Magma types of volcanic rocks and crustal history in the Japanese pre-Cenozoic geosynclines

Abstract

Magma types and temporal variations of the volcanic rocks erupted in the Japanese pre-Cenozoic geosynclines are examined using the petrochemical data of almost 550 new analyses. It is assumed that the alteration of these rocks was isochemical in nature. The characteristic rock type is basalt, but some variations of rock type are observed in certain areas: 1. (1) In western Japan, the parental magmas of Permian age are mostly inferred to be alkali basalt series and high-alkali tholeiite rock series. The rocks of low-alkali tholeiite are few if any. High-alumina basalts are observed in limited localities. 2. (2) The rocks of the Mikabu metamorphic belt, probably products of Permian submarine volcanism, lack any appreciable high-alumina basalt. These petrochemical features support the conclusion that the geosynclinal basalt from western Japan is much closer to the oceanic basalt than are the Japanese Cenozoic volcanics (Sugisaki et al., 1970). 3. (3) The changes of rock characteristics in the southern Kitakami area with time show that the magma has progressively changed since the Devonian from alkali basalt, high-alkali tholeiite and low alkali tholeiite, through high-alumina basalt and high-alkali tholeiite rock series finally to low-alkali tholeiite rock series. The several magma types show significant differences in both major and minor elements: 1. (1) the Japanese Paleozoic geosynclinal tholeiite is the petrochemical equivalent of the abyssal tholeiite of the Mid-Atlantic Ridge and East Pacific Rise, which was studied by several authors: Engel et al. (1965), Gast (1968), Hart (1969), and Miyashiro et al. (1970). 2. (2) the geosynclinal tholeiite was differentiated in a manner similar to that of the abyssal tholeiite. The weathering effect of sea-water was not significant in the geosynclinal basalts, implying that the volcanic rocks were confined under thick sediments soon after the eruption. These lithologic changes in the Japanese pre-Cenozoic geosynclines may record: 1. (1) the genesis of the geosynclines. 2. (2) the variations in crustal thicknesses in the early stage of the geosynclines; and (3) the crustal evolution (continental and oceanic) of the Japanese Islands.