During the 1984 “Natsushima” cruise to the North Tonga arc, fresh volcanic rocks were recovered from the Tongan forearc. Olivine phenocrysts in some of the dredged rocks are highly magnesian (Fo94) and contain large (up to 0.2 mm) glass inclusions. The glass inclusion chemistry, as analysed by electron microprobe, is characterized by high CaO (> 13 wt.%) and low Na2O (< 0.8 wt.%) contents, resulting in very high CaO/Na2O ratios (18–29). Such high SCV002SCV0 CaO/Na2O ratios in glasses provide a ready explanation for the occurrence of extremely calcic plagioclase (up to SCV002SCV0 An100) in the Tongan arc lavas. The prevalence of magma mixing and the existence of a spectrum of liquids, ranging to the extremely refractory glasses analysed, is a characteristic of this intra-oceanic island arc environment. High PH2O is not important in precipitation of extremely calcic plagioclase from common low CaO/Na2O liquids, although water SCV002SCV0 plays a significant role in controlling melting in refractory source compositions.