© Terra Antartica Publication Terra Antartica 2005, 12(2), 69-86

Stratigraphy and Structure of the Southern , East

G. PHILLIPS1*, C.J.L. WILSON1 & I.C.W. FITZSIMONS2

1School of Earth Sciences, The University of Melbourne, Victoria 3010 - Australia 2Tectonics SRC, Department of Applied Geology, Curtin University of Technology, GPO Box U1987 Perth, WA 6845 - Australia Received 22 April 2005; accepted in revised form 3 October 2005

Abstract- Stratigraphic and structural data support the existence of a thick and extensive low-grade sedimentary cover sequence of relatively young age at Cumpston Massif and Mount Rubin compared with the underlying Archaean-Palaeoproterozic basement rocks of the southern Prince Charles Mountains. The stratigraphy of these sequences suggests sediment deposition was within a shallowing-up marine basin. Folding within the basin displays a simple structural style in comparison to older, multiply deformed rocks in

the adjacent nunataks such as Mount Stinear and Mount Ruker. D1 folding and fabric development within the basin has formed in response to a northeast-southwest shortening. This stress regime may be responsible for late stage mylonite zones, thrust faults and transposed fabrics that overprint earlier structures in the basement that represent the basin margins. At Cumpston Massif, the base of the basin sediments is incorporated into a 100 m wide low-angle shear zone, with the meta-sediments ramping over deformed felsic gneiss. Similar relationships at Mount Maguire suggesting continuation of the basin further south, yet absent at Mount Rubin, creating uncertainties in extrapolating basin margins to the west. Early deformation within basement sequences prior to basin deposition suggests at least two phases of non-coaxial deformation, preserved in the folded banded iron formations at Mount Ruker. Such structures are partially obscured at Mount Stinear due to mylonitisation, and possible exhumation of the underlying crystalline basement.

INTRODUCTION Terrane and incorporates multiple phases of

deformation (D1-D4; Carson et al., 2000; Boger et al., Workers within the Prince Charles Mountains 2000, cf. D1-D8; Fitzsimons & Thost, 1992). (Fig. 1a) (Tingey, 1982; Grew, 1982; Hofmann, 1982; Rocks of the Fisher Terrane are characterised by Kamenev et al., 1993; Mikhalsky et al., 2001) have bimodal mafic to felsic intrusive rocks that have established a basic geological framework for the area preserved a greenschist to amphibolite facies (Tab. 1). The tectonic subdivision as outlined by metamorphic grade. U-Pb zircon geochronology Kamenev et al., (1993) divides the region into three analyses yields crystallisation ages of the basic to terranes; the northern high-grade Beaver-Lambert intermediate lithologies between ca. 1300 - 1200 Ma Terrane, the meta-volcanic Fisher Terrane in the (Beliatsky et al., 1994, Kinny et al., 1997; Mikhalsky central Prince Charles Mountains, and the low- to et al., 1999) overprinted by a second stage of felsic medium-grade Ruker Terrane in the south (Fig. 1b). magmatism between ca. 1050-1020 Ma (Mikhalsky et Discrimination between terranes is primarily based on al., 2001; Kinny et al., 1997). An east-west to isotopic dating. The Beaver-Lambert Terrane is northeast-southwest structural grain with up to four characterised by felsic orthogneiss and paragneiss phases of deformation is outlined by Crowe (1994)

(Mikhalsky et al., 2001) interleaved with pelite, and Mikhalsky et al., (1999). D1 and D2 events are psammite and calcareous meta-sediments. The Terrane associated with foliation development, folding and is discriminated by upper-amphibolite to granulite metamorphism while D3 and D4 are recorded as facies mineral assemblages that are attributed to early regional warping, ductile shearing and faulting Neoproterozoic (ca. 990 – 900 Ma) tectonism (Kinny (Crowe, 1994). et al., 1997; Boger et al., 2000; Carson et al., 2000). The Ruker Terrane has been interpreted as a Discrete mylonite zones, pegmatite emplacement complex greenstone-granite terrane composed of (ca. 550-480 Ma; Manton et al., 1992; Boger et al., Archaean cratonic fragments, overlain by 2002) and greenschist-amphibolite facies metamorphic Palaeoproterozoic-Neoproterozoic(?) cover sequences assemblages (Fitzsimons & Thost, 1992; Thost & and deformed throughout the Proterozoic to Hensen, 1992) overprint the early structures. A Palaeozoic (Tab. 1; Halpern & Grikurov, 1975; Grew dominant east-west structural grain (Fitzsimons & & Manton, 1983; Tingey, 1991; Boger et al., 2001; Harley, 1992) is reported from the Beaver-Lambert Mikhalsky et al., 2001). Mafic dykes and sills intrude

*Corresponding author ([email protected]) © Terra Antartica Publication 70 G. Phillips et al.

Fig. 1 – Locality map of the southern Prince Charles Mountains with respect to Antarctica. (a) Antarctica displaying the position of the Amery Ice Shelf – Lambert Glacier Region. (b) The Prince Charles Mountains divided into three distinct terranes: Beaver-Lambert, Fisher and Ruker. (c) The southern Prince Charles Mountains indicating position of the Beaver-Lambert/Ruker Terrane boundary. The younger Sodruzhestvo Series overlies rocks of the Ruker Terrane. Presence of ca. 550 - 500 Ma pegmatite and granite discriminates Lambert from Ruker Terrane rocks.

throughout the region. The boundary between the THE RUKER TERRANE – PREVIOUS WORK Ruker Terrane to the south and Lambert Terrane to the north (Fig. 1c) is exposed in the southern The crystalline basement of the Ruker Terrane Mawson Escarpment and is discriminated by the comprises a quartz-K feldspar-biotite±horn- presence of early Palaeozoic pegmatite and granite blende±epidote orthogneiss that is reported to crop (ca. 550 – 490 Ma; Tingey 1991; Boger et al., 2001). out at Mount Ruker, Cumpston Massif, Mount Such high-grade features highlight the Palaeozoic Maguire, Mount Stinear and Mount Rymill (Fig. 1c) geology of the Lambert Terrane, while the Ruker (Grew, 1982; Mikhalsky et al., 2001). The age of Terrane is characterised by the development of orthogneiss emplacement has been interpreted as discrete low-temperature mylonite zones (Boger et al., > 3.0 Ga based on an upper intercept age calculated 2001). from U-Pb dating of zircon (Kovach & Beliatsky, In this paper we present field observations from 1991). the Ruker Terrane to better constrain the depositional Three meta-sedimentary sequences have been and structural evolution of the southern Prince distinguished in the southern Prince Charles Charles Mountains. Detailed examination of the Mountains. Amphibolite facies meta-sediments overlie stratigraphy and structure at Mount Stinear, Mount the basement orthogneiss in the north (Fig. 1c) (Grew, Ruker, Cumpston Massif and Mount Rubin (Fig. 1c) 1982), and are partially equivalent to the Menzies highlight the contribution. Data was collected on the Series as defined by Kamenev et al. (1993) (Tab. 1). Prince Charles Mountains Expedition of Germany and These meta-sediments comprise amphibolite facies Australia (PCMEGA) over the Austral summer of quartzite and mica schist, amphibole rocks and meta- 2002/03. conglomerate (Grew, 1982). Rock types associated © Terra Antartica Publication Stratigraphy and Structure of the Southern Prince Charles Mountains, 71

Tab. 1 – Summary of previous stratigraphic and structural interpretation within the southern Prince Charles Mountains (revised from Mikhalsky et al. 2001). Periods of sediment deposition are poorly constrained.

with this sequence are reported to crop out at Mount boundaries of the Sodruzhestvo series into an Stinear and Mount Rymill. Relatively high-grade extensive cover sequence (Fig. 1c). metamorphic assemblages are highlighted by the presence of staurolite, sillimanite, kyanite and garnet (Grew, 1982) within the schists. Rb/Sr dating of STRATIGRAPHY AND STRUCTURE AT muscovite from an intruding pegmatite at Mount MOUNT STINEAR Stinear constrains sedimentation of these units to pre- date 2580 Ma (Tingey, 1991). Rocks exposed at Mount Stinear can be divided Greenschist facies meta-sediments in the south into four broad categories: (1) the northern granite; (Fig. 1c) (Grew, 1982; Hofmann, 1982) correspond to (2) the northern Menzies series quartzite package; (3) the Ruker Series of Kamanev et al. (1993) (Tab. 1) the central felsic gneiss (basement?) and; (4) the and comprise a basal package of banded iron southern Menzies series pelite, conglomerate and formation and slate, overlain by conformable units of quartzite package (Figs. 2b, 3). Stratigraphic quartzite, conglomerate and chlorite-schist (Kamenev correlation along the western flank of Mount Stinear et al., 1993). Rock types associated with this is complicated by mylonitic or fault related contacts sequence are reported to primarily outcrop at Mount between units, and the tectonically interleaved central Ruker, Mount Bird and Mount Newton (Tingey, 1991) felsic gneiss (Figs. 2b, 3). Mafic dykes and sills cut and are interpreted as Palaeo- to Mesoproterozoic in all rock units and are most prominent proximal to age (Thost et al., 1998) based on the occurrence of contacts. the banded iron formation. The northern biotite-amphibole-magnetite granite The Sodruzhestvo series (Kamanev et al., 1993) displays low to moderate degrees of ductile (Fig. 1c) comprise significant low-grade rocks that deformation along the contact with the quartzite have been reported at Mount Rubin, Mount Dummett package to the south. The nature of contact between and Goodspeed Nunataks (Kamenev et al., 1993) the units and the age of the granite remains (Tab. 1). The age of deposition attributed to these unresolved. The northern quartzite package (Fig. 2b) sequences ranges from Palaeoproterozoic to late comprises a thick (ca. 2 km) package of biotite-rich Neoproterozoic (Halpern & Grikurov, 1975; quartzite interleaved with thin (1-10 cm) pelite Mikhalsky et al., 2001). A bottom to top cross-section horizons that parallel bedding. Overlying the biotite- of stratigraphy within a low-grade sequence at Mount quartzite is a 200 m thick package of garnet-biotite- Rubin (Mikhalsky et al., 2001) consists of; carbonate- staurolite quartzite. Lenticular layering within the rock quartz schist, chlorite-sericite-quartz phyllite, suggests this unit could represent a modified calcareous quartz and meta-sandstone, quartzite and conglomerate (Fig. 4a). Overlying the garnet-rich meta-conglomerate, calcareous quartz meta-sandstone, quartzite is a fuchsite-bearing quartzite, which does carbonate meta-siltstone and quartzite/meta-siltstone not outcrop at the level of the glacier due to the lithologies. Original sedimentary structures such as tectonically interleaved central felsic gneiss (Fig. 3). ripple marks, cross beds and mud cracks are locally The central felsic gneiss consists of a quartz-feldspar- observed (Mikhalsky et al., 2001). We include biotite groundmass with fine (5 cm) leucosome veins sequences examined at Cumpston Massif into the that parallel the biotite-rich dominant foliation (S1) Sodruzhestvo series. This correlation extends the (Fig. 4b). © Terra Antartica Publication 72 G. Phillips et al.

Fig. 2 – Geology exposed along the western flank and southern tip of Mount Stinear. (a) Locality map indicating regions mapped and legend. (b) Plan view map and stratigraphic logs displaying distribution of units within the northern quartzite and southern meta- sedimentary package. Abbreviations: M – mylonite zone; F – fault.

The southern meta-sedimentary package (Figs. 2b, north of the conglomerate, and in direct contact with 3) comprises a thick (ca. 2 km) conformable the central felsic gneiss. Associated with the contact sequence of biotite-rich pelite that is interleaved with are sub-vertical mylonite zones that comprise highly- quartzite and muscovite-rich pelite. Tectonically strained schists that have developed a strong contact- juxtaposed to the north of the biotite-rich pelite is a parallel fabric. Higher-grade mineral assemblages are thick debris flow/conglomerate (ca. 1 km) (Figs. 3, characteristic of these zones with kyanite-muscovite- 4c) that consists of clasts of quartzite and biotite-rich garnet assemblages observed proximal to the central material. Clasts are generally angular and up to felsic gneiss contact (Fig. 4d). 30 cm in size. Garnet-rich, mylonite zones (ca. 5 – Limited structural features within the northern 10 m) that are parallel to the margins of the central quartzites indicate the sequence has been folded into tight F folds plunging to the northwest and southeast felsic gneiss cut the conglomerate (Figs. 2b, 3). 1 Muscovite-rich and biotite-rich pelite is found to the (Figs. 3, 5a). Bedding (and possible S1 traces) trend

Fig. 3 – Structural section along the western flank of Mount Stinear (refer figure 2a). Legend is the same as in figure 2. Section displays locations of figures 4-7. © Terra Antartica Publication Stratigraphy and Structure of the Southern Prince Charles Mountains, East Antarctica 73

Fig. 4 – Mount Stinear (a) Meta-conglomerate exposed in the northern quartzite package. Tectonically overlying the meta-conglomerate is a section of the interleaved central felsic gneiss. (b) Tight upright folding (D2) of S1/leucosome fabric within the central felsic gneiss. (c) Mylonite zone within the conglomerate/debris flow. (d) Kyanite-bearing mylonite zone from a zone of high strain within muscovite-rich schists near the southern margin of the central felsic gneiss.

north-south to east-west due to a secondary warping There is a consistent ca. 280 – 300˚ trending that may be related to localised north-east trending composite S0/S1 fabric that defines structural features crenulation cleavages (D2). within the southern section (Figs. 3, 6a). F1 folds Evidence of early deformation in the central felsic plunge 30˚-65˚ to the north and northwest and a gneiss is a metre scale recumbently folded pegmatite strong mineral lineation on the plane of S0/S1 (Fig. 5b) with an axial surface that parallels the plunging 65-50˚ to the northwest; similar to the dominant fabric in the gneiss (S1). D2 is preserved as stretching lineations within mylonite zones and the small-scale east-west trending gently plunging (10- central felsic gneiss. Meter-scale mylonite zones cut 20˚) folds (Figs. 4b, 5c), that deform an east-west the meta-sediments within the proximity of contacts trending alignment of biotite and parallel leucosomes (Figs. 3, 4c-d, 6b). These zones display a strongly

(S1). Two prominent east-west trending 100 m scale developed northwest plunging stretching lineation that folds crop out high on the escarpment (Fig. 5d) and implies hanging-wall transport to the south. The dip are possibly related to the D2 event. Measurements of of the mylonite zones flattens southwards from the mesoscopic folds suggest D2 deformation of the central felsic gneiss (Fig. 3), ranging from sub- central felsic gneiss is driven by north-south directed vertical zones within the gneiss and contact compared shortening possibly associated with shearing along to moderately north-dipping zones in the debris flow ≥ sub-vertical mylonite zones ( D2) (Fig. 5e), as a (dashed great circles; Fig. 6b). Overprinting all strong stretching lineation plunging 60-75˚ to the features are localised small-scale crenulations (>D2) northwest parallels F2 fold hinges (Fig. 5c). Late flat- that strike northeast and plunge 70-60˚ to the north lying, east-west trending shear zones that thrust top to (Fig. 6a). the north-east are exposed through the centre of the At the southern end of Mount Stinear there are complex and are associated with the development of large reclined D2 structures that fold the composite quartz in-filled tension gashes. The age of the gneiss S0/S1 bedding/metamorphic fabric into tight, shallow is unknown. dipping north-plunging folds (Fig. 7a). The © Terra Antartica Publication 74 G. Phillips et al.

Fig. 6 – Mount Stinear (a) Geometric data from southern meta- sedimentary package. (b) Geometric data from mylonite zones that cut the southern meta-sediments at various localities. Dashed great circles represent lower-angled zones distant from the central felsic gneiss/southern meta-sedimentary package contact.

most meta-sediments (Figs. 3, 7b). In the direct

footwall of this fault a strong S2 parallel slaty cleavage is preserved in biotite schists. Traversing to

the northeast away from the fault (Fig. 3), S0/S1, the mylonite zones and D2 structures rotate into a more sub-vertical orientation.

STRATIGRAPHY AND STRUCTURE AT MOUNT RUKER

Figures 8a-b and 9a show the distribution of the Ruker series meta-sediments that crop out on the northern side of Mount Ruker. They comprise a basal banded iron formation interleaved with thin siliceous/carbonate, phyllite and volcanic horizons Fig. 5 – Mount Stinear (a) Geometric data from the northern quartzite package. (b) Early deformation within the central felsic (lower sequence: Figs. 8c, 9a), overlain by a gneiss preserved as localised, recumbently folded pegmatite. conformable sequence of quartzite, agglomerate, (c) Geometric data within the central felsic gneiss. (d) Regional chlorite-biotite-quartz schist, quartzite, agglomerate antiform exposed high up on the escarpment within the central felsic gneiss. (e) Sub-vertical mylonite zone and geometric data; and biotite-rich psammite (upper sequence: Figs. 8d, zone cuts the central felsic gneiss and northern quartzite package. 9a). Siliceous horizons within the banded iron formations are composed of fine-grained quartz- sericite and carbonate-rich porphroblasts. Dolerite and northwest/southeast trending S2 axial surface of these minor vesicular basalt intrude the banded iron folds parallels a large two-metre thick gouge zone formation as both dykes and sills (Fig. 9a-b). In the associated with a major thrust fault in the southern- overlying agglomerate, (Fig. 9d) the clasts are well © Terra Antartica Publication Stratigraphy and Structure of the Southern Prince Charles Mountains, East Antarctica 75

basement complex. Five to ten metre wide mafic dykes were sighted within the granite at the far eastern end of the nunatak. Siliceous and iron-rich horizons in the banded iron formation highlight the fold structures (Fig. 9c). Early

F1 folds within the banded iron formation are tight to isoclinal and trend east-west to north-south (Fig. 8c).

The general strike of the S1 fabric is southeast, dipping moderately to the southwest, yet south to south-west trends are evident through the centre of

the banded iron formation sequence (S1 stereogram Fig. 8c). F1 folds have variable plunges that are attributed to a re-orientation by a secondary non-

penetrative folding event (D2). D2 is preserved as localised small-scale north and south plunging

crenulations (S2 stereogram Fig. 8c) that warp of the F1 linear elements (Fig. 9c) and is attributed to east- west shortening. East-west trending calcite-filled shear

zones related to a northeast-southwest (D3 or D4) shortening are observed in the dolerite but do not overprint the banded iron formation. Localised large-

scale top to the east flattening of large F1 fold structures in the banded iron formation is observed (Fig. 8b) but is not measurable on a mesoscopic scale. Massive undeformed dolerites conceal the contact between the upper and lower sequences (Fig. 8a-b) thus the relationship between the two stratigraphic levels is unknown. The chlorite-rich schists within the upper stratigraphic levels preserve a much more complex deformation history than in the lower sequences. At the macroscopic scale bedding appears to be dipping

Fig. 7 – Mount Stinear (a) Reclined, regional D2 fold and gently to the southwest and trending to the southeast. geometric data. Fold is defined by quartzite beds (limb 1 and 2) within biotite-rich schist and psammite. (b) Southern late-stage Within the schists the observed fabric is a composite ≥ east-west trending foliation comprising S -S fabrics thrust fault ( D3) and orientation of stress regime as indicated by 0 3 the right the di-hedra method. Geologist circled for scale. (Fig. 8d). Early fabrics and F2 isoclines are transposed along S3 shear planes that display reverse and normal indicators (Fig. 9e-f). East-west trending upright kinks (F : Fig. 8d) plunging moderately to the rounded to angular and predominantly quartzite, 4 west and south-east refold the composite fabric. D - banded iron formation and granite within a fine- 1 D events are pervasive through the whole upper- grained quartz-sericite-chlorite matrix. The chlorite- 4 sequence with the preservation of the S -S rich schists display a two-stage metamorphic history 1 4 overprinting relationships relating to mica content and with retrogressed garnets in a chlorite-quartz-rich matrix. A thick package (ca.1 km) of quartzite thus most prominent in the schists. overlies the chlorite schist. The meta-sedimentary Difficulties in correlating structure between the sequence that overlies the quartzite comprises lower and upper stratigraphic levels arise from chlorite-rich schist (ca. 100 m), agglomerate discrepancies in overprinting styles of the F1 and F2 (ca. 1 km), chlorite-rich schist (ca. 200 m) and structures (Tab. 2). In the banded iron formation F1 biotite-rich psammite (Fig. 9a). and F2 folds have formed non-coaxially (Fig. 8c), Below the banded iron formation is a massive with F2 folds trending north-south. In the upper biotite-hornblende granite displaying minimal sequence S0-S2 have been transposed and re-orientated deformation. The contact between the base of the along northwest trending S3 shear planes (Fig. 8d) banded iron formation and the eastern granite is forming S^C fabrics (Fig. 9f). Low-angle top to the concealed by a north-south trending glacial cirque east re-orientation of upright F1 structures in the (ca. 73˚35’S/64˚35’E) (Fig. 8a). U-Pb data (Kovach banded iron formation (Fig. 8b) may correlate with

& Beliatsky, 1991) suggests a crystallisation age of the S3 shearing in the upper sequence but this 3.0 Ga for the granite and attribute this to a correlation remains as yet inconclusive. © Terra Antartica Publication 76 G. Phillips et al.

Fig. 8 – Geology exposed on the northern flank of Mount Ruker. (a) Locality map showing the distribution of units. (b) Structural section along the northern flank of Mount Ruker. Inset shows overprinting orientations between regional F1 folding and late-stage (> D2) simple shearing. Section displays locations of photos in figure 9. (c) Geometric data from lower stratigraphic banded iron formation. (c) Geometric data from upper stratigraphic meta-sediments. Abbreviations: M – mylonite zone; F – fault.

STRATIGRAPHY AND STRUCTURE Cumpston Massif (Figs. 10a-c, 11a). This cover AT CUMPSTON MASSIF sequence overlies a quartz-feldspar-biotite-amphibole- epidote gneiss that crops out in the north, and along A thick package of Sodruzhestvo series meta- the western escarpment of the massif (Figs. 10a, 12a- sediment crop out along the eastern flank of c). The age of the gneiss remains unknown, and any

Tab. 2 – Comparison between structures in the upper and lower stratigraphic levels of the Ruker series, exposed at Mount Ruker. © Terra Antartica Publication Stratigraphy and Structure of the Southern Prince Charles Mountains, East Antarctica 77

Fig. 9 – Mount Ruker (a) Stratigraphic log of meta-sediments. Legend is the same as figure 8. (b) Dolerite intruding the banded iron formation within lower stratigraphic levels. (c) Refolding of upright F1 isoclines by non-co-axial F2 folding. (d) Agglomerate within upper stratigraphic levels. (e) Tight F2 folds within chlorite schist within upper stratigraphic levels. Axial planar to folds is the composite S0-S2 fabric. (f) S^C fabrics defining normal shear sense on S3 shear planes.

correlation with other suspect crystalline basement The package of Sodruzhestvo series rocks exposed complexes of the southern Prince Charles Mountains along the eastern escarpment of Cumpston Massif (i.e. central orthogneiss at Mount Stinear, eastern (Fig. 10a) consists of a ca. 6 km thick basin sequence granite at Mount Ruker) remains unresolved. Structure of conformable meta-sediments (Fig. 12a). The lower is defined by differential layering of a light (quartz- levels of the exposed sequence, comprise a feldspar) and grey (quartz-feldspar-amphibole-chlorite- conformably interleaved package (ca. 1 km) of epidote) gneiss and an alignment of biotite and quartzite and shale, with localised garnet-rich pelite hornblende (S1). Biotite-hornblende mafic sills and fine conglomerate beds (clasts < 5 cm). This unit (Figs. 10b-c, 11b, 12a) and dykes (ca. 5 - 50 m) is characterised by centimetre-scale beds (Fig. 11e) intrude planar to the S1 and along low-angle normal between the quartzite and shale (thickest was up to a faults. The northern margin of the basin (Figs. 10a; metre wide), and good schistosity development in the 11c-d; 12a-b) is identifiable by a 100 metre thick pelite layers. Overlying the quartzite/shale sequence is northwest trending, southwest dipping (70-40˚) shear a thick package (ca. 1 - 2 km) of low-grade well- zone. The shear zone is characterised by sheared bedded (beds ca. 10 - 60 cm) quartzite. An array of felsic gneiss intercalated with muscovite-rich quartzite sedimentary structures within the quartzite such as and biotite-hornblende-garnet (with localised feldspar ripple marks, flame structures and cross-bedding coronas) schist that displays a strong planar fabric indicates the sequence is the right way up. A parallel to the contact (Fig. 11c). Interleaved within quartzite/shale package (ca. 500m), distinguished by the shear zone is a bed of calcareous marble (ca. 20 - finely bedded (ca. 5 - 20 cm) shale and quartzite 30 m) (Fig. 11d) that has preserved a strong slaty horizons overlies the quartzite. The total thickness of fabric that parallels the basin margin. To the south of the latter unit is unknown due to a kilometre wide the marble unit is the overlying low-grade glacier truncating the sequence (Fig. 10b). Sodruzhestvo series cover sequence. Metamorphic assemblages (muscovite-chlorite-garnet) © Terra Antartica Publication 78 G. Phillips et al.

Fig. 10 – Exposed geology along the eastern flank of Cumpston Massif. (a) Locality map displaying regional geology and distribution of the basin sediments. (b) Map (A-A’) and legend of the eastern escarpment displaying the distribution of units. Map shows localities of photographs in figure 11. (c) Structural section along the eastern flank of Cumpston Massif. (d) Geometric data from the basement gneiss. (e) Geometric data from the northern contact shear zone. (f) Geometric data from basin meta-sediments. (g) Geometric data from southern basin contact. © Terra Antartica Publication Stratigraphy and Structure of the Southern Prince Charles Mountains, East Antarctica 79

Fig. 11 – Cumpston Massif (a) Stratigraphic log of the basement and cover sequences from section A-A’ of figure 10a-b. Meta-sediments are disrupted by fault F-F. Star indicates projection of pegmatite dykes onto the northern shear zone exposed on the eastern escarpment. Legend is the same as figure 10. (b) Reclined fold defined by mafic sills within the northern (basement?) gneiss. Geologist circled for scale. (c) Northern contact shear zone displaying boudins within the intercalated gneiss. (d) Calcareous rich unit within the sedimentary sequence directly overlying the shear zone. (e) Tight, mesoscopic folding in quartzite and pelite sequence. (f) Large scale folds in upper level quartzite. © Terra Antartica Publication 80 G. Phillips et al.

Fig. 12 – Detailed cross-sections across the basin margins shear zones at Cumpston Massif. For section localities refer to figure 10a. (a) Cross-section across the northern basin contact/shear zone exposed on the eastern escarpment (B-B’). (b) Detailed cross-section of the northern basin contact/shear zone exposed on the western escarpment (C-C’). (c) Detailed cross-section of the southern basin contact/shear zone exposed on the western escarpment (C-C’). Mineral abbreviations; mu – muscovite, bi – biotite, g – garnet. preserved within the shale units indicate low- to mid Three types of intrusion are observed at Cumpston greenschist metamorphic conditions were reached. Massif: (1) Biotite-hornblende mafic sills, that intrude Up sequence, outcropping along the southern the lower stratigraphic levels of the meta-sediments extent of the massif, are well-bedded packages of (10 - 50 m wide sills) and basin margin shear zones pure (ca. 200 - 500 m) and then biotite-rich (ca. 2- (5 - 10 m wide sills) (Figs. 10b, 11a, 12a); (2) late- 3 km) quartzite (Fig. 11f) interleaved with localised, stage plagioclase-beryl-bearing pegmatite dykes that thin pelite horizons (10 cm - 2 m). In the quartzites, locally intrude along the basin margins on the western cross-bedding indicates that the sequence is the right exposure of the massif (Figs. 11a, 12b-c) and; (3) an way up, and mud-cracks, ripple marks and flame undeformed solitary dyke composed of course-grained structures indicate an intra-tidal deposition amphibole within a fine-grained epidote-rich matrix environment. Tectonically interleaved within the that intrudes the upper stratigraphic quartzite biotite-rich quartzite are packages (up to 500 m) of (Fig. 11a). The biotite-hornblende sills are finely laminated (ca. 3 - 10 cm) muscovite-rich distinguished by blocky coarse-grained interiors with quartzite that display non-planar contacts with the fine-grained margins in the meta-sediments and surrounding biotite-rich quartzite. appear as schists within the basin margin shear zones. The southern margin of the basin abuts against the Thin zones (10 - 40 cm) of contact metamorphism gneissic basement as a northeast-dipping, southwest overprint the host rock with radiating, randomly trending shear zone (Fig. 12c). This feature crops out orientated biotite in proximity to the sills. The metre on the northwestern escarpment, but is not exposed wide pegmatite dykes that intrude the northern and on the eastern escarpment (Fig. 10a). This southern southern basin margin shear zones on the western shear zone is characterised by interleaved packages of escarpment (Fig. 12b-c) parallel the southern margin, garnet-rich amphibolite, marble, and biotite-rich schist yet crosscuts the northern. No pegmatite is observed that grades into muscovite-rich schist nearer the intruding the basin meta-sediments. The meter wide contact with the basement to the south. Chloritoid- amphibole-epidote-bearing dyke cuts the meta- seams within the quartz-rich layers are interleaved sediments that are exposed along the south-eastern with mafic sills, similar to those observed in the bluff (Fig. 10b). Along the dyke margins, large northern shear zone. In the foot-wall of the southern amphibole crystals overprint the quartzite within margin there are sheared packages of biotite- zones of contact metamorphism. hornblende mafic and quartz-biotite-feldspar gneiss. Northeast-southwest D2 directed shortening, folds With no correlative contact exposed along the eastern the differentiated biotite-rich layering (S1) and mafic escarpment, it is inferred that the southern basin sills in the basement orthogneiss (Fig. 11b). margin trends north-south along the granitic margin of Deformation is preserved as reclined tight F2 folds the western escarpment (Fig. 10a). This north-south that trend to ca. 280 – 300˚ and plunge shallowly to orientation of the contact may be responsible for the ca. 290˚ and ca. 110˚ (Fig. 10d). A weakly defined S2 variable trend of S1 within central section of the basin fabric has formed axial planar to folds, distinguished sediments (Fig. 10b). by an alignment of biotite. Traversing north, away © Terra Antartica Publication Stratigraphy and Structure of the Southern Prince Charles Mountains, East Antarctica 81

supports the suggestion that the southern margin of the cover sequence/basin continues south of Cumpston Massif and encompasses a wide expanse of the southern Prince Charles Mountains.

STRATIGRAPHY AND STRUCTURE AT MOUNT RUBIN

Up to ca. 8 km of Sodruzhestvo series meta- sediment comprising carbonate-rich schist and Fig. 13 – Photograph from helicopter reconnaissance at Mount quartzite, thick sequences of biotite-rich quartzite, Maguire. Possible basement/cover relationships may crop out along muscovite-rich pelite/psammite and conglomerate, is the southern flank of Mount Maguire. exposed along the north-western flank of Mount Rubin (Fig. 14a-b). At the base of the exposed sequence (Fig. 15a), is low-grade interleaved from the contact with the cover sequence (Figs. 10b, carbonate and psammite layers that define large 12a) there is a marked reduction in fold intensity with reclined fold structures (Fig. 15b). Overlying the structures in a more upright orientation (S2: 45 - 50˚ carbonate schists are thin units of tectonically S). interleaved biotite-rich quartzite, conglomerate and Within the northern shear zone/cover sequence quartzite (Fig. 14b). Well-defined mud-cracks and contact there is strong development of a northwest- ripple marks were consistent throughout the trending, southwest-dipping (40 - 70˚) planar fabric carbonaceous/quartzite sediments indicating intra-tidal (Fig. 10e) in all lithologies. Stretching lineations and deposition. Localised overturned cross-beds indicate fold structures plunge dominantly 10-30˚ to the that some sections of the carbonaceous/quartzite northwest and southeast, yet down-dip lineations (80- package are downward facing. A very thick package 90˚ to the southwest) were also observed. Large-scale (ca. 4 km) of relatively massive biotite-bearing extensional features such as mafic and gneissic quartzite overlies these low-grade sediments (Fig. boudins (Fig. 11c) and metre wide S^C fabrics are 15a). This unit displays little internal deformation, also evident within the shear zone. Limited structural fabric development or preservation of sedimentary data gathered from the southern shear zone/cover structures. sequence margin (Figs. 10g, 12c) indicate a steeply A conformable package of muscovite-rich northeast dipping (80 - 60˚), northwest/southeast pelite/psammite (Fig. 15c), quartzite and conglomerate trending planar fabric that parallels the contact form the upper sequence. The quartzite preserve between the meta-sediments and the basement gneiss. cross-bedding indicating sediments are the right way Low-angle and down-dip stretching lineations up, and ripple marks (Fig. 15d) that suggest the (Fig. 10g) indicate an overall top-to-the-southwest sediments were derived from the south. Cross-bedding transport direction. is observed up-sequence and suggests a more Deformation within the cover sequence is energetic depositional environment. The conglomerate preserved as tight to open, upright folds (Fig. 11e-f) consist of poorly sorted boulder beds that are with the development of an axial planar, micaceous interleaved with localised thin (ca. 20 – 50 cm) fabric (S1) and localised steeply plunging (60 - quartzite horizons (Fig. 15e). The contacts between 70˚ NW) stretching lineation. The trend of units along the quartzite and conglomerate display a very high the eastern escarpment at Cumpston Massif is proportion of clasts, characteristic of a debris flow, or dominantly northwest-southeast, (Fig. 10f) but trends glacial deposit (Tingey et al., 1981). Ripple marks on almost north-south through the centre of the sequence the surface of the quartzite beds suggest rapid

(Fig. 10b). The consistent nature of the S1 fabric and deposition after periods of relatively calm conditions. F1 elements suggest that there is a single folding Away from the debris flows are poorly sorted, event preserved in the basin driven by D1 northeast- relatively low pebble conglomerates that comprise the southwest shortening. Thrust faults that parallel majority of the units thickness. bedding and fold limbs indicate deformation of the Deformation within the lower stratigraphic levels now exposed sequences occurred at relatively low- (Fig. 15a) is observed as reclined F1 folds plunging to temperatures. This is supported by the low- to mid- the east and west (Fig. 14c). Axial planar to these greenschist metamorphic assemblages (muscovite- folds is a micaceous S1 fabric that trends to the east chlorite-garnet±biotite). and generally dips shallowly to the southwest

At Mount Maguire there is a similar (shallow S1: Fig. 14c). Overprinting the early basement/cover sequence as at Cumpston Massif structures is a localised flat-lying crenulation cleavage

(Fig. 13). The presence of low-grade, structurally (Fig. 14d) that buckles L1 intersection lineations. S2 simple meta-sediments overlying a granite complex axial traces trend to the northwest and plunge 5-10˚ © Terra Antartica Publication 82 G. Phillips et al.

Fig. 14 – Geology exposed along the northern flank of Mount Rubin. (a) Locality map and legend. (b) Structural section along the northern flank of Mount Rubin. Section displays localities of selected photographs from figure 15. (c) Geometric data displaying the orientation of D1 structures. (d) Localised D2 structures in reclined F1 structures in the lower stratigraphic levels.

to the south-west. These localised D2 structures are deformation of the basement Menzies and Ruker only in the very reclined sections of F1 structures series, exposed at Mount Stinear and Mount Ruker (Fig. 15b). Deformation within the upper stratigraphic (Tab. 3). The younger Sodruzhestvo sediments have levels has developed upright open to closed F 1 preserved a single dominant phase of D1 deformation, structures (Fig. 15c) that trend 280 – 300˚, and whereas, the older basement sequences have preserved plunge shallowly to the east and west (Fig. 14c). S 1 up to four phases of deformation. We correlate D1 traces are in a more upright orientation and dip preserved in the Sodruzhestvo series with at least D3-4 steeply to the north and south when compared to the in the Menzies and Ruker series. This assumption is lower stratigraphic levels (Fig. 14c). No flat-lying D2 based on overprinting relationships from the Menzies crenulation structures have been observed in the upper and Ruker series rocks and orientation of structural stratigraphic levels. features.

DEPOSITION OF THE SODRUZHESTVO BASIN SERIES DISCUSSION Incomplete meta-sedimentary sequences exposed at Deposition and deformation of the Sodruzhestvo Cumpston Massif, include a thin unit of calcareous basin series rocks at Cumpston Massif, Mount Rubin sediment, overlain by well-bedded shale/psammite and and possibly Mount Maguire, post-dates initial then a thick package of quartzite. A shallowing-up © Terra Antartica Publication Stratigraphy and Structure of the Southern Prince Charles Mountains, East Antarctica 83

Fig. 15 – Mount Rubin (a) Stratigraphic log of the low-grade Sodruzhestvo series sediments disrupted by fault F-F. Legend

is the same as figure 14. (b) Reclined F1 folds within carbonaceous schists. (c)

Upright F1 folds within quartzite/muscovite schists. (d) Ripple marks within quartzite. Palaeo-current direction is to the north. (e) Conglomerate beds in the upper stratigraphic levels of the Mount Rubin sequence with coarse conglomerate grading into quartzite. These units may represent extensive glacial deposits.

cycle due to a prominent regression could explain this indicate periods of significant extension during the sequence. The shale/psammite package is initial phases of sediment deposition. A similar characteristic of a shallow water environment, sequence was observed at Mount Rubin, also possibly attributed to marine conditions due to the indicative of a transgressive/regressive environment. locating of the calcareous unit. Sedimentary structures This is supported by intra-tidal sedimentary structures and thickness of the overlying quartzite indicates a and carbonate bearing units in the lower stratigraphic stable depositional environment of possible shelf levels, massive thick quartz-rich packages in the origin. Up-sequence, this unit becomes more massive intermediate levels, and intra-tidal features in the and lacks sedimentary structures. Thick mafic sills upper levels. Overlying the transgressive/regressive exposed at the base of the sedimentary package may sediments are the conglomerate units that are © Terra Antartica Publication 84 G. Phillips et al.

Tab. 3 – Summary of the relative order of key geological events preserved at each studied area. Age estimates are from the relative chronology of events from this study coupled with limited geochronology from the literature. NW/SE*, directions relate to orientation of shortening associated with each event (if known).

characterised by poorly sorted, moderately rounded Menzies and Ruker series suggest a more complex clasts. These units could have been deposited during and thus older deformation history, with two north-directed fluvial transport from the interior of the significant events (D1-D2) to have occurred prior to continent or as thick glacial deposits as suggested in shortening preserved within the basin. Structures that

Tingey et al., (1981). Poorly constrained Rb-Sr are attributed to D3 shortening within the Menzies isotopic analyses from conglomerate and surrounding and Ruker series are: (1) mylonite zones (Figs. 4c-d, schists from Mount Rubin suggest this section of the 5e) and; (2) large scale brittle/ductile faulting Sodruzhestvo series was deposited during the mid- to (Fig. 7b). Correlating these structures with D1 late Neoproterozoic (Halpern & Grikurov, 1975). deformation within the basin implies a pervasive The syn-rift deposition of the exposed regional northeast/southwest shortening regime that Sodruzhestvo series rocks places the Cumpston- produced a strong northwest-southeast structural grain Rubin-Maguire region in a marine environment that (Fig. 16) affected the region. has experienced a significant regression. Sequence D1 and D2 structural development within the stratigraphy at Cumpston Massif and Mount Rubin basement units of the southern margin of the basin is could therefore be explained by the closure of a basin recognised as non-coaxial deformation, preserved or rift. The lack of oceanic crust exposed within the within the banded iron formation at Mount Ruker. southern Prince Charles Mountains suggest an intra- Unlike the overlying meta-sediments, the banded iron continental setting, and thus considers the sequences formation does not preserve the northeast-southwest structural grain attributed to D in the basin and D in at Mount Stinear and Mount Ruker as rift margins. 1 3 Similar deformation histories preserved within units at the basement rift margins. Thus the banded iron Mount Stinear and Mount Ruker support an intra- formation may preserve early structural features due to limited overprinting by the D /D (Fig. 16) event. continental setting, and imply both have been part of 1 3 the same continental block during early deformation Early structural features preserved in the basement events (i.e. D -D ). This relative chronology suggests units could be associated with a deformation event at 1 2 ca. 2.8 Ga, constrained through U-Pb geochronology the Cumpston-Rubin region was the location of from the southern Mawson Escarpment (Boger et al., significant intra-continental extension between the D 2 2001; Tab. 3). It is difficult to assign the timing of and D3 events preserved in rocks of the Ruker and orogenic events to structural features within the Menzies series (Tab. 3). Menzies and Ruker series rocks due to no control concerning their age of deposition. Thus correlating STRUCTURAL IMPLICATIONS FROM THE SODRUZHESTVO structural features between nunataks is problematic BASIN SERIES until further geochronological constraints are published. Similar problems arise when determining A dominant late-stage northeast/southwest directed the age of folding within the Sodruzhestvo series. The shortening event affected the southern Prince Charles simple folding style preserved in these sequences

Mountains, producing F1 folding and S1 cleavage suggests deformation was during the final major development within rocks of the Sodruzhestvo series. orogenic event within the region. The youngest Multiple phases of deformation recorded in the orogenic event reported from the southern Prince © Terra Antartica Publication Stratigraphy and Structure of the Southern Prince Charles Mountains, East Antarctica 85

CONCLUSION

Local stratigraphic constraints and structural observations suggest at least two-phases of deformation occurred prior to deposition of the Sodruzhestvo series sediments. The banded iron formation, and thus older basement stratigraphic units at Mount Ruker have preserved a possible example of the early stage deformation. Sequences of carbonate- rich rocks overlain by shallow water psammites/shales deposited onto the felsic basement, imply marine conditions that underwent a prominent regression. This could be explained through the closure of a intra-continental oceanic basin. Ripple marks measured from the basin suggest sediment transport was directed from the south within the interior of the Antarctic continent. Deformation within the

Sodruzhestvo basin series correlates with at least D3 deformation within the Menzies and Ruker series.

This D1basin/D3basement deformation event has generated the dominant northwest/southeast structural grain across the southwest Prince Charles Mountains. Constraining this event places the timing of

D1basin/D3basement after basin inversion and subsequent sedimentation into the southern Prince Charles Mountains. We suggest that the

D1basin/D3basement event may be associated with ca. 550 - 490 Ma tectonism (Boger et al., 2001) reported from the central Mawson Escarpment within Fig. 16 – Regional structure of the southern Prince Charles the southern Prince Charles Mountains. Mountains attributed to the prominent D1 basin/D3 basement event. We suggest this event occurred during the Cambro-Ordovician. Acknowledgements - The Australian Antarctic Division, the BGR (Bundesanstalt für Geowissenschaften und Rohstoffe) Charles Mountains is preserved as ca. 550 – 490 Ma and AAS Grant 1215 are thanked for logistical and tectonic reworking of the central Mawson financial support for the 2002-03 PCMEGA field season. Escarpment, constrained through U-Pb, U-Th-Pb and G.P. acknowledges the support by an Australian Rb-Sr dating techniques (Halpern & Grikurov 1975; Postgraduate Award and Baragwanath Scholarship held at the University of Melbourne. E.S. Grew is thanked for his Grew & Manton 1983, Tingey 1991, Boger et al. review and suggestions to improve the manuscript. This is 2001). The effects of this event are preserved as TSRC Publication No. 339. deformation, pegmatite and granite emplacement as well as amphibolite-granulite metamorphic assemblages. Structural features associated with this REFERENCES event display a similar northwest structural grain as to

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