THREE TYPES OF BASALT OF THE BASALTIC ANDéSITE MEDVEZHYA CALDERA, ITURUP, SOUTHERN KURIL ISLANDS

Abstract

Melt inclusions were studied in olivine, plagioclase, and orthopyroxene from nine basaltic andésite samples collected on young cones of the Medvezh'ya caldera on Iturup Island, southern Kuril Archipelago. Melts of the inclusions were classified into three types: (I) magnesian basalt (47-49% SiO2, 10.4-5.0% MgO, 0.3% K2O) was detected in inclusions in olivine and anorthite; (II) highly potassic dacite (64-69% SiO2,0.7-1.4% MgO, 5.4-6.8% K2O), in inclusions in plagioclase (An 83-71) from the youngest lava flows of the Kudryavyi cone; and (III) the most abundant inclusions, which occur in medium-potassic dacite melt (67-42% SiO2, 0.7-1.4% MgO, 1.2-2.0% K2O), in plagioclase (An 72-58) in all of thé examined samples and in orthopyroxene. The melt of type I is much higher in S (0.12 wt %) than the other melts (0.03-0.04 wt %). The basalt melt is also rich in Cl (0.12 wt %), whereas the dacite melts vary broadly in Cl contents: from 0.02 wt % in type II to 0.16 wt % in type III. The melts are similar in low water contents (<1 wt %). The homogenization temperatures of the melts are as follows: 1200-1300°C for type 1,1160-1170°C for type II, and 1070-1170°C for type III.