Strontium isotopes in Cenozoic volcanic rocks from southeastern Papua New Guinea

Strontium isotope data are presented from 28 rock samples representing four distinct episodes of late Cenozoic volcanism in southeastern Papua. Eocene tholeiitic basalts have initial ⁸⁷Sr/⁸⁶Sr ratios (0.7037) which are higher than those in chemically comparable basalts and are thought to have been enhanced by rock-sea water interaction. Late Cenozoic are trench type volcanoes in the Papuan islands have initial ⁸⁷Sr/⁸⁶Sr ratios which show little variation (0.7041±2) in contrast to those from the contiguous Papua New Guinea mainland (0.7036-0.7054). This isotopic discontinuity does not appear to be due to contamination by immediately underlying sialic metamorphics. High-K trachytes in the Lusancay Islands north of the late Cenozoic are have comparable initial ⁸⁷Sr/⁸⁶Sr ratios. Wide variation (0.703-0.710) in the initial ratios measured in a group of apparently closely related rocks ranging in composition from transitional basalt to peralkaline rhyolite cannot be explained by differences in age or by late magmatic fractionation and continue to pose an enigma.