Complexity in the behavior and recrystallization of monazite during high-T metamorphism and fluid infiltration

Chemical Geology 322–323 (2012) 192–208

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Complexity in the behavior and recrystallization of monazite during high-T metamorphism and fluid infiltration Nigel M. Kelly a,⁎, Simon L. Harley b, Andreas Möller c a b c

Department of Geology & Geological Engineering, Colorado School of Mines, Golden, CO 80401, USA School of GeoSciences, University of Edinburgh, Kings Buildings, West Mains Rd., Edinburgh, EH9 3JW, United Kingdom Department of Geology, University of Kansas, 1475 Jayhawk Blvd., Lawrence, KS 66045, USA

a r t i c l e

i n f o

Article history: Received 27 October 2011 Received in revised form 29 June 2012 Accepted 1 July 2012 Available online 7 July 2012 Editor: K. Mezger Keywords: Monazite Recrystallization Electron microprobe dating Trace element Rayner Complex Antarctica

a b s t r a c t A detailed study of monazite grains in silica-undersaturated and quartz-bearing Mg–Al metapelite from the Oygarden Group of islands, east Antarctica, reveals a complex history of growth and recrystallization during two separate events in the Neoproterozoic and earliest Cambrian. Monazite grains from garnet-poor and garnet-rich metapelite preserve core domains that have ages corresponding to growth and/or recrystallization at granulite facies (P ≈ 9–1.0 GPa, T ≥ 850–900 °C) conditions during the Rayner Structural Episode between 930 and 890 Ma. High-Th rims (≤22 wt.% ThO2) that are in textural equilibrium with sapphirine– orthopyroxene symplectites formed after garnet during late-Rayner decompression, occur on monazite grains in garnet-rich assemblages, and give electron microprobe ages (883 ± 18 Ma) within error of core domains (903 ± 14 Ma). These high-Th rim domains are interpreted to have formed through recrystallization of liberated inclusions via a process dominated by coupled dissolution–reprecipitation reactions facilitated by transient fluid films on the large surface areas in the symplectite and not by new growth. In garnet-poor metapelite, monazite grains that have grain boundaries in textural equilibrium with the granulite-facies assemblage also show alteration to higher-Th compositions on rims and along vein-like fractures. This compositional shift is accompanied by partial- to complete-resetting of ages to ~ 500 Ma, along with minor modification of core domain ages. Textures and patterns of chemical age resetting are interpreted to also be the result of a coupled dissolution–reprecipitation reaction process, but in contrast to garnet-bearing assemblages, this alteration occurred during a fluid influx at lower temperatures during regional ‘Pan African’ tectonism. This study highlights the susceptibility of monazite to resetting, with fluid-present conditions dramatically increasing the potential for recrystallization. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The utility of monazite ([LREE,Th]PO4) as a chronometer of crustal events has been well demonstrated (Parrish, 1990; Spear and Pyle, 2002). Ubiquitous to many crustal rock compositions, the reactive nature of monazite makes it more sensitive to thermal events and may record the timing and nature of processes such as diagenesis (Rasmussen and Fletcher, 2002), prograde metamorphic reactions (Smith and Barreiro, 1990; Kingsbury et al., 1993; Bingen et al., 1996; Foster et al., 2002; Janots et al., 2008), fabric development (Williams and Jercinovic, 2002), crustal melting (Watt and Harley, 1993; Bea, 1996; Pyle and Spear, 2003), crystallization of magmas (Montel, 1993; Wark and Miller, 1993; Harrison et al., 1995) and hydrothermal fluid flow (Rasmussen et al., 2006). Detailed, in situ studies of monazite have revealed that the compositional zoning commonly seen within individual grains is a preserved record of growth or post-growth history (Poitrasson et al., 1996, 2000; Foster ⁎ Corresponding author. E-mail address: [email protected] (N.M. Kelly). 0009-2541/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2012.07.001

et al., 2000; Hetherington and Harlov, 2008). While the processes governing the growth of monazite are reasonably well described, the mechanisms that drive recrystallization of existing monazite are less well understood. Experimental (Smith and Giletti, 1997; Cherniak et al., 2004; Gardes et al., 2006, 2007; Cherniak and Pyle, 2008) and empirical studies (Bingen and van Breemen, 1998; Crowley and Ghent, 1999; McFarlane and Harrison, 2006) have shown that monazite is robust to volume diffusion of major and trace cation constituents (e.g. Th, REE, U, Pb). However, despite its perceived robustness under dry conditions, compositional zoning and age-resetting patterns in natural monazite indicates that modification through recrystallization may be common in many crustal rocks (Ayers et al., 1999; Zhu and O'Nions, 1999; Harlov and Forster, 2002; Möller et al., 2003; McFarlane and Harrison, 2006; St-Onge et al., 2007) and could be linked to fluid infiltration (Poitrasson et al., 1996; Seydoux-Guillaume et al., 2002; Harlov et al., 2011; Williams et al., 2011), resulting in chemical and isotopic changes to pre-existing monazite. The process of mineral recrystallization is becoming better understood (Putnis, 2002, 2009) and valuable experimental evidence is

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from the chemical compositions of and relationships between modally abundant minerals in a mineral assemblage.

linking such processes to modification of monazite (Hetherington and Harlov, 2008; Hetherington et al., 2010; Harlov et al., 2011; Tropper et al., 2011; Williams et al., 2011). For example, where observed, boundaries between altered and unaltered domains in recrystallized monazite are commonly curved or lobate, and do not appear to be associated with modification of the original grain shapes. Moreover, compositional changes in altered domains can be associated with partial to complete resetting of ages (Seydoux-Guillaume et al., 2002, 2003). Preservation of original grain shapes despite compositional modification in newly developed rims suggests that in these cases rims do not form through new growth, but possibly by fluid-mediated, coupled dissolution–reprecipitation along reaction fronts (Putnis, 2002, 2009; Harlov et al., 2011). The reaction front itself may be on the sub-micron scale, with removal or addition of material controlled by transport in a thin film of fluid that separates the reactant and product phases, leading to the development of sharp compositional boundaries that may appear much like overgrowths. As our recognition of monazite modification improves, of particular interest is the ability to link specific compositional signatures in modified monazite to geologic events and the compositions of fluids that accompanying them. For example, recent experimental work (Harlov et al., 2011) has demonstrated the relative effects of different fluid compositions common to crustal rocks, but what is needed are more detailed studies on natural monazite in the geologic record that have undergone modification. This paper presents the results of an integrated imaging and micro-analytical study of monazite hosted within silica-undersaturated metapelite from the Oygarden Group, east Antarctica. The monazite grains display complex textural features and chemical zoning patterns that suggest partial recrystallization during the prograde and retrograde path of a high-temperature metamorphic event at ~ 930–890 Ma. In addition, some monazite grains display chemical zoning patterns and textures that suggest further recrystallization in the presence of a low-water activity fluid between ~ 520 and 500 Ma. The latter results indicate that monazite is a sensitive monitor of fluid events that may not be readily identified

2. Geological setting The Oygarden Group of islands lies in the Neoproterozoic Rayner Complex of central Kemp Land, adjacent to its boundary with the Archaean Napier Complex (Fig. 1A). This part of Kemp Land is interpreted to represent Archaean crust that was reworked by the Rayner Structural Episode (RSE: Sandiford and Wilson, 1984; Sheraton et al., 1987). The Rayner Complex, which consists of both reworked Archaean and Proterozoic crust, experienced orogeny between ~ 1.0 and 0.9 Ga (Sheraton and Black, 1983; Clarke, 1987; Sheraton et al., 1987; Grew et al., 1988; Kelly et al., 2002) during convergence between an Indo-Napier craton (Napier Complex and peninsular India) and part of what now constitutes east Antarctica (Yoshida et al., 1996; Mezger and Cosca, 1999). The Oygarden Group of islands was affected by at least 5 episodes of deformation (D1–D5) over a ~ 1.8 Ga period during the Archaean (~ 3.6 Ga) and Proterozoic (~ 1.6, 0.93 Ga; Kelly, 2000; Kelly et al., 2000, 2002, 2004; summarized in Table 1). Structures in the islands are dominated by those that formed during the RSE (D3 and D4). The D3 deformation event is characterized by the effects of east-directed thrusting that occurred at medium to high-P granulite facies conditions (P ≈ 0.9–1.0 GPa, T ≥ 850–900 °C; Kelly et al., 2000, 2004; Halpin et al., 2007). D4 resulted in a 2–3 km wide extensional shear zone that recrystallized rocks in the south of the island group (e.g. Shaula Is, Fig. 1B), occurring at conditions similar to those that accompanied D3 (Kelly et al., 2000, 2004). Peak conditions were followed by decompression of up to 0.4–0.5 GPa, describing a clockwise P–T path for the RSE in the Oygarden Group (Kelly, 2000; Kelly and Harley, 2004). On the basis of U–Pb zircon dating (Kelly et al., 2002) and electron microprobe (EMP) dating of monazite (Halpin et al., 2007), the D3–D4 event is interpreted to have occurred as two closely spaced events in the period ~ 0.93–0.89 Ga.

60° E 50° E 65°S

La nd Enderby

Ke mp L and

MacRoberts

70° E

on Land

65°S

90°E

Oygarden Group Amundsen Bay

Mawson

Maw son

Casey Bay

0°

Coa st

180°

Stillwell Hills

50° E

Ice Shelf Northern Prince Charles Mountains

200km

Archaean rocks Archaean protoliths reworked during Rayner Structural Episode

A

90°W

70° E Amery

70°S

Oygarden Group Siri us

N

Rayner Complex with dominantly Proterozoic protoliths Inferred boundary between Archaean & Proterozoic crust

Is lan ds

Alphard Is Shaula Is

B

Karm Is

Fig. 1. (A) Location of the Oygarden Group of islands in east Antarctica. The dashed line designates the boundary between orthogneiss basement areas with predominantly Archaean and Proterozoic protolith ages. (B) Inset: central islands of the Oygarden Group.

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Table 1 U–Pb zircon age summary for the Oygarden Group of islands. Rock type

Inherited

Layered composite orthogneiss Homogeneous felsic orthogneiss A Homogeneous felsic orthogneiss B Syn-D3 diatextite Syn-D3 pegmatite Syn-D4 pegmatite

Crystallization age

Age(s) from rims

≥3650 Mat, ≥3470 Ma⁎ ≥2700 Ma ≥2700 Mat 904 ± 20 Ma ≥884 Ma 931 ± 14 Ma

~3500–3600 Mat >2000 Ma >2400 Ma, ~1630 Ma >2400 Ma, 1600 ± 21 Ma

~2500 Mat, ~1630 Ma, 926 ± 24 Ma ~2500 Mat, 929 ± 12 Ma 884 ± 24 Ma

Ages after Kelly et al. (2002), tKelly et al. (2004) and ⁎Halpin et al. (2005).

(quartz-absent), garnet-rich (quartz-present) and garnet-poor (quartzabsent) metapelite (assemblages are summarized in Table 2). Both quartz-absent assemblage types are dominated by orthopyroxene with variable abundances of sapphirine. Garnet-rich (quartz-absent) assemblages contain garnet porphyroblasts (up to several millimeters in diameter) that are embayed by randomly oriented intergrowths of orthopyroxene and sapphirine, with or without sillimanite, plagioclase and cordierite (e.g. Fig. 3A) and are interpreted to reflect high-T decompression following peak metamorphism in the Oygarden Group (Kelly and Harley, 2004). Garnet-rich (quartz-present) metapelite contains large euhedral grains of orthopyroxene in contact with plagioclase, which contains myrmekitic inclusions of quartz, suggesting these minerals grew in the presence of melt. Adjacent to biotite and quartz the garnet porphyroblasts have reacted to produce orthopyroxene overgrowths on biotite and moats of cordierite on garnet.

The last event recognized, D5, is characterized by narrow and discrete mylonites and ultramylonites that cut RSE structures and have a south over north sense of thrusting. These shear zones locally preserve amphibolite facies assemblages (P ≈ 0.6 GPa, T ≈ 600–650 °C; Kelly, 2000), with associated recrystallization and rehydration typically constrained to within 1 or 2 cm of these mylonites. These structures are not as yet constrained in age, but may be related to ~ 550–500 Ma Gondwana amalgamation in east Antarctica (Tingey, 1991; Zhao et al., 1992; Kinny et al., 1993; Carson et al., 1996; Shiraishi et al., 1997; Harley et al., 1998; Boger et al., 2001). 3. Sample context Monazite is hosted in zoned pods of high Mg–Al, silicaundersaturated metapelite, and lenses of quartz-bearing, Mg-rich pelitic gneiss that are adjacent to the quartz-absent rocks. These pods and lenses occur within and are enveloped by high D4 strain layering on Shaula Island (Figs. 1 and 2). Pods display a concentric zoning pattern characterized by anhydrous S3 mineral assemblages in the cores and 10–15 cm wide phlogopite-rich rinds that contain minerals aligned with S4 at pod boundaries (Kelly and Harley, 2004). Formation of rinds was associated with a metasomatic event that occurred prior to the development of S4 (>930 Ma; Kelly et al., 2002; Kelly and Harley, 2004). Three main assemblage/compositional types occur: garnet-rich

4. Accessory mineral textures The dominant accessory mineral assemblages identified in the samples investigated were zircon, monazite and rutile, with or without xenotime and apatite (Table 2). Minor fine-grained (b5 μm) thorite and uraninite were found as inclusions in zircon, and more rarely in monazite. Monazite occurs in a number of textural settings: inclusions in garnet, single grains in matrix domains,

(3) Garnet-rich, Quartz-absent

N

3

0

1

2

S4

2m

layered gneiss

schistose rind core B

(1) Garnetpoor

1

(2) Garnetpoor

S3

core 42 core A 3

1

core

sillimanite-rich gneiss

2

schistose rind

core B

core schistose rind

core A

6 Sillimanite-Sapphirine gneiss Sapphirine-absent gneiss

sillimanite-rich gneiss

53

biotite sapphirine

sillimanitesapphirine gneiss

schistose rind Location: 66°57.922'S 057°23.850'E

Fig. 2. Outcrop sketches and context of the metapelitic pods within S4 layering. Sketches numbered (1) Garnet-poor, (2) Garnet-poor and (3) Garnet-rich (quartz-absent) are zoned pods from which samples were taken for this study; sample locations within each pod are indicated by numbered dark boxes. 1: OG522, 2: OG524; 3: OG575, 4: OG576, 5: OG577; and 6: OG582A. Note: the Garnet-rich (quartz-present) sample (OG562), from a layer within the layered gneiss, is not pictured.

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Table 2 Summary of samples, giving lithological context, major mineral assemblages and the main monazite apparent age populations. Sample Garnet-poor, quartz-absent metapelite ‘pods’ OG522 OG524 OG575 OG576 OG577 Garnet-rich, quartz-absent metapelite ‘pods’ OG580 OG582 Quartz-present metapelite OG562

Lithology (see Fig. 2)

Silicate mineral assemblage

Accessory mineral assemblage

Pod 1 — core domain Pod 1 — edge of core Pod 2 — core domain Pod 2 — schistose rind Pod 2 — Sil-rich gneiss layer

Opx–Spr–Crn–Sil–Bt–(Grt) Opx–Spr–Crn–Sil–Crd–Bt–(Grt) Opx–Spr–Crn–Sil–Bt–(Grt) Opx–Spr–Crn–Sil–Bt–(Grt) Opx–Spr–Sil–Bt–Crd–(Grt)

Mnz–Xen–Zrc–Apt–Rt Mnz–Xen–Zrc–Apt–Rt Mnz–Xen–Zrc–Apt–Rt Mnz–Xen–Zrc–Apt–Rt Mnz–Xen–Zrc–Apt–Rt

Pod 3 — edge of core Pod 3 — Sil–Spr gneiss (rind)

Grt–Spr–Opx–Crd–Sil–(Pl–Bt) Grt–Spr–Opx–Crd–Sil–(Pl–Bt)

Mnz–Xen–Zrc–Apt–Rt Mnz–Xen–Zrc–Apt–Rt

Pelitic layer within layered gneiss

Grt–Opx–Plag–Kfs–Qtz–Crd–Bt–Sil

Mnz–Zrc–Rt

(Grt) = garnet only occurs as inclusions in peak minerals or in symplectite textures; (Pl–Bt) = minor occurrences. Abbreviations used in figures, tables and text: Grt = garnet, Qtz = quartz, Opx = orthopyroxene, Spr = sapphirine, Sil = sillimanite, Bt = biotite, Pl = plagioclase, Kfs = K-feldspar, Crd = cordierite, Crn = corundum, Mnz = monazite; Xen = xenotime, Zrc = zircon, Apt = apatite, and Rt = rutile.

intergrown with the reaction products of garnet breakdown, and as partial overgrowths on or intergrowths with other accessory minerals. Monazite in garnet-rich (quartz-absent) assemblages may occur as inclusions in porphyroblastic garnet (Fig. 3D), or as partially liberated inclusions, proximal to, or within cracks in resorbed garnet (Fig. 3A). Monazite is most abundant as grains within symplectites formed during post-peak garnet breakdown, either intergrown with orthopyroxene (Fig. 3B) or in textural equilibrium with sapphirine needles (Fig. 3C, F–I). The garnet-rich (quartz-present) metapelite studied contains monazite that may occur as rare inclusions in garnet (Fig. 4A, C), but predominantly as xenoblastic grains in the matrix intergrown with orthopyroxene, cordierite and/or biotite. The inferred former presence of melt in this rock may indicate that some growth or recrystallization of monazite occurred in the presence of melt. Monazite in garnet-poor (quartz-absent) metapelite occurs as xenoblasts (Fig. 5A) interstitial to matrix minerals, typically orthopyroxene, or in textural equilibrium with peak S3 minerals (e.g. sapphirine; Fig. 5B, C). Monazite grains may contain inclusions of apatite, or be overgrown by narrow rims of xenotime or apatite. In some samples, monazite occurs as overgrowths on zircon (Fig. 5D, F) or intergrown with xenotime (Fig. 5D).

5. Microanalysis of monazite and garnet 5.1. Analytical methods Accessory minerals and their textural settings were imaged in situ in polished thin sections using a Phillips XL30 Scanning Electron Microscope (SEM) at the School of GeoSciences, University of Edinburgh. Backscattered Electron (BSE) images were collected at 20 kV and 40–50 μA. Electron microprobe (EMP) analysis of monazite was conducted using a Cameca Camebax Microbeam electron microprobe located in the School of GeoSciences, University of Edinburgh, with additional data for some samples (OG582 and OG562) collected at the National Institute of Polar Research, Tokyo. Edinburgh data were collected with a beam width of b3 μm, an accelerating voltage of 20 kV, a beam current of 100 nA and reduced using the PAP method (after Pouchou and Pichoir, 1984). Data from Tokyo were collected at 15 kV, 200 nA and a beam size of b 3 μm. Data were reduced using ZAF correction procedures, with no systematic differences detected with data collected in Edinburgh. X-ray intensity maps of monazite were collected using the Cameca SX-100 electron microprobe (School of GeoSciences, University of Edinburgh), operating at 20 kV, 100 nA, a beam size of ~ 1 μm, and dwell time of ~ 200 m/s. Trace elements in garnet were determined using the Cameca ims-4f located in the Edinburgh Ion Microprobe Facility, University of Edinburgh, using methods covered in detail in Hinton and Upton (1991) and Kelly and Harley (2005).

X-ray data for monazite were calibrated against well-characterized natural and synthetic standards (see Supplementary data tables). Along with U, Th and Pb, a spectrum of elements (P, Si, Ca, Y, La, Ce, Pr, Nd, Sm, Gd and Dy) was analyzed to provide a more complete characterization of grain compositions, particularly important when interpreting data from complex populations. X-ray lines selected for EMP age analysis were: U Mβ, Th Mα and Pb Mβ, each measured on PET crystals. Interference of Th Mγ on U Mβ was corrected empirically by analyzing U Mβ on U-absent synthetic Thorite. Representative background positions (see Supplementary data tables) were selected using multiple WDS spectra run on monazite to be analyzed during the study, taking care to avoid major interferences and in the case of U Mβ, the Ar K absorption edge. During analysis, U, Th and Pb standards were routinely re-analyzed to check for spectrometer drift. U, Th and Pb were measured for 120, 120 and 240 s on peak, and 60, 120 and 240 s total count time on background, respectively. Detection limits were 0.02 wt.% for UO2, 0.01 wt.% for ThO2, and 0.02 wt.% for PbO. Representative monazite analyses are presented in Table 3 (full analytical data in Supplementary data tables). Prior to and during analytical sessions, secondary age standards (known age monazite) were analyzed to check for overall accuracy and reproducibility of results (Moacyr Brazilian Pegmatite monazite: ~506 Ma, Th–Pb TIMS age, J-M. Montel, pers. comm.; FG Madagascan monazite: 540± 15 Ma, EMP age, J-M. Montel, pers. comm.; 560 ± 1 Ma, U–Pb TIMS age, A. Möller, pers. comm.). Individual monazite EMP dates were calculated using a spreadsheet generated by T. Hokada, following the now well-established total-U–Th–Pb technique (Suzuki et al., 1991; Montel et al., 1996; Suzuki and Adachi, 1998; Williams et al., 1999; Williams and Jercinovic, 2002). Minimum age uncertainties for each analysis were calculated by propagating analytical errors on U, Th and Pb (relative standard deviation on the k-ratio) through the age equation as a linear combination, as previously described in Pyle et al. (2002). Cumulative probability and histogram plots, and grouped age calculations were carried out using Isoplot/Ex 3 (Ludwig, 2003), with grouped age uncertainties quoted at the 2σ level. Due to major scattering in total-Pb dates from the monazite grains discussed here, few samples were amenable to true grouped “age” calculation. Instead, weighted average “apparent” ages have been calculated for some data groupings. Data are presented in cumulative probability histograms on which the positions of major peaks are also given. These clusters of data and calculated group apparent dates are not necessarily considered true ages and care is taken to discuss geologic implications within that context. 5.2. Monazite compositional zoning Monazite compositional variations in garnet-rich and garnet-poor associations are dominated by the cheralite substitution (2REE 3+ =

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N.M. Kelly et al. / Chemical Geology 322–323 (2012) 192–208

B

A

C Mnz

Opx

Mnz

Mnz Zrc

Pl Mnz

Opx

Mnz Pl

Spr

Mnz Spr

Opx

Grt

Spr

Grt

E

D

F

1256 1147

922

884 875

855 897 905 932 909 892 951 922 887 919 882 916 908

999

883 890

G

880 985 927 944 922 926 877 929 929 927 937 731

H

971

864

883

900 861

894 885

885 894

Spr Opx

I

855 834

829

841 869

915

914

814 832 923 880 936 928 928 927

810 100 µm

Fig. 3. Backscattered Electron images of monazite and silicate-mineral textures in garnet-rich (quartz-absent) metapelite (sample OG582A). Numbers are EMP dates in millions of years. Corresponding concentrations of ThO2 and Y2O3 for each analysis are given in Supplementary Data Files. (A–C) Orthopyroxene + sapphirine ± plagioclase–cordierite symplectites formed at the expense of garnet, illustrating the textural context of monazite grains pictured in (E, G, I). Note the ‘straight’ edges between sapphirine and monazite, and intergrowth of monazite with sapphirine reaction products in (B) and (C); (D) monazite inclusion in garnet; (E–I) zoned monazite from reaction textures around garnet. Note the predominance of dark BSE cores and bright BSE rims. Inset in (F) is the context of the monazite grain with respect to sapphirine needles. Image (H) represents a zoomed view of the upper part of the monazite in (G), with dates omitted for image clarity. Mineral abbreviations are given in Table 2.

Th 4+ + Ca 2+), with an influence of the huttonite substitution (REE 3+ + P 5+ = Th 4+ + Si 4+). Monazite in each assemblage association defines contrasting compositional trends (e.g. Fig. 6A). Monazite from garnet-rich assemblages (OG562, OG582A) is dominantly HREEand Y-poor in comparison to monazite from garnet-poor assemblages (Fig. 6A), which is interpreted to reflect their growth concurrent with or following substantial growth of garnet. Monazite grains in garnet-rich (quartz-absent) assemblages typically preserve simple zoning of BSE-darker cores grading to BSE-bright rims that embay cores along curved, lobate fronts (Fig. 3E–I). The higher BSE brightness correlates with significant Th-enrichment in these rim domains (compare Fig. 3 with Fig. 6B–D) and these domains are typically slightly poorer in Y in comparison to cores and inclusions (Figs. 6C, 7A, B); LREE do not vary systematically (Fig. 6D). Rare grains (e.g. Fig. 7B, D) have very thin (b 10 μm), discontinuous outer rims elevated in Y (≤2.09 wt.% Y2O3). These rims embay or cut domains elevated in Th suggesting they overprint the high-Th rim generation. The overprinting Y-rich domains are especially pronounced in the monazite pictured in Fig. 7D, where domains of patchy Th- and Y-rich monazite truncate the rim domain. Monazite from garnet-rich (quartz-present) assemblages (OG562) preserve similar zoning patterns to those observed in other assemblage

types — homogeneous to patchy-zoned cores that may be rimmed by continuous or discontinuous rims of variable BSE-brightness (Fig. 4D–F). Locally, systematic core–rim compositional zoning patterns are preserved, where Y-rich and Th-poor rims overprint homogeneous or patchy-zoned cores (Figs. 4D, E, 7E, F). Other grains preserve more complex patchy zoning (Figs. 4f, 9G–I). Monazite grains from garnet-poor assemblages (samples listed in Table 2) are characterized by patchy zoning in cores (in BSE images) that are highly variable in composition, and with no systematic patterns between grains. Compositional data for these grains show a negative correlation between LREE and either HREE or Y (Fig. 6A), which suggests a decoupling of the REE instead of simple huttonite or cheralite substitution. Grains typically exhibit BSE-bright rims (Fig. 5A, E) that embay core domains along curved or irregular fronts. In addition, BSE-bright ‘vein-like’ features that can be traced to narrow rim zones may cut grain cores (Figs. 8D, 9). These veins are typically b2 μm wide and may preserve open pores, and more rarely, solid inclusions along their length (Fig. 9I–K). Where two or more veins occur in proximity, a broader bright zone is formed (Fig. 9I–J). These BSE-bright domains (rims and veins) correlate with higher Th concentrations compared to the cores on which they have formed (Fig. 8A, D). Rare grains have rims that are BSE-dark relative to cores (Fig. 8I).

N.M. Kelly et al. / Chemical Geology 322–323 (2012) 192–208

A

B

197

C

Pl

1255 1187

Pl

Grt Grt

1031

Bt

1268

Mnz

1179

Mnz

D

E 866

916

916

F

831 1179

961

912 937

698

729

1349 1276

924 917 2211 916 2034 801 912 1001 828 822 885

1252 1222 892 865

836

992 960

909

1184 1108

1473 861

812

1004

1135 895 905 947 803 869 790 880 942

819 741

6a

Fig. 4. Backscattered Electron images of monazite and silicate-mineral textures in garnet-rich (quartz-present) metapelite (sample OG562). Numbers are EMP dates in millions of years. Corresponding concentrations of ThO2 and Y2O3 for each analysis are given in Supplementary Data Files. (A–B) Textural context of monazite grains in (C, D, E). Dashed box in (A) is the location of monazite in (C). (C) Monazite inclusion in garnet. (D) Rayner-aged monazite with small ‘inherited’ core. (E) Monazite with broad core domain with pre-Rayner ages and narrow rim domain with predominantly Rayner ages. (F) Patchy zoned monazite. Mineral abbreviations are given in Table 2.

Surrounding many monazite grains associated with orthopyroxene in garnet-poor assemblages are narrow alteration zones (Fig. 9L). These zones appear to partially resorb the monazite and orthopyroxene, and do not appear when monazite is in contact with sapphirine. The existence

A

B

613

Spr

of these alteration haloes is equally pronounced in samples taken from anhydrous core and hydrous rind domains of the silica-undersaturated metapelite pods. Optical microscope investigation suggests the halo is composed of fine-grained sheet silicates and SEM-EDS analysis reveals a

C

Bt

552

578

984

898

925 1015 916

1430

Mnz

1051

Opx

1016 935

1016 898

1817

Zrc

1402

OG524

D

OG522

OG524

F

E

Mnz

Xen

851

Zrc

Zrc

Mnz

905

OG576

OG576

903

OG577

Fig. 5. Backscattered Electron images of monazite from garnet-poor (quartz-absent) metapelite. Numbers are EMP dates in millions of years. Corresponding concentrations of ThO2 and Y2O3 for each analysis are given in Supplementary Data Files. (A) Zoned monazite preserving pre-Rayner ages in the core and partially recrystallized rims (bright in BSE); occurs interstitial to matrix orthopyroxene (not visible); (B) monazite intergrown with S3 sapphirine and biotite (see Fig. 6A–C for more detail); (C) patchy zoned monazite grain with mixed Rayner and younger dates; (D) intergrowth of monazite (bright — M), xenotime (middle gray — X) and zircon (dark — Z); (E) patchy zoning in monazite (left half grain in D); and (F) monazite overgrowth (bright) on zircon. Mineral abbreviations are given in Table 2.

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N.M. Kelly et al. / Chemical Geology 322–323 (2012) 192–208

Table 3 Representative electron microprobe analyses of monazite. Sample

OG522

OG522

OG522

OG524B

OG524B

OG582A

OG582A

OG582A

OG562

OG562

Analysis #

MON-8a

MON-9a

MON-17

MON-16

MON-17

MON-31

MON-32

MON-24

MON-13

MON-12

1.11 29.55 2.79 1.46 0.69 10.12 0.32 14.57 25.27 2.32 8.46 1.32 1.34 0.61 99.91

1.33 28.79 2.79 1.47 0.70 10.18 0.29 14.53 25.10 2.43 8.61 1.25 1.43 0.68 99.59

0.79 29.23 5.22 1.88 1.60 9.33 0.66 13.49 24.61 2.20 7.71 1.26 1.63 1.42 101.03

0.66 30.30 3.95 1.42 0.88 8.10 0.30 14.60 25.55 2.38 8.55 1.41 1.79 0.94 100.83

0.46 30.94 5.38 1.74 1.62 7.80 0.48 13.70 24.06 2.21 7.86 1.40 1.97 1.34 100.96

0.91 29.16 0.33 2.44 1.71 12.00 0.70 15.04 25.65 2.36 8.23 1.12 0.72 0.11 100.46

2.76 26.07 2.09 1.61 1.52 16.57 0.75 12.78 22.42 2.12 7.64 1.40 1.63 0.57 99.92

3.46 25.25 0.19 2.34 1.34 22.10 1.06 12.39 22.06 2.23 7.65 0.91 0.45 0.00 101.42

0.68 28.66 1.88 0.78 0.20 5.18 0.53 16.63 30.54 2.75 10.21 1.33 0.99 0.41 100.76

1.17 27.79 0.28 1.53 0.61 9.11 0.44 14.71 28.09 2.57 10.50 2.35 1.15 0.15 100.45

Cations per 4 oxygens Si 0.04 P 0.97 Y 0.06 Ca 0.06 U 0.01 Th 0.09 Pb 0.00 La 0.21 Ce 0.36 Pr 0.03 Nd 0.12 Sm 0.02 Gd 0.02 Dy 0.01 Total 1.99

0.05 0.96 0.06 0.06 0.01 0.09 0.00 0.21 0.36 0.03 0.12 0.02 0.02 0.01 2.00

0.03 0.96 0.11 0.08 0.01 0.08 0.01 0.19 0.35 0.03 0.11 0.02 0.02 0.02 2.01

0.03 0.98 0.08 0.06 0.01 0.07 0.00 0.21 0.36 0.03 0.12 0.02 0.02 0.01 2.00

0.02 0.99 0.11 0.07 0.01 0.07 0.00 0.19 0.33 0.03 0.11 0.02 0.02 0.02 2.00

0.04 0.97 0.01 0.10 0.01 0.11 0.01 0.22 0.37 0.03 0.12 0.02 0.01 0.00 2.00

0.11 0.89 0.04 0.07 0.01 0.15 0.01 0.19 0.33 0.03 0.11 0.02 0.02 0.01 2.01

0.14 0.86 0.00 0.10 0.01 0.20 0.01 0.18 0.33 0.03 0.11 0.01 0.01 0.00 2.01

0.03 0.96 0.04 0.03 0.00 0.05 0.01 0.24 0.44 0.04 0.14 0.02 0.01 0.01 2.02

0.05 0.94 0.01 0.07 0.01 0.08 0.00 0.22 0.41 0.04 0.15 0.03 0.02 0.00 2.02

SiO2 P2O5 Y2O3 CaO UO2* ThO2 PbO La2O3 Ce2O3 Pr2O3 Nd2O3 Sm2O3 Gd2O3 Dy2O3 Total

Age Err (%, 1σ)

608 8

537 8

1031 2

638 7

912 4

major element composition close to that of the host orthopyroxene, with a slightly elevated Al Kα peak in EDS spectra. 5.3. Monazite age distributions Fig. 10 presents the general distribution of individual EMP total-Pb dates. To rely on dates calculated using this non-isotopic method, a number of assumptions are made (see Montel et al., 1996). Two significant assumptions are that: 1) insignificant common-Pb is present in the analyzed monazite relative to radiogenic-Pb, and 2) the isotopic system has not been disturbed, as concordance versus discordance cannot be evaluated. The distribution of EMP dates from the Oygarden samples, both on individual grain and sample scales, suggests both inheritance of old Pb and moderate to high degrees of disturbance of chemical and (by inference) isotopic systems in monazite. As such, the distributions of dates are interpreted within a framework provided by less disturbed zircon U–Pb isotopic ages (Kelly et al., 2002; Table 1) and with reference to patterns in the data. The utmost care is therefore taken before attempting to calculate weighted average “ages” and appropriate caution is applied to interpreting the significance of these ages. Monazite dates in the samples investigated here show a large variation that may or may not correlate with textural position or internal zoning patterns. Ages fall into three main age groupings: >1000 Ma, 800–1000 Ma, and 500–700 Ma (Fig. 10). Analyses with EMP dates that are >1000 Ma are not abundant and generally do not define specific age populations. However, these dates range from ~1000 Ma to as old as ~2935 Ma, and are found either as monazite inclusions within garnet (Figs. 3D, 4C), as distinct embayed cores within matrix monazite grains (Fig. 4D, E, 5A), or within patchy domains that also preserve ~900–1000 Ma ages (Fig. 4F). The ages are older than the timing of high-grade metamorphism associated with the RSE (~930–890 Ma in

914 4

810 4

927 3

2034 8

912 4

the Oygarden islands; Kelly et al., 2002), and where constrained by textural position and/or zoning patterns, are interpreted to reflect partially recrystallized, inherited monazite material. Pre-930 Ma dates from monazite analysis points that are inconsistent with these textural and chemical criteria are thus interpreted as artifacts possibly associated with inherited Pb components or U/Th–Pb fractionation during metamorphic recrystallization (e.g. Figs. 5C and 4F where immediately adjacent repeated analyses in the same BSE zone yield dates higher and lower than the age of the RSE). Across all samples the dominant peaks in data occur at ~ 900 Ma, although there is variation between samples (Fig. 10). These dates are taken from both matrix grains and monazite overgrowths on zircon (e.g. Fig. 5F). In garnet-rich samples (OG582A, OG562) ~ 900 Ma dates occur in both core and rim domains on the same grain (as defined by zoning in BSE and compositional maps; Figs. 3E, F, G, I, 4D), and also in rims on grains with older cores (Fig. 4E). In samples OG575, 576 and 577, which were taken from an anhydrous core, schistose rind and outer sillimanite–gneiss layer, respectively, of a single zoned garnet-poor metapelite pod (see Fig. 2, pod 2) the position of the main peak of dates is displaced from ~ 910 Ma (core domain), to ~ 875 Ma (rind domain) and ~ 845 Ma (sillimanite–gneiss; Fig. 10A–C). This gradual shift in age populations in these samples may suggest a gradual resetting of ~ 910 Ma monazite during a subsequent event. Another core domain sample (OG522; Pod 1, Fig. 2) shows a spread in dates that form a peak at ~ 1020 Ma (Fig. 10D). This spread in dates can be seen on an individual grain scale (Fig. 5C) and age zoning is rarely systematic with respect to core– rim position or zoning in BSE images or compositional maps. These dates, which are slightly older than the oldest recorded age for the high-grade RSE in the Oygarden Group, may reflect pockets of inherited common or radiogenic Pb within younger monazite (e.g.

N.M. Kelly et al. / Chemical Geology 322–323 (2012) 192–208

0.08

0.95 Grt-poor

0.85

OG582A

OG522 OG524 OG575 OG576 OG577

Grt-rich

0.90

OG582A OG562

0.07 0.06 0.05

0.80

Y (pfu)

LREE (pfu)

199

0.75

0.04 0.03

0.70 0.02 0.65

0.60

0.01

A 0

C

0.05

0.10

0.15

0 0.05

0.20

0.10

Y (pfu) 2.0

0.20

0.25

140

Inclusions BSE dark cores Intermediate BSE BSE bright rims

OG582A - Grt-rich, qtz-absent 1.8 1.6

120

OG575

OG522

OG576

OG524

OG577 100

1.4

Rims

Th/U (ppm)

1.2

Th+U+Si

0.15

Th (pfu)

1.0 0.8 Incl/ Cores

0.6

80

60

40

0.4 20 0.2 0

B 6

6.5

7

7.5

8

Y+REE+P

0

D 500

600

700

800

900

1000

Age (Ma)

Fig. 6. Compositional variation in monazite grains (analyzed by EMP, presented in cations per formula unit). (A) LREE (=∑La–Nd) vs Y illustrating compositional variation within and between samples. (B–C) Compositional trends between textural domains in monazite from sample OG582A (garnet-present, quartz-absent metapelite; REE = ∑La–Lu). (D) Th/U vs EMP age diagram emphasizing the relative increase in Th and decrease in U with age (garnet-absent metapelite). Mineral abbreviations are given in Table 2.

Seydoux-Guillaume et al., 2003). Importantly, no ages > 940 Ma occur in monazite that forms overgrowths on zircon or xenotime (e.g. Fig. 5F). Monazite analyses from sample OG524 (Fig. 10E), which was taken from the edge of the core of pod 1 (Fig. 2; transition zone of Kelly and Harley, 2004), display a displacement of the dominant “Rayner” peak to ~ 860 Ma, similar to that seen in outer zones of pod 2 (Fig. 10). EMP dates from monazite in garnet-poor samples also form broad distributions of data ranging from b900 Ma down to ~ 500 Ma. Within individual grains, these dates are commonly observed for BSE bright (Fig. 8A, D), or more rarely, dark (Fig. 8I) rims on ~ 900 Ma or older monazite cores (e.g. Fig. 5A). However, entire grains with minimal evidence for older preserved ages also give these younger apparent ages (Fig. 8J, K). These grains do not belong to any systematic textural association, nor location within the zoned pods. 5.4. Compositional and age zoning relationships Monazite grains from garnet-poor assemblages display a general correlation between composition and decreasing apparent age. This

is best illustrated within individual grains where young (b600 Ma) rim domains have higher Th, Si, LREE and Th/U, and lower Y, Ca, HREE and U, when compared with the cores on which they have formed (e.g. Fig. 8A–H). The compositional changes recorded in younger age domains correlate well with the composition of vein-like textures that are best observed in X-ray intensity maps (compare Figs. 8 and 9). On a sample scale, the correlation between age and compositional zoning is less well pronounced, interpreted to reflect initial compositional heterogeneity in Rayner-aged monazite, particularly those from samples OG575, OG576 and OG577. In comparison to monazite from garnet-poor samples, monazite grains from garnet-rich samples show less scatter in EMP dates, and importantly, only record minimal evidence for the ~ 600–500 Ma domains prevalent in populations from garnet-poor samples. In sample OG582A (garnet-rich, quartz-absent), inclusion monazite typically preserves dates older than events associated with the RSE (>1000 Ma), and trend to higher Y + HREE compositions than the dominant core populations (Fig. 6C). EMP dates from the high-Th rims developed on matrix monazite overlap with the main population, but some are younger. If treated as separate populations on the

200

N.M. Kelly et al. / Chemical Geology 322–323 (2012) 192–208

A

Th Mα

OG582A

339

Cts

100 μm

B

100 μm

G

Th Mα

200 μm

OG562

Cts

E

Th Mα

OG562

Cts

251

235

297

328

219

276

286

203

255

244

187

234

202

171

213

160

155

192

118

139

100 μm

D

200 μm

76 1069

F

Y Lα

Y Lα

123

Cts

406

582

964

382

529

859

359

476

754

336

423

649

313

370

543

289

317

438

266

264

333

243

211

352

412

370

635

Cts

Th Mα OG582A

318

171

Y Lα

C

100 μm

H Y Lα

200 μm

228

Cts

802

I

Ce Lα

219

Cts

2313

308

702

2258

264

602

2202

220

501

2147

176

401

2091

132

301

2036

88

200

1981

44

100

1925

0

200 μm

0

200 μm

1870

Fig. 7. X-ray intensity maps (Th, Y and Ce) for representative monazite grains from garnet-rich (quartz-absent) sample OG582A (A–D) and OG562 (E–I). Note that Y-enrichment in OG582A monazite only occurs as very narrow, incomplete rims except for a rare enriched middle domain in (C, D). Corresponding BSE images are presented in Figs. 3E, G, H and 5F.

basis of compositional differences and BSE zoning patterns, the monazite in OG582A gives weighted average ages of 903 ± 14 Ma (95% conf.; MSWD = 0.47; n = 38) for cores, and 883 ± 18 Ma (95% conf.; MSWD = 0.62; n = 23) for rims. These population ages are overlapping, but can be taken as minimum ages for the growth or recrystallization of monazite in this sample. On single grain scales compositional zoning may correlate with age for monazite from sample OG562 (garnet-rich, quartz-present; e.g. Fig. 4D), but this is not systematic in all grains throughout the sample. Analyses from relatively homogeneous core domains of matrix monazite give an average age of 879 ± 12 Ma (95% conf.; MSWD = 1.11; n = 56), excluding data points considered to reflect inherited monazite domains (>1000 Ma; see Fig. 4D–F). This population has significant scatter between individual dates (Fig. 10G), and therefore this calculated age is considered to be a minimum for the growth or recrystallization of monazite during high-T metamorphism.

5.5. Garnet compositional zoning Garnet porphyroblasts in garnet-rich (quartz absent) assemblages (OG582A) have reacted extensively to form symplectites of orthopyroxene and sapphirine, with or without plagioclase and cordierite. These garnet grains preserve core plateaus unzoned in major elements. They locally display subtle rimward increases in XAlm (core = 0.30, rim = 0.33–0.34) and decreases in XPyr (core = 0.64, rim= 0.61) within ~50–100 μm of the grain boundary, but are unzoned in XGrs (0.04–0.05) and XSps (0.01). In contrast, these grains are strongly zoned in many trace elements, showing an increase in Y and HREE and a weaker decrease in LREE–MREE within 400 μm of their rims (Fig. 11), with Gd remaining essentially unchanged. In garnet-rich (quartz-present) assemblages (OG562) garnets display limited reaction and resorption adjacent to matrix biotite, forming moats of cordierite and overgrowths of orthopyroxene on biotite. Garnet zoning is only

N.M. Kelly et al. / Chemical Geology 322–323 (2012) 192–208

A

Y Lα cts

B

8.4 2.27

36

505

1.57 7.4 2.93 8.7

499

Age (Ma) Y2O3

788 884 906 2.26 7.7

1.97 7.5

Th2O3

D

Zrc

9.68 1.33

Xen

100 µm

OG524

73

28

68

24

64

20

60

17

56

13

61 47

9

487

4.40 908 8.26

5.27 856 8.18

77

32

Y Lα cts

E

Th Mα cts

C

100 µm

5

521

Xen

201

43

Th Mα cts

F

850

430

812

373

775

316

737

259

700

202

662

145

625

88

587

50 µm

50 µm 31

OG576 U Mβ cts

G

503

Ce Lα cts

H

1890

444

1818

386

1745

327

1673

268

1600

210

1528

151

1456

93

1383

50 µm

50 µm 34

J

1311

550

I 4.43 8.4 886 3.19

828 7.9

5.56 8.1807 0.57 7.9 822

0.67

451 7.8

OG576

K Apt 2.94 533 9.2 3.41 535 9.0

2.42

1.72

744 8.7

589 9.1 2.95

476 8.9

1.59 9.5

561

3.77 622 8.9

1.34

548 9.2 1.61

581 9.7 584 3.29

0.99 9.1 820

8.4

OG575

553

8.9 2.01

OG577

Fig. 8. Backscattered Electron images and X-ray intensity maps for altered monazite from garnet-poor (quartz-absent) metapelite (EMP dates in millions of years), along with ThO2 (yellow) and Y2O3 (blue, italics) concentrations. (A–C) Monazite grain that has a weakly Th-enriched rim (slightly brighter BSE) with ~500 Ma dates developed on a core with partially reset Rayner dates. (D–H) Zoned monazite preserving a Th- and Ce-enriched, Y- and U-depleted rim. Very high Y X‐ray intensities in the peripheral areas of the grain are xenotime.

seen adjacent to localized resorbed domains, showing a near-rim increase in XAlm (core = 0.36, rim = 0.41) and decrease in XPyr (core= 0.61, rim =0.55), and a marked increase in Y + HREE and decrease in LREE–MREE within ~100 μm of the resorbed rim. Zoning in trace elements parallels the resorbed grain boundaries, and is not truncated by those boundaries. Moreover, in sample OG562, zoning is asymmetrical and only occurs adjacent to domains in garnet that have been resorbed. This zoning is therefore interpreted to have formed as a result of diffusion of Y + HREE into the garnet during decompression-driven garnet breakdown, and diffusive loss of

the LREE–MREE. Importantly, rimward enrichment of Y + HREE occurs in both xenotime-bearing (OG582A) and xenotime-absent (OG562) assemblages. 6. Discussion 6.1. Formation of monazite during the Rayner Structural Episode Monazite grain boundaries in garnet-poor (quartz-absent) metapelite pods are locally in textural equilibrium with S3/4 sapphirine (Fig. 5B, C),

202

N.M. Kelly et al. / Chemical Geology 322–323 (2012) 192–208

cts

BSE B

A

818

5.59 8.1

917 6.14 8.0 5.73 8.2

939

cts

C

414

792

379

766

344

740

310

714

275

688

240

662

205

887

900

5.72 8.2

915

25 µm

636

6.51 8.0

OG575

cts

D

1710

Ce Lα

Th Mα

610

cts

E

947

170

Y Lα

135

cts

F

292 263

1669

829

1628

711

235

1587

592

207

1546

474

178

1505

355

1464

237

121

1423

118

93

1382

G

Si Kα

0

150

U Mβ

L

H 9.2

Opx

8.7 2.09

840

Mnz

844

1.33

1.47

65

928 9.1 9.3 2.14

8.7 831 1.38 1.71 8.9

1.76

773

876 9.0

907

8.4 4.45

1.86 8.7

8.5 2.21

868

838

833 8.6

815

842

1.15

OG577

I

586

1.52 8.9 8.8 1.42

OG577

J

K

Fig. 9. Backscattered Electron images and X-ray intensity maps for altered monazite from garnet-poor, quartz-absent metapelite (EMP dates in millions of years), along with ThO2 (yellow) and Y2O3 (blue, italics) concentrations. (A–F) Rayner aged monazite with healed fractures that are Th and Ce enriched. (I–J) Enlargements from monazite grains in (A–F), (G) and (H), highlighting narrow rims (BSE—bright) that penetrate into the grains along now healed fracture networks (indicated by arrows). Some healed fractures preserve empty pores. (L) Alteration zone in orthopyroxene around monazite.

indicating that the age of at least part of the grains may reflect the timing of S3 and S4 fabric development in the Oygarden Group. A cluster of dates that form a peak at ~1020 Ma (OG522), which based on isotopic

dates from zircon are considered to be too old to represent growth during the Rayner event (Kelly et al., 2002), and therefore likely represent inherited monazite domains or artifacts caused by recrystallization.

N.M. Kelly et al. / Chemical Geology 322–323 (2012) 192–208

203

Resetting 15

OG575 ~910 Ma

10

A

24

Timing of high-T Rayner metamorphism from U/Pb zircon ages

~560 Ma 5

20 16

(n=42)

0 15

OG576

12

Cores ~905 Ma

B

~875 Ma

Rims ~885 Ma

8

Inclusions

10

Number

~575 Ma 5

Number

0 15 10

(n=110) 0

~845 Ma

OG562

Grt-rich, Qtz-present

C

~600 Ma

16

G

~890 Ma

12

5

8

0

(n=51)

15

OG522

10

4

(n=60) OG577

F

OG582

Grt-rich, Qtz-absent

Inclusions

4

~607 Ma

D

~1021 Ma

(n=88) 0 300

500

700

900

1100

1300

1500

Age (Ma) 5 (n=56) 0 15

Resetting OG524

10 ~605 Ma

E

~860 Ma

5 (n=42) 0 300

500

700

900

1100

1300

1500

Age (Ma) Fig. 10. Cumulative probability curves and histograms for EMP total-Pb dates and apparent ages for monazite. The wide stippled band through each figure indicates the U–Pb zircon age range for Rayner-aged metamorphic zircon from the Oygarden Group. Dashed lines in (A, B, C) indicate the relative offsets of the apparent age peaks between these three samples. Dates quoted in these figures (A–E) are not calculated ages, but approximate positions of peaks in the cumulative probability curves. Ages in quoted in (F–G) are weighted average ages calculated using data from texturally and compositionally defined core and rim domains.

The oldest data population that can be considered to reflect the Rayner event (~910 Ma) occurs in the anhydrous core domain of pod 2 (sample OG575; Fig. 2). In garnet-rich samples, monazite occurs both in the matrix (quartz-present: OG562) and in symplectites around decomposed garnet (quartz-absent: OG582A). Both samples have broad core domains giving Rayner ages (903±14 Ma and 879 ±12 Ma, respectively). Rims on monazite from sample OG582A are in textural equilibrium with garnet breakdown textures, and give an average age that is within error of the cores on which they form (883 ±18 Ma). These ages are close to estimates of metamorphic zircon formation in orthogneiss (~930 Ma) and crystallization of zircon from diatexite (~904 and ~884 Ma; Kelly et al., 2002). The data suggest that most monazite formed by new growth or recrystallization of pre-existing monazite during the RSE (~930–890 Ma in the Oygarden Group: Kelly et al., 2002; Halpin et al., 2007). In garnet-rich rocks, the preservation of older monazite domains both as inclusions in garnet (Figs. 3D, 4C) and small relics in the cores of matrix grains (Fig. 4D), suggests that at least some of these Rayner-aged

monazite grains also formed through in situ recrystallization of pre-existing monazite. In sample OG562, textural and mineral assemblage evidence indicates that the rocks partially melted during high-grade metamorphism, and we suggest that these monazite grains approximately date the timing of melt crystallization in the metapelite. This interpretation is consistent with zircon ages from Oygarden diatexite (~904 Ma and ~884 Ma; Kelly et al., 2002). Narrow rims developed on these grains are typically elevated in Y and depleted in Th relative to core domains (Fig. 7E, F), and may reflect the final stages of melt crystallization and/or growth during garnet decomposition. The origin of monazite in sample OG582A, in particular high-Th rims, will be discussed in detail below along with their significance to dating garnet breakdown. 6.2. High-temperature recrystallization in garnet-rich, quartz-absent metapelite Monazite grains from sample OG582A show systematic core–rim compositional variation. Cores have ages (903 ± 14 Ma) that overlap

204

N.M. Kelly et al. / Chemical Geology 322–323 (2012) 192–208

700 Rim

600

Core

500 400 300

Y

200

Concentration (ppm)

120 100 80 Dy

60 40

Yb 20 Sm

2.5 2.0 1.5 1.0

Nd

0.5

0

500

1000

1500

2000

Distance from grain edge (µm) Fig. 11. Selected REE data for an ion microprobe traverse from a garnet grain edge towards the interior of the grain (sample OG582A). Note the distinct increase in HREE(+Y) towards the rim and overall decrease in LREE towards the rim.

with rim ages (883 ± 18 Ma), but rims have distinctly higher Th and Si (Fig. 6B, C). These grains predominantly occur within symplectites around decomposed garnet porphyroblasts, and commonly have grain boundaries in textural equilibrium with symplectic phases (e.g. Fig. 3F–I). The location of monazite grains within the symplectites indicates that the cores either: 1) grew during garnet breakdown, or 2) grew during prograde metamorphism prior to occlusion by the growing garnet. Substantial growth beyond minor grain boundary migration (many grains are > 200 μm across) during garnet breakdown is unlikely because there is no evidence for breakdown of a reactant phase (garnet included) that contains the required LREE and Th to form appreciable amounts of monazite. One could consider the possibility of components being supplied in a fluid, but evidence is lacking to link monazite growth to such a fluid influx during garnet breakdown. Therefore, it is suggested that prograde growth (or recrystallization) formed the Rayner-aged monazite cores with only minor preservation of inherited domains. On the other hand, textural equilibrium between monazite grain boundaries and symplectite phases indicates that following release of monazite inclusions (i.e. their ‘exhumation’ from the interior of host garnets) during decompression and garnet-breakdown, these grains developed compositionally distinct rims that transgress and embay the core domains. The Y + HREE-poor nature of these monazite rims is consistent with our observations that garnet rims became progressively Y + HREEenriched during breakdown (Fig. 11). This has occurred despite the

presence of apatite and/or xenotime, possible competitor or receptor phases for these key elements. These Y + HREE relationships demonstrate that the often quoted interpretation that Y-rich domains in monazite indicate growth during a period garnet resorption (Pyle and Spear, 1999; Kohn and Malloy, 2004) does not hold for all high-T metamorphic rocks. Although the major mineral symplectite textures can be readily explained by the isochemical breakdown of garnet (see Kelly and Harley, 2004), the growth of Th-rich monazite (locally up to 22 wt.% ThO2) within these textures cannot be explained thus. The monazite grains occur in a symplectic intergrowth of anhydrous minerals (apart from cordierite) and the mineral assemblage and texture of the rock indicates that melt was not present during garnet-breakdown. The tightly constrained geometry and phase proportions in the symplectite point to diffusion-dominated and interface-controlled reaction, with fluid infiltration therefore playing a minor role, at least once reaction was initiated. It is most likely that these rims formed as the result of a re-distribution of material during decompression at high-T. Summarized in Fig. 12A is a suggested process where coarse-grained monazite liberated from the reacting garnet underwent a combined process of 1) grain boundary dissolution and minor regrowth to achieve textural equilibrium with newly growing reaction products of garnet, along with 2) a coupled dissolution/reprecipitation reaction (after Putnis, 2002) in the presence of low volume-percent fluid that enriched rim domains in Th. During grain boundary resorption (Fig. 12A-stage1a), some Y, HREE and P was liberated from monazite and scavenged by other accessory phases competing more effectively for these elements (e.g. xenotime or garnet). In the absence of a competitor phase, excess Th left following grain boundary readjustment would be incorporated into existing monazite during dissolution/reprecipitation reactions (Fig. 12A-stage1b). Therefore, in this scenario the composition of the resulting monazite rim is controlled by partitioning between monazite and other phases in the system that more effectively partition elements such as Y and HREE (e.g. garnet or xenotime). In some cases the recrystallization of grains has been extensive, dominating the grain volume (e.g. parts of monazite in Figs. 3G, 7C) whereas it has been more limited in others (e.g. Fig. 3E, F, I). The curved, inward embaying nature of these rims are interpreted as coupled dissolution–precipitation reaction fronts acting on the crystal, with sharp boundaries between Th-rich rims and Th-poor cores suggesting that any volume diffusion associated with this process was limited. Narrow (b10 μm) and discontinuous, Y-rich outer rims also occur on some monazite grains within the symplectites. These rims are irregular and do not correlate systematically with the thickness or location of Th-rich domains (Fig. 7B). Locally, these Y-rich domains are more extensive and may cut across Th-rich rims (Fig. 7C, D). These rims are interpreted to have formed during the latest stages of monazite recrystallization, concurrent with the breakdown of the outermost Y-HREE-enriched garnet rim. It is likely that as temperatures decreased and diffusion of the REE in garnet became limited, the continuing breakdown of garnet would contribute significant amounts of these elements into the local reaction environment. It is probable that at this time temperatures were not high enough for a large equilibration volume, restricting the supply of Y to those domains close to the garnet and allowing Y-enriched monazite to form (Fig. 12A-stage2). The textural and compositional patterns preserved in monazite from this study, and the processes they reflect, suggest that partitioning behavior of Y and HREE do not follow that predicted by empirical studies of garnet-monazite equilibrium (Pyle et al., 2001). Instead, they indicate that disequilibrium of the trace element inventory was significant within the symplectite despite temperatures in excess of 900 °C, conditions that are high enough to allow REE to diffuse within the garnets analyzed.

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B. Grt-poor / Qtz-absent -

A. Grt-rich / Qtz-absent - monazite in symplectite Cores: ~900 Ma Rims: ~885 Ma

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monazite in Opx matrix Spr

Stage 1a - resorption of original monazite rim

Cores: ~900 Ma Rims: ≥500 Ma

Spr

Th-rich

Opx

Th-rich Y-rich Stage 1b - recrystallization, Th-enrichment of rim

Stage 2 - late Y-enrichment

Y, HREE Y, HREE, P Th, Si Y, HREE Th, Si

Th, Si Y, HREE

Th, Si

Fig. 12. Schematic illustration of monazite grains undergoing recrystallization during post-peak metamorphic development of (A) garnet-reaction textures at ~890 Ma, and (B) fluid-mediated recrystallization at ~520–500 Ma.

6.3. Recrystallization during fluid infiltration The presence of monazite domains with post-Rayner ages indicates that recrystallization involving age resetting affected these monazite grains during an event unrelated to high-grade metamorphism during the RSE. Many grains in which localized or pervasive age-resetting has occurred preserve un-modified grain boundary relationships with peak, high-T minerals that equilibrated during the ~ 930–890 Ma Rayner event, suggesting that young domains did not form by mineral resorption followed by regrowth. Also, resetting of ages during a post-Rayner event cannot be solely explained by volume diffusion of Pb (Cherniak et al., 2004; Gardes et al., 2006, 2007; McFarlane and Harrison, 2006). Pb diffusion has been shown to be too slow to allow appreciable resetting even at relatively high temperatures. Moreover, experimental and empirical data from monazite (Ewing, 1975; Black et al., 1984; Meldrum et al., 1998; Seydoux-Guillaume et al., 2002) suggest that under normal crustal conditions and temperatures above 60–150 °C, amorphization due to radiation damage cannot be induced (Seydoux-Guillaume et al., 2003) and therefore Pb-loss is not suspected to be enhanced by this mechanism. Alternative mechanisms must therefore be invoked to explain age resetting in these monazite grains. A prior study (Kelly and Harley, 2004) concluded that the development of schistose rinds due to hydrous Si + K metasomatism occurred prior to, or during, the high-temperature RSE. Therefore, metasomatism associated with the formation of these rinds does not explain alteration of monazite subsequent to peak metamorphism at ~ 930–900 Ma. This is further supported by the fact that in garnet-poor metapelitic pods resetting of monazite occurred both within anhydrous core and schistose rind domains, with pervasive recrystallization affecting isolated grains in both domains. On the other hand, monazite grains from garnet-rich rocks show minimal effects of resetting, despite being from schistose rind domains (OG582A) or from layers within the D4 shear zone that envelops the pods (OG562). With any effect on major mineral assemblages absent or at least cryptic, textural and compositional data from the monazite grains may be used to gain some insight into processes causing age and compositional resetting.

The following discussion will therefore primarily focus on monazite from the garnet-poor assemblages in which post-RSE alteration textures are most pronounced. BSE-bright domains that occur in the monazite grains either as healed fractures, rims or more pervasively throughout grains are characterized by enrichment in Th, Si and LREE, and depletion in Y, HREE, Ca, U and P. BSE-bright rims are characterized by irregular, but commonly curved shapes (Fig. 8a, d), indicative of a reaction front that progressively altered the grains from the grain boundary inward. Furthermore, whole grains that have pervasively reset ages commonly show patchy zoning, similar to textures previously reported in monazite (e.g. Poitrasson et al., 1996, 2000; Townsend et al., 2000; Spear and Pyle, 2002) and zircon (Tomaschek et al., 2003; Geisler et al., 2007) that have been interpreted to represent the effects of fluid-mediated recrystallization. The proposition that alteration/recrystallization was driven in part by fluid ingress is supported by the occurrence of BSE-bright vein-like textures within some monazite grains that preserve open pore spaces (Fig. 9). These textures resemble healed and partly healed fractures that could have acted as conduits for fluids. In cases where such textures are preserved, recrystallization or new growth of monazite may have sealed the fractures, thereby limiting fluid access and further recrystallization. Further evidence for fluid infiltration is seen in the occurrence of alteration haloes around some monazite grains (Fig. 9l), a texture interpreted to reflect alteration of the silicate host mineral damaged by irradiation from monazite. Similar alteration haloes have been reported around thorianite (Seydoux-Guillaume et al., 2006). However, these zones of alteration need not be of the same age as the internal reaction fronts in monazite. The textural and chemical evidence presented above would suggest that the dominant mechanism that operated to alter monazite during the post-Rayner event was one that also involved dissolution– reprecipitation (after Putnis, 2002, 2009; summarized in Fig. 12b), similar to that invoked for the development of high-Th rims on monazite from the garnet-rich sample (OG582A). However, in contrast to those rims, the post-Rayner rims must have formed at lower temperatures and in the presence of a flux of fluid rather than a finite and small

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local fluid reservoir. In the absence of independent indicators (e.g. fluid inclusions), estimates of fluid composition, volume or flux, are purely speculation. However, recent experimental evidence (Harlov et al., 2011; Williams et al., 2011) suggests that fluid-driven recrystallization of monazite and development of Th-rich rims readily occurs in alkali-rich fluids, and such a mechanism could be invoked here. In contrast to the scenarios suggested by Poitrasson et al. (1996) where alteration resulted in overall REE-depletion, here REE behavior was decoupled producing depletion in HREE and relative enrichment in LREE. Therefore, we would suggest that although both the light and heavy rare earth elements are fluid-soluble, mineral-fluid partitioning – the LREE preferentially toward monazite while HREE potentially toward xenotime or other unrecognized phases – is a simpler explanation for relative depletion of LREE and enrichment of HREE in the affected (recrystallized) monazite. Although Th is considered immobile in most fluid compositions and across a wide temperature range (Bailey and Ragnarsdottir, 1994; Cuney and Mathieu, 2000; Seydoux-Guillaume et al., 2002), the absence of a likely competitor phase (e.g. thorite) would result in it strongly partitioning toward the recrystallized monazite. The process of recrystallization of monazite also resulted in apparent resetting of ages and therefore probable Pb-loss. Although mobility of Pb is not expected in fluids, Pb-loss and age resetting has been recorded in experimental studies of monazite alteration (e.g. Williams et al., 2011). In the Oygarden monazite grains Pb-loss appears locally incomplete, likely explaining the spread in dates both in domains with distinct compositional changes and domains adjacent to these. Incomplete recrystallization has been used to explain spurious ages in some studies (e.g. Baldwin et al., 2006) and has been suggested in some cases to result in Pb concentration in small volume defects (Seydoux-Guillaume et al., 2003). The excess scatter in data from multiple samples dictates caution with the geological meaning of retrieved dates. However, it is likely possible to extract meaningful ages from isolated domains of altered monazite through careful imaging and compositional analysis. In this study, select monazite grains preserve distinct core–rim relationships where the rims have consistent ages between 520 and 500 Ma (e.g. Fig. 8a, d), suggesting that this is the timing of recrystallization. 6.4. ~ 530–500 Ma events in east Antarctica Except for pegmatite intrusion in the eastern Napier Complex (~ 580 Ma and ~ 510 Ma; Pieters and Wyborn, 1977), geochronological data for a ~ 520–500 Ma event is previously unrecorded for Kemp Land. Evidence for events affecting the Oygarden Group following the RSE include the intrusion of pegmatite dykes and subsequent development of discrete mylonite–ultramylonite zones (Kelly, 2000). The intrusion of post-Rayner, pre-mylonite pegmatites could be considered as a potential source for alkali-rich fluids, and fluids from pegmatites have been suggested as being highly reactive (Harlov & Hetherington, 2010). The apparent effects of late shear zones should also be considered, with ambient temperatures during this event estimated to be in the amphibolite facies (Kelly, 2000). However, development of hydrous mineral assemblages and altered major element compositions are restricted to within a few centimeters of the shear zones. Further confidence in the interpretation of timing for an alteration event can be provided through comparison with event histories from adjoining areas. The common rim ages at ~ 520–500 Ma correlate well with regional tectonothermal activity between ~ 550 and 500 Ma in the western Rayner Complex (Enderby Land; Shiraishi et al., 1997; Carson et al., 2002), the northern and southern Prince Charles Mountains (Tingey, 1991; Manton et al., 1992; Boger et al., 2001; Corvino et al., 2008) and Prydz Bay (Zhao et al., 1992; Kinny et al., 1993; Carson et al., 1996; Harley et al., 1998; Kelsey et al., 2007). The effects of this event are locally or regionally pervasive in these areas. It is interesting

to note that Ar–Ar ages from detrital hornblende grains sampled off the Kemp Land coast are characterized by a dominant peak at ~ 550–500 Ma (Roy et al., 2007). This predominance of Pan African ages may hint towards more widespread overprint of mineral Ar–Ar systematics in rocks across Kemp Land, a thermal event that we hypothesize to have accompanied localized fluid-driven resetting of monazite. 7. Summary and conclusions Monazite grains in quartz-absent and quartz-present metapelites from the Oygarden Group, east Antarctica, preserve direct evidence for similar mechanisms of recrystallization at contrasting tectonometamorphic and fluid conditions. Integrated SEM and in situ (thin section) microanalysis revealed that: • Monazite in quartz-absent assemblages equilibrated texturally with peak (S3) minerals at ≥ 900 Ma, while monazite in quartz-present metapelite most likely records crystallization from melt at ≥ 880 Ma, ages that are consistent with previous estimates from U–Pb zircon geochronology. Despite the high temperatures associated with this event, older monazite domains are preserved. • Monazite rims in textural equilibrium with symplectites formed during high-T breakdown of garnet by recrystallization of liberated inclusions via a combined process of minor dissolution on grain boundaries and inward-propagating dissolution–precipitation reactions, facilitated by transient fluid films on the large surface areas in the symplectite, and at low fluid/rock ratio and negligible fluid flux, not by significant new growth. The ages of these rims confirm that the symplectites formed as a result of garnet breakdown at ≥ 880 Ma, and are not the result of a younger, overprinting low-P event. • During a fluid infiltration event that most likely occurred between ~ 550 and 500 Ma, monazite grains were partially to completely recrystallized. This caused changes in composition and distinct younging of ages, but did not impact the major mineral assemblages or apparent major element composition of the host rock. Recrystallization during both the high-T and lower-T fluid events led to monazite becoming depleted in Y, HREE, Ca, P and U, and relatively enriched in Th, Si and LREE. • This study demonstrates that monazite can be highly susceptible to resetting in certain environments, with fluid-present conditions dramatically increasing the potential for recrystallization. Importantly, if fluid types causing the specific chemical effects can be better constrained, monazite has the potential to be used to monitor the timing and compositions of fluid-related events, such as retrogression of high-grade metamorphic rocks, shear zone development, and hydrothermal ore body formation. Also, even in areas of significant recrystallization, the ages of previous crystallization events may be extracted. Supplementary data to this article can be found online at http:// dx.doi.org/10.1016/j.chemgeo.2012.07.001. Acknowledgments The analytical work for this study was completed while NMK was in receipt of a Royal Society of Edinburgh SEELLD Postdoctoral Fellowship at the University of Edinburgh. Samples used in the study were collected during the 1996/97 and 1997/98 austral summers (ASAC projects. 2214 and 1150) — NMK would like to thank the Australian Government Antarctic Division and expeditioners of ANARE's 49, 50 and 51 for support while in the field and at Mawson Station. Analyses were funded in part by Royal Society grant to SLH, a NERC-EMIF steering committee grant-in-kind and a NERC New Investigators award to NMK. NMK wishes to acknowledge the hospitality and support of the National Institute of Polar Research, Tokyo, and Tomokazu

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Hokada for assistance with EMP analysis. Nicola Cayzer is thanked for assistance with SEM imaging, Peter Hill and David Steele for assistance with the electron microprobe, and Richard Hinton for assistance with ion probe analyses. The manuscript was significantly improved with the aid of helpful and constructive reviews by Mike Williams and an anonymous reviewer, and through continuing lively discussion with other members of the metamorphic geochronology community. References Ayers, J.C., Miller, C., Gorisch, B., Milleman, J., 1999. Textural development of monazite during high-grade metamorphism; hydrothermal growth kinetics, with implications for U, Th–Pb geochronology. American Mineralogist 84, 1766–1780. Bailey, E.H., Ragnarsdottir, K.V., 1994. Uranium and thorium solubilities in subduction zone fluids. Earth and Planetary Science Letters 124, 119–129. Baldwin, J.A., Bowring, S.A., Williams, M.L., Mahan, K.M., 2006. 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