PETROLOGY AND GEOCHEMISTRY OF THE SOUTH KAUA‘I SWELL VOLCANO, HAWAI‘I

The large, pinnate bathymetric swell south of the Island of Kauai has recently been recognized as a low-relief shield volcano rather than a landslide deposit (Ito et al. 2013). Over 100 rocks were collected from 25 seamounts in water depths from 3 to 4 km during four Jason dives and three dredgehauls on the South Kauai Swell (SKS) volcano. These rocks range from pillow lavas with fresh glassy rims to moderately altered volcanic breccias. Two geochemical groups are present: Tholeiites and alkalic lavas (90:10%). The SKS tholeiites range in age from 5.4 to 3.9 Ma and are from the shield stage. The alkalic lavas are younger (1.9-0.1 Ma) and represent the rejuvenation stage. These young ages and the occurrence of coeval rejuvenated volcano along a 350 km segment of the Hawaiian Islands are inconsistent with the flexure model for the origin of rejuvenated volcanism.

The SKS tholeiites are geochemically similar to lavas from nearby northern Hawaiian Island shield volcanoes. Most SKS tholeiitic lavas exhibit Loa-like radiogenic Pb ratios (^208*Pb/^206*Pb>0.95) consistent with the geographical location of the SKS on an extension of the Loa trend (e.g. Weis et al. 2011). Pyroxenite is considered an important component in Loa trend volcanoes. High precision olivine analyzes indicate a major pyroxenite source component for SKS shield (70-90%) and rejuvenated (40-60%) lavas (using equation of Gurenko et al. 2010). This component decreased for younger samples from both groups. Estimates of the pyroxenite component based on the trace element modeling method (Pietruszka et al. 2013) yielded values of 50-53% for shield lavas with no temporal variation. These values are similar to those estimated for Kilauea lavas, which have comparable Sr, Nd and Pb isotopes. The cause of different values for the olivine and trace element methods is unclear but the large pyroxenite ranges for individual samples using olivine is problematic. No trace element solution was found for rejuvenated lavas.

Our work continues the Frey tradition (e.g., 1990) of thoroughly studying Hawaiian lavas.

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