© Terra Antartica Publication Terra Antartica 2005, 12(2), 69-86 Stratigraphy and Structure of the Southern Prince Charles Mountains, East Antarctica 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, East Antarctica 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