Introduction Directory UMM :Data Elmu:jurnal:P:Precambrian Research:Vol102.Issue3-4.2000:

Precambrian Research 102 2000 155 – 183 Geochronological constraints for a two-stage history of the Albany – Fraser Orogen, Western Australia D.J. Clark a , B.J. Hensen a, , P.D. Kinny b a Department of Applied Geology, Uni6ersity of NSW, Sydney 2052 , Australia b Tectonics Special Research Centre, School of Applied Geology, Curtin Uni6ersity of Technology, GPO Box U 1987 Perth 6845 , Australia Received 4 March 1998; accepted 16 January 2000 Abstract Based on structural, petrographic and geochronological work SHRIMP zircon, monazite and rutile, the Mesoproterozoic Albany – Fraser Orogeny is divided into two discrete thermo-tectonic stages, between c. 1345 and 1260 Ma Stage I and c. 1214 and 1140 Ma Stage II. The existence of a two-stage history is confirmed by the discovery of 1321 9 24 Ma detrital zircons and 1154 9 15 Ma metamorphic rutiles in metasedimentary rocks from Mount Ragged. The detrital zircons demonstrate that the Mount Ragged metasedimentary rocks unconformably overly, and were derived from, Stage I basement. Metamorphic rutile formed as a consequence of overthrusting by high-grade early-Stage II rocks along an inferred NE-SW striking structure the Rodona Fault. This interpretation is supported by zircon geochronology, which demonstrates that granulite facies metamorphism on the northwestern side of the structure predates that on the southeastern side by 100 Ma. Rocks to the northwest record a low-grade imprint relating to the younger Stage II event. The two-stage thermo-tectonic history of the Albany – Fraser Orogen correlates with adjacent Grenville-age orogenic belts in Australia and East Antarctica, implying that Mesoproterozoic Australia assembled in two stages subsequent to the amalgamation of the North Australian and West Australian cratons. Initial collision between the combined West Australian – North Australian craton and the South Australian – East Antarctic continent at c. 1300 Ma was followed by intracratonic reactivation affecting basement and cover at c. 1200 Ma. Two comparable and contemporaneous compressional orogenies controlled the formation of the Kibaran Belt in Africa and the Grenville Belt in Canada, suggesting that tectonic events in Mesoproterozoic Australia follow a similar pattern to that recognised for Rodinia amalgamation world-wide. © 2000 Elsevier Science B.V. All rights reserved. Keywords : Grenvillian; Rodinia; Albany – Fraser Orogen; Geochronology; Plate tectonics www.elsevier.comlocateprecamres

1. Introduction

By the early Mesoproterozoic, Australia con- sisted of three relatively stable regions; the North, South and West Australian cratons Myers et al., Corresponding author. E-mail address : b.hensenunsw.edu.au B.J. Hensen 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 6 3 - 2 1996; Fig. 1a. At this time the South Australian Craton was contiguous with the East Antarctic Shield Fanning et al., 1996. Tectonic activity between c. 1300 and 1000 Ma Chin and de Laeter, 1981; Pidgeon, 1990; Black et al., 1992a; Bruguier et al., 1994; Camacho and Fanning, 1995; Nelson et al., 1995 led to the assembly of Proterozoic Australia as a component of the su- percontinent Rodinia. The sutures between the three Australian cratons are defined by ‘Grenville- age’ orogenic belts i.e. broadly correlatable to the c. 1300- to 950-Ma Grenville belts of the North American Shield containing high-temperature, medium to low-pressure polymetamorphic rocks with complex histories. By studying these oro- genic belts an understanding can be gained of the tectonothermal processes by which Mesoprotero- zoic Australia assembled, and their timing. This contribution focuses primarily on the Al- bany – Fraser Orogen, which defines the suture between the West Australian and combined South Australian – East Antarctic Craton Fig. 1a. The principal aims of this study were two-fold: firstly, to gain an understanding of the thermo-tectonic history of the Albany – Fraser Orogen through a detailed structural, metamorphic and geochrono- logical study of a key area in the eastern part of the orogen, within a unit called the Nornalup Complex Myers, 1990; and secondly, to investi- gate whether the sequence of events recognised in the Albany – Fraser Orogen is of local significance only, or reflects processes on a larger scale. Reconnaissance fieldwork and a supporting geochronological study conducted by the Geologi- cal Survey of Western Australia in the eastern part of the Albany – Fraser Orogen Nelson et al., 1995; Myers, 1995a identify the eastern Nornalup Complex as of major significance to the develop- ment of the orogen. The Nornalup Complex is geologically complicated, comprising three or more distinct fault-bounded rock units and sev- eral felsic intrusive suites Myers, 1995a; Fig. 2. Thus far, dating has been mainly limited to the major felsic intrusive suites Nelson et al., 1995, with the result that many of the relationships between geological units remain propositional, Fig. 1. a Tectonic map of Mesoproterozoic Australia adapted largely after Myers et al., 1996. Australia – East Antarctica fit after Veevers and Ettreim 1988. Abbreviations are: North Australian Craton NAC; Western Australian Craton WAC; South Australian Craton SAC; East Antarctic Shield EAS; Albany – Fraser Orogen AFO; Musgravian Orogen MO Proto-Pinjarra Orogen PPO; Windmill Islands WI; Bunger Hills BH. b Basement geology map of the Albany – Fraser Orogen after Myers, 1990, 1993. Area of Fig. 2 is marked. Fig. 2. a Basement geology of the eastern Nornalup Complex showing major lithological units and structures adapted after Myers, 1995a. The Rodona Fault is inferred from the findings of this study. Rocks to the southeast of this fault are denoted the Salisbury Gneiss. Numbers represent sample localities discussed in the text and presented in Table 2. b Schematic NW-SE cross-section showing the relationships between the major lithological units. Dimensions are to scale. The dips and movement senses of major non-outcropping faults are inferred from sympathetic structures within the lithological units. derived by correlation of isolated outcrops across large non-outcropping fault structures. Of partic- ular interest to the present study were the ages of a sequence of quartzose cover rocks near the eastern margin of the orogen around Mount Ragged, and high-grade rocks occurring on is- lands off the eastern coast the Salisbury Gneiss; Fig. 2, which remained completely unknown. Geometric considerations Fig. 1a suggest that the events described in the Albany – Fraser Orogen should be manifest in some form in other Australian and East Antarctic Grenville- age orogenic belts. The second part of this study presents a review of Mesoproterozoic geochrono- logical data from contiguous Australian and East Antarctic orogenic belts in order to estab- lish the degree of correlation with the Albany – Fraser Orogen. The Albany – Fraser Orogen, as part of Mesoproterozoic Australia, is placed into the context of the supercontinent Rodinia and the worldwide Grenville-age orogenesis that marked its assembly. Such a correlation, tests the continental reconstruction presented in Fig. 1a, and furthermore provides an estimate of the scale of the driving mechanisms of orogene- sis. In this paper we describe the structural and metamorphic history of the Mesoproterozoic rocks of the Nornalup Complex in the eastern- most Albany – Fraser Orogen Figs. 1 and 2 using age constraints provided by U-Pb geochronology. On the basis of SHRIMP II ion microprobe data the Albany – Fraser Orogeny is divided into two distinct periods of tectonothermal activity similar to those previously identified by Myers 1995a,b and Nelson et al. 1995. Here, in addition, we recognise an episode of intracratonic sedimenta- tion and dyke intrusion between the two periods of tectonothermal activity, and determine a more detailed sequence of time constrained events. Cor- relation of the Albany – Fraser Orogen with con- tiguous Australian and East Antarctic orogenic belts, and with Grenville-age belts worldwide, provides insight into the scale and nature of the tectonic processes responsible for the assembly of the Mesoproterozoic supercontinent Rodinia.

2. The Nornalup Complex and its regional context