Cogenetic volatile-rich alkali basalts and gabbros, hawaiites, and mugearites occur in the late Miocene age epizonal Porgera intrusive complex, which is temporally and spatially associated with rich gold-silver mineralization. The least evolved rocks show enrichments in light rare earth elements ([La/Yb]cn = 15-19) and other incompatible elements (e.g., Ba/La ≈ 8-10, La/Nb ≈ 0.6-0.7, Sr/Nd ≈ 25) characteristic of intraplate alkalic basalts and have isotopic compositions (87Sr/86Sr ≈ 0.7035, ϵNd ≈ +6, 206Pb/204Pb ≈ 18.66, 207Pb/204Pb ≈ 15.56, 298Pb/204Pb ≈ 38.55) consistent with derivation from a time-averaged depleted mantle source. The Porgera intrusive complex was emplaced at 6 Ma in Jurassic-Cretaceous shelf-facies sedimentary rocks near the edge of the Australasian plate. Intrusion appears to have occurred in a back-arc environment during subduction of an oceanic microplate segment on two sides, beneath the continental margin and an island arc. We suggest that this unusual tectonic setting promoted partial melting of asthenospheric source materials that were perhaps modified by deep (>150 km) subduction processes to form alkalic magmas with intraplate character.

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