Tectonic division of the Southwestern terrane at the western Sør Rondane Mountains, Dronning Maud Land, East Antarctica, from a viewpoint of zircon U-Pb ages

Abstract

The Sør Rondane Mountains (SRM) in East Antarctica, lie within the late Neoproterozoic-early Paleozoic collision zone between East and West Gondwana. Many studies have been carried out in the eastern SRM, whereas fundamental questions on the western SRM remain unanswered, for example, tectonic division and detailed metamorphic age. This paper describes zircon U-Pb ages from gneisses in the western SRM, and the tectonic division of this area is discussed.

The rocks of the study area are divided into units 1 to 3 based on their lithology and structural position. Units 1 and 2, which are divided by the Kanino-tume shear zone (KSZ), are mainly composed of greenschist/amphibolite-facies gneisses and amphibolite/granulite-facies gneisses, respectively. Unit 3 is mostly occupied by the Proterozoic GTTG. It is, in this study, revealed that the metamorphic ages of units 1 and 2 are, respectively, ∼ 600-550 and ∼ 670-600 Ma. The distribution of the KSZ has been inferred by mapping, but the present results on the distribution of metamorphic ages confirm this. The KSZ of the low-angle shear zone may be a critical tectonic boundary dividing the two gneiss units of ∼ 600-550 and ∼ 670-600 Ma within the East African-Antarctic orogen.

INTRODUCTION

The Sør Rondane Mountains, Dronning Maud Land, East Antarctica are situated in the East African-Antarctic orogenic belt. The results of metamorphic and igneous research from this region have revealed late Neoproterozoic to early Paleozoic collision processes between East and West Gondwana (e.g., Shiraishi et al., 1994; Stern, 1994; Jacobs et al., 1998; Bauer et al., 2003; Jacobs et al., 2003; Meert, 2003; Jacobs and Thomas, 2004; Asami et al., 2005; Shiraishi et al., 2008; Grantham et al., 2013; Osanai et al., 2013; Mieth et al., 2014; Elburg et al., 2015; Jacobs et al., 2015; Ruppel et al., 2015; Elburg et al., 2016; Ruppel et al., 2020) (Fig. 1). The rocks of the Sør Rondane Mountains are divided into the Northeastern terrane (NE terrane) and the Southwestern terrane (SW terrane) on the basis of their provenance and metamorphic evolution, and the boundary of these terranes was defined as the Main Tectonic Boundary (MTB), which corresponds to a collision boundary between the East and West Gondwana (Osanai et al., 2013) (Fig. 1). The NE terrane is largely occupied by granulite-facies gneisses with minor amounts of amphibolite-facies rocks with the clockwise P-T-t path. Meanwhile, the SW terrane is mainly composed of granulite-, amphibolite-, and greenschist-facies gneisses with the counter-clockwise P-T evolution (Osanai et al., 2013). The SW terrane is further divided into units C, D, and D’. Units C and D consist mainly of granulite-facies and greenschist/amphibolite-facies gneisses, and unit D’ is made up of greenschist/amphibolite-facies gabbro-tonalite-trondhjemite-granodiorite complex (GTTG) (Osanai et al., 2013) (Fig. 1). Osanai et al. (2013) illustrated that unit C tectonically overlies unit D with a medium/high angle north-dipping fault. A ductile shear zone running along the boundary between units D and D’ was defined as the Main Shear Zone (MSZ, Kojima and Shiraishi, 1981; Shiraishi et al., 1992) (Fig. 1). Ruppel et al. (2015) asserted that the MSZ, showing dextral shearing, is a significant tectonic boundary separating the East African and Indo-Antarctic crusts.

Figure 1. Index map of the study area. (a) The locality of the Sør Rondane Mountains. (b) Simplified tectonic division of the western Sør Rondane Mountains. The study area is shown by the broken line. The metamorphic ages were from Takigami and Funaki (1991), Shiraishi et al. (2008), Adachi et al. (2013b), and Osanai et al. (2013). Tectonic divisions by Osanai et al. (2013) (units C, D, and D’) and Tsukada et al. (2017) (units 1, 2, and 3) are also shown. SR, Sør Rondane Mountains; MSZ, Main Shear Zone; KSZ, Kanino-tume shear zone; NE terrane, Northeastern terrane; SW terrane, Southwestern terrane; MTB, Main Tectonic Boundary; GTTG, gabbro-tonalite-trondhjemite-granodiorite complex.

Recently, Tsukada et al. (2017) tentatively divided the rocks of the SW terrane in the western Sør Rondane Mountains (called Widerøefjellet area hereinafter) into unit 1 (mainly greenschist/amphibolite-facies gneisses), unit 2 (mainly granulite-facies gneisses), and unit 3 (mainly of GTTG) based on their lithology and structural position (Fig. 1). Units 1, 2, and 3 lithologically corresponds to units D, C, and D’, respectively. Tsukada et al. (2017) suggested that, unlike the structural concept by Osanai et al. (2013), units 2 and 3, which had been juxtaposed by dextral movement along the MSZ, rode together onto unit 1 along a low-angle shear zone (Kamino-tume shear zone: KSZ) by top-to-the southward movement during the late Neoproterozoic-early Paleozoic. The distribution of the KSZ was inferred from geological mapping based on strike and dip data from the cataclasite outcrops in the Widerøefjellet, however it is not yet well established.

Many studies have been conducted on the metamorphic/magmatic history, chronology, and structural geology of units C and D’ (e.g., Asami et al., 1991; Grew et al., 1992; Osanai et al., 1992; Shiraishi and Kagami, 1992; Shiraishi et al., 1992; Ishizuka et al., 1993; Toyoshima et al., 1995; Osanai et al., 1996; Asami et al., 2005; Shiraishi et al., 2008; Adachi et al., 2013a, 2013b; Baba et al., 2013; Kamei et al., 2013; Osanai et al., 2013; Elburg et al., 2015; Ruppel et al., 2015, 2020). In contrast, only a few studies have been conducted on unit D (e.g., Kojima and Shiraishi, 1981; Tsukada et al., 2017) (Fig. 1). Turning to the study area, the Widerøefjellet area, studies have been made for unit D’ (Tsukada’s unit 3), but little attention has been given to units C and D (units 1 and 2 in Tsukada et al., 2017) (e.g., Takahashi et al., 1990; Takigami and Funaki, 1991; Shiraishi et al., 2008; Kamei et al., 2013; Elburg et al., 2015; Ruppel et al., 2015; Tsukada et al., 2017). The metamorphic age and tectonic division of the SW terrane in the western Sør Rondane Mountains are critical factors in understanding the development process of the rocks of the entire Sør Rondane Mountains and the East African-Antarctic orogenic belt. This paper describes the zircon U-Pb ages from the Widerøefjellet area and examines the validity of the tectonic division by Tsukada et al. (2017).

GEOLOGICAL OUTLINE OF THE STUDY AREA

The geological division of the study area is based on the observations by Tsukada et al. (2017) in this section. The rocks of the study area, embedded within the SW terrane (Osanai et al., 2013), were divided into units 1 to 3 based on their lithology and structural position (Tsukada et al., 2017) (Fig. 1). Units 1 and 2 are composed mainly of gneisses, and unit 3 consists mainly of mylonitized GTTG. Foliation, which trends east-west and dips steeply to moderately northward/southward, is developed in the rocks throughout the study area. The stretching lineation within unit 2 suggests dip-slip shearing, whereas that within unit 1 indicates strike-slip shearing (Tsukada et al., 2017). The rocks of units 2 and 3, divided by the MSZ, tectonically overlie unit 1 along the KSZ (Tsukada et al., 2017) (Fig. 1). On the east bank of the Ketelersbreen at the base of the Kanino-tume peak, a cataclasite zone of the KSZ is exposed. Clasts and blocks of red, pink, grey, and black granite-derived ultramylonite are embedded in the low-angle black foliated/non-foliated cataclasite. The Y-shears of the foliated cataclasite are sub-horizontal (Tsukada et al., 2017).

The rocks in unit 1 are predominantly composed of biotite-hornblende banded gneiss (Figs. 2a and 2b). The gneisses commonly include biotite and epidote, and minorly include muscovite and blue-green hornblende. Although the metamorphic condition has not been estimated in the study area, the peak metamorphism of the rocks for unit D, corresponding to unit 1, at Mefjell was inferred to have been at 550-620 °C and 3-6 kbar, and then underwent retrograde metamorphism around 400-500 °C and <4 kbar (Shiraishi et al., 1992; Osanai et al., 2013). Besides, Adachi et al. (2013a) reported P-T conditions of 550-650 °C, 5-8 kbar from the rocks of unit D in Lunckeryggen. Garnet in these rocks preserves prograde zoning, and the timing of metamorphism was estimated as ∼ 550 Ma (Adachi et al., 2013b). SHRIMP dating of zircon grains from the biotite orthogneiss in unit 1 suggests 653 ± 11 and 571 ± 5 Ma as igneous and metamorphic ages, respectively (Shiraishi et al., 2008). Unit 2 is largely composed of banded gneiss, with minor amounts of amphibolite, felsic schist, and marble (Figs. 2c and 2d). The gneisses commonly include biotite and green/brown hornblende, and minorly include clinopyroxene. In part, the gneiss and amphibolite yield orthopyroxene (Shiraishi et al., 1992; Osanai et al., 2013). The mafic to intermediate gneiss at Vengen underwent metamorphism of the upper amphibolite-facies (Shiraishi and Kojima, 1987). Although there is no estimation of the P-T condition on unit 2 rocks in the study area, the orthopyroxene-bearing gneisses of unit C at the Brattnipane which may correspond to unit 2 is considered to have reached granulite-facies (6-7 kbar and 800-900 °C), followed by retrogression and mylonitization (Adachi et al., 2013a; Baba et al., 2013; Osanai et al., 2013). The southern part of the study area mainly exposes the GTTG of unit 3. The GTTG is largely composed of biotite-hornblende meta-tonalite, including mafic enclaves and schist/gneiss/amphibolite blocks, and biotite meta-tonalite is locally exposed at Nils Larsenfjellet (Kamei et al., 2013). The GTTG is, in places, deformed and mafic enclaves are occasionally elongated to form thin layers, up to 1 m wide, parallel to the tectonic foliation. The GTTG was strongly mylonitized and includes schist/gneiss/amphibolite blocks along the MSZ. Ages around 1000 and 960-940 Ma were obtained from the GTTG as its igneous age (Takahashi et al., 1990; Tsukada et al., 2017). A biotite K-Ar age of 662 ± 23 Ma was reported from the gneiss within unit 3 at the Nils Larsenfjellet (Takigami and Funaki, 1991).

Figure 2. Photographs of the field occurrences in the study area. (a) and (b) Field occurrences of the gneiss of unit 1 in the north of Vengen. The gneiss was intruded by granite, then mylonitized together. The samples T08120401b and T08120501a were collected from the gneiss of these outcrops. (c) and (d) Field occurrence of the gneiss with a syn-tectonic granitic dike in unit 2 at Tanngarden. Both the gneiss and dike were mylonitized together. The samples T08122801d-D, T08122801d-W, and T09013101a were collected from the gneiss of these outcrops. (e) Field occurrence of the grayish granite-derived mylonite and cataclasite zone on the west bank of the Ketelersbreen. See Figure 1 for the locality. (f) Close-up view of the cataclasite zone on the west bank of the Ketelersbreen. Gn, gneiss; Gr, mylonitized granite; Pg, pegmatite vein; Gm, grayish granite-derived mylonite; C, cataclasite; B, block of red/reddish white granite-derived mylonite.

Kamei et al. (2013) clarified that the GTTG in unit D’, all through the Sør Rondane Mountains, is divided into 998-995 Ma tholeiitic rocks and 945-920 Ma and 772 Ma calc-alkaline rocks, which have been formed along a juvenile oceanic arc. Elburg et al. (2015) also recognized 995-975 and 965-925 Ma subduction-related igneous activities in the GTTG. Owada et al. (2013) and Elburg et al. (2015) suggested that the high-Ti back-arc basin mafic magmatism at 960-925 Ma that attributed to mantle activation, occurred simultaneously with the calc-alkaline magmatism. Additionally, the GTTG older than 1015 Ma was locally exposed (Elburg et al., 2015).

The metamorphosed and mylonitized rocks mentioned above were intruded by granitoid produced in several stages. The granitoid and its host together underwent some mylonitization events (Fig. 2). The granitic stock, Vengen/Vikinghøgda granite, yielding a magmatic age of ∼ 560-550 Ma (e.g., Shiraishi et al., 1992; Ikeda et al., 1995; Shiraishi et al., 2008; Elburg et al., 2016) intrudes into the gneiss and GTTG. The granitic stock cuts the MSZ (Shiraishi et al., 2008; Osanai et al., 2013; Tsukada et al., 2017) and is mylonitized. Elburg et al. (2016) and Ruppel et al. (2015) suggested that the MSZ-related movement lasted after the intrusion of the Vengen/Vikinghøgda granite on the ground of its mylonitization. Ruppel et al. (2020) suggested the biotite/amphibole ⁴⁰Ar/³⁹Ar ages of ∼ 600-525 Ma from the mylonitized granitoid and schist/gneiss in unit 3 beside the MSZ at the south of Ketelersbreen, and deduced that it records the time of cooling during and after late-stage ductile shearing along the MSZ. The foliated dike of monzogranite cutting mylonite along the MSZ gives zircon U-Pb ages of ∼ 585-570 Ma for its magmatism (Ruppel et al., 2020).

The KSZ cuts the MSZ and is intruded by felsic and mafic dikes. Biotite K-Ar age of 563 ± 14 Ma and zircon SHRIMP age of 564 ± 2 Ma were obtained from lamprophyre dikes at the Widerøefjellet (Owada et al., 2010, 2013). The K-Ar and ⁴⁰Ar/³⁹Ar methods for the mafic dikes at Brattnipane, Utnibba, and Vesthaugen give ages of ∼ 530-440 Ma (Takigami et al., 1987; Takigami and Funaki, 1991; Shiraishi et al., 1997). Ruppel et al. (2015) presented biotite/amphibole ⁴⁰Ar/³⁹Ar ages of ∼ 495-491 Ma from the deformed/non-deformed plutonic rocks of northern SW terrane. The plateau and total ages of ⁴⁰Ar/³⁹Ar method for biotite in the non-deformed granite at Pingvinane are 499 ± 9 and 487 ± 9 Ma respectively (Takigami and Funaki, 1991). Elburg et al. (2016) reported zircon U-Pb igneous ages of 575-506 Ma from granitoid all through the SW terrane at the Widerøefjellet area.

On the west bank of the Ketelersbreen, grayish granitoid-derived mylonite striking NE and dipping 35-90° S is contacted with a cataclasite zone including clasts and blocks of mainly red and reddish white granitoid-derived mylonite (Figs. 1, 2e, and 2f). The cataclasite is foliated/non-foliated. The foliation of the cataclasite strikes N 38°-58° E and sub-vertical (Fig. 2f). At the Vengen, 11 km northeast of this outcrop, a gently dipping shear zone, probably of the KSZ, is cut by a sub-vertical fault (Tsukada et al., 2017). This sub-vertical fault might be the NE extension of the cataclasite zone described here (Fig. 1).

ZIRCON U-Pb AGES FROM THE WIDERØEFJELLET AREA

LA-ICP-MS U-Pb dating of zircons in the gneisses of units 1 and 2 was carried out. In many places, the gneisses in this area are deformed together with younger granitic intrusive rocks to form black-and-white stripes (Fig. 2c). Therefore, samples with course-grained white bands probably derived from younger granitoid were avoided, and six samples from unit 1 (T08120401b, T08120401c, T08120501a, T08120502a, T08120702b, and T09013102a) and seven samples from unit 2 (T08122801c, T08122801d-D, T08122801d-W, T08122802a, T08122802b, T09013101a, and T09013101b) were selected to examine the metamorphic age of the gneisses (Fig. 1).

The zircons were concentrated using conventional mineral separation techniques. The zircon grains were mounted in epoxy resin and diamond-polished to expose the interior. Cathode luminescence (CL) images were obtained to investigate the interiors of individual zircons. The zircons were analyzed using ICP-MS (Agilent 7700x) connected to an NWR-213 laser ablation system (Electro Scientific Industries, Inc.) at Nagoya University. The 91500 standard zircons (1062.4 Ma; Wiedenbeck et al., 1995) and NIST Standard Reference Material 610 (Horn and von Blanckenburg, 2007) were used for calibration. The details of the analytical procedure are described in Kouchi et al. (2015). The 91500 standard zircon was measured for each 9-spot analysis, and the relative standard deviation from the ideal age (1062.4 Ma) was less than ±5%. The data were plotted on a Concordia diagram using Isoplot 3.70 software (Ludwig, 2008), and the data falling on the Concordia curve were selected for examination. Inappropriate data for examination, e.g., less than about 0.1 of probability for the concordant dates, were excluded. Based on beam spot evaluation using an optical microscope, data obtained from spots containing inclusions such as apatite and/or that significantly protruded from the grain were excluded. The data used in this study are available at http://www.num.nagoya-u.ac.jp/english/data/index.html. The petrographic property, internal structure of examined zircons at CL images, and dates of each sample are described below. The terms ‘core’ and ‘rim’ are generally used to describe the internal position and structure of the individual zircons at the CL images. However, some zircon crystals might have been broken or obliquely polished to the crystal axis. Besides, many of the zircons in this study do not suggest a clear internal structure in the CL images. Thus, since it could not be sure if the marginal domains of the CL images are really the ‘rim of the crystals’, this study uses the term ‘central and marginal domains of the image’ instead of ‘core and rim’ to avoid confusion. A few zircons show continuous concentric/sector zoning from central to marginal domains on the CL image. In such cases, the ‘marginal domain on the CL image’ is probably a part of the zircon core, and therefore they were counted as the ‘central’ that means nearly same as the core of the crystal.

Unit 1

Sample T08120401b. This sample, collected from psammitic gneiss, north of Vengen (72°0′46.8′′S, 23°25′14.81′′E), is composed mainly of quartz, plagioclase, biotite, blue-green hornblende, epidote, and apatite (Figs. 1, 2a, and 3a). Granular epidote and apatite are generally embedded in larger biotite and hornblende. The biotite and hornblende show a parallel orientation that defines the foliation. Hornblende was rarely replaced by chlorite. Signs indicating a reaction between hornblende and biotite are not seen. A marked gneissic structure, defined by alternating bands of mafic and felsic minerals, is common. Quartz ribbons showing typical dynamic recrystallization, such as sub-grain rotation and bulging (Passchier and Trouw, 2005) are embedded parallel to the foliation. The analyzed zircons are colorless and subhedral/euhedral. These zircons are generally dark and un-zoned/unclearly zoned in the CL images. In some cases, structures in which a relatively brighter irregularly-zoned central area is surrounded by an un-zoned darker area appear. There are even some grains that are darker in the central area than in the marginal one. The boundary between the central and marginal areas is generally obscure and parallel/oblique to the structure of the central area (Fig. 5). A few grains show obscure concentric zoning from central to marginal domains. Apatite and iron oxide inclusions are, in some grains, occurred. A total of 161 grains (259 spots) were examined, and the concordant dates (n = 155) range from 1039 to 562 Ma, and their Th/U ratios range from 0.041 to 0.88 (Figs. 6a and 7a). Twenty concordant dates from the marginal domain of the individual zircons range from 600 to 562 Ma (weighted mean age: 585 ± 5 Ma, MSWD = 2.9) (Figs. 6a and 7a). The Th/U ratio of the above analysis is 0.041-0.31, and 0.16 on average.

Figure 3. Photomicrographs of the psammitic gneiss of unit 1. (a) Sample number T08120401b. See Figure 2a for the field occurrence. (b) Sample number T08120401c. (c) Sample number T08120501a. See Figure 2b for the field occurrence. (d) Sample number T08120502a. (e) Sample number T08120702b. (f) Sample number T09013102a. Ap, apatite; Bt, biotite; Hbl, hornblende; Kfs, potassium feldspar; Ms, muscovite; Pl, plagioclase; Qz, quartz; Ttn, titanite; Zrn, zircon. Abbreviations for minerals are after Whitney and Evans (2010).

Sample T08120401c. This sample was collected from a psammitic gneiss, north of Vengen (72°0′46.8′′S, 23°25′14.81′′E) (Fig. 1). The main constituents of the present sample are quartz, plagioclase, potassium feldspar, and biotite, with subordinate amounts of epidote, apatite, and zircon (Fig. 3b). Biotite is rarely replaced by chlorite. The biotite shows a parallel orientation, which defines foliation, to form a gneissic structure of alternating bands of mafic and felsic minerals. Quartz ribbons are embedded parallel to the foliation. The asymmetric structure exhibits a dextral sense of shear. The analyzed zircons are colorless and subhedral/euhedral. High-CL central with irregular zoning and low-CL un-zoned marginal domains are recognized in many zircons in the CL image (Fig. 5). The boundary between the central and marginal domains is generally clear and crosses the zonal structure of the central region in most cases (Fig. 5). Some grains are darker in the central than in the marginal. Entirely dark and un-zoned/unclearly zoned zircons are also included. Besides, there are a few grains showing indistinct concentric zoning through whole CL image. Apatite and iron oxide inclusions are included in some grains. A total of 483 grains (680 spots) were examined; the concordant dates range from 748 to 559 Ma (n = 457), and their Th/U ratios range from 0.01 to 0.47 (Figs. 6b and 7b). Ninety-six concordant analyses from the dark marginal domain of individual grains range from 612 to 559 Ma (weighted mean age: 591 ± 3 Ma, MSWD = 3.0) (Figs. 6b and 7b). The range of the Th/U ratio is 0.02 to 0.31, with an average of 0.13.

Sample T08120501a. The sample was from a psammitic gneiss, north of Vengen (72°0′20.74′′S, 23°28′28.78′′E), consisting largely of quartz, plagioclase, potassium feldspar, biotite, and epidote with subordinate amounts of muscovite, apatite, and zircon (Figs. 1, 2b, and 3c). Chloritized biotite and sericitized plagioclase are hardly recognized. The parallel arrangement of biotite defines foliation, and alternating bands of mafic and felsic minerals form gneissic structures. Quartz exhibited dynamic recrystallization, such as sub-grain rotation and bulging (Passchier and Trouw, 2005). The analyzed zircons are colorless and subhedral/euhedral. For some zircons, the CL image shows disturbed obscure zoning or irregular patterns (Fig. 5). Some grains are composed of bright high-CL central and dark low-CL marginal domains. The central domain generally shows patchwork/irregular zoning, whereas the marginal domain is not structured (Fig. 5). The boundary between the central and marginal domains is generally clear and crosses the structure of the central domain in most cases. Small amounts of grains, that are darker in the central domain than in the marginal one, are also included. The grains showing continuous indistinct concentric zoning from the central to the marginal domains are also recognized. Apatite and iron oxide are commonly embedded in zircon grains. A total of 108 grains (149 spots) were examined; the concordant dates range from 857 to 571 Ma (n = 68), and their Th/U ratios range from 0.04 to 0.90 (Figs. 6c and 7c). Seven concordant data from the dark marginal domain of individual zircons range from 601 to 571 Ma (weighted mean age: 586 ± 10 Ma, MSWD = 3.0) (Figs. 6c and 7c). The range of Th/U ratio is 0.04 to 0.23, 0.13 on average.

Sample T08120502a. The main constituent of this sample was collected from an orthogneiss, north of Vengen (72°1′39.12′′S, 23°28′40.72′′E). The sample T08120502a consists mainly of quartz, plagioclase, potassium feldspar, biotite, and blueish green hornblende. Titanite, apatite, muscovite, and zircon are included as accessory minerals, but muscovite should not directly contact with hornblende (Fig. 3d). Replacement of mafic minerals by such as chlorite and actinolite is less recognized. Signatures showing replacement/reaction among the minerals are not seen. Biotite is arranged parallel to the quartz ribbons to form weak foliation. The quartz exhibits dynamic recrystallization. Mica-fish suggests a dextral sense of shear. The analyzed zircons are colorless and subhedral/euhedral. Many zircons show dark and obscure concentric/irregular oscillatory zoning in the CL image (Fig. 5). Some zircons are composed of bright central and dark marginal domains in the CL image (Fig. 5). The central domain commonly shows disturbed concentric zoning, whereas the marginal domain crossing the zoning of the central domain shows dark very obscure concentric zoning or no structure (Fig. 5). The dark marginal domain, in some cases, bulges/intrudes into the central domain. Minor amounts of zircon that are dark un-zoned in the central area and bright oscillatory-zoned in the marginal area are included. A total of 210 grains (282 spots) were examined, and the concordant dates range from 1798 to 544 Ma (n = 101), and their Th/U ratios range from 0.02 to 1.24 (Figs. 6d and 7d). Twenty-four data points obtained from the marginal domain of individual grains are grouped as 772 ± 12 Ma (concordant date of 1 grain) and 590-550 Ma (weighted mean age: 569 ± 5 Ma, MSWD = 3.9, n = 23) (Figs. 6d and 7d). Th/U ratios of the latter range from 0.06 to 0.18, and 0.10 on average.

Sample T08120702b. This sample was collected from psammitic gneiss, north of Vengen (72°0′48.04′′S, 23°24′58.74′′E) (Fig. 1). It is composed mostly of quartz, plagioclase, and potassium feldspar with minor amounts of biotite, muscovite, zircon, allanite, and stilpnomelane (Fig. 3e). Although a few amounts of biotite are chloritized, mineral replacement of the mafic minerals is hardly recognized. Biotite has a preferred orientation that is parallel to the structural plane. Quartz in places shows dynamic recrystallization, such as sub-grain rotation and bulging (Passchier and Trouw, 2005). The analyzed zircons are colorless and subhedral/euhedral. Most zircon grains are entirely dark and un-zoned in the CL images (Fig. 5). Some grains are composed of a bright central domain with oscillatory zoning and a dark un-zoned marginal domain. The zonal structure in the central region does not extend to the rim. A total of 132 grains (202 spots) were examined, and concordant dates range from 1092 to 549 Ma (n = 28) (Figs. 6e and 7e). Their Th/U ratios range from 0.01 to 0.74. Five concordant analyses from the dark marginal domain (622-573 Ma) are grouped as 622 ± 10 Ma (concordant date of 1 grain) and 609-573 Ma (weighted mean age: 589 ± 25 Ma, MSWD = 6.8, n = 4) (Figs. 6e and 7e). The Th/U ratios of the latter range from 0.021 to 0.16, and are mostly less than 0.04.

Sample T09013102a. This sample is a psammitic gneiss, east of Pingvinane (72°1′46.18′′S, 22°57′2.18′′E) (Fig. 1). It is composed mostly of quartz, plagioclase, biotite, and epidote, and minorly includes apatite and zircon (Fig. 3f). A minor amount of biotite is chloritized. Biotite has a subparallel orientation to form a foliation. Quartz shows dynamic recrystallization. The analyzed zircons are colorless and subhedral/euhedral. Many zircons are entirely dark and un-zoned in the CL image (Fig. 5). Some grains show disturbed concentric/irregular oscillatory zoning at their central domain. The zonal structure is clear or obscure. The zoned central domain is surrounded by a dark un-zoned domain (Fig. 5). The boundary between the central and marginal domains is clear/obscure, and parallel to or clearly crosses the structure of the central domain. Very a few grains show indistinct concentric zoning continuous from the center to the edge. A total of 173 grains (269 spots) were examined, and the concordant dates range from 827 to 549 Ma (n = 108) (Figs. 6f and 7f). The Th/U ratios range from 0.0050 to 0.57. Dates from 55 concordant analyses for the marginal domain of individual grains (820-549 Ma) are grouped as 820-786 Ma (n = 6), 642-611 Ma (n = 5), and 598-549 Ma (weighted mean age: 571 ± 4 Ma, MSWD = 4.6, n = 44) (Figs. 6f and 7f). The Th/U ratios of the youngest cluster range from 0.0050 to 0.43, with 0.22 on average.

Unit 2

Sample T08122801c. This sample was obtained from a psammitic gneiss in Tanngarden (72°2′19.97′′S, 22°47′19.58′′E) (Fig. 1). The sample consists of quartz, plagioclase, potassium feldspar, and biotite with minor amounts of apatite and zircon (Fig. 4a). The biotite is clear without chloritization. Parallel arrangements of biotite and apatite, and quartz ribbons form a foliation. The quartz exhibits dynamic recrystallization. The analyzed zircons are colorless and subhedral/euhedral. Many zircons are entirely dark or unclearly zoned in the CL image (Fig. 5). Some grains show bright disturbed obscure irregular, patchwork, or vermicular zoning in the center and entirely dark and non-structured in the marginal. The boundary between the dark marginal and bright central domains crosses the structure of the central domain. One grain seems to show very indistinct concentric zoning continuous from the center to the edge. A total of 279 grains (414 spots) were examined, and concordant dates range from 1176 to 595 Ma (n = 101) and their Th/U ratios range from 0.040 to 0.70 (Figs. 6g and 7g). Five concordant analyses from marginal domain range from 667 to 595 Ma (weighted mean age: 638 ± 35 Ma, MSWD = 27) (Figs. 6g and 7g). The Th/U ratios range from 0.037 to 0.27, 0.13 on average.

Figure 4. Photomicrographs of the psammitic gneiss of units 2. (a) Sample number T08122801c. (b) Sample number T08122801d-W. See Figure 2c for the field occurrence. (c) Sample number T08122801d-D. See Figure 2c for the field occurrence. (d) Sample number T08122802a. (e) Sample number T08122802b. (f) Sample number T09013101a. See Figure 2d for the field occurrence. (g) Sample number T09013101b. Ap, apatite; Bt, biotite; Cpx, clinopyroxene; Hbl, hornblende; Kfs, potassium feldspar; Pl, plagioclase; Qz, quartz; Zrn, zircon. Abbreviations for minerals are after Whitney and Evans (2010).

Figure 5. Representative CL images of zircon grains from the samples of the study area. Analytical spots are shown with open circles. The concordant ²⁰⁶Pb/²³⁸U dates with the 2σ error of each analytical spot are shown. Scale bars denote 100 µm.

Figure 6. Concordia diagrams of LA-ICP-MS zircon U-Pb dating (left: all concordant data, right: concordant data from the marginal domain of each grain). The gray and red ellipses in the left diagram indicate the dates obtained from the central and marginal domains, respectively. The solid black and dotted orange-colored ellipses in the right diagram show the dates assigned as the metamorphic and inherited ages, respectively, (a) T08120401b, (b) T08120401c, (c) T08120501a, (d) T08120502a, (e) T08120702b, (f) T09013102a, (g) T08122801c, (h) T08122801d-W, (i) T08122801d-D, (j) T08122802a, (k) T08122802b, (l) T09013101a, and (m) T09013101b.

Figure 7. Distribution of concordant dates of each sample. The gray and red bars are the data obtained from the zircon’s central and marginal domains respectively. The blue line shows 600 Ma. The histograms (blue rectangles) and probability density curves (red lines) of the youngest date cluster of metamorphic rim and MD are also shown for each sample. (a)-(f) Samples from unit 1. (g)-(m) Samples from unit 2.

Sample T08122801d-W. This sample, collected from Tanngarden (72°2′19.97′′S, 22°47′19.58′′E) (Figs. 1 and 2c), consists largely of quartz, plagioclase, and potassium feldspar, with subordinate amounts of green hornblende, biotite, apatite, and zircon (Fig. 4b). The hornblende and biotite are less altered. The boundary between these mafic minerals and the surrounding minerals is distinct, and a reaction rim is not recognized. The parallel orientation of the biotite crystals and quartz ribbons defines the foliation. Quartz generally exhibits dynamic recrystallization. The analyzed zircons are colorless and subhedral/euhedral. Zircons generally show dark obscure disturbed concentric/irregular/vermicular zoning, which is mantled by a comparatively darker/brighter very unclearly zoned/un-zoned domain (Fig. 5). The marginal domain, in some cases, bulges/intrudes into the central domain to cut its zoning. Some of the zircons are entirely un-zoned (Fig. 5). The zircons commonly contain apatite inclusions. A total of 189 grains (258 spots) were examined, and their concordant dates range from 843 to 593 Ma (n = 116), and their Th/U ratios range from 0.018 to 0.53 (Figs. 6h and 7h). Dates of the marginal domain from 32 concordant analyses (736-593 Ma) are grouped as 736-700 Ma (n = 4) and 666-593 Ma (weighted mean age: 638 ± 7 Ma, MSWD = 7.3, n = 28) (Figs. 6h and 7h). The Th/U ratios of the latter range from 0.018 to 0.51 and is mostly less than 0.10.

Sample T08122801d-D. This sample was from a psammitic gneiss alternating with the gneiss of T08122801d-W at Tanngarden (72°2′19.97′′S, 22°47′19.58′′E) (Figs. 1 and 2c). The sample consists mainly of biotite, green hornblende, and clinopyroxene with minor amounts of quartz, plagioclase, titanite, and apatite (Fig. 4c). The preferred orientation of biotite and hornblende defines a foliation. A minor chloritization of hornblende is recognized. The boundary among biotite, hornblende, and clinopyroxene is distinct, lacking a sign of replacement and reaction to each other. The analyzed zircons are colorless and subhedral/euhedral. Most zircons show comparatively brighter obscure concentric/irregular/vermicular zoning at the central domain, which is mantled by a darker unclearly zoned/un-zoned domain to cut the zoning of the central domain (Fig. 5). The dark marginal domain, in some grains, bulges/intrudes into the central domain cutting the structure of the central. A few grains, which have a brighter marginal domain than the central one, are included. Continuous obscure oscillatory zoning from central to marginal domains is recognized in some grains. The zircons commonly contain apatite inclusions. A total of 222 grains (290 spots) were examined, and the concordant dates range from 849 to 611 Ma (n = 154) (Figs. 6i and 7i). Their Th/U ratios range from 0.015 to 0.44. The concordant dates from the low-CL marginal domain of 39 grains range from 666 to 611 Ma (weighted mean age: 645 ± 5 Ma, MSWD = 5.4) (Figs. 6i and 7i). The Th/U ratios of the marginal domain range from 0.015 to 0.43 and 0.10 on average.

Sample T08122802a. This sample was collected from psammitic-pelitic gneiss, Tanngarden (72°2′27.25′′S, 22°47′42.27′′E) (Fig. 1). The sample, showing a stripe of black and white layers, is composed mainly of quartz, plagioclase, potassium feldspar, green/brown hornblende, and biotite with subordinate amounts of apatite and zircon (Fig. 4d). The black layers are enriched with hornblende and biotite. The boundary between biotite and hornblende is distinct, lacking a sign of a replacement for each other. The parallel orientation of the biotite and hornblende in black layers and quartz ribbons define foliation. Quartz generally exhibits dynamic recrystallization. Asymmetric structures show top to the north of the movement. The analyzed zircons are colorless and subhedral/euhedral. Many of the examined zircons are entirely dark and un-zoned in the CL image (Fig. 5). Another shows dark obscure concentric/irregular/vermicular zoning at the central domain, which is mantled by a dark un-zoned domain to cut the zoning of the central domain (Fig. 5). The marginal domain, in some grains, bulges/intrudes into the central domain cutting the structure of the central. A few grains, which have a brighter marginal domain than the central, are included. The zonal structure in the central region does not extend to the dark marginal region. The zircons commonly contain apatite inclusions. A total of 229 grains (348 spots) were examined, and the concordant dates range from 725 to 608 Ma (n = 186), with a Th/U ratio ranging from 0.0080 to 0.42 (Figs. 6j and 7j). The concordant data from the dark marginal domain of 59 individual zircons range from 676 to 608 Ma (weighted mean age: 644 ± 4 Ma, MSWD = 7.6) (Figs. 6j and 7j). The Th/U ratios of the marginal domain range from 0.0080 to 0.32, and 0.15 with an average.

Sample T08122802b. This sample was collected from a psammitic gneiss in Tanngarden (72°2′27.25′′S, 22°47′42.27′′E) (Fig. 1). The sample is composed mainly of quartz, plagioclase, and potassium feldspar with subordinate amounts of apatite and zircon (Fig. 4e). Thin layers made up of biotite and green/brown hornblende are intercalated in this sample. The boundary between biotite and hornblende is clear, and they have not been replaced by each other. The parallel orientation of the biotite and quartz ribbons defines the foliation. Quartz generally exhibits dynamic recrystallization. Asymmetric structures show top to the north of the movement. In the CL image, most of the grains show dark irregular/patchwork/vermicular zoning in their central portion (Fig. 5). The zoned central domain is commonly surrounded by a dark un-zoned domain that crosses the central structure (Fig. 5). The dark marginal domain bulges/intrudes into the central domain in some grains. Some zircons consist of entirely dark un-zoned central and brighter marginal domain. The marginal domain of such zircon, in some cases, shows indistinct zoning. The zircons commonly contain apatite inclusions. A total of 178 grains (279 spots) were examined; the concordant dates range from 692 to 616 Ma (n = 130), and their Th/U ratios range from 0.017 to 0.36 (Figs. 6k and 7k). The concordant dates from the marginal domain of 48 individual grains range from 671 to 620 Ma (weighted mean age: 642 ± 4 Ma, MSWD = 5.0) (Figs. 6k and 7k). The Th/U ratios of the 48 zircons range from 0.090 to 0.29, with an average value of 0.16.

Sample T09013101a. This sample was obtained from felsic gneiss, Pingvinane (72°1′52.76′′S, 23°0′0.61′′E) (Figs. 1 and 2d). It mainly consists of quartz, plagioclase, and potassium feldspar (Fig. 4f). Allanite, green/brown hornblende, biotite, titanite, and zircon are also included. The boundary between hornblende and biotite is clear, and no concrete evidence of replacement and reaction to each other is recognized. The parallel orientation of the quartz ribbons defines the foliation. The quartz exhibits dynamic recrystallization. The analyzed zircons are colorless and subhedral/euhedral. Most of the grains are dark, very indistinctively zoned, or un-zoned. Disturbed indistinct irregular/vermicular zoning, which is mantled by darker un-zoned domains, is recognized in some grains (Fig. 5). The marginal, in rare cases, is brighter than the central. The marginal domain bulges/intrudes into the central region in some grains. The boundary between the marginal and the central is parallel or oblique to the zonal structure of the central domain. Apatite inclusions are embedded in zircon. A total of 175 grains (269 spots) were examined, and concordant dates range from 693 to 601 Ma (n = 112) (Figs. 6l and 7l). Their Th/U ratios range from 0.08 to 0.40. The dates from 21 concordant analyses in the marginal domain (684-611 Ma) are grouped as 684-664 Ma (n = 3) and 651-611 Ma (weighted mean age: 632 ± 5 Ma, MSWD = 2.5, n = 18) (Figs. 6l and 7l). Th/U ratios of the 21 grains range from 0.19 to 0.40, and 0.29 on average.

Sample T09013101b. This sample was collected from a psammitic gneiss, Pingvinane (72°1′52.76′′S, 23°0′0.61′′E) (Fig. 1). The sample is composed mainly of quartz, plagioclase, potassium feldspar, green/brown hornblende, and biotite with subordinate amounts of apatite and zircon (Fig. 4g). The boundary between hornblende and biotite is distinct and signs of their interaction such as replacement and reaction rim are not seen. The parallel orientation of the hornblende and biotite defines foliation. Quartz commonly exhibits dynamic recrystallization. The analyzed zircons are colorless and subhedral/euhedral. All grains are composed of comparatively brighter central domain with irregular/patchwork/vermicular zoning and darker un-zoned marginal domain (Fig. 5). The boundary between the central and the marginal domains commonly crosses the structure of the central. Small number of grains show unclear oscillatory concentric/sector zoning continuous from the center to the edge. The dark marginal domain, in some cases, bulges/intrudes into the central domain. Apatite inclusions are embedded in zircon. A total of 228 grains (280 spots) were examined; the concordant dates range from 752 to 606 Ma (n = 185), and their Th/U ratios range from 0.012 to 0.56 (Figs. 6m and 7m). The concordant dates from the marginal domain of 32 individual grains (676-615 Ma) can be divided into 676-672 Ma (n = 3) and 659-615 Ma (weighted mean age: 633 ± 5 Ma, MSWD = 4.8, n = 29) (Figs. 6m and 7m). The Th/U ratios of the latter range from 0.018 to 0.42, with an average of 0.18.

DISCUSSION

Metamorphic ages of the gneisses in units 1 and 2

The Th/U ratio is widely used to estimate the event to which the age of the zircon points. Generally, zircons with high Th/U ratios (>0.2) are thought to be magmatic in origin, while those with low Th/U ratios (<0.1) have undergone secondary processes such as metamorphism (e.g., Schaltegger et al., 1999; Hartmann et al., 2000; Hoskin and Black, 2000; Rubatto et al., 2001; Rubatto, 2002; Hartmann and Santos, 2004). On the other hand, high Th/U ratios (>0.15) have been recorded also from recrystallized zircon, and also from zircon grown during high-temperature metamorphism (e.g., Pidgeon, 1992; Vavra et al., 1999; Carson et al., 2002; Kelly and Harley, 2005). Thus, since the ‘low Th/U ratio’ does not always evidence of the metamorphic event, the metamorphic ages are discussed basically on the ground of the internal structure of the zircons in this paper. The observation and interpretation of zircon CL images are critical for understanding the isotopic data. For example, clear concentric oscillatory zoning in CL image is generally regarded as evidence of the crystallization of zircon from magma. In contrast, it is generally known that the concentric oscillatory zoning of zircons formed through the magmatic process would be disturbed to form obscure zoning or tends to be modified to patchwork, sector, or irregular zoning by later alteration such as metamorphism and hydrothermal alteration (e.g., Kröner et al., 2000; Pidgeon et al., 2000; van Breemen et al., 1986; Vavra et al., 1999). Kröner et al. (2000) noted that the ‘butterfly structure’, in which the marginal domain of zircon bulges/intrudes into the central domain to cut its zoning, as evidence that the zircon was reabsorbed by metamorphism of granulite-facies. Besides, Vavra et al. (1999) mentioned, based on the study of amphibolite- to granulite-facies pelitic gneiss from the Ivrea zone, Southern Alps, that the anatexis of metasediments produced prograde zircon overgrowths on detrital cores and the metamorphosed zircons can be divided into several types of growth morphologies.

Some zircons in this study have a structure that comparatively brighter concentrically/irregularly/patchwork-like/vermicularly zoned central domain is surrounded by the darker obscurely zoned/un-zoned marginal domain. Besides, the boundary between these domains of such zircons, in some cases, clearly cuts the structure of the central, and the dark marginal domain bulges/intrudes into the central domain (Fig. 5). Furthermore, the marginal domain of the zircons generally gives younger dates than the central domain (Fig. 5).

Taking these into the consideration, it is probable that the central and marginal domains above represent inherited cores and metamorphic rims respectively (Fig. 5). Some zircons are entirely dark and unclearly zoned/un-zoned in the CL image (Fig. 5). The dates from the marginal domain of these zircons are commonly similar to those of the metamorphic rims above from the same sample. Furthermore, in some cases, the central dates of these zircons are much older than the marginal ones (Fig. 5). Therefore, the dates of the marginal domains similar to those from the metamorphic rim (called the dates from MD hereinafter) of such zircons also likely represent the timing of metamorphism. So, we took them as well into account to estimate the metamorphic age. The date-distribution of all zircon shows that the data from the above metamorphic rims and MDs occupy, in most cases, the youngest portion of each sample (Fig. 7). This probably supports the view that the dates from the metamorphic rim and MD basically represent the time of the later event. Incidentally, the metamorphic rim and MD having Th/U <2.0, which has a low possibility of magmatic origin, are 75 and 70% for units 1 and 2, respectively.

Some of the dates obtained from the central domain are almost identical to those from the MD (Fig. 7). Most of such zircons do not show clear oscillatory zoning but are entirely dark un-zoned or very obscure concentric/irregular zoning in CL images (Fig. 5). It appears, from the CL image, that the central domain of the zircons had also suffered metamorphism, and therefore the age-coincide between the central and marginal domains is likely attributed to the complete resettlement of the U-Pb system in the zircon by metamorphism.

The polished surface of zircon might not always cross its internal structure, for example, when polished obliquely to the elongation of the mineral. In these cases, the date of the core might be obtained from the apparent ‘marginal domain’. Thus, the existence of marginal dates that are much older than the youngest date cluster of the metamorphic rim and the MD in samples T08120401c, T08120502a, T08120702b, T09013102a, T08122801d-W, T09013101a, and T09013101b is perhaps due to the oblique polishing of the zircons (Fig. 7). The marginal dates older than the youngest date cluster of the metamorphic rim and the MD are more likely to be inherited ages recorded in the zircon grains. Therefore, the youngest cluster of the dates of the metamorphic rim and the MD of each sample are regarded to represent the metamorphic event of the gneiss samples, here.

It is possible that what appears to be continuous metamorphism is actually the summation of pulsed metamorphisms. The histograms of dates and the probability density curves accounting for their errors for the youngest date cluster of the metamorphic rim and the MD are shown in Figure 7. The probability density curves of the samples with a number of data (T08120401b, T08120401c, T08120501a, T08120502a, and T09013102a) seem to show the nearly single-peak normal distribution in unit 1 (Fig. 7). When data have multiple clusters, mixture modeling (Sambridge and Compston, 1994) is, in some cases, used to isolate the clusters. Therefore, the cluster analyses of each sample of unit 1 were performed using the Unmix Ages routine of Isoplot 3.70 software (Ludwig, 2008).

Unmixing the youngest date cluster of the metamorphic rim and the MD at the number of components when the relative misfit value is minimized following Kawakami et al. (2022) resulted in approximately, T08120401b: 580 and 595 Ma (relative misfit: 0.90), T08120401c: 576 and 597 Ma (relative misfit: 0.90), T08120501a: 579 and 592 Ma (relative misfit: 0.91), T08120502a: 559 and 577 Ma (relative misfit: 0.83), and T09013102a: 561 and 585 Ma (relative misfit: 0.78) (Table 1). Generally, a smaller misfit value point to a preferable fit. For example, Takeuchi et al. (2017) mentioned that, though there is no strict criterion, when the relative misfit value is less than about 0.2 the results obtained by the unmixing can be considered to fit well with the actual data distribution. For the present samples, it could hardly say that the relative misfit values (0.78-0.91) are enough low. Therefore, it may not be always said that the data is subdivided into several clusters.

Table 1. Results of mixture model processing of the dates from each sample

Unit	Area	Sample No.	Unmix age (Ma)	Fractions	Relative misfit
Unit 1	North of Vengen	T08120401b	580 ± 4	0.69	0.90
			595 ± 6	0.31
		T08120401c	576 ± 5	0.51	0.90
			597 ± 3	0.49
		T08120501a	579 ± 7	0.53	0.91
			592 ± 8	0.47
		T08120502a	559 ± 4	0.40	0.83
			577 ± 4	0.60
	Pingvinone	T09013102a	561 ± 3	0.37	0.78
			585 ± 3	0.63
Unit 2	Tanngarden	T08122801d-W	605 ± 7	0.10	0.65
			633 ± 5	0.52
			654 ± 5	0.37
		T08122801d-D	618 ± 9	0.14	0.73
			641 ± 4	0.49
			659 ± 4	0.37
		T08122802a	617 ± 6	0.08	0.65
			633 ± 5	0.26
			645 ± 6	0.33
			664 ± 4	0.33
		T08122802b	626 ± 4	0.33	0.75
			645 ± 4	0.52
			662 ± 6	0.15
	Pingvinone	T09013101a	620 ± 4	0.28	0.93
			642 ± 3	0.72
		T09013101b	561 ± 3	0.37	0.76
			585 ± 3	0.63

In the samples which have sufficient data numbers of unit 2, the probability density curves of the data do not show distinct multimodal peak patterns, but nearly single-peak patterns (Fig. 7). The results of the unmixing of the youngest date cluster of the metamorphic rim and the MD, in the same way as the samples of unit 1, were approximately T08122801d-W: 605, 633, and 654 Ma (relative misfit: 0.65), T08122801d-D: 618, 641, and 659 Ma (relative misfit: 0.73), T08122802a: 617, 633, 645, and 664 Ma (relative misfit: 0.65), T08122802b: 626, 645, and 662 Ma (relative misfit: 0.75), T09013101a: 623 and 638 Ma (relative misfit: 0.93), and T09013101b: 620 and 642 Ma (relative misfit: 0.76) (Table 1). The relative misfit values of the samples are not low enough (0.65-0.93) suggesting a less favorable fit between the results of the mixture model processing and the actual data distribution.

Thus, several metamorphic events might be recognized according to the cluster analysis, but these events are not always clearly distinguished and could also be interpreted as nearly continuous. Besides, the high-misfit values of the samples imply that the data may not be divided into distinctive multiple segments. Furthermore, for each sample, the probability density curves show nearly single-peak patterns (Fig. 7). Therefore, it is not possible to make a definitive conclusion if the data show pulsed metamorphism, and we only mention it as a reference here. Then, in this paper, we infer that the metamorphisms of units 1 and 2 had continuously lasted at once respectively, not the summation of pulsed metamorphisms. For the sample T08122801c, although it is difficult to say anything definitive due to the very small numbers of data, the date of ∼ 590 Ma seems to be markedly younger than the others. It might be possible to consider the age-gap implies the pulsed metamorphism, but it will be clear when further data are available.

Incidentally, Adachi et al. (2013b), for example, reported that the rocks of the Sør Rondane Mountains had suffered retrograde hydration at 570-550 Ma after their peak metamorphism of 640-600 Ma. Thus, if the ‘metamorphic age’ of this study points to peak metamorphism or later retrograde hydration is discussed here. It is known that, for instance, the replacement of pyroxene by hornblende, the replacement of hornblende by chlorite and/or actinolite, the replacement of garnet and biotite by chlorite, the replacement of plagioclase by sericite or epidote, and others evidence the retrograde hydration (e.g., Yardley et al., 2014). Besides, the rocks that suffered retrograde hydration in some cases have a reaction rim adjacent to the preceding mineral (e.g., Baba, 2003; Baba et al., 2013). The boundaries of mafic minerals such as clinopyroxene, hornblende, and biotite are generally distinct in the samples. And concrete evidence suggesting significant retrograde hydration, e.g., mineral replacement and reaction rim, could hardly be recognized in the samples, though biotite/hornblende was minorly chloritized (Figs. 3 and 4). Therefore, the retrograde hydration may be considered insignificant in the samples, and the assumption that the ‘metamorphic age’ of this study represents retrograde hydration likely be unnecessary.

The ranges of the youngest date cluster of the metamorphic rim and MD of the individual zircons in the gneiss are summarized as ∼ 600-550 Ma for unit 1 and ∼ 670-600 Ma for unit 2 (Figs. 6 and 8). The metamorphic ages of the gneisses of units 1 and 2 are, therefore, deduced as ∼ 600-550 and ∼ 670-600 Ma respectively. Shiraishi et al. (2008) reported a metamorphic age of 571 ± 5 Ma from a biotite orthogneiss in the north of Vengen which is in unit 1 of this study. Besides, the peak metamorphic age for the gneisses of unit C, which may correspond to unit 2 of this study, is considered to be 650-600 Ma (Adachi et al., 2013b; Osanai et al., 2013) (Figs. 1 and 9). Thus, the metamorphic ages of the gneisses in this study are in close agreement with the previous studies.

Figure 8. Ranges of youngest date cluster of metamorphic rim and MD of zircons in the gneiss of this study. Pm., psammitic; fel., felsic.

Figure 9. Overview of the LA-ICP-MS zircon U-Pb dating in the study area. The lithology and tectonic division are basically after Shiraishi et al. (1992) and Tsukada et al. (2017). Division in Osanai et al. (2013) is also shown. The ages published by previous studies, excluding the igneous and MSZ-affected ages younger than 600 Ma (e.g., Shiraishi et al., 2008; Elburg et al., 2016; Ruppel et al., 2020), are also shown. GTTG, gabbro-tonalite-trondhjemite-granodiorite complex. 1, Takigami and Funaki (1991); 2, Takahashi et al. (1990); 3, Kamei et al. (2013); 4, Tsukada et al. (2017); 5, Shiraishi et al. (2008); 6, Elburg et al. (2015).

Tectonic division of the Widerøefjellet area

Tsukada et al. (2017) tentatively redefined the rocks in the Widerøefjellet area into units 1-3, based on their lithology and structural position. Unit 1 is mainly composed of greenschist/amphibolite-facies gneisses, while unit 2 is composed mainly of amphibolite/granulite-facies gneisses (e.g., Shiraishi and Kojima, 1987; Shiraishi et al., 1992; Osanai et al., 2013; Tsukada et al., 2017). Unit 3 consists largely of metamorphosed GTTG of greenschist/amphibolite-facies (e.g., Shiraishi et al., 1992; Kamei et al., 2013; Osanai et al., 2013; Tsukada et al., 2017). The rocks of unit 2 are in contact with those of unit 3 and are separated by the MSZ. Units 2 and 3 overlie unit 1 along the KSZ (Tsukada et al., 2017). Although the outcrop of the KSZ has not been found in the west of the Ketelersbreen, it is estimated to extend toward Pingvinane based on mapping (Tsukada et al., 2017) (Figs. 1 and 9).

According to this study, a clear difference is recognized in the metamorphic ages of the rocks in units 1 and 2. That is, unit 1 documents metamorphic ages younger than ∼ 600 Ma (∼ 600-550 Ma); in contrast, the metamorphic ages of unit 2 are older than ∼ 600 Ma (∼ 670-600 Ma). This may imply that the KSZ of the low-angle shear zone is a critical tectonic boundary dividing the two gneiss units, unit 1 is exposed at a lower elevation and unit 2 at a higher elevation (Tsukada et al., 2017) (Fig. 9). Another possible scenario is that units 1 and 2 are divided by an east-west trending high-angle fault running from Pingvinane (between the samples T09013102a and T09013101a and b) to north Vengen (Figs. 1 and 9). At present, there is no concrete evidence of the ‘east-west trending high-angle fault’, it is a necessary matter to be confirmed further.

ACKNOWLEDGMENTS

We thank Mr. S. Yogo at Nagoya University and Dr. Manchuk N. at the Mongolian University of Science and Technology for their technical support in the thin section making of the samples. We appreciate the critical comments from anonymous reviewers and the Guest Editor, Prof. T. Hokada at the National Institute of Polar Research, Japan, who greatly improved the earlier draft. We are indebted to Prof. M. Takeuchi and Prof. H. Yoshida at Nagoya University for their valuable comments. We would like to thank Editage (https://www.editage.jp) for the English language review.

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