Blackwell Science, LtdOxford, UKIARThe Island Arc1038-48712002 Blackwell Science Asia Pty Ltd.
112June 2002
358
Thermal evolution of the Ryoke Metamorphic Belt
T. Okudaira
10.1046/j.1038-4871.2001.00358.x
Comment and Reply146148BEES SGML

The Island Arc (2002) 11, 146–148

Comment and Reply
Thermal evolution of the Ryoke metamorphic belt, southwestern Japan:
Tectonic and numerical modeling: Reply
TAKAMOTO OKUDAIRA*
Department of Geosciences, Osaka City University, Osaka 558-8585, Japan (email: oku@sci.osaka-cu.ac.jp)

INTRODUCTION
Concerning the thermal model for the lowpressure high-temperature Ryoke Metamorphic
Belt, Yasutaka et al. (2002) suggest that thethermal effects of the emplacement of a thicksubhorizontal granitic sill (Gamano granodiorite
occurred in the Yanai district as one of the Older
Ryoke granitoids) are large only for the rocks
located near to the intrusion and are much lessimportant at shallow crustal depth. Furthermore,they emphasize that the main cause of the Ryoke
metamorphism is the presence of a very highgeothermal gradient at the time of low-pressure
metamorphism. I appreciate the comments ofthese colleagues and take this opportunity toclarify some of my thoughts and arguments onthe topics.
Their comments can be summarized as reflecting concern for and debate over: (i) the evaluationof the peak metamorphic temperature of thecordierite zone (potassium feldspar (K-feldspar)–cordierite zone of Ikeda 1998); (ii) the discordance
between the model results and the observed metamorphic conditions; and (iii) the reliability of theinitial and boundary conditions of the numericalmodel of Okudaira (1996).

METAMORPHIC CONDITIONS OF THE
CORDIERITE ZONE
The metapelitic mineral assemblage of the cordierite zone is as follows:*Correspondence.
Received 15 October 2001. Accepted for publication 18 October 2001.
© 2002 Blackwell Publishing Asia Pty Ltd.

biotite+ muscovite+K-feldspar+cordierite± (andalusite or sillimanite or both)(1)+ quartz+ plagioclase

Andalusite occurs in the northern part of thecordierite zone. Sillimanite occurs in close spatial
association with the Younger Ryoke granitoids(Nureki 1974) and with the granitic rocks in theTengatake–Nagano migmatite zone (Okudairaet al. 1993). Muscovite grains are recognized astwo types: matrix grains and inclusions within
K-feldspar or cordierite porphyroblasts. The
matrix muscovite was stable in the cordierite zone,
because the matrix muscovite grains showed preferential alignment parallel to the tectonic foliation
formed at the peak metamorphism. Therefore,
the pressure–temperature (P–T) field of most of
the cordierite zone did not exist beyond the curveof the breakdown reaction of muscovite + quartz.
Based on such microstructure, Okudaira (1996)
suggested the porphyroblast formation reaction:
biotite+ muscovite+quartz= cordierite+K-feldspar+H 2 O(2)This is a divariant reaction and its location inthe P–T space depends on mineral chemistry(Pattison 1989). The ratios of XMg [= Mg/(Mg + Fe)] of biotite and cordierite in the cordierite zone are 0.38–0.45 and 0.55–0.57, respectively.
Using the computer program THERMOCALC(version 3.1; Powell & Holland 1988), the divariant
curve of this reaction for such mineral chemistry
passes the temperature of ∼610°C at 300 MPa,
when the activity of the H2O component (aH2O) ofcoexisting fluid is unity. The aH2O of coexisting
fluid in the metamorphic rocks of the cordieritezone has not been estimated but may be lessthan unity, as the aH2O in the rocks of the garnet–

Thermal evolution of the Ryoke Metamorphic Belt 147

cordierite zone was considered to be much lessthan unity (Okudaira et al. 1993). This is also
supported by the presence of graphite grains asaccessory minerals in the rocks of the cordieritezone. Under such a low aH2O condition, the location of the dehydration reactions shifts to a lower
temperature region in the P–T space. Based onthese facts, I conclude that the peak metamorphic temperature of the cordierite zone did notreach up to 640°C regionally, the value suggested
by Yasutaka et al. (2002).

DISCORDANCE BETWEEN THE MODEL RESULTS
AND THE OBSERVED METAMORPHIC CONDITIONSAs the peak temperature of the cordierite zone islower than that proposed by Yasutaka et al. (2002),
the peak temperature calculated by Okudaira(1996) is not much lower than the observed temperature. In contrast, in order to simplify thethermal model in Okudaira (1996), the emplacement of single intrusion of the Older Ryoke
granitoid magma was considered. In the cordierite zone there are small granitic sills of theTengatake–Nagano migmatite zone. Their intrusions are contemporaneous with that of the Oldergranitoids (Higashimoto et al. 1983; Okudaira
et al. 1993). Multiple intrusions of the granitic
magma in the study area during the time ofregional low-pressure metamorphism can reasonably be postulated, so that the simulated peaktemperature of the cordierite zone might increase.
However, as pointed out by Wells (1980) andRothstein and Hoisch (1994), there are many
uncertainties in the multiple intrusion model, suchas the time intervals of the intrusions. Theseparameters strongly affect the model’s results.
Because of this, I will not incorporate these parameters into my model until reliable petrologicalconstrains on the granitic rocks in the Tengatake–
Nagano migmatite zone have been definedadequately. Therefore, Okudaira (1996) calculated
the minimum thermal effects due to the intrusionof the Older granitoid magmas at the time ofregional low-pressure metamorphism.

INITIAL AND BOUNDARY CONDITIONS:CAUSE OF HIGH GEOTHERMAL GRADIENTFirst, it is necessary to correct the sentence concerning the boundary condition in the model ofOkudaira (1996). The sentence, ‘In this study, theboundary conditions are constant temperature(0°C) at z = 0 and constant mantle heat flux, qm, atthe bottom of the lithosphere’ (p. 381), should bechanged to ‘In this study, the boundary conditions
are constant temperature (0°C) at z = 0 and constant temperature (945°C) at the bottom of thelithosphere’. This correction, however, does notaffect the results of the 1996 model strongly, as
discussed by Yasutaka et al. (2002), and this point
has been modified in the revised model (Okudaira
et al. 2001).
The initial geothermal gradient of ∼30°C/km
was assumed in the model of Okudaira (1996),
based on the mean geothermal gradient of anactive continental margin or island arc (Sugimura& Uyeda 1973) and on the results of the semiquantitative petrological analysis for metapelite xenoliths in the Tengatake–Nagano migmatite zone
(Okudaira et al. 1993). As suggested by Yasutaka
et al. (2002), the value of the geothermal gradientis outside the bounds of normally inferred values
in continental regions. Thus, it is necessary for
us to define the thermal structure of the crust
before the intrusion of the Older Ryoke granitoids
accurately. One possible cause for the high geothermal gradient of ∼30°C/km is that during themid-Cretaceous, some young oceanic plates oractive spreading ridge related to the activity ofa Pacific superplume was probably subducted at
the Eurasian continental margin (Larson 1991;
Kinoshita 1995). The subduction of a hot young
oceanic plate and an active spreading ridge couldhave increased the geothermal gradient andresulted in the generation of granitic magma at
a continental margin or island arc (Iwamori 2000;
T. Okudaira, unpubl. data, 2000).

CONCLUSION
The results of the simple thermal model of
Okudaira (1996) are roughly consistent with thepetrologically observed thermal structure of themetamorphic sequence of the Yanai district, where
the direct cause of the regional low-pressure
metamorphism is attributed to the thermal effectsof the intrusion of the Older Ryoke granitoidmagmas. In the Ryoke Metamorphic Belt a hugeamount of the Older granitoids have been recognized. Although the thermal model of Okudaira(1996) was applied only to the Yanai district, I
believe that the magma-intrusion model can beapplied appropriately to other districts.

148 T. Okudaira

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