The Island Arc (2002) 11, 142–145 Blackwell Science, LtdOxford, UK IARThe Island Arc1038-48712002 Blackwell Publishing Asia Pty Ltd. 112June 2002 357 Thermal modeling of Ryoke metamorphism Y. Yasutaka et al. 10.1046/j.1038-4871.2001.00357.x Comment142145BEES SGML *Correspondence. Received 17 September 2001. Accepted for publication 18 October 2001. © 2002 Blackwell Publishing Asia Pty Ltd. Comment and Reply Thermal evolution of the Ryoke metamorphic belt, southwestern Japan: Tectonic and numerical modeling: Comment YOKO YASUTAKA, 1, * SIMON WALLIS 1 AND TAKESHI IKEDA 2 1 Department of Earth and Planetary Sciences, Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8602, Japan (email: yasutak@mbox.media.nagoya-u.ac.jp) and 2 Department of Earth and Planetary Sciences, Graduate School of Science, Kyushu University, Fukuoka 812-8581, Japan INTRODUCTION Thermal modeling is an indispensable part of understanding the evolution of metamorphic belts and has been used to help understand the whole range of different types of metamorphic environ- ments (England & Thompson 1984; De Yoreo et al. 1991; Peacock 1996). The processes responsible for regional high-temperature low-pressure metamorphism are particularly elusive and this topic is still the focus of much debate. The Ryoke Belt of south-western Japan is one of the classic high-temperature low-pressure metamorphic belts formed in an island arc setting (Miyashiro 1973); there has, however, been surprisingly little attempt to use thermal modeling to investigate the evolution of this region. One exception is Okudaira (1996). In this work, Okudaira proposes that the Ryoke metamorphism in the Yanai district can be accounted for by the emplacement of a thick sub-horizontal sill, the Gamano granodiorite. Okudaira’s study makes an important contribution to the discussion about the origin of the Ryoke Belt. However, the present commentary makes the following two points: (i) recent petrological data shows that the thermal structure of the Yanai district is incompatible with the Okudaira model; and (ii) even adaptations to the general model cannot account for this dis- crepancy. These results suggest alternative models need to be considered to understand the causes of the Ryoke metamorphism. GENERAL POINTS The first point to make is that the discussion of Okudaira (1996) is only applicable to the Yanai district. Intrusions similar to the Gamano grano- diorite would have to be present throughout the Ryoke Belt for this to be a viable cause of the regional Ryoke metamorphism. Second, Okudaira emphasizes the thermal importance of the grano- diorite sill; however, close inspection of the model shows this intrusion has only secondary localized importance. The main cause of the Ryoke meta- morphism in Okudaira’s model is not the intrusion but the presence of a very high background geo- thermal gradient. The main concern of the present commentary is to show that Okudaira’s model is incompatible with petrological studies of the Yanai district. PRESSURE–TEMPERATURE ESTIMATES AND PRESSURE–TEMPERATURE PATH OF THE POTASSIUM FELDSPAR–CORDIERITE ZONE Metamorphic petrology can be used to estimate pressure–temperature (P–T) conditions of forma- tion and, in some cases, P–T paths of individual rocks. This information is one of the primary tests of any thermal model. Okudaira (1996) com- pares the calculated temperature–time profiles at several depths with the temperature estimates for the corresponding metamorphic zones and concludes that they show a close match. However, geothermobarometers do not always give reliable estimates of peak conditions, particularly in high- temperature environments, because of the loss of information by subsequent diffusion. In these cases, more reliable estimates of peak P–T condi- tions may be made using mineral assemblages.