Lithostratigraphy of the Chewore Ophiolite Geological outline

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 524 9 16 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 parallelsub-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 1393 9 22 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 oceanicmar- 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

The ophiolite and island-arc rocks are best exposed within the Maunde Region of the Chewore Inliers. The Ophiolite Terrane has been subdivided by Johnson 1999 into the Maunde Fig. 3. Sketch geological map illustrating the sub-division of the Ophiolite Terrane after Johnson 1999. Also shown are meta-basalt samples localities and their interpreted tectonic origin. Box illustrates the position of Fig. 4. Fig. 4. Detailed geological map of the Maunde River section illustrating the tectono-stratigraphic layout of the Maunde Ophiolite Group. A – B represents the section through which the lithostratigraphic section Fig. 5 is drawn. 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 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, andor 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. This lithology occurs as upto 50 cm wide, par- allel sided, fine to medium grained, amygdaloidal and non-amygdaloidal sheets. The upper portion of the section is highly strained and is dominated by amygdaloidal sheets which contain between 30 and 60 highly-deformed, elongate, plagioclase-, quartz- and epidote-filled lensoid amygdales. The margins of the sheets are difficult to identify due to their highly strained nature and lack of distin- guishing features between differing sheets; how- ever, the lower portions of this formation are less strained and primary features such as dyke boundaries and texturally preserved chilled mar- Fig. 5. Lithostratigraphic section of the Maunde River section illustrating the tectono-stratigraphic layout of the Maunde Ophiolite Group. The location of the section is illustrated in Fig. 4. T = tectonic contact Thrust??. 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-cuttingsub-parallel ‘feeder dykes’ indi- cates that this is not a sill complex. Fig. 6. Field photographs of various units from the Maunde Ophiolite Group. a A well preserved pillow lava from the Mvuu Meta-Volcanic Formation MR[9169 3770]. Notice that the pillow is selvaged by a 5 cm thick, fine grained band interpreted as a texturally-preserved chilled margin. The hand lens is 6 cm in length and the view is toward the south. b Planar bounded, non-amygdaloidal sheeted dykes of the Mbizi Sheeted Dyke Formation MR[9152 3778]. The hammer is 50 cm in length and the view is toward the southeast. c Close-up of a dyke boundary from the Mbizi Sheeted Dyke Formation MR[9152 3778]. The tectonic foliation is sub-parallel to the dyke margins. The hammer is 50 cm in length and the view is toward the northeast. d Close-up view of the contact between a medium and fine grained unit of the Mbizi Sheeted Dyke Formation MR[9152 3778]. The finer grained unit left of picture fines over a distance of 10 mm, toward the medium grained unit, the contact of which is marked by a leucocratic vein. This finer grained margin is interpreted to be a texturally-preserved chilled margin. The pencil is 7 cm long and the view is toward the north. 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 + CO 2 + H 2 O = 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. The massive meta-gabbro is a coarse grained 2 – 5 mm, granular, gabbroic textured unit Fig. 7a, comprising an amphibolite facies mineralogy of hornblende porphyroblasts set within an oligo- clase matrix. The layered meta-gabbros are simi- larly textured but differ due to a gross, centimetre scale, parallel banding formed from a variable hornblende porphyroblast content Fig. 7b and which is interpreted to represent primary igneous layering. These layered units are 10 – 20 mm thick and are laterally extensive along the length of the outcrop, i.e. 2 m. They contain upto 30 porphy- roblast rich bands, with each porphyroblast-rich layer containing upto 90 hornblende. The con- tact between the layered- and massive-textured meta-gabbros is gradational over a distance of 5 – 10 mm. Layered units can contain upto 5 por- phyroblast-rich layersmetre. 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, upto 30 cm thick, occasionally boudinaged pla- giogranite sheets, both of which are sub-parallel to the strong tectonic foliation Fig. 7c. The transition between the coarser grained sheets, with textures similar to those of the massive meta-gab- bro, and the finer grained, highly strained meta- gabbro is gradational over a distance of less than 5 mm. Since the fine grained unit does not display coarse grained, gabbroic-type textures, this unit might be interpreted to represent lithologies of the Mvuu Meta-Volcanic: or Mbizi Sheeted Dyke Formation Oliver et al., 1998; however, geo- chemical analyses indicates that this unit has an 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 basalticgabbroic 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 Fig. 8. MORB-normalised spider plot after Pearce, 1983 illustrating the similarity in high field strength elements HFSE trace element concentrations between the highly- strained, massive, layered meta-gabbros of the Twiza Meta- Gabbro Formation and the meta-mafic cumulates of the Ingwe Meta-Mafic Cumulate Formation for analyses, see Table 1. 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