Scholarly article on topic 'Petrology and phase equilibrium modeling of sapphirine + quartz assemblage from the Napier Complex, East Antarctica: Diagnostic evidence for Neoarchean ultrahigh-temperature metamorphism'

Petrology and phase equilibrium modeling of sapphirine + quartz assemblage from the Napier Complex, East Antarctica: Diagnostic evidence for Neoarchean ultrahigh-temperature metamorphism Academic research paper on "Earth and related environmental sciences"

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Abstract of research paper on Earth and related environmental sciences, author of scientific article — Hisako Shimizu, Toshiaki Tsunogae, M. Santosh

Abstract A synthesis of the petrological characters of granulite facies rocks that contain equilibrium sapphirine + quartz assemblage from two localities (Tonagh Island (TI) and Priestley Peak (PP)) in the Napier Complex, East Antarctica, provides unequivocal evidence for extreme crustal metamorphism possibly associated with the collisional orogeny during Neoarchean. The reaction microstructures associated with sapphirine + quartz vary among the samples, probably suggesting different tectonic conditions during the metamorphic evolution. Sapphirine and quartz in TI sample were probably in equilibrium at the peak stage, but now separated by corona of Grt + Sil + Opx suggesting near isobaric cooling after the peak metamorphism, whereas the Spr + Qtz + Sil + Crd + Spl assemblage replaces garnet in PP sample suggesting post-peak decompression. The application of mineral equilibrium modeling in NCKFMASHTO system demonstrated that Spr + Qtz stability is lowered down to 930 °C due to small Fe3+ contents in the rocks (mole Fe2O3/(FeO + Fe2O3) = 0.02). The TI sample yields a peak p-T range of 950–1100 °C and 7.5–11 kbar, followed by cooling toward a retrograde stage of 800–950 °C and 8–10 kbar, possibly along a counterclockwise p-T path. In contrast, the peak condition of the PP sample shows 1000–1050 °C and >12 kbar, which was followed by the formation of Spr + Qtz corona around garnet at 930–970 °C and 6.7–7.7 kbar, suggesting decompression possibly along a clockwise p-T trajectory. Such contrasting p-T paths are consistent with a recent model on the structural framework of the Napier Complex that correlates the two areas to different crustal blocks. The different p-T paths obtained from the two localities might reflect the difference in the tectonic framework of these rocks within a complex Neoarchean subduction/collision belt.

Academic research paper on topic "Petrology and phase equilibrium modeling of sapphirine + quartz assemblage from the Napier Complex, East Antarctica: Diagnostic evidence for Neoarchean ultrahigh-temperature metamorphism"

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Research paper

Petrology and phase equilibrium modeling of sapphirine + quartz assemblage from the Napier Complex, East Antarctica: Diagnostic evidence for Neoarchean ultrahigh-temperature metamorphism

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Hisako Shimizua, Toshiaki Tsunogaea,b*, M. Santosh

a Graduate School of Life and Environmental Sciences, University ofTsukuba, Ibaraki 305-8572, Japan b Department of Geology, University of Johannesburg, Auckland Park 2006, South Africa c China University of Geosciences (Beijing), No. 29, Xueyuan Road, Haidian District, Beijing 100083, China d Division of Interdisciplinary Science, Faculty of Science, Kochi University, Kochi 780-8520, Japan

ARTICLE INFO

Article history:

Received 7 July 2012

Received in revised form

30 August 2012

Accepted 20 September 2012

Available online 27 September 2012

Keywords:

Ultrahigh-temperature granulite Petrology

Pseudosection modeling Napier complex Antarctica

ABSTRACT

A synthesis of the petrological characters of granulite facies rocks that contain equilibrium sapphirine + quartz assemblage from two localities (Tonagh Island (TI) and Priestley Peak (PP)) in the Napier Complex, East Antarctica, provides unequivocal evidence for extreme crustal metamorphism possibly associated with the collisional orogeny during Neoarchean. The reaction microstructures associated with sapphirine + quartz vary among the samples, probably suggesting different tectonic conditions during the metamorphic evolution. Sapphirine and quartz in TI sample were probably in equilibrium at the peak stage, but now separated by corona of Grt + Sil + Opx suggesting near isobaric cooling after the peak metamorphism, whereas the Spr + Qtz + Sil + Crd + Spl assemblage replaces garnet in PP sample suggesting post-peak decompression. The application of mineral equilibrium modeling in NCKFMASHTO system demonstrated that Spr + Qtz stability is lowered down to 930 °C due to small Fe3+ contents in the rocks (mole Fe2O3/(FeO + Fe2O3) = 0.02). The TI sample yields a peakp-Trange of950-1100 °C and 7.5-11 kbar, followed by cooling toward a retrograde stage of 800-950 °C and 8-10 kbar, possibly along a counterclockwise p-T path. In contrast, the peak condition of the PP sample shows 1000-1050 °C and >12 kbar, which was followed by the formation of Spr + Qtz corona around garnet at 930-970 °C and 6.7-7.7 kbar, suggesting decompression possibly along a clockwise p-T trajectory. Such contrasting p-T paths are consistent with a recent model on the structural framework of the Napier Complex that correlates the two areas to different crustal blocks. The different p-T paths obtained from the two localities might reflect the difference in the tectonic framework of these rocks within a complex Neoarchean subduction/collision belt.

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1. Introduction

* Corresponding author. Graduate School of Life and Environmental Sciences,

University of Tsukuba, Ibaraki 305-8572, Japan.

E-mail address: tsunogae@geol.tsukuba.ac.jp (T. Tsunogae).

Peer-review under responsibility of China University of Geosciences (Beijing).

The Napier Complex is known as a classic example of Neoarchean granulite terrane exposed around an area of ca. 300 km x 200 km in the Enderby Land of East Antarctica (e.g., Sheraton et al., 1987; Harley and Hensen, 1990; Boger, 2011; Horie et al., 2012; among others). The complex has excellent exposures of anhydrous granu-lites that were formed by deep crustal metamorphism at extreme thermal conditions of T > 900 °C and p = 7—13 kbar, referred to as ultrahigh-temperature (UHT) metamorphic rocks (e.g., Harley, 1985, 1998, 2008; Brown, 2007; Kelsey, 2008; Santosh and Kusky, 2010). The rocks formed through such UHT metamorphism, particularly at T > 1000 ° C, are often characterized by the occurrence of several diagnostic minerals or mineral assemblages such as sapphirine + quartz, orthopyroxene + sillimanite + quartz, Al-rich

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orthopyroxene, scapolite + wollastonite, mesoperthite, and inverted pigeonite (e.g., Harley, 1998; Harley and Motoyoshi, 2000; Hokada, 2001; Kelsey, 2008; and references therein; Tsunogae et al., 2011; Zhang et al., 2012). Among the assemblages of UHT meta-morphism, sapphirine + quartz has been regarded as the most robust evidence for extreme temperatures, formed above 950 °C, and sometimes exceeding 1000 °C (e.g., Hensen and Green, 1973; Bertrand etal., 1991; Harley, 1998; Kelsey et al.,2004; Kelsey, 2008). The Amundsen Bay and Casy Bay areas, located in the western part of the Napier Complex, are regarded as the highest grade region of the complex, where sapphirine + quartz assemblages have been widely reported (e.g., Dallwitz, 1968; Ellis, 1980; Ellis et al., 1980; Grew, 1980,1982; Motoyoshi and Matsueda, 1984; Sheraton et al., 1987; Motoyoshi and Hensen, 1989; Harley and Hensen, 1990; Osanaietal., 2001a,b; Tsunogae et al., 2002; Hokada et al., 2008; and references therein). The peak temperature conditions of T > 1100 ° C or even >1150 ° C have been suggested on the basis of the stability of sapphirine + quartz and related mineral assemblages as well as geothermometry (e.g., Harley et al., 1990; Harley and Motoyoshi, 2000; Hokada, 2001; Tsunogae et al., 2002). Recently Taylor-Jones and Powell (2010) reported lowering of the stability temperature of sapphirine + quartz assemblage down to 850 ° C in highly oxidized states in FMASTO system and provided a new activity-composition (a—x) model for sapphirine that includes ferric iron. Similar lowering of the sapphirine + quartz stability temperature was reported by Korhonen et al. (2012) based on pseudosection analyses in KFMASHTO and NCKFMASHTO systems. Korhonen et al. (2012) suggested that the stability of sapphirine + quartz over a range of natural rock compositions should be investigated on a case-by-case basis using the modeling approach of chemical systems including Fe2O3. Although there are many sapphirine + quartz localities so far reported in the Napier Complex, and pseudosection approach has been tested to some of these sapphirine granulites (e.g., Hokada et al., 2008), no attempt has yet been made to evaluate the stability of sapphirine + quartz assemblage considering the effect of Fe2O3. In this study, we synthesize the petrographic and mineral-ogical characters of sapphirine + quartz from Tonagh Island and Priestley Peak in Amundsen Bay, and draw inferences on the p-T evolution of this area. We adopted mineral equilibrium technique in a complex chemical system including TiO2 and Fe2O3 and evaluate the peak p-T condition of sapphirine + quartz assemblage from the localities. The sapphirine + quartz bearing assemblages from the two localities show distinct reaction textures that probably indicate different metamorphic evolution under different tectonic settings within the UHT orogen, possibly associated with subduction/collision events during Neoarchean.

2. General geology

The Amundsen Bay area in the Napier Complex is composed mainly of layered quartzo-feldspathic, pelitic, and psammitic gneisses with subordinate orthopyroxene-quartz-feldspar gneiss (charnockite or enderbite), mafic and ultramafic granulites, and magnetite quartzite (Sheraton et al., 1987). p-T estimates of the various lithologies from this region based on different methods such as conventional geothermometry (e.g., Ellis, 1980; Harley, 1985), phase analysis (e.g., Grew, 1980; Harley and Hensen, 1990; Harley et al., 1990; Harley, 1998), Al solubility in orthopyroxene (e.g., Harley and Motoyoshi, 2000), inverted pigeonite (e.g., Sandiford and Powell, 1986; Harley, 1987), oxygen isotope geothermometry (e.g., Farquhar et al., 1996), and ternary feldspar geothermometry (e.g., Hokada, 2001) suggest that the rocks were subjected to UHT metamorphism. The available geochronological data suggest that the granulite-facies peak metamorphism took place during Neoarchean to early Paleoproterozoic (ca. 2.5 Ga; e.g.,

Harley and Black, 1997; Carson et al., 2002; Hokada et al., 2003, 2004, 2008; Horie et al., 2012). The geological features of the locations examined in this study are summarized below.

2.1. Tonagh Island

Detailed regional geological framework and structural characteristics of Tonagh Island are given in Osanai et al. (2001a). Tonagh Island is dominantly composed of various metasedimentary and metaigneous rocks such as pelitic granulite, magnetite-quartzite (meta-BIF), mafic to ultramafic granulites, felsic garnet granulite, and felsic to intermediate orthopyroxene granulite (charnockite or enderbite). The rocks show dominant E—W trending foliation. The lithologies are divided into several blocks by E—W trending vertical shear zones. Mafic granulite and charnockite are dominant in the blocks in the north, whereas metasediments are abundant in the south. Peak metamorphic conditions of Tonagh Island were determined as T > 1100 °C using ternary-feldspar equilibrium (Hokada, 2001), phase equilibria on sapphirine granulites (Osanai et al., 1999), and Al solubility in orthopyroxene (Tsunogae et al., 2002). A counterclockwise p-T path has been proposed for the area based on geothermobarometry, phase analysis, and fluid inclusion study of Mg-Al rocks (Tsunogae et al., 2002).

A sapphirine + quartz bearing quartzo-feldspathic granulite from the eastern part of Tonagh Island was analyzed in this study. The lithology occurs as a thin (~30 cm) layer of garnet-pyroxene-rich brownish gray rock intercalated with felsic garnet gneiss, charnock-ite, quartzo-feldspathic sillimanite gneiss, and mafic granulite.

2.2. Priestley Peak

Priestley Peak is located about 10 km south of Tonagh Island (Fig. 1). No detailed field geological survey has been done so far in this locality. The studied area, which corresponds to the northwestern region of the exposure, is composed of layered gneisses of quartzo-feldspathic, mafic, and pelitic compositions, which are principally similar to the lithologies in the northern part of Tonagh Island. The analyzed sample of sapphirine granulite corresponds to a felsic layer within mafic to intermediate granulite.

3. Petrography and metamorphic reactions

Below we will summarize petrography and reaction textures related to sapphirine + quartz in the two localities described in the earlier section. The samples were collected during field geological survey of Amundsen Bay area undertaken by the 39th Japanese Antarctic Research Expedition (JARE-39) in 1998. Representative textures of the samples are shown in Figs. 2 and 3.

3.1. Sample B98021104AA (Tonagh Island)

The sample is composed of mesoperthite/perthite, quartz, garnet (Grt1), sapphirine, and orthopyroxene (Opx1). Sillimanite (Sil1), ilmenite and rutile occur as accessory minerals. The rock shows weak foliation defined by quartzo-feldspathic layers and garnet-rich layers. Sapphirine, Opx1, and Grt1 occur as medium-grained subidioblastic minerals in the matrix of quartz and mesoperthite/perthite (Fig. 2a). Sil1 occurs as needles or idioblasts only as inclusions in Grt1 and sapphirine (Fig. 2a), probably as a prograde phase. The peak mineral assemblage of the rock is therefore inferred to be Kfs + Qtz + Grt1 + Opx1 + Spr + Ilm + Rt ± melt. The sapphirine is separated from quartz by corona of garnet (Grt2), sillimanite (Sil2), and orthopyroxene (Opx2) (Figs. 2b, 3a and b). The width of Grt2 + Sil2 + Opx2 coronae is 30—60 mm, and Sil2 is always formed on the sapphirine side, while Grt2 and Opx2 on the quartz side (Fig. 3a and b). This texture suggests that

50°00' 50°30' 51°00'E

Figure 1. Location map of Tonagh Island and Priestley Peak in Amundsen Bay area, East Antarctica. Block boundaries are after Toyoshima et al. (2008).

sapphirine and quartz were once in equilibrium, but later separated by the progress of the following retrograde reaction (1).

Spr + Qtz / Grt2 + Sil2 + Opx2 (1)

Similar reaction textures have been reported from several localities in the Napier Complex (e.g., Sheraton et al., 1987). The sapphirine is in places mantled by fine-grained aggregates of orthopyroxene (Opx3), sillimanite (Sil3), biotite, and quartz, suggesting the progress of further retrograde reaction (2) possibly with melt phase (Figs. 2c, 3c and d).

Spr + melt / Opx3 + Sil3 + Bt + Qtz (2)

3.2. Sample TS98022407 (Priestley Park)

The sample is composed of alternating quartzo-feldspathic and ferromagnesian layers. The quartzo-feldspathic layer is composed of mesoperthite/antiperthite, quartz, sapphirine, sillimanite, and garnet, while the ferromagnesian layer contains garnet, silli-manite, mesoperthite/antiperthite, quartz, sapphirine, spinel, cordierite, and rutile. Sapphirine (Spr1) in the quartzo-feldspathic layer is xenoblastic, medium grained (0.2—0.5 mm), and often surrounded by plagioclase and K-feldspar (Figs. 2d and 3e), suggesting that Spr1 is a prograde to peak phase. In contrast, in the ferromagnesian layer, sapphirine occurs as a retrograde mineral around garnet (Figs. 2e, 3f—h). As shown in Fig. 3f garnet is surrounded by aggregates of sapphirine (Spr2) + sillimanite (Sil2) + quartz + cordierite + spinel. Quartz is rare, but occurs as

inclusions in Spr2, and boundary between the Spr2 and quartz is sharply defined. The probable peak assemblage in the ferromag-nesian layer is inferred as garnet + Sil1 + mesoperthite/ antiperthite + quartz (Fig. 2f), which is coarse-grained and idio-blastic to subidioblastic.

4. Mineral chemistry

Chemical analyses of all minerals were carried out using an electron microprobe analyzer (JEOL JXA8530F) at the Chemical Analysis Division of the Research Facility Center for Science and Technology, the University of Tsukuba. The analyses were performed under conditions of 15 kV accelerating voltage and 10 nA sample current, and the data were regressed using an oxide-ZAF correction program supplied by JEOL. Below we summarize the salient results from mineral chemistry data from the analyzed rocks. Representative compositions of minerals are given in Tables 1—3. Fe3+ of sapphirine and spinel was calculated based on stoichiometry.

4.1. Sapphirine

Sapphirine in the examined samples exhibits magnesian compositions (XMg = 0.81—0.85) close to the 7:9:3 ideal composition in terms of (Mg, Fe)O:Al2O3:SiO2 ratio (Table 1, Fig. 4). For example, XMg range of subidioblastic sapphirine in B98021104AA (Tonagh Island; Fig. 3a) is 0.81—0.84. Spr1 in sample TS98022407 (Priestley Peak) has consistentXMg of 0.81—0.85, and shows a slight rimward increase in XMg and w(Cr2O3) content from 0.81—0.82 to 0.84—0.85 and from 1.1%—1.3% to 1.4%—1.5%, respectively. Retrograde

Figure 2. Photomicrographs showing reaction microstructures in sapphirine granulites discussed in this paper. Mineral abbreviations are discussed in the text. (a) to (c) are textures in sample B98021104AA (Tonagh Island), while (d) to (e) are in sample TS98022407 (Priestley Peak). All photographs are taken in polarized light. (a) Coarse-grained subidioblastic garnet, orthopyroxene, and quartz as a probable peak mineral assemblage. Sillimanite is present only as inclusions in garnet. Biotite occurs around the rim of some ferromagnesian minerals as a retrograde phase. Sapphirine is surrounded by reaction coronae as shown in Fig. 3b and c. (b) Corona of garnet + sillimanite between sapphirine and quartz, suggesting that sapphirine and quartz were once in equilibrium but subsequently separated by near isobaric cooling as discussed in the text. (c) Fine-grained symplectite of orthopyroxene + sillimanite + biotite between sapphirine and quartz probably related to a retrograde hydration event. (d) Subidioblastic sapphirine mantled by thin film of plagioclase and K-feldspar in quartz- and mesoperthite (Mpth)-rich portion of the rock. (e) Coronae of sillimanite + sapphirine + quartz and sillimanite + cordierite + spinel partly replacing prograde garnet, probably related to decompression during a retrograde stage. (f) Idioblastic to subidioblastic garnet + sillimanite + quartz + perthite as a probable peak mineral assemblage in garnet-rich layer of the rock.

Spr2 shows consistent XMg of 0.83—0.84 and w(Cr2O3) content of -1.2%.

4.2. Garnet

Garnet in the samples is essentially a solid solution of pyrope and almandine (XMg = 0.51—0.58) with low grossular (<5 mol%) and spessartine (<2 mol%) contents (Table 2). It is compositionally nearly homogeneous, possibly due to diffusion during UHT metamorphism. In the Tonagh sample, matrix prograde Grt1 and retrograde Grt2 around sapphirine have nearly consistent compositions of Alm48—52 Pyr46—51 Grsi—2 Sps0—i and Alm48 Pyr50—51 Grsi Sps0—1,

respectively. Garnet in the Priestley Peak sample is slightly rich in pyrope component as Alm41—45 Pyr53—57 Grs1 Sps2.

4.3. Orthopyroxene

Three generations of orthopyroxene occur in sample B98021104AA. Subidioblastic coarse-grained orthopyroxene (Opx1) has the highest w(Al2O3) content of up to 7.6% (Al = 0.32 pfu) with XMg ratio of 0.74—0.77 (Table 2, Fig. 5). Opx2 has lower w(Al2O3) content (5.3%—5.5%, Al = 0.22—0.30 pfu) with consistent XMg ratio of 0.74—0.75. Symplectic Opx3 shows the lowest XMg of 0.72 and intermediate w(Al2O3) content of 6.5%—6.8% and Al = 0.27—0.29 pfu.

Figure 3. Back-scattered electron images showing detailed reaction microstructures in sample B98021104AA (Tonagh Island; Fig. 3a—d) and sample TS98022407 (Priestley Peak; Fig. 3e—h). (a), (b) Garnet + sillimanite + orthopyroxene corona between sapphirine + quartz. (c), (d) Orthopyroxene + sillimanite + biotite symplectite between sapphirine and quartz. (e) Subidioblastic sapphirine mantled by thin film of plagioclase and K-feldspar in quartz- and mesoperthite-rich portion of the rock. (f) Various corona textures in garnet probably related to decompression. (g) Sapphirine + quartz + sillimanite + cordierite corona around garnet. (h) Sillimanite + cordierite + spinel corona around garnet.

4.4. Other minerals

Spinel in sample TS98022407 (Priestley Peak) is Mg-rich (XMg = 0.59—0.66), and contains some Cr2O3 (w(Cr2O3) = ~6.7%)

and ZnO (w(ZnO) = ~ 6.9%). The calculated Fe3+/(Fe2+ + Fe3+) ratio is low, 0.09—0.13. Plagioclase in TS98022407 occurs in several modes (lamella in mesoperthite, thin film around Spr1, fine-grained phase with Spr2 and Sil2), but all of these show consistent

Table 1

Representative electron microprobe analyses of sapphirine, spinel, and sillimanite.

Mineral name Sapphirine Spinel Sillimanite

Sample Nos. B98021104AA TS98022407 TS98022407 TS98022407 B98021104AA B98021104AA TS98022407 TS98022407

Oa 20 20 20 4 5 5 5 5

Remarks Matrix Spri Spr2 Sil1 Sil2 Sil1 Sil2

SiO2 13.33 13.68 13.09 0.02 37.17 37.77 36.42 37.05

M2O3 60.39 59.74 61.17 57.14 62.37 62.96 62.07 62.61

TiO2 0.02 0.04 0.06 0.00 0.01 0.00 0.00 0.00

Cr2O3 0.26 1.27 1.17 6.71 0.01 0.04 0.17 0.08

Fe2O3 2.19 1.35 1.52 2.15 0.42 0.53 0.42 0.97

FeO 6.35 6.56 5.71 16.20

MnO 0.04 0.10 0.03 0.02 0.06 0.01 0.03 0.00

MgO 16.49 16.53 16.77 13.01 0.01 0.05 0.03 0.09

ZnO 0.05 0.10 0.05 4.89 0.07 0.00 0.00 0.00

CaO 0.02 0.03 0.03 0.01 0.01 0.00 0.01 0.01

Na2O 0.00 0.00 0.00 0.12 0.01 0.00 0.01 0.01

K2O 0.00 0.02 0.01 0.00 0.02 0.02 0.01 0.00

Total 99.14 99.41 99.58 100.26 100.15 100.15 99.17 100.82

Si 1.605 1.645 1.564 0.000 1.002 1.007 0.992 0.991

Al 8.564 8.463 8.616 1.819 1.980 1.978 1.992 1.974

Ti 0.002 0.003 0.005 0.000 0.000 0.000 0.000 0.000

Cr 0.025 0.120 0.110 0.143 0.000 0.001 0.004 0.002

Fe3+ 0.199 0.122 0.136 0.044 0.008 0.011 0.009 0.020

Fe2+ 0.639 0.660 0.570 0.366

Mn 0.004 0.010 0.003 0.000 0.001 0.000 0.001 0.000

Mg 2.957 2.961 2.986 0.524 0.001 0.001 0.001 0.003

Zn 0.004 0.009 0.004 0.098 0.001 0.000 0.000 0.000

Ca 0.002 0.004 0.004 0.000 0.000 0.000 0.000 0.000

Na 0.000 0.000 0.000 0.006 0.000 0.000 0.001 0.001

K 0.000 0.002 0.001 0.000 0.001 0.001 0.000 0.000

Total 14.000 14.000 14.000 3.000 2.996 2.998 2.998 2.991

Mg/(Fe2+ + Mg) 0.82 0.82 0.84 0.59

Zn/(Fe2+ + Mg + Zn) 0.10

Fe3+/(Fe3+ + Fe2+) 0.24 0.16 0.19 0.11

a Number of oxygens.

composition of Ab77—80. K-feldspar in the sample is also nearly homogeneous in composition as Or86—92. Sillimanite in the two samples is close to the ideal chemistry, but it sometimes contains small amount of Fe2O3 (w(Fe2O3) = 0.32%—0.97%). Cordierite in sample B98022407 is Mg-rich as XMg = 0.89—0.90.

5. Mineral equilibrium modeling

Metamorphic p-T conditions of the stability of Spr + Qtz assemblages in the UHT granulites from Tonagh Island and Priestley Peak were constrained using THERMOCALC 3.33 (Powell and Holland, 1988, updated October 2009) with the internally consistent data set of Holland and Powell (1998; data set tcds55s, file created November 2003). The computations using this software are based on the stable mineral assemblage and phase compositions from Gibbs Free Energy minimization for a given bulk composition at specified p-T conditions, and the results are used to construct rock-specific equilibrium assemblage diagrams (also called pseudosections). Calculations were undertaken in the system Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3 (NCKFMASHTO) (White et al., 2003,2007), which provides a realistic approximation to model the examined rocks. The phases considered in the modeling and the corresponding a—x models used are garnet, biotite, and melt (White et al., 2007), orthopyroxene (White et al., 2002), plagioclase and K-feldspar (Holland and Powell, 2003), spinel and magnetite (White et al., 2002), sapphirine (TaylorJones and Powell, 2010), cordierite (Holland and Powell, 1998), and ilmenite-hematite (White et al., 2000). Aluminosilicates, quartz, and rutile are treated as pure end-member phases. For the analysis, slabs of relatively homogeneous part of the examined granulites were used for thin-section preparation, and the counterpart of these was used for chemical analysis. Bulk compositions for the rocks were determined

by X-ray fluorescence spectroscopy and FeO/Fe2O3 ratio by titration at Activation Laboratories, Canada. The chemical composition of sample B98021104AA(Tonagh Island) is w(SiO2) = 68.06%, w(Al2O3) = 12.67%, w(FeO) = 6.70%, w(MgO) = 8.41%, w(MnO) = 0.027%, w(CaO) = 0.14%, w(Na2O) = 0.04%, w(K2O) = 0.72%, w(TiO2) = 0.60%. Fe2O3 is taken into account for the calculations because the rock contains 0.43% of w(Fe2O3), which we regard not negligible. Sample TS98022407 (Priestley Peak) shows slightly Al-rich composition of w(SiO2) = 57.20%, w(Al2O3) = 23.74%, w(Fe2O3) = 0.14%, w(FeO) = 3.50%, w(MgO) = 2.92%, w(MnO) = 0.047%, w(CaO) = 2.09%, w(Na2O) = 4.94%, w(K2O) = 3.52%, w(TiO2) = 0.77%. Analysis of the Priestley Peak sample was done on the ferromagnesian layer of the sample to infer the stability of sapphirine + quartz.

5.1. Tonagh Island

Sample B98021104AA contains Kfs + Qtz + Grt1 + Spr + Opx1 + Ilm + Rt ± inferred melt, representing the probable peak assemblage as discussed in the petrography section. Fig. 6a shows a p-T pseudosection for the sample calculated using the compositional factors listed in Fig. 6 a and at mole H2O ratio of the rock (M(H2O)) of 0.2 mol%. The stability field of the peak mineral assemblage of the rock plotted in the pseudosection suggests a p-T range of 950—1100 °C and 7.5—11 kbar for the assemblage. The upper p-T stability limit of the assemblage is defined by the absence of K-feldspar, whereas the lower limit is set by the absence of sapphirine. Spr + Qtz has a minimum stability field of 940 °C and 7 kbar, although at the condition, Spr + Qtz should coexist with cordierite, which is not the case of this rock. If we adopted higher M(H2O) values such as 1.0 mol% (Fig. 6b), the minimum stability of Spr + Qtz slightly increases as >950 °C and >7 kbar, but the stability field of the peak mineral

Table 2

Representative electron microprobe analyses of garnet, orthopyroxene, cordierite, and biotite.

Mineral name Garnet Orthopyroxene Cordierite Biotite

Sample B98021 B98021 TS980 TS980 B98021 B98021 B98021 B98021 TS980 B98021

Nos. 104AA 104AA 22407 22407 104AA 104AA 104AA 104AA 22407 104AA

Oa 12 12 12 12 6 6 6 6 18 22

Remarks Grt1 Grt2 Core Rim Opx1, core Opx1, rim Opx2 Opx3

SiO2 40.25 41.03 40.64 40.95 51.95 51.38 51.61 51.33 49.57 40.42

Al2O3 22.89 23.03 23.15 23.48 7.56 6.93 6.99 6.50 33.74 14.08

TiO2 0.03 0.00 0.01 0.03 0.07 0.05 0.05 0.00 0.00 3.72

Q2O3 0.00 0.00 0.07 0.13 0.04 0.01 0.05 0.00 0.01 0.00

FeOb 22.81 22.82 21.21 20.66 14.34 14.50 15.60 17.10 2.61 5.96

MnO 0.07 0.10 0.28 0.37 0.05 0.02 0.00 0.05 0.06 0.00

MgO 13.55 13.37 14.96 14.80 26.12 26.26 25.34 24.99 12.10 20.81

ZnO 0.00 0.05 0.00 0.00 0.00 0.00 0.01 0.17 0.00 0.00

CaO 0.59 0.45 0.66 0.58 0.00 0.03 0.00 0.02 0.01 0.01

Na2O 0.00 0.00 0.00 0.00 0.02 0.02 0.00 0.02 0.04 0.19

K2O 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.01 9.74

Total 100.19 100.19 100.97 101.00 100.14 99.20 99.64 100.18 98.15 94.93

Si 3.001 3.033 2.988 2.999 1.851 1.853 1.860 1.856 4.960 5.785

Al 2.011 2.005 2.005 2.026 0.317 0.294 0.297 0.277 3.978 2.375

Ti 0.002 0.000 0.000 0.002 0.002 0.001 0.001 0.000 0.000 0.400

Cr 0.000 0.000 0.004 0.007 0.001 0.000 0.001 0.000 0.001 0.000

Fe2+ 1.421 1.410 1.303 1.265 0.427 0.437 0.470 0.517 0.197 0.713

Mn 0.004 0.006 0.018 0.023 0.002 0.000 0.000 0.001 0.005 0.000

Mg 1.504 1.472 1.638 1.615 1.386 1.410 1.360 1.346 1.804 4.436

Zn 0.000 0.003 0.000 0.000 0.000 0.000 0.000 0.004 0.000 0.000

Ca 0.047 0.036 0.052 0.045 0.000 0.001 0.000 0.001 0.001 0.001

Na 0.000 0.000 0.000 0.000 0.001 0.001 0.000 0.001 0.007 0.052

K 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.002 1.778

Total 7.992 7.992 8.008 7.982 3.988 3.999 3.990 4.005 10.955 15.542

Mg/(Fe + Mg) 0.51 0.51 0.56 0.56 0.76 0.76 0.74 0.72 0.90 0.86

Pyr(%) 50.5 50.3 54.4 54.8 XAlc 0.16 0.15 0.15 0.14

Alm (%) 47.7 48.2 43.3 42.9 y(Opx)c 0.17 0.15 0.16 0.13

Grs (%) 1.6 1.2 1.7 1.5

Sps (%) 0.1 0.2 0.6 0.8

a Number of oxygens.

b Total Fe as FeO.

c Xai = Al/2, y(Opx) = Si + Al - 2.

assemblage is not available at the H2O content, as the stability field of Kfs shifts to lower temperature, while melt phase increases the stability field. Therefore, M(H2O) value during the peak meta-morphism should be lower than 1.0 mol%, which is consistent with the dry mineral assemblage during the peak metamorphism.

As the peak Spr + Qtz assemblage is replaced by Grt2 + Sil2 + Opx2 corona formed by the progress of reaction (1), we regard that the p-T condition shifted toward the stability of Grt + Sil + Opx without sapphirine. The stability field of the inferred assemblage (Grt + Opx + Sil + Kfs + Ilm + Rt + Qtz + Liq) is 800—950 °C and 8—10 kbar, suggesting near isobaric cooling from the peak stage to form the retrograde assemblage.

5.2. Priestley Peak

The ferromagnesian layer of sample TS98022407 contains the probable peak assemblage of Grt + Kfs + Qtz + Pl + Sil + Qtz + Rt ± inferred melt as discussed previously. Fig. 7a shows a p-T pseudosection for the sample calculated using the compositional factors listed in Fig. 7a and at M(H2O) = 0.2 mol%. The stability field of the assemblage occurs at p > 12 kbar and T = 1000—1050 °C. The upper-T stability limit of the assemblage is defined by the occurrence of K-feldspar while the lower-T limit is defined by the lack of spinel. As garnet in the sample is replaced by aggregates of Spr2 + Sil2 + Qtz + Crd + Spl, the metamorphic condition of the rock should have shifted to the stability of the assemblage, which corresponds to the stability field of Grt + Kfs + Qtz + Pl + Spr + Sil + Spl + Crd + Rt in Fig. 7a at 6.7—7.7 kbar and 930—970 °C

within the stability field of sapphirine + quartz. We constructed a pseudosection at higher M(H2O) values such as 1.0 mol%, which is shown in Fig. 7b. However, the stability field of the peak mineral assemblage is absent at such high water content, as the stability field of Kfs shifts to lower temperature, while the stability of melt phase expands toward higher temperature. Therefore, M(H2O) value during the peak metamorphism is inferred to be lower than 1.0 mol%.

6. Discussion

6.1. UHT metamorphism in the Napier Complex

This study examined the stability of Spr + Qtz assemblage from two localities in the Napier Complex based on phase equilibrium approach in NCKFMASHTO system and confirmed that the mineral assemblage was stable at the peak UHT metamorphism of T = 950—1100 °C (Tonagh Island) and 930—970 °C (Priestley Peak). Although there are several reports on Spr + Qtz from the Napier Complex (e.g., Dallwitz, 1968; Ellis et al., 1980; Grew, 1980; Sheraton et al., 1987; Harley, 1998, 2004, 2008; Osanai et al., 2001a; Tsunogae et al., 2002, 2008; Hokada et al., 2008), this is the first attempt to apply pseudosection approach in a complex system including TiO2 and Fe2O3. Earlier phase equilibrium studies placed the stability of Spr + Qtz at T > 1030 °C and p = 9.5 kbar (Hensen and Green, 1973) or T > 1050 °C and 11 kbar (Bertrand et al., 1991). Recent thermodynamic modeling in the KFMASH system also supports the high-temperature stability of this assemblage

Table 3

Representative electron microprobe analyses of feldspars.

Mineral name

K-feldspar

Plagioclase

Sample B980-21 TS980- TS980- TS980- TS980-

Nos. 104AA 22407 22407 22407 22407

Oa 8 8 8 8 8

Remarks Around Spr1 Around Spr1 With Kfs In Spr

SiO2 63.86 64.91 62.54 63.69 62.98

Al2O3 18.75 18.91 22.57 22.78 22.99

TiO2 0.04 0.00 0.01 0.00 0.04

Cr2O3 0.02 0.00 0.01 0.00 0.00

FeOb 0.05 0.04 0.09 0.00 0.13

MnO 0.02 0.02 0.00 0.00 0.05

MgO 0.01 0.00 0.01 0.00 0.00

ZnO 0.00 0.00 0.05 0.02 0.00

CaO 0.03 0.27 4.61 3.77 4.25

Na2O 1.43 1.70 9.06 8.74 8.95

K2O 15.17 13.95 0.10 0.20 0.23

Total 99.37 99.81 99.06 99.20 99.62

Si 2.969 2.982 2.796 2.826 2.796

Al 1.027 1.023 1.189 1.191 1.203

Ti 0.001 0.000 0.000 0.000 0.001

Cr 0.001 0.000 0.000 0.000 0.000

Fe2+ 0.002 0.002 0.003 0.000 0.005

Mn 0.001 0.001 0.000 0.000 0.002

Mg 0.000 0.000 0.001 0.000 0.000

Zn 0.000 0.000 0.002 0.001 0.000

Ca 0.002 0.013 0.221 0.179 0.202

Na 0.129 0.152 0.785 0.752 0.770

K 0.899 0.817 0.006 0.011 0.013

Total 5.030 4.991 5.004 4.960 4.992

An (%) 0.2 1.4 21.8 19.0 20.5

Ab (%) 12.5 15.4 77.6 79.8 78.1

Or (%) 87.3 83.2 0.6 1.2 1.3

a Number of oxygens. b Total Fe as FeO.

(T > 1005 °C, Kelsey et al., 2004; Kelsey, 2008). Our results demonstrated that the Spr + Qtz stability of the samples is lowered down to 940 °C (Tonagh Island) and 930 °C (Priestley Peak) at M(H2O) = 0.2 mol.% due to small Fe3+ contents in the rocks (mole Fe2O3/(FeO+Fe2O3) = 0.02 and 0.03, respectively), which is consistent with the observations by Taylor-Jones and Powell (2010) and Korhonen et al. (2012). The results of our mineral equilibrium modeling on sapphirine granulites therefore conformed lowering of Spr + Qtz stability by addition of minor components such as Fe2O3 and TiO2. The high geothermal gradient estimated from the

7® 8.75

¡.60 5.00

Tonagh Island

+ EE spr

A + Priestley Peak

[3 Spn

H Spr2

- 1 Am

5.20 Si+Fe2++Mg

Figure 4. A compositional diagram showing sapphirine chemistry.

Mg/(Fe+Mg)

Figure 5. A compositional diagram showing orthopyroxene chemistry.

peakp-Tconditions (30—40 °C/km) provides unequivocal evidence for the formation of extreme metamorphic rocks within Amundsen Bay area of the Napier Complex possibly associated with the Neo-archean orogenic event.

6.2. Implications for p-T evolution of the Napier Complex

Although the occurrences of Spr + Qtz assemblages in granulites from the two localities in the Napier Complex suggest peak UHT metamorphism, reaction microstructures related to the diagnostic UHT assemblage discussed in this study indicate two contrasting p-T trajectories: near-isobaric cooling and near-isothermal decompression. Sil2 + Grt2 + Opx2 corona between sapphirine and quartz in sample B98021104AA from Tonagh Island is consistent with cooling from ca. 1000 °C to <950 °C at nearly consistent pressure of 8—10 kbar possibly along a counterclockwise p-T path (Fig. 6a). Such a counterclockwise p-T path has been reported from Tonagh Island by Tsunogae et al. (2002) on the basis of fluid inclusion analysis. Corona texture of Grt + Qtz after Cpx + Pl in mafic granulites from Tonagh Island (Tsunogae et al., 1999) as well as other localities (e.g., Ellis and Green, 1985; Harley, 1985; Sheraton et al., 1987) also supports isobaric cooling of Tonagh Island. In contrast, Spr2 + Qtz + Sil2 + Crd + Spl corona around garnet in sample TS98022407 from Priestley Peak suggests decompression from >12 kbar and 1000—1050 °C to 6.7—7.7 kbar and 930—970 °C toward the stability field of Spr + Qtz along a clockwise p-T path. Such a clockwise p-T evolution of the studied region is consistent with previous studies on Bunt Island (e.g., Osanai et al., 2001b) located about 25 km east from Priestley Peak.

Macroscopic structural study of the Napier Complex by Toyoshima et al. (2008) suggested that the Napier Complex is separated into at least two types of metamorphic units or crustal bocks by fault/shear zones; clockwise p-T block (Block-1) and counterclockwise p-T block (Block-2). Block-1 is characterized by the occurrence of peak Opx + Sil assemblage and retrograde osu-milite and cordierite in Mg-Al and pelitic granulites, that are indicative of clockwise p-T path. On the other hand, granulites in Block-2 contain Opx + Grt or Opx + Sil coronae between sapphirine and quartz, suggesting that sapphirine and quartz were once in equilibrium, but then corona was formed by near-isobaric cooling possibly along a counterclockwise p-T path. According to the classification of Toyoshima et al. (2008), our studied localities are separated by a major crustal break (Amundsen Bay Fault) as shown in Fig. 1, and Tonagh Island belongs to Block-2, while Priestley Peak is a part of Block-1. As discussed previously, the sapphirine gran-ulite from Tonagh Island discussed in this study contains Sil2 + Grt2 + Opx2 corona between sapphirine and quartz, which suggests near isobaric cooling from the stability of Spr + Qtz (ca. 1000 °C) to <950 "Cat 8—10 kbar possibly along a counterclockwise

Figure 6. p-T diagrams showing calculated pseudosections of mineral assemblages in sample B98021104AA from Tonagh Island at M(H2O) = 0.2 mol% (a) and M(H2O) = 1.0 mol% (b). Hatched areas in (a) show peak and retrograde mineral assemblages. q: quartz, pl: plagioclase, ksp: K-feldspar, g: garnet, opx: orthopyroxene, sa: sapphirine, bi: biotite, sill: sillimanite, ky: kyanite, sp: spinel, cd: cordierite, mt: magnetite, ilm: ilmenite, ru: rutile, liq: inferred melt.

Figure 7. p-T diagrams showing calculated pseudosections of mineral assemblages in sample TS98022407 from Priestley Peak at M(H2O) = 0.2 mol% (a) and M(H2O) = 1.0 mol% (b). Hatched areas show peak and retrograde mineral assemblages.

p-T path (Fig. 6a). On the other hand, Spr + Qtz corona around garnet from Priestley Peak suggests decompression along clockwise p-T history, which is a unique character for Block-1. The results of our p-T paths based on pseudosection approach on

Spr + Qtz granulite in NCKFMASHTO system therefore support the model that the Amundsen Bay area in the Napier Complex is composed of discrete crustal blocks that contain contrasting p-T histories.

The different p-T paths obtained from different rock units within a single metamorphic complex are common in many subduction-accretion-collision belts in the world. For example, in the Neoproterozoic—Cambrian Southern Granulite Terrane in India, isobaric cooling from very high-T condition or possible counterclockwise p-T path is inferred from sapphirine granulites in the Madurai Block (e.g., Santosh and Sajeev, 2006; Tsunogae and Santosh, 2006, 2010), whereas prograde high-pressure and peak ultrahigh-temperature metamorphisms along a clockwise p-T path have been illustrated for mafic granulites and Mg-Al-rich rocks from adjacent Palghat-Cauvery Suture Zone (e.g., Shimpo et al., 2006; Collins et al., 2007; Nishimiya et al., 2010). Santosh et al. (2009) interpreted the evolution of the Southern Granulite Terrane through a model involving a progressive sequence from Pacific-type subduction-accretion orogeny to Himalayan-type collisional orogeny. This model envisages the Madurai Block as a long-lived Neoproterozoic magmatic arc with accretionary belts and exhumed hot orogens, and regarded such isobaric cooling along a possible counterclockwise p-T path inferred from the Madurai Block as a result of input of heat related to magmatic underplating. In contrast, the clockwise p-T evolution through high-pressure and UHT metamorphism within the Palghat-Cauvery Suture Zone reflects deep subduction of supracrustal materials, which were metamorphosed and extruded during the final continent-continent collisional stage. Although detailed tectonic framework of the Napier Complex is not known due to limited rock exposure as well as complete recrystallization of the rocks at peak UHT stage, the different p-T paths obtained from the two localities might reflect the difference in the tectonic framework of these rocks within the complex subduction/collision belt. Further detailed petrological, structural, and geochronological investigations on the UHT granulites are necessary to fully understand the evolution of Neoarchean orogenic event in the Napier Complex.

7. Concluding remarks

Although the occurrence of equilibrium sapphirine + quartz assemblage from several localities in the Neoarchean Napier Complex, East Antarctica, provides unequivocal evidence for regional UHT metamorphism, the reaction microstructures associated with sapphirine + quartz from two localities (Tonagh Island and Priestley Peak) suggest different tectonic evolution within the complex. Petrography and mineral equilibrium modeling in NCKFMASHTO system suggest that sapphirine and quartz in Tonagh Island sample are separated by corona of Grt + Sil + Opx suggesting near isobaric cooling after the peak metamorphism (from 950—1100 °C and 7.5—11 kbar to 800—950 °C and 8—10 kbar) possibly along a counterclockwise p-T path, whereas the Spr + Qtz + Sil + Crd + Spl assemblage replaces garnet in Priestley Peak sample suggesting post-peak decompression (from 1000—1050 ° C and >12 kbar to 930—970 °C and 6.7—7.7 kbar) possibly along a clockwise p-T trajectory. Such contrasting p-T paths are consistent with a recent model on the structural framework of the Napier Complex that correlates the two areas to different crustal blocks. The different p-T paths obtained from the two localities might reflect the difference in the tectonic framework of these rocks within a complex Neo-archean subduction/collision belt.

Acknowledgments

We express our sincere thanks to the members of JARE-39 and the crew of the icebreaker Shirase for giving us the opportunity for geological field investigation of the Napier Complex, and for their helpful support. Especially we thank Professors Y. Osanai, T. Toyoshima, M. Owada, K. Shibuya, K. Moriwaki, K. Shiraishi,

Y. Motoyoshi, Drs. T. Hokada, and W. Crowe for their field support and discussion. Shimizu thanks Drs. Fawna J. Korhonen and Chris Clark for their guidance of pseudosection calculations. Dr. N. Nishida is acknowledged for his assistance on microprobe analyses. Drs. C.V. Dharma Rao and Shoujie Liu provided valuable comments and suggestions to the earlier version of this manuscript. We thank these reviewers as well as Prof. Xiaoqiao Wan for his editorial comments. Partial funding for this project was produced by a Grant-in-Aid for Scientific Research (B) from Japan Society for the Promotion of Science (JSPS) to Tsunogae (Nos. 20340148,22403017) and a Grant-in-Aid for JSPS Fellows to Shimizu (No. 23-311).

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