Petrogenesis of the Toba Tuffs, Sumatra, Indonesia

During the past 1.2 my, at least 3400 km3 of magma have been erupted in four ash flow tuff units from the Toba Caldera Complex. This activity culminated at 74 ka with the fourth eruption, which produced 2800 km3 of magma and formed the 100 km × 30 km caldera visible today. A relatively homogeneous two-pyroxene dacite was erupted during the first phase of activity. Magma erupted during each successive eruption was compositionally zoned, generally ranging from rhyodacite to rhyolite. The youngest three tuff deposits contain up to 40 wt % crystals of quartz, sanidine, plagioclase, biotite, and amphibole. Minor minerals are magnetite, ilmenite, allanite, zircon, fayalite, and orthopyroxene; inclusions of apatite and pyrrhotite are common. Extensive fractional crystallization in double-diffusive convecting magma bodies is suggested for creation of the compositional variation. Although similar in composition and mineralogy, the tuffs can be distinguished by subtle variations in their mineral, whole-rock, and glass compositions. Intensive parameter determinations suggest that much of the crystallization of the quartz-bearing tuffs occurred between 700 and 760°C at depths of 10 km. Water pressure estimates indicate that Ptot > PH2O; thus volatile oversaturation probably did not initiate the eruptions. Individual pumice blocks and fiamme collected from the youngest three units record simultaneous eruption across compositional boundaries. Low-energy ring fracture eruptions resulted in dense welding of all units except for the top of the youngest unit, and thick accumulations of rhyodacitic magma in the collapsing calderas.