Implications of high-Mg# adakitic magmatism at Hunter Ridge for arc magmatism of the Fiji - Vanuatu region

Abstract

The mostly submarine Hunter Ridge, located in the SW Pacific records a ∼12 Myr to present history of magmatism related to the opening of the North Fiji Basin and subduction of oceanic lithosphere of the South Fiji Basin. Although the Hunter Ridge is probably composed primarily of an older Vitiaz-related basement, young volcanic features are present from Matthew Island to Kadavu Island. Some dredged volcanic rocks from these features have low-FeO and high-Mg# affinities, ranging from picrites to high-Mg# andesites and dacites. Elevated Sr (500 – 3400 ppm) and Sr/Y (50 – 240) coupled to fractionated (adakitic) rare-earth element patterns (La/Yb = 5 – 40, Gd/Yb = 1.5 – 5.7) indicate a garnet-signature derived from the melting of eclogite-facies basalt. Pacific-type MORB Nd-Hf-Pb isotopic ratios of these rocks contrast with the Indian-type MORB nature of the underlying North Fiji mantle but match closely the subducted South Fiji ocean crust. Low values of Th/La (< 0.15), Ba/La (< 22), unradiogenic 87Sr/86Sr (0.7026 – 0.7032) and Pacific-MORB Nd-Hf-Pb isotopic ratios indicate that sediment is a minor contributor to the source. The isotopic data clearly connect Hunter Ridge arc rocks of all compositions (picrites, low- to medium K2O arc lavas, basalts, high-Mg# andesites and dacites) to source components predominantly within the subducting plate. Unradiogenic 87Sr/86Sr (0.7026 – 0.7029) at high Sr abundances (700 – 1400 ppm) are common in hot-slab localities and are interpreted to reflect flux-melting of MORB under eclogite-facies conditions driven by dehydration in the underlying mantle of the subducting plate. Such an adakitic slab-melt component can be detected in more common (non-adakitic) arc rocks along the Hunter Ridge and Vanuatu arc as well. Evidence of slab melting along the western Pacific indicates that melting of subducting oceanic lithosphere is likely a common occurrence at convergent margins.