High-alumina Basalt

Abstract

Aphyric basalts of central Honsyū and the Izu Islands, Japan, are classified into three types: tholeiite with low A12O3 and alkalis, alkali basalt with variable A12O3 and higher alkalis, and high-alumina basalt with higher A12O3 and intermediate alkalis. The tholeliite invariably yields normative quartz whereas the high-alumina basalt may yield a little normative quartz or normative olivine. The alkali basalt is undersaturated with SiO2. Mineralogically, the high-alumina basalt is intermediate between the tholeiite and alkali basalt. Thus the high-alumina basalt is transitional between the other two basalt types. The high-alumina basalt occurs in a zone extending between the tholeliite provinces and alkali province of the Japanese islands. There is a complete gradation from the tholeiite provinces, passing through the high-alumina basalt zone, to the alkali province. The high-alumina basalt is not a derivative of the tholeiite magma or of the alkali olivine basalt magma, but represents a primary magma. It is suggested that the high-alumina basalt magma is produced by partial melting of the mantle peridotite at depths intermediate between those of the tholeiite and alkali olivine basalt magma production, say at about 200 km. The high-alumina basalt is common in other orogenic belts of the world, but is absent.from the oceanic regions. It is rarely present in non-orogenic continental regions such as Manchuria and Skaergaard, Greenland. The fractionation trend of the high-alumina basalt magma is generally similar to that of the tholeiite magma, although higher concentration of alkalis in the middle stage may take place. From distribution of granitic xenoliths in volcanic rocks it is concluded that the limit of the sialic crust or ‘an-desite line’ passes a little to the south-east of the Pacific coast of Honsyu and not along the east of the Izu Islands as previously considered.