Continental Reworking during Overprinting Orogenic ... - Oxford Journals

JOURNAL OF PETROLOGY

VOLUME 50

NUMBER 11

PAGES 2017^2041

2009

doi:10.1093/petrology/egp065

Continental Reworking during Overprinting Orogenic Events, Southern Prince Charles Mountains, East Antarctica G. PHILLIPS1*, D. E. KELSEY2, A. F. CORVINO3 AND R. A. DUTCH2 1

DISCIPLINE OF EARTH SCIENCES, SCHOOL OF ENVIRONMENTAL AND LIFE SCIENCES, UNIVERSITY OF NEWCASTLE,

CALLAGHAN, NSW 2308, AUSTRALIA 2

CONTINENTAL EVOLUTION RESEARCH GROUP, SCHOOL OF EARTH AND ENVIRONMENTAL SCIENCES, UNIVERSITY

OF ADELAIDE, ADELAIDE, SA 5005, AUSTRALIA. 3

SCHOOL OF EARTH SCIENCES, THE UNIVERSITY OF MELBOURNE, MELBOURNE, VIC. 3010, AUSTRALIA

RECEIVED DECEMBER 11, 2008; ACCEPTED SEPTEMBER 3, 2009 ADVANCE ACCESS PUBLICATION OCTOBER 9, 2009

In situ electron microprobe monazite dating and mineral equilibria modelling of amphibolite^granulite-facies metapelites from the southern Prince Charles Mountains, East Antarctica has been carried out to unravel the P^Tconditions, spatial extent and structural style of two overprinting orogenic records.This study shows that: (1) rocks of the northern Palaeoproterozoic Lambert Complex were pervasively reworked at peak conditions (6·5^7·1 kbar and 790^ 8108C) during the Early Neoproterozoic Rayner orogenic event; (2) rocks of the southern Lambert Complex experienced pervasive deformation and metamorphism at peak conditions (5·8^6·1 kbar and 625^6358C) during Early Palaeozoic Prydz orogenic activity; (3) in regions of the Lambert Complex reworked during the Rayner orogenic event, Prydz-aged orogenesis was highly localized. The distribution of orogenic activity pertaining to the Rayner and Prydz orogenic events in the southern Prince Charles Mountains can be attributed to (1) the development of a southward directed (current coordinates) orogenic front that propagated from an Early Neoproterozoic collision between India and Antarctica, and (2) rock fertility (i.e. availability of free fluid) during Early Palaeozoic intraplate orogenesis that was driven by far-field stresses generated by a collision of India^Antarctica with the Mawson Craton.

I N T RO D U C T I O N

continental reworking; mineral equilibria modelling; monazite; Prydz orogenic event; Rayner orogenic event

The MacRobertson to Princess Elizabeth Land sector of East Antarctica is a collage of Archaean^Mesoproterozoic crustal fragments. The geological, geographical and spatial arrangement of these crustal fragments is chiefly the result of the Rayner (990^880 Ma) and Prydz (570^480 Ma) orogenic events (Fig. 1a and b). Many previous studies have investigated the structural, metamorphic and temporal evolution of rocks affected by these orogenic events, in turn, delineating extensive belts of crustal reworking (e.g. Clarke et al., 1989; Hand et al., 1994; Carson et al., 1995; Dirks & Wilson, 1995; Fitzsimons, 1996; Kelsey et al., 2003a, 2005, 2007; Kelly & Harley, 2004; Boger & Wilson, 2005; Halpin et al., 2007a, 2007b). However, in many places, Rayner- and Prydz-aged belts of reworking are superimposed, resulting in longstanding ambiguity as to the role of each reworking event in the development of metamorphic mineral assemblages and structural fabrics (e.g. Fitzsimons & Harley, 1991; Carson et al., 1995; Fitzsimons, 1996; Harley, 1998; Kelsey et al., 2003a, 2003b; Boger & Wilson, 2005; Tong & Wilson, 2006). In situ geochronology offers an approach that can be used to constrain the age of specific metamorphic mineral assemblages and structural elements in rocks affected by poly-tectonism. The benefit of this approach is that mineral

*Corresponding author. Telephone: þ61 (0)2 4921 5410. Fax: þ61 (0)2 49216925. E-mail: [email protected]

ß The Author 2009. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@ oxfordjournals.org

KEY WORDS:

JOURNAL OF PETROLOGY

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NOVEMBER 2009

Fig. 1. Locality maps of the Ruker Province in East Antarctica and East Gondwana. (a) MacRobertson to Princess Elizabeth Land region of East Antarctica. The focus of this study is the Ruker Province in the southern Prince Charles Mountains. (b) Map of the continental elements assembled in East Gondwana by the Early Palaeozoic (after Fitzsimons, 2000; Kelsey et al., 2008).

grains selected for geochronology are in their microstructural context within the rock sample (e.g. Williams & Jercinovic, 2002; Kelsey et al., 2003b, 2007). This study focuses on the southern Prince Charles Mountains, where previous geochronological studies have demonstrated that the Rayner and Prydz orogenic events reworked the region (Fig. 1; Tingey, 1991; Boger et al., 2001, 2006; Mikhalsky et al., 2006; Phillips et al., 2007a; Corvino et al., 2008). However, as in situ geochronology has not yet been undertaken in this region it is commonly unclear which event was responsible for the preserved metamorphic mineral assemblages, reaction microstructures and structural fabrics in a given rock mass. As a consequence, the spatial extent, structural style and metamorphic conditions associated with the Rayner and Prydz events remains somewhat unclear. It is therefore unknown whether polyphase structures and metamorphic mineral reaction microstructures reflect a single protracted event or multiple events separated in time (Boger & Wilson, 2005). In this paper we integrate thin-section petrography, mineral equilibria modelling and (Th þ U)^Pb electron probe microanalysis (EPMA) dating of monazite to establish a link between the temporal and tectono-metamorphic evolution of rocks from the southern Prince Charles Mountains. The over-arching purpose of the study is to address the outstanding problem of the spatial extent, P^ Tconditions and structural style of the Rayner and Prydz

orogenic events in the region. As a result, this study provides constraints for geodynamic models of the assembly of East Antarctica.

B AC KG RO U N D G E O L O G Y From north to south, the geology of the Prince Charles Mountains can be subdivided into: (1) the Neoproterozoic Rayner Complex; (2) the Mesoproterozoic Fisher Complex; (3) the Archaean^Neoproterozoic Ruker Province (or Ruker and Lambert terranes of Kamanev et al., 1993) (Crohn, 1959; Sheraton et al., 1980; Mikhalsky et al., 2001; Fig. 1a). In the north, rocks of the Rayner Complex preserve evidence of granulite-facies metamorphism and deformation between 990 and 880 Ma (Young & Black, 1991; Kinny et al., 1997; Boger et al., 2000; Kelly et al., 2002; Halpin et al., 2007a, 2007b). Along the Mawson Coast, regional metamorphism from 990 to 970 Ma was largely associated with an anticlockwise P^T path and peak conditions of 5·4^6·2 kbar and 850^9008C (Halpin et al., 2007a). Further to the south in the northern Prince Charles Mountains, metamorphism during the Early Neoproterozoic was associated with isobaric cooling from peak conditions of 6·0^7·9 kbar and 740^8808C (Fitzsimons & Harley, 1991; Thost & Hensen, 1992; Hand et al., 1994; Boger & White, 2003). Based on Rb^Sr biotite and U^Pb zircon dating, Early Palaeozoic pegmatite

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veins locally intrude and cut Early Neoproterozoic structures (Black et al., 1983; Clarke, 1988; Carson et al., 2000; Mikhalsky et al., 2001; Boger et al., 2002). The Fisher Complex (Fig. 1a) consists of mafic to intermediate metavolcanic rocks (Beliatsky et al., 1994; Mikhalsky et al., 1996, 1999; Kinny et al., 1997). Peak metamorphism occurred at conditions of 6·0^8·0 kbar and c. 6708C during the late Mesoproterozoic (1120^1000 Ma; Mikhalsky et al., 2001). Rocks of the Ruker Province are exposed in the southern Prince Charles Mountains and are the main focus of this study (Figs 1a and 2a). The geology of the southern Prince Charles Mountains has recently been described in a number of papers (Boger et al., 2006; Mikhalsky et al., 2006; Phillips et al., 2006; Corvino et al., 2008) and can be subdivided into three main units: (1) the Archaean Tingey Complex (or Ruker Terrane of Boger et al., 2001); (2) the Palaeoproterozoic Lambert Complex; (3) the Neoproterozoic Sodruzhestvo Group (Fig. 2a and b).

Tingey Complex Rocks of the Mawson Gneiss were emplaced between 3390 and 3370 Ma and are the oldest exposed units of the Tingey Complex (Fig. 2b; Boger et al., 2006; Mikhalsky et al., 2006). Tectonically interleaved with the Mawson Gneiss is the metasedimentary Menzies Group, which was deposited after c. 3100 Ma (Phillips et al., 2006). U^Pb sensitive high-resolution ion microprobe (SHRIMP) analysis of zircon rims from grains extracted from the Mawson Gniess yield ages of 2790^2770 Ma, which are suggested to record a period of tectonism (Boger et al., 2006). Based on detrital zircon age data, deposition of the Stinear Group (Fig. 2b) occurred after c. 2780 Ma (Phillips et al., 2006). An undeformed pegmatite dated at c. 2645 Ma is suggested to constrain a minimum deformation age for the Tingey Complex (Boger et al., 2001). Mineral equilibria modelling indicates that the rocks of the Tingey Complex were pervasively reworked at P^T conditions of 6·3^6·6 kbar and 590^6208 (Phillips et al., 2007a), which, based on the geochronological constraints, probably occurred between 2770 and 2645 Ma. During this tectonism, rocks of the Tingey Complex were folded into kilometre-scale nappe structures that were folded around upright, open folds (Fig. 3; Boger et al., 2006; Phillips et al., 2007b). Localized development of high-strain zones at metamorphic conditions of 4·0^5·0 kbar and 565^5808C occurred during the Early Palaeozoic (523^488 Ma), constrained by 40Ar/39Ar mica thermochronology (Phillips et al., 2007a).

Lambert Complex The Lambert Complex consists of voluminous packages of 2490^2420 Ma grey gneiss (Fig. 2b; Mikhalsky et al., 2006; Corvino et al., 2008). Metasedimentary rocks of the Ruker Group include grains of detrital zircon that are in the age

range 2600^2480 Ma, indicating that their deposition was either coeval with, or occurred after the emplacement of the Palaeoproterozoic grey gneiss (Phillips et al., 2006). Based on these chronological constraints, the Ruker Group is classified as a component of the Lambert Complex (cf. Phillips et al., 2006). Zircon grains from the grey gneiss comprise 2490^2420 Ma cores mantled by 2180^2080 Ma rims that are interpreted as evidence of reworking of the Lambert Complex during the Middle Palaeoproterozoic (Corvino et al., 2008). In addition, U^Pb dating of zircon grains from augen gneiss gives a mean age of 2152 10 Ma, which is comparable with zircon rim ages from the dominant grey gneiss (Fig. 2b; Corvino et al., 2008). Metasedimentary rocks located in the southernmost Prince Charles Mountains (Wilson Bluff and Blake Nunataks; Fig. 2a) also contain populations of 2660^2590 Ma and 2160^1830 Ma detrital zircon grains, which requires deposition of these units after c. 1830 Ma (Fig. 2b; Phillips et al., 2006). In the northern Lambert Complex (north of Harbour Bluff; Fig. 3a), Early Neoproterozoic strain manifests as isoclinal folds that are refolded around open^tight upright hinges (Corvino et al., 2007). Leucogranite dykes axial planar to the upright folds record intrusion ages of 907 15 Ma and 905  9 Ma (Corvino et al., 2008). Reworking of the southern Lambert Complex (south of Harbour Bluff; Fig. 3a) during the Early Palaeozoic is evident along the central Mawson Escarpment (Boger et al., 2001). U^Pb dating of zircon grains from folded leucogneiss reveals a crystallization age of 508 11 Ma (Boger et al., 2008). Following tectonism, cooling of the Lambert Complex below the closure of 40Ar/39Ar diffusion in biotite occurred between 525 and 478 Ma (Phillips et al., 2007a). A P^T path illustrating the evolution of metapelitic rocks from the central Mawson Escarpment is clockwise and has peak conditions of 6·0^7·0 kbar and 650^7008C (Boger & Wilson, 2005).

Sodruzhestvo Group The Sodruzhestvo Group consists of interlayered packages of quartzite, metasiltstone, metasandstone, metaconglomerate and minor carbonate-bearing layers (Fig. 2a and b; Phillips et al., 2005). Deposition of these rocks occurred after c. 950 Ma, which is constrained by the youngest population of detrital zircon (Phillips et al., 2006). A single phase of shortening folded the Sodruzhestvo Group into upright to reclined NW-trending structures. Metamorphism of the lower structural levels at conditions of P43·5 kbar and T45508C coincided with deformation (Phillips et al., 2005, 2007a). The timing of folding and metamorphism is constrained to 488^504 Ma by 40Ar/39Ar mica thermochronology (Phillips et al., 2007a).

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(b)

(a)

Fig. 2. (a) Simplified geological map of the southern Prince Charles Mountains. Geological subdivisions are based on the isotopic studies of Boger et al. (2006, 2008), Mikhalsky et al. (2006), Phillips et al. (2006) and Corvino et al. (2008). Outlines of geological units are from McLean et al. (2008). Locations of samples used for monazite dating and thermodynamic modelling are shown. (Refer to Fig. 1a for map location.) (b) Time^space plot of emplacement and depositional ages for the rock units of the Ruker Province.

Early Palaeozoic granites A kilometre-scale peraluminous granite pluton located at Harbour Bluff has an estimated emplacement age of 551 74 Ma (Figs 2a and 3a; Tingey, 1991). Further afield, pegmatite dykes, small (55 km2) felsic plutons and leucocratic veins are sporadically located throughout the southern Prince Charles Mountains. The emplacement of these rocks is constrained to the Early Palaeozoic by U^Pb zircon dating and 40Ar/39Ar thermochronology (Boger et al., 2001, 2008; Mikhalsky et al., 2006; Phillips et al., 2007a; Corvino et al., 2008).

Geodynamic models for the Rayner and Prydz orogenic events It is widely supported that tectonism during the Rayner orogenic event was driven by collision between India and

Antarctica (Sheraton et al., 1996; Mezger & Cosca, 1999; Kelly et al., 2002). Conversely, there are competing geodynamic models to explain the cause of Prydz-aged orogenic activity in the Prince Charles Mountains. This is particularly the case for the southern Prince Charles Mountains, where models involving collision (Boger et al., 2001) or intraplate orogenesis (Phillips et al., 2007a; Kelsey et al., 2008) have been invoked. The collision model requires a collision between India (and the attached Rayner Complex) and Antarctica during the assembly of Gondwana (Fig. 1a and b). The location of this collision zone is suggested to be in the Gibbs Bluff region in the southern Prince Charles Mountains (Fig. 2a; Boger et al., 2001). For an intraplate setting, far-field stresses generated during collision between India^Antarctica and the Mawson continent (Fig. 1b; Kelsey et al., 2008) are argued

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Fig. 3. Schematic cross-sections along the Mawson Escarpment (a) and Mount Stinear (b). Cross-sections illustrate the structural architecture of the outcropping basement rocks along with the sample locations. (For cross-section locations, refer to Fig. 2a.) HSZ, high-strain zone.

to have inverted the Neoproterozoic-aged sedimentary basins that contain rocks of the Sodruzhestvo Group (Phillips et al., 2007a).

SA M P L E DE SC R I P T ION, P E T RO G R A P H Y A N D M I N E R A L C H E M I S T RY Six metapelite samples were chosen for mineral equilibria modelling and (Th þ U)^Pb monazite microprobe dating. Of these six samples, four are from the Lambert Complex (samples 73, 94, 147, 326), one is from a tectonic contact between the Lambert and Tingey Complexes (sample 238) and one is from the Tingey Complex (sample 37; Figs 2a and 3a, b). Samples were selected to provide in situ information on the timing of metamorphism and deformation in rock units preserving metamorphic reaction microstructures and contrasting strain patterns. Coordinates for each sample location, the structural setting, the peak metamorphic mineral assemblages and post-peak mineral growth, along with the results of the mineral equilibria and geochronological study are presented in Table 1. Sample 73 is a migmatitic paragneiss that is tectonically interleaved with the dominant 2490^2420 Ma grey gneiss of the Lambert Complex. Geographically, the sample is

from the Waller Hills area along the northern Mawson Escarpment (Figs 2a and 3a). The mineral assemblage consists of feldspar (40^45%), biotite (15^20%), garnet (5^10%), sillimanite (5^10%), quartz (52%), spinel (52%) and accessory ilmenite and magnetite. A discontinuous foliation is defined by prismatic sillimanite (5200 mm in length) that is interspersed with finer flakes of biotite (5100 mm). Centimetre-thick layers of decussate coarse biotite (5500 mm) and rare idioblastic emeraldgreen spinel are wrapped by the sillimanite and biotite folia (Fig. 4a). Anhedral^subhedral poikiloblastic garnet grains up to 1cm in diameter contain biotite and feldspar inclusions and overgrow the biotite and spinel horizons (Fig. 4b). Sillimanite-rich inclusion trails in some garnet grains show overall continuity with sillimanite in the matrix, which can be interpreted to reflect the synchronous growth of these phases (Vernon et al., 2008). Feldspar is both plagioclase and K-feldspar. Blebs of fine-grained white mica locally replace feldspar along cleavage planes and fractures (Fig. 4c). Rare quartz generally has curved and embayed interfaces with surrounding minerals. Sample 94 is a metapelite that is representative of a large package of metasedimentary rocks found throughout the Sulzberger Bluff region of the central Mawson Escarpment (Figs 2a and 3a). As these rocks are structurally underlain by augen-gneiss that was emplaced at 2152 10 Ma they probably represent part of the Lambert

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66814·266’E

73805·801’S

Tingey Complex

Tingey Complexes

Lambert and

High-strain

High-strain

limb of regional fold

Low-strain, on the

and dykes

leucogranite bodies

Low-strain, close to

package of rocks

Medium-strain folded

package of rocks

Medium-strain folded

Structural setting

q þ pl þ bi þ st þ mu þ ilm

q þ bi þ sill þ g þ st þ cd þ pl

q þ bi þ chl þ mu þ g þ pl þ ilm

pl þ cd þ st þ bi þ g þ pl þ ilm

pl þ bi þ g þ sill þ q þ ksp þ liq

pl þ bi þ g þ sill þ q þ sp þ ksp þ liq

Peak metamorphic assemblage

cd

cd, mu

cd

mu

minerals

Post-peak

565–5808C

4·0–5·0 kbar

590–6508C

4·7–7·3 kbar

570–6008C

4·8–7·3 kbar

620–6358C

5·8–7·2 kbar

700–8208C

44·3 kbar

790–8108C

6·8 kbar

Peak P–T

Early Palaeozoic

Early Palaeozoic

Early Palaeozoic

Early Palaeozoic

rare Early Palaeozoic (rims)

Early Neoproterozoic,

Early Neoproterozoic

Neoproterozoic (rims)

(cores) and Early

Early Palaeoproterozoic

Monazite age population(s)

Metamorphism

Metamorphism

Metamorphism

Metamorphism

Polymetamorphism

Metamorphism

detrital(?)

Metamorphism or

Interpretation of ages

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bi, biotite; cd, cordierite; g, garnet; ilm, ilmenite; ksp, K-feldspar; pl, plagioclase; q, quartz; sill, sillimanite; sp, spinel; st, staurolite.

37

Contact between

68819·290’E

Lambert Complex

Lambert Complex

Lambert Complex

73828·730’S

68812·200’E

73814·260’S

68804·834’E

73810·917’S

68810·100’E

72858·633’S

Lambert Complex

Rock unit

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238

326

147

94

72850·217’S

73

68803·250’E

Coordinates

Sample

Table 1: Summary of sample localities, structural setting of samples, mineral assemblages, post-peak minerals, calculated peak P^T conditions and geochronological results

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Fig. 4. Photomicrographs illustrating key petrographic relationships used to constrain P^T conditions and construct P^T paths for rocks affected by the Rayner orogenic event. (a) Sample 73: sillimanite and biotite foliation wrapping around biotite and spinel. (b) Sample 73: sillimanite and biotite foliation wrapping garnet. (c) Sample 73: breakdown of alkali feldspar to muscovite (crossed polars). (d) Sample 94: garnet with sillimanite inclusions encased in a biotite corona and wrapped by sillimanite þ biotite foliation. (e) Sample 94: sillimanite breakdown in cordierite porphyroblast.

Group (Fig. 2b). The mineral assemblage consists of biotite (25^30%), quartz (20^25%), cordierite (20^25%), garnet (10^15%), feldspar (5^10%), sillimanite (5^10%) and minor white mica (51%). The dominant foliation is defined by prismatic (5100 mm in length) and fibrous sillimanite interlayered with flakes of brown biotite (550 mm).

Larger grains (5350 mm) of decussate biotite are oblique to the foliation. Garnet grains are generally poikiloblastic, anhedral^subhedral in shape and up to 1cm in size. They are generally mantled by biotite coronae and can contain inclusions of sillimanite, biotite and feldspar (Fig. 4d). Garnet grains also commonly contain aligned quartz

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Fig. 5. Photomicrographs illustrating key petrographic relationships used to constrain P^T conditions and construct P^T paths for rocks affected by the Prydz orogenic event. (a) Sample 147: garnet encased by staurolite that is surrounded by a cordierite corona. (b) Sample 147: staurolite replaced by cordierite (crossed polars). (c) Sample 326: subhedral garnet overgrowing chlorite and biotite foliation. (d) Sample 326: elongate garnet parallel to the main foliation.

inclusions that are oblique to the main foliation in the rock. In places, cordierite that is partially recrystallized to pinite and fine-grained white mica replaces sillimanite (Fig. 4e). Polygonal to embayed aggregates of quartz that contain sub-grain boundaries form most of the rock matrix. Feldspar is predominantly plagioclase with minor K-feldspar. Sample 147 is from a package of folded metapelites located in the Harbour Bluff region of the Mawson Escarpment (Figs 2a and 3a). Abundant, non-foliated leucogranite bodies and dykes intrude the metapelite package. The metapelites have a dominant schistosity that has been folded into kilometre-scale open, upright folds. The mineral assemblage of sample 147 consists of quartz (30^ 35%), cordierite (15^20%), staurolite (15^20%), biotite (5^10%), garnet (5^10%), feldspar (5%) and ilmenite (52%). A weak and discontinuous foliation in the sample is defined by aligned fine-grained biotite (5100 mm), ilmenite and polygonal quartz. The foliation both wraps around and overgrows euhdral garnet that is rich in

elongate and aligned inclusions of quartz and, to a lesser extent, ilmenite and chlorite (Fig. 5a). Large porphyroblasts (510 cm) of staurolite containing garnet are generally oblique to the main foliation but are also rarely parallel to the foliation (Fig. 5a). Throughout the sample, staurolite has been partially replaced by muscovite and cordierite, with the latter forming centimetre-scale coronae (Fig. 5a and b). The remaining rock matrix is composed of polygonal aggregates of quartz and rare simply twinned feldspar. Sub-grain boundary development in large embayed quartz crystals is common. Sample 326 is from a package of low-strain metapelites that are located in the Gibbs Bluff region of the southern Mawson Escarpment. Structurally, they are located on the northern limb of a kilometre-scale fold (Figs 2a and 3a). The mineral assemblage consists of quartz (25^30%), biotite (10^15%), chlorite (10^15%), white mica (10^15%), garnet (10^15%), feldspar (5^10%), ilmenite (52%) and titanite (52%). The dominant foliation is defined by aligned aggregates of intergrown chlorite, biotite,

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muscovite and titanite (Fig. 5c and d). Euhedral grains of garnet up to 300 mm in size contain elongate quartzrich inclusions that are aligned with the foliation. These garnet grains both overgrow and are wrapped by the mica-rich folia. In addition, elongate grains of garnet up to 500 mm in length are aligned parallel to the foliation (Fig. 5d). In the rock matrix, quartz generally has curved interfaces with surrounding minerals, whereas simply twinned feldspar grains are defined by sharp linear boundaries. Sample 238 is from a high-strain package of metapelites that are located in tectonic contact between the Lambert and Tingey Complexes. Geographically this contact is located in the Gibbs Bluff region of the southern Mawson Escarpment (Figs 2a and 3a). A detailed petrographic description of sample 238 was previously published in Boger & Wilson (2005) and is summarized here. The mineral assemblage consists of quartz, biotite, sillimanite, garnet, staurolite, cordierite and plagioclase. Elongate grains of biotite intergrown with quartz, plagioclase and fibrous mats of sillimanite define the main foliation. Large poikiloblasts (52 cm) of cordierite overgrow the foliation and additionally contain inclusions of relict staurolite and small (5100 mm) inclusion-free garnet grains (Boger & Wilson, 2005, fig. 7a and b). Sample 37 is from a sequence of metapelites located in a 400 m wide high-strain zone at Mt Stinear. The highstrain zone marks the contact between metasedimentary and gneissic rocks of the Tingey Complex (Figs 2a and 3b). A detailed petrographic description of sample 37 was published previously in Phillips et al. (2007b) and is summarized here. The mineral assemblage consists of quartz, plagioclase, biotite, staurolite, muscovite and ilmenite. A strong foliation is defined by aligned fine-grained (300 mm long) white mica, along with elongate grains of quartz and plagioclase (Phillips et al., 2007b, fig. 5e). Poikiloblastic staurolite, biotite and ilmenite up to 2 cm in size preserve quartz inclusions that parallel the foliation (Phillips et al., 2007b, fig. 5f).

M I N E R A L C H E M I S T RY Mineral chemical data were collected over two sessions using (1) a CAMECA SX-50 Electron Probe Microanalyser (operating conditions 15 kV and 25 nA) at the School of Earth Sciences, The University of Melbourne, Australia, and (2) a PHILIPS XL30 scanning electron microscope with attached IS200 energy-dispersive spectrometer (operating conditions 15 KV and 0·2 nA) at the EM-X-ray facility, The University of Newcastle. Data were collected as weight per cent oxides and stoichiometric formulae were calculated using standard techniques outlined by Deer et al. (1992). Where applicable, Fe3þ contents were estimated using the technique of Droop (1987). New mineral composition data are summarized in Table 2.

For completeness, pre-existing mineral composition data from the studies of Boger & Wilson (2005) and Phillips et al. (2007b) are also shown in Table 2. Garnet analyses are normalized to 12 oxygens per formula unit (p.f.u.), with Fe3þ estimated by normalizing analyses to eight cations. Garnet in sample 73 is an almandine-rich, almandine^pyrope solid solution with XAlm ¼ Fe2þ/(Fe2þ þ Mg þ Mn þ Ca) ¼ 0·72 and XPyp ¼ Mg/(Fe2þ þ Mg þ Mn þ Ca) ¼ 0·20. Minor grossular XGrs ¼ Ca/(Fe2þ þ Mg þ Mn þ Ca) ¼ 0·05 and spessartine XSps ¼ Mn/(Fe2þ þ Mg þ Mn þ Ca) ¼ 0·03 are also present. Garnet in sample 73 is compositionally unzoned. Garnet in sample 94 is a almandine^pyrope solid solution with XAlm ¼ 0·70^0·76 and XPyp ¼ 0·18^0·24. Garnet cores are subtly richer in Mg, whereas Fe and Ca increase towards the rims. Two textural settings exist for garnet in sample 147: (1) garnet grains located in the rock matrix; (2) garnet grains located as inclusions in staurolite. Compositionally, in both textural settings garnets are an almandine-rich, almandine^pyrope solid solution with XAlm ¼ 0·73^0·81 and XPyp ¼ 0·11^0·14. Garnet inclusions in staurolite are slightly enriched in Fe2þ compared with those in the rock matrix. In addition, they show subtle compositional zoning with rimward enrichment of Fe2þ and a decrease in Ca. Garnet in sample 326 is also an almandine^pyrope solid solution with XAlm ¼ 0·65^0·68 and XPyp ¼ 0·24^0·26 and has a similar composition irrespective of textural setting and habit (e.g. Fig. 5d). Biotite formulae are normalized to 22 oxygens with Fe calculated as Fe2þ. In sample 73, biotite is Fe-rich [XMg ¼ Mg/(Mg þ Fe) ¼ 0·43] with 0·37 Ti p.f.u. Biotite in sample 94 has XMg ¼ 0·49^0·50 with 0·27^0·32 Ti p.f.u. Comparable biotite compositions in sample 147 have XMg ¼ 0·47 and 0·29 Ti p.f.u. Biotite in sample 326 has slightly higher Mg contents (XMg ¼ 0·59). Ti contents for the same grains range between 0·32 and 0·35 p.f.u. White mica analyses are also normalized to 22 oxygens. Compositional data from sample 37 indicate that white mica is largely muscovite in composition [XK ¼ (K/ (K þ Na)) ¼ 0·59]. Stoichiometric formulae for chlorite are normalized to 28 oxygens with total Fe assumed to be Fe2þ. Ferroan-chlorite is dominant in sample 147 [XMg ¼ Mg/(Mg þ Fe þ Mn) ¼ 0·29], whereas chlorite in sample 326 is magnesium-rich (XMg ¼ 0·60) Ilmenite analyses are normalized to three oxygens with Fe calculated as Fe2þ. IImenite in samples 326 and 37 is slightly enriched in pyrophanite with Mn contents of 2·01^2·46 and 1·59^1·79 p.f.u., respectively. By comparison, ilmenite in sample 147 is close to the pure end member with 0·35 Mn p.f.u and 0·26 Mg p.f.u. Feldspar is dominantly albite in samples 73, 94 and 326 [XAn ¼ (Ca/ (Na þ Ca) ¼ 0·24^0·35]. In contrast, feldspar in sample 147 is anorthite (XAn ¼ 0·88).

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Table 2: Representative mineral chemistry data Sample 73 g (core)

Sample 94 g (rim)

bi

sp

fsp

g (core)

g (rim)

bi (coarse)

bi (fine)

cd

fsp

55·14

SiO2

36·38

36·80

33·11

0·00

57·67

37·39

37·87

35·70

35·09

48·78

TiO2

0·02

0·02

3·18

0·04

0·01

0·02

0·00

2·42

2·74

0·00

0·02

Al2O3

21·16

21·22

18·91

56·20

25·54

21·63

21·27

19·10

19·98

32·84

28·12

0·02

0·03

0·11

0·76

0·00

0·04

0·00

0·07

0·04

0·00

0·00

FeO

Cr2O3

33·55

33·93

21·09

36·12

0·02

32·71

33·84

19·00

17·83

9·03

0·02

MnO

1·40

1·44

0·03

0·23

0·00

0·63

0·98

0·00

0·00

0·11

0·00

MgO

4·95

4·61

8·82

3·56

0·00

6·15

4·58

10·22

10·00

8·26

0·01

CaO

1·56

1·76

0·00

0·00

7·95

1·60

2·07

0·00

0·00

0·03

10·54

Na2O

0·04

0·05

0·33

0·05

7·14

0·03

0·03

0·06

0·14

0·11

5·85

K2O

0·01

0·02

9·27

0·00

0·18

0·01

0·02

9·66

9·49

0·02

0·14

ZnO

0·01

0·13

0·01

1·74

0·02

0·00

0·01

0·00

0·00

0·00

0·00

Total

99·09

100·00

94·85

98·70

98·52

100·19

100·68

96·23

95·31

99·19

99·84

Oxygen

12

22

32

32

22

22

18

32

12

12

12

Si

2·94

2·95

5·13

0·00

10·08

2·95

3·00

5·37

5·29

4·99

9·65

Ti

0·00

0·00

0·37

0·01

0·00

0·00

0·00

0·27

0·31

0·00

0·00

Al

2·01

2·00

3·45

15·42

5·26

1·99

1·98

3·38

3·55

3·96

5·80

Cr

0·00

0·00

0·01

0·14

0·00

0·00

0·00

0·01

0·00

0·00

0·00

Fe2þ

2·09

2·11

2·73

7·03

0·00

2·10

2·19

2·39

2·25

0·77

0·00

Fe3þ

0·18

0·16

0·06

0·05

Mn

0·10

0·10

0·00

0·04

0·00

0·04

0·07

0·00

0·00

0·01

0·00

Mg

0·60

0·55

2·04

1·24

0·00

0·72

0·54

2·29

2·25

1·26

0·00

Ca

0·14

0·15

0·00

0·00

1·49

0·14

0·18

0·00

0·00

0·00

1·98

Na

0·01

0·02

0·20

0·05

4·84

0·00

0·01

0·03

0·08

0·04

3·97

K

0·00

0·00

1·83

0·00

0·04

0·00

0·00

1·85

1·83

0·00

0·03

Zn

0·00

0·01

0·00

0·30

0·00

0·00

0·00

0·00

0·00

0·00

0·00

Total

8·06

8·06

15·78

24·24

21·72

8·03

8·02

15·61

15·57

11·05

21·44

0·43

0·49

0·50

XMg

0·20

0·19

XFe

0·72

0·73

XCa

0·05

XMn

0·03

—

—

—

XAn

0·76

XMg

0·24

0·18

XAb

0·24

XFe

0·70

0·74

0·05

XCa

0·04

0·06

0·03

XMn

0·01

0·02

—

XMg

—

—

XFe

0·38

—

XAn

0·67

XAb

0·33

(continued)

T EXT U R A L S ET T I NG OF MONA Z I T E A N D ( T h þ U ) ^ P b DAT I N G Textural relationships between monazite, tectonic fabrics and metamorphic mineral assemblages provide a critical link between the kinematic^metamorphic record and geochronology. The dating of monazite grains is routinely carried out by measuring the total amounts of U, Th and Pb by electron microprobe (Montel et al., 1996; Williams & Jercinovic, 2002). Monazite is typically rich in the radioactive elements Th and U, and therefore radiogenic Pb

accumulates at a rate such that measurable quantities (4300 ppm) are reached in c. 100 Myr (Montel et al., 2000). Previous studies (e.g. Parrish, 1990) have shown that monazite contains negligible common Pb compared with the radiogenically produced component and therefore it can be reasonably assumed that the measured Pb is dominantly the result of radiogenic breakdown of Th and U (Jercinovic & Williams, 2005; Pyle et al., 2005). Owing to a c. 500 Myr interval between the Rayner and Prydz orogenic events, a reconnaissance and time-effective dating protocol, namely chemical dating by EPMA, was employed. The details of the analytical protocol are

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PHILLIPS et al.

OVERPRINTING OROGENIC EVENTS

Table 2: Continued Sample 147 g (core)

g (rim)

g (core) in st

g (rim) in st

SiO2

37·37

37·09

37·35

36·21

TiO2

0·01

0·00

0·00

0·01

Al2O3

21·27

21·09

20·86

Cr2O3

0·08

0·11

0·08

FeO

34·20

34·17

MnO

1·77

0·74

MgO

3·24

CaO

3·28

st

bi

chl (in g)

cd

fsp

ilm

27·76

35·57

23·51

48·55

43·86

0·00

0·67

2·57

0·08

0·00

0·00

52·31

20·42

53·53

18·08

21·19

32·80

35·03

0·01

0·00

0·00

0·06

0·02

0·00

0·00

0·09

35·38

36·11

15·12

19·74

36·13

8·60

0·03

46·54

0·74

1·47

0·09

0·04

0·10

0·06

0·00

0·35

3·21

3·45

2·68

1·96

9·93

8·16

8·33

0·00

0·26

4·14

2·95

1·65

0·05

0·00

0·00

0·01

18·92

0·00

Na2O

0·01

0·11

0·06

0·00

0·03

0·41

0·07

0·37

0·72

0·00

K2O

0·02

0·00

0·04

0·00

0·02

8·72

0·00

0·02

0·03

0·01

ZnO Total Oxygen

0·00

0·00

0·00

0·00

0·80

0·01

0·00

0·01

0·00

0·07

101·25

100·66

100·91

98·55

100·03

95·13

89·26

98·75

98·59

99·64

22

28

18

32

12

12

12

12

48

3

Si

2·97

2·96

2·98

2·98

7·95

5·41

5·17

4·98

8·19

0·00

Ti

0·00

0·00

0·00

0·00

0·14

0·29

0·01

0·00

0·00

1·00

Al

1·99

1·99

1·96

1·98

18·08

3·24

5·50

3·97

7·71

0·00

Cr

0·01

0·01

0·01

0·00

0·00

0·01

0·00

0·00

0·00

0·00

Fe2þ

2·18

2·11

2·22

2·41

3·62

2·51

6·65

0·74

0·01

0·99

Fe3þ

0·09

0·17

0·14

0·08

Mn

0·12

0·05

0·05

0·10

0·02

0·01

0·02

0·01

0·00

0·01

Mg

0·38

0·38

0·41

0·33

0·84

2·25

2·68

1·27

0·00

0·01

Ca

0·28

0·35

0·25

0·15

0·02

0·00

0·00

0·00

3·78

0·00

Na

0·00

0·03

0·02

0·00

0·03

0·24

0·06

0·15

0·52

0·00

K

0·00

0·00

0·00

0·00

0·01

1·69

0·00

0·00

0·01

0·00

Zn

0·00

0·00

0·00

0·00

0·17

0·00

0·00

0·00

0·00

0·00

Total

8·03

8·06

8·05

8·03

30·88

15·64

20·09

11·11

20·22

2·00

XMg

0·13

0·13

0·14

0·11

XFe

0·74

0·73

0·76

0·81

XCa

0·09

0·12

0·09

0·05

XMn

0·04

0·02

0·02

0·03

—

—

0·47

—

—

XFe

0·37

—

XAn

0·12

XAb

0·88

—

(continued)

presented in the Appendix. The total (Th þ U)^Pb dataset is available as Supplementary Data, at http:// www.petrology.oxfordjournals.org/. In sample 73, two distinct textural settings of monazite are evident. First, large (200 mm) rounded to angular embayed grains are intergrown with the dominant biotite and sillimanite foliation (Fig. 6a). Second, small to large (20^100 mm), rounded to angular grains can be located along the grain boundaries of, or included within, biotite, plagioclase or quartz (Fig. 6b and c). Late Palaeoproterozoic ages were dominantly recorded in the large monazite grains, whereas Early Neoproterozoic ages were located on the rims of the larger monazite grains, or

within small grains grown along the boundaries of biotite, feldspar, sillimanite, garnet and quartz (Fig. 7a). Monazite in sample 94 is typically lenticular in shape, between 50 and 100 mm in length, has corroded grain boundaries, and occurs within the sillimanite and biotite foliation (Fig. 6d). In addition, larger angular grains (c. 200 mm) occur within coarse-grained biotite (Fig. 6e), and rounded grains (up to 100 mm) that occur in sillimaniteand K-feldspar-rich patches in the rock matrix are also present (Fig. 6f). Monazite grains from all three textural settings record Early Neoproterozoic ages (Fig. 7b). A smaller population of Early Palaeozoic ages was located on the outer rims of some monazite grains (Fig. 7b).

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Table 2: Continued Sample 326 g (core)

g (rim)

bi

chl

fsp

ilm

SiO2

37·97

38·08

36·54

27·72

58·69

0·25

TiO2

0·00

0·13

3·00

0·09

0·05

53·16

Al2O3

21·26

21·26

16·60

19·45

26·60

0·17

Cr2O3

0·12

0·07

0·13

0·05

0·00

0·00

FeO

30·19

31·70

16·31

21·45

0·00

44·15

MnO

0·90

1·08

0·07

0·23

0·03

2·46

MgO

6·88

6·22

13·23

18·47

0·00

0·20

CaO

2·34

2·15

0·01

0·04

8·56

0·09

Na2O

0·02

0·23

0·15

0·13

6·60

0·09 0·03

K2O

0·00

0·03

9·22

0·03

0·01

ZnO

0·00

0·00

0·00

0·00

0·00

0·00

Total

99·68

100·95

95·26

87·66

100·54

100·60

Oxygen

24

22

28

32

3

Si

2·99

24 2·98

5·48

5·71

10·07

0·01

Ti

0·00

0·01

0·34

0·01

0·01

1·00

Al

1·97

1·96

2·93

4·72

5·38

0·00

Cr

0·01

0·00

0·02

0·01

0·00

0·00

Fe2þ

1·91

1·86

2·05

3·69

0·00

0·92

Fe3þ

0·07

0·21

Mn

0·06

0·07

0·01

0·04

0·00

0·05

Mg

0·81

0·72

2·96

5·67

0·00

0·01

Ca

0·20

0·18

0·00

0·01

1·57

0·00

Na

0·01

0·07

0·09

0·10

4·39

0·01

K

0·00

0·00

1·76

0·01

0·00

0·00

Zn

0·00

0·00

0·00

0·00

0·00

0·00

Total

8·03

8·07

15·63

19·97

21·43

2·00

XMg

0·27

0·26

0·59

XFe

0·64

0·66

XCa

0·07

0·06

XMn

0·02

0·03

—

—

—

XAn

0·74

XAb

0·26

—

(continued)

There are three main textural settings of monazite in sample 147. Elongate ovoid grains 50^100 mm in length occur parallel to the biotite foliation (Fig. 6g) and along biotite and garnet interfaces. Larger (c. 200 mm) angular grains that are oblique to the foliation are also present. Additionally, rounded to angular embayed grains up to 100 mm in size occur within clots of texturally late cordierite (Fig. 6h). Irrespective of textural setting, all monazite ages are in the range Late Neoproterozoic^Early Palaeozoic (Fig. 7c). Three distinctive habits characterize monazite in sample 326. Elongate grains between 50 and 100 mm in length are parallel to the foliation and occur along grain boundaries

of biotite, chlorite, plagioclase and/or garnet (Fig. 6i and j). In addition, small (c. 20 mm) elongate and rounded grains are located as inclusions in garnet (Fig. 6k). Finally, larger (60^80 mm), irregularly shaped, rounded grains that are not parallel to the main foliation occur as inclusions within plagioclase and quartz in the rock matrix. EPMA analyses of monazite grains representing each of the microstructural settings give Late Neoproterozoic^ Early Palaeozoic ages (Fig. 7d). Grains of monazite in sample 238 have two main morphologies. First, small (20^50 mm), embayed and irregularly shaped grains are either located along the grain boundaries or totally encased in foliation-parallel biotite

2028

PHILLIPS et al.

OVERPRINTING OROGENIC EVENTS

Table 2: Continued Sample 238

Sample 37y

g (rim)

g (core)

st

cd

bi

SiO2

37·39

TiO2

0·03

36·84

26·77

47·87

35·26

0·01

0·92

0·00

1·80

Al2O3 Cr2O3

21·56

21·32

53·65

33·88

0·07

0·01

0·17

0·06

FeO

35·79

35·38

14·27

MnO

2·23

1·86

0·12

MgO

2·85

3·97

CaO

1·43

1·31

fsp

st

bi

mu

fsp

ilm

59·39

27·57

37·02

46·35

66·11

0·20

0·00

0·49

1·69

0·44

0·00

54·78

19·76

25·99

54·08

19·71

36·39

22·78

0·18

0·09

0·00

0·14

0·12

0·11

0·03

0·00

8·46

18·97

0·02

13·11

16·21

0·32

0·03

44·81

0·13

0·04

0·02

0·29

0·10

0·04

0·01

1·79

1·87

8·46

10·22

0·01

2·42

12·27

0·36

0·01

0·17

0·00

0·00

0·00

6·65

0·00

0·00

0·04

3·54

0·01

Na2O

0·01

0·01

0·07

0·22

0·24

7·95

0·01

0·39

1·86

9·53

0·00

K2O

0·01

0·00

0·00

0·00

8·66

0·08

0·01

8·75

8·10

0·03

0·03

ZnO Total Oxygen

0·00

0·00

1·17

0·00

0·00

0·06

0·00

0·00

0·00

0·00

0·00

101·37

100·71

99·01

99·08

95·04

100·17

98·13

96·25

94·01

102·07

101·97

48

18

22

48

22

22

12

12

32

32

3

Si

2·98

2·95

7·75

4·90

5·34

10·14

7·95

5·44

6·09

10·86

0·00

Ti

0·00

0·00

0·20

0·00

0·20

0·00

0·11

0·19

0·04

0·00

1·01

Al

2·02

2·01

18·31

4·09

3·52

5·23

18·37

3·41

5·63

4·41

0·01

Cr

0·00

0·00

0·04

0·00

0·01

0·00

0·03

0·01

0·01

0·00

0·00

Fe2þ

2·36

2·23

3·45

0·72

2·40

0·00

3·16

1·99

0·04

0·00

0·92

Fe3þ

0·02

0·14

Mn

0·15

0·13

0·03

0·01

0·01

0·00

0·07

0·01

0·00

0·00

0·04

Mg

0·34

0·47

0·81

1·29

2·31

0·00

1·04

2·69

0·07

0·00

0·01

Ca

0·12

0·11

0·00

0·00

0·00

1·22

0·00

0·00

0·01

0·62

0·00

Na

0·00

0·00

0·08

0·09

0·14

5·26

0·01

0·22

0·95

6·07

0·00

K

0·00

0·00

0·00

0·00

1·67

0·02

0·00

1·64

1·36

0·01

0·00

Zn

0·00

0·00

0·25

0·00

0·00

0·01

0·00

0·00

0·00

0·00

0·00

Total

8·01

8·05

30·92

11·10

15·60

21·88

30·75

15·59

14·20

21·97

1·98

XMg

0·11

0·16

XFe

0·79

0·76

XCa

0·04

0·04

XMn

0·05

0·04

—

—

XFe

0·36

—

XMg

—

0·49

XAn

0·81

XAb

0·19

—

—

XMg

0·57

—

—

XAn

0·91

XAb

0·09

—



Data from Boger & Wilson (2005). yData from Phillips et al. (2007b).

(Fig. 6l). Second, smaller (c. 30 mm) rounded grains are included in the rock matrix minerals plagioclase and quartz (Fig. 6m). Ages obtained from both of these grain morphologies and microstructural settings are Late Neoproterozoic^Early Palaeozoic (Fig. 7e). Monazite in sample 37 is irregular in shape, 30^100 mm in diameter and occurs either as inclusions or along the grain boundaries of staurolite, biotite, plagioclase or quartz (Fig. 6n and o). Ages from monazite grains located within and along the boundary of staurolite, biotite, plagioclase and quartz are Late Neoproterozoic^Early Palaeozoic (Fig. 7f).

MINERAL EQU ILIBRIA MODELLI NG Calculations of mineral equilibria for each of the dated samples were made in the system MnNCKFMASHTO using the computer software THERMOCALC version 3.26 (Powell & Holland, 1988; Powell et al., 1998). The following a^x relationships were used: silicate melt (White et al., 2001); garnet and biotite (White et al., 2005); cordierite and staurolite (Mahar et al., 1997); chlorite (Mahar et al., 1997; Holland et al., 1998); muscovite and paragonite (Coggon & Holland, 2002); plagioclase and alkali feldspar

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Fig. 6. Backscattered electron images showing the morphology and textural setting of monazite. (a) Sample 73: large angular to rounded monazite (bright) in contact with foliation sillimanite. (b) Sample 73: small angular monazite grain intergrown with plagioclase and foliationparallel biotite. (c) Sample 73: rounded^angular monazite encased in plagioclase. (d) Sample 94: monazite intergrown with dominant biotite þ sillimanite foliation. (e) Sample 94: angular monazite grain. (f) Sample 94: large rounded monazite located in K-feldspar rock matrix. (g) Sample 147: elongate monazite intergrown with biotite foliation. (h) Sample 147: embayed grain of monazite associated with cordierite. (i) Sample 326: large angular grain aligned parallel to biotite foliation. (j) Sample 326: elongate monazite in contact with garnet and aligned parallel to foliation. (k) Sample 326: small monazite grain encased by garnet. (l) Sample 238: small angular monazite grains intergrown with biotite foliation. (m) Sample 238: small angular monazite grain surrounded by plagioclase. (n) Sample 37: embayed monazite grain encased within coarse-grained biotite. (o) Sample 37: angular monazite grains in contact with foliation-parallel biotite.

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PHILLIPS et al.

8

16

Sample 73

14

n = 61

Frequency

Frequency

10

OVERPRINTING OROGENIC EVENTS

6 4

12

Sample 94 n = 108

10 8 6 4

2

2 200

600

(a)

1000

1400

1800

12

Frequency

Frequency

n = 95

8 6 4

1400

1800

Sample 326 n = 87

6 2

200

600

1000

1400

200

1800

600

(d)

Age (Ma)

14

1000

1800

Sample 37

n = 86

10

1400

Age (Ma)

Sample 238 Frequency

Frequency

1000

Age (Ma)

10

2

(c)

600

14

Sample 147

10

200

(b)

Age (Ma)

6

15

n = 93

10 5

2 200

(e)

600

1000

1400

1800

(f)

Age (Ma)

200

600

1000

1400

1800

Age (Ma)

Fig. 7. Frequency histograms of EPMA monazite age data. (a) Sample 73. (b) Sample 94. (c) Sample 147. (d) Sample 326. (e) Sample 238. (f) Sample 37.

(Holland & Powell, 2003); epidote (Holland & Powell, 1998; White pers comm.); spinel and magnetite (White et al., 2002); and ilmenite (White et al., 2000). Bulk-rock chemical compositions were obtained by X-ray fluorescence, which, based on the thoroughly recrystallized nature of the samples, has been assumed to accurately represent the equilibrium volume during metamorphism. Estimates of rock H2O contents for mineral equilibria calculations were based on the assumption that the rock contained free water until the solidus was reached. Estimates of ‘O’ (representing ferric iron) were based on the modal proportion of Fe3þ-bearing phases in samples. To further test the veracity of bulk ferric iron estimates

used in the phase diagram calculations, reaction lines defining key stability fields were calculated considering lower (0·1mol%) and higher (1·0 mol%) ‘O’ contents. In general, the reaction lines did not deviate until higher ferric (8^10% of total Fe) contents were used in calculations. However, such ferric-iron-rich bulk compositions cannot be justified because of the low modal abundance of Fe3þ-bearing minerals in the rocks. The results of the phase equilibria modelling are discussed in light of the monazite age data, and therefore, discussed in order of age. For sample 73, the peak mineral assemblage occurs at P45·2 kbar and T ¼ 775^8208C (highlighted by bold

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ovoid in Fig. 8a). To further refine an estimate of P^T, isopleths were calculated and plotted for the almandine [THERMOCALC variable x(g) ¼ Fe2þ/(Fe2þ þ Mg þ Ca þ Mn)] and grossular [THERMOCALC variable z(g) ¼ Ca/(Fe2þ þ Mg þ Ca þ Mn)] contents of garnet, as these are known to be sensitive to changes in P^T. Of these compositional isopleths, x(g) ¼ 0·72 (Table 2) plots outside, and z(g) ¼ 0·05 plots within the peak mineral assemblage field (Fig. 8a). Nevertheless, owing to the proximity of this isopleth intersection to the stability field of the inferred peak assemblage, P^T conditions of c. 6·8 kbar and 790^8108C are a reasonable estimate for peak metamorphism. Retrograde metamorphism as indicated by the growth of minor muscovite at the expense of K-feldspar can be illustrated on the phase diagram by crossing into a muscovite-stable field from the Kfeldspar stability region of the diagram (Fig. 8a). The absolute trajectory of this retrograde P^T path is non-unique and therefore cannot be established. Metamorphic mineral reaction microstructures in sample 94 involve the partial replacement of sillimanite by cordierite. The peak, sillimanite-bearing assemblage in sample 94 (Fig. 8b) occurs over a large P^T range of 44·3 kbar and 700^8208C. In comparison with sample 73, compositional isopleths in garnet could not be used to refine the peak P^T conditions because the calculated mineral (phase) compositions do not overlap with the measured mineral compositions. Mineral reaction microstructures involving the replacement of sillimanite by cordierite can be shown on the pseudosection by crossing into the lower-P stability field that predicts the stability of biotite, cordierite, garnet, plagioclase, quartz, ilmenite, magnetite and silicate melt (Fig. 8b). Metamorphic conditions during the growth of cordierite can potentially be refined to 2·7^5·2 kbar and 750^7908C by using the compositional isopleth of Fe/ (Fe þ Mg) in cordierite [THERMOCALC variable x(cd) ¼ 0·38; Fig. 8b]. Peak P^Tconditions of 5·8^7·2 kbar and 620^6358C are estimated for the development of the inferred peak mineral assemblage in sample 147 (Fig. 9a). The interpreted replacement of staurolite by cordierite coronae can be shown on the pseudosection by crossing the staurolite-out and cordierite-in reaction lines. Additionally, as there is no sillimanite preserved in the sample, the retrograde path probably involved decompression to below 4·5 kbar (Fig. 9a). Metamorphic conditions during the growth of cordierite are constrained by the relatively large biotite, plagioclase, garnet, cordierite, ilmenite and quartz stability field (highlighted by the bold dashed outline in Fig. 9a). Refinement of this estimate to 3·7^4·2 kbar and 580^6108C can be achieved through calculating and plotting the compositional isopleth Fe/(Fe þ Mg) in cordierite [x(cd) ¼ 0·37; Fig. 9a].

NUMBER 11

NOVEMBER 2009

In sample 326, P^T conditions required for the stability of the peak mineral assemblage are estimated at 44·8 kbar and 550^6008C (Fig. 9b). To provide a maximum limit of P during the growth of this assemblage, compositional isopleths of the almandine content in garnet were calculated and plotted [x(g) ¼ 0·68; Fig. 9b; Table 2]. This limits P^T conditions to 4·8^7·3 kbar and 570^6008C. Because of a lack of mineral reaction microstructures preserved in this sample, inferences on the retrograde P^T path cannot be made. A revised pseudosection from that originally published in Boger & Wilson (2005, fig. 8c) was calculated for sample 238 (Fig. 9c). This was done to accommodate the effects of MnO, TiO2 and Fe2O3 on mineral assemblage stability. A dominant metamorphic microstructure in sample 238 comprises sillimanite, biotite, plagioclase, ilmenite, staurolite and garnet inclusions within poikiloblastic cordierite (Boger & Wilson, 2005). As a stability field consisting of staurolite, sillimanite and cordierite does not exist, the relict inclusion assemblage is interpreted to be persisting metastably (Fig. 9c). Peak metamorphic conditions for the growth of biotite, staurolite, plagioclase, ilmenite and garnet are constrained to 4·7^7·3 kbar and 590^6508C (Fig. 9c). Similar to the interpretation made by Boger & Wilson (2005), the growth of sillimanite and cordierite can be explained by crossing the sillimanite-in, staurolite-out, and finally, cordierite-in reaction lines (Fig. 9c). However, owing to the topology of the pseudosection, the trajectory of such a retrograde P^T path is nonunique (e.g. Vernon, 1996; Kelsey et al., 2003c; Nicollet & Goncalves, 2005) and could be consistent with heating followed by decompression (i.e. P^T path 1; Fig. 9c), or decompression^cooling (i.e. P^T path 2; Fig. 9c). Compositional isopleths for garnet and cordierite could not be used to constrain the P^T path because the calculated phase compositions do not overlap with the measured mineral compositions. The pseudosection for sample 37 shown in Fig. 9d is from Phillips et al. (2007b). The inferred peak mineral assemblage corresponds to a small stability field on the calculated pseudosection (Fig. 9d). As a result, the P^Tstability of this assemblage is estimated at 4·0^5·0 kbar and 565^5808C (Fig. 9d). Because of a lack of reaction microstructures in this sample, a post-peak P^T path cannot be established.

DISCUSSION Integrating the geochronological and metamorphic records: P^T^t paths for the Rayner and Prydz orogenic events The main aim of this study was to constrain the temporal evolution of the tectono-metamorphic record within the southern Prince Charles Mountains by conducting in situ

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PHILLIPS et al.

OVERPRINTING OROGENIC EVENTS

Sample 94

6

k

z

= (g)

bi g mu pl sill q ilm mt v=4

= x(g)

bi g pl ksp sill ilm mt sp liq

6

5

0.72

0.7

bi g pl sill q ilm mt H2O v=5

3

bi g cd pl ksp ilm mt sp liq v=4

750

cd g pl sill q ilm mt liq v=4

cd = ill

=

0 cd g pl q ilm mt liq v=5

0

3 s

v=

bi cd pl q ilm mt H2O

bi cd pl q ilm mt liq v=5

bi cd g pl q ilm mt liq v=4

cd g mt pl liq v=7

(b)

800

cd g pl ilm mt liq

8 0.3

700

bi g pl sill q ilm mt liq

d) = x(c

(a)

si g liq cd ilm

4

bi g cd sp pl ilm mt sill H2O v=4 bi g cd sp pl ilm mt H2O sill =0 v=5

Peak assemblage

k

Peak assemblage

3

0

7

5 0.0

x(g) =

4

y=

bi g ksp pl ilm sp liq

bi g pl ilm mt sill H2O v=6

5

bi g pl q ilm liq v=6 ll q

+ksp

bi g pl ilm mt sp sill liq v=5

0

us

y=

sol id

7

bi g st mu pl ilm mt ky H2O

solidus

PRESSURE (kbar)

Sample 73

700

750

800

TEMPERATURE (˚C) Fig. 8. Phase diagrams calculated in the chemical system MnNCKFMASHTO for samples metamorphosed during the Rayner orogenic event. Mineral abbreviations are shown in the caption to Table 1. The variance (v) for stability fields is shown. (a) Sample 73, normalized bulk-rock composition: MnO 0·25, Na2O 4·11, CaO 4·41, K2O 1·76, FeO 12·00, MgO 4·53, Al2O3 18·66, SiO2 53·21, H2O 4·50, TiO2 0·85, Fe2O3 0·60. Isopleths: x(g) ¼ Fe2þ/(Fe2þ þ Mg þ Ca þ Mn); z(g) ¼ Ca/(Fe2þ þ Mg þ Ca þ Mn) for garnet. (b) Sample 94, normalized bulk-rock composition: MnO 0·08, Na2O 0·88, CaO 0·79, K2O 4·00, FeO 7·33, MgO 5·18, Al2O3 13·71, SiO2 62·45, H2O 4·67, TiO2 0·53, Fe2O3 0·40. Isopleth: x(cd) ¼ Fe/(Fe þ Mg) for cordierite.

monazite geochronology. Geochronological analysis of monazite grains from the six samples from the Lambert and Tingey Complexes reveals a range of Late Palaeoproterozoic, Early and Late Neoproterozoic, and Early Palaeozoic ages (Table 1). Based on the geochronological data, the temporal metamorphic evolution of the following samples can be considered together: (1) 73 and 94 (Rayner orogenic event); (2) 147, 238, 326 and 37 (Prydz orogenic event). A Venn-diagram type of approach consisting of superimposing the peak assemblage fields from the pseudosections of different samples is used to further constrain the peak P^T conditions and propose P^T paths for the Rayner and Prydz orogenic events (e.g. Kelsey et al., 2003a; Halpin et al., 2007b).

Late Palaeoproterozoic ages Late Palaeoproterozoic ages were recorded only within sample 73 and were restricted to the cores of large, randomly oriented monazite grains. Because of this spatial arrangement, it is difficult to ascertain whether these ages reflect the timing of metamorphic mineral growth within this sample, or represent a detrital signature. For example, an age of 1740 Ma has been reported from low Th/U zircon grains from a leucosome located in close proximity to sample 73 (Mikhalsky et al., 2007). This supports a

metamorphic origin for monazite growth. Alternatively, a maximum deposition age of c. 1830 Ma has previously been suggested for the rocks of the Lambert Group on the basis of detrital zircon ages (Phillips et al., 2006), which, in addition to the textural setting of the monazite grains (i.e. non-aligned with the foliation), supports a detrital origin. The significance of the Late Palaeoproterozic monazite age is therefore unresolved.

Early Neoproterozoic ages: Rayner orogenic event In sample 94, monazite grains dominantly record Early Neoproterozoic ages, which are interpreted to reflect peak metamorphism during the Rayner orogenic event. Small monazite grains and overgrowths on the Late Palaeoproterozoic-age grains also define Early Neoproterozoic ages in sample 73 (Table 1). This age is suggested to constrain the timing of inferred peak metamorphism in samples 73 and 94. Importantly, the stability fields for the inferred peak assemblages in both samples overlap (Fig. 10a). This overlap is also close to intersecting compositional isopleths for unzoned garnet in sample 73. Combined, we propose that the overlapping stability fields, which define conditions of P ¼ 6·5^7·1 kbar and T ¼ 790^8108C, provide a plausible estimate of peak metamorphic conditions during the Rayner orogenic event.

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Fig. 9. Phase diagrams calculated in the chemical system MnNCKFMASHTO for samples metamorphosed during the Prydz orogenic event. Mineral abbreviations are shown in the caption to Table 1. The variance (v) for labelled stability fields is shown. (a) Sample 147, normalized bulk-rock composition: MnO 0·14, Na2O 0·79, CaO 2·48, K2O 1·34, FeO 9·00, MgO 6·09, Al2O3 10·55, SiO2 68·43, TiO2 0·69, Fe2O3 0·50, H2O in excess. Isopleths: x(cd) ¼ Fe/(Fe þ Mg) for cordierite. (b) Sample 326, normalized bulk-rock composition: MnO 0·17, Na2O 1·69, CaO 2·63, K2O 1·32, FeO 9·50, MgO 9·20, Al2O3 10·05, SiO2 64·23, TiO2 0·72, Fe2O3 0·50, H2O in excess. Isopleths: x(g) ¼ Fe/ (Fe þ Mg þ Ca þ Mn) for garnet. (c) Sample 238, normalized bulk-rock composition: MnO 0·04, Na2O 1·91, CaO 2·04, K2O 1·59, FeO 6·03, MgO 4·71, Al2O3 8·24, SiO2 74·49, TiO2 0·55, Fe2O3 0·40, H2O in excess. (d) Sample 37, normalized bulk-rock composition: MnO 0·06, Na2O 1·94, CaO 0·70, K2O 2·51, FeO 6·50, MgO 7·19, Al2O3 12·79, SiO2 67·24, TiO2 0·73, Fe2O3 0·10, H2O in excess.

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OVERPRINTING OROGENIC EVENTS

In addition, texturally late cordierite overprints the peak mineral assemblage in sample 94 (Table 1). The small population of Late Neoproterozoic^Early Palaeozoic ages recorded by monazite in this sample may correspond to cordierite growth during the Prydz orogenic event (Fig. 10a).

Early Palaeozoic ages: Prydz orogenic event Monazite ages from samples 147, 326 and 238 are entirely Late Neoproterozoic to Early Palaeozoic. Metamorphic mineral parageneses and associated tectonic fabrics in these three samples can therefore be confidently linked to the Prydz orogenic event. Characteristic of the metamorphic mineral assemblages in samples 147 and 238 is the early growth of biotite, garnet and staurolite with or without plagioclase, ilmenite and chlorite. The mineral assemblage fields containing these phases overlap for both of these samples, which, in addition to the garnet compositional data from sample 147 [i.e. z(g) shown in Fig. 10b], provide an estimate of 5·8^6·1 kbar and 625^6358C for peak metamorphism (Fig. 10b). These thermal conditions are slightly higher than those calculated for the peak metamorphic mineral assemblage in sample 326, which may reflect metamorphism at different structural levels. However, as the calculated peak pressure conditions are comparable across samples 147, 238 and 326, which potentially indicates metamorphism at similar crustal levels, we tentatively suggest that the peak P^Tconditions calculated for sample 326 represent a prograde section of a regional P^T path (transparent arrow in Fig. 10b). Following peak metamorphism, decompression can be inferred for the generation of cordierite coronae in samples 147 and 238. Mineral equilibria modelling and cordierite compositional data from sample 147 indicate that cooling probably coincided with decompression after peak metamorphism (Fig. 9a). The presence of sillimanite in sample 238 was, however, used by Boger & Wilson (2005) to suggest that heating probably took place prior to decompression (i.e. P^T path 1 in Fig. 9c; Boger & Wilson, 2005). As the growth of sillimanite in sample 238 can also be explained by decompression^cooling (i.e. P^T path 2 in Fig. 9c), which is similar to the retrograde P^T path constructed for sample 147 (Fig. 9a), we favour a retrograde history that involved decompression^cooling (transparent arrow in Fig. 10b). P^T conditions during the growth of cordierite can be constrained to 3·7^4·2 kbar and 580^ 6108C by superimposing the relevant mineral assemblage fields from samples 147 and 238 and the cordierite compositional data [i.e. x(cd) shown in Fig. 10b] from sample 147. Monazite age data from sample 37 indicate that the localized development and/or reactivation of high-strain zones in the Tingey Complex occurred during the Early Palaeozoic. Peak P^T conditions associated with the development of these localized high-strain zones are constrained to 4·0^5·0 kbar and 565^5808C (Fig. 9d).

As these conditions are lower than those estimated for peak conditions during the Prydz orogenic event (i.e. 5·8^ 6·1 kbar and 625^6358C), we suggest that sample 37 was probably metamorphosed in a higher structural position than samples 147, 238 and 326.

The spatial extent of Rayner and Prydz orogenesis in the southern Prince Charles Mountains In comparison with previous U^Pb zircon studies (Boger et al., 2001; Mikhalsky et al., 2006; Corvino et al., 2008), new in situ monazite geochronology provides timing constraints that definitively link the geochronological, metamorphic and kinematic records. In addition, the geographical locations of the selected samples provide constraints on the spatial extent of crustal reworking related to the Rayner and Prydz orogenic events in the southern Prince Charles Mountains. The structural setting of each sample is also used to infer the pervasiveness of crustal reworking during these events. In situ monazite geochronology and mineral equilibria modelling of samples 73 and 94 (Table 1) indicate that the rocks of the northern Lambert Complex (exposed to the north of Harbour Bluff; Fig. 11) were pervasively recrystallized at granulite-facies conditions during the Early Neoproterozoic Rayner orogenic event. These rocks were also reworked during the Early Palaeozoic Prydz orogenic event; however, tectonism at this time was manifested by localized shear zone development and/or reactivation and the sporadic emplacement of pegmatite and granitic sheets (Mikhalsky et al., 2006; Corvino et al., 2008). Metamorphic evidence for this event is the local growth of corderite (e.g. sample 94). Samples 147, 238 and 326 provide constraint on the tectonothermal evolution of the southern Lambert Complex (exposed to the south of Harbour Bluff and north of Gibbs Bluff; Fig. 11). Preserved in these rocks is a record of pervasive recrystallization and amphibolite-facies metamorphism during the Prydz orogenic event. Metamorphism is consistent with a clockwise P^T path, whereas deformation involved the development of kilometre-scale upright open folds and subvertical highstrain zones (Boger & Wilson, 2005; this study). Sample 326 is located on the limb of an upright fold structure (Fig. 3a), and provides constraint on the P^T conditions within a low-strain package of rocks. By contrast, sample 238 is from a high-strain zone (Fig. 3a). Similarities in the calculated P^T conditions and geochronological data for both of these samples (Table 1) support a model of strain localization in the southern Lambert Complex during the Prydz orogenic event. In situ monazite geochronology from sample 37 indicates that rocks of the Tingey Complex (exposed to the south of Gibbs Bluff along the Mawson Escarpment and at Mount

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RAYNER OROGENIC EVENT Sample 94 3 ple 7 sam z(g) RAYNER OROGENIC EVENT

sam ple 73

7

Overprinting orogenic cycles and geodynamic implications

x( g )

6

3) ample 7 cd = 0 (s

PRYDZ OROGENIC EVENT

4 3

(a) 700

750

800

TEMPERATURE (˚C)

PRYDZ OROGENIC EVENT Sample 326

Sample 238

Sample 147

PRESSURE (kbar)

47

6

1 le mp a )s

z(g

38 le 2 amp in’ s ‘sill-

7

5

4 d) x(c

le mp sa

7 14

3

(b) 550

600

NOVEMBER 2009

development or reactivation of local high-strain zones and localized injection of pegmatite (Boger et al., 2006; Phillips et al., 2007b). Furthermore, rocks of the Tingey Complex also preserve evidence of high-grade metamorphism prior to the Prydz Orogenic event (Fig. 11ç ArchaeançPalaeoproterozoic(?); Phillips et al., 2007b).

) x(cd 94 ple s am

PRESSURE (kbar)

Sample 73

NUMBER 11

650

TEMPERATURE (˚C) Fig. 10. Venn-diagram approach in which stability fields are overlain to further refine the P^T conditions during the Rayner and Prydz orogenic events. (a) Rayner orogenic eventçcomposite phase diagram and regional P^T path constructed from mineral equilibria modelling of samples 73 and 94. (b) Prydz orogenic eventçcomposite phase diagram and regional P^T path constructed from mineral equilibria modelling of samples 147, 238 and 326.

Stinear) were locally reworked during the Prydz orogenic event (Fig. 11; Boger et al., 2006; Phillips et al., 2006). Similar to the rocks of the northern Lambert Complex, Early Palaeozoic reworking was associated with the

Based on similarities in age and P^T conditions, rocks affected by a pervasive belt of Rayner-aged tectonism can be traced from the Mawson Coast through the northern Prince Charles Mountains and as far south as the central Mawson Escarpment in the southern Prince Charles Mountains (Fig. 1a; Clarke et al., 1989; Manton et al., 1992; Hand et al., 1994; Boger et al., 2000; Halpin et al., 2007a, 2007b; Corvino et al., 2008; this study). In addition to the dominant Early Neoproterozoic age signature recorded by these rocks, there are subtle differences between the isotopic records preserved by the rocks of the northern and southern Prince Charles Mountains. For example, rocks of the Rayner Complex in the northern Prince Charles Mountains almost entirely preserve Early Neoproterozoic isotopic ages (Manton et al., 1992; Boger et al., 2000; Carson et al., 2000). By contrast, rocks of the northern Lambert Complex in the southern Prince Charles Mountains have complex isotopic records that are characterized by protolith emplacement at c. 2450 Ma, tectonism during the Palaeoproterozoic (c. 2100 Ma) and deformation^metamorphism during the Rayner orogenic event (Corvino et al., 2008; this study). Rayner-aged collision between Antarctica and India would have caused substantial crustal thickening and thinning of the mantle lithosphere, in turn, elevating the geothermal gradient in the crust (Sandiford & Powell, 1991). It is reasonable to argue that the locus of mantle lithospheric thinning would be below regions of maximum crustal meltingçwhich, in this case, would be represented by the voluminous packages of Early Neoproterozoic felsic igneous rocks located along the Mawson Coast and in the northern Prince Charles Mountains (Young & Black, 1991; Manton et al., 1992; Carson et al., 2000). Conversely, there are limited Early Neoproterozoic-age magmatic rocks in the southern Prince Charles Mountains. Instead, pervasive granulite-facies metamorphism and deformation occurred (Corvino et al., 2008; this study). We suggest that this apparent decrease in felsic magmatism from the Mawson Coast to the southern Prince Charles Mountains reflects the diminishing effects of crustal thickening and associated mantle lithospheric thinning away from the collision zone. Based on this model, it can be argued that the rocks of the northern Lambert Complex, probably represent the southernmost extent (present coordinates) of an orogenic front propagating from a collision zone between India and East Antarctica.

2036

PHILLIPS et al.

OVERPRINTING OROGENIC EVENTS

Fig. 11. Summary map showing the distribution of rocks affected by the Rayner and Prydz orogenic events. P^T information is from this study, Boger & Wilson (2005) and Phillips et al. (2007a, 2007b). The intensity of the orogenic overprint is shown by the darkness of the fill: dark indicates pervasive; light indicates localized. The colour legend for the geological subdivisions can be seen in the online version of the paper.

It is now apparent that all the rocks exposed in the southern Prince Charles Mountains were reworked during the Prydz orogenic event (Boger et al., 2001; Phillips et al., 2007a; Corvino et al., 2008; this study). In the southern Lambert Complex (samples 147 and 326), reworking was pervasive. In contrast, reworking in the northern Lambert and Tingey Complexes was localized in nature, manifested as local high-strain zones (samples 238 and 37) and minor pegmatite intrusion. Previous workers have suggested that a collision zone in the southern Prince Charles Mountains was the cause of this Early Palaeozoic reworking (Fig. 11; Boger et al., 2001). In contrast to the work of Boger et al. (2001), we propose that the spatial distribution of Early Palaeozoic continental reworking in the southern Prince Charles Mountains can be related to rock fertility. It is well known that an available fluid phase is required to drive metamorphic and recrystallization processes in rocks (Etheridge et al., 1983). It is also highly probable that rocks that had experienced high-grade metamorphism prior to the Prydz

orogenic event would have lost significant volumes of free fluid or silicate melt (e.g. White & Powell, 2002). Without rehydration of these rock volumes, metamorphism and recrystallization during successive orogenic events would be inhibited (Thompson, 1983; Carson et al., 1999; Guiraud et al., 2001; White & Powell, 2002) and reworking would probably be localized in nature (Phillips et al., 2007b). Support for this model is that the rocks of the southern Lambert Complex do not preserve isotopic evidence of Rayner-aged metamorphism (Corvino et al., 2008). As a result, a free fluid phase was probably available during the Prydz orogenic event, which allowed pervasive metamorphism and recrystallization to proceed. Associating the distribution and style of Prydz orogenic activity to rock fertility supports the hypothesis that Early Palaeozoic tectonism in the southern Prince Charles Mountains was intracratonic and probably related to the inversion of Neoproterozoic sedimentary basins (Phillips et al., 2007a). The inversion of these basins was probably caused by far-field stress associated with the final suturing

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of India^Antarctica and the Mawson Craton during Gondwana assembly (Fig. 1b; Fitzsimons, 2003; Kelsey et al., 2008).

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are thanked for logistical and financial support of the 2002^2003 PCMEGA field season in Antarctica. Analytical work was funded by a University of Newcastle New Staff Grant to G.P. D.E.K. is funded by Australian Research Council grant DP0665094.

CONC LUSION Reconnaissance in situ monazite geochronology has provided a time-effective approach to unravel the temporal evolution of the complex metamorphic and kinematic records preserved by rocks within the southern Prince Charles Mountains. Based on these data, we have been able to link metamorphic mineral growth, reaction microstructures and structural fabrics to the Rayner and Prydz orogenic events. The distribution of selected samples has allowed definitive boundaries on the spatial extent of Rayner- and Prydz-aged tectonism to be constructed. Furthermore, field observations have contributed towards understanding the nature of orogenic activity and whether it was pervasive or localized in nature. Geochronology and mineral equilibria studies show that rocks of the northern Lambert Complex were pervasively metamorphosed and recrystallized at granulite-facies conditions during the Early Neoproterozoic Rayner orogenic event. Prydz-aged tectonism is highly localized (e.g. highstrain zones) in these regions. Rocks of the southern Lambert Complex, which do not preserve evidence of tectonism during the Rayner orogenic event, experienced pervasive recrystallization and metamorphism at amphibolite-facies conditions during the Early Palaeozoic Prydz orogenic event. This relationship between high-grade metamorphism and localization of tectonism during successive orogenic events supports a potential feedback between rock fertility and styles (i.e. localized vs pervasive) of crustal reworking. Based on this interpretation, we support a model in which Early Palaeozoic orogenesis in the southern Prince Charles Mountains was intracratonic.

AC K N O W L E D G E M E N T S Dave Phelan (EM/X-Ray unit, The University of Newcastle) assisted with SEM-EDS analyses and Angus Netting (Adelaide Microscopy) provided valuable assistance during the collection of EMPA monazite data. Dr Steve Boger is thanked for providing sample material. Thorough reviews from Professor Mike Williams, Dr Jacqueline Halpin and Dr Chris Carson substantially improved the manuscript. Professor Geoff Clarke is thanked for his editorial guidance. This is TraX contribution #24.

F U N DI NG The Australian Antarctic Division (AAS Grant 1215) and the Bundesanstalt fu«r Geowissenschaften und Rohstoffe

S U P P L E M E N TA RY DATA Supplementary data for this paper are available at Journal of Petrology online.

R E F E R E NC E S Beliatsky, B. V., Laiba, A. A. & Mikhalsky, E. V. (1994). U^Pb zircon age of the metavolcanic rocks of Fisher Massif (Prince Charles Mountains, East Antarctica). Antarctic Science 6, 355^358. Black, L. P., James, P. R. & Harley, S. L. (1983). Geochronology and geological evolution of metamorphic rocks in the Field Islands area, East Antarctica. Journal of Metamorphic Geology 1, 277^303. Boger, S. D. & White, R. W. (2003). The metamorphic evolution of metapelitic granulites from Radok Lake, northern Prince Charles Mountains, east Antarctica; evidence for an anticlockwise P^T path. Journal of Metamorphic Geology 21, 285^298. Boger, S. D. & Wilson, C. J. L. (2005). Early Cambrian crustal shortening and a clockwise P^T^t path from the southern Prince Charles Mountains, East Antarctica: implications for the formation of Gondwana. Journal of Metamorphic Geology 23, 603^623. Boger, S. D., Carson, C. J., Wilson, C. J. L. & Fanning, C. M. (2000). Neoproterozoic deformation in the Radok Lake region of the northern Prince Charles Mountains, east Antarctica; evidence for a single protracted orogenic event. Precambrian Research 104, 1^24. Boger, S. D., Wilson, C. J. L. & Fanning, C. M. (2001). Early Paleozoic tectonism within the East Antarctic craton: The final suture between east and west Gondwana? Geology 29, 463^466. Boger, S. D., Carson, C. J., Fanning, C. M., Hergt, J. M., Wilson, C. J. L. & Woodhead, J. D. (2002). Pan-African intraplate deformation in the northern Prince Charles Mountains, east Antarctica. Earth and Planetary Science Letters 195, 195^210. Boger, S. D., Wilson, C. J. L. & Fanning, C. M. (2006). An Archaean province in the southern Prince Charles Mountains, East Antarctica: U^Pb zircon evidence for c. 3170 Ma granite plutonism and c. 2780 Ma partial melting and orogenesis. Precambrian Research 145, 307^328. Boger, S., Maas, R. & Fanning, C. (2008). Isotopic and geochemical constraints on the age and origin of granitoids from the central Mawson Escarpment, southern Prince Charles Mountains, East Antarctica. Contributions to Mineralogy and Petrology 155, 379^400. Carson, C. J., Dirks, P. H. G. M., Hand, M., Sims, J. P. & Wilson, C. J. L. (1995). Compressional and extensional tectonics in low^ medium pressure granulites from the Larsemann Hills; East Antarctica. Geological Magazine 132, 151^170. Carson, C. J., Powell, R. & Clarke, G. L. (1999). Calculated mineral equilibria for eclogite in CaO^Na2O^FeO^MgO^Al2O3^SiO2^ H2O: application to the Poue¤bo Terrane, Pam Peninsula, New Caledonia. Journal of Metamorphic Geology 17, 9^24. Carson, C. J., Boger, S. D., Fanning, C. M., Wilson, C. J. L. & Thost, D. E. (2000). SHRIMP U^Pb geochronology from Mount Kirkby, northern Prince Charles Mountains, East Antarctica. Antarctic Science 12, 429^442.

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Clarke, G. L. (1988). Structural constraints on the Proterozoic reworking of Archaean crust in the Rayner Complex, MacRobertson and Kemp Land Coast, East Antarctica. Precambrian Research 40/41, 137^156. Clarke, G. L., Powell, R. & Guiraud, M. (1989). Low-pressure granulite facies metapelitic assemblages and corona textures from MacRobertson Land, east Antarctica: the importance of Fe2O3 and TiO2 in accounting for spinel-bearing assemblages. Journal of Metamorphic Geology 7, 323^335. Coggon, R. & Holland, T. J. B. (2002). Mixing properties of phengitic micas and revised garnet^phengite thermobarometers. Journal of Metamorphic Geology 20, 683^696. Corvino, A. F., Boger, S. D., Henjes-Kunst, F., Wilson, C. J. L. & Fitzsimons, I. C. W. (2008). Superimposed tectonic events at 2450 Ma, 2100 Ma, 900 Ma and 500 Ma in the North Mawson Escarpment, Antarctic Prince Charles Mountains. Precambrian Research 167, 281^302. Corvino, A. F. & Wilson, C. J. L. (2007). Tectonic transposition of Palaeo-Mesoproterozoic rocks at 1000 Ma in the Waller Hills area of the Mawson Escarpment, Antarctica Southern Prince Charles Mountains. In: Antarctica: A Keystone in a changing World ^ Online Proceedings of the 10th ISEAS, edited by A. K. Cooper and C. R. Raymond et al., USGS Open-File Report 2007-1047, Extended Abstract 048, 6p. Crohn, P. W. (1959). A contribution to the geology and glaciology of the western part of Australian AntarcticTerritory. ANARE Reports, Series A, III. Deer, W. A., Howie, R. A. & Zussman, J. (1992). An introduction to The Rock Forming Minerals, 2nd Edition., 696pp., Pretince-Hall, London. Dirks, P. H. G. M. & Wilson, C. J. L. (1995). Crustal evolution of the East Antarctic Mobile Belt in Prydz Bay: Continental collision at 500 Ma? Precambrian Research 75, 189^207. Droop, G. T. R. (1987). A general equation for estimating Fe3þ concentrations in ferromagnesian silicates and oxides from microprobe analysis, using stoichiometric criteria. Mineralogicial Magazine 51, 431^435. Etheridge, M. A., Wall, V. J. & Vernon, R. H. (1983). The role of the fluid phase during regional metamorphism and deformation. Journal of Metamorphic Geology 1, 205^226. Fitzsimons, I. C. W. (1996). Metapelitic Migmatites from Brattstrand Bluffs, East Antarcticaçmetamorphism, melting and exhumation of the mid crust. Journal of Petrology 37, 395^414. Fitzsimons, I. C. W. (2000). A review of tectonic events in the East Antarctic Shield and their implications for Gondwana and earlier supercontinents. Journal of African Earth Sciences 31, 3^23. Fitzsimons, I. C. W. (2003). Proterozoic basement provinces of southern and southwestern Australia, and their correlation with Antarctica. In: Yoshida, M., Windley, B. F. & Dasgupta, S. (eds) Proterozoic East Gondwana: Supercontinent Assembly and Breakup. Geological Society, London, Special Publications 206, 93^130. Fitzsimons, I. C. W. & Harley, S. L. (1991). Geological relationships in high-grade gneiss of the Brattstrand Bluffs coastline, Prydz Bay, East Antarctica. AustralianJournal of Earth Sciences 38, 497^519. Guiraud, M., Powell, R. & Rebay, G. (2001). H2O in metamorphism and unexpected behaviour in the preservation of metamorphic mineral assemblages. Journal of Metamorphic Geology 19, 445^454. Halpin, J. A., Clarke, G. L., White, R. W. & Kelsey, D. E. (2007a). Contrasting P^T^t paths for Neoproterozoic metamorphism in MacRobertson and Kemp Lands, east Antarctica. Journal of Metamorphic Geology 25, 683^701. Halpin, J. A., White, R. W., Clarke, G. L. & Kelsey, D. E. (2007b). The Proterozoic P^T^t evolution of the Kemp Land Coast, East

Antarctica; constraints from Si-saturated and Si-undersaturated metapelites. Journal of Petrology 48, 1321^1349. Hand, M., Scrimgeour, I., Powell, R., Stu«we, K. & Wilson, C. J. L. (1994). Metapelitic granulites from Jetty Peninsula, east Antarctica: Formation during a single event or by polymetamorphism? Journal of Metamorphic Geology 12, 557^573. Harley, S. L. (1998). Proterozoic granulites from the Rauer Group, East Antarctica. I: Decompressional pressure^temperature paths deduced from mafic and felsic genisses. Journal of Petrology 29, 1059^1095. Holland, T. J. B. & Powell, R. (1998). An internally-consistent thermodynamic data set for phases of petrological interest. Journal of Metamorphic Geology 16, 309^343. Holland, T. & Powell, R. (2003). Activity^composition relations for phases in petrological calculations: an asymmetric multicomponent formulation. Contributions to Mineralogy and Petrology 145, 492^501. Holland, T., Baker, J. & Powell, R. (1998). Mixing properties and activity^composition and relationships of chlorites in the system MgO^FeO^Al2O3^SiO2^H2O. European Journal of Mineralogy 10, 395^406. Jercinovic, M. J. & Williams, M. L. (2005). Analytical perils (and progress) in electron microprobe trace element analysis applied to geochronology: Background acquisition, interferences, and beam irradiation effects. American Mineralogist 90, 526^546. Kamenev, E. N., Andronikov, A. V., Mikhalsky, E. V., Krasnikov, N. N. & Stu«we, K. (1993). Soviet geological maps of the Prince Charles Mountains, East Antarctic Shield. Australian Journal of Earth Sciences 40, 501^517. Kelly, N. M. & Harley, S. L. (2004). Orthopyroxene^corundum in Mg^Al-rich granulites from the Oygarden Islands, East Antarctica. Journal of Petrology 45, 1481^1512. Kelly, N. M., Clarke, G. L. & Fanning, C. M. (2002). A two-stage evolution of the Neoproterozoic Rayner Structural Episode: new U^Pb sensitive high resolution ion microprobe constraints from the Oygarden Group, Kemp Land, East Antarctica. Precambrian Research 116, 307^330. Kelsey, D. E., White, R. W., Powell, R., Wilson, C. J. L. & Quinn, C. D. (2003a). New constraints on metamorphism in the Rauer Group, Prydz Bay, east Antarctica. Journal of Metamorphic Geology 21, 739^760. Kelsey, D. E., Powell, R., Wilson, C. J. L. & Steele, D. A. (2003b). (U þ Th)^Pb monazite ages from Al^Mg-rich metapelites, Rauer Group, east Antarctica. Contributions to Mineralogy and Petrology 146, 326^340. Kelsey, D. E., White, R. W. & Powell, R. (2003c). Orthopyroxene^sillimanite^quartz assemblages: distribution, petrology, quantitative P^T^X constraints and P^T paths. Journal of Metamorphic Geology 21, 439^453. Kelsey, D. E., White, R. W. & Powell, R. (2005). Calculated phase equilibria in K2O^FeO^MgO^Al2O3^SiO2^H2O for silica-undersaturated sapphirine-bearing mineral assemblages. Journal of Metamorphic Geology 23, 217^239. Kelsey, D. E., Hand, M., Clark, C. & Wilson, C. J. L. (2007). On the application of in situ monazite chemical geochronology to constraining P^T^t histories in high-temperature (48508C) polymetamorphic granulites from Prydz Bay, East Antarctica. Journal of the Geological Society, London 164, 667^683. Kelsey, D. E., Wade, B. P., Collins, A. S., Hand, M., Sealing, C. R. & Netting, A. (2008). Discovery of a Neoproterozoic basin in the Prydz belt in East Antarctica and its implications for Gondwana assembly and ultrahigh temperature metamorphism. Precambrian Research 161, 355^388.

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Kinny, P. D., Black, L. P. & Sheraton, J. W. (1997). Zircon U^Pb ages and geochemistry of igneous and metamorphic rocks in the northern Prince Charles Mountains, Antarctica. AGSO Journal of Australian Geology and Geophysics 16, 637^654. Mahar, E. M., Baker, J. M., Powell, R., Holland, T. J. B. & Howell, N. (1997). The effect of Mn on mineral stability in metapelites. Journal of Metamorphic Geology 15, 223^238. Manton, W. I., Grew, E. S., Hofmann, J. & Sheraton, J. W. (1992). Granitic rocks of the Jetty Peninsula, Amery Ice Shelf area, east Antarctica. In: Yoshida, Y., Kaminuma, K. & Shiraishi, K. (eds) Recent Progress in Antarctic Earth Science. Tokyo: Terra Scientific Publishing Company, pp. 179^189. McLean, M., Rawling, T. J., Betts, P. G., Phillips, G. & Wilson, C. J. L. (2008). Three-dimensional inversion modelling of a Neoproterozoic basin in the southern Prince Charles Mountains. Tectonophysics 456, 180^193. Mezger, K. & Cosca, M. A. (1999). The thermal history of the Eastern Ghats Belt (India) as revealed by U^Pb and 40Ar/39Ar dating of metamorphic and magmatic minerals: implications for the SWEATcorrelation. Precambrian Research 94, 251^271. Mikhalsky, E. V., Sheraton, J. W., Laiba, A. A. & Beliatsky, B. V. (1996). Geochemistry and origin of Mesoproterozoic metavolcanic rocks from Fisher Massif, Prince Charles Mountains, East Antarctica. Antarctic Science 8, 85^104. Mikhalsky, E. V., Laiba, A. A., Beliatsky, B. V. & Stu«we, K. (1999). Geology, age and origin of the Mount Willing area (Prince Charles Mountains, East Antarctica). Antarctic Science 11, 338^352. Mikhalsky, E.V., Sheraton, J. W., Laiba, A. A., Tingey, R. J., Thost, D. E., Kamenev, E. N. & Federov, L. V. (2001). . Geology of the Prince Charles Mountains, Antarctica. AGSO Bulletin 247. Mikhalsky, E. V., Beliatsky, B. V., Sheraton, J. W. & Roland, N. W. (2006). Two distinct Precambrian terranes in the Southern Prince Charles Mountains, East Antarctica: SHRIMP dating and geochemical constraints. Gondwana Research 9, 291^309. Mikhalsky, E. V., Henjes-Kunst, F. & Roland, N. W. (2007). Early Precambrian mantle dervied rocks in the southern Prince Charles Mountains, East Antarctica: age and isotopic constraints. In: Cooper, A. K. & Raymon, C. R. (eds) Antarctica: A Keystone in a Changing WorldçOnline Proceedings of the 10th ISAES, Open-File Report 2007^1047, Short Research Paper 039, doi:10.3133/of20071047.srp039. Montel, J. M., Foret, S., Veschambre, M., Nicollet, C. & Provost, A. (1996). Electron microprobe dating of monazite. Chemical Geology 131, 37^53. Montel, J. M., Kornprobst, J. & Vielzeuf, D. (2000). Preservation of old U^Th^Pb ages in shielded monazite; example from the Beni Bousera Hercynian kinzigites (Morocco). Journal of Metamorphic Geology 18, 335^342. Nicollet, C. & Gonclaves, P. (2005). Two contrasted P-T-time paths of coronitic metanorites of the French Massif Central: are reaction textures reliable guides to metamorphic histories? Journal of Metamorphic Geology 23, 97^105. Parrish, R. R. (1990). U^Pb dating of monazite and its application to geological problems. CanadianJournal of Earth Sciences 27, 1431^1450. Phillips, G., Wilson, C. J. L. & Fitzsimons, I. C. W. (2005). Stratigraphy and structure of the southern Prince Charles Mountains, East Antarctica. Terra Antartica 12, 69^86. Phillips, G., Wilson, C. J. L., Campbell, I. A. & Allen, C. M. (2006). U^Th^Pb detrital geochronology from the southern Prince Charles Mountains, East AntarcticaçDefining the Archaean to Neoproterozoic Ruker Province. Precambrian Research 148, 292^306.

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Phillips, G., Wilson, C. J. L. & Phillips, D. (2007a). Early Palaeozoic basin inversion within the southern Prince Charles Mountains, East Antarctica: Implications for East Gondwana. Journal of the Geological Society, London 164, 771^784. Phillips, G., White, R. W. & Wilson, C. J. L. (2007b). On the roles of deformation and fluid during rejuvenation of a polymetamorphic terrane: inferences on the geodynamic evolution of the Ruker Province, East Antarctica. Journal of Metamorphic Geology 25, 855^871. Powell, R. & Holland, T. J. B. (1988). An internally consistent thermodynamic dataset with uncertainties and correlations: Application, methods, worked examples and a computer program. Journal of Metamorphic Geology 6, 173^204. Powell, R., Holland, T. J. B. & Worley, B. A. (1998). Calculating phase diagrams involving solid solutions via non-linear equations, with examples using THERMOCALC. Journal of Metamorphic Geology 16, 577^588. Pyle, J. M., Spear, F. S., Wark, D. A., Daniel, C. G. & Storm, L. C. (2005). Contributions to precision and accuracy of monazite microprobe ages. American Mineralogist 90, 547^577. Sandiford, M. & Powell, R. (1991). Some remarks on high-temperatureçlow-pressure metamorphism in convergent orogens. Journal of Metamorphic Geology 9, 333^340. Sheraton, J. W., Tindle, A. G. & Tingey, R. J. (1996). Geochemistry, origin and tectonic setting of granitic rocks of the Prince Charles Mountains, Antarctica. AGSO Journal of Australian Geology and Geophysics 16, 345^370. Sheraton, J. W., Tingey, R. J., Black, L. P. & Ellis, D. J. (1980). Enderby Land, Antarctica ^ an unusual Precambrian high grade terrain. Journal of the Geological Society Australia 27, 1^18. Thompson, A. B. (1983). Fluid-absent metamorphism. Journal of the Geological Society, London 140, 533^547. Thost, D. E. & Hensen, B. J. (1991). Gneisses of the Porthos and Athos Ranges, Northern Prince Charles Mountains, East Antarctica: Constraints on the Prograde and Retrograde P^T path. In: Yoshida, Y., Kaminuma, K. & Shiraishi, K. (eds) Recent Progress in Antarctic Earth Science. Tokyo: Terra Scientific Publishing Company, pp. 93^102. Tingey, R. J. (1991). The regional geology of Archaean and Proterozoic rocks in Antarctica. In: Tingey, R. J. (ed.) The Geology of Antarctica. Oxford: Oxford University Press, pp. 1^73. Tong, L. & Wilson, C. J. L. (2006). Tectonothermal evolution of the ultrahigh temperature metapelites in the Rauer Group, east Antarctica. Precambrian Research 149, 1^20. Vernon, R. H. (1996). Problems with inferring P-T-t paths in low-P granulite facies rocks. Journal of Metamorphic Geology 14, 143^153. Vernon, R. H., White, R. W. & Clarke, G. L. (2008). False metamorphic events inferred from misinterpretation of microstructural evidence and P^Tdata. Journal of Metamorphic Geology 26, 437^449. White, R.W. & Powell, R. (2002). Melt loss and the preservation of granulite facies mineral assemblages. Journal of Metamorphic Geology 20, 621^632. White, R. W., Powell, R. & Clarke, G. L. (2002). The interpretation of reaction textures in Fe-rich granulites of the Musgrave Block, central Australia: constraints from mineral equilibria calculation in the system K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3. Journal of Metamorphic Geology 20, 41^55. White, R. W., Powell, R., Holland, T. J. B. & Worley, B. A. (2000). The effect of TiO2 and Fe2O3 on metapelitic assemblages at greenschist and amphibolite facies conditions: mineral equilibria calculations

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in the system K2O^FeO^MgO^Al2O3^SiO2^H2O^TiO2^Fe2O3. Journal of Metamorphic Geology 18, 497^511. White, R. W., Powell, R. & Holland, T. J. B. (2001). Calculation of partial melting equilibria in the system Na2O^CaO^K2O^FeO^ MgO^Al2O3^SiO2^H2O (NCKFMASH). Journal of Metamorphic Geology 19, 139^153. White, R. W., Pomroy, N. E. & Powell, R. (2005). An in situ metatexite^diatexite transition in upper amphibolite facies rocks from Broken Hill, Australia. Journal of Metamorphic Geology 23, 579^602. Williams, M. L. & Jercinovic, M. J. (2002). Microprobe monazite geochronology: putting absolute time into microstructural analysis. Journal of Structural Geology 24, 1013^1028. Young, D. N. & Black, L. P. (1991). U^Pb zircon dating of Proterozoic igneous charnockites from the Mawson Coast, East Antarctica. Antarctic Science 3, 205^216.

APPENDIX Method for (Th þ U)-Pb monazite dating using a Cameca SX51 Electron Microprobe Analyses of monazite were conducted using a Cameca SX51 Electron Microprobe at the Adelaide Microscopy unit in The University of Adelaide. The analyses were run at an accelerating voltage of 20 kVand a 100 nA beam current. Th, U, Pb and Ce were analysed concurrently with PET crystals using the Ma line for Th, Mb lines for Pb and U, and La line for Ce. The standards used were huttonite (Th), UO2, synthetic Pb glass (K227) and singleelement REE glasses. Elements analysed for each monazite spot analysis were O, Al, Si, P, Ca, Y, La, Ce, Pr, Nd, Sm,

Gd, Dy, Er, Pb, Th and U. Offline corrections were made to account for the overlap of the second-order Ce La escape peak with the required Pb Mb peak (Pyle et al., 2005), whereas online corrections were made for a Th Mg overlap on U and a U Mz2 overlap on Pb. Spectrometer calibration was tested against the elemental standards, with analysed concentrations for U, Th, Pb and Ce being 0·5% of the standard values. All other elements were within 1% of the standard composition. Probe performance was monitored by comparison with the MAD monazite standard with known U^Th^Pb concentrations (518 Ma). Reproducibility of the standard during the course of the analytical sessions was 527  8 Ma (n ¼ 34). This indicates that on average our results are probably 1·7% too old. In the scheme of defining either Early Palaeozoic or Early Neoproterozoic ages, this degree of error is of no concern. For data processing, initial verification of age analyses was based on the following criteria: (1) Al content should be low (50·1wt % element) to screen analyses in which neighbouring Al-bearing K-rich phases were partially targeted by the beam; (2) O content should be between 26·0 and 27·0 wt %; (3) the total measured oxides should be in excess of 95·0 wt %. Data were then plotted as frequency histograms to identify populations. As a result, three main populations were identified: (1) Late Palaeoproterozoic; (2) Early Neoproterozoic and (3) Early Palaeozoic.

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