Geotectonic evolution of the West Philippine Basin: Petrological and geochemical characterization and interaction of spreading ridge with mantle plume upwelling

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The West Philippine Basin (WPB) represents the oldest back-arc basin in the Philippine Sea Plate. To address unsolved question about its tectono-magmatic evolution, we have conducted two scientific survey cruises (KR03-04 and KR04-14). Integrative data by multi-narrow beam mapping reveals the complicated morphology in the northwestern part of WPB. The study area can be divided into two domains and separated by the NE-SW trending Okinawa-Luzon Fracture Zone (OLFZ), which appears to be right-lateral offset. The eastern domain of the OLFZ is characterized by series of relict overlapping spreading center (OSC). The OSC systems developed with the spreading segment located north of the system retreating, being progressively cancelled on the benefit of a new propagating rifts located more to the sough, implying successive southward ridge jumps. All observed propagating rift systems have propagated from east to west, i.e., away from the plume (Uldaneta Plateau). By contrast, the western domain of OLFZ demonstrates rather simple spreading fabrics. Whole-rick Ar-Ar dating for recovered basalts revealed reliable plateau-ages of ~32-40 Ma, which are ~10m. y. younger than previously suggested formation ages. Lavas from eastern domain of OLFZ have enriched geochemical compositions similar to E-MORB and/or ocean island basalts (OIB), which are compatible with morphological features and suggest contribution of deep source component to magma genesis. In contrast, basalts from western domain of OLFZ bear N-MORB affinity with Pb isotopic characteristics of the Indian Ocean MORB-type mantle, suggesting involvement of Indian ocean MORB-type asthenospheric mantle during early stage of the formation of the Philippine Sea Plate.