Collaborative study between Japan and Philippine on volcanic and geothermal history of Irosin caldera, south Luzon, Philippine

伊罗辛火山口位于菲律宾吕宋岛东南端。布卢桑火山是一个后破火山锥。为了揭示Irosin破火山口喷发的序列和破火山口后火山活动的喷发历史，我们与PHIVOLCS进行了一个合作项目。一个小的熔岩圆顶和火山灰沉积物被喷发，作为主要的破火山口形成喷发的先驱。破火山口喷发的第一阶段是普利尼安期，发生在主熔结凝灰岩喷发之前不久。喷发的第二阶段是伊罗辛熔结凝灰岩的形成，它可分为两个单元：下细单元和上粗单元。在距火山口80 km的伊那斯坎火山锥顶火山口出露的细流纹岩灰存款被认为是与伊罗辛熔结凝灰岩共生的熔结凝灰岩灰。在伊罗辛熔结凝灰岩及相关矿床中发现了钙镁矾斑晶。从破火山口获得的岩芯沉积物主要由拉哈尔沉积物组成。流纹岩至安山质灰与沉积物互层。其中一个具有与伊罗辛熔结凝灰岩相似的岩石学特征。从Bulusan火山的现代火山灰组成的辅助材料，其中包括石膏。布卢桑火山周围温泉的化学和同位素组成表明，所有温泉都以HCO_3为主，属蒸汽加热型。

Irosin caldera is located at the southeastern end of Luzon Island, Philippines. Bulusan volcano is a post-caldera cone. To reveal the sequence of the Irosin caldera eruption and the eruptive history of post-caldera volcanism, we conducted a collaborative project with PHIVOLCS. A small lava dome and ash-fall deposits were erupted as forerunners of the main caldera-forming eruption. The first stage of the caldera eruption was the plinian, which occurred shortly before the main ignimbrite eruption. The second stage of the eruption was the formation of the Irosin ignimbrite which can be divided into two units ; the lower fine unit and the upper coarse unit. The fine rhyolitic ash-fall deposit exposed in the summit crater of Inascan cone 80 km from the caldera is regarded as a co-ignimbrite ash-fall associated with the Irosin ignimbrite. Cummingtonite phenocrysts were identified in the Irosin ignimbrite and related deposits. Cored sediments obtained from the caldera are composed mainly of lahar deposits. Rhyolitic to andesitic ashes are intercalated with the sediments. One of them has similar petrographic characteristics with the Irosin ignimbrite. Recent volcanic ash from Bulusan volcano is composed of accessory materials, which involves gypsum. Chemistry and isotopic composition of hot springs distributed around Bulusan volcano suggests that all of the hot springs are dominant in HCO_3 suggesting a steam-heated type of water.