Australia and Indonesia in collision: geochemical sources of magmatism

The islands of Alor, Lirang, Wetar and Romang are located in the extinct section of the Sunda-Banda arc, where the collision with the Australian continent has brought subduction to a halt. Intrusive and extrusive igneous samples show a wide range of Sr, Nd and Pb isotopic characteristics. Samples from the northeast coast of Alor extend the trend of increasing 206Pb/204Pb ratios along the arc in an easterly direction, with values as high as 19.6. Samples from Alor's south coast, Lirang, Wetar and Romang have appreciably lower 206Pb/204Pb ratios (≤19.1), and 143Nd/144Nd ratios down to 0.5119. The Pb isotope data are interpreted as reflecting mixing between two internally variable end members within the subducting Australian continent, either the upper and lower crust, or two upper crustal end members of different ages. These melts may come up virtually unmodified, giving rise to the felsic, low 143Nd/144Nd samples, or may interact with the mantle, of which the partial melts and the fractionation products thereof give rise to basalts to rhyodacites with more intermediate Nd isotopic characteristics. Mixing modelling of the latter samples' isotopic ratios constrains the amount of crustal material that has been added to the mantle wedge to reach up to 9%. The isotopic and trace element heterogeneity in the samples studied is likely to reflect inhomogeneity of the crustal sources contributing to magmatism.