Sedimentary protolith and high-P metamorphism of oxidized manganiferous quartzite from the Lanterman Range, northern Victoria Land, Antarctica

. We investigated the mineral assemblage, mineral and bulk-rock chemistry, and zircon U–Pb age of a manganiferous quartzite layer in the Lanterman Range, northern Victoria Land, Antarctica. The mineral assemblage consists primarily of phengite and quartz, along with spessartine-rich garnet, Mn 3 + and rare earth element–yttrium (REY)-zoned epidote-group minerals


Introduction
Oxidized, deep-sea siliceous sediments rich in manganese are commonly found as a precursor of spessartine-rich quartzitic rocks (e.g., Mottana, 1986;Abs-Wurmbach and Peters, 1999).Occurrences of Mn-rich quartzose rock have been reported particularly in high-to ultrahigh-P terranes, such as the Hellenides blueschist belt on the island of Andros, Greece (Reinecke et al., 1985;Reinecke, 1986), and the meta-ophiolite belt in the Zermatt-Saas zone of the Western Alps (Reinecke, 1998;Rubatto et al., 1998;Tumiati et al., 2010).However, fundamental questions still remain largely unsolved concerning the petrologic and tectonic nature of the sedimentary protoliths and metamorphism.This is because of (1) rare occurrence, (2) diverse source reservoirs, (3) variable oxidation states and (4) metamorphic overprinting (e.g., Reinecke et al., 1985;Tumiati et al., 2010).Further complexity arises from the different names used to describe the spessartine-quartz-rich metasedimentary rocks, "coticules" in Belgium (Herbosch et al., 2016) and "gondites" in India (Melcher, 1995).Coticules are fine-grained, mica-rich yellowish rocks which have likely been used for whetstones since the time of the Roman Empire.Gondites, on the other hand, are meta-arenaceous and meta-argillaceous rocks commonly associated with ores.In order to avoid any confusion, we use the term Mn-rich quartzose metasedimentary rock or manganiferous quartzite rather than these historical or descriptive ones.
Manganese is known to be mobile during water-rock interactions, with high solubility in hydrothermal fluids (Edmond et al., 1982).The primary source of silica in the Mnrich quartzose (meta)sedimentary rocks is generally considered to be biogenic, particularly radiolarian chert (Mottana, 1986).Variable contributions of detrital siliciclastics also played a role in the formation of these peculiar sedimentary rocks (Coombs et al., 1985;Reinecke et al., 1985;Mottana, 1986).Hydrogenous precipitation is another significant process, where Mn-rich sedimentary rocks could serve as key beds in stratigraphic correlation (e.g., Thomson, 2001;Herbosch et al., 2016).The combination of these variable source components in various proportions contributed to the formation of the Mn-rich siliceous sedimentary rocks.
In this study, we analyzed the bulk-rock and mineral chemistry, conducted pseudosection modeling, and determined the U-Pb geochronology of manganiferous quartzite.This quartzite occurs as centimeter-to decimeter-thick layers hosted within mafic eclogites and phengite-bearing quartzofeldspathic schists/gneisses from the Lanterman Range in northern Victoria Land (NVL), Antarctica.The aims of this study are (1) to unravel the origin and depositional setting of the protolith; (2) to demonstrate high-P paragenesis of the epidote-group mineral, phengite and garnet; (3) to constrain the oxidation state of the Mn 3+ -Fe 3+ -bearing mineral assemblages; (4) to estimate the depositional and metamorphic ages from the U-Pb dating of zircon; and (5) to finally discuss tectonic implications of the petrogenesis.

Geological background
At the final stage of the Gondwana assembly, subduction of the Paleo-Pacific and Iapetus plates along the periphery of Gondwana resulted in the formation of the Andean-type Terra Australis orogen (Fig. 1a) (Boger and Miller, 2004;Cawood, 2005;Foden et al., 2006).This accretionary tectonics was represented by the Ross orogeny in East Antarctica, giving rise to a prolonged continental arc system spanning the late Neoproterozoic to Early Ordovician times (e.g., Kleinschmidt and Tessensohn, 1987;Rocchi et al., 2011;Goodge, 2020).The exposed crustal root of the Ross orogen forms the Transantarctic Mountains, which currently serve as a physiographic boundary between East and West Antarctica (e.g., Stump, 1995;Faure and Mensing, 2010).
The low-P /T gneisses predominate to the west of the Wilson terrane and transform into medium-to high-P /T schists and gneisses to the east in the mountainous regions.It is inferred that the Rennick Glacier overlies a profound disconti-nuity of metamorphic grade in the Wilson terrane (Figs.1b, 2a) (Grew et al., 1984;Kleinschmidt and Tessensohn, 1987).Such an increase in the P /T ratio is particularly noticeable in the Lanterman, Salamander and Mountaineer ranges.The Lanterman Range is located in the central part of the Wilson-Bowers terrane boundary, which is defined by the Lanterman Fault (Figs. 1b,2a).The Dessent Ridge unit to the west of the Lanterman Fault comprises medium-to high-P /T rocks and is mainly composed of medium-P amphibolites and metapelites in the southeast (e.g., Scambelluri et al., 2003;Palmeri et al., 2012) and eclogite-facies rocks of the Lanterman Range in the northwest (e.g., Di Vincenzo et al., 1997;Palmeri et al., 2003) (Fig. 2a).Three metamorphic units are distinguished from west to east in the Lanterman  Mineral abbreviations are after Whitney and Evans (2010).
eclogites and record high-P zircon growth at ca. 500 Ma (Kim and Lee, 2023) (Fig. 3).The celadonite-rich phengite in the (ultra)high-P schists and gneisses yielded 40 Ar/ 39 Ar ages slightly younger than 500 Ma, whereas the relatively celadonite-poor neoblasts which are aligned parallel to or discordantly to the foliation gave ca.482-478 Ma in dating the greenschist-facies retrogression (Di Vincenzo et al., 2001) (Fig. 3).Di Vincenzo et al. (2016) extended a time frame of the eclogite-facies metamorphism of the quartzofeldspathic schists and gneisses on the basis of scattered zircon spot dates ranging from ca. 520 Ma to ca. 500 Ma (Fig. 3).The tectonic setting of the protoliths of the (ultra)high-P quartzofeldspathic schists and gneisses has been suggested to be arc/back arc; this proposition is based upon largely siliciclastic compositions, their interlayering with meta-mafic rocks of possibly basalt flow/dike origin and the varying geochemical affinities of the meta-mafic rocks ranging from E-MORB (enriched mid-ocean ridge basalt) or T-MORB (transitional mid-ocean ridge basalt) to calcalkaline signatures (Palmeri et al., 2003;Rocchi et al., 2011).Capponi et al. (1997) mapped the occurrence of garnetbearing pinkish manganiferous quartzites as thin layers ranging from centimeters to decimeters in thickness, aligned parallel to the regional foliation trending NW to NNW.Garnet is idioblastic and rich in spessartine, and quartz is granoblastic showing weak alignment parallel to the regional foliation.Capponi et al. (1997) speculated that the relationships of intercalation with meta-mafic and meta-ultramafic rocks suggest an ophiolite affinity of the manganiferous quartzites.

Sample description and petrography
Manganiferous quartzite sample 23-1F was collected from the northern part of "outcrop 4" according to the geological map of the Lanterman Range produced by Capponi et al. (1997).The sample location matches that of Di Vincenzo et al. (2016) and Kim et al. (2019) where medium-grained colder eclogites were reported.It is situated several hundred meters northward of the locality of fine-grained hotter eclogite (Di Vincenzo et al., 1997) (Fig. 2a).The Mn-rich quartzose metasedimentary rocks are volumetrically minor relative to the meta-mafic blocks and the surrounding garnetphengite quartzofeldspathic and hornblende-biotite gneisses/schists in the study area and are characterized by a pinkish tinge visible to the naked eye (Fig. 2b, c).The centimeterto decimeter-thick concordant layers of the manganiferous quartzite are interleaved within the garnet-phengite quartzofeldspathic and amphibolitic schists (Fig. 2b, c; see also Capponi et al., 1997).These schists of contrasting lithologies commonly show intensive high-T deformation features such as isoclinal folding and mafic boudin formation, whereas the pinkish medium-grained manganiferous quartzites are moderately foliated (Fig. 2b, c).The major foliation of the quartzite layer is well-defined by flaky phengite and elongate-shaped titanohematite.

Analytical methods
Concentrations of the major and trace elements in manganiferous quartzite sample 23-1F were determined using inductively coupled plasma optical emission system and mass spectrometer methods, respectively, employing lithium metaborate-tetraborate fusion at Activation Laboratories Ltd., Ontario, Canada.Mineral compositions were analyzed using a JEOL JXA-8530F field-emission (FE) electron microprobe housed at the Korea Polar Research Institute (KOPRI), with 15 kV accelerating voltage, 10 nA beam current, 3-5 µm beam diameter and 20 s counting times.The compositions of epidote-group minerals particularly rich in rare earth element and yttrium (REY) were measured following the analytical conditions of Kim et al. (2009).Standard materials used for the calibration include natural silicates and synthetic phosphates and oxides.Matrix correction was performed using the ZAF method.The X-ray mapping was conducted with an accelerating voltage of 15 kV, beam current of 200 nA, dwell time per pixel of 150 ms, step size of 3 µm and focused beam.Representative analyses of garnet, epidote-group mineral, phengite, chlorite, feldspars and oxides are given in Table S1.
A thin section of sample 23-1F cut parallel to lineation and perpendicular to foliation was coated with 10-15 nm thick carbon after mechanical and chemical polishing.The lattice preferred orientations (LPOs) of quartz were measured using electron backscatter diffraction (Oxford Symmetry S2) attached to a JEOL JSM-7200F-LV FE scanning electron microscope (SEM) housed at KOPRI.Analytical settings include an accelerating voltage of 20 kV, working distance of 23 mm and beam current of 10 nA.Automated montage mapping (10 µm step size) was performed on the whole area of the section (ca.3.5 × 2.5 cm), and the raw data were processed to compile montaged maps using the AZtec software.Noise reduction was conducted by filling the non-indexed pixels from five to eight identical neighbors using the AZtecCrystal software.Pole figures of one point per grain data were plotted on lower-hemisphere equal-area projections employing an orientation distribution function with half width of 15°.
Zircon separates were collected from sample 23-1F, using the conventional heavy-liquid technique of Cheong et al. (2013), and were mounted in 25.4 mm epoxy disks together with standard materials: zircon SL13 and FC-1 for measuring the U concentration (µg g −1 ; Claoué-Long et al., 1995) and for calibrating 206 Pb/ 238 U ratios ( 206 Pb * / 238 U = 0.1859; Paces and Miller, 1993), respectively.Cathodoluminescence (CL) images of individual crystals were obtained using a JEOL JSM-6610LV SEM housed at the Korea Basic Science Institute (KBSI).The U-Th-Pb isotopic compositions of zircon were measured using the KBSI sensitive high-resolution ion microprobe, SHRIMP-IIe.The analytical protocol for zircon followed that of Williams (1998).All isotopes were measured using a primary O − 2 beam with ∼ 3-5 nA and a ∼ 25 µm diameter spot.Common Pb contributions to total Pb counts were corrected using the 204 Pb and 207 Pb correction methods for ages older and younger than 1.0 Ga, respectively, with a model Pb composition (Cumming and Richards, 1975;Williams, 1998).The software programs SQUID2 (Ludwig, 2009) and Isoplot/Ex (Ludwig, 2008) were used for the age calculation and data processing, respectively.Individual spot analyses and weighted mean 206 Pb/ 238 U ages of zircon are quoted at 1σ and 2σ confidence levels, respectively.
5 Mineral chemistry and quartz LPO

Epidote-group minerals
Epidote-group minerals commonly exhibit systematic zoning patterns, with compositions spanning piemontite to epidote (Armbruster et al., 2006).Ce and Mg zoning patterns in Xray maps are similar to each other but antithetic to that of Ca (Fig. 6).The inner domains are divided into two compositional groups: piemontite (Pmt = Mn 3+ / (Fe 3+ + Mn 3+ + Al VI − 2) = 0.46-0.59;Ep = Fe 3+ / (Fe 3+ + Mn 3+ + Al VI  − 2) = 0.37-0.43)and epidote (Pmt = 0.19-0.48;Ep = 0.50-0.66)(Figs. 6,7b).These inner domains are low in REY abundances ( REY = La + Ce + Pr + Nd + Gm + Gd + Y = 0.02-0.10apfu) (Fig. 8) and relatively high in SrO and PbO contents (0.57 wt %-0.92 wt % and 0.17 wt %-0.32 wt %, respectively).The outer domains with https://doi.org/10.5194/ejm-36-323-2024 Figure 5. X-ray maps and compositional profiles of garnet porphyroblasts (a Grt I and b Grt II).The Ti X-ray map for titanohematite and its inclusions is shown with bluish colors at the top left in the Mn X-ray map in (a).Minerals with X-ray counts that are too high and low are shown in black for clarity; for example, titanohematite (Hem) and other Fe-rich oxides in the Fe map of (a).Rutile inclusion shows a similar intensity to that of host Mn-rich garnet in the Fe map because garnet is extremely low in FeO (< 1 %; Table S1 in the Supplement).In (a), two small lower-Mn inclusions and one higher-Fe inclusion near the garnet center are impurities.Mineral abbreviations are after Whitney and Evans (2010).

Chlorite
Chlorite is clinochlore in composition with 1.68 wt %-2.37 wt % MnO.The X Fe of chlorite is in the range of 0.20-0.28.

Quartz fabric
Quartz LPOs of the analyzed quartzite sample exhibit a single cluster of [0001] around the y axis, and great circles of a axes ([10-10]) and poles to m planes ([11-20]) (Fig. 9).The LPO strength was quantified as the J and M index; the former is calculated as the second moment of the orientation distribution function (Bunge, 1982) and the latter as the difference between the distributions of uncorrelated misorientation angles for sample data and a random fabric (Skemer et al., 2005).The J index ranges theoretically from 1 to infinity for random and single-crystal LPO, respectively, and the M index from 0 to 1.The calculated J and M indices of quartz are 3.00 and 0.19, respectively.

Pseudosection modeling
In order to complement previous experimental studies on the Mn-Si-O (± Al) system (Anastasiou and Langer, 1977;Keskinen and Liou, 1979; Abs-Wurmbach and Peters, 1999), we conducted thermodynamic modeling of the Mn-Ca-Fe-Al-Si-O system (Fig. 10).We calculated pseudosections in the Mn-Ca-Fe-Al-Si-O model system, using the Perple_X program (Connolly, 2005).The thermodynamic database of Holland and Powell (1998), updated in 2004 (hp04ver.dat),has been further modified by Tumiati et al. (2015) to include manganese oxides and end-member piemontite.Ideal mixing was assumed for epidote-piemontite solid solution.The garnet solution model is from Holland and Powell (1998).Pure end-member phases include braunite, tephroite, pyroxmangite, hematite, magnetite, quartz, kyanite, margarite, hedenbergite and H 2 O. Two f O 2 -X pseudobinary diagrams were calculated at 8 kbar and 500 °C and 18 kbar and 550 °C for upper greenschist-facies (Fig. 10a) and prograde eclogite-facies conditions, respectively (Fig. 10b), the latter being obtained from the colder eclogites in the study area (Kim et al., 2019).The model bulk-rock composition (X) ranges from pure SiO 2 (X = 0) to a compound (X = 1) consisting of 1 mol of garnet (Sps 81 Grs 19 ) + 1 mol of epidote-group mineral (Pmt 50 Ep 50 ) + 0.2 mol of tephroite which was estimated on the basis of inclusion relationships and mineral chemistries (Table S1).The amount of tephroite is apparently exaggerated in comparison to that observed in the thin section as micro-inclusions (Fig. 6) in order to increase bulk-rock MnO content and to highlight the phase relationships of Mnbearing minerals (Tumiati et al., 2015).
Both modeling results under two different P -T conditions yielded a similar topology (Fig. 10).The spessartinegrossular-almandine garnet solid solution is stable over a wide range of both f O 2 and X values, except for extremely high f O 2 fields under the greenschist-facies condition (Fig. 10a).The stability of piemontite-epidote solid solution is limited to relatively oxidized fields.The tephroite-bearing, silica-undersaturated assemblages are stable at high-Mn bulk-rock compositions (X > 0.8) and in more reduced environments below the hematite-magnetite redox buffer (the f O 2 / 1 bar ratio relative to the fayalite-magnetite-quartz buffer (i.e., FMQ) is lower than +2.25) in both conditions, whereas the epidote-group mineral-garnet-quartz-bearing assemblages are stable under more oxidized conditions above the hematite-magnetite buffer ( FMQ = +6.96-15.76 and +8.26-14.60 in Fig. 10a and b, respectively).The latter assemblages are less oxidized than braunite-bearing assemblages ( FMQ > 16.44; Fig. 10) (cf., Tumiati et al., 2015).The pure piemontite-spessartine redox buffer in Tumiati et al. (2015) is absent in this study because the addition of the Fe component enlarges the stability of the epidote-group solid solution towards lower f O 2 so that an epidote-group mineral could coexist with a garnet solid solution in our Mn-Ca-Fe-Al-Si-O system (Fig. 10).
MnO contents distinctly higher than those of the chert and pelagic clay in the Al 2 O 3 -MnO-Fe 2 O T 3 diagram (Fig. 11a), except for the talc-free piemontite-quartz schists of Sanbagawa.The extent of Mn contents in the quartzite sample is comparable with and slightly higher than those of the Otago schists, piemontite ± spessartine quartzites on the island of Andros, talc-bearing piemontite-quartz schists of Sanbagawa and garnet-rich rocks in Venezuela but lower than the piemontite-free spessartine quartzites on the island of Andros, Praborna Mn ores, Belgian coticules and Fe-Mn nodules/crusts.Transition metal contents of the Mn-rich quartzose rocks are distinctly low, belonging to the hydrothermal field in the (Cu + Ni + Co)-Fe T -Mn diagram, compared to that of the Fe-Mn nodules/crusts that plot in the hydrogenous field (Fig. 11b; Bonatti et al., 1972).

Discussion
9.1 Sedimentary formation, depositional setting and formation age of the Mn-rich protolith Deep-sea cherts or pelagic siliceous mudstones are known as protoliths of Mn-rich quartzose metasedimentary rocks, with a variety of inputs from biogenic, hydrogenous, hydrothermal and terrigenous sources (Coombs et al., 1985;Reinecke et al., 1985;Mottana, 1986;Izadyar et al., 2003;Tumiati et al., 2010).The major-element concentrations of the manganiferous quartzite sample in this study lead to chert as its probable protolith, together with minor input from pelagic clay (Fig. 11a), like other Mn-rich quartzose metasedimentary rocks except for the Belgian coticules (Herbosch et al., 2016) and Venezuelan garnet-rich rocks (Maresch et al., 2022) of limy mud turbidite and exhalite origin, respectively.The high MnO content (1.78 wt %) and pronounced positive Ce anomaly (Ce/Ce * = 2.96) of the manganiferous quartzite sample (Fig. 11a, c) are possibly accounted for by the incorporation of the Fe-Mn nodules/crusts which are common on the seafloor (e.g., Elderfield et al., 1981;Usui and Someya, 1997).On the basis of geochemical constraints, Reinecke et al. (1985) concluded that the Mn-nodulebearing sediments were the primary source for the piemontite ± spessartine quartzites from the island of Andros, Greece; the nodules are now metamorphosed braunite-rich ore lenses hosted by the piemontite ± spessartine quartzites.It is noteworthy that the field occurrence of the manganiferous quartzites in the Lanterman Range as centimeterto decimeter-thick layers is comparable with those of the piemontite ± spessartine quartzites from Andros (Reinecke et al., 1985) and piemontite quartzites in the Zermatt-Saas zone, the Western Alps (Chopin, 1978;Reinecke, 1998).Similar Mn (micro)nodules would have been present in the quartzite protolith and might have been obliterated by Cambrian high-P metamorphism during the Ross orogeny.
In the (Cu + Ni + Co)-Fe T -Mn diagram (Fig. 11b), the manganiferous quartzite belongs to the hydrothermal deposit field where the concentration of transition metal by hydrogenous processes could be relatively weak (Bonatti, 1975;Usui and Someya, 1997).These contrasting affinities of the hy-drogenous and hydrothermal sources could be accounted for by the dependence of element enrichment on the combination of the accumulation rate, redox condition of seawater, proximity of hydrothermal source and dilution effects from other components (e.g., Hein et al., 1997;Bau and Koschinsky, 2009).Tumiati et al. (2010) suggested a ratio of hydrogenous to hydrothermal input of 1 : 4 in the Praborna ore rocks, primarily on the basis of the degree of positive Ce anomaly (Ce/Ce * = 1.13) in the REE pattern.Thus, the proportion of the hydrogenous component in the manganiferous quartzite of this study could be higher than 1 : 4 of the Praborna case because of the far stronger positive Ce anomaly (Ce/Ce * = 2.96; Fig. 11c).The volumetrically overwhelming garnet-phengite schists and gneisses which enclose the manganiferous quartzites in the Lanterman Range originated from continentally derived sediments in an arc/back-arc setting (Palmeri et al., 2003;Rocchi et al., 2011).Because the manganiferous quartzite is a minor lithologic unit and would share a common provenance with the host siliciclastic metasedimentary rocks, it is likely that the deep-sea chert protolith formed at a continental margin.This interpretation is supported by terrigenous input exemplified by the Paleoproterozoic (ca.2150-1890 Ma) and late Ediacaran (ca.570-550 Ma) detrital zircons (Fig. 12).In order to reconcile a deep-sea origin with a continental margin setting of the protolith deposition, we propose incipient extension in an arc/back-arc setting.This is partly in line with depositional settings of other Mn-rich quartzose sediments, such as a small ocean basin near a continental margin for manganiferous quartzite in Lago di Cignana, the Western Alps (Rubatto et al., 1998); a continental shelf to slope shallower than the abyssal-plain environment for the Belgian coticules (Herbosch et al., 2016); and a passive margin associated with mature back-arc extension for the Venezuelan meta-exhalites (Maresch et al., 2022), though the depositional setting differs from the oceanic spreading center environment interpreted for the Praborna Mn ore in the Saint-Marcel valley meta-ophiolite, the Western Alps (Tumiati, 2005).
The depositional timing of the chert-dominated protolith, as well the surrounding garnet-phengite quartzofeldspathic schists/gneisses, is bracketed between the youngest detrital age (ca.546 Ma) and the metamorphic zircon age (ca.506 Ma) (Fig. 12).This time interval does not correspond to a glacial period during Neoproterozoic (Meert and Van Der Voo, 1994), ruling out the presence of far-traveled glaciogenic detritus transported by icebergs (e.g., Porêbski et al., 2019).Our results are consistent with the detrital zircon age data of Gibson et al. (2011), who proposed an upper limit of the protolith deposition of amphibolite-facies paragneisses in the Lanterman Range to be early to middle Cambrian (ca.520-500 Ma).In addition, it is noteworthy that the latest Ediacaran maximum depositional age of the manganiferous quartzite protolith overlaps with an early phase of arc-related magmatism in NVL (e.g., Bomparola et al., 2007;Rocchi et al., 2011).This suggests possible arc-derived local sources such as the Wilson terrane and inland cratonic Antarctica (Estrada et al., 2016;Paulsen et al., 2016;Kim et al., 2017).

High-P metamorphism, oxidation state and tectonic implications
The Mn-rich mineral assemblages of manganiferous quartzite sample 23-1F are, unfortunately, not very informative for P -T estimation (e.g., Reinecke, 1998;Izadyar et al., 2000;Tumiati et al., 2020).Therefore, we infer metamorphic stages on the basis of the mineral assemblage and compositional zonation.The Mn-rich cores of the porphyroblastic epidote-group mineral (Pmt = 0.19-0.59;Figs. 6, 7b) denote prograde metamorphism (M 1 ) in high-f O 2 conditions (e.g., Chopin, 1978;Bonazzi and Menchetti, 2004).This is supported by the extremely low X Fe (0.00-0.07) of the spessartine-rich garnet core (Figs. 5,7a).A rare occurrence of tephroite and rutile inclusions in the piemontite core and spessartine-rich garnet core, respectively, is consistent with blueschist-facies or low-T eclogite-facies conditions for M 1 , largely depending on the oxidation state and bulk-rock composition (Fig. 10); these are based upon experimentally derived phase relationships (Abs-Wurmbach and Peters, 1999) and petrologic work on natural occurrences (e.g., Mottana, 1986;Reinecke, 1986;Tumiati, 2005).Two sequential growth zones of epidote-group minerals (the outer domains consisting of REY-rich piemontite outer zone 1 and REY-rich epidote outer zone 2) represent prograde M 2 and peak M 3 , respectively (Figs. 6,7b,8).The allanite enrichment trend from outer zone 1 to 2 is comparable with that reported in the eclogite-facies Mn-rich metacherts from the Praborna ore deposit, the Western Alps (Tumiati et al., 2020).These outer domains were likely produced during the prograde metamorphism at the expense of REY-Thrich minerals such as monazite, which commonly occurs in the matrix.The paragenetic relationships between Mn-REY-rich epidote-group mineral and monazite are reminiscent of those in the eclogite-facies mica schists (Gabudianu Radulescu et al., 2009) and Barrovian metapelitic schists (Kim et al., 2009) where allanite is stable in higher-P conditions than monazite.The metamorphic peak M 3 is also represented by the celadonite-rich inner segment of phengite (Si = 3.23-3.34apfu).The abrupt increase in X Fe of garnet between the core (0.00-0.07) and rim (0.49-0.50) (Figs. 5, 7a) is tentatively attributed to a decrease in the epidote / piemontite ratio of the epidote-group mineral during M 2-3 metamorphism and an accompanying decrease in the spessartine / almandine ratio of garnet.The relatively large grain size (∼ 500 µm on average), largely rectilinear grain boundaries and shape preferred orientation of quartz (Fig. 4) suggest high-T grain boundary migration recrystallization (dislocation creep regime 3 of Hirth and Tullis, 1992) possibly at peak M 3 .It is corroborated by the single cluster of c axes in the center and peripheral maxima of a axes typical https://doi.org/10.5194/ejm-36-323-2024Eur.J. Mineral., 36, 323-343, 2024 of dominant prism a slip of quartz at ∼ 500-650 °C (e.g., Stipp et al., 2002).
The REY-poor epidote of the outermost domain or rim in the porphyroblastic epidote-group mineral marks retrograde stage M 4 (Tumiati et al., 2020).The exsolution of pyrophanite in titanohematite is indicative of retrograde cooling (Nakyak and Mohapatra, 1998).The chlorite growth at the expense of garnet is also a retrograde feature, which is coeval with the relatively low-celadonite outer segment of phengite (Si = 3.14-3.27apfu).It is noteworthy that a marginal decomposition texture of phengite into symplectites (Palmeri et al., 2003;Kim and Lee, 2023) is absent in the manganiferous quartzite sample (Fig. 4).This suggests that the quartzite escaped significant retrogression during rapid decompression demonstrated in the surrounding garnet-phengite gneisses (Di Vincenzo and Palmeri, 2001).The aligned bands of Kfeldspar and albite of near-end-member compositions parallel to the major foliation (Fig. 4a, b, c) were likely produced after the porphyroblastic epidote-group mineral or phengite during the latest stage of retrogression under ∼ 300 °C (e.g., Kroll et al., 1993).The presence of two feldspars is similar to that with the low-P greenschist-facies assemblages in the late-stage fracture of the Praborna ore deposits (Tumiati et al., 2010).
The pseudosection calculation allowed us to delineate the evolution of and linkage among mineral assemblage, bulkrock composition and oxidation state of the manganiferous quartzite (Fig. 10).The epidote-group mineral-garnetquartz-hematite assemblage is stable under highly oxidized conditions above the hematite-magnetite buffer.The FMQ values (+6.96 to +15.76 and +8.26 to +14.60) of the oxidized conditions exceed those typically observed in normal subduction settings (see Tumiati et al., 2015, and references therein) but are lower than those represented by the braunitebearing assemblages (Tumiati, 2005;Tumiati et al., 2015).The Mn-rich, quartz-free assemblages including tephroite, which occurs as inclusions inside the epidote-group mineral (Fig. 6), are stable under relatively reduced conditions ( FMQ < 2.25; Fig. 10), comparable with those of the hausmannite-rhodochrosite-bearing assemblages involving the quartz-free domain of the upper levels at Praborna (Tumiati et al., 2015).Taken together, the reactive bulk-rock composition changed from Mn-rich (quartz-undersaturated, tephroite-stable) to silica-rich (quartz-present, epidote-group mineral-stable).This scenario is in harmony with the Fe-Mn nodules which would have been embedded in the chert-dominated protolith and likely homogenized with the quartzite matrix (Reinecke et al., 1985).
The metamorphic zircon age of ca.506 Ma overlaps with those of the eclogite-facies peak stages recorded in the high-P rocks of the Lanterman Range, such as ca.500 Ma of the boudin-shaped mafic eclogites (Di Vincenzo et al., 1997;Kim et al., 2019) and megacrystic phengite-bearing quartz vein (Kim and Lee, 2023) but is apparently younger than the ca.530-515 Ma of prograde stages of the colder eclog-ites and garnet-phengite quartzofeldspathic schists/gneisses (Di Vincenzo et al., 2016) (Fig. 3).The timing of the peak metamorphism is noteworthy because the juxtaposition of the Wilson and Bowers terranes also took place in the middle Cambrian in the context of the Ross orogeny (Fig. 1) (e.g., Di Vincenzo et al., 1997;Rocchi et al., 2011).Finally, we note the common depositional setting, i.e., arc/back arc, of the metachert layers and enclosing continent-derived quartzofeldspathic schists/gneisses (e.g., Palmeri et al., 2003).The deficiency of arc-derived detrital zircons in the manganiferous quartzite (Fig. 12) suggests that the deposition of the chert-like protoliths took place distally from the leading edge of the downgoing lithosphere.This paleo-position of the peculiar high-P metasedimentary rock is more amenable to exhumation in comparison to the leading edge, as suggested by numerical modeling (Warren et al., 2008), because crustal materials of the leading edge are commonly thin and cold and thus easily resist detachment.It is also consistent with the predominance of the siliciclastic metasedimentary rocks which facilitates buoyancy-driven detachment and exhumation (e.g., Ernst, 2001).

Conclusions
Our petrological, geochemical and geochronological results, combined with a compilation of available data on Mn-rich quartzose rocks from the literature, allowed us to characterize the sedimentary protolith and high-P metamorphism of the manganiferous quartzite layer in the Ross orogen, Antarctica.High-P metamorphism is represented by the Mn 3+ -REY-rich epidote-group mineral-Mn 2+ -rich garnetphengite-bearing assemblages; porphyroblasts of the former mineral exhibit multiple growth zones, attesting to prograde to peak evolution in a highly oxidized environment.The protolith is most likely chert mixed with pelagic clay and Fe-Mn nodules, which is silica-rich and highly oxidized with a strong positive Ce anomaly.The reactive bulkrock composition changed from Mn-rich and silica-poor to quartz-rich composition.This results in a transition from a reduced tephroite-bearing mineral assemblage to an oxidized epidote-group mineral-bearing assemblage.Zircon U-Pb ages dated the maximum age of deposition and timing of the peak eclogite-facies metamorphism (M 3 ) to be the latest Neoproterozoic (ca.546 Ma) and middle Cambrian (ca.506 Ma), respectively.The deep-sea sedimentary protolith shares a common depositional setting with the surrounding schists/gneisses of siliciclastic composition, most likely in an arc/back-arc setting of a continental margin.A lack of the Ross-arc detrital zircons further suggests that the deposition of the chert-like protolith took place distally from the leading edge of the subducting plate.This paleo-position of the protolith is amenable to exhumation of the manganiferous quartzite.

Figure 1 .
Figure 1.(a) Schematic map exhibiting the distribution of the Terra Australis orogen along the Gondwana margin.The Antarctic segment of the orogen is the Ross orogen of the Transantarctic Mountains.A box denotes the location of northern Victoria Land (NVL) enlarged in (b).(b) Tectonic province map of NVL modified from Läufer et al. (2011), showing key lithotectonic elements including three terranes and two intervening metamorphic belts.The region in the black box is enlarged in Fig. 2a.The post-Ross rocks were omitted for clarity.

Figure 2 .
Figure 2. (a) Geological map of the Lanterman Range, northern Victoria Land, modified from GANOVEX Team (1987) and Talarico et al. (1998), put on a © Google Earth image.A blue star denotes the Eclogite Ridge where a manganiferous quartzite sample was collected (70°40.56S, 163°27.726E). (b, c) Outcrop photographs showing field occurrence of the quartzite and associated mafic-felsic layers.A pinkish quartzite horizon intercalated with (garnet) amphibolite and quartzofeldspathic schist layers is in (b).Complex fold structures and alternations of the mafic and felsic rocks are consistent with pervasive ductile deformation.A box is enlarged in (c).Medium-grained quartzite showing pinkish garnet, brownish phengite and dark titanohematite (Hem) embedded in the quartz-dominant matrix is in (c).Mineral abbreviations are after Whitney and Evans (2010).

Figure 4 .
Figure 4. Photomicrographs (a-c) and backscattered electron image (d) of manganiferous quartzite sample 23-1F showing mineral parageneses and occurrences of porphyroblasts.Photomicrographs were taken under cross-polarized light (a) and plane-polarized light (b, c).White arrows denote albite-K-feldspar-rich layers.(a) Representative mineral assemblage mainly consisting of garnet, epidote-group mineral (Epgr.min.),phengite, titanohematite (Hem) and quartz.(b) Subhedral epidote-group mineral porphyroblast zoned in Mn 3+ .The X-ray chemical maps of the grain are shown in Fig. 6.Porphyroblasts of subhedral Grt I, flaky phengite and titanohematite are also shown.(c) Xenoblastic Grt II partly rimmed by chlorite.The X-ray chemical maps of the grain are shown in Fig. 5b.(d) Subhedral, fine-grained Grt III.The grains are elongate in shape and oriented subparallel to the major foliation (horizontal).Mineral abbreviations are after Whitney and Evans (2010).

Figure 6 .
Figure 6.Backscattered electron (BSE) image, X-ray maps and compositional profile of a subhedral porphyroblast of epidote-group mineral.Four distinct compositional domains are defined on the basis of Mn 3+ , Fe 3+ , and rare earth element and yttrium (REY) contents: inner domain (i; piemontite and epidote), outer zone 1 (o1; REY-rich piemontite), outer zone 2 (o2; REY-rich epidote) and outermost domain (om; epidote).A box in the BSE image shows tephroite inclusions enlarged in the lower-right part of the image (asterisks).White arrows in the Mg map denote the celadonite zonation of phengite.Mineral abbreviations and epidote-group mineral nomenclatures are after Whitney and Evans (2010) and Armbruster et al. (2006), respectively; apfu, atoms per formula unit.

Figure 9 .
Figure9.Lattice preferred orientations of quartz in quartzite sample 23-1F.The east-west direction corresponds to lineation (X), and the north-south direction is normal to foliation (Z).J , J index; M, M index; n, number of grains.

Figure 12 .
Figure 12.(a) Representative cathodoluminescence images of zircon from manganiferous quartzite sample 23-1F.Spot numbers for the U-Pb isotopic analyses listed in Table S3 are shown for individual zircon grains, followed by their Th/U ratios in parentheses.The bottom lines list the 206 Pb/ 238 U or 207 Pb/ 206 Pb dates (in Ma).Ellipses denote the SHRIMP analytical spots.(b, c) Tera-Wasserburg concordia diagrams showing U-Pb isotopic compositions of zircon.Error ellipses are 2σ .Dashed ellipses denote spot analyses that are excluded from the mean age calculation.Inset diagram in (c) shows the error bars (in 2σ ) of each 206 Pb/ 238 U date for the weighted mean age calculation; MSWD, mean square weighted deviation.