Low‐K tholeiites and high‐K igneous rocks from Woodlark Island, Papua New Guinea

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Abstract
Woodlark Island, the largest above‐sea portion of the Woodlark Rise, has an exposed basement of pre‐Miocene (?Cretaceous‐Eocene) low‐K tholeiitic basalt and dolerite, and minor sediments. The basement is unconformably overlain by Early Miocene limestone and volcaniclastic sediments and later Miocene high‐K volcanics and comagmatic intrusives. Pleistocene to Recent sediments partly blanket the Tertiary sequence. Basement low‐K tholeiites vary only slightly in composition and are interpreted as ocean floor or possible marginal basin material. The high‐K suite appears to be chemically similar to late Tertiary to Recent high‐K igneous rocks of mainland Papua New Guinea. It includes porphyritic hornblende‐, clinopyroxene‐, biotite‐ and magnetite‐bearing shoshonite, latite and toscanite, and intrusive equivalents that range from olivine normative to strongly quartz normative compositions (S1Q2 46% to 75%). Computer mixing models indicate that separation of the pheno‐crysts in the shoshonites, particularly pargasitic hornblende, is a feasible mechanism for producing the more silica‐rich monzonites and latites. The low‐K tholeiitic basement rocks of Woodlark Island are inferred to be part of an ophiolitic slab en echelon with the Papuan Ultramafic Belt, thrust over equivalents of the Cretaceous Owen Stanley Metamorphics or, in part, onto existing oceanic crust. High‐K igneous rocks on Woodlark Island appear to form an eastward extension of a province of calcalkaline to shoshonitic volcanic and intrusive rocks, which stretches from Mount Lamington to the Louisiade Archipelago. Late‐middle Miocene high‐K magmatism at Woodlark Island is consistent with the observation that activity commenced earlier in the E and became progressively younger westwards towards mainland Papua New Guinea. Periodicity in the magmatism was apparently synchronous with major rifting episodes that formed the Woodlark Basin. The data on the Woodlark Island high‐K suite support the currently accepted. concept of delayed partial melting of a mantle source previously modified by the introduction of water and LILE from an earlier subduction zone (Johnson et al., 1978b).