Isotopic compositions of late Cenozoic volcanics from southeast Papua New Guinea: Evidence for multi-component sources in arc and rift environments

Abstract

Transition from collisional to extensional tectonics in eastern Papua New Guinea (PNG) is reflected in cessation of arc-type volcanism and eruption of rifting-related transitional basalts. Pb, Sr and Nd isotopic compositions for 28 samples representing these volcanic suites and subordinate high-K trachytes of unknown origin reflect melting and mixing of three distinct magma sources. The arc volcanics have high La/Nb, low Ce/Pb and overabundances of LILE relative to the LREE consistent with melting of subduction-modified sources. 87 Sr 86 Sr and 143 Nd 144 Nd ratios are moderately heterogeneous ( ϵ Nd = + 2.8 to + 6.3) and plot in the upper OIB field. Pb isotopic ratios are similar to modern Pacific sediment. Melting of a depleted MORB-type mantle containing up to 1.5% of Pacific sediment can account for the isotopic trends. The modeled amount of sediment is higher than that generally observed in Pacific arcs but lower than that suggested for Indonesia. Rift basalts have 87 Sr 86 Sr and 143 Nd 144 Nd ( ϵ Nd = + 5.3 to + 7.8) intermediate between Pacific MORB and associated arc volcanics. 207 Pb 204 Pb and 206 Pb 204 Pb ratios form a mixing trend and define Pacific MORB and sediment as end-members. High La/Nb and low Ce/Pb are further evidence for an important influence of subduction-modified sources at mantle and/or crustal levels. 143 Nd 144 Nd and 87 Sr 86 Sr ratios in the high-K trachytes plot near those of the most enriched arc volcanics but low 206 Pb 204 Pb and high 208 Pb 204 Pb indicate an old source that had evolved with low 238 U 204 Pb and high Th/U. These characteristics are similar to those in inferred lower crust from northeast Australia and EM 1 mantle sources; however, high La/Nb and low Ce/Pb ratios suggest additional source modification. Moderately radiogenic 207 Pb 204 Pb ratios preclude a significant contribution from crustal material much older than 2 Ga. We suggest that the trachytes inherited their isotopic signatures either from lower crust that was recycled into the upper mantle possibly during Mesozoic rifting of the Australian craton, or from a yet not recognized ancient continental block within the Late Cretaceous basement of PNG.