THE LATE MESOZOIC ACIDIC IGNEOUS ROCKS OF THE NORTHERN ASHIO MOUNTAINS

Abstract

The acidic rocks in northern Ashio mountains area are classified into following groups, based on their stuctural state and chemical composition of feldspars, and scale of the rock bodies. Matsuki and Sori granitic rocks are larger bodies, the lowest temperature type (430-480°C apparent temperature after Barth), the lowest content of Ab (9-14 wt%) in K-feldspars and low temperature plagioclase. Adamellites and granodiorite in Chuzenji acidic rocks are of intermediate scale, containing high temperature type (530-580°C) and high Ab content (17-24%), and low-intermediate plagioclase. Porphyries and welded tuffs contain the highest type (530-730°C) and highest Ab content (18-33%), and structurally high state plagioclase. Then, it is expected that adamellites and granodiorite are not related in origin to Matsuki and Sori granitic rocks, but porphyries and welded tuffs. This opinion agrees with field occurrences, ages of emplacement and others (Yanai, 1972, 1973). Igneous activities of Chuzenji acidic rocks was taken place as follows (Fig. 2), formation of the magma reservoir, eruption of rhyolite lava, consolidation of vents and increasing vapor pressure in magma reservoir, eruption of voluminous acidic pyroclastic flows, caldera collapse, intrusion of serverl porphyries, and intrusion of granitic rocks at the latest stage. Formation of caldera coincides well with the negative Bouguer anomaly.