Geochemistry of Post-Shield and Secondary Volcanism on and Around the Island of Kaua`i

The Koloa Volcanics are the most voluminous (~ 58 km3) and long-lived (2.4 m.y.) example of Hawaiian rejuvenated volcanism. The recent discovery of extensive Koloa volcanism offshore heightens their importance in helping to determine the mechanism(s) responsible for downstream plume-related volcanism, as do post-shield lavas. We collected a suite of K-Ar dated, post-shield and Koloa lavas and new offshore samples for major and trace element, and Pb, Sr, and Nd isotope characterization for comparison with well characterized shield lavas. Previous studies of Koloa lavas were compromised by rock alteration. X-ray fluorescence analyses of new, unaltered to weakly altered rocks indicate the Koloa Volcanics are exclusively alkalic, ranging from alkali basalts to foidites, while the post-shield stage lavas range from tholeiites to hawaiites and basanites. Preliminary results show that four of five offshore blocks are tholeiitic, and may be related to the Kaua`i shield. The other is geochemically similar to Koloa Volcanics. The post-shield stage is primarily defined by its age range, rather than by its geochemistry. These lavas were erupted between 4.0-3.6 Ma vs. 5.14-4.0 Ma for shield lavas, and were followed by the rejuvenated stage after a 1.2 Ma hiatus. Major and trace element data show overlapping compositions for the post-shield stage and the shield or rejuvenated stages. Based on trace element ratios, two geochemically distinct groups can be distinguished within the post-shield stage. This distinction is shown clearly on plots of Ba/Ce vs. Zr/Nb and Ba/Sr vs. Zr/Nb, where the alkalic samples plot near to, but not within, the Koloa lavas field and the tholeiitic samples, plot in a separate field, within the tholeiitic shield field. No temporal gradational trend exists in trace element ratios from shield stage through post-shield to rejuvenated stages. Whereas the shield and rejuvenated stages form separate fields, post-shield stage lavas do not form a transitional field; rather it is sharply divided into two groups indicating the source of the rejuvenated lavas is probably related to the Hawaiian plume rather than the lithosphere. Preliminary Sr and Nd data for the tholeiitic post-shield stage lavas are typical of shield lavas. The forthcoming isotopic data for the alkalic post-shield lavas will supplement this characterization and possibly allow for a geochemical distinction between the post-shield and rejuvenated stages.