三畳紀美禰層群産火成岩・変成岩礫のK-Ar年代とその地質学的意義 (続・日本列島の基盤) [in Japanese] K-Ar ages of some igneous and metamorphic pebbles from Triassic Mine Group, West Chugoku, Southwest Japan, and their geologic meaning [in Japanese]
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K-Ar ages of two pebbles of volcanic rocks and four of metamorphic rocks (three hornfelses and one crystalline schist) in the conglomerates from Triassic Mine Group, were determined. They show 125-212 Ma in the metamorphic rocks, while 93-101 Ma in the volcanic ones. Taking into consideration for the alteration and weathering observed, 200 and 212 Ma is thought to be the most reliable ages. Some of the pelitic hornfels pebbles contain porphyroblastic andalusite and cordierite crystals indicating the formation at relatively low pressure and low to medium temperature. Constituent minerals of the crystalline schist investigated also have a feature suggesting that they were subjected to low-grade metamorphism. These data lead to the conception of the intimate genetical relation of the hornfelses with the hypabyssal or shallow type of plutonic rocks, althought hehypabyssal pebbles occur only in small amount in the Mine Group, Granite and gneiss pebbles are also accompanied, though not so plentiful. They are possibly unrelated to the thermal metamorphism of the hornfelses and older than the volcanic rocks, because they show the comparatively deep facies and are characterized by the marked cataclastic texture lacking in the hornfelses as well as volcanic pebbles.
- The Memoirs of the Geological Society of Japan.
The Memoirs of the Geological Society of Japan. (21), p117-125, 1982-04