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Precambrian Research 103 (2000) 125 – 146

Mesoproterozoic oceanic subduction, island-arc formation

and the initiation of back-arc spreading in the Kibaran Belt

of central, southern Africa: evidence from the Ophiolite

Terrane, Chewore Inliers, northern Zimbabwe

S.P. Johnson *, G.J.H Oliver

Crustal Geodynamics Group,School of Geography and Geoscience,Uni6ersity of St Andrews,Fife,Scotland KY16 9ST,UK

Received 15 February 2000; accepted 20 March 2000

Abstract

The Ophiolite Terrane of the Southern Chewore Inliers is comprised of two related, but lithologically- and geochemically-distinct groups. The Maunde Ophiolite Group comprises a suite of lithologies similar to those within Phanerozoic ophiolites. The Kaourera Island Arc Group contains a suite of silica-variable, extrusive lithologies similar to those within present-day island-arcs. Geochemical analyses (HFSE and REE) of meta-basalts from the Maunde Ophiolite Group coupled with the ophiolite-type lithostratigraphy, indicate that this group represents an ophiolite which formed as part of an immature, back-arc marginal basin (the Chewore Ophiolite). Meta-basalts of the Kaourera Island Arc Group predominantly display tholeiitic, island-arc geochemical signatures, indicating that this group represents an associated island-arc (the Kaourera Arc); however, some meta-basalts display oceanic within-plate geochemistries. These oceanic within-within-plate meta-basalts are interleaved with both arc-lavas and marginal basin-lavas and are interpreted to result either from the interleaving of a within-plate seamount with the arc/marginal basin during accretion/Pan African tectonism or from the contamination of the arc-/marginal basin-lavas by the subduction and dehydration of a within-plate seamount under the arc/marginal basin. The Chewore Ophiolite has been dated at 1393922 Ma and is at present the oldest dated ophiolite (senso stricto) in Africa. The Mesoproterozoic age of this marginal basin and island-arc complex make this the first description of recognised oceanic-type crust of this age within the Kibaran orogenic system of central, southern Africa. The location of the Ophiolite Terrane (on the northern margin of the Zimbabwe Craton and between that of the Congo Craton), indicates that Kibaran-aged crust extends further south than had previously been indicated by other authors and that a regional scenario at this time must involve ocean – ocean collision, oceanic subduction, the formation of an island-arc and the initiation of juvenile back-arc basin spreading. At 1393 Ma the two cratons must have therefore been separated by an ocean (the Chewore Ocean) of unknown extent. © 2000 Elsevier Science B.V. All rights reserved.

Keywords:Chewore Inliers; Island-Arc; Kibaran Belt; Mesoproterozic; Ophiolite; Zambezi Belt

www.elsevier.com/locate/precamres

* Present Address: Department of Geology and Geophysics, Tectonics Special Research Centre, The University of Western Australia, Nedlands, WA 6907, Australia. Tel.:+61-8-9380-7849; fax:+61-8-9380-7848.

E-mail address:[email protected] (S.P. Johnson).

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1. Introduction

The Neoproterozoic Pan African Belt of central southern Africa is an east – west trending transcon-tinental orogenic belt located within central, southern Africa and is interpreted to transect the cratonic assemblage of West Gondwana (Shackle-ton, 1996). The belt is comprised of several oro-genic systems which, from west to east include the Damara Orogenic Belt of western, southern Africa; the Lufilian Arc of central, southern Africa; the Zambezi Belt of southern Zambia and northern Zimbabwe and the north – south trending Mozambique Belt of eastern Africa (Fig. 1). Like other Proterozoic orogenic belts, the Pan African Belt and its many orogenic systems are comprised of a mosaic of structurally and metamorphically contrasting terranes most of which invariably en-train tracts of older, re-tectonised basement.

Mapping by Goscombe et al. (1994), Goscombe et al., (1998), Oliver et al. (1998) and Johnson (1999) in the Zambezi Belt of northern Zimbabwe has revealed such a region of re-mo-bilised basement (Fig. 2). The Chewore Inliers are a group of isolated horsts, comprised of four lithologically contrasting, tectonically juxtaposed terranes, i.e. Granulite, Quartzite, Zambezi and Ophiolite Terranes. The most southerly terrane of this basement tract, the Ophiolite Terrane, is interpreted by Oliver et al. (1998) and Johnson (1999) to represent a relict, highly dismembered oceanic/marginal basin (the Chewore Ophiolite) and island-arc complex (the Kaourera Arc). The discovery of the ophiolite-type lithologies were documented in brief by Oliver et al. (1998) and are subsequently described in detail here and by Johnson (1999).

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S.P.Johnson,G.J.H.Oli6er/Precambrian Research103 (2000) 125 – 146 127

Fig. 2. Simplified geological map of the Chewore Inliers after Oliver et al. (1998). Box illustrates the position of Fig. 3.

Ophiolite Group that comprises lithologies associ-ated with the Chewore Ophiolite and the Kaour-era Island Arc Group that comprises lithologies associated with the Kaourera Arc (Fig. 3).

Rocks in both the Maunde and Kaourera groups have undergone lower to upper amphibo-lite facies re-crystallisation, static annealing and intense deformation during the Pan African Orogeny (Goscombe et al., 1998), the peak of which is dated in the Chewore Inliers at 524916 Ma from high precision, SHRIMP zircon ages (Goscombe et al., 1998). This tectono-metamor-phic event has altered the primary igneous miner-alogy and has obliterated most igneous textures. Meta-basaltic lithologies have been altered to epi-dote-bearing amphibolites; ultramafics to tremo-lite- and talc-bearing serpentinites and the intermediate island-arc lithologies to biotite- and hornblende-bearing quartzofeldspathic schists. All lithologies accommodate a tectonic fabric, both foliation and lineation, of which the foliation is parallel/sub-parallel to the boundaries of the dif-fering lithologies. The foliation and tectonic boundaries dip moderately (30 – 50°) toward the southeast while the lineation plunges shal-lowly (10 – 20°) toward the south (Fig. 3). These fabrics share a similar orientation to Pan African aged fabrics within northern margin of the Zimbabwe Craton (Barton et al., 1993) and are interpreted by Goscombe et al. (1994, 1998) to be related to the same Pan African tectonother-mal cycle. This deformation episode has dismem-bered both the ophiolite and island-arc stratigraphy such that both lithological and map-ping group contacts are tectonic (Oliver et al., 1998; Johnson, 1999).

3. Lithostratigraphy of the Chewore Ophiolite

The Chewore Ophiolite is best exposed in an 800 m long, east to west striking structural section along the Maunde River from MR [ST 9105 3772] to MR [ST 9174 3762] (Anon, 1989) (Figs. 4 and 5). This section is of the lowest metamorphic grade (lower amphibolite facies) and deforma-tional strain thus allowing the preservation of some primary structures such as pillow lavas, Oliver et al. (1998) have obtained a SHRIMP

zircon age for the ophiolite at 1393922 Ma, making this the oldest dated ophiolite (senso stricto) in Africa. The Mesoproterozoic age of the Chewore Ophiolite and the Kaourera Arc are consistent with it being a remnant of oceanic/ mar-ginal basin crust and island-arc associated with the Kibaran Orogenic system of central, southern Africa. This is the first description of recognised oceanic-type crust of this age, within this orogenic system.

This paper outlines the various lithostrati-graphic units of the Maunde Ophiolite Group in order to formalise its stratigraphy as that of an ophiolite and to present the major and trace element geochemistry of both the ophiolite and island-arc-type lithologies in order to determine the tectonic environment of formation.

2. Geological outline

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relict chilled margins and sedimentary lamina-tions.

Since most ophiolites contain a well defined igneous stratigraphy as proposed by the Geologi-cal Society of America, Penrose conference on ophiolites (Anon, 1972) (i.e. mafic pillow lavas and extrusive mafic volcanics; sheeted dolerite dykes; massive and layered gabbros with pla-giogranite screens; mafic cumulates and serpen-tinised mantle peridotites), division of the Maunde Ophiolite Group into similar, distinctive, stratigraphic units (i.e. formations) is proposed here. Subdivision is in accordance with the recom-mendations proposed by Whittaker et al. (1994) and has been formalised by Johnson (1999). Fig. 5 is a tectono-stratigraphic column through the Maunde River section of the Maunde Ophiolite Group illustrating the sub-division into the dis-tinctive lithological formations. Since all the for-mation contacts are tectonic, the thickness of each stratigraphic unit is therefore unrepresentative of the original pre-tectonic stratigraphy.

3.1. Nzou Meta-Greywacke Formation

This formation outcrops in the east of the section at MR [ST 9173 3762] and is interpreted to be 25 m in structural thickness. Since the lithostratigraphic units dip toward the southeast, this formation is structurally at the top of the Maunde River section (Figs. 4 and 5). The forma-tion is predominated by meta-greywackes with subordinate pelites. The meta-greywakes display millimeter scale laminations due to a variation in biotite content from 5 up to 10% and are inter-preted to represent relict sedimentary laminations. The pelites occur as semi-continuous (up to 1.5 m long, 20 cm thick), isolated lenses within the predominant meta-greywacke. Contacts between the two are sharp and straight. The lack of alu-minium silicate metamorphic minerals indicates the lower amphibolite facies nature of metamorphism.

3.2. M6uu Meta-Mafic Volcanic Formation

This formation is comprised of fine grained epidote- and sphene-bearing amphibolites which

outcrop 30 m to the northwest of the Nzou meta-greywacke formation at MR [ST 9169 3770], and lie structurally below the meta-greywakes in the section (Figs. 4 and 5). The contact between the two formations is obscured by lack in outcrop. The formation is interpreted to be 100 m in structural thickness.

This formation is characterised by massive, ho-mogeneous meta-basalts which comprise 95% of the outcrop and meta-basaltic pillow lavas which comprise the remaining 5%. The pillow lavas are variably-deformed from recognisable pillow shapes to highly-deformed lenses. Pillows occur either as isolated units or in trains as at MR [ST 9169 3770]. At this locality, each pillow is roughly 50 cm in diameter, contains abundant epidote-and calcite-filled amygdales epidote-and are selvaged by a 1 – 5 cm thick darkly coloured, fine grained am-phibolitic band (Fig. 6a), which are tentatively interpreted to represent relict chilled margins. The homogeneous meta-basalts might represent mas-sive, and/or sheet flows, the margins of which have been obscured by (Pan African) deformation.

3.3. Mbizi Sheeted Dyke Formation

This formation is characterised by planar bounded, occasionally amygdaloidal, fine grained amphibolites which have a maximum structural thickness of 250 m. The first occurrence of this unit within the section is at MR [ST 9158 3779] which is 30 m to the northwest of the overlying Mvuu Meta-Volcanic Formation. The contact be-tween the formations is unexposed.

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mar-S.P.Johnson,G.J.H.Oli6er/Precambrian Research103 (2000) 125 – 146 131

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gins are well preserved, allowing the identification of at least 20, metre scale, amygdale-free, parallel dykes (Fig. 6b).

The contact between all the dykes are sharp and straight (Fig. 6c) and at MR [9152 3778] the matrix of a 30 cm thick, fine grained meta-basalt dyke is observed to grade from 0.25 mm to less than 0.1 mm, over a distance of 10 mm, towards the contact with a medium grained meta-dolerite dyke (Fig. 6d). This structure is traceable along the length of the outcrop (3 m), and is inter-preted to be a texturally preserved chilled margin.

The orientation of the dykes are sub-parallel to the regional foliation and to the meta-basaltic sheet flows and pillow lavas of the Mvuu-Meta Volcanic Formation (Figs. 4 and 5). Within unde-formed ophiolite examples, the dyke complexes are orthogonal to the sheet flows and pillows. Due to the highly attenuated nature of the Chewore Ophiolite, it is interpreted that this unit has been re-oriented and rotated toward the regional shear plane during the high shear strain event. The lack of cross-cutting/sub-parallel ‘feeder dykes’ indi-cates that this is not a sill complex.

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3.4. Ngwena Ultramafic Formation

This formation is comprised of metamorphosed and highly altered and variably-serpentinised, ul-trabasic lithologies which crop out within the section between MR [9126 3763] and MR [9123 3767], have a maximum structural thickness of 25 m and occur structurally below the Mbizi Sheeted Dyke Formation (Figs. 4 and 5). The formation is highly attenuated due to deformation and all con-tacts are interpreted to be tectonic. Three distinc-tive meta-ultramafic lithologies are identified. The predominant lithology is tremolite-rich. This unit contains a fine grained tremolite- and serpentine-rich matrix with upto 2 mm diameter tremolite porphyroblasts that contain abundant, very fine grained chrome-spinel inclusions. These inclusion rich porphyroblasts might result from the break-down of chrome diopside in a reaction similar to Eq. (1):

chromium diopside+CO2+H2O

=tremolite+calcite+quartz+chrome-spinel (1)

Another variety of this lithology contains abun-dant chrome-spinel (upto 20%) which aggregate with the chromite inclusion-rich, tremolite porphy-roblasts into discontinuous layers (less than 5 mm thick) which contain upto 70% chrome-spinel. These aggregates are interpreted to represent once continuous cumulate layers of chrome-spinel and chrome-diopside within a predominantly diopside-rich matrix. The lack of metamorphic minerals such as talc suggest that orthopyroxene may not have been a significant component within these two lithologies. The third lithology, however, is a talc-rich serpentinite that contains abundant chrome-spinel grains suggesting the presence of both olivine and orthopyroxene; however, since these lithologies have undergone intense alteration and metasomatism during metamorphism it is difficult to interpret their original mineralogy.

3.5. Ingwe Meta-Mafic Cumulate Formation

This formation is characterised by porphyrob-lastic hornblendites which occur structurally below

the Ngwena Ultramafic Formation at MR [9120 3771]. The approximate structural thickness of the formation within the Maunde River section is 50 m (Figs. 4 and 5).

This lithology is comprised of upto 70%, 1 – 8 mm diameter, augen-shaped hornblende porphy-roblasts set within a fine grained (upto 1 mm long) hornblende matrix. The porphyroblasts seldom touch. At MR [9122 3769] this lithology is ob-served to sharply grade (over a distance of less than 2 mm) into a meta-gabbroic body of the Twiza Meta-Gabbro Formation (Fig. 7a). The hornblende porphyroblasts within this unit occur without loss of grain shape, size or distribution into the meta-gabbro, while the hornblende domi-nant matrix is gradually replaced by plagioclase. The contact is sharp and straight along the length of the outcrop, i.e. 2 m and is interpreted to be a cumulate rock (adcumulate) resulting from the initial gravity settling of an igneous gabbroic magma.

3.6. Twiza Meta-Gabbro Formation

This formation is characterised by massive, lay-ered and highly strained meta-gabbros with inti-mately related, variably-abundant, plagiogranite sheets and veins. The maximum structural thick-ness of this formation within the Maunde River section is 75 m (Figs. 4 and 5).

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Fig. 7. Field photographs of the various units from the Maunde Ophiolite Group. (a) Sharp contact between the meta-gabbro, sample number SJ 213 Hb, (Twiza Meta-Gabbro Formation, for analyses see Table 1), bottom of photograph, and the mafic-cumulate, sample number SJ 213 Ha (Ingwe Meta-Mafic Cumulate Formation, for analyses, see Table 1), top of photograph (MR[9122 3769]). The contact is sharp and straight. The compass dial is 4 cm in diameter and the view is toward the east. (b) Layered meta-gabbro of the Twiza Meta-Gabbro Formation (MR[9010 37051). This unit contains hornblende-rich (upto 90%) bands upto 5 mm thick. The compass is 10 cm in length and the view is toward the north. (c) Overview of the highly-strained meta-gabbro of the Twiza Meta-Gabbro Formation (MR[9114 3772]). The dark, fine grained amphibolitic matrix (highly-strained meta-gabbro) contains continuous, upto 15 cm thick, leocucratic bands which display textures similar to the massive meta-gabbros (right of picture) and discontinuous, boudinaged sheets of plagiogranite (left of picture). The field of view is 2 m and the view is toward the north. (d) A network of fine grained plagiogranite veins cutting through massive meta-gabbro of the Twiza Meta-Gabbro Formation (MR[8990 3687]). The field of view is 50 cm and the view is toward the northwest.

A highly strained meta-gabbro is restricted in outcrop to the Maunde River section (MR [9114 3772]) where it crops out at the base of the ophiolite between the Ingwe Meta-Mafic Cumu-late Formation and the basement granitic gneisses of the Zambezi Terrane (Figs. 4 and 5). The contact between both the granitic gneisses and meta-basic cumulates are unexposed, the width of this nonexposure being 75 m. The highly-strained meta-gabbro is fine grained, i.e. less than 0.5 mm, and contains 10 – 40 mm thick, coarser grained (upto 5 mm), leucocratic, massive-gabbroic tex-tured sheets and medium grained (0.5 – 1 mm),

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identical trace element geochemistry to that of the massive meta-gabbros (Fig. 8) and thus represents mylonitised metagabbros at the base of the sec-tion. This interpretation is important since a pla-giogranite sheet from this unit was dated by Oliver et al. (1998) and the results used to infer the age of the ophiolite. These elevated shear strains are localised at the base of the section and indicate the position of the ductile, Maunde Thrust which separates the Ophiolite Terrane from the basement gneisses of the Zambezi Ter-rane. These dilated strains are also reflected for some 10 m below the Maunde Thrust, within the Zambezi Terrane granitic gneisses which display intense grain size reduction and ultramyolinitisa-tion (Fig. 5).

Plagiogranite sheets and fine anastamosing net-works of plagiogranite occur throughout the Twiza Meta-Gabbro Formation; however, the majority occur within the highly strained meta-gabbro. These sheets occur as medium grained (0.5 – 1 mm), upto 30 cm thick, sugary textured, equigranular bodies which are sub-parallel to the dominant tectonic foliation and are frequently boudinaged (Fig. 7c). The contact between these

sheets and the surrounding meta-gabbros are sharp, straight and do not display any com-positional or textural grading. The plagiogranite veins form an anastamosing network of veins and veinlets which are observed within most of the Twiza Meta-Gabbro Formation (Fig. 7d), the boundaries between both are gradational. Gerlach et al. (1981) interpret such relationships as hy-drous, in-situ, partial melts of a basaltic/gabbroic source.

3.7. Discussion

The Maunde River section, albeit only 800 m in length and having undergone amphibolite facies tectono-metamorphism, contains a suite of varied lithologies which include mafic-pillow lavas, ex-trusive mafic-volcanics, sheeted dolerite dykes, massive and layered meta-gabbros with pla-giogranite screens, mafic-cumulates, serpentinised massive- and layered-ultramafics. The sequence in which these lithologies occur is similar to that within Phanerozoic ophiolites (Anon, 1972). These lithologies occur in two successions, sepa-rated from one-another by a significant (un-mamed) thrust: from east to west (Fig. 4), the upper-crustal sections of the ophiolite stratigra-phy are preserved in the correct way-up sequence, i.e. sediments resting upon extrusive volcanics and pillow lavas which in turn rest upon sheeted meta-dolerite dykes. The lower-portion of the ophiolite stratigraphy is preserved as an overturned se-quence with the ultramafics resting in tectonic (thrusted) contact with the sheeted meta-dolerite dykes. These ultramafics are interleaved with tec-tonised, but once layered ultramafics; these are in tectonic contact with layered meta-mafic cumu-lates which in turn rest upon layered and massive meta-gabbros with discontinuous zones and sheets of plagiogranite (Fig. 5). Such a predictable se-quence of lithologies suggests that all units of the lithological sequence are interrelated, i.e. the cu-mulates grade into the meta-gabbros indicating that they are the possible result of gravity settling of the gabbroic magma chamber. The lack of a metamorphic derivative of orthopyroxene (talc) and the very attenuated outcrop of the ultramafics suggests that these may not represent mantle

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lithologies. They may however represent ultra-mafic cumulates. Their position in the stratigra-phy below that of the mafic-cumulates suggests that this might be the case. Thus the lowercrustal portion of the Maunde Ophiolite Group repre-sents a stratified magma chamber, consisting of massive and layered ultramafics and mafic-cumu-lates at the base which grade into massive and layered meta-gabbro representing the main por-tion of the chamber.

The major difference between the Maunde Ophiolite Group and that of a typical Phanero-zoic ophiolite is the thickness of each lithostrati-graphical unit, with Phanerozoic examples containing considerably thicker units. According to Brown and Mussett (1981) the thinnest igneous unit within an ophiolite is the pillow lavas which are generally between 300 and 700 m thick. This is in fact as thick as the whole of the type-section of the Maunde Ophiolite Group. This attests to the fact that the Maunde Ophiolite Group has been extensively attenuated during tectono-meta-morphism.

4. Geochemistry of the Chewore Ophiolite and the Kaourera-Arc

4.1. Analytical procedures

After the removal of weathered surfaces the samples were crushed in a jaw crusher and pow-dered in a Tungsten Carbide ‘Tema mill’ until the grain size of the powder was less than 200 mm.

Major and trace elements were analysed using a Phillips(c) PW 1450/20 X-ray fluorescence spec-trometer with a side-window Rhodium tube at the University of St Andrews. Major elements were analysed on glass discs prepared by fusing the sample with Johnson-Matthey Spectroflux(c) 105 in a sample to flux ratio of 5.33:1. Trace elements were analysed using pressed powder pellets, bound with approximately ten drops of polyvinyl alcohol (Movial). Calibration of analyses were based on 30 (majors) and 30 (traces) international rock standards based on the procedure of Norrish and Chappell (1977). The rare-earth elements of 11 samples were analysed by ICP-AES [Phillips(c)

simultaneous sequential PV 8060 spectrometer] at Royal Holloway, University of London according to the procedure of Walsh et al. (1981).

4.2. Geochemical alteration

Since all the rocks in the Ophiolite Terrane have undergone amphibolite facies metamor-phism, the more mobile elements such as the alkali major elements (Na2O, CaO and K2O) and

the large lithophile elements (Sr, Rb, Ba, Th) are likely to have been altered and thus do not repre-sent the original concentrations prior to metamor-phism. It is generally considered (Humphris and Thompson, 1977; Brekke et al., 1984; Brouxel et al., 1989) that the high field strength elements (HFSE) Nb, Ce, Zr, Y, Sc, Cr, Ni and the rare earth elements (REE) are relatively immobile dur-ing hydrothermal alteration and metamorphism. These elements can be considered with some cau-tion to represent original, pre-metamorphic concentrations.

4.3. The Maunde Ophiolite Group (The Chewore

Ophiolite)

4.3.1. Meta-basalts

All of the meta-basaltic lithologies (both the Mvuu Meta-Volcanic Formation and the Mbizi-Sheeted Dykes Formation) have SiO2 contents

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S

Data table presenting representative analyses for Maunde Ophiolite and Kaourera Island Arc lithologiesa

Kaourera Group Maunde Ophiolite Group

Meta-basalts Talc- Massive/ High- Meta- Island-arc meta-basalts Oceanic within-plate meta-basalts Andesite Dacite Rhyolite

strain cumulate

48.60 47.48 47.55 41.10 46.34 45.24

SiO2 46.77 48.51 49.73 48.09 49.11 49.48 50.65 60.85 66.14 78.89 SiO2

0.78 0.74 0.80 0.22 0.51 0.72 0.70 0.70

TiO2 0.68 0.66 2.98 2.54 1.94 1.80 3.00 0.65 TiO2

14.66 17.00 17.93 2.69 16.68 13.48 11.80 16.75

Al2O3 15.20 16.54 12.52 12.63 13.08 12.60 15.92 8.94 Al2O3

11.33 10.93 11.77 14.29 6.68 14.03 9.84 11.13

Fe2O3 10.78 11.10 18.28 18.01 15.48 11.90 1.96 3.73 Fe2O3

0.22 0.24 0.16 0.13 0.13 0.23 0.23 0.19 0.21 0.18 0.25 0.25 0.22 0.22 0.05 0.05 MnO

MnO

10.50 8.47 7.72 30.57 10.53 11.11 15.17 8.55

MgO 8.64 8.20 4.97 4.37 5.28 1.93 0.23 1.01 MgO

CaO 9.35 8.65 9.59 2.09 17.60 12.41 12.86 10.87 9.70 11.05 9.30 9.16 9.84 3.48 3.05 2.16 CaO

Na2O 2.99 2.51 3.07 0.01 0.58 1.38 1.27 2.58 3.39 2.75 1.35 1.33 1.95 3.22 8.35 0.11 Na2O

0.32 2.66 0.61 0.00 0.20 0.64 0.34 0.43

K2O 0.23 0.27 0.46 0.54 0.54 2.22 0.24 2.44 K2O

P2O5 0.09 0.13 0.10 0.00 0.03 0.03 0.04 0.10 0.05 0.07 0.40 0.32 0.19 0.52 0.46 0.24 P2O5

2.00 1.90 1.30 8.60 1.10 1.10 1.40 0.90 2.10

LOI 1.80 1.20 1.90 0.70 1.30 1 2 LOI

100.84 100.71 100.60 99.70 100.38 100.37 100.42 100.71

Total 100.28 100.71 100.79 100.50 99.94 100.02 100.40 100.22 Total

1 1 1 0 2 0 1 1

270 211 151 1739 365 224 542 204

Ni 237 206 39 40 56 6 8 7 Ni

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Fig. 9. Major element variation plots for the Maunde Ophiolite and Kaourera Island Arc Groups. For representative analyses, see Table 1. Key to abbreviations: A, andesite; B, basalt; BA, basaltic andesite; BTA, basaltic trachyandesite; D, dacite; P, phonolite; PB, picro basalt; PT, phonotephrite; R, rhyolite; TA, trachyandesite; TB, trachy-basalt; TD, trachydacite; TP, tephriphonolite. (a) Total alkali vs. silica (TAS) diagram for meta-basalts within the Maunde Ophiolite Group (pillow lavas, massive sheet flows and sheeted dykes are undifferentiated). (b) TAS diagram for all lithologies within the Kaourera Island Arc Group. (c) K2O vs. SiO2plot for the Kaourera Island Arc Group samples. Notice that there is a poorly defined calc-alkaline series. Since K2O is readily mobile during metamorphism, this poorly defined calc-alkaline series is probably an artifact resulting from the alteration of once, low-K lithologies.

that of an ophiolite (i.e. oceanic-type crust), indi-cate that they formed within an extensional (spreading) tectonic regime, i.e. a back-arc mar-ginal basin and not that of an arc/fore-arc. This is not altogether surprising since the majority of Phanerozoic ophiolitic fragments are interpreted to be relict marginal basins (Saunders et al., 1979; Wilson, 1989; Taylor et al., 1992; Smith, 1993; Windley, 1994). Within Fig. 10a and d the fields for basalts from modern-day marginal basins (data includes analyses from the Bransfield Strait, Guyamas Basin, Mariana Basin and Scotia Sea) (Brekke et al., 1984; Gribble et al., 1998; Hawkins

et al., 1990) has been superimposed and a good correlation is evident.

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back-arcs where spreading has yet to dominate over rifting (Gribble et al., 1998). As the mar-ginal basin matures from the rifting to spreading stage, adiabatic decompression melting of the mantle becomes predominant and the input from the subducting slab decreases. Trace element con-centrations of the spreading centre marginal basin basalts therefore become distinct from those produced within the associated arc. The Maunde Ophiolite Group meta-basalts might therefore represent a young, immature marginal basin.

4.3.2. Ultramafics, meta-gabbros and

meta-cumulates

All Ngwena Ultramafic Formation lithologies contain less than 44 wt.% SiO2(for representitive

analyses, see Table 1) and are thus classified as ultramafic (Le Maitre, 1989). The highly altered nature of this formation is illustrated in Fig. 11 which is a plot of the MgO wt.% versus MgO/

CaO wt.% after Meisel et al. (1997). The figure shows the field for all known, unaltered mantle xenoliths and massive peridotites which fall on a line intersecting the position of primitive upper

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Fig. 11. Whole rock MgO vs. MgO/CaO wt.% variation diagram after Meisel et al. (1997). The Ngwena Utramafic Formation ( ) plot away from that of unaltered mantle and primitive upper mantle (PUM) values indicating alteration due to metasomatism and serpentinisation. For representitive analyses, see Table 1.

which range from picro-basalt to rhyolite (Fig. 9b and Table 1). These rocks also are variable in total alkalis (Fig. 9c); however, due to amphibolite facies tectono-metamorphism and the mobile na-ture of the alkali elements it is likely that this does not reflect the original pre-metamorphic concen-trations. It is impossible therefore to determine if this rock suite displays both a low-K and calc-al-kaline trend as tentatively indicated in Fig. 9c. The trace element composition of the meta-basalts re-veal the presence of two distinct geochemical sig-natures (Fig. 10b and c). The first has HFSE-concentrations lower than that of N-MORB (depleted) and display a negative Nb anomaly compared with Ce, while the second have HFSE-concentrations greater than that of N-MORB (enriched) and no significant Nb anomaly. The HFSE depleted meta-basalts are spatially associated only with the with the silica-variable suite of lithologies (Fig. 3). When normalised against chondritic values of Nakamura (1974), the REE display a relatively flat pattern (Fig. 10e) with La/Yb ratios of between 0.7 and 1.5 (Table 1) indicating slight, HREE enrichment. Such geo-chemical signatures are characteristic of basalts produced within destructive plate margin settings (Saunders and Tarney, 1979; Basaltic Volcanism Study Project, 1981; Pearce, 1982; White and Patchett, 1984; Wilson, 1989). Again such basalts may have formed within a variety of destructive plate margin settings, i.e. within the arc, fore-arc or marginal basin. The association of these meta-basalts with the silica-variable, volcanic suite sug-gests that they may have formed within the main portion of an island-arc. In Fig. 10b and e, the field for modern-day, ensimatic island-arc, low-K, tholeiitic basalts has been superimposed (data in-cludes analyses from the Mariana Arc, Northern New Hebrides and the Paulau-Kyushu Range) (Barsdell et al., 1982; Brekke et al., 1984; Gribble et al., 1998), and a close match is evident.

The lack of plutonic material within the Kaour-era Island Arc Group indicates that during either arc accretion, or later deformation and metamor-phism, only the upper crustal section of the Kaourera Arc have been preserved/exposed.

The HFSE-enriched meta-basalts are located toward the southern margin of the Kaourera Is-mantle (PUM), (McDonough and Sun, 1995). The

Ngwena Ultramafic Formation lithologies plot outside this field towards lower MgO and MgO/

CaO wt.% values indicating that all lithologies have undergone metasomatic alteration and re-moval of major elements due to serpentinisation. Both the meta-gabbroic (Twiza Meta-Gabbro Formation) and meta-cumulate (Ingwe Meta-Mafic Cumulate Formation) rocks have SiO2

con-tents less than 51% (Table 1) and range from picro-basalt to basalt. When normalised against N-MORB, both groups display similar HFSE con-centrations (Fig. 8) indicating that the two share a similar parental source. This enforces the field interpretation that the cumulate formation is derived by the gravity settling of the original meta-gabbroic magma.

4.4. The Kaourera Island-Arc Group (Kaourera

Arc)

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land-Arc Group where meta-basalt lithologies predominate over the more silica-rich lithologies, and in the west of the region where they are tectonically interleaved with Maunde Ophiolite Group, marginal basin meta-basalts (Fig. 3). When normalised against chondritic values of Nakamura (1974), the REE display LREE en-richment with La/Yb ratios of between 3.5 and 6.9 (Table 1), similar to those of oceanic within-plate basalts. In Fig. 10c and f the field of mod-ern-day oceanic withinplate and ridge-centred hot spot basalts has been superimposed (data in-cludes analyses from Hawaii, Kerguelen Plateau and Iceland) (Basaltic Volcanism Study Project, 1981; Schilling et al., 1983; West et al., 1992; Mahoney et al., 1995), and a good correlation is evident. Oceanic within-plate volcanic suites asso-ciated with mid-plate hot spots such as Hawaii or ridge-centred hot spots such as Ascension Island and Iceland are characterised by alkalic fraction-ation trends and the production of trachybasalts

(Wilson, 1989). The lack of such lithologies within the Kaourera Island Arc Group, especially those associated with the oceanic within-plate meta-basalts, suggests that these basalts were not formed within these environments. Also superim-posed on Fig. 10f is the field for the seamount/

large igneous province of Kerguelen, which also displays oceanic within-plate geochemical trace element signatures and which is dominated early in its history by simple, tholeiitic volcanics (Ma-honey et al., 1995; Kerr et al., 1996). A good correlation is evident. Fig. 12 is an Y/Nb versus Zr/Nb discrimination plot, which distinguishes between P-type, T-type and N-type MORB. The fields for oceanic mid-plate (Hawaii), ridge-cen-tred (Ascension) (Wilson, 1989) and seamount/

large igneous provinces (Kerguelen) (Mahoney et al., 1995) basalts are plotted. Both the Kaourera within-plate basalts and Kerguelen basalts plot as T-MORB while the mid-plate (Hawaii) and ridge-centred (Ascension) basalts plot clearly as P-MORB. The Kaourera Island Arc HFSE-depleted meta-basalts plot as N-MORB. The sim-ilarity in trace element geochemistry of the Kaourera oceanic within-plate basalts with those of Kerguelen and the similar HFSE and REE trace element concentrations suggest that these meta-basalts might represent the remnants of a seamount-type, within-plate feature. The spatial volume of these meta-basalts compared to the Ophiolite Terrane is relatively small and thus not what one would expect from an accreted mid-plate seamount; however, this again might be due to attenuation and interleaving during tectono-metamorphism.

It is possible that the geochemistry of these oceanic within-plate-type meta-basalts may be the result of contamination of the Kaourera arc-type meta-basalts from the subduction and dehydra-tion of an oceanic seamount/Hawaiian-type plateau under the are. Turner and Hawksworth (1998) have illustrated that marginal basin and arc-type basalts within the northern sector of the Lau Back Arc Basin display oceanic within-plate characteristics. This is interpreted to be the result of contamination of these lavas, from the interac-tion of mantle plume material and the subducinterac-tion of oceanic seamounts below the marginal basin

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and island-arc. Such basalts are spatially re-stricted to the northern sector of the arc where these three components interact. If the Kaourera oceanic within-plate-type metabasalts result from a similar process, this might account for the re-stricted volume and sporadic/interleaved nature of these meta-basalts.

5. Discussion

The lithological variation within the Maunde Ophiolite Group can be equated with similar lithologies taken to represent an ophiolite, i.e. oceanic-type crust. The geochemistry of the metabasalts indicates that formation of this crust was within a back arc, marginal basin-type envi-ronment, while the strong arc-type, geochemical signature suggests that the marginal basin was relatively immature, i.e. was characterised by ju-venile spreading and rifting. The associated is-land-arc (the Kaourera Island Arc Group) comprises a range of silica-variable extrusive vol-canics and contains both arc-type and oceanic within-plate-type meta-basalts. The presence of all three (ophiolite, island-arc and within-plate) suites leads to a regional scenario involving ocean – ocean collision, oceanic subduction, the formation of an island-arc and the initiation of juvenile back-arc basin spreading (Fig. 13). The location of the Ophiolite Terrane between the Zimbabwe and Congo cratons indicates that these two cratons were at some point in time separated by an ocean of unknown size (but un-doubtedly large enough to develop within-plate seamounts and induce ocean – ocean collision and subduction). The age of final collision and amal-gamation of the Zimbabwe (Kalahari) and Congo Cratons is unknown and is the subject of much debate (Dalziel, 1992; Hanson et al., 1994; Rogers 1996; Unrug, 1997; Weil et al., 1998; Johnson, 1999, 2000; Kampunzu et al., 1999; Vinyu et al., 1999). Thus, the presence of an island-arc/marginal basin complex between these two cratons has direct implications for the re-gional palaeogeographical interpretation at this time and the constraint in the timing of the colli-sion between these two cratons.

Oliver et al. (1998) has obtained a SHRIMP age for magmatic zircons from a plagiogranite dyke within the Maunde Ophiolite Group of 1393922 Ma, i.e. Kibaran, and have interpreted this to be the age of the ophiolite. Although the Kaourera Arc is part of the same tectonic frame-work it is not necessarily of the same age, since when one considers the longevity of the present-day, Circum-Pacific region, the age of an ob-ducted/subducted ophiolitic fragment need not be the same age as the arc.

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Fig. 13. Cartoon style cross sections illustrating the interpreted tectonic relationship between the Ophiolite Terrane groups at ca. 1393922 Ma (top) and the present day, post Pan African arrangement (bottom).

6. Conclusions

The Ophiolite Terrane which is situated on the southerly margin of the Chewore Inliers is com-prised of two lithostratigraphical-/ geochemically-related groups. The Maunde Ophiolite Group

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indicate that the ophiolite is of marginal basin origin and that the strong island-arc geochemical signature indicates that the basin was relatively immature, i.e. dominated by both spreading and rifting. Meta-basalts from the Kaourera Arc dis-play both island-arc and oceanic within-plate geo-chemical signatures. The within-plate meta-basalts are interpreted to represent either, a tectonically interleaved oceanic-seamount, or arc-basalts which have been contaminated from the subduc-tion of a seamount under the arc.

The Mesoproterozoic (Kibaran) age of the Chewore Ophiolite indicates that:

1. the regional tectonics of this part of the belt involved ocean-ocean collision, subduction of oceanic crust, the formation of an island-arc and the initiation of back-arc basin spreading, 2. Kibaran-aged crust extends to the present-day

northern margin of the Zimbabwe Craton, 3. there was possibly significant crustal growth

by arc-accretion,

4. and that the Zimbabwe and Congo cratons were separated by an ocean of unknown ex-tent at this time. The maximum age for the final collision/accretion of the Zimbabwe and Congo (Kalahari) cratons must be 1393 Ma.

Acknowledgements

The research was funded by a scholarship to St Andrews University and grants from the Irvine and Welch bequests to St Andrews. Special ac-knowledgements go to the Geology Department of the Royal Holloway College London, ICP-AES facility who conducted the analyses of the Rare Earth Elements. A. Calder of St Andrews Univer-sity carried out the major and trace element XRF analyses. Many thanks go to G.J.H. Oliver for supervising the research, Professor Boudier and Professor Marsh for greatly improving the manuscript. Special acknowledgements go the Re-search Council and National Parks of Zimbabwe for appropriate permits and to Harare University for logistical support during fieldwork. This is a contribution to IGCP 418.

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