Tangkil and Rajabasa volcanoes are neighboring subduction-zone volcanoes located within Sunda volcanic arc on the southeast tip of Sumatra Island, Lampung Province, Indonesia. These volcanoes are situated in the Sunda Strait, where Panaitan Island, Krakatau Island, Sebesi and Sebuku Islands, and the Sukadana basalt plateau form a volcanic lineament. This region is unique in term of its tectonic and volcanic activity. Tangkil and Rajabasa volcanoes are not well studied compared to other volcanoes in Sunda Strait. The age determination and geochemical analyses of volcanic rocks from the two volcanoes have not been conducted. Detailed observation on mineral textures and zoning patterns, which could reflect the magmatic history for some extents, also has not been done. These data can give a valuable information about volcanic history and evolution of magma plumbing system. Thus, these reasons provide a good opportunity to study Tangkil and Rajabasa volcanoes. This study introduces a new stratigraphy of lavas, radiometric ages, whole-rock chemistry, detailed observation on mineral textures and zoning, and a new set of mineral chemistry data. Stratigraphic correlation of lavas in the Tangkil-Rajabasa volcanic area is established from field observations, morphological analysis, and K-Ar dating analyses. Detailed petrography and geochemical data of 13 lava units are then integrated with the stratigraphy to show temporal petrological variations. The description of petrological variations by a careful observation on mineral textures, compositional zoning patterns, and mineral assemblages, helps to identify the magmatic processes. The endmember magmas are identified via whole-rock and mineral chemistry, and their T-P conditions are estimated using thermobarometry models. Early stage (> 4.3 Ma) effusives of Tangkil volcano are dacitic to rhyolitic (67-71 wt% SiO2; Tklf), whereas later (c. 4.3 Ma) rocks are basaltic to basaltic andesite (c. 52 wt% SiO2; Tklm). Then, it took c. 4.0 Ma to resume volcanic activity at Rajabasa volcano. Lavas of Rajabasa volcano are comparatively younger (c. 0.3 to 0.1 Ma) with composition ranging from basalt to andesite (51-62 wt% SiO2; Rbs).Akita University The rocks from Tangkil (last Tklf) and most Rajabasa (all Rbs, except for one sample) indicate open system processes. These rocks contain plagioclase and pyroxene phenocrysts that show chemically evolved cores with resorption textures and less evolved mantles or rims. The resorption textures can be formed by processes of heating, hydration, compositional change of ambient magma, or decompression. The multiple zones of dissolution-overgrowth in plagioclase crystal and the fluctuating trend in temporal whole-rock variation suggest that the changes of magmatic condition in T, H2O, or chemical composition were repetitive. The chemical variations of Rajabasa are accounted for by the interactions of at least three endmembers: Mg-rich medium-K basalt magma, low-Mg medium-K basalt magma, and high-K andesitic magma. Mg-rich medium-K basalt magma is a primitive magma with high contents of Cr (>250 ppm) and Ni (>114 ppm). The mixing of basalt magmas with andesite magma is indicated by two linear trends in whole-rock chemistry diagrams and supported by chemical modelling performed using MELTS. The felsic endmember magma of Rajabasa is fixed in composition (at ~62 wt% SiO2; ~2.2 wt% MgO). Tangkil also involves bimodal magma system of basalt and felsic magma. Although the majority of samples from Tangkil do not show evidence of open system magmatic processes, one sample (last Tklf) contain phenocrysts that show disequilibrium features. The origin of the phenocrysts is determined by core-rim compositional variations of the phenocrysts. Tangkil is divided into series of Tklf, last Tklf, and Tklm; Rajabasa is divided into series of high-Mg basaltic andesite, low-Mg basaltic andesite, andesite, and transitional basalt based on geochemical characteristics. The assemblages of early-formed crystals in an endmember magma are established by finding the mutual chemical equilibrium between series. In accordance with the bulk geochemical results, the compositional variations of phenocrysts in last Tklf, high-Mg basaltic andesite, low-Mg basaltic andesite, and andesite series suggest contributions of various endmember magmas. The multiple crystallization origins of phenocrysts are also indicated by the variations in T-P estimates obtained by thermobarometry calculation. The last Tklf series shows a distinct of T estimates (934 and 1069 °C), of which the lower temperature is similar with that of Tklf series. The series of high-Mg basalt, low-Mg basalt, and andesite also exhibit a wide range of T within series, but the T estimate of high-Mg basalt magma is averagely higher (1174 °C) than that of low-Mg basalt magma (972 °C) and andesite magma (932 °C). The magma storage region depths beneath Tangkil and Rajabasa are 11 km to 22 km, at mid- to low-crustal levels, whereas high-Mg basalt magma resided in crustal-mantle interface level (c. 25 km). The magmatism in Tangkil was initially sourced from rhyolite magma at mid-crustal level. At later stage, a deeper and hotter basalt magma injected and mixed with the rhyolite magma. Ascent of another unmixed basalt magma occurred at c. 4.33 Ma, close before Tangkil volcano ceased its activity. The magmatism in Rajabasa during Upper Pliocene (c. 0.3 to 0.1 Ma) was originated from four distinct magmas. During its evolution, the upper mantle-origin high-Mg basalt magma and lower crust-origin low-Mg basalt magma repetitively replenished the middle crust-origin andesite magma. At one occasion, though, a basalt magma ascent and did not mix with the other three magmas.博士（理学）秋田大
