P. MENDONIDIS, R.A. ARMSTRONG, B.M. EGLINGTON, G.H. GRANTHAM AND R.J. THOMAS 325

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1 P. MENDONIDIS, R.A. ARMSTRONG, B.M. EGLINGTON, G.H. GRANTHAM AND R.J. THOMAS 325 Metamorphic history and U-Pb Zircon (SHRIMP) geochronology of the Glenmore Granite: Implications for the tectonic evolution of the Natal Metamorphic Province P. Mendonidis Department of Metallurgical Engineering, Vaal Triangle Technikon, P/Bag X021, Vanderbijlpark, R.A. Armstrong Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia. B.M. Eglington and G.H. Grantham Council for Geoscience, P/Bag X112, Pretoria, R.J. Thomas British Geological Survey, Kingsley Dunham Centre, Keyworth, Nottingham NG12 5GG, U.K. ABSTRACT A new SHRIMP U-Pb zircon age of 1091 ± 9 Ma has been acquired for the gneissic, S-type Glenmore Granite from the Margate Terrane of the Natal Metamorphic Province. The Glenmore Granite contains two foliations and mineral textures indicate it underwent two metamorphic episodes separated by a period of retrogression. The presence of a folded S 1 foliation in the Glenmore Granite indicates that it was either pre- or syn-tectonic relative to D 1, thus providing a maximum age constraint of 1091 ± 9 Ma on the D 1 event in the Margate Terrane. This is ~60 Ma later than the completion of the main tectonism (D 1-3 ) documented from the Tugela Terrane, suggesting diachronous arc accretion. Syntectonic granitoids with ages of ~1090 Ma have also been documented from the Mzumbe terrane, as well as from Western Dronning Maud Land (east Antarctica), and the Cape Meredith Complex in the Falklands, which on reconstruction of Gondwanaland, lie adjacent to the Natal Metamorphic Province. Introduction The intrusive and tectonic history of the Margate Terrane of the Natal Metamorphic Province (NMP) has been studied since 1942 (Gevers and Dunne, 1942; du Toit, 1946). Although great strides have been made in unraveling the history (e.g. McIver, 1963, 1966; Grantham, 1983; Talbot and Grantham, 1987; Thomas, 1988a; 1989; Jacobs et al., 1993), most lithological contacts are concordant to the regional tectonic fabric and age relationships remain equivocal. Moreover, many contacts are not visible due to deep soils and Phanerozoic cover, so the relative timing of intrusion of the different types of granitoids, which has implications for the evolution of the tectonic environment into which they were emplaced, has been controversial. The foliated Glenmore Granite has had a checkered history regarding its perceived stratigraphic position, which has ranged from the youngest intrusive in the Margate Terrane (McIver, 1963; 1966) to the oldest (Grantham, 1983; Talbot and Grantham, 1987; Mendonidis, 1989). Adding to the confusion, Rb-Sr whole-rock radiometric data (Eglington, et al., 1986) provided a relatively young date, which was initially interpreted as an age of intrusion (Eglington, et al., 1986) and later as a metamorphic cooling age (Thomas et al., 1993a). The U-Pb zircon SHRIMP data presented in this study aims to precisely determine the age of intrusion of the Glenmore Granite which will throw light on the timing of events in the tectonic evolution of the Margate Terrane. Regional Geology The NMP flanks the southern margin of the Archaean Kaapvaal Craton in southeastern Africa. It is considered to be an extension of the Namaqualand Metamorphic Province in southwestern Africa and the gneiss terranes of Western Dronning Maud Land of East Antarctica (Nicholaysen and Burger, 1965; De Beer and Meyer, 1984; Jacobs et al., 1993). The southern boundary of the NMP with the Pan-African Saldania Province is obscured by Phanerozoic cover, but has been modelled to be broadly coincident with major geophysical anomalies known as the Beattie Magnetic Anomaly and the Southern Cape Conductive Belt (De Beer and Meyer, 1984) (Figure 1). Thomas (1989) subdivided the exposed NMP into three tectonostratigraphic terranes, namely, from north to south: the Tugela, Mzumbe and Margate terranes. The Tugela terrane comprises a stack of thrust nappes, which contain transported oceanic and arc material among other rock-types, northwards onto the Kaapvaal Craton, 2002,VOLUME 105, PAGE

2 326 U-PB ZIRCON (SHRIMP) GEOCHRONOLOGY OF THE GLENMORE GRANITE Figure 1. Locality map. The major lithologies of the Margate Terrane are shown.

3 P. MENDONIDIS, R.A. ARMSTRONG, B.M. EGLINGTON, G.H. GRANTHAM AND R.J. THOMAS 327 Table 1. Summary of the lithostratigraphy and tectonic history of the Margate Terrane U-Pb zircon date Lithostratigraphy Structure 1026 ± 3 Ma Mbizana Microgranite: microgranite dykes, locally dumortierite-bearing Undeformed 1025 ± 8 Ma Oribi Gorge Suite: Syn- to late-tectonic granitoid charnockitoid intrusion with D3 D5: Open N-S folds and shearing A- type geochemical characteristics. D2: Northward verging F2 folds and pervasive S2 foliation. No data Margate Granite Suite: A number of syn-tectonic granitic and charnockitic plutons and sheets (?) with S-type geochemical characteristics. No data No data This study No data The Nicholson s Point Granite, Belmont Pluton, Palm Bearch Granite, Marina Beach Granite and Portobello Granite, which were described from coastal exposures by Grantham (1983) and Mendonidis (1989) and inland exposures by Thomas (1988a) have been grouped with the Margate Granite Suite (Thomas et al., 1991). Munster Suite: calc-alkaline suite of mafic mangeritic compositions. Banana Beach Tonalite: Tonalitic orthogneiss, age relations to other units unknown Glenmore Granite: foliated calc alkaline, megacrystic granite sheet. Mzimkulu Group, including 3 granulite grade supracrustal gneiss formations, one of which, the Leisure Bay Formation, forms wall rocks of Glenmore Granite: granulitic metapelites, metasemipelites and calc silicates. D1: Isoclinal folds (Matthews, 1972; 1981; Smalley, 1980; Arima et al., 2001; Johnston et al., 2001). The Mzumbe and Margate terranes, are separated from the Tugela terrane by a major sinistral transcurrent ductile shear zone, and distinguished by distinctive lithological assemblages and different metamorphic grades (Thomas, 1989). The Glenmore Granite forms part of the southernmost Margate Terrane. Thomas (1988a) provided a geological history of the Mzumbe and Margate Terranes. In terms of that work and later updates (Thomas et al., 1993a; Thomas et al., 1994), the sequence of events in the Margate Terrane was as shown in Table 1. The first deformational event (D 1 ) of the Mzumbe and Margate terranes was initially reported to have deformed only rocks of the supracrustal sequences and the Glenmore Granite (Grantham, 1983; Evans et al., 1987; Thomas 1988a). However, Mendonidis (1989) and Mendonidis and Strydom (1989) subsequently reported deformed S 1 fabrics in the Margate Granite. A pervasive S 2 foliation in the Margate terrane is generally oriented east-west to northeast-southwest striking with a gentle to steep southward dip. It is axialplanar to northward-verging F 2 folds which deform the S 1 foliation in the Mzimkulu Group supracrustal gneisses and the Margate and Glenmore Granites (Talbot and Grantham, 1987; Thomas, 1988a; Mendonidis, 1989). Similar oriented fabrics have been documented from the Mzumbe terrane (Thomas, 1989; Thomas et al., 1995) and from within thrust-bounded zones in the Tugela terrane (Smalley, 1980; Johnston et al., 2001). The gneisses of the Mzumbe and Margate terranes of the NMP are intruded by large granitoid plutons (Oribi Gorge Suite), which collectively form a rapakivi granite - charnockite association with A-type geochemical characteristics (Grantham, 1983; Kerr, 1985; Eglington et al., 1989; Thomas, 1988a; b; Grantham et al., 2001). Some of these plutons contain a penetrative foliation correlated with S 2 (e.g. Port Edward pluton - Grantham, 1983), whereas others have undeformed cores, but apophyses emanating from them are deformed by F 2 folds (Thomas, 1988b). Thomas et al. (1993a) considered these plutons to be late- to post-tectonic relative to D 2. U-Pb zircon ages (SHRIMP and TIMS) of between ~ Ma for the various plutons of the Oribi Gorge Suite have been taken as emplacement ages (Thomas et al., 1993a). Zircons from some plutons within this suite have inherited zircons of ~1070 Ma and metamorphic rims of ~1030 Ma (Eglington et. al., 2000) The published tectonic model for the NMP (Jacobs, et al., 1993) involves the formation of juvenile crust in an island arc environment. Jacobs, et al. (1993) attribute all the southward dipping planar fabrics with down dip lineations in the NMP to a protracted northeast accretion of the island arc terranes against the Kaapvaal Craton. Ar-Ar dating on hornblende grains from the Tugela terrane indicates a minimum age of 1135 ± 9 Ma for this collisional event, but similar dating in the Mzumbe

4 328 U-PB ZIRCON (SHRIMP) GEOCHRONOLOGY OF THE GLENMORE GRANITE Figure 2. Summed probability graph of the ages for various lithologies from the Natal Namaqua belt (after Eglington et al., 2000). The light grey areas represent the data of the Namaqua Metamorphic Province and the black areas are the data from the granitoids of the Natal Metamorphic Province. Three main periods of activity are evident. These may be interpreted as follows: 1 = formation; 2 = accretion and intrusion of various granitoids; 3 = main metamorphism and granite formation. terrane indicates that cooling to below 550 C occurred only at ~1005 Ma (Jacobs, et al., 1997). Thomas (1989) and Jacobs et al. (1993) recognised, in the Mzumbe and Margate terranes, regional, transpressive, sinistral-sense shear zones defined by a foliation parallel but steeper than the S 2. Jacobs et al. (1993) interpret these structures as a response to indentation and lateral escape tectonics associated with northeast collision of the NMP with the Kaapvaal Craton. The Oribi Gorge Suite plutons mentioned above may have intruded during this transpressional event (Jacobs et al., 1993; Thomas et al., 1993a). Ar-Ar dating on hornblende grains from these shear zones indicate that shearing began between ~ Ma and ended at ~990 Ma (Jacobs, et al., 1997). A compilation of previous geochronological data from the NMP (Eglington et al., 2000) indicates three broad periods of tectonothermal activity at ca. 1200Ma (early supracrustal rocks and arc-related TTG magmatism), 1150Ma (syntectonic granites in Mzumbe terrane) and Ma (A-type granitoids in the Mzumbe and Margate terranes) (Figure 2). Field relationships and relative age of the Glenmore Granite The Glenmore Granite is a coarse-grained, foliated, K-feldspar megacrystic, biotite garnet granite that crops out between the Mpenjati and Mtongazi rivers near Glenmore on the Natal south coast (Mendonidis et al., 1991) (Figure 1). Field relationships of the Glenmore Granite are relatively well established despite poor inland outcrops and Quaternary cover. The best exposures of the granite occur along the coast between Glenmore beach in the north and Leisure Bay in the south. Here, Talbot and Grantham (1987) interpreted the Glenmore Granite as a sheet-like body occupying the core of an E-W trending, F 2, synform. At the southern margin of this exposure, the Glenmore granite intrudes paragneisses of the Leisure Bay Formation (Mzimkulu Group) and xenoliths of Leisure Bay Formation granulites are common within the Glenmore Granite (Grantham, 1983). The deformed S 1 foliation (folded by F 2 ) is best developed near the contact with the Leisure Bay Formation. In other parts of the granite, S 1 is completely overprinted by the southward dipping regional S 2 foliation. At Glenmore Beach the granite is in contact with the calc alkaline, two-pyroxene metabasitic rocks of Munster Suite. The lithological boundary is concordant to the regional fabric, however, xenoliths of Glenmore Granite in the metabasites (Mendonidis and Strydom, 1989) and intrusive sheets of Munster metabasites in the Glenmore Granite (Talbot and Grantham, 1987) establish that the Munster metabasites post-dates the Glenmore Granite. In its northern extremities, the Glenmore Granite is juxtaposed against the calc-alkaline, Margate Granite Suite with the contact concordant to the regional foliation (Thomas, 1988a). However, the relative ages of the Margate Suite granites and the Glenmore Granite are indicated by enclaves of Glenmore Granite in the Margate granite reported from inland exposures by Thomas (1988a) who concluded that the Glenmore granite was intruded by the Margate granite. Supporting, indirect evidence is that the Margate Suite granites intrude the Munster Suite. Other intrusive lithologies in the Margate terrane, which are not in contact with the Glenmore Granite, such as the Nicholson s Point Granite, Port Edward Enderbite, and the Palm Beach granite, all post-date the Munster Suite (Grantham, 1983; Mendonidis, 1989). Since the Munster Suite intrudes the Glenmore Granite, these age relationships imply that the Glenmore Granite is the oldest intrusive lithology in the Margate terrane. In summary, therefore, the Glenmore Granite intrudes the supracrustal rocks of the Leisure Bay Formation and is intruded by rocks of both the Munster Suite and Margate Granite Suite. Moreover, the intrusion of all these units (Glenmore Granite, Munster Suite and Margate Granite Suite) pre-dated the D 1 event. Mineral Textures of the Glenmore Granite Megacrysts of K-feldspar are 2-3cm in length, anhedral, and oblate within the plane of the S 2 foliation. The groundmass is coarse grained (averaging 3mm grainsize) and comprises perthitic K-feldspar, plagioclase (An ), quartz, biotite, garnet, opaques, apatite and zircon. Sillimanite was reported from one sample (Grantham, 1983). Microcline and orthoclase are microperthitic and poikilitically enclose rounded inclusions of quartz, biotite, plagioclase, and zircon. Myrmekite is a common feature at the margins of K-feldspar grains. Localised

5 P. MENDONIDIS, R.A. ARMSTRONG, B.M. EGLINGTON, G.H. GRANTHAM AND R.J. THOMAS 329 Figure 3. Biotite grains defining the S 2 foliation. Note the biotite quartz symplectites on the right side of the picture. Photomicrograph, plane polarised light, 5mm across. sericitisation of K-feldspar is evident. Plagioclase grains are anhedral, display patch antiperthitic texture, carlsbad-albite twinning, and contain rounded inclusions of quartz. Some plagioclase grains contain calcite patches indicative of low temperature alteration. Strained quartz has a patchy distribution and forms lenticular composite grains up to 1mm in size with inclusions of orthoclase, apatite and zircon. Biotite is red-brown in colour and strongly pleochroic. It occurs as stringers of subhedral grains of various sizes up to 3mm, which anastamose around larger feldspar grains in the general direction of the foliation (Figure 3). Small grains of biotite are scattered along minor fractures where they have replaced other ferromagnesian minerals through which the fractures pass. Biotite grains that are near such fractures are partly altered to muscovite and chlorite by isovolumetric replacement along cleavage planes (Figure 4). Biotite also occurs as biotite quartz symplectites that are Figure 4. Replacement of biotite by chlorite and muscovite along fractures. Photomicrograph, plane polarised light, 5 mm across. frequently spatially associated with garnet and chlorite (Figures 3 and 5). Garnet occurs as anhedral grains up to 3mm in size. These grains contain inclusions of biotite, quartz, zircon, chlorite and magnetite (Figures 6 and 7). Rare euhedral faces are developed where garnet embays biotite grains (Figure 8). Garnet quartz intergrowths are also present. These have two habits, one with rounded quartz and anhedral garnet (Figure 9) and the other having very fine symplectitically intergrown laminae of quartz and garnet (Figure 10). The latter occur in close proximity to biotite quartz symplectites where the garnet appears to be replacing biotite. Some garnet grains have been replaced by chlorite along internal fractures. Patches of chlorite and calcite intergrowths occur in close proximity to pristine biotite quartz symplectites and garnet grains (Figure 5). The chlorite - calcite intergrowths are probably pseudomorphs of an earlier calcic ferromagnesian mineral phase. No unaltered grain Figure 5. Biotite quartz symplectitic intergrowth replacing psuedomorphic chlorite. Photomicrograph, plane polarised light, 3mm across. Figure 6. Garnet with inclusions of biotite. Indicative of prograde metamorphism. Photomicrograph, plane polarised light, 3mm across.

6 330 U-PB ZIRCON (SHRIMP) GEOCHRONOLOGY OF THE GLENMORE GRANITE a Figure 7. Inclusion of chlorite in garnet. Photomicrograph, plane polarised light, 1mm across. of this mineral was seen in any of the thin sections studied. There are also chlorite pseudomorphs without any calcite. The retrogressive sericite, chlorite and calcite associated with biotite, feldspar and garnet are mostly developed in zones with abundant fractures (Figure 3). In most areas, the garnet, biotite and feldspars are pristine. The chlorite and chlorite-calcite pseudomorphs, on the other hand, are spatially unrelated to the retrogressive fracture zones. Inclusions of pseudomorphic chlorite occur in garnet (Figure 7). Many biotite-quartz symplectites terminate against the pseudomorphic chlorite and seem to have replaced portions of the mineral now pseudomorphed by chlorite (Figure 5). Metamorphic Interpretation Of The Mineral Textures There are no relict grains of the unidentified ferromagnesian mineral mentioned above. It has been completely pseudomorphically replaced by chlorite. However, the occurrence of biotite quartz symplectites that embay the pseudomorphic chlorite (Figure 5) suggests that the relict mineral may have been orthopyroxene, because biotite quartz symplectites can be derived by the reaction: Orthopyroxene + K-feldspar = biotite + quartz (e.g. Waters, 2001; Grantham and Mendonidis, 1995). Biotite quartz symplectites from granulite terranes have been attributed to retrogressive replacement of resitite phases on cooling and in situ solidification of anatectic melts which released water (Waters, 2001; Kriegsman, 2001). The chlorite pseudomorphism indicates retrogressive greenschist facies metamorphism. Biotite, feldspar and garnet grains are partly replaced by chlorite, sericite and calcite near fractures, but are otherwise in pristine condition. This suggests that the complete and pervasive retrogression of the ferromagnesian mineral (orthopyroxene?) predated the garnet biotite paragenesis. The early, ferromagnesian mineral b Figure 8. Euhedral crystal faces of garnet embay biotite. Photomicrographs, plane polarised light, (a) 0.3mm across, (b) 1mm across. (orthopyroxene?) may have been a primary magmatic phase or one representing an earlier metamorphic event. The presence of a deformed S 1 foliation in the Glenmore Granite (Talbot and Grantham, 1987) suggests that the pseudomorphic chlorite may have replaced a phase (orthopyroxene?) formed during an earlier metamorphic event (M 1 ). The majority of biotite grains anastamose around larger feldspar grains in the general direction of the foliation indicating that they were recrystallised during D 2, the fabric forming deformation. This alignment of biotite with S 2 indicates a prograde metamorphic episode (M 2 ) that coincided with the D 2 event. Textures provide the following evidence that garnet developed by prograde replacement of biotite after or late during the D 2 event: Garnet porphyroblasts enclose biotite, magnetite, pseudomorphic chlorite and quartz (Figures 6, 7 & 9). Garnet overgrowths mimic the textures of biotite quartz symplectites producing fine-grained garnetquartz intergrowths (Figure 10).

7 P. MENDONIDIS, R.A. ARMSTRONG, B.M. EGLINGTON, G.H. GRANTHAM AND R.J. THOMAS 331 Figure 9. Garnet quartz symplectites with inclusions of biotite. Photomicrograph, plane polarised light, 2mm across. Figure 10. Finely intergrown garnet quartz symplectite. This is interpreted to have replaced biotite- quartz symplectite. Photomicrograph, plane polarised light, 1mm across. Euhedral faces are developed in the portions of garnet grains that embay biotite (Figure 8). The preservation of such sharp crystal edges at reaction sites shows that garnet replaces biotite and not vice versa. Experimental modelling of the breakdown of biotite shows that biotite reacts with quartz to produce an anhydrous phase such as orthopyroxene or garnet plus fluid (Bucher and Frey, 1994). The temperature range of these reactions depends on the Mg/(Fe+Mg) ratio of the biotite and the partial pressure of water in the fluid phase (Stevens et al., 1997). The production of garnet or orthopyroxene has been shown to be pressure dependant but is also dependant on the whole-rock composition (Green and Ringwood, 1967). Moreover, at pressures above ±1 kb, biotite breakdown occurs within the subliquidus P-T field of granitic compositions, at C (Fyfe, 1973; Stevens et al., 1997; Clemens and Vielzeuf 1987; Le Breton and Thompson, 1988). This implies that the garnet formed by the breakdown of biotite would have been in equilibrium with a melt phase of granitic composition. Although, no leucosomes were observed in the Glenmore Granite, evidence of partial melting associated with garnet development is very abundant in the leucogranites of the Margate Granite Suite and the Leisure Bay granulites (Thomas, 1988a; Mendonidis 1989). Such a syn- to late-d 2 metamorphic event (M 2 ), which produced garnetiferous leucosomes was also described from the Leisure Bay granulites by Grantham and Mendonidis (1995). These authors further noted that M 2 partly overprinted an earlier granulite facies event characterised by isobaric cooling, which they concluded, was probably associated with D 1. In conclusion, the microtextures and fabrics suggest that the Glenmore Granite experienced two periods of prograde metamorphism separated by a period of retrogression. Temperatures of around 750 C were reached during M 2, which was coeval with D 2. Previous Geochronological Data A Rb-Sr whole-rock isochron date of 946 ±31Ma was determined by Eglington et al. (1986) for the Glenmore Granite. A significant number of whole rock and mineral (biotite) Rb-Sr ages from the Natal Metamorphic Province fall between 950 and 880Ma (Thomas et al., 1993a). Many of the lithologies that produced these Rb-Sr ages have also produced U-Pb ages in the order of ± 100Ma older (Thomas et al., 1993a). For example, unfoliated granitic dykes (Mbizana microgranite) in the Margate terrane gave a Rb-Sr age of 960 ± 32 Ma (Grantham and Eglington, 1992) and a U-Pb zircon age of 1026 ± 3 Ma (Thomas et al., 1993b). Therefore, the Rb- Sr system appears to have remained open until ~950 Ma. K-Ar dates on muscovite of 950 to 910 Ma (Jacobs and Thomas, 1996) fall into the same range as the Rb Sr ages. Muscovite is generally believed to be of retrogressive origin in the Natal metamorphic rocks (Jacobs et al., 1995; Grantham, 1983; Mendonidis, 1989) although the specific population of muscovite crystals dated by Jacobs and Thomas (1996) represent a post-d 2 growth of new skeletal muscovite cutting across the retrogressed D 2 fabric, first recorded by Evans et al. (1987). Thomas et al. (1993a) suggested that the Rb-Sr ages of Ma could be accounted for by a thermal perturbation at about 950Ma, which left no petrographic imprint other than muscovite growth. On the other hand, Jacobs and Thomas (1996) linked various K-Ar dates of ~ 910 Ma on muscovite (including data from the Highbury Pegmatite, a phase of the Margate Granite Suite), to localised K-flushing and thermal rejuvenation. Ion Microprobe (Shrimp) U-Pb Zircon Dating Sampling A 10kg sample (PM98/1) of megacrystic Glenmore Granite, free of any veining and weathering was collected from littoral exposures south of Glenmore Beach within the lithostratigraphic type area described by Mendonidis et al. (1991).

8 332 U-PB ZIRCON (SHRIMP) GEOCHRONOLOGY OF THE GLENMORE GRANITE Figure 11. A Wetherill U-Pb Concordia plot of the SHRIMP analyses for the Glenmore Granite. The error ellipses are 1 and the concordia curve is calibrated in millions of years. The dashed line represents the regressed best-fit line for all of the data. Zircon Separation The sample was broken into small pieces, crushed and pulverised. The resulting m size-fraction was separated by sieving. The heavy minerals were concentrated by centrifugal gravity separation. About twenty zircon grains were hand picked from the concentrate under stereoscopic microscope and submitted to PRISE at the Research School of Earth Sciences at the Australian National University, Canberra. At PRISE, zircon grains were mounted in epoxy, sectioned approximately in half, polished and examined by cathodoluminescence. Zircon Morphology The zircons are generally anhedral and fractured with only occasional subhedral grains preserving prism faces. All zircons show metamorphic rounding and resorption features. Although all zircons are strongly zoned, they are structurally complex with zones of high U and Th contents and metamictisation contrasting with central less structured areas. These areas show up as relatively bright cathodoluminescent zones that also preserve some magmatic concentric compositional zoning. Although they appear to be magmatic and probably represent an early phase of growth, it is possible that some of these areas are, in fact, inherited cores. Analytical Procedures Standards used for SHRIMP analyses were the zircon standard AS3 (Duluth Complex gabbroic anorthosite; Paces and Miller, 1989) and the RSES standard SL13. The SHRIMP data were reduced in a manner similar to that described by Compston et al. (1992) and Williams and Claesson (1987). U/Pb ratios in the unknowns were normalised to a 206* Pb/ 238 U value of (equivalent to an age of Ma) for AS3. The U and Th concentrations were determined relative to those measured in the SL13 standard. Correction for common Pb was made from the measured 204 Pb and the appropriate common Pb isotopic compositions assuming the Cumming and Richards (1975) model. Results Eighteen analyses were done on fifteen different grains. These data are reported in Table 2 and are plotted on a Wetherill U-Pb concordia diagram (Figure 11). Uncertainties in the isotopic ratios and ages in the data (Table 2) are reported at the 1 level and the weighted mean ages as 95% confidence limits. All age calculations and statistical assessments of the data were carried out with the software Isoplot/Ex (Ludwig, 1999). Analyses sited in the strongly zoned, high U areas are highly discordant, and have elevated common Pb contents (e.g. analysis 11.1).

9 P. MENDONIDIS, R.A. ARMSTRONG, B.M. EGLINGTON, G.H. GRANTHAM AND R.J. THOMAS 333 Table 2. Summary of SHRIMP U-Th-Pb zircon results. Radiogenic Ratios Ages (in Ma) Grain U Th Th/U Pb* 204 Pd/ f Pb/ 207 Pb/ 207 Pb/ 206 Pb/ 207 Pb/ 207 Pb/ Conc. spot (ppm) (ppm) (ppm) 206 Pb % 238 U ± 235 U ± 206 Pb ± 238 U ± 235 U ± 206 Pb ± % * * * * * * Notes: 1. Uncertainties given at the one a level. 2. f~ % denotes the percentage of ~Pb that is common Pb. 3. Correction for common Pb made using the measured ~ Pb/~Pb ratio. 4. For % Conc., 100% denotes a concordant analysis. 5. ~ * data excluded from weighted mean age calculation Regression analysis of all data gives an upper intercept date of 1097 ± 12 Ma (MSWD = 2.3; probability = 0.002) with a lower intercept date of 209 ± 33 Ma. The best estimate of the age, however, is given by the calculation of a weighted 207 Pb/ 206 Pb date on the most concordant grains. For analyses greater than 90% concordant, this weighted mean date is 1091 ± 9 Ma (MSWD = 0.87, probability = 0.57, n = 12). This 1091 ± 9 Ma date is probably a reliable estimate of the crystallisation age of the Glenmore Granite. Implications For Tectonic Evolution The geochronological and metamorphic data presented in this paper provide new constraints on the tectonic evolution of the NMP. The metamorphic textures in the Glenmore Granite suggest two prograde metamorphic events separated by a period of retrogression. Similarly, Grantham and Mendonidis (1995) have interpreted two prograde events from the metamorphic textures of the Leisure Bay Formation, which is in contact with the Glenmore Granite. The first metamorphic event (M 1 ) evident in the Leisure Bay Formation is characterised by low-pressure, granulitic mineral assemblages and microtextures supporting an isobaric cooling path (Grantham, 1983; Mendonidis, 1989; Grantham et al., 1994; Grantham and Mendonidis, 1995). This type of metamorphism is associated with magmatic underplating and/or extensional tectonics rather than crustal thickening associated with accretion and collision (Bohlen, 1987; Schreurs, 1985; Shreurs and Westra, 1986; Wickham and Oxburgh, 1985; Sandiford and Powell, 1986; Waters, 1990; 1989; 1988; Waters and Whales, 1984; Raith and Harley; 1998). Furthermore, the Leisure Bay Formation has a S 1 fabric defined by hypersthenebearing mineral assemblages (Grantham, 1983; Mendonidis, 1989) that is partially overprinted by a M 2 - D 2 high-grade garnet-bearing assemblage (Grantham and Mendonidis, 1995) indicating that granulitic conditions were related to D 1, not the regional D 2 event or the intrusion of the Oribi Gorge Suite. This sequence of metamorphic events is the reverse of that documented for the Mzumbe Terrane (Cornell et al., 1996) and the Namaqua section of the orogen (Waters, 1989; Robb et al., 1999). M 2 in the Leisure Bay Formation was interpreted to be a syn- to late D 2, moderate-pressure, high-grade (anatectic) event which partially overprinted the M 1 granulite facies paragenesis (Grantham and Mendonidis, 1995). The nature of M 2 is compatible with a collisional model (Grantham and Mendonidis, 1995). The M 2 assemblage in the Glenmore Granite is also synto late D 2, and may, therefore, be correlated with the M 2 in the Leisure Bay Formation. The presence of chloritic pseudomorphs, which completely replaced the

10 334 U-PB ZIRCON (SHRIMP) GEOCHRONOLOGY OF THE GLENMORE GRANITE M 1 assemblage in the Glenmore Granite, but did not affect the M 2 -D 2 biotite garnet assemblage suggests a period of cooling between M 1 and M 2, which also supports a punctuated metamorphic history rather than a single protracted event in the Margate Terrane. The new age of 1091 ± 9 Ma for the Glenmore granite is also a maximum constraint on the age of D 1 in the Margate Terrane since the Glenmore granite contains a deformed S 1 fabric in addition to the regional, southward-dipping, S 2 foliation (Talbot and Grantham, 1987). This maximum age for D 1 in the Margate terrane is much younger than the age constraints on the early deformational events in the Tugela terrane. Johnston et al. (2001) constrained the timing of the deformational events in the Tugela terrane with U-Pb zircon ages of 1209 ± 5 Ma for the pre-d 1 Kotongweni gneiss, ~1181 Ma for the syn-d 2 Mkodeni diorite, and 1155 ± 1 Ma for the syn- to post-d 3 Wozi granitoids. This indicates that the main tectonism (D 1-3 ) in the Tugela terrane took place some 60 Ma before the earliest recorded deformational event in the Margate terrane. The Mzumbe Suite has an age of 1207 ± 11 Ma (Thomas and Eglington, 1990) indicating that the Mzumbe Terrane has a longer magmatic history than the Margate Terrane where the Glenmore Granite is the oldest intrusive lithology (Talbot and Grantham, 1987). However, there may be some overlap in the tectonomagmatic histories of the two terranes. Lithologies in the Mzumbe terrane with similar ages to the Glenmore granite, include the foliated Mzimlilo Suite which has a zircon evaporation age of ~1090 Ma (Thomas et al., 1995), and the post-d 1 pre-d 2 Equeefa Metabasites with an age of ~1083 Ma (Johnston et al., 2000). However, it is not known from what time onwards the two terranes shared a common tectonomagmatic history. The main constraint on the time of jaxtapositioning of the two terranes is the ~ Ma ages for the Oribi Gorge Suite plutons which occur in both the Mzumbe and Margate terranes (Thomas, 1989). Gondwana reconstructions place the gneiss terranes of Western Dronning Maud Land (WDML) in East Antarctica adjacent to the NMP, with the Falkland Islands wedged in between, on the southern margin (Grantham et al., 1997). Jacobs et al. (1999) recognised three phases of granitoid intrusion in the Cape Meredith Complex (CMC) in the Falkland Islands, and determined SHRIMP ages of ~1090 Ma for the oldest granitoids, 1067 ± 9 Ma for syn-tectonic granitoids, and 1003 ± 14 Ma for posttectonic granitoids. These ages are comparable to those in the Margate terrane as well as to those in WDML where syn-tectonic granite sheets and plutons give ages of c Ma (Jacobs et al., 1998). Jacobs et al. (1999) suggested that the rocks of the CMC may represent younger outboard arc terranes relative to those exposed in the NMP. Since the Margate terrane is now known to also be young, a model of diachronous arc accretion may be suggested for the NMP. Conclusions The U-Pb zircon age of 1091 ± 9Ma for the Glenmore Granite is probably an emplacement age. This age is similar to the ages of pre- and syn-tectonic granitoids documented from the Cape Meredith Complex in the Falklands and Western Dronning Maud Land in east Antarctica. Since the Glenmore Granite contains a S 1 foliation, this new age is also a maximum constraint on the D 1 tectonic event in the Margate Terrane. This is considerably younger than the early deformational events documented from the Tugela terrane, suggesting a protracted period of diachronous arc accretion, which formed the NMP. Metamorphic textures in the Glenmore Granite indicate that there were two prograde metamorphic events separated by a period of retrogression, suggesting a punctuated metamorphic history for the Margate Terrane. The M 2 -D 2 event, which may have involved compression and crustal thickening, is considered to have been responsible for the major pervasive S 2 fabric in the Margate Terrane. Acknowledgements Funding for the zircon age was provided by the Central Research Committee of Vaal Triangle Technikon. Brent Combrink assisted in the field and with zircon grain extraction. Reviews by Joachim Jacobs and Steve McCourt helped improve this paper. 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