ABSTRACT

INTRODUCTION
Recent CHIME monazite geochronology revealed that the Ryoke gneiss in the Mikawa area attained to the metamorphic grade of monazite formation (lower amphibolite facies) at ca. 100 Ma (Suzuki et al, 1994a,b). The Ryoke gneiss underwent subsequently the peak metamorphism and the emplacement of granitic rocks.  Identification of granitic plutons and their intrusive order had been established in early 1970's on the basis of the field relations, petro- graphic features and morphology of accessory zircon (e.g. Sakai et al., 1960; Ryoke Research Group, 1972).  The oldest intrusive in the Mikawa area is the Kamihara Tonalite which is followed successively by the Tenryukyo Grano- diorite, Kiyosaki Granodiorite, Shinshiro Tonalite, Mitsuhashi Granodiorite, Inagawa Granodiorite and Busetsu Granite. Unlike the KamiharaTonalite, Tenryukyo Granodiorite and Kiyosaki Granodiorite, the Shinshiro Tonalite forms a discordant pluton with a contact aureole (Asami and Hoshino, 1980). This indicates the Shinshiro Tonalite to be a post-tectonic intrusive. Its em¬ placement age, therefor, may provide a constraint on the duration of the Ryoke metamorphism.  Previous K-Ar dating of the Shinshiro Tonalite yielded 73.3±2.9 Ma hornblende age and 68.0±2.1 Ma biotite age (Uchiumi et al., 1990). These ages, however, are not considered to represent the emplacement time, because they are younger than the subsequent Mitsuhashi Granodiorite (ca. 84 Ma CHIME monazite ages, Suzuki et al., 1994a,b) and the Busetsu Granite (82.5±3.9 Ma Rb-Sr whole-rock isochron ages, Shibata and Ishihara, 1979; ca. 77 Ma CHIME monazite ages, Suzuki et al.,1994a).  To know the emplacement age by an alternative method, we have made CHIME age deter- minations of monazites from the leucocratic varieties of the Shinshiro Tonalite. Although our CHIME geochronological research on the Shinshiro Tonalite is in progress, we here present the dating results and discuss their meaning.
OUTLINE OF THE SHINSHIRO TONALITE
The Shinshiro Tonalite forms a stock-like body with an area of 14 x 6 km on the north of the Median Tectonic Line (Fig. 1).  It intruded discordantly in to the Ryoke metamorphic rocks which show a general trend of NE-SW direction and consist mainly of pelitic, psammitic and siliceous gneisses. Asami and Hoshino (1980) divided the gneiss on the western side of the p lu t on in to three m ineral zones; and a lu s it e zone, silli manite zone and sillimanite-orthoclase zone southward.  Atsumi (1984) reported a paragenesis of andalusite-sillimanite-muscovite from the northeast of the pluton. The plu- ton forms a contact aureole of 2 km in width and yields the andalusite-ortho- clase assemblage overprinted on the regional assemblages (Asami and Ho- shino, 1980).  The assemblage of the contact metamorphism is stable on lower pressure conditions than those for the regional assemblage. This suggests that the Shinshiro Tonalite emplaced after a significant uplift of the Ryoke belt. The Shinshiro Tonalite is intruded by the Busetsu Granite (Fig. 1). The pluton of the Shinshiro Tonalite comprises coarse-grained hornblende¬ biotite tonalite in the center and biotite tonalite to granite in the margin (Ohmoto, 1985; see Fig. 1).  The central tonalite, characterized by the presence of prismatic hornblende of 8-11 mm long, grades into the marginal biotite tonalite, biotite granodiorite and muscovite-bearing biotite granite within a narrow zone of several dozens of meters.  Some marginal facies contain garnet as accessory phase.
SAMPLE DESCRIPTION
We examined a number of samples, and found monazite from three sam¬ ples from the northern margin of the pluton. The sample localities are given in Fig. 1. Sample Gs17 (137°28'35"E, 3457'40"N), medium-grained biotite tonalite, was collected from a dike extended from the margin of the pluton at the granite¬ gneiss boundary.  It consistts mainly of quartz (30%), plagioclase (60%), potash feldspar (0.1%) and biotite (9.4%).  Plagioclase forms euhedral grains of 6-7 mm in length, and shows marked compositional zoning.  Quartz occurs as anhedral grains and as interstitials. Sample Gs20 (13728'25"E, 3457'52"N) was collected from a ca. 15 m thick dike intruding into psammitic gneiss. The sample, medium-grained garnet- bearing two-mica adamellite, consists mainly of quartz (39%), plagioclase (24%), potash feldspar (23%), biotite (7.9%) and muscovite (4.7%).  Potash feld- spar, more than 1 cm in size, shows a poikilitic texture and includes euhedral grains of plagioclase, biotite and muscovite. Sample Gs31 (13728'13"E, 3458'21"N), medium-grained garnet-bearing bio- tite tonalite, was collected from the immediate contact with psammitic gneiss. It consists of quartz (32%), plagioclase (60%), potash feldspar (1.2%), biotite (6.6%), garnet (trace) and muscovite (trace).  Plagioclase, ca. 2 mmin size, displays a marked compositional zoning and its calcic center is usually seri¬ citized. Garnet forms euhedral grains of smaller than 1 mmin diameter. All samples contain monazite, zircon, apatite, pyrrhotite and pyrite as accessories. Monazite grains are yellow-colored and euhedral, and range in size from 0.1 to 0.4 mm.  They differ in grain shape and size from monazites in adjacent Ryoke gneiss, which are largely anhedral and smaller than 0.15 mm in size. We consider that monazite grains in the samples crystallized from magmatic melt.
T. Morishita and K. Suzuki
Electron microprobe analyses of ThO 2 , UO 2 and PbO in monazites from samples of the Shinshiro Tonalite.  ThO 2 *: sum of the measured ThO 2 and ThO 2 equivalent of the measured UO 2 .

RESULTS
Crushing of samples, separation of monazite, preparation of polished thin sections were done through the method described by Suzukiand Adachi (1991a,b).  Monazite were analyzed on a JEOL JXA-733 electron microprobe equipped with three wavelength dispersive-type spectrometers.  Intensities of ThMα, UMβ and PbMα lines only were measured. The background was measured at two optimum positions on both sides of each line peak position. The X-ray intensities for both the line peak and two backgrounds were integrated for 300s.  The measurement was repeated three times, and the arithmetic average was taken. After the correction of intensity data with an average composition of monazite, the CHIME ages were calculated through the method proposed by Suzuki and Adachi (1991a,b). The microprobe analyses of ThO 2 , UO 2 and PbO together with apparent age and ThO 2* are listed in Table 1.  Plots of PbO vs. ThO 2* are given in Figs. 2a, 2b and 2c.  A total of 51 analyses on 22 monazite grains from sample Gs17 are regressed with an isochron of 86.0±4.7 Ma (Fig. 2a).  monazite grains from sample Gs20 show a wide compositional variation; the ThO 2* content ranges from 5.8 to 11.1 %. A total of 44 analyses on 14 grains yield a well-defined isochron of 85.2±3.3 Ma (Fig. 2b).  Monazite grains from sample Gs31 are low in the ThO 2* content than those from the rest samples. The 22 data points for 8 grains can be combined with an isochron of 85.5±5.5 Ma (Fig. 2c).  The CHIME monazite ages for the three samples coincide well with one another. Since the blocking temperature of Pb-diffusion in monazite (650-700℃) is close to solidus temperatures of granitic magmas (Suzuki et al., 1994a), we consider the ca. 86 Ma CHIME ages as the time for the emplacement of the Shinshiro Tonalite.
CONCLUDING REMARKS
The present CHIME monazite ages are placed in the context of the field evidence in the Mikawa area (Fig. 3), together with the previously reported CHIME ages for the Ryoke gneiss and granitoids (Suzuki et al., 1994a,b) in the area.  Our CHIME age data are in accord with the field relations. The CHIME ages of monazite from the Ryoke gneiss are ca. 100 Ma (Suzuki et al., 1994a,b).  Smith and Barreiro (1990) found that monazite formed at the lower amphibolite facies conditions records the time since the formation even with subsequent excursions into the upper amphibolite facies.  Following this observation, Suzuki et al. (1994a) considered the ca. 100 Ma monazite ages as the time of the first attainment to the lower amphibolite facies during the progressive stage of the Ryoke metamorphism.  Since the Shinshiro Tonalite emplaced after a significant uplift of the Ryoke belt, about 15 Ma is the maxi- mum estimate for the duration of the peak metamorphism. The mineral ages of the Shinshiro Tonalite were dated by the K-Ar method (Uchiumi et al., 1990); they are 73.3±2.9 Ma for hornblende and 68.0±2.1 Ma for biotite.  The closure temperatures of these minerals are estimated to be 510±25 and 300±50°C, respectively (Dodson and McClelland-Brown, 1985).  If we assume the temperature for the emplacement (monazite crystallizaion) to be 700°C, we can obtain average cooling rates of 16°C/Ma for the temperature range between 700 and 510°C and 43°C/Ma for the range between 510 and 300℃ (Fig.4). This cooling curve can be hard ly explained by a simple conductive cooling. Alternatively, we consider that the Shinshiro Tonalite was reheated above the closure temperature for hornblende by the emplace¬ ment of the Busetsu Granite at ca. 77 Ma ago.  In this case, the area appears to have cooled at a rate of about 40°C/Ma after the thermal input. This high cool¬ ing rate may be owing to the regional uplift and cooling of the Ryoke belt be¬ fore the emplacement of the Busetsu Granite. The K-Ar and Rb-Sr biotite ages for the Ryoke gneiss and granitic rocks in the Mikawa area concentrate at around 68 Ma (see the complication of Suzuki et al., 1994b). These ages have been interpreted as the time of the regional uplift (e.g. Suzuki et al., 1994b).  The present study, however, revealed that the area had undergone a significant uplift before the emplacement of the Shin- shiro Tonalite at ca. 86 Ma age. The ca. 68 Ma K-Ar biotite ages should be interpreted as the time of cooling to the closure temperature of biotite after the regional reheating caused by the emplacement of the Busetsu Granite.
ACKNOWLEDGEMENTS
We thank Prof. K. Shibata and Prof. M. Adachi of Nagoya University for their critical reading of this manuscript and constructive comments. We also thanks Mr. S. Yogo for his technical assistance.
REFERENCES
Asami, M. and Hoshino, M. (1980) Staurolite-bearing schists from the Hongu-san area in the Ryoke metamorphic belt, Central Japan. J. Geol. Soc. Japan, 86, 581-591. Atsumi, H. (1984) Finding of margarite in the Ryoke metamorphic rocks from the Kansa- gawa area, Aichi Prefecture. J. Geol. Soc. Japan, 90, 505-508 (in Japanese). Dodson, M. H. and McClelland-Brown, E. (1985) Isotopic and paleomagnetic evidence for rates of cooling, uplift and erosion. Geol. Soc. Mem., 10, 315-325. Kagami, H. (1973) A Rb-Sr geochronological study of the Ryoke granites in Chubu district, Central Japan. J. Geol. Soc. Japan, 79, 1-10. Ohmoto, Y. (1985) Zonal structure of Shinshiro tonalitic mass. MAGMA , 73, 69-73 (in Japanese). Ryoke Research Group (1972) THe mutual relations of the granitic rocks of the Ryoke Metamorphic Belt in Central Japan. Earth Sci. (Chikyu Kagaku), 26, 205-216 (in Japanese with English abstract.). Sakai, E., Nakai, Y., Sakazaki H., Itakura, K., Kawai, Y., Tsuruta, M. and Nomura, R. (1962) Preliminary note on the order of the intrusion of the granitic bodies in the Mikawa district, Aichi Prefecture. Bull. Aichi Gakugei Univ., 9, 169-185 (in Japanese with English abstract). Shibata, K. and Ishihara, S. (1979) Rb-Sr whole-rock and K-Ar mineral ages of granitic rocks in Japan. Geochem. J., 13, 113-119. Smith, H. A. and Barreiro, B. (1990) Monazite U-Pb dating of staurolite grade metamor- phism in pelitic schists. Contrib. Mineral. Petrol., 105, 602-615. Suzuki, K. and Adachi, M. (1991a) Precambrian provenance and Silurian metamorphism of the Tsubonosawa paragneiss in the South Kitakami terrane, Northeast Japan, revealed by the chemical Th-U-total Pb isochron ages of monazite, zircon and xenotime. Geochem. J., 25, 357-376. Suzuki, K. and Adachi, M. (1991b) The chemical Th-U-Pb isochron ages of zircon and monazite from the Gray Granite of the Hida terrane, Japan. J. Earth Sci. Nagoya Univ., 38, 11-38. Suzuki, K., Adachi, M. and Kajizuka, I. (1994a) Electron microprobe observations of Pb diffusion in metamorphosed detrital monazites. Earth Planet. Sci. Lett., 128, 391-405. Suzuki, K., Morishita, T., Kajizuka, I., Nakai, Y., Adachi, M. and Shibata, K. (1994b) CHIME ages of monazites from the Ryoke metamorphic rocks and some granitoids in the Mikawa-Tono area, central Japan. Bull. Nagoya Univ. Furukawa Museum , 10 , 17-38 (in Japanese with English abstract). Uchiumi, S., Uto, K. and Shibata, K. (1990) K-Ar age results-3 - New data from the Geological Survey of Japan -. Bull. Geol. Surv. Japan, 41, 567-575 (in Japanese with English abstract).