Geology and geochemistry of early arc-volcanic rocks from Guam

Abstract

Research Article| June 01, 1984 Geology and geochemistry of early arc-volcanic rocks from Guam MARK K. REAGAN; MARK K. REAGAN 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar AREND MEIJER AREND MEIJER 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1984) 95 (6): 701–713. https://doi.org/10.1130/0016-7606(1984)95<701:GAGOEA>2.0.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation MARK K. REAGAN, AREND MEIJER; Geology and geochemistry of early arc-volcanic rocks from Guam. GSA Bulletin 1984;; 95 (6): 701–713. doi: https://doi.org/10.1130/0016-7606(1984)95<701:GAGOEA>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract 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. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.