南東阿武くま山地，水石山超苦鉄質‐苦鉄質複合深成岩体 特に不透明鉱物について

Abstract

An ultramafic-mafic plutonic complex at Mt. izuishi can be divided into the following three emplaced units. Unit I: Dunite, wehrlite and olivine clinopyroxenite (serpentinized). Unit II: Hornblende gabbro and clinopyroxene hornblende gabbro with or without orthopyroxene. Unit III: Hornblende gabbro. Unit IV: Biotite-hornblende diorite.Some small-scale iron ore deposits are embedded mainly in Unit I. Bulk chemical analyses are presented for twenty-one rocks and two iron ores (magnetitites). All the constituents of the complex have been thermally metamorphosed owing to the emplacement of Cretaceous granitic rocks, and therefore their original petrographic chracteristics are not well preserved. However, a most plausible explanation of the origin of the com-plex is a hypothesis of successive injections differentiating from a basaltic magma. Microprobe data are presented for olivines and oxide minerals in uitramafic rocks and ironores, and for oxide and sulfide minerals in mafic rocks. At the contact with the granitic mass the equilibrium temperature of contact metamorphism, based on the olivine-spinel geothermometer, can be assumed to be 650-700°C. At a distance from the contact, the compositions of coexisting magnetite and ilmentie in representative mafic rocks indicate consistently temperature and oxygen fugacity of about 550°C and 10-20 bars. The magnetite+ilmenite+pyrite+pyrrhotite and magnetite+ilmenite+pyrite assemblages in mafic rocks also give approximate values of fo2=10-19 and fs2=10-3 bars at 550°C for metamorphic conditions.