| Summary: | Abstract The modern western Pacific basins and arcs formed during the Cenozoic are considered to be underlain by Indian‐type asthenospheric mantle. Nevertheless, Mesozoic volcanic arcs are rare in the western Pacific, and as a result, the nature of the mantle source and subduction history in the western Pacific remains unclear. The Gagua Ridge constitutes the western boundary of the Philippine Sea Plate; however, its geologic nature and genesis are unclear. Here, we determine whole‐rock major and trace elements and Sr‐Nd‐Pb‐Hf‐Mg isotopic compositions and emplacement ages of the volcanic rocks from the Gagua Ridge. These rocks are classified as trachy‐andesites and basaltic andesites. They are characterized by depletion in Nb‐Ta and enrichment in large ion lithosphile elements, confirming their arc affinity. 40Ar‐39Ar analyses of plagioclase yields ages of 124‐123 Ma for the basaltic andesite group and ∼115 Ma for the trachy‐andesite group. The elevated Zr‐Hf‐K of these rocks coupled with high Th/La ratios indicate melting of subducted sediments. The anomalously high Nb/Ta (mean ∼33) can be explained by slab melting with residual rutile. These rocks also show the highest δ26Mg (−0.14 to 0.4‰) for arc lavas ever reported and the δ26Mg values are coupled with rare earth element compositions, which can be explained by interaction of sediment melts with altered peridotite/altered oceanic crust in the subduction zone. These volcanic rocks have Nd‐Hf isotopes similar to the Pacific mantle, indicating the presence of Pacific‐type mantle in the Early Cretaceous western Pacific.
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