Abstract <p>Klumba basaltic andesite volcano is the only postcaldera volcano in the eruptive center of one of the most powerful explosive eruptions that occurred at the end of the Late Pleistocene in the Great Kuril Arc (GKA) and formed a thick sequence of dacitic pumice tuffs on the Vetrovoy Isthmus on Iturup Island. A detailed study of the mineralogy of the basaltic andesites of Klumba volcano and olivine-hosted fluid and melt inclusions showed that the feeding magma evolved within the arc crust at depths between 15.5 and 7 km and was related to Mg-rich (up to 9.8 wt % MgO), low-K and low-Al basaltic andesite melts that initially contained about 5–6 wt % H₂O. Olivine and Cr–Al spinel were the first to crystallize in the magma and later were joined by plagioclase and pyroxenes. The phenocrysts crystallized at temperatures of about ~1000–1200°C. The melt was saturated with CO₂ fluid with minor amounts of SO₂. Pleistocene basaltic andesitic magmatism in the central part of Iturup Island was predominantly intrusive and resulted in the formation of a large transcrustal magmatic system (TCMS), which could include the dacitic chamber that fed the explosive eruption of the Vetrovoy Isthmus. The plumbing system of Klumba volcano is considered to be a part of this TCMS, which was intermittently recharged by variously differentiated basaltic andesite magmas. It is assumed that such systems may have developed on the scale of the whole island. The duration of the processes and the amount of intruded magma may have been sufficient to cause partial melting in the upper parts of the island-arc crust and to form magma reservoirs of powerful explosive caldera-forming eruptions.</p>