PRE-CALDERA BASEMENT ROCKS AT TOBA CALDERA, SUMATRA, INDONESIA: CONSTRAINING VENT AREAS IN NORTHERN TOBA

The Toba Caldera Complex erupted 4 ignimbrites known as the Haranggaol Dacite Tuff (HDT), Oldest Toba Tuff (OTT), Middle Toba Tuff (MTT) and Youngest Toba Tuff (YTT) at 1.2, 0.84, 0.50, and 0.074 Ma respectively. Because the youngest eruption was the largest, its caldera (100 X 30 km) engulfs the eruption sites or calderas of the 3 older eruptions. The general vent/caldera locations for these older eruptions are poorly constrained and largely inferred from the distribution of their tuff units. Rocks exposed in the northern walls of the Toba caldera indicate that Paleozoic metasedimentary and Tertiary sedimentary basement is locally overlain by intermediate lavas, and the relatively small volume HDT, and MTT. The extensive YTT caps all of these rocks at the caldera rim. Conformable sequences and petrologic similarities of the lavas and HDT suggest that they may have had a common source, possibly an intermediate stratovolcano. Due to similar distribution patterns, the HDT and MTT are also postulated to have erupted from coincident or nearby vents/calderas. Using the premise that the tuffs should contain lithic fragments derived from the local source rocks, a lithic population study was done in order to better constrain the vent areas for the HDT and MTT. To provide comparative reference to the lithics found within the tuffs, a suite of pre-caldera basement rocks was studied petrographically and geologic maps were consulted. Preliminary results of the HDT and MTT lithic populations indicate that metasedimentary and sedimentary fragments were predominantly incorporated into these welded tuffs, and volcanic fragments are scarce. Thus, if an intermediate stratovolcano did exist prior to eruption of the tuffs, it may not have been their source. Furthermore, the dominant metasedimentary/sedimentary fragment types found in the HDT and MTT are distinctly different, suggesting that the two tuffs had different source areas. Detailed characterization of the fragment lithologies within the HDT and MTT, assessment of lithics within the YTT, and petrologic comparisons between the intermediate lavas and HDT are expected to provide additional insights into the pre-YTT volcanic terrain of northern Toba that now lies beneath tuffs, lake sediments, and Lake Toba.