Subduction-related magmatism at the southern tip of the North Fiji backarc basin

Abstract

The North Fiji backarc basin (NFB) began opening ~12 Ma in response to subduction reversal along the Vanuatu section of the Proto-Vitiaz arc. The triangular shape of the basin reflects ongoing (since ~5 Ma) southward propagation of the backarc spreading centre in response to westward roll-back of the Vanuatu trench and eastward roll-back of the Tonga trench. During this time (5-3 Ma), a short-lived intraoceanic arc, the Hunter Ridge, formed along the southeast margin of the NFB. This arc magmatism occurred in response to a short period of NNW-directed subduction of the South Fiji basin crust under the NFB. The southern tip of the North Fiji backarc basin (22°S, 174°E) is a volcanically active submarine triple junction between the backarc basin spreading centre, Vanuatu trench and Hunter Fracture zone. This area has been mapped and sampled during the SS10/2004 cruise of R/V ``Southern Surveyor'. Volcanic rocks erupted within 100 km cover the entire spectrum of subduction-related magmas from backarc basin basalts to boninites, arc tholeiites, calc-alkaline basalts, high-Mg andesites and adakites. Subduction-related lavas in the area are dominated by olivine-clinopyroxene-phyric basalts with high-Mg phenocrysts (Mg# to 94). We will present detailed geology, geochemistry and mineralogy (including melt inclusion compositions) of all volcanic rock from this region and discuss their origin.