Staurolite－bearingJournal gneiss of Mineralogical and re－examination and Petrological of metamorphic Sciences, zonal Volume mapping 99 ,of page the Higo1─ 18, metamorphic2004 terrane 1 Staurolite－bearing gneiss and re－examination of metamorphic zonal mapping of the Higo metamorphic terrane in the Kosa area, central Kyushu, Japan * * ** Kenshi MAKI , Yoshihisa ISHIZAKA and Tadao NISHIYAMA *Graduate School of Science and Technology, Kumamoto University, 2－39－1 Kurokami, Kumamoto 860－8555, Japan **Department of Earth Sciences, Faculty of Science, Kumamoto University, 2－39－1 Kurokami, Kumamoto 860－8555, Japan This paper describes the finding of staurolite－bearing gneiss from the Higo metamorphic terrane and proposes new mineral zones in the Kosa area, Kumamoto Prefecture. Previous mineral zones of the Higo metamorphic terrane proposed by Obata et al. (1994) consist of five zones: Zone A characterized by Chl + Ms, Zone B by Bt + Ms + And, Zone C by Kfs + Sil + Bt, Zone D by Grt + Crd + Bt, and Zone E by Opx in metapelites. They identified three zones from B to D in the Kosa area. However, we found that sillimanite appears together with Grt + Crd, hence this paper shows that Zone C is absent in the Kosa area. New finding of Opx in the southern－ most part of the area made it possible to define three mineral zones; Bt zone, Grt－Crd zone, and Opx zone, in the order of increasing grade from north to south in the Kosa area. Analysis of staurolite－bearing assemblage in a KFMASH system together with textural evidence reveals that the following reactions occurred in the stauro- － lite bearing gneiss in the excess of Kfs, Qtz and H2O: [Sil] Str + Bt = Grt + Crd [Bt] Str = Grt + Crd + Sil Chemographic analysis of these reactions together with Grt－Bt geothermometers shows the metamorphic condition of P = 200 MPa and T = 600－620°C, which is much lower in pressure than that estimated by Osanai et al. (1996) in the Toyono area, about 10 km west of Kosa. Introduction phic rocks into three metamorphic zones with increasing grade from north to south based on mineralogy of metaba- The Higo terrane is located in the western part of central sites and axial color of hornblende. Nagakawa (1991) and Kyushu (Fig. 1) and is bordered to the south by the Usuki － Obata et al. (1994) presented a new zonal mapping, which Yatsushiro tectonic line. The terrane consists of Mano- defined five mineral zones based on pelitic mineral assem- tani metamorphic rocks, Higo metamorphic rocks, Higo blages (Fig. 1). The metamorphism was first considered plutonic rocks and Ryuhozan metamorphic rocks from to be of the andalusite－sillimanite type (Yamamoto, 1962; north to south, forming a narrow belt trending east－west Tsuji, 1967). However, several lines of evidence have (nomenclature of the rock units after Yamamoto, 1962). come out since then, showing the possibility of a medium The Manotani metamorphic rocks gradually change to P type metamorphism. Karakida and Yamamoto (1982) the Higo metamorphic rocks with no faults between them reported the occurrence of garnet amphibolite from the (Okamoto et al., 1989; Karakida et al., 1989), and they Higo metamorphic rocks in the Ogawa area, about 17 represent the lower grade part of the Higo metamorphic km west of Kosa, although it is not in itself a conclusive rocks (Karakida et al., 1989). In this paper, we discuss evidence of medium P. Osanai et al. (1995) reported the the nature of the metamorphism of the Higo metamorphic occurrence of staurolite inclusion in garnet porphyloblasts rocks. from the Toyono area, about 10 km west of Kosa. Kano Yamamoto (1962) first divided the Higo metamor- and Uruno (1995) reported the occurrence of kyanite from river sands in the Kosa area. These lines of evidence sug- K. Maki, [email protected]－u.ac.jp Corresponding author gest that the Higo metamorphic rocks may have suffered T. Nishiyama, [email protected]－u.ac.jp a medium P type metamorphism. Furthermore, Karakida 2 K. Maki, Y. Ishizaka and T. Nishiyama Staurolite－bearing gneiss and re－examination of metamorphic zonal mapping of the Higo metamorphic terrane 3 Figure 1. Metamorphic zonal mapping of the Higo metamorphic belt reprinted from Obata et al. (1994) with permission from Elsevier, Copy- right (1994). The study area is indicated by an envelope. et al. (1989) found occurrences of lawsonite and crossite microstructures from a garnet －biotite －cordierite gneiss from the Manotani metamorphic rocks, suggesting a high in Zone E (Obata et al., 1994) and interpreted the cores P / T metamorphism as a precursor of the low to medium and the rims as detrital zircons and recrystallized zircons, P type metamorphism. We therefore need further petro- respectively. They dated cores to be 2155－184 Ma U－Pb logical studies to clarify the nature of the metamorphism. ages and rims to be 116.5 ± 18.7 Ma U－Pb age by means The age of the Higo metamorphic and plutonic rocks of a SHRIMP. They interpreted that the Higo metamor- has been under debate. Nakajima (1995) dated biotites phic rocks reached the peak of metamorphism at 116.5 ± from both the Shiraishino granodiorite and the Higo meta- 18.7 Ma. This age is comparable with those of the Higo morphic rocks to be 100 －106 Ma by K －Ar dating. He plutonic rocks with a SHRIMP (Sakashima et al., 2003). also determined the same ages for these rocks by Rb－Sr This paper describes the finding of staurolite from whole rock isochron method. Osanai et al. (1996; 1998; the Kosa area and presents the result of a re －examina- 2001) dated zircons from garnet－cordierite－biotite tonal- tion of the metamorphic zonal mapping in the Kosa area ite to be about 250 Ma for the core by SHRIMP method based on new occurrences of orthopyroxene and garnet + and reported a similar age for garnet－orthopyroxene－bio- cordierite from metapelites. This paper also discusses the tite tonalite using a Rb －Sr whole rock isochron method. pressure－temperature condition of the metamorphism by They interpreted that the Higo metamorphic rocks reached considering breakdown reactions of staurolite and garnet－ the peak of metamorphism at about 250 Ma and were then biotite geothermometers. locally re －heated by the intrusion of the Shiraishino granodiorite at about 120 Ma (the age after Kamei et al., Geological setting 1997). They stated that the age of 100 Ma reported by Nakajima (1995) represents the cooling age of the Shi- The Higo and the Manotani metamorphic rocks occur in raishino granodiorite and the Higo metamorphic rocks. the central part of the Kosa area, which is in fault contact On the contrary, Nagakawa et al. (1997) obtained very with the Mizukoshi Formation (Paleozoic) in northwest similar age (103－108 Ma) for biotite from Zones B to E (Fig. 2). A thin body of serpentinite occurs along the of Obata et al. (1994) using K －Ar dating. They stated fault. In the south of the Kosa area, the Shiraishino that this fact cannot be explained by the local re－heating granodiorite intrudes almost concordantly into the Higo suggested by Osanai et al. (1993) and concluded that the metamorphic rocks trending nearly east －west. The grano- peak of the Higo metamorphism was at about 105 Ma. diorite includes a pelitic xenolith of about 50 m in diam- Sakashima et al. (2003) found zircons with core －rim eter (Fig. 2). The metamorphic and plutonic complex is 2 K. Maki, Y. Ishizaka and T. Nishiyama Staurolite－bearing gneiss and re－examination of metamorphic zonal mapping of the Higo metamorphic terrane 3 Figure 2. Geologic map of the Higo terrane in the Kosa area. 4 K. Maki, Y. Ishizaka and T. Nishiyama Staurolite－bearing gneiss and re－examination of metamorphic zonal mapping of the Higo metamorphic terrane 5 Figure 3. Revised metamorphic zonal mapping of the Higo terrane in the Kosa area. Mineral abbreviations are after Kretz (1983). 4 K. Maki, Y. Ishizaka and T. Nishiyama Staurolite－bearing gneiss and re－examination of metamorphic zonal mapping of the Higo metamorphic terrane 5 locally covered by the Aso －4 pyroclastic flow deposits an accelerating voltage of 20 kV, a beam current of 0.6 nA (Quaternary) along the Midorikawa river and its branches. and a beam width of 2 μm. The Higo metamorphic rocks in the Kosa area consist mainly of psammitic －pelitic gneisses with small Biotite zone amounts of metabasites (amphibolites), marble and gneisses intermediate between pelitic and basic in com- The biotite zone is characterized by biotite + K－feldspar. positions. They show an east－west strike, dipping steeply Representative mineral assemblage of metapelites in the to the north. A large body of marble occurs in the central biotite zone is as follows: part of the Kosa area, which is in fault contact with the biotite + K－feldspar ± garnet + plagioclase + quartz (a) gneisses. Many small bodies of the marble occur as The biotite －rich melanocratic layer alternates with the lenses or as thin layers in the gneisses. quartz －rich leucocratic layer with distinct gneissosity. Biotite occurs as flaky crystal parallel to the gneissoisty. Metamorphic zonal mapping K－feldspar and plagioclase occur as anhedral crystals of about 0.1 mm in size. Myrmekite sometimes occurs be- The Higo metamorphic terrane has been divided into five tween K－feldspar and plagioclase. Muscovite shows two zones, A, B, C, D, and E in the order of increasing grade modes of occurrence in the Kosa area. One is flaky crys- form north to south based on mineral assemblages of tal parallel to the gneissosity, and the other is randomly pelitic and psammitic rocks (Obata et al., 1994). Zone A oriented crystal. We consider the former as primary and is characterized by chlorite and muscovite in metapelites, the latter as secondary phase.