SUBMARINE LAVAS FROM MAUNA-KEA VOLCANO, HAWAII - IMPLICATIONS FOR HAWAIIAN SHIELD STAGE PROCESSES

The island of Hawaii is composed of five voluminous shields but only t he youngest, active and well-exposed shields of Mauna Loa and Kilauea have been studied in detail. The shield lavas forming Kohala, Hualalai and Mauna Kea are largely covered by postshield lavas with geochemica l characteristics that differ from the shield lavas. In order to deter mine the geochemical characteristics of the Mauna Kea shield which is adjacent to the Kilauea and Mauna Loa shields, 12 Mauna Kea shield bas alts dredged from the submarine east rift were analyzed for major and trace element contents and isotopic (Sr, Nd, and Pb) ratios. The lavas are MgO-rich (11 to 20%), submarine erupted, tholeiitic basalts, but they are not representative of crystallized MgO-rich melts. Their whol e rock and mineral compositions are consistent with mixing of an evolv ed magma, <7% MgO, with a magma containing abundant olivine xenocrysts , probably disaggregated from a dunitic cumulate. At a given MgO conte nt, some of the Mauna Kea whole rocks have lower abundances of CaO and higher abundances of incompatible elements. The evolved melt componen t in these lavas reflects significant fractionation of plagioclase and clinopyroxene and in some cases even the late crystallizing phases or thopyroxene and Fe-Ti oxide. Although these Mauna Kea lavas are not is otopically homogeneous, in general their Sr, Nd and Pb isotopic ratios overlap with the fields for lavas from Loihi and Kilauea volcanoes. L avas from these three shields define the low Sr-87/Sr-86, high Pb-206/ Pb-204, and low Zr/Nb ranges for Hawaiian shield lavas, and they may c ontain a larger proportion of a component with high U-238/Pb-204. Most of these Mauna Kea shield lavas, however, are distinguished from Kila uea and Loihi tholeiitic basalts by having lower La/Ce and La/Yb. Thes e data provide additional evidence that each Hawaiian shield has disti nctive geochemical characteristics.