2012 Volume 118 Issue 11 Pages 723-740
The Poroshiri ophiolite is exposed on the western side of the Hidaka metamorphic belt in central Hokkaido, Japan. Here, we present a study of metamorphism in the northern part of the ophiolite, in the Chiroro River area, to determine the relationship between metamorphism and large-scale folding and faulting.
Along the northern E–W transect, the ophiolite succession in the western area forms an isoclinal anticline, whereas the eastern area of the transect shows an open syncline. Along the southern E–W transect, the upper sequence of the ophiolite is exposed in the eastern part of the transect along a NW–SE trending thrust fault.
The metamorphic grades in these areas change sharply from greenschist to amphibolite facies at the western margin, then gently increases to 600 °C towards the central fault in the northern transect, whereas the metamorphic temperature to the east of this central fault is around 700 °C, indicating a temperature gap of about 100 °C at the central fault. The folded structure of the ophiolite along the northern transect is discordant with the metamorphic thermal structure, indicating that peak metamorphism occurred after folding. This also indicates that the central fault must have been active after peak metamorphism to cause the temperature gap recorded in the northern transect. The metamorphic temperature of the upper sequence of the ophiolite in the eastern section of the southern transect is about 600 °C, similar to the eastern margin of the western section of the ophiolite within the northern transect. This suggests that the upper sequence along the southern transect was transported from the west after peak metamorphism along the NW–SE trending thrust fault.
Amphibole porphyroclasts record pervasive prograde but no retrograde zoning, probably resulting from rapid exhumation after peak metamorphism. The metamorphism observed in the Poroshiri ophiolite is vastly different from ordinary regional metamorphism.
