Breaking Up Is Hard to Do: Magmatism During Oceanic Arc Breakup, Subduction Reversal, and Cessation

The formerly continuous Vitiaz Arc broke into its Vanuatu and Fijian portions during a reversal of subduction polarity in the Miocene. Basaltic volcanism in Fiji that accompanied the breakup ranged from shoshonitic to low‐K and boninitic with increasing distance from the broken edge of the arc that, presumably, marks the broken edge of the slab. The Sr‐Pb‐Nd isotope ratios of the slab‐derived component in the breakup basalts most closely match those of the isotopically most depleted part of the Samoan seamount chain on the Pacific Plate that was adjacent to the site of breakup at 4–8 Ma, and differ from those of subsequent basalts in spreading segments of the surrounding backarc North Fiji and Lau Basins. Subduction of the Samoan Chain along the Vitiaz Trench Lineament may have controlled the limit of polarity reversal and, hence, where the double saloon doors (Martin, 2013) opened. Prior to breakup, Fijian volcanics were more similar isotopically to the Louisville Seamount Chain. Plain Language Summary The subduction zone that included Tonga and Fiji was once connected to Vanuatu. We attribute the arc breakup to subduction of the Samoan Seamount Chain. Volcanism in Fiji accompanying breakup ranges from shoshonitic closest the tear in the arc, to low‐K and boninitic farthest from it. The ambient mantle source of magma during breakup was the same as earlier in arc history but the slab‐derived component changed during breakup. Post‐breakup volcanism came from different mantle unaffected by subduction and derived from beneath the Pacific Plate. Key Points The breakup between Fiji and Vanuatu may have been triggered by subduction of Samoan seamounts Shoshonitic to low‐K and boninitic volcanism accompanied breakup with increasing distance from the break The mantle source of later basalts in surrounding backarc basins and islands came from beneath the Pacific Plate north of the breakup site