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Terra Antartica Publication © 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.
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