Analysis of the Relationship Between the Gosaisyo and Takanuki Metamorphic Rocks in the Abukuma Metamorphic Terrane, Japan

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The Abukuma metamorphic rocks are lithologically and structurally divided into the Gosaisyo and Takanuki Series.1. It has revealed that these two units are in structurally discordant relation. Moreover, decollement blocks of the Takanuki Series occur in the Gosaisyo Series.2. The Rb-Sr whole-rock isochron are of the Jumonji plutonic mass, which is intruded along the boundary between the Gosaisyo and Takanuki Series, is 122+20 Ma. The age may be close to the time when the Gosaisyo Series thrust up onto the Takanuki Series.3. Sector-zoned garnet, which was considered to be peculiar to the Takanuki Series, has been revealed to occur in the pelitic-psammitic rocks of the Gosaisyo Series close to the Takanuki Series. This clearly shows that the high-temperature rapid loading and subsequent unloading are in common to these two geological units and that the loading was due to the overthrusting of the Gosaisyo Series onto the Takanuki Series.4. The homogenization of growth-zoned garnet porphyroblasts by the intracrystalline diffusion of elements suggests about 4 m. Y. As the duration of high-temperature conditions. This suggests 5 mm/year as the rate of subduction and subsequent uplift, because at least 3 kb loading and unloading took place over that time span.5. The mica K-Ar dating yields ages of 90-105 Ma. The lowerーgrade rocks tend to have older ages.6. The ion microprobe dating of zircon from meta-quartz porphyry, which is intruded into the Gosaisyo Series, yields about 120 Ma as the time of crystallization from magma and a few m. y. Younger age as the metamorphic time.Thus the two distinct metamorphic units in the Abukuma terrane are well-documented, and now we may be able to construct a new tectonic model in the arc-trench system.