Geology and geochemistry of early arc-volcanic rocks from Guam

Two volcanic units: middle Eocene Facpi Formation and the late Eocene to early Oligocene Alutom Formation. The Facpi Formation is composed largely of interbedded boninite series pillow lavas, pillow breccias, and dikes, although arc tholeiite series rocks cap the formation in some areas. The Alutom Formation is composed of interbedded volcanic breccias, tuffaceous sandstones, lava flows, and sills. Calc-alkaline, arc tholeiite, and boninite series compositions are all found in the Alutom Formation. Chemical variation in the boninite series can be explained by early fractionation of olivine and clinopyroxene followed by plagioclase, clinopyroxene, and orthopyroxene. The arc tholeiite series chemical trends can be explained by early olivine fractionation followed by plagioclase, clinopyroxene, orthopyroxene, and magnetite. Basalts were erupted in an arc setting. The boninite series magmas were produced by hydrous partial melting of depleted mantle at relatively shallow levels, whereas the arc tholeiite and calc-alkaline series magmas were generated by partial melting of less depleted mantle at deeper levels. Because the calc-alkaline series parent basalts most likely were richer in incompatible high field-strength elements and SiO (sub 2) than were the tholeiitic parents, they were generated by lower degrees of melting.--Modified journal abstract.