Stratigraphy and Lithofacies of the Toya Ignimbrite in Southwestern Hokkaido, Japan: Insights into the Caldera-forming Eruption at Toya Caldera

摘要

A stratigraphic study of the Toya Ignimbrite in southwestern Hokkaido, Japan, was performed to clarify the sequence of caldera-forming eruption at Toya caldera. The Toya Ignimbrite (thickness ＜ 80 m) is rhyolitic in composition and comprises six stratigraphic units: (1) a finegrained ash-fall deposit at the base; (2) a base surge deposit and an overlying, voluminous, pum-iceous pyroclastic flow deposit, both of which contain accretionary lapilli; (3) a number of base surge deposits and associated ash-fall deposits; (4) a pumiceous pyroclastic flow deposit that contains large lithic clasts up to 50 cm in diameter; (5) a pumiceous pyroclastic flow deposit with a basal lithic-rich layer (lag breccia); and (6) a pumiceous pyroclastic flow deposit at the top. The stratigraphy suggests that the caldera-forming eruption at Toya caldera commenced with a phreatomagmatic explosive eruption (forming unit 1)，followed by violent phreatomagmatic eruptions that generated a voluminous pyroclastic flow (unit 2)，and small-scale phreatomagmatic eruptions that generated a number of base surges (unit 3). The next phreatomagmatic eruption triggered caldera collapse (unit 4), which reached the climax with a violent phreatomagmatic eruption (unit 5) and ended with a magmatic eruption (unit 6). These eruptions occurred continuously without any significant time breaks. Component analysis of non-juvenile lithic clasts suggests that the vent-opening phase of the caldera-forming eruption involved a single vent. Pyroclastic flows during the caldera collapse may have erupted from multiple vents. Textural studies of pumice clasts suggest that white pumice was ejected during the initial to final stages, while banded pumice and grey pumice were ejected during the final stage. Geochemical data indicate that there was no significant change in magma composition during the caldera-forming eruption, with the exception of a small amount of mafic magma was mixed into the rhyolitic magma during the final stage.