Introduction Directory UMM :Data Elmu:jurnal:P:Precambrian Research:Vol104.Issue1-2.2000:

Precambrian Research 104 2000 1 – 24 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 www.elsevier.comlocateprecamres

1. Introduction

The margin of the east Antarctic craton, includ- ing the northern Prince Charles Mountains nPCMs, has traditionally been considered part of an extensive Neoproterozoic orogenic belt 1300 – 900 Ma that has been correlated with metamorphic belts of similar age in India, parts of east Africa, Sri Lanka, and Australia Fig. 1a. These belts were thought to represent a major accretionary system that led to the formation of Corresponding author. E-mail address : s –sbogereduserv.its.unimelb.edu.au S.D. Boger. 0301-926800 - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 9 2 6 8 0 0 0 0 0 7 9 - 6 east Gondwana during the growth and consolida- tion of Rodinia Grew and Manton, 1986; Katz, 1989; Moores, 1991; Clarke et al., 1995; Rogers, 1996. East Gondwana was thought to have then remained intact and generally internally unde- formed until rifting in the Mesozoic Yoshida et al., 1992. However, the more recent recognition of extensive Palaeozoic tectonism within east Antarctica Zhao et al., 1992; Shiraishi et al., 1994; Hensen and Zhou, 1995; Carson et al., 1996; Fitzsimons et al., 1997 has lead to a num- ber of authors questioning the validity of this model. Instead, it has been suggested that east Gondwana may represent a collage of continental fragments that accreted during the Palaeozoic 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. The nPCMs, together with the Mawson Coast and the Rayner Complex, separate Prydz and Lu¨tzow-Holm Bays Fig. 1c. With the recogni- tion of high-grade Palaeozoic tectonism within these terrains, the nPCMs has received consider- able attention regarding the extent of possible Palaeozoic reworking. A number of authors have postulated that a late Proterozoic to early Palaeozoic accretionary belt may have linked Prydz and Lu¨tzow-Holm Bays Kriegsman, 1995; Hensen and Zhou, 1997 effectively crossing the nPCMs – Mawson Coast – Rayner Complex re- gion. Within the nPCMs, this inference has been supported by Sm – Nd age data presented by Hensen et al. 1997 from which they infer two significant tectonothermal events overprinting the widely recognised 1000 Ma orogen; one at 800 Ma and a second at 630 – 500 Ma. Similarly, Scrimgeour and Hand 1997 suggest that the complex pressure – temperature paths ob- served along the eastern edge of the nPCMs reflect thermal interference between two unrelated tectonic events. They infer that 1000 Ma tec- tonism is overprinted in the east by the affects of 550 – 500 Ma orogenesis recognised to the northeast in Prydz Bay. These studies contrast with that of Kinny et al. 1997, who argue that the lack of new zircon growth or Pb-loss discon- cordia post-dating 1000 Ma indicate that late Proterozoic to early Palaeozoic tectonism in the nPCMs was of relatively minor importance. This interpretation is more consistent with earlier studies from the area Tingey, 1982, 1991; Man- ton et al., 1992. These different hypotheses arise primarily due to a paucity of structurally well- constrained geochronologic data from the nPCMs, an issue that we have aimed to address in this study. In this paper, we refine the temporal framework of high-grade deformation and metamorphism in the nPCMs. We describe the sequence of high- grade structural events recognised, and couple our geometric observations with structurally con- strained geochronological data obtained from fel- sic intrusives and locally derived leucosomes. New SHRIMP age data from four structurally con- strained samples collected in the vicinity of Radok Lake are presented, and the relative contributions of Neoproterozoic and possible post-Proterozoic orogenesis in the nPCMs are assessed.

2. Regional geologic setting