Geology and Arc Magmatism of the Eastern Sunda Arc
The Eastern Sunda Arc of Indonesia that consists of the islands of Java, Bali, Lombok and Sumbawa represents one of the most complex arc magmatism settings in the world. Cenozoic magmatism associated with a subduction along the Java trench started since Eocene in west Java to Pacitan section, Oligocene for the easternmost Java to probably west Lombok, and Miocene for most of Lombok and Sumbawa. Cenozoic volcanoes demonstrate common features of island arc magmatism, such as a wide variation in SiO2 contents, high A12O3, with low TiO2, Na2O and MgO contents.
Magma alkalinity increased towards younger volcanoes and towards the backarc-side. On the other hand, the arc also demonstrates several unique features that differ to idealized temporal and spatial schemes of subduction zone magmatism. For example, the region holds the widest range of K2O contents among all subduction-related magmatic arcs in the world. Volcanic centers and arc have also experienced migrations towards the backarc-side during its Cenozoic history, which is different from a typical trench-side migration of a subduction zone.
Along the whole section, there are at least two distinctive petrochemical (or lithochemical) arc sectors to be present, i.e. west Java and east Java to Sumbawa. Such differences are products of different crustal types and source components for the magma generation that may produce different mineralization styles. West Java is dominated by low-sulfidation Au-Ag epithermal system associated with Neogene-Quaternary high-K to shoshonitic volcanism at continental crust setting. Meanwhile, eastern Java to Sumbawa sector is dominated by porphyry- related Cu-Au mineralization system associated with middle Tertiary-Neogene, low- to moderate-K magmas with high Sr/Y affinity, suggesting immature arc magmas with minimum crustal contamination.