Geochemical Correlation Between Metasediments of the Mfongosi Group of the Natal Sector of the Namaqua-Natal Metamorphic Province, South Africa and the Ahlmannryggen Group of the Grunehogna Province, Antarctica

Gondwana Research, c! 7, No. I, pp. 57-73. 0 2004 International Association for Gondwana Research, Japan. ISSN: 1342-937X

Geochemical Correlation Between Metasediments of the Mfongosi Group of the Natal Sector of the Namaqua-Natal Metamorphic Province, South Africa and the Ahlmannryggen Group of the Grunehogna Province, Antarctica Ian J. Basson’, Samantha Perritt2, Michael K. Watkeys3and Andrew H. Menzies4

’ Department of Geological Sciences, University of Cape Town,Private Bag Rondebosch, 7701, South Africa *

Council for Geoscience, c/o University of Natal, l? 0.Box 18091, Dalbridge, 4014, South Africa School of Geological and Computer Sciences, University of Natal, l? 0.Box 18091, Dalbridge, 4014, South Africa Mineral Services, P.O. Box 38668, Pinelands, 7430, South Afyica

(Manuscript received August 30,2002; accepted June 5,2003)

Abstract The whole-rock geochemistry of metamorphosed greywackes, arenites and arkoses within the Mesoproterozoic Namaqua-Natal-Maudheim Province is interpreted with the aim of establishing geochemical correlations and defining common sediment source terrains. Metasediments of the Mfongosi Group of the Natal Sector of the Namaqua-Natal Metamorphic Province were sampled from their type area in the Mfongosi Valley. Metagreywackes from the northern limits of the Mfongosi Valley, directly adjacent to the Kaapvaal Craton, show ocean island arc signatures while metagreywackes from the southern limits of the Mfongosi Valley, near the contact with the Madidima Thrust of the Natal nappe zone, show mainly active continental margin signatures. Interleaved, geochemically distinct low-Ca+Na, high-K metamorphosed arkoses to lithic arkoses indicate a minor passive margin sediment component. Geochemical classification of low-grade Ahlmannryggen Group greywackes, arenites and arkoses of the Grunehogna Province, Antarctica, indicates both active and passive continental margin sediment sources. An oceanic island arc signature is not evident in Ahlmannryggen Group data. The active continental margin signature in both Natal Sector and Grunehogna Province metasediments potentially provides for a common link between these terranes. Discriminant Function Analysis, using three pre-defined provenance sub-sets within the Mfongosi Group and two pre-defined provenance sub-sets within the Ahlmannryggen Group, indicate that metasediments with active continental margin signatures from both groups are geochemically identical, implying that the active continental margin of the Grunehogna Province shed immature sediments westwards (African azimuths) into the developing, narrow or restricted Mesoproterozoic ‘Mfongosi Basin.’ This was accompanied by minor sediment influx from a stable continental platform, potentially the Kaapvaal Craton. Oblique and diachronous collision, initiated in the southwestern portions of the combined Natal Sector/ Grunehogna Province system produced a laterally variable Mfongosi Group, which formed in the ‘Mfongosi Basin’. Coarse-grained sediments dominated in its eastern portions while basalts with thin sapropelite units dominated in its western portions. Key words: South Africa, Antarctica, Mfongosi Group, Ahlmannryggen Group, metasediment geochemistry.

Introduction Mesoproterozoic mobile belts preserved mainly in South America, Africa, Antarctica and Canada formed a significant part of the supercontinent of Rodinia (Fig. 1).Several researchers (e.g., Groenewald et al., 1991; Moores, 1991; Jacobs et al., 1995; Dalziel et al., 2000) have emphasized the central role of the NamaquaNatal-Falkland-Maudheim belt in the formation and break-up of Rodinia (Fig. 1). The Namaqua-Natal

Metamorphic Province (NNMP), which was laterally continuous with the Grenvillian terranes of Laurentia and East Antarctica, represents this collisional event in southern Africa between 1250 Ma and 950 Ma (e.g., Moores, 1991; Dalziel et al., 2000). The NNMP, which formed between the southern margin of the Archaean Kaapvaal Craton and a plate or sub-plate to the south (Matthews, 1959, 1972, 1981a, b), is typically subdivided into a western Namaqua Sector and an eastern Natal Sector.

58

I.J. BASSON ET AL.

Both the Mfongosi Group of the Natal Sector, South Africa and the Ahlmannryggen Group of the Grunehogna Province of western Dronning Maud Land (WDML), Antarctica contain greywackes, arkoses and arenites that have undergone low-grade o r greenschist-facies metamorphism. While the sedimentary hinterlands of the Ahlmannryggen Group (Perritt, 2001) and the Mfongosi Group (Basson, 2000) have been defined, a stratigraphic or geochemical correlation between the metasediments of respective groups in potentially adjacent terranes has not been attempted, indeed, the fingerprinting and correlation of syn-tectonic sedimentary sequences has yet to find more widespread application in studies on the Mesoproterozoic Namaqua-Natal-Falkland-Maudheim Belt. The tectonic provenance of metagreywackes, metaarkoses and meta-arenites is potentially instructive in determining the proximity and influence of crustal fragments or arcs and the configuration of their intervening sedimentary basins during the final stages of Rodinia amalgamation.

%pe

n

Fold Belt

Innnkm

Regional Geology

‘2

Natal Sector - Ntingwe and Mfongosi Groups The Natal Sector is a complex collection of terranes, including a northern deformed melange of basin sediments (Natal thrust front - NTF), partly overlain by the Natal nappe zone, often referred to as the Tugela Terrane (Mathews, 1959, 1972; Fig. 2). The NTF consists of the Ntingwe and Mfongosi groups. The former comprises greenschist-facies metasediments with dolomite, limestone, mudstone, shale and conglomerate/breccia protoliths (Matthews, 1959, 1972; Matthews and Charlesworth, 1981). The Mfongosi Group invariably occurs to the south of minor inliers of the Ntingwe Group and is largely bimodal, consisting of schistose metatholeiites interbanded with metamorphosed shallow water sapropelites to coarse-grained metasediments with unknown palaeo-transport directions (Matthews, 1959; Basson, 2000; Basson and Watkeys, 2000). The NTF is tectonicallyjuxtaposed at its southern margin, partly along the Manyane Thrust, against the base of the hinterlanddipping Natal nappe zone, which contains four NE-verging nappes of oceanic crust with a minor ophiolitic component in the form of the Tugela nappe (Mathews and Charlesworth, 1981; Bisnath et al., 2002; Johnstone et al., 2002; Fig. 2). The accreted Mzumbe and Margate terranes, the final configurations of which were complicated by late-tectonic oblique collision and indentor tectonics at the southern tip of the Kaapvaal Craton (Fig. 2; Thomas, 1989a, b; Jacobs et al., 1993) occur to the south of this nappe zone. Although it is known that

-ape

Fold Belt

-\

Fig. 1. (a) A ‘tight-fit’ reconstruction (e.g., Roeser et al., 1996), in which the Grunehogna Province represents a fragment of the Kaapvaal Craton. Note the nature of the Pan-African strike-slip system identified in western Dronning Maud Land. (b) A ‘loosefit’ or extended Kaapvaal Craton reconstruction (e.g., de Wit et al., 1988), necessitating a dramatic change in orientation of the N-S strilung Mozambique Belt. (c) A ‘loose-fit’reconstruction in which the Grunehogna Province exists as a separate microplate is favoured in this study (Perritt, 2001). This implies that the region between the Kaapvaal Craton and Grunehogna Province was occupied by Proterozoic crust. The positions of figures 2a and 2c are indicated. DB=Damara Belt, GP= Grunehogna Province, KC=Kaapvaal Craton, MB=Mozambique Belt, MP=Maudheim Province, MR=Mozambique Ridge, NNMP=Namaqua-Natal Metamorphic Province (containing a western Namaqua Sector and a n eastern Natal Sector), ZC=Zimbabwe Craton.

Gondwana Research, K 7, No. 2, 2004

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59

the NTF represents the northern or leading edge of collision in the Natal Sector (African azimuths; Fig. l ) , the source terranes for sediments in the 'Mfongosi Basin' and this basin's original tectonic setting has not been addressed in the literature.

Grunehogna Province - Ahlmannryggen Group At approximately the same time as the Natal Sector was forming, collision between the Grunehogna Province and an 'East-Antarctic' craton formed a Proterozoic fold-thrust belt, termed the Maud Belt, which is contained within WDML (Amdt et al., 1991; Groenewald et al., 1991, 1995; Moyes et al., 1993; Grantham et al., 1995; Figs. 1, 2). In response to this continental collision, a peripheral foreland basin developed on the margin of the Grunehogna Province, in which the Ahlmannryggen Group sediments accumulated (Groenewald et al., 1995; Moyes and Harris, 1996; Perritt, 2001). A model of the evolution of the Ahlmannryggen Group has emerged in the last few years, enabling the origin of its constituent metasediments to be proposed. The Ahlmannryggen Group, which forms the oldest known/exposed unit of the Grunehogna Province cover sequence (Wolmarans and Kent, 1982), represents detritus shed into the peripheral foreland basin from the uplifted orogen, the stable craton and the active arc system in the central Kirwanveggen to H. U. Sverdrupfjella (Fig. 2; Perritt, 2001). The dominant sediment transport direction varied along the length of the belt, being approximately E-W in the Heimefrontfjella, and changing to NW-SE in the remainder of the belt. The evolution of the western side of the uplifted orogen is largely unconstrained. The volcaniclastic Jutulstraumen Group overlies the Ahlmannryggen Group, while extensive sills from the Borgmassivet suite have invaded both the Ahlmannryggen Group sediments and Jutulstraumen Group volcaniclastics (Fig. 2; Wolmarans and Kent, 1982; Krynauw et al., 1990, 1991, 1994; Watters et al., 1991). These sills exhibit a continental tholeiitic signature and are considered contemporaneous with the lavas of the capping Straumsnutane Group (Krynauw et al., 1991; Peters et al., 1991; Watters et al., 1991; subdivisions introduced in Perritt, 2001).

Metasediment Descriptions Mfongosi Group The Mfongosi Group contains a mixed sequence of metabasites, coarse- to medium-grained metasediments and metapelites. These have undergone some degree of cataclasis, that is, extreme mechanical grain breakage and grain size reduction at low metamorphic grades. These lithologies are therefore termed cataclastics to Gondwana Research, K 7, No. 1, 2004

w

.&

31's

Durban

Margate Terrane

Y

31's

0

x x x x x x x ' X X X Y X X X

Fig. 2. (a) Map of the Natal Sector (Thomas, 1989a, b). MfV denotes the position of the Mfongosi Valley, containing type examples of the Mfongosi Group. The section A-B is shown in figure 2b. (b) North-South cross-section of the nappe zone and thrust front of the Natal Sector, demonstrating the relatively restricted extent of the Natal thrust front at the leading edge of deformation against the Kaapvaal Craton (after Matthews and Charlesworth, 1981) (c) Position and extent of the Ahlmannryggen Group within Western Dronning Maud Land (after Perritt, 2001, modified after Jackson, 1997). (d) Schematic cross-section of the configuration of the Ahlmannryggen, Jutulstraumen and Straumsnutane Groups and the Borgmassivet Suite (after Perritt, 2001).

60

I.J. BASSON ET AL.

ultracataclastics, depending on their degree of tectonicallyinduced grain-size reduction. Cataclastictextures are more evident in the northern limits of the Mfongosi Valley, where the Mfongosi Group is juxtaposed against the Kaapvaal Craton. Metasediments in the northern portions of the type area for the Mfongosi Group, the Mfongosi Valley, consist of massive cataclastics to ultracataclastics (Fig. 3) with rare banding defined by minor concentrations of phyllitic minerals, siderite or ankerite. The main cataclastic rock type is a light brown to khaki phyllitic quartzite (Fig. 3a). Small lenses of amphibole-bearing phyllitic quartzite occur in the northern portions of the Mfongosi Valley, along with thinner layers of epidote-bearing and epidote-rich cataclastics, the latter indicating tectonic juxtaposition of the southernmost areas of the Mfongosi Valley wherein metabasites are typically more abundant. Rootless fold hinges and disrupted segments of folded quartz veins, combined with a uniform cataclastic texture with an average grain size of 0.02 mm in their host rocks, support the proposition that portions of the Mfongosi Group comprise a melange (Fig. 3b; Basson, 2000). Kink folding indicates late-tectonic, left-lateral transpressional movement within the sequence (op. cit.). The metasediments of the northern Mfongosi Valley grade southwards into banded quartz-muscovite (Fig. 3c) schists and metatholeiitic epidote-actinolite schists (Basson, 2000). Metasedimentary schists in the southern Mfongosi Valley contain sutured and annealed saccharoblastic quartz, feldspar and aligned muscovite or talc, with an average grain size of 0.03 mm (Fig. 3d). Epidote and opaque mineral-rich porphyroblasts display (T and 6 kinematic indicators that verify top-to-north and left-lateral shearing (Basson, 2000). Metasediments in the southern portions of the Mfongosi Valley also show lateral variants, in the form of pinkish to yellow rocks wherein talc or muscovite dominate over quartz. Due to grain size reduction during cataclasis, this variation is cryptic and not readily observable in outcrop (Basson, 2000). These geochemically and often petrographically distinct metasediments are indicated by an asterix in Table 1. Due to cataclasis and subsequent low-grade metamorphism, point counting is not possible for Mfongosi Group metasediments, hence their geochemistry is relied on as a means of division, classification and attempted correlation with Ahlmannryggen Group metasediments. Although it is impossible to directly determine the validity and applicability of geochemical and tectonic provenance classifications of low-grade, heterogeneous and immature meta-sediments, it should be noted that the original magmatic ratios of immobile-incompatibleelements in the closely interbanded metabasites of the Mfongosi Group

have been preserved (Basson et al., 1998; Basson, 2000; Basson and Watkeys, 2000>, even in the higher-grade, higher-strain areas to the west of the Mfongosi Valley (the Ngubevu area), thereby indicating minimal or significant fluid flow or metasomatism. Veining is minimal or restricted to individual units. Furthermore, the Chemical Index of Alteration (CIAs) of ‘type’, unmetamorphosed greywackes (Table 1; Reed, 1957; Pettijohn, 1975; Steed and Morris, 1986; Floyd et al., 1989 and CamirC et al., 1993) are very similar to those of the samples considered in this study. The ratio of coarse-grained to fine-grained metasediments generally decreases from east to west across the Mfongosi Group within the NTE Coarsergrained detrital metasediments grade laterally into very thin metapelite bands, denoting a change in original depositional environment from an eastern, high-energy, sediment-dominated area to a western metabasite 2 metapelite dominated area (Basson et al., 1998; Basson, 2000). The embryonic ‘Mfongosi Basin’ therefore experienced laterally variable sedimentary conditions, with the added complication of major basalt incursions in its westernmost portions.

Ahlmannryggen Group Petrological studies of metamorphosed sandstones from the Ahlmannryggen Group indicate pervasive greenschist facies alteration, resulting in an abundance of secondary minerals. These include sericite, chlorite, zoisite, epidote, actinolite, calcite and microcrystalline silica, which comprise a low-grade metamorphic matrix. These secondary minerals also pseudomorph many of the original detrital grains. Current-deposited sediments such as the Ahlmannryggen Group sandstones contain minimal primary detrital matrix (Pittman, 1979). The abundant metamorphic matrix therefore cannot be solely attributed to the alteration of original interstitial material, as it may have formed by in-situ breakdown of unstable detrital mineral grains and rock fragments (Cummins, 1962). An abundance of this metamorphic matrix adversely affects provenance and tectonic discrimination schemes based on petrographic data, as these tend to be critically sensitive to unstable components such as feldspar and lithic fragments (Ingersoll, 1978; Dickinson and Suczek, 1979; Suttner et al., 1981; Ingersoll et al., 1984; Dickinson, 1985; Suttner and Basu, 1985; Harwood, 1988). Preferential removal of these phases results in a shift towards more mature, quartz-rich compositions, and incorrect interpretation of provenance and tectonic setting (Cox and Lowe, 1996). As such, the petrographic data is not used for tectonic discrimination. Rather, the geochemistry of a representative suite of samples is used to establish a correlation with Mfongosi Group metasediments. Gondwana Research, V. 7, No. 1, 2004

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61

Fig. 3. (a) The dominant lithology of the northern Mfongosi Valley - a light brown banded phyllitic quartzite/cataclastic. Kink folding, indicating left-lateral movement along the NTF, is common. Length of hammer (top left) is 40 cm. (b) Photomicrograph of typical northern Mfongosi Valley phyllitic quartzite/cataclastic, showing scattered ankerite and minor sub-parallel shears in a cataclastic matrix that consists primarily of quartz and lesser albite. Crossed polars, horizontal field of view = 2.5 mm. (c) The dominant lithology of the southern Mfongosi Valley - a porphyroblastic calcite-bearing quartz-muscovite/talc-albite schist surrounds a tectonised lens of quartz/feldspar-epidote-chlorite schist, which in turn shows relict pillow basalts. Scale on camera lens is in centimeters. (d) Photomicrograph of typical calcite-poor quartz-albitemuscovite schist. Small muscovite laths are scattered around a @-structure (indicating left-lateral movement - section is viewed looking down dip and is oriented east-west) cored by slightly coarse-grained quartz and epidote. The matrix consists of partially annealed, mosaictextured quartz and albite. Crossed polars, horizontal field of view = 2.5 mm. (e) The Ahlmannryggen Group is a sandstone- to finegrained mudstone-dominated sequence exposed in several Nunantaks. The height of the outcrop shown is approximately 80 m. (0 As for (el. (g) Hydraulically fractured heavy mineral grains (HM) within typical Ahlmannryggen Group sandstone/greywacke. Scale bar = 250 microns. (h) A sedimentary lithic (Ls) grain in which the individual sub-grains are visible under plain polarized light, due to the presence of a remnant grain coating. Scale bar = 250 microns.

Gondwana Research, V. 7, No. 1, 2004

53.92 14.27 10.23 1.26 11.37 12.63 0.20 4.18 9.27 3.74 0.41 1.26 0.17

100.06

14.0 35.0 89.6 0.0 64.2 8.8 77.4 19.0 0.0 4.7 0.0 4.8 16.1 7.3 31.0 41.8 153.7 4.7 0.6 384.7 29.2 103.7 91.1 1.9 51.5

53.13 14.61 10.97 1.35 12.19 13.54 0.20 4.07 8.13 4.33 0.40 1.48 0.21

100.09

17.0 15.0 61.4 10.5 59.6 12.2 57.2 19.1 0.0 4.7 7.1 5.6 16.6 7.6 81.0 46.2 193.7 4.4 0.5 411.3 30.1 102.2 104.2 2.8 53.2

bld

72.0

99.92

63.04 21.57 4.62 0.57 5.13 5.70 0.02 1.10 0.04 0.98 6.13 1.18 0.16

343.1 1510.4 0.0 98.2 57.1 55.2 178.0 110.5 0.0 161.9 14.3 27.5 0.0 24.5 2.1 18.2 0.0 46.4 22.6 22.7 11.1 135.4 15.3 82.1 28.0 514.0 33.2 33.3 137.8 60.9 2.7 10.2 1.1 8.8 124.7 297.2 8.6 34.3 53.3 46.6 53.5 242.4 65.9 10.8 59.2 75.1

bld

7.0

100.82

53.21 19.44 6.94 0.86 7.71 8.57 0.14 5.83 9.13 3.03 1.22 0.22 0.04 0.11

68.20 16.22 3.80 0.47 4.22 4.69 0.13 0.69 2.90 4.55 2.09 0.46

SMF-G grey wacke

bld

5.0

bld

2.0

99.66

73.32 13.66 2.28 0.28 2.54 2.82 0.09 0.84 4.05 3.49 0.99 0.31 0.09

SMF-H grey wacke

1069.9 1494.5 1351.4 26.7 29.3 19.9 62.9 81.3 117.8 6.6 7.1 14.1 6.3 1.5 6.8 14.0 10.0 15.5 8.6 13.2 1.6 5.9 7.9 6.8 7.7 9.1 11.1 0.0 0.0 0.0 5.4 10.0 7.7 39.7 29.7 15.0 20.0 0.0 4.0 12.9 10.7 7.2 126.5 227.5 238.5 3.7 6.8 5.3 0.1 1.2 0.0 23.1 10.6 27.7 31.4 46.1 31.8 29.8 52.0 21.5 178.8 222.6 171.8 1.8 1.0 2.7 61.8 63.0 61.6

bld

2.0

99.24 100.03

67.51 16.13 3.32 0.41 3.69 4.10 0.07 0.98 2.96 4.29 2.72 0.36 0.11

SMF-F grey wacke

NMF-32 grey wacke

NMF-2 WY wacke

NMF-45* arkose/ lithic arkose

Natal Natal Natal Natal Natal Sector Sector Sector Sector Sector Mfongosi Mfongosi Mfongosi Mfongosi Mfongosi

Natal Sector Mfongosi Grun.

1067.2 25.9 85.0 16.6 0.0 7.4 0.0 6.7 10.8 0.0 10.7 60.4 388.0 12.2 44.2 5.1 2.8 58.8 43.4 5.6 170.0 3.3 75.3

bld

8.0

99.72

75.64 14.51 3.12 0.39 3.46 3.85 0.01 0.60 0.03 0.34 4.38 0.32 0.03

-

857.2 65.7 35.4 18.5 1.5 4.2 25.2 11.6 29.3 35.0 8.7 67.7

na 16.4 179.9 12.8 2.0 98.3 34.5 50.6 199.8 1.4 63.2

na 18.0 162.1 10.2 3.8 104.0 26.3 57.7 159.3 6.7 61.1

na

4.0

99.79

69.66 14.14 3.76 0.46 4.17 4.62 0.07 2.28 2.45 3.32 2.48 0.56 0.21

Ahl. Gruneh ogna A.JE.3 lithic grey wacke

Grun.

568.0 34.5 61.0 68.1 4.6 9.4 12.8 8.2 6.1 248.5 0.0 35.9

na

0.0

98.40

68.64 10.19 5.14 0.63 5.71 6.32 0.12 5.83 3.12 2.30 1.06 0.67 0.14

Ahl. Pyra miden SMF-LBQ A.PM.6 arkose/ feldspa lithic thicgrey arkose wacke

Natal Sector Mfongosi

7.3 158.9 8.2 2.2 69.2 24.0 18.2 121.6 5.2 59.9

na

892.3 25.6 80.9 42.5 1.8 6.4 14.7 7.5 15.1 36.4 3.9 70.5

na

1.1

99.52

78.44 9.77 2.15 0.30 2.72 3.01 0.02 1.20 1.90 1.98 2.66 0.41 0.13

Ahl. Veten B.NL. 7 H.M. lithic arrnite

Grun.

12.0 462.9 12.6 4.5 91.6 27.3 56.0 144.1 3.4 61.1

na

651.8 39.6 32.9 42.6 4.2 4.9 20.9 12.2 13.2 14.5 17.6 91.5

na

7.8

99.74

64.09 16.80 3.85 0.47 4.28 4.74 0.08 2.64 4.09 3.98 2.63 0.50 0.19

feldspa thic grey wacke

Ahl. Veten B.PR.1

Grun.

Gruneh

Grun.

Grun.

Gmn.

Grun.

18.0 333.3 8.1 3.0 211.7 27.2 61.6 130.6 10.8 56.7

na

268.0 54.4 79.3 87.7 4.3 4.3 20.5 11.2 18.9 45.9 27.1 27.2

na

7.1

99.65

66.85 10.49 9.08 1.12 10.09 11.17 0.12 1.51 5.08 2.26 0.66 1.26 0.25

feldspa thicgrey wacke

13.4 981.8 5.4 4.6 148.0 23.7 68.7 99.8 10.9 57.3

na

72.9 37.7 80.8 59.0 30.7 0.7 13.1 6.5 14.5 87.7 26.4 0.9

na

14.8

99.69

69.64 11.81 5.26 0.65 5.84 6.47 0.19 1.98 6.27 2.50 0.02 0.62 0.18

lithic grey wade

na

16.2

99.51

68.99 16.61 2.70 3.00 0.33 3.32 0.03 1.77 0.80 2.09 5.58 0.26 0.06

lithic arenite

9.0 263.3 11.8 4.3 58.5 33.4 21.0 147.0 0.7 63.3

na

13.3 167.6 30.0 4.6 57.4 63.8 54.1 245.7 0.8 66.2

na

678.6 1404.2 42.8 104.1 64.2 16.4 7.9 23.2 0.3 4.1 2.1 1.3 1.9 36.4 11.1 18.6 15.8 41.9 40.8 9.2 1.0 2.7 88.5 236.5

na

2.9

99.25

72.21 13.91 2.42 2.69 0.30 2.98 0.04 1.47 2.95 2.70 2.40 0.45 0.14

lithic grey wacke

na

6.4

99.80

63.95 16.22 3.76 4.17 0.46 4.62 0.11 2.07 5.44 3.14 3.50 0.60 0.15

lithic grey wacke

23.2 99.6 12.2 3.6 78.5 37.6 50.1 164.5 3.5 64.1

na

9.7 356.0 9.4 4.4 88.0 27.4 71.5 145.8 6.8 57.3

na

1545.8 1131.0 69.9 62.3 37.1 31.6 43.3 64.2 0.9 11.1 1.9 4.8 21.5 24.3 13.3 9.8 31.8 26.8 85.3 7.9 12.2 8.9 237.5 121.8

na

2.2

99.70

64.08 15.66 5.46 6.07 0.67 6.72 0.04 3.59 0.92 1.05 6.81 0.66 0.17

arkosic arenite

Ahl. Ahl. Ahl. Ahl. Ahl. Ahl. Framyggen Framyggen Hogfonna Hogfonna Brapiggen Brapiggen B.ST.3H.M. B.VE.3 B.0B.7 B.Hh.2* B.RY.2 B.BP.4*

Gmn.

(Grun.=Grunehogna; NMF=Northem Mfongosi Valley; SMF=Southem Mfongosi Valley; CIA=Chemical Index of Alteration; bld=below limit of detection; na=not analysed).

Zn Zr Crflh CIA

Y

Ga La Nb Nd Ni Pb Rb S sc Sr Th U V

cu

TiO, PZO, TOTAL As Au Ba Ce co Cr

K,O

SiO, Al,O, FeO Fe,O, FeO(t) Fe,O,(t) Mi0 MgO CaO Na,O

Province/ Natal Sector Sector Mfongosi Group Forma tion Sample NMF-I Rock WY Twe wacke

Table 1. Major (wt%) and Minor Element (ppm) data for selected Mfongosi and Ahlmannrygen Group metasediments.

GEOCHEMICAL CORRELATION BETWEEN METASEDIMENTS IN SOUTH AFRICA AND ANTARCTICA

Despite the extensive alteration present in the metamorphosed sandstone samples, the results of petrographic analysis are considered sufficient for a preliminary assessment of possible source terranes for the Ahlmannryggen Group (Perritt, 2001; Fig. 3e-h). Framework grain point-count d a t a from the metamorphosed sandstone samples suggest a dominance of magmatic and sedimentary over metamorphic source terrains, as indicated by high feldspar concentrations, the ubiquitous presence of chert and a low concentration of metamorphic lithics (Perritt, 2001). The limited metamorphic lithic component is restricted to sandstones originally deposited in NE draining fluvial systems in the Borgmassivet region. This suggests a localised metamorphic terrain situated to the S/SW of the Ahlmannryggen Group basin. Petrographic data indicate that contributions from two distinct magmatic terrains are represented (Perritt, 2001). These include a felsic plutonic source, from which K-feldsparswere derived, and a contemporaneously active volcanic terrain, characterised by explosive acid volcanism, from which a volcanic fraction was sourced (Perritt, 2001). Unlike the plutonic component, which does not exhibit any marked stratigraphic or geographic variations, the volcanic fraction is typically better represented in the more southerly exposures of the Borgmassivet region of the Grunehogna Province, suggesting this area was more proximal to the volcanic source. This source terrain evidently consisted of a combination of quartzites and chemical precipitates, such as chert and jasper, suggesting a banded ironstone association. Detritus sourced from this terrain is represented in all samples and evidently lacked geographic restrictions, suggesting that banded ironstone was extensive in pre-existing cover rocks. The Annandagstoppane basement granites constitute a potential plutonic magmatic source, while the magmatic arc system of the Maudheim Province that docked against the southern margin of the Grunehogna Province during the Mesoproterozoic (Grantham et al., 1995; Groenewald et al., 1995; Jacobs et al., 1996) represents a likely source of pyroclastics. Uplifted orogenic regions of the latter may have supplied the metamorphic component. The sedimentary source terrain is either no longer preserved, or is obscured by the extensive ice cover, but was likely part of an older (possibly early Proterozoic) stable continental cover sequence similar to that of the Transvaal Basin of South Africa or the Hammersley Basin of Western Australia (q.v. Perritt, 2001).

Geochemical Classification Data show internal consistency on a variety of majorand minor-element plots (Table 1). A general Mfongosi Gondwana Research, V. 7, No. I , 2004

63

Group trend towards the A-K join is evident on a ternary A-CN-K plot of Nesbitt and Young (1984, 1988; Fig. 4a). Low-Ca+Na, high-K metasediments plot almost on the A-K join in the ‘Muscovite+Illite’ zone. Ahlmannryggen Group metasediments show a trend that is slightly oblique to that of the Mfongosi Group, with compositions that are ‘intermediate’ to the two well-defined groups of the Mfongosi Group in A-CN-K space. The Chemical Index of Alteration (CIA) of Mfongosi metagreywackes (49.0265.27) is comparable to that of the Annandagstoppane Granite, the Kirwanveggen Basement and the H.U. Sverdrupfjella Basement (Fig. 4), while the CIA of Mfongosi arkoses and lithic arkoses (75.10-76.94) is slightly higher than the CIA range of Ahlmannryggen Group metasediments, at 54.28-69.63. The low-Ca+Na, high-K samples display a separate trend to the bulk of the samples on the A-CNK-FM plot of Nesbitt and Young (1984, 1988; Fig. 4b). Both trends converge close to the ‘Muscovite+ Illite’ field on the ACNK join. The Annandagstoppane Granite, Kirwanveggen Basement and the H.U. Sverdrupfjella Basement form a less altered group compared to the Ahlmannryggen Group metasediments. There is an overlap between the southern Mfongosi Group metagreywackes and the low-Fe+Mg end of the Ahlmannryggen Group distribution. It is evident from both the A-CN-K plot and the A-CNK-FM plot that weathering trends, from source rock to final product, are not exhibited by the data, rather, the arrays represent a slight, primary difference in source rock composition, with their compositions being extremely close to those of typical, pristine rocks, directly supporting the application of geochemical provenance classification systems. Conventional major element binary and ternary plots indicate a predominant greywacke protolith for the bulk of the Mfongosi Group metasediments, while the lowCaSNa, high-K samples are meta-arkoses to meta-lithic arkoses, or are merely classified as ‘mature’ sediments (Fig. 5a, b). Geochemically, the Ahlmannryggen Group metasediments are classified as a mixture of low-grade greywackes, arkoses and lithic arkoses, with a minor component of mature and possibly calcareous sedimentary rocks. Both the Mfongosi and Ahlmannryggen Group metasediments define &O/Rb ratio trends parallel to the magmatic trend of Shaw (1968; Fig. 5c), the trend of unmetamorphosed arkosic sands (van de Kamp et al., 1976) and typical low-grade metagreywackes (Caby et al., 1977), although the Mfongosi Group metasediments display slightly raised Y O values and extend to more felsic compositions. This conclusion is supported by the TiO, vs. Ni plot (after Floyd et al., 1989), which emphases the dominant magmatogenic nature of Mfongosi Group metasediments. Ahlmannryggen Group metasediment

I.J. BASSON ET AL.

64

Mfongosi Group data as they indicate at least some influence of a mafic hinterland. The felsic granitoid nature of the source of the Ahlmannryggen Group metasediments is emphasized by the very low Cr values (7.9-95.8 ppm), while the Ni values (1.6-85.3 ppm), although low, are not entirely indicative of a felsic hinterland. The Cr/Th ratio is also used to gauge mafic rock contribution: a ratio of 0-75 indicates a hinterland dominated by felsic granitoids/volcanics (Condie et al., 1991). The Cr/Th ratio

protoliths are more variable, displaying poorly-defined subsidiary ‘calcareous’and ‘sandstone’ trends (Fig. 5d). Condie et al. (1991) used the Cr, Ni and Th concentrations of metagreywackes as indicators of a mafic source rock contribution to sedimentation. Low Cr values (0-150 ppm) and low Ni values (0-35 ppm) imply dominantly felsic granitoids in the source rock area. As such, samples NMF14, 32 and 46 (northern Mfongosi Valley), which have high Cr and/or Ni contents, are possible exceptions within

A

aolinite, Gibbsite. Chlorite

A Kaoiinite, Gibbsite, Chlorite

bl m uands c o v ~ liiite

/ K $,Kaohk,

\

Basalt 4

CNK

Gibbsite. Chlorite

c)

Chlorite Basalt

CN

K

CNK

0 Ahlmannryggen Group meta-arenite Ahlmannryggen Group meta- sub-arkose and arkosic arenite 1 3 Mfongosi Group metagreywacke A Mfongosi Group meta-arkose and lithic arkose

/

”

m

”

”

FM

(Barberton granite-greenstone)

4- Annandagstoppane granite 0 Kirwanveggen basement

V

H U Sverdrupfjella basement

Fig. 4. A-CN-K (a, c) and A-CNK-FM (b, d) ternary plots. A=Al,O,, C=CaO, N=Na,O, K=&O, F=total Fe as FeO, and M=MgO (in mole fraction); arrows indicate general weathering trends exhibited by various rock types (after Nesbitt and Young, 1984, 1988). Mfongosi Group metasediments from the southern portion of the Natal thrust front (active continental margin signatures) have similar compositions to Kirwanweggen basement and Fe and Mg-poorer Ahlmannryggen Group metasediments. Mfongosi Group metasediments from the northern portion of the Natal thrust front (island arc signatures) show only slight overlap with Kinvangweggen and H. U. Sverdrupfjella basement data, while the Mfongosi Group arkoses/lithic arkoses show no obvious compositional similarities with similar rocks from Dronning Maud Land. (Annandagstoppane granites: unpublished data from J. Krynauw; Maudheim Basement lithologies - Kirwanveggen and H.U. Sverdrupfjella - from Grantham, 1992 and Groenewald, 1995).

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for all Ahlmannryggen Group samples is extremely low, ranging from 0.51 to 10.93. Only sample NMF-14 from the Mfongosi Group has a Cr/Th ratio greater than 75 (Cr/Th = 224). Protoliths to metasediments of the Mfongosi and Ahlmannryggen Groups therefore originated in a felsic granitoid dominated, magmatogenic hinterland(s). A minor component of mature metasediments, with low Ca+Na and high K (and Rb) in the Mfongosi Group correspond to arkoses to lithic arkoses that have greater proportions of illite and/or muscovite. Due to the restricted

F e 2 0 3 (t)

a)

+ MgO (wt Yo)

65

nature of sampling imposed by limited fresh outcrop and the extremely complex tectonic interleaving of lithologies (Basson and Watkeys, 2000) in the Mfongosi Valley, and limited outcrop in Antarctica, it is impossible to estimate the exact proportions of the constituent metasedimentary types in each group.

Provenance Geochemistry The geochemical signatures of the constituents of the

calcareous I/ sediments I/

bl

I

1000-

0: greywacke

log Sr (PPm) 100-

0

mature sediments 10-

Na20 (wt Yo)

/ 100

K 2 0 (wt Yo)

1000

log B a (PPml

CI

3-

0 arkosic sands @

magmatic

greywackes 0

10. 2-

5.

log K 2 0 (wt Yo)

/

magmatogenic 0 greywackes 1

Ti02

(wt 740)

1' 0.5.

1-

0.1

1

10

&O 100

500 1000

log Rb (PPm) Ahlmannryggen Group meta-arkose, greywacke, sub-arkose and arkosic arenite 111 Mfongosi Group meta-arkose and lithic arkose

0 Mfongosi Group metagreywacke

Fig. 5. (a) (Fe,O,+MgO) - (Na,O) - (YO) plot, indicating that the samples are a mixture of (meta-) greywackes, arkoses and lithic arkoses. (b) Log Sr vs. log Ba plot, indicating a trend from (meta)-greywackes to mature sediments (Figs. a and b from Blatt et al., 1980; Blatt, 1992 and Pettijohn et al., 1973). (c) Log Y O vs. log Rb plot, indicating that data are parallel to the magmatic trend defined by Shaw (19681, the trend of unmetamorphosed arkosic sands (van de Kamp et al., 1976) and typical low-grade metagreywackes (Caby et al., 1977) (d) TiO, vs. Ni plot, indicating clustering in the magmatogenic field (after Floyd et al., 1989).

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I.J. BASSON ET AL.

Mfongosi and Ahlmannryggen groups are complex due to the mixing of detritus from multiple source terranes. Geochemical classification indicates that greywackes predominate, although arenites and arkoses are also represented. The immaturity of the sediments is emphasised by the moderate to high CIA values. The spread of points in tectonic discrimination diagrams is therefore considered an effect of the possible mixing of detritus from multiple source terranes, particularly in the vicinity of volcanic arcs. The Al,O,/(CaO+Na,O) vs. Fe,O,(t) +MgO plot of Bhatia (1983) shows two distinct groups for the Mfongosi Group metasediments; northern MfongosiValley metagreywacke protoliths formed from an oceanic island arc while the southern Mfongosi Valley metagreywacke protoliths formed from an active continental margin (Fig. 6 a ) . The arkoses/lithic arkoses show high Al,O,/(CaO-tNa,O) values and are therefore not represented on Fig. 6a, however, they do fall within the proximal extrapolation of Field D on this plot, probably also indicating a passive margin setting. The classification of Ahlmannryggen Group metasediments as either active continental margin or passive margin in origin is less clear on the Al,O,/(CaO+Na,O) vs. Fe,O,(t)+MgO plot where data are not coherently grouped and overlap with intervening groups, although data mainly cluster in the active continental margin and continental island arc fields. In a classification that caters for high-A1 samples, the northern Mfongosi Valley metagreywacke protoliths formed in an oceanic island arc, while the southern Mfongosi Valley metagreywacke protoliths formed in a dominantly active continental margin setting (YO/Na,O vs. SiO, plot of Roser and Korsch, 1986; Fig. 6b). In contrast, the interleaved arkoses/lithic arkoses were derived from a passive continental margin. Ahlmannryggen Group metasediments were sourced mainly from an active continental margin with a minor component originating in a passive margin setting, although the latter compositions are notably different to those of the passive margin metasediments of the Mfongosi Group (Fig. 6b). Minor element classification diagrams are not as definitive as major element plots (e.g., Fig. 6c), which show the Mfongosi Group metasediments transgressing the active continental margin field at low Th values. Basson (2000) found that interleaved U- and S-rich sapropelites (which are probably geochemically linked to their adjacent metabasites) are common approximately 30 km to the west of the sampling area, potentially obscuring a clearer tectonic signature in adjacent areas. In contrast, Ahlmannryggen Group data, unaffected by anomalous metapelites, display good clustering in the continental island arc and active continental margin fields (Perritt, 2001). Fig. 6b displays the best separation of the

units mapped at Mfongosi and in Antarctica; tectonic provenance diagrams clearly define the Ahlmannryggen Group metasediment protoliths as having either a passive margin source or an active continental margin source. The former displays little compositional overlap with the arkoses/lithic-arkoses of the Mfongosi Group, while the latter active continental margin metasediments have YO/ Na,O vs. SiO, ratios that are similar to Ahlmannryggen active continental margin metasediments. This correlation is, however, not entirely definitive on its own and consequently a rigorous statistical method, namely Discriminant Function Analysis was applied in order to better constrain the potential correlation.

Discriminant Function Analysis Geochemistry-based tectonic provenance diagrams provide only ‘circumstantial’ evidence of a common sedimentary source for the Mfongosi and Ahlmannryggen groups. Multi-element Discriminant Function Analysis (DFA) was applied to the data in order to definitively match sub-sets established by tectonic provenance interpretation. All statistical procedures were performed using the software package Statistica. The full dataset and the tables resulting from stepwise analysis are available from the first author (IJB). A detailed discussion on the statistical procedures employed is beyond the scope of this study, however the reader is referred to Stevens (1986), Harman (1967) and Lawley and Maxwell (1971) for a fuller description of DFA. DFA is widely used in geology, for example in the discrimination and grouping of kimberlites (Ferguson et al., 1975; Danchin et al., 1975; Smith et al., 1985a, b) or in grouping and distinguishing garnets from a variety of rock sources (Dawson and Stephens, 1975). DFA first detects any structure between the selected variables using Stepwise Discriminant Analysis. The best possible combination of non-correlated variables that effectively maximises the variance between the pre-assigned groups is then established by maximising the ratio of betweengroup to within-group variance. Variables are entered into (or removed from) the statistical analysis in a stepwise manner until the best group separation is achieved, usually when the addition of variables to the classification no longer improves group separation. The resultant derivation can then potentially be used to classify unknown samples into one of the pre-assigned groups. Alternatively, whole groups of samples from different formations or groups can be correlated with each other by consideration of a broad range of major and minor elements. Whole-rock major and minor element compositions of metagreywackes from the Mfongosi Group were statistically compared to the Ahlmannryggen Group using Gondwana Research, V. 7, No. 1, 2004

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GEOCHEMICAL CORRELATION BETWEEN METASEDIMENTS IN SOUTH AFRICA AND ANTARCTICA

DFA, primarily in order to establish compositional similarities between active continental margin metagreywackes from either group. Samples were broadly divided into their source groups. Each group was given equal weighting for the statistical analysis and a 'F-to-enter' value of 3 was used. Two classificationmodels, containing different sets of elemental oxides and elements, were considered for statistical analysis: Classification 1: SO,, Al,O,, Fe,O,(t) (for total Fe), MnO, MgO, CaO, Na,O, YO, TiO,, and P,O,, these being commonly employed in discrimination or bivariate plots for metasediments in the literature; and Classification 2: Cr, Zr, Y, Nb, Sc, Th and La in addition to the major element oxides used in Classification 1. Geological formations in the Ahlmannryggen Group were used as the grouping variable for each classification of the DFA. There are eight major element oxides useful for the discrimination, namely (in order of entry or importance) YO, SO,, MgO, Na,O, Al,O,, TiO,, Fe,O,(t), and MnO. When the whole rock major and minor elements are

considered jointly, there are fourteen dominant elements in the classification, namely (in order of entry) K,O, Sc, La, MgO, Na,O, Al,O,, Y, Th, Cr, Zr, P,O,, Fe,O,(t), SO,, and TiO,. Both major element and the major+minor element DFA's achieved correct classificationand excellent correlation with Mfongosi Group provenance subdivisions. In contrast, the Ahlmannryggen Group formations display internal overlap. Significantly, the Mfongosi Group metasediments, which have protoliths that formed adjacent to an active continental margin, are geochemically very similar, if not identical, to the active continental margin components of the Ahlmannryggen Group (Fig. 7 - ACM).

Discussion The key to explaining and correlating the nearsimultaneous development and proximity of common source terranes of the Mfongosi and Ahlmannryggen Groups lies in the concept of oblique or diachronous collision, whereby collision initiated at approximately 1180

30

A n

10

n

0 N CJ

Y 00

o 0

Th

8

-

0.1

-

.

B 0

AhlrnannryggenGroup rneta-arkose,greywacke, sub-arkose and arkosic arenite Mfongosi Group metagreywacke A Mfongosi Group meta-arkose and lithic arkose

Fig. 6. (a) Log Na,O/K,O vs. log SiO,/Al,O, (Bhatia, 1983). Northern Mfongosi Valley samples have oceanic island arc signatures; southern Mfongosi Valley samples have active continental margin signatures. The metamorphosed arkoses/lithic arkoses fall on the extrapolation of Field D. Ahlmannryggen Group metasediments show mainly continental island arc and active continental margin signatures, with only minor passive margin and oceanic island arc signatures. (b) K,O/Na,O vs. SiO, (Roser and Korsch, 1986), clarifymg the discrimination and classification of Figure 6a. The Ahlmannryggen Group samples display mainly active continental margin and passive margin signatures. (c) Th - Sc - (Zr/lO) (Bhatia and Crook, 1986), with the Mfongosi Group samples displaying spurious results, possibly due to the variable Th contents in metasediments and adjacent metabasites. Ahlmannryggen Group samples yield primarily continental island arc and active continental margin signatures.

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I.J. BASSON ET AL

Ma in the SW portions of the embryonic Natal Sector and then proceeded northeastwards (African azimuths), into Dronning Maud Land, as allowed for by the model of Jacobs et al. (1993). Jacobs et al. (1997) summarise the ages for the evolution of the Natal Sector, which are shown with the broadly corresponding ages for Dronning Maud Land events in Table 2. Subduction of the Tugela Ocean and the formation of the Mzumbe and Margate Terranes, which followed deposition and extrusion of supracrustal rocks, occurred from approximately 1180 to 1135 Ma (Cornell et al., 1996; Jacobs et al., 1997; Thomas et al., 1999). This was followed by largely oblique emplacement of the obducted Natal nappe zone or Tugela Terrane (Jacobs et al., 1997; Basson, 2000; Johnston et al., 2002), probably contemporaneous with the initiation of deformation in the western and central parts of the NTF, from at least 1135 Ma (Thomas and Eglington, 1990; Jacobs et al., 1997; Thomas et al., 1999). The Mfongosi Basin probably acted as a receptacle for sediments prior to 1135 Ma, deepening in its NE parts and accumulating a substantial portion of medium- and coarse-grained, immature sediments. Tectonic deformation in the easternmost parts, as observed in the Nkandlha area (Basson, 2000), produced a thick metagreywacke melange with no intercalated metabasite. D, and possibly D, in the Natal Sector comprised sustained NE-directed collision in the

southern terranes, and amphibolite to greenschist facies metamorphism (Jacobs et al., 1993; Johnstone et al., 2002) from ca. 1090 Ma to 980 Ma. This oblique collision served to create a triangular Mfongosi Basin (Fig. 8>, dominated by Grunehogna Province active continental margin greywackes in its easternmost parts, but with an increasing proportion of Natal Sector basalt in its westernmost parts. The localised growth of muscovite at 900 Ma provides a marker for the end of tectonism. The Ahlmannryggen Group is dated at 1 1 3 9 i l l Ma (Grunehogna Formation) and 1 1 3 7 t 10 Ma (Hogfonna Formation), based on U/Pb SHRIMP analysis of detrital zircon grains introduced from contemporaneous volcanic activity in the adjacent Maud Belt (Perritt, 2001). This timing of sedimentation within the Ahlmannryggen Group basin implies a close relationship with Mesoproterozoic tectonism in the adjacent Maudheim Province. The earliest recorded phases of a prolonged period of plate convergence involving folding, thrusting and high-grade metamorphism of this island arc terrain are dated at ca. 1135 Ma (Jacobs et al., 1996; Jackson and Armstrong, 1997). The peripheral foreland basin in which the Ahlmannryggen Group was deposited was evidently well established by this stage, implying that docking of the island arc against the margin of the Grunehogna Province must have been initiated prior to 1135 Ma. Unfortunately, the lack of exposure of underlying basin fill sequences

Classification-1

Classification-2

j 0 0

I ~

I a

AO 8

I

0

0

0 0

;00%

PM

4

10

A

I 41

I

1 7 10

-1

6

2

2

6

10

A

A -

I

I

d Pyramiden

0 Framryggen

0 I - Oceanic islandarc (ARC]

2

2

6

Canonical Score - 1 Ahlmannryggen Group formation Mfongosi Group c,asslf,catlon

0

ARC

A

6

10

Canonical Score - 1 0 Gunehogna

+

Hogfonna

rn 2 Actlve conCnentalmargin (ACMJ

0

Veten A

*

Brapiggen 0

Llterature greywackes

3 - Passive continental margin (PM)

Fig. 7. Results of Discriminant Function Analysis using a) K,O, SO,, MgO, Na,O, AI,O,, TiO,, Fe,O,(t), and MnO; b) K,O, Sc, La, MgO, Na,O, A1,0,, Y,Th, Cr, Zr, P,O,, Fe,O,(t), SO,, and TiO,. Mfongosi Group samples which have oceanic island arc or passive margin protoliths are distinct from metagreywackes with active continental margin protoliths, which in turn overlap with the Ahlmannryggen Group field, most of which were derived from the active continental margin in the western portions of the Grunehogna Province (African azimuths). The Ahlmannryggen Group has been divided into its component formations for the purposes of statistical analysis. The single sample from the Pyramiden Formation and greywackes from the literature were not included in the DFA but these are plotted, by using the resultant Canonical Scores, for comparison purposes, in figure 7a.

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GEOCHEMICAL CORRELATION BETWEEN METASEDIMENTS IN SOUTH AFRICA AND ANTARCTICA

precludes accurate determination of the timing of basin formation. Lithification of the Ahlmannryggen Group is estimated at 1080 Ma, from Rb-Sr and Sm-Nd data (Moyes et al., 1995). According to Krynauw et al. (1988), the Borgmassivet Suite sills, which are correlated with the 1102 Ma Umkondo dolerites of Mozambique (U/Pb SHRIMP; Wingate, 2001), were intruded into wet, unconsolidated sediment (i.e., pre-1080 Ma), suggesting that the currently accepted age of the Borgmassivet Suite (ca. 1000-800 Ma - Rb-Sr) is probably incorrect. Deformation in the Kirwanveggen occurred between 1070 and 1035 Ma (Jackson and Armstrong, 1997) and at approximately 1035 Ma in the Sverdrupfjella (Board, 2001). The preserved sedimentary sequence of the Ahlmannryggen Group represents medial to distal deposits within the basin, which was aligned longitudinally, subparallel to the dominant structural grain in the orogenic belt (Perritt, 2001). The basin axis developed approximately 60 km inland from the orogenic front, with a depocentre to the easvnortheast near the Sverdrupfjella. In turn, the Sverdrupfjella is considered to be a direct effect of Pan-African deformation and shearing and/or rotation of an originally linear belt (Allen, 1991; Perritt, 2001). Reconstruction of Rodinia requires the Maud Belt

69

to be a continuous linear feature from the Kirwanveggen to central DML, with the Grunehogna Province extending notably further eastward.

Conclusion The initiation of Mfongosi Group deposition, with a probable minimum age of 1135 Ma, pre-dated and was partly contemporaneous with the deposition of the Ahlmannryggen Group (minimum age 1137 & 10 Ma; Perritt, 2001; Fig. 8; Table 2). Oblique collision from 1135 to 980 Ma potentially created a triangular Mfongosi Basin. Extreme deformation of metabastites and minor intercalated sapropelites in the westernmost portions of the Mfongosi Group, with strain ratios of R=12 at the Ngubevu area at the extreme western extent of the NTF inlier (Basson, 2000) suggests high-strain or prolonged left-lateral shearing in the west of the Natal Sector. This contrasts with the relatively intact metasedimentary sequence, showing strain ratios of only 1.5 in the Mfongosi Valley in the central portions of the NTF inlier (Fig. 3a) and massive metagreywackes in the easternmost portions of the Mfongosi Group (Fig. 8, Nkandlha area). Oblique or diachronous collision, starting in the SW of the

Table 2. Summary of metamorphic. deformation and sedimentarv events in the evolution of the Natal Sector and western Dronninrr Maud Land. Natal sector event Localized growth of post-tectonic muscovite Widespread transpressional motion and leftlateral shearing. Closed folding and lowpressure amphibolite to granulite grade local metamorphism

NE-directed collision, refolded isoclinal folds, thrusts, amphibolite to greenschist grade regional metamorphism Obduction of the 'hgela ophiolite / initiation of nappe emplacement Formation of Manyane Thrust cataclasis and juxtaposition of the 'hgela Nappe and the Mfongosi Group?

Age range/ Approx. age (Ma)

Dronning Maud Land event

Age range/ Approx. age (Ma)

Deformation in Sverdrupfjella

1035 g

Deformation in Kinvanveggen Lithification of Ahlmannryggen Gp

1070-1035 1080

Collision in Heimfrontfjella Deposition of Hogfonna Fmn. Ahlmannryggen Gp.

1135 2 11j 1137 t 10 *,k

900 a 1050-980 b.c

1090

1135

Deposition of Grunehogna Fmn. Ahlmannryggen Gp. Arc-related volcanism in Kinvanveggen

1139 k 11

*sk

-1135

h,j

Initiation of Mfongosi Group with sediment input from adjacent crustal fragments? Subduction of the 'hgela Ocean, formation 1180-1135 c , e , f , * of the Mzumbe and Margate Terranes (juvenile arcs) Data from a Jacobs and Thomas, 1996; Thomas et al., 1993; Jacobs et al., 1997; Thomas et al., 1995; Thomas et al. 1999; Cornell et al., 1996; g Board, 2001; Jackson and Armstrong, 1997; Moyes et al., 1995; j Jacobs et al., 1996; Perritt, 2001. (*) denotes present study.

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1.J. BASSON ET AL.

Fig. 8. Schematic diagram of the proposed tectonic evolution, with an indication of progressive, NE-directed diachronous deformation in the combined configuration. The proposed positions of the sedimentary hinterlands in the relevant portion of the Mesoproterozoic NamaquaNatal-Falkland-Maudheim Belt is shown.

developing Natal Sector and progressing through to Dronning Maud Land in the north-east, caused deepening and closure of the Mfongosi Basin, bringing the easternmost parts of the basin within sedimentary range (60 km?) of the active continental margin of the Grunehogna Province. This tectonic feature contributed to the sedimentary package, which includes sediments sourced from both a passive continental margin and an oceanic island arc. As deformation continued northeastwards into the Maud Belt, deposition of the Ahlmannryggen Group was initiated at ca. 1135 Ma and continued in a peripheral foreland basin established on the margin of the Grunehogna Province, (Fig. 8; Jacobs et al., 1996; Perritt, 2001) until ca. 1080 Ma. This diachronous collision is reflected in the range of event ages in the combined Natal Sector/Grunehogna Province system, the provenance of sediments found in the Mfongosi Group and the lateral variation in sediment:basalt ratio across the Natal thrust front.

Acknowledgments IJB expresses his gratitude to Gold Fields of South Africa for their generous support during mapping and sample collection during the early stages of this study. The National Research Foundation (NRF) is thanked for financial support for IJB during the final stages of a more regional study at M.Sc. and Ph.D. levels. Prof. Allan Wilson and Roy Seyambu are thanked for their assistance with geochemical analysis. SP gratefully acknowledges financial and logistical support provided by the Department of Environmental Affairs and Tourism -

Directorate: Antarctica and Islands, and the support provided by the airforce personnel of 22 Squadron, South African Airforce (SAAF). Mawson Croaker’s assistance in the field is greatly appreciated. Bob Thomas and Bryan Storey provided very helpful reviews of the manuscript. This study was completed while IJB held an NW PostDoctoral ResearchFellowshipat the Universityof Cape Town.

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