Petrogenesis of igneous rocks from the Yap Arc, west Pacific: implications for its tectonic evolution

Abstract

The Yap arc-trench system was formed by the westward subduction of the Pacific/Caroline Plate beneath the Philippine Sea Plate (PSP) and was subsequently influenced by the jam of the Caroline Plateau into the Yap Trench. This makes it an ideal area for deciphering the geological evolution of an island arc when an oceanic plateau approaches a subduction zone. In the present study, we analyzed the mineralogical, geochemical, and isotopic compositions of peridotite, gabbroic diorite, and volcanic rocks collected from the southern Yap Arc. The results show that: (1) the peridotites are forearc peridotites, representing residues after 10–15 % partial melting of the sub-arc mantle wedge, caused by fluids released from the subducted slab during the subduction of the oceanic plateau. These peridotites were subsequently exposed in the Yap Arc due to tectonic erosion of the subducting plate. (2) The Yap Arc gabbro diorites share similarities with infant arc lavas and may have formed through a relatively low degree of partial melting of sub-arc basic rocks. (3) The Yap Arc basalts were produced by 2–5 % partial melting of the southern Yap peridotites under the influence of approximately 0.5 % sediment input. These basalts exhibit characteristics typical of the Pacific MORB-type mantle. (4) The andesites of the Yap Arc were formed through partial melting of preexisting basic rocks at the arc front, influenced by 0.5–1 % sediment melt. Hf-Nd isotopic characteristics of the southern Yap Arc basalts have validated the entry of Pacific MORB-type mantle into the Yap sub-arc mantle via a slab gap.