Early volcanic history of the Rabaul area

Abstract

We conducted an extensive program of 40Ar–39Ar age determinations on a suite of 27 volcanic rock samples from key stratigraphic units at Rabaul, Papua New Guinea in order to improve understanding of the early eruption history of the multiple volcanic systems present in the area. Analyses of whole rock, plagioclase and groundmass separates yielded statistically significant ages for 24 samples. Replicate analyses (groundmass, plagioclase) for 17 of the samples provided concordant ages. The oldest systems in the Rabaul area (>1 Ma to ≈300 ka) are in the south, associated with the caldera-like Varzin Depression, and in the north, at the stratovolcanoes Watom and Tovanumbatir. The earliest known activity of the Rabaul system occurred between about 330 and 200 ka and involved emplacement of lava flows and scoria deposits. Major explosive activity at the Rabaul system commenced at about 200 ka and produced a sequence of dacitic ignimbrites that culminated with the emplacement of the large-volume Malaguna Pyroclastics at about 160 ka. Calderas may have been formed as a consequence of the large volumes of tephra produced during some of these eruptions. Products of the early activity are found in the northern and northeastern walls of Rabaul Caldera and on the northeastern flank of Tovanumbatir. This leads to the conclusion that the source of the early activity at Rabaul probably was located in the northern part of the present caldera complex. A shift in the focus of activity at the Rabaul system took place between about 160 and 125 ka. All of the younger (<125 ka) major pyroclastic formations, including the Karavia Welded Tuff, the Barge Tunnel Ignimbrite and the Latlat Pyroclastics, which make up the bulk of the exposure in the southern and western walls of Rabaul Caldera, were erupted from a source or sources in the south-central part of the complex. The stratovolcanoes Palangiangia and Kabiu, which flank the northeastern part of the complex, had commenced activity by about 100 ka while their neighbour to the southeast, Turagunan, was active by about 70 ka. There is little stratigraphic and chronological information about the Tavui system, immediately north of Rabaul. At present, the products of only two eruptions in the Rabaul area can be attributed to Tavui: the ≈79 ka Tokudukudu Ignimbrite and the ≈7 ka Raluan Ignimbrite. However, this evidence of activity and its timing is sufficient to show that the Tavui and Rabaul systems are co-eval. The average interval between major eruptions at the Rabaul system over the last ≈18 ky is in the range 2.6 to 6 ky. Eruption frequency likely varied over the life of the system; however, the return period during the early phase of ignimbrite eruptions appears to have been similar to that of the last ≈18 ky.