nearly contemporaneous ages suggest that the Kolekole Volcanics do not represent a completely independent
or separate volcanic event from earlier postshield activity; thus, the Kolekole Volcanics are reduced in rank, becoming the
Kolekole Member of the Waianae Volcanics. Magmas of the Palehua and Kolekole Members have similar incompatible element ratios,
and both suites show evidence for early crystallization of clinopyroxene consistent with evolution at high pressures below
the edifice. However, lavas of the Kolekole Member are less fractionated and appear to have evolved at greater depths than
the earlier Palehua hawaiites. Postshield primary magma compositions of the Palehua and Kolekole Members are consistent with
formation by partial melting of mantle material of less than 5–10% relative to Waianae shield lavas. Within the section of
Palehua Member lavas, an increase with respect to time of highly incompatible to moderately incompatible element ratios is
consistent with a further decrease in partial melting by approximately 1–2%. This trend is reversed with the onset of eruption
of Kolekole Member lavas, where an increase in extent of partial melting is indicated. The relatively short time interval
between the eruption of Palehua and Kolekole Member lavas appears to date the initial formation of Lualualei Valley, which
was accompanied by a marked change 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 Waianae
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 extent of melting between the youngest Palehua
magmas and the posterosional magmas of the Kolekole Member. The association between magmatic change and a giant landsliding
event suggests that there may be a general relationship between large mass-wasting events and subsequent magmatism in Hawaiian
volcano evolution.
Received: 1 September 1996 / Accepted: 26 November 1996 |
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| Keywords: | Petrology Alkalic lavas Postshield volcanism Submarine landslide |
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