Geochemistry of quaternary volcanism in the Sunda-Banda arc, Indonesia, and three-component genesis of island-arc basaltic magmas

Abstract

Volcanic rocks of the Sunda and Banda arcs range from tholeiitic through calcalkaline and shoshonitic to leucititic, the widest compositional span of mafic magmatism known from an active arc setting. Mafic rocks in our data set, which includes 315 new analyses of volcanic rocks from twelve Quaternary volcanoes, including Batu Tara in the previously geochemically unknown Flores-Lembata arc sector, are generally similar to those from other island arcs: most contain <1.3 wt. % TiO 2 and 16–22 wt. % Al 2O 3, and have characteristically high K/Nb and La/Nb values. Abundances of P, Ba, Rb, Sr, La, Ce, Nd, Zr and Nb increase sympathetically with increasing K 2O contents of mafic rocks but those of Na, Ti, Y and Sc vary little throughout the geochemical continuum from low-K tholeiitic to high-K leucititic rocks. Excluding Sumatra and Wetar, which possess mainly dacitic and rhyolitic volcanics, the Sunda-Banda arc is divisible into four geochemical arc sectors with boundaries that correlate with major changes in regional tectonic setting and geological history. From west to east, the West Java, Bali and Flores arc sectors each comprise volcanoes which become progressively more K-rich eastwards, culminating in the leucitite volcanoes Muriah, Soromundi and Sangenges, and Batu Tara, respectively. In the most easterly Banda sector, the volcanics vary from high- to low-K eastwards around the arc. Correlations between geochemistry and 87Sr/ 86Sr values show separate trends for each of the four arc sectors, believed to be the result of involvement of at least three geochemically and isotopically distinct components in the source regions of the arc magmatism. A dominant source component with a low K content and a low 87Sr/ 86Sr value, and common to all sectors, is probably peridotitic mantle. A second component, with low K content but high 87Sr/ 86Sr value, appears to be crustal material. This component is most apparent in the Banda sector, in keeping with that sector's tectonic setting close to Precambrian Australian continental crust, but it is also present to lesser extents in the West Java and Flores sectors. However, the most marked geochemical and isotopic variations shown by the arc volcanics are primarily due to the involvement of a third component, which is rich in K-group elements but has relatively low 87Sr/ 86Sr values. This component appears to be mantle-derived and is least overprinted by crustal material in the Bali sector volcanics where the Pb, Be, U-Th and O isotope characteristics of the rocks support the suggestion that their genesis has not involved incorporation of recently subducted, continent-derived sialic material. The high, regionally persistent, Th/U value (about 4.3) of the Sunda subarc mantle, obtained from U-Th isotopic data, suggests a close association could exist between the K-rich component and the southern hemisphere ‘DUPAL’ mantle isotopic anomaly.