Blind orogen: Integrated appraisal of multiple episodes of Mesoproterozoic deformation and reworking in the Fowler Domain, western Gawler Craton, Australia

Precambrian Research 166 (2008) 263–282

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Blind orogen: Integrated appraisal of multiple episodes of Mesoproterozoic deformation and reworking in the Fowler Domain, western Gawler Craton, Australia Jane L. Thomas 1 , Nicholas G. Direen ∗ , Martin Hand Continental Evolution Research Group, School of Earth and Environmental Sciences, University of Adelaide, SA 5005, Australia

a r t i c l e

i n f o

Article history: Received 1 May 2006 Received in revised form 1 April 2007 Accepted 20 May 2007 Keywords: Fowler Domain Gawler Craton Gravity Magnetics Monazite Proterozoic Rodinia Transpression

a b s t r a c t The Fowler Domain in the western Gawler Craton in southern Australia is a poorly exposed region that can only presented be explored using a combination of information from drilling and potential field geophysics. Regional maps of the Total Magnetic Intensity (TMI) field of the Fowler Domain highlight an anastomosing system of terrain-scale shear zones that bound four crustal-scale tectonic blocks: from west to east, the Colona, Barton, Central and Nundroo blocks. Integrated thermobarometry and electron microprobe chemical dating of metamorphic monazites from drillholes in the Fowler Domain suggest that the Colona Block in the west underwent two mid-crustal amphibolite grade metamorphic events at ca. 1643 and 1600 Ma. The younger age corresponds to the timing of regional high-grade metamorphism in the Barton Block. Together, the age data suggest that the western Fowler Domain underwent a major tectonothermal event at ca. 1600 Ma. In contrast, regional lower crustal metamorphism in the Nundroo Block, which forms the eastern Fowler Domain, occurred at ca. 1545 Ma. In both the Barton and Nundroo Blocks, petrological relationships, mineral zoning, and pressure–temperature (P–T) modelling suggest the terrains cooled in the mid- to lower crust, rather than undergoing exhumation immediately following peak metamorphism. Age data from the geophysically defined shear zone systems that bound the blocks suggest that exhumation of these lower crustal domains occurred between ca. 1470 and 1450 Ma and was associated with transpressional reactivation of the terrain during the Coorabie Orogeny. A key finding of this study is that the tectonic evolution of the crustal blocks in the Fowler Domain was not in concert until at least 1500 Ma. Thus, the evolution of the individual blocks is unlikely to be related to the macroscopic character of the terrain defined by the regional-scale shear zone systems, which are one of the youngest tectonic imprints on the Fowler Domain. Coincident gravity and magnetic forward modelling of significant bounding faults suggests the shear zones form a steeply dipping transpressional array, consistent with the observed metamorphic field character of the different blocks. This study provides a demonstration of the integration of geophysical and petrological approaches to investigate the time-integrated tectonic evolution of poorly exposed basement terrains. © 2008 Elsevier B.V. All rights reserved.

1. Introduction The assembly and subsequent dispersal of Proterozoic supercontinents such as Rodinia is relatively widely accepted, but exact configurations remain elusive and controversial (e.g. Burrett and Berry, 2001; Giles et al., 2004; Karlstrom et al., 2001; Wingate et al., 2002). This is largely due to the complexities entailed

∗ Corresponding author. Current address: FrOG Tech Pty Ltd., P.O. Box 145, Blackmans Bay, TAS 7052, Australia. Fax: +61 2 62834801. E-mail address: [email protected] (N.G. Direen). 1 Current address: YTC Resources Ltd., Orange, NSW 2800, Australia. 0301-9268/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.precamres.2007.05.006

in reconstructing continental margins, arising from uncertainties regarding both the original geometry and the diachronous nature of rifted margins (e.g. Direen and Crawford, 2003a,b), or because of the poor constraints on potential piercing points (Direen et al., 2005b). To circumvent these problems in continental reconstruction, Karlstrom et al. (2001) advocated the use of “time-integrated piercing points”—understanding the tectonic evolution of orogenic systems rather than a single event or rock package. The Proterozoic belts of Australia figure prominently in many continental reconstructions (e.g. Betts and Giles, 2006; Brookfield, 1993; Burrett and Berry, 2001; Li and Powell, 2001; Myers et al., 1996), and their role in assembly and dispersal of Rodinia has been widely debated (e.g. Payne et al., 2006; Direen and Crawford,

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Fig. 1. Location map and interpreted subsurface geology of the Gawler Craton, South Australia (after Daly et al., 1998; Swain et al., 2005a). Fowler Domain is region contained within inset square. kfz: Karari Fault Zone; tsz: Tallacootra Shear Zone; clfz: Colona Fault Zone; crfz: Coorabie Fault Zone. Inset, top left: distribution of outcrop in the Gawler Craton. Box shows study area. Inset, top right: location of Gawler Craton (box).

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2003a,b; Wingate and Giddings, 2000; Wingate et al., 2002). In particular, the relationship of Proterozoic terranes now resident in southern Australia to those now found in northern Australia remains contentious (Betts and Giles, 2006; Dawson et al., 2002; Direen et al., 2005b; Giles et al., 2004). In this regard, Proterozoic systems in southern Australia such as the Gawler Craton, form “keystones” in many models of supercontinent assembly and dispersal (e.g. Collins and Pisarevsky, 2005; Fitzsimons, 2003; Giles et al., 2004). Understanding of the timeintegrated evolution of the Gawler Craton (Fig. 1) continues to remain problematic, due to the widespread presence of younger cover that obscures much of the craton. Given the extent of the cover, the Proterozoic orogenic systems of the Gawler Craton are largely blind tectonic systems. Moreover, they are also “blind” in the sense that they are cryptic in the geological record, and many extant reconstructions rely heavily on radically incomplete datasets. In the southern Australian Proterozoic, workers have approached the problem of detecting and appraising blind orogenic systems through integrating metamorphic and geochronologic data (e.g. Swain et al., 2005a,b; Teasdale, 1997; Tomkins et al.,

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2004) from drillcores, with geometric information from areally extensive gravity and magnetic datasets. Additional forward modelling of the potential field datasets is valuable for determining three-dimensional geometry, large-scale structural orientation and overprinting relationships, especially when combined with rock property analysis, and field-based structural observations (Direen et al., 2001, 2005a,b; Direen and Lyons, 2002). The combination of geometric information from the potential fields with the timing of deformation, metamorphism and exhumation produces more robust, time-integrated tectonic models. In this paper, we present results of applying these techniques to the Fowler Domain in the western Gawler Craton. We then discuss a time-integrated tectonic evolution for the Coorabie Orogen, a blind orogenic system within the keystone Gawler Craton of Proterozoic Australia. 2. Geological setting The Gawler Craton is a complex late Archean (ca. 2550 Ma)–Mesoproterozoic (ca. 1450 Ma) metasedimentary and

Table 1 Time–space plot for domains in the western Gawler Craton

Time–space plot for major terranes within the study area (see Fig. 1 for locations), modified after Direen et al. (2005a). References for events: (1) Sayers et al. (2001); (2) Parker (1993); (3) Myers et al. (1996); (4) Wingate et al. (2002); (5) Swain et al. (2005a); (6) Daly et al. (1998); (7) Hand et al. (2007); (8) Ferris and Schwarz (2004).

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metaigneous collage (Fig. 1). It has been subdivided into a number of geophysically distinct regions, which represent different levels of tectonic exhumation (Direen et al., 2005b; Payne et al., 2006; Teasdale, 1997); different tectonic histories (Swain et al., 2005a,b); or both. Table 1 summarizes our current understanding of the tectonic evolution of the northern and western Gawler Craton. The core of the Gawler Craton comprises two related Archean complexes, the Mulgathing and Sleaford Complexes (Swain et al., 2005b). The northern and western Gawler Craton contains a large expanse of sparsely exposed late Archean Mulgathing Complex, dominated by the Christie Gneiss. This migmatitic paragneiss is interlayered with felsic orthogneiss; garnet–biotite gneiss; calcsilicate rock; mafic orthogneiss; and metamorphosed banded iron formation (BIF). All of these rocks were deformed by the Paleoproterozoic Sleaford Orogeny between ca. 2.5 and 2.42 Ga (Teasdale, 1997; Tomkins et al., 2004; Hand et al., 2007). The Mulgathing Complex was reworked by the ca. 1730–1700 Ma Kimban Orogeny along the eastern margin of the proto-Gawler Craton (Vassallo and Wilson, 2002; Swain et al., 2005a; Hand et al., 2007). Tomkins et al. (2004) identified patchily developed, lower greenschist to amphibolite facies reworking of parts of the western Gawler Craton that were geochrologically identical to the Kimban Orogeny. Teasdale (1997) had shown previously that Kimban reworking was spatially linked to presently north-east trending shear zones. Direen et al. (2005b) postulated that the present distribution of Kimban structures in the western Gawler Craton could be explained by post-Kimban reactivation of a craton-scale NE-trending shear zone system. The Fowler Domain in the western Gawler Craton (Fig. 1) forms a geophysically distinctive, NNE- to NE-trending belt of magnetic metamorphic and igneous rocks, dissected by a number of these NE-trending anastomosing shear zones (Daly et al., 1998; Teasdale, 1997) (Fig. 2). The Fowler Domain is bounded by the Archean Christie Domain to the northwest and the Paleo-Mesoproterozoic Wilgena and Nuyts Domains to the east (Fig. 1). It is separated from the Christie Domain by a branch of the Tallacootra Shear Zone, and from the Wilgena and Nuyts Domains by the Coorabie Fault Zone (Fig. 2). Although almost completely concealed beneath younger, 25–150 m thick, flat-lying sedimentary successions (Fig. 1 inset), drilling (e.g. Fig. 2) has shown the Fowler Domain contains a range of rock types (Martin et al., 1993). These include mafic and ultramafic rocks, intermediate-felsic igneous, and metasedimentary rocks (Teasdale, 1997; Daly et al., 1998). Two distinct tectonothermal events have been previously identified in the Fowler Domain. These were accompanied by granitoid plutonism at ca. 1690–1670 and 1590–1575 Ma (Daly et al., 1998). The 1690–1670 Ma event resulted in intrusion by the felsic to mafic plutons of the Tunkillia (aka. Ifould) Suite (Ferris and Schwarz, 2004; also Teasdale, 1997), which has been speculatively linked to magmatic arc processes (Betts and Giles, 2006). Direen et al. (2005b) suggested that the ca. 1590 Ma event saw the amalgamation (or reamalgamation) of the high-grade block NawaMabel Creek Terrane (Fig. 1) with the Christie Domain in northern Gawler Craton. This event was contemporaneous with widespread emplacement of the high temperature felsic Hiltaba Suite (Creaser, 1996), which occurred during active deformation (Swain et al., 2005a), eruption of the comagmatic mafic–felsic Gawler Range Volcanics (GRV) (Blissett et al., 1993), and geophysically distinctive Iron-Oxide Copper Gold mineralisation (Heinson et al., 2006). Previous work (Teasdale, 1997), divided the Fowler Domain into four blocks with contrasting geological characteristics: from east to west, the Nundroo, Central, Barton and Colona Blocks (Fig. 2). Geochronological data suggests the western Gawler Craton was reworked over the interval 1550–1450 Ma. This interval has been

Fig. 2. (A) Total Magnetic Intensity (TMI) image of the Fowler Domain showing the Colona, Barton, Central and Nundroo Blocks, their bounding shear zones, locations of drillholes used for petrological and geochronological analysis in this study (circles). Lines show cross sections used to forward model potential field responses. (B) Uninterpreted 1st Vertical Derivative of the TMI to highlight geological structures imaged in the dataset.

referred to as the Coorabie Orogeny by Direen et al. (2005b), and was characterized by major reactivation of craton-scale northeasttrending faults and shear-zones (Swain et al., 2005a; Fraser and Lyons, 2006). Reactivated structures include the Tallacootra Shear Zone and Coorabie Fault Zone, which bound the Fowler Domain, and cross-cut Hiltaba Suite plutons (Fig. 1). The Karari Fault Zone (Figs. 1 and 2) which truncates the other shear zones was also reactivated at this time (Direen et al., 2005b; Swain et al., 2005a; Fraser and Lyons, 2006).

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lowed by deposition of extensive, but often thin, Neoproterozoic to Cenozoic successions (Parker, 1993). 3. Methods 3.1. Geophysical basement mapping We have used regional aeromagnetic and gravity data from Primary Industries and Resources South Australia (PIRSA) to construct a series of filtered images, highlighting textural variations due to structural fabrics and/or lithological contrasts in the covered basement of the Fowler Domain (Figs. 2 and 3a–c). The adopted aeromagnetic grid comprises data mostly acquired in 1993, with a flight line spacing of 400 m at a drape height of ca. 80 m above the ground surface. The grid also contains some merged high resolution datasets from other sources. The data has been subjected to standard pre-gridding processing and leveling techniques such as those described by Minty et al. (2003). Geophysical imagery was divided into domains of cognate signal character, and then systematically interpreted for lithological and structural information, to construct a basement geology interpretation. Units on the map were assigned identities based on existing stratigraphy from outcrop and drillhole databases (Daly et al., 1994; Martin et al., 1993; Morris et al., 1994; Schwarz et al., 2002). 3.2. Metamorphic analysis Teasdale (1997) undertook a reconnaissance study of the metamorphism in the Fowler Belt, and showed that rocks in the Nundroo and Barton Blocks had undergone upper amphibolite to granulite grade metamorphism. We have estimated exhumation levels across major domain-bounding structures using equilibrium metamorphic mineral assemblage data (e.g. Goscombe et al., 2003). Although the construction of regional metamorphic field gradients is best done in regions with extensive outcrop, it is still useful in terrains such the Fowler Domain, where there are drill holes that intersect basement. Sample localities for all petrographic and age data are provided in Table 2. In total, 26 samples representing the dominant rock types, as well as being suitable for metamorphic pressure–temperature (P–T) and geochronological work, were analysed. Mineral compositions were obtained using a Cameca SX 51 Electron Microprobe at the University of Adelaide. Quantitative analyses were run using an accelerating voltage of 15 kV and a beam current of 20 nA. Representative mineral compositions can be found in Supplementary data Table 1. Pressures and temperatures were calculated using both conventional cation-exchange thermobarometers (Berman, 1990; Hodges and Spear, 1982; Thompson, 1976), and average pressure–temperature (P–T) approaches as implemented in THERMOCALC v3.1 (Powell and Holland, 1988). Mineral activities used in THERMOCALC were calculated using the computer program AX 2000 (Powell et al., 1998). Fig. 3. Interpreted images of the 1st Vertical Derivative of the TMI in the (a) Colona; (b) Barton; and (c) Central and Nundroo Blocks with interpreted structural trends, shear zones, and the location of the modelled potential field traverses.

Following the Coorabie Orogeny, the northern and western Gawler Craton appears to have been tectonically stable with, as yet, no record of the 1100–1050 Ma Musgravian Orogeny aside from minor disturbance of Ar systems (Fraser and Lyons, 2006), which occurred to the north (Wade et al., 2006). Cratonisation was fol-

3.3. Event timing Event timing of metamorphic and kinematic events in amphibolite to granulite-grade felsic and metapelitic rocks has been evaluated using electron microprobe analysis (EMPA) Th–U–Pb chemical age dating of monazite. The in situ determination of EMPA analyses provides texturally specific geochronologic data that places absolute time constraints on metamorphic mineral growth and associated deformation (e.g. Dutch et al., 2005; Rutherford et al., 2006; Kelsey et al., 2007).

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Fig. 4. Representative population density plots from (a) Colona Block sample COL20D 44 m; (b) Barton Block sample BAC33; and (c) Nundroo Block sample NDR1 41 m. Representative monazite grains are shown in backscattered electron mode (BSE) and scanning electron mode (SE), with host minerals. Mineral abbreviations: bi, biotite; qtz, quartz. In all cases, the population density plots show one age population, with the number of analyses at each particular age represented on the left axis of the plot. Ages are reported at 95% confidence.

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Table 2 Summary of location, petrography and chemical age data Sample

Easting

Northing

Description

Mineralogy

BAC13 BAC17 BAC18 BAC23 BAC26 BAC28 BAC33 BAC41 TAL 4 TAL 13 TAL 20 COL20D 44 m COL20D 46.2 m COL20D 46.5 m COL20D 47 m NDDH1 52 m NDDH2 139 m

246771 248252 248614 250347 251152 251864 253552 255670 246028 252328 247028 782282 782282 782282 782282 782282 238453

6574953 6574386 6574437 6573841 6573783 6573519 6573288 6573106 6561171 6555871 6560271 6536363 6536363 6536363 6536363 6536363 6477760

Metapelite Granulite Metapelite Protomylonite Amphibolite Pelitic gneiss Mylonite Pelitic gneiss Pelitic gneiss Amphibolite Pelitic gneiss Metapelite Metapelite Metapelite Metapelite Amphibolite Granulite

gt–bi–sill–plag–kfsp gt–hbl–plag–ilm–cpx gt–bi–sill–plag gt–bi–sill gt–bi–hbl–plag–ilm gt–bi–sill gt–bi–sill gt–bi–sill–plag gt–bi–plag gt–bi–hbl–plag–ilm gt–bi–kfsp gt–bi–mus–plag–ilm gt–bi–mus–plag gt–bi–mus–plag gt–bi–mus–plag gt–bi–hbl–plag–ilm gt–hbl–plag–opx–cpx–bi(minor)–ilm

NDDH2 207 m NDDH2 273 m

238453 238453

6477760 6477760

Granulite Granulite

gt–cpx–hbl–plag gt–hbl–plag–opx–bi–ilm

NDDH3 72 m NDDH3 108 m NDDH3 129 m NDR1 40 m NDR1 41 m NDR 3 NDR 5 45 m

236403 236403 236403 238062 238062 235690 240516

6480093 6480093 6480093 6478212 6478212 6480880 6474390

Metapelite Metapelite Gt–hbl gneiss Metapelite Metapelite Metapelite Pelitic gneiss

gt–bi–plag–kf gt–bi–plag–kf gt–bi–hbl–plag gt–bi–plag gt–bi–plag gt–bi–plag–kf gt–bi–sill–plag–kf–ilm

Age (Ma) (95% confidence)

No. of analyses

1606 ± 17

123

1592 ± 18

69

1638 ± 15 1608 ± 13

115 96

1648 ± 15

122

1471 ± 14

135

1557 ± 15

97

Mineral abbreviations: gt, garnet; bi, biotite; sill, sillimanite; plag, plagioclase; kf, alkali feldspar; hbl, hornblende; ilm, ilmenite; mus, muscovite; opx, orthopyroxene; cpx, clinopyroxene. Coordinates are GDA Zone 54.

Monazite imaging was undertaken at the University of Adelaide using a Philips XL30 FESEM in backscattered mode (BSE) at 15 kV. BSE imaging of monazites allows assessment of chemical heterogeneity and its correlation with crystallographic domains. In selecting spots for analysis, a range of textural settings was analysed, avoiding microfracturing, which may result in age resetting (Carson et al., 2004). Electron microprobe analysis was carried out at Adelaide Microscopy, University of Adelaide, using a Cameca SX51 microprobe. The analytical procedure for chemical U–Th–Pb monazite dating technique using EMPA follows that outlined in detail by Cocherie et al. (2005). The technique used at the University of Adelaide was described by Rutherford et al. (2007). Operating conditions were 15 kV accelerating voltage, 100 nA cup current and a spot size of approximately 3 ␮m. Mean ages were calculated using the weighted average function in ISOPLOT (Ludwig, 2003). The concordant 514 Ma Madagascan monazite (MAD) (Fitzsimons et al., 2005) is used as our in-house standard. Repeat analyses (n = 120) of MAD yielded an age of 509 ± 10 Ma (2␴). A summary of the chemical age data is listed in Table 2, with all the chemical U–Th–Pb monazite compositional data in Supplementary data Table 2. Fig. 4 shows the probability density plots of the monazite EMPA U–Th–Pb age data for each sample. 3.4. Modelling of structural geometry at depth The trends in the geophysical images, combined with the differing ages between the main blocks and the observed steep metamorphic field gradients, suggest the presence of major structures within and between lithotectonic blocks. The subsurface geometries of major structures were tested using 2.5D and 3D forward modelling software (Encom Technology, 2002). We simultaneously modeled the gravity and magnetic fields of simplified structural cross-sections derived from the geophysical image interpretation that honor the observed metamorphic field gradients. Further details of the methods used to build and constrain the cross-

sections can be found in Direen et al. (2005a), but have been omitted here for reasons of brevity. 4. Gross architecture of the Fowler Domain In the Fowler Domain, shear zones are typically demagnetised. The shear zones separate areas that are generally characterised by a higher (300–1300 nT), but homogeneous, magnetic responses (Fig. 3). Granite plutons have been identified as characteristically ovoid elements with demagnetised central zones (e.g. Fig. 2). The Colona Block is characterised by a generally high amplitude magnetic response (up to 1300 nT), bounded by two poorly magnetised branches of the Tallacootra Shear Zone (Fig. 3a). A regional-scale fold is recognisable, which appears to close and plunge towards the north–northeast (Fig. 3a). The western limb of this fold is thicker than the eastern limb, but is obliquely truncated by a branch of the Tallacootra Shear Zone (Fig. 3a). The Barton Block exhibits high magnetisation (ca. 2000 nT; Fig. 3b). It is bounded by the poorly magnetised Tallacootra Shear Zone to the west, and the Colona Fault Zone to the east (Figs. 2 and 3b). A number of narrow (ca. 100 m) shear zones can be identified within the block, with panels of very high (1000–2000 nT) magnetic intensity rocks separating them. A number of granitoid plutons (ca. 5 km in diameter) have also been interpreted (Fig. 3b), cross-cutting fabrics within the shearbounded blocks. In places, these granitoids are elongated within the shear zones. The Central Block exhibits a lower average magnetic response than the other blocks in the Fowler Domain (−400 to −100 nT; Figs. 2 and 3b). It is bound by two low magnetic response shear zones (ca. −500 nT), and appears to be characterised by a uniform magnetic signature (Fig. 3b). Although there is limited information from drilling within this block, the uniform magnetic patterns of the Central Block indicate that the observed rocks are likely to persist over most of the block. A younger, cross-

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cutting granitoid pluton has been interpreted to intrude this block (Fig. 3b). The Nundroo Block contains a number of north–northeast trending shear zones with low magnetisation (−200–100 nT), which range from 1 to 7 km in width (Fig. 3c). These separate more magnetised panels (up to 1500 nT).

quartz + garnet + biotite + plagioclase ± sillimanite ± muscovite ± alkali feldspar, with minor zircon, monazite, ilmenite and magnetite. Metabasites generally include quartz + garnet + hornblende + plagioclase-bearing assemblages (± clinopyroxene ± orthopyroxene) with minor biotite, ilmenite and magnetite. 5.1. Garnet-bearing metapelites

5. Petrography of metamorphic rocks in the Fowler Domain Metamorphic rocks in the Fowler Domain can be broadly grouped into garnet-bearing metabasites, and garnetbearing metapelites. Generally the metapelites contain

The Colona Block metapelites are weakly foliated. COL20D 46.5 m shows two generations of garnet and plagioclase growth. First generation garnet is typically anhedral and, contains inclusions of sillimanite which is absent in the matrix (Fig. 5a). The second-generation of garnet is smaller in size 0.5 mm and euhedral,

Fig. 5. Photomicrographs of typical mineral assemblages in the Fowler Domain. (a) COL20D 46.5 m first generation garnet with inclusions of fine-grained sillimanite. (b) COL20D 44 m muscovite stability within foliation, possibly contemporaneous with biotite. (c) BAC33 mylonitic fabric defined by biotite–sillimanite foliation. Garnet is elongated by shearing. (d) NDDH3 72 m foliated pelite with medium grained garnets wrapped by biotite foliation, note the absence of garnet resorption. (e) NDDH2 139 m mafic granulite, medium grained garnet with fine grained matrix of cpx, opx, plag, qtz, hbl and mag. (f) TAL13 amphibolite with undeformed reaction texture consisting of fine-grained hornblende and quartz, suggestive of post deformational breakdown of clinopyroxene. Mineral abbreviations: gt, garnet; sill, sillimanite; bi, biotite; qtz, quartz; kspar, alkali feldspar; plag, plagioclase; mus, muscovite; cpx, clinopyroxene; opx, orthopyroxene; hbl, hornblende; mag, magnetite.

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forming aggregates in the matrix with biotite, muscovite, feldspar and quartz. First generation plagioclase is comparatively coarsegrained, and has been partly replaced by finer-grained granoblastic aggregates of second-generation plagioclase, which is also intergrown with biotite, muscovite and quartz in the weakly foliated matrix (Fig. 5b). Sampled Barton Block metapelites all contain foliationdefining sillimanite and biotite (Fig. 5c). The fabric commonly has a mylonitic appearance with ribbon quartz paralleling the sillimanite–biotite folia. Garnets are anhedral and in some samples show evidence of conversion into porphyroclasts within the fabric (Fig. 5c). Metapelites within the Nundroo traverse samples typically have medium sized (1–5 mm) anhedral garnet porphyroblasts that are wrapped by a biotite foliation (Fig. 5d). Garnets have inclusions of biotite, ilmenite and magnetite. Both plagioclase and quartz are medium grained (1–5 mm), and define polygonal aggregates within the foliated matrix. 5.2. Garnet-bearing metabasites Mafic granulites occur in the Nundroo and Barton Blocks, and are characterised by the presence of coexisting clinopyroxene and orthopyroxene (Fig. 5e). Assemblages are typically medium- to fine-grained (1–3 mm) with coarser-grained anhedral garnet. The

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weakly foliated matrix assemblage enclosing garnet consists of granoblastic plagioclase, quartz orthopyroxene and clinopyroxene, and hornblende with accessory ilmenite and magnetite. Amphibolites within the Barton and Nundroo Blocks typically contain fine-medium grained hornblende (1–2 mm), and biotite that define the tectonic foliation (Fig. 5d). Plagioclase and quartz occur as weakly aligned minerals interspersed with the main foliation defining minerals. Garnet porphyroblasts occasionally contain inclusions of quartz, hornblende and biotite. In rare instances (e.g. TAL13; Fig. 5d), undeformed reaction textures consisting of fine-grained hornblende and quartz, suggest post-metamorphic breakdown of clinopyroxene (e.g. Spear, 1993). 6. Mineral chemistry 6.1. Garnet-bearing metapelites Mineral compositions between the blocks do not differ significantly. Biotite XFe2+ (annite) values range from 0.13 to 0.48, XNa values range from 0.002 to 0.02, XTi from 0.15 to 0.77, and AlVI 0.085 to 0.42 (based on 11 oxygens). Muscovite has an XNa of ca. 0.05 and an XFe of ca. 0.07. Alkali feldspar XNa ranges from 0.07 to 0.23. Plagioclase compositions range from anorthite 0.29 to 0.47 (based on 8 oxygens), with the most calcium-rich samples coming from the Barton Block. In the Colona Block, first generation plagioclase shows

Fig. 6. Qualitative X-ray compositional garnet maps. Numbers refer to mole fractions Xalmandine, Xpyrope, Xgrossular, Xspessartine (in Fe, Mg, Ca and Mn maps, respectively). (a–d) COL20D 46.5 m, metapelite. Ca zoning in (c) is indicative of prograde growth. (e–h) BAC33, proto-mylonite. Mg zoning in (f) and (j) is indicative of retrogression. (i–l) NDR1 41 m, metapelite.

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significant zoning, with Ca-rich cores (XCa = 0.45) and comparatively Ca-poor (XCa = 0.35) rims. Second-generation plagioclase is unzoned, with XCa corresponding to the rim compositions of the earlier plagioclase. Garnet XFe2+ values range from 0.22 to 0.68, and XMn values range from 0.03 to 0.3. The most Fe-rich garnet compositions come from the Nundroo Block. XCa increases from core to rim in some metapelitic samples. XMn generally increases slightly from core to rim (e.g. Sample NDDH3 72 m XMn: 0.055–0.058). This style of zoning is characteristic of re-equilibration on the hightemperature segment of retrograde P–T paths (e.g. Spear et al., 1990). In contrast, samples from the Colona Block, show a rimward decrease in XMn (see Section 6.3), consistent with prograde mineral growth (e.g. Spear et al., 1990). Second-generation garnet in the Colona Block shows little zoning, with compositions corresponding to the rims of first generation garnet. 6.2. Garnet-bearing metabasites Plagioclase in amphibolites has a similar range of anorthite values (0.29–0.47) to the metapelites. However the granulites have a restricted range of plagioclase compositions (XCa ca. 0.44). XFe2+ in orthopyroxene is ca. 0.44 and in clinopyroxene ranges from 0.13 to 0.17 with XCa ranges from 0.4 to 0.45, and AlVI ca. 0.04 (based on 6 oxygens). Hornblende XNa ranges from 0.32 to 0.53, except for sample NDDH2 207 m, which has an XNa value of 0.97. XFe ranges from 0.29 to 0.45; the most iron rich sample is TAL13. AlVI ranges from 0.23 to 0.27. Garnet XFe2+ values range from 0.50 to 0.58 with XCa ranging between 0.18 and 0.24. XMn values are within the same range as for the metapelites. Garnets show weak zoning with rims enriched in XFe by around 0.04 relative to the core. Biotite within amphibolites has XFe2+ (annite) values ranging from 0.41 to 0.55, XNa values that range from 0.006 to 0.03, XTi from 0.5 to 0.95, and an AlVI of 0.017 to 0.185 (based on 11 oxygens). 6.3. Qualitative X-ray compositional garnet mapping To further explore the compositional character of garnets from the Fowler Domain, individual garnets in selected metapelitic samples were compositionally mapped for the elements Mg, Fe, Mn and Ca (Fig. 6). Garnet from the Colona Block (sample COL20D 46 m, Fig. 6a–d) shows a significant increase in XCa towards the

rim (Fig. 6c). This Ca zoning coupled with rimward enrichment in XFe (Fig. 6a) and decrease in Mn (Fig. 6d), suggests garnet growth during prograde pressure increase (e.g. Spear et al., 1990; Vance and Mahar, 1998). Sample BAC33 from the Barton Block also shows a significant increase in XFe (Fig. 6e) and XCa (Fig. 6g) from core to rim, and a decrease in XMg (Fig. 6f). BAC33 shows no significant zoning in XMn (Fig. 6h), indicating there has been no garnet resorption by Mn-poor minerals such as hornblende and plagioclase (e.g. Kohn and Spear, 2000). Similar zoning is evident in NDR141 m from the Nundroo Block (Fig. 6i–l). 7. P–T constraints on metamorphism in the Fowler Domain The results of average P–T calculations (Powell and Holland, 1994) on 21 peak and retrograde metamorphic assemblages obtained using the software package THERMOCALC (Powell and Holland, 1988; Powell et al., 1998) are summarised in Table 3. The results give P–T information about the Colona, Barton, and Nundroo Blocks. No thermobarometrically useful mineral assemblages were found in the Central Block. In the present study, water activity (aH2 O) exerts an important control on calculations of metamorphic pressures which, in general, are less well constrained. Water activity was constrained for most samples by heuristically estimating aH2 O values and matching resultant temperature calculations using the average-temperature mode in THERMOCALC with results from garnet–biotite Fe–Mg exchange thermometry which is independent of aH2O (e.g. Berman, 1990; Hodges and Spear, 1982; Thompson, 1976). The optimal aH2 O value was then utilised in calculations to determine pressures. P–T calculations were undertaken using both core and rim mineral compositions (Table 3). In the Colona Block, calculated temperatures using mineral core compositions from COL20D samples range between 533 ± 27 and 618 ± 37 ◦ C with corresponding pressures in the range 4.3 ± 1 to 5.6 ± 1 kbar. Assuming that the mineral assemblages with this single drill hole formed during a single metamorphic event, the average of the core composition P–T results gives 561 ± 30 ◦ C and 4.8 ± 1.1 kbar. P–T conditions for the analysed rim/secondary assemblage compositions range between 529 ± 28 to 589 ± 31 ◦ C and 5.3 ± 1.2 to 5.8 ± 1.0 kbar, respectively, giving a pooled estimate of 546 ± 29 ◦ C and 5.6 ± 1 kbar.

Table 3 Summary of THERMOCALC pressure–temperature calculations using both core and rim mineral compositions at 95% confidence interval Block

Sample

Rock type

Core T ◦ C ± 1

Colona

COL20D 44 m COL20D 46.2 m COL20D 46.5 m COL20D 47 m

Metapelite Metapelite Metapelite Metapelite

550 533 618 569

± ± ± ±

29 27 37 32

5.6 4.3 5.0 4.5

± ± ± ±

1 1 1.3 1.1

529 525 545 589

± ± ± ±

28 31 27 31

5.8 5.3 5.4 5.7

± ± ± ±

1 1.2 1 1.1

Barton

BAC13 BAC17 BAC18 BAC26 BAC41 TAL4 TAL13

Metapelite Granulite Metapelite Amphibolite Pelitic gneiss Gt–bi gneiss Amphibolite

678 846 793 766 650 750 819

± ± ± ± ± ± ±

28 65 128 59 171 50 105

6.9 10 7.8 8.9 6.8 9.3 9.3

± ± ± ± ± ± ±

1.2 1.6 2.2 1.4 2.6 3.6 1.7

610 809 777 654 512 750 729

± ± ± ± ± ± ±

26 60 126 73 83 50 107

6 9.2 7.6 7.8 4.5 10.1 8.5

± ± ± ± ± ± ±

1.1 1.4 2.2 1.5 1.5 2.5 1.7

Nundroo

NDDH1 52 m NDDH2 139 m NDDH2 207 m NDDH2 273 m NDDH3 129 m NDR1 41 m NDR3 NDR5

Amphibolite Granulite Granulite Granulite Gt–hbl gneiss Metapelite Metapelite Pelitic gneiss

757 ± 857 ± 780 ± 848 ± 758 ± 750a 672 ± 666 ±

56 47 45 63 73

8.5 9.5 7.8 9.4 8.5 8.2 7.0 6.1

± ± ± ± ± ± ± ±

1.3 1.2 1.4 1.3 1.5 4.8 1.5 1.3

759 ± 839 ± 790 ± 800 ± 708 ± 600a 602 ± 610 ±

56 42 48 59 68

8.8 9 8.1 8.5 7.7 6 6.4 5.4

± ± ± ± ± ± ± ±

1.3 1.2 1.2 1.5 1.4 1.3 1.4 1.2

52 43

Core P kbar ± 1

Rim T ◦ C ± 1

46 37

Rim P kbar ± 1

a Insufficient independent endmember reactions for average P–T calculations. A core temperature of 750 ◦ C and a rim temperature of 600 ◦ C were chosen to calculate average P.

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In the Barton Block, calculated P–T results from core compositions obtained from the BAC traverse show a wide range, from 650 ± 171 to 846 ± 65 ◦ C and 6.9 ± 1.2 to 10.0 ± 1.6 kbar respectively. Conceivably, this wide range could reflect partial re-equilibration of peak assemblages; however, there is no obvious reason to exclude any of the P–T results, since the associated errors define a continuous spectrum of results. Assuming that samples from the BAC traverse formed during the same event, the average P–T result from mineral core compositions is 716 ± 85 ◦ C and 8.1 ± 1.4 kbar. P–T results from rim compositions are also highly variable, ranging between 512 ± 83 to 809 ± 60 ◦ C and 4.5 ± 1.5 to 9.2 ± 1.4 kbar. Excluding the result from BAC 41, which is below the stability field of the sillimanite-bearing mineral assemblage (e.g. Spear, 1993), the average P–T result from the rim compositions is 647 ± 130 ◦ C and 7.4 ± 1.4 kbar. Only two samples were analysed from the TAL traverse in the Barton Block. P–T conditions obtained from core compositions range between 750 ± 50 to 819 ± 105 ◦ C at 9.3 ± 3.6 to 9.3 ± 3.6 kbar. Average results are 763 ± 90 ◦ C and 9.3 ± 3.0 kbar. Rim compositions in the TAL sample yield a range of P–T conditions between 729 ± 107 to 750 ± 50 ◦ C and 8.5 ± 1.7 to 10.1 ± 2.5 kbar, giving averages of 746 ± 91 ◦ C and 7.2 ± 1.2 kbar. The results from the Nundroo traverse appear to define two discrete groups. DDH1 and DDH3 appear to record significantly higher-grade conditions than samples from NDR1 41 m, NDR3 and NDR5, which lie at the eastern end of the Nundroo traverse. Core results from DDH1–DDH3 range between 750 ± 50 to 857 ± 47 ◦ C and 7.8 ± 1.5 to 9.5 ± 1.2 kbar, giving averages of 781 ± 38 ◦ C and 8.7 ± 1.0 kbar. Rim compositions in DDH1–DDH3 give results that range between 708 ± 68 to 839 ± 42 ◦ C and 7.7 ± 1.4 to 9.0 ± 1.2 kbar, yielding average results of 779 ± 37 ◦ C and 8.4 ± 1.0 kbar. Temperatures obtained from core compositions in samples from NDR3 and NDR5 range from 666 ± 43 to 672 ± 52 ◦ C with corresponding pressures of 6.1 ± 1.3 to 7.0 ± 1.5 kbar. Rim results from NDR3 and NDR5 range from 602 ± 46 to 610 ± 37 ◦ C and 5.4 ± 1.2 to 6.4 ± 1.4 kbar. There were insufficient independent endmember reactions for average P–T calculations in sample NDR1 41 m. Consequently, it was only possible to calculate average P or average T by assigning a value to either P or T. Average P calculations were undertaken, with a T of 750 ◦ C, yielding 8.2 ± 4.8 kbar. Significant zoning of garnets in mineral composition data (Fig. 6i–l, Table 3) prompts the assumption that the rim temperature would be somewhat lower than the core. Using an estimated rim temperature of 600 ◦ C yields a theoretical pressure of 6.0 ± 1.3 kbar.

8. Geochronology Monazite is present in most metapelites in this study, but is particularly abundant in the samples from the Colona Block. Monazite is often associated with biotite–muscovite foliation, but can also be found as rare inclusions in garnet and quartz. Three samples from the Colona Block, and two samples each from the Barton and Nundroo Blocks were analysed (Table 2). The combined weighted average from each sample, as well as the population density plots support a single age population for each sample (Fig. 4). In all samples, monazites in both the foliated matrix and within garnet porphyroblasts were analysed. Due to volumetric considerations, the bulk of the analysed monazites come from the matrix; however, within each sample there was no observed statistical age difference between matrix monazite grains compared to those within garnets. Back-scattered electron images of the monazites indicate chemical variation within some samples (Fig. 4b and c). This zoning has no correlation with specific age spots. Significant variation in the

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amount of Th was not observed in individual samples, with concentrations of ThO2 in monazites ranging from 3 to 10 wt.%. Samples from the Colona Block come from a 3 m interval in drillhole COL20D. Samples COL20D 44 and 47 m give statistically equivalent ages of 1638 ± 15 and 1648 ± 15 Ma, respectively. These ages are marginally older than the age of 1608 ± 13 Ma obtained from COL20D 46.5 m. In comparison with the other samples from COL20D, COL20D 46.5 m contains a well-developed secondary garnet–plagioclase-bearing assemblage. The two samples from the Barton Block produced statistically similar ages of 1606 ± 17 and 1592 ± 18 Ma, respectively. In contrast, samples from the Nundroo Block produced significantly different ages. NDR1 41 m gives an age of 1471 ± 14 Ma and NDR5 45 m gives an age of 1557 ± 15 Ma. 9. Interpretation to depth In this section, we describe cross-sections derived from interpreting and modelling geophysical data over two significant structural features in the Fowler Domain. The orientation of these sections was influenced by the locations of drillholes (Fig. 2), which provide lithological, kinematic and limited petrophysical constraints on the interpretations. Models were constructed with three-dimensional strike extents to attempt to account for obliquity introduced into modelling the geophysical anomalies by the spread of drillholes. 9.1. Barton Traverse As shown by results described in prior sections, the Barton Block is characterised by a high frequency, linear magnetic pattern that we interpret as a series of shear zones. The block contains rocks exhibiting high strain and variable metamorphic pressures and temperatures, suggesting significant differential exhumation between shear zones. In order to correctly interpret the kinematics (extension vs. shortening vs. strike-slip) which produced the observed juxtapositions, it is important to constrain the dip and dip directions of the shear zones—thus establishing hangingwall/footwall relationships. This can be done by modelling gravity and magnetic contrasts that arise at structural and lithological contacts controlled by the structural architecture (e.g. Direen, 1998; Direen et al., 2005b). A region of lower magnetic intensity (ca. 200 nT) lies between two regions of high magnetic intensity (1000–2000 nT) in the Barton Traverse (Fig. 7a). A gravity high is observed to ca. 26 km. This coincides with a magnetic high zone, cut by sinuous magnetic lows, coincident with drilled shear zones (Fig. 3b). The magnetic field can be modelled with a network of steeply dipping, branching shear zones (Fig. 7c). At ca. 28 km, there is a distinct magnetic and gravimetric low (Fig. 7a and b), along strike from granitoids drilled to the north of the section (Morris et al., 1994). Both magnetic intensity and gravity responses increase again subtly from ca. 36 km, where there is an interpreted macroscopic fold (Fig. 3b and 7). The model suggests the western limb of this structure is more gently dipping than the eastern limb, which is truncated by a branch of poorly magnetised Tallacootra Shear Zone (Fig. 6). Modelled densities required to fit anomalies over regions of drilled metapelitic rocks are bimodal, either ranging from 2.77 to 2.81 × 103 kg m−3 or 2.87 to 2.91 × 103 kg m−3 (Fig. 7). This distribution may explain the large standard deviation associated with measured densities (Table 4). Amphibolites have modelled densities of 2.82 × 103 kg m−3 , which is towards the lower end of the observed values (Table 4). The modelled granitoid has a density of 2.65 × 103 kg m−3 , suggesting a more felsic endmember, con-

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Fig. 7. Barton Traverse. (a) Bouguer Gravity response (␮m s−2 ). (b) TMI response (nT). (c) Forward modelled cross section showing major structural features and lithologies. Shear zones are modelled as discrete regions of lower magnetic intensity.

sistent with the plagiogranite to adamellite compositions drilled along strike (Morris et al., 1994). Interpreted upper crustal gneisses have densities of (2.85–2.88) × 10−3 kg m−3 , consistent with either para- or orthogneissic origins. Overall, the densities are sufficiently similar through the section to prevent resolution of all but the edges of the interpreted granitoid, which has a significant contrast ( = 0.17 × 103 kg m−3 ) with the surrounding country rock. Magnetically, more of the crustal architecture at depth can be resolved. The metapelitic rocks display two groups of modelled magnetic susceptibility: higher density metapelites have magnetic susceptibility >0.07 SI, whereas lower density metapelites have susceptibilities <0.04 SI. Amphibolites have high magnetic susceptibilities ca. 0.08 SI (Fig. 7), but can be discriminated from the metapelites on the basis of density. Although the magnetic susceptibility of shear zones ranges from 0.01 to 0.07 SI (Fig. 7), in all cases they have lower susceptibility than their wall-rocks, implying retrograde magnetite destruction during shearing. Analysis of the model (Fig. 7) indicates susceptibility contrasts of 0.05–0.09 SI on the walls of seven shear zones, and of these, three have contrasts >0.05 SI on both walls. Perturbation of the model dips around these

shear zones in the manner described by Direen et al. (2005b) suggests that the best fit is achieved when the entire shear array dips steeply (ca. 80–85◦ ) to the WNW. 9.2. Nundroo Traverse Our geological results (above) show the Nundroo Block juxtaposes the highest grade metamorphic domain in the Fowler Belt (to the west) with the low grade Nuyts Domain to the east. As well, the Nundroo Block is cut by a series of NNE-trending, linear to sinuous, variably magnetised shear zones that separate internal domains of contrasting metamorphic evolution (Figs. 2, 5 and 6). These characteristics strongly suggest differential exhumation. In order to determine the dip of the shear array, and thus the exhumation kinematics, we have modelled a NW–SE traverse crossing the Nundroo and Central Blocks into the Munjeela terrane (Fig. 2). Petrophysical values for the Nundroo Traverse were constrained using lithological information from the Nundroo Drilling Report (Martin et al., 1993), direct measurements from drillcore (this study), and published regional data (Direen et al., 2005b). Modelled properties show a tripartite distribution. West of the branch of the

Table 4 Measured densities and magnetic susceptibilities of rock types in the Fowler Domain Rock type

Number of samples

Average density (×10−3 kg m−3 ) ± 1

Number of samples

Magnetic susceptibility (SI) (range; median)

Amphibolite Mafic granulite Felsic granitoids (e.g. Hiltaba Suite) Intermediate metagranitoids (e.g. Tunkillia Suite—inclusions in diorite, tonalite and plagiogranite)a S-type granitoids (e.g. Munjeela Suite) Felsic granulite Felsic gneiss Pelites/pelitic gneiss Mylonites

10 5 10 3

2.90 ± 0.08 2.75 ± 0.02 2.63 ± 0.02 2.90 ± 0.01

9 35 4 5

0.004–0.4; 0.1 0–0.25; 0.015 0.0005–0.15; 0.03 0.024–0.36; 0.1

NA 8 10 7 NA

NA 2.83 ± 0.12 2.76 ± 0.10 2.80 ± 0.10 NA

2 12 6 17 18

0.0023–0.025 0–0.25; 0.0012 0–0.25; 0.015 0.0005–0.008; 0.0015 0–0.006; 0.0012

a Density data incorporates some data for the Tunkillia Suite from Direen et al. (2005b); magnetic susceptibility data incorporates data from this study and that of Teasdale (1997).

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Fig. 8. Nundroo Traverse. (a) Bouguer Gravity response (␮m s−2 ). (b) TMI response (nT). (c) Forward modelled cross section showing major structural features and lithologies.

Coorabie Shear Zone (CSZ) which occurs between x ≈ 20 and 22 km (Figs. 3c and 8), amphibolites and felsic gneisses in drillholes have density values (2.72–2.85) × 10−3 kg m−3 , and very low susceptibilities (0.001–0.005 SI). The exception to this is an ovoid anomaly likely due to a denser, magnetised granitoid (2.80 × 10−3 kg m−3 , 0.02 SI) which cuts across the shear zone fabric (Fig. 3c), and may be part of the Tunkillia Suite. East of this, and west of the edge of the CSZ bounding the Nuyts Domain, values are uniformly higher, ranging between 2.82 and 2.93 × 10−3 kg m−3 , and 0.02–0.07 SI. Thickening of the low-density regolith over the shear zone has eliminated a strong gravity anomaly over this part of the CSZ, however, the strong magnetisation remains. The Nuyts Domain to the east of the CSZ has been modelled with a density of 2.65, and a lower magnetic susceptibility of 0.03 SI, commensurate with a felsic granitic composition (Table 4). The main magnetic contrasts in the model are at the eastern edge of the western CSZ (ca. 22 km: 0.017 SI); within the CSZ at ca. 27.5 km (0.03 SI) where orthopyroxene–hornblende bearing gneiss forms the wall rock to a mylonitic retrograde shear zone; and within the easternmost CSZ, around 35.5 km (0.019 SI), not far from the boundary with the Nuyts Domain. Perturbation of the dips of these 3 boundaries indicate the best model results are achieved by (1) a steep (ca. 80◦ ) dip of the western branch of the CSZ; (2) shallower (ca. 58◦ ) dip of the central main branch of the CSZ; and shallow (ca. 30◦ ) dip of the eastern, leading edge of the CSZ. This W–E fanning of dips across a major shear zone is typical of the transition from the internal to frontal zones of a duplex system (e.g. Mitra, 1986). Overall, the modelled solution is consistent with the results from the metamorphic analysis: middle to lower crustal high grade, dense, magnetised rocks form the hanging-wall of a steep reverse fault (frontal Coorabie SZ) overlie a lower-density, poorly magnetised footwall (Nuyts Domain). The immediate hanging-wall (west of the westernmost Coorabie SZ: Fig. 8) is passively overridden by eroded, lower-density, less-magnetised rocks, most likely representing earlier and higher crustal-level imbricates.

10. Discussion 10.1. Implications of the thermobarometric data from the Fowler Domain The Colona Block records the lowest grade metamorphism of all the blocks in the Fowler Domain (Fig. 9, Table 3), as well as evidence for significant, crustal-scale, shortening in the form of large-amplitude (␭ ≈ 30 km) folding (Fig. 3a). P–T calculations on paragneiss from drillhole COL20D in the core of this fold (Fig. 9), and thus at the locus of maximum exhumation, yield pressures of ca. 4–6 kbar, for both core and rim mineral compositions (Table 3). This implies exhumation of mid-crustal material (ca. 14–16 km depth) in the core of this fold. Sample COL20D 46.5 m shows evidence for two distinct amphibolite-grade mineral assemblages (Fig. 5), pointing to the possibility of polymetamorphism in the Colona Block. On the basis of the P–T calculations, both events were of similar metamorphic grade. The Barton Block consists of high-grade metapelites and amphibolites. The metapelitic rocks give pressures of ca. 7–9 kbar, and temperatures of ca. 650–790 ◦ C, whereas the mafic rocks give slightly higher average pressures of ca. 9–10 kbar, and temperatures of ca. 770–850 ◦ C (Table 3). This metamorphic difference between metapelitic and metabasic rocks may reflect retrograde strain partitioning. Mafic rocks, in general, are more competent than metapelitic rocks (e.g. Talbot and Sokoutis, 1992), and a number of studies have shown that retrograde strain is commonly partitioned into metapelites (Goodwin and Tikoff, 2002; Holdsworth et al., 2002; Talbot and Sokoutis, 1992). This is consistent with the petrology of Barton Block samples, whose metapelites commonly have a mylonitic appearance. If retrograde strain partitioning did occur, then the difference in P–T conditions between mafic and metapelitic rocks may provide clues about the retrograde P–T path. Following this logic, the retrograde P–T evolution of the Barton Block may have been dominated by comparatively high-pressure cooling, as opposed to decompression.

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Fig. 9. Summary of the metamorphic P–T data across the Fowler Domain. Note the higher average pressures and temperatures in the Nundroo and Barton Blocks compared with the Colona Block. Lines indicate modelled cross-sections.

Further constraints on the retrograde evolution of the Barton Block can be inferred from garnet zoning. Garnet zoning in sample BAC33 shows an increase in Ca and Fe from core to rim (Fig. 6i and k), while Mg contents decrease (Fig. 6e and h). This zoning is typical of high-temperature retrogression (e.g. Spear et al., 1990). The lack of Mn enrichment at the garnet rim suggests that the retrograde path was not associated with significant garnet resorption (Kohn and Spear, 2000). The lack of garnet retrogression also indicates that the retrograde path was associated with relatively high-pressure cooling, as opposed to decompression, since decompressional (exhumation) paths characteristically lead to garnet breakdown (Kohn and Spear, 2000; Wei and Powell, 2004). The inference that the retrograde path may have involved cooling at depth as opposed to decompression is consistent with the rimward Ca-enrichment (Spear, 1993), and also with the P–T calculations from the Barton Block which show comparatively little difference between core and rim pressure estimates. This implied P–T path has important implications, since it implies that a separate event to was needed to exhume the high-grade rocks of the Barton Block. Peak metamorphism in mafic granulites of the Nundroo Block records the highest estimated P–T conditions in the Fowler Domain (Fig. 9). P–T conditions obtained from metapelitic rocks from the Nundroo Block record pressures ranging from ca. 6 to 8.5 kbar, and temperatures ranging from ca. 660 to 760 ◦ C, while those of mafic compositions give average pressures ranging from ca. 8 to

9.5 kbar, and temperatures ranging from ca. 780 to 860 ◦ C (Table 3). If this difference in P–T conditions between mafic and metapelitic rocks reflects retrograde strain partitioning as argued for the Barton Block, then the retrograde P–T evolution of the Nundroo Block also appears to be dominated by cooling at depth as opposed to exhumation. This is consistent with the small difference in core and rim P–T conditions and the lack of garnet resorption in the high-grade parts of the Nundroo Block. As with the Barton Block, these relationships suggest relatively late tectonic exhumation of the Nundroo Block. The lower grade conditions recorded in samples from NDR3–5 may be related to different processes. NDR5 comes from east of the highest-grade part of the Nundroo Block, and is separated from it by shear zones with an inferred southeast-directed transport direction (Fig. 8). Viewed in this context, the lower P–T conditions of NDR5 may reflect a mid-crustal expression of the Nundroo metamorphism. The lower grade NDR3 comes from within the high-grade core of the Nundroo Block, and may represent a mid-crustal expression of retrograde metamorphism related to the exhumation of the core of the Nundroo Block. 10.2. Implications of geochronological data Chemical dating of monazites across the Fowler Domain, combined with existing geochronological data, highlights apparent

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277

Fig. 10. Summary of the geochronological data across the Fowler Domain, from this and previous studies. Note the systematic difference in ages across the Fowler Belt, in general, younging eastwards.

separate evolutionary histories for the Colona, Barton and Nundroo Blocks (Fig. 10). Three metapelites containing monazite from the COL20D drillhole yield U–Th–Pb ages of 1638 ± 15, 1608 ± 13, and 1648 ± 15 Ma (Table 2). Since the monazite forms part of the metapelitic mineral assemblage, the ages are interpreted to reflect the timing of metamorphism. The two oldest ages are within error, combining to give a mean apparent metamorphic age of 1643 ± 11 Ma. The youngest age comes from COL20D 46.5 m, which contains a well-developed secondary garnet–plagioclase–biotite–muscovite-bearing mineral assemblage. The younger age may therefore reflect partial or complete resetting of ca. 1643 Ma monazite by a younger event. The age obtained from COL20D 46.5 m is within error of the ages obtained from the Barton Block, suggesting the metamorphic overprint in the Colona Block may be associated with tectonism recorded in the Barton Block. Assuming the crustal scale folding in the Colona Block

(Fig. 3a) is associated with regional metamorphism, the timing of that deformation probably occurred between ca. 1640 and 1600 Ma. The two metapelites from the Barton Block (BAC20 and BAC33) give unimodal age populations with mean ages 1606 ± 17 and 1595 ± 17 Ma, respectively (Table 2). These ages are within error, and are interpreted to reflect the timing of regional metamorphism in the Barton Block. The similarity in the ages obtained from the Barton Block and the age of reworking obtained from COL20D 46.5 m suggests that the western Fowler Domain may have undergone a regional medium to high-grade tectonothermal thermal event at around 1600 Ma. In the Barton Block, this event was associated with high-grade lower crustal metamorphism that effectively obliterated the record of any earlier event, whereas in the Colona Block, there was a less intense mid-crustal metamorphic response that led to partial replacement of ca. 1640 Ma assemblages, and the observed polymetamorphic parageneses.

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The structural trends in the Barton Block broadly parallel the crustal-scale fold trace in the Colona Block. If the structural trends in the Barton Block relate to ca. 1600 Ma tectonism, then the regionalscale folding in the Colona Block may also be contemporaneous. This implies the existence of ca. NW–SE directed shortening in the western Fowler Domain at ca. 1600 Ma. This is consistent with the findings of Direen et al. (2005b) and Swain et al. (2005a). No age data were obtained from the Central Block in this study. Previous work has recorded concordant SHRIMP U–Pb zircon ages of granites in three locations in this block (Fig. 10). Crystallisation ages of 1672 ± 12, 1567 ± 14 and 1584 ± 9 Ma were obtained for granitoids in the White Gin Rockhole, Unnamed Rockhole, and drillhole NDR13, respectively (Fig. 10; Teasdale, 1997). Granite at White Gin Rockhole is included within the Tunkillia Suite (Ferris and Schwarz, 2004), whereas younger granites at Unnamed Rockhole and NDR13 may be correlatives of the voluminous Hiltaba Suite, which intruded much of the Gawler Craton in the interval 1600–1575 Ma (Daly et al., 1998). Importantly, in the context of this study, all the dated granites in the Central Block are deformed (Teasdale, 1997). This implies that at least some of the regional deformation in the Central Block must be younger than ca. 1570 Ma. The samples analysed within the Nundroo Block record unimodal age populations of 1471 ± 14 and 1557 ± 15 Ma from a garnet–biotite–plagioclase metapelite (NDR1) and garnet–biotite–sillimanite–plagioclase–K-feldspar metapelitic gneiss (NDR5) respectively. The age of 1557 ± 15 Ma obtained from NDR5 is consistent with timing of peak metamorphism (1543 ± 9 Ma) recorded by SHRIMP U–Pb analysis of metamorphic zircons in mafic granulite in NDDH2 (Fig. 10; Daly et al., 1998). The age obtained from NDR5 in this study is older than ages reported by Swain et al. (2005a) from the same drill hole (Fig. 10). Swain et al. (2005a) obtained an age of 1516 ± 18 Ma from monazite inclusions in garnet and 1468 ± 12 Ma from monazites in the strongly deformed matrix. The sample analysed by Swain et al. (2005a) is structurally reworked, resulting in extensive fracturing of garnets and the development of a secondary biotite–sillimanite-bearing foliation. Conceivably the age of 1516 ± 18 Ma reflects partial restting of older monazite during matrix recrystallisation at ca. 1470 Ma. In contrast, the sample analysed in this study shows no evidence for structural reworking, and the age of 1557 ± 15 Ma is interpreted to correspond to the timing of the regional peak metamorphism in the Nundroo Block. The age of 1471 ± 14 Ma obtained from NDR1 requires some evaluation. Texturally, the sample appears to be an equilibrated peak metamorphic assemblage, with the comparatively coarse-grained paragenesis showing little evidence for retrogression. In this case, the preferred interpretation is that 1471 ± 14 Ma is the timing of regional peak metamorphism in the Nundroo Block. This would imply that there are two high-grade metamorphic events in the Nundroo Block, one at ca. 1545 Ma and one at ca. 1470 Ma. However, the silicate mineral chemistry in NDR1 suggests that the sample has undergone significant post-peak modification. Garnets in the sample are strongly zoned in Ca and Mg, with the Mg zoning in particular pointing to down temperature re-equilibration. P–T calculations using garnet rim compositions together with the unzoned matrix minerals suggest the matrix assemblage re-equilibrated at around 615 ◦ C and 6 kbar (Table 3). These conditions are substantially lower than the recorded peak and retrograde conditions recorded elsewhere in the Nundroo Block (Table 3). Significantly, the majority of the analysed monazites in NDR1 come from the reequilibrated matrix. This suggests that the age of 1471 ± 14 Ma may reflect the timing of metamorphic re-equilibration at mid-crustal levels. This age is identical to EMPA ages obtained from shear zone assemblages associated with the terrain-scale Coorabie Shear Zone,

which forms the eastern boundary of the Nundroo Block (Swain et al., 2005a). Age constraints on shear zone movement and cooling histories within the Fowler Domain have also been derived independently by a number of workers, using a variety of thermochronometers (Fraser and Lyons, 2006; Swain et al., 2005a). EMPA chemical dating of monazites from two locations on the Tallacootra Shear Zone give ages of ca. 1680 Ma for the inferred timing of mylonitic deformation (Swain et al., 2005a; Fig. 10). 40 Ar/39 Ar cooling ages of biotite and muscovite from within the Tallacootra Shear Zone give ages of ca. 1450 Ma, suggesting cooling of the shear zone through ca. 300 ◦ C at that time (e.g. Dunlap et al., 1995). If the cooling is linked to exhumation (e.g. Dunlap and Teyssier, 1995) then the 40 Ar/39 Ar ages suggest that the Tallacootra Shear Zone was reactivated at around 1450 Ma. The ca. 1450 Ma reactivation of the Tallacootra Shear Zone is similar to EMPA monazite ages of ca. 1460 Ma obtained from the Coorabie Shear Zone that defines the eastern margin of the Nundroo Block (Swain et al., 2005a). It is also consistent with the EMPA monazite age of 1471 ± 14 Ma obtained from a retrograde shear fabric within the Nundroo Block (this study). These ca. 1470–1450 Ma ages associated with the terrain-scale shear zones in the Fowler Domain are significantly younger than the inferred timing of regional deformation and metamorphism in the crustal blocks between the shear zones e.g. the Barton and Nundroo Blocks. These retrograde evolutions indicate that these blocks cooled at depth, and therefore must have required a younger (1470–1450 Ma) major event to exhume them from the lower crust. The macroscopic pattern of shear zones in the Fowler Domain is similar to that of major transpressional terrains, which have been demonstrated to achieve these kinds of evolutionary patterns (e.g. Goscombe et al., 2003; Tavarnelli and Holdsworth, 1999; Teyssier and Tikoff, 1998). 10.3. Implications of potential field modelling of structural architecture In using magnetic data to place constraints on geological processes, it is important to consider what the measured magnetic response represents. Magnetic susceptibility of a rock is approximately proportional to its magnetite content (Clark, 1997). For rocks with magnetite contents <10%, there is a simple linear relationship between the magnetite content of the rocks and magnetic susceptibility, and hence the amplitudes of regional magnetic anomalies associated with these rocks (Clark, 1997). Since the observed magnetite contents of rocks in the Fowler Domain are generally ca. 1–5%, it can be assumed that this relationship holds. Magnetite content is affected by a number of factors such as total iron content, oxidation state, and major element chemistry (Clark, 1997; Grant, 1985). Apart from lithological variation, one of the main causes of increased magnetite content is the progressive breakdown of hydrous, iron-rich (Fe, Mg) silicates such as biotite and amphibole toward magnesium-rich orthosilicates such as pyroxene (Grant, 1985; Skilbrei et al., 1991) during regional metamorphism. These breakdown reactions follow the general pattern of: Hydrous (Fe, Mg) Al-silicates ± SiO2 ± O2 ↔ K-feldspar + (Fe, Mg) silicates ± magnetite + H2 O

(1)

This is often seen to occur in rocks that have undergone prograde transition from amphibolite to granulite facies (Grant, 1985; Skilbrei et al., 1991). These observations are consistent with thermodynamic modelling of metamorphic mineral assemblages

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Fig. 11. Forward modelled structural traverses for the Barton (a) and Nundroo (b) blocks, showing metamorphic field gradients, timing of structural fabic formation from geochronology, and implied net sense of motion on major structures. (a) Geochronological data collected in this study. (b) Data from previous studies in the area (Teasdale, 1997).

which show that in Fe3+ -bearing metapelitic bulk compositions, increasing metamorphic grade leads to the stabilisation of magnetite (White et al., 2000). If this general relationship between increasing metamorphic intensity and magnetite content is combined with the relationship between magnetic susceptibility and magnetite content, it follows that in a metapelitic rock unit of a given bulk composition, higher magnetic anomalies map rocks of higher metamorphic grade. Conversely, exhumation of material within shear zones may lead to demagnetisation by consumption of magnetite in retrograde metamorphic reactions.

Consequently, crustal scale fault structures that juxtapose metapelites with significantly different metamorphic grades may show a magnetic anomaly corresponding to the metamorphic field gradient. From this, a sense of movement on these fault structures may be deduced, with the more magnetised block representing the more deeply exhumed block. Using these principles, multiple generations of faults (and shear zones) in the Fowler Domain can be identified, and the relative displacements and transport directions inferred. The sense of dip-slip movement has thus been inferred by change in metamorphic grade or magnetic susceptibility, where

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more magnetised, higher-grade metapelitic panels are differentially exhumed and juxtaposed against lower-grade panels. The Barton traverse is inferred to show both strike-slip and dipslip movement on shear zones (Fig. 11a). Interpretation of regional aeromagnetic imagery (Fig. 3b) suggests that that the strike-slip fault segments in the Tallacootra SZ have sinistral displacements; however, the net dip-slip movement on these segments is ambiguous, due to the distribution of P–T information. P–T data is sufficient to infer a significant dip-slip displacement on several segments in the TSZ, however. The fault at ca. 12 km along the traverse, for example, juxtaposes high-pressure (ca. 10 kbar) rocks in BAC17 to the east of the shear zone with lower pressure (ca. 7 kbar) rocks to the west, implying some 9 km of net east-side up displacement. Similarly, at ca. 20 km, a complex shear zone juxtaposes rocks of ca. 7 and 9 kbar, indicating around 6 km of west-side up motion. The Nundroo Traverse crosses a number of discrete, curvilinear magnetic lows that truncate structural trends. As these lows appear to be magnetite-destructive, relative to the wall rocks, we infer that they are retrogressive shear zones (Fig. 3c and 8). Forward modelling implies that these shear zones have moderate to steep northwest dips (Fig. 8). Strike-slip movement on the shear zones has been inferred where the wall rocks do not display a large difference in magnetic susceptibility or metamorphic grade. The sense of movement along shear zones intersected on the Nundroo Traverse has been interpreted as having a large component of sinistral displacement (see also Daly et al., 1998; Teasdale, 1997). This is based on the sense of large-scale drag of foliations into the shear zones, and also on the strike length of the shear zones (between 100 and 300 km: Fig. 3c), which strongly suggests a strike-slip dominated regime. The strike length of these shear zones implies significant displacement (e.g. D’Lemos et al., 1997), in addition to the substantial vertical displacements implied by the metamorphic field gradients on some of them. Differential exhumation of crustal blocks is best expressed on the eastern margin of the Nundroo Block, where highly magnetic lower crustal granulites that formed at depths of around 30 km (ca. 9 kbar) are now juxtaposed against mid-crustal rocks (ca. 6 kbar) to the east at around 30 km along the traverse (Fig. 11b), suggesting around 9–10 km of west-side up, east vergent exhumation. The combined evidence from the sections suggests that the shear zones in the Fowler Domain have accommodated significant strike-slip and dip-slip movement, consistent with regional-scale transpression. 10.4. Time-integrated tectonic evolution of the Fowler Domain The protracted history revealed by our integrated analysis of the metamorphism and architecture (Figs. 10 and 11) of the Fowler Domain is consistent with the tectonic model for the western Gawler Craton proposed by Direen et al. (2005b). In that model, at ca. 1450 Ma, the eastern edge of Fowler Domain was exhumed and stabilised as the leading imbricate edge of a left-lateral transpressional stack which included the Archean rocks of the Christie Domain. Direen et al. (2005b) termed this event the Coorabie Orogeny, and on the basis of available geochronological data, suggested it occurred over the interval 1550–1450 Ma. However there is growing evidence for a major high-grade event in the Gawler Craton between 1580 and 1550 Ma (Hand et al., 2007; Fanning et al., 2007; Holm, 2004). Hand et al. (2007) referred to this as the Kararan Orogeny. Based on Ar–Ar and monazite EMPA data (Fraser and Lyons, 2006; Swain et al., 2005a), we suggest that the Coorabie Orogeny has a more restricted interval (ca. 1470–1450 Ma). The data obtained in this study, coupled with existing data, point to a complex evolution of the Fowler Domain, with different blocks having different tectonic histories (Fig. 11). The oldest metamorphic ages (ca. 1680 Ma) come from the Tallacootra Shear zone in

the northern Barton Block which reworks ca. 2450 Ma rocks belonging to the Mulgathing Complex and the ca. 1690–1670 Ma Tunkillia Suite. The age data obtained in this study imply that the Barton Block underwent high P–T reworking at around 1600 Ma (Fig. 11). The Colona Block in the western Fowler Domain appears to have undergone mid-crustal prograde metamorphism at ca. 1640 Ma, and reworking at ca. 1600 Ma, coincident with reworking of the Barton Block. The Nundroo Block underwent granulite-grade lower crustal deformation at ca. 1545 Ma (Fig. 11). Conceivably, the timing of this event also corresponds to the timing of deformation in the Central Block, which is constrained to be younger than ca. 1570 Ma (Teasdale, 1997). The juxtaposition of these contrasting crustal blocks is consistent with exhumation linked to terrain-scale transpression between ca. 1470 and 1450 Ma (Direen et al., 2005a; Swain et al., 2005a; Fraser and Lyons, 2006). The development of this transpressional system is in all likelihood unrelated to the events that metamorphosed the protoliths of the exhumed crustal blocks. The recognition of relatively late reworking of the northern and western Gawler Craton during the Coorabie Orogeny has important consequences for regional tectonic models. Firstly, as highlighted by Direen et al. (2005b), this event could have dissected, and subsequently translated elements of the Kimban Orogen (Hoek and Schaefer, 1998; Vassallo and Wilson, 2002; Hand et al., 2007). This appearance of “cryptic” Kimban-aged structures (Teasdale, 1997; Tomkins et al., 2004) in the western Gawler Craton has not been incorporated in models seeking to reconstruct the position or tectonic setting of the Kimban Orogen (e.g. Betts and Giles, 2006; Dawson et al., 2002; Giles et al., 2004). Moreover, the apparently pronounced “bend” in the Paleoproterozoic structures of the Gawler Craton, which is incorporated into many models as a major salient during early orogenic events (e.g. Betts and Giles, 2006; Dawson et al., 2002) is likely a young feature related to the Coorabie orogenic reworking and translation. There is thus no need for kinematically awkward salients within models seeking to reconstruct the Paleoproterozoic tectonic setting. Following this logic, restoration of major left-lateral translations within the Fowler Domain leads to palinspastic restoration of the Tunkillia Suite as a much more linear feature than currently apparent. The Tunkillia Suite is a critical element in many tectonic models, although datasets allowing unambiguous characterisation of the tectonic setting of the Tunkillia Suite, and what controlled this melting, have not yet been published (cf. Betts and Giles, 2006). Terminal exhumation during the Coorabie Orogeny will have removed much of the upper and mid-crustal expression of the Tunkillia magmatic system within the Fowler Domain. The occurrence of correlative Archean crustal blocks on both sides of the Fowler Domain (Mulgathing Complex to the NW, Sleaford Complex to the SE: Swain et al., 2005b) suggests that the Fowler Domain was relatively narrow and its metapelite–BIF–metabasalt association suggests a relatively deep basin developed on thinned continental crust: a failed rift? The 1690–1670 Ma Tunkillia Suite was therefore very likely intruded into this basin on a substrate of continental crust. This would also permit interpretation of the Tunkillia Suite as a back-arc or intracontinental suite, not just an arc suite, as proposed by Betts and Giles (2006).

11. Conclusion—towards a time-integrated evolution of the Gawler Craton The combination of thermobarometric and geochronologic data with potential field modelling has enabled the development of a

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time-integrated tectonic model for the Fowler Domain in the Western Gawler Craton. The conclusions of this study are: (1) The western Fowler Domain underwent a major tectonic thermal event at ca. 1600 Ma, resulting in regional lower crustal high-grade metamorphism in the Barton Block and mid-crustal amphibolite-grade metamorphism in the Colona Block, which defines the westernmost Fowler Domain. This event at 1600 Ma overprinted an earlier metamorphic system in the Colona Block that occurred at 1640 Ma, and was associated with around significant crustal shortening that produced large-scale folds with amphibolite facies cores. (2) Peak metamorphism in the eastern Fowler Domain occurred at around 1545 Ma, and is recorded by the formation of regional lower crustal granulite-grade mineral assemblages. In the central Fowler Domain, deformation of 1570 Ma granites suggests that the 1545 Ma event may have also affected the central Fowler Domain. (3) Retrograde P–T paths of rocks in the Barton and Nundroo Blocks suggest that both terrains cooled at depth following peak metamorphism. (4) Age data and the macroscopic structural patterns of the shear zone systems that bound the crustal blocks in the Fowler Domain suggest that exhumation of the lower crustal domains occurred during the latest stages of transpressional reworking in the Coorabie Orogeny (1470–1450 Ma). (5) Geophysical modelling indicates that the anastomosing pattern of terrain-scale shear zones that define the architecture of the Fowler Domain have steep dips. These geometries are consistent with a combination of early SE-directed thrusting (in the current coordinates); sinistral strike-slip translation; and late SE-directed exhumation. All of these characteristics are consistent with the youngest event in the Fowler Domain being of a transpressional character. (6) The late transpression in the Fowler Domain is not required to be genetically related to the events that shaped the geological character of the exhumed crustal blocks. (7) The combination of petrological, geochronological and geophysical approaches offers a useful way to construct timeintegrated tectonic evolution models of poorly exposed basement terrains. Acknowledgements We would like to thank Angus Netting and John Terlet at Adelaide Microscopy, and Mike Schwarz and Sue Daly from Primary Industries and Resources, South Australia (PIRSA). Jon Teasdale collected many of the original samples used in this study. Financial and logistical support for this project was provided by the Australian Research Council, and PIRSA. JLT was the recipient of the Bruce Webb Scholarship from the University of Adelaide. Reviews by Tony Meixner and Mike Dentith helped improve the manuscript. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.precamres.2007.05.006. References Berman, R.G., 1990. Mixing properties of Ca Mg Fe Mn garnets. American Mineralogist 75, 328–344. Betts, P.G., Giles, D., 2006. The 1800–1100 Ma tectonic evolution of Australia. Precambrian Research 144, 92–125. Blissett, A.H., Creaser, R.A., Daly, S.J., Flint, R.B., Parker, A.J., 1993. Gawler range volcanics. In: Drexel, J.F., Preiss, W.V., Parker, A.J. (Eds.), The Geology of South

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