ALKALIC MAGMA MODIFIED BY INCORPORATION OF DIVERSE THOLEIITIC COMPONENTS - COMPLEX HYBRIDIZATION ON KAHOOLAWE ISLAND, HAWAII

Kahoolawe Island is a similar to 1.4 to 1 Ma shield volcano composed o f shield, caldera-fill, and postshield tholeiitic lavas, and postshiel d alkalic basalt and hawaiite lavas. One postshield vent erupted alkal ic lavas (K2O 1.1-1.6 wt.%) with resorbed olivines of wide composition al range (cores, Fo(86-71)), resorbed Na-plagioclase (An(57-30); large ly andesine) and clinopyroxene (evolved; Mg#s 76-71), and groundmass o rthopyroxene (Mg# less than or equal to 62). They also contain tholeii tic gabbro xenoliths, which, as a suite, have a continuum of mineral c ompositions - clinopyroxene Mg#s 83-74, orthopyroxene Mg#s 83-76, and plagioclase An(69-35) (e.g., includes andesine gabbro). Lava compositi ons do not fall on expected 'Hawaiian' fractionation trends due to MgO 'enrichment' (e.g., CaO similar to 7 wt.% @ MgO similar to 6 wt.%) Th is assortment of mineral and rock components within alkalic lavas with apparent Mg enrichment is owed to a complex history that began with p rotracted mixing among primitive and differentiated tholeiitic magmas, probably near the end of shield building. These hybrid magmas crystal lized a compositional variety of olivines that were resorbed during re servoir replenishments, and also crystallized in situ to form orthopyr oxene-bearing gabbro on reservoir walls. When magma production rates d eclined during the shield to postshield transition of tholeiitic to al kalic magmatism, the tholeiitic hybrids in reservoirs fractionated to yield highly evolved phases such as andesine and clinopyroxene with Mg # <75. When postshield hawaiite magmas subsequently entered reservoirs , alkalic-tholeiitic hybridization occurred; the resulting 'complex' m ixture of hawaiite+tholeiitic hybrids resorbed andesine and clinopyrox ene crystals and, upon eruption, entrained xenoliths of gabbro. Mass b alancing suggests that the alkalic-tholeiitic hybridization involved s imilar to 44% hawaiite mixed with a nearly equal amount of tholeiitic hybrid (MgO similar to 9.5 wt.%) plus olivine and andesine. This type of complex hybridization is a logical process for magmatism associated with tholeiitic to alkalic transitions and waning magma production, a nd this Kahoolawe example is the first to document such mixing in Hawa iian reservoirs.