Precambrian Research 104 (2000) 1–24 www.elsevier.com/locate/precamres Neoproterozoic deformation in the Radok Lake region of the northern Prince Charles Mountains, east Antarctica; evidence for a single protracted orogenic event S.D. Boger a,*, C.J. Carson b, C.J.L. Wilson a, C.M. Fanning c a School of Earth Sciences, The Uni6ersity of Melbourne, Park6ille, Vic. 3010, Australia b Department of Geology and Geophysics, Yale Uni6ersity, New Ha6en, CT 06511, USA c Research School of Earth Sciences, The Australian National Uni6ersity, Canberra, ACT. 0200, Australia Received 7 May 1999; accepted 14 April 2000 Abstract Ion microprobe dating of structurally constrained felsic intrusives indicate that the rocks of the northern Prince Charles Mountains (nPCMs) were deformed during a single, long-lived Neoproterozoic tectonic event. Deformation evolved through four progressively more discrete phases in response to continuous north–south directed compression. In the study area (Radok Lake), voluminous granite intrusion occurred at 990 Ma, contemporaneous with regionally extensive magmatism, peak metamorphism, and sub-horizontal shearing and recumbent folding. Subse- quent upright folding and shear zone development occurred at 940 Ma, while new zircon growth at 900 Ma constrains a final phase of deformation that was accommodated along low-angle mylonites and pseudotachylites. This final period of deformation was responsible for the allochthonous emplacement of granulites over mid-amphibolite facies rocks in the nPCMs. The age constraints placed on the timing of deformation by this study preclude the high-grade reworking of the nPCMs as is postulated in some of the recent literature. Furthermore, 990–900 Ma orogenesis in the nPCMs is at least 50 Myr younger than that recognised in other previously correlated Grenville aged orogenic belts found in Australia, east Africa and other parts of the Antarctic. This distinct age difference implies that these belts are probably not correlatable, as has been previously suggested in reconstructions of the supercontinent Rodinia. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Northern Prince Charles Mountains; East Antarctica; Granulites; Rodinia; Gondwana; Orogenesis 1. Introduction (nPCMs), has traditionally been considered part of an extensive Neoproterozoic orogenic belt The margin of the east Antarctic craton, includ- (1300–900 Ma) that has been correlated with ing the northern Prince Charles Mountains metamorphic belts of similar age in India, parts of east Africa, Sri Lanka, and Australia (Fig. 1a). * Corresponding author. These belts were thought to represent a major E-mail address: s–[email protected] (S.D. Boger). accretionary system that led to the formation of 0301-9268/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S0301-9268(00)00079-6 2 S.D. Boger et al. / Precambrian Research 104 (2000) 1–24 east Gondwana during the growth and consolida- Antarctica (Zhao et al., 1992; Shiraishi et al., tion of Rodinia (Grew and Manton, 1986; Katz, 1994; Hensen and Zhou, 1995; Carson et al., 1989; Moores, 1991; Clarke et al., 1995; Rogers, 1996; Fitzsimons et al., 1997) has lead to a num- 1996). East Gondwana was thought to have then ber of authors questioning the validity of this remained intact and generally internally unde- model. Instead, it has been suggested that east formed until rifting in the Mesozoic (Yoshida et Gondwana may represent a collage of continental al., 1992). However, the more recent recognition fragments that accreted during the Palaeozoic of extensive Palaeozoic tectonism within east (Hensen and Zhou, 1997). Fig. 1. (a) Traditional reconstruction of Rodinia at 1000 Ma showing the location of East Gondwana within this reconstruction (after Hoffman, 1991; Unrug, 1997). In these models, east Gondwana is inferred to have formed though the accretion of parts of Australia, India and east Africa along a single laterally extensive Meso-Neoproterozoic orogenic belt thought to have rimmed the east Antarctic coastline. (b) Gondwana at 500 Ma with the continents of east and west Gondwana illustrated. The position of the nPCMs is highlighted and enlarged in (c). Traditional models for the construction of Gondwana suggest that it remained intact from Rodinian times and formed a keystone onto which west Gondwana accreted. (c) Expanded section shows the gross geology of the region of interest. NC, Napier Complex; VH, Vestfold Hills; sPCMs, southern Prince Charles Mountains; RC, Rayner Complex; nPCMs, northern Prince Charles Mountains; LHB, Lu¨tzow-Holm Bay; PB, Prydz Bay. The more complicated tectonic frame work arising from the dissection of the Proterozoic mobile belt exposed in the nPCMs by Palaeozoic terrains recognised in Prydz and Lu¨tzow Holm Bays are highlighted. S.D. Boger et al. / Precambrian Research 104 (2000) 1–24 3 The nPCMs, together with the Mawson Coast of Neoproterozoic and possible post-Proterozoic and the Rayner Complex, separate Prydz and orogenesis in the nPCMs are assessed. Lu¨tzow-Holm Bays (Fig. 1c). With the recogni- tion of high-grade Palaeozoic tectonism within these terrains, the nPCMs has received consider- 2. Regional geologic setting able attention regarding the extent of possible Palaeozoic reworking. A number of authors have The nPCMs are exposed as a series of isolated postulated that a late Proterozoic to early inland ranges and massifs located on the western Palaeozoic accretionary belt may have linked margin of the Amery Ice Self (Fig. 2). They form Prydz and Lu¨tzow-Holm Bays (Kriegsman, 1995; part of an east–west trending orogenic belt, dom- Hensen and Zhou, 1997) effectively crossing the inated by granulite facies felsic and mafic gneisses, nPCMs–Mawson Coast–Rayner Complex re- interleaved with subordinate metasedimentary gion. Within the nPCMs, this inference has been and calc-silicate units (Crohn, 1959; Tingey, 1982, supported by Sm–Nd age data presented by 1991; McKelvey and Stephenson, 1990; Fitzsi- Hensen et al. (1997) from which they infer two mons and Thost, 1992; Thost and Hensen, 1992; significant tectonothermal events overprinting the Kamenev et al., 1993; Hand et al., 1994b). The widely recognised 1000 Ma orogen; one at sequence as a whole was intruded episodically by 800 Ma and a second at 630–500 Ma. significant volumes of granitic and charnockitic Similarly, Scrimgeour and Hand (1997) suggest magma, as well as by locally derived partial melts that the complex pressure–temperature paths ob- (Munksgaard et al., 1992; Sheraton et al., 1996; served along the eastern edge of the nPCMs Kinny et al., 1997; Zhao et al., 1997). At Beaver reflect thermal interference between two unrelated Lake (Fig. 2), the high-grade gneisses are overlain tectonic events. They infer that 1000 Ma tec- by relatively undeformed Permo-Triassic sedi- tonism is overprinted in the east by the affects ments (Crohn, 1959; Mond, 1972; Webb and of 550–500 Ma orogenesis recognised to the Fielding, 1993; Fielding and Webb, 1995, 1996; northeast in Prydz Bay. These studies contrast McLaughlin and Drinnan, 1997a,b). These are with that of Kinny et al. (1997), who argue that thought to lie in a sub-basin on the western side the lack of new zircon growth or Pb-loss discon- of the Lambert Graben, an inferred rift system cordia post-dating 1000 Ma indicate that that separates the nPCMs from the Palaeozoic late Proterozoic to early Palaeozoic tectonism in (ca. 550–500 Ma) granulite facies terrain of Prydz the nPCMs was of relatively minor importance. Bay (Ren et al., 1992; Zhao et al., 1992; Carson et This interpretation is more consistent with earlier al., 1995; Dirks and Wilson, 1995; Harley and studies from the area (Tingey, 1982, 1991; Man- Fitzsimons, 1995; Hensen and Zhou, 1995; Car- ton et al., 1992). These different hypotheses arise son et al., 1996; Fitzsimons, 1997; Fitzsimons et primarily due to a paucity of structurally well- al., 1997). To the north and west of the nPCMs, constrained geochronologic data from the the extent of the terrain is unconstrained. How- nPCMs, an issue that we have aimed to address in ever, it probably extends to at least the Mawson this study. Coast (Fig. 3), where rocks of similar age and In this paper, we refine the temporal framework grade are exposed (Young and Black, 1991; of high-grade deformation and metamorphism in Young et al., 1997), and has been tentatively the nPCMs. We describe the sequence of high- correlated with the Rayner Complex still further grade structural events recognised, and couple our to the west (Black et al., 1987). The terrain is geometric observations with structurally con- bounded in the south by exposures of older strained geochronological data obtained from fel- Meoproterozoic volcanics at Fisher Massif sic intrusives and locally derived leucosomes. New (Kamenev et al., 1993; Beliatsky et al., 1994; SHRIMP age data from four structurally con- Mikhalsky et al., 1996; Kinny et al., 1997; Laiba strained samples collected in the vicinity of Radok and Mikhalsky, 1999), and by granitic Archaean Lake are presented, and the relative contributions basement complex overlain by two or more super- 4 S.D. Boger et al. / Precambrian Research 104 (2000) 1–24 Fig. 2. Schematic map of the northern Prince Charles Mountains showing the study area, extent of outcrop and the distribution of the Proterozoic basement and Permo-Triassic strata. Localities of existing U–Pb zircon geochronolgical data are also illustrated. Data from Mt McCarthy, Loewe Massif, Mt Collins and the Fisher Massif are after Kinny et al. (1997); data from Jetty Peninsula are after Manton et al. (1992). Locality of cross-section illustrated in Fig. 3 is also shown. Insert shows the geographic position of the northern Prince Charles Mountains along the western margin of the Amery Ice Self. Mawson and Davis refer to Australian Antarctic Stations. crustal sequences in the southern Prince Charles (Hand et al., 1994a; Nichols 1995; Scrimgeour and Mountains (Grew, 1982; Tingey, 1982, 1991; Hand, 1997).