Source characteristics of the basement rocks from the Sulu and Celebes basins (western Pacific): Chemical and isotopic evidence

New Sr- Nd- and Pb-isotopic and trace element data are presented on basalts from the Sulu and Celebes Basins, and the submerged Cagayan Ridge Arc (Western Pacific), recently sampled during Ocean Drilling Program Leg 124. Drilling has shown that the Sulu Basin developed about 18 Ma ago as a backarc basin, associated with the now submerged Cagayan Ridge Are, whereas the Celebes Basin was generated about 43 Ma ago, contemporaneous with a general plate reorganisation in the Western Pacific, subsequently developing as an open ocean receiving pelagic sediments until the middle Miocene. In both basins, a late middle Miocene collision phase and the onset of volcanic activity on adjacent arcs in the late Miocene are recorded. Covariations between Sr-87/Sr-86 and Nd-143/Nd-144 show that the seafloor basalts from both the Sulu and Celebes Basins are isotopically similar to depleted Indian mid-ocean ridge basalts (MORE), and distinct from East Pacific Rise MORE, defining a single negative correlation. The Cagayan Are volcanics are different, in that they have distinctly lower epsilon(Nd)(T) for a given epsilon(Sr)(T), compared to Sulu and Celebes basalts. In the Pb-207/Pb-204 and Pb-208/Pb-204 versus Pb-206/Pb-204 diagrams, the Celebes, Sulu and Cagayan rocks air plot distinctly above the Northern Hemisphere Reference Line, with high Delta 7/4 Pb (5.3-9.3) and D8/4 Pb (46.3-68.1) values, They define a single trend of radiogenic lead enrichment from Celebes through Sulu to Cagayan Ridge, within the Indian Ocean MORE data field. The data suggest that the overall chemical and isotopic features of the Sulu, Cagayan and Celebes rocks may be explained by partial melting of a depleted asthenospheric N-MORB-type (''normal'') mantle source with isotopic characteristics similar to those of the Indian Ocean MORE source. This asthenospheric source was slightly heterogeneous, giving rise to the Sr-Nd isotopic differences between the Celebes and Sulu basalts and the Cagayan Ridge volcanics, In addition, a probably slab-derived component enriched in LILE and LREE is required to generate the elemental characteristics and low (Nd)(T) Of the Cagayan Ridge island are tholeiitic and calcalkaline lavas, and to contribute to a small extent in the backarc basalts of the Sulu Sea. The results of this study confirm and extend the widespread Indian Ocean MORE signature in the Western Pacific region. This signature could have been inherited by the Indian Ocean mantle itself during the rupture of Gondwanaland, when fragments of this mantle could have migrated towards the present position of the Celebes, Sulu and Cagayan sources.