Abstract
AbstractThe puzzling <7 Ma old “postsubduction” arc magmatism of New Guinea contains geochemical subduction‐type signatures yet did not occur above an active subduction zone. Here we show that these arc magmas formed at the North Australian continental lithospheric edge when it plowed northward through mantle above the detached Arafura slab remnant. This mantle preserved its subduction signature and the edge plowing process generated new melts that ascended via an active transform fault. Arafura slab subduction occurred at an intraoceanic subduction zone that ended ~30–25 Ma ago, when the Australian continental edge was still ~1,000 km to the south. Our absolute plate tectonic reconstruction of continental‐edge plowing suggests that ancient mantle wedges remain semistationary in the upper mantle and can preserve their geochemical signature for tens of Ma, explaining previously enigmatic “postsubduction” arc magmatism.
Highlights
The subduction of oceanic lithosphere introduces fluids into the mantle, causing hydration‐type or “wet” melting and the formation of distinct “arc‐type” magmas, that is, calc‐alkaline magmatic rocks with an enrichment in large ion lithophile relative to high field strength elements that are found above almost all subduction zones (McCulloch & Gamble, 1991; Wilson, 1989)
The puzzling
Our absolute plate tectonic reconstruction of continental‐edge plowing suggests that ancient mantle wedges remain semistationary in the upper mantle and can preserve their geochemical signature for tens of Ma, explaining previously enigmatic “postsubduction” arc magmatism
Summary
The subduction of oceanic lithosphere introduces fluids into the mantle, causing hydration‐type or “wet” melting and the formation of distinct “arc‐type” magmas, that is, calc‐alkaline magmatic rocks with an enrichment in large ion lithophile relative to high field strength elements that are found above almost all subduction zones (McCulloch & Gamble, 1991; Wilson, 1989). Eocene ophiolite obduction marked the arrest of a northward dipping subduction zone consuming oceanic lithosphere of the Australian plate formerly present to the north of the New Guinea passive margin (Hall, 2002; Schellart & Spakman, 2015). This gap between Eocene subduction and the much‐later Plio‐Pleistocene genesis of arc‐type magmas on New Guinea indicates that new insights and observations are required to determine the geodynamic processes that triggered the genesis of these magmas by melting of a previously subduction‐enriched region of the asthenosphere. Our work benefits from previous research that determined the subduction‐enriched mantle origin and its composition directly underpinning the magmatic units and mineral deposits in this region
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