Middle Precambrian and Older Apparent Age Values in Basement Gneisses of South Korea, and Relations with Southwest Japan
Rb-Sr whole-rock model age dates on Precambrian gneisses in South Korea have values that are greatly scattered owing to lower Mesozoic thermotectonic activity which caused an isotopic homogenization at about 180 m.y. The Rb-Sr data indicate, however, that the general original age of the province was within the range 2,000 ± 500 m.y. This belt is bordered on the southeast by a region invaded by Cretaceous granitic plutons that are approximately 100 m.y. old.

This sequence may be correlated with a similar sequence in southwest Japan, proceeding from north to south across the section: 1. A now-submerged source-region for the 2,000-m.y.-old pebbles in the Kamiaso conglomerate; 2. a 200-m.y. metamorphism of the Hida gneiss complex; and 3. a belt of Cretaceous and younger granite plutons. This matching yields an estimate of 2.6° lat as the total southward shift of southwest Japan from a former Precambrian position. This shift almost exactly matches the north-south opening of the Japan Basin, and suggests that the Kita-Yamato Bank and sea floor to the south of it is a submerged platform of 2,000-m.y.-old continental crust.