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 (1999) 263–288

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 Mirjam Schaller a, Oliver Steiner a, Ivan Studer a, Lorenz Holzer a,1, Marco Herwegh b, Jan D. Kramers a, * a Gruppe Isotopengeologie, Mineralogisch–Petrographisches Institut, Universita¨t Bern, Erlachstr. 9a, CH-3012 Bern, Switzerland b Geologisches Institut, Universita¨t Bern, Baltzerstr. 1, CH-3012 Bern, Switzerland Received 3 February 1999; accepted 19 March 1999

Abstract The Palala Shear Zone is part of a major tectonic lineament in southern Africa, which extends over 1000 km from central Botswana to the Soutpansberg in South Africa. The lineament is situated between the Central Zone of the Limpopo Belt and the Kaapvaal Craton. At its main exposure, in the Koedoesrand Window 50 km E of Ellisras, Northern Province, the Palala Shear Zone comprises a c. 15 km wide mylonite zone which can be subdivided into a Northern, Central and Southern Domain. This study addresses the tectono-metamorphic history of the Palala Shear Zone in the Koedoesrand Window and its role in the tectonics of the Limpopo Belt. In order to do this, structural and petrographical work was combined with geochronology. Age data from relic mineral parageneses show that metapelites from the Central Domain underwent granulite facies metamorphism around 2.6 Ga. However, because of an intense Proterozoic overprint, field relationships of the Archean event are largely disrupted. During a second highgrade event at c. 2.0 Ga, granulite facies metamorphism was reached in the Northern Domain and the adjacent Limpopo Central Zone. Charnockites were emplaced in the Northern Domain after 2.1 Ga and were affected by retrograde amphibolite and greenschist facies metamorphism around 1.97 Ga. Post-orogenic transcurrent faulting under (sub)greenschist facies conditions characterizes the Central and Southern Domains. Field observations and microstructural investigations indicate a dextral shear sense for the retrograde mylonitisation. These findings document that the Palala Shear Zone played an important role during the exhumation of the Proterozoic granulites in the Limpopo Central Zone, associated with a transpressive orogeny between 2.05 and 1.95 Ga. Low-grade post-orogenic transcurrent faulting in the Palala Shear Zone at c. 1.9 Ga may be associated with the transtensional opening of the Soutpansberg and Palapye grabens. On the basis of the tectono-metamorphic history in the Koedoesrand Window and regional geophysical, geochemical and sequence-stratigraphic data, the Palala Shear Zone is interpreted as a Proterozoic suture zone, along which the Archean Kaapvaal and Zimbabwe Provinces were juxtaposed. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Granulite facies metamorphism; Limpopo Belt; Polyphase deformation; Proterozoic

* Corresponding author. Tel.: +41-31-631-8789; fax: +41-31-631-4988. E-mail address: [email protected] (J.D. Kramers) 1 Present address: EMPA Du¨bendorf, Eidgeno¨ssische Material Pru¨fungs- und Forschungsanstalt, Ueberlandstrasse 129, CH-8021 Du¨bendorf, Switzerland. 0301-9268/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S0 3 0 1- 9 2 68 ( 9 9 ) 0 00 1 5 -7

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1. Introduction Together the Kaapvaal and Zimbabwe Cratons in southern Africa form one of the world’s largest areas of Archean crust. The ENE trending granulite facies Limpopo Belt is wedged between the Kaapvaal and Zimbabwe Cratons ( Figs. 1 and 2). In most tectonic models high-grade metamorphism in the Limpopo Belt is interpreted as a product of a late Archean continent–continent collision at 2.7–2.65 Ga between the Kaapvaal and Zimbabwe Cratons (e.g. Light, 1982; Van Reenen et al., 1990; Barton and Van Reenen, 1992a; Treloar et al., 1992; Roering et al., 1992). This ‘Limpopo Orogeny’ is thus suggested to mark the final stages of the formation of a Southern African Archean continent (De Wit et al., 1992).

However, recent studies provide evidence for a much younger tectono-metamorphic event in the Limpopo Belt. High-pressure granulite metamorphism with a clockwise PT loop is dated to 2.0 Ga (e.g. Barton et al., 1994; Jaeckel et al., 1997). This type of PT evolution has previously been described as a characteristic feature of the Archean ‘Limpopo Orogeny’ as a whole (e.g. Horrocks, 1983; Droop, 1989), but the latest geochronological investigations have now shown that there is also a highpressure granulite event at 2020 Ma which is confined to the Limpopo Central Zone (Holzer et al., 1997). The tectonic setting of the Proterozoic highgrade event is still a matter of controversy, and two opposite interpretations exist. One group of geologists argues that the Proterozoic high-grade metamorphism might

Fig. 1. Terrane interpretation of Southern Africa adapted from Cheney and Winter (1995). On the basis of sequence-stratigraphic correlations two Archean provinces can be distinguished: the Kaapvaal and Zimbabwe Provinces. The Palala–Zoetfontein lineament defines the boundary between these two provinces. No sedimentary sequences older than 2.0 Ga can be correlated across the Palala– Zoetfontein lineament.

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Fig. 2. Geological sketch map of the Limpopo Belt and the adjacent Kaapvaal and Zimbabwe Cratons showing the location of the Koedoesrand Window. Curved lines in the Limpopo Central Zone mark the trend of foliations. Abbreviations: NLTZ, North Limpopo Thrust Zone; HRSZ, Hout River Shear Zone. Proterozoic intrusions: 1, Schiel Alkaline Complex; 2, Entabeni Pluton; 3, Palala Granite; 4, Palala Charnockite (retrogressed ); 5, Mahalapye Granite; 6, Mokgware Granite. Archean Plutons: 7, Bulai Pluton; 8, Matok Pluton. Greenstone Belts (GB): 9, Sutherland GB; 10, Rhenosterkoppies GB; 11, Buhwa GB; 12, Gwanda GB; 13, Umzingwane GB; 14, Belingwe GB. Sedimentary basins: 15, Waterberg; 16, Palapye; 17, Karroo sediments. 18, Profile line from Fig. 3.

reflect an anorogenic thermal event, probably related to the emplacement of the Bushveld Igneous Complex at 2060 Ma (e.g. McCourt et al., 1995), or to a mantle plume ( Treloar et al., 1997). It is further argued that the near isothermal decompression might result from delamination of the subcontinental lithospheric roots ( Treloar et al., 1997). In the case of such an anorogenic thermal event the structural pattern of the entire Limpopo Belt would be the product of earlier tectonism. This viewpoint strongly supports ‘Archean-only’ tectonic models for the Limpopo Belt. On the other hand, an increasing number of geologists believe that the Proterozoic event reflects a major orogenic cycle (e.g. Barton et al., 1994; Kamber et al., 1995a; Holzer et al., 1996, 1998; Jaeckel et al., 1997). The inferred crustal thickening which led to the high-grade metamorphism

shortly before 2 Ga is explained as a product of transpressive movements between the Kaapvaal and Zimbabwe Cratons ( Kamber et al., 1995a). Such transpressive tectonism at 2.0 Ga has been documented for the boundary between the Limpopo Central and Northern Marginal Zones, i.e. the Triangle Shear Zone ( Kamber et al., 1995b). Important Proterozoic deformation also occurred within the other bounding shear zones of the Limpopo Central Zone, i.e. the Palala, Sunnyside, Magogaphate and Lepokole Shear Zones (Holzer et al., 1999; see Fig. 14 later). The results of these recent studies question the ‘Archean-only’ tectonic models. The controversy culminates in the question whether the collision between the Kaapvaal and Zimbabwe Cratons was Archean or Proterozoic (or both). Holzer et al. (1997) considered the Palala Shear

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Zone as a possible Proterozoic suture zone between the two cratons. The Palala Shear Zone (also called ‘Palala–Zoetfontein lineament’) represents a major tectonic break which can be traced over several hundreds of kilometres along the northern edge of the Kaapvaal Craton. It separates two sedimentary provinces, the Kaapvaal and Zimbabwe Provinces (Cheney and Winter, 1995; see Fig. 1). In the Koedoesrand Window (our study area, see Fig. 2) the Palala Shear Zone marks the boundary between the granulite facies Limpopo Central Zone and the low-grade rocks of the adjacent Kaapvaal Craton. Most Palala mylonites formed under (sub)greenschist facies conditions, but relic parageneses at granulite and amphibolite grade are also frequent. Under the view of an ‘Archean-only’ Limpopo history, the high-grade parageneses have been interpreted as a product of the Archean ‘Limpopo Orogeny’. However, because the Palala mylonites cross-cut the Bushveld Igneous Complex (2060 Ma), later reactivation must have occurred. It was argued that this Proterozoic reactivation had ‘‘little influence on the creation of the high-grade Limpopo Belt’’ ( Van Reenen et al., 1988) because it was interpreted as a low-grade post-orogenic episode. However, the recent perception of a Proterozoic high-grade event in the Limpopo Central Zone also questions these tectonic interpretations, and therefore we have re-investigated the Palala Shear Zone. In this paper we have three main aims. First we intend to unravel the polymetamorphic history in the Koedoesrand Window. In the main part of the paper we therefore describe structural geometries and petrographical observations and combine them with new geochronological data. A second point of interest is the comparison of the tectonic evolution in the Koedoesrand Window with the recent findings of a Proterozoic event in the entire Limpopo Central Zone. The third point considers the Palala Shear Zone as a potential suture zone between the Kaapvaal and Zimbabwe Cratons. In order to test possible scenarios of the juxtaposition of the two cratons, various large-scale geological relationships have to be (re)considered. We therefore briefly discuss some geophysical, geochemical

and sequence stratigraphic aspects from the terrains N and S of the Palala Shear Zone.

2. Geological setting 2.1. The Limpopo Belt The Limpopo Belt (Fig. 2, c. 550 km in length, with a width of c. 250 km) is commonly divided into three subzones: the Northern Marginal Zone (NMZ), the Limpopo Central Zone (LCZ ), and the Southern Marginal Zone (SMZ ). The two marginal zones are regarded as lower crustal highgrade equivalents of the adjacent cratons which have been exhumed mainly in the late Archean along inward dipping thrust zones: the North Limpopo Thrust Zone (Blenkinsop et al., 1995; Mkweli et al., 1995) and the Hout River Shear Zone (Smit et al., 1992). In contrast, the LCZ is made up of high-grade metasediments (the Beit Bridge Complex) with a preceding tonalitic basement (the Sand River Gneisses) and mafic as well as felsic plutonic rocks of various ages (e.g. the Messina Layered Intrusion and the Singelele and Bulai granitoids). The LCZ is bounded by two strike–slip shear zones, the Triangle Shear Zone in the N and the Palala Shear Zone in the S. Recent studies document that the three Limpopo subzones underwent distinct metamorphic histories. This is schematically summarized in the upper part of Fig. 3 (simplified after Holzer et al., 1998). In the SMZ granulite facies metamorphism occurred shortly after 2.7 Ga, with a clockwise PT evolution, and has been attributed to orogenic tectonics (e.g. Van Reenen et al., 1990). In contrast, the granulites in the NMZ are characterized by anticlockwise PT evolution and a late thermal peak was reached at 2.58 Ga ( Kamber and Biino, 1995; Kamber et al., 1995a; Rollinson, 1989) — 100 Ma after peak metamorphism in the SMZ. Closely related to this high-grade metamorphism is the widespread granitoid magmatism in the NMZ (Razi Granites), which occurred between 2.65 and 2.5 Ga (Blenkinsop and Frei, 1997). The LCZ has a unique, polymetamorphic history. A first phase of TTG formation occurred around 3.2 Ga and a first metamorphic event predates

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Fig. 3. Interpretation of a seismic profile through the Limpopo Belt (after De Beer and Stettler, 1992) showing the main seismic reflectors. The profile line is shown in Fig. 2. The upper part of the figure schematically summarizes the metamorphic evolution in each of the Limpopo subzones and indicates the timing of major magmatic activity (for discussion and references see text). Abbreviations: SMZ, Southern Marginal Zone; LCZ, Limpopo Central Zone; NMZ, Northern Marginal Zone; KC, Kaapvaal Craton; HRSZ, Hout River Shear Zone; TSZ, Triangle Shear Zone.

mafic dykes at 3.0 Ga (e.g. Barton et al., 1990; Kro¨ner et al., 1999). The late Archean history of the LCZ is very similar to that of the NMZ. For the period between 2.7 and 2.6 Ga, mainly magmatic activity is documented ( Kro¨ner et al., 1999). High-grade metamorphism, anatexis and felsic magmatism persisted after 2.6 Ga. A final thermal pulse of the late Archean event occurred at 2.52 Ga (Holzer et al., 1998). However, as mentioned above, the main metamorphic event in the LCZ is now dated at 2 Ga, when high-pressure granulite conditions were reached (Barton et al., 1994; Kamber and Biino, 1995; Kamber et al., 1995a,b; Jaeckel et al., 1997). During this Proterozoic event important transpressive deformation was localized in the bounding strike–slip shear zones of the LCZ ( Kamber et al., 1995b; Holzer et al., 1999). The deep structure of the Limpopo Belt is shown in the lower part of Fig. 3, which represents

the main reflectors of a seismic reflection line (after De Beer and Stettler, 1992). The trace of the seismic profile is indicated in Fig. 2. A striking feature of the deep seismic profile across the Limpopo Belt is the lack of seismic reflectors within the LCZ. This might be attributed to the observed complex structural pattern which was produced during polymetamorphism and polyphase ductile folding. The Triangle Shear Zone at the northern boundary of the LCZ can be correlated with a southward dipping zone of reflectors continuing to lower crustal depth. In contrast, ‘‘the southern boundary of the LCZ, which is correlated with the Palala Shear Zone, has no direct seismic signature… The gravity anomaly (not shown here) is indicative of a steeply dipping feature. The Palala fault therefore can be interpreted as a near-vertical fault that penetrates the crust’’ (De Beer and Stettler, 1992).

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2.2. The Palala Shear Zone The Palala Shear Zone is a continental scale tectonic lineament which defines an ENE trending tectonic break at the northern edge of the Kaapvaal Craton (compare Figs. 2 and 14). It extends for c. 900 km from the Kalahari line in Botswana, to the Vivo area (60 km W of Louis Trichard ) where it is lost underneath the Soutpansberg trough. It is best exposed in the Koedoesrand Window (c. 300 km2) which is situated 50 km NE of the town of Ellisras (Fig. 2). The Koedoesrand Window was first mapped by Visser (1953) and re-investigated by McCourt (1983), McCourt and Vearncombe (1987, 1992), Brandl and Reimold (1990) and Broekhuizen et al. (1995). In Botswana it is entirely covered by Phanerozoic sediments ( Karroo and Kalahari). Reactivation of the Palala Shear Zone led to the formation of the Zoetfontein fault, which cuts the Phanerozoic cover. The entire structure is therefore also called the Palala–Zoetfontein lineament. Between the intersection with the Limpopo river and the Soutpansberg trough it marks the boundary between the Kaapvaal Craton and the LCZ. Although the Palala Shear Zone is poorly exposed over large parts along strike, it is clear from aeromagnetic data that this lineament forms a prominent structure of crustal scale. Field evidence indicates that several deformational episodes have affected the rocks of the Palala Shear Zone. A major phase of transcurrent faulting is bracketed between the Bushveld Igneous Complex intrusion age (2.06 Ga, Walraven et al., 1990) and the deposition of the Soutpansberg sediments (between 1.95 and 1.85 Ga, Barton et al., 1996). The gabbros of the Bushveld Igneous Complex in the Koedoesrand Window are strongly affected by low-grade mylonitisation ( Visser, 1953), while the <1.95 Ga old Soutpansberg sediments locally overlie the Palala mylonites in the Vivo area (Brandl, 1981). The Soutpansberg graben, however, is formed in a dextral transtensive regime which may be associated with a late phase of transcurrent faulting along the Palala Shear Zone (Barker, 1983). The Palala Shear Zone in the Koedoesrand Window is represented by a c. 15 km wide myloni-

tic belt situated between the granulite facies LCZ to the N and the northernmost limb of the Bushveld Igneous Complex to the S (Fig. 4). The boundary between the Palala Shear Zone and the LCZ is conventionally defined by the Melinda fault, a large-scale ENE trending brittle fracture which also affects Karroo sediments (Fig. 4). The southern margin of the PSZ is marked by a southward transition from extensive mylonites to isolated shear zones within the otherwise littledeformed Bushveld Igneous Complex rocks. The shear zones decrease in width and frequency to the S. To the E and to the W of the Koedoesrand Window the shear zone is poorly exposed or covered by clastic sediments younger than 2.0 Ga. According to McCourt (1983) the Palala Shear Zone can be subdivided into a Northern, Central and Southern Domain. In our description we further define a Pyroxenite Subdomain within the southern part of the Central Domain. Northern Domain (ND): Quartzo-feldspathic gneisses, xenolith-bearing strongly deformed and retrogressed charnockites (‘Hornblende-granites’ of previous authors), intermediate to felsic mylonites and layers of calcsilicates are the dominant lithologies of this domain. Relic, moderately to steeply dipping mineral stretching lineations occur in the northernmost part. These ductile structures are obliterated by amphibolite to greenschist facies shears with subhorizontal mineral elongation lineations. Central Domain (CD): Lenses of high-grade metasedimentary gneisses are hosted in felsic, mafic and enderbitic mylonites and ultramylonites. This domain contains an ENE striking foliation, a predominantly subhorizontal mineral elongation lineation and occasional isoclinal folds with subhorizontal axes and N dipping axial planes. The transition into the ND is marked by a continuous change of lithologies and style of deformation. In the CD, deformation is generally more intense (ultramylonites) and occurred at lower grade. This transition zone is intruded by late tectonic granitoids, which we termed ‘Northern Granite’. Pyroxenite Subdomain: Pyroxenite bodies are hosted within mafic mylonites. The mylonitic foliations in this subdomain are bent from ENE to N trending between the pyroxenite lenses. Mineral

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Fig. 4. Simplified geological map of the Palala Shear Zone in the Koedoesrand Window, Northern Province. The small inset (bottom right) shows the subdivision into five tectonic units (from N to S): Limpopo Central Zone, Palala Northern, Central and Southern Domains, Bushveld Igneous Complex/Kaapvaal Craton. In the southern part of the Central Domain we define a Pyroxenite Subdomain. Each of these domains underwent a distinct tectono-metamorphic evolution during the late Archean and mid/early Proterozoic events (discussed in text).

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lineations remain subhorizontal within the foliations. Southern Domain (SD): Cataclastic and lowgrade mylonitic deformation are a characteristic feature of this domain. The main lithologies are the Palala Granite, the Koedoesrand sediments and deformed Bushveld Igneous Complex members. The southern boundary of this domain is defined by the (sub)greenschist facies Abbottspoort Shear Zone which runs between the adjacent Bushveld Igneous Complex and the Palala Granite. Its northern boundary is formed by the sheared northern edge of the Palala Granite. This ‘Beaufort Shear Zone’ (Brandl and Reimold, 1990) in fact marks the southern limit of the c. 10 km wide penetrative mylonite zone which is the CD. The interpretations of the Palala Shear Zone are controversial. McCourt and Vearncombe (1987, 1992) suggested that the Sunnyside–Palala Shear Zones and Triangle–Tuli–Sabi Shear Zones (see Fig. 14 later) are complementary systems and interpreted them as lateral ramps on an Archean crustal-scale thrust zone. The kinematics of the bounding shear zones would suggest NE to SW emplacement of the LCZ relative to both margins, thus implying a sinistral shear sense for the Palala Shear Zone. McCourt and Vearncombe (1992) describe kinematic indicators in all but the extreme NE section of the Palala Shear Zone that are compatible with sinistral strike–slip motion. In the NE section of the Koedoesrand Window ( Farm Alexanderfontein) movement sense indicators of the mylonites with steeply plunging mineral lineation have proved elusive and either a contractional or extensional origin may apply. Furthermore, these mylonites show definitive evidence for two phases of mylonitisation. McCourt and Vearncombe (1987) and Brandl and Reimold (1990) document discrete shear bands in the Palala Granite with dextral shear sense. This observation agrees with the description of dextral shear zones in the southern part of the Palala Shear Zone by Broekhuizen et al. (1995). Whilst McCourt and Vearncombe (1987, 1992) postulate several phases of reactivation in pre- and post-Bushveld times, Brandl and Reimold (1990) describe a single post-Bushveld deformation event which occurred over a prolonged period of time

(>100 Ma). Mylonitisation of the ND and CD is interpreted to have taken place shortly after the emplacement of the Palala Granite. Deformation in the SD is thought to have occurred at higher crustal levels, and also later than the events in the other domains.

3. Deformation and metamorphism in the Koedoesrand Window In the following sections we summarize structural and petrographic observations which are described in greater detail by Schaller et al. (1997b). The data are presented as a N–S profile starting in the LCZ through the PSZ domains into the Bushveld Igneous Complex. Structural data for each domain are given in Fig. 5. 3.1. Limpopo Central Zone Mineral assemblages in the different lithologies of the LCZ reflect granulite facies metamorphic conditions, e.g. mafic lenses (ortho- and clinopyroxene, amphibole and plagioclase), quartzo-feldspathic gneisses (quartz, plagioclase, K-feldspar, garnet and pyroxene), metapelites (garnet, sillimanite, biotite, cordierite, quartz) and calcsilicates (scapolite, pyroxene, titanite, garnet). In general the granoblastic textures indicate recrystallisation under high-grade conditions and provide evidence for equilibrium between the different phases of the granulite assemblages. Thermobarometric calculations with TWEEQU (Berman, 1991; Lieberman and Petrakakis, 1991) and the hornblende plagioclase thermometer of Holland and Blundy (1994) indicate temperatures of 700°C up to c. 830°C and pressures of 5 to 10 kbar (Schaller et al., 1997b). Locally, the growth of muscovite, talc, chlorite and epidote document retrograde overprint at greenschist facies conditions. A complex pattern with different generations of ductile folds can be observed within the migmatized gneisses of the LCZ. Due to the lack of structural time markers (such as magmatic bodies of known age) it cannot be distinguished whether these fold generations are part of a single continuous event, or whether they were produced during distinct events.

Fig. 5. Compilation of structural data from the Koedoesrand Window. Poles of foliations and lineations are plotted on the lower hemisphere, equal area projections; contouring on the projection sphere. Contours in multiples of a uniform distribution.

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Fig. 6. (a) Thin section showing the granoblastic texture of a granulite facies, titanite-bearing calcsilicate (sample 95/645) from the Limpopo Central Zone in the Koedoesrand Window. The PbSL mineral age of a titanite is 2017±6 Ma. (b) In retrogressed charnockite (sample 95/653) from the Northern Domain relics of orthopyroxene are overgrown by hornblende and biotite. (c) Symplectites consisting of cordierite and orthopyroxene formed at the expense of resorbed garnet in a metapelitic mylonite (sample 95/609) from the Central Domain. The symplectite texture indicates pressure decrease at high temperatures. (d) Rotated orthopyroxene clasts are embedded in an ultramylonitic matrix (feldspar, quartz and chlorite; sample 95/299 from the Central Domain).

Holzer et al. (1999) have dated titanite from a granoblastic calcsilicate [Fig. 6(a)] using the Pb stepwise leaching (PbSL) method. In recent studies evidence was presented that titanite closes at a much higher temperatures than previously suggested (e.g. Scott and St-Onge, 1995). Therefore Holzer et al. (1999) interpreted the titanite age of 2017±6 Ma to reflect the time of the (youngest) high-grade metamorphism in the northern part of the Koedoesrand Window and pointed out that this is contemporaneous with peak granulite metamorphism at 2020 Ma in other parts of the LCZ, e.g. in the Messina area (see Jaeckel et al., 1997; Holzer et al., 1998). We have sampled a titanite-bearing calcsilicate

from the LCZ domain in order to test the reproducibility of the PbSL titanite age. 3.2. Northern Domain The magmatic charnockite in the ND represents an important structural and metamorphic time marker. Banded mafic xenoliths (e.g. two pyroxene–garnet–amphibolite) within the charnockite contain relics of early granulite facies parageneses (M ), which predate the charnockite intrusion. 1 Pyroxene in both the charnockite and the mafic xenoliths is partly replaced by hornblende [see Fig. 6(b)]. In a later stage hornblende and pyroxene are overgrown by biotite, chlorite and epidote.

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These retrograde minerals define flow textures and indicate that the transition from granulite through amphibolite to greenschist facies conditions was contemporaneous with the main fabric forming event (D retrograde). Thermobarometric calcula2 tions of peak granulite conditions are made difficult by the strong amphibolite to greenschist facies retrogression. The calculated temperatures between 600 and 750°C and pressures of c. 5 kbar can merely indicate a minimum estimation of the peak conditions (Schaller et al., 1997b). Early structures (D ) which have been formed 2a during the high-grade metamorphic stage (M ) 2a include ductile folds of calcsilicate layers (up to kilometre scale) which are preserved in the western part of the ND. Steeply dipping lineations (L ) 2a within quartzo-feldspathic gneisses (e.g. Locality Alexanderfontein, Fig. 4) are marked by stretched and dynamically recrystallised plagioclase. E–W striking foliations and fold axial planes of isoclinal folds with a steep N dip may indicate a N–S shortening direction. However, the few relic highgrade structures which are preserved in the ND do not give a sufficiently coherent picture to draw further conclusions about the kinematic evolution during the high-grade deformation (D ). These 2a early deformations have been overprinted by a strong shearing, during which the predominantly WSW to W trending mylonitic foliations (S ) and 2b associated subhorizontal stretching lineations (L ) were formed ( Fig. 7). Rotated clasts of mafic 2b minerals which are embedded in (ultra)mylonitic matrices are further evidence for a strong lateral shear component. These structures thus document the strike–slip character of the D phase. Lower 2b amphibolite facies conditions associated with this deformation are reflected by the dynamic recrystallisation (grain boundary migration and subgrain rotation) of feldspar. The synkinematic growth of biotite, chlorite and clinozoisite/epidote documents a further temperature decrease during D to 2b greenschist facies conditions. 3.3. Central Domain The CD mainly consists of a c. 10 km wide band of mylonites and ultramylonites (M /D ) 2 2 with lenses of less deformed gneisses. In these

Fig. 7. At Locality Olifantsdrift the foliation S is folded and 2a dissected by a younger foliation S . The latter is characterized 2b by stronger shear intensity. Fold axes and stretching lineations plunge moderately westwards. The deformational succession from D to D is interpreted as the product of a continuous 2a 2b tectonic activity under (slightly) decreasing metamorphic conditions (for discussion see text).

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lenses relic assemblages of an early granulite metamorphic event are preserved (defined as M ). 1 Characteristic M textures consist of resorbed gar1 nets in metapelites, which are rimmed by symplectitic coronas of orthopyroxene, cordierite and rare spinel [Fig. 6(c)]. These textures, which are preserved in pressure shadows and in cracks of garnet grains, document a pressure dependent reaction that occurred during the transition from high to low pressure granulite facies conditions. Thermobarometric calculations from these pelitic parageneses with TWEEQU yielded a PT decrease from 6–8 kbar and 800°C down to values around 5–5.5 kbar and 650–700°C (details in Schaller et al., 1997b). The Ti versus AlVI distribution (Schreurs, 1985) in biotite inclusions in garnet indicates the same temperatures. In the CD, the high-grade relic textures are cut by greenschist facies mylonitic foliations (S ). The 2 D mylonitic event produced predominantly WSW 2 striking foliations (S ), which mainly dip steeply 2 towards N ( Fig. 5). The mineral stretching lineations are subhorizontal, plunging at shallow angles towards ENE in the eastern part and towards WSW in the western part of the CD. Mineral fragments of high-grade parageneses (e.g. garnet or pyroxene) are embedded in the (ultra)mylonitic matrices [Fig. 6(d )]. The clasts have been brittly deformed and/or synkinematically rotated, thus indicating non-coaxial deformation. The metamorphic grade prevailing during D is reflected by the 2 synkinematic growth of biotite, white mica, amphibole, chlorite, clinozoisite and epidote. Thus deformation occurred under greenschist facies conditions. Compared to the ND, K-feldspar and plagioclase underwent stronger grain size reduction by subgrain rotation, recrystallisation and grain boundary migration, which corroborates the lower grade kinematic conditions in the CD. Throughout the CD isoclinal folds ( F ) with 2 axial planes (S ) subparallel to the foliations S 2b 2a can be observed. The S foliations are spaced in 2b the 1 to 100 m scale. Fig. 7 documents such fold structures from the Olifantsdrift Locality. The cross-cutting relationships between S and S can 2a 2b only be identified in fold hinges, because the two foliations are otherwise co-planar. In the ND as well as the CD, aplitic dykes and the above men-

tioned ‘Northern Granite’ intrude preferentially along shear bands and S axial planes. At some 2b localities cross-cutting relationships between more and less deformed granitic dykes can be observed, indicating different stages of synkinematic melt injection. Although at some localities relative age relationships between two planar structures (S , 2a S ) can be identified, we do not interpret them as 2b a product of two distinct phases. Rather the above observations document a long lasting continuous (D ) strike–slip shearing. However, in contrast to 2 the ND, no evidence for a continuous decrease of metamorphic grade (from granulite through amphibolite to greenschist facies) during the ongoing D deformation can be observed. The conspicu2 ous absence of (intermediate) amphibolite facies mineralogy provides evidence that the granulite metamorphism (M ) in the CD and the greenschist 1 facies mylonitisation (M /D ) are two distinct 2 2 events. A Pb–Pb stepwise leaching (PbSL) age of 2602±40 Ma (Schaller et al., 1997a) was obtained for garnet of M assemblages within metapelites 1 (Melinda Locality). This age indicates that the M assemblages in the paragneisses of the CD 1 have been formed during an Archean high-grade event. Schaller et al. (1997b) further present a zircon age of 1931±33 Ma for the ‘Northern Granite’. 3.4. Pyroxenite Subdomain In the southern part of the CD a c. 2 km wide band consisting of mafic mylonites contains pyroxenite lenses. The pyroxenites form boudin lenses up to kilometre size which have been cataclastically deformed. Syntectonic talc (D ) is locally over2 grown by post-tectonic chlorite and calcite. To the N and to the S of the Pyroxenite Subdomain, the mafic mylonites strike parallel to the ‘Palala trend’ ( ENE). Between the pyroxenite lenses the foliations and subhorizontal lineations are oriented N– S (see Fig. 5). The orientation of subhorizontal lineations changes together with that of the foliations. The oblique orientation of mylonitic foliations in this domain relative to the ‘Palala trend’ may be attributed to a large rheological contrast between relatively rigid pyroxenite ‘mega-clasts’

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hosted within a less competent matrix of mafic mylonites. 3.5. Southern Domain In contrast to the other Palala domains, the SD is not characterized by a penetrative deformation. Primary magmatic textures dominate in the Palala Granite and sedimentary structures are well preserved in the Koedoesrand conglomerates and quartzites. Discrete shear zones transect both lithologies. The Koedoesrand formation outcrops in two main ridges striking parallel in ENE direction and defining a synclinal structure. The foliations in the entire SD trend E to ENE and generally

275

dip steeply towards N (Fig. 5). Mineral elongation lineations are subhorizontal. Synkinematic growth of biotite and white mica in the Palala Granite as well as the recrystallisation of quartz indicate deformation under greenschist facies conditions or even lower, as indicated by the formation of epidote. Quartz dominated Koedoesrand sediments show weak dynamic recrystallisation of quartz and synkinematic growth of white mica. Along the Abbottspoort Shear Zone, the Palala Granite as well as the Bushveld Igneous Complex are deformed in a cataclastic manner. In this shear zone steeply dipping stretching lineations may indicate a late tectonic phase with subvertical movements. Cataclastic deformation under (sub)-

Fig. 8. Compilation of all shear sense indicators for the D event in the Palala Shear Zone. Microstructural data (shape preferred 2 orientation, c-axis patterns measured on U-stage), asymmetric sigma-clasts and shear bands provide evidence for a dextral shear component.

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greenschist facies conditions are also recognized in the Bushveld Igneous Complex gabbroids S of the Abbottspoort Shear Zone. The Bushveld Igneous Complex is dated by Rb–Sr whole rock to 2061±21 Ma ( Walraven et al., 1990). Zircon ages from the Palala Granite scatter between 2040 Ma (SHRIMP, S. McCourt and R. Armstrong, personal communication) and 1972±69 Ma (G. Brandl, personal communication), providing upper age limits on the last shearing in the SD. 3.6. Late brittle deformation Several deformational phases that are characterized by brittle faulting and dyke intrusions (aplitic and doleritic) postdate the D event in the 2 PSZ. The eastward dip of the lineations in the eastern part and the westward dip in the western part of the Koedoesrand Window (Fig. 5) is attributed to tilting during a late phase of doming (Schaller et al. 1997b). 3.7. Shear sense determination The PSZ underwent polyphase deformation. The high-grade history recorded in the ND is characterized by ductile folding and variable orientation of mineral elongation lineations. Steeply dipping lineations imply a general N–S directed

Fig. 9. Thin section from a quartz-dominated band in a felsitic mylonite from the Southern Domain (Palala Shear Zone). The shape preferred orientation (SPO) indicates a dextral shear sense.

shortening and shear sense determinations are difficult to carry out. In this work we have focused on the sense of subhorizontal shearing which occurred during the predominant low-grade deformation (D ). Thereby we have used microstruc2 tural parameters such as shape preferred orientation and quartz c-axis patterns, but also shear bands and rotated clasts (see Fig. 8). Fig. 9 shows a typical example of a shape preferred orientation in a quartz-rich mylonite from the CD. Both dextral as well as sinistral structures have been observed in the SD and CD, whereby the dextral ones are more frequent (2:1). Quartz c-axis patterns: Textural c-axis patterns can be used as reliable shear sense indicators (see Simpson and Schmid, 1983). Under the first order assumption of plane strain deformation, symmetric c-axes cross-girdles are characteristic for coaxial deformation whereas oblique single girdles reflect simple shear (non-coaxial ) configurations. Oblique c-axes cross-girdles can represent either a first transient stage of simple shear (Herwegh and Handy, 1996), or general shear conditions (see Schmid and Casey, 1986). In this study, c-axis orientations were measured using standard universal stage microscope techniques ( Turner and Weiss, 1963) and projected in a pole figure using the computer program stereoplot (Mancktelow, 1987). We studied five samples from the PSZ including two metasedimentary mylonites (95/237 and 95/242) from the SD, a felsic (95/260) and an enderbitic mylonite (95/630) from the CD and a charnockitic mylonite (95/299) from the ND. In each of these samples about 250 quartz c-axes were measured within monomineralic ribbons or bands. For samples 95/237 and 95/299 asymmetric cross-girdles indicate dextral sense of shearing [shown in Fig. 10(a) for 95/237]. In sample 95/242 a homogeneous texture with a symmetrical crossgirdle is recognized, indicating pure shear deformation [Fig. 10(b)]. In sample 95/260 [Fig. 10(c)] texture varies in different microstructural domains. Domains with oblique single girdles are dextral (domain A, B and C ), weakly defined cross-girdles also have a slight dextral asymmetry (domain D). Sample 95/630 (not shown in Fig. 10) contains domains with sinistral single girdles as well as

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Fig. 10. Thin sections and c-axes stereoplots of different samples from the Palala Shear Zone. Locations are shown in Fig. 8. (a) Sample 95/237 is a muscovite and K-feldspar bearing quartzitic mylonite from the Southern Domain. The c-axes pattern is characterized by an oblique cross-girdle with homogeneous c-axes distribution. The textural obliqueness indicates a dextral shear sense. (b) Sample 95/242 from the Southern Domain is also a quartz-rich mylonite with minor K-feldspar and sericite. It is characterized by a homogeneous texture. The c-axis pattern reflects a symmetric cross-girdle, indicating a coaxial deformation regime. (c) Sample 95/260 is a quartzo-feldspathic mylonite with monomineralic quartz lenses. Distinct c-axes patterns can be distinguished in the different quartz bands. In domain A, B and C the point maxima define an oblique single girdle with a dextral shear sense. In domain D the point maxima indicate a ( less well defined) oblique cross-girdle with dextral asymmetry. Further c-axis patterns from Palala mylonites are described by Schaller et al. (1997b).

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dextral asymmetric cross-girdles. This textural heterogeneity implies strain partitioning and is typical for heterogeneous, polymineralic mylonites (Pauli et al., 1996). Overall, girdles indicate a predominance of dextral sense of shearing. Observed shear bands are very heterogeneous and do not provide a single unequivocal shear sense indication. Shear bands trending subparallel to the main foliation preferentially have a distinct dextral orientation. Conjugate shear bands oblique to the main foliation include shear bands with a sinistral as well as a dextral shear sense. Overall a ratio of 2:1 for dextral to sinistral shear bands was observed. Rotated clasts: In previous studies of the PSZ a sinistral shear sense was postulated based on the asymmetry of rotated clasts. However, at Localities Melinda and Rooipoort in the CD, a sufficient number of asymmetric clasts (amphibole and feldspar in felsic mylonites; garnet and feldspar in metapelitic mylonites) yielded a ratio of 3:1 for dextral to sinistral clasts. This shows that shear sense determination from rotated clasts is not unequivocal. In the worst case isoclinal folding which postdates the shearing can change the apparent shear sense. Based on the dextral predominance of different shear sense indicators (Fig. 8) we postulate a regional dextral strike–slip for the D deformation. 2 Nevertheless, the additional occurrence of sinistral shear senses may imply a N–S shortening, resulting in conjugate shear systems with opposite sense of shears.

4. Geochronology 4.1. Sample description and analytical techniques U–Pb: Conventional U–Pb zircon dating was performed on four grain-size fractions from a retrogressed charnockite in the ND (sample 95/655). Blank, spike and fractionation corrections as well as the general analytical procedure are identical with those described by Holzer et al. (1999). The charnockite consists of quartz, plagioclase, K-feldspar, orthopyroxene, biotite and hornblende. Biotite and hornblende grew at the expense

of pyroxene during the retrograde deformation [D , see also Fig. 6(b)]. SEM imaging (backscat2 tered electron modus) of the accessory zircons revealed the presence of complex zonations with round cores and euhedral outer zones (Schaller et al., 1997b). Pb/Pb: Pb stepwise leaching (PbSL) was performed on titanite from a calcsilicate in the LCZ (sample 95/647). The subhedral, pale brown titanite is embedded in a matrix dominated by orthopyroxene and scapolite, and contains inclusions of opaque phases [compare Fig. 6(a)]. It was treated following the Pb stepwise leaching (PbSL) technique of Frei and Kamber (1995) with modifications of leaching intervals (see Table 1). Rb–Sr: Rb–Sr dating was carried out on a biotite-rich metapelitic mylonite (sample 95/423). Dissolution of very fine-grained, dark brown biotite and a whole rock powder, chemical extraction and analyses of Rb and Sr followed routine laboratory procedures (see Gilg and Frei, 1994). Data listed in Table 2 are corrected for fractionation, spike and blank. Biotite is the main fabric forming mineral in this mylonite and it coexists with chlorite, epidote and feldspar. The texture indicates that biotite completely recrystallised during the mylonitisation under greenschist facies conditions. 4.2. Geochronological data U–Pb: The four zircon fractions of the (retrogressed ) charnockite neither define an isochron in the Pb–Pb diagram (207Pb/204Pb vs. 206Pb/204Pb) nor a discordia line in the conventional concordia diagram ( Table 3 and Fig. 11). The common Pb content of all fractions was considerable and Pb isotope ratios of feldspar from the same rock ( Table 1) were used for correction. All data are discordant (up to 25%), and the pattern most likely reflects multiple discordancy due to both inherited cores (as observed) and probably subrecent Pb loss. The age of 2117±17 Ma given by a cord forced through zero is thus a maximum estimate for the crystallisation of the magmatic rims. Pb/Pb: The PbSL data of Table 1 are plotted in the uranogenic [207Pb/204Pb vs. 206Pb/204Pb,

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M. Schaller et al. / Precambrian Research 96 (1999) 263–288 Table 1 PbSL data of titanite from sample 95/647 Sample

Mineral

Acida

Time

Codeb

206Pb/204Pb

± 2sc

207Pb/204Pb

± 2s

208Pb/204Pb

± 2s

rd 1

re 2

95/647 95/647 95/647 95/647 95/647

ttn ttn ttn ttn ttn

mix 4 N HBr 8 N HBr 15 N HNO 3 HF, residue

15 min 4h 12 h 1d 2d

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

19.19 221.2 1330 1159 1041

0.54 10.9 73 86 59

15.87 40.88 179.0 157.5 142.9

0.45 2.02 9.9 11.7 8.1

38.63 187.6 954 849 772

1.09 9.3 52 63 44

0.997 0.999 1.000 0.995 1.000

0.999 1.000 1.000 1.000 1.000

a Mix=1.5 N HBr–2 N HCl 12:1 mixture used in the first leach step. b Step leaching codes according to Table 3 (Frei and Kamber, 1995; with slight modifications). c Errors are two standard deviations absolute after Ludwig (1988). d r =206Pb/204Pb vs. 207Pb/204Pb error correlation after Ludwig (1988). 1 e r =206Pb/204Pb vs. 208Pb/204Pb error correlation after Ludwig (1988). 2 Table 2 Rb–Sr data of biotite from sample 95/423 Sample

Rb (ppm)

2SE (%)

Sr (ppm)

2SE (%)

87Rb/86Sr

2SE (%)

87Sr/86Sr

2SE (%)

Biotite Whole rock

463.9 36.6

0.03 0.06

35.1 265.2

0.06 0.12

42.78 0.40

0.09 0.18

1.929 0.727

0.024 0.010

Fig. 12(a)] and in the thorogenic [208Pb/204Pb vs. 206Pb/204Pb, Fig. 12(b)] Pb isotope diagrams. The first three steps yielded increasingly uranogenic as well as thorogenic Pb compositions followed by decreasing values for steps [4] and [5]. The PbSL data define an isochron of 2020±8 Ma (MSWD= 0.07). In the thorogenic vs. uranogenic diagram [Fig. 12(b)] a quasi-linear array is formed by the five leach steps. This indicates that effectively all Pb was leached from one phase (titanite) and none was derived from inclusions. As shown by Frei et al. (1997), monazite inclusions, if present, would have increased the 208Pb/204Pb ratio of step [2], whereas zircon inclusions would have influenced step [5] by highly uranogenic and at the same time low thorogenic values. Recent geochronological studies have shown that the U–Pb isotopic system of titanite remains closed up to temperatures above 700°C (Scott and St-Onge, 1995). The age of 2020±8 Ma is identical within error to the titanite age of 2017±6 Ma from a nearby locality (Holzer et al., 1998). The excellent colinearity of the data and the uniformity of the two titanite ages from different grain size fractions rules out mixing or cooling age interpretations, and thus these titanite ages are interpreted as reflecting the timing of a

high-grade metamorphism in the northern part of the Koedoesrand Window (LCZ). Rb–Sr: Rb–Sr data listed in Table 2 from biotite and the whole rock of sample 95/423 are plotted in Fig. 13. These Rb–Sr data define a two-point age of 1971±26 Ma and a Sr initial value of 0.7151±3. This age is interpreted to reflect the synkinematic growth of biotite under low-grade metamorphic conditions.

5. Discussion 5.1. The tectono-metamorphic evolution in the Koedoesrand Window According to previous studies the main highgrade tectono-metamorphic event in the Koedoesrand Window was assigned to the Archean Limpopo Orogeny (e.g. McCourt and Vearncombe, 1992). Our age data allow us now to distinguish two granulite facies events. From the first event (D /M ), which is dated to 1 1 2602±40 Ma (PbSL on garnet, Schaller et al., 1997a), only relic parageneses could be identified. These relics are preserved in the CD, where the

>100 80–100 60–80 <60 1.94 2.72 1.56 3.58

467 475 537 521

161 152 168 168

204 192 209 215

201 211 217 200 14.98

0.3878 0.3355 0.3680 0.3426 2.3847

0.1321 0.1379 0.1314 0.1327 1.0126f

1.9 2.6 1.0 1.4 0.02

5.3619 5.3813 4.8808 5.2323

3.1 3.2 1.3 1.9

0.2945 0.2829 0.2694 0.2861

2.2 1.1 0.7 1.1

1879 1882 1799 1858

57 59 24 35

1664 1606 1538 1622

36 17 11 17

2129 2205 2120 2137

32 44 17 24

33 45 17 24

0.790 0.650 0.697 0.698

±2s 207Pb/ ±2s 206Pb/ ±2s 207Pb/ ±2s 206Pb/ ±2s 207Pb/ +2s −2s re 1 (%) 235U (%) 238U (%) 235U (%) 238U (%) 206Pb (Ma) (Ma) (Ma)

a Data corrected for fractionation (0.068±11%/AMU ), blank (80±30 pg) and common Pb. b Pb rad.=radiogenic Pb. c Pb tot.=total Pb. d Meas.=measured. e r =correlation coefficient 207Pb/235U vs. 206Pb/238U (Ludwig, 1988). 1 f Measured value.

zrn zrn zrn zrn fsp

Sample Fraction Weight U Pb Pb 206Pb/ 208Pb/ 207Pb/ (mm) (mg) (ppm) rad.b tot.c 204Pb 206Pb 206Pb (ppm) (ppm) (ppm) (meas.)d

Table 3 U–Pb isotope compositions from zircons of sample 95/655a

280 M. Schaller et al. / Precambrian Research 96 (1999) 263–288

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Fig. 11. Conventional concordia and common Pb diagram for four zircon fractions from a retrogressed charnockite (sample 95/655) in the Northern Domain of the Palala Shear Zone.

Proterozoic metamorphism did not exceed upper greenschist to lower amphibolite facies conditions. The main tectono-metamorphic activity evolved at c. 2.0 Ga. During this Proterozoic event each domain in the Koedoesrand Window underwent a distinct metamorphic (M ) and deformational 2 (D ) history ( Table 4). 2 Titanite from granulite facies gneisses in the LCZ give ages of 2020±8 Ma and 2017±6 Ma. We therefore interpret the two titanite ages to reflect the time of the latest granulite facies event in the northern part of the Koedoesrand Window. Another important time marker is the charnockitic intrusion in the ND. We interpret the apparent age of 2117±17 Ma from zircons as a maximum age for the charnockite intrusion. This clearly points to a temporal (and genetic) relationship between the charnockite formation and the

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Fig. 12. Pb isotopic compositions from a stepwise leaching (PbSL) experiment with titanite. This titanite is part of a highgrade parageneses in a granulite facies calcsilicate (95/647) from the Limpopo Central Zone in the Koedoesrand Window.

Proterozoic granulite event shortly before 2.0 Ga. In the southern part of the Koedoesrand Window metamorphic conditions did not exceed greenschist facies temperatures. The Bushveld Igneous Complex (2061±21 Ma) is affected by the lowgrade mylonitic and cataclastic deformations (D ) and hence this intrusion gives a maximum 2 age for these deformations. For the early M stages 2 (c. 2060 to ≤2020 Ma) a steep metamorphic gradient can thus be documented from granulite facies in the N to (sub)greenschist facies in the S of the Palala Shear Zone. In the gneisses of the ND a continuous transition from granulite through amphibolite to greenschist facies conditions is manifested by a succession of distinct synkinematic and faciesindicative parageneses. The fact that these retro-

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found. Evidence is also found for a simultaneous N–S directed pure shear component. This implies a transpressive tectonic regime which persisted during the main greenschist facies D strike–slip 2 episode. The 1971±26 Ma Rb–Sr age on synkinematic biotite places a constraint on the timing of this transpression. 5.2. The role of the Palala Shear Zone during the Proterozoic high-grade event in the LCZ

Fig. 13. 87Rb/86Sr vs. 87Sr/86Sr ‘isochron’ diagram of biotite– whole rock pair from a greenschist facies metapelitic mylonite (95/423) in the Central Domain (Palala Shear Zone). The data are interpreted to give the age of the synkinematic biotite growth during D . 2

grade mineral assemblages are found within the Proterozoic charnockites disproves earlier hypotheses that the granulites have been exhumed in the course of the Archean Limpopo Orogeny. These observations rather indicate that the steep metamorphic gradient of the early Proterozoic phase (>2.0 Ga) has been re-equilibrated shortly after 2.0 Ga. Two amphibole Ar/Ar ages of rocks from the Northern Domain in the Palala Shear Zone and the adjacent Limpopo Central Zone (Belluso et al., 1999) are in accord with this. The exhumation and associated cooling of the high-grade gneisses in the northern part of the Koedoesrand Window is bracketed between the titanite ages of 2020 Ma and a Rb–Sr biotite age of 1971±26 Ma from a greenschist facies mylonite. The latter is compatible with a large number of Rb–Sr mica ages from the entire LCZ which scatter around 1960±40 Ma (Barton and Van Reenen, 1992b). Intense low-grade mylonitisation strongly affected the entire Koedoesrand Window so that the kinematic evolution during the high-grade deformational phase (D early >2.0 Ga) is difficult 2 to assess. Most preserved D structures thus 2 formed under retrograde metamorphic conditions. All of these mid- to low-grade mylonites are characterized by a strong strike–slip component for which mainly dextral shear sense indicators are

The perception of a granulite facies event at 2 Ga, followed by a retrograde evolution during which the northern part of the Koedoesrand Window was exhumed, raises questions about the relationship between the tectonic history of the Palala Shear Zone and the Proterozoic high-grade event in the LCZ. In recent studies a large amount of geochronological data from the entire LCZ and parts of the NMZ has been produced (Barton et al., 1994; Kamber and Biino, 1995; Kamber et al., 1995a; Jaeckel et al., 1997; Holzer et al., 1997; Kro¨ner et al., 1999). Holzer (1998) and Holzer et al. (1998, 1999) reviewed these geochronological data and combined them with structural and petrographic aspects. As a synthesis a model was postulated which describes the Proterozoic event as a transpressive orogeny. We briefly summarize the four stages of this geodynamic model and compare them with the findings in our study area. According to the model the LCZ was tectonically thickened due to the convergence of the Kaapvaal and Zimbabwe Cratons (stage 1, >2030 Ma). As a consequence, the LCZ underwent prograde metamorphism. At 2020 Ma peak granulite facies conditions were reached throughout the LCZ. Between 2030 and 2010 Ma (stage 2) the tectonic setting changed from N–S compression to NW–SE directed shortening. This transpressive setting caused ENE-directed dextral movements in the shear zones N and S of the LCZ. Dextral strike–slip movements persisted after 2010 Ma (stage 3). The ongoing SE shortening produced a transpressive pop-up so that the LCZ underwent several kilobars of isothermal decompression. This was followed by near isobaric thermal relaxation at mid- to upper-crustal levels.

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283

Table 4 Compilation of metamorphic grades related to the deformation phases in the Koedoesrand Windowa

a Abbreviations: LCZ, Limpopo Central Zone; ND, Northern Domain; CD, Central Domain; SD, Southern Domain; BIC, Bushveld Igneous Complex; GF, Granulite facies; AF, Amphibolite facies; GSF, Greenschist facies; M , Archean metamorphism; M , 1 2 Proterozoic metamorphism. 1 Schaller et al. (1997a). 2 Belluso et al. (1999). 3 This study.

The entire LCZ cooled down to greenschist facies temperatures until 1960 Ma. After the exhumation of the LCZ the dextral ENE-directed displacements between the Kaapvaal and Zimbabwe Cratons persisted. The low-grade strike–slip movements (stage 4) were localized within relatively narrow shear zones. Locally the tectonic regime changed to a transtensive setting which led to the post-orogenic opening of the Soutpansberg and Palapye grabens between 1950 and 1850 Ma. Fig. 14 shows the large-scale tectonic relationships during stage 4 at the end of the Proterozoic orogeny. In the northern part of the Koedoesrand Window the high-grade structures (D early) show 2 steep N dipping lineations and ductile isoclinal folds with steeply NNW dipping axial planes. The kinematics of the high-grade deformations could not be characterized satisfactorily, however, the observed structures do not contradict a N–S convergence between the two cratons (stage 1,

>2030 Ma). The high-grade metamorphism at 2020 Ma in the northern part of the Koedoesrand Window is contemporaneous with peak granulite conditions in the entire LCZ (stage 2). Furthermore, it could be documented that the exhumation of high-grade rocks in the Koedoesrand Window and the subsequent cooling to greenschist facies conditions occurred between 2020 and 1970 Ma. The timing of the retrograde evolution is thus comparable with the regional pop-up (stage 3) of the entire LCZ. During this period we have also identified a transpressive tectonic regime which led to ENE dextral displacements along the Palala Shear Zone. These movements are considered as a period of postorogenic transcurrent faulting which is related to the transtensional opening of the Soutpansberg and Palapye grabens at c. 1900 Ma (stage 4). The extrapolation of the findings in the Koedoesrand Window and the comparison with large-scale tectonic relationships indicates that the

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Fig. 14. Tectonic map illustrating the post-orogenic transtensional tectonic setting in the Limpopo Central Zone around 1.9 Ga. Adapted from Holzer et al. (1999), who explain the Proterozoic high-grade metamorphism (including high-pressure granulites) in the Limpopo Central Zone as a product of a dextral transpressive orogeny between 2050 and 1950 Ma. Contemporaneously, the Magondi and Kheis Fold Belts formed at the western rims of the Zimbabwe and Kaapvaal Cratons. After 1950 Ma post-orogenic dextral shearing is localized mainly within the Palala and Sunnyside Shear Zones. Associated with this post-orogenic transcurrent faulting between 1.95 and 1.85 Ga is the dextral transtensional opening of the Soutpansberg and the Palapye grabens. Abbreviations: 1, Mahalapye Granite; 2, Mokgware Granite; 3, Entabeni Granite; 4, Schiel Alkaline Complex; 5, Palala Charnockite; 7, Palala Granite. NLTZ, North Limpopo Thrust Zone; NMTZ, Northern Marginal Thrust Zone; HRSZ, Hout River Shear Zone. NMZ, Northern Marginal Zone; LCZ, Central Zone; SMZ, Southern Marginal Zone. F, Francistown; M, Messina; P, Pietersburg; S, Selebi; V, Vivo.

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Palala Shear Zone played an active role during the (formation and) exhumation of the Proterozoic granulites in the LCZ. The Palala Shear Zone represents a relatively sharp terrane boundary along which the Proterozoic granulites have been juxtaposed with the low-grade rocks of the Kaapvaal Craton and the Archean granulites of the SMZ. The juxtaposition includes considerable vertical movements (≥2020 to c. 1970 Ma) and lateral displacements (after c. 2.0 Ga). Before the Proterozoic event, the units which are now separated by the Palala Shear Zone were situated at different geographical positions and possibly also at different crustal levels. 5.3. The Palala Shear Zone and the juxtaposition of the Kaapvaal and Zimbabwe Cratons The Kaapvaal and Zimbabwe Cratons are characterized by distinct Archean histories, reflected by different periods of shield formation (amalgamation and obduction of oceanic terranes) and craton stabilization (orogenic accretion) (De Wit et al., 1992). It was assumed that a collision of the two cratons produced the c. 2.7 Ga old Limpopo Orogeny. However, the events in which the late Archean granulites were formed in the different subzones of the Limpopo Belt are clearly separated in time and reflect different tectonic regimes (see Introduction and Fig. 3). These new findings are in strong conflict with the model of a late Archean Limpopo Orogeny and alternative models that could explain the juxtaposition of the two cratons should be tested. Of first importance hereby is the identification of a possible suture zone. In order to test whether the Palala Shear Zone could represent a suture zone, we briefly compare geochemical signatures, stratigrafic successions and paleomagnetic data from the tectonic units N and S of this lineament. The Kaapvaal and Zimbabwe Cratons form two distinct geochemical provinces. This is manifested for example by the Pb isotopic signatures. Whereas the SMZ and the Kaapvaal Craton are characterized by datapoints below the Stacey– Kramers (S+K ) evolution line in the 207/208Pb/ 204Pb vs. 206Pb/204Pb plot, those of the LCZ, NMZ and Zimbabwe Craton plot above the S+K line

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(Barton et al., 1983; Berger et al., 1995; Kreissig and Holzer, 1997). This documents a sharp terrane boundary which is located between the SMZ and LCZ, i.e. in the Palala lineament (compare Fig. 14). The Kaapvaal and Zimbabwe Cratons also represent two distinct sedimentary provinces. Cheney and Winter (1995) defined the Palala– Zoetfontein lineament as a province boundary based on sequence stratigraphy (see Fig. 1). It is important to note that no sequences older than 2.0 Ga can be correlated across the Palala lineament. For example the Transvaal Supergroup which consists of four large unconformity bounded sequences, deposited between 2.5 and 2.05 Ga on the Kaapvaal Craton, has no equivalent in the Zimbabwe Province. The first sequences which can be correlated across the Palala lineament are the Soutpansberg–Palapye formations, deposited shortly after 1.95 Ga (e.g. Barton et al., 1995; Walraven et al., 1990). Thus the juxtaposition of the two provinces might be as young as 2.0 Ga. Unfortunately, no palaeomagnetic data are available which could indicate whether or not the two cratons have been amalgamated already during the late Archean. The first paleomagnetic data which allow us to reconstruct the contemporaneous relative positions of the Kaapvaal and Zimbabwe Cratons originate from the Bushveld Igneous Complex (2060 Ma) and the southernmost Great Dyke satellites in the NMZ. The remanent magnetic poles of these Dykes have been reset during a metamorphic event in the Limpopo Belt at about 2.0 Ga (Jones et al., 1974). The virtual polar wander paths give evidence for considerable eastward migration of the Zimbabwe Craton relative to the Kaapvaal Craton (Jones et al., 1974). This is compatible with a tectonic model of a Proterozoic transpressive orogeny with considerable dextral displacements in the shear zones N and S of the LCZ.

6. Conclusions The main tectonic activity in the Palalal Shear Zone occurred between 2050 Ma and 1900 Ma. Early high-grade shearing along the Palala Shear Zone is intimately associated with granulite facies

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metamorphism in the Limpopo Central Zone, which is the product of a transpressive orogeny. Dextral strike–slip shearing occurred during the exhumation of the Limpopo Central Zone shortly after 2.0 Ga and graded into post-orogenic transcurrent faulting. The latter persisted during the formation of the Soutpansberg and the Palapye grabens at c. 1.9 Ga in a dextral transtensive environment. The Archean metamorphic history of the gneisses incorporated in the Palala Shear Zone is only preserved as relic parageneses and the Archean structural relationships are almost completely erased by the strong Proterozoic overprint. There is thus hardly any evidence for an Archean Limpopo Orogeny preserved in the Koedoesrand Window. However, the Palala lineament is a subvertical, crust-penetrating lineament, which is characterized by geological features that are comparable with suture zones of recent orogenies. The combined geochemical, geochronological, sedimentological and geophysical data provide strong evidence for a juxtaposition of the Zimbabwe and Kaapvaal Cratons during the transpressive event at 2.0 Ga. We therefore regard the Palala Shear Zone as part of the main suture between the Zimbabwe and Kaapvaal Cratons, formed by their juxtaposition at 2.0 Ga.

Acknowledgements This project was founded by the Swiss National Foundation (Grant 20-47157.96). We further acknowledge support of the electron microprobe at the University of Bern by the Swiss National Foundation (Grant 21-26579.89). We thank F. Corfu, M. De Wit and K. Karlstrom for their constructive comments on an earlier version of the manuscript.

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