Abstract
Abstract. A key component of subduction initiation rock suites is boninite, a high-magnesium andesite that is uniquely predominant in western Pacific forearc terranes and in select Tethyan ophiolites such as Oman and Troodos. We report, for the first time, the discovery of low-calcium, high-silica boninite in the middle Eocene Zambales ophiolite (Luzon Island, Philippines). Olivine–orthopyroxene microphyric high-silica boninite, olivine–clinopyroxene–phyric low-silica boninite and boninitic basalt occur as lapilli fall deposits and pillow lava flows in the upper volcanic unit of the juvenile arc section (Barlo locality, Acoje Block) of the Zambales ophiolite. This upper volcanic unit overlies a lower volcanic unit consisting of basaltic andesite, andesite to dacitic lavas and explosive eruptive material (subaqueous pahoehoe and lobate sheet flows, agglutinate and spatter deposits) forming a low-silica boninite series. The overall volcanic stratigraphy of the extrusive sequence at Barlo resembles holes U1439 and U1442 drilled by IODP Expedition 352 in the Izu–Ogasawara (Bonin) trench slope. The presence of depleted proto-arc basalts in the Coto Block (45 Ma) (Geary et al., 1989), boninite and boninite series volcanics in Barlo (Acoje Block (44 Ma)) and simultaneous and post-boninite moderate-Fe arc tholeiites in Sual and Subic areas of the Acoje Block (44–43 Ma) indicate that the observed subduction initiation stratigraphy in the Izu–Ogasawara–Mariana forearc is also present in the Zambales ophiolite. Paleolatitudes derived from tilt-corrected sites in the Acoje Block place the juvenile arc of northern Zambales ophiolite in the western margin of the Philippine Sea plate. In this scenario, the origin of Philippine Sea plate boninites (IBM and Zambales) would be in a doubly vergent subduction initiation setting.
Highlights
As the surface manifestation of a convecting mantle, subduction zones play a fundamental role in material transfer from the surface to the deep interior and vice versa
Motivated by how the ophiolite concept progressed through studies of the Izu–Ogasawara (Bonin)–Mariana (IBM) forearc, we present field, petrologic and geochemical characterization of a juvenile arc section in northern Zambales ophiolite to evaluate subduction initiation and boninite petrogenesis in a regional geodynamic context
Zambales boninite and boninite series volcanics are marked by low trace element abundances relative to mid-ocean ridge basalt with hydrous fluid mobile element enrichment and heavy rare earth element depletion, which is comparable with boninite from Troodos and Oman
Summary
As the surface manifestation of a convecting mantle, subduction zones play a fundamental role in material transfer from the surface to the deep interior and vice versa. Numerical models show that plate tectonics or mantle overturn in terrestrial planets can be triggered by narrow plumes impinging on weakened crust inducing gravitational instability and slab descent (Crameri and Tackley, 2016; Gerya et al, 2015). Driven mechanisms such as plume-induced subduction initiation (Whattam and Stern, 2015), are fundamentally different from end-member models of Cenozoic subduction initiation which rely on preexisting zones of weakness such as fracture zones and transform faults or require lithospheric collapse Perez et al.: Boninite and boninite-series volcanics in northern Zambales ophiolite
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