Electron-microprobe Th–U–Pb monazite dating in Early-Palaeozoic high-grade gneisses as a completion of U–Pb isotopic ages (Wilson Terrane, Antarctica)

    Electron-microprobe Th-U-Pb monazite dating in Early-Paleozoic high-grade gneisses as a completion of U-Pb isotopic ages (Wilson Terrane, Antarctica) B. Schulz, U. Sch¨ussler PII: DOI: Reference:

S0024-4937(13)00169-2 doi: 10.1016/j.lithos.2013.05.008 LITHOS 2991

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LITHOS

Received date: Accepted date:

18 May 2012 16 May 2013

Please cite this article as: Schulz, B., Sch¨ ussler, U., Electron-microprobe Th-U-Pb monazite dating in Early-Paleozoic high-grade gneisses as a completion of U-Pb isotopic ages (Wilson Terrane, Antarctica), LITHOS (2013), doi: 10.1016/j.lithos.2013.05.008

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Electron-microprobe Th-U-Pb monazite dating in Early-Paleozoic high-grade gneisses

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as a completion of U-Pb isotopic ages (Wilson Terrane, Antarctica)

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B. Schulz a*, U. Schüssler b

Institut für Mineralogie, TU Bergakademie Freiberg/Sachsen, Brennhausgasse 14, D-09596

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Freiberg, Germany, [email protected]

Institut für Geographie und Geologie, Universität Würzburg, Am Hubland, D-97074

Würzburg, Germany, [email protected]

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*corresponding author

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Abstract: The electron microprobe (EMP) Th-U-Pb monazite bulk chemical dating method was applied to granulite-facies rocks of the Wilson Terrane in Antarctica. A combination of

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this method to isotopic U-Pb-SHRIMP ages for the evaluation of metamorphic processes

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required the analysis of reference monazites. These can be subdivided into three groups: a) Monazite with variable total Pb at constant Th, (e.g.VK-1), is unsuitable for EMP data evaluation. b) Monazite with highly variable total Pb and Th, but with at least some Th/Pb approximating an apparent isochrone, (e.g. MPN), is partly useful. c) Monazite with constant Th/Pb at high Th, (e.g. Madmon monazite) is best suitable for the combined approach and can be additionally used to improve the Th calibration for EMP. Study of monazite in grain mounts and in thin sections led to partly different but complementary results: Older monazites with EMP ages up to 680 Ma occur mainly in a grain mount from diatexite and metatexite and are interpreted as detrital relics. Some of these monazites show structures and mineralchemical zonation trends resembling metasomatism by alkali-bearing fluids. A marked mobility of Th, P, Ce, Si and U is observed. The age of the metasomatic event can be

ACCEPTED MANUSCRIPT bracketed between 510 and 450 Ma. Furthermore, in the grain mount and in numerous petrographic thin sections of migmatites and gneisses, the EMP Th-U-Pb and SHRIMP U-Pb

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monazite data uniformly signal a major metamorphic event with a medium-pressure granulite

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facies peak between 512 and 496 Ma. Subsequent isothermal uplift and then amphibolitefacies conditions between 488 and 466 Ma led to crystallisation of pristine monazite. The

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than 600 km along strike in the Wilson Terrane.

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high-grade metamorphic event, related to the Ross Orogeny, can be uniformly traced more

Keywords: monazite metasomatism; electron microprobe Th-U-Pb dating; granulite facies;

Introduction

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Early Paleozoic

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The in-situ "chemical" Th-U-Pb dating of monazite by electron microprobe (EMP) analysis

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(CHIME monazite dating, Montel et al., 1996) has become a well evaluated and common tool for the dating of protolith ages in leuco-granites (Djouka-Fonkwe et al., 2008) and, more important, for the temporal resolution of thermal events in metamorphic terranes (Finger et al., 2002; Fitzsimons et al., 2005; Krenn and Finger, 2007; Pyle et al., 2005). The potential of the EMP-monazite dating method has been demonstrated by numerous case studies in polymetamorphic greenschist- and amphibolite-facies terranes (Schulz et al., 2007a; Schulz and von Raumer, 2011). The decomposition of igneous or metamorphic monazite into apatite and epidote (Finger et al., 1998, Spear, 2010) and its recrystallisation with different compositions (Finger et al., 2002) have further potential to recognize and to date postmagmatic and retrogressive processes. Particularly, monazite can crystallize and/or undergo alteration during hydrothermal or metasomatic events (Poitrasson et al., 1996;

ACCEPTED MANUSCRIPT Schandl and Gorton, 2004; Harlov and Hetherington, 2010; Harlov et al., 2011). As a consequence, monazite could be used for the dating of deformational processes if fluids also

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participate (Just et al., 2010; Williams et al., 2011).

At temperatures exceeding 600 - 700 °C, it would appear that new monazite can form through

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the recrystallization (dissolution-reprecipitation) from previous magmatic and amphibolitefacies metamorphic grains. Such high-temperature replacement of monazite through monazite

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often leads to significant element fractionations on a small scale, creating composite grains with complex zoning patterns (Harlov et al., 2007; 2011). Unreacted domains can remain as relic cores (Finger and Krenn, 2007). This implies that monazite has the potential to record several thermal events even in high-grade and granulite-facies terrains. In granulite-facies

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rocks, monazite cores or inclusions in garnet porphyroblasts can preserve ages from earlier

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stages of metamorphic evolution. Furthermore, the metamorphic overprint at lower grades in such terrains can be recognised by EMP-monazite dating. On these grounds, EMP monazite

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dating has repeatedly been applied in Precambrian granulite-facies terrains (e.g. Braun et al.,

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1998; Cocherie et al., 1998; Ayers et al., 1999; Montel et al., 2000; Mahan et al., 2006; Motoyoshi et al., 2006; Simmat and Raith, 2008).

Although a considerable and growing list of successful applications exists, EMP-monazite Th-U-Pb dating is a non-isotopic method which has its limitations and pitfalls, especially with the respect to the large error on the ages, as summarised by Williams et al. (2006), Jercinovic et al. (2008) and Spear et al. (2009). In igneous and meta-igneous rocks, the various in-situ UPb isotopic dating methods of zircon by SHRIMP and LA-MC-ICPMS appear to be the preferred choice. In overprinted high-grade metapelites, detrital igneous zircon may not have recorded successive metamorphic subsolidus thermal events. In this situation, the isotopic insitu U-Pb isotopic dating of monazite by SHRIMP and LA-MC-ICPMS provides an

ACCEPTED MANUSCRIPT alternative (e.g. Catlos et al., 2002). However, these alternatives also have their limits, as evidenced by complex calibration procedures, reduced spatial resolution, instrument access

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and availability, and cost. As a consequence it appears interesting to apply both approaches, i.

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e. non-isotopic EMP-monazite Th-U-Pb and SHRIMP U-Pb isotopic dating, in a complementary way. As discussed in this study, the most challenging task is the comparison

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and calibration of the EMP-Th-U-Pb method to the SHRIMP and TIMS U-Pb monazite data. This approach would allow age data in-situ from single monazite grains to be gained and also

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a much larger number of samples at the regional scale of a metamorphic terrane to be measured. As a consequence, the geodynamic interpretation of the age data is ascertained not only by precision and single observation but also on process-based mineral-chemical trends

Geology and sample provenance

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and sampling on a regional scale.

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The Wilson Terrane, about 600 km in longitude, is located in northern Victoria Land and Oates Coast (sometimes also named the Oates Land) at the Pacific margin of the Transantarctic Mountains (Fig. 1). It belongs to the Late Pan-African Ross Orogenic Belt and has been interpreted as the active continental margin along the Paleo-Pacific coast of the East Antarctic Continent in the Early Paleozoic (e.g. Tessensohn, 1997). The dominating metasedimentary sequences were formed under varying metamorphic conditions and were extensively intruded by the syn- to late-orogenic Granite Harbour Intrusives. The northernmost part of the Wilson Terrane was basically investigated for the first time during the German Antarctic North Victoria Land Expeditions GANOVEX V and VII in 1988/89 and 1992/93. There, a subdivision of the crystalline basement into three different NNW-SSE trending zones was recognized: a central zone with granulite-facies gneisses and migmatites is

ACCEPTED MANUSCRIPT flanked by one eastern and one western zone with gneisses which were formed under lower amphibolite- to lowest granulite-facies conditions (Schüssler, 1996; Schüssler et al., 1999;

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2004). The zones are confined by the prominent Wilson, Exiles, and Lazarev thrust systems

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(Flöttmann and Kleinschmidt, 1991; 1993; Flöttmann et al., 1993; Läufer et al., 2006) except

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the boundary between the eastern and central zone which is still of unclear character.

The petrography of the metamorphic rocks in the northern Wilson Terrane and estimates of

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metamorphic conditions have been detailed in Schüssler et al. (2004): Metamorphic rocks of the central zone were formed in the course of one single clockwise P-T evolution including a medium-pressure/high-temperature granulite-facies stage at about 8 kbar and >800 °C, a subsequent isothermal decompression, and a final amphibolite-facies stage with retrograde

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formation of biotite and muscovite gneiss. In the eastern and western zones the majority of

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metamorphic rocks experienced clockwise oriented P-T paths at somewhat lower P-T conditions of about 4 - 5.5 kbar and 700 – 800°C. While some parts of both zones did not

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reach the upper stability limit of biotite + sillimanite and muscovite + quartz, granulite-facies

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rocks detected locally in the western zone were formed under P-T conditions similar to those of the central zone.

Previously, monazite U-Pb isotopic data have been published from four samples collected in the central, granulite-facies zone of the Wilson Terrane. Two of them were analysed by SHRIMP (Henjes-Kunst et al., 2004), the other two by TIMS (Schüssler et al., 1999; 2004). The SHRIMP analysed samples are G8-57.4 from Mt. Blowaway, a leucocratic diatexite consisting of biotite + K-feldspar + plagioclase + quartz, and G8-58.1 from Mt. Dalton, a metatexite of garnet + biotite + K-feldspar + plagioclase + quartz. Both samples are mediumto coarse-grained, but contain fine-grained secondary muscovite. For recent EMP Th-U-Pb dating, the monazite grain mount used in the U-Pb SHRIMP study by Henjes-Kunst et al.

ACCEPTED MANUSCRIPT (2004) was re-investigated. It contains hundreds of large monazite grains from the samples mentioned above, together with monazite SHRIMP reference standards. Samples for TIMS

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are US-380, a migmatitic gneiss from Thompson Peak with a garnet + biotite + sillimanite +

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K-feldspar + plagioclase + quartz mineral assemblage, with sillimanite included in garnet, and US-501, a diatexitic migmatite from Mt. Archer with the mineral assemblage garnet + biotite

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+ sillimanite + cordierite + hercynite + K-feldspar + plagioclase + quartz, with sillimanite and hercynite included in garnet. For the EMP Th-U-Pb dating of these samples in the present

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study, monazites in polished thin-sections were measured.

In addition, monazites in seven metapsammitic to metapelitic paragneiss and migmatite samples from the northern Wilson Terrane (locations are indicated in Fig. 8, descriptions are

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detailed in Henjes-Kunst and Schüssler, 2003) were newly analysed by the EMP Th-U-Pb

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dating method. These are the samples US-227 from Mt. Shield, and US-273 (Mt. Gorton), both migmatites from the eastern zone of the Wilson Terrane with biotite + cordierite ±

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sillimanite + K-feldspar + plagioclase + quartz, with sillimanite included in cordierite, and

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with retrograde formation of muscovite. Sample US-242b (Mt. Send) is a xenolith of the same petrographic type which occurs in the Granite Harbour granite, to the East of the Wilson Thrust. Samples US-387, US-388 and US-477b are from DeRemer Nunataks, Stanwyx Ridge, and Celestial Peak, respectively, in the central zone. They are granulite-facies migmatitic gneisses and migmatites with garnet + biotite ± cordierite ± sillimanite ± hercynite + Kfeldspar + plagioclase + quartz. Sillimanite occurs as inclusions in cordierite and garnet. US378 was collected in the Rescue Nunataks in the western zone and contains the mineral assemblage of biotite + sillimanite + muscovite + plagioclase + quartz. Metamorphic conditions are therefore within the stability fields of muscovite + quartz and biotite + sillimanite, i.e. in the upper amphibolite-facies.

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locations in the Wilson Terrane were analysed by Th-U-Pb EMP on polished thin-sections.

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Some sampling locations are also along the Oates Coast region in the northern part of the Wilson Terrane (Fig. 1). In the Daniels Range a single metamorphic event with a strongly

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increasing grade is observed in the Penseroso Bluff and Thompson Spur (Kleinschmidt, 1981; Läufer et al., 2006). Metamorphic conditions in the metasediments collected by Schubert

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(1987) and Ulitzka (1987) increase from a low-to-medium grade andalusite zone in the west to a high-grade sillimanite zone with migmatites to the east. Amphibolite-facies gneisses in the Kavrayskiy Hills to the east of the Wilson Thrust show no signs of migmatisation (Schubert et al., 1984). In the northern part of the Lanterman Range, the studied samples are

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amphibolite-facies biotite gneisses with sillimanite (Roland et al., 1984; Talarico et al., 1998;

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Henjes-Kunst and Schüssler, 2003). These samples show no further indications of high-

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pressure metamorphism as described from other parts of the region (Di Vincenco et al., 1997).

Protocol for EMP monazite analysis

EMP-monazite Th-U-Pb dating is based on the observation that concentrations of common Pb in monazite (LREE, Th)PO4 are negligible when compared to radiogenic Pb resulting from the decay of Th and U (Parrish, 1990; Montel et al., 1996; Cocherie et al., 1998; Cocherie and Albarede, 2001). Diffusion experiments by Cherniak et al. (2004) demonstrated that monazite is the most retentive of Pb among accessory minerals. As Th concentrations in magmatic and metamorphic monazite are commonly high (3 - 14 wt %), a sufficient amount of radiogenic Pb, detectable by electron microprobe analysis, can accumulate in monazite within > 100 million years. As a consequence, in-situ electron microprobe analysis of the bulk Th, U and

ACCEPTED MANUSCRIPT Pb concentrations in monazite without the discrimination between Pb and U isotopes, allows for the calculation of a chemical model age (CHIME) with a considerable error (Suzuki et al.,

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1994; Montel et al., 1996; Rhede et al., 1996; Dahl et al., 2005; Pyle et al., 2005; Jercinovic

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and Williams, 2005; Williams et al., 2006; Jercinovic et al., 2008; Suzuki and Kato, 2008;

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Spear et al., 2009).

In-situ analysis of Th, U, and Pb for the calculation of monazite model ages, as well as for Ca,

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Si, LREE, and Y for the correction and evaluation of the mineral chemistry were carried out for two samples in the monazite grain mount and for 50 thin sections using a JEOL JXA 8200 at Geozentrum Nordbayern of the University Erlangen-Nürnberg. The Mα1 lines for Th and Pb and the Mβ1 line for U were measured on the same PETH crystal in the sequence Th-U-

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Pb. For the analysis of the Pb Mα1 line, the background positions were evaluated by linescans

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on samples of the Madmon monazite with known compositions (Schulz et al., 2007b), vanadinite, REE-orthophosphates, and a Paleoproterozoic monazite with PbO contents at ~3

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wt %. A negative background position far from the Pb-Mα1 peak (-9.650 mm in JEOL

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position scale), was chosen. A suitable choice of the positive background position also far from the Pb-Mα1 peak (+7.500 mm) minimizes the problem of uncertainty of possible background curvature at the Pb-Mα1 position. Combined with a long counting time on background, this provided the best results with regard to the arguments on background positions described by Williams et al. (2006), Jercinovic et al. (2008), and Spear et al. (2009). Repeated control of background positions on Madmon revealed stable counting rates for long periods of time. As a consequence, no modifications and composition-dependent modeling of the background curvature were made. A linear interpolation was applied for the comparison of monazites with different Paleozoic ages. The measurements were performed during several sessions with the same operating conditions and using the same spectrometer and crystal for each element. Resulting absolute errors (2) at 20 kV acceleration voltage, 100 nA beam

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Th. Error for Pb increases significantly when measurement times on peak and background are

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reduced. The chosen counting times for Pb provided a balance among the error and the stability of monazite under the electron beam. The lines Lα1 for La, Y and Ce, Lβ1 for Pr,

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Sm, Nd and Gd, and Kα1 for P, Si and Ca were chosen. Orthophosphates from the Smithsonian Institution were used as standards for REE analysis (Jarosewich and Boatner,

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1991; Donovan et al., 2003). Calibration of PbO was carried out on a vanadinite standard. The U was calibrated on an appropriate glass standard with 3.85 wt% UO2. The age of the Madmon monazite (Schulz et al., 2007b), dated by SHRIMP at 496 ± 9 Ma and by numerous Pb-Pb-TIMS monazite evaporation data (K. Bombach, Freiberg, unpublished analytical

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method) at 497 ± 2 Ma, was also determined at 503 Ma by the EMP monazite dating routines

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established at facilities in Salzburg and BRGM Orléans (Cocherie et al., 1998; Finger and Helmy, 1998). The Madmon contains ThO2 at around 11 wt%, as determined by ICP-OES,

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LA-ICPMS and by electron-microprobe analysis at University of Salzburg. Madmon was used

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for calibration and offline re-calibration of ThO2 as well as for the control of data. Interference of YLγ on the PbM line was corrected by linear extrapolation after measurement of several standards with different Y-contents, as proposed by Montel et al. (1996). An interference of ThMγ on UMβ was also corrected by using a Th-glass standard. Interference of a Gd-line on UMβ needs correction when Gd2O3 in monazite is > 5 wt %. These parameters matched the analytical problems and limits of the method discussed in Williams et al. (2006), Jercinovic et al. (2008), Suzuki and Kato (2008), and Spear et al. (2009).

The monazite chemical model ages were determined by following two approaches. First, for each single analysis, an age was calculated using the equations given by Montel et al. (1996).

ACCEPTED MANUSCRIPT The error resulting from counting statistics was typically on the order of ± 20 to ± 40 Ma (1) for Early Paleozoic ages. Using these apparent age data, weighted average ages for monazite

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populations in the samples were then calculated using Isoplot 3.0 (Ludwig, 2001) and are

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interpreted as the time of closure for the Th-U-Pb system of monazite during growth or recrystallisation in the course of metamorphism. Second, the ages were determined using the

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ThO2*-PbO isochrone method (CHIME) of Suzuki et al. (1994) and Montel et al. (1996), where ThO2* is the sum of the measured ThO2 plus ThO2 equivalent to the measured UO2.

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The age is calculated from the slope of the regression line in ThO2* vs PbO coordinates forced through zero. In all analysed samples, the model ages obtained by the two different

Results

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methods agree exceptionally well.

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4.1 SHRIMP-dated reference monazites analysed by electron microprobe Th-U-Pb

A grain mount block containing more than 100 single monazite grains plus reference monazites (Henjes-Kunst et al., 2004) and the Madmon reference monazite (Schulz et al., 2007b) were previously analysed using the SHRIMP II at Curtin University of Technology at Perth. The two reference monazites on the grain mount block are labelled MPN and VK-1. As outlined in Henjes-Kunst et al. (2004), the MPN and VK-1 reference monazites were used for calibration of the U/Pb and Th/Pb ratios as well as the U and Th abundances in the SHRIMP analytical method for monazite, closely following Foster et al. (2000). The MPN reference monazite is of metamorphic origin with an age of 2150 Ma. It has 6 wt% Th and 0.05 wt% U which are similar to most of the sample grains (Rasmussen and Fletcher, 2002). The reference monazite VK-1 is of magmatic origin. It was reported to have an age of 488 Ma and Th

ACCEPTED MANUSCRIPT abundances of 14.75 wt% and 5.8 wt%, respectively (c.f. Rasmussen et al., 2002). Analytical details of these reference monazites are given in the electronic appendix in Henjes-Kunst et

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al. (2004). The Madmon reference monazite is from a pegmatite in Madagascar. A SHRIMP

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age has been determined at 496 ± 10 Ma. According to LA-ICPMS and wet chemical analyses

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the Th ranges around 11 wt% and the U is about 0.3 - 0.4 wt% (Schulz et al., 2007b).

The reference monazites (Fig. 2, Table 1) were analysed by electron microprobe following the

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protocol reported above. They display a different appearance in backscattered electron (BSE) imaging. Madmon-D shows a homogeneous shade of gray with large inclusion-free domains, which are separated by larger cracks associated with alteration (Fig. 2a). These altered parts were avoided during analysis. Mean values from 30 analyses through the sessions are PbO

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0.25 wt%, ThO2 10.44 wt%, UO2 0.41 wt% and Y2O3 0.99 wt%, yielding a well-defined

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weighted average age of 504 ± 6 Ma. All mounted grains of the MPN monazite display large domains of irregular shape with signs of alteration, inclusions, and cracks (Fig. 2b). The

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means of 80 analyses are PbO 0.52 wt%, ThO2 5.70 wt%, UO2 0.08 wt%, and Y2O3 0.05

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wt%. In domains with visible alterations the Th-U-Pb ages are younger (< 2000 Ma) than in the unaltered domains with ~2000 - 2100 Ma (Fig. 2b). A weighted average age, based on 50 selected analyses in domains without alteration is 2113 ± 12 Ma. The VK-1 monazite shows a homogeneous gray surface with no signs of alteration or inclusions (Fig. 2c). Judging from the perfect appearance of the VK-1 grains in the BSE image, one would expect a homogeneous age and element distribution. This is supported by a very narrow range in Y2O3 around 2.62 wt%, UO2 at 0.72 wt%, and ThO2 at 14.59 wt%. In contrast, the PbO displays considerable variation between 0.19 and 0.36 wt% with a mean of 0.263 wt%. Correspondingly, the Th-U-Pb ages are highly variable, resulting in a poorly defined weighted average of 368 ± 38 Ma.

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XCher, the VK-1 has a high XCher, and the MPN displays considerable variability for both

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endmembers (Fig. 2d - f). Similarly in terms of Th+U vs Ca and Th+U vs Si, the MPN and Madmon follow a huttonite substitution trend, whereas the VK-1 displays a cheralite

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substitution trend.

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The relationships among Th, U, and radiogenic Pb in monazite are important for the EMP-ThU-Pb dating method and can be evaluated in the U/Pb-Th/Pb diagram proposed by Cocherie and Albarède (2001). In this diagram the analyses from Madmon-D are arranged and centered on an isochrone (Fig. 2g). Data from unaltered domains in the MPN monazite are also

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arranged along an isochrone, whereas the data from the altered domains are distributed across

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several isochrones. Data from monazite VK-1 are distributed without clustering and do not allow for definition of an isochrone (Fig. 2g). This situation is also obvious in the ThO2* -

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PbO diagram proposed by Suzuki et al. (1994) and Montel et al. (1996). Data from Madmon-

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D are situated on an isochrone forced through zero (Fig. 2h). Analyses from the unaltered domains in MPN also define an isochrone and a weighted average age at 2113 ± 12 Ma which is lower than the nominal SHRIMP age of 2150 Ma reported in Rasmussen and Fletcher (2002). The analyses from altered parts display similar ThO2* at lower PbO. This signals a partial loss of Pb or monazite recrystallization during the alteration process. However, it appears possible that some of the analyses selected for the calculation of the isochrone also underwent subtle Pb loss. One should also be aware that the excitation bulb of the electron beam may hit altered domains beneath the polished surface of the grains. The SHRIMP age of MPN can be approached by calculation of EMP ages after rigorous elimination of analyses with comparably lower PbO. The ThO2*-PbO data from VK-1 are not suitable for the determination of a Th-U-Pb age. The calculation of the weighted average age of 368 ± 38 Ma

ACCEPTED MANUSCRIPT is a pure arithmethic mean which is not supported by a cluster or linear arrangement of data

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points along an isochrone.

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As a consequence, natural reference monazites involved in the calibration and adjustment of the monazite U-Pb SHRIMP or U-Pb SIMS age dating protocols are not necessarily suitable

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as standards for Th-U-Pb EMP monazite dating. Reference monazites with large variation in PbO at constant Th + U are inadequate as exemplified by monazite VK-1. Its apparently

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perfect appearance under BSE imaging is no indication for its acceptability as a reference material. The MPN monazite can be used under the restriction that only the unaltered domains are considered. Judging from its ThO2* - PbO characteristics, monazites like the Madmon appear suitable for the correlation of U-Pb isotopic and Th-U-Pb bulk chemical age dating

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SHRIMP-dated monazites from migmatites analysed by EMP-Th-U-Pb

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4.2

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methods.

The monazite grain mount used in the U-Pb SHRIMP study by Henjes-Kunst et al. (2004) contains hundreds of large monazite grains from a diatexite (sample G8-57.4, Mt. Blowaway) and a metatexite (G8-58.1, Mt. Dalton) from the northern Wilson Terrane. In BSE imaging mode at an acceleration voltage of 20 kV (SEM Quanta 600 FEG by FEI), monazite from sample G8-57.4 displays a patchy distribution of domains. There are no sharp boundaries but rather diffuse transitions between the different domains (Fig. 3a - c). The monazites from sample G8-58.1 also display a patchy distribution of domains. However, the contrasts between the domains are often sharp and the domain boundaries tend to have a cauliflower shape (Fig. 3d - f).

ACCEPTED MANUSCRIPT The overall mineral chemical characteristics of the monazites from both samples also display some differences (Table 1). Monazite analyses from sample G8-57.4 plot in a closed linear

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cluster with an accumulation in the intermediate part in the XHutt-XCher diagram (Fig. 4a).

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This contrasts with a more scattered distribution characterised by a marked cluster at low XHutt in data from sample G8-58.1 (Fig. 4d). These differences are also obvious in the Th+U

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vs Ca and Th+U vs Si diagrams (Fig. 4b, e). The diagram XGdPO4 vs XYPO4 was suggested by Pyle et al. (2001) for the evaluation of monazites in metapelites belonging to distinct

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mineral zones of regional metamorphism. Under these coordinates the monazites from G857.4 display a straight cluster at comparably low XGdPO4 and XYPO4 (Fig. 4c). Monazites from sample G8-58.1 cover a large cluster from low to high XGdPO4 and XYPO4 with an accumulation at XGdPO4 of 0.02 and XYPO4 of 0.04 (Fig. 4f). Judging from the comparison

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of monazite compositions in this diagram in Pyle et al. (2001) where XYPO4 increases with

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metamorphic grade, the monazites in sample G8-58.1 should have (re)crystallised during a

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migmatisation at higher metamorphic grades as in sample G8-57.4.

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Single grains from samples G8-57.4 and G8-58.1 display a considerable variation in the ThU-Pb EMP ages. In monazite grains with subtle and transitional domains in the BSE image, one can observe a large variation from 638 to 455 Ma (Fig. 3a). Other grains display a narrower range (Fig. 3b) or a quite homogeneous distribution of ages, which allowed for the calculation of weighted averages (Fig. 3c). Apart from a large variation in single analyses between 647 and 405 Ma in sample G8-58.1, single grains with a quite homogeneous age distribution around 630 ± 32 Ma (Fig. 3d), 530 ± 24 Ma (Fig. 3e) and 477 ± 17 Ma (Fig. 3f) occur. When compiled in the ThO2*-PbO diagrams (Fig. 5) one can distinguish an older group of grains with ages of 658 - 619 Ma from a younger group at 512 - 484 Ma. Data points belonging to these groups show a large scatter. The isochrones are not sharply defined, with low values of R2 and high MSWD of weighted average ages. However, the overall trend is

ACCEPTED MANUSCRIPT clear, as the older 658 - 619 Ma and the younger 512 - 484 Ma groups are also defined by single grains (Fig. 5a, b, d, e). In both samples there are also examples with a continuous

Monazite with signatures of metasomatism by alkali-bearing fluids

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distribution of ages between the older and younger groups (Fig. 5c, f).

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Many of the large monazite grains in sample G8-58.1 show complex internal structures in the BSE images (Fig. 3d - f, 6a). As described from other monazites in migmatites (Cocherie et al., 2005), one observes dark rim domains with cauliflower-like embayments into light gray domains. Also more complex distributions of light and dark domains, always with an

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embayment of the dark zones into the light gray zones can be found (Fig. 3e). Quite similar

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internal structures have been reported from monazites which were experimentally metasomatised by alkali-bearing fluids (Harlov et al., 2011; Williams et al., 2011). These

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experimental observations are supported by distinct element variations in the monazites. They

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were explained by a dissolution-reprecipitation process during a fluid-mediated partial alteration (Harlov and Hetherington, 2010; Harlov et al., 2011; Williams et al., 2011). Dependent on the fluid compositions and conditions, the experiments with sodium- and fluorine-bearing fluids by Williams et al. (2011) led to dark zones with loss of Th and Ca, whereas the experiments by Harlov et al. (2011) led to light alteration zones with enrichment of Th and Si and depletion of Ce and P. As both types of experimental fluid-mediated alterations concern the Th contents, the authors suggested that this could offer means for recognition of and potentially dating fluid-rock interaction.

A set of 16 monazite grains, most of them with the typical structures described above, represents a minority population and has been selected for detailed presentation. The ThO2* -

ACCEPTED MANUSCRIPT PbO data from these monazites are enclosed in Fig. 5f. The dark and light domains have a sharp border. When the single point EMP analyses are plotted versus ThO2, the dark domains

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have comparably low ThO2 from 6 - 9 wt%. The light domains have ThO2 from 9 - 19 wt %

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(Fig. 6b - g). As analyses next to the dark-to-light boundaries were avoided, an apparent gap of ThO2 compositions appears around 9 wt%. The monazite Ce2O3 and P2O5 contents are high

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in the dark domains and decrease with increase of ThO2 (Fig. 6b, c). In contrast, the SiO2 and UO2 are low in the dark domains and increase with the ThO2 in the light domains (Fig. 6d, e).

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In the light domains, the CaO at about 1 wt% is constant with increase of ThO2. In the dark domains the CaO is slightly higher (1.5 wt%) as in the light domains (Fig. 6f). For Y2O3 one observes a considerable variation (0.5 to 3.5 wt%) in the dark domains. In the light domains the Y2O3 is less variable (1.0 - 2.5 wt%) and no distinct trend with increasing ThO2 can be

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recognised (Fig. 6g). The mineral-chemical zonations from dark to light domains in single

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monazite grains confirm these bulk observations. ThO2 and SiO2 significantly decrease, and P2O5 and Ce2O3 increase from light domains in the monazite cores towards dark domains in

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the rims (Fig. 6h, i). For CaO and UO2 the single zonation profiles display both cases with

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decreasing as well as increasing contents from light to dark domains (Fig. 6h, i). For control, a dark grain with no visible light domains has been analysed. The concentrations are constant and similar as in the dark domains of the zoned grains (Fig. 6k). It has been checked if the dark and light domains are related to distinct EMP-Th-U-Pb monazite age populations. In single zonation profiles there is no clear trend toward younger ages in the dark domains (Fig. 6a, h), as it could be presumed according to the experiments by Williams et al. (2011). Also the analyses from the light domains are not related to a distinct age population. In the bulk plot of age vs ThO2 (Fig. 6l) the ages in both domains cover a large range which is similar to those observed from monazite grains with no marked domains (Fig. 9a, b). According to the experiments (Harlov et al., 2011; Williams et al., 2011), both, the light and the dark domains could have been generated during metasomatism by alkali-bearing fluids. The embayments of

ACCEPTED MANUSCRIPT the dark domains allow us to conclude that the depletion of ThO2 should represent the late stage of the process. As shown in Figs. 5f and 6l, the alkali-fluid metasomatism affected an

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apparently older (~650 Ma) monazite population. It is difficult to evaluate the effects of a

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partial Th-enrichment and subsequent depletion even when pre-existent radiogenic PbO remains immobile. However, the bulk effect should be a general rejuvenation of the grains.

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Following the majority of measured single ages (Figs 5f, 6l), this rejuvenation should have

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been between 500 and 450 Ma.

Monazite in migmatites and gneisses analysed by EMP Th-U-Pb

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Monazite in medium- to high-grade metapsammitic to metapelitic gneisses and migmatites

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from the eastern, central and western zones of the northern Wilson Terrane (Oates Coast) were analysed by Th-U-Pb EMP on polished thin-sections and compared to samples G8-57.4

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and G8-58.1. From samples US-380 and US-501, also U-Pb TIMS monazite ages have been

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reported (Schüssler et al., 1999) and are listed in Fig. 8.

In the XHutt-XCher coordinates the monazite compositions from the various high-grade gneisses and migmatites differ from monazites in samples G8-57.4 and G8-58.1 (Fig. 4, Table 2). The XHutt are considerably lower at XCher between 0.14 and 0.08 (Fig. 4g). Most of the data plot on the cheralite substitution vector in the Th+U vs Ca diagram (Fig. 4h). The distribution of XGdPO4 vs XYPO4 also differs from that of samples G8-57.4 and G8-58.1. There is a linear cluster at low XYPO4 and there is another cluster at high XYPO4 with somewhat higher XGdPO4 (Fig. 4i).

ACCEPTED MANUSCRIPT Despite some intragranular domains in BSE imaging, the Th-U-Pb EMP ages from single grains display quite homogeneous ages, ranging between 495 and 465 Ma (Fig. 3g - m). In

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some parts of the monazite grains one can find higher ages up to 534 Ma and even older (Fig.

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3i). In the ThO2*-PbO diagram, the monazite data from high-grade gneisses usually allow for the definition of isochrones with a high R2 and a low MSWD for the weighted averages (Fig.

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7a - i). As for single grains, the ages for single samples (thin section scale) range between 466 and 486 Ma. The Th-U-Pb EMP monazite age at 486 ± 10 Ma from the garnet-cordierite

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diatexite sample US-501 corresponds, within error, to the U-Pb TIMS ages between 491 - 494 Ma. The 484 ± 9 Ma Th-U-Pb age from the metatexite sample US-380 corresponds to the 484 - 488 Ma U-Pb TIMS age reported in Schüssler et al. (1999).

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Only 3 samples with a significant number of pre-480 Ma monazite ages are reported in ThO2*

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vs PbO coordinates (Fig. 7k - m). In the mylonitic orthogneiss sample US-215 from the Eastern Zone of the Oates Coast, some monazites define a separate isochrone at 595 ± 19 Ma

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apart from the main population at 469 ± 12 Ma (Fig. 7k). In a mylonitic migmatite sample

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US-334, also from the Eastern Zone of the Oates Coast, some monazites have ages at 585 ± 36 Ma. This group of older monazites can be considered as outliers of the 482 ± 7 Ma main population (Fig. 7l). In the Oates Coast Central Zone, a partly retrograded migmatite also shows an older monazite population at around 582 ± 19 Ma apart from the main population at 482 ± 10 Ma (Fig. 7m). In all the other samples from Oates Coast, Kavrayskiy Hills, Daniels Range and Lanterman Range, the monazite analyses plot along well-defined isochrones between 460 and 510 Ma. The data from single analyses in monazite grains in several samples from a region are compiled in histograms (Fig. 9c - h). Samples from the Daniels Range vary in their monazite ages between 480 and 506 Ma (Fig. 9c, g). These ages are similar to that of the Central Zone in Oates Coast (Fig. 9e). They are also similar to the Eastern Zone in Oates Coast, where the average of the main monazite population ranges around 480 Ma (Fig. 9f). In

ACCEPTED MANUSCRIPT the Daniels Range, no systematic variation of the age data can be observed from amphibolitefacies to granulite-facies rocks along the west-east profile across the Thompson Spur. With

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respect to the north-south direction, samples from Penseroso Bluff and Thompson Spur also

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yielded similar ages. From structural position, from petrological arguments, and from age data, the eastern part of the Daniels Range can be interpreted as the continuation of the

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granulite-facies core complex in the Central Zone of Oates Coast. The new data fill the gap between Oates Coast and the Deep Freeze Range (Fig. 1). In the Kavrayskiy Hills, age data

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range between 485 and 505 Ma and average 494 Ma (Fig. 9d). This is distinctly older than in the other metamorphic complexes, and also slightly older if compared to the Lanterman Range, where an average 490 Ma is seen (Fig. 9h).

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Summary and Conclusions

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A Th-U-Pb monazite dating protocol designed for an electron microprobe JEOL JXA8200 has

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been applied to reference monazites and samples from the high-grade Wilson Terrane in

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Antarctica. Data from U-Pb SHRIMP and U-Pb TIMS dating of monazite in grain mounts and in mineral separates from the same samples were used for controls on the TH-U-Pb EMP protocol. The reference monazites used for the adjustment of the U-Pb SHRIMP protocol are not necessarily suitable for controls on the Th-U-Pb EMP analyses. Despite its unaltered, homogeneous and inclusion-free appearance under BSE imaging, the reference monazite VK1 is unsuitable due to its large variation in PbO at constant ThO2 and ThO2*. Natural reference monazites with nominally suitable Th-U-Pb characteristics can also display depletions in PbO at given ThO2* as exemplified by the MPN monazite. When a suitable number of analyses exists, which allow for the definition of an isochrone, such monazites can be used as controls for Th-U-Pb EMP dating. This is possible, if based on a rigorous elimination of analyses plotting at PbO values below the nominal isochrone. In this case, the

ACCEPTED MANUSCRIPT bulk chemical Th-U-Pb data provide chemical evidence for monazite alteration and/or

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recrystallization.

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An ideal reference monazite which connects U-Pb isotopic and Th-U-Pb bulk chemical dating methods should have ThO2 contents higher than 10 wt %. Such high-Th reference monazites

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are also preferrable for the EMP calibration. A fairly homogeneous spatial distribution of Th within grains or parts of them are a requirement. In contrast, the Th metal standards are very

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sensitive to oxidation. Glass standards are hampered by low Th concentrations and inadequate matrix characteristics. In ThO2*-PbO plots such high-Th reference monazite analyses should define an isochrone forced through zero, supported by a Pb isotopic evaluation with negligible amounts of common 204Pb. Although there are altered domains along cracks, most parts of the

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Madmon monazite satisfy these pre-conditions. The Madmon (can be obtained by request

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from the corresponding author) appears most promising as a reference monazite, especially

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for metamorphic monazites of Paleozoic to Neoproterozoic age.

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The electron microprobe enables analysis of numerous single <5 microns points on monazite grains and provides mineral-chemical data in addition to the Th-U-Pb ages. This allows for the classification and discrimination of monazites in grain mount samples G8-57.4 and G858.1. These show deviant behaviour, if compared to monazites in thin section samples. A younger group of ages in both samples scatter between 512 and 484 Ma (i.e. 503 ± 10; 488 ± 12 and 484 ± 10 Ma for G8-57.4, and 512 ± 8; 505 ± 11 and 496 ± 8 Ma for G8-58.1). These derive partly from poorly defined isochrones with low R2 and high MSWD values (Fig. 5). The age data from the thin section samples from Oates Coast migmatites as well as from the enlarged sample set are based on much better defined isochrones and range from 486 to 466 Ma (Fig. 7, 9b - h). Furthermore, samples G8-57.4 and G8-58.1 display an older, Upper Proterozoic thermal history between 658 and 619 Ma (i.e. 658 ± 46; 638 ± 25 and 619 ± 25

ACCEPTED MANUSCRIPT Ma for G8-57.4, and 646 ± 13 and 644 ± 21 Ma for G8-58.1). Older generation monazites are mainly found among the large (150 - 250 µm) monazite grains. Such large grains are

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preferrably hand-picked from heavy-mineral concentrates which are extracted from 5 - 10 kg

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of rock. In a thin section a much smaller and more arbitrary selection of mainly small monazite grains is present. As a consequence, the other migmatite samples do not reflect these

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Upper Proterozoic monazite ages. From the enlarged sample set only three samples show a

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small monazite age population at around 595 - 582 Ma (Fig. 7k - m).

Hints to older events in the northern Wilson Terrane (Daniels Range) have been previously indicated by SHRIMP zircon data from a S-type granite in the Wilson plutonic complex (Black and Sheraton, 1990). A series of four different episodes of zircon growth during the

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Proterozoic was followed by two further stages at 544 ± 4 and 469 ± 4 Ma. Between 1100 and

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500 Ma, however, a high amount of concordant or slightly discordant data outlines the concordia. A similar situation is described by Henjes-Kunst et al. (2004) for inherited zircons

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in the Central Zone from the northern Wilson Terrane. These zircons, too, gave concordant or

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slightly discordant ages in the range between 1100 and 500 Ma. Particularly, one dominant zircon group from the DeRemer Nunataks yielded ages of 680 – 545 Ma. These inherited zircons are interpreted as detrital grains from a continental crust of very late Neoproterozoic age and were deposited within a short time span during the Cambrian (Henjes-Kunst et al. 2004). Following this interpretation, the Upper Proterozoic parts of the recently dated monazites in samples G8-57.4 and G8-58.1 (Fig. 7b) and three further samples from Oates Coast may display relics of detrital monazites and reflect age data from the source region.

Some large monazites in the grain mounts display structures of dark embayments into light domains in BSE images. The dark domains have lower ThO2 (<9 wt %), lower SiO2 and UO2, but higher Ce2O3 and P2O5 when compared to the light domains.The same mineral-chemical

ACCEPTED MANUSCRIPT trends and structures have been observed in monazites which were experimentally metasomatised by alkali-bearing fluids (Harlov et al., 2011; Williams et al., 2011). In the

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studied case, the Th-U-Pb ages from the dark and light domains as well as the zonations are

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not sharply related to distinct generations and trends of ages. In all grains some single analyses belong to an older (>510 Ma) population. The majority of ages in such grains is a

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younger (<510 Ma) population. It can be concluded that the metasomatism by alkali-bearing fluids could have occurred between 510 and 450 Ma and was only recorded by pre-existent

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older monazites.

In thin sections, a different and more arbitrary selection of monazite grain sizes, with smaller monazite grain sizes compared to the grain mount, is present. All analysed samples from the

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Wilson Terrane have isochrones between 512 and 466 Ma, i.e. the Upper Cambrian to Lower

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Ordovician. No typical structures of metasomatism are observed. As a consequence, these grains should have been recrystallised as pristine new monazite during a prominent thermal

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event. Metasomatism by an alkali-bearing fluid resulting in monazite dissolution and re-

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precipitation overlaps with this thermal event. However, the data between 512 and 496 Ma in non-migmatic samples probably refers to a medium pressure granulite-facies event, whereas the data between 488 and 466 are due to isothermal uplift and subsequent amphibolite-facies overprint, as suggested by Henjes-Kunst et al. (2004). None of the monazites in the five mylonite samples from the Wilson Thrust along the eastern zone of the Oates Coast basement yielded younger ages, if compared to the neighbouring non-mylonitic high-grade metamorphic samples. This may indicate that the tectonic movement along the Wilson Thrust occurred synchronously with the high-grade metamorphism. The new EMP-Th-U-Pb monazite ages from the Daniels Range fill the gap between Oates Land and the Deep Freeze Range (Fig. 1), left by Talarico et al. (2004). They document the continuity of metamorphic evolution and concurrent geochronology in the low to medium pressure/high temperature belt

ACCEPTED MANUSCRIPT all over the whole of the western Wilson Terrane, i.e. a distance of more than 600 km. This metamorphism in the Wilson Terrane is uniform in space and time and associated with the

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late Pan-African Ross Orogeny. Segments of the Ross orogenic metamorphic P-T paths can

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be labelled by an interpretation of the corresponding monazite ages (Fig. 9i).

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As a consequence, the isotopic U-Pb SHRIMP, TIMS, ICP-MS analyses and non-isotopic EMP Th-U-Pb monazite dating can be understood as complementary methods for the

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evaluation of geological ages in high-grade metamorphic rocks. When adjusted and involving suitable reference monazites, isotopic methods provide indispensable means of obtaining few but precise ages. These can be extended to regional scales by non-isotopic EMP Th-U-Pb monazite dating. When embedded into the mineral-chemical record, the EMP Th-U-Pb

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Acknowledgements

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monazite ages allow for an interpretation in terms of geological processes.

Our colleague Friedhelm Henjes-Kunst from the Federal Institute for Geosciences and Natural Resources BGR at Hannover kindly provided the monazite grain mount and a lot of corresponding information to the study. The electron-microprobe monazite dating required long-term analytical sessions which were made possible through support by Matthias Göbbels at the Geozentrum Nordbayern at Erlangen. Technical assistance during the electronmicroprobe sessions was provided by Andreas Richter. Polished thin sections of the gneisses were carefully produced by Peter Spaethe at the Chair of Geodynamics and Geomaterialscience at Würzburg. The authors acknowledge the constructive and extensive reviews by D. Harlov and E. Krenn to successive versions of the manuscript. The study was

ACCEPTED MANUSCRIPT funded by a grant SCHU 873/6 to the authors within the frame of the Priority Program 1158

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Antarctic Research of the Deutsche Forschungsgemeinschaft.

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monazite from Forefinger Point granulites, East Antarctica: implications for Pan-African overprint, in: Fütterer, D.K., Damaske, D., Kleinschmidt, G., Miller, H., Tessensohn, F.

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Canadian Jounal of Earth Sciences 27, 1431-1450.

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mechanisms and their geochemical implications. Earth and Planetary Science Letters 145, 79-

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Pyle, J.M., Spear, F.S., Rudnick, R.L., McDonough, W.F., 2001. Monazite-xenotime-garnet equilibrium in metapelites and a new monazite-garnet thermometer. Journal of Petrology 42, 2083–2107.

Pyle, J.M., Spear, F.S., Wark, D.A., Daniel, C.G., Storm, L.C., 2005. Contribution to precision and accuracy of monazite microprobe ages. American Mineralogist 90, 547-577.

ACCEPTED MANUSCRIPT Rasmussen, B., Fletcher, I.R., 2002. Indirect dating of mafic intrusions by SHRIMP U-Pb analysis of monazite in contact metamorphosed shale: an example from the Palaeoproterozoic

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Capricorn Orogen, Western Australia. Earth and Planetary Science Letters 197, 287-299.

Rasmussen, B., Bengtson, S., Fletcher, I.R., McNaughton, N.J., 2002, Discoidal impressions

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Rhede, D., Wendt, I., Forster, H.J., 1996: A three-dimensional method for calculating independent chemical U/Pb- and Th/Pb-ages of accessory minerals. Chemical Geology 130, 247253.

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Roland, N.W., Gibson, G., Kleinschmidt, G. & Schubert, W., 1984. Metamorphism and

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structural relations of the Lanterman metamorphics, north Victoria Land, Antarctica.

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Geologisches Jahrbuch B60, 319-361.

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Schandl, E., Gorton, M.P., 2004. A textural and geochemical guide to the identification of hydrothermal monazite: criteria for selection of samples for dating epigenetic hydrothermal ore deposits. Economic Geology 99, 1027-1035.

Schubert, W., 1987. Petrography of the Eastern Thompson Spur, Daniels Range, North Victoria Land, Antarctica. Geologisches Jahrbuch B66, 131-143.

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ACCEPTED MANUSCRIPT Schüssler, U., 1996. Metamorphic rocks in the Northern Wilson Terrane, Oates Coast,

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Antarctica. Geologisches Jahrbuch B 89, 247-269.

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Schüssler, U., Bröcker, M., Henjes-Kunst, F., Will, T., 1999. P-T-t evolution of the Wilson

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Terrane metamorphic basement at Oates Coast, Antarctica. Precambrian Research 93, 235-258.

Schüssler, U., Henjes-Kunst, F., Talarico, F., Flöttmann, T., 2004. High-grade crystalline

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basement of the northwestern Wilson Terrane at Oates Coast: new petrological and geochronological data and implications for its tectonometamorphic evolution. Terra Antartica 11, 15-34.

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Schulz, B., von Raumer, J.F., 2011. Detection of a pre-Variscan metamorphic event by EMP

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monazite dating and thermobarometry of garnet metapelites in the Alpine External Aiguilles

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Rouges Massif. Swiss Journal of Geosciences 104, 67-79.

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Schulz, B., Krenn, E., Finger, F., Brätz, H., Klemd, R., 2007a. Cadomian and Variscan metamorphic events in the Léon Domain (Armorican Massif, France): P-T data and EMP monazite dating, in: Linnemann, U., Nance, D., Kraft, P., Zulauf, G. (Eds.), The Evolution of the Rheic Ocean: From Avalonian-Cadomian active margin to Alleghenian-Variscan collision. Geological Society of America, Special Paper 423, pp. 267-285.

Schulz, B., Brätz, H., Bombach, K., Krenn, E., 2007b. In-situ Th-Pb dating of monazite by 266 nm laser ablation and ICP-MS with a single collector, and its control by EMP analysis. Zeitschrift für Angewandte Geologie 35, 377-392.

ACCEPTED MANUSCRIPT Simmat, R., Raith, M.M., 2008. U-Th-Pb monazite geochronometry of the Eastern Ghats Belt, India: Timing and spatial disposition of poly-metamorphism. Precambrian Research

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162, 16-39.

Spear, F.S., 2010. Monazite-allanite phase relations in metapelites. Chemical Geology 279,

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55-62.

Chemical Geology 266, 218-230.

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Spear, F.S., Pyle, J.M., Cherniak, D., 2009. Limitations of chemical dating of monazite.

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in metamorphosed detrital monazites. Earth and Planetary Science Letters 128, 391-405.

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Talarico, F., Palmeri, R., Ricci, C.A., 2004. Regional metamorphism and P-T evolution of the Ross Orogen in northern Victoria Land (Antarctica): a review. Periodico di Mineralogia 73, 185-196.

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12.

Ulitzka, S. (1987): Petrology and Geochemistry of the Migmatites from Thompson Spur,

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Daniels Range, North Victoria Land, Antarctica. Geologisches Jahrbuch B66, 81-130.

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Williams, M.L, Jercinovic, M.J, Goncalves, P., Mahan, K., 2006. Format and philosophy for collecting, compiling, and reporting microprobe monazite ages. Chemical Geology 225, 1–15.

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Figures and Tables

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Resetting monazite ages during fluid-related alteration. Chemical Geology 283, 218-225.

Fig. 1. Geological sketch map of northern Victoria Land and Oates Coast (Oates Land) in the Transantarctic Mountains. The major units of the Early-Paleozoic Ross Orogen are the Robertson Bay Terrane (RBT), the Bowers Terrane (BT), and the Wilson Terrane (WT). The Western Zone (WZ), Central Zone (CZ), and Eastern Zone (EZ) in the Wilson Terrane as defined in Schüssler

et al. (1999) and Henjes-Kunst et al. (2004). The Lazarev Thrust (L), the Exiles Thrust (E), and the Wilson Thrust (W) are noted. Abbreviations of mountain ranges, partly with sampling locations include the Deep Freeze Range (DFR), Daniels Range (DR), Kavrayskiy Hills (KH), Lanterman Range (LR), Penseroso Bluff (PB), and Thompson Spur (TS).

ACCEPTED MANUSCRIPT Fig. 2. Backscattered electron images (BSE), mineral chemistry and electron microprobe (EMP) Th-U-Pb data of reference monazites. (a) Pegmatite monazite Madmon-D from

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Madagascar. (b) Selected grains of metamorphic monazite MPN. (c) Selected grains from

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magmatic monazite VK-1. Numbers are single point ages in Ma. Weighted average ages with 2 sigma error refer to analyses also from other grains. Altered parts (alt) of monazite with

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numerous inclusions are indicated. (d) Reference monazite compositions in mole fractions of huttonite (XHutt) and cheralite (XCher) endmembers, calculated according to Pyle et al.

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(2001). (e) Compositions in Th+U vs Si coordinates with huttonite substitution trend. (f) Compositions in Th+U vs Ca coordinates with cheralite substitution trend. (g) Analyses from reference monazites in the U/Pb - Th/Pb diagram of Cocherie and Albarède (2001). (h) Total PbO vs. ThO2* (wt %) isochrone diagram; ThO2* is ThO2 + UO2 equivalents expressed as

ED

ThO2, after Suzuki et al. (1994). Isochrones are calculated from the regression being forced

PT

through zero as proposed by Montel et al. (1996). Isochrone ages match weighted average ages with error calculated according to Ludwig (2001). Analyses of monazite VK-1 allow no

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CE

definition of an isochrone.

Fig. 3. Backscattered electron images (BSE) of monazites from the northern Wilson Terrane, Antarctica. Numbered points are analyses with EMP Th-U-Pb ages in Ma. Weighted average ages from the single grains are calculated according to Ludwig (2001). (a) - (c) Monazite grains from sample G8-57.4 (diatexite) with subtle transitional domains. Locations of SHRIMP analyses in (c) are next to age data in white letters. (d) - (f) Monazite grains from sample G8-58.1 (metatexite) with cauliflower-like domains separated by sharp boundaries. (g) - (m) Selected monazite grains from various gneiss samples from eastern, central, and western zones of the northern Wilson Terrane (see Fig. 8 for the locations).

ACCEPTED MANUSCRIPT Fig. 4. Mineral chemistry of monazites from the northern Wilson Terrane, Antarctica. (a), (d), (g) Monazite compositions in mole fractions of huttonite (XHutt) and cheralite (XCher)

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endmembers, calculated according to Pyle et al. (2001). (b), (e), (h) Monazite compositions in

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Th + U vs Ca with the cheralite substitution trend. (c), (f), (i) Monazite compositions in mole fractions of GdPO4 and YPO4, calculated according to Pyle et al. (2001). The lines mark

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maximal mole fractions of YPO4 in monazites from metapelite mineral zones of metamophism reported in Pyle et al. (2001). YPO4 in monazite increases with metamorphic

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grade: Grt garnet zone; Sil sillimanite zone; Mig migmatite zone.

Fig. 5. ThO2* vs total PbO (wt %) isochrone diagrams of monazites in grain mounts of diatexite sample G8-57.4 (Mt. Blowaway) and metatexite sample G8-58.1 (Mt. Dalton), see

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Fig. 8 for locations; ThO2* is ThO2 + UO2 equivalents expressed as ThO2, after Suzuki et al.

PT

(1994). Isochrones are calculated from the regression being forced through zero as proposed by Montel et al. (1996). Isochrone ages match weighted average ages with error calculated

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according to Ludwig (2001). Note high MSWD values of weighted average ages and low R2

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values of isochrones. Groups A-G in the grain mount are assorted by grain size of monazites.

Fig. 6. Mineral chemistry in monazite grains with signs of alteration by metasomatism. (a) BSE image of typical monazite grain with embayment of dark domains into light domains. Numbers are EMP ages in Ma. Core (c) to rim (r) profile by analyses. (b) - (g) ThO2 vs element oxide plots for dark and light domains in monazite grains; for distinct trends see text. (h) - (k) Typical mineral-chemical zonation profiles from light to dark domains in monazite grains; profile in (h) refers to grain in (a). (l) EMP monazite age vs ThO2 plot for dark and light domains in monazite grains.

ACCEPTED MANUSCRIPT Fig. 7. ThO2* vs total PbO (wt %) isochrone diagrams of monazites in thin sections from biotite gneisses and migmatites of the northern Wilson Terrane, see Fig. 6 for locations;

T

ThO2* is ThO2 + UO2 equivalents expressed as ThO2, after Suzuki et al. (1994). Isochrones

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are calculated from the regression being forced through zero as proposed by Montel et al. (1996). Isochrone ages match weighted average ages with error calculated according to

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Ludwig (2001). Note low MSWD values of weighted average ages and high R2 values of

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isochrones.

Fig. 8. Sketch map of outcrops (Nunataks) and sample locations in the northern Wilson Terrane, Ross Orogen, Pacific Ocean side of Antarctica. Samples with U-Pb SHRIMP and UPb-TIMS data in Schüssler et al. (1999) and Henjes-Kunst et al. (2004) are indicated.

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Monazite EMP Th-U-Pb age data in the current study are in bold numbers. The Lazarev

PT

Thrust (L), Exiles Thrust (E), and Wilson Thrust (W) are shown. The Central Zone (CZ), Eastern zone (EZ), and Western Zone (WZ) are defined in Schüssler et al. (1999) and

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Henjes-Kunst et al. (2004).

Fig. 9. (a) Summary of monazite age vs mole fraction YPO4 in grain mounts G8-57.4 and G858.1, and selected migmatite gneiss samples from the northern Wilson Terrane. Note the different distribution pattern between grain mount samples and thin sections. (b) Histogram of the Th-U-Pb EMP monazite ages, same sample set as in (a). (c) - (h) Histograms summarising monazite ages from several locations in the Wilson Terrane; see Figs. 1 and 8 for the locations. (i) Interpretative correlation of monazite ages and metamorphic P-T paths for the Ross Orogeny in the Wilson Terrane. Monazite ages label segments of the P-T paths. Broken lines mark inferred segments of the P-T paths; arrows mark P-T path segments reconstructed by geothermobarometry of cordierite-garnet-biotite-K-feldspar assemblages. (1) Sample US495 from the Western Zone (Schüssler et al., 1999); (2) Sample US-501 from the Central

ACCEPTED MANUSCRIPT Zone (Schüssler et al., 1999). (3) Sample PB-7 from Penseroso Bluff, Daniels Range (Schulz, unpublished data). (4) Sample US-477 from the Central Zone (Schulz, unpublished data). The

T

broken lines +Mnz (1.0) and +Mnz (4.3) mark the low temperature limit of stability fields of

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monazite for different CaO contents (wt%) in bulk rock, according to Janots et al. (2007) and

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Spear (2010).

Table 1 Electron microprobe analyses of SHRIMP-dated monazites in grain mount of

not weighted average. Age is in Ma.

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diatexite and metatexite. Error is 2σ on each analysis. Error on Madmon analysis is mean and

Table 2 Electron microprobe analyses of monazites in petrographic thin sections of

AC

CE

PT

ED

migmatites and gneisses. Error is 2σ on each analysis. Age is in Ma.

AC

CE

PT

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T

ACCEPTED MANUSCRIPT

SiO2

P2O5 CaO Y2O3

La2O3

Ce2O3

Pr2O3 Sm2O3

Nd2O3

Gd2O3 ThO2

UO2

PbO Total

Th

U

Pb

Th*

ThO2*

2σ

Age

18

1.72 28.73 1.93

2.68

11.08

23.76

2.27

2.23

7.86

1.44 14.53 0.73

0.42

99.37 12.772 0.642 0.389 14.899 16.947

581 ±

30

VK-01-05-mz3

17

1.67 28.74 1.94

2.59

11.16

23.85

1.45

2.26

6.70

1.47 14.59 0.75

0.20

97.37 12.824 0.659 0.182 14.956 17.011

273 ±

30

VK-01-02-mz2

24

1.69 28.54 1.88

2.57

11.06

24.03

2.22

2.28

8.03

1.42 14.28 0.67

0.25

98.91 12.546 0.591 0.233 14.474 16.463

360 ±

31

VK-01-03-mz4

14

1.76 28.90 1.92

2.62

11.15

23.87

2.20

2.28

8.00

1.46 14.58 0.71

0.27

99.72 12.812 0.629 0.248 14.863 16.906

373 ±

30

MPN-02-01m3

174

1.01 29.57 0.67

0.87

15.50

28.91

2.76

1.71

10.82

0.95

5.90 0.13

0.56

99.36

5.185 0.116 0.524

5.627

6.401

1998 ±

76

MPN-03-07-2

18

1.07 29.26 0.59

0.55

16.51

29.57

2.78

1.41

10.71

0.76

6.22 0.10

0.62 100.16

5.466 0.090 0.580

5.814

6.615

2132 ±

74

MPN-03-06-4

13

1.20 28.83 0.62

0.55

16.26

29.52

2.97

1.49

11.01

0.76

6.54 0.07

0.63 100.43

5.743 0.060 0.582

5.972

6.795

2086 ±

72

MPN-03-06-2

11

1.49 28.62 0.73

0.39

17.20

28.75

2.75

0.76

10.52

0.55

6.79 0.09

0.66

99.28

5.963 0.077 0.613

6.261

7.124

2096 ±

68

MPN-03-08-2

4

1.00 29.38 1.11

0.58

15.92

29.67

2.93

1.56

11.17

0.86

5.55 0.07

0.52 100.31

4.876 0.061 0.481

5.109

5.813

2020 ±

84

E57-4-02-04mz3

46

1.36 29.11 1.01

0.74

15.09

28.85

2.66

1.30

E57-4-02-04mz4

47

1.35 29.08 1.14

0.86

14.51

28.18

2.63

1.42

G57-4-01-11-11

90

1.18 29.02 1.09

0.88

14.34

28.40

2.69

1.43

G57-4-01-11-12

91

1.39 28.65 1.16

0.85

14.47

27.97

2.79

G57-4-01-11-13

92

1.63 28.36 1.26

0.84

14.80

27.68

2.85

G57-4-01-03-5

75

1.76 28.10 0.82

0.57

14.63

28.36

G57-4-01-03-1

71

1.39 29.19 0.94

0.73

14.77

28.61

G57-4-01-03-7

77

1.78 28.34 0.82

0.56

14.79

28.56

F57-4-01-06-1

115

1.25 28.84 0.97

0.60

14.98

28.69

F57-4-01-06-2

116

1.23 28.83 0.98

0.60

15.04

28.62

D58-1-01-01m2

64

0.31 31.08 1.64

2.90

13.17

D58-1-01-01m1

63

0.74 30.06 1.70

0.97

D58-1-01-01-2

77

0.30 31.02 1.44

2.11

D58-1-01-01-4

79

0.33 30.73 1.58

B58-1-01-02m1

130

B58-1-01-02m2

MA

NU S

VK-01-05-mz4

CR I

Monazite

PT

ACCEPTED MANUSCRIPT

0.70

9.30 0.09

0.23 100.27

8.175 0.082 0.215

8.448

9.613

566 ±

53

0.82

9.93 0.11

0.28 100.37

8.726 0.096 0.260

9.046 10.292

638 ±

49

10.31

0.80

9.34 0.09

0.21

99.76

8.204 0.079 0.196

8.466

9.633

515 ±

53

TE

D

9.83

10.08 10.03

0.76

9.90 0.09

0.23

99.64

8.701 0.082 0.215

8.973 10.209

534 ±

50

1.15

10.23

0.75

9.42 0.09

0.19

99.24

8.281 0.080 0.179

8.545

9.723

467 ±

52

0.61 10.58 0.14

0.23

99.68

9.295 0.119 0.209

9.687 11.022

482 ±

46

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1.35 1.16

9.94

2.93

1.27

10.00

0.68

9.69 0.08

0.21 100.49

8.512 0.075 0.197

8.758

9.965

501 ±

51

2.76

1.21

10.04

0.67 10.39 0.14

0.22 100.30

9.135 0.126 0.205

9.548 10.863

479 ±

47

2.76

1.21

9.90

0.64

9.07 0.10

0.20

99.21

7.972 0.090 0.181

8.269

9.409

488 ±

54

2.72

1.23

9.70

0.62

8.97 0.11

0.19

98.81

7.881 0.094 0.178

8.191

9.319

483 ±

54

26.13

2.50

2.02

9.96

1.42

7.83 0.42

0.24

99.64

6.885 0.367 0.226

8.106

9.219

620 ±

55

13.13

26.71

2.63

1.96

10.65

1.25

9.65 0.42

0.24 100.11

8.479 0.372 0.227

9.705 11.040

522 ±

46

13.14

26.93

2.68

2.01

10.76

1.43

6.92 0.34

0.23

99.30

6.080 0.303 0.215

7.090

8.065

674 ±

38

2.97

12.65

25.68

2.50

2.03

10.03

1.46

7.62 0.41

0.24

98.23

6.697 0.366 0.221

7.914

9.001

622 ±

34

1.70 28.66 1.08

2.56

15.21

25.91

2.34

1.45

8.42

1.08 10.76 0.50

0.27

99.94

9.459 0.440 0.247 10.911 12.410

505 ±

41

131

0.51 30.70 1.58

1.76

13.45

26.82

2.78

1.93

10.55

1.34

8.33 0.34

0.22 100.29

7.322 0.297 0.201

8.304

9.446

539 ±

54

B58-1-01-04m3

136

0.45 31.26 1.45

2.35

13.29

27.04

2.83

2.01

10.30

1.35

7.44 0.48

0.18 100.42

6.536 0.426 0.169

7.936

9.026

476 ±

56

D58-1-02-08m5

101

1.21 29.39 1.11

1.56

12.99

27.03

2.78

1.82

10.90

1.15

9.43 0.20

0.21

99.76

8.285 0.176 0.193

8.864 10.085

485 ±

50

D58-1-02-08m6

102

1.25 29.02 1.03

1.39

13.42

27.51

2.85

1.77

10.87

1.06

9.31 0.16

0.20

99.85

8.183 0.137 0.186

8.636

9.825

481 ±

52

D58-1-02-08m7

103

1.32 28.65 1.03

1.29

13.65

27.63

2.86

1.67

10.83

0.99

9.50 0.16

0.23

99.80

8.353 0.143 0.212

8.827 10.042

534 ±

50

2.80 26.14 0.13

1.00

7.56

24.82

3.73

4.50

15.54

2.00 10.44 0.41

0.25

99.32

9.176 0.360 0.234 10.361 11.786

504

37

Madmon/30

AC

2.79

±

ACCEPTED MANUSCRIPT

AC

CE P

TE

D

MA

NU S

CR I

PT

Schulz-Schuessler Table 1

AC

CE

PT

ED

MA NU

SC

RI P

T

ACCEPTED MANUSCRIPT

AC

CE

PT

ED

MA NU

SC

RI P

T

ACCEPTED MANUSCRIPT

Monazite

SiO2

P2O5

CaO

Y2O3

La2O3

Ce2O3

Pr2O3 Sm2O3

Nd2O3

Gd2O3

ThO2

PT

ACCEPTED MANUSCRIPT

Total

Th

U

Pb

Th*

ThO2*

Age

2σ

UO2

PbO

7.69

0.11

0.17 100.30

6.760 0.100 0.157

7.088

8.064

494 ±

63

5.85

0.66

0.17 100.21

5.138 0.583 0.154

7.056

8.022

486 ±

63

5.78

0.58

0.16 100.34

5.081 0.507 0.148

6.749

7.675

490 ±

66

5.16

0.45

0.14

99.93

4.537 0.396 0.127

5.841

6.642

485 ±

76

140

1.20

28.45

0.72

0.14 15.72

29.81

3.10

1.22

11.37

0.58

US501-mz2-10

141

0.30

30.47

1.31

0.70 14.29

28.19

3.02

1.93

11.97

1.37

US501-mz3-3

145

0.18

30.81

1.34

0.34 14.16

28.27

3.07

1.96

12.45

1.23

US501-mz3-4

146

0.14

30.39

1.22

0.18 14.26

28.80

3.13

2.11

12.77

1.18

US380-mz2-1

116

0.29

30.37

1.24

2.49 13.84

27.05

2.86

1.77

11.24

1.43

5.80

0.60

0.16

99.14

5.093 0.529 0.153

6.835

7.772

499 ±

65

US380-mzsm10

133

0.39

30.09

1.23

2.32 13.38

27.16

2.81

1.96

11.59

1.43

5.96

0.66

0.17

99.14

5.237 0.584 0.156

7.160

8.141

487 ±

62

US380-mzsm26

157

0.35

29.89

1.21

2.46 13.37

26.85

2.84

1.90

11.46

1.36

6.09

0.51

0.16

98.45

5.349 0.453 0.148

6.841

7.779

483 ±

65

US380-mzsm27

162

0.27

30.28

1.50

1.23 13.29

27.01

2.92

1.97

11.73

1.45

7.01

0.56

0.18

99.39

6.158 0.497 0.171

7.795

8.865

489 ±

57

US477b-mz7-5

38

2.29

26.88

0.77

0.25 14.25

26.86

2.73

1.45

11.31

0.79

11.56

0.64

0.27 100.03 10.157 0.561 0.253 12.001

13.649

471 ±

37

US477b-mz8-4

45

0.18

30.17

1.47

0.49 13.38

28.18

3.01

2.14

12.32

1.34

6.47

0.51

0.16

99.81

5.682 0.451 0.152

7.165

8.148

473 ±

62

US477b-mz8-5

46

0.19

30.46

1.40

0.45 13.67

28.38

2.89

2.05

12.54

US477b-mzigt2-1

57

0.36

29.91

0.66

0.45 14.99

30.05

3.22

1.89

US-387-mz5-4

95

0.40

30.86

1.10

1.74 13.39

28.01

3.05

US-387-mz6igt

96

0.39

30.86

0.94

2.29 13.56

28.13

2.97

US-387-mz7-6

102

0.40

30.91

1.53

0.81 13.12

27.84

3.05

US-387-mz8-1

103

0.31

30.71

1.43

0.61 13.44

28.53

US378-mz2-1

132

0.25

30.56

1.26

2.06 14.09

27.55

US378-mz2-2

133

0.15

30.38

1.34

2.39 14.24

US378-mzsm

146

0.13

30.98

1.39

2.50 13.69

US378-mz4-8

157

0.12

31.24

1.30

2.68 13.66

US-388-mz5-2

150

0.39

28.35

1.51

0.47 13.38

US-388-mz6-1

151

0.27

29.91

1.50

US-388-mz9-3

164

0.25

29.10

1.58

US-288-mz14igt1

188

0.31

28.26

US242b-mz2-2

64

0.16

US242b-mz5-3

81

US242b-mz9-4 US242b-mz9-17

6.30

0.52

0.16 100.29

5.535 0.456 0.152

7.037

8.002

483 ±

63

1.20

3.53

0.58

0.12 100.30

3.102 0.508 0.108

4.776

5.429

503 ±

94

2.00

12.29

1.37

4.96

0.58

0.14

99.87

4.358 0.508 0.132

6.029

6.855

490 ±

74

1.38

12.14

1.36

4.74

0.57

0.14

99.46

4.170 0.499 0.129

5.814

6.610

495 ±

77

1.87

12.08

1.34

6.60

0.82

0.19 100.57

5.804 0.724 0.178

8.186

9.306

487 ±

55

3.10

2.00

12.40

1.34

6.22

0.63

0.17 100.87

5.465 0.555 0.156

7.291

8.290

478 ±

61

2.85

1.90

11.29

1.29

5.51

0.96

0.18

99.73

4.842 0.844 0.164

7.618

8.659

482 ±

59

27.32

2.87

1.87

10.92

1.34

5.20

1.21

0.19

99.43

4.572 1.070 0.179

8.096

9.200

493 ±

55

26.95

2.87

1.93

11.27

1.43

5.39

1.17

0.19

99.89

4.738 1.029 0.179

8.126

9.235

491 ±

55

27.13

2.94

2.00

11.59

1.49

5.20

0.91

0.17 100.40

4.569 0.805 0.157

7.217

8.203

487 ±

62

28.14

3.05

1.82

11.92

1.16

6.98

0.54

0.17

97.87

6.133 0.474 0.159

7.691

8.746

463 ±

58

0.46 13.64

28.59

3.11

1.98

12.02

1.19

7.17

0.40

0.17 100.40

6.305 0.349 0.160

7.455

8.479

479 ±

60

0.61 13.49

28.11

2.96

1.90

11.42

1.21

6.54

1.50

0.23

98.90

5.749 1.325 0.213 10.104

11.482

472 ±

44

1.50

0.36 13.49

28.36

2.96

1.74

11.74

1.11

6.91

0.66

0.18

97.57

6.076 0.581 0.167

7.984

9.079

466 ±

56

30.58

1.52

0.73 14.05

27.89

2.96

1.73

11.48

1.37

6.59

0.72

0.18

99.94

5.788 0.632 0.170

7.868

8.946

482 ±

57

0.83

29.81

1.65

1.41 12.86

25.75

2.66

1.92

10.68

1.46

9.87

0.50

0.27

99.67

8.672 0.445 0.248 10.143

11.537

544 ±

44

100

0.13

29.95

1.36

1.54 13.83

27.57

2.83

1.96

11.68

1.51

5.71

0.68

0.16

98.90

5.021 0.602 0.151

7.000

7.959

481 ±

64

113

0.20

30.10

1.10

2.10 12.78

27.49

3.15

2.14

12.89

1.48

4.56

0.89

0.15

99.02

4.011 0.782 0.143

6.586

7.485

486 ±

68

US227-mz4-1

13

0.12

31.54

1.56

2.75 12.88

26.28

2.85

2.09

11.09

1.47

6.12

1.45

0.22 100.42

5.378 1.280 0.205

9.587

10.894

478 ±

47

US227-mz6-4

27

0.09

31.51

1.50

3.11 13.10

26.06

2.81

1.94

11.15

1.59

5.91

1.22

0.21 100.19

5.197 1.074 0.193

8.732

9.924

494 ±

51

US227-mz17-1

60

0.15

32.34

1.51

3.20 12.51

25.77

2.78

2.07

11.24

1.62

6.29

1.12

0.20 100.82

5.529 0.990 0.190

8.787

9.988

483 ±

51

CE P

TE

D

1.30

13.34

AC

MA

NU S

CR I

US501-mz2-9

ACCEPTED MANUSCRIPT

US227-mz17-2

61

0.13

32.18

1.42

2.78 13.05

26.49

2.90

2.02

11.26

1.49

5.61

AC

CE P

TE

D

MA

NU S

CR I

PT

Schulz-Schuessler Table 2

1.27

0.20 100.81

4.933 1.123 0.184

8.626

9.803

477 ±

52

AC

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PT

ED

MA NU

SC

RI P

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AC

CE

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ED

MA NU

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RI P

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ED

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AC

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ED

MA NU

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RI P

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ACCEPTED MANUSCRIPT

AC

CE

PT

ED

MA NU

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RI P

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ACCEPTED MANUSCRIPT

AC

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ED

MA NU

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ACCEPTED MANUSCRIPT

AC

CE

PT

ED

MA NU

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RI P

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ACCEPTED MANUSCRIPT

AC

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ED

MA NU

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ACCEPTED MANUSCRIPT

AC

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ED

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AC

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ED

MA NU

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Research highlights Monazite allows combined application of U-Pb isotopic and Th-U-Pb chemical age dating. Reference monazites for age dating measurements are classified in 3 categories. Reference monazite with constant Th/Pb and high Th are most suitable. Metasomatism by alkali-bearing fluids recognised in old monazites from grain mounts. Crystallisation of pristine monazite during high-grade metamorphism in the Wilson Terrane.