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[SMP43-04] Did the Ryoke belt form beneath the Cretaceous volcanic arc?
Keywords:Ryoke belt, volcanic arc, paired metamorphic belt
The morphology and tectonic location of the SSP is very similar to that of the Great Valley forearc basin in the West Coast of north America. The essence of my new hypothesis is that the SSP formed as accumulated accretionary complex at the tectonic location equivalent to the forearc basin. That is, the Ryoke belt formed in situ at the present position ( the past forearc basin). The protolith of the Ryoke metamorphic rocks are known as Jurassic accretionary complexes. During the formation of Jurassic accretionary prism, the thickening of accretionary complex was enhanced by out-of-sequence thrust (Kimura, 1998). The thickened complex developed laterally towards the arc from the subduction zone to make nappes on the arc. At the forearc, the thickened complex caused subsidence of the forearc region with a flexure of middle crust, finally making the SSP. The sediments within the SSP was then heated by radiogenic heat and also by basic magmas intruded into the lower part of the SSP, leading to the partial melting of the SSP sediments to form granitic magams. Thus the sediments in the SSP were converted into granites and low P metamorphic rocks, which are now recognized as the Ryoke belt. The fact that the Ryoke granites belong to I-type granites may contradict the above hypothesis, however, the ilmenite series nature of the Ryoke granites represents reduced condition for the genesis of Ryoke granites, which strongly suggests the partial melting or assimilation of sediments.
The Ryoke metamorphic rocks represents the burial depth of about 15 km (e.g. Ikeda, 2004), therefore, the original depth of the SSP may have reached 35 km. Such thick sediments in the SSP may pressurized the structurally lower Sambgawa belt to make it squeezed out to the surface. Thus the formation of the SSP may be a cause of the exhumation of the Sambagawa belt. The thick SSP may have uplifted due to buoyancy to keep isostacy, then the upper portion of the SSP may have been eroded out to crop the lower portion, consisting of metamorphic rocks and plutonic rocks, out to the surface.