ISOTOPE CHARACTERISTICS OF SUBMARINE LAVAS FROM THE PHILIPPINE SEA - IMPLICATIONS FOR THE ORIGIN OF ARC AND BASIN MAGMAS OF THE PHILIPPINE TECTONIC PLATE

Igneous rocks from the Philippine tectonic plate recovered on Deep Sea Drilling Project Legs 31, 58 and 59 have been analyzed for Sr, Nd and Pb isotope ratios. Samples include rocks from the West Philippine Basin, Daito Basin and Benham Rise (40-60 m.y.), the Palau-Kyushu Ridge (29-44 m.y.) and the Parece Vela and Shikoku basins (17-30 m.y.). Samples from the West Philippine, Parece Vela and Shikoku basins are MORB (mid-ocean ridge basalt)-like with Sr-87/Sr-86 = 0.7026-0.7032, Nd-143/Nd-144 = 0.51300-0.51315, and Pb-206/Pb-204 = 17.8-18.1. Samples from the Daito Basin and Benham Rise are OIB (oceanic island basalt)-like with Sr-87/Sr-86 = 0.7038-0.7040, Nd-143/Nd-144 = 0.51285-0.51291 and Pb-206/Pb-204 = 18.8-19.2. All of these rocks have elevated Pb-207/Pb-204 and Pb-208/Pb-204 compared to the Northern Hemisphere Regression Line (NHRL) and have delta-Pb-207 values of 0 to +6 and delta-Pb-208 values of +32 to +65. Lavas from the Palau-Kyushu Ridge, a remnant island arc, have Sr-87/Sr-86 = 7032-0.7035, Nd-143/Nd-144 = 0.51308-0.51310 and Pb-206/Pb-204 = 18.4-18.5. Unlike the basin magmas erupted before and after them, these lavas plot along the NHRL and have Pb-isotope ratios similar to modern Pacific plate MORB's. This characteristic is shared by other Palau-Kyushu Arc volcanic rocks that have been sampled from submerged and subaerial portions of the Mariana fore-arc. At least four geochemically distinct magma sources are required for these Philippine plate magmas. The basin magmas tap Source 1, a MORB-mantle source that was contaminated by EMI (enriched mantle component 1 [31]) and Source 2, an OIB-like mantle source with some characteristics of EMII (enriched mantle component 2 [31]). The arc lavas are derived from Source 3, a MORB-source or residue mantle including Sr and Pb from the subducted oceanic crust, and Source 4, MORB-source or residue mantle including a component with characteristics of HIMU (mantle component with high U/Pb [31]). These same sources can account for many of the isotopic characteristics of recent Philippine plate arc and basin lavas. The enriched components in these sources which are associated with the DUPAL anomaly were probably introduced into the asthenosphere from the deep mantle when the Philippine plate was located in the Southern Hemisphere 60 m.y.b.p.