Volume and Petrologic Characteristics of the Koloa Volcanics, Kaua`i, Hawai`i

The Koloa Volcanics on the island of Kaua`i are the most voluminous example of the rejuvenated-stage in Hawai`i. They provide an excellent opportunity to evaluate the cause(s) of rejuvenated-stage eruptions through the study of their volume and petrologic characteristics. We have conducted an extensive field study of the Koloa Volcanics, collecting 162 samples and measuring sections from most regions of the island where Koloa rocks are exposed. Published ages for Koloa Volcanics lava range from 3.65 to 0.375 Ma. These moderately olivine-phyric lavas are alkalic basalts, basanites, foidites, and melilite foidites; they show no temporal correlation between rock types (e.g., alkalic basalts and basanites were erupted for the duration of Koloa volcanism). Most olivine compositions (Fo 83-86) are too low to be in equilibrium with the whole rock compositions (Mg#) reflecting variable amounts of olivine accumulation and/or resorption of ultramafic xenoliths. Olivine compositions indicate parental magmas with Mg#s of 62-64 for many lavas. Major-oxide and trace element variation diagrams show trends indicating low but variable degrees of partial melting from a somewhat heterogeneous source with no correlation of Nd or Sr isotopes with rock type. When compared with other incompatible elements, Zr and Nb are less enriched in foidic rocks, which may reflect a residual oxide phase in the source. Data from 41 measured sections, 85 water well logs and previous geologic studies of Kaua`i were synthesized using ARC/GIS to create a subsurface image of the Koloa Volcanics for volume calculations. Division of the island into quadrants yielded a minimum volume estimate of 50 km3. This is the first quantitative volume estimate for Hawaiian rejuvenation volcanism. When compared with a new, larger estimate for the total volume of Kaua`i (57,000 km3; Robinson and Eakins, in press), the Koloa Volcanics are 0.1% of the total, which is consistent with a previous guesstimate (<<1% of the total volume). Current models for rejuvenated stage volcanism provide predictable consequences for the volumes and age range of the magmas produced. Flexure-induced decompressional melting and convective mantle plume upwelling models predict a hiatus between shield and rejuvenated volcanism and relatively small volumes, which are not observed on Kaua`i. Lithospheric melting by conductive heating predicts little or no age gap and relatively large volumes of magma. When coupled with effects of small scale convection eroding the overlying lithosphere, the conductive heating model best explains the observed age-volume relations for Koloa Volcanics. Roinson, J.E. and Eakins, B.W., 2005, J. Volcanol. Geotherm. Res., in press.