Geology and geochemistry of basaltic lava flows and dikes from the Trans-Koolau tunnel, Oahu, Hawaii

A 200-m section of Koolau basalt was sampled in the 1.6-km Trans-Koolau (T- K) tunnel. The section includes 126 aa and pahoehoe lava flows, five dikes and ten thin ash units. This volcanic section and the physical characterist ics of the lava flows indicate derivation from the nearby northwest rift zo ne of the Koolau shield. The top of the section is inferred to be 500-600 m below the pre-erosional surface of the Koolau shield. Therefore, compared with previously studied Koolau lavas, this section provides a deeper, presu mably older, sampling of the shield. Shield lavas from Koolau Volcano defin e a geochemical end-member for Hawaiian shields. Most of the tunnel lavas h ave the distinctive major and trace element abundance features (e.g, relati vely high SiO2 content and Zr/Nb abundance ratio) that characterize Koolau lavas. In addition, relative to the recent shield lavas erupted at Kilauea and Mauna Loa volcanoes, most Koolau lavas have lower abundances of Sc, Y a nd Yb at a given MgO content; this result is consistent with a more importa nt role for residual garnet during the partial melting processes that creat ed Koolau shield lavas. Koolau lavas with the strongest residual garnet sig nature have relatively high Sr-87/Sr-86, Os-187/Os-188, O-18/O-16, and low Nd-143/Nd-144. These isotopic characteristics have been previously interpre ted to reflect a source component of recycled oceanic crust that was recrys tallized to garnet pyroxenite. This component also has high La/Nb and relat ively low Pb-206/Pb-204, geochemical characteristics which are attributed t o ancient pelagic sediment in the recycled crust. Although most Koolau lava s define a geochemical endmember for Hawaiian shield lavas, there is consid erable intrashield geochemical variability that is inferred to reflect sour ce characteristics. The oldest T-K tunnel lava flow is an example. It has t he lowest Sr-87/Sr-86, Zr/Nb and La/Nb, and the highest Nd-143/Nd-144 ratio found in Koolau lavas. In most respects it is similar to lavas from Kilaue a Volcano. Therefore, the geochemical characteristics of the Koolau shield, which define an end member for Hawaiian shields, reflect an important role for recycled oceanic crust, but the proportion of this crust in the source varied during growth of the Koolau shield.