Geochemical data are presented for a sequence of spilitic pillow basalts (Tumu River basalts) associated with peridotites and gabbros of the Marum ophiolite complex in northern mainland Papua New Guinea. The basalts are strongly differentiated from relatively magnesian types (Mg-value = 70) to ferrobasalts (Mg-value = 30) characterised by high levels of Fe, Ti, Zr, Nb, Y. The Tumu River basalts are enriched in large ion lithophile elements such as REE, Zr, Hf, Nb, P2O5, and compare with tholeiites from oceanic islands. Major and trace elements suggest extensive fractionation involving olivine, pyroxene, and plagioclase, followed by pyroxene, plagioclase, titanomagnetite, and ilmenite. Trace-element plots are used to examine fractionation processes and to estimate abundances in the parent magma. The calculated initial concentrations are compared with abundances and abundance ratios in least fractionated enriched and depleted tholeiites. The abundances in the parent magma are used to calculate source abundances for large (20-30%) degrees of partial melting. The levels range from 2-3 times chrondites for HREE, Ti, Y, Zr, Sc, and P2O5, to 3.5-5.5 times for LREE, and are similar to those inferred for other LREE-enriched tholeiites from both oceanic and continental areas. The chemistry of the basalts therefore reflects the mantle-source composition rather than a particular tectonic setting within an ocean basin.