Major and trace elements, mineral chemistry, and Sr-Nd isotope ratios are reported for representative igneous rocks drilled from the Sulu Basin basement (Site 768) and Cagayan Ridge (Sites 769, 771) during Ocean Drilling Program Leg 124.The Sulu Basin basement rocks, cored for about 220 m beneath late early Miocene pelagic sediments, consist, downhole, of pillow basalts (Unit 1), dolerite (Unit 2), and two-pyroxene microgabbro (Unit 3), followed by pillowed and massive basalts (Units 4, 5, 6, 7, and 8). Basalts and dolerites are relatively homogeneous in petrochemical features, except for LOI and elements such as Na, K, Rb, Cs, Li, and Ti, which suffered intense and variable mobilization due to seawater alteration and low-grade greenschist facies oceanic metamorphism. Ca and Mg contents also appear significantly influenced by halmyrolysis in some samples.Some basalts of Unit 1 have a picritic character (mg = 77), which mainly reflects the composition of the primary magma as evidenced by their quenched texture. The high mg (81-77) of the two-pyroxene microgabbro in Unit 3 reflects not only a primary picritic composition, but also olivine accumulation.Basalts and dolerites of Units 1, 2, 4, 5, 6, and 7 have mg (74-64), Ni (234-46) and Cr (490-47), variations compatible with moderate fractionation of mantle-derived primary magmas.Clinopyroxene chemistry, together with the presence of orthopyroxene, indicate a subalkaline nature of Sulu Basement magmas. Their relatively high ratios between LFSE (K, Rb, Ba, Th) and HFSE (Nb, Zr, Hf, Ti, Y) and the REE distribution (CeN/YbN = 1.6-1.0) coherently indicate normal-MORB/IAT transitional features. Consistent isotope ratios were obtained for picritic basalts and basalts from Unit 1 and microgabbro from Unit 3 (143Nd/144Nd = 0.51297-0.51301 and 87Sr/86Sr = 0.70308-0.70340). These petrochemical characteristics imply that oceanic crust creation in the Sulu Basin developed from a basaltic magmatism generated from MORB-like mantle sources, which were modified by subduction-related geochemical components.Basaltic to andesitic lava flows and clasts in tuffs cored beneath late early to early middle Miocene sediments in the Cagayan Ridge suffered only seafloor alteration, which did not substantially affect their primary composition. Petrographical and geochemical features are completely comparable with tholeiitic (CeN/YbN = 1.5-2.9) and calc-alkaline (CeN/Ybn 2.8-2.9) island arc magmas, except for unusually low Nd isotope ratios (143Nd/144Nd = 0.51286-0.51280 with 87Sr/86Sr = 0.70292-0.70309).The data obtained in this paper, while confirming the arc-magma affinity for Cagayan Ridge volcanics, further indicate that Sulu Basin back-arc magmatism with MORB/IAT transitional character was also generated from subduction-modified intraoceanic mantle sources. The older (middle Eocene) Celebes Basin oceanic crust, generated by a pure, normal MORB magmatism thus represents a distinct magmatic system with respect to that of the Sulu Sea.

Supplement to: Spadea, Piera; Beccaluva, L; Civetta, L; Coltorti, Massimo; Dostal, J; Sajona, F G; Serri, G; Vaccaro, C; Zeda, O (1991): Petrology of basic igneous rocks from the floor of the Sulu Sea. In: Silver, EA; Rangin, C; von Breymann, MT; et al. (eds.), Proceedings of the Ocean Drilling Program, Scientific Results, College Station, TX (Ocean Drilling Program), 124, 251-269