The Nornalup Complex and its regional context

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

The Albany – Fraser Orogen Myers, 1990 is a Proterozoic orogenic belt outcropping along the southern and southeastern margins of the Ar- chaean Yilgarn Craton in Western Australia Fig. 1b. Myers 1990 subdivided the orogen into Biranup and Nornalup complexes Fig. 1b, defined mainly on the basis of apparent differ- ences in structure shown by regional aeromagnetic data Myers, 1990, 1995a,b and brief observation of discontinuous coastal outcrop in the western part of the orogen. The division was subsequently substantiated Myers, 1995a following further fieldwork supported by a U-Pb zircon geochrono- logical study of major felsic intrusives in the east- ern part of the orogen Nelson et al., 1995. Both complexes are dissected by a number of non-out- cropping structures, which are defined by aero- magnetics and are inferred to have formed as thrust faults Myers, 1995a,b; Fig. 1b and Fig. 2. In the eastern part of the orogen, the Biranup Complex comprises strongly deformed Archaean, Palaeoproterozoic and Mesoproterozoic felsic plu- tons and minor metasedimentary gneiss Myers, 1995a; Nelson et al., 1995. The northeastern part of the Biranup Complex is dominated by a tecton- ically disrupted layered basic intrusion called the Fraser Complex Myers, 1985; Fig. 1b, which is Mesoproterozoic in age Fletcher et al., 1991 and is recrystallised in granulite to garnet amphibolite facies. The Mesoproterozoic Nornalup Complex is dominated by scattered outcrops of Recherche Granite Myers, 1995a and comprises several in- ferred tectonostratigraphic units Fig. 2. The Malcolm Gneiss, in the far east of the complex, comprises highly deformed ortho- and parag- neisses intruded by Recherche Granite and nu- merous generations of felsic and mafic dykes. The Mount Ragged metasedimentary rocks, formerly called the Mount Ragged Beds Lowry and Doe- pel, 1974 and the Mount Ragged schist Myers, 1995a, are a sequence of massive quartzites and subordinate metapelites recrystallised in upper greenschistlower amphibolite facies that outcrop northwest of the Malcolm Gneiss Fig. 2. Off- shore islands southeast of the Malcolm Gneiss form a unit here called the Salisbury Gneiss, which we infer to be separated from rocks on the mainland by a fault the Rodona Fault, based upon the geochronological data presented in this paper. Late- to post-tectonic Esperance Granite plutons Myers, 1995a outcrop throughout the Nornalup Complex. U-Pb zircon geochronological studies con- ducted in the early 1990s Pidgeon, 1990; Black et al., 1992a; Nelson et al., 1995 succeeded in brack- eting the major period of tectonothermal activity in the Albany – Fraser Orogen, denoted the Al- bany – Fraser Orogeny, to between c. 1300 and 1100 Ma. Based on this information, Myers 1995a introduced a structuralmetamorphic framework for the Albany – Fraser Orogen. Three major Mesoproterozoic tectonothermal episodes D 1 – D 3 M 1 – M 3 were proposed. D 1 and D 2 were considered to have occurred under granulite facies conditions M 1 and M 2 at c. 1300 Ma, resulting in pervasive fabric formation, crustal thickening and thrust stacking. D 3 – M 3 was broadly con- strained to the interval c. 1200 – 1100 Ma, and was argued to be related to the westward transport of thrust sheets onto the Yilgarn Craton margin under both ductile and brittle deformation conditions. 2 . 1 . A synopsis of the structuralmetamorphic framework for the eastern Nornalup Complex In order to put the new geochronological data into context, an overview of the structural and metamorphic history of the area illustrated in Fig. 2 is presented in this section. The structural and metamorphic framework developed for the Al- bany – Fraser Orogen by Myers 1995a provides a foundation for the present study of the eastern part of the Nornalup Complex. A more detailed analysis of the structural and metamorphic data will be published elsewhere. Five distinct deformation episodes are recog- nised in the Nornalup Complex Table 1: 1 formation of a first gneissic fabric in the Malcolm Gneiss D 1 ; 2 transposition of that fabric into a second composite recumbent fabric and the folia- tion of Recherche Granite plutons D 2 ; 3 open upright folding D 3 ; 4 high grade deformation and metamorphism of the Salisbury Gneiss, folia- tion development and folding of the Mount Ragged metasedimentary rocks, and reactivation of the Malcolm Gneiss in discrete shear zones D 4 ; and 5 a further generation of open folding D 5 . 2 . 1 . 1 . D 1 A pervasive layer-parallel foliation S 1 is devel- oped in both metasedimentary and meta-igneous rocks of the Malcolm Gneiss. S 1 -parallel stro- matic migmatites occur locally, suggesting upper amphibolite facies conditions M 1 prevailed dur- ing D 1 deformation. Pelitic mineral assemblages are characterised by the stability of garnet, biotite and sillimanite. Textural and mineralogical evi- dence from migmatites, inferred to have formed by melting reactions involving muscovite and bi- otite, suggest peak M 1 conditions in the vicinity of 750°C and 4 kbar. 2 . 1 . 2 . D 2 Further deformation of the Nornalup Complex occurred during D 2, after the intrusion of the Recherche Granite two nearby plutons are dated at 1330 9 14 and 1314 9 21 Ma, Nelson et al., 1995. The S 1 fabric in the Malcolm Gneiss was isoclinally folded and transposed into a second planar, recumbent S 1 S 2 fabric, containing root- less intrafolial isoclinal folds. D 2 did not result in the formation of a new axial planar fabric. Post D 1 , pre- to syn-D 2 Recherche Granite plutons commonly outcrop as elongate trains of prominent hills. There is a progressive increase in strain intensity from weakly foliated cores to moderately gneissic margins. The first-formed fab- ric in these rocks S 2 is defined by oriented biotite and amphibolite boudins, and is associated with a variably-oriented stretching lineation L 2 . Late- plutonic aplite dykes are tightly folded F 2 with their axial planes parallel to S 2 . F 2 fold axes coincide with L 2 . The S 2 fabric is truncated by numerous small ductile shears S 2b . These struc- tures rarely exceed a centimetre in width and are generally continuous over a metre or two. Both sinistral and dextral shear sets are represented, across which S 2 is offset up to 15 cm. Quart- zofeldspathic leucosomes commonly occupy the plane of S 2b shears. Shear orientations are consis- tent with extension parallel to the S 2 fabric. 2 . 1 . 3 . D 3 D 3 is characterised by significant horizontal shortening. F 3 folds are ubiquitous at all scales, ranging in style from open kilometre-scale struc- tures to tighter, shorter wavelength folds. Dextral asymmetry of F 3 folds is consistent with approxi- mately NW-SE bulk shortening during D 3 . Axial planes trend northeasterly and dip steeply to the southeast. An axial planar fabric is best developed in mica-rich metasedimentary rocks. Fold axes trend generally NE-SW and plunge variably ac- cording to their position on later folds. F 3 folds commonly have thickened hinges and attenuated limbs. Clearly recrystallised boudin necks on at- tenuated F 3 limbs preserve amphibolite facies mineral assemblages. Subsequent to D 3 , vertical NE-trending dolerite dykes intruded the Malcolm Gneiss and Recherche Granite. These intrusions are typically no more than 2 m in width and may be continu- ous over hundreds of metres. They provide a locus for post-D 3 strain. D .J . Clark et al . Precambrian Research 102 2000 155 – 183 Table 1 Summary of the structuralmetamorphic framework proposed by this study a Myers 1995a Deformation Metamorphism Structure U-Pb age Ma equivalent event this paper Salisbury Gneiss Mt Ragged Mseds. Recherche Granite Malcolm Gneiss D 1 Gneissosity S 1 + c. 1330 D 1 Peak M 1 : upper stromatic amphibolite facies migmatites D 2 Isoclinal folding Foliation S 2 +con- Retrograde M 1 : c. 1330–1310 F 2 +transposition amphibolite facies jugate shears S 2b of S 1 S 1 S 2 D 3 Open-tight NW-SE Open NW-SE fold- D 2 Retrograde M 1 : amphibolite facies ing F 3 folding F 3 n.r. Gneissosity S 1S + n.r. b D 4a n.r. c. 1215-1180 M 2a : granulite isoclinal folding facies F 1S NE-trending thrust ?D 3 M 2b : greenschist to D 4b D 4c NE-trending thrust NW-SE open fold- Layer parallel c. 1180-1140 zones S 4b strike- ing F 2S +shear shearing S 1R + zones S 4b strike- amphibolite facies fabric S 2S slip reactivation slip reactivation NW-SE open fold- S4 c S4 c ing F 2R +cleavage S 2R Open NE-SW fold- Open NE-SW fold- D 5 n.r. n.r. ? open NE-SW fold- M 3 ? ing F 5 , local ing F 5 ing F 5 crenulation cleav- age S 5 a Time constraints are based on geochronology presented in Nelson et al. 1995 and the results of this study. b n.r., not recognised. 2 . 1 . 4 . D 4 The D 4 deformation episode heterogeneously affected the eastern part of the Nornalup Com- plex and comprises several distinct phases of de- formation D 4a,b,c . During these events strain was partitioned into large fault structures transecting the complex e.g. Tagon, Wininup and Rodona faults, Fig. 2, which acted as thrusts. Rocks of the Malcolm Gneiss and Recherche Granite were reactivated in discrete northeasterly-trending zones of intense shearing deformation during D 4 . L 4 lineations are steep in these subvertical to southeast-dipping zones, indicating a dominant vertical component to motion, probably related to the larger thrusts. Mineral assemblages in the shear zones are recrystallised in mid-amphibolite facies. 2 . 1 . 5 . D 4 a New age constraints presented in the following section see also Fig. 4 suggest that the first deformation fabric recognised in the Salisbury Gneiss S 1S formed at the onset of the D 4 struc- tural episode. S 1S is a pervasive foliation in all Salisbury Gneiss lithologies and is defined by peak-M 2a medium-pressure granulite facies assem- blages characterised by the stable coexistence of biotite, sillimanite and garnet in metapelites and the occurrence of orthopyroxene in metabasic rocks. S-planes trend to the northeast and dip subvertically. In migmatitic metapelitic rocks, leu- cosomes are oriented within S 1S . Thin leucosomes are often isoclinally folded on a centimetre-scale F 1S , reflecting progressive non-coaxial deforma- tion during D 4a . F 1S axial planes are sub-vertical and fold axes plunge moderately to the northeast parallel to a pervasive sillimanite lineation L 1S . Coronitic textures consistent with decompression from peak conditions 800°C and \ 5 kbar formed in metapelites subsequent to D 4a . The S 1S fabric is folded around a later generation of open asymmetric folds F 2S , which are associated with shearing deformation D 4b , and overprint the decompression textures. Biotite, sillimanite and quartz are stable within the sheared matrix. 2 . 1 . 6 . D 4 b We propose that the deformation in the Mount Ragged metasedimentary rocks can be correlated to D 4b in the Salisbury Gneiss Table 1, Fig. 4. Metapelitic layers contain a pervasive bedding- parallel schistosity S 1R defined by monomineral- lic quartz segregations, sheets of opaque minerals and oriented mica. The development of S 1R pre- dates the growth of randomly-oriented porphy- roblastic minerals e.g. andalusite, gahnite-rich spinel and is related to strain partitioning into the more micaceous rocks in preference to in- terbedded massive quartzites in the early stages of D 4b . Continued NW-SE horizontal shortening folded S and S 1R around open NW-verging F 2R similar folds, which are ubiquitous at all scales. F 2R fold axes plunge shallowly towards the north- east in the northern parts of the Mount Ragged metasedimentary rocks and plunge shallowly southwestwards in the south. An axial planar fracture cleavage S 2R is pervasively developed throughout the Mount Ragged metasedimentary rocks. In schistose layers, bedding and the S 1R fabric are crenulated by S 2R . These planes trend northeasterly and dip steeply to the southeast. The peak, uppermost greenschist-lower amphibo- lite facies metamorphic paragenesis M 2b post- dates deformation and comprises the assemblage muscovite, chlorite, margarite, quartz and rare kyanite overprinting andalusite. Aluminosilicate- bearing quartz-mica veins postdate the formation of S 2R and commonly occupy the plane of small shears with centimetre-scale offsets. Viridine an- dalusite is the stable aluminosilicate polymorph in most veins, but sillimanite has been observed in the southernmost exposures of the Mount Ragged metasedimentary rocks. Bodies of undeformed granite and pegmatites, which correlate with the c. 1140-Ma Esperance Granite, intrude the Mount Ragged metasedimentary rocks. 2 . 1 . 7 . D 4 c The evidence for thrust movement in D 4 shear zones is typically obscured by pervasive reactiva- tion D 4c at a lower metamorphic grade, typically upper greenschist to lower amphibolite facies. Shear sense indicators in reactivated D 4 shear zones indicate dextral displacement in the hori- zontal plane. Oblique fold axes plunge shallowly to the southwest, suggesting a dominant horizon- tal component of movement. 2 . 1 . 8 . D 5 Fold axes vary systematically in plunge throughout the mainland exposures of the Nor- nalup Complex, consistent with folding by a later generation of regional-scale folds F 5 with half- wavelengths in the order of 20 km. F 5 folds plunge shallowly to the northwest and reflect moderate NE-SW horizontal shortening. A crenu- lation cleavage S 5 related to this deformation is developed in amphibolites in the Malcolm Gneiss.

3. Geochronology