Zircon geochronology of high-grade metamorphic rocks from outcrops along the Prince Olav Coast, East Antarctica: Implications for multi-thermal events and regional correlations

Ippei KITANO; Tomokazu HOKADA; Sotaro BABA; Atsushi KAMEI; Yoichi MOTOYOSHI; Prayath NANTASIN; Nugroho I. SETIAWAN; Davaa-ochir DASHBAATAR; Tsuyoshi TOYOSHIMA; Masahiro ISHIKAWA; Takuma KATORI; Nobuhiko NAKANO; Yasuhito OSANAI

doi:10.2465/jmps.221220

Abstract

This paper reports laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U-Pb ages of a suite of high-grade metamorphic rocks collected from Sinnan Rocks, Akebono Rock, Niban Rock, Gobanme Rock, Tenmondai Rock, Akarui Point, Cape Omega, and Oku-iwa Rock along the Prince Olav Coast, in the Lützow-Holm Complex (LHC), East Antarctica. The dating results indicate that a newly detected ∼ 990 Ma metamorphism of garnet-sillimanite-biotite gneiss from Niban Rock. A thermal event at 931.7 ± 9.8 Ma is recorded in zircon from staurolite-bearing garnet-gedrite-biotite-chlorite gneiss in Akebono Rock. The metamorphic zircon grains in other analyzed samples provide Ediacaran to Cambrian ages. Their multi-growth textures and age populations are possibly interpreted to exhibit three metamorphic stages during >600-580, 580-550, and 550-500 Ma. Combined with previous reports, the metamorphic rocks in Cape Hinode, Niban-nishi Rock of Niban Rock, and Akebono Rock might have experienced earlier high-temperature metamorphism at ∼ 990-930 Ma without younger overprinting. Extensive high-grade metamorphism during ∼ 650-500 Ma is recorded from not only the granulite-facies zone in the west of the LHC but also the amphibolite-facies zone in its east. The main metamorphic episode in the LHC is likely to be subdivided into a preceding thermal event (either independent single metamorphic event or prograde stage) at pre-580 Ma, near-peak condition stage during 580-560 Ma, and subsequent retrograde stage after 550 Ma. In regional context this indicates that the assembly at the central Gondwana started with the collision of early and late Neoproterozoic terranes prior to 580 Ma, as a part of the East Africa-Antarctic Orogeny. Subsequent collisions took place among late Neoproterozoic igneous terrane, above terranes collided at pre-580 Ma, and Neoarchean terrane, which were probably driven by the Kuunga Orogeny.

INTRODUCTION

The Lützow-Holm Complex (LHC) is mainly composed of late Neoproterozoic to Cambrian high-grade metamorphic rocks, sporadically exposing along the Prince Olav Coast and around the Lützow-Holm Bay of East Antarctica (e.g., Shiraishi et al., 1994). It is characterized by a southwestward increase in metamorphic grade from amphibolite- to granulite-facies and high-temperature (HT) to ultrahigh-temperature (UHT) metamorphic conditions. These rocks underwent metamorphism with clockwise pressure (P)-temperature (T) trajectories (e.g., Hiroi et al., 1983a; Shiraishi et al., 1994; Motoyoshi and Ishikawa, 1997; Yoshimura et al., 2008; Kawasaki et al., 2011), with the exception of the early Neoproterozoic metamorphic rocks of Cape Hinode (Dunkley et al., 2014). The complex has long been recognized as one of key regions to any reconstructions of the Gondwana supercontinent, which can be viewed as a single long-lived orogenesis, the ‘East Africa-Antarctic Orogen (EAAO)’ (Jacobs and Thomas, 2004) or a series of orogens, ‘East African Orogeny (EAO)’ and ‘Kuunga Orogeny (KO)’ (Meert, 2003) (Fig. 1). The large-scale continent-continent collision between the west and east Gondwana produced the EAAO during ∼ 650-500 Ma (Jacobs and Thomas, 2004) (Fig. 1a). Jacobs et al. (2015) suggested the protracted metamorphic history during 630-500 Ma as follows. The westward collision of the ∼ 1130-1040 Ma metamorphic terrane of the Maud Belt to the Proto-Kalahari Craton in the south part of the EAAO was followed by the accretion of the Tonian Oceanic Arc Super Terrane which is a series of juvenile oceanic arcs formed around 1000-900 Ma (Fig. 1a). An alternative proposal by Meert (2003) (Fig. 1b) is that the Gondwana assembly was initiated by the N-S trending EAO during ∼ 750-620 Ma, and was completed by the E-W trending KO at some 570-530 Ma. Thus, it is obvious that an understanding of the geological events that have affected the LHC is central to constraining any proposed models of the tectono-thermal history in the central Gondwana supercontinent.

Figure 1. The reconstruction models of the Gondwana supercontinent after Jacobs et al. (2015) (a) and Meert (2003) (b). The Lützow-Holm Complex (LHC), East Antarctica, is located within both (a) the East Africa-Antarctic Orogen and (b) Kuunga Orogen. M, Madagascar; NC, Napier Complex; RC, Rayner Complex; S, Schirmacher Oasis; SL, Sri Lanka; SRM, Sør Rondane Mountains; TOAST, Tonian Oceanic Arc Super Terrane; WRC, Western Rayner Complex. met, metamorphic ages; pro, protolith igneous ages.

The geochronological methodology of U-Pb dating of zircon has proved to provide reasonably precise dates in the high-grade metamorphic rocks. This is due to the high-closure temperature and durability of zircon, which often preserves the timeline of multiple thermal events (e.g., Harley and Kelly, 2007; Taylor et al., 2016). By the application of this method, our understanding of the geochronological framework in the LHC has improved steadily over the last three decades, as summarized by Dunkley et al. (2020). The primary sensitive high resolution ion microprobe (SHRIMP) zircon dating by Shiraishi et al. (1994) revealed that a phase of amphibolite- to granulite-facies metamorphism occurred in the LHC at ∼ 550-520 Ma. The long-lived thermal event of >600-520 Ma was clarified from several outcrops of the granulite-facies zone around the Lützow-Holm Bay (e.g., Hokada and Motoyoshi, 2006; Dunkley et al., 2014). The thermal history of the transitional zone along the Prince Olav Coast has also been investigated (e.g., Takamura et al., 2018). One of the key insights is the recognition that the Tonian granulite-facies metamorphic rocks outcrop at Cape Hinode as allochthonous blocks within the Cambrian amphibolite-facies zone on the Prince Olav Coast (e.g., Motoyoshi et al., 2005; Hiroi et al., 2006). A separate thermal event at ∼ 940 Ma was recently reported by Baba et al. (2022) from amphibolite-facies pelitic gneisses in Akebono Rock and by Mori et al. (2023) from a upper amphibolite-facies pelitic gneiss in Niban-nishi Rock of Niban Rock next to Cape Hinode. Therefore, over time it has become increasingly apparent that the high-grade metamorphic rocks along the Prince Olav Coast record a complicated multi-thermal history, which must be explained by any tectonic models that attempt to explain how the various terrains that comprise the central part of the Gondwana supercontinent were amalgamated. Although many authors have investigated the geochronological affinities of metamorphic rocks in this region (e.g., Shiraishi et al., 1994, 2003; Dunkley et al., 2014; Tsunogae et al., 2015; Takamura et al., 2018), the recent finding of areas affected by a ∼ 940 Ma thermal event, as mentioned above, indicates the necessity of further geochronological surveys along the Prince Olav Coast. This should include outcrops where no or less U-Pb zircon dating has been conducted.

This study will provide the results of U-Pb zircon dating as well as petrography from high-grade metamorphic rocks in 8 outcrops along the Prince Olav Coast in the LHC, namely Sinnan Rocks, Akebono Rock, Niban Rock, Gobanme Rock, Tenmondai Rock, Akarui Point, Cape Omega, and Oku-iwa Rock (Fig. 2). This includes the first such zircon age data from Gobanme Rock, Cape Omega, and Oku-iwa Rock. These outcrops were investigated through the operations of the 58th and 60th Japanese Antarctic Research Expeditions. The abbreviations of mineral names in the text, figures, and tables in this study follow Whitney and Evans (2010).

Figure 2. The distribution of outcrops (red) in the Lützow-Holm Complex. The metamorphic zonation and lithological division of the Lützow-Holm Complex are after Hiroi et al. (1991) and Dunkley et al. (2020), respectively. The representative outcrop names are also described in red. INH, Innhovde Suite; RVG, Rundvågshetta Suite; SKV, Skallevikshalsen Suite; LHV, Langhovde Suite; EOG, East Ongul Suite; AKR, Akarui Suite; HB, Hinode Block.

GEOLOGICAL OUTLINE

The LHC is a Neoproterozoic-Cambrian plutono-metamorphic complex along the Prince Harald, Sôya, and Prince Olav Coasts of eastern Dronning Maud Land, East Antarctica (Fig. 2). It is composed of various metamorphic rocks, including felsic to mafic orthogneisses and granulites, ultramafic rocks, pelitic and psammitic gneisses, marbles, and quartzites, together with intrusive rocks (e.g., Shiraishi et al., 2008; Dunkley et al., 2020). The metamorphic grade increases from upper amphibolite-facies in the east to granulite-facies in the west (Fig. 2), although granulite-facies rocks also occur in Cape Hinode in the eastern part of the LHC (Hiroi et al., 1991, 2006). The high-grade metamorphic rocks in the LHC are interpreted to record a clockwise P-T path, based on the presence of relict prograde kyanite and sillimanite in the matrix (e.g., Hiroi et al., 1983a; Shiraishi et al., 1989; Hiroi et al., 1991; Suzuki and Kawakami, 2019) and supported by pseudosection analyses (e.g., Iwamura et al., 2013; Takamura et al., 2020; Baba et al., 2022). UHT conditions have been reported in Rundvågshetta and Skallevikshalsen along the Sôya Coast (e.g., Motoyoshi and Ishikawa, 1997; Yoshimura et al., 2008; Kawasaki et al., 2011, 2013) (Fig. 2). Recent petrological analyses of pelitic gneisses and mafic to ultramafic rocks from Akarui Point and Tenmondai Rock, located in the transitional zone of the LHC, indicate these rocks nearly reached UHT granulite-facies conditions (Iwamura et al., 2013; Suzuki and Kawakami, 2019; Takamura et al., 2020). Although metamorphic ages range from ∼ 650-500 Ma across the LHC, the majority of samples to date have produced ages of ∼ 550-520 Ma (e.g., Shiraishi et al., 1994; Hokada and Motoyoshi, 2006; Dunkley et al., 2014; Tsunogae et al., 2015; Takahashi et al., 2018; Takamura et al., 2018, 2020). The exception is Cape Hinode, where rocks were metamorphosed at ∼ 970-960 Ma (Motoyoshi et al., 2005; Dunkley et al., 2014, 2020). Two distinct metamorphic events, based on monazite and zircon analyses, at ∼ 650-580 and 560-500 Ma, affected pelitic gneisses in Skallen (Hokada and Motoyoshi, 2006) and Skallevikshalsen (Kawakami et al., 2016) within the granulite-facies zone. Takamura et al. (2020) proposed that the onset of post-peak decompression processes that affected the mafic granulites of Sudare Rock and Tenmondai Rock occurred at ∼ 560 Ma. Compiling all of the U-Pb ages obtained from protolithic zircon up to that time, Dunkley et al. (2020) divided the protoliths of late Neoproterozoic orthogneisses into 6 suites. These are the Innhovde Suite (1070-1040 Ma), the Rundvågshetta Suite (2520-2470 Ma), the Skallevikshalsen Suite (1830-1790 Ma), the Langhovde Suite (1100-1050 Ma), the East Ongul Suite (630 Ma), and the Akarui Suite (970-800 Ma) with the early Neoproterozoic orthogneisses of the exotic Hinode Block (1020 Ma) (Fig. 2). Recently, another thermal event at ∼ 940 Ma in the LHC was discovered by Baba et al. (2022) from zircon and monazite in Ky-bearing Grt-Bt gneiss of Akebono Rock and by Mori et al. (2023) from monazite in Sil- and Grt-bearing pelitic gneiss of Niban-nishi Rock in the amphibolite-facies zone.

Geological maps for each of the eight studied exposures along the Prince Olav Coast, showing the sampling locations, are provided in Supplementary Figures S1-S8 (Supplementary Figures S1-S8 are available online from https://doi.org/10.2465/jmps.221220). All of these areas belong to the Akarui Suite of Dunkley et al. (2020), and are part of either amphibolite-facies or transitional zones of Hiroi et al. (1991) (Fig. 2). There follows a brief synopsis of the geology and U-Pb geochronology of each of these areas.

Sinnan Rocks consist mainly of two-mica granitic migmatite, Bt gneiss, and Bt-Hbl gneiss with subordinate Sil-Bt gneiss, amphibolite, and minor calcsilicate, which are intruded by granite and pegmatite (Hiroi et al., 1983b) (Fig. S1). Corundum porphyroblasts up to several cm in diameter are reported from a thin layer of Sil-Bt gneiss in the northeast of the exposed area (Hiroi et al., 1983b). Shiraishi et al. (1994) obtained metamorphic age of 553 ± 6 Ma from zircon in a Grt-Bt-Sil gneiss by SHRIMP U-Pb dating.

At Akebono Rock, Bt-Hbl gneiss, Bt gneiss, Grt-Bt gneiss, amphibolite (amphibolite I), and Grt-bearing leucogranite are dominant, being well layered with NW-SE trending foliation (Hiroi et al., 1986) (Fig. S2). The gneisses are intruded by granite, pegmatite, and a thoroughly recrystalized mafic dyke (amphibolite II), of basaltic to andesitic composition (Hiroi et al., 1986). Baba et al. (2022) conducted SHRIMP U-Pb zircon dating from Ky-bearing Grt-Bt gneiss in the western part of Akebono Rock and reported not only metamorphic age of ∼ 940 Ma but also detrital zircon ages of ∼ 1120-1010 Ma. They also provided electron microprobe (EMP) U-Th-Pb monazite ages of 977-917 Ma from pelitic gneisses in the same part of Akebono Rock.

The exposures at Niban Rock are divided into Niban-higashi Rock in the northeast and Niban-nishi Rock in the southwest (Fig. S3). The main lithologies are Bt gneiss and Bt-Hbl gneiss in the former, with Sil-Grt-Bt gneiss and Bt gneiss in the latter. In both areas the gneisses are associated with minor metabasites and calcsilicate rocks and are intruded by granite and aplite (Kizaki et al., 1983) (Fig. S3). The orthogneisses and a metagranitic dyke in Niban Rock were dated by Dunkley et al. (2014), who determined their magmatic ages to be ∼ 940 and 550 Ma with inherited U-Pb Zrn ages of ∼ 960 and 1100-900 Ma, respectively.

The geology of Gobanme Rock and neighbouring Byôbu Rock was initially mapped by Satish-Kumar et al. (2006). They provided a dataset of structure, bulk chemistry, P-T conditions, and EMP U-Th-Pb monazite ages. The predominant lithologies of Gobanme Rock are migmatized Grt-Bt quartzofeldspathic gneiss, and Hbl-Bt gneiss, both with and without interlayered amphibolite (Satish-Kumar et al., 2006) (Fig. S4). Subordinate Bt gneiss and layer parallel Grt leucogranite and granite intrusions also occur (Fig. S4). Satish-Kumar et al. (2006) reported finding of Ky-bearing Grt-Bt-Sil-Gr gneiss at Byôbu Rock but not at Gobanme Rock. They determined a U-Th-Pb monazite age of 557 ± 33 Ma from Grt-Bt-Sil-Gr gneiss, Grt-Bt quartzofeldspathic gneiss, and Grt-Opx granulite in Byôbu Rock, which they interpreted to reflect the timing of granulite-facies metamorphism.

Tenmondai Rock is comprised predominantly of migmatitic, well-layered Bt-Hbl and Hbl-Bt gneisses, intercalated with amphibolite and Grt-Bt gneiss, with later intrusions of gneissose granite, granite, and pegmatite (Shiraishi et al., 1985) (Fig. S5). In the eastern part, Spl- and Ky-bearing Sil-Grt-Bt gneiss has been reported and investigated petrologically (Shiraishi et al., 1985; Baba et al., 2023). U-Pb zircon analysis of Grt-Bt-Sil gneiss indicates ages of ∼ 2020-900 Ma from relic cores and ∼ 610-530 Ma from rims and mantles (Takamura et al., 2018). Takamura et al. (2020) obtained similar U-Pb zircon ages of ∼ 630-480 Ma from mafic granulite, which indicates that the thermal event was long-lived. Dunkley et al. (2014) showed that a Sil-bearing metadyke at Tenmondai Rock was emplaced at ∼ 580 Ma and subsequently metamorphosed at ∼ 540 Ma.

Akarui Point is mainly composed of migmatitic Grt-Bt gneiss, Hbl-Bt gneiss, and Bt-Hbl gneiss, with subordinate amphibolites and ultramafic rocks occurring as thin layers or blocks, with extensive intrusions of granite and pegmatite (Yanai et al., 1984) (Fig. S6). U-Pb dating of detrital zircon in quartzite by Takamura et al. (2018) produced a range of ages between ∼ 1210-630 Ma, while Kazami et al. (2016) found that a felsic orthogneiss has an igneous protolith age of ∼ 850 Ma. Both lithologies were metamorphosed at ∼ 610-510 Ma.

The exposures at Cape Omega are subdivided into Omega-higashi Rock, Omega-naka Rock, and Omega-nishi Rock. These consist mainly of Hbl gneiss and plutonic rocks such as gneissose granite, pink granite, and pegmatite in association with Grt-Bt gneiss, amphibolite, and calcsilicates (Suzuki and Moriwaki, 1979) (Fig. S7). Suzuki (1984) reported the occurrences of Grt-Opx-Bt gneiss from Omega-higashi Rock and Sil-Crd-Bt gneiss from Omega-nishi Rock and determined that they were metamorphosed at ∼ 680 °C and 5 kbar.

The main lithologies in Oku-iwa Rock are migmatized and folded Bt gneiss, Bt-Hbl gneiss, and leucocratic Bt gneiss, which have been intruded by granite, aplite, and pegmatite (Nakai et al., 1981) (Fig. S8).

SAMPLE DESCRIPTION

The locations and mineral assemblages of samples analyzed in this study are summarized in Table 1. Figures 3 and 4 provide field photographs of sample outcrops and photomicrographs of thin sections, respectively.

Table 1. The mineral assemblages of analyzed samples from the Prince Olav Coast, East Antarctica

Sample No.	Locality	Rock type	Grt	Opx	Spl	Crn	St	Ky	Sil	Hbl	Ged	Ath	Bt	Ms	Chl	Pl	Kfs	Qz	Ilm	Mag	Gr	Rt	Ap	Mnz	Zrn	2nd phase
IK1612 3001A	Sinnan Rocks	Crn-Sil-Bt-Ms gneiss				○			○				△	△		+	○		△				+	+	+	Ms
IK1612 3003A	Sinnan Rocks	Grt-bearing Bt gneiss	+										○			○	△	+		+			+	+	+	Ms, Chl
IK1612 3102	Akebono Rock	St-bearing Grt-Ged-Bt- Chl gneiss	△				+				○		○		△	○		△	+				+	+	+
IK1901 1606A	Niban Rock	Grt-Sil-Bt gneiss	△						△				○			△	+	○	+		+		+	+	+
IK1901 1805A	Gobanme Rock	Grt-Sil-Bt gneiss	△						△				△			△	△	○	+		+		+		+
IK1701 1002A	Tenmondai Rock	Grt-Spl-Sil-Bt gneiss	△		+			+	△				△			△	△	○	+		+	+	+	+	+
IK1701 0601	Akarui Point	Grt-Bt gneiss	△										△			△	△	○		+			+	+	+	Ms, Chl, Cal
IK1701 0904A	Cape Omega	Sil-bearing Grt-Bt gneiss	△						+				○			○		○	△				+		+	Ms
IK1901 1404A	Oku-iwa Rock	Grt-Opx granulite	△	+	+					△	○	○	△			○				△		+	+		+	Ged, Ath, Bt, Hbl

○, abundant; △, moderate; +, minor

Figure 3. Photographs of outcrops of the analysed samples. (a) Crn-Sil-Bt-Ms gneiss (IK16123001A) from Sinnan Rocks. (b) Grt-bearing Bt gneiss (IK16123003A) from Sinnan Rocks. (c) St-bearing Grt-Ged-Bt-Chl gneiss (IK16123102) from Akebono Rock. (d) Grt-Sil-Bt gneiss (IK19011606A) from Niban Rock. (e) Grt-Sil-Bt gneiss (IK19011805A) from Gobanme Rock. (f) Grt-Spl-Sil-Bt gneiss (IK17011002A) from Tenmondai Rock. (g) Grt-Bt gneiss (IK17010601) from Akarui Point. (h) Sil-bearing Grt-Bt gneiss (IK17010904A) from Cape Omega. (i) Grt-Opx granulite (IK19011404A) from Oku-iwa Rock.

Figure 4. Representative photomicrographs of analyzed samples (a), (c), (e), (g), (i), (k), (m), (o), and (q) and zircon occurrence in each sample (b), (d), (f), (h), (j), (l), (n), (p), and (r). (a) and (b) Crn-Sil-Bt-Ms gneiss (IK16123001A) from Sinnan Rocks. (c) and (d) Grt-bearing Bt gneiss (IK16123003A) from Sinnan Rocks. (e) and (f) St-bearing Grt-Ged-Bt-Chl gneiss (IK16123102) from Akebono Rock. (g) and (h) Grt-Sil-Bt gneiss (IK19011606A) from Niban Rock. (i) and (j) Grt-Sil-Bt gneiss (IK19011805A) from Gobanme Rock. (k) and (l) Grt-Spl-Sil-Bt gneiss (IK17011002A) from Tenmondai Rock. (m) and (n) Grt-Bt gneiss (IK17010601) from Akarui Point. (o) and (p) Sil-bearing Grt-Bt gneiss (IK17010904A) from Cape Omega. (q) and (r) Grt-Opx granulite (IK19011404A) from Oku-iwa Rock.

Crn-Sil-Bt-Ms gneiss (IK16123001A) from Sinnan Rocks

This sample was collected from a micaceous layer within pelitic gneiss in the northern part of Sinnan Rocks (Fig. 3a). The micaceous gneiss is characterized by large, euhedral to subhedral corundum porphyroblasts (Fig. 4a), several cm in diameter, with elongate sillimanite, biotite, and muscovite having a preferred orientation. The mineral assemblage is corundum, sillimanite, biotite, muscovite, K-feldspar, and plagioclase, with accessory ilmenite, apatite, monazite, and zircon (Table 1). The porphyroblastic corundum is colorless and includes orientated biotite, muscovite, and ilmenite, while being surrounded by Ms ± Ilm (Fig. 4a). The matrix consists of lepidoblastic subhedral dark brown biotite (up to 2.7 mm in length) and brownish to colorless muscovite (up to 3.5 mm in length), with acicular crystals or fine-grained aggregates of sillimanite. Zircon occurs both in the matrix between grains of biotite and quartz, or as inclusions in K-feldspar and muscovite (Fig. 4b).

Grt-bearing Bt gneiss (IK16123003A) from Sinnan Rocks

This quartzofeldspathic gneiss sample is a part of the rock hosting the micaceous layer sample IK16123001A described above (Fig. 3b). It is composed mainly of medium-grained biotite, plagioclase, K-feldspar, and quartz with accessory minerals of garnet, magnetite, apatite, monazite, and zircon (Table 1). The garnet is irregular in shape, up to 3.3 mm in length, and partly replaced by euhedral to subhedral biotite and plagioclase (Fig. 4c). Biotite is dark brown, fine- to coarse-grained (up to 2.8 mm), euhedral to subhedral tabular in shape and weakly oriented, except biotite replacing garnet (Fig. 4c). Some of biotite is partially altered to chlorite and muscovite. Zircon grains are included in biotite, plagioclase, K-feldspar, quartz, and chlorite, as well as occurring in the matrix between biotite and plagioclase (Fig. 4d).

St-bearing Grt-Ged-Bt-Chl gneiss (IK16123102) from Akebono Rock

This is a sample of a migmatitic garnetiferous layer within the layered gneisses of eastern Akebono Rock (Fig. 3c). In the field, a well-developed foliation and large garnet porphyroblast are recognized. Its mineral assemblage is garnet, gedrite, biotite, chlorite, plagioclase, and quartz (Fig. 4e) with minor staurolite, ilmenite, apatite, monazite, and zircon (Table 1). Garnet is an anhedral, porphyroblastic, up to 9.3 mm in diameter, and includes grains of gedrite, biotite, chlorite, plagioclase, quartz, and ilmenite (Fig. 4e). A foliation is defined by biotite and chlorite, while elongate gedrite defines a lineation (Fig. 4e). Gedrite is dark gray in color, subhedral, prismatic, up to 7.5 mm in length and often intergrown with biotite. Biotite is dark brown, subhedral, with flakes 0.05-2.1 mm in length. Some of chlorite flakes are surrounded by Bt ± Ged ± Ilm. Anhedral staurolite grains occur only as inclusions in plagioclase, together with quartz, gedrite, biotite, and ilmenite (Fig. 4e inset). Zircon grains are included in both biotite and plagioclase (Fig. 4f).

Grt-Sil-Bt gneiss (IK19011606A) from Niban Rock

This pelitic gneiss in Niban-nishi Rock of Niban Rock is intruded by deformed leucocratic granite (Fig. 3d). The mineral assemblage is garnet, sillimanite, biotite, plagioclase, and quartz (Fig. 4g), with accessory K-feldspar, ilmenite, graphite, apatite, monazite, and zircon (Table 1). Garnet occurs as anhedral porphyroblasts (up to 1.6 mm in size) and contains inclusions of biotite, sillimanite, and quartz (Fig. 4g). The acicular to fibrolitic sillimanite is aligned with the preferred orientation of biotite (Fig. 4g). Biotite is brown, subhedral to anhedral, with a maximum length of ∼ 2.0 mm (Fig. 4g). Zircon grains occur in the matrix in contact with biotite, sillimanite, or quartz and as inclusions in biotite, sillimanite, and quartz (Fig. 4h).

Grt-Sil-Bt gneiss (IK19011805A) from Gobanme Rock

This sample was taken from relatively thick layer within a quartzofeldspathic gneiss (Fig. 3e). Although this layer as a whole is weathered, the part where this sample was collected is relatively fresh. The pelitic gneiss layer consists mainly of garnet, sillimanite, biotite, K-feldspar, plagioclase, and quartz (Fig. 4i) with accessory apatite, ilmenite, graphite, and zircon (Table 1). Garnet is porphyroblastic, anhedral, up to 3.4 mm in diameter, and includes sillimanite, biotite, quartz, and ilmenite (Fig. 4i). Subhedral brown biotite (up to 2.2 mm in length), together with prismatic to acicular sillimanite defines a foliation (Fig. 4i). Zircon gains are included in biotite, quartz, plagioclase, and K-feldspar (Fig. 4j).

Grt-Spl-Sil-Bt gneiss (IK17011002A) from Tenmondai Rock

This pelitic gneiss from eastern Tenmondai Rock shows compositional layering (Fig. 3f). It contains garnet, spinel, kyanite, sillimanite, biotite, plagioclase, K-feldspar, and quartz (Fig. 4k) with accessory rutile, apatite, ilmenite, graphite, monazite, and zircon (Table 1). Subhedral garnet porphyroblasts (up to 2.3 mm in size) occur throughout and contain kyanite, sillimanite, spinel, biotite, quartz, ilmenite, rutile, and zircon (Fig. 4k). Garnet is partly replaced by Sil + Bt (Fig. 4k). Occasionally vermicular green spinel occurs between garnet and sillimanite in the matrix (Fig. 4k). The subhedral to anhedral kyanite is also present in the matrix, in direct contact with garnet, spinel, biotite, or plagioclase. Most of the sillimanite form aggregates of subhedral, fine- to medium-grained crystals (up to 2.2 mm in length). However, some sillimanite in contact with spinel is anhedral (Fig. 4k). Biotite is brown, subhedral, and up to 0.9 mm in length. Zircon is included in garnet, sillimanite, and quartz. It is also present in the matrix at the grain boundaries between quartz, biotite, sillimanite, K-feldspar, or plagioclase (Fig. 4l).

Grt-Bt gneiss (IK17010601) from Akarui Point

This is a sample of a quartzofeldspathic, migmatitic, Grt-Bt gneiss which is intruded by pegmatite dyke in eastern Akarui Point (Fig. 3g). The mineral assemblage consists of garnet, biotite, plagioclase, K-feldspar, and quartz (Fig. 4m), with accessory apatite, magnetite, monazite, and zircon (Table 1). Subhedral garnet with up to ∼ 4.9 mm in diameter occurs sporadically throughout the sample (Fig. 4m). It includes quartz and is partly replaced by an intergrowth of subhedral Bt + Pl (Fig. 4m). Biotite in the matrix is dark brown (up to 1.0 mm), subhedral tabular to flaky in shape and weakly oriented. It is noted that the biotite replacing garnet does not follow this orientation (Fig. 4m). Biotite in the matrix is partially replaced by secondary chlorite, muscovite, and calcite. Mesoperthite and antiperthite textures remain in K-feldspar and plagioclase, respectively. Zircon occurs as inclusions in biotite, plagioclase, K-feldspar, and quartz (Fig. 4n).

Sil-bearing Grt-Bt gneiss (IK17010904A) from Cape Omega

This sample is from a migmatitic Grt-Bt gneiss that is intercalated with a mafic layer of amphibolite in western Cape Omega (Omega-nishi Rock) (Fig. 3h). The gneiss is composed mainly of fine- to medium-grained garnet, fine- to coarse-grained biotite (up to 6.8 mm), plagioclase (up to 5.7 mm), and quartz (Fig. 4o) with minor sillimanite, apatite, ilmenite, and zircon (Table 1). The subhedral to anhedral garnet (up to 2.0 mm in size) is inclusion free (Fig. 4o). The biotite is dark brown and includes quartz and zircon. The acicular sillimanite forms a preferred orientation that defines a foliation with biotite (Fig. 4o). Zircon occurs as inclusions in biotite, plagioclase, and quartz, or in the matrix at their grain boundaries (Fig. 4p).

Grt-Opx granulite (IK19011404A) from Oku-iwa Rock

This sample was collected from the central part of a large lenticular body of garnet and orthopyroxene mafic granulite within a migmatitic felsic gneiss in Oku-iwa Rock (Fig. 3i). It has a granoblastic texture and a mineral assemblage of garnet, gedrite, anthophyllite, hornblende, biotite, and plagioclase with small amounts of orthopyroxene, spinel, rutile, apatite, magnetite, and zircon (Fig. 4q and Table 1). In the central part of the mafic granulite block, anhedral garnet, up to 5.5 mm across, is surrounded by symplectite that can be observed in hand specimen (Fig. 3i). The garnet porphyroblasts are being replaced by Bt + Pl ± Mag ± Spl ± Hbl (Fig. 4q). Spinel, magnetite, rutile, plagioclase, and quartz are included in the garnet (Fig. 4q). Remnant orthopyroxene in the matrix is pleochroic reddish to pale brownish, anhedral, and surrounded by Ged ± Ath ± Bt ± Pl (Fig. 4q). Anhedral green spinel occurs as inclusions in garnet and magnetite, or as aggregates with Mag + Pl ± Bt. The colorless anthophyllite, green brownish gedrite, deep green hornblende, and brown biotite are subhedral and up to 3.1, 5.0, 2.3, and 2.0 mm in length, respectively (Fig. 4q). Zircon is included in plagioclase and biotite (Fig. 4r).

ZIRCON U-Pb DATING

Analytical method

The zircon gains were separated from crushed powders of samples through elutriation, panning, and magnetic separation following Kitano et al. (2014). The extracted zircon was mounted in a resin disc and polished with a 1 µm diamond paste. Cathodoluminescence (CL) images of zircon were obtained using a scanning electron microprobe (JEOL JSM-6390) at Kyushu University. U-Pb dating of zircon was performed using an Agilent 7500cx quadrupole ICP-MS with LA system of Photon Machines Analyte G2 193 nm ArF excimer laser at Kyushu University. The details of the analytical procedure and condition were described by Nakano et al. (2021). The isotopes ²⁰²Hg, ²⁰⁴Hg + ²⁰⁴Pb, ²⁰⁶Pb, ²⁰⁷Pb, ²⁰⁸Pb, ²³²Th, and ²³⁸U were monitored during laser ablation of zircon with a 20 µm spot size, 8 Hz repetition and 3.2 J/cm². Isotopes and Th/U ratios were correlated using the 91500 zircon standard (Wiedenbeck et al., 1995) and NIST SRM-611 glass standard, respectively. The reference FC 1 zircon (1099 Ma; Paces and Miller, 1993) was utilized to check data accuracy. The calculation of isotopic ratios and ages were carried out by using GLITTER software (Griffin et al., 2008). The Isoplot/Ex 3.7 software (Ludwig, 2008) was used to draw concordia diagrams, probability density plots and to estimate weighted mean and unmixed ages. In this study data is regarded as concordant data if it plots on a concordia curve within 2 sigma error with −5% < discordance < 5%. Discordance was defined as a value of (²⁰⁷Pb/²³⁵U age − ²⁰⁶Pb/²³⁸U age)/(²⁰⁶Pb/²³⁸U age)* 100. The analyses of the FC 1 zircon in this study resulted in a weighted mean ²⁰⁶Pb/²³⁸U age of 1102.3 ± 8.5 Ma (n = 15, 95% confidence interval). The analytical results are listed in Supplementary Tables S1-S9 (Supplementary Tables S1-S9 are available online from https://doi.org/10.2465/jmps.221220).

Results

Crn-Sil-Bt-Ms gneiss (IK16123001A) from Sinnan Rocks. Zircon crystals in this sample are elongated with rounded terminations. They are 20-90 µm in length, with aspect ratios of 2.5-3.4. CL images show that they have almost homogeneous bright cores, dark mantles, and gray rims (Fig. 5a). All three domains have low Th/U ratios of 0.00-0.03. The obtained ²⁰⁶Pb/²³⁸U dates of concordant data were 637 ± 34-569 ± 21 Ma from bright parts, 592 ± 22-489 ± 22 Ma from dark parts, and 528 ± 22-486 ± 24 Ma from gray parts (Fig. 5a).

Figure 5. Concordia diagrams of analyzed samples. The representative CL images of the analyzed zircon grains with apparent ²⁰⁶Pb/²³⁸U ages, Th/U ratios, analyzed spots (20 µm) and 50 µm scale bars are shown. The yellow dash lines in the CL images represent the boundaries among core, mantle, rim, and outermost rim. M, metamorphic domain; D, detrital domain; I, Inherited domain. The open light gray ellipses indicate discordant data. (a) Crn-Sil-Bt-Ms gneiss (IK16123001A) from Sinnan Rocks. (b) Grt-bearing Bt gneiss (IK16123003A) from Sinnan Rocks. (c) St-bearing Grt-Ged-Bt-Chl gneiss (IK16123102) from Akebono Rock. (d) Grt-Sil-Bt gneiss (IK19011606A) from Niban Rock. (e) and (f) Grt-Sil-Bt gneiss (IK19011805A) from Gobanme Rock. (g) and (h) Grt-Spl-Sil-Bt gneiss (IK17011002A) from Tenmondai Rock. (i) Grt-Bt gneiss (IK17010601) from Akarui Point. (j) Sil-bearing Grt-Bt gneiss (IK17010904A) from Cape Omega. (k) Grt-Opx granulite (IK19011404A) from Oku-iwa Rock.

Grt-bearing Bt gneiss (IK16123003A) from Sinnan Rocks. The zircon grains from this gneiss are ovoid to elongated and rounded in shape, 20-140 µm in length and have aspect ratios of 1.3-3.0. In CL images they display bright sector zoned cores and dark homogeneous rims (Fig. 5b). Some of zircon have an inner core with bright banded zoning (Fig. 5b), which has a high Th/U ratio of 0.33 and gave ²⁰⁶Pb/²³⁸U date of 885 ± 41 Ma. In contrast, the other cores and rim domains have low Th/U ratios below 0.05. The concordant data obtained from these outer bright CL cores and dark CL rims indicated dates of 663 ± 30-552 ± 19 and 568 ± 17-511 ± 15 Ma, respectively (Fig. 5b).

St-bearing Grt-Ged-Bt-Chl gneiss (IK16123102) from Akebono Rock. The morphology of zircon in this sample is ovoid to squat or elongated, with lengths of 20-110 µm with aspect ratios of 1.2-3.0. Most of their CL images consist of bright sector to patchy zoning with/without dark, structureless cores and thin gray rims (Fig. 5c). Some grains are indicative of homogeneous and low CL emission, and overgrown by a thin outermost rim (Fig. 5c). These gray rims were too narrow to be analyzed. The Th/U ratios of dark CL cores (0.01-0.14) were slightly higher than sector to patchy zoned domains (0.01-0.05), while their dates of 974 ± 35-921 ± 33 Ma for the former and 998 ± 43-902 ± 37 Ma for the latter overlap. All concordant data yielded a weighted mean age of 931.7 ± 9.8 Ma (n = 25, MSWD = 1.6) (Fig. 5c).

Grt-Sil-Bt gneiss (IK19011606A) from Niban Rock. Zircon grains from this gneiss are predominantly elongate, with minor ovoid examples. They are 20-110 µm in length and have aspect ratios of 1.3-3.3. CL images show that most zircon have distinct core, mantle, and rim texture (Fig. 5d). The internal textures of the core are complex, variously displaying either bright banding to oscillatory, patchy zoning or a uniform gray to dark gray, with occasional faint hints of zoning (Fig. 5d). The mantles are mostly bright and uniform, while the rims are gray to dark and structureless (Fig. 5d). Analyses of cores yielded ²⁰⁶Pb/²³⁸U dates of 1940 ± 56-1005 ± 38 Ma (Fig. 5d) and Th/U ratios of 0.01-1.69. The dark CL homogeneous domains of the rim portion and a single grain have Th/U ratios of 0.01-0.28 and younger dates of 1021 ± 29-984 ± 26 Ma (Fig. 5d). The weighted mean age of 994 ± 11 Ma (n = 6, MSWD = 0.99) was calculated from the younger concordant data (Fig. 5d).

Grt-Sil-Bt gneiss (IK19011805A) from Gobanme Rock. Zircon grains from this sample are predominant ovoid to squat in shape, with a few elongated examples. They are 20-90 µm in length, with aspect ratios of 1.2-2.2. The CL images show a core-rim texture (Figs. 5e and 5f), with the cores varying in brightness. The darker cores are unzoned, but the brighter cores show banding, sector zoning, or faint concentric zoning (Figs. 5e and 5f). The rims are dark and unzoned or faint zoned (Figs. 5e and 5f). Some grains are homogeneous, with low CL intensity (Fig. 5f). The range of concordant dates obtained from the cores was between 1861 ± 54 and 936 ± 29 Ma, with high Th/U ratios of 0.32-1.07 (Figs. 5e and 5f). The darker rims and the homogeneous grains yielded younger dates of 629 ± 21-532 ± 16 Ma and low Th/U ratios (0.01-0.02) (Fig. 5f).

Grt-Spl-Sil-Bt gneiss (IK17011002A) from Tenmondai Rock. Zircon separated from this gneiss is either elongated and rounded, ovoid or occasionally equant in shape, ranging 20-110 µm in length, with aspect ratios of 1.0-2.8. The CL images show brighter cores with oscillatory, banded, sector, patchy to faint zoning, dark unzoned mantles, and gray to bright unzoned to faintly zoned rims (Figs. 5g and 5h). The dates obtained from cores ranged from 3331 ± 124 to 798 ± 28 Ma (Figs. 5g and 5h) with Th/U ratios of 0.18-1.30. The mantles and rims yielded younger dates of 620 ± 25-550 ± 20 and 542 ± 23-528 ± 22 Ma, with low Th/U ratios of 0.00-0.01 and 0.02-0.04, respectively (Fig. 5h).

Grt-Bt gneiss (IK17010601) from Akarui Point. Most zircon grains extracted from this sample are ovoid to equant shape with a few being elongate. Crystal lengths are 20-140 µm with aspect ratios of 1.0-3.6. CL images show that most of the zircon grains have distinct cores, which exhibit broad, bright banding, or are oscillatory to faint patchy zoned. These are surrounded by darker, unzoned mantles and thin, grayish, unzoned rims (Fig. 5i). Some zircon grains do not have a bright core, but rather are homogeneous, rimmed by a thin, gray unzoned domain (Fig. 5i). Analysis of the cores gave concordant dates of 619 ± 22-516 ± 16 Ma (Fig. 5i), with low Th/U ratios of 0.00-0.03. Similar dates of 576 ± 19-517 ± 16 Ma were obtained from the dark CL domains, while the gray CL rims yielded younger dates of 535 ± 17-496 ± 17 Ma (Fig. 5i). The Th/U ratios of these domains are 0.02-0.13 and 0.02-0.15, respectively, which are higher than those of cores.

Sil-bearing Grt-Bt gneiss (IK17010904A) from Cape Omega. Most of the zircon grains from this gneiss are subhedral and elongated, 30-130 µm in length, with aspect ratios of 1.4-2.9. The main internal CL texture is cores with brighter banding and oscillatory, sector, or patchy zoning, surrounded by dark unzoned rims (Fig. 5j). Occasional grains have a dark and faint zoned core, with thin bright structureless mantles (Fig. 5j). The cores have high Th/U ratios of 0.26-0.67 and yielded concordant dates of 1025 ± 52-863 ± 41 Ma (Fig. 5j). In contrast, the rims have low Th/U ratios of 0.01-0.03 and provided younger concordant dates of 632 ± 18-517 ± 15 Ma (Fig. 5j).

Grt-Opx granulite (IK19011404A) from Oku-iwa Rock. The zircon grains obtained from this granulite sample are predominantly squat to equant in shape, although a minor component is elongated. Size and aspect ratios of these zircon grains are 10-110 µm and 1.0-2.3, respectively. Their CL images generally display brighter sector or patchy zoning with/without dark unzoned to faint zoned cores and thin gray rims (Fig. 5k). Some of the cores show brighter CL and patchy or banded zoning (Fig. 5k). Scattered dates of 1015 ± 29-813 ± 25 Ma and wide range in Th/U ratios of 0.04-1.13 were obtained by analyses of eight core portions (Fig. 5k). Two concordant data obtained from the brighter cores provided the oldest dates of 1015 ± 29 and 973 ± 33 Ma, with relatively high Th/U ratios of 1.13 and 0.27, respectively. However, six concordant data obtained from dark CL cores provided slightly younger dates of 967 ± 28-813 ± 25 Ma and Th/U ratios of 0.04-1.00. The dominant bright sector or patchy zoned domains yielded even younger concordant dates of 613 ± 33-482 ± 19 Ma (Fig. 5k) and Th/U ratios of 0.13-0.35.

DISCUSSION

Interpretation and significance of zircon U-Pb geochronology

The analyzed zircon grains from the sampled high-grade metamorphic rocks are generally rounded, producing CL images that usually exhibit a core(-mantle)-rim texture. The core domain is often irregular shaped and truncated by the mantle and/or rim domains (Fig. 5). Their mantles and rims characteristically display lower CL intensity and are unzoned or sector zoned, with a narrow range of derived U-Pb ages and Th/U ratios below 0.1 (Fig. 5). The features of these domains are considered indicative of a metamorphic origin (Hoskin and Schaltegger, 2003; Kohn and Kelly, 2018). Although the bright sector or patchy zoned rims of zircon in the Grt-Opx granulite sample IK19011404A from Oku-iwa Rock exhibited higher Th/U ratios over 0.10 (Fig. 5k), it is considered likely that this might be caused by the absence of Th-rich phase such as monazite in this sample. Alternatively, it might also have resulted from interaction with metamorphic fluid or melt (Kohn and Kelly, 2018). The cores of zircon in pelitic gneisses of samples IK19011606A from Niban Rock, IK19011805A from Gobanme Rock, IK17011002A from Tenmondai Rock, and IK17010904A from Cape Omega, as well as an inner core of zircon in sample IK16123003A from Sinnan Rocks, show a wide variety of CL textures. They also have older U-Pb ages and higher Th/U ratios than their mantles and rims (Figs. 5b, 5d-5h, and 5j). Therefore, the cores are considered to be relic detrital zircon grains derived from their protoliths. On the other hand, the zircon cores of other gneisses, samples IK16123001A and IK16123003A from Sinnan Rocks, IK16123102 from Akebono Rock, and IK17010601 from Akarui Point, display CL textures that are rather monotonous, without the obvious that would indicate detrital cores. These particular zircon cores yielded U-Pb ages and Th/U ratios comparable with those of their mantles and rims (Figs. 5a-5c and 5i). This suggests that these cores are metamorphic domains rather than detrital one. Regarding the cores of zircon in the Grt-Opx granulite sample IK19011404A from Oku-iwa Rock, those with the bright CL, patchy or sector zoning yielded ages of ∼ 1020-970 Ma, while those with the dark unzoned to faint zoned CL textures yielded relatively younger and scattered ages between ∼ 970 and 810 Ma (Fig. 5k). As the mafic granulite most likely originated from igneous rock judging from its mode of occurrence (Fig. 3i) and mineral assemblage (Fig. 4q and Table 1), the cores of its zircon are likely to be igneous in origin. However, magmatic zircon generally exhibits a peak cluster of ages, reflecting the timing of magmatism. We consider that the scattered ages of these cores may originally have been clustered, reflecting the time of intrusion, and later become scattered due to partial Pb loss during the Ediacaran to Cambrian high-grade metamorphism. A similar phenomenon was reported from mafic granulite in Tenmondai Rock (Takamura et al., 2020). Therefore, the oldest concordant data of 1015 ± 29 Ma detected from a bright patchy zoned core of zircon is plausible as the protolith igneous age of the mafic granulite.

As discussed above some domains of zircon in samples IK16123001A and IK16123003A from Sinnan Rocks, IK19011805A from Gobanme Rock, IK17011002A from Tenmondai Rock, IK17010601 from Akarui Point, IK17010904A from Cape Omega, and IK19011404A from Oku-iwa Rock are considered to have characteristics indicating metamorphic origins and record the Ediacaran to Cambrian thermal event. The other distinct early Neoproterozoic metamorphic events, at ∼ 990 and 930 Ma, were also detected from zircon derived from pelitic gneisses in Niban Rock and Akebono Rock, respectively.

This study newly confirmed a major metamorphic event occurred at ∼ 550-520 Ma from Gobanme Rock, Cape Omega, and Oku-iwa Rock in the LHC. It also derived evidence that a pre-600 Ma metamorphic event affected the entire Akarui Suite of Dunkley et al. (2020) (Fig. 2) including the amphibolite-zone samples from Sinnan Rocks (Figs. 5a, 5b, and 5e-5k). Probability density plots (Fig. 6) of the concordant data obtained from metamorphic domains of zircon show thermal metamorphic events during the late Neoproterozoic to Cambrian when zircon newly recrystallized. At Sinnan Rocks the gneiss sample (IK16123001A) contains zircon of which metamorphic domains show ages of 640-490 Ma (Fig. 6a). The sample has three age peaks each, corresponding to a triple-layer core-mantle-rim textural pattern (Figs. 5a and 6a). The trimodal age spectra were also identified from multi-growth metamorphic domains of zircon in sample IK16123003A from Sinnan Rocks, IK17011002A from Tenmondai Rock, and IK17010601 from Akarui Point (Figs. 5b, 5g-5i, 6b, 6d, and 6e). Zircon from Gobanme Rock (IK19011805A), Cape Omega (IK17010904A), and Oku-iwa Rock (IK19011404A) all have simple CL textures in the domains surrounding their more complex cores (Figs. 5e, 5f, 5j, and 5k). These outer domains are interpreted to be metamorphic in origin. Given their uniform CL texture, with no evidence of multiple zoning, they might be expected to have formed in a single metamorphic event. However, they show a broad range of ages between ∼ 630-480 Ma, with trimodal or bimodal age peaks (Figs. 6c, 6f, and 6g). The observation suggests the possibility of multiple thermal events where Pb is lost without modification of the internal texture of the zircon, as investigated by Kohn and Kelly (2018). Our interpretation of these observations is that these rocks were affected by multiple thermal events during a long-lived metamorphic period, spanning the Ediacaran to Cambrian. Each of the thermal events possibly invoked recrystallization of parts of the zircon and various degrees of Pb loss. This sort of phenomenon has been encountered in other granulite facies terranes (Taylor et al., 2016; Kunz et al., 2018 and references therein).

Figure 6. The probability density plots of the concordant data obtained from metamorphic domains of zircon in late Neoproterozoic to Cambrian metamorphic rocks. The orange lines and bands represent the centered ages and errors calculated by unmix ages function of isoplot, respectively. (a) Crn-Sil-Bt-Ms gneiss (IK16123001A) from Sinnan Rocks. (b) Grt-bearing Bt gneiss (IK16123003A) from Sinnan Rocks. (c) Grt-Sil-Bt gneiss (IK19011805A) from Gobanme Rock. (d) Grt-Spl-Sil-Bt gneiss (IK17011002A) from Tenmondai Rock. (e) Grt-Bt gneiss (IK17010601) from Akarui Point. (f) Sil-bearing Grt-Bt gneiss (IK17010904A) from Cape Omega. (g) Grt-Opx granulite (IK19011404A) from Oku-iwa Rock.

In order to determine the most likely number of components in an age spectrum, Sambridge and Compston (1994) recommended examining the relationship between the calculated relative misfit values and the assumed number of age components. In the case of the present study (Table 2), the reduction in relative misfit in the age spectra derived from the above samples is relatively small when assuming three or at most four age components, excluding sample IK19011805A from Gobanme Rock, for which the assumption of two age components is more suitable. Using this approach, we suggest that there were probably three thermal events at >600-580 (∼ 625-586), 580-550 (∼ 572-549), and 550-500 Ma (∼ 541-504 Ma) during the Ediacaran to Cambrian that affected the high-grade rocks from Sinnan Rocks, Gobanme Rock, Tenmondai Rock, Akarui Point, Cape Omega, and Oku-iwa Rock (Fig. 6 and Table 3). Takamura et al. (2020) investigated the rare earth elements chemistry of garnet and zircon in mafic granulite from Tenmondai Rock. These contain garnet being replaced by Opx + Pl symplectite which they proposed was formed during decompression exhumation soon after peak granulite-facies condition (850-860 °C and 7.8-8.4 kbar) at around 560 Ma. Dunkley et al. (2014) found that a Sil-bearing metamorphosed dyke at Tenmondai Rock had been intruded at ∼ 580 Ma. Based on these results it is likely that peak granulite-facies conditions were reached at Tenmondai Rock around 580-560 Ma. This time span overlaps with the age peak at 580-550 Ma (∼ 572-549 Ma) identified from zircon from Sinnan Rocks, Gobanme Rock, Tenmondai Rock, Akarui Point, Cape Omega, and Oku-iwa Rock. Thus, the age peak may represent the duration of near peak HT metamorphic conditions in those areas. Zircon ages within the time span of the youngest peak at 550-500 Ma (∼ 541-504 Ma) are commonly encountered in studies of the LHC. These include not only the ages of major metamorphic events, such as 550-520 Ma (e.g., Shiraishi et al., 1994, 2008; Dunkley et al., 2014), but also the magmatic period of intrusion of post-tectonic, unmetamorphosed igneous rocks at <530 Ma (e.g., Dunkley et al., 2014). Based on chronological studies for intrusive rocks using whole-rock or mineral isochron methods on Nd-Sm or Rb-Sr system, post-tectonic intrusion is likely to have continued from ∼ 530 to 490 Ma (Kawano, 2018, and references therein). Originally this time period was interpreted as the timing of peak high-grade metamorphism in the LHC (e.g., Shiraishi et al., 1994; Hokada and Motoyoshi, 2006). However, the consensus of recent studies (Dunkley, 2007; Kawakami et al., 2016; Takamura et al., 2020) is that post-peak retrograde metamorphism occurred after 550 Ma. In the present study, the U-Pb ages of ∼ 550-480 Ma are prominent in metamorphic zircon within a mafic granulite (IK19011404A) from Oku-iwa Rock (Fig. 6g). This sample underwent the intense retrograde hydration decomposing garnet and orthopyroxene (Fig. 4q), with some zircon also occurring near the hydration zone (Fig. 4r). We also note that the <550 Ma rims of metamorphic zircon in samples IK17010601 from Akarui Point and IK17011002A from Tenmondai Rock showed slightly bright CL emission and high Th/U ratios in comparison with their dark CL unzoned mantle domains (Figs. 5g-5i). Kawakami et al. (2016) reported a similar relationship between the internal textures and Th/U ratios of metamorphic zircon in a granulite-facies pelitic gneiss from Skallevikshalsen. They assumed that the rim of zircon crystallized from melt during the retrograde stage at 524 ± 7 Ma. The gneiss samples IK17010601 from Akarui Point and IK17011002A from Tenmondai Rock exhibit textures indicating the breakdown garnet into subhedral Bt + Pl and Bt + Sil, respectively (Figs. 4k and 4m). In both cases zircon grains occur near these coronas (Figs. 4l and 4n). We interpret these decomposition textures produced by a back-reaction with residual hydrous melt during the cooling stage (e.g., Holness et al., 2011). Thus, the younger rims of zircon in these samples might also have been formed by the interaction with melt at the same time. From the above discussion we suggest that the available evidence that after 550 Ma the LHC entered a stage of retrograde metamorphism associated with post-tectonic magmatism. Currently we find the event associated with the observed oldest age peak of >600-580 Ma (∼ 625-586 Ma) is difficult to explain. The pre-580 Ma metamorphic ages are predominantly reported from high-grade metamorphic rocks in the granulite-facies and transitional zone of the LHC, notably the UHT locations of Rundvågshetta and Skallevikshalsen, and near-UHT localities of Skallen (Hokada and Motoyoshi, 2006; Dunkley et al., 2014; Kawakami et al., 2016; Takahashi et al., 2018; Takamura et al., 2018). Kawakami et al. (2016) proposed that the Skallevikshalsen area underwent poly-metamorphism at ∼ 650-580 and ∼ 560-500 Ma. This conclusion was based on detail analyses of inclusions and trace elements chemical mapping of garnet in a khondalitic gneiss, which also showed two age populations of the EMP U-Th-Pb monazite ages and LA-ICP-MS U-Pb zircon ages. In contrast Dunkley et al. (2014) and Takamura et al. (2020), based on the wide range of metamorphic zircon ages obtained, suggested the same area underwent a long-lived high-grade metamorphism extending 630 to 510 Ma. However, the present study has no evidence to discuss the relative merits of whether the thermal episode associated with the oldest peak ages (>600-580 Ma) was an independent metamorphic event or rather the prograde stage of single and long-lived metamorphic event.

Table 2. The relation between the number of age components and relative misfit for unmixing broad metamorphic ages

Location	Sample No.	Rock type	Number of age components	Relative misfit (%)
Sinnan Rocks	IK16123001A	Crn-Sil-Bt-Ms gneiss	2 3 4	60.1 55.9 54.1
Sinnan Rocks	IK16123003A	Grt-bearing Bt gneiss	2 3 4	67.0 58.8 56.6
Gobanme Rock	IK19011805A	Grt-Sil-Bt gneiss	2 3 4	60.1 58.8 55.5
Tenmondai Rock	IK17011002A	Grt-Spl-Sil-Bt gneiss	2 3 4	70.8 66.9 no solution
Akarui Point	IK17010601	Grt-Bt gneiss	2 3 4	77.5 71.0 69.8
Cape Omega	IK17010904A	Sil-bearing Grt-Bt gneiss	2 3 4	47.0 30.5 no solution
Oku-iwa Rock	IK19011404A	Grt-Opx granulite	2 3 4	84.2 78.7 75.9

Table 3. The summary of zircon U-Pb ages of metamorphic rocks on the Prince Olav Coast, East Antarctica

Location	Sample No.	Rock type	U-Pb zircon ages (Ma)
Location	Sample No.	Rock type	Metamorphic domain	Detrital domain	Inherited domain
Sinnan Rocks	IK16123001A	Crn-Sil-Bt-Ms gneiss	586.3 ± 11 539.5 ± 15 504 ± 11
Sinnan Rocks	IK16123003A	Grt-bearing Bt gneiss	611.2 ± 9.5 567 ± 9.4 529.9 ± 10	890
Akebono Rock	IK16123102	St-bearing Grt-Ged-Bt-Chl gneiss	931.7 ± 9.8
Niban Rock	IK19011606A	Grt-Sil-Bt gneiss	994 ± 11	1940-1760, 1300, 1160-1010
Gobanme Rock	IK19011805A	Grt-Sil-Bt gneiss	605.9 ± 10 549.3 ± 6.5	1860-1710, 1050-930
Tenmondai Rock	IK17011002A	Grt-Spl-Sil-Bt gneiss	608.2 ± 11 572.1 ± 13 540.7 ± 15	3330, 2400, 2090-1730, 1080-800
Akarui Point	IK17010601	Grt-Bt gneiss	619 ± 22 555.1 ± 6.2 522.3 ± 5.5
Cape Omega	IK17010904A	Sil-bearing Grt-Bt gneiss	625.1 ± 8.6 571 ± 6.9 525.3 ± 8.6	1030-860
Oku-iwa Rock	IK19011404A	Grt-Opx granulite	595.1 ± 19 533.5 ± 8.8 505.1 ± 10		1020∼

Our analysis of a St-bearing Grt-Ged-Bt-Chl gneiss (IK16123102) from Akebono Rock indicates it was affected by a thermal event at 931.7 ± 9.8 Ma (Fig. 5c and Table 3). This is consistent with Baba et al. (2022), who reported a weighted mean age of 936.8 ± 6.2 Ma from sector zoned overgrown rim of zircon in Ky-bearing Grt-Bt gneiss also from Akebono Rock. The pelitic gneiss analyzed by Baba et al. (2022) contains minor amounts of kyanite in the matrix but no sillimanite. They estimated peak metamorphic conditions for this pelitic gneiss to be 674-679 °C at 7 kbar, with a clockwise P-T path. The pelitic gneisses in Akebono Rock described by the present study and Baba et al. (2022) appear to be quite unique in the LHC in terms of not only an early Neoproterozoic metamorphic age (∼ 940-930 Ma), but also the presence of primary chlorite or kyanite and the absence of sillimanite.

This study indicates that the Grt-Sil-Bt gneiss sample (IK19011606A) collected from Niban-nishi Rock of Niban Rock (Fig. 5d and Table 3) underwent HT metamorphism at 994 ± 11 Ma. This age is comparable with a monazite EMP age of ∼ 940 Ma obtained from pelitic gneiss in Niban-nishi Rock (Mori et al., 2023). It is also consistent with monazite EMP ages of 1007-935 Ma (Motoyoshi et al., 2005) and a U-Pb age of metamorphic zircon at ∼ 970 Ma (Dunkley et al., 2014) obtained from Sil- and retrograde Ky-bearing pelitic gneiss in the southwestern part of Cape Hinode.

Regional terrane correlations based on similar thermal histories

Many studies have pointed out that the LHC and the high-grade metamorphic rocks exposed in Sri Lanka and southern India were all formed during the assembly of Gondwana and were geographically juxtaposed at that time (e.g., Shiraishi et al., 1994) (Fig. 1). The correlation of protolith, metamorphic evolution, and structural history among these three metamorphic regions has been convincingly demonstrated by many workers (e.g., Shiraishi et al., 1994; Osanai et al., 2016a, 2016b; Kitano et al., 2018; Takahashi et al., 2018; Takamura et al., 2018, 2020; Dunkley et al., 2020; Durgalakshmi et al., 2021). Based on the accumulated database of U-Pb geochronology, metamorphic grade and lithology, Dunkley et al. (2020) proposed the LHC be divided into the following tectonic suites: Innhovde (INH), Rundvågshetta (RVG), Skallevikshalsen (SKV), Langhovde (LHV), East Ongul (EOG), Akarui (AKR) Suites, and Hinode Block (HB) (Fig. 2). The INH consists mainly of orthogneisses with inherited ages of 1070-1040 Ma. The other suites are composed of both metasedimentary and metaigneous rocks, with protolithic igneous ages of 2520-2470 Ma in RVG, 1830-1790 Ma in SKV, 1100-1050 Ma in LHV, 630 Ma in EOG, and 970-800 Ma in AKR. The HB is an exotic block within the AKR on the Prince Olav Coast, metamorphosed during the early Neoproterozoic. Dunkley et al. (2020) also discussed the correlation of the protoliths of the suites of the LHC with terranes in Sri Lanka and southern India, which are shown in Figure 7. According to Dunkley et al. (2020), the geological and geochronological features of the INH are compared with those of the Vijayan Complex in Sri Lanka. The SKV could be a part of the Highland Complex of Sri Lanka and the Trivandrum Block of southern India. The Sri Lankan Wanni Complex and Indian Southern Madurai Block can be extended to the LHC as the LHV and AKR. The Northern Madurai Block of southern India and the Western Rayner Complex, which is a Cambrian granulite-facies metamorphic complex juxtaposing to the LHC, both contain metaigneous rocks that originated from ∼ 2500 Ma protoliths. The HB is regarded as an enclave within the LHC of the Tonian age Rayner Complex. Based on the results of this study and taking into account many other studies (Plavsa et al., 2012; Kröner et al., 2013; Collins et al., 2014; Plavsa et al., 2014; Osanai et al., 2016a, 2016b; Takamura et al., 2016; Kitano et al., 2018; Dunkley et al., 2020) the central part of Gondwana represented by the LHC, Sri Lanka, and southern India, can be divided into three terrains metamorphosed at different times (Fig. 7). A central area consisting of the LHC of Antarctica, the Wanni and Highland Complexes of Sri Lanka, together with the Trivandrum and Southern Madurai Blocks of India metamorphosed at 650-500 Ma. Two areas flanking this central area both metamorphosed at 580-500 Ma. The INH of Antarctica and the Vijayan Complex of Sri Lanka on one side, with the Western Rayner Complex of Antarctica and the Northern Madurai Block on India on the other side (Fig. 7). The third area being the Rayner Complex and the HB as well as Niban-nishi Rock of Niban Rock and Akebono Rock of Antarctica, metamorphosed at 1000-900 Ma. Timing of late Neoproterozoic to Cambrian metamorphic events and trending orientation of the correlated terranes (Fig. 7) imply that the almost E-W collision happened before 580 Ma as an eastmost part of the EAAO (Fig. 1a), prior to the final amalgamation by the KO terminated around 500 Ma (Fig. 1b). The early Neoproterozoic metamorphic rocks in the HB, Akebono Rock, and Niban Rock might have been detached from the Rayner Complex during Cambrian orogenesis corresponding to the KO, as Nogi et al. (2013) proposed.

Figure 7. The regional correlation of metamorphic ages among East Antarctica (LHC, Lützow-Holm Complex; WRC, Western Rayner Complex; RC, Rayner Complex), Sri Lanka (WC, Wanni Complex; HC, Highland Complex; VC, Vijayan Complex), and southern India (TB, Trivandrum Block; SMB, Southern Madurai Block; NMB, Northern Madurai Block). INH, Innhovde Suite; RVG, Rundvågshetta Suite; SKV, Skallevikshalsen Suite; LHV, Langhovde Suite; EOG, East Ongul Suite; AKR, Akarui Suite; HB, Hinode Block. The terrane divisions follow Shiraishi et al. (1994), Plavsa et al. (2012), Collins et al. (2014), Osanai et al. (2016a, 2016b), Kitano et al. (2018), and Dunkley et al. (2020). Study areas: SR, Sinnan Rocks; AR, Akebono Rock; NR, Niban Rock; GR, Gobanme Rock; TR, Tenmondai Rock; AP, Akarui Point; CO, Cape Omega; OR, Oku-iwa Rock.

CONCLUDING REMARKS

The new data presented in this study, when combined with that of previous reports, provides a more detailed and integrated view of the multi-thermal history of the high-grade metamorphic rocks in the LHC than was previously available. The oldest metamorphic event in the area at 990-930 Ma, which previously had only reported from Cape Hinode and the Rayner Complex further to the east, has now been shown to have also affected Niban-nishi Rock of Niban Rock and Akebono Rock. Thus, these areas combine (Fig. 7) to form a bigger inlier of this older metamorphic terrain than previously mapped. High-grade metamorphism took place across the LHC, excluding the inlier discussed above, through the Ediacaran to Cambrian (>600-500 Ma). This long period of metamorphism can be divided into three stages: pre-580 Ma (an independent metamorphic episode or prograde stage), ∼ 580-550 Ma (near-peak stage), and post-550 Ma (retrograde stage). The geochronological correlation of terranes in central Gondwana implies the amalgamation might have involved by the EAAO during >600-580 Ma and the subsequent KO after 580 Ma.

ACKNOWLEDGMENTS

This study was part of the Science Program of the Japanese Antarctic Research Expedition (JARE). It was supported by the National Institute of Polar Research (NIPR) under MEXT. The fieldwork in this study was conducted by the operations of the 58th and 60th JARE. We thank the members of the 58th and 60th JARE and the crew of the icebreaker SHIRASE. We also acknowledge all members of the JARE geology group for their fruitful discussions and J. Booth for his extensive and helpful English correction of the manuscript. We greatly appreciate two anonymous reviewers for their extensive constructive reviews and T. Kawakami for the editorial support. This work was supported by Fujiwara Natural History Foundation to I. Kitano and JSPS KAKENHI Grants Numbers JP21H01182 to T. Hokada, JP21253008, JP22244063, and JP16H02743 to Y. Osanai, and JP15K05345, JP18H01316, and JP21K18381 to N. Nakano.

SUPPLEMENTARY MATERIALS

Supplementary Figures S1-S8 and Tables S1-S9 are available online from https://doi.org/10.2465/jmps.221220.

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