Rifting-related volcanism in an oceanic post-collisional setting: the Tabar-Lihir-Tanga-Feni (TLTF) island chain, Papua New Guinea

The Tabar-Lihir-Tanga-Feni (TLTF) volcanic island chain occurs in a zone of lithospheric extension superimposed on a post-collisonal tectonic setting along the Pacific and Indo-Australian plates northeast of Papua New Guinea. We present geochemical and Sr, Nd, and Pb isotope data for volcanic rocks from these islands and three recently discovered seamounts located at Lihir island. Major element data document an alkalic affinity of the sample suit e and trachybasalts as the predominant rock type. Negative Nb-anomalies in extended trace element patterns, enrichment of the light rare earth element s, and Ce/Pb ratios of about 4 are typical of the values in calc alkaline i sland are volcanics and support an origin from subduction-modified mantle. Sr-87/Sr-86 ratios of 0.7037 to 0.7044 and epsilon(Nd) values of +5.6 to +6 .8 indicate that the upper mantle evolved with a time-integrated depletion in LREE, however, not as severe as that recorded in basalts from the East P acific Rise. Variable Sr-87/Sr-86 ratios at less variable Nd-143/Nd-144 rat ios suggest that Sr-87/Sr-86 ratios of the melts were modified by secondary processes, such as assimilation of seawater Sr from crustal rocks. The Pb isotope ratios are uniform, moderately radiogenic (Pb-206/Pb-204 ca. 18.7 t o 18.8), and similar to those reported for the active Mariana are. Elevated Pb-207/Pb-204 ratios relative to Pacific MORE suggest melting of small amo unts of subducted sediments (ca. 1-2 wt.%). An important control of subduct ed sediment on the chemistry of the melts can also be inferred from the rat ios of highly incompatible trace elements (e.g., Th, U, Pb, La, and Nc). Ad ditional mantle enrichment by subduction derived fluids is reflected in hig h values of highly incompatible trace element ratios between fluid mobile ( e.g., Ba) and fluid immobile elements (e.g., Th, Nb). The results of this s tudy document that the chemical composition of igneous rocks from post-coll isional tectonic settings are strongly influenced by previous plate tectoni cs. This conclusion implies that the information conveyed by tectonic discr imination diagrams for these rocks must be interpreted with care. (C) 1998 Elsevier Science B.V. All rights reserved.