Abstract

Active island-arc volcanoes west of the arc/continent collision between the Sunda arc and northwest Australia are erupting mafic volcanics that range from calc-alkaline basalts through shoshonitic trachybasalts to leucitites. These mafic arc volcanics display a threefold enrichment in K, Rb, Sr, Ba, La, and Nb contents along the arc from Bali to Flores, increasing toward the collision zone. The enrichment is very well correlated with increasing 87Sr/86Sr and decreasing 143Nd/144Nd values. The geochemical and isotopic data imply that the arc volcanics are being generated from two source materials; one yielding material poor in K-group elements, with relatively low 87Sr/86Sr (<0.704) values and high 143Nd/144Nd values, the other yielding material rich in K-group elements, with relatively high 87Sr/86Sr (>0.707) values and low 143Nd/144Nd values.

The K-rich material appearing in the arc volcanoes is being derived from ancient subcontinental mantle involved in the collision zone. Eastern Sunda K-rich mafic volcanism first appeared after the collision began. Before the collision, ancient northwest Australian mantle erupted K-rich, diamond-bearing ultramafics with high 87Sr/86Sr values and low 143Nd/144Nd values. These form part of a typical ultrapotassic continental volcanic association, yet they have Ba/La and Ba/Nb values characteristic of arc volcanism, showing their parent mantle to be geochemically and isotopically capable of supplying the K-rich material appearing in the eastern Sunda arc volcanoes. Terranes possibly underlain by similar “Gondwanaland” mantles surround the collision zone, which is the site of large-scale mixing between oceanic and continental lithospheres. Other petrogenetic theories seem incapable of accounting for the regional variations in eastern Sunda arc geochemistry.

Striking geochemical and isotopic resemblances between eastern Sunda leucitites and K-rich orogenic volcanics elsewhere in the world, such as those of the Roman region, imply similar origins involving K-rich subcontinental mantle material. Where a “K-h” correlation occurs, it might only be a signal that subduction of a lithospheric slab beneath a continental margin is inducing melting in old mantle, deep in the continental lithosphere.

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