北上山地宮守超苦鉄質岩体の斑れい岩類に含まれる角閃石のＫ‐Ａｒ年代

Abstract

K-Ar ages were determined on hornblende in 4 gabbroic rocks, one amphibolite, and one granodiorite porphyry from the Miyamori ultramafic complex of the Kitakami Mountains, Northeast Japan. The obtained K-Ar ages for gabbroic rocks range from 421 to 484 Ma. Most of them are Ordovician (445-484 Ma), excepting the age for one clinopyroxene hornblendite, which is middle Silurian (421 Ma). These K-Ar ages indicate that the Miyamori complex began to cool through the emplacement from the upper mantle to the shallower crustal level along the “Hayachine Tectonic Belt” in or before Ordovician. The amphibolite from a fenster in the complex gives a K-Ar age of 369 Ma. It contains considerable amount of secondary amphibole, which was probably formed by thermal effect of the Cretaceous Hitokabe granodiorite mass. Thus, the metamorphic age of the amphibolite is interpreted to be before middle Devonian. One hornblende gabbro from the “Hyachine Tectonic Belt” also gives an Ordovician age of 453 Ma. These fairly old K-Ar ages and arc-related petrological characteristics of the Miyamori complex suggest that the “Hayachine Tectonic Belt” dividing the North and South Kitakami Mountains was originated already in Ordovician as uplifted ophiolitic complexes formed at a consuming plate margin. The granodiorite porphyry gives a whole-rock K-Ar age of 112 Ma, suggesting that it is related to the early Cretaceous acidic volcanic rocks distributed to the south of the Miyamori complex.