Scholarly article on topic 'Petrology and SHRIMP zircon geochronology of granulites from Vesleknausen, Lützow-Holm Complex, East Antarctica: Neoarchean magmatism and Neoproterozoic high-grade metamorphism'

Petrology and SHRIMP zircon geochronology of granulites from Vesleknausen, Lützow-Holm Complex, East Antarctica: Neoarchean magmatism and Neoproterozoic high-grade metamorphism Academic research paper on "Earth and related environmental sciences"

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{Granulite / Charnockite / "SHRIMP zircon geochronology" / Pseudosection / Rundvågshetta / "Gondwana supercontinent"}

Abstract of research paper on Earth and related environmental sciences, author of scientific article — Toshiaki Tsunogae, Daniel J. Dunkley, Kenji Horie, Takahiro Endo, Tomoharu Miyamoto, et al.

Abstract We report new petrological data and geochronological measurements of granulites from Vesleknausen in the highest-grade section of the Lützow-Holm Complex, part of the Gondwana-assembling collisional orogen in East Antarctica. The locality is dominated by felsic to intermediate orthogneiss (charnockite and minor biotite gneiss), mafic orthogneiss, and hornblende-pyroxene granulite, with deformed and undeformed dykes of metagranite and felsic pegmatite. Pseudosection analysis of charnockite in the system NCKFMASHTO, supported by geothermometry of mafic orthogneiss, was used to infer peak metamorphic temperatures of 750–850 °C, approximately 150 °C lower than those estimated for metasedimentary gneisses from Rundvågshetta, 6 km to the northeast. SHRIMP U-Pb analysis of zircons from feldspar-pyroxene gneiss, which corresponds to a partially molten patch around mafic orthogneiss, yielded a Concordia upper intercept ages of 2507.9 ± 7.4 Ma, corresponding to the time of formation of the magmatic protolith to the orthogneiss. Partial melting during peak metamorphism probably took place between 591 and 548 Ma, as recorded in rims overgrew around magmatic zircon. Our results suggest that Rundvågshetta-Vesleknausen-Strandnibba region in southwestern Lützow-Holm Bay, where orthogneisses are dominant, consists of a single crustal block, possibly formed by ca. 2.5 Ga arc magmatism. The Neoarchean magmatic terrane was tectonically mingled with other fragments (such as metasedimentary units in northern Lützow-Holm Bay) by subduction/collision events during the assembly of Gondwana supercontinent, and subsequently underwent ∼850 °C granulite-facies metamorphosed during Neoproterozoic to Cambrian final collisional event.

Academic research paper on topic "Petrology and SHRIMP zircon geochronology of granulites from Vesleknausen, Lützow-Holm Complex, East Antarctica: Neoarchean magmatism and Neoproterozoic high-grade metamorphism"

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Petrology and SHRIMP zircon geochronology of granulites from Vesleknausen, Lützow-Holm Complex, East Antarctica: Neoarchean magmatism and Neoproterozoic high-grade metamorphism

Toshiaki Tsunogaea,b'*, Daniel J. Dunkleyc,d, Kenji Horied, Takahiro Endoa, Tomoharu Miyamoto e, Mutsumi Katof

a Graduate School of Life and Environmental Sciences, University ofTsukuba, Ibaraki 305-8572, Japan b Department of Geology, University of Johannesburg, Auckland Park 2006, Johannesburg, South Africa

c Department of Applied Geology, Western Australian School of Mines, Curtin University, GPO Box U1987, Perth, WA 6845, Australia d National Institute of Polar Research, Tachikawa, Tokyo 190-8518, Japan

e Department of Earth and Planetary Sciences, Faculty of Sciences, Kyushu University, Fukuoka 812-8581, Japan f Department of Earth Sciences, Graduate School of Science, Chiba University, Chiba 263-8522, Japan

ARTICLE INFO

Article history: Received 29 January 2013 Received in revised form 8 April 2013 Accepted 9 April 2013 Available online 28 April 2013

Keywords: Granulite Charnockite

SHRIMP zircon geochronology Pseudosection Rundvâgshetta Gondwana supercontinent

ABSTRACT

We report new petrological data and geochronological measurements of granulites from Vesleknausen in the highest-grade section of the Lutzow-Holm Complex, part of the Gondwana-assembling collisional orogen in East Antarctica. The locality is dominated by felsic to intermediate orthogneiss (charnockite and minor biotite gneiss), mafic orthogneiss, and hornblende-pyroxene granulite, with deformed and undeformed dykes of metagranite and felsic pegmatite. Pseudosection analysis of charnockite in the system NCKFMASHTO, supported by geothermometry of mafic orthogneiss, was used to infer peak metamorphic temperatures of 750—850 °C, approximately 150 °C lower than those estimated for met-asedimentary gneisses from Rundvâgshetta, 6 km to the northeast. SHRIMP U-Pb analysis of zircons from feldspar-pyroxene gneiss, which corresponds to a partially molten patch around mafic orthogneiss, yielded a Concordia upper intercept ages of 2507.9 ± 7.4 Ma, corresponding to the time of formation of the magmatic protolith to the orthogneiss. Partial melting during peak metamorphism probably took place between 591 and 548 Ma, as recorded in rims overgrew around magmatic zircon. Our results suggest that Rundvâgshetta-Vesleknausen-Strandnibba region in southwestern Lutzow-Holm Bay, where orthogneisses are dominant, consists of a single crustal block, possibly formed by ca. 2.5 Ga arc mag-matism. The Neoarchean magmatic terrane was tectonically mingled with other fragments (such as metasedimentary units in northern Lutzow-Holm Bay) by subduction/collision events during the assembly of Gondwana supercontinent, and subsequently underwent ~850 °C granulite-facies metamorphosed during Neoproterozoic to Cambrian final collisional event.

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1. Introduction

* Corresponding author. Graduate School of Life and Environmental Sciences, University of Tsukuba, Ibaraki 305-8572, Japan. Tel.:+81 29 853 5239. E-mail address: tsunogae@geol.tsukuba.ac.jp (T. Tsunogae).

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

The late Neoproterozoic to early Cambrian East African-Antarctic Orogen (EAAO) is regarded as one of the largest collisional orogenic belts in the Earth's history, thought to have been formed during the amalgamation of Gondwana supercontinent. Although classic models invoked the formation of the orogen by a single collision of pre-existing East and West Gondwana continents (e.g., Stern, 1994; Shackleton, 1996), recent studies propose that the EAAO is formed by a series of orogenic events of various proto-East Gondwana fragments against a coherent West Gondwana between 750 and 530 Ma (e.g., Meert and Voo, 1997; Meert, 2003; Jacobs and

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Thomas, 2004; Collins and Pisarevsky, 2005; Collins et al., 2007a,b; Meert and Lieberman, 2008; Santosh et al., 2009a,b). Many recent studies on high-grade metamorphic rocks in southern India, Sri Lanka, Madagascar, and East Antarctica, which occupy the central region of Paleozoic Gondwana supercontinent, have focused on unraveling the structural and metamorphic processes in this region as the juxtaposition of these crustal blocks is critical in understanding the history of Gondwana amalgamation (e.g., Collins and Windley, 2002; Sajeev and Osanai, 2004; Santosh et al., 2009a; Tsunogae and Santosh, 2011).

The Lutzow-Holm Complex, exposed along the Prince Olav and Prince Harald Coasts of East Antarctica (Fig. 1a), is known for exposures of regionally metamorphosed amphibolite- to granulite-facies rocks formed during the late Neoproterozoic to early Cambrian 'Gondwana orogeny' associated with the final phase of amalgamation of continental fragments within the Gondwana assembly ( ~0.55 Ga, e.g., Shiraishi et al., 2003,2008). It is bordered to the east by the Meso-to Neoproterozoic Rayner Complex, while the western and southern extent of the complex is unknown due to thick ice cover (Fig. 1a). The dominant lithologies of the complex are orthogneiss (charnockite

and biotite gneiss) and various metasedimentary and metavolcanic rocks, which increase in metamorphic grade from amphibolite-facies in the northeast to granulite-facies in the southwest (e.g., Hiroi et al., 1991). The highest-grade metamorphic rocks of the complex are exposed at Rundvàgshetta (Fig. 1) in the southwestern part of the Lutzow-Holm Complex, where assemblages such as spinel + quartz (Motoyoshi et al., 1985a; Kawasaki et al., 2011), orthopyroxene + sillimanite + quartz (Kawasaki et al., 1993; Motoyoshi and Ishikawa, 1997; Fraser et al., 2000), sapphirine + quartz (Yoshimura et al., 2008a), and osumilite-bearing assemblages (Kawasaki et al., 2011 ), that are diagnostic of metamorphism at T > 900 °C, have been reported. The peak P— Tcondition of Rundvàgshetta has been estimated as 1040 °C at 13—15 kbar (Kawasaki et al., 2011). Such granulites formed by ultrahigh-temperature (UHT) metamorphism (e.g., Harley, 1998; Brown, 2007; Kelsey, 2008) have also been reported from adjacent localities such as Skallen and Skallevikshalsen, about 30 km northeast of Rundvàgshetta (Osanai et al., 2004; Yoshimura et al., 2008b). The peak UHT event in Rundvàgshetta was followed by retrograde stages of 950 °C at 8 kbar and 830 °C at 6.1 kbar along a clockwise P—T path (Kawasaki et al., 2011 ). Fraser et al. (2000) argued

Figure 1. (a) Location of representative rock exposures in the Lutzow-Holm complex.

:. (b) Geological map of Vesleknausen.

that the granulites cooled to temperatures between ca. 350 and 300 °C by ca. 500 Ma based on K/Ar and 40Ar/39Ar ages from hornblende and biotite.

Recently we performed a detailed geological field survey of the granulite-facies region of the Lutzow-Holm Complex on the Prince Harald coast of Lutzow-Holm Bay, during the 52nd Japanese Antarctic Research Expedition (JARE52), and examined granulites from a new locality named Vesleknausen, from which no geological observations have been reported. Although Vesleknausen is a small (0.7 km x 1 km) rock exposure, the location is only 6 km southwest of Rundvagshetta, so that it would be expected that the rocks there underwent similar degrees ofUHT metamorphism. Detailed studies on structural and metamorphic processes of Vesleknausen therefore provide valuable new information on the tectono-thermal evolution of this region. In this study, we present the first geological and petrographic observations of granulites from Vesleknausen, and discuss the P—Tevolution of this locality based on mineral chemistry and pseudosection analysis of charnockite. We also performed SHRIMP U-Pb analysis of zircon to determine the time of peak metamorphism as well as protolith formation. Our new petrological data are important in understanding the metamorphic history of the Lutzow-Holm Complex as well as in correlating the regional extent ofUHT metamorphic belts in the EAAO. Such information is vital for evaluating the tectonics of continent—continent collision during the Pan-African orogeny in East Antarctica, associated with the final assembly of the Gondwana supercontinent.

2. Geology of Vesleknausen

Vesleknausen ("little knoll" in Norwegian) is a 1 km wide rocky outcrop located near the southern end of the Soya Coast of Lutzow-Holm Bay (69°57'30"S, 38°54'10"E; Fig. 1a). It is geomorphologically divided into a low, flat terrace mostly covered by snow in the north and a craggy rise about 100 m above sea level in the south. Lithol-ogies comprise mainly felsic to intermediate orthogneiss (mostly charnockite), intercalated with mafic orthogneiss, and lesser amounts of melanocratic hornblende + two-pyroxene (Hbl-Px) granulite, fine-grained metagranitic and metamafic dykes, and un-deformed felsic pegmatite (Fig. 1b). Lithological varieties and field relationships are principally similar to those found in the southern part of Rundvagshetta (Motoyoshi et al., 1986), located 6 km northeast of Vesleknausen, and Strandnibba (Motoyoshi et al., 1985b) approximately 2 km southwest from Vesleknausen (Fig. 1a).

The predominant felsic to intermediate orthogneiss is composed mainly of charnockite, along with minor biotite gneiss. Charnockite is olive-colored on fresh faces and pale brown on weathered faces (Fig. 2a and b), and contains orthopyroxene as a stable phase. Although the foliation is sometimes weak in granoblastic hand specimens (Fig. 2a), charnockite has a well-developed gneissosity that is intercalated with mafic orthogneiss Fig. 2b). Biotite gneiss occurs in small amounts, and can be distinguished by being gray on fresh surfaces, fine-grained, and lacking in orthopyroxene. It occurs in layers and boudins 30—100 cm thick in association with charnockite and mafic

Figure 2. Field occurrences of representative lithologies in Vesleknausen. (a) Dark grayish felsic to intermediate orthogneiss (FO) is the most abundant lithology in this area. Gneissosity is not obvious in hand specimen. (b) Compositional layering defined by felsic to intermediate orthogneiss and intercalated mafic orthogneiss (MO). Pinkish pegmatite (P) (K-feldspar + quartz ± biotite) that cuts foliation of all lithologies as a later product. (c) Elongated boudins of grayish biotite gneiss (BG) within mafic orthogneiss. (d) Layers of mafic orthogneiss in felsic orthogneiss (charnockite). (e) Boudins of mafic orthogneiss surrounded by coarse-grained leucocratic Fs-Px rock (FPR) of possibly partial-melting origin. (f) Dark greenish brown Hbl-Px granulite that occurs as layers distributed parallel to the foliation of matrix felsic to mafic orthogneiss. (g) Discordant rock bodies of Hbl-Px granulite apparently intruded in and cutting foliation of felsic to intermediate orthogneiss. (h) Boudins of concordant Hbl-Px granulite in felsic and mafic orthogneisses association. (i) Pink, fine-grained metagranite (PG) dykes and other bodies that cut gneissosity in orthogneiss, and are deformed into isoclinal folds with axial planar foliations defined by biotite and feldspar parallel to the dominant gneissosity. These isoclinal folds are in turn deformed by mesoscopic open folds with sub-vertical E-trending axes.

orthogneiss (Fig. 2c). Mafic orthogneiss is dark gray to brown and occurs in layers a few cm to ~ 5 m thick, and as chains of boudins that can be traced for hundreds of meters along strike (Fig. 2b and d). It is often accompanied by feldspathic layers, pods, and nebulitic networks with migmatitic textures that are variably deformed. For example, in Fig. 2e, a boudin of mafic orthogneiss in charnockite is surrounded by a coarse-grained leucocratic network containing K-feldspar, plagioclase, and clinopyroxene. The coarse-grained, unfoliated appearance of this nebulitic network suggests an origin as melt derived from felsic and/or mafic orthogneiss, post-dating deformation that produced foliation and gneissosity in the latter. As discussed in a later section, this coarsegrained rock (feldspar-pyroxene rock) was dated by SHRIMP to examine the timing of these relationships.

Vesleknausen is also host to abundant discrete layers and discontinuous bodies of black, massive, hornblende-pyroxene-rich granulite, broadly distributed along gneissosity in the orthogneiss (Fig. 2f). Some bodies have discordant contacts with the host orthogneiss (Fig. 2g), whereas others are boudinaged within felsic orthogneiss (charnockite) (Fig. 2h), suggesting emplacement as dykes and stocks before and/or during high-strain deformation. All varieties are strongly metamorphosed and deformed.

Pink, fine-grained metagranite occurs as strongly deformed dykes and networks that intrude orthogneiss and hornblende-pyroxene granulite. In a few localities, complex structural relationships are revealed by metagranite dykes that cut gneissosity in orthogneiss, and are deformed into isoclinal folds with axial planar foliations defined by biotite and feldspar parallel to the dominant gneissosity (Fig. 2i). These isoclinal folds are in turn deformed by mesoscopic open folds with sub-vertical E-trending axes (Fig. 2i). As the metagranite occurs in small amounts and as thin layers, it is not shown on the geological map (Fig. 1b).

Vesleknausen is also host to sparse sets of NNE to NNW-trending, sub-vertical pegmatite dykes that intrude all lithologies (Fig. 1b). Pegmatite is composed of pink K-feldspar and quartz in crystals up to several centimeters across, with or without biotite. The dykes slightly hydrate mafic minerals in host lithologies, producing biotite (Fig. 2b). The pegmatites are undeformed, but in places are associated with minor faults and discrete shear zones.

The strong gneissosity in felsic, intermediate, and mafic orthogneiss records the earliest recognizable stage of high-strain deformation, herein defined as Dn. Orthogneisses typically have granoblastic fabrics, with a variably intense Sn foliation defined by trains of granoblastic orthopyroxene (Fig. 2b). In places intrafolial folding of discontinuous felsic laminations are visible in felsic orthogneiss. Mesoscopic isoclinal folds with an axial planar foliation define Dn + 1 structures, that can only be distinguished from Dn structures where metagranitic dykes were intruded between the events (Fig. 2i); elsewhere, Sn and Sn + 1 fabrics follow gniessosity, and cannot be distinguished. Some or all hornblende-pyroxene granulite protoliths may also have been intruded after Dn, and both metagranitic and metamafic dykes are boudinaged during Dn + 1. The variable attitude of Sn/Sn + 1 gneissosity and foliation is controlled by meso- to macro-scale open Fn + 2 folds, which have E-trending sub-vertical axial planes and a weakly developed Sn + 2 defined by biotite flakes. Pegmatite dykes postdate ductile deformation, but are occasionally associated with Dn + 3 semi-ductile shear zones and faults with only cm-scale displacements, which are steep to sub-vertical and trend NNW to NNE.

3.1. Felsic to intermediate orthogneiss (charnockite)

Orthopyroxene-bearing and coarse-grained felsic to intermediate orthogneiss (or charnockite) is the most dominant lithology in Vesleknausen. A representative sample Tsll012409-1 is composed of quartz (40-50%), K-feldspar (30-40%), plagioclase (5-10%), and orthopyroxene (5-10%) with accessory clinopyroxene, biotite, apatite, and zircon. Foliation defined by alternation of the orthogneiss and mafic orthogneiss layers can be seen in outcrops (Fig. 2b) as well as in regional scale (Fig. lb), but it is not obvious in thin section (Fig. 3a). Although quartz is sometimes present as elongated coarse-grained (up to 9 mm) minerals, lack of undulatory extinction suggests that the quartz grains recrystallized at high-temperature stage after the main deformation event. K-feldspar (0.2-1.1 mm) is subhedral and perthitic, and shows granoblastic texture. Orthopyr-oxene (0.3-2.0 mm) is subhedral to anhedral and scattered in the quartzo-feldspathic matrix. Myrmekite is often present along the grain boundaries of plagioclase. Biotite occurs as medium-grained (0.2-0.8 mm) flakes mainly in orthopyroxene-poor portions of the rock, and not a product of hydration after orthopyroxene; it is therefore regarded as a stable phase at high-grade metamorphism.

3.2. Biotite gneiss

Biotite gneiss is a medium-grained feldspathic rock with abundant biotite. A representative sample Ts11012402 (Fig. 3b) is composed of plagioclase (80-90%), K-feldspar (5-10%), and biotite (5-10%) with minor apatite and zircon. Quartz is absent or occurs in accessory amounts along feldspar grain boundaries. No foliation can be seen in hand specimen, although it is intercalated with mafic orthogneiss at outcrop scale (Fig. 2c). Plagioclase (0.2-1.5 mm) is subhedral in a granoblastic fabric. Minor fine-grained K-feldspar (0.2-0.5 mm) fills the grain boundary of the plagioclase. Biotite (0.1 -0.9 mm) occurs as randomly oriented flakes.

3.3. Mafic orthogneiss

Mafic orthogneiss (mafic granulite) is a medium-grained dark grayish rock containing predominantly orthopyroxene, clinopyrox-ene and plagioclase. All of the mafic orthogneiss samples examined have similar mineral assemblages and textures. A representative sample Ts11012401B (Fig. 3c) is composed of plagioclase (40-50%), orthopyroxene (10-20%), hornblende (5-15%), biotite (5-15%), and clinopyroxene (5-10%), with accessory ilmenite and apatite. Quartz is absent in the rock. It is characterized by medium-grained (0.3-0.7 mm) sub- to xenoblastic orthopyroxene and clinopyrox-ene in the matrix of plagioclase. The plagioclase is medium grained (0.3-1.1 mm) and granoblastic. Dark brownish hornblende often occurs as coarse-grained (up to 3.5 mm) porphyroblast and is aligned along weak foliation defined by interlayering of plagioclase-rich and hornblende-pyroxene-rich layers. Brownish biotite often accompanies with hornblende and is sometimes intergrown.

Mafic orthogneiss is often associated with coarse-grained feld-spathic rocks, probably formed by partial melting during high-grade metamorphism (Fig. 2e). Such coarse-grained felsic pyroxene granulite, discussed later in Section 3.5, was adopted for the U-Pb geochronology in order to identify the time of high-grade metamorphism.

3. Petrography

3.4. Hornblende-pyroxene (Hbl-Px) granulite

Photomicrographs of representative lithologies in Vesleknausen discussed below are shown in Fig. 3. Approximate modal abundance of rock-forming minerals in the rocks is summarized in Table 1. Mineral name abbreviations are after Spear (1993).

Hbl-Px granulite is a coarse-grained, massive, and dark greenish brown rock type associated with mafic orthogneiss and felsic to intermediate orthogneiss. Although both concordant and discordant types have been recognized (Fig. 2f and g), there are no petrographic

Figure 3. Photomicrographs showing representative mineral assemblages and textures of high-grade metamorphic rocks from Vesleknausen. All photographs were taken by polarized light. Opx: orthpyroxene, Cpx: clinopyroxene, Pl: plagioclase, Kfs: K-feldspar, Qtz: quartz, Ilm: ilmenite, Bt: biotite, Hbl: hornblende. (a) Subidioblastic pyroxenes in a matrix of feldspars and quartz in felsic orthogneiss (charnockite) (sample Ts11012409-1). (b) Biotite flakes and feldspathic matrix in biotite gneiss (sample Ts11012402). (c) Granoblastic plagioclase, pyroxenes, and hornblende in mafic orthogneiss (sample Ts11012401B). (d) Coarse-grained clinopyroxene, hornblende, and biotite in Hbl-Px rock (sample B98021103A). (e) Plagioclase-bearing and biotite-rich portion of Hbl-Px rock (sample Ts11012403B). (f) Coarse-grained granoblastic K-feldspar and subidioblastic clinopyroxene in Fs-Px rock (sample Ts11012406B), which are the product of partial-melting of charnockite/mafic orthogneiss.

differences between them. A representative sample Ts11012403A (Fig. 3d) is composed of hornblende (40—50%), orthopyroxene (20—30%), clinopyroxene (15—25%), and biotite (2—5%). Plagioclase is present in some samples (e.g., Ts11012403B, Fig. 3e). No obvious

foliation can be seen in hand specimen or thin section. Hornblende is coarse-grained (0.5—1.9 mm) and pale brownish. Biotite (0.2—1.4 mm) occurs along the grain boundary of hornblende and orthopyroxene, and is probably retrograde in origin.

Table 1

Modal abundance of minerals in granulites discussed in this study.

Sample No. Lithologya Qtz Pl Kfs Opx Cpx Bt Hbl Ilm Accessory

Ts11012401A MO +++ ++ ++ + ++ +

Ts11012401B MO +++ ++ ++ + ++ + Ap

Ts11012401C MO +++ ++ ++ + ++ + Ap, Chl

Ts11012402 BG +++ ++ ++ Ap, Zrn

Ts11012403A Hbl-Px ++ ++ + +++ Ap

Ts11012403B Hbl-Px + ++ ++ ++ +++ Ap, Mag

Ts11012404B MO +++ + + ++ + Ap

Ts11012406A MO +++ + + ++ ++ + Ap

Ts11012406B Fs-Px + + +++ ++ ++ Ap

Ts11012406C MO +++ + + ++ ++ + Ap

Ts11012407A FO ++ + ++ + + Ap, Zrn

Ts11012407B MO +++ ++ + ++ ++ + Ap

Ts11012407C Hbl-Px + ++ ++ +++

Ts11012408A BG +++ + ++ ++ Ap, Zrn

Ts11012409 MO +++ ++ + + + Ap

Ts11012409-1 FO +++ + +++ + + Ap, Zrn

++: abundant; ++: moderate; +: rare.

a MO: mafic granulite; BG: biotite gneiss; Hbl-Px: hornblende-pyroxene rock; FO: felsic to intermediate orthogneiss; Fs-Px: feldspar-pyroxene rock.

3.5. K-feldspar-plagioclase-clinopyroxene (Fs-Px) rock 4.1. Pyroxenes

This is a coarse-grained feldspathic rock present around boudins of mafic orthogneiss within felsic orthogneiss (Fig. 2e). Sample Tsll012406B (Fig. 3f) is composed of K-feldspar (40-50%), plagio-clase (20—30%), clinopyroxene (10-20%), and orthopyroxene (5—10%). Accessory minerals are magnetite, zircon, and apatite. Quartz is absent in the rock. It has granoblastic texture and lacks obvious foliation in thin section. K-feldspar is coarse-grained (~3.3 mm) and shows obvious exsolution texture with abundant thin lamellae of plagioclase. Plagioclase (0.1—0.3 mm) also occurs as a matrix phase. Pyroxenes (0.5—4 mm) are subhedral, scattered in the sample, and occur as aggregates to form pyroxene-rich domains. Myrmekite is commonly seen along grain boundaries of feldspars. The relatively high contents of K and Ca in this lithology, as well as the petrographical observations discussed above (coarse grained, unfoliated nature, appearance as nebulitic networks), might suggest that the Fs-Px rock is a partial melting product of the metasomatic zone between mafic orthogneiss and charnockite.

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 2 and 3.

Orthopyroxene in various granulites from Vesleknausen is enstatite-rich in composition. Orthopyroxene in sample Ts11012409-1 (felsic to intermediate orthogneiss) has XMg (= Mg/ (Fe + Mg)) ratio of 0.62—0.63. Al content in the mineral is very low as 0.04 pfu (0.70—0.92 wt.%), which is a common feature of char-nockite in the Gondwana orogeny (e.g., Rajesh et al., 2011). Orthopyroxene in other felsic to intermediate orthogneiss samples (e.g., Ts11012407A) principally has similar composition of XMg = 0.63-0.64 and Al2O3 = 0.64-0.66 wt.%. Orthopyroxene in Hbl-Px granulite (e.g., Ts11012403A) shows the highest XMg of 0.69—0.72, while that in mafic orthogneiss (e.g., Ts11012401B) is slightly enriched in Fe and Al as XMg = 0.58-0.59 and Al2O3 = 1.11.2 wt.%. Orthopyroxene in Fs-Px rock (Ts11012406B) has moderate XMg and Al2O3 content (sample Ts11012406B; XMg = 0.61-0.62, Al2O3 = 0.62-0.71 wt.%).

All clinopyroxene in felsic to intermediate orthogneiss (XMg = 0.70—0.73), mafic orthogneiss (XMg = 0.65—0.71), Hbl-Px granulite (XMg = 0.78—0.79), and Fs-Px rock (XMg = 0.68—0.70) are compositionally classified as augite. Its Al2O3 content varies from 1.6—1.9 wt.% (felsic to intermediate orthogneiss) to 2.02.2 wt.% (Hbl-Px granulite and Fs-Px rock), and up to 3 wt.% (mafic orthogneiss).

4.2. Calcic amphibole

Calcic amphiboles in the mafic orthogneiss and Hbl-Px granulite from Vesleknausen show notable compositional variation depending on the host rocks. Brownish subhedral amphibole in Hbl-Px granulite in samples Ts11012403A is compositionally par-gasite (XMg = 0.69-0.71, Si = 6.6-6.7, Na + KA = 0.69—0.71) after the

Table 2

Representative electron microprobe analyses of orthopyroxene, clinopyroxene, and biotite.

Lithology3 Opx (O = 6) Cpx (O = 6) Bt (O = 22)

FO FO Hbl-Px MO Fs-Px FO Hbl-Px MO Fs-Px FO FO Hbl-Px MO BG

Sample No. TS110-12409-1 TS110-12407A TS110-12403A TS110-12401B TS110-12406B TS110-12409-1 TS110-12403A TS110-12401B TS110-12406B TS110-12409-1 TS110-12407A TS110-12403A TS110-12401B TS110-12402

SiO2 52.87 53.48 54.92 53.39 52.52 52.36 54.00 51.49 51.78 39.51 38.90 37.60 38.37 38.40

Al2Û3 0.73 0.64 0.90 1.17 0.71 1.76 2.02 3.06 2.14 13.65 13.81 13.57 14.26 14.08

TiO2 0.04 0.05 0.07 0.03 0.05 0.26 0.26 0.38 0.25 3.34 4.72 5.54 5.03 5.03

Q2O3 0.01 0.01 0.09 0.00 0.05 0.08 0.22 0.00 0.00 0.07 0.00 0.39 0.04 0.03

FeOb 23.68 23.31 18.59 25.49 24.59 9.38 7.43 12.58 10.59 11.65 14.65 11.52 15.67 16.15

MnO 0.50 0.83 0.43 0.67 0.65 0.19 0.15 0.33 0.24 0.04 0.12 0.02 0.13 0.07

MgO 22.25 22.06 25.92 20.50 21.95 13.18 14.52 12.95 13.04 17.46 14.85 16.00 13.72 14.12

CaO 0.35 0.63 0.48 0.48 0.53 21.75 21.31 20.13 21.52 0.00 0.00 0.05 0.00 0.00

Na2O 0.05 0.01 0.05 0.00 0.00 0.88 1.00 0.45 0.86 0.05 0.05 0.40 0.02 0.01

K2O 0.01 0.01 0.01 0.00 0.01 0.00 0.00 0.00 0.00 8.08 7.87 9.71 9.81 9.95

Total 100.49 101.01 101.45 101.74 101.05 99.87 100.90 101.37 100.43 93.85 94.97 94.79 97.04 97.84

Si 1.972 1.982 1.977 1.979 1.960 1.962 1.976 1.918 1.941 5.824 5.750 5.586 5.648 5.624

Al 0.032 0.028 0.038 0.051 0.031 0.078 0.087 0.135 0.095 2.372 2.405 2.375 2.473 2.429

Ti 0.001 0.001 0.002 0.001 0.001 0.007 0.007 0.011 0.007 0.370 0.524 0.619 0.557 0.554

Cr 0.000 0.000 0.002 0.000 0.001 0.002 0.006 0.000 0.000 0.008 0.000 0.045 0.004 0.004

Fe2+ 0.738 0.722 0.559 0.790 0.767 0.294 0.227 0.392 0.332 1.435 1.810 1.430 1.928 1.978

Mn 0.016 0.026 0.013 0.021 0.020 0.006 0.005 0.011 0.008 0.005 0.015 0.003 0.016 0.008

Mg 1.236 1.218 1.389 1.132 1.220 0.736 0.791 0.719 0.728 3.834 3.270 3.540 3.008 3.080

Ca 0.014 0.025 0.018 0.019 0.021 0.873 0.835 0.803 0.864 0.000 0.000 0.008 0.000 0.000

Na 0.004 0.000 0.003 0.000 0.000 0.064 0.071 0.032 0.062 0.015 0.015 0.114 0.005 0.003

K 0.001 0.000 0.001 0.000 0.000 0.000 0.000 0.000 0.000 1.519 1.484 1.841 1.842 1.857

Total 4.013 4.003 4.003 3.994 4.023 4.023 4.006 4.020 4.036 15.383 15.273 15.562 15.480 15.536

Mg/(Fe + 0.63 0.63 0.71 0.59 0.61 0.71 0.78 0.65 0.69 0.73 0.64 0.71 0.61 0.61

a FO: felsic orthogneiss; Hbl-Px: hornblende-pyroxene rock; MO: mafic orthogneiss; Fs-Px: feldspar-pyroxene rock; BG: biotite gneiss. b Total Fe as FeO.

Table 3

Representative electron microprobe analyses of hornblende and feldspars.

Hbl (O = 23) Pl (O = 8) Kfs (O = 8)

Lithologya Hbl-Px Hbl-Px MO FO BG Fs-Px MO FO FO BG Fs-Px

Sample TS110- TS110- TS110- TS110- TS110- TS110- TS110- TS110- TS110- TS110- TS110-

No. 12403A 12403B 12401B 12409-1 12402 12406B 12401B 12409-1 12407A 12402 12406B

SiO2 45.81 43.98 43.01 62.00 63.34 61.84 53.48 64.61 64.02 65.10 64.45

Al2O3 9.90 10.18 11.50 22.85 23.24 23.41 29.74 18.42 18.42 18.80 18.44

TiO2 2.47 2.64 2.34 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.09

Q2O3 0.44 0.34 0.08 0.01 0.01 0.03 0.04 0.06 0.02 0.00 0.00

FeOb 10.88 11.27 15.48 0.08 0.10 0.10 0.17 0.06 0.15 0.08 0.01

MnO 0.07 0.11 0.07 0.00 0.00 0.00 0.00 0.01 0.01 0.02 0.00

MgO 14.35 14.33 10.87 0.00 0.00 0.01 0.00 0.00 0.01 0.00 0.00

CaO 11.28 11.73 11.15 4.46 5.13 4.82 12.91 0.07 0.03 0.04 0.05

Na2O 2.54 2.03 1.29 8.98 8.43 8.80 4.12 1.75 0.38 0.69 1.61

K2O 0.97 1.13 1.91 0.24 0.56 0.17 0.24 11.45 14.76 15.72 14.32

Total 98.83 97.74 97.70 98.63 100.80 99.18 100.70 96.43 97.78 100.44 98.98

Si 6.621 6.466 6.443 2.785 2.786 2.764 2.408 3.023 3.003 2.989 2.991

Al 1.686 1.764 2.029 1.210 1.205 1.233 1.578 1.015 1.018 1.017 1.008

Ti 0.268 0.291 0.263 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.003

Cr 0.050 0.039 0.009 0.000 0.000 0.001 0.001 0.002 0.001 0.000 0.000

Fe2+ 1.315 1.385 1.938 0.003 0.004 0.004 0.007 0.002 0.006 0.003 0.001

Mn 0.009 0.014 0.009 0.000 0.000 0.000 0.000 0.001 0.000 0.001 0.000

Mg 3.090 3.139 2.425 0.000 0.000 0.001 0.000 0.000 0.000 0.000 0.000

Ca 1.746 1.847 1.789 0.214 0.242 0.231 0.622 0.003 0.001 0.002 0.003

Na 0.710 0.579 0.375 0.782 0.719 0.762 0.360 0.159 0.035 0.061 0.145

K 0.179 0.211 0.365 0.014 0.031 0.010 0.014 0.683 0.883 0.920 0.847

Total 15.687 15.736 15.645 5.008 4.986 5.005 4.989 4.889 4.947 4.993 4.998

Mg/(Fe + 0.70 0.69 0.56 An (%) 0.77 0.72 0.76 0.36 0.19 0.04 0.06 0.15

Ab (%) 0.21 0.24 0.23 0.62 0.00 0.00 0.00 0.00

Or (%) 0.01 0.03 0.01 0.01 0.81 0.96 0.94 0.85

a FO: felsic orthogneiss; Hbl-Px: hornblende-pyroxene rock; MO: mafic orthogneiss; Fs-Px: feldspar-pyroxene rock; BG: biotite gneiss. b Total Fe as FeO.

classification of Leake et al. (1997). Slightly fine-grained variety of the lithology (sample Ts11012403B) also shows similar composition ofXMg = 0.69-0.70, Si = 6.4-6.5, and Na + KA = 0.73-0.79. Greenish amphibole in mafic orthogneiss (e.g., sample Ts11012401B) is also pargasite, but it is slightly Fe rich (XMg = 0.53-0.56, Si = 6.4-6.5, Na + KA = 0.65—0.70).

4.3. Feldspars

Plagioclase is a dominant mineral in most lithologies in Vesleknausen. Plagioclase in felsic to intermediate orthogneiss, biotite-gneiss and Fs-Px rock is albite-rich as An2o-24, yet it is significantly enriched in anorthite molecule in mafic orthogneiss as An57-62. K-feldspar in biotite gneiss and Fs-Px rock is orthoclase rich as Or94 and Or78-85, respectively.

4.4. Biotite

Biotite in Hbl-Px granulite is characterized by high TiO2 content (5.0-5.7 wt.%) and XMg (0.69—0.71). Biotite in felsic to intermediate orthogneiss is slightly TiO2-poor (3.3-3.4 wt.%) but more magne-sian (XMg = 0.72—73), while that in biotite gneiss shows the lowest Mg content of XMg = 0.61-0.62.

5. Metamorphic condition

5.1. Hornblende-plagioclase geothermometer

As garnet is absent throughout all lithologies in Vesleknausen, we could not calculate metamorphic conditions by applying conventional geothermometers and geobarometers to mineral

assemblages in the granulites of this region except hornblende-plagioclase pairs. As hornblende occurs as coarse-grained por-phyroblastic mineral in mafic orthogneiss, and the hornblende commonly coexists with plagioclase (e.g., Fig. 3c), the mineral pair has been used for conventional geothermometry. Based on hornblende solid-solution models and well-constrained natural and experimental studies, two geothermometers were calculated by Holland and Blundy (1994) based on the edenite-tremolite reaction, which is applicable to quartz-bearing rocks, and edenite-richterite reaction, which is applicable to both quartz-bearing and quartz-free rocks. As quartz is absent in mafic orthogneiss of Vesleknausen, the latter thermometer is used for the calculation of temperature of mafic orthogneiss (e.g., sample Ts11012401B). The calculated results are 780—790 °C at 3—5 kbar, which is consistent with the results of our pseudosection analysis as discussed below.

5.2. Mineral equilibrium modeling

We employed mineral equilibrium modeling technique to estimate metamorphic P—T conditions of the stability of mineral assemblages in felsic to intermediate orthogneiss (sample Ts11012409-1), a dominant lithology of Vesleknausen. Recently Endo et al. (2012a,b) applied phase equilibrium modeling technique to incipient charnockite from two localities in southern India, and successfully determined the P—T condition of charnockitization. We therefore attempted to apply the technique to massive charnockite in Vesleknausen. Although charnockite resulting from both igneous and metamorphic processes have been reported from different ter-ranes (e.g., Janardhan et al., 1979; Srikantappa et al., 1985; Frost and Frost, 1987, 2008; Hansen et al., 1987; Kramers and Ridley, 1989; Santoshetal.,1990; Bhattacharyya and Goswami,2009; Rajeshetal.,

2011; Rajesh, 2012; Rajesh and Santosh, 2012; Touret and Huizenga, 2012), our field and petrographic data indicate that charnockite was metamorphosed together with intercalated mafic to ultramafic granulites, thereby suggesting that the mineral assemblage in charnockite might record the P—T condition of high-grade meta-morphic event.

For the phase equilibrium calculations, we adopted THERMO-CALC 3.33 software (Powell and Holland, 1988, updated October 2009) with an internally consistent dataset of Holland and Powell (1998; dataset 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 the most realistic approximation to model the rocks examined in this study. The phases considered in the modeling and the corresponding a-x models used are garnet, biotite, and melt (White et al., 2007), cordierite (Holland and Powell, 1998), plagioclase (Holland and Powell, 2003), clinopyrox-ene (Green et al., 2007), amphibole (Diener et al., 2007), orthopyroxene, spinel, and magnetite (White et al., 2002), and ilmenite-hematite (White et al., 2000). The aluminosilicates, quartz, and H2O are treated as pure end-member phases. For the analysis, a slab of relatively homogeneous part of the examined granulite was used for thin-section preparation, and the counterpart of the same slab was used for chemical analysis. Bulk rock compositions for the rocks were determined by X-ray fluorescence spectroscopy at Activation Laboratories, Canada. The chemical composition (in wt.%) of sample Ts11012409-1 (charnockite) is SiO2 = 72.18, Al2O3 = 12.76, Fe2O3 = 0.10, FeO = 3.38, MgO = 1.18, CaO = 1.32, Na2O = 3.10, K2O = 5.22, TiO2 = 0.19. Mn is neglected in the modeling because the MnO content of this sample is low (0.04 wt.%). Fe2O3 is taken into account for the calculations because the rock contains small but un-negligible quantity of Fe2O3. The charnockite sample contains 0.06 wt.% P2O5, due to which the sample contains minor apatite. As we neglect P2O5 from the system, the CaO content equivalent to apatite should be extracted from the calculation. The corrected CaO content (1.24 wt.%) is adopted for the pseudosection calculation.

Sample Ts11012409-1 contains K-feldspar + quartz + plagioclase + orthopyroxene + biotite + clinopyroxene + ilmenite ± inferred melt. As orthopyroxene and biotite occurs as subidioblastic minerals in the matrix of granoblastic quartz and feldspars, we regard the mineral assemblage as the probable peak assemblage. Fig. 4a shows a P—T pseudosection for the sample calculated using the compositional factors listed in the figure. Water content of the rock in mole (M(H2O)) was fixed to be 0.2 mol.% (see discussion below). The stability field of the peak mineral assemblage of the rock plotted in the NCKFMASHTO pseudosection suggests a narrow P—T range of 750 ° C at 1 kbar and 850 °C at 10 kbar (hatched area in Fig. 4a) for the assemblage. The upper temperature stability limit of the assemblage is defined by the disappearance of biotite, while the lower limit by the absence of magnetite. The upper pressure stability limit of the assemblage is defined by the absence of garnet. The stability field of the peak assemblage slightly (about 10 °C) shifts toward lower temperature if we adopted higher M(H2O) value such as 1.0 mol.% (Fig. 4b), but not significant.

In this study we also attempted to construct NCKFMASHTO pseudosections for a charnockite from Rundvagshetta, where UHT metamorphic conditions of T ~1000 °C have been reported, using the rock composition (in wt.%) (SiO2 = 65.37, Al2O3 = 15.22,

Fe2O3 = 1.23, FeO = 3.10, MgO = 3.23, CaO = 3.11, Na2O = 4.50, K2O = 2.65, TiO2 = 0.45, P2O5 = 0.22) and mineral assemblage (Cpx + Opx + Bt + Pl + Kfs + Qtz + Opq with apatite) data of sample RH111 (charnockite) reported by Motoyoshi et al. (1986). The calculated results in Fig. 4c at M(H2O) = 0.2 mol.% demonstrate that the mineral assemblage in charnockite was stable at wide P-T ranges of ~800 °C at 2 kbar and 870 °C at 9 kbar (hatched area in Fig. 4c). The stability field shifts to slightly higher P—T of ~ 13 kbar and ~890 °C if M(H2O) of 1.0 mol.% is applied (Fig. 4d). The implications of the pseudosections will be discussed in a later section.

6. Geochronology

In order to determine the timing of high-grade metamorphism as well as protolith formation, we analyzed zircons from sample Ts11012406B of coarse-grained feldspar-pyroxene granulite, as this lithology was interpreted as having formed through the partial melting of charnockite/mafic orthogneiss during high-grade metamorphism.

6.1. Analytical methods

The sample was crushed and zircon grains were concentrated using heavy liquid and magnetic separation techniques. Zircon crystals were hand-picked under optical microscope for mounting. The zircon grains were set in Petropoxy epoxy resin along with grains of the age reference zircon TEMORA2 (416 Ma; Black et al., 2004) and the compositional reference zircon SL13 (U concentration of 238 ppm; Claoue-Long et al., 1995). The mount was polished, cleaned and evaporation-coated with high-purity gold. In order to investigate the internal structures of zircon grains, back-scattered electron (BSE) and cathodoluminescence (CL) images were obtained using scanning electron microscope (JEOL JSM-5900LV) at the National Institute of Polar Research (NIPR). U-Pb isotopic analyses of zircon were performed using the sensitive high-resolution ion microprobe (SHRIMP II) at NIPR. A primary beam of O2~ ions with a current of 4.7 nA was used to sputter analytical spots 25 mm wide on polished zircon. The secondary mass spectrometer was focused to a resolution of 5300 (M/DM at 1% of peak height) and yielded a sensitivity of approximately 17 counts per second per nA per ppm of 206Pb in zircon SL13. The analytical procedure for U-Pb dating of zircon was adapted from Compston et al. (1984), Williams (1998), and Horie et al. (2006). The U-Pb data were reduced in a manner similar to that described by Williams (1998), using the SQUID Excel macro (version 2.50; Ludwig, 2009). Corrections for common Pb were made using measured 204Pb and the model for common Pb compositions proposed by Stacey and Kramers (1975). Concordia and intercept ages were calculated from isotopic U-Pb datasets using Isoplot/Ex version 3.71 (Ludwig, 2003).

6.2. Results

Zircon grains from the sample analyzed are subhedral and rounded, with internal complexity most clearly revealed by CL imaging (Fig. 5). Grains consist mostly of unzoned, weakly concentrically zoned or sector-zoned zircon with a relatively low CL response. Many, but not all, grains also contain distinct irregular cores of varied appearance, including oscillatory and euhedral growth zones (Fig. 5a), which are truncated, partially replaced and mantled by thin zones of very high CL response that separate the cores from low-CL zircon (Fig. 5b and c). High-CL zircon is also found as distinct rims and grains of varying thickness, surrounding low-CL zircon. Growth zones in this generation of zircon are typically simple and anhedral (Fig. 5d), tend to be thin but are present to some degree on almost all grains.

Figure 4. P—T diagrams showing calculated pseudosections of mineral assemblages in charnockite from Vesleknausen (sample TS11012409-1) and Rundvagshetta. Hatched areas show the peak mineral assemblage. q: quartz, pl: plagioclase, ksp: K-feldspar, g: garnet, opx: orthopyroxene, bi: biotite: sill: sillimanite, ky: kyanite, mu: muscovite, sp: spinel, cd: cordierite, mt: magnetite, ilm: ilmenite, ru: rutile, liq: inferred melt. (a) Vesleknausen charnockite at M(H2O) = 0.2. (b) Vesleknausen charnockite at M(H2O) = 1.0. (c) Rundvagshetta charnockite at M(H2O) = 0.2. (d) Rundvagshetta charnockite at M(H2O) = 1.0.

Results of SHRIMP analysis are presented in Fig. 6 and Table 4. A total of 101 spots on 86 zircon grains were obtained. Considering the truncated cores, low-CL and outer high-CL zones as distinct generations of zircon growth, data for each type are presented and

assessed separately. Spots that include zircon from cores yielded for the most part discordant U-Pb and Pb-Pb ages that scatter along a linear array, with only two concordant analyses yielding 207Pb/206Pb ages of ca. 2.5 Ga (Fig. 6a). Together, 32 core data define

Figure 5. CL images with 206Pb/238U ages of selected zircons analyzed by SHRIMP. Numbers in parenthesis indicate analyzed spot numbers shown in Table 3. Ages with asterisk indicate discordant age data.

a Model 2 mixing line that intercepts the Concordia at 572 ± 15 and 2506 ± 15 Ma (95% confidence, MSWD = 3.2). The latter age is interpreted as the original age of zircon cores derived from a common source.

Age data for the low-CL type zircon are concordant within 2s precision, and do not define a single statistical age population, but scatter along the Concordia between ca. 610 and 540 Ma (Fig. 6b). The interpretation of this data is problematic; such distributions may be generated by a continuous process of zircon growth within this age range, or by the overlapping of two or more discrete periods of zircon growth; they could also be the product of Pb disturbance in zircon grown about the older end of the age distribution. However, in all three cases, the extraction from the oldest ages in the dataset of a Gaussian population, here defined as having a probability of equivalence of >0.05, provides a statistically robust estimate for the maximum age of zircon growth. Out of 46 analyses on low-CL zircon, the 28 oldest data (approximately two thirds of the dataset) define a Concordia age of 591.4 ± 2.6 Ma (95% confidence, probability of equivalence = 0.10, probability of concordance = 0.004). This age is that of the earliest stage of zircon growth. Younger ages from low-CL zircon, such as in analysis 63.1 (Fig. 5c), represent later growth and/ or isotopic disturbance after ca. 591 Ma.

Age data for the high-CL grains and rims scatter along the Concordia in the same age range as the low-CL zircon (Fig. 6c), with most analyses yielding younger ages than the latter (Fig. 5d). Since in most grains high-CL zircon occurs as overgrowths on low-CL metamorphic zircon, we can consider a Gaussian population of the youngest ages from high-CL zircon as representing the minimum age of growth. Out of 16 analyses on high-CL grains and rims, the 10 youngest data define a Concordia age of 547.5 ± 6.8 Ma (95% confidence, probability of equivalence = 0.16, probability of concordance = 0.6).

A consideration of the compositions of each type of zircon can help in understanding the significance of these age results. Analyses on zircon cores yielded variable U and Th contents (Fig. 6d), with outlying U contents as high as 1600 ppm and Th/U values between

0.1 and 1.8. However, most analyses concentrate around a mode of around 300 ppm U and Th/U = 0.6. Since zircon cores yielded a consistent upper intercept age of ca. 2506 Ma, and host rock is interpreted as a melt derived from the host orthogneiss, we interpret the age and compositions as those of the magmatic protolith of the orthogneiss. For the low-CL type of zircon, U and Th compositions have a very similar distribution to the majority of cores, suggesting that this generation was formed through dissolution and regrowth of zircon in the orthogneiss, in the absence of compositional changes or co-existing U- and Th-bearing phases, as early as ca. 591 Ma. This is in contrast to the high-CL zircon rims that overgrow low-CL zircon, which are strongly depleted in U (<100 ppm) and, to a lesser degree, Th, with Th/U values between 1.1 and 2.7. These compositions indicate a change in mineral or bulk chemistry between the time of growth of low-CL and high-CL zircon. This change may be a result of zircon crystallizing out of the melt-derived Fs-Px rock, as late as ca. 547 Ma. This interpretation is consistent with the undeformed nature of the Fs-Px rock (Fig. 2e). Alternatively, high-CL zircon may be the product of fluid activity after the crystallization of the melt, although there is no substantiating evidence for this hypothesis. In either case, the zircon ages constrain growth during high-grade metamorphism to 591-548 Ma.

7. Discussion

7.1. Petrology and metamorphic conditions

This is the first report on geology, petrology, and geochronology of granulites from Vesleknausen, located in the highest-grade region of the Lutzow-Holm Complex, which corresponds to the late Neo-proterozoic/Cambrian collisional orogeny in East Antarctica. The dominant lithology of this region comprises felsic to intermediate orthogneiss, mafic orthogneiss, and Hbl-Px granulite with subordinate biotite gneiss and pegmatite. Similar lithological sequences have also been reported from adjacent localities such as Strandnibba (Motoyoshi et al., 1985b) and Rundvagshetta (Motoyoshi et al., 1986)

Figure 6. (a) Tera-Wasserburg Concordia diagram showing U-Pb and Pb/Pb ratios of oscillatory-zoned zircons in sample TS11012406B and their discordance. (b) Concordia diagram showing U-Pb and Pb/Pb ratios of low-CL zircons in the sample. (c) Concordia diagram showing U-Pb and Pb/Pb ratios of high-CL zircons in the sample. (d) U vs. Th/U diagram showing variations of Th and U contents in the three types of zircon.

(Fig. 1a), suggesting that the southwestern part of the Lutzow-Holm Complex is composed dominantly of orthogneiss including char-nockite and mafic-ultramafic granulites, while metasediments are more abundant in the region to the north such as Skallen and Skal-levikshalsen (see Fig. 1a). The peak metamorphic condition of Vesleknausen has been estimated to be 750—850 °C and ~ 10 kbar based on pseudosection analysis of the mineral assemblage within felsic to intermediate orthogneiss (charnockite) in NCKFMASHTO system (Fig. 4a and b). The temperature range is consistent with an estimate of 780—790 °C using hornblende-plagioclase equilibria (Holland and Blundy, 1994) in mafic orthogneiss. This is more than 150 °C lower than the peak temperatures at Rundvàgshetta, located immediately northeast of Vesleknausen, from which peak UHT metamorphism at ~1000 °C has been inferred from sapphirine + quartz and orthopyroxene + sillimanite + quartz assemblages in pelitic granulites (e.g., Motoyoshi and Ishikawa, 1997; Yoshimura et al., 2008a; Kawasaki et al., 2011 ). However, it has to be noted that such diagnostic mineral assemblages of UHT meta-morphism have been reported only from various layers of metasediment on the northern margin of Rundvàgshetta; elsewhere, felsic to mafic orthogneisses are predominate (Motoyoshi et al., 1986), similar to Vesleknausen. No P—T data has been reported from Rundvàgshetta orthogneisses.

Accordingly, we constructed NCKFMASHTO pseudosections for felsic orthogneiss (charnockite) from Rundvàgshetta and compared the P—T conditions with those from Vesleknausen. The P—T pseudosection in Fig. 4c, calculated at M(H2O) = 0.2 mol.% demonstrates

that the mineral assemblage in charnockite is stable over a wide P—T range, from ~800 °C at 2 kbar to 870 °C at 9 kbar (hatched area in Fig. 4c). The stability field shifts to slightly higher conditions of ~ 13 kbar and ~ 890 °CifM(H2O) of 1.0 mol.% is applied (Fig. 4d). The P—T conditions are still more than 100 °C lower than the peak P—T conditions (~1000 °C) obtained from sapphirine granulites in the area, and more comparable with those at Vesleknausen. Our results therefore suggest that orthogneisses in the two regions underwent similar high-T metamorphism during late Neoproterozoic. The T > 1000 ° C UHT metamorphic condition reported from the northern margin of Rundvàgshetta, therefore, might be a local high-temperature event, although we cannot neglect a possibility that P—T conditions estimated in this study correspond to a retrograde stage.

7.2. Timing of magmatism and metamorphism

SHRIMP U-Pb analysis of oscillatory-zoned cores of zircons in Fs-Px rock, which cuts foliation of mafic orthogneiss (Fig. 2e) and was probably formed by partial melting at high-grade stage, yielded Concordia intercept ages of 2507.9 ± 7.4 Ma (upper intercept), which is regarded as the timing of protolith formation. Lack of ages older than 2.5 Ga suggests that cores of zircons in this sample were formed by single Neoarchean magmatic event, although we cannot completely reject a possibility that the 2.5 Ga zircon grains are detrital or xenocrystic, and the main magmatism that formed the protoliths of felsic to mafic orthogneisses in Vesleknausen took

Table 4

Zircon SHRIMP U-Pb-Th rations and ages from sample Tsll012406B (Fs-Px rock) from Vesleknausen.

Spot 206pbc (%) U (ppm) Th (ppm) 232Th/23sU 206Pb* (ppm) 204pb/206pb ±% 207pb/2C

High CL grains and rims

8.1 0.40 69 116 1.73 5.2 2.2E-4 50 0.0590

6.1 0.75 37 67 1.86 2.8 4.2E-4 50 0.0659

24.2 0.15 45 69 1.59 3.3 8.5E-5 100 0.0600

60.1 0.70 49 128 2.71 3.7 3.9E-4 45 0.0625

18.2 0.72 71 80 1.16 5.4 4.0E-4 35 0.0646

7.1 0.38 54 117 2.23 4.1 2.1E-4 58 0.0623

4.1 0.11 54 97 1.84 4.2 6.3E-5 100 0.0607

20.1 0.17 42 100 2.48 3.3 9.3E-5 100 0.0637

37.1 0.18 38 80 2.14 3.0 1.0E-4 100 0.0641

43.1 - 59 121 2.10 4.7 - - 0.0571

50.2 0.32 42 93 2.27 3.4 1.8E-4 71 0.0652

64.1 0.76 53 97 1.88 4.3 4.3E-4 41 0.0687

85.1 0.13 55 116 2.17 4.5 7.6E-5 100 0.0573

11.1 0.56 44 95 2.23 3.6 3.2E-4 50 0.0659

22.1 0.41 47 75 1.65 4.0 2.3E-4 58 0.0639

36.2 0.00 28 50 1.85 2.4 - - 0.0618

Low CL grains, mantles and rims

53.1 0.06 461 347 0.78 34 3.3E-5 50 0.0595

69.1 0.06 242 139 0.59 18 3.5E-5 71 0.0600

30.1 0.22 286 159 0.57 22 1.2E-4 33 0.0618

24.1 - 308 187 0.63 24 — 1.1E-5 100 0.0603

58.1 0.00 362 179 0.51 28 - - 0.0596

68.1 0.13 402 236 0.61 32 7.2E-5 38 0.0592

56.1 0.13 342 206 0.62 27 7.3E-5 38 0.0603

63.1 0.12 279 174 0.64 22 6.8E-5 45 0.0601

79.1 - 311 196 0.65 25 -6.4E-5 45 0.0583

51.1 0.12 357 85 0.24 28 7.0E-5 38 0.0596

74.2 0.07 290 94 0.33 23 3.9E-5 58 0.0582

72.1 0.13 289 153 0.55 23 7.4E-5 41 0.0605

66.1 - 418 249 0.62 33 -8.6E-6 100 0.0594

71.1 0.07 380 232 0.63 30 4.1E-5 50 0.0604

57.1 0.14 439 256 0.60 35 7.6E-5 33 0.0596

80.1 0.11 255 167 0.68 20 6.2E-5 50 0.0605

38.1 0.04 323 166 0.53 26 2.2E-5 71 0.0594

86.1 - 401 278 0.72 32 —2.1E-5 71 0.0599

75.1 0.02 418 327 0.81 34 8.7E-6 100 0.0594

34.1 0.02 306 193 0.65 25 1.1E-5 100 0.0593

35.1 0.07 284 211 0.77 23 3.8E-5 58 0.0592

9.1 - 326 199 0.63 27 —4.2E-5 50 0.0593

54.1 0.07 426 246 0.60 35 4.1E-5 45 0.0596

60.2 0.25 366 272 0.77 30 1.4E-4 26 0.0624

55.1 0.07 351 185 0.54 29 4.0E-5 50 0.0597

46.1 0.00 389 294 0.78 32 - - 0.0599

52.1 0.11 345 225 0.67 28 6.1E-5 41 0.0612

16.1 - 473 172 0.37 39 —2.2E-5 58 0.0583

59.1 0.13 204 132 0.67 17 7.2E-5 50 0.0589

18.1 0.12 216 141 0.68 18 6.5E-5 50 0.0610

42.1 - 370 233 0.65 30 —2.0E-5 71 0.0589

15.1 0.04 491 374 0.79 40 2.2E-5 58 0.0589

33.1 - 363 194 0.55 30 -9.0E-6 100 0.0585

81.1 0.02 315 193 0.63 26 1.3E-5 100 0.0603

6.2 0.22 342 203 0.61 28 1.2E-4 29 0.0616

3.1 0.16 413 240 0.60 34 8.9E-5 30 0.0599

±% 238U/206Pb* ±% 207рЬ,/206рЬ, ±% 206рь„/238ц 3g£ геПр^гОбр^ 3g£ % Discordant NB

2.4 11.52 1.3 0.0557 3.9 536 ±7 440 ±86 -23 Rim

3.1 11.49 1.6 0.0598 6.2 538 ±8 596 ±134 ±10 Rim

10.0 11.46 1.5 0.0588 10.4 539 ±8 559 ±228 ±4 Rim

2.7 11.44 1.5 0.0567 5.5 540 ±8 480 ±121 -13 Rim

2.2 11.30 4.0 0.0587 4.3 547 ±21 557 ±93 ±2 Rim

2.6 11.21 1.4 0.0592 4.1 551 ±7 573 ±89 ±4 Rim

2.5 11.04 5.2 0.0598 2.9 559 ±28 595 ±63 ±6 Grain

2.9 10.95 1.5 0.0624 3.7 563 ±8 687 ±79 ±19 Grain

3.1 10.91 1.6 0.0626 4.0 565 ±9 695 ±85 ±20 Rim

2.6 10.90 5.6 0.0571 2.6 566 ±30 497 ±58 -14 Grain

2.9 10.58 1.5 0.0625 4.2 582 ±9 693 ±90 ±17 Rim

2.5 10.56 1.5 0.0625 4.9 583 ±8 691 ±105 ±16 Rim

2.8 10.52 1.5 0.0562 3.4 586 ±8 459 ±76 -29 Rim

2.7 10.50 1.5 0.0613 4.7 586 ±8 649 ±102 ±10 Grain

2.7 10.03 1.5 0.0605 4.2 613 ±9 623 ±91 ±2 Rim

3.5 9.87 1.8 0.0618 3.5 622 ±11 666 ±75 ±7 Rim

0.9 11.50 0.9 0.0590 1.0 537 ±4 569 ±22 ±6

1.3 11.31 1.0 0.0595 1.4 546 ±5 586 ±31 ±7

1.1 11.02 0.9 0.0600 1.5 560 ±5 604 ±33 ±8

4.1 11.02 0.9 0.0604 4.1 560 ±5 619 ±88 ±10

1.0 10.99 2.0 0.0596 1.0 561 ±11 590 ±22 ±5 Rim

1.0 10.96 0.9 0.0582 1.2 563 ±5 536 ±27 -5 Rim

1.0 10.91 1.9 0.0592 1.2 565 ±10 574 ±27 ±2

1.1 10.90 0.9 0.0591 1.4 566 ±5 570 ±30 ±1 Rim

1.1 10.90 0.9 0.0592 1.3 566 ±5 574 ±28 ±2

1.0 10.86 0.9 0.0585 1.2 568 ±5 550 ±26 -3

1.1 10.83 0.9 0.0577 1.3 569 ±5 517 ±28 -11

1.1 10.81 0.9 0.0594 1.3 570 ±5 581 ±29 ±2

0.9 10.81 0.9 0.0595 0.9 571 ±5 586 ±20 ±3

1.0 10.73 0.9 0.0598 1.1 574 ±5 595 ±24 ±4

0.9 10.69 2.2 0.0585 1.1 576 ±12 547 ±24 -6

1.2 10.69 0.9 0.0596 1.5 576 ±5 588 ±32 ±2

1.0 10.68 0.9 0.0591 1.1 577 ±5 572 ±24 -1

1.0 10.63 0.9 0.0602 1.1 580 ±5 610 ±23 ±5

0.9 10.63 1.9 0.0593 0.9 580 ±10 577 ±21 0

1.1 10.59 0.9 0.0591 1.1 582 ±5 571 ±24 -2

1.1 10.59 0.9 0.0586 1.3 582 ±5 554 ±27 -5

1.1 10.55 0.9 0.0599 1.2 584 ±5 599 ±27 ±3

0.9 10.51 0.9 0.0590 1.0 586 ±5 567 ±22 -3

0.9 10.51 0.9 0.0604 1.3 586 ±5 618 ±28 ±6

1.0 10.49 0.9 0.0591 1.1 587 ±5 570 ±24 -3

3.3 10.48 0.9 0.0599 3.3 587 ±5 600 ±71 ±2

1.0 10.48 0.9 0.0603 1.2 588 ±5 616 ±25 ±5

0.9 10.46 0.9 0.0586 0.9 589 ±5 552 ±20 -7

1.3 10.46 1.0 0.0578 1.6 589 ±5 524 ±36 -13 Rim

1.2 10.45 1.0 0.0600 1.5 589 ±5 604 ±32 ±3

1.0 10.45 2.1 0.0592 1.0 589 ±12 575 ±22 -2

0.9 10.43 2.1 0.0586 0.9 590 ±12 551 ±20 -8

4.9 10.41 2.0 0.0586 4.9 591 ±11 552 ±108 -7

1.1 10.41 0.9 0.0601 1.2 591 ±5 607 ±25 ±3

1.0 10.38 2.7 0.0598 1.3 593 ±15 596 ±29 ±1

0.9 10.35 0.9 0.0586 1.1 594 ±5 554 ±25 -8

48.1 0.13 282 150 0.55 23 7.5E-5 41 0.0608 1.1 10.34 2.2 0.0597 1.3 595 ±12 593 ±29 0

22.2 0.09 278 158 0.59 23 4.9E-5 50 0.0596 1.1 10.33 0.9 0.0588 1.3 596 ±5 561 ±28 —6

73.1 — 334 154 0.48 28 —3.3E-5 58 0.0599 1.0 10.25 0.9 0.0604 1.1 600 ±5 618 ±24 +3

65.1 0.09 331 204 0.64 28 5.3E-5 45 0.0593 1.0 10.25 0.9 0.0585 1.2 600 ±5 549 ±26 —10

36.1 0.12 353 231 0.68 30 6.8E-5 38 0.0608 3.8 10.21 0.9 0.0598 3.9 603 ±5 596 ±85 —1

62.1 0.02 328 195 0.61 28 1.1E-5 100 0.0598 1.0 10.20 0.9 0.0597 1.1 603 ±5 592 ±23 —2

26.1 0.00 428 237 0.57 36 — — 0.0593 0.9 10.19 0.9 0.0593 0.9 604 ±5 579 ±19 —4

25.1 0.09 316 89 0.29 27 5.3E-5 45 0.0599 1.0 10.18 0.9 0.0592 1.2 604 ±5 573 ±26 —6

45.1 — 504 385 0.79 43 —7.2E-6 100 0.0591 0.8 10.18 1.9 0.0592 0.9 604 ±11 573 ±19 —6

78.1 0.10 256 133 0.54 22 5.6E-5 50 0.0599 1.2 10.15 3.2 0.0591 1.4 606 ±18 572 ±30 —6

Magmatic cores and core-rim mixtures

32.1 0.20 296 50 0.17 25 1.2E-4 32 0.0640 1.0 10.19 0.9 0.0623 1.4 603 ±5 685 ±29 +12

40.1 0.10 358 240 0.69 33 5.7E-5 41 0.0843 0.8 9.31 0.9 0.0835 0.9 658 ±6 1282 ±18 +51

2.1 0.16 300 131 0.45 31 8.9E-5 32 0.0938 0.8 8.20 2.3 0.0925 0.9 741 ±16 1478 ±17 +53

19.1 0.05 307 38 0.13 39 3.1E-5 50 0.1069 0.7 6.73 0.9 0.1064 0.7 893 ±7 1739 ±13 +52

28.1 0.09 220 116 0.54 31 5.3E-5 41 0.1187 0.7 6.03 1.0 0.1180 0.8 990 ±9 1926 ±14 +52

29.2 0.10 274 132 0.50 40 6.1E-5 38 0.1170 3.3 5.95 2.2 0.1162 3.3 1001 ±20 1899 ±60 +51

71.2 0.05 314 265 0.87 56 3.2E-5 41 0.1297 2.4 4.85 0.9 0.1292 2.5 1208 ±10 2088 ±43 +46

84.1 0.05 332 229 0.71 60 2.8E-5 45 0.1356 0.5 4.78 0.9 0.1352 0.5 1224 ±10 2167 ±9 +48

69.2 0.03 644 155 0.25 130 2.1E-5 35 0.1392 2.1 4.25 4.7 0.1389 2.1 1362 ±58 2214 ±37 +43

12.1 0.06 776 760 1.01 158 3.6E-5 21 0.1390 2.0 4.22 4.0 0.1385 2.0 1372 ±50 2209 ±34 +42

77.1 0.05 354 204 0.59 79 3.1E-5 35 0.1442 2.7 3.86 0.9 0.1438 2.7 1486 ±12 2273 ±47 +39

34.2 0.01 1484 2017 1.40 349 7.8E-6 32 0.1452 0.2 3.65 1.2 0.1451 0.2 1561 ±17 2289 ±3 +36

8.2 0.07 301 87 0.30 74 4.5E-5 30 0.1448 0.4 3.51 2.7 0.1442 0.5 1618 ±39 2278 ±8 +33

83.1 0.02 297 177 0.61 77 1.3E-5 58 0.1530 3.4 3.31 0.9 0.1528 3.4 1703 ±14 2378 ±58 +32

14.1 0.25 115 109 0.97 32 1.6E-4 24 0.1483 3.6 3.12 6.1 0.1462 3.7 1794 ±95 2302 ±63 +25

10.1 0.03 359 248 0.71 104 1.8E-5 38 0.1532 2.8 2.98 0.9 0.1530 2.8 1867 ±15 2380 ±47 +25

35.2 0.01 582 502 0.89 168 5.3E-6 58 0.1543 0.3 2.97 0.8 0.1543 0.3 1870 ±14 2394 ±5 +25

29.1 0.03 364 308 0.88 105 1.9E-5 38 0.1566 1.4 2.97 4.4 0.1563 1.4 1872 ±71 2416 ±23 +26

5.1 0.03 334 123 0.38 103 1.9E-5 38 0.1544 0.3 2.80 0.9 0.1542 0.4 1969 ±15 2393 ±6 +21

39.1 0.04 440 134 0.32 136 2.5E-5 29 0.1546 1.5 2.77 2.0 0.1542 1.5 1988 ±35 2394 ±26 +20

50.1 0.02 484 318 0.68 151 1.3E-5 38 0.1581 0.3 2.75 0.8 0.1580 0.3 2000 ±15 2434 ±5 +21

27.1 0.07 171 210 1.26 54 4.9E-5 33 0.1557 2.5 2.72 1.1 0.1550 2.5 2016 ±18 2402 ±42 +19

63.2 0.06 467 298 0.66 149 3.8E-5 23 0.1578 1.7 2.70 2.5 0.1573 1.7 2034 ±43 2427 ±28 +19

91.1 0.12 210 109 0.53 67 8.1E-5 100 0.1663 1.8 2.69 3.1 0.1653 2.0 2039 ±54 2511 ±33 +22

89.1 — 477 331 0.72 153 — — 0.1609 2.4 2.68 1.0 0.1609 2.4 2043 ±17 2465 ±40 +20

41.1 0.01 232 139 0.62 75 8.0E-6 71 0.1587 0.4 2.67 0.9 0.1586 0.4 2050 ±17 2441 ±7 +19

49.1 — 264 133 0.52 87 —1.3E-5 50 0.1573 2.9 2.62 4.6 0.1575 2.9 2083 ±82 2429 ±48 +17

23.1 0.01 1537 376 0.25 504 7.3E-6 28 0.1572 0.2 2.62 0.8 0.1571 0.2 2086 ±14 2425 ±3 +16

70.2 0.02 771 1299 1.74 253 1.1E-5 32 0.1571 1.3 2.61 0.8 0.1569 1.3 2089 ±15 2423 ±23 +16

1.1 0.02 217 87 0.41 73 1.4E-5 50 0.1605 1.4 2.55 0.9 0.1603 1.4 2131 ±17 2459 ±23 +16

13.1 0.03 307 155 0.52 104 2.0E-5 41 0.1620 0.6 2.55 1.7 0.1617 0.6 2135 ±31 2474 ±10 +16

61.1 0.08 139 63 0.47 47 5.7E-5 33 0.1592 2.0 2.53 1.0 0.1585 2.0 2144 ±19 2439 ±34 +14

74.1 — 335 155 0.48 114 —5.2E-6 71 0.1574 2.2 2.52 0.9 0.1575 2.2 2155 ±16 2429 ±37 +13

82.1 — 276 192 0.72 95 — — 0.1603 0.4 2.49 0.9 0.1603 0.4 2175 ±17 2459 ±7 +14

67.1 0.04 155 77 0.51 54 2.7E-5 45 0.1617 4.2 2.47 2.9 0.1613 4.2 2190 ±54 2470 ±71 +13

47.1 0.02 314 204 0.67 111 1.3E-5 45 0.1625 1.3 2.43 0.9 0.1624 1.3 2218 ±17 2481 ±21 +12

44.1 0.01 289 160 0.57 102 8.8E-6 58 0.1602 0.4 2.43 0.9 0.1601 0.4 2225 ±17 2457 ±6 +11

76.1 0.02 227 178 0.81 92 1.7E-5 45 0.1669 0.4 2.12 0.9 0.1667 0.4 2488 ±20 2525 ±6 +2

21.1 0.05 173 94 0.56 71 3.3E-5 35 0.1655 0.4 2.08 1.0 0.1651 0.4 2532 ±21 2509 ±7 —1

Conc. Conc.

Errors are 1s; Pbc and Pb* indicate the common and radiogenic portions, respectively.

Error in standard calibration was 0.24% (not included in above errors but required when comparing data from different mounts). Common Pb corrected using measured 204Pb. NB. conc. = concordant ages.

place at ca. 600 Ma. However, similar Neoarchean ages obtained from Sil-Bt-Grt gneiss from Rundvagshetta (2.53 Ga Sm-Nd TDM age; Shiraishi et al., 2008) and also from Botnnuten (2.5 Ga SHRIMP zircon age, Dunkley, unpublished data) within the southwestern Lutzow-Holm Complex suggest regional Neoarchean magmatism.

The timing of peak metamorphism of the Lutzow-Holm Complex has been inferred to be 520-550 Ma by SHRIMP zircon dating (e.g., Shiraishi et al., 1994; Fraser et al., 2000) and 500-560 Ma by monazite electron microprobe (EMP) dating (Hokada and Motoyoshi, 2006). In this study, we obtained a maximum age of ca. 591 Ma U-Pb ages for high-U (low-CL) zircons in Fs-Px rock. U and Th compositions of the zircons have a very similar distribution to the majority of oscillatory-zoned 2.5 Ga magmatic cores (Fig. 6d), suggesting that this generation was formed through dissolution and regrowth of zircon in the orthogneiss. As the Fs-Px rock cuts foliation of matrix mafic orthogneiss, the major deformation event in Vesleknausen probably took place before ca. 591 Ma. The ca. 2.5 Ga and 591 Ma zircons are often surrounded by low-U (high-CL) rims with a minimum age of growth of ca. 548 Ma, suggesting that the youngest zircons might have grown by crystallization from low-U melt or by fluid activity after the crystallization. The high-grade metamorphism of Vesleknausen thus took place between 591 and 548 Ma.

Suda et al. (2008) argued that, based on geochemical and isotopic studies of metabasites, the Lutzow-Holm Complex is divided into two lithological domains: a younger (1.0-1.8 Ga) amphibolite-granulite terrane in the Prince Olav Coast and northern Lutzow-Holm Bay areas, and an older (2.3-2.9 Ga) granulite terrane in the southern Lutzow-Holm Bay area (see Fig. 1a), and the two units were assembled by multiple subduction of Pan-African and/or pre-Pan-African age. Although few magmatic age data were reported from the granulite terrane in the southern Lutzow-Holm Complex, our new SHRIMP zircon ages confirm that the southwestern part of the Lutzow-Holm Complex is composed of Neoarchean crust, and there is a major geochronological, lithological, and P—T gap around Rundvagshetta, which may correspond to the unit boundary discussed in Suda et al. (2008).

Similar Neoarchean (ca. 2.5 Ga) magmatic ages and Neo-proterozoic (ca. 530-590 Ma) metamorphic ages were reported from the Madurai Block in southern India (Collins et al., 2013). Although the Madurai Block has been regarded as a single granulite block separated from other blocks (such as Nilgiri and Trivandrum Blocks) by the Palghat-Cauvery and Achankovil suture zones, recent detailed geochronological investigations suggest that the block is composed of three fragments on the basis of magmatic ages (Neoarchean Northwestern Madurai Block, Paleoproterozoic Northern Madurai Block, and Meso- to Neoproterozoic Southern Madurai Block), and that they were amalgamated before the final collision stage and subsequently metamorphosed during the late Neoproterozoic (Collins et al., 2013). The similarity of geochronology (Neoarchean magmatic age and Neoproterozoic metamorphic age) as well as li-thology (dominantly composed of granulite-facies felsic to intermediate orthogneisses) between the Northwestern Madurai Block and southwestern Lutzow-Holm Complex suggests that the two regions might have been formed by related tectonic events. Santosh et al. (2009a) argued that the Madurai Block is a long-lived magmatic arc formed by Pacific-type subduction-accretion orogeny, which collided with the Archean Dharwar Craton and subsequently been metamorphosed during Neoproterozoic to Cambrian times. The calc-alkaline affinity of felsic to intermediate orthogneiss in Vesleknausen (see chemical compositions in Fig. 4a) might also support that Rundvagshetta-Vesleknausen-Strandnibba region in the southwestern Lutzow-Holm Complex was a single crustal block formed, at least in part, by ca. 2.5 Ga arc magmatism. The Neoarchean terrane might have amalgamated with other fragments (such as metasedimentary units in the northern Lutzow-Holm Bay area) by

subduction/collision events during the assembly of Gondwana Supercontinent, and subsequently metamorphosed at ca. 591-548 Ma related to the final collisional event.

8. Conclusion

The dominant lithologies of Vesleknausen in the highest-grade region of the Lutzow-Holm Complex in East Antarctica, comprises felsic to intermediate orthogneiss (mainly charnockite), mafic orthogneiss, and Hbl-Px granulite. The strong gneissosity in the li-thologies suggests high-strain deformation during prograde stage. The peak metamorphic condition of this area has been estimated by hornblende-plagioclase geothermometry of mafic granulite (780-790 °C) and pseudosection analysis of felsic orthogneiss in NCKFMASHTO system (750—850 °C). These conditions are approximately 150 °C lower than the peak conditions at Rundvàgshetta, located immediately northeast of Vesleknausen, from which peak UHT metamorphic conditions of ~1000 °C has been inferred from sapphirine + quartz and orthopyroxene + sillimanite + quartz assemblages in pelitic granulites. We therefore performed pseudosection analysis of felsic orthogneiss (charnockite) from Rundvàgshetta, and obtained lower temperature conditions of ~870 °C comparable to those at Vesleknausen. Our results therefore suggest that orthogneisses in the two regions underwent similar high-T metamorphism, and the T > 1000 °C UHT metamorphic condition reported from the northern margin of Rundvàgshetta might be a local thermal event. SHRIMP U-Pb analysis of zircons in a Fs-Px rock, which corresponds to a partially molten patch around mafic orthogneiss, yielded Concordia intercept ages of2507.9 ± 7.4 Ma (upper intercept) which probably derive from orthogniesses metamorphosed much later to produce the melt. Peak metamorphism took place between 591 and 548 Ma, as obtained from rims around magmatic zircon. Our results suggest that the Rundvàgshetta-Vesleknausen-Strandnibba region in the southwestern Lutzow-Holm Complex, where orthog-neisses are dominant, corresponds to a single crustal block formed by ca. 2.5 Ga arc magmatism. This Neoarchean magmatic terrane amalgamated with other crustal fragments (such as metasedimentary units in northern Lutzow-Holm Bay) by subduction/collision events during the assembly of Gondwana Supercontinent, and subsequently underwent ~850 ° C granulite-facies metamorphosed during Neoproterozoic to Cambrian (ca. 591-548 Ma) orogenesis.

Acknowledgments

We express our sincere thanks to the members ofJARE-52 and the crew of the icebreaker SHIRASE for giving us the opportunity for geological field investigation of the Lutzow-Holm Complex, and for their helpful support. We are grateful to members of the Japanese Antarctic Research Group, particularly Profs. T. Kawasaki, K. Shiraishi, Y. Motoyoshi, and Dr. T. Hokada, for their continuous discussion and encouragement. Dr. Y. Suda and an anonymous reviewer provided constructive comments to the earlier version of this manuscript. We thank these referees as well as Prof. M. Santosh for his editorial suggestions and encouragements. Dr. Li Cheng and Ms. Lily Wang at the GSF editorial office in China University of Geosciences (Beijing) are acknowledged for their editorial assistance. This work was partly supported by a Grant-in-Aid for Scientific Research (B) from Japan Society for the Promotion of Science (JSPS) to Tsunogae (Nos. 20340148, 22403017).

References

Bhattacharyya, C., Goswami, B., 2009. Discussion on the paper "On charnockite"

published in Gondwana Research, volume 13, 30—44 (2008) by B. Ronald Frost

and Carol D. Frost. Gondwana Research 15, 216—217.

Black, L.P., Kamo, S.L., Allen, C.M., Davis, D.W., Aleinikoff, J.N., Valley, J.W., Mundil, R., Campbell, I.H., Korsch, R.J., Williams, I.S., Foudoulis, C., 2004. Improved 206Pb/238U microprobe geochronology by the monitoring of a trace element related matrix effect: SHRIMP, ID-TIMS, LA-ICP-MS, and oxygen isotope documentation for a series of zircon standards. Chemical Geology 205,115—140.

Brown, M., 2007. Metamorphism, plate tectonics, and the supercontinent cycle. Earth Science Frontiers 14,1—18.

Claoue-Long, J.C., Compston, W., Roberts, J., Fanning, C.M., 1995. Two carboniferous ages: a comparison of SHRIMP zircon dating with conventional zircon ages and 40Ar/39Ar analysis. In: Berggren, W.A., Kent, D.V., Aubrey, M.P., Hardenbol, J. (Eds.), Geochronology Time Scales and Global Stratigraphic Correlation, SEPM (Society for Sedimentary Geology) Special Publication 54, pp. 3—21.

Collins, A.S., Pisarevsky, S.A., 2005. Amalgamating eastern Gondwana: the evolution of the Circum-Indian orogens. Earth Science Reviews 71, 229—270.

Collins, A.S., Windley, B.F., 2002. The tectonic evolution of central and northern Madagascar and its place in the final assembly of Gondwana. Journal of Geology 110, 325—340.

Collins, A.S., Santosh, M., Braun, I., Clark, C., 2007a. Age and sedimentary provenance of the Southern Granulites, South India: U-Th-Pb SHRIMP secondary ion mass spectrometry. Precambrian Research 155,125—138.

Collins, A.S., Clark, C., Sajeev, K., Santosh, M., Kelsey, D.E., Hand, M., 2007b. Passage through India: the Mozambique ocean suture, high-pressure granulites and the Palghat-Cauvery Shear System. Terra Nova 19,141—147.

Collins, A.S., Clark, C., Plavsa, D., 2013. Peninsula India in Gondwana: the Tectono-thermal evolution of the southern granulite terrane and its Gondwanan counterparts. Gondwana Research. http://dx.doi.org/10.1016Zj.gr.2013.01.002.

Compston, W., Williams, I.S., Meyer, C.E., 1984. U-Pb geochronology of zircons from lunar breccia 73217 using a sensitive high-mass resolution ion microprobe. Journal of Geophysical Research B89, 525—534.

Diener, J.F.A., Powell, R., White, R.W., Holland, T.J.B., 2007. A new thermodynamic model for clino- and orthoamphiboles in Na2O-CaO-FeO-MgO-Al2O3-SiO2-H2O-O. Journal of Metamorphic Geology 25, 631—656.

Endo, T., Tsunogae, T., Santosh, M., Shimizu, H., Shaji, E., 2012a. Granulite formation in a Gondwana fragment: petrology and mineral equilibrium modeling of incipient charnockite from Mavadi, southern India. Mineralogy and Petrology. http://dx.doi.org/10.1007/s00710-012-0214-x.

Endo, T., Tsunogae, T., Santosh, M., Shaji, E., 2012b. Phase equilibrium modeling of incipient charnockite formation in NCKFMASHTO and MnNCKFMASHTO systems: a case study from Rajapalaiyam, Madurai Block, southern India. Geo-science Frontiers 3, 801—811.

Fraser, G., McDougall, L., Ellis, D.J., Williams, I.S., 2000. Timing and rate ofisothermal decompression in Pan-African granulites from Rundvagshetta, East Antarctica. Journal of Metamorphic Geology 18, 441—454.

Frost, B.R., Frost, C.D., 1987. CO2, melts, and granulite metamorphism. Nature 327, 503—506.

Frost, B.R., Frost, C.D., 2008. On charnockite. Gondwana Research 13, 30—44.

Green, E.C.R., Holland, T.J.B., Powell, R., 2007. An order-disorder model for omphacitic pyroxenes in the system jadeite-diopside-hedenbergite-acmite, with applications to eclogite rocks. American Mineralogist 92,1181—1189.

Hansen, E.C., Janardhan, A.S., Newton, R.C., Prame, W.K.B.N., Kumar, G.R.R., 1987. Arrested charnockite formation in southern India and Sri Lanka. Contributions to Mineralogy and Petrology 96, 225—244.

Harley, S.L., 1998. On the occurrence and characterization of ultrahigh-temperature crustal metamorphism. Geological Society, London, Special Publication 138, 81—107.

Hiroi, Y., Shiraishi, K., Motoyoshi, Y., 1991. Late Proterozoic paired metamorphic complexes in East Antarctica, with special reference to the tectonic significance of ultramafic rocks. In: Thomson, M.R.A., Crame, J.A., Thomson, J.W. (Eds.), Geological Evolution of Antarctica. Cambridge University Press, Cambridge, pp. 83—87.

Hokada, T., Motoyoshi, Y., 2006. Electron microprobe technique for U-Pb-Th and REE chemistry of monazite and its implication for pre-, peak- and post-metamorphic events in the Lutzow-Holm Complex and Napier Complex, East Antarctica. Polar Geoscience 19, 118—151.

Holland, T., Blundy, J., 1994. Non-ideal interactions in calcic amphiboles and their bearing on amphibole-plagioclase thermometry. Contributions to Mineralogy and Petrology 116, 433—447.

Holland, T.J.B., Powell, R., 1998. An enlarged and update internally consistent thermodynamic dataset with uncertainties and correlations: the system K2O-Na2O-CaO-MgO-MnO-FeO-Fe2O3-Al2O3-TiO2-SiO2-C-H2-O2. Journal of Meta-morphic Geology 8, 89—124.

Holland, T.J.B., Powell, R., 2003. Activity-composition relations for phases in petrological calculations: an asymmetric multicomponent formulation. Contributions to Mineralogy and Petrology 145, 492—501.

Horie, K., Hidaka, H., Gauthier-Lafaye, F., 2006. Elemental distribution in zircon: alteration and radiation-damage effects. Physics and Chemistry of the Earth 31, 587—592.

Jacobs, J., Thomas, R.J., 2004. Himalayan-type indenter-escape tectonics model for the southern part of the late Neoproterozoic-early Paleozoic East African-Antarctic orogen. Geology 32, 721—724.

Janardhan, A.S., Newton, R.C., Smith, J.V., 1979. Ancient crustal metamorphism at low pH2O: charnockite formation at Kabbaldurga, south India. Nature 278, 511—514.

Kawasaki, T., Ishikawa, M., Motoyoshi, Y., 1993. A preliminary report on cordierite-bearing assemblages from Rundvagshetta, Lutzow-Holm Bay, East Antarctica:

evidence for a decompressional P—T path? Proceedings of NIPR Symposium on Antarctic Geosciences 6, 47—56.

Kawasaki, T., Nakano, N., Osanai, Y., 2011. Osumilite and a spinel + quartz association in garnet-sillimanite gneiss from Rundvagshetta, Lutzow-Holm Complex, East Antarctica. Gondwana Research 19, 430—445.

Kelsey, D.E., 2008. On ultrahigh-temperature crustal metamorphism. Gondwana Research 13,1—29.

Kramers, J.D., Ridley, J.R., 1989. Can Archean granulites be direct crystallization products from a sialic magma layer? Geology 17, 442—445.

Leake, B.E., Woolley, A.R., Arps, C.E.S., Birch, W.D., Gilbert, M.C., Grice, J.D., Hawthorne, F.C., Kato, A., Kisch, H.J., Krivovichev, V.G., Linthout, K., Laird, J., Mandarino, J.A., Maresch, W.V., Nickel, E.H., Rock, N.M.S., Schumacher, J.C., Smith, D.C., Stephenson, N.C.N., Ungaretti, L., Whittaker, E.J.W., Youzhi, G., 1997. Nomenclature of amphiboles: report of the subcommittee on amphiboles of the International Mineralogical Association, commission on new minerals and mineral names. American Mineralogist 82,1019—1037.

Ludwig, K.R., 2003. User's Manual for Isoplot 3.00. Berkeley Geochronology Center Special Publication, No. 4, 70p.

Ludwig, K.R., 2009. Squid 2.50: a User's Manual. Berkeley Geochronology Center, Berkeley, California, USA, 95p (unpublished manual).

Meert, J.G., 2003. A synopsis of events related to the assembly of eastern Gondwana. Tectonophysics 362, 1—40.

Meert, J.G., Lieberman, B.S., 2008. The Neoproterozoic assembly of Gondwana and its relationship to the Ediacaran-Cambrian radiation. Gondwana Research 14, 5—21.

Meert, J.G., Voo, R.V.D., 1997. The assembly of Gondwana 800-550 Ma. Journal of Geodynamics 23, 223—235.

Motoyoshi, Y., Ishikawa, M., 1997. Metamorphic and structural evolution of granu-lites from Rundvagshetta, Lutzow-Holm Complex, East Antarctica. In: Ricci, C.A. (Ed.), The Antarctic Region: Geological Evolution and Processes. Terra Antarctica, Siena, pp. 65—72.

Motoyoshi, Y., Matsubara, S., Matsueda, H., Matsumoto, Y., 1985a. Gar-net—sillimanite gneiss from the Lutzow-Holm complex, east Antarctica. Memoirs of National Institute for Polar Research Special Issue 37, 82—94.

Motoyoshi, Y., Matsubara, S., Matsumoto, Y., Moriwaki, K., Yanai, K., Yoshida, Y., 1985b. Explanatory Text of Geological Map of Strandnibba, Antarctica. Antarctic GeologicalMap Series, 26. National Institute of Polar Research, Japan.

Motoyoshi, Y., Matsueda, H., Matsubara, S., Sasaki, K., Moriwaki, K., 1986. Explanatory Text of Geological Map of Rundvagskollane and Rundvagshetta, Antarctica. Antarctic Geological Map Series 24. National Institute of Polar Research, Japan.

Osanai, Y., Toyoshima, T., Owada, M., Tsunogae, T., Hokada, T., Crowe, W.A., Ikeda, T., Kawakami, T., Kawano, Y., Kawasaki, T., Ishikawa, M., Motoyoshi, Y., Shiraishi, K., 2004. Explanatory Text of Geological Map of Skallen, Antarctica (Revised Edition). Antarctic Geological Map Series, Sheet 39 Skallen (Revised Edition). National Institute of Polar Research, Japan.

Powell, R., Holland, T.J.B., 1988. An internally consistent thermodynamic dataset with uncertainties and correlations: 3. Application, methods, worked examples and a computer program. Journal of Metamorphic Geology 6, 173—204.

Rajesh, H.M., 2012. A geochemical perspective on charnockite magmatism in Peninsular India. Geoscience Frontiers 3, 773—788.

Rajesh, H.M., Santosh, M., 2012. Charnockites and charnockite. Geoscience Frontiers 3, 737—744.

Rajesh, H.M., Santosh, M., Yoshikura, S., 2011. The Nagercoil charnockite: a magnesian, calcic to calc-alkalic granitoid dehydrated during a granulite-facies metamorphic event. Journal of Petrology 52, 375—400.

Sajeev, K., Osanai, Y., 2004. Ultrahigh-temperature metamorphism (1150°C, 12 kbar) and multistage evolution of Mg-, Al-rich granulites from the Central Highland Complex, Sri Lanka. Journal of Petrology 4, 1821—1844.

Santosh, M., Harris, N.B.W., Jackson, D.H., Mattey, D.P., 1990. Dehydration and incipient charnockite formation: a phase equilibria and fluid inclusion study from South India. Journal of Geology 98, 915—926.

Santosh, M., Maruyama, S., Sato, K., 2009a. Anatomy of a Cambrian suture in Gondwana: Pacific-type orogeny in southern India? Gondwana Research 16, 321—341.

Santosh, M., Maruyama, S., Yamamoto, S., 2009b. The making and breaking of supercontinents: some speculations based on superplumes, superdownwelling and the role of tectosphere. Gondwana Research 15, 324—341.

Shackleton, R.M., 1996. The final collision zone between East and West Gondwana: where is it? Journal of African Earth Sciences 23, 271—287.

Shiraishi, K., Ellis, D.J., Hiroi, Y., Fanning, C.M., Motoyoshi, Y., Nakai, Y., 1994. Cambrian orogenic belt in east Antarctica and Sri Lanka: implications for Gondwana assembly. Journal of Geology 102, 47—65.

Shiraishi, K., Hokada, T., Fanning, C.M., Misawa, K., Motoyoshi, Y., 2003. Timing of thermal events in eastern Dronning Maud Land, east Antarctica. Polar Geo-science 16, 76—99.

Shiraishi, K., Dunkley, D.J., Hokada, T., Fanning, C.M., Kagami, H., Hamamoto, T., 2008. Geochronological constraints on the late Proterozoic to Cambrian crustal evolution of Eastern Dronning Maud Land, East Antarctica: a synthesis of SHRIMP U-Pb age and Nd model age data. Geological Society, London, Special Publications 308, 21 —67.

Spear, F.S., 1993. Metamorphic Phase Equilibria and Pressure-Temperature-time Paths. Mineralogical Society of America, Washington, D.C., 799pp.

Srikantappa, C., Raith, M., Spiering, B., 1985. Progressive charnockitization of a leptynite-khondalite suite in southern Kerala, India: evidence for formation of

charnockite through a decreasing fluid pressure? Journal of Geological Society of India 26, 62—83.

Stacey, J.S., Kramers, J.D., 1975. Approximation of terrestrial lead isotope evolution by a two-stage model. Earth and Planetary Science Letters 26, 207—221.

Stern, R.J., 1994. Arc assembly and continentalcollision in the Neoproterozoic East African Orogen: implications for the consolidation of Gondwana. Annual Review of Earth and Planetary Sciences 22, 319—351.

Suda, Y., Kawano, Y., Yaxley, G., Korenaga, H., Hiroi, Y., 2008. Magmatic evolution and tectonic setting of metabasites from Lutzow-Holm complex, East Antarctica. Geological Society, London, Special Publications 308, 211—233.

Touret, J.L.R., Huizenga, J.M., 2012. Charnockite microstructures: from magmatic to metamorphic. Geoscience Frontiers 3, 745—753.

Tsunogae, T., Santosh, M., 2011. Sapphirine + quartz assemblage from the Southern Granulite Terrane, India: diagnostic evidence for ultrahigh-temperature meta-morphism within the Gondwana collisional orogen. Geological Journal 46, 183—197.

Williams, I.S., 1998. U-Th-Pb geochronology by ion microprobe. In: McKibben, M.A., Shanks, W.C. (Eds.), Applications of Microanalytical Techniques to Understanding Mineralizing Processes. Reviews in Economic Geology 7, pp. 1 —35.

White, R.W., Powell, R., Holland, T.J.B., Worley, B.A., 2000. The effect of TiO2 and Fe2O3 on metapelitic assemblages at greenschist and amphibolite facies

conditions: mineral equilibria calculations in the system K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3. Journal of Metamorphic Geology 18, 497—511.

White, R.W., Powell, R., Clarke, G.L., 2002. The interpretation of reaction textures in Fe-rich metapelitic granulites of the Musgrave Block, central Australia: constraints from mineral equilibria calculations in the system K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3. Journal of Metamorphic Geology 20, 41—55.

White, R.W., Powell, R., Clarke, G.L., 2003. Prograde metamorphic assemblage evolution during partial melting of metasedimentary rocks at low pressures: migmatites from Mt Stafford, Central Australia. Journal of Petrology 44, 1937—1960.

White, R.W., Powell, R., Holland, T.J.B., 2007. Progress relating to calculation of partial melting equilibria for metapelites. Journal of Metamorphic Geology 25, 511—527.

Yoshimura, Y., Motoyoshi, M., Miyamoto, T., 2008a. Sapphirine + Quartz association in garnet: implication for ultrahigh-temperature metamorphism at Rundväg-shetta, Lützow-Holm complex, East Antarctica. Geological Society, London, Special Publications 308, 377—390.

Yoshimura, Y., Motoyoshi, M., Miyamoto, T., 2008b. High-grade metamorphic rocks from Skallevikshalsen in the Lützow-Holm Complex, East Antarctica: meta-morphic conditions and possibility of partial melting. Polar Geoscience 17, 57—87.