GEOCHEMISTRY OF BASALTS FROM THE WESTERN WOODLARK, LAU AND MANUS BASINS - IMPLICATIONS FOR THEIR PETROGENESIS AND SOURCE-ROCK COMPOSITIONS

Geochemical compositions are reported for back-arc and marginal basin basalts sampled during the 1990 cruise of R/V Akanemik Mstislav Keldys h 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 weste rn Woodlark Basin (Dobu Seamount region, WD), continental rifting segm ent; East Basin region, WE, the boundary between continental and sprea ding segments; and Franklin Seamount region, WF, spreading rifting seg ment). Two main geochemical types of basalts have been distinguished w ithin 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 Iherzo lite (N-MORB type) for LC, LE, MRS basalts; the N-MORB source was meta somatized by adding LILE and volatiles ('arc-source' components) for e nriched lavas. Direct observations from Mir submersibles in the area o f the King's triple junction (Lau Basin) established that LILE-deplete d massive sheet-flow basalts overlie vesicular pillow basalts. The wes tern Woodlark Basin is a tectonic transition zone between typical ocea nic spreading in the central part of the basin and continental rifting in easternmost Papua-New Guinea. The WD basalts show strong LILE, LRE E, P, and Zr enrichment and are spatially associated with alkaline rhy olites. The WE and WF lavas demonstrate moderate enrichment in these e lements. The western Woodlark lavas formed from multiple sources, incl uding N-MORB mantle, 'arc-source' components and enriched mantle like OIB. (C) 1997 Elsevier Science B.V.