Journal ofUnusual Mineralogical cooling ofand the Petrological Middle Miocene Sciences, Ichifusayama Volume 101 Granodiorite, page 23─ 28, 2006 23

Unusual cooling of the Middle Miocene Ichifusayama Granodiorite, Kyushu,

* * * ** Teruki OIKAWA , Koji UMEDA , Sunao KANAZAWA and Tatsuji MATSUZAKI

*Tono Geoscience Center, Japan Atomic Energy Agency, 959－31, Jorinji, Izumi, Tokishi, Gifu 509－5102, Japan **Suncoh Consultants Co. Ltd., 1－8－9, Kameido, Koto－ku, 136－8522, Japan

K－Ar and FT analyses were carried out on biotite, zircon and apatite from the Middle Miocene Ichifusayama Granodiorite pluton in Kyushu, Japan in order to reveal the cooling history. The core and rim of the pluton revealed K－Ar biotite ages of 13.39－13.36 and 13.31－13.54 Ma, FT zircon ages of 13.3－13.1 and 12.7－12.3 Ma, and FT apatite ages of 13.1－13.7 and 10.6－10.8 Ma respectively, suggesting that the core cooled, to 100 °C from 300 °C, faster than the rim, of which the cooling rate is ~ 100 °C/Ma. The track－length analyses also sug- gest extremely rapid cooling without a late stage annealing process. The faster cooling of the core in compari- son with the rim is unusual. The difference in cooling velocity between the core and rim was possibly caused by an influence of paleotopography.

Keywords: Thermochronology, K－Ar dating, Fission－track dating, Paleotopography, Ichifusayama Grano- diorite, Southwest Japan

INTRODUCTION in the region (Saito et al., 1996). The Ichifusayama Gran- odiorite may therefore be expected to have a simple cool- Emplacement of the Middle Miocene granitic rocks in the ing history. In this study, we attempt to clarify its cooling Outer Zone of Southwest Japan occurred at 14 ± 1 Ma history using a thermochronological method based on K－ (Shibata, 1978; Sumii, 2000) just after the opening of the Ar and fission－track (FT) dating, and we discuss its rela- Japan Sea with the clockwise rotation of Southwest Japan tionship to the Miocene tectonic events in the Outer Zone (e.g., Otofuji et al., 1985), and in association with forma- of Southwest Japan. tion of the Setouchi Volcanic Rocks characterized by the high magnesium andesite (e.g., Tatsumi, 2001). There are SAMPLES AND ANALYTICAL PROCEDURES many studies on the origin and magmatism of the granitic rocks (e.g., Nakada and Takahashi, 1979; Shinjoe, 1997). Biotite K－Ar analyses with zircon and apatite FT dating There is, however, no previous study of their cooling his- were carried out on samples from four localities on the tory to give information on the intrusion and emplace- Ichifusayama Granodiorite (Fig. 1): two from the center ment processes and the relationship between tectonic evo- (Ic－2 and －3), and two from the rim (Ic－1 and －4). All of lution and igneous activity in the Outer Zone of Southwest the samples were collected at approximately the same Japan. altitude above sea level (1100－800 m). Rock types are The Ichifusayama Granodiorite (Saito et al., 1996) is shown in Table 1. a member of the Middle Miocene granitic rocks exposed in southern Kyushu (Fig. 1). This pluton intruded into the K-Ar dating Cretaceous to Paleogene mélange of the Shimanto Super- group (Saito et al., 1996). It is a gently gradationally Rock samples were crushed and sieved to obtain the 60 zoned pluton composed of fine－ to coarse－grained biotite (250 μm) to 120 mesh (125 μm) fractions. Biotite was granodiorite with many mafic microgranular enclaves and separated using conventional heavy liquid and magnetic xenoliths (Saito et al., 1996). After cessation of the Mid- techniques. The potassium and argon analyses of biotite dle Miocene plutonism, igneous activity did not reoccur were carried out at the Hiruzen Institute for Geology and Chronology Co. Ltd. The analytical and age calculation T. Oikawa, [email protected] Corresponding author methods were based on those described by Nagao et al. 24 T. Oikawa, K. Umeda, S. Kanazawa and T. Matsuzaki Unusual cooling of the Middle Miocene Ichifusayama Granodiorite 25

(Hurford and Green, 1983), using the zeta value of 352 ± 3 for zircon (Iwano and Danhara, 1997) and 299 ± 4 for apatite (Iwano and Danhara, 1998). The errors in zeta val- ues are given to one sigma. FT lengths in this study were measured on horizontal confined fission tracks (Laslett et al., 1982) following the methods described by Iwano et al. (1996). As the samples were obtained from plutonic rock, the influence of the extra grains was ignored in the track length measurements. The number of measurement grains in a sample was therefore about 300, more than those for the age determination.

RESULTS AND DATA PRESENTATION

The results of the K－Ar and FT dating are summarized in Tables 1 and 2, respectively. The errors for all ages are shown to the one－sigma level. The K－Ar biotite ages for Ic－1 and Ic－4 on the rim of the pluton and for Ic－2 and Ic－3 in the core are: 13.54 ± 0.22 and 13.31 ± 0.22 Ma, and 13.39 ± 0.22 and 13.36 ± 0.22 Ma, respectively (Table 1). Zircon FT ages from the same locations are: 12.7 ± 0.5 Ma (Ic－1), 12.3 ± 0.4 Ma (Ic－4), 13.3 ± 0.5 Ma (Ic－2) and 13.1 ± 0.5Ma (Ic－3), and apatite ages are: 10.8 ± 0.5 Ma (Ic－1), 10.6 ± 0.7 Ma (Ic－4), 13.7 ± 0.6 Ma (Ic－2), 13.1 ± 0.5 Ma (Ic－3) (Table 2). The 12 new ages are con- sistent with the results from previous work that indicated a K－Ar biotite age of 14 ± 1 Ma (Miller et al., 1962) and a zircon FT age of 12.0 ± 0.5 Ma (Miyachi, 1985). Figure 1. Geological map showing sample locations. Map simpli- fied after Teraoka et al. (1981). Inset shows the distribution of In general, the nonaligning, spontaneous and induced Miocene igneous rocks (patterned area) in the Outer Zone of track lengths are characterized by a narrow unimodal dis- Kyushu, based on Geological Survey of Japan (1995). Abbrevia- tribution with a mean confined FT length of ~ 10.7 μm tion; I, Ichifusayama Granodiorite. and standard deviation of ~ 0.8 μm for zircon (Hasebe et al., 1994), and a mean confined FT length of about 15－16 (1984) and Itaya et al. (1991). For calculation of the K －Ar μm for apatite (Gleadow et al., 1986). The confined FT −10 −10 40 age, λe = 0.581 × 10 /yr, λβ = 4.962 × 10 /y and K/K lengths for all samples are characterized by a narrow uni- = 0.0001167 (Steiger and Jäger, 1977) were used. The modal distribution with a mean length of 10－11 μm (mode: argon isotope analyses were replicated for all the samples 10－11 μm) for zircon and a mean length of 14－15 μm to check the reproducibility of experiments. Average ages (mode: 14－16 μm) for apatite (Fig. 2). These FT length were calculated using the formula proposed by Tsukui et data do not show evidence of noticeable shortening or al. (1985). annealing. None of the apatite and zircon FT data from loca- Fission-track (FT) dating tions Ic－3 and Ic－4 passed the χ2－test at the 5% criterion (Galbratith, 1981). The ED1 (Gleadow, 1981) data are sus- Zircon and apatite samples were separated using conven- ceptible to variation in addition to Poisson variation in tional heavy liquid and magnetic techniques. FT dating track counts (Danhara et al., 1991). The χ2－test indicates was carried out at the Kyoto FT Co. Ltd. by the external that sample failure could be attributed to contamination detector method using the geometry factors of 0.5 for 4π by a xenocryst, or variation in the uranium content of /2π (ED1, internal surface: Gleadow, 1981), following the each grain or inner grain. In the context of these age data procedures described by Danhara et al. (1991), Iwano and obtained from plutonic rocks, it is not considered that an Danhara (1997), and Iwano and Danhara (1998). Track inmix of tracked exotic material would survive the heat of length was also analyzed at the same laboratory. FT ages granitic magma. were determined with the zeta calibration approach A failure to pass the χ2－test is often observed in ED1 24 T. Oikawa, K. Umeda, S. Kanazawa and T. Matsuzaki Unusual cooling of the Middle Miocene Ichifusayama Granodiorite 25

Table 1. Sample description and K－Ar biotite dating results for the Ichifusayama Granodiorite

* See Figure 1. ** Based on the WGS84 Datum.

Table 2. FT zircon and apatite dating results for the Ichifusayama Granodiorite

* ρ and N are the density and the total number of fission tracks counted, respectively. ** Analyses were made by the external detector method using geometry factors of 0.5 for 4π/2π (ED1, internal sur- face: Gleadow, 1981). Ages were calculated using a dosimeter glass NBS－SRM612 and age calibration factors zeta (ED1) = 352 ± 3 (Zircon; Iwano and Danhara, 1997), zeta (ED1) = 299 ± 4 (Apatite; Iwano and Danhara, 1998). Errors for zeta values are given at one sigma. Zircon samples were irradiated using a TRIGA MARK II nuclear reactor at Rikkyo University, Japan and apatite samples were irradiated using a TRIGA nuclear reactor at Oregon State University, USA. *** P (χ2) is the probability of obtaining the χ2－value for n degrees of freedom (where ν = number of crystals－1). **** r is the correlation coefficient betweenρ s and ρi.

data (e.g., Hurford and Green, 1983; Danhara et al., (> 25 grains: Green, 1981). 1991). Danhara et al. (1991) showed that the main reason The fission－track lengths in all of the samples are, for failure of the χ2－test in ED1 measurement could be furthermore, not reduced, and the ages obtained are con- attributed to the difference in uranium content of each cordant with ages determined by other methods and mate- grain or inner grain. It was observed that all of the apatite rials. It therefore seems that the sampled material cooled and zircon FT data from locations Ic－3 and Ic－4, which uniformly through the closure temperatures of the K－Ar failed the χ2－test, were obtained from samples that con- biotite system (300 ± 50 °C: Dodson and McClelland－ tained one or several grains that had a relatively old Brow, 1985), the FT zircon system (230－330 °C: Tagami appearance age compared to that of other grains. Hence, and Shimada, 1996), and the FT apatite system (70－130 the failed χ2－test for the FT data can be attributed to the °C: Gleadow et al., 1983). difference in uranium content. The reliability of the ages obtained is, however, considered to be high because the number of grains measured was more than sufficient 26 T. Oikawa, K. Umeda, S. Kanazawa and T. Matsuzaki Unusual cooling of the Middle Miocene Ichifusayama Granodiorite 27

Figure 3. Cooling history of Ichifusayama Granodiorite. The age errors are shown to the two－sigma level. The closure temperature or the temperatures of partial annealing zone: biotite K－Ar 300 ± 50 °C (Dodson and McClelland－Brow, 1985), zircon FT 230－ 330 °C (Tagami and Shimada, 1996), apatite FT 70－130 °C (Gleadow et al., 1983).

pluton can be expected to cool more slowly than the periphery; therefore the apatite FT age of the central part Figure 2. Histograms of confined fission tracks for zircon and apa- tite samples, Ichifusayama Granodiorite. should be younger than that of the rim because the center theoretically cools through the closure temperature for apatite later. There are two possibilities to explain the DISCUSSION unusual cooling pattern in this study. The first interpreta- tion is that there was rejuvenation of the ages by some The relationship between the ages obtained and the clo- thermal event after cooling of the pluton. However, evi- sure temperatures is shown in Figure 3. This indicates that dence of postintrusion thermal events, such as igneous the biotite K－Ar ages are the same but the apatite ages activity or hydrothermal alteration, has not been recog- show a significant difference between the core (Ic－2 and nized in and around the pluton (Saito et al., 1996). In －3) and rim (Ic－1 and －4) samples. The rim is younger addition, a shortening of the FT track lengths is not ob- than the core, suggesting that the core and rim had differ- served in any of the samples (Fig. 2) and, therefore, this ent cooling histories. The rim cooled down to 100 °C possibility is ruled out. from 300 °C with a cooling rate of ~ 100 °C/Ma (80－130 The second interpretation is that the central part of °C/Ma). This cooling rate is of the same order and in the the pluton approached ground surface earlier than the rim. same temperature range as those of the Takidani Grano- If so, the central part would cool down more rapidly than diorite (Bando et al., 2003), which was emplaced and the rim. The underground temperature profile on the low－ exposed in the Quaternary (Harayama, 1992). temperature shallow crust is changed by the influence of An issue arises because the core part of the topography (House et al., 1998). These authors showed Ichifusayama Granodiorite cooled down more rapidly that the subsurface temperature at < ~ 100 °C at ~ 1 km than the rim over a period of ~ 13 Ma (Fig. 3). In the depth can be remarkably changed by the influence of an cooling of a simple plutonic body, the central part of the incised topography. Moreover, they showed that the apa- 26 T. Oikawa, K. Umeda, S. Kanazawa and T. Matsuzaki Unusual cooling of the Middle Miocene Ichifusayama Granodiorite 27

tite U－Th/He age (closed temperature: ~ 70 °C) is remark- Dodson, M.H. and McClelland－Brow, E. (1985) Isotopic and pal- ably changed by influence of paleotopography. We thus eomagnetic evidence for rates of cooling, uplift and erosion. Geological Society of London Memoirs, no. 10, Published suggest the possibility that the difference in the apatite FT for the Geological Society by Blackwell Scientific Publi- ages in the Ichifusayama Granodiorite was caused by cations, Oxford, 315－325. influence of the paleotopography. In the Middle Miocene Galbratith, R.F. (1981) On statistical models for fission track (~ 13 Ma), there may have existed a NE－trending deep counts. Journal of Mathematical Geology, 13, 471－488. valley crossing above the core of the pluton such that the Geological Survey of Japan (1995) Geological Map of Japan 1 : 1000000 3rd Edition CD－ROM Version; Digital Geo- central part of the pluton was closer to the ground surface science Map G－1. that the rim. Gleadow, A.J.W. (1981) Fission－track dating methods: what are Based on obtained apatite FT ages of 7.1 ± 2.3, 7.6 ± the real alternatives? Nuclear Tracks: Methods, Instruments 4.1, 8.8 ± 6.5 and 13.3 ± 3.7 Ma, ± 2σ level, Hasebe and and Applications, 5, 3－14. Tagami (2001) showed that the temperature of the Gleadow, A.J.W., Duddy, I.R. and Lovering, J.F. (1983) Fission track analysis: a new tool for the evaluation of thermal histo- Shimanto Supergroup in the eastern part of Southern ries and hydrocarbon potential. Australian Petroleum Ex- Kyushu decreased to ~ 100 °C at approximately 10 Ma plorer Association, 23, 93－102. when the Southwest Japan block obducted onto the sub- Gleadow, A.J.W., Duddy, J.R., Green, P.F. and Lovering, J.F. ducting Philippine Sea Plate. Miyachi (1985) showed that (1986) Confined fission track lengths in apatite: a diagnostic the zircon FT ages from the four granitic bodies of Osumi, tool for thermal history analysis. Contributions to Mineralogy and Petrology, 94, 405－415. Takakumayama, Ichifusayama, and Shibi in the Outer Green, P.F. (1981) A new look at statistics in fission－track dating. Zone of Southwest Japan in Kyushu are ~ 12 Ma, indicat- Nuclear Tracks, 5, 77－86. ing that the four plutons cooled down to the closure tem- Harayama, S. (1992) Youngest exposed granitoid pluton on Earth: perature (230－280 °C) at virtually the same time. The Cooling and rapid uplift of the Pliocene — Quaternary Taki- obduction of the Southwest Japan block may have led to dani Granodiorite in the Japan Alps, central Japan. Geology, 20, 657－660. the rapid exhumation of the Shimanto Supergroup and its Hasebe, N., Tagami, T. and Nishimura, S. (1994) Towards zircon igneous intrusive rocks resulting in responsive erosion fission track thermochronology: Reference framework for and rapid dissection that incised a deep NE－trending val- confined track length measurements. Chemical Geology, 112, ley crossing the core part of the Ichifusayama Grano- 169－178. diorite. Hasebe, N. and Tagami, T. (2001) Exhumation of an accretionary prism — results from fission track thermochronology of the Shimanto Belt, southwest Japan. Tectonophysics, 331, 247－ ACKNOWLEDGMENTS 267. House, M.A., Wernicke, B.P and Farley K.A. (1998) Dating topo- We would like to thank Mr. G. McCrank, ex－JNC In- graphy of the Sierra Nevada, California, using apatite U－ ternational Fellow, for his valuable editorial help. The Th/He ages. Nature, 396, 66－69. Hurford, A.J. and Green, P.F. (1983) The zeta age calibration of Kyoto Fission－Track Co. Ltd. undertook the FT dating fission track dating. Isotope Geoscience, 1, 285－317. and track length analysis, and the Hiruzen Institute for Itaya, T., Nagao, K., Inoue, K., Honjo, Y., Okada, T. and Ogata, A. Geology and Chronology Co. Ltd. carried out the K－Ar (1991) Argon isotope analysis by a newly developed mass dating. The submitted manuscript was improved follow- spectrometric system for K－Ar dating. Mineralogical Journal, ing constructive comment by Professor T. Itaya of Oka- 15, 203－221. － yama University of Science and Associate Professor H. Iwano, H., Yamashita, T. and Danhara, T. (1996) Three dimen- sional analysis of fission track length in minerals? A measur- Shinjoe of Tokyo Keizai University. Additional sugges- ing system and its application. Fission Track News Letter, no. tions by Associate Professor M. Nakamura of Tohoku 9, 13－22. University were much appreciated. Iwano, H. and Danhara, T. (1997) Zeta calibration of fission－track dating system using high－resolution monitor and comparison REFERENCES of reported zeta values. Fission Track News Letter, no.10, 11－20 (in Japanese with English abstract). Iwano, H. and Danhara, T. (1998) A re－investigation of the geom- Bando, M., Sekine, K., Bignall, G. and Tsuchiya, N. (2003) Pe- etry factors for fission－trak dating of apatite, sphene and zir- trography and uplift history of the Quaternary Takidani Gran- con. In Advances in Fission－Track Geochoronology (Haute, P.; odiorite: Could it have hosted a supercritical (HDR) geother- De Corte, F. Ed.). pp. 352, Kluwer Academic Publishers, mal reservoir? Journal of Volcanological and Geothermal Dordrecht, 47－66. Research, 120, 215－234. Laslett, G.M., Kendall, W.S., Gleadow, A.J.W. and Duddy, I.R. Danhara, T., Kasuya, M., Iwano, H. and Yamashita, T. (1991) Fis- (1982) Bias in measurement of fission－track length distribu- － sion track age calibration using internal and external sur- tions. Nuclear Tracks, 6, 79－85. faces of zircon. Journal of the Geological Society of Japan, Miller, J.A., Shibata, K. and Kawachi, Y. (1962) Potassium－argon 97, 977－985. ages of granitic rocks from the Outer Zone of Kyushu, Japan. 28 T. Oikawa, K. Umeda, S. Kanazawa and T. Matsuzaki

Bulletin of the Geological Survey of Japan, 13, 712－714. prism. Chemical Geology, 134, 237－255. Miyachi, M. (1985) Fission track ages of some granitic rocks in Steiger, R.H. and Jäger, E. (1977) Subcommission on geochronol- the Outer Zone of Kyushu, Japan. The Journal of the Jap- ogy: convention on the use of decay constants in geo－ and anese Association of Mineralogists, Petrologists, and Econom- cosmochronology. Earth and Planetary Science Letters, 36, ic Geologists, 80, 406－409. 359－362. Nagao, K., Nishido, H., Itaya, T. and Ogata, K. (1984) An age de- Sumii, T. (2000) K－Ar ages of the Miocene granites in the Ta- termination by K－Ar method. Bulletin of the Hiruzen Re- katsuki－yama and the surrounding areas, southwestern search Institute, Okayama College of Science, no. 9, 19－38 (in . Japanese Magazine of Mineralogical and Petrol- Japanese with English abstract). ogical Sciences, 29, 67－73 (in Japanese with English ab- Nakada, S. and Takahashi, M. (1979) Regional variation in chem- stract). istry of the Miocene intermediate to felsic magmas in the Tagami, T. and Shimada, C. (1996) Natural long－term annealing Outer Zone and the Setouchi Province of Southwest Japan. of the zircon fission track system around a granitic pluton. Journal of the Geological Society of Japan, 85, 571－582 (in Journal of Geophysical Research, 101, 8245－8255. Japanese with English abstract). Tatsumi, Y. (2001) Geochemical modeling of partial melting of Otofuji, Y., Matsuda, T. and Nohda, S. (1985) Paleomagnetic evi- subducting sediments and subsequent melt－mantle interac- dence for the Miocene counter－clockwise rotation of North- tion: Generation of high－Mg Andesites in the Setouchi volca- east Japan — rifting process of the Japan arc, Earth and nic belt, southwest Japan. Geology, 29, 323－326. Planetary Science Letters, 75, 265－277. Teraoka, Y., Imai, I.Okumura, K., Suda, Y. and Watanebe, S. (1981) Saito, M., Kimura, K., Naito, K. and Sakai, S. (1996) Geology of Geological Map of Japan, 1 : 200000, Nobeoka. Geological the Shiibamura district. With Geological Map Sheet at Survey of Japan (in Japanese). 1:50000, Geological Survey of Japan, pp. 133 (in Japanese Tsukui, M., Nishido, H. and Nagao, K. (1985) K－Ar ages of the with English abstract). Hiruzen volcano group and Daisen volcano. Journal of the Shibata, K. (1978) Contemporaneity of Tertiary granites in the Geological Society of Japan, 91, 279－288 (in Japanese with Outer Zone of Southwest Japan. Bulletin of the Geological English abstract). Survey of Japan, 29, 551－554 (in Japanese with English ab- stract). Manuscript received; March 16, 2005 Shinjoe, H. (1997) Origin of the granodiorite in the forearc region Manuscript accepted; September 30, 2005 of southwest Japan: Melting of the Shimanto accretionary Manuscript handled by Michihiko Nakamura