Fig. 1 Distribution of the Sambagawa and Ryoke belts in SW Japan with the localities of Seba and Kotsu regions (MTL = Median Tectonic Line ; ITSZ = It

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1 Journal of Geography 113 (5) Derivation of Subduction-Stage Pressure-Temperature Path of Sambagawa Eclogite and its Tectonic Significance: Towards the Goal of a Comprehensive Petrology Mutsuki AOYA * Abstract In general, derivation of subduction-stage P-T (pressure-temperature) paths for high-p metamorphic rocks is difficult, because in most cases they are affected by subsequent metamorphism, during which the pre-peak P mineral assemblages are significantly modified. However, petrological studies during the last 10 years in the Sambagawa belt have recognized unusually well-constrained sections of the subduction-stage P-T path. In particular, derivation of the subduction-stage P-T path of the Kotsu glaucophane eclogite was backed-up by insights obtained from a combination of petrological and structural analyses. Application of a new thermal model shows that the derived subduction-stage P-T paths are most likely to have been generated in a setting where an active spreading ridge is close to being subducted. This tectonic interpretation suggests that exhumation of high-p/t metamorphic rocks in oceanic subduction zones is triggered by regional heating events such as ridge subduction. A review of recent studies on the Sambagawa belt indicates the significance of promoting a 'comprehensive petrology', which encompasses the full set of constraints provided by disciplines such as petrology, structural geology, geochronology and thermal modeling. Key words : metamorphic petrology, P-T path, structural geology, thermal modeling, ridge subduction, Sambagawa belt Research Fellow, Graduate School of Environmental Studies, Nagoya University. present adress : Institute of Geoscience National Institute of Advanced Industrial Science and Technology (AIST)

2 Fig. 1 Distribution of the Sambagawa and Ryoke belts in SW Japan with the localities of Seba and Kotsu regions (MTL = Median Tectonic Line ; ITSZ = Itoigawa-Shizuoka Tectonic Line).

3 Fig. 2 Outcrop sketch of fold overprint pattern observed in the Seba eclogite unit (taken from Aoya, 2002). Three distinct stages of deformation (named as Do, DA and DB in Aoya and Wallis (1999)) can be recognized.

6 Fig. 3 (a), (b) Back-scattered electron images of amphibole inclusions within garnet in Kotsu glaucophane eclogite (Aoya et al., 2003). (c) Chemical compositions of amphibole inclusions on Si-c31 Na diagram. Amphibole inclusions are classified into 3 types focusing on their relationship to cracks developed in the host garnet (Aoya et al., 2003).

7 Fig. 4 Compositional change of clinopyroxene inclusions within garnet from Kotsu glaucophane eclogite (left figure). The horizontal axis is taken to represent the Mg content of garnet adjacent to clinopyroxene inclusions. In this way the compositional change from the core to the rim can be recognized because the Mg content of the host garnet monotonously increases from the core to the rim (right figure). Both figures are modified from Aoya et al. (2003).

8 Fig. 5 Compilation of subduction-stage P-T paths of the Sambagawa metamorphic rocks. Metamorphic - facies boundaries are taken from Peacock (1993).

9 Fig. 6 Comparison between the Sambagawa subduction - stage P - T paths (shaded arrows) and model P-T paths which generate during steady-state subduction of young slab. Fig. 7 Two factors that can cause curvature of subduction-stage P-T paths. (a) Geotherms of oceanic lithosphere calculated by the half-space diffusion model (Turcotte and Schubert, 1982). (b) Schematic illustration showing progressive younging of the subducting slab in the ridgeapproaching model (upper figure). An example of the steady-state model is also presented for comparison (lower figure).

10 Fig. 8 (a) Framework of calculations incorporating the oceanic-ridge approaching to the trench after initiation of younging of subducting slab. (b) An example of the calculation result (Aoya et al., 2003). The Sambagawa subductionstage P-T paths are shown by shaded arrows.

12 Fig. 9 Comparison between petrologically derived P - T conditions for high - P/T metamorphic belts worldwide (Hacker, 1996 ; Okay, 2002) and modeled equilibrated thermal conditions along subduction boundaries (Peacock et al., 1994 ; Iwamori, 2000).

Aoya, M. (2001) : P-T-D path of eclogite from the Sambagawa belt deduced from combination of petrological and microstructural analyses. J. Petrology, 42, 1225-1248. Aoya, M.

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地質調査研究報告/Bulletin of the Geological Survey of Japan

地質調査研究報告/Bulletin of the Geological Survey of Japan Takayuki Uchino and Makoto Kawamura (2010) Glaucophane found from meta-basalt in the Nedamo Terrane, Northeast Japan, and its geologic significance. Bull. Geol. Surv. Japan, vol. 61 (11/12), p. 445-452,