The Irosin co-ignimbrite ash-fall deposit: A widespread tephra marker in the Bicol arc, south Luzon, Philippines

摘要

The Irosin caldera, which is located in the province of Sorsogon, southern Luzon, Philippines, represents the largest extrusion of highly silicic magmas in the Bicol arc at ca. 41 cal ka BP. The 41 cal ka BP rhyolitic eruption led to a collapse and formation of the 11 km-wide Irosin caldera. This paper presents the results of the stratigraphy, grain assemblage, morphology, and geochemistry of the recently discovered rhyolitic fine ash which is exposed in the crater of Inascan scoria cone, 80 km from Irosin caldera. The morphology of glass shards obtained at Inascan cone is not only pumice-type but also bubble-wall-type glass shards which are typical of a co-ignimbrite ash. The mineral assemblage of the fine ash is quite similar to those of Irosin pumice. The refractive index, measured using the thermal immersion method, together with geochemical analyses of glass shards from the fine rhyolitic ash deposits and ignimbrite deposits from Irosin caldera, both indicate a strong geochemical similarity between the ignimbrite and fine ash deposits. Thus, the tephra sequence at Inascan scoria cone is interpreted as co-ignimbrite ash falls sourced from the 41 cal ka BP catastrophic eruption that formed the Irosin caldera. The Irosin co-ignimbrite ash-fall deposit, which measures 1.3 m thick and 80 km away from its source volcano, represents the most explosive eruption in the Bicol arc. The identification of the Irosin co-ignimbrite ash-fall deposit is a valuable contribution to the establishment of a chronological framework of widespread tephra in the Philippines as well as a potential regional tephra marker.