中部地方水窪地域の中央構造線沿いの領家変成岩類からのカリ長石の三斜度

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  • チュウブ チホウ ミサクボ チイキ ノ チュウオウ コウゾウセン ゾイ ノ リ
  • The obliquities of K-feldspars from Ryoke metamorphic rocks along the Median Tectonic Line in the Misakubo area, central Japan

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The Ryoke metamorphic rocks along the Mechan Tectonic Line in the Misakubo area connsist of fine-grained gneisses, migmatitic granites and mylonitic rocks. The fme-grained gneisses are pelitic, psamitic and silicious. The migmatitic granites are leucocratic, and occur as veined or dykelike forms ranging from a few centimeters to several tens of centimeter in the fine-grained gneisses. These rocks were metamorphosed under the moderate to higher temperature part of the amnphibolite facies. The mylonitic rocks are close to the Mechan Tectonic Line and consist of mylonitized finegrained gneisses, K-feldspar porphyroblast mylonite gneisses and mylonitized diorite or quartz diorite. We present the obliquities of 113 K-feldspars from the migmatitic granites and K-feldspar porphyroblast mylonite gneisses. From the obliquities of K-feldspars , two events can be recognized in the metamorphic process : in the first event, monoclinic K-feldspars (types I and II ) were formed by regional metamorphism related to migmatization, and triclinic K-feldspars (types IV and V with mean Δ value of 0.83) formed successively by retrogressive dislocation metamorphism closely related with the formation of the Mechan Tectonic Line. The incompleteness of inversion of K-feldspars in the latter might suggest faster upheaval of the metamorphic rocks in the area in comparison with those in the northeastern part of the Hida metamorphic belt. The obliquity study of K-feldspar grains from the K-feldspar porphyroblast mylonite gneisses shows that different grains within one rock specimen have different obliquities. The detailed investigations of 3-5 mm size K-feldspar for a large single crystal reveal that the inversion of monoclinic to triclinic K-feldspar occurs at random in a single grain unlike the suggestion by TOURET (1967) that in a single grain it takes place systematically from the core towards the periphery of the porphyroblast.

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