北部北上山地，田野畑深成複合岩体接触帯の変成作用 白亜紀深成岩類マグマ溜りの深度の解明に向けて

Abstract

The Tanohata plutonic complex, northern Kitakami Massif, Northeast Japan, is an early Cretaceous zoned pluton mainly composed of hornblende-biotite granodiorite. The complex is in a wall contact relation in its western margin and in a roof contact relation in the northern part of the complex, with surrounding sedimentary rocks of the Jurassic accretionary complex. The width of thermal aureole is about 5-7 km in the “western marginal area”. Sedimentary rocks in the “northern roof pendant area”, with an extension about 10×8 km, are thoroughly metamorphosed extensively. The aureole is divided into four progressive mineral zones, namely, the biotite, cordierite, cordierite-K-feldspar and garnet-cordierite zones, based on the paragenetic changes of pelitic rocks in the western marginal area. In the northern roof pendant area, only the cordierite-K-feldspar and garnet-cordierite zones are recognized, and the width of each zone is several times larger than that in the western marginal area. The distinct development of high-temperature zones in the northern roof pendant area indicates a selective upward heat transfer during cooling of a granodioritic magma chamber of the Tanohata complex. Conditions of metamorphism are estimated from the biotite-muscovite-cordierite-sillimanite-K-feldspar equilibrium at the sillimanite isograd, the Fe-cordierite-almandine-sillimanite equilibrium in the garnet-cordierite zone and the garnet-biotite geothermometry. The estimated temperatures are at about 560°C at the sillimanite isograd, and in a range of 650-750°C in the garnet-cordierite zone, at pressures of 2-3 kbar. The maximum temperatures of the Tanohata aureole are similar to those of the highest temperature zone of the Tono contact metamorphic aureole in the southern Kitakami Massif. The result implies that the thermal properties of magma and the surrounding wall rocks are quite similar between the Tanohata and Tono plutons. The estimated pressures of contact metamorphism are also quite similar between Tanohata and Tono aureoles, indicating that these two granodioritic bodies both emplaced and solidified in a deep part of upper crust (7-11 km) during early Cretaceous magmatism in the Kitakami Massif.