The Tonga-Kermadec Ridge, Lau Basin, and Lau-Colville Ridge are, respectively, a frontal arc, interarc basin, and remnant arc at the Australian-Pacific plate boundary. Basement rocks of the Lau-Colville Ridge (Lau Volcanics) are 9- to 6-m.y.-old basaltic andesites to dacites with 55 to 66 percent SiO2, K60 = 1.0 to 1.5 percent, little Fe enrichment, Sr87/Sr86 = 0.7030 to 0.7034, and enrichment in light rare-earth elements. Westward increases in K, Rb, Th, and U suggest that subducted lithosphere was un-derthrust from the east. Variations in rock composition are consistent qualitatively with derivation from basalt by low-pressure crystal-liquid fractionation involving removal of phenocryst phases: plagioclase + clinopyroxene + orthopyroxene + magnetite. Volcaniclastic turbidites of the same age and derived from western sources are found in Tonga. These andesitic vol-canogenic recks are overlain on both ridges by Pliocene limestones, which are capped on the Lau-Colville Ridge by 3.9- to 3.5-m.y.-old olivine + hypersthene normative tholeiites (Korombasanga Volcanics) having minor 56 to 60 percent SiO2 andesitic differentiates. Lau Basin basalts are transitional between ocean-floor and island-arc tholeiites, sharing with the latter their higher Rb, Ba, light rare-earth element, and Sr87 contents and lower Ti, Zr, and Hf contents. These data support Karig's idea that the Lau and Tonga Ridges represent a once-united island arc now dismembered by rifting, which has formed the intervening Lau Basin. This rifting began about 5 m.y. B.P. The change in volcanism on the Lau-Colville Ridge reflects its removal from a subduction site.