The island of Guam is located at the southern end of the Mariana fore-arc and was the site of early arc volcanism between 43 and 32 m.y. ago. Two volcanic units were erupted during the early evolution of Guam: the late 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.

Primitive boninite series rocks are relatively high in SiO2, MgO, and Ni and low in A12O3 and TiO2 contents compared to basaltic rocks from the arc tholeiite series. Additionally, they have low Ti/Zr ratios and high K/Zr, Rb/Zr, and Ba/Zr ratios. Although no calc-alkaline basalts are found in the early arc formations of Guam, basalts with chemical characteristics transitional between boninite and tholeiite series rocks may be genetically linked to the calc-alkaline series. Among the analyzed samples here are a few with anomalously high Y and rare-earth element (REE) contents.

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. The composition of the least silicic calc-alkaline series andesite can be modeled by fractional crystallization of olivine, clinopyroxene, plagioclase, and magnetite from a transitional basalt. Derivation of more silicic calc-alkaline series rocks requires magma mixing or crustal assimilation in addition to crystal fractionation involving hornblende.

All basalts from the early volcanic series of Guam 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. We suggest that because the calc-alkaline series parent basalts most likely were richer in incompatible high field-strength (HFS) elements and SiO2 than were the tholeiitic parents, they were generated by lower degrees of melting.

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