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Abstract

The mantle and crustal sections of the Permian Dun Mountain ophiolite (DMO) in New Zealand's South Island have similar and potentially related tectonic histories. The mantle section records a four-part sequence of mid-ocean ridge basalt, and boninite melt extraction, followed by refertilization from supra-subduction melts and, finally, the intrusion of mafic–felsic island-arc tholeiite dykes. The crustal sequence represents a progression from mid-ocean ridge basalts to transitional island-arc tholeiites and, finally, mafic–felsic, possibly calc-alkaline, lavas; overlain by the 6 km-thick sedimentary Maitai Group. Plagiogranite zircon geochronology shows that the crustal ocean-ridge volcanism had commenced by 277.6 ± 3.3 Ma, and the youngest granites and granodiorites crystallized by 269.3 ± 4.5 Ma. All lithologies of the DMO are inferred to have formed in a broad forearc setting, representing a sequence from subduction initiation to magmatic arc formation. Sedimentary blocks within the structurally underlying ophiolitic Patuki Melange are similar to the Maitai Group, supporting a formational relationship with the DMO.

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