Metamorphic P–T evolution of the Gotsu blueschists from the Suo metamorphic belt in SW Japan: Implications for tectonic correlation with the Heilongjiang Complex, NE China

Abstract

In the Gotsu area of the c. 200 Ma high-P/T Suo metamorphic belt in the Inner Zone of southwest Japan, blueschists occur as lenses or layers within pelitic schists. Prograde, peak, and retrograde stages are distinguished in the blueschists, and the prograde and the peak metamorphic conditions are determined using pseudosection modelling in the NCKFMASHO system. The prograde metamorphic stage is defined by inclusions in porphyroblastic epidote and glaucophane, such as phengite, chlorite, albite, epidote and glaucophane/winchite, and the estimated metamorphic conditions are <325 °C and < 4–5 kbar at the boundary between the glaucophane schist facies and the greenschist facies. The peak metamorphic stage is well-defined by the schistosity-forming minerals, i.e. epidote, glaucophanic amphibole, phengite, and chlorite, suggesting the glaucophane schist facies conditions of 475–500 °C and 14–16 kbar. Actinolite/magnesiohornblende, chlorite, and albite replacing the peak stage minerals suggest the retrograde metamorphism into the greenschist facies. The metamorphic facies series of the Suo belt is defined by pumpellyite–actinolite facies to epidote–blueschist facies, and it has a relatively lower-P/T compared with the c. 300 Ma Renge belt in the Inner Zone of southwest Japan, which is defined by a sequence of lawsonite–blueschist facies to glaucophane–eclogite facies. The P– $$ {M}_{{\mathrm{H}}_2\mathrm{O}} $$ pseudosection and water isopleth show that the rocks were dehydrated during the initial stage of the exhumation and remained in water-saturated conditions. Similarities of the detrital zircon and peak metamorphic ages of the blueschists from the Suo metamorphic belt in southwest Japan and the Heilongjiang Complex in northeast China suggest that both metamorphic belts were probably formed in the same Paleo-Pacific subduction system in the Late Triassic to Jurassic period.