－bearingJournal 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 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 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 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 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 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 and crossite microstructures from a garnet － －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 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. 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 with small Biotite zone amounts of metabasites (), 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 + (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 parallel to the gneissoisty. Metamorphic zonal mapping K－feldspar and plagioclase occur as anhedral 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. But, in many cases, it is and Zone B by the occurrence of biotite. Muscovite very difficult to distinguish them. remains stable with andalusite in this zone. Zone C is de- Metabasites in the biotite zone have the assemblage: fined by the disappearance of muscovite and the appear- hornblende + plagioclase + quartz + clinopyroxene ance of K－feldspar and sillimanite. Zone D is character- ± (b). ized by the stability of garnet + cordierite assemblage, and The intermediate gneisses have the following assem- Zone E by the presence of orthopyroxene (hypersthene) in blages: the absence of sillimanite and cordierite. In the Kosa area hornblende + K－feldspar + biotite + plagioclase + quartz three zones B, C and D have been identified by Obata et ± garnet (c), al. (Fig. 1). hornblende + K－feldspar + biotite + plagioclase + quartz We newly found that sillimanite coexists with garnet + clinopyroxene ± garnet (d). + cordierite and hence Zone C is absent in the Kosa area (Fig. 3). We also found a new occurrence of orthopyrox- Garnet－cordierite zone ene in the southernmost part of the area (Fig. 3). These lines of evidence lead us to a re－examination of the meta- The first appearance of garnet + cordierite in metapelites morphic zonal mapping. defines the garnet －cordierite isograd. Representative Through this study, we re－define the following three assemblages in the metapelites are as follows, in which zones: biotite zone, garnet－cordierite zone, and orthopy- quartz is in excess: roxene zone, in the order of increasing grade from north biotite + K－feldspar + plagioclase (e), to south in the Kosa area (Fig. 3). The garnet－cordierite biotite + K－feldspar + garnet + plagioclase (f), zone and the orthopyroxene zone correspond to Zone biotite + K－feldspar + sillimanite + garnet D and Zone E of Obata et al. (1994), respectively. The + plagioclase (g), biotite zone includes both Zone B and Zone C of Obata biotite + K－feldspar + cordierite + plagioclase (h), et al. (1994), because the latter two zones cannot be dis- biotite + K－feldspar + sillimanite + cordierite criminated in the Kosa area. Furthermore, the garnet － + plagioclase ± garnet (i), cordierite isograd in this study, which defines the bound- biotite + K－feldspar + cordierite + garnet ary between the biotite and the garnet－cordierite zones, is + plagioclase (j). located about 1.6 km north of the same isograd between K－feldspar and plagioclase occur as anhedral crystals Zones C and D defined by Obata et al. (1994). up to 0.7 and 0.9 mm, respectively, and they progressively We will describe mainly metapelites in each min- become coarser towards south. Myrmekite often devel- eral zone below, and a detailed study on the metabasites ops between K －feldspar and plagioclase. Plagioclase is in progress. were analyzed using an electron sometime occurs as an inclusion up to 0.1 mm within K－ －probe microanalyser (JEOL, PC －SEM 5600 combined feldspar. Sillimanite shows two modes of occurrence: with LINK ISIS) at Kumamoto University, operating at fibrolite and coarse euhedral to anhedral crystal (Figs. 4 6 K. Maki, Y. Ishizaka and T. Nishiyama Staurolite－bearing gneiss and re－examination of metamorphic zonal mapping of the Higo metamorphic terrane 7

Figure 4. Photomicrograhs showing occurrences of sillimanite in garnet－biotite gneiss from the garnet－cordierite zone. (a) Fibro- lite included in plagioclase. (b) Breakdown texture of sillimanite surrounded by cordierite. Cordierite is anhedral granular crystal and free from inclusion. and 5a). Fibrolite ubiquitously occurs in this zone and is included in cordierite, biotite, plagioclase, quartz and muscovite (Fig. 4a). Sillimanite is euhedreal prismatic to rhombic (Fig. 5a) in the northern part of the garnet－ cordierite zone, and anhedral porphyloblast (Fig. 4b) in the southern part. Cordierite occurs in three modes. One is anhedral porphyloblast including biotite, plagioclase, quartz, apatite, tourmaline and opaque minerals in the Figure 5. Photomicrograhs showing occurrences of minerals in northern part of the zone (Fig. 5b). The second type is staurolite －bearing gneiss from the garnet －cordierite zone. (a) anhedral granular crystal free from inclusions (Figs. 4b Sillimanite occurs as euhedral prismatic to rhombic crystal in and 5c), which occurs throughout the zone. The third biotite－rich layer. Parallel alignment of crystals elongated to c－ type fills the interstices of sillimanite crystals (Fig. 5a). axis is common. Cordierite fills the interstices of sillimanite crystals. (b) Cordierite occurs in the melanocratic layer as Garnet is subhedral porphylobalst up to 1 cm including anhedral porphylobrast with abundant inclusions of biotite, plagioclase and quartz. Some show a resorption plagioclase, quartz, apatite, tourmaline and opaque minerals. (c) texture. Both primary and secondary muscovites occur Staurolite is completely surrounded by cordierite which is anhe- in this zone. Prehnite occurs as lenses in the cleavages of dral granular crystal and free from inclusions (Samples Str and biotite or as a radial aggregate between minerals such as Crd in Table 2). quartz and plagioclase. It also occurs commonly in the higher grade gneisses. Staurolite occurs in a metapelite 6 K. Maki, Y. Ishizaka and T. Nishiyama Staurolite－bearing gneiss and re－examination of metamorphic zonal mapping of the Higo metamorphic terrane 7

in this zone and will be described in detail below. Other particular rock types from this zone, andalusite－silliman- ite－bearing gneiss and garnet－orthopyroxene－biotite rock will be also described separately in Petrography. Metabasites in the garnet －cordierite zone have the following assemblages, in which quartz is in excess: hornblende + plagioclase (k), hornblende + plagioclase + clinopyroxene ± epidote (l). The intermediate gneisses have the following assem- blages, in which quartz is in excess: hornblende + K－feldspar + biotite + plagioclase ± garnet (m), hornblende+K－feldspar + biotite + plagioclase + clinopyroxene ± garnet (n), hornblende + K－feldspar + biotite + plagioclase + epidote (o), hornblende + K－feldspar + biotite + plagioclase + epidote + clinopyroxene (p).

Orthopyroxene zone

The orthopyroxene isograd is defined by the first appear- ance of orthopyroxene in metapelites with one exception of garnet －orthopyroxene －biotite rock from the garnet － cordierite zone, which will be discussed later in Petrog- raphy. We found two localities of orthopyroxene in the southernmost part of the area (Fig. 3). The critical assem- blage in this zone is: biotite + K－feldspar + garnet + orthopyroxene + plagioclase + quartz (q) Orthopyroxene free assemblages are the same as those in the garnet－cordierite zone. － Orthopyroxene (XFe = 0.57 0.59) occurs in a leuco- cratic layer as anhedral crystal of about 0.8 mm in size (Fig. 6a, b), forming a thin orthopyroxene －rich layer parallel to the gneissosity. It often occurs around garnet, but is never in contact with garnet. Orthopyroxene some- times includes quartz and plagioclase crystals. Biotite near the orthopyroxene usually shows a resorption texture (Fig. 6a, b). Garnet occurs as anhedral porphyloblast of about 1 mm in size, usually showing a resorption texture Figure 6. Photomicrograhs showing occurrences of minerals in (Fig. 6). It is generally surrounded by a fine aggregate of garnet －orthopyroxene －biotite gneiss from the orthopyroxene plagioclase but sometimes included in a large plagioclase zone. (a) Garnet occurs as anhedral and is sur- － － crystal. Garnet is rich in almandine (Xalm = 0.67 0.68) rounded by coarse grained crystals of plagioclase (～0.4 mm) (Sample Grt1 in Table 1). (b) Garnet crystals completery includ- and poor in grossular (Xgrs = 0.03) (Table 1). It consists of ed in plagioclase. (c) Garnet occurs as anhedral porphyroblast homogeneous core and outermost rim where Mg and Fe and is surrounded by fine－grained crystals of plagioclase (～0.1 decrease and Mn increases outward (Fig. 7a, b). Ca is al- mm) (Sample Grt2 in Table 1). most constant throughout the grain. Plagioclase included － in garnet is XAn = 0.48. It is higher than that (XAn = 0.31 0.34) of plagiolcase in the matrix (Table 1). Metabasites in the orthopyroxene zone have the fol- Occurrences of other minerals in metapelites are lowing assemblages, in which quartz is in excess: almost the same as those in the garnet－cordierite zone. hornblende + plagioclase (r), 8 K. Maki, Y. Ishizaka and T. Nishiyama Staurolite－bearing gneiss and re－examination of metamorphic zonal mapping of the Higo metamorphic terrane 9

Table 1. Representative analyses of minerals in garnet－orthopyroxene－biotite gneiss in the orthopyroxene zone

* ** *** **** FeO , total iron as FeO. XMg = Mg / (Mg + Fe). Pl, in the matrix; Pl, included in garnet. c, core; n.r., near rim; r, rim. Mineral abbreviations are after Kretz (1983).

hornblende + plagioclase + clinopyroxene ± epidote (s). gneiss, andalusite－sillimanite－bearing gneiss and garnet－ The intermediate gneisses have the following assem- orthopyroxene－biotite rock. blage: hornblende + K－feldspar + biotite + plagioclase Staurolite－bearing gneiss + quartz + clinopyroxene ± garnet (t). Staurolite－bearing gneiss occurs in a roadside outcrop at Petrography about 1 km northwest of Mt.Kosadake (Figs. 3 and 8). This is the first report of occurrence of staurolite in the This section describes petrography of three particular rock matrix of metapelite from the Higo metamorphic rocks, types from the garnet－cordierite zone: staurolite－bearing although Osanai et al. (1995; 1996) have reported oc- 8 K. Maki, Y. Ishizaka and T. Nishiyama Staurolite－bearing gneiss and re－examination of metamorphic zonal mapping of the Higo metamorphic terrane 9

Table 2. Representative analyses of minerals in staurolite－bearing gneiss in the garnet－cordierite zone

* ** 3 + *** FeO , total iron as FeO. Fe is assumed to be about 3.5 % of total Fe (Holdaway et al., 1991). XMg = Mg / (Mg **** ***** + Fe), XZn = Zn / (Mg + Fe + Zn). Pl, included in staurolite. c, core; n.r., near rim; r, rim.

currence of staurolite inclusion in garnet porphyloblast apatite, zircon and ilmenite. The gneiss shows distinct from metapelites in the Toyono area, about 10 km west of gneissosity consisting of melanocratic (biotite－rich) layer Kosa. The outcrop is located at about 300 m south of the and leucocratic (plagioclase + quartz－rich) layer. garnet－cordierite isograd newly defined in this study. In staurolite is fine grained (～0.3 The mineral assemblage of this rock is biotite + K－ mm) anhedral crystal in a thin melanocratic layer which feldspar + sillimanite + garnet + cordierite + plagioclase + consists mainly of biotite, cordierite, tourmaline and quartz + staurolite with minor amounts of tourmaline, plagioclase and is free from quartz (Fig. 5c). Staurolite 10 K. Maki, Y. Ishizaka and T. Nishiyama Staurolite－bearing gneiss and re－examination of metamorphic zonal mapping of the Higo metamorphic terrane 11 bearing gneiss. bearing － : : grossular components in mole fractions. (a) (r A Sample = 0.56 mm) (Sample Grt1 1) in Table grs : spessartine, : X spessartine, sps : : pyrope, X prp : : almandine, X alm biotite gneiss. Samples C and D are in the same thin section from the staurolite the from section thin same the in are D and C Samples gneiss. biotite － orthopyroxene zoning Compositional of garnets. X － (b) Sample B (r = 0.43 mm) (Sample Grt2 in Table 1) (c) Sample C (r = 1.00 mm) (d) Sample D (r = 0.22 mm) (Sample Grt in Table 2). Samples A and B are in the same thin section from the the from section thin same the in B are and A 2). Samples Table in Grt (Sample mm) D = (r 0.22 Sample mm) (d) C = (r 1.00 1) Sample (c) Table in Grt2 (Sample mm) B = (r 0.43 Sample (b) garnet Figure Figure 7. 10 K. Maki, Y. Ishizaka and T. Nishiyama Staurolite－bearing gneiss and re－examination of metamorphic zonal mapping of the Higo metamorphic terrane 11

sometimes includes plagioclase crystals. Staurolite is occuring in the melanocratic layer (Fig. 5b). Biotite chemically homogeneous, containing up to 53.5 －55.9 flakes surround the porphyloblast parallel to its outward － wt% Al2O3 and 0.82 1.38 wt% ZnO (Table 2). Its XMg form, while biotite crystals included in the porphyloblast ranges from 0.16 to 0.19, which is lower than that (0.26－ are randomly oriented. The third type of cordierite fills 0.30) of staurolite inclusions in garnet reported by Osanai interstices of sillimanite crystals (Fig. 5a). Composition 3 + － et al. (1996). Fe in staurolite is assumed to be about of the cordierite (XMg = 0.63 0.65; Table 2) is almost the 3.5% of total Fe following Holdaway et al. (1991). same among the three types. Garnet occurs in the biotite －rich layer as subheral Plagioclase occurs as anhedral crystal of about 0.1 porphyloblast up to 2.0 mm in diameter and is free from mm in size. Plagioclase in the matrix has a homogeneous － inclusions. It is rich in almandine (Xalm = 0.71) and poor composition with XAn = 0.26 0.36, while plagioclase in- － in grossular (Xgrs = 0.03 0.04) components (Table 2). cluded in staurolite shows higher content: XAn = － Xprp ranges from 0.13 to 0.15, and Xsps from 0.11 to 0.13. 0.42 0.49 (Table 2). Two types of chemical zoning are observed in the garnet Sillimanite occurs as euhedral prismatic to rhombic from this specimen. Outward zonal structure consisting crystal of about 0.17 mm in size in the biotite－rich layer of increase in Fe and Mg and decrease in Mn is observed (Fig. 5a). Parallel alignment of crystals elongated to c－ in coarse grains of garnet (Fig. 7c). The outermost rim axis is common. Fibrolite does not occur in this sample. shows reverse zoning with decreasing Mg and increas- ing Mn towards rim. Ca is almost constant throughout Andalusite－sillimanite－bearing gneiss the grain. This type of zoning indicates growth zoning. The other type of zonal structure consists of almost ho- We found an andalusite－sillimanite －bearing gneiss from mogeneous core and mantle with outermost rim where the same outcrop as that of the staurolite－bearing gneiss Mg decreases and Mn increases, and is observed in fine (Fig. 8). The gneiss consists of andalusite, sillimanite grains of garnet (Fig. 7d). This type of zonal structure has (prismatic crystals and fibrolite), biotite, K －feldspar, been interpreted to mean that the original growth zoning plagioclase and muscovite with accessory tourmaline and has been homogenized by diffusion (Yoshimura, 1995). opaque minerals and is free from quartz. Andalusite oc- Although Yoshimura (1995) considered that the growth zoning and the homogenized zoning are well observed in garnets from lower grade and higher grade zones, re- spectively, our observation of the coexistence of the two types of zoning in the same rock implies that the degree of homogenization due to diffusion depends on the grain size of garnet in the garnet －cordierite zone. There are many samples having garnets with two types of zoning in the same rock (as described in Fig. 15) in the Kosa area. Biotite is subhedal tabular crystal with distinct . It occurs in two different mode: one in the melanocratic layer (the biotite －rich layer) and the other surrounding staurolite. The chemical compositions of biotite differ depending on the mode of occurrence. Bio- － tite from the melanocratic layer has XFe = 0.52 0.55 and － XTi = 0.06 0.08 (Fig. 9). XTi becomes smaller at rims in contact with garnet. Biotite surrounding staurolite shows － － slightly larger variation: XFe = 0.50 0.55 and XTi = 0.03

0.08 (Fig. 9). There is a positive correlation between XFe

and XTi as shown in Figure 9, but such a correlation is not very clear for biotite from the melanocratic layer. Cordierite occurs in three different mode. One is anhedral granular crystal (Fig. 5c) in the melanocratic layer, about 0.1 mm in size and free from inclusions. The second type is coarse grained porphyroblast (about 2.3 Figure 8. Geologic sketch map showing occurrences of the stau- mm in size) with abundant inclusions of biotite, plagio- rolite－bearing gneiss and andalusite－sillimanite－bearing gneiss. clase, quartz, apatite, tourmaline and opaque minerals, The star represents the locality of these rocks. 12 K. Maki, Y. Ishizaka and T. Nishiyama Staurolite－bearing gneiss and re－examination of metamorphic zonal mapping of the Higo metamorphic terrane 13

－ Figure 9. Plots of XFe versus XTi in biotite from the staurolite bearing gneiss in the Kosa area.

Figure 11. Photomicrograhs showing occurrences of minerals in garnet －orthopyroxene －biotite rock from the garnet －cordierite zone. (a) Anhedral crystals of orthopyroxene (Sample Opx in Table 3). (b) A skeletal crystal of garnet (Sample Grt in Table 3).

Garnet－orthopyroxene－biotite rock

Figure 10. Photomicrograph showing occurrences of minerals in andalusite－sillimanite－bearing gneiss from the garnet－cordierite This is a particular rock exceptionally involving orthopy- zone. Andalusite occurs as subhedral prismatic to rhombic por- roxene found from the middle part of the garnet －cordierite phyloblast. The porphyloblast includes sillimanite, biotite, tour- zone (Fig. 3). The rock occurs as a thin metamorphosed maline and opaque minerals. The c－axis of sillimanite inclusion dyke of about 1 m width and pods of about 1 m in size is parallel to that of mother andalusite. Sillimanite also occurs as in garnet －bearing metapelites. The rock is massive and euhedral prismatic to rhombic crystal up to 0.4 mm in the matrix. fine grained without any obvious layering. The mineral assemblage is biotite + K－feldspar + garnet + orthopyrox- curs as subhedral prismatic to rhombic porphyloblast (Fig. ene + plagioclase + quartz. 10) of about 1 mm in size. The porphyloblast includes Orthopyroxene occurs as anhedral crystal of about sillimanite, biotite, tourmaline and opaque minerals. The 0.05 mm in size, much smaller than that in the orthopy- c －axis of sillimanite inclusion is parallel to that of host roxene zone (Fig. 11a). Its modal composition is 4.6%, andalusite. Sillimanite also occurs as euhedral prismatic which is higher than that from the orthopyroxene zone (0.8 － to rhombic crystal of up to 0.4 mm in the matrix. XAn of %). It shows XFe = 0.52 0.53, which is more magnesian － plagioclase is 0.31. Biotite compositions are XFe = 0.53 than that from the orthopyroxene zone (Tables 1 and 3). － 0.55 and XTi = 0.07 0.08. Garnet occurs in a small amount as anhedral skeletal crys- tal of about 1.6 mm in size with abundant quartz inclu- sions (Fig. 11b). It is richer in almandine and grossular 12 K. Maki, Y. Ishizaka and T. Nishiyama Staurolite－bearing gneiss and re－examination of metamorphic zonal mapping of the Higo metamorphic terrane 13

Table 3. Representative analyses of minerals in garnet－ortho- poorer in FeO than metapelites in the orthopyroxene zone pyroxene－biotite rock in the garnet－cordierite zone (Fig. 12). This indicates that garnet + orthopyroxene + biotite assemblage can be stable in this zone and that or- thopyroxene may occur depending on the bulk composi- tion.

Discussion

Reactions involving staurolite

Osanai et al. (1995; 1996) found a staurolite inclusion in garnet from Zone D metapelite at Toyono, about 10 km southwest of the staurolite locality of this study. They proposed the following reactions for the breakdown of staurolite based on the find of inclusions of spinel, sil- limanite and corundum in the cordierite, which coexists with the garnet including staurolite: staurolite + quartz = garnet + cordierite + sillimanite

+ H2O (1), staurolite + corundum = spinel + sillimanite (2), and staurolite = cordierite + spinel + sillimanite

+ H2O (3). Neither spinel nor corundum occurs in the present staurolite－bearing gneiss from the study area, and, there- fore, reactions (2) and (3) do not apply to this case. We will employ here a chemographic analysis of the staurolite－bearing assemblage from Kosa using Schreine- marker’s method to clarify the reaction relations (Fig. 13). The assemblage in the staurolite －bearing gneiss, staurolite + garnet + cordierite + sillimanite + biotite + K－ feldspar + quartz, can be treated in a KFMASH system. The five phase assemblage, staurolite + garnet + cordierite + sillimanite + biotite, represents an invariant point in a － － P T diagram in excess of K feldspar, quartz and H2O, and there are five univariant reactions emanating from the invariant point as follows:

[Str] garnet + cordierite = sillimanite + biotite [Grt] staurolite + cordierite = sillimanite + biotite [Crd] staurolite = sillimanite + garnet + biotite

* ** [Sil] staurolite + biotite = garnet + cordierite FeO , total iron as FeO. XMg = Mg / (Mg + Fe). c, core; r, [Bt] staurolite = garnet + cordierite + sillimanite rim. (reaction (1)) Although the staurolite －bearing gneiss contains all － － (Xalm = 0.64 0.65; Xgrs = 0.13 0.14) and poorer in pyrope the five minerals, they are not equilibrium assemblage, － (Xpyp = 0.13 0.14) components than that from ordinary because staurolite is completely surrounded by cordierite metapelites in this zone (Table 3). Biotite occurs as subhe- crystals and is not in direct contact with other minerals － － dral tabular crystal with XFe = 0.49 0.52 and XTi = 0.07 (Fig. 5c). This occurrence suggests that cordierite is a

0.09 (Table 3). XAn in plagioclase ranges from 0.63 to reaction product, and so the two reactions, [Sil] and [Bt], 0.78, which is much higher than that in plagioclase from are possible for the staurolite breakdown in Kosa. Further ordinary metapelites (Table 3). A’FM diagram shows that studies are required to resolve the precise reaction for

the bulk composition of the rock is poorer in Al2O3 than staurolite breakdown. ordinary metapelites in the garnet －cordierite zone and The reaction [Str] is placed on the higher tempera- 14 K. Maki, Y. Ishizaka and T. Nishiyama Staurolite－bearing gneiss and re－examination of metamorphic zonal mapping of the Higo metamorphic terrane 15

Figure 12. A’FM diagram projected from K－feldspar in Thompson’s AKFM system for (a) the garnet－cordierite and (b) the orthopyroxene zones.

Figure 13. Schematic Schreinemakers’ net showing reactions involving staurolite. 14 K. Maki, Y. Ishizaka and T. Nishiyama Staurolite－bearing gneiss and re－examination of metamorphic zonal mapping of the Higo metamorphic terrane 15

Figure 14. P－T diagram giving the estimated condition of the Higo metamorphic rocks, the estimated P－T path of the Higo metamorphic rocks from the garnet－cordierite zone (large arrow) in this study, the P－T path of Zone D in the Toyono area (small arrow) obtained by Osanai et al. (2001). Relevant reaction curves after Spear and Cheney (1989).

－ ture side of the reaction, muscovite + quartz = Al2SiO5 + The garnet cordierite isograd － K feldspar + H2O (4), in Figure 14 following Spear and Cheney (1989) who preclude the association muscovite + Discontinous reaction which characterizes the garnet + cordierite to be consistent with field observations isograd between the biotite and the garnet－cordierite zones by Holdaway and Lee (1977). Because one of excess can be deduced from the tie －line switching reaction in phases is K－feldspar at the invariant point, the invariant A’FM daigram (Fig. 12a) as follows: biotite + sillimanite － point should be on the higher temperature side of reaction + quartz = garnet + cordierite + K feldspar + H2O. As (4) or just on reaction (4). Furthermore, the point should described above, this reaction [Str] is one of five reactions be placed on or near the univariant curve of sillimanite = emanating from the invariant point. This reaction is sup- andalusite in the sillimanite field (Fig. 14), because the ported by the following petrographic evidence: occur- staurolite －bearing gneiss contains sillimanite, and also rences of cordierite filling the interstices of sillimanite because andalusite －sillimanite －bearing gneiss occurs at crystals in the northern part of the garnet－cordierite zone the same outcrop as described above. This gives a strong (Fig. 5a), some biotite included in cordierite porphylo- constraint on the pressure－temperature conditions of the blasts in the northern part of the garnet －cordierite zone metamorphism such that pressure should be about 200 (Fig. 5b) and fibrolite inclusion in minerals (Fig. 4a). MPa and temperature 600－620°C. Obata et al. (1994) deduced the reaction defining We have no direct evidence for the staurolite－form- the K －feldspar isograd between Zones B and C as fol- ing reaction in the Kosa area. The fact that no chloritoid lows: muscovite + quartz = K －feldspar + sillimanite +

occurs in the Higo metamorphic rocks implies that H2O (4). As discussed above, the reaction is located near reactions consuming chloritoid to produce staurolite is the invariant point in a P－T diagram and the P－T condi- unlikely to have occurred in the Kosa area, although such tion of the garnet－cordierite isograd is near the invariant reactions are responsible for staurolite formation in many point of the five mineral assemblage (staurolite + garnet metamorphic terrane (e.g. Hiroi, 1983; Spear, 1993). The + cordierite + sillimanite + biotite) which will be placed only potential candidate for the staurolite －forming reac- on or near the univariant curve sillimanite = andalusite in tion in the Kosa area is: the sillimanite field (Fig. 14). This implies that any P－T

garnet + chlorite = staurolite + biotite + H2O (5), paths passing through or near the invariant point will give which is consistent with mineral assemblages of only a very small temperature difference between reaction metapelites from the Kosa area. (4) and [Str]. Thus Zone C will be very narrow if ever 16 K. Maki, Y. Ishizaka and T. Nishiyama Staurolite－bearing gneiss and re－examination of metamorphic zonal mapping of the Higo metamorphic terrane 17 existed and cannot be defined practically in the Kosa area. Pressure－temperature conditions

Orthopyroxene isograd Peak metamorphic temperature for the garnet －cordierite zone is estimated by employing various computations The orthopyroxene isograd between the garnet－cordierite of the garnet －biotite geothermometer (Ferry and Spear, and the orthopyroxene zones is defined by the appearance 1978; Hodges and Spear, 1982; Ganguly and Saxena, of orthopyroxene in metapelites. As described above, 1984; Indares and Martignole, 1985; Williams and Gram- both biotite and garnet from the garnet －orthopyroxene － bling, 1990; Dasgupta et al., 1991). We did not apply biotite gneiss in this zone show resorption textures and other geothermometers to garnet －cordierite and garnet － garnet is completely surrounded by plagioclase crystals. orthopyroxene pairs, because garnets are in contact nei- Furthermore, there is no such textural evidence in the gar- ther with cordierite nor with orthopyroxene. In the case － － － net orthopyroxene biotite rock in the garnet cordierite of garnets with growth zoning, we used the highest XPrp zone. These facts suggest that the orthopyroxene－form- composition that represents the peak metamorphic condi- ing reaction in the orthopyroxene zone differs from that in tion. Core compositions of biotite in contact with garnet － the garnet cordierite zone. We deduce the following de- are used. If we use the lowest XPrp content (core com- hydration melting reaction for the orthopyroxene isograd position) of the zoned garnet, the estimated temperature

(Fig. 12b); biotite + garnet = orthopyroxene + plagioclase is much lower than that for the highest XPrp composition + melt. Obata et al. (1994) proposed the following reac- (Fig. 15). The garnet rim－biotite core pair for the stauro- tions: biotite + quartz = orthopyroxene + K －feldspar + lite －bearing gneiss gives temperatures between 590 and liquid according to Peterson and Newton (1989) and Viel- 690°C, depending on the selected geothermometers (Fig. zeuf and Clemens (1992) and stated that the dehydration 14). The temperature is consistent with the equilibrium melting reaction may be important and responsible for condition (200 MPa and 600－620oC) constrained by the the large degree of melting as observed in the pervasive staurolite breakdown reaction and the andalusite －silli- occurrence of diatexite because the melt produced by the manite transition. This temperature is also consistent with reaction is nearly equal to the volume % of initial content a widespread occurrence of migmatites in the garnet － of biotite (Peterson and Newton, 1989). This reaction is cordierite zone (Obata et al., 1994). also possible as the orthopyroxene －forming reaction in Sillimanite inclusions in andalusite indicate the this zone, however, we have no textural evidence for it. nucleation and growth of sillimanite in andalusite mother crystal, because all crystals of sillimanite have the same orientation governed by of andalusite (Fig. 10). This implies the P－T path from the andalusite

Figure 15. Temperature conditions of the metamorphism for the Higo metamorphic rocks increasing towards south from the garnet －cordierite isograd. Temperatures are estimated using the garnet －biotite geothermometer of Indares and Martignole (1985) and assuming pressure to be 200 MPa. Data based on rim compositions of garnets with growth zoning show a south- ward increment, because they will retain the original composition. Data based on homogenized garnets will show retrograde condition due to Fe－Mg exchange. 16 K. Maki, Y. Ishizaka and T. Nishiyama Staurolite－bearing gneiss and re－examination of metamorphic zonal mapping of the Higo metamorphic terrane 17

to the sillimanite field (Fig. 14). The estimated P－T path The estimated P －T path for the garnet －cordierite is shown in Figure 14. zone shows heating at low pressures (～200 MPa), which The P －T path obtained by Osanai et al. (2001) in is quite different from that in the Toyono area estimated the Toyono area is also shown in Figure 14 for compari- by Osanai et al. (2001). Further studies are required to son. They considered that Zone D of Obata et al. (1994) reconcile these two results. was locally re－heated by the intrusion of the Shiraishino The peak metamorphic temperature estimates using granodiorite at about 120 Ma following the peak of meta- garnet－biotite geothermometers show an increasing trend morphism at about 250 Ma. The difference between the southward from the garnet －cordierite to the orthopyrox- two P－T paths may represent either a local difference in ene zones, providing an estimate of geothermal gradient metamorphic conditions or a lack of enough information as 62°C per 1 km. The cause for this high gradient re- to reconcile the two results. Further studies will be re- mains unresolved. quired in either case. For the estimate of temperature in the orthopyroxene Acknowledgments zone we take Indares and Martignole’s (1985) garnet － － biotite geothermometer which incorporates non ideal in- We greatly appreciate Dr. M. Obata for his constructive teractions between Ca and Mn in garnet and also between informal reviews of the manuscript and encouragement － Ti and Al in biotite, because it is known that such non throughout the work. Constructive reviews by Drs. S. idealities give significant effects on geothermometry for Banno and T. Tsunogae are also appreciated. We are granulite facies conditions. Computations using this ther- grateful to Dr. H. Isobe for his kind assistance in micro- － mometer yield temperatures of 780 830°C. probe analysis. T.N. extends his sincere thanks to Drs. The peak metamorphic temperature estimated by Y. Osanai and M. Owada for their guide in the field trip geothermometer increases towards south (Fig. 15). The to the Toyono area and also for discussions on the Higo geothermal gradient calculated by this result gives 62°C metamorphic terrane. This work has been financially sup- per km, which is higher than that of the Ryoke metamor- ported by Grant －in Aid of JSPS (Grant No.11304038 to － phic belt (～40 50°C / km; e.g. Okudaira, 1996). At pres- T.N.). ent we have no explanation for such an abnormally high geothermal gradient. The thermal effect of granodiorite intrusion is only minor as discussed by Obata et al. (1994). References We also confirmed this view by examining the tem- Dasgupta, S., Sengupta, P., Guha, D. and Fukuoka, M. (1991) A perature condition of a pelitic xenolith in the Shiraishino refined garnet－biotite Fe－Mg exchange geothermometer and granodiorite. The estimated temperature by garnet－biotite it application in amphibolites granulites. Contributions to geothermometer is about 820°C, which is consistent with Mineralogy and Petrology, 109, 130－137. that of metapelites from the orthopyroxene zone. Ferry, J.M. and Spear, F.S. (1978) Experimental calibration of the partitioning of Fe and Mg between biotite and garnet. Con- tributions to Mineralogy and Petrology, 66, 113－117. Conclusion Ganguly, J. and Saxena, S.K. (1984) Mixing properties of alumi- nosilicate garnets: constraints from natural and experimental The Higo metamorphic rocks in the Kosa area can be data, and applications to geothermo －barometry. American divided into three zones in the order of increasing grade Mineralogist, 69, 88－97. from north to south: biotite, garnet－cordierite, and ortho- Hiroi, Y. (1983) Progressive metamorphism of the Unazuki Pelitic in the Hida Terrane, Central Japan. Contributions to pyroxene zones. Mineralogy and Petrology, 82, 334－350. We newly report the occurrence of staurolite in the Hodges, K.V. and Spear, F.S. (1982) Geothermometry, geobarom- － matrix of metapelite from the garnet cordierite zone in etry and the Al2SiO5 triple point at Mt. Moosilauke, New the Kosa area. Chemographic analysis of the staurolite－ Hampshire. American Mineralogist, 67, 1118－1134. bearing assemblage shows the following two reactions as Holdaway, M.J. and Lee, S.M. (1977) Fe－Mg cordierite stability in high－grade pelitic rocks based on experimental, theoreti- candidates for staurolite breakdown reaction in the Kosa cal, and natural observations. Contributions to Mineralogy area: [Sil] staurolite + biotite = garnet + cordierite and [Bt] and Petrology, 63, 175－198. staurolite = garnet + cordierite + sillimanite in excess of Holdaway, M.J., Mukhopadhyay, B., Dyar, M.D., Dutrow, B.L., － K feldspar, quartz and H2O in a KFMASH system. The Rumble III, D. and Grambling, J.A. (1991) A new perspec- rare occurrence of staurolite from the Higo metamorphic tive on staurolite crystal chemistry: Use of stoichiometric and chemical end－members for a mole fraction model. American rocks may be due to bulk compositions of metapelites, Mineralogist, 76, 1910－1919. most of which are poor in FeO as deduced from A’FM Indares, A. and Martignole, J. (1985) Biotite －garnet geother- diagram. mometry in the granulite facies: the influence of Ti and Al in 18 K. Maki, Y. Ishizaka and T. Nishiyama

biotite. American Mineralogist, 70, 272－278. H. (1996) High －temperature metamorphism and crustal Kamei, A., Owada, M., Osanai, Y., Hamamoto, T. and Kagami, H. evolution of the Higo metamorphic terrane, central Kyushu, (1997) Solidification and cooling ages for the Higo plutonic Japan. Tectonics and Metamorphism (The Hara Volume). rocks in the Higo metamorphic terrane, central Kyhushu. pp.404, SOUBUN Co., Ltd., 113－124. Journal of Mineralogy, Petrology and Economic Geology, 92, Osanai, Y., Hamamoto, T., Maishima, O. and Kagami, H. (1998) 316－326. Sapphirine－bearing granulites and releted high －temperature Kano, H. and Uruno, K. (1995) Discovery of kyanite residual metamorphic rocks from the Higo metamorphic terrane, west － from the Higo metamorphic belt in addition to the occurrenc- central Kyushu, Japan. Journal of Metamorphic Geology, 16, es of zircon and garnet. Abstracts. The102th Annual Meeting 53－66. of the Geological Society of Japan, 288. Osanai, Y., Kamei, A., Owada, M. and Hamamoto, T. (2001) Tec- Karakida, Y. and Yamamoto, H. (1982) Garnet amphibolites in the tono －metamorphic evolution and related igneous activities Higo metamorphic terrane, middle Kyushu. The Memoirs of of the Higo metamorphic terrane, west－central Kyushu (ex- the Geological Society of Japan, 21, 173－187. planatory text and field guide). Reports of tectonics working Karakida, Y., Yamamoto, H. and Hayama, Y. (1989) The Mano- group in Kyushu, 5, 1－62. tani metamorphic rocks, Kumamoto Prefecture and its rever- Peterson, J.W. and Newton, R.C. (1989) Reversed experiments on sion. The Memoirs of the Geological Society of Japan, 33, biotite－quartz－feldspar melting in the system KMASH: im- 199－215. plications for crustal anatexis. Journal of Geology, 97. 465－ Kretz, R. (1983) Symbols for rock－forming minerals. American 485. Mineralogist, 68, 277－279. Sakashima, T., Terada, K., Takeshita, T. and Sano, Y. (2003) Large － Nagakawa, K. (1991) Metamorphic zonal mapping of the Higo scale displacement along the Median Tectonic Line, Japan: Metamorphic Belt in the Toyono region, Kumamoto Prefec- evidence from SHRIMP zircon U－Pb dating of granites and ture. Bachelor’s thesis, Kumamoto University. gneisses from the South Kitakami and paleo －Ryoke belts. Nagakawa, K., Obata, M. and Itaya, T. (1997) K－Ar ages of the Journal of Asian Earth Sciences, 21, 1019－1039. Higo metamorphic belt. The Jounal of the Geological Soci- Spear, F.S. and Cheney, J.T. (1989) A for pelitic － － － － － － ety of Japan, 103, 943 952. schists in the system SiO2 Al2O3 FeO MgO K2O H2O. Nakajima, T., Nagakawa, K., Obata, M. and Uchiumi, S. (1995) Contributions to Mineralogy and Petrology, 101, 149－164. Rb －Sr and K －Ar ages of the Higo metamorphic rocks and Spear, F.S. (1993) Metamorphic phase equilibria and pressure － related granitic rocks, southwest Japan. Jounal of the Geo- temperature －time paths. pp.799, Mineralogical Society of logical Society of Japan, 101, 615－620. America Monograph. Mineralogical Society of America, Obata, M., Yoshimura, Y., Nagakawa, K., Odawara, S. and Osanai, Washington, D.C., 352－356. Y. (1994) Crustal anatexis and melt migrations in the Higo Tsuji, S. (1967) Petrology of the Higo metamorphic complex in metamorphic terrane, west －central Kyushu, Kumamoto, the Kosa－Hamamachi area, Kumamoto Prefecture, Kyushu. Japan. Lithos, 32, 135－147. Japanese Journal of Geology and Geography, 38, 13－25. Okamoto, K., Hara. I. and Suzuki, M. (1989) A preliminary report Vielzeuf, D and Clemens, J.D. (1992) The fluid －absent melting on geological structure of the Manotani－Higo metamorphic of phlogopite + quartz: Experiments and models. American rocks of the Kosa district, Kyushu. The Memoirs of the Geo- Mineralogist, 77, 1206－1222. logical Society of Japan, 33, 187－198. Yamamoto, H. (1962) Plutonic and metamorphic rocks along the Okudaira, T. (1996) Thermal evolution of the Ryoke metamorphic Usuki－Yatsushiro tectonic line in the western part of central belt, southwestern Japan: tectonic and numerical modeling. Kyushu. Bulletin of the Fukuoka Gakugei University, 12, III, The Island Arc, 5, 373－385. 93－172. Osanai, Y., Masao, S. and Kagami, H. (1993) Rb－Sr whole rock Yoshimura, Y. (1995) Compositional zoning of garnets from the isochron ages of granitic rocks from the central Kyushu, Higo metamorphic belt, west－central Kyushu, Japan. Kuma- Japan. The Memoirs of the Geological Society of Japan, 42, moto Journal of Science, Earth Sciences, 14, 1－18. 135－150. Williams, M.L. and Grambling, J.A. (1990) , ferric Osanai, Y., Hamamoto, T. and Maishima, O. (1995) High temper- iron, and the equilibrium between garnet and biotite. Ameri- ature metamorphism and crustal anatexis in the Higo meta- can Mineralogist, 75, 886－908. morphic terrane, central Kyushu, Japan. Abstracts. The102th Annual Meeting of the Geological Society of Japan, 52. Manuscript received; 3 October, 2003 Osanai, Y., Hamamoto, T., Kamei, A., Owada, M. and Kagami, Manuscript accepted; 4 February, 2004