© Terra Antartica Publication

Terra Antartica 2005, 12(2), 55-68

Geology and SHRIMP U-Pb Zircon Chronology of the Clemence Massif, Central Prince Charles Mountains, East Antarctica

1 1 1 2 A.F. CORVINO *, S.D. BOGER , C.J.L. WILSON & I.C.W. FITZSIMONS

1School of Earth Sciences, The University of Melbourne, Victoria, 3010 - Australia 2Tectonics Special Research Centre, Department of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth, WA 6845 – Australia

Received 22 April 2005; accepted in revised form 28 September 2005

Abstract- New single zircon SHRIMP U-Pb analyses from Clemence Massif reveal that both Proterozoic and Cambrian ages are ubiquitous in rocks of pre-, syn- and post-tectonic origin. Igneous zircon cores from a pre-tectonic felsic orthogneiss yielded a mean 207Pb/206Pb age of 1062 ±9 Ma, whereas metamorphic rims all gave Cambrian-Ordovician ages of c. 535-464 Ma. Zircons from a syn-tectonic leucogneiss gave scattered 207Pb/206Pb ages of c. 1079-782 Ma with a high density at c. 910 Ma, along with a single 238U/206Pb age of 527 ±10 Ma. An undeformed pegmatite dyke yielded zircons with core ages of c. 1116-828 Ma about a mean age of 905 ±6 Ma, along with discrete rim ages of c. 552-494 Ma. The available evidence suggests that Clemence Massif witnessed a high-grade tectono-metamorphic event during the Cambrian Period similar to nearby domains of the Mawson Escarpment, Prydz Bay and the Grove Mountains. Although there is some overlap of Early Neoproterozoic zircon ages (c. 990-900 Ma) with those from the northern Prince Charles Mountains (PCM) of the Rayner Complex, the data do not show clear affinities with this region. Consequently, the tectonic setting of Clemence Massif, and possibly that of the central PCM, appears to have been different from the Rayner Complex during Neoproterozoic-Cambrian times. A model that considers deposition of paragneiss protoliths at Clemence Massif during the Neoproterozoic is consistent with the field and isotopic data.

INTRODUCTION quartz, potash feldspar, biotite, magnetite and apatite, with plagioclase showing alteration to muscovite and Clemence Massif (72° 12’ S, 68° 40’ E) is calcite (McLeod, 1959). Garnet and biotite-rich situated in the northernmost sector of the southern quartzo-feldspathic gneisses, magnetite-bearing Prince Charles Mountains (PCM) and at the pegmatites and hornblende-bearing mafic layers were northeastern end of the Lambert Glacier (Fig. 1). The later observed near the northernmost peak of massif trends north-northeast to south-southwest and Clemence Massif (Tingey et al., 1981). is 28 km in length by 8 km in width. It has peaks The earliest regional geology correlations grouped rising to over 1200 m above the ice, which at this Clemence Massif with the northern PCM and Grove location has a reduced level of c. 100 m above sea Mountains based on the predominance of ‘granite- level. textured gneiss’ at all three localities (McLeod, 1959, Clemence Massif was first visited in 1958 by an 1964). A preliminary whole rock Rb-Sr isotopic age Australian field party that landed alongside the small of 816 ±298 Ma was later published for felsic nunatak, Gora Otdelënnaja, located 4 km to its south gneisses collected on the eastern side of Clemence (McLeod, 1959; Fig. 2). The outcrop was described Massif (P. Arriens unpub. in Tingey, 1991) along with Sr by McLeod (1959) as consisting of ‘migmatitic, a T Ur model age of 1079 Ma. Whilst these initial mainly pink medium-grained granite gneisses’ with an geochronological data have poor precision and context irregular small scale banding defined by biotite. With they do suggest affinities with Meso- to increasing mafic content the granitic gneiss was Neoproterozoic age metamorphic rocks of the observed to grade into a finer grained, well foliated, northern PCM (Tingey, 1991; Manton et al., 1992, ‘hornfelsic looking rock’. Concordant pegmatites and Kinny et al., 1997; Boger et al., 2000; Carson et al., shear structures were also noted. A representative 2000). This paper builds upon the early work of specimen of the granitic gneiss revealed a mineral McLeod (1959), Tingey et al. (1981) and Tingey assemblage consisting of antiperthitic oligoclase, (1991) by presenting geological observations of the

*Corresponding author ([email protected]) © Terra Antartica Publication 56 A.F. Corvino et al.

Fig. 1 – Location map of Clemence Massif within the Prince Charles Mountains.

rocks from Clemence Massif together with SHRIMP igneous rocks that show chemical characteristics U-Pb age data from three samples collected during indicative of formation in an active continental the 2002-03 Prince Charles Mountains Expedition of margin (Mikhalsky et al., 1996). Various isotopic Germany and Australia (PCMEGA). With these new studies have indicated that a series of igneous events data we assess the significance of Clemence Massif occurred in this area between c. 1300-1000 Ma with within the broader tectonic framework of the PCM. peak amphibolite-facies metamorphism at c. 1010 Ma (Beliatsky et al., 1994; Kinny et al., 1997; Mikhalsky et al., 1999). To the immediate north, the Rayner REGIONAL GEOLOGIC CONTEXT Complex consists of granulite-facies gneisses that The PCM are divided into at least three tectonic were deformed and metamorphosed between 990 Ma provinces (Fig. 1). These consist of: (i) the and 900 Ma (Manton et al., 1992; Kinny et al., 1997; predominately Archaean Ruker Terrane of the Boger et al., 2000; Carson et al., 2000). southern PCM; (ii) the Meso-to Neoproterozoic Metamorphism was of the high-temperature, low- to Rayner Complex that crops out in the northern PCM, mid-pressure type (800-900°C and 6-7 kbar) and and; (iii) the geologically distinct Fisher Province characterised by an isobaric cooling P-T path located in the southern sector of the northern PCM (Fitzsimons and Harley, 1992, 1994; Hand et al., (Mikhalsky et al., 2001b). 1994; Stephenson and Cook, 1997; Boger and White, The Fisher Province is characterised by an 2003). In the extreme south of the PCM, the Ruker abundance of c. 1300 Ma intermediate and mafic Terrane consists of interleaved granitic gneiss © Terra Antartica Publication Geology and SHRIMP U-Pb Zircon Chronology of the Clemence Massif, East Antarctica 57

Fig. 2 – Geological map of Clemence Massif (Gora Iskristaja). The simplest interpretation of a broadly folded sequence is shown. Key: 1. Ice and snow cover (undifferentiated); 2. Magnetite bearing felsic orthogneiss; 3. Intermediate grey-green pyroxene bearing gneiss; 4. Undifferentiated paragneiss (leucocratic garnet-biotite gneiss, garnet-biotite semipelite and sillimanite-garnet pelite); 5. biotite-quartz-feldspar ± garnet gneiss; 6. Area not visited but possibly 'migmatitic granite-textured gneiss' after descriptions by McLeod (1959, 1964); 7. Gradational lithological boundary inferred; 8. S1 strike and dip; 9. S1 strike and dip after Tingey and Convine (1982); 10. Linear element; 11. Open to tight mesoscopic upright folds; 12. PCMEGA field camp; 13. Rock specimen location and identification; 14. Elevation contour (m); 15. Spot height (m). Contours reproduced after the 1:200,000 Gora Iskristaja Topographical Map, Ministry of Merchant Fleet of the U.S.S.R., 1978.

(c. 3160 Ma) and metasediments deformed prior to considerable debate. It is either included as part of 2650 Ma (Boger et al., 2001). Staurolite and kyanite the Rayner Complex (Mikhalsky et al., 2001b) or is bearing assemblages from the metasediments considered part of a separate Lambert Province. This (Mikhalsky et al., 2001b) imply lower peak latter hypothesis was first proposed by Kamenev et temperatures when compared to the rocks of the al. (1993) who considered the Lambert Province to Rayner Complex. consist of a tectonic mixture of progressively The origin of the region between the Fisher metamorphosed rocks of the Ruker Terrane and Province and the Ruker Terrane (Fig. 1) is subject to retrogressed rocks of the granulite-facies northern © Terra Antartica Publication 58 A.F. Corvino et al.

Fig. 3 – Paragneiss-type rocks along the eastern margin of Clemence Massif. A strong mesoscopic banding is defined by alternating leucocratic gneiss (equivalent to specimen CLEM-153) and weathered semipelite. Cliffs are c. 80-100m high. Photo looking north- northwest.

PCM. Although this model has not been supported by Escarpment (Boger et al., 2001; Fig. 1). Recent whole rock Rb-Sr isotopic data (Tingey, 1991), the workers have suggested that a Pan-African age concept of a separate Lambert Province can be tectonic event was significant in the southern PCM inferred using recent U-Pb zircon data from the (e.g. Fitzsimons, 2000; Boger et al., 2001; Boger & central Mawson Escarpment (Boger et al., 2001). Miller, 2004) although the impact of such an event on From this region, detrital zircons with age populations the central PCM is unknown. of c. 2790 Ma and c. 2130 Ma and some evidence of new zircon growth at c.1800 Ma and 1600 Ma are obtained from Early Cambrian intrusions. Both the ROCK TYPES intrusion ages and the detrital populations from these rocks are dissimilar to the main metamorphic and Clemence Massif consists of seven main intrusive ages from either the Ruker Terrane, Fisher lithologies that are either of intermediate to felsic Province or Rayner Complex. However, the northern intrusive origin (i.e. orthogneiss and pegmatite) or extent of these isotopically distinct rocks (Lambert have a siliceous paragneiss protolith (Tab. 1). The Province) is unconstrained and the felsic gneisses mid-eastern margin of Clemence Massif is dominated from Mt. Johns, Shaw Massif and Clemence Massif by leucocratic garnet-biotite gneiss (leucogneiss) and have revealed provisional ages closer to c. 1000 Ma brown coloured garnet-biotite semipelite that are (Tingey, 1991) suggesting a closer affinity to the interlayered at the metre to tens-of-metres scale Rayner Complex. (Fig. 3). Both rock types are rich in medium-grained At present there is a genuine paucity of geological quartz and K-Feldspar (commonly perthite) and information for the central section of the PCM (i.e. include variable amounts of fine biotite and Clemence and Shaw Massifs, Mt. Izabelle, Mt. Johns porphyroblastic garnet. A ubiquitous centimetre or plus Boose and Ely Nunataks, Fig. 1) that makes any sub-centimetre gneissic foliation is defined by reliable correlation between the northern and the alternating quartzo-feldpathic layers and biotite ± greater southern PCM difficult. In this context the garnet rich layers. The semipelite is distinguished geographical position of Clemence Massif makes it a from the leucogneiss by a greater modal occurrence valuable link; it is the largest nunatak immediately of garnet, biotite and the presence of minor south from the geologically distinct Fisher Province, sillimanite. Microfabrics are dominated by a and north from the recently proposed Pan-African age granoblastic, equigranular to inequigranular- (c. 500 Ma) mobile belt that transects the southern interlobate, quartzo-feldspathic grain structure and are margin of the Lambert Province across the Mawson inferred to have formed during a high-grade tectono- © Terra Antartica Publication Geology and SHRIMP U-Pb Zircon Chronology of the Clemence Massif, East Antarctica 59

Tab. 1 - Provisional classifications for Clemence Massif rock types and their dominant mineral assemblages. Mineral abbreviations after Kretz (1983): bt, biotite; cpx, clinopyroxene; grt, garnet; ilm, ilmenite; Kfs, K-feldspar; mag, magnetite; mc, microcline; or, orthoclase; pl, plagioclase; qtz, quartz; sil, sillimanite.

metamorphic event. Narrow horizons of aluminous possibly imply that low-grade hydrothermal alteration pelite are interlayered with this sequence and exhibit post-dated pegmatite emplacement. Previously a mineral paragenesis indicative of high-temperature greenschist-facies metamorphism and igneous metamorphism, including sillimanite + garnet + K- intrusive activity in the central PCM area have been feldspar + quartz along with minor biotite and ascribed to an event at c. 500 Ma (e.g. Tingey, 1991). ilmenite. Medium-grained, equigranular, grey-pink coloured biotite-quartz-feldspar gneiss with minor garnet is also STRUCTURE a widespread lithology. This rock type dominates the high outcrops of Gora Tri Vershiny and also areas to Although a minimum of two separate phases of the south of where the semipelite and pelite are found ductile deformation are identified at Clemence Massif (Fig. 2). Locally it is interlayered with an (Tab. 2) the structural geologic interpretation of this intermediate grey-green coloured pyroxene-bearing massif is straightforward. All units strike north- gneiss of assemblage clinopyroxene + plagioclase + northwest and dip moderately to the southwest. This orthoclase + microcline + biotite + opaque + quartz primary orientation is recorded by both lithological

(Fig. 2). The microfabric of the pyroxene-bearing layering (S0) and a sub-parallel, penetrative gneissic gneiss is variable; some parts appear igneous (i.e. a foliation (S1) that formed during a first, major hypidiomorphic-granular matrix of euhedral deformation (D1). The gneissic layering varies as a plagioclase and pyroxene interspersed with quartz), function of ferromagnesium mineral concentration while others are typically metamorphic (i.e. (e.g. biotite or pyroxene) and this is particularly granoblastic plagioclase-pyroxene intergrowth). evident in magnetite-bearing felsic orthogneiss, The northern end of Clemence Massif is biotite-quartz-feldspar gneiss and intermediate grey- dominated by a medium-grained, migmatitic green pyroxene-bearing gneiss (a phenomenon also magnetite-bearing felsic orthogneiss that is well noted by McLeod, 1959). In addition, S1 is banded at the decimetre to metre scale. In hand commonly defined by the preferred orientation of specimen this gneiss tends to be massive in biotite laths or by sillimanite prisms lying in a plane. appearance, although where present a weak foliation The widespread occurrence of leucosome (partial may be defined by the preferred orientation of fine- melt) parallel to S1 suggests that D1 peak grained biotite. This unit, as implied, is considered to metamorphism occurred at suprasolidus temperatures. have an igneous origin and correlates with a high Isolated layer-parallel, sigmoidal shear bands (c. 1m positive anomaly on magnetic intensity maps of the thick) and rare intrafolial isoclines probably signify

PCM (Golynksky et al., 2002). In contrast, the area that simple shear domains existed during D1. of lower magnetic intensity in the east-central sector At the macroscopic level the orientation of S1 of the massif corresponds to the paragneiss dominated shows very little variation across Clemence Massif; sequence described above. from c. 340°/40° SW in the southeast through to c. All gneisses are intruded by planar, up to 1m 310°/40° SW in the northwest (Fig. 2). The mean thick, sub-vertical pegmatite dykes or veins and show strike and dip of S1 is equal to 330°/33° SW (as some affects of retrograde metamorphism. The most calculated via the mean axial vector for 82 common alteration products include: chlorite, sericite, measurements of S1 poles). A best-fit great circle to muscovite, calcite and epidote after biotite; chlorite, S1 poles may indicate gentle folding about a muscovite and sericite after garnet, and; serisitisation moderately plunging, southwest trending axis (040° to of feldspars. Retrograde minerals in the pegmatite 222°, Fig. 2). This is, to some extent, in agreement © Terra Antartica Publication 60 A.F. Corvino et al.

Fig. 4 – Rare outcrop of tight, moderately inclined folds possibly associated with late shearing. A strong north-northeast to south-southwest lineation is associated with this structure. Person provides scale. Photo looking north. with fold axis data (Fig. 2) measured on a few Although this does not reveal whether the shear uncommon mesoscopic F2 folds (Fig. 4). These necessarily pre-dates the pegmatite, it does suggest locally reorient S1 about a shallow, south-southwest that local folding (F2) and shearing may have been plunging axis and are associated with a strong, sub- allied (Tab. 2). The limited geometric data show parallel linear fabric that is not obvious elsewhere. pegmatite dykes are oriented either north-northeast or They are moderately inclined and have moderate to east-southeast (Fig. 2). tight interlimb angles. The timing relationship between the local F2 folds and the inferred macroscopic folding has not been established and it is GEOCHRONOLOGY plausible that both structures are expressions of a secondary deformation event (Tab. 2). Three rock specimens were selected for Sensitive The timing of late pegmatite emplacement relative High Resolution Ion MicroProbe (SHRIMP) U-Pb to F2 is also uncertain as no direct overprinting zircon analysis with the aim of acquiring pre-, syn- relationships have yet been discovered. However, a and post-tectonic ages. Specimen CLEM-157 is from pegmatite vein occupying a shear adjacent to ‘highly a foliated magnetite-bearing felsic orthogneiss thought contorted gneiss’ was noted by McLeod (1959). to be derived from an igneous protolith emplaced

Tab. 2 – Inferred succession of geologic events at Clemence Massif. © Terra Antartica Publication Geology and SHRIMP U-Pb Zircon Chronology of the Clemence Massif, East Antarctica 61

prior to formation of S1. Specimen CLEM-153 is Ludwig (2001). All single spot and mean ages are from a several-metre-thick layer of leucocratic quoted in the text at the 95% confidence-level. garnet-biotite gneiss that may represent either partial melt derived from adjacent semipelitic and pelitic ORTHOGNEISS SPECIMEN (CLEM-157) units or an originally arenaceous rock recrystallised during peak metamorphism. Specimen CLEM-152 is Zircons from the felsic orthogneiss are translucent from a late undeformed pegmatite dyke that is pale pink coloured or clear. All grains preserve a superimposed onto, and post-dates, S . subhedral exterior, with sub-rounded pyramidal 1 terminations, and may have notched prismatic faces. Aspect ratios (length:width) vary between 2:1 and ANALYTICAL METHODS 3:1. Grain sizes are relatively uniform and have an Zircons were separated from crushed rock estimated mean length of c. 175 mm, although grains specimens using standard magnetic and heavy liquid up to 300 mm are present. In CL imagery more than procedures. For each specimen c. 100 grains were 90% of grains reveal anhedral, oscillatory zoned cores handpicked and mounted in epoxy resin discs along that have embayed or lobed boundaries. The cores with fragments of the zircon standard CZ3. The discs have low U content (263-479 ppm) and comparatively were then polished, gold coated and imaged using a high Th/U ratios (0.39–0.60). They are surrounded by 10-50 mm thick rims that have high CL emission and cathodoluminescence (CL) detector. U-Pb isotopic low Th/U ratios (c. 0.02). The irregular interface analyses were made using the SHRIMP II based in geometries between the cores and rims probably the John De Laeter Centre of Mass Spectrometry, originates from recrystallisation or new zircon growth Curtin University, Perth, Australia. Seven mass scans during high-grade metamorphism (e.g. Corfu et al., were made during each analysis of the isotope species 2003). Zr O+ (2 secs), 204Pb+ (10 secs), 206Pb+ (10 secs), 2 Eleven analyses on nine zircons were obtained 207 + 208 + 238 + Pb (20 secs), Pb (10 secs), U (5 secs), from the felsic orthogneiss (Tab. 4; Fig. 5a). Analyses 232 + 238 + ThO (5 secs) and UO (2 secs). In order to of five cores yielded 207Pb/206Pb ages of c. 1108-1039 remove surface contamination the primary beam was Ma and a weighted mean age of 1062 ±9 Ma rastered across the sample surface for 2 minutes prior (MSWD = 1.44). In contrast, two cores and four rims to each analysis. Isotope ratios were corrected for with lower Th/U ratios yielded younger Cambrian- common lead using measured 204Pb for which Stacey- Ordovician ages of c. 535-464 Ma. Analyses 3.1, 6.2 Kramers compositions based on the age were assumed and 9.2 lie close to the concordia curve and record (Stacey and Kramers, 1975). Pb/U ratios for the 206Pb/238U ages of 512 ±11 Ma, 530 ±10 Ma and 535 unknowns were adjusted using a calibration curve ±10 Ma respectively (Fig. 5a). Two further analyses (Pb+/U+ versus UO+/U+) determined by interspersed (spots 2.1 and 5.1) record 206Pb/238U ages of 513 ±16 analyses of the standard CZ3 (206Pb/238U = 0.0914; and 510 ±16 Ma, however these data are highly 564 Ma; U content = 551 ppm). Data were examined, discordant (70-80%). The remaining analysis (spot processed and reduced using SQUID software of 4.1, with anomalously high U content of 3497 ppm)

Tab. 3 - SHRIMP U-Pb isotopic data for specimen CLEM-152. Pbc and Pb* indicate common and radiogenic portions respectively. 2σ error in standard calibration was 1.05% (not included in errors shown). © Terra Antartica Publication 62 A.F. Corvino et al.

Fig. 5 – Standard Wetherill concordia plots (a and b) for the leucogneiss and felsic orthogneiss, and Tera-Wasserburg concordia diagram (c) for the pegmatite. Error ellipses are 1σ. Weighted mean ages are shown for shaded ellipses. Relative probability diagrams (d,e and f) correspond to zircon analyses from the felsic orthogneiss, leucogneiss and pegmatite respectively. yielded a considerably younger 206Pb/238U age of 464 910 ±7 Ma (MSWD = 0.59). Two further analyses ±13 Ma. taken from relatively high-U, low-Th/U rim areas yielded 206Pb/238U ages of 527 ±10 Ma (10.1) and LEUCOGNEISS SPECIMEN (CLEM-153) 584 ±12 Ma (4.2). Whilst the former analysis is highly concordant, the latter is 16% discordant and Zircons from the leucogneiss comprise subhedral thus the age obtained is questionable. A further to anhedral rounded fragments that are either analysis taken in the mid section of an unstructured transparent or translucent brown. They have an grain gave a 207Pb/206Pb age of 782 ±65 Ma (8.1). aspect ratio of c. 3:1 and a mean grain length of The significance of this age is unknown, however it c. 100-300 mm. Most grains (c. 95%) have poor CL may represent mixing between core (c. 1000 Ma) and emission and appear dark, homogenous and unzoned. rim (c. 530 Ma) components of this particular zircon. In general their U contents are high (c. 1000-5000 ppm) and Th/U ratios are low (0.02-0.13). The PEGMATITE SPECIMEN (CLEM-152) remaining grains (c. 5%) show discrete core fragments with weak to moderate CL emissions. Zircons from the pegmatite are translucent pale These have both variable Th/U ratios (0.06-0.53) and pink-brown in colour. They preserve euhedral to U concentrations (631-2409 ppm). subhedral forms with sub-rounded terminations, and Thirteen analyses of ten zircons from the have stubby to elongate aspect ratios between 2:1 and leucogneiss were obtained (Tab. 4; Fig. 5b). Analyses 5:1. Grain size varies with length, with a range of c. of three structurally distinct cores gave 207Pb/206Pb 150-500 mm. CL imagery reveals that most zircons ages of 1079 ±28 Ma, 1010 ±53 Ma and 969 ±128 have two main zones: an older core (sometimes with Ma. Seven analyses, taken from both rim and core oscillatory zoning) and a younger, homogeneous rim. areas of homogeneous grains, represent a single The majority of cores retain a subhedral outline and population with a 207Pb/206Pb weighted mean age of are, in general, highly cathodoluminescent although a © Terra Antartica Publication Geology and SHRIMP U-Pb Zircon Chronology of the Clemence Massif, East Antarctica 63

Tab. 4 - SHRIMP U-Pb isotopic data for specimens CLEM-153 and CLEM-157. Pbc and Pb* indicate common and radiogenic portions respectively. 2σ error in standard calibration was 1.17% (not included in errors shown).

few are dark. In this case the difference in CL 10.2 at 494 ±9 Ma and by spots 2.1 and 3.1 at 525 emission is attributable to variation in U content; ±8 Ma. The remaining analyses produced discrete bright cores have relatively low U content (less than 206Pb/238U ages of 506 ±9 Ma (6.2), 514 ±9 Ma (5.2) 162 ppm) and higher Th/U ratios (0.41-0.95) and 552 ±12 Ma (7.1). A further rim analysis, around compared with their dark counterparts (1153-10538 the c. 1105 Ma core, yielded a 206Pb/207Pb age of ppm and 0.02-0.29 respectively; Tab. 3). In contrast 845 ±43 Ma whilst another revealed an extremely all rims show poor CL emission and have high U high common Pb count (40%) and a highly discordant content (c. 1076-3830 ppm) and low Th/U ratios 206Pb/238U age of 689 ±61 Ma (spot 1.2). (0.06-0.27). A small percentage of zircons are homogeneous, unzoned and have a uniform low CL emission. These have comparatively high U content DISCUSSION and low Th/U ratios similar to the rims. Seventeen isotope analyses were made in total. Zircons from both the pegmatite and felsic Eight analyses of zircon cores yielded near orthogneiss, and a few from the leucogneiss, are concordant 207Pb/206Pb ages between c. 1116-828 Ma characterised by magmatic cores containing moderate (Tab. 3). Of these, five represent a dominant single concentrations of U and comparatively high Th/U population with a 207Pb/206Pb weighted mean age of ratios (c. 0.40-1.00) mantled by compositionally and 905 ±6 Ma (MSWD = 0.87). The remaining analyses structurally distinct high-U, low- Th/U rims (c. 0.01- yielded 207Pb/206Pb ages of 1105 ±188 Ma, 858 ±64 0.30). These qualities are thought to result from the and 828 ±79 Ma (Tab. 3). In contrast, seven analyses preferential expulsion of Th over U during of zircon rims or homogeneous grains plot close to recrystallisation processes, and thus low Th/U ratios the concordia curve and record 207Pb/238U ages in the can be symptomatic of metamorphic zircon formed by range of c. 552-494 Ma (Tab. 3; Fig. 5c). Although such phenomena (e.g. Williams and Claesson, 1987; these data produce a weighted mean age of 515 ±17 Maas et al., 1992; Hoskin and Black, 2000; Rubatto, Ma, they are too limited and dispersed to define a 2002). On the other hand, igneous zircons are single population (MSWD = 16). Concordia ages commonly identified by their oscillatory zoning (after Ludwig, 1998) are defined by spots 9.1 and patterns and Th/U ratios of around 0.5 or greater © Terra Antartica Publication 64 A.F. Corvino et al.

(Hoskin and Schaltegger, 2003). Given these is consistent with observations from both the northern observations, it is probable that the c. 1060 Ma zircon and southern PCM where Cambrian intrusions are population in the orthogneiss, the c. 1100-950 Ma mostly defined by discrete pegmatite or granite dykes zircon cores in the leucogneiss, and the c. 905 Ma (Manton et al. 1992; Mikhalsky et al., 2001b; Boger zircon population in the pegmatite are of igneous et al., 2002). For the leucogneiss, it is possible that origin, whilst the c. 910 Ma population in the the concordant age of c. 530 Ma records the time of leucogneiss is likely to be metamorphic. This would peak metamorphism and that all older metamorphic- be consistent with the following history: (i) igneous ages, scattered in the range c. 1079-782 Ma, crystallisation of the felsic orthogneiss protolith at represent detrital grains. To account for the paucity, 1062 ±9 Ma; (ii) partial melting and metamorphism but not absence, of Cambrian ages we would of sedimentary protoliths, perhaps containing some c. envisage a fluid-undersaturated partial melt source for 1100-950 detrital grains, at c. 910 Ma, and; (iii) the leucogneiss in which zircon solubility was limited intrusion of the pegmatite dyke at 905 ±6 Ma. This (by analogy to the work of Watt and Harley, 1993). interpretation constrains D to have occurred after c. 1 This scenario constrains D1 to have occurred during 1060 Ma and prior to c. 905 Ma, with peak the Early Cambrian, with peak metamorphism at c. metamorphism occurring at c. 910 Ma. However, 530 Ma followed by igneous intrusive activity in the there are also a younger set of ages indicating a high- Late Cambrian. temperature overprint during the Cambrian Period (c. A hypothesis that is consistent with our data 530-510 Ma) as recorded by variably developed assumes that deposition of paragneiss protoliths metamorphic zircon of this age in all three rock occurred at Clemence Massif during the types. Neoproterozoic and preceded a high-temperature event A similar interpretation drawn from earlier at c. 530 Ma. Firstly, such an explanation accounts SHRIMP U-Pb zircon studies from the Rauer Islands for the absence of Early Neoproterozoic ages (c. 990- (Kinny et al., 1993) was later shown to underestimate 900 Ma) from the felsic orthogneiss and the spectrum the significance of Cambrian orogenesis in this region of such ages from both the leucogneiss and (e.g. Hensen and Zhou, 1995; Dirks and Wilson, pegmatite. Secondly, the simple structural geometry of 1995; Carson et al., 1996; Kelsey et al., 2003). Kinny Clemence Massif and current lack of identified et al. (1993) found that c. 500 Ma metamorphic overprinting tectono-metamorphic fabrics (on all overgrowths were common to zircons from both c. scales) is congruent with one ubiquitous high-grade 1000 Ma meta-supracrustal rocks and tectonically episode at c. 530 Ma. Inherited zircons within the interleaved Archaean orthogneiss. Although they failed sedimentary protoliths, and consequently the to detect Proterozoic age growth in the Archaean leucogneiss, would most likely have provenance from zircons, Kinny et al. (1993) nevertheless argued for a the metamorphic-igneous rocks of the adjacent Rayner dominant tectonic episode at c. 1000 Ma and Complex. If we accept this idea, then it is tempting attributed the c. 500 Ma ages to ‘lower-grade to speculate that the felsic orthogneiss provides a hydrothermal processes’. The difficulty with their basement that both pre-dates and structurally underlies interpretation, as pointed out by Zhao et al. (2003), much of the presumed paragneiss cover sequence. was that if the meta-supracrustal rocks of the Rauer One advantage of this interpretation is that it partly Islands experienced a pervasive tectonic event at c. removes the conundrum of having two sections of the 1000 Ma then some evidence for this should have Rayner Complex, reworked from 990 Ma to 900 Ma, been recorded by the Archaean rocks (assuming these rocks were juxtaposed prior to c. 500 Ma). With this located on either side of the discrete Fisher Province. in mind, the absence of 990-900 Ma zircon ages from Although a model involving Middle our felsic orthogneiss specimen (CLEM-157) casts Neoproterozoic sedimentation is appealing and cannot some doubt on Clemence Massif having witnessed a be precluded, we recognise that it is not convincingly major tectono-metamorphic episode during the Early supported by age data from the leucogneiss which Neoproterozoic. suggest that a c. 910 Ma event was dominant. Thus, we offer an alternative interpretation of our Ultimately this age must represent one of three data. Pegmatite emplacement occurred during the Late options: (i) a high-grade metamorphic event that did Cambrian with magmatic zircon growth at about 495 not impact upon zircon U-Th-Pb systematics in the Ma. Some evidence for this is recorded by two felsic orthogneiss; (ii) a coincidentally high zircons that have moderate, and possibly igneous- occurrence of c. 910 Ma detrital grains in a type, U/Th ratios (spots 9.1 and 10.2, Table 3). sedimentary protolith that underwent anatexis at c. Moreover, inheritance of xenocrystic Proterozoic 530 Ma; or (iii) contrary to our field observations, the grains is represented by the spectrum of core ages emplacement of a concordant felsic dyke overprinted from c. 1116 Ma to c. 828 Ma. Given that many of by metamorphism at c. 530 Ma. the Early and Middle Cambrian age zircon rims A major feature of the isotopic data is that zircon appear to have a metamorphic origin it follows that ages of both c. 1100-800 Ma and c. 550-460 Ma they may also form part of the xenocrysts and signify occur together in rocks of pre-, syn- and post-tectonic metamorphism of the source rocks. This interpretation origin, an observation that is unique within the PCM. © Terra Antartica Publication Geology and SHRIMP U-Pb Zircon Chronology of the Clemence Massif, East Antarctica 65

The absence of ages older than c.1100 Ma Escarpment as a mechanism for merging the Lambert automatically precludes correlation of any of these Province and Ruker Terrane (Fig. 1). Whilst the rocks with the Archaean Ruker Terrane in the evidence for a bona fide suture may be conjectural, it southern PCM. Similarly, the absence of Palaeo- and seems clear that Cambrian tectonism was significant Mesoproterozoic inheritance precludes a direct along the Mawson Escarpment and correlates with the correlation with the Lambert Province as defined by timing of a high-grade metamorphic event at rocks exposed along the central Mawson Escarpment Clemence Massif. It is perhaps noteworthy that both (Boger et al., 2001). The crystallisation age of the Clemence Massif and the Mawson Escarpment felsic orthogneiss (c. 1060 Ma) corresponds with Late occupy a position along the easternmost margin of the Mesoproterozoic ages from a suite of other granitoids PCM. Although a c. 500 Ma mobile belt might located throughout the central and northern PCM latitudinally transect the PCM, as indicated by the (these are summarised by Mikhalsky et al., 2001b). dominant structural trend, we highlight the fact that Sr This age also correlates with the T Ur model age of there is yet no supporting geochronological evidence 1079 Ma originally obtained from Clemence Massif for this beyond proximity to the Lambert Graben (a felsic gneisses (Tingey, 1991) that we presume are point previously made by Fitzsimons, 2003). similar to our felsic orthogneiss. In addition, there are Beyond the PCM, extensive Cambrian tectonism several notable similarities in Early Neoproterozoic has been reported from both Prydz Bay and the isotopic ages from both the leucogneiss and pegmatite Grove Mountains. In southern Prydz Bay, c. 400 km that suggest affinities with the Rayner Complex. For northeast from Clemence Massif, the Pan-African example, the mean c. 910-900 Ma ages from these event is characterised by pervasive ductile rocks correlate precisely with late syn-orogenic ages deformation, granulite-facies metamorphism, in-situ reported from the Radok Lake area (900 ±28 Ma; anatexis and syn-tectonic granite emplacement (Stüwe SHRIMP U-Pb zircon age; Boger et al., 2000), Mt. & Powell, 1989; Dirks et al., 1993; Hensen & Zhou, Kirkby (910 ±18 Ma; SHRIMP U-Pb zircon age; 1995; Carson et al., 1996; Fitzsimons, 1997; Carson et al., 2000) and Cape Bruce (c. 910 Ma; U- Fitzsimons et al. 1997; and others). Current models Pb zircon age; Dunkley et al., 2002). However, syn- suggest peak metamorphic conditions reached c. 800°- to late-orogenic zircons of Cambrian age are unknown 860° C at 5.5-7 kbar during a collisional event at from the northern PCM (Kinny et al., 1997; Boger et c. 530 Ma, followed by tectonically driven al., 2000; Carson et al., 2000). Their presence at exhumation and extensional shearing synchronous Clemence Massif indicates either a separate tectono- with the emplacement of granite and leucosome at c. metamorphic history, or at least a significantly higher 517-486 Ma (summarised by Zhao et al., 2003 and temperature Cambrian overprint than is recognised in Harley, 2003). Early Palaeozoic zircon ages from the northern PCM. This is in accord with differing Clemence Massif correlate highly with those from deformation histories from the two regions. Early Prydz Bay and it is possible they represent the same Neoproterozoic (c. 990-900 Ma) orogenesis in the tectonic event. northern PCM and extended Rayner Complex can be Similarly, the Grove Mountains, c. 220 km east- readily associated with pervasive multi-phase folding southeast from Clemence Massif, preserve some and episodic magmatism (Fitzsimons & Thost, 1992; evidence for having experienced a single granulite Boger et al., 2000; Kelly et al., 2000; Dunkley et al., facies event from c. 530-510 Ma with syn- to post- 2002). In comparison, observations from Clemence tectonic granitoid and charnokite emplacement from Massif reveal a simple structural architecture that is c. 534-501 Ma (Zhao et al., 2000; Mikhalsky et al., exemplified by one major foliation and a notable lack 2001a; Liu et al., 2002, 2003). In particular, Zhao et of folds. Hence, we find it difficult to accept that al. (2000) have reported SHRIMP U-Pb zircon Clemence Massif and the northern PCM share a analyses from a felsic gneiss that yielded similar tectonic evolution and again highlight the lack metamorphic rim ages of c. 529 Ma and inherited of Neoproterozoic ages from the felsic orthogneiss. scattered core ages from c. 953-870 Ma. Whilst we Boger et al. (2001) have recognised Pan-African acknowledge the paucity of Cambrian ages from the age tectonism, associated with some structural leucogneiss dated in our study, the idea that the reworking, medium- to high-grade metamorphism and c. 530 age reflects peak metamorphism with a voluminous felsic dyke emplacement, in the central spectrum of inherited ages from 1079 Ma to 782 Ma Mawson Escarpment of the southern PCM (Fig. 1). correlates well with available data from the Grove Pre-, syn- and post-tectonic felsic dykes from this Mountains. Interestingly, Zhao et al. (2003) have also area preserve a unique record of inherited suggested that Neoproterozoic ages from the Grove Palaeoproterozoic zircon ages, coupled with new Mountains felsic gneiss might reflect detrital grains zircon growth ages of c. 550-490 Ma, that are with provenance from the Rayner Complex. Similar incongruent to the exclusively Archaean age rocks of interpretations within East Antarctica have been made the adjacent Ruker Terrane (Boger et al. 2001). This from Lützow-Holm Bay rocks (Shiraishi et al., 1992) observation prompted Boger et al. (2001) to propose and the Prydz Bay region (Zhao, et al., 1995; Hensen a Pan-African age suture zone across the Mawson & Zhou, 1995). © Terra Antartica Publication 66 A.F. Corvino et al.

CONCLUSIONS REFERENCES

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