Paleoproterozoic orogenesis in the southeastern Gawler Craton, South Australia*

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1 Australian Journal of Earth Sciences (2008) 55, ( ) Paleoproterozoic orogenesis in the southeastern Gawler Craton, South Australia* A. REID 1{, M. HAND 2, E. JAGODZINSKI 1, D. KELSEY 2 AND N. PEARSON 3 1 Geological Survey Branch, Minerals and Energy Resources, PIRSA, GPO Box 1671, Adelaide, SA 5001, Australia. 2 Continental Evolution Research Group, Geology and Geophysics, University of Adelaide, SA 5005, Australia. 3 ARC National Key Centre for Geochemical Evolution and Metallogeny of Continents (GEMOC), Department of Earth and Planetary Sciences, Macquarie University, NSW 2109, Australia. INTRODUCTION Integrated structural, metamorphic and geochronological data demonstrate the existence of a contractional orogen preserved in the ca 1850 Ma Donington Suite batholith along the eastern margin of the Gawler Craton, South Australia. The earliest structures are a pervasive gneissic foliation developed in the Donington Suite and interleaved metasedimentary rocks. This has been overprinted by isoclinal and non-cylindrical folding, and zones of pervasive non-coaxial shear with north-directed transport, suggesting that deformation was the result of orogenic contraction. SHRIMP U Pb zircon data indicate that a syn-contractional granitic dyke was emplaced at Ma. Overprinting the contractional structures are a series of discrete, migmatitic high-strain zones that show a normal geometry with a component of oblique dextral shear. U Pb zircon data from a weakly foliated microgranite in one such shear zone give an emplacement age of Ma. Rare aluminous metasedimentary rocks in the belt preserve a granulite-grade assemblage of garnet þ biotite þ plagioclase þ K-feldspar þ silicate melt that formed at *600 MPa and *7508C. Peak metamorphic garnets are partially replaced by biotite þ sillimaniteþ cordierite assemblages suggesting post-thermal peak cooling and decompression, and are indicative of a clockwise P T evolution. Chemical U Th Pb electron microprobe ages from monazites in retrograde biotite yield a minimum estimate for the timing of retrogression of ca 1830 Ma, indicating that decompression may be linked to the development of the broadly extensional shear zones and that the clockwise P T path occurred during a single tectonothermal cycle. We define this ca 1850 Ma phase of crustal evolution in the eastern Gawler Craton as the Cornian Orogeny. KEY WORDS: Cornian Orogeny, Donington Suite, Gawler Craton, orogenesis, Paleoproterozoic. Paleoproterozoic magmatism at ca 1850 Ma occurs in a number of Australian Proterozoic terranes (Wyborn 1988), including the Halls Creek Orogen (Page & Hancock 1988), the Pine Creek Orogen (Needham et al. 1988) and the Mt Isa Inlier (Wyborn & Page 1983). The Gawler Craton, the central component of the South Australian Craton, also contains a significant magmatic suite of this age (Parker et al. 1993). In each of these terranes, 1850 Ma magmatism occurs at or near the onset of prolonged magmatic, sedimentary and deformational histories (Page 1988; Myers et al. 1996). In the case of the Gawler Craton, an 1850 Ma magmatic suite forms the basement to Paleoproterozoic bimodal volcanosedimentary successions and Mesoproterozoic magmatism, both of which contain a variety of mineralisation types and significant mineralisation potential (Figure 1) (Daly et al. 1998). Understanding the evolution of the basement to these sequences may provide clues as to the tectonic framework within which these younger thermal and sedimentary events occurred. This contribution is a study of the ca 1850 Ma orthogneisses of the southeastern Gawler Craton. GEOLOGICAL SETTING The Gawler Craton preserves a record of Archean to Mesoproterozoic continental evolution (Figures 1, 2) (Drexel et al. 1993; Daly et al. 1998; Fanning et al. 2007). The oldest units are Late Archean ( Ma) volcano-sedimentary rocks that were deformed during the ca Ma Sleafordian Orogeny (Daly et al. 1998; Swain et al. 2005). Intrusion of felsic magmas occurred in the southern Gawler Craton at ca 2000 Ma *Data Tables 1 and 2 [indicated by an asterisk (*) in the text and listed at the end of the paper] are Supplementary Papers; copies may be obtained from the Geological Society of Australia s website (5http:// or from the National Library of Australia s Pandora archive (5http://nla.gov.au/nla.arc ). { Corresponding author: reid.anthony@saugov.sa.gov.au ISSN print/issn online Ó 2008 Geological Society of Australia DOI: /

2 450 A. Reid et al. (Miltalie Gneiss: Fanning et al. 1988), although little is known about the tectonic setting of this event. Following this, sedimentation of Hutchison Group occurred, consisting of psammite and pelite with lesser carbonate and iron-formation (Parker & Lemon 1982). An intercalated volcanogenic unit within the upper Hutchison Group (the Bosanquet Formation) has a zircon U Pb crystallisation age of Ma (Fanning et al. 2007) and provides an upper bound on the timing of sedimentation. It is noted that Vassallo & Wilson (2001) have suggested that the Hutchison Group may be younger than ca 1850 Ma and that the Bosanquet Formation may represent a basement thrust slice that has been tectonically juxtaposed with the Hutchison Group. Recent detrital zircon investigations have revealed a rock with a maximum depositional age ca 1790 Ma within units mapped as Hutchison Group (Jagodzinski et al. 2006), although the majority of detrital zircon investigations from the group yield maximum depositional ages ca 2000 Ma (Fanning et al. 2007). The younger age may indicate infolding of a younger sequence within the Hutchison Group during the Kimban Orogeny. Nevertheless, the provenance and stratigraphy of the Hutchison Group requires further investigation. Dominantly granitic and monzonitic rocks of the Donington Suite were then emplaced at 1850 Ma (Parker et al. 1993). Subsequent to this, the region preserves a complex Late Paleoproterozoic to Early Mesoproterozoic orogenic record with numerous cycles of sedimentation, magmatism, metamorphism and deformation lasting until around 1500 Ma when granites of the Spilsby Suite were emplaced in the southern Gawler Craton (Figure 1) (Daly et al. 1998). The Donington Suite (Schwarz 2003) is an intrusive magmatic complex that occupies a north southtrending belt around 600 km long and up to 80 km wide along the eastern margin of the Gawler Craton (Figure 2). The Donington Suite consists of rocks that range in composition from gabbro, gabbronorite, charnockite, granodiorite to alkali granite and includes within it the rocks of the Colbert Granite, formerly known as the Colbert Suite (Mortimer et al. 1988a; Hoek & Schaefer 1998; Schwarz 2003). Early workers considered the Donington Suite to belong to the Lincoln Complex, a rock association that was originally proposed to encompass magmatism that occurred synchronous with the Kimban Orogeny (Parker et al. 1993). However, recent revision of this nomenclature (Schwarz 2003) has seen the Donington Suite excluded from the Lincoln Complex as the timing and duration of the 3 Figure 1 Temporal evolution of the southeastern Gawler Craton, showing magmatic, volcano-sedimentary, deformation/metamorphism and mineralisation events. For details on individual events and rock units see Drexel et al. (1993), Daly et al. (1998) and Fanning et al. (2007). Note that the 1850 Ma event is termed the Cornian Orogeny rather than previous descriptors (see Discussion). Ages quoted are U Pb zircon determinations from Fanning et al. (1988, 2007). Mineralisation is indicated as elemental occurrences rather than deposit/prospect styles for simplicity: for details on mineralisation styles in the Gawler Craton, see Daly et al. (1998 and references therein).

3 Paleoproterozoic orogenesis, Gawler Craton 451 Figure 2 Geology of the Gawler Craton, South Australia. (a) Interpreted solid geology of the Gawler Craton, highlighting the Sleaford Complex, Miltalie Gneiss, Hutchison Group, Donington Suite together with the widespread Gawler Range Volcanics and Hiltaba Suite; the remaining Paleoproterozoic to Mesoproterozoic units are not differentiated and shown in grey. Major mines are highlighted. (b) Outcrop map showing the locations of Donington Suite and other Proterozoic stratigraphic units of the southeastern Gawler Craton (after Drexel et al. 1993).

4 452 A. Reid et al. Kimban Orogeny have been clarified (see below). Previous U Pb geochronology of the Donington Suite indicates that magmatic crystallisation of a range of lithologies including a quartz gabbronorite gneiss used as a SHRIMP zircon age standard (Black et al. 2003) occurred within error of 1850 Ma (Creaser & Cooper 1993; Parker et al. 1993; Jagodzinski 2005). Geochemically, Donington Suite granitoids show LREE enrichment, negative Nb, Sr, P and Ti anomalies and have end 1850 Ma values between 72 and 74 (Schaefer 1998). The Donington Suite is thought to be derived from a mixture of melt from a mafic parent possibly a mafic underplate and crustal material (Mortimer et al. 1988a; Schaefer 1998). The major Paleoproterozoic orogenic phase recognised in the southeastern Gawler Craton is the Kimban Orogeny. Many workers considered the Kimban Orogeny to have been long lived, occurring between 1850 and 1700 Ma (Thompson 1969; Glen et al. 1977; Daly et al. 1998; Zang & Fanning 2001). However, recently Hoek & Schaefer (1998) and Vassallo & Wilson (1999, 2002) have shown that a tectonic foliation developed within the Donington Suite prior to Ma reworking. Mortimer et al. (1988a) also identified a foliation in the Donington Suite that developed before the emplacement of the Colbert Granite, which also indicated a pre-1730 Ma timing for this foliation. From these observations, Hoek & Schaefer (1998) suggested that the earlier foliation indicated the occurrence of a separate tectonothermal event or orogeny, and that the latter, Ma event alone should be considered as the Kimban Orogeny. This notion is supported by the development of several phases of sedimentation and volcanism between 1850 Ma and 1740 Ma (Figure 1) in the region, interpreted by earlier workers (Parker et al. 1993; Daly et al. 1998) to record the effects of the Kimban Orogeny. Subsequently, the importance of this earlier ca 1850 Ma event has been recognised and has been variously termed the Lincoln Orogeny (Vassallo & Wilson 1999) or the Neill Event (Ferris et al. 2002). Despite this recognition, and the fact that the Donington Suite dominates the crustal architecture along the eastern margin of the Gawler Craton, little work has focused on evaluating the structural and metamorphic expression of 1850 Ma tectonism. One of the major difficulties in evaluating the significance and character of the 1850 Ma event in the eastern Gawler Craton is the intensity of Kimban reworking of Archean and Paleoproterozoic rocks on Eyre Peninsula (Parker 1980; Parker et al. 1993; Vassallo & Wilson 2002; Tong et al. 2004). Fortunately, Donington Suite granitoids are also exposed along the southwestern coast of Yorke Peninsula, some 80 km east of the main zone of known Kimban-aged deformation (Figure 2). Confirmation that the earlier ca 1850 Ma phase of tectonism is preserved here was provided by Zang & Fanning (2001), who obtained a SHRIMP zircon U Pb metamorphic age of Ma from a garnet biotite sillimanite granulite facies paragneiss intercalated with the Donington Suite. However, aside from this age determination, the extent to which the Yorke Peninsula gneisses as a whole preserve the effects of an 1850 Ma tectonothermal event has not been previously evaluated. PALEOPROTEROZOIC GEOLOGY OF SOUTHERN YORKE PENINSULA Rock types On Yorke Peninsula, Paleoproterozoic basement is exposed only on coastal platforms due to a thick Cambrian to Holocene sedimentary cap (Figure 3) (Zang 2006). Metasedimentary rocks of the Corny Point Paragneiss (Zang & Fanning 2001) make up less than *5% of the outcropping Paleoproterozoic rocks on southwestern Yorke Peninsula. The principal exposure of metasedimentary rocks is at Corny Point, where garnetiferous cordierite-bearing, migmatitic quartzo-feldspathic gneiss is the dominant lithology. A discussion of the metamorphic evolution of these critical outcrops is presented below. The dominant Proterozoic rock types on southwestern Yorke Peninsula, the Donington Suite (Figure 3), are composed of two magmatic units: the Gleesons Landing Granite and the Royston Granite (Zang 2002, 2006). SHRIMP U Pb zircon geochronology indicates that the Gleesons Landing Granite was emplaced at Ma (Zang 2006), Ma and Ma (Jagodzinski et al. 2006). Two samples of the Royston Granite yielded ages of Ma and Ma (Zang 2006), which are essentially identical to the emplacement age for the Gleesons Landing Granite. The Gleesons Landing Granite is volumetrically dominant and contains a suite of rock types including syenogranite, adamellite, granodiorite and augen orthogneiss that is everywhere foliated and migmatised. Recrystallised, foliated and rafted mafic dyke remnants within the Gleesons Landing Granite show back veining from the enclosing felsic lithologies, suggesting they were emplaced soon after or during the crystallisation of the felsic host. We refer to these dykes as Type 1 dykes. The Royston Granite has an adamellite to syenogranite composition (Zang 2002) and intrudes the Gleesons Landing Granite as a series of discrete dykes. The intrusion chronology of the Royston Granite relative to the structural elements in the Gleesons Landing Granite is particularly important in unravelling the deformation history of the Donington Suite and is discussed in detail below. Intruding both the Gleesons Landing Granite and Royston Granite are a suite of mafic dykes that are generally straight-sided and variably recrystallised, which we refer to as Type 2 dykes. These dykes cross-cut or strike subparallel to the foliation of the host-rock. In places, such dykes are cross-cut by pegmatite dykes, which appear on field criteria to be related to the Royston Granite. However, elsewhere cross-cutting relationships between straight-sided mafic dykes and Royston Granite equivalents are not observed, and it is possible that multiple generations of younger dykes are present, as has been documented in the Donington Suite on Eyre Peninsula (Mortimer et al. 1988b).

5 Paleoproterozoic orogenesis, Gawler Craton 453 Whole-rock geochemistry and Hf isotopic composition We have investigated the whole-rock geochemistry and zircon Hf isotopic composition of the Gleesons Landing Granite and Royston Granite. Whole-rock samples from a range of rock types were analysed for major- and trace-element element composition at a commercial geochemical operation, Amdel Laboratories in Adelaide (Data Table 1*). The Hf isotopic signature of felsic units was obtained through laser ablationmulticollector inductively coupled plasma-mass spectrometry (ICPMS) of individual zircons using facilities in the Geochemical Analysis Unit at the GEMOC National Key Centre, Macquarie University, Sydney. Analytical methods followed those of Knudsen et al. (2001) and Griffin et al. (2002). From the selected ICPMS trace, integrated 177 Hf/ 176 Hf, 176 Lu/ 177 Hf and 176 Yb/ 177 Hf ratios were calculated from which an epsilon (e) Hf value was derived based on the decay constant for Lu of y 71 (Patchett et al. 1981; Tatsumoto et al. 1981). Depleted mantle Hf model ages have been calculated based on measured 176 Hf/ 177 Hf compared with a model depleted mantle with a present-day 176 Hf/ 177 Hf ¼ and 176 Lu/ 177 Hf ¼ (after Griffin et al. 2002). Samples from the Gleesons Landing Granite and Royston Granite show a trend towards higher MgO, Al 2 O 3, TiO and CaO with lower silica content (Figure 4a), consistent with fractionation of a single magmatic suite. REE patterns for felsic members of the Gleesons Landing Granite and Royston Granite show light REE enrichment, similar to the trend expected for average continental crust (Figure 4b). Both Type 1 and Type 2 mafic dykes are broadly tholeiitic in composition (Figure 4c). They also show LREE enrichment compared with typical MORB (Figure 4d). These data may indicate either a component of crustal contamination in these magmas or that the lithospheric source was enriched in these elements. Zircons from the Gleesons Landing Granite have ehf 1850 Ma values that range from 74.0 to 5.3 (sample R639091; Table 1), with a mean value of Depleted mantle model ages calculated for these zircons are in the range 2.0 to 2.4 Ga (Figure 4e; Table 1). A sample of the Royston Granite shows similar ehf 1850 Ma values, with a range from 71.7 to 5.8 (sample R639097: 3 Figure 3 Proterozoic rocks of southwestern Yorke Peninsula. (a) Coastal outcrops of Proterozoic gneisses, showing the Donington Suite and Hutchison Group equivalent metasediments along with regional trends of the S 1 //S 2 and S 4 fabrics. (b) Total magnetic intensity (TMI) image. (c) Solid geology interpretation. Low magnetism is shown by the metasedimentary rocks of the Corny Point Paragneiss in the north. The Gleesons Landing Granite, which occupies the bulk of the area, can be differentiated into the more highly magnetic region in the south, which corresponds to layered gneiss and discordantly migmatised granodiorite gneiss, and an area of less strongly differentiated magnetic signal that is interpreted as megacrystic granite gneiss. Section marked X Y shown in Figure 6.

6 454 A. Reid et al. Figure 4 Geochemical and isotopic composition of the Donington Suite on Yorke Peninsula. (a) Harker plot of selected elements vs SiO 2, to illustrate compositional trends in various members of the Gleesons Landing Granite and Royston Granite. (b) REE plot for samples of the Gleesons Landing Granite and Royston Granite, normalised to chondrite (Boynton 1984). Shown for comparison are the average continental crustal values of Weaver & Tarney (1984). (c) AFM classification plot of Irvine & Baragar (1971) for the mafic rocks of the Donington Suite. (d) REE plot for samples of the Type 1 and Type 2 mafic dykes, normalised to chondrite (Boynton 1984). Shown for comparison are REE values for MORB from Sun & McDonough (1989). (e) Plot of 176 Hf/ 177 Hf vs age (Ma) showing location of Depleted Mantle curve (assuming present-day 176 Hf/ 177 Hf ¼ and 176 Lu/ 177 Hf ¼ after Griffin et al. 2002). The slope of the line of best fit between the analyses and the depleted mantle curve is determined by the assumed value of the crustal 176 Lu/ 177 Hf ratio of (Patchett et al. 1981). Table 1) and a mean of Depleted mantle model ages calculated for zircons of this sample are identical to that of the Gleesons Landing Granite sample, being in the range Ga (Figure 4e; Table 1). These data suggest that the Donington Suite rocks were derived from the fractionation of a mafic magma that incorpo-

7 Paleoproterozoic orogenesis, Gawler Craton 455 Table 1 Summary of LAM-ICPMS Hf isotopic results. Analysis Hf 176 /Hf 177 1s Lu 176 /Hf 177 Yb 176 /Hf 177 U Pb age Initial Hf ehf 1s TDM (Ga) R Gleesons Landing Granite [670258E, N (GDA 94 Zone 53)] R Royston Granite [668525E, N (GDA 94 Zone 53)] rated a component of pre-existing crustal material. The crustal contaminant is likely to be material at least 2.3 Ga in age, as suggested by the Hf depleted mantle model ages. These results are consistent with the conclusions of previous, more detailed, geochemical and isotopic investigations of the Donington Suite (Mortimer et al. 1988a, b; Schaefer 1998) and further strengthen the concept that the Donington Suite is a geochemically homogeneous batholith over its considerable extent (Hoek & Schaefer 1998). STRUCTURAL GEOMETRY, KINEMATIC AND TEMPORAL EVOLUTION OF THE DONINGTON SUITE Four phases of deformation, D 1 to D 4, can be distinguished in the Paleoproterozoic paragneisses and orthogneisses on Yorke Peninsula. In this section, we present integrated structural observations and U Pb zircon geochronological data from structurally constrained samples. Geochronological data were obtained via SHRIMP II instruments at Curtin University, Perth, and the Australian National University, Canberra. Zircon separates were acquired through standard density and magnetic-separation techniques. Hand-picked zircons were mounted into epoxy and imaged under backscatter electron and cathodoluminescence to determine internal structure of the grains. Details of the SHRIMP analytical methods are given in Appendix 1. D 1 : gneissic fabric and leucosome formation The first deformation event, D 1, resulted in the formation of the gneissic fabric, S 1, in both the Gleesons Landing Granite and the Corny Point Paragneiss. This fabric is defined by leucosomal segregations, and by planar alignment of biotite or hornblende. Aside from these fabric elements, no macroscopic D 1 structures are apparent. D 2 : north-directed, non-coaxial compressional deformation D 2 structures in the Gleesons Landing Granite vary from low-strain disruption of the S 1 fabric through foliation boudinage (Figure 5a), to higher strain intrafolial, isoclinal (Figure 5b) and non-cylindrical folding (Figure 5c) and zones of pervasive ductile shear (Figure 5d). The occurrence of intrafolial isoclinal folding suggests that the gneissic foliation in the Gleesons Landing Granite is a composite S 1 //S 2 fabric, which, at the regional scale, strikes *2908 and has a variable but dominantly southward dip (Figures 3, 6). D 2 is associated with regionally significant zones of planar

8 A. Reid et al. 456 Figure 5 Structures observed in the Donington Suite. (a) Discordantly migmatised granodiorite gneiss of the Gleesons Landing Granite, deformed by foliation boudinage at Royston Head. Notebook 19 cm long. Location E, N. (b) Layered granite gneiss of the Gleesons Landing Granite, Point Yorke. Pen is 14 cm long. Location E, N. Inset shows F2 isoclinal folding, which is only present in Unit A of the Gleesons Landing Granite. Width of view of inset *10 cm. (c). Down-plunge view of F2 non-cylindrical fold within layered granite gneiss of the Gleesons Landing Granite, Point Souttar. Pencil is 10 cm long. Location E, N. (d) D2 shear fabric in megacrystic granite gneiss of the Gleesons Landing Granite, Berry Bay. This regionally significant shear fabric shows north-directed kinematics and implies D2 resulted from compressional deformation. Photograph taken looking west. Pencil is 10 cm long. Location E, N. (e) Dyke of feldspar-rich megacrystic gneiss of the Royston Granite, which cross-cuts the S2 foliation within surrounding discordantly migmatised granodiorite gneiss, Royston Head. Pen is 12 cm long. Location: E, N. (f) Example of narrow D4 mylonite zone reworking the S2 fabric in the Gleesons Landing Granite, Royston Head. Pencil is 12 cm long. Location: E, N. All locations are given in GDA 1994 Zone 53 coordinates. ductile deformation as evidenced by a *10 km wide zone of augen orthogneiss (Figures 3, 6). D2 kinematic indicators including s-type porphyroclasts, C C0 shear fabrics (Figure 5d), and asymmetric folding show consistent north vergence (Figure 6b). Thus, D2 is characterised by non-coaxial, north-directed

9 Paleoproterozoic orogenesis, Gawler Craton 457 Figure 6 Structural observations across southwestern Yorke Peninsula. (a) Outcrop map of Proterozoic rocks on southwestern Yorke Peninsula with stereographic projections describing the principal structural elements of the region. All stereonets are lower hemisphere, equal-area projections. (b) Schematic cross-section along X Y.

10 458 A. Reid et al. contractional deformation and significant strain partitioning. Importantly, D 2 does not affect the Royston Granite. D 3 : north-directed contraction D 3 resulted in the formation of tight to open F 3 folds in all units of the Gleesons Landing Granite and interleaved metasedimentary rocks. F 3 folds generally plunge shallowly to the west or east. Granitic dykes of the Royston Granite cross-cut the D 2 structures, but are weakly foliated (Figure 5e), and in places appear to intrude along the axial plane of F 3 folds in the Gleesons Landing Granite. As these dykes are foliated, this implies they were emplaced post-d 2 to syn-d 3. A sample of K-feldspar-rich megacrystic granite dyke of the Royston Granite (sample R639097) yielded euhedral prismatic zircons, with generally blunt terminations (Figure 7a). The grains display oscillatory zoning typical of igneous crystallisation. Rare narrow, high-u rims are observed, that in the majority of cases were too thin to analyse, although a few grains with thicker overgrowths were targeted. Twenty-five cores and single-phase grains along with nine rims were analysed (Table 2). All analyses of cores and single-phase grains are concordant and yield a weighted mean 207 Pb/ 206 Pb age of Ma (MSWD ¼ 1.3; probability of fit ¼ 0.12: Figure 7b). Of the rim analyses, most are either within error of the grain centres and/or show high errors or high U. However, one rim (RH3.1), at Ma(1s), is concordant and has a low Th/U. We consider the 207 Pb/ 206 Pb age of Matobe the crystallisation age of this granite. The gneissic fabric in the granite is interpreted to relate to metamorphism and deformation soon after intrusion as evidenced by the analysis of seven zircon rims, which yield a weighted mean 207 Pb/ 206 Pb age of Ma (MSWD ¼ 1.8; probability ¼ 0.09) of near-identical age. There is limited evidence (one grain) of zircon regrowth or recrystallisation at ca 1770 Ma, although the significance of this analysis (RH3.1) is uncertain. D 4 : south-side-down extension with component of dextral strike-slip D 4 deformation is manifest as a series of shear zones that overprint all prior structural fabrics in all units of the Gleesons Landing Granite. D 4 shear zones vary from discrete metre-scale, mylonitic shear zones (Figure 5f) to zones of pervasive reworking of the S 1 //S 2 fabric with minimum widths in the order of tens of metres (Figures 8, 9). Discrete D 4 shear zones are defined by a gneissic fabric, S 4, and consistently show south-sidedown, normal kinematics and a shallow (158) to moderately (608) west-plunging stretching lineation, L 4 (Figures 6, 8). The geometry and stretching lineation orientation of these shear zones suggest that D 4 resulted from extension coupled with a component of dextral strike-slip deformation. The frequency of D 4 shear zones increases towards the south such that on southernmost Yorke Peninsula, the S 1 //S 2 fabric is almost completely overprinted by S 4. At the regional scale, S 4 dips moderately to the southwest, although we note that zones of local north dip are also observed (Figure 6). We interpret this as synchronous folding of the S 4 surface during the fabric development, akin to the apparent folding of the lower plate rocks observed in other extensional systems (e.g. Norwegian Caledonides: Fossen 1992). At a number of localities along the southernmost Yorke Peninsula, pegmatite and microgranite dykes of the Royston Granite are observed to truncate the gneissic fabric of D 4 shear zones, although in places these microgranite dykes display a weak foliation, subparallel to the shear zone boundaries (Figure 8). These observations imply that these microgranites were emplaced late-syn- to post-d 4. Importantly also, Type 2 mafic dykes are restricted to those areas in which D 4 deformation is most strongly expressed (Figure 6), where they intrude subparallel to the S 4 gneissic foliation. In order to constrain the timing of D 4 deformation, we have sampled one of the microgranite intrusions (sample R698218, Figure 8). The majority of zircons from this sample are euhedral to subrounded, show strong oscillatory zoning, and are commonly mantled by metamict rims up to 100 mm thick (Figure 7c). Twenty-nine SHRIMP analyses of cores and whole grains with no rims were collected from this sample (Table 2). Most are concordant or near-concordant, and the dominant population of near-concordant analyses produces a weighted mean 207 Pb/ 206 Pb age of Ma (MSWD ¼ 1.5; probability ¼ 0.09; n ¼ 16: Figure 7d). Attempts were made to analyse rims that occur on zircons from this population; however, these analyses are strongly discordant, have very high U and common Pb and do not yield reliable age information (Figure 7d; Table 2). On the basis of their high U content, these overgrowths may be related to late-stage residual magmatic fluids. Thus, on the available evidence, the late-syn-tectonic microgranite is considered to have crystallised at Ma. METAMORPHIC CONSTRAINTS Paragneisses at Corny Point are one of the few localities on Yorke Peninsula where diagnostic metamorphic mineral assemblages occur. Lithologies at this locality include garnet-bearing quartzo-feldspathic gneiss and pods of calc-silicate (Figures 9, 10a, b). All of these lithologies, except the calc-silicate, show complex networks of garnet-bearing leucosomes that both parallel and cross-cut the S 2 gneissic foliation (Figure 10c). These leucosomes occur in rocks that contain a biotitedefined foliation along with matrix of plagioclase and quartz. This suggests that the leucosomes may have formed via the general reaction: bi þ sill þ qz þ plag ¼ gt þ melt + K-spar (Spear 1993). The peak assemblage does not contain sillimanite, suggesting that this reaction was terminated by the exhaustion of sillimanite. In order to constrain the timing of leucosome formation, we have dated zircons preserved in an S 2 - parallel garnet-bearing leucosome in paragneiss units at Corny Point (sample R698216, Figure 7). The sample

11 Paleoproterozoic orogenesis, Gawler Craton 459 Figure 7 Results of SHRIMP zircon U Pb geochronology. (a) Cathodoluminescence image of zircons from sample R (b) Concordia plot for sample R (c) Cathodoluminescence image of zircons from sample R Inset shows transmitted light image of one typical zircon with metamict overgrowth. (d) Concordia plot for sample R (e) Cathodoluminescence image of zircons from sample R (f) Concordia plot for sample R contains abundant subhedral zircon, with oscillatory zoning present in many grains (Figure 7e). Cores are common, and are generally more translucent, with a brighter cathodoluminescence. Thirty analyses yielded generally concordant analyses that cluster around 1850 Ma along with a number of older ca Ma

12 460 A. Reid et al. Table 2 Summary of SHRIMP U Pb results. Spot U (ppm) Th (ppm) 232 T/ 238 U 206 Pb c (%) 206 Pb* (ppm) 206 Pb*/ 238 U 207 Pb*/ 235 U 207 Pb*/ 206 Pb* Disc. (%) +1s +1s +1s Age (Ma) 207 Pb/ 206 Pb +1s R [668525E, N (GDA 94 Zone 53)] RH RH RH RH RH RH RH RH RH R [668871E, N (GDA 94 Zone 53)] TG TG TG3.1r TG3.2c TG (continued)

13 Paleoproterozoic orogenesis, Gawler Craton 461 Table 2 (Continued) Spot U (ppm) Th (ppm) 232 T/ 238 U 206 Pb c (%) 206 Pb* (ppm) ages that are interpreted to be detrital zircons scavenged from the surrounding metasedimentary rocks (Table 2). A weighted mean 207 Pb/ 206 Pb age of Ma can be calculated from the 14 most concordant analyses (MSWD ¼ 1.4; probability ¼ 0.14), which is interpreted to record the timing of zircon crystallisation in the leucosome. This age is within error of the metamorphic zircon age reported by Zang & Fanning (2001), thus confirming that high-temperature metamorphism and leucosome formation in the Corny Point Paragneiss is associated with the 1850 Ma event. The peak garnet has been partially replaced by intergrowths of biotite þ cordierite + sillimanite (Figure 10d, e, f). In many instances, cordierite coronae isolate garnet from matrix quartz (Figure 10f), while in other examples, biotite and cordierite form pseudomorphs of garnet (Figure 10e, f). This retrograde assemblage is similar to those imaged on numerous P T pseudosections (Harley & Carrington 2001; White et al. 2001) and recorded in field studies (Clarke & Powell 1991; Norlander et al. 2002). In the cited examples, the progression from the upper amphibolite to granulite facies garnet-bearing assemblages to retrograde, biotite þ cordierite + sillimanite-bearing assemblages occurs in response to high-temperature decompression, and in some instances, workers have inferred that 206 Pb*/ 238 U 207 Pb*/ 235 U 207 Pb*/ 206 Pb* Disc. (%) +1s +1s +1s Age (Ma) 207 Pb/ 206 Pb +1s R [684154E, N (GDA 94 Zone 53)] decompression was essentially isothermal (Clarke & Powell 1991). In order to evaluate the conditions of metamorphism in the Corny Point Paragneiss, a calculated P T pseudosection has been constructed in the model system Na 2 O CaO K 2 O FeO MgO Al 2 O 3 SiO 2 H 2 O TiO 2 Fe 2 O 3 (NCKFMASHTO; White et al. 2003) (Figure 11). Mineral equilibria calculations were undertaken using THERMOCALC 3.0 (Powell & Holland 1988) and the internally consistent thermodynamic dataset of Holland & Powell (1998). The bulk-rock composition used in the calculations was taken from whole-rock XRF analyses of a garnet cordierite biotite-bearing metapelite from Corny Point (see Figure 11 for composition). The inferred bi þ sill breakdown reaction is observed to occur at conditions around 7308C and MPa (Figure 11). This reaction produces a garnetiferous assemblage and occurs in the presence of silicate melt, as is indicated by the extensive network of garnetbearing leucosomes at Corny Point. The progression to cordierite-bearing assemblages due to the breakdown of garnet observed in the Corny Point Paragneiss is indicated on the pseudosection to occur due to decompression to MPa at elevated temperatures. The locally evident complete breakdown of garnet

14 A. Reid et al. 462 Figure 8 Outcrop map of Donington Suite rocks at (a) Meteor Bay and (b) The Gap, southern Yorke Peninsula. Base map and some structural data from Meteor Bay modified from Pedler (1976); otherwise all data from our observations. All stereonets are upper hemisphere, equal-area projections.

15 Paleoproterozoic orogenesis, Gawler Craton 463 Figure 9 Outcrop map of Corny Point. Base map modified from Richardson (1978). Note that the southwestern outcrops at Corny Point show zones of intense D 4 ductile shear associated with a strongly downdip stretching lineation, L 4. Kinematic indicators including s- and d-type mantled porphyroclasts and C C 0 fabrics show south-side-down kinematics (see photograph inset). We note, however, that local apparent reversals in kinematic shear sense are observed, which may be associated with isoclinal folding of the S 4 mylonitic layering during progressive south-side-down ductile shear. In addition, although in many cases south-side-down kinematics can be shown, a number of shear zones lie parallel to the axial plane of tight F 3 folds and some lack convincing kinematic indicators. Thus, the shear sense across some of these shear zones is uncertain, and it is possible that some may in fact have developed during D 3. All stereonets are equal-area lower hemisphere projections and mineral elongation lineations plot as dots, fold axes as crosses. to biotite þ cordierite suggests that the retrograde path continued with further decompression and cooling into the biotite þ cordierite þ K-feldspar þ plagioclase (þliquid þ quartz þ ilmenite) field, suggesting that pressures may have reached as low as *300 MPa (Figure 11). Thus, we suggest that the Corny Point Paragneiss experienced a clockwise P T evolution. In order to constrain the timing of post-peak mineral growth, we have dated monazite that is intergrown with biotite around partially replaced garnet porphyroblasts

16 A. Reid et al. 464 Figure 10 Metamorphic characteristics of metapelites in the Corny Point Paragneiss. (a) Garnet biotite quartzo-feldspathic gneiss, with garnet-bearing leucosomes and leucocratic layering. Pen lid is 4 cm long. Location: E, N. (b) Calcsilicate pod in quartzo-feldspathic gneiss. Location E, N. (c) Example of a garnet-bearing leucosome parallel to lithological layering, similar to sample R dated at Ma with SHRIMP U Pb zircon. Location E, N. (d) Cordierite corona surrounding large garnet porphyroblast. Plane-polarised light; field of view 8 mm. Sample number MB Location E, N. (e) Cordierite þ biotite pseudomorph of garnet. Minor sillimanite is also present in this example. Plane-polarised light; field of view 12 mm. Sample number MBCP-6A. Location E, N. (f) Biotite corona around garnet. Plane polarised light; field of view 8 mm. Sample number MBCP03-9. Location E, N. All locations are given in GDA 1994 Zone 53 coordinates. Thin-sections stored at Geological Survey, PIRSA. (Figure 12a). Analyses were undertaken using the electron microprobe in situ chemical U Th Pb method at the University of Adelaide using a CAMECA SX51 Electron Probe Micro Analyser following the methods of Clark et al. (2005), Swain et al. (2005) and Rutherford et al. (2006). Analysis of the Malagasy monazite (MAD, 514 Ma: Fitzsimons et al. 2005) during the analytical session yielded a weighted mean age of Ma

17 Paleoproterozoic orogenesis, Gawler Craton 465 Figure 11 P T pseudosection for Corny Point Paragneiss. Bulk composition for these calculations from XRF data of Howard (2006) and is indicated in the top left of the figure. Arrow indicates inferred P T path for the Corny Point Paragneiss based on mineral paragenesis described in the text. (MSWD ¼ 1.15; n ¼ 48), giving confidence in the accuracy of the age data for the Corny Point samples. Results (Data Table 2*) show a predominance of ages ca 1830 Ma, with two samples (R and R674765) giving weighted mean ages of Ma and Ma from 35 and 33 analytical points, respectively (Figure 12a; Table 3). The monazite chemical data also show a scatter of younger ages towards ca 1300 Ma, although given that the technique cannot detect discordance, these younger ages probably reflect modification of the mean population of ca 1830 Ma. Chemical mapping of two monazite grains reveals a complex internal structure, with irregular and patchy zonation (Figure 12b), which may suggest these grains have been hydrothermally altered (cf. Poitrasson et al. 1996) and thus account for Pb loss and younger ages. The age peak at ca 1830 Ma thus provides a minimum constraint on the timing of retrograde biotite growth and therefore the down-pressure evolution of the paragneiss. Consequently, the retrograde evolution is interpreted to reflect the waning effects of ca 1850 Ma orogenic processes rather than younger (e.g. Kimban: Ma) changes in thermobarometric conditions. DISCUSSION 1850 Ma orogenesis in the southeastern Gawler Craton: the Cornian Orogeny The 1850 Ma event is characterised by the emplacement of the Donington Suite into a compressional tectonic environment within which contractional strain and granulite-facies peak metamorphism were synchronous. Contractional deformation was terminated by hightemperature, extensional deformation and the emplacement of Type 2 mafic dykes. Syn-extensional microgranite was emplaced at Ma, within error of syn-contractional granite ( Ma), suggesting that phases of contraction and extension occurred within a short period of time, probably less than *10 million

18 466 A. Reid et al. Figure 12 Results of chemical U Th Pb electron microprobe monazite geochronology from Corny Point Paragneiss. (a) Cumulative probability plot and histogram of ages derived from U Th Pb analyses, showing peak at ca 1830 Ma and younger Pb loss. Inset shows setting of some of the monazites analysed (in centre of small radiation damage halos) within retrograde biotite around a garnet porphyroblast. Plot of monazite results using AgeDisplay 2.25 (Sircombe 2004). (b) Chemical maps of selected monazites for U, Th, Y and Pb. Field of view is 700 mm wide. years. Our geochronological constraints on the timing of metamorphic decompression are also within error of those for the extensional deformation, and we suggest that decompression is likely to be directly related to the onset of extension. Thus, our interpretation of the field evidence and available geochronology is that contractional deformation was transient and that the entire tectonothermal cycle occurred within *10 million years. Detrital zircons analysed from the Corny Point Paragneiss (Zang & Fanning 2001; Howard et al. 2006; this study) yield ages as young as ca 1870 Ma, suggesting that a maximum depositional age for the sedimentary precursor may well approach the timing of emplacement of the Donington Suite to within at most 20 million years. These data suggest that the tectonic setting for the 1850 Ma event is one that was characterised by an