Successive Eruption of Alkaline and Tholeiite Magmas in a Japanese Palaeozoic Geosynclinal Basalt Body with Special Reference to Rare Earth Element Features

Rare-earth element (REE) abundances and major chemical compositionsof six late Palaeozoic geosynclinal basalts at Nakaoku, theKii peninsula, southwest Japan are discussed from the generalviewpoint of geosynclinal basalt magma. The REE patterns ofbasalt samples are smoothly and progressively enriched relativeto Leedey chondrite. The lighter REE are considerably fractionated,whereas concentrations of heaviest REE remain approximatelyconstant. Each fractionation pattern probably corresponds toresidual liquid at different stages during the solidificationof magma in depth. The partition coefficients of REE betweenmagma (i.e. liquid) and crystallizing solid can be calculated,assuming that the partition coefficient of Lu is nearly unity,because the Lu contents show little change among samples. Byusing the REE contents and partition coefficients, solidifiedpercentages for various stages of the magmatic process werecalculated; the percentage shows a good correlation with thesolidification index calculated from major chemical compositions.Some major compositions are also correlative with the solidifiedpercentage calculated from REE data. The Nakaoku basalts when plotted on a silica-alkali diagramshow a change of type from tholeiitic to alkali basalt duringthe solidifying process in depth. These petrochemical aspectsof the Nakaoku basaltic body are compatible with the resultsof experimental melting study at moderate pressures of about10 kb carried out by Green & Ringwood (1967). The spatialcoexistence of tholeiitic and alkali basalt in the Japaneselate Palaeozoic geosyncline found by Sugisaki & Tanaka (1971b)and disclosed here in the Nakaoku basalts, is not uncommon phenomenon.