Petrogenesis of Batur caldera, Bali, and the geochemistry of Sunda-Banda arc basalts

The active Batur volcano On the island of Bali in the eastern Sunda arc, Indonesia, has produced a wide variety of basaltic to dacitic volcanic rock types during its evolution and now possesses a large, well-formed, double caldera. A new basaltic stratovolcano is forming centrally within the caldera. The evolution of Batur can he divided into precaldera, caldera and postcaldera stages from the volcanic stratigraphy. Beginning more than 500,000 years ago, the precaldera stage produced basaltic to andesitic lava flows and pyroclastics, forming a stratovolcano possibly 2500-3000 m high above sea-level. The subsequent caldera stage produced widely distributed deposits of dacitic pyroclastids and less abundant lava flows, which contrast markedly in lithology, mineralogy and composition with the precaldera products. Following the formation of the first Batur caldera by collapse about 100,000 years ago, and its subsequent partial filling by a thick sequence of near-vent pyroclastics and lavas during continued explosive dacitic activity, the caldera stage culminated about 23,.670 years ago with the generation of the Bali ignimbrite, which covers at least 450 km2 of southern Bali. This eruption led to formation of the second, inner Batur caldera. Basaltic and basaltic andesite material, including nine lava flows since 1949, has been erupted from within the caldera during the postcaldera stage which may have begun about 20;0.00 years ago.