Geochronological problems related to polymetamorphism in the Limpopo Complex, South Africa

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Geochronological problems related to polymetamorphism in the Limpopo Complex, South Africa D.D. Van Reenen a , R. Boshoff a,⁎, C.A. Smit a , L.L. Perchuk a,b,c , J.D. Kramers d , S. McCourt e , R.A. Armstrong f a

Department of Geology, University of Johannesburg, P.O. Box 524, Auckland Park, 2006, Johannesburg, South Africa b Department of Petrology, Geological Faculty, Moscow State University, Vorobievy Gory, Moscow, 119899, Russia c Institute of Experimental Mineralogy, Russian Academy of Sciences, Chernogologvka, Moscow district, 142432, Russia d Isotope Laboratory for Geology, University of Bern, Erlachstrasse 9a, Bern, Switzerland e School of Geological Sciences, University of KwaZulu-Natal Durban, South Africa f Research School of Earth Sciences, The Australian National University, Mills Road, Canberra, 0200, A.C.T., Australia Received 26 October 2006; accepted 15 January 2008 Available online 21 February 2008

Abstract The integration of new and published geochronologic data with structural, magmatic/anatectic and pressure–temperature (P–T) process information allow the recognition of high-grade polymetamorphic granulites and associated high-grade shear zones in the Central Zone (CZ) of the Limpopo high-grade terrain in South Africa. Together, these two important features reflect a major high-grade D3/M3 event at ~ 2.02 Ga that overprinted the N2.63 Ga high-grade Neoarchaean D2/M2 event, characterized by SW-plunging sheath folds. These major D2/M2 folds developed before ~ 2.63 Ga based on U–Pb zircon age data for precursors to leucocratic anatectic gneisses that cut the high-grade gneissic fabric. The D3/M3 shear event is accurately dated by U–Pb monazite (2017.1 ± 2.8 Ma) and PbSL garnet (2023 ± 11 Ma) age data obtained from syntectonic anatectic material, and from sheared metapelitic gneisses that were completely reworked during the high-grade shear event. The shear event was preceded by isobaric heating (P = ~6 kbar and T = ~ 670–780 °C), which resulted in the widespread formation of polymetamorphic granulites. Many efforts to date high-grade gneisses from the CZ using PbSL garnet dating resulted in a large spread of ages (~ 2.0–2.6 Ga) that reflect the polymetamorphic nature of these complexly deformed high-grade rocks. © 2008 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. Keywords: Limpopo high-grade terrain; PbSL and U–Pb age dating; Polymetamorphic granulites; High-temperature shear zones; Southern Africa

1. Introduction The Limpopo high-grade terrain (HGT) of Southern Africa (Fig. 1), situated between the Archaean Kaapvaal Craton (KC) to the south, and the Zimbabwe Craton (ZC) to the north, is characterized by a tectono-metamorphic evolution that extended from ~ 3.34 Ga to ~ 2.0 Ga (e.g. Van Reenen et al., 1992; Kramers et al., 2006). The high-grade gneiss terrain is internally subdivided into Southern (SMZ) and Northern (NMZ) Marginal Zones that are separated from a Central Zone (CZ) by ~ 2.0 Ga inward-dipping mylonitic strike-slip shear zones: the Palala ⁎ Corresponding author. Tel.: +27 11 559 3852; fax: +27 11 559 43361. E-mail address: [email protected] (R. Boshoff).

Shear Zone in the south, and the Magagophate-Triangle Shear Zone in the north (e.g. Kramers et al., 2006) (Fig. 1). The two Marginal Zones are interpreted to be the lower crustal equivalents of the granite-greenstone terrains of the adjacent cratons (Kreissig et al., 2000), The rocks in these Marginal Zones underwent a single granulite facies metamorphism in the Neoarchaean (Kreissig et al., 2001; Blenkinsop et al., 2004) and are separated from the cratons by ~ 2.6 Ga inward-dipping gneissic dip-slip shear zones (Fig. 1) referred to as the North Limpopo Thrust Zone and the Hout River Shear Zone (Roering et al., 1992; Kramers et al., 2006). Whereas all researchers agree on the tectono-metamorphic history of the marginal zones, there is little agreement on the nature and timing of tectono-metamorphic events that affected

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Fig. 1. Subdivision of the Limpopo high-grade terrain into a Central Zone (CZ), Northern Marginal Zone (NMZ), and a Southern Marginal Zone (SMZ). Major bounding shear zones are shown, and the localities of the west Alldays and Musina areas (modified after Van Reenen et al., 1990).

the CZ. One group of researchers (e.g. Van Reenen et al., 1987, 1990; McCourt and Vearncombe 1992; Roering et al., 1992; McCourt and Armstrong, 1998) argue that the entire Limpopo HGT formed during a single high-grade crustal-thickening event that pre-dated 2.6 Ga; based on the crystallisation age of uplift related granitoids. Another group (e.g. Barton et al., 1994, 2006; Kamber et al., 1995a,b; Holzer et al., 1995, 1998; Schäller et al., 1999) is of the opinion that the majority of the high-grade gneisses in the CZ reflect a single tectono-metamorphic event at ~ 2.0 Ga, based on Pb–Pb stepwise age data for minerals from these gneisses. Hoffman et al. (1998) and Kröner et al. (1998, 1999), on the basis of U–Pb SHRIMP and SIMS age data for the protoliths of various ortho-gneisses, suggested that although the main folding event occurred in the Neoarchaean, peak metamorphic conditions in the CZ were reached at ~ 2.0 Ga. We believe that the controversy concerning the timing and nature of high-grade events that affected the CZ is because previous studies of the high-grade fabric-forming events have not integrated precise geochronology with detailed pressure (P)–temperature (T) process information (P–T-t paths). Boshoff et al. (2006) adopted this approach in the west Alldays area of the CZ (Fig. 2) and as a result were able to demonstrate that a system of N–S trending high-grade ductile shear zones were related to a tectono-metamorphic event at ~ 2.02 Ga, superimposed onto the regional Neoarchaean sheath-type fold pattern (Roering et al., 1992). Perchuk et al. (2006a,b, in press) subsequently showed that the ~ 2.0 Ga shear deformational event was immediately preceded by an isobaric-heating event that resulted in the widespread development of polymetamorphic granulites throughout the CZ. The aim of this paper is to discuss new and published age data for rocks from throughout the CZ in South Africa, and to demonstrate that by integrating these data

with structural, petrologic, and magmatic/anatectic data; it is possible to argue that the entire CZ was affected by two temporally distinct high-grade events, one in the Neoarchaean and the other in the Palaeoproterozoic. This paper also contributes to petrologic-structural-geochronologic studies on African and related terranes (e.g. Agbossoumondé et al., 2007; Tucker et al., 2007; Goscombe and Gray, 2008). 2. Geological setting The CZ of the Limpopo HGT comprises a large variety of complexly deformed high-grade gneisses of meta-sedimentary and meta-igneous character. These include the supracrustal rocks of the Beit Bridge Complex (BBC) comprising marble and calcsilicate gneisses, metaquartzite, banded iron formation, mafic gneisses, and various garnet-biotite-bearing paragneisses. Metapelitic gneisses are a minor component of the BBC. The rocks of the BBC are intruded (e.g. Bahnemann, 1972; Brandl, 1983; Hofmann et al., 1998; Kröner et al., 1998) by a number of units that includes meta-gabbro and anorthosite of the layered Musina Suite, tonalitic to trondhjemitic and granodioritic (TTG) gneisses (e.g. the Sand River and Alldays Gneiss Suites), and leucocratic quartzofeldspathic gneisses. A distinctive garnetbearing pink to orange–brown weathering type of these quartzofeldspathic gneisses is referred to as the Singelele Gneiss (Söhnge, 1945; Bahnemann, 1972; Brandl, 1983; Watkeys, 1984). Another important group of intrusive rocks are granitic in composition and typically lack penetrative deformation fabrics. These include the Bulai granitic pluton (Watkeys et al., 1983) north and northwest of Musina (Figs. 1 and 2), as well as small and undeformed veins and melt patches of granitic composition (e.g. Jaeckel et al., 1997) that destroy the high-grade fabric of the

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Fig. 2. Regional structural pattern of the CZ comprising mainly of sheath folds (the D2/M2 Ha-Tshansi, Bellevue and Avoca sheath folds), and large N–S trending foldlike structures (e.g. the Campbell and Baklykraal structures) (modified after Van Kal, 2004). Localities (1–6) refer to the samples discussed in the text.

rocks in which they are developed. The age of the magmatic events that produced these intrusions and the relationship between the intrusions and specific fabric-forming (S) and metamorphic (M) events, is crucial to understanding the tectonometamorphic (D/M) history of this extremely complex highgrade polymetamorphic terrain. Geological maps of the CZ are characterized by three distinct, regional-scale, structural patterns (Figs. 1 and 2): (i) large (kmscale) fold structures that deform gneissic banding and have near-circular outcrop patterns and an intense SW-plunging linear fabric (e.g. the Ha-Tshansi and Bellevue structures near Musina, and the Avoca structure west of Alldays). These structures are interpreted as regional-scale sheath folds (Roering et al., 1992; Van Kal, 2004), (ii) Kilometre scale N–S-trending structural domains, (e.g. the Campbell structure near Musina, and the Baklykraal structure west of Alldays) referred to in the literature as cross-folds (Pienaar, 1985; Pretorius, 1986; Feldtmann, 1996), (iii) a N20 km-wide ENE–WSW trending zone of subparallel outcrop termed the Tshipise Straightening Zone (Söhnge, 1945; Bahnemann, 1972; Horrocks, 1983; Watkeys, 1984) that helps define the southern boundary of the CZ. This complex deformational pattern has been interpreted to reflect a single high-grade tectono-metamorphic event, either in the Neoarchaean (e.g. Roering et al., 1992; McCourt and Armstrong, 1998) or in the Palaeoproterozoic (Holzer et al., 1998; Schäller et al., 1999). The notion of a single major event is supported by the observation that regional-scale sheath folds within both the CZ (Roering et al., 1992) and the Tshipise Straightening Zone (Horrocks, 1983) share the same geometry including SW plunge and top-to-the-NE vergence. In addition, the high-grade gneissic fabric of the N–S trending Campbell structure in the Musina area (Fig. 2) can be traced into the

Tshipise Straightening Zone without disruption, implying a single continuous foliation and thus a single deformation event. However, a recent study (Boshoff et al., 2006) of the N–S trending Baklykraal structure in the west Alldays area of the CZ (Fig. 2) concluded that this was not a cross-fold as previously described but a high-grade ductile shear zone superimposed on the Neoarchaean fold pattern of the area. Precise geochronology indicated that this shear zone system formed in the Palaeoproterozoic and that in the Alldays area the sheath-folds and the N– S cross-folds represent temporally distinct deformation events. 3. Published age data from the CZ of the Limpopo HGT 3.1. Introduction U–Pb age data have been used to determine the time of crystallisation of magmatic/anatectic rocks in the CZ and the minimum age of the deformation fabrics into which they intrude (e.g. McCourt and Armstrong, 1998; Kröner et al., 1999), while Pb–Pb step-leaching (PbSL) age data and Ar/Ar ages determined from a variety of metamorphic minerals (garnet, titanite, staurolite, kyanite, sillimanite, amphibole, etc.) in highgrade paragneisses have been used to directly date metamorphic events (e.g. Holzer et al., 1998). The interpretation of previously published age data by the relevant authors for the CZ is next discussed with reference to both magmatic/anatectic and metamorphic events. 3.2. Age data relating to magmatic/anatectic events The earliest deformation (Mesoarchaean) D1/M1, has only been recognized in the Musina area. It is preserved as compositional

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banding in both the SRG and the infolded supracrustal rocks of the BBC (e.g. Hofmann et al., 1998). D1/M1 has been dated at ~3.3 Ga (e.g. Kröner et al., 1999). Precursors to the tonalitic to trondhjemitic and granodioritic (TTG) Alldays and Verbaard gneisses intruded into the supracrustal gneisses of the BBC during the Neoarchaean (~2.65–2.64 Ga, Table 1; Kröner et al., 1999). These TTG rocks were subsequently metamorphosed and deformed together with their host supracrustal gneisses during D2/M2. The timing of D2/M2 is constrained by the magmatic event that produced the precursors to a variety of foliated but not banded, leucocratic gneisses over the time period ~2.68–2.56 Ga (Table 1). These distinct quartzofeldspathic gneisses are grouped under the general term Singelele-type gneisses (Table 1) (e.g. Söhnge, 1945; Bahnemann, 1972, Fripp et al., 1979; Kröner et al., 1999), while a specific garnet-bearing pink to orange–brown weathering variety at the type locality of Singelele Hill near Musina has been named the Singelele Gneiss (Söhnge, 1945; Bahnemann, 1972). The Bulai granitoid pluton (Fig. 2) intrudes gneisses of the BBC and the Singelele Gneiss and contains xenoliths of migmatised metapelitic gneiss of the BBC (Watkeys, 1984; Holzer, 1995). The unequivocal field relationship between the Bulai pluton and its host rocks makes the Bulai pluton an important time marker in the CZ (e.g. Watkeys, 1984; Barton and Van Reenen, 1992; Roering et al., 1992; McCourt and Armstrong, 1998; Holzer et al., 1998). Zircon from the Bulai pluton have yielded U–Pb data which provide ages of 2605 ± 2 Ma and 2572 ± 4 Ma for the enderbitic and granitic phases respectively (Barton et al., 1994). These ages are interpreted to indicate the crystallisation age of the Bulai pluton and thus the maximum age of the crustal-thickening event responsible for the deformation fabrics in

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the metapelitic xenoliths and, by implication, the wall rocks of the pluton (McCourt and Armstrong; 1998). Hofmann et al. (1998) and Kröner et al. (1998, 1999) argue that single zircon ages of 2569± 0.3 Ma and 2582.5± 0.3 Ma for two compositionally different samples of quartzofeldspathic gneiss from the Musina area, can be used to constrain the main deformational event (termed D2) in the CZ to ~2.58 Ga. This conclusion is based on the interpretation of field evidence (Hofmann et al., 1998) that the emplacement of precursors to the Singeleletype gneisses took place syntectonically with the main D2 deformation event, thus constraining the timing for both D2 and M2 (Kröner et al., 1998, 1999). Kröner et al. (1999) present evidence that a slightly foliated garnet-bearing quartzofeldspathic gneiss (Fig. 2, locality 4), mapped as Singelele Gneiss by Holzer, (1995) cuts the high-grade gneissic fabric (garnet-cordieritesillimanite-biotite-k-feldspar-quartz-plagioclase) of a metapelitic gneiss. Single zircon grains from this slightly foliated gneiss (sample TR136) yielded a precise U–Pb age of 2623 ± 6 Ma. The same authors also published a precise U–Pb zircon age of 2681 ± 8 Ma (sample TR130) for a sample of the Singelele Gneiss ~15 km SE of Musina. These data strongly suggests that the Neoarchaean event termed D2/M2 probably commenced as early as ~2.68 Ga. Undeformed anatectic melt patches and veins of granitic composition are well-documented from the Musina area, and destroy the high-grade fabric of the rocks in which they are developed (Watkeys, 1984; Jaeckel et al., 1997; Holzer et al., 1998; Kröner et al., 1999). Zircon U–Pb dating of such melt patches and veins near Musina yielded a mean age of 2005.6 ± 4.4 Ma (Jaeckel et al., 1997). Kröner et al. (1999) link this event to peak metamorphic conditions in the CZ but do not present petrologic data to support their interpretation.

Table 1 Compilation of age data and field relations relating to post 3.0 Ga magmatic/anatectic events in the Central Zone Rock type SRG/Dorothy gneiss

Nature and field relations

Age (in billions of years)

Discussion

Precursors to these banded grey gneisses intruded rocks ~3.18–3.31 Lithologic layering preserved in the SRG and the infolded of the BBC (Hoffman et al., 1998) (e.g. Kröner et al., 1999) rocks of the BBC presents the earliest deformation recognized (D1/M1) Alldays and The banded grey gneisses are interfolded with, and intrude ~2.65–2.637 The well foliated Alldays and Verbaard gneisses were Verbaard gneisses rocks of the BBC, and the Dorothy gneiss (Hoffman et al., (e.g. Kröner et al., 1999) emplaced during a relatively short time-interval during the 1998) main D2/M2 event. The Verbaard Gneiss is often characterized by xenoliths of high-grade paragneisses. Singelele-type A variety of anatectic quartzofeldspathic gneisses that ~2.68–2.56 (e.g. Kröner Singelele-type gneisses are always less foliated than gneisses include the garnet-bearing pink to orange–brown et al., 1999) the high-grade paragneisses, which they intrude, and weathering rock termed the Singelele Gneiss (Söhnge, are considered to represent late-tectonic granitoids. 1945; Bahnemann, 1972). These slightly foliated rocks occur Researchers agree that these rocks represent the product of interlayered with rocks of the BBC and can always be shown more or less in situ melting (e.g. Fripp et al., 1979), during a to also intrude the main high-grade S2 gneissic fabric. very long-lived Neoarchaean high-grade D2/M2 event. The Singelele-type gneisses are considered to be the most important time markers for the D2/M2 high-grade event in the CZ. The Bulai Rocks of the Bulai Pluton intrude Singelele-type gneisses ~2.61–2.57 (Barton The almost undeformed Bulai Pluton is characterized by charnockitic to and paragneisses of the BBC, and often include xenoliths et al., 1994) numerous small (cm wide) ductile shear zones, and is an granitic Pluton of high-grade gneisses. important time marker for the D2/M2 high-grade tectonometamorphic event in the CZ. Development of These small and undeformed bodies destroy the high- ~2.01 (Jaeckel Small granitic melt patches documents the final melting granitic melt grade D2/M2 gneissic fabric of the rocks within which et al., 1997) episode related to the ~ 2.02 D3/M3 high-grade event. patches and veins they occur.

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In summary, age data related to magmatic/anatectic rocks in the CZ therefore reflect high-grade events at ~ 3.3 Ga (D1/M1); ~ 2.6 Ga (D2/M2), and ~ 2.0 Ga (D3/M3) (Hofmann et al., 1998; Kröner et al., 1998, 1999). 3.3. Age data relating to high-grade metamorphic events Holzer et al. (1998) produced a PbSL sillimanite-garnet isochron age of 2521 ± 4 Ma from metapelitic gneiss of the BBC near Musina and, without referencing published petrologic data, interpreted this age to reflect a distinct low-pressure-hightemperature Neoarchaean metamorphic event at that time, which they related to an anti-clockwise P–T evolution. Jaeckel et al. (1997) dated a near-spherical grain of metamorphic zircon from similar metapelitic gneiss and obtained a near-concordant age of 2575 ± 4 Ma, which they attributed to the same metamorphic event. The PbSL mineral age published by Holzer et al. (1998) is however some 50 My younger than the 2572 ± 4 Ma U–Pb zircon age for the emplacement of the Bulai pluton and their interpretation of the PbSL age is at variance with the well-documented field relationships between the Bulai pluton and the metapelitic rocks of the BBC. U–Pb SHRIMP data from zircon and monazite (Jaeckel et al., 1997) as well as PbSL data from garnet, sillimanite, titanite, and clinopyroxene (Holzer et al., 1998) have been interpreted to indicate a high-grade metamorphic event in the CZ at ~ 2.0 Ga. Holzer et al. (1998) published ages of 2010 ± 17 Ma and 2007 ± 5 Ma of what they termed synkinematic recrystallized garnet and titanite from supracrustal gneisses of the BBC and interpreted these as the age of near-isothermal decompression during a Palaeoproterozoic tectono-metamorphic event. These PbSL age data were supported by an age of 2011 ± 20 Ma on monazite from the same sample. Metamorphic zircons from garnet-cordierite-sillimanite gneisses exposed in the Sand River southeast of Musina yielded an age of 2026 ± 7 Ma (Jaeckel et al., 1997) which was interpreted to reflect the same high-grade P–T event. These ages all cluster close to ~ 2.0 Ga, are often within error, and are best interpreted to reflect a high-grade metamorphic event at ~ 2.0 Ga. However, the suggested link of this ~ 2.0 Ga high-grade event with the proposed isothermal decompression event in the CZ by Holzer et al. (1998) and Schäller et al. (1999) is at variance with the field evidence that the 2572 Ma Bulai pluton (Figs. 1 and 2) intrudes metapelitic gneisses that preserve evidence for this decompression event (e.g. Kramers et al., 2006). 4. New data relating to the evolution of the CZ 4.1. Introduction In this section we integrate new geochronologic data with structural data (different high-grade fabric-forming events, S), petrologic data (pressure (P)–temperature (T) process information), and magmatic/anatectic data. These integrated data show that the CZ was affected by superimposed Neoarchaean (~ 2.63 Ga) and Palaeoproterozoic (~ 2.02 Ga) high-grade tectono-metamorphic events. The Neoarchaean event, termed

D2/M2, is linked to the formation of SW-plunging sheath folds, while the Palaeoproterozoic event is linked to discrete N–S trending shear zones, termed D3/M3. 4.2. The age of the regional S2 gneissic fabric and the formation of D2/M2 sheath folds The studies of Roering et al. (1992) on the Avoca structure west of Alldays (Fig. 2) and of Van Kal (2004) on the Ha-Tshansi and Bellevue structures near Musina (Fig. 2) established that these structures have an almost identical geometry (Fig. 3a–c). The structures, which have a closed sub-circular outcrop pattern, deform the regional high-grade gneissic foliation and are characterised by an intense mineral elongation lineation which plunges consistently and moderately to the SW (Fig. 3). They are interpreted as sheath folds and their formation has been related (Roering et al., 1992) to a regional-scale deformation event involving top-to-the-NE displacement of CZ gneisses along high-grade ductile shear zones. The Avoca sheath fold in the western part of the CZ (Fig. 4a) was selected for detailed study to try and establish the age of the tectono-metamorphic event responsible for the development of sheath folds. The Avoca sheath fold: strongly foliated and lineated banded gneisses, termed the Avoca gneiss (Fig. 4b), are interlayered with foliated and lineated two-pyroxene mafic granulites probably representing metamorphosed mafic dykes, in the boulder-strewn rim of this sheath fold (Fig. 4a). The core of the fold is defined by an intensely lineated quartzofeldspathic gneiss, termed the Avoca L-tectonite (Fig. 4c). This homogenous, pink to orange–brown weathering rock has been correlated (Pienaar, 1985) with the Singelele Gneiss from the Musina area. Mineral elongation lineations on the Avoca gneiss and the mafic-granulite plunge 55° to the SW parallel to the intense fabric of the L-tectonite (Fig. 3a). The Avoca gneiss and the L-tectonite (Singelele Gneiss) carry deformation fabrics that are unequivocally related to the geometry of the Avoca sheath fold; the age of the precursor rocks to these gneisses will therefore provide a maximum age for the development of both the high-grade gneissic foliation and the sheath fold. Detailed field studies of the Avoca fold have established the presence of xenoliths of Avoca gneiss in the Ltectonite (Fig. 4e). We interpret these field relationships, supported by the observation that veins of the L-tectonite also intrude the high-grade mafic dykes (Fig. 4d) to indicate that the precursor to the L-tectonite intruded the Avoca gneiss synkinematically with the formation of the Avoca sheath fold. The age of the precursor to the rock forming the L-tectonite will therefore provide a minimum age for the formation of the gneissic foliation defining the Avoca sheath fold, and a direct age on the deformation event responsible for the formation of the sheath fold. Hand picked grains of zircon were separated from the Avoca gneiss (sample Avoca) and Avoca L-tectonite (sample RB8) for U–Pb age dating on the SHRIMP-RG at the Research School of Earth Sciences at the Australian National University in Canberra (Boshoff, 2004). The techniques are described in the Appendix. Zircons from the Avoca gneiss are ~ 300 μm in length and mostly subhedral, but also include occasional euhedral grains. Most

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Fig. 3. Stereographic projection of structural data for sheath folds (a, b, and c) and N–S trending structures (d and e) (Fig. 2) (Roering et al., 1992; Holzer et al., 1998; Boshoff, 2004; Van Kal, 2004; this study). Crosses are poles to foliation, and dots are lineations. The squares in (a) represent lineations measured for the Avoca gneiss, while the dots represent lineations for the Avoca L-tectonite. Major sheath folds (a, b, and c) are characterized by mineral stretching lineations, which plunge moderately to the SW. The Triangle in (b) represents lineations (n = 4) measured from locality 4 (Fig. 2). Structural data for the N–S trending shear zones developed within the Campbell (d) and Baklykraal (e) structures are also shown.

grains comprise a weakly zoned or unzoned central domain that grades into outer domains characterized by oscillatory compositional zoning. These zircons are clearly magmatic with no observed inherited cores or xenocrysts. Reflected light and cathodoluminescence imaging show that all grains are strongly

metamict. The U–Pb analyses are widely discordant, generally in proportion to the elevated U and/or Th contents (Table 2, and Fig. 5a). Clearly, these zircons, most with extremely high U contents, have a long and complicated history of Pb-loss and most data are unsuitable for geochronology. However, the

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Fig. 4. The Avoca sheath fold in the west Alldays area (Fig. 2). (a) Aerial photograph of the oval shaped sheath fold. (b) Banded Avoca gneiss in the rim of the fold. (c) Avoca L-tectonite in the core of the fold. (d) A mafic granulitic dyke in the Avoca L-tectonite. Note vein from L-tectonite cutting the dyke. (e) Xenolith of Avoca gneiss in the Avoca L-tectonite. White dot in (a) represents localities (d) and (e).

analyses of relatively undamaged areas did yield some useful data. Analyses AV1-3.2 and 4.2, (Fig. 5a), both having low U concentrations, plot within error of concordia, and give a weighted mean 207 Pb/206 Pb age of 2651 ± 8 Ma (2σ error). This is the best estimate for the age of crystallisation of the protolith, to the Avoca gneiss but it is a minimum age (Boshoff, 2004). Subhedral to anhedral zircons with few or no inclusions separated from the L-tectonite (sample RB8) are clearly of igneous origin, with many grains showing internal zoning. Some zones have high U contents and the resultant metamictisation has produced radial cracking in the outer portions of the grains. Apart from the magmatic zoning, cathodoluminescence images are generally bland and uniform, and no unambiguous cores or inherited zircons, or metamorphic overgrowths were observed.

The U–Pb analyses show a wide range in U (340–5071 ppm), Th (59–1240 ppm), and Th/U (Table 3). Nevertheless, all zircons appear to belong to a single population, albeit with some significant scatter and some severe discordance. Regression of all points gives a Model 2 upper intercept age of 2594 ± 38 Ma, with the high MSWD of 35 reflecting the unsatisfactory scatter of the data (Fig. 5b). A better estimate of the age of crystallisation of this “granite” can be gained from the 207 Pb/206Pb age of 2626.8 ± 5.4 Ma (1σ) for the least discordant analysis (#3.2, 1% discordant) (Fig. 5b) (Boshoff, 2004). Note that there is no evidence for metamorphic recrystallisation or overgrowths at ~ 2.0 Ga in these zircon data. The younger U–Pb zircon age of the Avoca L-tectonite (~2.63 Ga) compared with the age of the Avoca gneiss (~2.65 Ga)

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Table 2 Summary of SHRIMP U–Pb monazite data for sample RB24 Th/238U Ppm (1) 206Pb/238U (1) 207Pb/206Pb % Discordant (1) 207Pb⁎ / ±% 206 206 Age Pb* age Pb⁎

Grain spot

(1) % Ppm Ppm 206 Th Pbc U

232

AV1-1.1 AV1-1.2 AV1-3.1 AV1-3.2 AV1-2.1 AV1-2.2 AV1-4.1 AV1-4.2 AV1-5.1 AV1-5.2 AV1-7.1 AV1-6.1 AV1-6.2 AV1-8.1 AV2-1.1 AV2-2.1 AV2-2.2 AV2-3.1 AV2-4.1 AV2-4.2 AV2-4.3 AV2-5.1 AV2-5.2 AV2-6.1 AV2-7.1 AV2-7.2 AV2-8.1 AV2-8.2 AV2-8.3 AV2-9.1 AV2-10.1

0 0.03 0.17 0 0.25 0.49 1.63 0.08 2.33 2.43 0.45 0.1 0.08 0.93 6.7 0.24 0.25 0.23 0.63 0.33 0.35 0.22 0.13 0.04 1.2 0.49 1.14 0.12 0.22 0.06 0.44

0.49 0.47 0.28 0.5 0.26 0.1 0.31 0.84 0.58 0.38 0.04 0.32 0.5 0.37 0.07 0.33 0.3 0.71 0.31 0.11 0.11 0.3 0.57 0.48 0.31 0.36 0.49 0.47 0.37 0.69 0.26

419 909 824 98 918 570 4313 447 3073 1257 1108 1371 759 1004 2233 2058 1085 469 1630 2130 874 1163 3922 973 2663 1193 3991 2972 1998 322 945

198 417 226 48 229 57 1302 363 1738 461 47 431 370 359 158 652 310 320 481 226 92 333 2173 453 800 417 1884 1365 718 216 236

164 94.5 200 42.8 86.5 150 503 196 457 161 146 177 120 149 263 440 167 161 231 360 145 145 632 306 332 114 301 396 212 129 218

2421 736 1603 2647 669.8 1717 808 2660 1007 876 914 904 1090 1017 777 1430 1062 2161 978 1154 1132 874 1107 2008 865 677 536.6 929 751 2465 1528

±29 ±10 ±20 ±34 ±9.4 ±22 ±85 ±35 ±23 ±12 ±12 ±12 ±14 ±15 ±12 ±21 ±15 ±30 ±14 ±17 ±16 ±13 ±16 ±29 ±14 ±10 ±9.1 ±14 ±11 ±33 ±22

2622.80 1932.50 2478.80 2671.50 1483 2387.20 2251 2647.00 1335 1791 1551 1561 1882 1855 2103 2210.30 2256.40 2544.10 2183.50 2030.60 2188.80 2043 1824.20 2651 1828 1759 1378 1338 1370 2643.50 2333.50

±4.5 ±6.7 ±5.4 ±9.6 ±17 ±7.4 ±240 ±4.1 ±470 ±46 ±15 ±33 ±29 ±14 ±33 ±6.5 ±5.9 ±7.6 ±7.7 ±7.6 ±7.8 ±15 ±9.1 ±11 ±46 ±14 ±31 ±13 ±15 ±6.7 ±6.3

8 62 35 1 55 28 64 0 25 51 41 42 42 45 63 35 53 15 55 43 48 57 39 24 53 62 61 31 45 7 35

0.17677 0.11842 0.16221 0.182 0.09277 0.15368 0.142 0.17936 0.086 0.1095 0.09617 0.0967 0.1151 0.11341 0.1304 0.13864 0.14238 0.16863 0.13652 0.12513 0.13693 0.126 0.11151 0.1798 0.1118 0.10759 0.0878 0.08597 0.08745 0.17899 0.14892

0.27 0.37 0.32 0.58 0.92 0.44 14 0.24 24 2.5 0.78 1.8 1.6 0.76 1.9 0.37 0.34 0.45 0.44 0.43 0.45 0.85 0.5 0.65 2.5 0.76 1.6 0.67 0.77 0.4 0.37

(1) 207Pb⁎/ ±% (1) 206Pb⁎ / ±% Err 238 U U corr

235

11.11 1.974 6.314 12.74 1.401 6.467 2.61 12.63 2 2.198 2.019 2.007 2.925 2.672 2.303 4.749 3.517 9.26 3.084 3.383 3.625 2.523 2.881 9.06 2.211 1.641 1.05 1.837 1.489 11.49 5.492

1.5 1.5 1.5 1.7 1.7 1.5 18 1.6 24 2.9 1.6 2.3 2.1 1.8 2.5 1.7 1.6 1.7 1.6 1.6 1.6 1.8 1.7 1.8 3.1 1.8 2.4 1.7 1.8 1.7 1.7

0.4559 0.1209 0.2823 0.5077 0.1095 0.3052 0.133 0.5107 0.1691 0.1456 0.1523 0.1505 0.1843 0.1709 0.1281 0.2484 0.1792 0.3983 0.1638 0.1961 0.192 0.1452 0.1874 0.3655 0.1435 0.1107 0.0868 0.155 0.1235 0.4657 0.2675

1.4 1.5 1.4 1.6 1.5 1.4 11 1.6 2.4 1.4 1.4 1.4 1.4 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.7 1.7 1.6 1.8 1.6 1.6 1.6 1.6

0.983 0.97 0.976 0.938 0.849 0.956 0.622 0.989 0.099 0.495 0.876 0.622 0.659 0.905 0.651 0.975 0.978 0.963 0.963 0.965 0.962 0.883 0.953 0.933 0.561 0.902 0.742 0.921 0.9 0.971 0.976

Errors are 1 − σ; Pbc and Pb* indicate the common and radiogenic portions, respectively. Error in Standard calibration was 0.49% (not included in above errors but required when comparing data from different mounts). (1) Common Pb corrected using measured 204Pb.

is in accord with the field relationships (Fig. 4d and e). The strong linear fabric is interpreted to have formed at high temperature during which some zircon recrystallisation or overgrowths would be expected to occur. The absence of ~2.0 Ga dates in the zircon population therefore precludes a ~2.0 Ga age for the formation of the Avoca L-tectonite and by implication the Avoca sheath fold. The field, structural, and zircon SHRIMP age data thus constrain the formation of the D2/M2 Avoca high-grade sheath fold to ~2.63 Ga. The pristine two-pyroxene mafic granulite in the rim of the sheath fold provides proof that the S2 gneissic fabric developed during the D2 granulite facies event. The strong geometrical link between the Avoca sheath folds and those further east, is well-documented (Roering et al., 1992; van Kal, 2004). We interpret this as indicating that the age constraints for the Avoca sheath fold are also applicable to the Ha-Tshansi and Bellevue structures near Musina (Fig. 2). Support for this interpretation comes from a study of Singelele Gneiss located some 150 km east of the Avoca sheath fold. The Singelele Gneiss (sample RB24) at this locality intrudes across the high-grade gneissic fabric of garnet-biotite gneiss, while Singelele Gneiss (sample TR 136) also intrudes metapelitic gneiss of the BBC at another locality close by, and has previously been dated and yielded an U–Pb zircon age of 2620 ± 8 (Kröner et al., 1999).U–Pb

monazite SHRIMP and garnet PbSL data obtained from sample RB24 were used to establish the time of crystallisation of the precursor to the Singelele Gneiss. The study also provided a comparison between the age of the Singelele Gneiss and that of the L-tectonite in the Avoca sheath fold and by implication between the age of the high-grade foliation in the metapelitic gneiss near Musina and that of the Avoca gneiss some 150 km to the west. Fifteen SHRIMP U–Pb analyses were obtained from monazite in sample RB24. The data are plotted on a conventional concordia diagram (Fig. 6b) and presented in Table 4. All fifteen grains analyzed give concordant or near concordant data points (Fig. 6b), with a mean 207Pb/206Pb age of 2627.0 ± 2.6 Ma, or a concordant 207Pb/206Pb age (on 9 points) of 2627.3 ± 2.5 Ma. We interpret this concordant age as the time of crystallisation of the precursor to the Singelele Gneiss, and thus the minimum age of the high-grade gneissic fabric in the associated metapelitic gneiss. PbSL data for garnet from the Singelele Gneiss sample RB24 are plotted in Fig. 6c, and d, and listed in Table 5. The analytical techniques for the Pb–Pb stepwise leaching of garnets are described in the Appendix. The Pb released is increasingly radiogenic from step [1] to step [4]. After step [4] the radiogeneity decreases, but the least radiogenic Pb (206Pb/204Pb ~23) was

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MSWD value of 50 (Fig. 6c), interpreted to reflect the time of crystallisation of the igneous garnet, with uncertainty limits overlapping the more precise monazite U–Pb age. The 2627.3 ± 2.5 Ma age of the precursor to the Singelele Gneiss (sample RB24) at locality 4 (Fig. 2) is virtually identical to that of the L-tectonite (2626.8 ± 5.4 Ma), in the core of the Avoca sheath fold (Fig. 4a). These data clearly link the time of formation of the high-grade S2 gneissic fabric at two localities, more than 150 km apart (Fig. 2). 4.3. Age of the high-grade S3 shear fabric related to N–S trending D3/M3 structures Two large N–S trending structures were studied to determine the time of formation of the associated high-grade shear fabrics: the Baklykraal structure, located in the west Alldays area, and the Campbell structure, located SW of Musina (Fig. 2).

Fig. 5. U–Pb concordia diagrams of SHRIMP analyses for the foliated Avoca gneiss (a), and the Avoca L-tectonite (b). The corresponding data are shown in Tables 2 and 3.

measured for the short leaching step [1]. The most radiogenic one (206Pb/204Pb ~63) was measured in step [4]. All PbSL data together define a less precise isochron age of 2602 ± 53 Ma with a

4.3.1. The N–S trending Baklykraal structure in the west Alldays area Boshoff et al. (2006) showed that the N–S trending Baklykraal structure in the west Alldays area (Fig. 2) is not a fold as previously mapped (e.g. Brandl and Pretorius, 2000; Van Reenen et al., 2004), but reflects the superimposition of N–S trending high-grade D3/M3 shear zones onto the earlier D2/M2 fold pattern of the area. The N–S trending Baklykraal structure (Boshoff et al., 2006) is characterized by a strongly developed sub-vertical N–S-trending S3 foliation and associated subhorizontal lineation (Fig. 3b). These fabrics are mainly developed in marble and calcsilicate gneisses of the BBC. Within the Baklykraal structure, discrete N–S trending D3 shear zones with S3 foliation wraps Km-scale lenses of mafic gneisses and of

Table 3 Summary of SHRIMP U–Pb zircon data for the Avoca L-tectonite sample RB8 Grain (1) % Ppm Ppm spot 206Pbc U Th

232

1.1 2.1 2.2 3.2 3.3 4.1 4.2 5.1 5.2 5.3 6.1 7.1 8.1 8.2 9.1 9.2 10.1 11.1 12.1 12.2

0.66 0.25 0.2 0.15 0.2 0.61 0.15 1.02 0.19 0.45 0.35 0.13 0.07 0.18 0.16 0.38 0.14 0.1 0.33 0.44

0.8 11.46 2.83 0.78 1.95 1.46 4.47 0.28 2.16 0.15 2.97 1.25 3.9 1.03 9.38 1.89 4.87 1.52 8.11 7.3

342 5071 734 411 981 263 2674 350 4292 353 1003 3013 2528 723 3502 1225 1107 1056 1075 1199

220 1240 141 59 190 154 379 345 775 154 343 393 171 124 544 447 147 106 341 509

Th/238U Ppm (1) 206Pb/238U 206 Pb* age 114 548 202 178 308 106 698 142 270 137 226 1140 370 231 412 253 202 154 212 168

2092 659.2 1732 2607 1966 2444 1622 2492 444.9 2400 1452 2312 968 2016 733 1356 1169 993 1209 891

±26 ±9.6 ±22 ±35 ±24 ±30 ±21 ±30 ±6.8 ±30 ±23 ±37 ±13 ±25 ±11 ±25 ±17 ±13 ±16 ±23

(1) 207Pb/206Pb age

% Discordant (1) 207Pb⁎ / ±% 206 Pb⁎

2624.30 2182 2475 2626.80 2514.90 2595 2457 2572.80 1670 2443.30 2357 2470.00 2203 2524 2336 2255 2357 2148.90 2260 2223

20 70 30 1 22 6 34 3 73 2 38 6 56 20 69 40 50 54 46 60

±8.4 ±21 ±21 ±5.4 ±9.0 ±10 ±24 ±5.9 ±18 ±5.1 ±20 ±5.7 ±30 ±13 ±51 ±10 ±23 ±9.4 ±37 ±230

0.17693 0.1364 0.1619 0.17719 0.16573 0.1738 0.1601 0.17154 0.10249 0.15884 0.151 0.16137 0.138 0.1666 0.1491 0.14229 0.151 0.13384 0.1427 0.14

0.5 1.2 1.3 0.32 0.53 0.62 1.4 0.36 0.95 0.3 1.2 0.34 1.7 0.75 3 0.59 1.4 0.54 2.1 13

(1) 207Pb⁎ / ±% U

235

9.38 2.094 6.94 12.21 8.2 11.1 6.41 11.17 1.014 9.89 5.31 9.64 3.117 8.46 2.549 4.617 4.202 3.084 4.16 2.91

Errors are 1 − σ; Pbc and Pb* indicate the common and radiogenic portions, respectively. Error in standard calibration was 0.47% (not included in above errors but required when comparing data from different mounts). (1) Common Pb corrected using measured 204Pb.

1.5 1.9 1.9 1.6 1.5 1.6 2 1.5 1.8 1.5 2.1 1.9 2.2 1.6 3.3 2.1 2.1 1.5 2.6 14

(1) 206Pb⁎ / ±% U

Err corr

0.3843 0.1113 0.3111 0.4998 0.3589 0.4633 0.2902 0.4724 0.0717 0.4514 0.2549 0.4331 0.1638 0.3683 0.124 0.2353 0.2019 0.1671 0.2114 0.1513

0.945 0.77 0.75 0.981 0.935 0.92 0.709 0.971 0.855 0.981 0.828 0.984 0.643 0.886 0.448 0.96 0.749 0.935 0.559 0.198

238

1.5 1.5 1.4 1.6 1.4 1.5 1.4 1.4 1.6 1.5 1.7 1.9 1.4 1.4 1.5 2 1.6 1.4 1.4 2.7

D.D. Van Reenen et al. / Gondwana Research 14 (2008) 644–662

653

Fig. 6. Locality 4 within the outcrop area of the Bulai pluton (Fig. 2). (a) Unoriented rafts of high-grade garnet-biotite paragneiss within slightly foliated garnetiferous Singelele Gneiss (sample RB24). The Singelele gneiss also cut the high-grade fabric of a metapelitic gneiss at a nearby locality. PbSL data of garnet in the uranogenic (c) and thorogenic (d) Pb isotope diagrams for sample RB24. The isochron age in the 206Pb/204Pb vs. 207Pb/204Pb space (c) includes all respective leach steps (data labels as in Table 4), indicating isotope equilibrium.

complexly folded metaquartzite, which still preserve the regional S2 gneissic fabric (Boshoff et al., 2006). The strong contrast in the geometry of the D3/M3 strike-slip shear zones (Fig. 3b) with that of the nearby SW-plunging D2/M2 Avoca

sheath fold (Fig. 3a) is support for the suggestion that these structures reflect distinctly different deformational events. Van Reenen et al. (2004) showed that highly sheared garnetcordierite-sillimanite-bearing metapelitic gneisses from the

Table 4 Summary of SHRIMP U–Pb monazite data for sample RB24 Grain (1) % Ppm U Ppm Th spot 206Pbc

232

1.1 2.1 3.1 3.2 4.1 5.1 6.1 7.1 8.1 10.1 11.1 12.1 13.1 14.1 15.1

11.9 14.9 19.6 19.5 12 11.9 11.6 10.9 10.1 45 11.7 12.1 25.3 20.6 10.9

0.03 0.02 0.22 0.04 0.02 0.02 0.02 0.02 0.02 0.1 0.03 0.09 0.17 0.44 0.06

8251 7335 3227 5222 7981 6564 7329 7821 8110 3366 6228 6655 3362 4233 3790

94,815 105,653 61,195 98,539 92,464 75,788 82,502 82,301 79,631 146,736 70,724 77,718 82,286 84,519 39,805

Th/238U Ppm (1) 206Pb/238U 206 Pb* age 3390 3100 1430 2180 3450 3040 3240 3420 3580 1420 2760 2870 1450 1880 1650

2519 2578 2680 2548 2626 2780 2673 2655 2671 2570 2680 2617 2626 2671 2646

±34 ±28 ±30 ±28 ±29 ±30 ±29 ±29 ±29 ±29 ±29 ±28 ±29 ±29 ±29

(1) 207Pb/206Pb age

% Discordant (1) 207Pb⁎ / ±% 206 Pb⁎

2630.10 2633.20 2625.30 2621.40 2626.00 2628.30 2629.20 2628.00 2621.70 2589.70 2625.30 2604.90 2620.60 2633.50 2609.90

4 2 −2 3 0 −6 −2 −1 −2 1 −2 0 0 −1 −1

±3.2 ±3.2 ±4.4 ±3.4 ±3.2 ±3.1 ±3.1 ±4.4 ±4.0 ±4.5 ±4.2 ±3.9 ±4.0 ±6.1 ±3.4

0.17755 0.17788 0.17704 0.17662 0.17711 0.17736 0.17746 0.17733 0.17665 0.1733 0.17704 0.17488 0.17654 0.17792 0.1754

0.19 0.19 0.26 0.2 0.19 0.19 0.19 0.26 0.24 0.27 0.26 0.23 0.24 0.36 0.2

(1) 207Pb⁎ / ±% U

235

11.7 12.06 12.58 11.81 12.28 13.18 12.57 12.46 12.5 11.7 12.58 12.07 12.24 12.59 12.27

Errors are 1 − σ; Pbc and Pb* indicate the common and radiogenic portions, respectively. Error in standard calibration was 0.54% (not included in above errors but required when comparing data from different mounts). (1) Common Pb corrected using measured 204Pb.

1.6 1.3 1.4 1.4 1.3 1.3 1.3 1.3 1.3 1.4 1.3 1.3 1.4 1.4 1.4

(1) 206Pb⁎ / ±% U

Err corr

0.4781 0.4916 0.5155 0.4848 0.5027 0.5391 0.5138 0.5096 0.5134 0.4898 0.5155 0.5008 0.5029 0.5134 0.5075

0.993 0.99 0.982 0.989 0.99 0.99 0.99 0.981 0.984 0.981 0.982 0.985 0.985 0.965 0.989

238

1.6 1.3 1.4 1.3 1.3 1.3 1.3 1.3 1.3 1.4 1.3 1.3 1.4 1.3 1.3

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Table 5 Pb isotope data of PbSL experiments on garnets from samples O6-19M, O6-19L, RB24 and TOV13 IC0 Gain and IC1 gain (Hg) corrected values Sample #

Mineral a

step

Acid

time

206/204

+/− 2SE+

207/204

+/− 2SE+

208/204

+/− 2SE+

[1] [2] [3] [4] [5]

4 N HNO3 4 N HNO3 4 N HNO3 b Aqua Regia b Aqua Regia b

15' 8h 8h 16 h 2d

20.715 22.643 53.998 331.646 78.876

0.028 0.031 0.07 0.151 0.101

16.297 16.686 20.827 61.872 24.307

0.023 0.023 0.027 0.028 0.031

39.426 45.551 103.462 628.888 42.247

0.058 0.065 0.14 0.29 0.055

sample O6-19L grt [1] grt [2] grt [3] grt [4] grt [5]

4 N HNO3 4 N HNO3 4 N HNO3 b Aqua Regia b Aqua Regia b

15' 8h 8h 16 h 2d

21.909 23.34 24.548 40.432 29.144

0.031 0.033 0.034 0.068 0.037

16.644 16.885 17.074 19.077 17.073

0.024 0.024 0.024 0.034 0.022

38.735 38.821 43.289 63.621 38.777

0.058 0.06 0.065 0.121 0.05

Metapelite sample O6-19M O6-19M-step1 grt O6-19M-step2 grt O6-19M-step3 grt O6-19M-step4 grt O6-19M-residue grt Intrafolial leucosome O6-19L-step1 O6-19L-step2 O6-19L-step3 O6-19L-step4 O6-19L-residue

Quartzo-feldspathic sample RB24 RB24-step1 grt RB24-step2 grt RB24-step3 grt RB24-step4 grt RB24-residue grt

[1] [2] [3] [4] [5]

4 N HNO3 4 N HNO3 4 N HNO3 b Aqua Regia b Aqua Regia b

15' 8h 8h 16 h 2d

23.746 25.402 33.188 63.349 26.053

0.035 0.034 0.179 0.096 0.032

16.489 16.854 18.135 23.406 16.826

0.024 0.023 0.104 0.037 0.021

44.184 49.026 81.455 187.08 41.45

0.067 0.071 0.544 0.319 0.052

Metapelite sample TOV13 TOV13-step1 grt TOV13-step2 grt TOV13-step3 grt TOV13-step4 grt TOV13-residue grt

[1] [2] [3] [4] [5]

4 N HNO3 4 N HNO3 4 N HNO3 b Aqua Regia b Aqua Regia b

15' 8h 8h 16 h 2d

70.344 34.253 236.328 261.78 144.738

0.097 0.05 0.332 0.338 0.247

23.614 18.77 43.563 46.021 32.982

0.033 0.029 0.063 0.06 0.058

55.656 52.846 510.933 638.372 67.161

0.08 0.085 0.769 0.837 0.119

mix1 = 1.5 N HBr + 2 N HCL in the relation 10:1. mix2 = HF + 15N HNO3 in relation 3:1. Aqua Regia = 6 N HCl + 15 N HNO3 in relation 2:1. + Errors are two standard deviations (2σ). a grt = garnet. b Leached on a hot plate.

Baklykraal shear zone record a single high-temperature decompression-cooling P–T path from 780 °C at 5.7 kbar to 600 °C at 3.3 kbar (Fig. 7, path DC2). This P–T path was attributed to a D3/ M3 event, interpreted to reflect the uplift of the high-grade rocks from the mid- to the upper crustal levels (Van Reenen et al., 2004). Boshoff et al. (2006) recorded a PbSL isochron age of 2023 ± 11 Ma for garnet from one of the studied samples (sample T73), and interpreted this precise age to accurately constrain the time of the high-grade S3 fabric-forming event in the CZ. Note that this Palaeoproterozoic age is within error of many PbSL ages obtained on metamorphic garnets and titanites from the CZ (e.g. Holzer et al., 1998, 1999; Schäller et al., 1999) and U–Pb dates on zircons in melt patches (e.g. Jaeckel et al., 1997). 4.3.2. The N–S trending Campbell structure in the Musina area New age, structural and metamorphic data, show that ductile shear zones developed within the N–S trending Campbell structure in the Musina area (Fig. 2), can be linked to the same D3/M3 Palaeoproterozoic shear deformational event recognised in the Alldays area. The Campbell structure (e.g. Van Kal, 2004) is characterised by numerous discrete high-grade N–S trending S3 shear zones

ranging from centimetre to metre-sized structures that dip moderately to the west, and are characterized by down-dip mineral lineations. The shear zones wrap centimetre to metresize lenses of para- and ortho gneiss that preserve evidence for an earlier (S2) gneissic fabric, very similar to the situation in the N–S trending Baklykraal shear zone (Boshoff et al., 2006). Van Kal (2004) also showed that an intensely sheared metapelitic gneiss (garnet-cordierite-sillimanite-biotite-k-feldspar-quartz) (sample O6-19) from a discrete D3/M3 shear zone on the farm Verbaard near the centre of the Campbell structure (Fig. 2, locality 5), reflects a decompression-cooling P–T path from ~ 5 kbar and ~ 750 °C to ~ 3.5 kbar and ~ 570 °C (Perchuk et al., in press). This D3/M3 P–T path is very similar to the D3/M3 P–T path constructed for sample T73 (Fig. 7, path DC2) from a D3 shear zone within the Baklykraal structure, located more than 100 km to the west (Fig. 2, locality 2). Metamorphic and structural data thus strongly suggest that the formation of the Campbell structure might be linked to the same D3/M3 shear deformational event that resulted in the formation of superimposed D3 shear zones within the Baklykraal structure (Boshoff et al., 2006). This suggestion is confirmed by new age data obtained from metapelitic sample O6-19.

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calcsilicate rock in a high-grade shear zone developed at the NE contact of the Ha-Tshansi sheath fold with the Bulai pluton. Integrated geochronologic, structural, and petrologic data thus conclusively show that two major N–S trending structural domains in the CZ, both associated with high-grade N–S trending shear zones, reflect the same high-grade shear deformational event, here dated at close to 2.02 Ga. 5. Solving the geochronologic problem in the CZ of the Limpopo HGT 5.1. Introduction Fig. 7. P–T diagram (van Reenen et al., 2004; Boshoff et al., 2006; Perchuk et al., in press) for metapelitic gneisses from the Baklykraal structure (see Fig. 2). Path DC1 reflects the relic D2/M2 Neoarchaean decompression-cooling event for unsheared gneisses (sample JC1) from this structure. The Palaeoproterozoic overprint is reflect in sample JC1 by the isobaric heating event (D3a/M3a), followed by decompression-cooling (D3b/M3b) (sheared sample T73). The dots represent individual P–T estimates calculated on the basis of the method of local equilibrium (e.g. van Reenen et. al., 2004; Perchuk et al., in press). The closely associated Ha-Tshansi and Campbell structures, located more than 100 km to the east (Fig. 2), are characterized by an identical configuration of P–T paths, in which a D3a/M3b isobaric heating path also links the D2/M2 (Ha-Tshansi) and D3b/M3b (Campbell) decompression-cooling P–T paths. See text for discussion.

Sample O6-19 is a banded gneiss that is comprised of a highly sheared melanosome (sample O6-19M) and a composite leucosome (sample O6-19L). The composite leucosome consists of a highly sheared garnet-bearing portion and a more homogenous garnet-free portion (Perchuk et al., in press). Zircon and monazite were separated from the leucosome (O6-19L) to date the time of crystallisation of the leucocratic material. U–Pb data for zircon and monazite from sample O6-19L are listed in Table 6 and the results are plotted in concordia diagrams (Fig. 8a and b). Most of the studied zircon grains are strongly discordant, but two clusters of data points are close to concordance. The one cluster of four grains defines a mean 207Pb/206Pb age of 2011 ± 11 Ma (MSWD = 0.095), while another cluster of four grains is b 1% discordant and defines a concordant age of 2610.3 ± 8.5 Ma. We interpret this data to show that metapelitic sample O6-19L records evidence for two distinct high-grade events. Twelve monazite grains were also analyzed on SHRIMP II and show b 8% discordance (Table 6). All the monazite data points yielded a combined mean 207Pb/206Pb age of 2017.1 ± 2.8 Ma (Fig. 8b), interpreted to reflect a very precise age for the D3/M3 metamorphic event. This age cannot be distinguished from the precise garnet PbSL age (2023 ± 11 Ma) recorded by sheared sample T73 from a discrete D3/M3 shear zone within the Baklykraal structure, strongly suggesting that these major N–S trending shear zones (Fig. 2), reflect the same D3/M3 Palaeoproterozoic shear deformational event (Van Kal, 2004; Boshoff et al., 2006; Perchuk et al., in press). Holzer et al. (1998) provided additional evidence, which can be used in support of the proposed time of formation of the S3 shear fabric in the Musina area. These authors obtained a U–Pb monazite age of 2011 ± 20 Ma and PbSL dates on garnet (2010 ± 17 Ma) and titanite (2007 ± 5 Ma) from a sheared

Published and new (this paper) age data for the b 3.0 Ga history of the Limpopo HGT are compiled in Fig. 9. Three important observations follow from the compiled age data: Firstly, U–Pb age data for magmatic/anatectic rocks reflect long-lasting (~2.56–2.69 Ga) Neoarchaean magmatic events in all three sub-zones of the Limpopo HGT. Secondly, the link between PbSL age data obtained from metamorphic minerals in high-grade paragneisses and Neoarchaean magmatic/anatectic events clearly reflects the predominant monometamorphic nature of the two Marginal Zones (Kreissig et al., 2001; Blenkinsop et al., 2004) (Fig. 9). Thirdly, and most important with reference to the geochronologic problem, is the fact that the CZ shows a wide spread of PbSL age data (~2.0 and ~2.65 Ga, as well as intermediate values), which strongly supports previous suggestions (Holzer et al., 1998) that this sub-zone had a polymetamorphic evolution. 5.2. Problems of high-grade polymetamorphism Perchuk et al. (2006a,b, in press) first documented petrologic evidence for high-grade polymetamorphism in the CZ. These authors showed that metapelitic sample JC1 (Fig. 2, locality 3), a garnet-cordierite-orthopyroxene-biotite-quartz bearing rock from a gneissic lens wrapped by a S3 shear fabric in the Baklykraal structure, is characterized by different generations of the same minerals. These different generations reflect two distinct P–T paths (Fig. 7, path DC1 and IH). The post-peak decompression-cooling P–T path (Fig. 7, path DC1) from ~ 850 °C and ~ 8.5 kbar to ~ 675 °C and 6 kbar is interpreted to reflect the uplift of the high-grade rocks from the lower- to the mid-crustal levels during the D2/M2 tectono-metamorphic event, while evidence for a subsequent sub-isobaric (~ 6 kbar) heating (T ~ 675 °C to ~ 770 °C) event, and thus for polymetamorphism, is recorded in the same sample by a different generation of the same mineral assemblage (Fig. 7, path IH). Furthermore, Fig. 7 shows that the isobaric heating path IH links the two decompression-cooling P–T paths DC1 and DC2 (Fig. 7). This observation suggests that the process of isobaric heating reflected by path IH (Fig. 7) immediately preceded the final uplift of the high-grade rocks as demonstrated by P–T path C (Fig. 2), which was constructed from sample T73. Isobaric heating resulted in the formation of polymetamorphic granulites (samples JC1 and RB1), which are preserved within D2/M2 gneissic lenses within the Baklykraal structure.

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Table 6 Summary of SHRIMP U/Pb monazite and zircon data for sample O6-19L Grain (1) % Ppm U Ppm spot 206Pbc Th

232

Th/ Ppm (1) 206Pb/238U 206 U Pb* age

238

(1) 207Pb/206Pb age

% Discordant (1) 207Pb⁎/ ±% 206 Pb⁎

(1) 207Pb⁎/ ±% U

235

(1) 206Pb⁎/ ±% U

Err corr

238

Monazite data 1.1 0.05 3995 2.1 0.24 2042 2.2 0.08 5735 3.1 0.47 2200 4.1 0.05 3938 5.1 0.04 5719 5.2 0.09 1954 6.1 0.06 4186 7.1 0.07 2769 4.2 0.28 1665 8.1 0.41 2261 9.1 0.04 2727

224,911 113,611 84,450 85,280 59,154 72,071 65,656 74,029 62,012 81,047 74,724 35,262

58.2 57.5 15.2 40.1 15.5 13.0 34.7 18.3 23.1 50.3 34.1 13.4

1150 627 1770 667 1250 1800 624 1310 883 534 759 869

1858 1967 1975 1941 2021 2014 2035 1998 2034 2042 2119 2033

±20 ±22 ±20 ±16 ±15 ±15 ±16 ±15 ±16 ±22 ±16 ±16

2012.1 2009.0 2020.9 2014 2021.9 2014.3 2011.6 2021.3 2022.9 2013.3 2007.8 2013.7

±5.5 ±8.3 ±4.5 ±11 ±4.1 ±3.7 ±5.6 ±4.1 ±4.3 ±8.9 ±8.4 ±4.1

8 2 2 4 0 0 −1 1 −1 −1 −6 −1

0.12383 0.12361 0.12444 0.12395 0.12451 0.12398 0.12379 0.12447 0.12459 0.12391 0.12353 0.12394

0.31 0.47 0.25 0.59 0.23 0.21 0.32 0.23 0.24 0.50 0.47 0.23

5.704 6.081 6.152 6.003 6.320 6.268 6.335 6.237 6.372 6.366 6.630 6.336

1.3 1.4 1.2 1.1 0.91 0.88 0.98 0.91 0.93 1.4 1.0 0.92

0.3341 0.3568 0.3586 0.3513 0.3681 0.3666 0.3712 0.3634 0.3709 0.3726 0.3892 0.3708

1.2 1.3 1.2 0.94 0.88 0.86 0.93 0.88 0.90 1.3 0.91 0.89

0.969 0.938 0.978 0.847 0.968 0.972 0.947 0.968 0.966 0.928 0.886 0.968

Zircon data 1.1 0.06 2.1 0.22 3.1 1.39 3.2 0.08 4.1 0.02 5.1 1.57 6.1 1.97 7.1 6.93 8.2 4.96 9.1 0.29 10.1 0.15 10.2 0.27 10.3 3.29 11.1 0.06 12.1 0.01 13.1 0.61 13.2 0.64 14.1 0.07 14.2 0.93 15.1 0.12 16.1 0.47 17.1 3.25 18.1 0.09 19.1 0.15 20.1 0.27 21.1 1.62 21.2 0.69

221 11 207 255 58 361 369 29 26 171 304 164 24 166 36 38 95 177 334 189 46 63 161 101 331 319 81

0.45 0.01 0.16 0.48 0.07 0.24 0.28 0.01 0.02 0.58 0.33 0.33 0.02 0.31 0.04 0.04 0.12 0.34 0.30 0.56 0.04 0.05 0.39 0.41 0.35 0.29 0.23

215 104 149 234 256 313 164 343 187 94.4 293 220 274 230 319 242 247 204 181 149 202 149 163 78.3 149 416 123

2602 554.7 775.1 2597 1987 1321 842.6 862.5 844.1 1982 1983 2580 1226 2533 1989 1511 1940 2366 1078 2606 1120.0 714.9 2388 1965 1057.1 2263 2135

±24 ±4.8 ±6.5 ±20 ±15 ±13 ±7.7 ±8.1 ±7.7 ±19 ±17 ±21 ±13 ±20 ±16 ±12 ±16 ±20 ±14 ±28 ±9.5 ±7.1 ±21 ±21 ±9.3 ±18 ±20

2617.0 1827 1687 2608.4 2010.0 2032 1736 1960 1907 1985 2408.7 2604 1708 2590.6 2010.3 2007 2018 2589.1 1790 2618 1828 1362 2557 2020 2181 2545 2507

±7.7 ±18 ±38 ±7.9 ±8.4 ±35 ±51 ±130 ±100 ±19 ±8.4 ±13 ±210 ±8.2 ±8.4 ±18 ±17 ±9.2 ±29 ±12 ±19 ±100 ±11 ±26 ±15 ±23 ±21

1 70 54 0 1 35 51 56 56 0 18 1 28 2 1 25 4 9 40 0 39 48 7 3 52 11 15

0.17615 0.1117 0.1035 0.17524 0.12368 0.1252 0.1062 0.1202 0.1168 0.1220 0.15563 0.1748 0.105 0.17339 0.12370 0.1235 0.1242 0.17323 0.1094 0.1762 0.1118 0.0871 0.1699 0.1244 0.1364 0.1687 0.1649

0.46 1.0 2.1 0.48 0.48 2.0 2.8 7.4 5.7 1.0 0.49 0.75 11 0.49 0.48 1.0 0.97 0.55 1.6 0.70 1.1 5.2 0.68 1.4 0.88 1.4 1.3

12.07 1.384 1.823 11.99 6.155 3.927 2.045 2.37 2.25 6.055 7.727 11.86 3.02 11.51 6.164 4.498 6.012 10.59 2.745 12.11 2.924 1.408 10.50 6.11 3.350 9.78 8.93

1.2 1.4 2.3 1.1 1.0 2.2 3.0 7.5 5.8 1.5 1.1 1.2 11 1.1 1.0 1.4 1.3 1.1 2.1 1.5 1.4 5.3 1.3 1.9 1.3 1.6 1.7

0.4971 0.08986 0.1278 0.4961 0.3609 0.2275 0.1396 0.1432 0.1399 0.3600 0.3601 0.4923 0.2094 0.4814 0.3614 0.2642 0.3510 0.4435 0.1820 0.4981 0.1897 0.1173 0.4483 0.3564 0.1782 0.4205 0.3925

1.1 0.90 0.90 0.94 0.90 1.1 0.97 10 0.97 1.1 0.97 0.98 1.2 0.97 0.91 0.92 0.94 1.0 1.4 1.3 0.92 1.0 1.1 1.2 0.96 0.93 1.1

0.922 0.666 0.397 0.893 0.885 0.473 0.328 0.134 0.167 0.724 0.891 0.793 0.104 0.891 0.886 0.669 0.697 0.876 0.662 0.882 0.655 0.196 0.843 0.649 0.736 0.564 0.668

504 1348 1340 550 826 1578 1337 2592 1476 304 946 519 1474 555 1027 1061 813 534 1145 348 1231 1432 423 255 970 1133 363

Errors are 1 − σ; Pbc and Pb⁎ indicate the common and radiogenic portions, respectively. Error in Standard calibration was 0.35% for monazite analysis and 0.28% for zircon analysis (not included in above errors but required when comparing data from different mounts). (1) Common Pb corrected using measured 204Pb.

Perchuk et al. (in press) further showed that metapelitic samples TOV13 and O6-19M, respectively from the D2/M2 HaTshansi sheath fold and from the adjacent N–S trending D3/M3 Campbell structure (Fig. 2), also record evidence for polymetamorphism, with both decompression-cooling and isobaric heating P–T paths occurring in the same sample. The situation is similar to the different P–T paths (Fig. 7, paths DC1, IH, and DC2) recorded from samples JC1 and T73 from the Baklykraal structure, located more than 100 km to the west (Fig. 2). The high-pressure post-peak decompression-cooling P–T path for sample JC1 in Fig. 7 (path DC1) compares with that of sample TOV 13, while the lower-pressure decompression P–T path of

sample T73 (Fig. 7, path DC2) compares with that of sample O6-19M. Perchuk et al. (in press) also showed that both samples TOV13 and O6-19M reflect petrologic evidence for isobaric heating (~ 5 kbar, 600 to 750 °C), very similar to the situation described for sample JC1 (Fig. 7, path IH). Polymetamorphism is thus petrologically recognized by the presence of two distinct P–T paths in the same sample. 5.3. Dating high-grade polymetamorphic rocks Polymetamorphism thwarted all efforts to directly date the time of formation of the high-grade S2 gneissic fabric of the CZ

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Fig. 8. PbSL data of garnet from the melanosome of metapelitic gneiss sample O6-19 (O6-19M), shown in uranogenic (a) and (b) thorogenic Pb isotope diagrams. The isochron age shown in (c) includes all respective leach steps (data labels as inTable 3), indicating isotope equilibrium. Uranogenic (c) and thorogenic (d) Pb isotope diagrams of the leucocratic material from the metapelitic gneiss sample O6-19 (sample O6-19L). The isochron age of 2103 ± 820 Ma in (c) includes all respective leach steps (data labels as in Table 5), indicating isotope equilibrium. U–Pb concordia diagrams of the SHRIMP monazite (e) and zircon (f) analyses of the leucocratic material from the metapelitic gneiss sample O6-19 (O6-19L). The corresponding data are shown in Table 6.

based on the garnet PbSL method, and thus the timing of the main D2/M2 tectono-metamorphic event (Boshoff et al., 2006). The same problem also applies to efforts to date the S3 fabric related to the N–S trending Campbell structure, using the same method. Boshoff et al. (2006) attempted to date garnet from high-grade para- and ortho-igneous gneisses from a D2/M2 gneissic lens, wrapped by the S3 shear fabric within the Baklykraal structure (Fig. 2). Garnet from a completely undeformed metapelitic rock (sample JC1, Fig. 2, locality 3) and from a garnet-biotite gneiss (sample RB1) from the same D2/M2 lens, recorded PbSL ages of 2120 ± 110 Ma (MSWD = 42) (sample JC1) and 2173 ± 79 Ma (MSWD = 1150) (sample RB1) respectively. A PbSL age for garnet obtained from sample RB38 (a Singelele-type gneiss) from the same D2/M2 gneissic lens yield a PbSL age of 2443 ± 34 Ma (MSWD = 17,500) (Boshoff et al., 2006). These imprecise ages

with large scatter in the data arrays were interpreted as “mixed ages”, reflecting evidence for both the Neoarchaean and the Palaeoproterozoic event due to the presence of two generations of garnet in the same sample (Boshoff et al., 2006). Efforts to date the precursor to the Singelele-type gneiss (sample RB38) based on U–Pb zircon SHRIMP data, also yield ages intermediate between 2600 and 2000 Ma (Boshoff, 2004). The data spreads were interpreted to be mostly due to growth of new domains, or overgrowths, during a younger metamorphic or igneous event, rather than loss of radiogenic Pb from the zircons (Boshoff et al., 2006). Attempts to date the S2 fabric-forming event associated with the formation of the Ha-Tshansi sheath fold (Fig. 2) near Musina, using the garnet PbSL method also proved unsuccessful due to polymetamorphism. PbSL step-leaching data for garnet from

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structure (Fig. 2, locality 5). Leach spectra for garnet following the PbSL procedure are plotted in Fig. 8a and c (uranogenic diagrams) and Fig. 8b and d (thorogenic diagrams), and the corresponding PbSL data are shown in Table 5. Lead measured from the step solutions of sample O6-19M are low but increasingly radiogenic from step 1 to step 3, with very high 206 Pb/204Pb released by the first Aqua Regia mixture during step 4 (Table 5). The very high 206Pb/204Pb released by step 4 is matched by similar high 208 Pb/204 Pb ratios (Fig. 8d). The residual step 5 is again characterized by a lower radiogenic value of ~ 78. All step-leaching data define an isochron age of 2310 ± 43 Ma (SMWD = 1250). For sample O6-19L, the first four leaching steps yielded increasingly uranogenic Pb compositions (Table 5) followed by the residual step 5 again characterized by a lower radiogenic value (~ 29). All PbSL data define an isochron age of 2103 ± 820 Ma (MSWD = 914) (Fig. 8a) Garnet from both the melanosome (O6-19M) and the leucosome (O6-19L) produced ages best interpreted as mixed ages that reflect the effect of the Palaeoproterozoic overprint onto the Neoarchaean Pb–Pb isotopic signature of the rock. Mixed age data obtained from PbSL of garnet thus strongly support petrologic evidence (Van Kal, 2004; Perchuk et al., 2006a,b, in press) for high-grade polymetamorphism in the CZ. 6. Discussion

Fig. 9. Compilation of published and new (this study) geochronological data relating to the post-3.0 Ga history of the Limpopo HGT (modified after Figs. 2 and 12, Kramers et al., 2006). Northern Marginal Zone: (1) Berger et al. (1995); (2) Blenkinsop et al. (2004); (3) Holzer et al. (1999); (4) Mkweli et al. (1995); (5) Frei et al. (1999); (6) Kamber et al. (1995b); (7) Blenkinsop and Frei (1996); (8) Kamber et al. (1998); (9) Kamber et al. (1996); Central Zone: (1) Barton et al. (1994); (2) Barton et al. (2003); (3) Boshoff (2004); (4) Boshoff et al. (2006); (5) Buick et al. (2003); (6) Chavagnac et al. (2001); (7) Holzer (1998); (8) Holzer et al. (1998); (9) Holzer et al. (1999); (10) Jaeckel et al. (1997); (11) Kamber et al. (1995b); (12) Kröner et al. (1999); (13) McCourt and Armstrong (1998); (14) Schäller et al. (1997); (15) Schäller et al. (1999); (16) This study. Southern Marginal Zone: (1) Kreissig et al. (2001); (2) Barton et al. (1992).

metapelitic sample TOV13 are shown in Table 5. The uranogenic spectrum is dominated by two very radiogenic values which are characterized by 206Pb/204Pb of ~ 236 (step 3) and ~ 261 (step 4) (Table 5, Fig. 8e). These values are matched by similar high 208Pb/204Pb ratios (Fig. 8f). All leach steps together yield an isochron age of 1956 ± 190 Ma (MSWD = 1450). This imprecise age is interpreted as a mixed age that reflects a strong Palaeoproterozoic overprint at ~ 2.0 Ga, similar to the situation described for sample JC1 from the Baklykraal structure (Fig. 2). The mixed age data thus suggest that sample TOV13 is also a polymetamorphic granulite, a suggestion confirmed by detailed petrologic studies (Van Kal, 2004; Perchuk et al., in press). Garnet for PbSL dating was also separated from both the melanosome (O6-19M) and the composite leucosome (O6-19L) of sample O6-19 from a discrete shear zone in the Campbell

The interpretation of geochronologic data is obviously as important as the data itself, and when dealing with high-grade rocks it is imperative that age data should be linked to detailed structural and petrologic data. Petrologic studies form the basis for the construction of well-constrained P–T paths that allow the recognition of high-grade polymetamorphic rocks characterized by different generations of the same high-grade minerals in the same rock. We have shown that integrated geochronologic (U–Pb and Pb–Pb), structural and petrologic data for magmatic/anatectic rocks and high-grade paragneisses provide conclusive evidence that the CZ of the Limpopo HGT was affected by two high-grade events. The major Neoarchaean D2/M2 tectono-metamorphic event (Pmax = ~ 9 kbar, Tmax = ~ 900 °C) occurred before ~2.63 Ga, and was overprinted at ~2.02 Ga by a D3/M3 isobaric (P = ~6 kbar) heating (T = ~670 to 780 °C) event, which resulted in the widespread formation of polymetamorphic granulites. The isobaric-heating event immediately preceded a high-temperature shear deformational event (Pmax = ~6 kbar, Tmax = ~780 °C), interpreted to reflect the final uplift of the high-grade CZ rocks from the mid- to the upper crustal level. The D2/M2 Neoarchaean and D3/M3 Palaeoproterozoic events respectively are linked to major SW-plunging sheath folds and to N–S trending structures (Fig. 2). U–Pb age data for monazite and zircon from syntectonic and late-tectonic leucocratic anatectic material were used to constrain the time of formation (~2.02 Ga) of the S3 gneissic fabric (sample O6-19L, Campbell structure, Fig. 2) related to the N–S trending structures, and the minimum time of formation (~2.63 Ga) of the S2 gneissic fabric associated with a CZ sheath fold (Avoca fold, Fig. 2) and with the regional S2 high-grade gneissic fabric of the CZ (locality RB24, Fig. 2). PbSL age data

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for garnet also provided an accurate age (~2.02 Ga, sample T73) for the S3 shear fabric associated with the N–S trending Baklykraal structure (Fig. 2). In contrast, all efforts to directly date the S2 and S3 gneissic fabrics, using the PbSL method provided to be unsuccessful, and yielded mixed age data, due to high-grade polymetamorphism. Geochronologic and petrologic problems related to polymetamorphism in the CZ of the Limpopo HGT are further complicated by the fact that high-grade shear zones were only recently (Boshoff et al., 2006) recognized as distinct deformational features, which are linked to the Palaeoproterozoic D3/M3 shear deformational event at ~2.02 Ga. The minimum age for the main D2/M2 tectono-metamorphic event (~ 2.63 Ga) suggested by the data from the Avoca Ltectonite is significantly earlier than the time of ~ 2.58 Ma previously interpreted to reflect the main D2 fold event in the Musina area (Hofmann et al., 1998; Kröner et al., 1999). It is also possible that the regional D2/M2 high-grade event could even have occurred as early as ~ 2.69 Ga, based on the U–Pb zircon age of ~ 2.69 Ga (Kröner et al., 1999) for a leucocratic anatectic garnet-bearing gneiss mapped as Singelele Gneiss (Horrocks, 1983) at a locality 15 km SE of Musina. Finally, our new data strongly suggest that the SW–NE trending Tshipise Straightening Zone, which bounds the CZ in the south (Fig. 2), is a composite high-grade shear zone (Holzer et al., 1998) that was active in both the Neoarchaean and in the Palaeoproterozoic. The link with the Neoarchaean event is suggested by the fact that movement indicators within this shear zone (Horrocks, 1983) are developed parallel to the SW-plunging D2/M2 sheath folds (Fig. 3). The link with the Palaeoproterozoic shear deformational event is suggested by the observation that fabrics developed within the N–S trending structures near Musina can be followed uninterrupted into the Tshipise Straightening Zone (Fig. 2). The kinematic relationships between the N–S trending Palaeoproterozoic shear zones and the suggested reactivation of the Tshipise Straightening Zone is the subject of our ongoing studies. 7. Conclusions The results of our integrated geochronologic, field, structural, and petrologic studies in the CZ of the Limpopo HGT strongly suggest that the ongoing debate (e.g. Kramers et al., 2006 and references therein) on the nature and timing of high-grade events that affected this sub-zone is a direct consequence of the interpretation of published geochronologic data. Previous interpretations did not account for two important features of the CZ, namely (i) the presence of high-grade polymetamorphic granulites, which explains the spread in PbSL age data for metamorphic minerals from high-grade gneisses (Fig. 9), and (ii) the presence of highgrade shear zones that reflect a distinct D3/M3 shear deformational event. Together, these two important features reflect a high-grade Palaeoproterozoic event at ~2.02 Ga that overprinted the longlived Neoarchaean D2/M2 event (N 2.63 Ga). The results of our integrated geochronologic, structural, petrologic, and magmatic/ anatectic studies (Van Reenen et al., 2004; Boshoff et al., 2006; Perchuk et al., 2006a,b, in press) can be summarized as follows: U–Pb SHRIMP age data for zircon were successfully applied to establish the minimum age (2.63 Ga) of the Neoarchaean

659

event reflected by the regional high-grade gneissic fabric that characteristically includes large SW-plunging sheath folds. This approach was used to date the precursors to Singelele-type gneiss, which clearly cuts the early high-grade S2 gneissic fabric at two different localities. U–Pb SHRIMP age data for monazite from syntectonic anatectic material yielded a precise age (2017.1 ± 2.8 Ma) for the time of formation of the S3 fabric-forming event related to the N–S trending Campbell structure SW of Musina. PbSL age dating of garnet was used successfully to directly date the S3 gneissic fabric related to the N–S trending D3/M3 Baklykraal structure in the west Alldays area (Boshoff et al., 2006). The metapelite (sample T73) used for age dating, is characterized by a new mineral paragenesis (garnet + cordierite + sillimanite + biotite + K-feldspar + quartz), which developed syntectonically during the high-temperature D3/M3 shear deformational event (Van Reenen et al., 2004). Garnet from this sample yielded a precise age of 2023 ± 11 Ma for the Palaeoproterozoic shear deformational event. All other efforts to date garnet from either sheared (D3/M3) or unsheared (D2/M2) high-grade metapelitic gneisses using the PbSL method resulted in mixed ages between ~ 2.0 and 2.6 Ga. These mixed ages are interpreted to reflect the presence of two distinct generations of the same mineral paragenesis in the same high-grade polymetamorphic gneisses (Perchuk et al., in press). Integrating age data for a specific fabric-forming event S2 or S3 with P–T process information (P–T-t paths) (Fig. 7), allowed a process of isobaric (~ 5.5 kbar) heating (~ 670 to 770 °C) to be recognized. Isobaric heating immediately preceded the D3/M3 shear deformation event (~ 2.02 Ga) (Perchuk et al., in press), and resulted in the widespread formation of polymetamorphic granulites in the CZ. Published data (Kreissig et al., 2001; Blenkinsop et al., 2004; Boshoff et al., 2006; this study) show that all three sub-zones of the Limpopo HGT (SMZ, CZ, and NMZ) experienced high-grade tectono-metamorphic events within the same time interval (~2.58 to 2.68 Ga). Whereas the SMZ and NMZ are largely monometamorphic high-grade terrains, the CZ is a high-grade polymetamorphic terrain in which the main Neoarchaean highgrade event was overprinted by a high-grade Palaeoproterozoic event (Fig. 9). The results of our studies thus strongly suggest that published models for the geologic evolution of the Limpopo HGT (Barton et al., 2006; Kramers et al., 2006 and references therein) need to be re-evaluated. Acknowledgements DDvR acknowledges financial support from the University of Johannesburg and for a National Research Foundation Grant (GUN: 2053192). Isotope geology research at Bern University is supported by the Swiss National Science Foundation. Kevin Mahan, Reiner Klemd and Leo Hartman are thanked for their valuable comments on an earlier version of the manuscript. Neels and Ada van Wyk (the farm Kilimanjaro), and Johan Wolvaardt (the farm Dorothy) are thanked for their hospitality during fieldwork.

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Appendix A Pb stepwise leaching (PbSL) of garnet was carried out at the University of Bern, Switzerland. Garnet separates were obtained from crushed hand specimens using standard magnetic separation and heavy liquid techniques. The final separates were hand picked under a high-power binocular microscope. PbSL was performed in modified form after Frei and Kamber (1995) and Frei et al. (1997). The separates were ultrasonically washed in doubly distilled water and subsequently 100–500 mg were treated in 7 ml Savillex screw-top beakers. Between steps the leachate was decanted and the remaining solids dried on a hot plate. Leach acids and times of leaching are given in Table 5. Pb was separated using conventional HCl–HBr anion exchange columns, contributing a Pb procedure blank less than 130 pg. The Pb was loaded on single Re-filaments and measured on a “Nu Instruments” (R) Multicollector ICP mass spectrometer. A CETAC “Aridus” desolvating nebulizer system was used and Pb was run in the static mode. The instrumental mass fractionation was corrected by doping with thallium and monitoring the 205/203 ratio. For small samples, the 204Pb was measured on a calibrated electron multiplier by ion counting, while other isotopes were measured on Faraday cups. Mercury was trapped from the sample and sweep gas argon by gold traps. Interference of remaining mercury on 204Pb was corrected by monitoring 202Hg. Two standard errors (σ2) are assigned to the isochrones. U–Pb zircon SHRIMP dating. Mineral separation was carried out at Rand Afrikaans University in Johannesburg using standard density and magnetic separation techniques. The final concentrate was handpicked under a binocular microscope and the zircon grains were mounted in epoxy together with the zircon standard SL13 of the RSES, Australian National University (ANU). The grains were then polished and photographed. Scanning Electron Microscope cathodoluminescence imaging was carried out to detect cores, rims and other complexities, which might be present, and to ensure no areas of mixed age were analyzed. The U–Pb analyses were carried out on the SHRIMP-II and SHRIMP-RG at the RSES. Most of the analyses are based on six scans through the mass stations. The primary beam intensity was between 6 and 7 nA. Reduction of the data were done in a manner similar to that described by Williams (1998, and references therein), using the SQUID Excel Macro of Ludwig (2000). For the Pb/U calibration the measured Pb/U ratios have been normalised relative to a value of 0.1859 for the 206⁎Pb/238U ratio of the FC1 reference zircons, equivalent to an age of 1099 Ma (Paces and Miller, 1989). U and Th concentrations were determined relative to the SL13 standard. Zircon data are corrected for common lead using the measured 204Pb contents and the relevant model initial common Pb compositions of Stacey and Kramers (1975). References Agbossoumondé, Y., Ménot, R.-P., Paquette, J.L., Guillot, S., Yéssoufou, S., Perrache, C., 2007. Petrological and geochronological constraints on the origin of the Palimé–Amlamé granitoids (South Togo, West Africa): a segment of the West African Craton Paleoproterozoic margin reactivated during the Pan-African collision. Gondwana Research 12, 476–488.

Bahnemann, K.P., 1972. A review of the structure, the stratigraphy and the metamorphism of the basement rocks of the Messina District, Northern Transvaal. D.Sc thesis, Univ. Pretoria. Barton Jr., J.M., Van Reenen, D.D., 1992. When was the Limpopo orogeny? \Precambrian Research 55, 7–16. Barton Jr, J.M., Doig, R., Smith, C.B., Bohlender, F., Van Reenen, D.D., 1992. Isotopic and REE characteristics of the intrusive charnoenderbite and enderbite geographically associated with the Matok Complex. Precambrian Research 55, 451–467. Barton, J.M., Holzer, L., Kamber, B., Doig, R., Kramer, J.D., Nyfeler, D., 1994. Discrete metamorphic events in the Limpopo belt, southern Africa: implications for the application of P–T paths in complex metamorphic terrains. Geology 22, 1035–1038. Barton Jr., J.M., Barnett, J.W.R., Barton, E.S., Barnett, M., Doorgapershad, A., Twiggs, C., Klemd, R., Martin, L., Mellonig, L., Zenglein, R., 2003. The geology of the area surrounding the Venetia kimberlite pipes, Limpopo Belt, South Africa: a complex interplay of nappe tectonics and granite magmatism. South African Journal of Geology 106, 109–128. Barton Jr., J.M., Klemd, R., Zeh, A., 2006. The Limpopo Belt: a result of Archean to Proterozoic, Turkic-type orogenesis? In: Reimold, W.U., Gibson, R.L. (Eds.), Processes on the Early Earth, vol. 405. Geological Society of America Special Paper, pp. 315–332. Berger, M., Kramers, J.D., Nägler, T.F., 1995. Geochemistry and geochronology of charnoenderbites in the Northern Marginal Zone of the Limpopo Belt, Southern Africa, and genetic models. Swiss Bulletin of Mineralogy and Petrology 75, 17–42. Blenkinsop, T.G., Frei, R., 1996. Archaean and proterozoic mineralization and tectonics at the Renco Mine (Northern Marginal Zone of the Limpopo Belt, Zimbabwe). Economic Geology 91, 1225–1238. Blenkinsop, T.G., Kröner, A., Chiwara, V., 2004. Single stage, late Archaean exhumation of granulites in the Northern Marginal Zone, Limpopo Belt, Zimbabwe, and relevance to gold mineralization at Renco Mine. South African Journal of Geology 107, 377–396. Boshoff, R., 2004. Formation of major fold types during distinct geological events in the Central Zone of the Limpopo Belt, South Africa: new structural, metamorphic and geochronologic data. M.Sc. Thesis, Rand Afrikaans University. Boshoff, R., Van Reenen, D.D., Smit, C.A., Perchuk, L.L., Kramers, J.D., Armstrong, R.A., 2006. Geologic history of the Central Zone of the Limpopo Complex: the west Alldays area. Journal of Geology 114, 699–716. Brandl, G., 1983. Geology and geochemistry of various supracrustal rocks of the Beit Bridge Complex east of Messina. In: Van Biljon, W.J., Legg, J.H. (Eds.), The Limpopo Belt. Special Publication - Geological Society of South Africa, vol. 8, pp. 103–112. Brandl, G., Pretorius, S.J., 2000. The geology of the Alldays area,. Council of Geoscience, p. 71. Buick, I.S., Williams, I.S., Gibson, R.L., Cartwright, I., Miller, J.A., 2003. Carbon and U–Pb evidence for a Palaeoproterozoic crustal component in the Central Zone of the Limpopo Belt, South Africa. Journal of the Geological Society of London 160, 601–612. Chavagnac, V., Kramers, J.D., Nägler, T.F., Holzer, L., 2001. The behaviour of Nd and Pb isotopes during 2.0 Ga migmatization in paragneisses of the Central Zone of the Limpopo Belt (South Africa and Botswana). Precambrian Research 112, 51–86. Feldtmann, F., 1996. The structural-metamorphic evolution of the marble and calc-silicate rocks of the Baklykraal quarry near Alldays, Central Zone, Limpopo Belt, South Africa. M.Sc. thesis, Rand Afrikaans University. Frei, R., Kamber, B.S., 1995. Single mineral Pb–Pb dating. Earth and Planetary Science Letters 129, 261–268. Frei, R., Villa, I.M., Nägler, F., Kramers, J.D., Przyblylowi, W.J., Prozesky, V.M., Hofmann, B.A., Kamber, B.S., 1997. Single mineral dating by the Pb–Pb step-leaching method: assessing the mechanism. Geochimica et Cosmochimica Acta 61, 393–414. Frei, R., Blenkinsop, T.G., Schönberg, R., 1999. Geochronology and late Archaean Razi and Chilimanzi suite of granites in Zimbabwe: implications for the late Archaean tectonics of the Limpopo Belt and Zimbabwe Craton. South African Journal of Geology 102, 55–63.

D.D. Van Reenen et al. / Gondwana Research 14 (2008) 644–662 Fripp, R.E.P., Lilly, P.A., Barton Jr., J.M., 1979. The structure and origin of the Singelele Gneiss, Limpopo mobile belt, South Africa. Transvaal Geological Society of South Africa 82, 161–167. Goscombe, B.D., Gray, D.R., 2008. Structure and strain variation at mid-crustal levels in a transpressional orogen: a review of Kaoko Belt structure and the character of West Gondwana amalgamation and dispersal. Gondwana Research 13, 45–85. Hofmann, A., Kröner, A., Brandl, G., 1998. Field relationships of mid- to late Archaean high-grade gneisses of igneous and sedimentary parentage in the Sand River, Central Zone of the Limpopo Belt, South Africa. South African Journal of Geology 101, 185–200. Holzer, L., 1995. The magmatic petrology of the Bulai Pluton and the tectonometamorphic overprint at 2.0 Ga in the central zone of the Limpopo belt (Messina-Beitbridge area. southern Africa). Diploma thesis, University of Berne. Holzer, L., 1998. The transpressive Orogeny at 2 Ga in the Limpopo Belt, South Africa. Unpublished Ph.D Thesis, University of Bern, Switzerland, pp.202. Holzer, L., Kamber, B.S., Kramers, J.D., Frei, R., 1995. The tectonometamorphic event at 2 Ga in the Limpopo Belt and the resetting behavior of chronometers at high temperature. Communications of the Geological Survey of Namibia 10, 129–140. Holzer, L., Frei, R., Barton Jr., J.M., Kramers, J.D., 1998. Unravelling the record of successive high grade events in the Central zone of the Limpopo belt using Pb single phase dating of metamorphic minerals. Precambrian Research 87, 87–115. Holzer, L., Barton, J.M., Paya, B.K., Kramers, J.D., 1999. Tectonothermal history of the western part of the Limpopo Belt: tectonic models and new perspectives. Journal of African Earth Sciences, 28, 383–402. Horrocks, P.C., 1983. The Precambrian geology of an area between Messina and Tshipise, Limpopo mobile belt. In: Van Biljon, W.J., Legg, J.H. (Eds.), The Limpopo Mobile Belt. Special Publication - Geological Society of South Africa, vol. 8, pp. 81–88. Jaeckel, P., Kröner, A., Kamo, S.L., Brandl, G., Wendt, J.I., 1997. Late Archaean to Early Proterozoic granitoid magmatism and high grade metamorphism in the central Limpopo belt, South Africa. Journal of the Geological Society of London 154, 25–44. Kamber, B.S., Blenkinsop, T.G., Villa, I.M., Dahl, P.S., 1995a. Proterozoic strike-slip deformation and uplift of the Central Zone, Limpopo Belt, Zimbabwe. Journal of Geology 103, 493–508. Kamber, B.S., Kramers, J.D., Napier, R., Cliff, R.A., Rollinson, H.R., 1995b. The triangle Shearzone, Zimbabwe, revisited: new data document an important event at 2.0 Ga in the Limpopo Belt. Precambrian Research 70, 191–213. Kamber, B.S., Biino, G.G., Wijbrans, J.R., Davies, G.R., Villa, I.M., 1996. Archean granulites of the Limpopo Belt, Zimbabwe: one slow exhumation or two rapid events? Tectonics 15, 1414–1430. Kamber, B.S., Frei, R., Gibb, A.J., 1998. Pitfalls and new approaches in granulite chronometry an example from the Limpopo Belt, Zimbabwe. Precambrian Research 91, 269–285. Kramers, J.D., McCourt, S., Van Reenen, D.D., 2006. The Limpopo Belt. In: Johnson, M.R., Anhaeusser, C.R., Thomas, R.J. (Eds.), The Geology of South Africa. Geological Society of South Africa. Johannesburg/Council for Geoscience, Pretoria, p. 691. Kreissig, K., Thomas, F.N., Kramers, J.D., Van Reenen, D.D., Smit, C.A., 2000. An isotopic and geochemical study of the northern Kaapvaal Craton and the Southern Marginal Zone of the Limpopo Belt: are they juxtaposed terranes? Lithos 50, 1–25. Kreissig, K., Holzer, L., Frei, R., Villa, I.M., Kramers, J.D., Kröner, A., Smit, C.A., Van Reenen, D.D., 2001. Geochronology of the Hour River Shear Zone and the metamorphism in the Southern Marginal Zone of the Limpopo belt, Southern Africa. Precambrian Research 109, 145–173. Kröner, A., Jaeckel, P., Hofmann, A., Nemchin, A.A., Brandl, G., 1998. Field relationships and age of supracrustal Beit Bridge Complex and associated ganitoid gneisses in the Central Zone of the Limpopo Belt, South Africa. South African Journal of Geology 101, 201–213. Kröner, A., Jaeckel, P., Brandl, G., Nemchin, A.A., Pidgeon, R.T., 1999. Single zircon ages for granitoid gneisses in the Central Zone of the Limpopo belt, southern Africa and geodynamic significance. Precambrian Research 93, 299–337.

661

Ludwig, K.R., 2000. SQUID 1.00, A User's Manual; Berkeley Geochronology Centre Special Publication, vol. 2. Berkeley, California. McCourt, S., Vearncombe, J.R., 1992. Shear zones of the Limpopo Belt and adjacent granitoids-greenstone terranes: implications for late Archaean collision tectonics in southern Africa. Precambrian Research 55, 553–570. McCourt, S., Armstrong, R.A., 1998. SHRIMP U–Pb zircon geochronology of granites from the Central Zone, Limpopo Belt, southern Africa: implications for the age of the Limpopo Orogeny. South African Journal of Geology 101, 329–338. Mkweli, S., Kamber, B., Berger, M., 1995. Westward continuation of the Craton-Limpopo Belt tectonic Break in Zimbabwe and new age constraints on the timing of the thrusting. Journal of the Geological Society of London 152, 77–83. Paces, J.B., Miller, J.D., 1989. Precise U–Pb ages of Duluth Complex and related mafic intrusions, Northeastern Minnesota: geochronological insights to physical, petrogenic, paleomagnetic and tectonomagmatic processes associated with the 1.1 Ga mid-continent rift system. Journal of Geophysical Research 98B, 13997–14013. Perchuk, L.L., Gerya, T.V., Van Reenen, D.D., Smit, C.A., 2006a. P–T paths and problems of high-temperature polymetamorphism. Petrology 14, 117–153. Perchuk, L.L., Varlamov, D.A., Van Reenen, D.D., 2006b. A unique record of P–T history of high-grade polymetamorphism. Doklady Earth Sciences 409A, 958–962. Perchuk, L.L., Van Reenen, D.D., Varlamov, D.A., Van Kal, S.M., Boshoff, R., Tabatabaeimanesh, S.M., in press. P–T record of two high-grade metamorphic events in the Central Zone of the Limpopo Complex, South Africa. Lithos. Pienaar, J.C., 1985. The geology of the area in the vicinity of Alldays in northern Transvaal. M.Sc. thesis, Rand Afrikaans University. In Afrikaans. Pretorius, S.J., 1986. Die geologie in die omgewing van Swartwater en die westelike deel van die distrik Messina in die sentrale sone van die Limpopogordel. M.Sc thesis, Rand Afrikaans University. Roering, C., Van Reenen, D.D., Smit, C.A., Barton Jr., J.M., De Beer, J.H., De Wit, M.J., Stettler, E.H., Van Schalkwyk, J.F., Stevens, G., Pretorius, S., 1992. Tectonic model for the evolution of the Limpopo Belt. Precambrian Research vol. 55, 539–552. Schäller, M., Steiner, O., Studer, I., Frei, R., Kramers, J.D., 1997. Pb stepwise leaching (PbSL) dating of garnet–addressing the inclusion problem. Swiss Bulletin of Mineralogy and Petrology 77, 83–91. Schäller, M., Steiner, O., Studer, I., Holzer, L., Herwegh, M., Kramers, J.D., 1999. Exhumation of Limpopo Central Zone granulites and dextral continent-scale transcurrent movement at 2.0 Ga along the Palala Shear Zone, Northern Province, South Africa. Precambrian Research 96, 263–288. Söhnge, P.G., 1945. The geology of the Messina copper mines and surrounding country. Memoirs of the Geological Survey of South Africa 40, 280. Stacey, J.S., Kramers, J.D., 1975. Approximation of terrestrial lead isotope evolution by a two-stage model. Earth and Planetary Science Letters 26, 207–221. Tucker, R.D., Kusky, T.M., Buchwaldt, R., Handke, M.J., 2007. Neoproterozoic nappes and superposed folding of the Itremo Group, west-central Madagascar. Gondwana Research 12, 356–379. Van Kal, S., 2004. Two distinct tectono-metamorphic events in the Central Zone of the Limpopo Complex, South Africa: evidence from the Mt Shanzi sheath fold and the Campbell cross fold near Musina. M.Sc thesis, Rand Afrikaans University. Van Reenen, D.D., Barton Jr., J.M., Roering, C., Smit, C.A., Van Schalkwyk, J.F., 1987. Deep crustal response to continental collision: the Limpopo belt of southern Africa. Geology 15, 11–14. Van Reenen, D.D., Roering, C., Brandl, G., Smit, C.A., Barton Jr., J.M., 1990. The granulite-facies rocks of the Limpopo belt, southern Africa. In: Vielqeuf, D., Vidal, P. (Eds.), Granulites and Crustal Evolution. . NATO ASI series. Series C, Mathematical and Physical Science, 311. Kluwer Academic Publication, Dordrecht, pp. 257–289. Van Reenen, D.D., Roering, C., Ashwal, L.D., De Wit, M.J. (Eds.), 1992. The Archaean Limpopo Granulite Belt: Tectonics and Deep Crustal Processes. Precambrian Research, vol. 55, p. 587. Van Reenen, D.D., Perchuk, L.L., Smit, C.A., Varlamov, D.A., Boshoff, R., Huizenga, J.M., Gerya, T.V., 2004. Structural and P–T evolution of a major

662

D.D. Van Reenen et al. / Gondwana Research 14 (2008) 644–662

Cross Fold in the Central Zone of the Limpopo High-grade Terrain, South Africa. Journal of Petrology, 45, 1413–1439. Watkeys, M.K., 1984. The Precambrian geology of the Limpopo belt north and west of Messina. PhD. Thesis, Univ. Witwatersrand. Watkeys, M.K., Light, M.P.R., Broderick, R.J., 1983. A retrospective view of the Central Zone of the Limpopo Belt, Zimbabwe. Special Publication Geological Society of South Africa 8, 65–80.

Williams, I.S., 1998. U–Th–Pb geochronology by ion microprobe. In: McKibben, M.A., SahnksIII III, W.C., Ridley, W.I. (Eds.), Applications of Microanalytical Techniques to Understanding Mineralizing Processes. Reviews in Economic Geology, vol. 7.