The youngest blueschist belt in SW Japan: implication for the exhumation of the Cretaceous Sanbagawa high-P/T metamorphic belt

摘要

The tectonic evolution of the Northern Shimanto belt, central Shikoku, Japan, was examined based on petrological and geochronological studies in the Oboke area, where mafic schists of the Kawaguchi Formation contain sodic amphibole (magnesioriebeckite). The peak P-T conditions of metamorphism are estimated as 4-4.5 kbar (15-17 km depth), and 240-270 degrees C based on available phase equilibria and sodic amphibole compositions. These metamorphic conditions are transitional between blueschist, greenschist and pumpellyite-actinolite facies. Phengite K-Ar ages of 64.8 +/- 1.4 and 64.4 +/- 1.4 Ma were determined for the mafic schists, and 65.0 +/- 1.4, 61.4 +/- 1.3 and 63.6 +/- 1.4 Ma for the pelitic schists. The metamorphic temperatures in the Oboke area are below the closure temperature of the K-Ar phengite system, so the K-Ar ages date the metamorphic peak in the Northern Shimanto belt. In the broad sense of the definition of blueschist facies, the highest-grade part of the Northern Shimanto belt belongs to the blueschist facies. Our study and those of others identify the following constraints on the possible mechanism that led to the exhumation of the overlying Sanbagawa belt: (i) the Sanbagawa belt is a thin tectonic slice with a structural thickness of 3-4 km; (ii) within the belt, metamorphic conditions varied from 5 to 25 kbar, and 300 to 800 degrees C, with the grade of metamorphism decreasing symmetrically upward and downward from a structurally intermediate position; and (iii) the Sanbagawa metamorphic rocks were exhumed from similar to 60 km depth and emplaced onto the Northern Shimanto metamorphic rocks at 15-17 km depth and 240-270 degrees C. Integration of these results with those of previous geological studies for the Sanbagawa belt suggests that the most probable exhumation mechanism is wedge extrusion.