Luzon, the Philippines, is wedged between east- and west-directed subducting slabs. Westward subduction has resulted in pre-Miocene igneous rocks from northern Luzon characterized by 87Sr/86Sr and 143Nd/144Nd ratios typical of intra-oceanic arcs where sediment involvement is minor. In addition, Pliocene to Holocene volcanic eruptions of Sr and Nd isotopic signatures similar to those of the pre-Miocene rocks have developed along the Bicol arc in southern Luzon. These rocks are also associated with the westward subduction. In contrast, Pliocene to Holocene volcanoes have been generated by eastward subduction along the western coast of central Luzon (the Bataan arc in the north and the Mindoro arc in the south) and are significantly higher in radiogenic Sr and lower in radiogenic Nd than the rocks discussed above.

The pre-Miocene and Bicol arc isotopic characteristics appear to reflect the mixing of small amounts of products from the Philippine Sea crust with the mantle wedge. Magmas erupted along the Bataan and Mindoro arcs may reflect that larger amounts of sediment are available for subduction in the South China Sea. Rocks containing the highest radiogenic Sr and lowest radiogenic Nd values so far noted in Philippine igneous rocks occur in the Mindoro arc and may have been contaminated by slivers of the continental Mindoro-North Palawan terrane, which apparently had been carried down to mantle depths after the collision of this terrane and the arc in the Miocene.

These interpretations imply that Nd (and, by inference, other rare-earth elements) can migrate from the subducted slab to the mantle wedge. Thus, variable Nd isotopic ratios in island-arc magmas (e.g., Sunda and Lesser Antilles arcs) may not be a primary feature of the mantle wedge, but a consequence of variations in the release of Nd from the subducted slab.

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