High-magnesium andesites: the example of the Papuan Volcanic Arc
The late Cenozoic arc-type volcanic arc in southeastern Papua New Guinea developed in an environment of complex tectonic processes including obduction, subduction, rifting and sea floor spreading. The volcanic arc extends from the Papuan Peninsula south-eastward through the D'Entrecasteaux Islands into the Louisiade Archipelago. Lithologies are predominantly basaltic andesite and andesite, but include basalt, dacite and rhyolite. The rocks have typical arc-type geochemical features but include a group ranging from basalt to dacite which, although comparable in most other aspects of their compositions, are higher in MgO, Cr and Ni. These high-Mg rocks are less porphyritic and have simple olivine- or clinopyroxene- dominated phenocryst assemblages compared with the associated low-Mg rocks. The low-Mg rocks are plagioclase-phyric and contain augite and hypersthene with or without olivine, hornblende and biotite phenocrysts. Boninites are spatially associated with, but genetically unrelated to the arc-type rocks in Papua. The high-Mg rocks represent magmas derived by partial melting of subduction-modified mantle which rose rapidly from their source. In contrast, the low-Mg lavas represent magmas which were modified by shallow processes. The unusual abundance of high-Mg lavas in southeastern Papua is related to extensional tectonics which allowed deep sourced magmas to rise without significant modification.