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Legs 59 and 60 of the International Phase of Oceanic Drilling (IPOD) were designed to study the nature and history of volcanism of the active Mariana arc, its currently spreading inter-arc basin (the Mariana Trough), and the series of inactive basins and intervening ridges that lie to the west (Fig. 1). The older basins and ridges were drilled during Leg 59 as the first part of a transect of single-bit holes drilled in each major basin and ridge. The eastern part of the transect—the technically active region—was drilled during Leg 60. The evolution of island-arc volcanos and magma genesis associated with lithospheric subduction remain some of the most complex petrologic problems confronting us. Many types of source material (mantle, oceanic crust, continental crust) and an unusually wide range of possible physical conditions at the time of magma genesis must be identified even before the roles of partial melting and subsequent magma fractionation, mixing, and contamination can be assessed. The present-day Mariana arc is characterized by a steeply dipping Benioff zone, which becomes essentially vertical at depths greater than 200 km (Kanamori, 1977). The slope of the Benioff zone is thought to be greatest below the active volcanos of the northern Mariana arc and to become shallower (to the south) beneath the inactive volcanic islands of Guam and Saipan. Subduction below the Mariana arc is relatively aseismic, and coupling between the upper and lower (subducted) plates is weak or effectively nonexistent (Kanamori, 1977). More recent results from drilling in the trench and fore-arc regions of the Mariana arc not only suggest the lack of an accretionary sedimentary prism, but extensional tectonics within the fore-arc sediments suggest a tensional stress regime within the trench region (Leg 60 Scientific Party, 1979). Thus there is clear evidence that the Pacific plate is subducting spontaneously with relatively little resistance from the upper plate. During the Pliocene, the Mariana Trough began to open about a north-south trending axis, effectively splitting the West Mariana Ridge from the active Mariana arc (Karig, 1975). The topography and magnetic fabric of the Parece Vela Basin suggest a similar origin—the sundering of the Palau-Kyushu Ridge in the late Oligocene (Karig, 1975). The origin of the West Philippine Basin is less clear, however, and the age and magnetic fabric of this basin remain controversial. Recent studies of rocks dredged from the Mariana Trough (Hart et al., 1972), the Lau Basin (Hawkins, 1976), and the Scotia Sea (Saunders and Tarney, 1979) all reveal that these basins are floored by basalts geochemically similar to basalts erupted at mid-ocean ridges. The formation of active back-arc basins (e.g., the East Scotia Sea, Barker 1970) in the Mariana Trough (Karig et al., 1978) is considered by many authors to be the result of seafloor spreading broadly similar to that associated with a mid-ocean ridge (e.g., Karig, 1971).

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The Geochemistry, Mineralogy, and Petrology of Basalts from the West Philippine and Parece Vela Basins and from the Palau-Kyushu and West Mariana Ridges, Deep Sea Drilling Project Leg 59