Onset of the Indian Ocean isotopic signature in the Philippine Sea Plate: Hf and Pb isotope evidence from Early Cretaceous terranes
Basalts from Paleocene-recent back arc basins within the Philippine Sea Plate have the Pb and Hf-Nd isotopic characteristics of Indian Ocean mid-ocean ridge basalts (MORB). We examined the isotopic composition of rocks from Early Cretaceous terranes within the Philippine Sea Plate, in order to understand the onset and origin of the Indian MORB isotopic signal. Igneous rocks from the Huatung Basin, a fragment of Early Cretaceous oceanic lithosphere sutured to the Paleocene-Oligocene West Philippine Basin, have an Indian MORB Hf-Nd isotopic signature, but Pb isotope ratios are intermediate between those of Indian and Pacific MORB. West Philippine Basin basalts have a stronger Indian Pb isotope signature than Huatung Basin rocks. Pre-subduction mantle sources for igneous rocks from the Amami Plateau, an Early Cretaceous island arc in the northernmost Philippine Sea Plate, have isotopic characteristics of Pacific MORB, and subducted components added to the mantle sources also have isotopic characteristics of Pacific MORB, plus sediment. The Indian MORB characteristics of Early Cretaceous Huatung Basin rocks lend support to the idea that mantle sources with this signature existed prior to the opening of the present day Indian Ocean and that Tethyan oceanic basalts, now found throughout southern Eurasia, shared them. As the West Philippine Basin opened, basalts formed from mantle sources with a more pronounced Indian Pb isotope signature than those tapped by the older Huatung Basin. Results for the Amami Plateau indicate that the Philippine Sea Plate also contains Pacific-type lithosphere, perhaps scattered within subduction terranes, such as the Philippine Islands, that were disrupted by basin opening. Taken together, results indicate that the lithosphere on which the proto-Izu-Bonin-Mariana arc formed was diverse, containing Pacific and Indian oceanic lithosphere, and old, depleted and subduction-conditioned arc mantle wedge.