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Journal of Petrology | Volume 44 | Number 2 | Pages 329-354 | 2003
© Oxford University Press 2003
Two Stages of Sapphirine Formation During Prograde and Retrograde Metamorphism in the Palaeoproterozoic Lewisian Complex in South Harris, NW Scotland
DEPARTMENT OF CRUSTAL STUDIES, NATIONAL INSTITUTE OF POLAR RESEARCH, KAGA 1-9-10, ITABASHI-KU, 173-8515 TOKYO, JAPAN
*Present address: Department of Natural Environment, University of the Ryukyus, Senbaru 1, Nishihara, 903-0213 Okinawa, Japan. Telephone: (+81) 98 895 8359. Fax: (+81) 98 895 8316. E-mail: baba{at}edu.u-ryukyu.ac.jp
RECEIVED December 2, 2001; ACCEPTED August 26, 2002
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ABSTRACT |
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Two types of sapphirine occurrences were found in the Lewisian complex in South Harris, NW Scotland: (1) inclusions within porphyroblasts; (2) symplectic grains together with secondary cordierite, plagioclase and orthopyroxene. The presence of sapphirine inclusions implies that sapphirine was stable at the early stage of ultra-high-temperature metamorphism, whereas symplectic sapphirine grains were formed during decompressional retrograde metamorphism. The sapphirine occurrences and compositions of associated minerals depend on the host rock composition. Sapphirine inclusions occur only in rocks with high bulk-XMg, and sapphirine is never present as porphyroblastic grains because of its breakdown in response to pressure increase. Sapphirine symplectites can be seen in the relatively low bulk-XMg rocks, and the texture suggests local equilibrium in the Mg–Al-rich domain that is formed by metamorphic segregation of the Mg–Al-rich minerals in response to partial melting. The various sapphirine occurrences observed in South Harris were controlled by not only protolith composition but also local, mineral-scale, composition in a continuous metamorphic history.
KEY WORDS: sapphirine; Lewisian complex; high-Mg garnet; partial melting
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INTRODUCTION |
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In many high-grade terrains, sapphirine occurs as a symplectic grain that is formed during isothermal decompression in a clockwise P–T path (Harley, 1998a); e.g. Forefinger Point in East Antarctica (Harley et al., 1990; Motoyoshi et al., 1994), Perumalmalai in the Eastern Ghats (Raith et al., 1997), In Hihaou, In Ouzzal (Bertrand et al., 1992; Mouri et al., 1996), Gruf Complex, Italian Alps (Droop & Bucher-Nurminen, 1984), Bamble sector, Norway (Kihle & Bucher-Nurminen, 1992), Sveconorwegian granulite, Sweden (Möller, 1999), Grenville Province, Canada (Grant, 1989), and Limpopo belt, South Africa (Windley et al., 1984). On the contrary, sapphirine breakdown, forming coronas or porphyroblasts, is interpreted as an isobaric cooling path or an anticlockwise P–T path (Harley, 1998a); e.g. Labwor Hills, Uganda (Sandiford et al., 1987), Strangways Range, Australia (Coscombe, 1992), and Napier Complex, East Antarctica (Harley, 1998a).
In the South Harris area in NW Scotland, an anticlockwise P–T path has been proposed on the basis of mineral textures, mineral compositions and P–T conditions estimated from pelitic, quartzofeldspathic and mafic gneisses, which are widely exposed in the Leverburgh belt (Baba, 1998). The proposed P–T path, involving ultra-high-temperature (UHT) metamorphism (Baba, 1999a), is also supported by the formation and breakdown reaction of surinamite (Baba et al., 2000). Two types of sapphirine-bearing rocks have been reported previously (Baba, 1999a), and this study reports another five types of sapphirine-bearing assemblages. The new sapphirine occurrences indicate that they were formed during both the prograde and retrograde stage of a continuous metamorphic history involving anticlockwise P–T paths.
This paper presents possible models of metamorphic processes responsible for sapphirine breakdown and formation in rocks of various compositions, and in response to the distinctive metamorphism in South Harris. All mineral abbreviations used in this discussion are after Kretz (1983).
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GEOLOGICAL SETTING |
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The Lewisian complex is a TTG (tonalite–trondhjemite– granodiorite) gneiss terrain with numerous enclaves of mafic to ultramafic rocks and rafts of supracrustal material. The evolutionary history of the Lewisian complex was summarized by Park et al. (1994). Within the mainland Lewisian, recent isotopic dating has established: (1) three TTG protolith ages—3·0 Ga for the central region, 2·8–2·7 Ga for the northern region (Kinny & Friend, 1997; Whitehouse et al., 1997) and
2·8 Ga for the southern region (Friend & Kinny, 2001); (2) two stages of high-grade metamorphism, at 2·7 and 2·5 Ga (Corfu et al., 1994; Zhu et al., 1997; Friend & Kinny, 2001); (3) 1·75–1·74 Ga amphibolite-facies metamorphism, involving kyanite and staurolite formation, in the Proterozoic Laxfordian shear zone (Zhu et al., 1997; Friend & Kinny, 2001).
The Lewisian complex in South Harris (South Harris complex), exposed in the central part of the Outer Hebrides (Fig. 1), is composed of the high-pressure granulite-facies Leverburgh belt, the amphibolite-facies Langavat belt and a Palaeoproterozoic plutonic igneous complex involving meta-anorthosite, metagabbro, metanorite and metadiorite (South Harris igneous complex: SHIC). Sm–Nd isotopic studies by Cliff et al. (1983, 1998) revealed that the protoliths of the South Harris metasediments were originally post-Archaean sediments deposited at c. 2·5–2·0 Ga, and the high-pressure granulite-facies metamorphism in South Harris occurred soon after the emplacement of the SHIC at c. >1·87–1·84 Ga. Recently, Friend & Kinny (2001) reported U–Pb single zircon protolith age of 1·88–2·78 Ga for the Leverburgh metasediment and of 1·88 Ga for the granulite-facies metamorphism.
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The Leverburgh belt is composed of pelitic, quartzofeldspathic and mafic gneisses with lensoid marble and meta-ultramafic rocks. Four metamorphic stages (M1–M4) were recognized in the Leverburgh belt (Baba, 1998), and trace an anti-clockwise P–T path. M1 represents a UHT event derived from the emplacement of the SHIC at 9–11 kbar and 900–980°C. M2, also a granulite event, progressed at c. 850–900°C with pressures increasing to 13–14 kbar. M3 was a retrograde event involving decompression under relatively dry conditions as evidenced by corona and symplectite formation of Opx–Crd, Opx–Pl and Hc–Crd replacing Grt porphyroblasts. M4, also a retrograde event, is characterized by the alignment of hydrous minerals modifying previous porphyroblastic assemblages. Geochemical studies of the mafic gneisses in the Leverburgh belt and their lithological assemblages suggest that the protoliths may have formed in an accretionary prism (Baba, 1997). Combining these results, the South Harris complex is considered to have formed by a continuous tectonothermal process involving subduction, magmatism, collision, exhumation and then uplift accompanied by deformation (Baba, 1998, 1999a, 1999b). On the basis of U–Pb zircon ages for the South Harris complex, Whitehouse & Bridgewater (2001) suggested the existence of a major tectonic boundary in the Outer Hebrides, interpreted as a result of collision. Similarly, Friend & Kinny (2001) proposed that the South Harris complex was a Proterozoic terrain, juxtaposed against the north Archaean Tarbert terrane after 1·65 Ga along a tectonic boundary at Langavat–Findsbay.
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FIELD OCCURRENCES AND PETROGRAPHY |
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The sapphirine-bearing granulites are found in seven localities, and termed here Grt–Opx granulite (sample 95913-7), Grt–Ky granulite (A) (95919-18), Grt–Ky granulite (B) (95908-B), Grt–Opx–Ky block (99921-1), Grt–Opx–Ky layer (99922-5), Opx–Sil granulite and Opx–Ky granulite. Sapphirine occurs either as inclusions within Grt or Als (aluminosilicate) or in secondary symplectites. As Baba (1999a) and Baba et al. (2000) have already described the Opx–Ky granulite (gneiss) and Opx–Sil granulite in some detail, this paper focuses on the occurrence of sapphirine in the new textures. Mineral assemblages of each lithology are summarized in Table 1.
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Sapphirine (inclusions)-bearing rock
Grt–Opx granulite (95913-7)
This rock occurs as a thin layer of up to 20 cm width embedded within Grt–Cpx-bearing mafic gneiss
500 m north from St. Clement church at Rodel [UK national grid reference, NG 048 873]. The rock consists mainly of Grt, Bt, Opx, Spr and Crd with small amounts of Pl, Sil, Crn, Ilm and Zrn. Garnet porphyroblasts include Spr, Opx and Bt in the core, and are partially replaced by secondary Crd, Opx–Crd symplectite (Fig. 2c) and Bt. Sapphirine inclusions (up to 0·8 mm in length) occur together with minor Crd, rounded Opx, Bt, and rarely Sil. In places, Spr inclusions further include tiny grains of Spl, Crd and Crn (Fig. 2a and b). Orthopyroxene porphyroblasts include biotite and form intergrowths with Grt. Secondary biotite has replaced marginal parts of the resorbed Grt and Opx, and forms a foliation that truncates the previous weak foliation marked by the elongation of Grt grains. Secondary Crd and Pl occur along the grain boundaries between Grt and Bt.
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Grt–Opx–Ky block (99921-1)
The rock, exposed 10 m south of the Opx–Ky granulite locality (Baba, 1999a) [NG 040 854], occurs as a rounded block or boudin,
50 cm in diameter, in the quartzofeldspathic gneiss. The central part of the block consists mainly of coarse-grained Grt and Opx, and rare Ky. The marginal part of the block is finer grained, and is enclosed in a quartz vein. Garnet, Opx, Ky, Bt and Crd are the main constituent minerals. Sapphirine occurs as inclusions within Grt together with Opx, Sil and Bt, and in secondary symplectites. Garnet and Opx form irregular grain boundaries. The Grt–Opx grain boundaries and Opx margins are replaced by fine-grained symplectites of Spr–Opx and Opx–Crd (Figs 3a and 4). Opx–Crd symplectites are widely developed throughout the thin section, and are replaced by secondary Bt (Fig. 3b).
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Opx–Sil and Opx–Ky granulite
These two rocks have been described previously in some detail (Baba, 1999a; Baba et al., 2000). The sapphirine textures are summarized below.
In Opx–Sil granulite, sapphirine occurs as inclusions within Sil, which in turn is surrounded by secondary Crd. Garnet, Al-rich Opx, Spr and Sil occur as inclusions in secondary large Crd corona, and are considered to have been stable before the growth of the Crd corona.
Opx–Ky granulite consists of Qtz, Kfs, Opx, Ky/Sil, Crd and Bt. Two types of Opx occur as porphyroblasts (Al2O3 up to 9·7 wt %) and irregular grains intergrown with Ky (Al2O3 up to 7·5 wt %). The latter contain Sil inclusions. The texture indicates early Opx–Sil and later Opx–Ky stability, consistent with a change in P–T conditions. Tiny Spr grains are enclosed by Sil/Ky, which in turn is surrounded by Qtz. In addition, Si-rich Spr, Opx and Sil are enclosed by surinamite (Sur), and the texture indicates an increase in pressure (Baba et al., 2000). Cordierite occurs as secondary grains replacing the Opx, Ky/Sil and Sur. Sapphirine inclusions within Sil in the quartz matrix imply the possible stable association of Spr and Qtz at moderately high fO2 condition at 950°C (Baba, 1999a).
Sapphirine (symplectite)-bearing rocks
Grt–Ky granulite (A) and (B)
In the Leverburgh belt, pelitic gneisses have been subdivided into two types, namely Na–K-rich and Fe–Al-rich pelitic gneisses (Baba, 1998). The latter type is typically observed on the southern marginal part of meta-anorthosite body (SHIC). Grt–Ky granulite (A) and (B) belong to the Fe–Al-rich pelitic gneiss.
Grt–Ky granulite (A) occurs as a thin lens (up to 5 cm) in a Grt-rich band, surrounded by the quartzofeldspathic gneiss (Grt–Cpx–Opx–Bt ± Hbl–Pl–Qt ± Kfs assemblage) near the contact of metagabbro (SHIC). The outcrop is located 800 m south of the summit of Benn na h-Air [NG 050 845]. The rock is coarse grained and crudely foliated, and consists chiefly of Grt, Pl, Qtz, Kfs, Bt, Ky, Sil and Crd, and subordinate Opx, Spr, Spl, Ti–Fe oxide (ilmenite and titanomagnetite), Ap and Zrn. Two domains are recognized: (1) Grt–Als–Crd domain; (2) Grt–quartzofeldspathic domain. Garnet in both domains includes Sil, Bt, Qtz, Pl, Spl and Ti-oxide. Sapphirine symplectites occur in the Grt–Als–Crd domain (Spr-present domain: up to 25 mm in diameter) (Fig. 5). In the Spr-present domain, Opx–Spl and Spl–Crd associations (Fig. 6a) and rounded Crd can be seen as inclusions within a single Grt grain. Sapphirine occurs (1) in symplectites replacing Sil and/or Ky together with Crd (Fig. 6b), and (2) as secondary grains on the margins of Ti–Fe oxide adjacent to Spl (Fig. 6c). Orthopyroxene generally occurs in secondary symplectites replacing Grt porphyroblasts together with Crd in the Spr-absent domain.
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Grt–Ky granulite (B) occurs as a discontinuous garnetiferous band within Grt–Ky-bearing pelitic gneiss. This rock, exposed
700 m SW of the Roinebhal summit [NG 038 854], is surrounded by quartzofeldspathic gneiss with alternating Grt–Cpx–Pl–Qtz ± Opx ± Kfs and Grt–Opx–Qtz–Kfs–Pl assemblages. The Grt–Ky granulite (B) has compositional bands formed by felsic (Qtz, Pl, Grt and Bt: Spr-absent domain) and garnetiferous bands (Grt, Bt, Crd, Opx and Ky/Sil: Spr-present domain). Distinctive retrograde coronas were developed in each domain. In the marginal part of the Spr-present domain an irregular Grt margin is replaced by Opx–Crd coronas or symplectites (Fig. 7b). In the central part of the domain, Spr symplectites occur replacing Ky. The Spr symplectites are observed only on the Ky margin (Fig. 7a). In the Spr-absent domain, Opx–Pl coronas are developed on the margin of Grt (Fig. 7c) in the presence of ilmenite. Garnet includes Sil in both domains, but Spl occurs only in the Spr-absent domains. Biotite is also present in both domains replacing Grt porphyroblasts, Opx–Crd and Opx–Pl symplectites, and forms a foliation.
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Grt–Opx–Ky layer (99922-5)
This lithology forms a very narrow band (0·5 m) at the contact between orthopyroxenite and Grt–Ky gneiss. The outcrop is 15 m NE of the Grt–Ky granulite (B). The band consists of Ky-Bt-rich, Grt–Opx-rich, Grt–Ky–Opx-rich and Opx–Ky-rich layers with gradual changes from orthopyroxenite to Grt–Ky gneiss over a half-metre zone. The orthopyroxenite is composed of Opx (90 modal %), secondary Bt, and Pl with trace Qtz and rounded Bt inclusions. An interstitial domain consisting of Pl, Kfs and Qtz can be seen in the hand specimen, and thus the rock is considered to be of metamorphic restitic rather than igneous origin.
Sapphirine symplectites occur in the Grt–Opx–Ky layer, which consists mainly of Opx, Grt, Bt and Ky. In places Qtz is present together with Opx and Ky. Orthopyroxene and Grt form an intergrowth, and their contact and the Opx margins are generally replaced by fine symplectites of Opx–Crd, and less commonly Opx–Spr–Crd (Fig. 8a). The Opx–Crd symplectites are well developed throughout the thin section and most Spr symplectites are developed on the margins of Ky (Fig. 8b). Biotite occurs as inclusions within Grt and Opx, but more commonly as a secondary minerals.
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WHOLE-ROCK GEOCHEMISTRY |
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Whole-rock compositions were determined by X-ray fluorescence spectrometry at Ehime University (Phillips PW2400) and the National Institute of Polar Research (Rigaku RIX3000). Rock samples 4·0 cm x 2·5 cm in size were crushed by WC mill and quartz ball mill. The results are presented in Table 2.
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With the exception of the Grt–Opx–Ky layer, the occurrence of Spr is related to bulk-rock XMg. The rocks that have Spr inclusions show high bulk-XMg (Grt–Opx granulite, Grt–Opx–Ky block), whereas the rocks that have Spr symplectites replacing Ky/Sil have low bulk-XMg [Grt–Ky granulite (A) and (B)]. This compositional relationship is probably associated with P–T conditions during the different Spr-forming stages. We can infer that Spr inclusions were formed at an early metamorphic stage in the high bulk-XMg rock, whereas Spr symplectites were formed at a later stage in the low bulk-XMg rock via compositional modification. Sapphirine symplectites in the Grt–Ky granulite (A) and (B) occur in restricted domains that are rich in Grt and Sil/Ky. Accordingly, reintegrated compositions for Spr-bearing and Spr-absent domains were determined using modal analyses of constituent minerals and their average chemical compositions (Table 3). The Spr-present domains are characterized by enrichment in MgO and Al2O3 and depletion in SiO2 with regard to the Spr-absent domain.
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MINERAL CHEMISTRY |
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Chemical analyses of constituent minerals were performed using an electron microprobe with a wavelength-dispersive X-ray analytical system (JEOL JXA-8800M) both at the National Institute of Polar Research and at the Ehime University, and at Osaka City University (SHIMAZU 8705). Mineral analyses were performed using an accelerating voltage of 15 kV and specimen current of 4–12 nA. Natural minerals and synthetic oxides were used as standards. Mineral compositional data are presented in Tables 4–9.
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Sapphirine
Sapphirine composition varies in relation to textural setting. Figure 9 shows compositional features of Spr. The Spr inclusions generally have SiO2-rich compositions with ranges of 12·6–13·8 wt % SiO2 for the Grt–Opx granulite (95913-7) and 13·7–13·9 wt % for the Grt–Opx–Ky block (99921-1). Sapphirine has similar compositional ranges in the Opx–Sil granulite and the Opx–Ky granulite (Baba, 1999a). In the case of the Opx–Ky granulite the SiO2 content reaches >13·95 wt % and sometimes nearly 20 wt %.
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Symplectic Spr is more aluminous and less SiO2 rich than Spr inclusions. In the Grt–Ky granulite (A) and (B) (95919-18, 95908-B) Fe3+ contents calculated using charge balance are greater than those of Spr in the other samples (Fe3+/Fetotal = 0·13–0·52 and 0·42–0·45, respectively). In addition, symplectic Spr in the Grt–Ky granulite (A) has higher XMg than that occurring in coronas on Ti–Fe oxide. Sapphirine in symplectites in the Grt–Opx–Ky layer shows high XMg (0·88–0·90) with low Fe3+/Fetotal values of up to 0·13. These Spr have higher XMg than the coexisting symplectic Opx (up to 0·85). The composition of fine Spr in the Spr–Opx–Crd symplectites in the Grt–Opx–Ky layer and block could not be determined because of their small size.
Garnet
Garnet porphyroblasts within Spr-bearing assemblages have higher pyrope contents than those in surrounding pelitic gneiss (up to XMg 0·47: Baba, 1998). Garnet composition are plotted in Fig. 10 in terms of XMg [Mg/(Fe2+ + Mg)] and Xgrs. All Grt shows decreases in XMg toward the rim.
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Garnets in the Grt–Opx granulite and the Grt–Opx–Ky block with Spr inclusions have higher XMg than those of the Grt–Ky granulite (A) and (B). Grt porphyroblasts in the Grt–Opx–Ky layer record the highest XMg and lowest Xgrs.
Garnet compositions in the Grt–Ky granulite (A) and (B) vary with textural setting. Grt porphyroblasts in the Spr-present domain from both rock types record higher XMg (<0·55 in A) than those in the Spr-absent domain (<0·51 in A) within the same thin section. Relatively idioblastic Grt porphyroblasts in Spr-absent domains preserve compositional zoning with decreases in pyrope content and increases in grossular content from core (Xprp = 0·42–0·45; Xgrs = 0·03–0·04) to rim (Xprp = 0·36; Xgrs = 0·08). Similar compositional zoning has been reported by Baba (1998) and is thought to represent a pressure increase during metamorphism.
Orthopyroxene
Orthopyroxene occurs as inclusions, porphyroblasts (sometimes forming intergrowths with Grt), and in symplectites with Crd, Pl and Spr. Figure 11 shows Opx compositions in terms of XMg [Mg/(Mg + Fe2+] and XAl (Al cations/2, per 6 oxygens).
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High XMg values are preserved in Opx porphyroblasts and inclusions in Grt from the Grt–Opx–Ky layer and block. They are also relatively rich in Al2O3, whereas those in Grt–Opx granulite are relatively poor in Al2O3 and MgO. Symplectic Opx are poorer in MgO compared with Opx in other textural settings, and their XAl values are variable especially in the Grt–Opx granulite.
In the Grt–Ky granulites (A) and (B), Opx symplectites have compositional differences reflecting their coexisting minerals. Orthopyroxene occurring in Opx–Crd symplectites is Mg rich, whereas that in Opx–Pl symplectite is Mg poor in the Grt–Ky granulite (B). In the Grt–Ky granulite (A), high-Mg and Al–Opx inclusions can be seen in the Grt porphyroblasts together with Crd and Spl, whereas the Opx symplectite has low Mg and Al contents.
Cordierite
Cordierite inclusions within Grt in the Grt–Opx granulite and the Grt–Ky granulite (A) have relatively higher XMg values compared with secondary Crd within any individual sample. Secondary Crd has fairly similar XMg values regardless of the coexisting phase (e.g. Opx–Crd, Spr–Crd).
Biotite
Biotite inclusions within Grt from all lithologies have higher XMg and TiO2 contents than secondary Bt. Fluorine contents of biotite are uniformly low (up to 0·53 wt %), and maximum contents are generally recorded in biotite inclusions.
Spinel
Spinel inclusions within Grt have higher XMg and ZnO contents (3·47 wt %) than secondary spinel in the Grt–Ky granulite (A). In the Grt–Opx granulite, Spl inclusions enclosed by Spr, itself included in Grt, have the highest XMg compared with those in other textural settings. The Cr2O3 contents are low (1·2–3·2 wt %), but inclusion spinel in Grt–Ky granulite (A) has higher Cr2O3 contents than secondary spinel.
Aluminosilicate
Aluminosilicate contains up to 1·08 wt % Fe2O3, except for the Ky and Sil in the Grt–Ky granulite (A) and (B) (0·52–2·26 wt %).
Plagioclase
Matrix plagioclase generally has low An with a small range of compositions [An = 0·27–0·29 (Grt–Opx granulite); 0·29–0·33 (Spr-absent domain in Grt–Ky granulite (A) and (B)], whereas secondary plagioclase, which occurs with Opx and Bt, shows higher An values (An = 0·40–0·48). It should be noted that secondary Pl in Spr-present domains in the Grt–Ky granulite (B) has extremely high An values (An = 0·92).
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INTERPRETATION OF REACTION TEXTURES |
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Sapphirine-forming reactions
Sapphirine-forming reactions are inferred from inclusion assemblages within Spr. Most of the textures can be interpreted in the FMAS system. Inclusions of Spl, Crd and Crn within Spr are inferred to have been stable before Spr formation in the Grt–Opx granulite (95913-7), and probably indicate the following FMAS divariant reaction:
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A possible divariant reaction in the KFMAS system suggested by the Bt inclusions is
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Sapphirine-breakdown reactions
Sapphirine occurs as inclusions enclosed by Grt porphyroblasts both in the Grt–Opx granulite and the Grt–Opx–Ky block. In the Grt–Opx granulite, Crd, Opx and trace Sil occur as inclusions in association with Spr in the Grt porphyroblasts, consistent with Spr breaking down to form Grt by following the
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The Grt–Opx–Ky block has similar mineral associations to the inclusion minerals of the Grt–Opx granulite, but is lacking in Crd. Thus the following FMAS divariant reaction is plausible as a Grt-forming reaction:
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Other prograde reactions
The Opx–Spl and Crd–Spl associations and rounded Crd included in Grt in the Grt–Ky granulite can be interpreted as stable minerals at an early stage. The inferred FMAS divariant reaction is
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The formation of Grt in the Spr-absent domain in the Grt–Ky granulite (A) and (B) might have occurred via the reactions Bt + Pl + Qtz + Sil = Grt + Kfs + H2O/melt or Hc + Qtz = Sil + Grt as previously proposed (Baba, 1998).
Biotite inclusions commonly show low F contents of up to 0·5 wt %. Thus early biotite may have broken down to form Grt and/or Opx during the early stage under lower-temperature conditions (c. 860°C, at 8 kbar) (e.g. Vielzeuf & Clements, 1992; Hensen & Osanai, 1994).
Sapphirine symplectite formation and other retrograde reactions
In the Grt–Ky granulite (A), Spr symplectites occur as follows: (1) replacing Sil/Ky in association with Spl and Crd; (2) forming coronas on Ti–Fe oxides. These mineral textures probably indicate the following FMAS divariant reactions:
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Spinel appears to grow at the expense of Spr and Ti–Fe oxide, and might indicate the following reaction:
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Reaction (8) was proposed by Powell & Sandiford (1988) in the system FMASTO in the presence of Opx and Qtz. In the Spr-present domain, Qtz and Opx do not coexist, although high fO2conditions and relatively lower T and P are plausible for the formation of the Spr–Spl association (Powell & Sandiford, 1988; Fig. 7b). The high fO2 is evidenced by high Fe3+/Fetotal ratios of Spr in the symplectites (<0·52) and high Fe2O3 content of the aluminosilicate. The Spr composition is consistent with previously reported Spr composition formed under high fO2 (e.g. Fe3+/Fetotal = 0·32–0·70, Gnos & Kurz, 1994; 0·40–0·42, Dasgupta et al., 1995).
In the Grt–Ky granulite (B), Spr symplectites occur in association with Crd and An-rich Pl (An = 0·92), consistent with the following reaction:
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Formation of Pl via Grt breakdown is supported by high-Xgrs Grt content of 0·05–0·10. The Spr symplectites occur in the central part of the Spr-present domain, and are restricted to the Ky margin, whereas Grt in the marginal part of the domain is replaced by Crd–Opx coronas. This texture is consistent with Grt–Qtz reaction in the marginal part of the domain via the FMAS divariant reaction
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Garnet in the Spr-absent domain is replaced by Opx–Pl coronas. Formation of retrograde Spr in the central part of the Spr-present domains may be a consequence of an Mg–Al-rich local bulk composition. Garnet in the Spr-present domain generally has high XMg (0·50–0·56 for both samples), compared with that in the Spr-absent domain (XMg = 0·40–0·51). Figure 12 shows composition-assemblage relations for retrograde parageneses in Grt–Ky granulite (A) and (B). The relationship between coexisting mineral phases and reintegrated domain composition supports the formation of different types of retrograde minerals in response to the domain composition. Sapphirine-present domains plot on the Mg–Al-rich, Si-deficient side of Grt–Spr–Crd join, whereas Spr-absent domains plot on the Grt–Opx–Qtz join. In the case of Grt–Ky granulite (A), the Spr-bearing domain is plotted outside the join, although the critical domain composition will be inside the Grt–Crd–Spr join. Because half the volume of the Ti–Fe oxide is enclosed by Grt grains, the reacting surface is expected to be the more Mg-rich side (see arrow in Fig. 12).
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The Grt–Opx–Ky layer and block contain fine symplectites of Opx–Spr–Crd replacing Grt, Opx and/or grain boundaries between them. In addition, Spr–Crd symplectites with Crd are also observed on the margin of Ky. These rocks lack Qtz in the Spr-symplectite-bearing domains. The above textural relationships are consistent with the following FMAS univariant and divariant reactions:
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Large amounts of secondary Opx–Crd in these rocks were presumably produced by reaction (10).
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METAMORPHIC PRESSURE–TEMPERATURE CONDITIONS |
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 TOPABSTRACTINTRODUCTIONGEOLOGICAL SETTINGFIELD OCCURRENCES AND...WHOLE-ROCK GEOCHEMISTRYMINERAL CHEMISTRYINTERPRETATION OF REACTION... METAMORPHIC PRESSURE-TEMPERATURE... DISCUSSIONCONCLUSIONSREFERENCES |
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The early-stage mineral reactions observed in the Spr-bearing lithologies can be analysed in the model low-fO2 FMAS and KFMAS systems (Fig. 13) modified after Hensen (1971), Hensen & Harley (1990) and reproduced in Harley (1998a) with experimental calibrations (Bertrand et al., 1991). The Spr-forming reactions (2) probably took place at rising temperature under low-pressure conditions before Grt-forming reactions. Corundum-bearing reaction (1) also progressed in similar conditions to (2), in light of the topological relationships proposed by Hensen (1987). On the basis of the petrographical evidence, the Grt-forming Spr breakdown reactions (3) and (4) proceeded with increasing pressure under high-temperature conditions of
950 ± 30°C (Fig. 13) (Hensen & Harley, 1990). The high temperatures are also evidenced by the high Al2O3 content of Opx in the Opx–Grt–Ky block and layer (Al2O3 <7·9 and 8·0 wt %) in light of the Opx isopleths of Harley (1998a).
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The P–T conditions of equilibration of the early and late metamorphic stages have been estimated using experimentally calibrated geothermobarometers based on Grt–Opx and Grt–Opx–Pl–Qtz equilibria. The composition of Grt and associated Opx inclusion pairs and Grt core and Opx core pairs were used for the early stage, and Grt rim and Opx rim pairs for the late-stage metamorphic conditions. The maximum temperature calculated from each set of analyses is given Table 10. Temperature calculated using Grt–Opx Fe–Mg exchange thermometry (Harley, 1984a; Lee & Ganguly 1988; Carswell & Harley 1989) is up to 750°C, 10 kbar, except for the Grt–Opx granulite (95913-7), and is slightly higher than the result obtained using rim–rim pairs from the Grt–Ky granulite (A) and (B). As shown in Figs 3 and 8, the Grt–Opx block (99921-1) and the Grt–Opx layer (99922-5) were intensely replaced by secondary symplectites of Opx–Crd. Thus the compositions are likely to have been modified by the development of these secondary minerals. The calculated results for core–core pairs and core–inclusion pairs are considerably lower than temperatures estimated from the petrogenetic grid. The results presumably represent re-equilibration temperatures of Grt–Opx (e.g. Fitzsimons & Harley, 1994). Pressures calculated using Grt–Opx barometry (Harley & Green, 1982) and Grt–Opx–Pl–Qtz barometry (Newton & Perkins, 1982) at 750°C are 8·5–9·7 kbar for the Opx–Pl coronas in Grt–Ky granulite (B).
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In the Grt–Ky granulite (A) the temperature conditions of the Spr symplectite-forming stage are estimated by the adjacent Spr–Spl association. An empirical Spr–Spl Fe–Mg exchange thermometer proposed by Owen & Greenough (1991) was applied to the Spr–Spl association, and the results show temperature ranges from 814 to 840°C. The relative P–T conditions for the early stage (M1) are also estimated using XMg [Mg/(Mg + Fe2+)] and XAl (Al/2, per 6 oxygen unit) isopleths for orthopyroxene (Hensen & Harley, 1990). The Opx–Crd associations, together with minor Qtz, occurring as Grt inclusions indicate P–T conditions of c. 900°C, 8·5 kbar. These conditions are broadly consistent with the petrogenetic grid for the M1 stage.
In the ideal system, retrograde reactions (6) and (7) could be used to estimate the retrograde P–T conditions using the petrogenetic grid of Fig. 13. However, this is not possible for the following reasons: (1) Fe3+/Fetotal of Spr in symplectites is high, and Ky contains high amounts of Fe2O3 (up to 1·96 wt %), both of which are indicative of high fO2; (2) Spr symplectites generally occur replacing Ky. These features result in a shift of the grid to lower temperatures.
Reactions (11)–(13), inferred from both the Grt–Opx–Ky layer and block, have been proposed as Spr-forming reactions in other Qtz-absent and Qtz–Sil-absent UHT granulites by Harley et al. (1990), Raith et al. (1997) and Harley (1998a). These workers interpreted the textures as indicative of UHT metamorphic conditions of >950°C. However, the Al2O3 content of associated Opx (up to 6·5 wt %) is lower than those of other UHT granulites, e.g. 6·9–7·8 wt % (Raith et al., 1997) and 7·0–7·8 wt % (Harley, 1998b). According to the experimental results of Aranovich & Berman (1996), a 1·0 wt % change in the Al2O3 content of Opx corresponds to a temperature change of 100°C. Thus, the Spr-bearing textures from the Grt–Opx–Ky layer and block may have formed at slightly lower temperatures of 850–900°C.
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DISCUSSION |
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Metamorphic conditions related to sapphirine formation
Sapphirine inclusions within aluminosilicate were recognized in the Opx–Ky and Opx–Sil granulite and are considered to have been stable during M1 (Baba, 1999a). Peak metamorphic temperatures of c. 950 ± 30°C and c. 930 ± 30°C have been estimated using Opx isopleths and petrogenetic grids, which are consistent with the stability of Spr-bearing associations during M1.
The mineral association of porphyroblast Grt–Opx–Ky is indicative of the subsequent high-P metamorphic event, M2, c. 850–900°C, >12 kbar (Baba, 1999a). Estimates from Fe–Mg exchange thermometry give lower-temperature conditions, interpreted as the product of late resetting. Conversely, P–T conditions estimated from the Opx isopleths for Opx inclusions in Grt from the Grt–Ky granulite (A), are broadly consistent with P–T conditions estimated from the petrogenetic grid.
Sapphirine symplectites are thought to have been formed during the decompressional stage (M3), although the estimated temperature is slightly higher than previously estimated (Baba, 1998: M3). Sapphirine symplectites on Ky margins have been reported from high-P granulites and eclogites (Grant, 1989; Möller, 1999), interpreted as the breakdown products of Grt–Ky ± Cpx and Cpx–Ky ± Crn at 700–800°C and 10 kbar. The Spr symplectites in the Grt–Ky gneiss (B) may have been formed at similar conditions on the basis of the coexistence of Ky and high-An Pl. The P–T conditions for the formation of the Spr symplectite, estimated by geothermobarometry and the mineral compositions of coexisting phases, are around 800°C, which is lower than the early stage UHT condition (M1).
Spr–Opx–Crd symplectites in the high-XMg rocks, the Grt–Opx–Ky layer and block indicate higher temperature conditions of 850–900°C. The possible explanation for the difference in temperature is that both rocks were formed at higher temperatures during the early stage than the other rock types, and they experienced high-T conditions of 850–900°C during the retrograde stage as a result of local temperature rise resulting from emplacement of the SHIC. Baba (1998, 1999a) proposed that the SHIC was a heat source for UHT metamorphism in South Harris on the basis of common mineral textures and compositional zoning of clinopyroxene in the SHIC metagabbro and mafic gneiss in the Leverburgh belt. The rocks with the Opx–Spr–Crd association are exposed about 200–300 m from the metagabbro body, and the Opx–Ky granulite, which records the highest temperature, is also nearby. The local temperature rise caused by emplacement of the SHIC (Baba, 1999a) may have continued during the retrograde stage as a result of emplacement of multiple igneous bodies (e.g. anorthosite first, then gabbro, and tonalite later). However, we cannot deny the possibility that rocks with the Opx–Spr–Crd association were metamorphosed at greater depth, and uplifted by the subsequent deformation.
The relative P–T path is broadly consistent with the anti-clockwise path proposed by Baba (1998). Spr breakdown and formation are compatible with a pressure increase between M1 and M2 and decompression during M3. However, these rocks experienced different temperatures, within a range of c. 800–900°C during the decompressional stage. This may have been a consequence of pulsed emplacement of the SHIC or of tectonic modification by later shearing.
Sapphirine formation related to whole-rock composition and partial melting
Sapphirine inclusions occur in rocks with high bulk-XMg, and the textural relationships are consistent with previous work on South Harris (Baba, 1999a). In the high bulk-XMg rock, Spr was formed during the early stage of metamorphism (M1), and has broken down to form Grt and Opx at high pressure during M2. This distinctive Spr occurrence is consistent with higher peak metamorphic pressure conditions than in other high-grade gneiss terrains in which Spr-bearing assemblages are stable [e.g. Wilson Lake, Canada; Napier Complex, East Antarctica, Forefinger Point, East Antarctica (Harley, 1998a, and references therein)].
The relationships between mineral assemblage and local reacting composition is illustrated for the Spr-bearing rocks using a P–µFeO diagram based on Hensen & Harley (1990) and Harley (1998b) (Fig. 14a and b). Sapphirine was formed at low pressure in the high bulk-XMg rock (XMg > 0·71), and is now observed as an inclusion phase, whereas no Spr-forming reaction occurred in the relatively low bulk-XMg rock [Grt–Ky granulite (A), XMg = 0·51, and (B), XMg = 0·64] (see Fig. 14).
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In the Grt–Ky granulites (A) and (B), which have lower bulk-XMg (<0·64) than rocks with Spr inclusions, Spr occurs as a symplectic phase. However, these granulites are relatively higher in XMg compared with the widely exposed Spr-absent pelitic gneiss (Baba, 1998). The field occurrence and textures indicate that these domains were formed in response to partial melting of originally moderately high bulk-XMg and Al-rich sediment (Fig. 5). In the Leverburgh belt, Grt–Ky/Sil-rich lenses or layers can be seen in the Grt–Ky-bearing pelitic gneiss, and are thought to represent metamorphosed restite. These domains are inferred to have formed at the peak or early stage of metamorphism. The Spr symplectites were probably formed late in the metamorphic history in Mg–Al-rich domains formed by partial melting. Initially, Grt and Sil were formed as restite phases during partial melting. Removal of melt from the system resulted in the formation of Mg–Al-rich domains (see Fig. 14b). The partial melts were probably rich in FeO relative to the restite based on the experimental studies of Montel & Vielzeuf (1997), who showed that the melt has low XMg (0·32–0·47; average 0·40) at 875–1040°C and 8 kbar. Thus, the XMg of the restite may become high. The expected compositional difference is recorded in the difference in XMg of Grt and the reintegrated composition between the Spr-present domains and the Spr-absent domains. Consequently, it can be concluded that formation of Mg–Al-rich domains via partial melting is an explanation for why Spr symplectite occurs in the low bulk-XMg rocks. These results imply that the formation of Spr was controlled not only by protolith composition but also by the formation of Mg–Al-rich restitic domains via partial melting during metamorphism.
Possible model for sapphirine formation and breakdown
Figure 15 shows a possible model for two stages of sapphirine formation in rocks of differing bulk-XMg and degrees of partial melting. The rocks with Spr inclusions might be derived from a high-Mg metasedimentary protolith. Grt–Opx granulite (95913-7) and Opx–Sil granulite are quartz undersaturated, and occur embedded within mafic gneiss. The protoliths for these rocks may have originally been a hydrothermal deposit derived from mafic–ultramafic rocks on the basis of their occurrence (e.g. Warren, 1983). The Grt–Opx–Ky block is very similar to the Opx–Ky granulite in terms of their localities and mineral assemblages, and the protolith is considered to be a high-Mg sediment. The mineral assemblage of the Grt–Ky gneiss (A) and (B) resembles the Grt–Ky-bearing pelitic gneiss and might be a member of these metasediments.
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In the Grt–Ky pelitic gneiss with low bulk-XMg (<0·50), early metamorphic reactions involved St, Hc, Sil and Bt (Baba, 1998). In the intermediate bulk-XMg rocks (0·50–0·65), Grt–Ky granulite (A) and (B), reactions were St and Hc absent, and instead involved Crd, Spl and Opx [e.g. reaction (5)]. Quartz-deficient Mg–Al-rich domains, now recognized as pods or lenses, were formed by partial melting during the prograde path in both rock types. Sapphirine symplectites were formed only in the domains derived from relatively higher bulk-XMg rocks, and the constituent minerals in these domains have high XMg. Instead of Spr symplectites, Crd–Opx or Crd–Spl symplectites/coronas were formed in the lower bulk-XMg rocks and Spr-absent domains, and the constituent minerals have lower XMg.
In the high bulk-XMg rocks (>0·70) Spr formed early in the metamorphic history (M1), e.g. the Opx–Ky granulite, Opx–Sil granulite (Baba, 1999a), Grt–Opx granulite and Grt–Opx–Ky block. This Spr broke down to form Grt, Opx and Ky/Sil during an increase in pressure (M2), and now occurs as inclusions within porphyroblasts. Grt–Opx–Ky-rich lithologies, e.g. the Grt–Opx–Ky block, may represent metamorphic restites from original high-Mg metasediments. However, late-stage shearing (M4) has made this relationship unclear. The Grt–Opx–Ky layer occurs between alternating layers of Ky–Bt, Grt–Opx and Opx–Ky, and is located at the contact between orthopyroxenite and Grt–Ky-bearing pelitic gneiss. These layers are likely to have formed by diffusion metasomatism, involving the exchange of Al and Si for Mg. This probably progressed after the formation of the orthopyroxenite which might have been derived from partial melting of relatively high-Mg and low-Al sediment, although this is ruled out by the interpretations shown in Fig. 15.
The origin of Spr-bearing rocks has been discussed by several workers. Possible protoliths are summarized as follows: (1) high-Mg metasediments formed from hydrothermally altered mafic or ultramafic igneous rocks or their weathering products or high-Mg clays (Sheraton, 1980; Warren, 1983; Arima & Barnett 1984; Harley et al., 1990); (2) metamorphic metasomatism (Herd et al., 1969; Vry & Cartwright, 1994; Dunkley et al., 1999); (3) restites formed via partial melting (Clifford et al., 1981). The sapphirine-bearing rocks from South Harris reported here have characteristics of all of the above protoliths, and were formed during a continuous metamorphic cycle. The distinctive occurrences indicating Spr formation, breakdown and then formation are attributed to the response of protoliths with differing bulk-XMg and the formation of domains to an anticlockwise P–T path.
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CONCLUSIONS |
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Two new sapphirine occurrences are reported from the Lewisian complex in South Harris. They formed at different metamorphic stages and from protoliths having different XMg. The Spr inclusions indicate that they were stable during early metamorphism (M1), and were broken down to produce Grt in response to pressure increase. The Spr-breakdown textures are observed only in rocks with high bulk-XMg (>0·70). In the low bulk-XMg rocks, Spr only occurs as a symplectic phase in Mg–Al-rich domains, which may be derived from partial melting of relatively low bulk-XMg rocks (XMg = 0·50–0·65). In these domains, Spr generally occurs at aluminosilicate margins, and coexisting minerals are relatively high in Fe2O3. This indicates that Al2O3 and fO2 are important factors in its petrogenesis. These Spr symplectites were formed during decompression after high-pressure granulite-facies metamorphism (M2). The inferred metamorphic P–T path and conditions derived from both petrogenetic grids and Opx isopleths are consistent with previous work. However, the occurrence of Spr–Opx–Crd symplectites is indicative of possible high-T conditions during retrograde metamorphism, and may be indicative of the thermal effects of late emplacement of the SHIC. Sapphirine formation and breakdown in South Harris is thought to have been dependent on (1) protolith bulk composition and (2) restite formation via partial melting during a continuous anticlockwise P–T path.
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ACKNOWLEDGEMENTS |
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I thank B. Hensen, H. Ishizuka, Y. Motoyoshi and K. Shiraishi for their valuable comments on the early draft of the manuscript and improvement, and Fukada Geological Institute and Japan Society for the Promotion of Science for their financial support. J. Hollis and an anonymous referee are thanked for their reviews of the manuscript and useful comments, and S. L. Harley for the helpful comments and editing of the manuscript.
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