Geochemistry of basalts from the western Woodlark, Lau and Manus basins: implications for their petrogenesis and source rock compositions

Abstract

Geochemical compositions are reported for back-arc and marginal basin basalts sampled during the 1990 cruise of R/V Akademik Mstislav Keldysh in the Lau Basin (North Lau, LN; King's Triple Junction, LK; Central Lau, LC; East Lau, LE), Manus Basin (Red Star region, MRS), and western Woodlark Basin (Dobu Seamount region, WD, continental rifting segment; East Basin region, WE, the boundary between continental and spreading segments; and Franklin Seamount region, WF, spreading rifting segment). Two main geochemical types of basalts have been distinguished within the Lau and Manus back-arc marginal basins: (i) depleted basalts comparable with N-MORB lavas (LC, LE, MRS); and (ii) enriched basalt lavas (LN, LK vesicular pillow basalts: LK, VPB) correspond to typical BABB. The sources of melts were depleted spinel or plagioclase lherzolite (N-MORB type) for LC, LE, MRS basalts; the N-MORB source was metasomatized by adding LILE and volatiles (‘arc-source’ components) for enriched lavas. Direct observations from Mir submersibles in the area of the King's triple junction (Lau Basin) established that LILE-depleted massive sheet-flow basalts overlie vesicular pillow basalts. The western Woodlark Basin is a tectonic transition zone between typical oceanic spreading in the central part of the basin and continental rifting in easternmost Papua-New Guinea. The WD basalts show strong LILE, LREE, P, and Zr enrichment and are spatially associated with alkaline rhyolites. The WE and WF lavas demonstrate moderate enrichment in these elements. The western Woodlark lavas formed from multiple sources, including N-MORB mantle, ‘arc-source’ components and enriched mantle like OIB.