the chirwa dome:granite emplacement during late archaean...

A. HOFMANN, O. JAGOUTZ. A. KRÖNER, P.H.G.M. DIRKS AND H. A. JELSMA

SOUTH AFRICAN JOURNAL OF GEOLOGY, 2002, VOLUME 105, PAGE 285-300

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IntroductionThe Zimbabwe craton is a granite-greenstone terrainwhich grades into a polymetamorphic high-grade gneissterrain along its northeastern margin. The granite-greenstone terrain formed during several episodes ofcrustal growth between ~3.6 and ~2.6 Ga ago.Stabilization of the central part of the Zimbabwe cratonwas achieved at ~2.6 Ga, but granitoid magmatism, high-grade metamorphism and intense deformation stillaffected the northern and eastern margins in contactwith late Archaean mobile belts (U-Pb zircon dates,Vinyu et al., 2001; Jelsma et al., 2002). The high-gradegneisses of the mobile belts were overprinted duringNeoproterozoic (~1000 to ~800 Ma) and earlyPhanerozoic (~500 Ma) tectono-metamorphic events,which involved terrane accretion and continent-continent collision (Pinna et al., 1993; Dirks and

Sithole, 1999). At the contact between the granite-greenstone terrain of the Zimbabwe craton and the lateArchaean high-grade gneiss terrain occurs a circulargranite intrusion, the Chirwa dome, which wasemplaced into a strongly deformed metasedimentarysequence that has been thrust westward ontogreenstones of the Zimbabwe craton. According toJohnson (1968), the Chirwa dome represents Archaeanbasement that was remobilized during Pan-Africanthrusting and metamorphism and emplaced as a mantledgneiss dome into younger sediments, presumed to be ofProterozoic age. In order to test this hypothesis, wepresent structural data combined with Pb/Pb zirconevaporation dates from the area around Chirwa dome to establish the age of the sediments and the temporalrelationship between granite emplacement anddeformation.

The Chirwa dome: granite emplacement during late Archaean thrusting along the northeastern margin

of the Zimbabwe craton

Axel HofmannSchool of Geosciences, University of the Witwatersrand, Private Bag 3, Wits 2050, South Africa

Department of Geology, University of Zimbabwe, P.O. Box MP 167, Harare, Zimbabwee-mail: [email protected]

Oliver JagoutzInstitut für Erdwissenschaften, ETH-Zürich, 8092 Zürich, Switzerland

e-mail: [email protected]

Alfred KrönerInstitut für Geowissenschaften, Universität Mainz, 55099 Mainz, Germany


Paul H.G.M. DirksSchool of Geosciences, University of the Witwatersrand, Private Bag 3, Wits 2050, South Africa


Hielke A. JelsmaCIGCES-Centre of Interactive Graphical Computing of Earth Systems

Department of Geological Sciences, University of Cape Town, Rondebosch, 7701, South Africae-mail: [email protected]

ABSTRACTThe Chirwa dome in northeast Zimbabwe is situated at the boundary between the Zimbabwe craton and Archaeangneisses of the Zambezi mobile belt. This circular granite intrusion has long been regarded as Archaean granite whichwas remobilized during Pan-African times and emplaced as a mantled gneiss dome into a Proterozoicmetasedimentary sequence. Field mapping, structural work and new zircon dates indicate that the Chirwa dome andsurrounding rocks underwent a very different history. The supracrustal sequence was deposited and moderatelydeformed between ~2613 and ~2601 Ma ago, as bracketed by a provenance date and an intrusive date, respectively.Shortly after deposition, the sequence was thrust westward and juxtaposed against a greenstone terrain of theZimbabwe craton. This active thrust stack was intruded by successive syntectonic granitoids at ~2601 and ~2593 Ma.The Chirwa dome granite, ~2570 Ma old, represents the last intrusive event before cessation of deformation. Our dataindicate that while the central Zimbabwe craton already behaved as a stable crustal block at ~2600 Ma, tectonism wasactive along its northeastern margin.

Regional geologyChirwa dome is situated along the contact between theZimbabwe craton and Neoproterozoic and reworkedArchaean gneisses, belonging to the Zambezi belt to thenorth and the Mozambique belt to the east (Figure 1). Abelt of migmatitic gneisses, the Migmatitic Gneiss Terrain(Figure 2; Barton et al., 1991), separates cratoniclithologies from Proterozoic gneisses and can beregarded as the Archaean part of the polyphase Zambeziand Mozambique belts.

The Zimbabwe craton consists of granitoid-gneissdomes surrounded by linear or arcuate outcrops ofvolcano-sedimentary sequences, ranging in age from~3.5 to ~2.6 Ga (Taylor et al., 1991; Wilson et al., 1995;Jelsma et al., 1996; Horstwood et al., 1999). A majorepisode of rock formation occurred between ~2.7 and~2.64 Ga and resulted in the deposition of thewidespread volcanic succession of the Upper BulawayanSupergroup and the metasedimentary ShamvaianSupergroup which overlies the Upper Bulawayan inseveral greenstone belts (Wilson et al., 1995). Theformation of the supracrustal sequence wasaccompanied by the intrusion of tonalites andgranodiorites, such as the ~2.7 Ga Sesombi and the~2.65 Ga Wedza Suites (Wilson et al., 1995). The lastmajor Archaean igneous event recorded in theZimbabwe craton was, first, the intrusion of crustallyderived granites of the Chilimanzi Suite (~2.6 Ga; Jelsmaet al., 1996) and, secondly, the intrusion of ultramafic tomafic magmas of the Great Dyke (~2575 Ma; Oberthüret al., 2002). Stabilization of the central part of theZimbabwe craton was thus achieved shortly after ~2.6Ga (Wilson et al., 1995; Jelsma et al., 1996).

Two models for the evolution of the Zimbabwecraton have been proposed. One model regards theZimbabwe craton as an example of vertically accretedcrust in which the greenstone volcanics and relatedsediments were laid down in rifts on older continentalbasement, so that the current greenstone beltstratigraphy reflects the stratigraphy as it was originallylaid down (Bickle et al., 1994; Wilson et al., 1995; Jelsmaet al., 1996). Compressional deformation is attributed tofar-field stresses related to the Limpopo orogeny (Treloarand Blenkinsop, 1995) or folding associated with granitediapirism (Jelsma et al., 1993). Dirks and Jelsma (1998)and Kusky (1998) proposed a different model for thetectonic evolution of the craton. They suggested that,prior to compressional deformation, ultramafic andmafic oceanic crust and plateaus coexisted with felsiccrustal fragments, such as volcanic arcs and mini-continents. Early compressional deformation involvedthe low-angle imbrication and lateral stacking of thecrustal segments.

The Migmatitic Gneiss Terrain (MGT) consists of acomplex suite of amphibolite-facies para- andorthogneiss units locally containing granulite remnants(Barton et al., 1991). Geochronological data indicate thatmid- to late Archaean crustal components weresubjected to granulite facies metamorphism during mid-Archaean times (~3.0 Ga) and reworked at amphibolitefacies metamorphism at ~2.6 Ga (Barton et al., 1991;Vinyu et al., 2001). The gneisses were stronglyrecrystallized during a Pan-African metamorphicoverprint at ~500 Ma (Barton et al., 1991; Dirks et al.,1998; Vinyu et al., 1999). The northeastern contact of theMGT is defined by a sharp lithological change between

SOUTH AFRICAN JOURNAL OF GEOLOGY

THE CHIRWA DOME: GRANITE EMPLACEMENT DURING LATE ARCHAEAN THRUSTING286

ZAMBEZI BELT

0 250km

N

Neoproterozoic Zambezi/Mozambique terrain

Mesoproterozoic Magondi terrain

Great Dyke

Archaean gneiss terrain

Central Zone of Limpopo Belt

Granitoid-gneiss complex

Archaean greenstone belt

3.5 Ga crustal segment

Phanerozoic cover sequence

Major tectonic boundary geophysical boundary of craton

ZIMBABWE CRATON

MOZAMBIQUE BELT

MAGONDI BELT

29o

16o

LIMPOPO BELT

Figure 2

Figure 1. Geological map of the Zimbabwe craton showing the main lithotectonic units.



287

migmatitic gneiss and structurally overlying supracrustalgneiss and associated Neoproterozoic granitoids (Bartonet al., 1991). Towards the craton, the migmatitic gneisssequence grades into the granitoid-greenstone terrain viaa 5 to 10km wide zone across which metamorphic gradeand the amount of strain decreases (Jelsma et al., 2002).

The Proterozoic part of the Zambezi belt representsan east-west trending crustal segment characterized bypenetrative ductile deformation and regionalamphibolite to granulite facies metamorphism. Thissegment mainly consists of Meso- to Neoproterozoicmetasediments and granites (Barton et al., 1991). Thenorth-south trending Mozambique Belt (Figure 1)represents a major continental collision zone betweeneast and west Gondwana and records a 500 Ma historyof accretion culminating in final collision around 550 Ma(Shackleton, 1986; Pinna et al., 1993; Stern, 1994).

Study areaThe study area is situated southwest of the triplejunction of the Zambezi and Mozambique belts(Johnson and Vail, 1965; Barton et al., 1985) at theboundary between the MGT and the Zimbabwe craton.It is located within the eastern extremity of the Makaha

greenstone belt (MGB). The study area comprises avariety of granitoid gneisses and metavolcanic andmetasedimentary rocks (Figure 3). The oldest rock unitrecognized comprises the southeast-trending volcano-sedimentary sequence of the MGB (Macgregor, 1935;Stocklmayer, 1980). Pre-existing rocks such as olderbasement gneisses have not been reported. Stocklmayer(1980) subdivided the rocks of the MGB into twostratigraphic units, the Lawleys Formation (felsic, maficand ultramafic lava flows and pyroclastic rocks) and theoverlying Mudzi Formation (monotonous sequence ofcommonly pillowed metabasalts) which are separatedby a strongly sheared ironstone horizon (Figure 3).

The MGB is intruded by various granitoid rocks suchas the Mtoko granodiorite in the north and the Chirwadome granite in the east (Stocklmayer, 1980). The Mtokogranodiorite (Figure 3) is part of a widespread, lateArchaean tonalite-trondhjemite-granodiorite suite ofnortheastern Zimbabwe which is commonly intrusiveint, 1980). Vail and Dodson (1969) reported whole-rockRb-Sr ages between ~2900 Ma and ~2206 Ma and a Rb-Sr biotite age of 473 ± 8 Ma for the Chirwa dome granite(Rb-Sr ages recalculated using l(87Rb)= 1.42*10-11a-1).The Chirwa dome was therefore regarded as an

Figure 2. Geological map of the northern margin of the Zimbabwe craton.

0 50km

Dindi greenstone belt

Mount Darwin greenstone belt

Bindura-Shamva greenstone belt

Great Dyke

MOZAMBIQUE

N

31˚30' 32˚00' 32˚30' 33˚00'31˚00'

17˚0

0'16

˚30'

17˚3

0'

Granite-greenstone terrain Zambezi mobile belt

Great Dyke

Granitoid-gneiss terrain

Shamvaian Supergroup metasediments

Bulawayan Supergroup metavolcanics

Major tectonic contact

Migmatitic gneissesof the Migmatitic Gneiss Terrain

Ortho- and paragneissesof Proterozoic age

Mesozoic/Cenozoicsediments

Zambezi rift valley

Zambezi mobile belt

Zambezi rift valley

Chirwa dome

Figure 3

Makahagreenstone belt

International border



Figure 3. Geological map of the eastern margin of the Makaha greenstone belt and adjoining granite-gneiss terrain (after Johnson, 1968;

Stocklmayer, 1980). Outcrop width of ironstone is exaggerated for clarity. Greenschist-amphibolite boundary after Stocklmayer (1980).

0 10km

Nyamvu Gneiss

Ruenya Gneiss

Metasediments/intrusives

Mica-schist, marble, quartzite

Undifferentiated granitoids/gneisses

Mudzi Formation

Xenolith-rich granodioritic gneissLawleys Formation

Granite

N

Granitoid intrusivesMassive/pillowed metabasalts

Sheared ironstone

Felsic to mafic metavolcanics

Makaha greenstone belt

Nyarugo granites

Mtoko granodiorite

Figure 4

Chirwa dome granite

greenschist facies amphibolite facies

62

52

35

4055

45

66

37

36

45 31

32

27

4030

59

30

25

2630

33

21

45

24

40

3624

1535

54

37

25

40

20

38

3235

19

30

30

5250

67

35

35

58

45

80

30

40

65

80

50

52

30

45

60

6040

50

60 50

68

45

70

65

57

40

2545

30 32

25

75

80

87

25

33

70

45

45

5060

44

70

75

70

65

85

60

50

60

74

85

80

50

76

89

62

65

85

68

80

78

46

45

42

70

Ruenya River

Paragneiss/intrusives (Nyamvu Gneiss)

High-grade gneiss (Ruenya Gneiss)

Talc schist

Xenolith-rich granodioritic gneiss

Intermediate/mafic metavolcanics

Granite

Metabasalt

N

S1L1

Sample locationIronstone17°20´

17°15´ 32°55´32°50´

0 2km

Hornblende schist/amphibolite

Zim 06

Zim J1

Zim 46, 47

Marble-schist suite

Metaconglomerate-schist suite

Figure 4. Geological map of the Chirwa dome area (after Johnson, 1968; Stocklmayer, 1980 and own mapping). Selected foliation (S1) and

lineation (L1) data are shown.



289

Archaean granite which was remobilized during the Pan-African Mozambique orogeny and emplaced as amantled gneiss dome (Johnson, 1968; Stocklmayer,1980).

Southeast of Chirwa dome occurs a complexsequence of ortho- and paragneiss units, the latterincluding metaconglomerate, quartzose metasediment,kyanite mica schist, migmatite and quartzo-feldspathicgneiss (Figure 3; Nyamvu Gneiss of Stocklmayer, 1980).To the east of Chirwa dome are similar but commonlymigmatitic ortho- and paragneiss units. This gneissic unithas been termed Ruenya Gneiss (Stocklmayer, 1980) andis the southeastern extension of gneisses of the MGT(Barton et al., 1991). Whereas Johnson (1968) correlated

the metasediments within the gneissic units with theShamvaian Group, Stocklmayer (1980) assumed insteadthat they represent remnants of a sequence ofProterozoic sediments that were deposited in a basinalong the edge of the Zimbabwe craton.

A narrow band of mica schist, calc-silicate rock and quartzite partly encircle Chirwa dome (Figure 3).Based on the observation of lithological similarity and a presumed unconformable basal contact, the metasediments have been correlated with theProterozoic supracrustal succession of the UmkondoGroup of eastern Zimbabwe and western Mozambique(Slater, 1965; Johnson, 1968; Chunnett, 1972;Stocklmayer, 1980).

Table 1. Description of lithologies in the study area.

Makaha greenstone belt Pillowed and massive metabasalts are most common. Pillows (0.2-1m in diameter) show well preserved chilled

margins and are variably stretched (Figure 5A). Massive metabasalt may represent lava flows or sills. Rare

intercalations of talc-chlorite schist may represent metamorphosed komatiitic lava flows, tuffs or ultramafic

intrusive rocks.

Hornblende schist envelops Chirwa dome and consists of hornblende, plagioclase and, locally, garnet. East of

Chirwa dome occurs a pinching-and-swelling, c. 20m thick band of talc schist.

North of Chirwa dome, amphibolite with rare remnants of pillow structures is tectonically intercalated with

strongly deformed tuff and agglomerate of intermediate composition. Banded and brecciated ferruginous

quartzite also occurs together with hornblende schist and biotite-rich gneiss. Contacts with surrounding rocks

are generally sheared.

Chirwa dome granite The dome centre comprises leucocratic, non-porphyritic granite, whereas the rim comprises porphyritic

granite, characterized by the presence of microcline phenocrysts up to 3cm in length. Pegmatites and aplites

occur throughout the dome but are most common in the centre. Xenoliths (up to 0.5m in diameter) are

lensoidal to irregular in shape and are aligned parallel to the foliation. They include biotite and biotite-

muscovite schist, minor granite and rare granitoid gneiss.

Metasedimentary Mica schist of the marble-schist suite occurs as different compositional types which are

sequence/ in gradational contact to each other. A common variety is a muscovite schist with garnet and staurolite

Nyamvu Gneiss porphyroblasts and, locally, with kyanite and/or sillimanite. Equally common are biotite-muscovite and biotite

schists, characteristically containing cm-sized lenses of vein quartz. Other varieties include quartzose

muscovite-biotite and biotite-hornblende schists. Continuous horizons of quartzite are locally intercalated with

mica schist. It consists of coarse-grained, recrystallized quartz, mica and minor garnet.

Calcareous rocks include banded marble, dolomitic marble and hornblende-rich calc-silicate. Banded marble

(Figure 5B) comprises mm to cm thick layers that vary in colour from light green to grey to white. Beside

calcite and dolomite occur variable amounts of garnet, epidote, diopside, minor microcline and plagioclase

and rare quartz. Dolomitic marble is a whitish, coarse-grained rock consisting mainly of calcite and dolomite

with minor quantities of quartz, tremolite and granular epidote. Dark green calc-silicate rocks with abundant

hornblende, diopside and granular epidote are intercalated with the marbles.

Metaconglomerate is clast-supported and sorting is moderate. The clasts include pebbles, cobbles and rare

boulders (up to 50cm in size) of granite, leucogranite, quartzite and biotite schist (Figures 5C and 5D). The

metaconglomerate is interleaved with kyanite-biotite gneiss, biotite-kyanite-garnet schist, quartz-biotite schist,

calc-silicate rock and amphibolite.

Xenolith-rich granodioritic This lithology is a medium grey, medium- to coarse-grained granodioritic rock with abundant biotite and

gneiss minor hornblende. The xenoliths are intermediate to mafic in composition and range from quartz-biotite

gneiss to amphibolite.

High-grade gneiss/ Paragneisses are the oldest observed rock suite and are tectonically intercalated with, or occur as enclaves

Ruenya Gneiss in, granitoid gneiss. They include metaconglomerate, quartz-biotite gneiss, hornblende gneiss, muscovite-

biotite schist and migmatized counterparts thereof (Figure 5G). The most common orthogneiss is a fine-

grained, granodioritic to trondhjemitic gneiss which is tectonically interleaved with, or intrusive into,

paragneiss (Figure 5G). The gneiss is intruded by the xenolith-rich granodioritic gneiss. A porphyritic granitic

gneiss locally occurs in the gneiss terrain and intrudes the xenolith-rich gneiss. Quartzo-feldspathic gneisses

are common and represent deformed granite bodies and pegmatite and aplite dykes.



Figure 5. Features of lithological units of the study area.

5A. Deformed pillow basalt of the MGB. Unambigious younging directions can rarely be deduced.

5B. Sheath folds in banded marble.

5C. Metaconglomerate consisting of clasts of granite, leucogranite, quartzite and biotite schist.

5D. Edge of high-strain zone in metaconglomerate.

5E. Mafic xenoliths in xenolith-rich granodioritic gneiss.

5F. Dyke of leucocratic granitic gneiss cutting the main fabric in the xenolith-rich granodioritic gneiss. Sample Zim 47 was taken from this

dyke.

5G. Migmatitic gneiss consisting of complexly folded intercalations of leucosome and biotite-restite layers intruded by granodioritic gneiss

(top).

5H. Schistose amphibolite, interpreted as a deformed mafic dyke, interleaved with xenolith-rich granodioritic gneiss. Hight of exposure

is c. 20m.

A

C D

FE

G H

B



291

LithologiesThe various lithologies encountered in the study areacan be grouped into five distinct lithotectonic units.These are from west to east (1) the volcano-sedimentaryrocks of the MGB, (2) the Chirwa dome granite, (3) ametasedimentary sequence, (4) a xenolith-richgranodioritic gneiss and (5) high-grade gneisses andmigmatites of the MGT (Figures 3 and 4). A detaileddescription of the lithologies of the study area ispresented in Table 1.

Makaha greenstone belt West of Chirwa dome is a monotonous sequence ofmassive and pillowed metabasalt flows (Figure 5A) thatare part of the Mudzi Formation of the MGB(Stocklmayer, 1980). Hornblende schist forms a broadenvelope around Chirwa dome and is in gradationalcontact to metabasalt towards the west, suggesting thatthe schist represents the strongly deformed equivalent ofmetabasalt. The contact of the hornblende schist withthe granite and the metasedimentary sequence is sharp.North of Chirwa dome there is a small, irregular unit ofcomplexly deformed mafic to intermediate volcanics thathas been correlated with the Lawleys Formation(Stocklmayer, 1980).

Chirwa dome graniteThe Chirwa dome granite is an ovoid body of four byfive km in plan view (Figure 4). The long axis of the

dome trends north-south and the short axis, east-west.The granite is in sharp contact with hornblende schist asdescribed above. The centre of the dome comprisesnon-porphyritic granite and common pegmatite dykes. The central granite is surrounded by porphyriticgranite and both granite types are variably foliated.Towards the contact with the surrounding schists the porphyritic granite grades into augen-gneiss. Schist and granite enclaves, interpreted as xenoliths, arerare.

Metasedimentary sequence/Nyamvu GneissA thin band of metasedimentary rocks surrounds Chirwadome on its eastern side. This band opens into twotriangular-shaped outcrops northwest and southwest ofthe dome, respectively (Figure 4). The sequence is madeup of aluminosilicate-rich mica schist with less abundantmarble (Figure 5B) and quartzite (marble-schist suite).The sequence is in tectonic contact with the hornblendeschist which separates them from the granite dome. Tothe east and south of the central schist band, the marble-schist suite grades lithologically into a suite of clast-supported, granite pebble metaconglomerate (Figures5C and 5D) and intercalated biotite schist(metaconglomerate-schist suite, Figure 4). Further to thesoutheast occur abundant sheet-like units of granitoidrock and quartzo-feldspathic gneiss thinly interleavedwith migmatized metasediment. Remnants of partlyassimilated metasediment occur as rafts in the quartzo-feldspathic gneiss. This gneiss suite is strongly shearedand isoclinally folded, locally giving rise to a bandedleucocratic rock. These rocks have previously beenmapped as a unit that is distinct from the othermetasediments (Nyamvu Gneiss of Stocklmayer, 1980).However, the gradational contact to the marble-schistsuite suggests that all metasedimentary lithologiesconstitute one sedimentary stack that has been morestrongly deformed and intruded by granitoid sheets tothe east.

Massive to weakly foliated amphibolite formslenticular bodies and continuous layers intercalated withthe metasedimentary sequence. Locally, it truncates thelayering of the host rocks at low angles, suggesting anintrusive origin (Johnson, 1968).

Xenolith-rich granodioritic gneissThe xenolith-rich granodioritic gneiss is an intrusivebody which occurs along the contact between theNyamvu and Ruenya Gneiss (Figures 3 and 4). It alsooccurs as irregular sheets in the Ruenya Gneiss (seebelow). The gneiss typically contains mafic enclaves,interpreted as xenoliths (Figure 5E), that are aligned and elongated parallel to the foliation. Various dykesand pods of granitoid rock are intrusive into thexenolith-rich granodioritic gneiss. These include agranodioritic augen-gneiss with feldspar porphyroclastsand a light grey homogeneous granitic gneiss (Figure5F). The intrusive rocks generally cross-cut the foliationof the xenolith-rich gneiss but were subsequently

Figure 6. Photomicrographs of structural features in the

metasedimentary sequence. Width of photomicrographs is 1.7 mm.

6A. Snowball garnet in garnet-muscovite schist (plane-

polarized light).

6B. Small-scale fold in muscovite schist (crossed-polarized light).

B

A



deformed together with the host rock as indicated bytheir gneissic fabric. An apophysis of xenolith-richgranodioritic gneiss occurs between the paragneiss andmetasediment units northwest of Chirwa dome (Figure 4).

High-grade gneiss/Ruenya GneissThe eastern part of the study area comprises sheet-likebodies of various granitoid gneiss units, includingxenolith-rich granodioritic gneiss, with enclaves and/ortectonic intercalations of various migmatitic ortho- andparagneisses. Contacts between paragneiss and granitoidgneiss are intrusive (Figure 5G). The boundary betweenthis gneiss suite, termed Ruenya Gneiss, and thexenolith-rich gneiss to the west is a several metres widezone of high-strain. Homogeneous, garnet-bearingamphibolite occurs as 10-20m wide horizons, whichstrike parallel to the gneissic fabric of adjacent rocks(Figure 5H). Local cross-cutting of the gneissic fabricdemonstrates an intrusive contact relationship,suggesting that the amphibolite precursor intruded asdykes. The paragneiss observed here is similar to that ofthe Nyamvu Gneiss described earlier, although itexperienced more intense deformation, migmatizationand granitoid intrusion.

StructureJohnson (1968) reported four deformational phases thataffected the area around Chirwa dome. A first event,assumed Archaean in age, resulted in folding of thegreenstone sequence about east-southeast-trendingaxes. This was followed by three events (F1, F2, F3;terminology of Johnson, 1968) interpreted to be relatedto the Pan-African Mozambique orogeny. F1 is the maindeformation event and its intensity increases from westto east. Accordingly, Johnson (1968) divided the areaaround Chirwa dome into three deformational domains:(1) an outer zone west of Chirwa dome, characterizedby nonpenetrative shearing along cm-scale, eastward-dipping shear zones separated by wide areas ofunsheared rocks, (2) an intermediate zone aroundChirwa dome in which the rocks have a penetrativefoliation due to a flattening-type deformation and (3) aninner zone, distinguished by constrictional deformationeast of the xenolith-rich granodioritic gneiss outcrop.Foliation and lineation, including the long axis of thedeformation ellipsoid, within all domains dips/plungesmoderately to steeply towards the east.

F2 is linked to the solid-state emplacement of theNyarugo and Chirwa granites causing deflection of pre-existing structures (Johnson, 1968). The Chirwa domewas interpreted as remobilized Archaean granite thatwas emplaced during the Pan-African as a mantledgneiss dome due to gravitational forces. During D2, aneast-west-trending anticline-syncline-pair developedwith the anticline crossing the Chirwa granite. Parasiticto these large scale folds are medium- to small (cm)-scale folds. The third deformation phase (F3) isrepresented by north-northwest striking fractures and

brittle shear zones as well as millimetre-scale mircofolds.Our study broadly confirms the findings of Johnson

(1968). We suggest that the area has mainly beenaffected by a progressive deformation event (D1) thatgave rise to a penetrative, north-south trending foliation(S1) and an eastward plunging lineation (L1). An earlierdeformation event (D0) is restricted to the greenstones ofthe MGB, as indicated by east-southeast orientedstructures. A second deformational event (D2) is relatedto dome emplacement and resulted in the deflection of earlier structural features and the formation ofcrenulations. A third deformational event (D3) gave riseto the formation of cataclastic shear zones. Structuralfeatures observed are described according to theiroccurrence in the different lithotectonic units.

Makaha greenstone beltThe foliation in the western and central parts of theMGB is trending east-southeast (S0), parallel to the elongation of the belt. At the eastern extremity of theMGB in the vicinity of Chirwa dome (c. 2km away fromthe contact) the foliation swings to a north-northwesttrend (Johnson, 1968; Stocklmayer, 1980). Stratigraphicyounging directions in the volcano-sedimentarysequence to the west of the dome show repeatedreversals, indicating complex folding (cf. Chunnet,1972). Foliation data west of Chirwa dome (Figure 7A)define a great circle with the pole plunging at moderateangles towards the northeast, identical to the foliationdistribution in the northern triangular body ofmetasediments (Figure 7C). A lineation (L1) is defined bythe preferred orientation of hornblende as well as by thelong axis of pillows (Figure 5A) and plunges steeplytowards the east (Figure 7A). This lineation can beobserved up to a minimum distance of 5km west ofChirwa dome. The hornblende mineral lineationindicates amphibolite facies metamorphic conditionsduring deformation. Greenschist facies mineralassemblages occur in the greenstone sequence c. 8-10km west of Chirwa dome (Figure 3; Stocklmayer, 1980).

Chirwa dome graniteThe central Chirwa granite is almost undeformed andshows a weak biotite foliation. A preferred orientation ofK-feldspar phenocrysts is locally preserved and isinterpreted as flow banding; it seems to parallel themarginal gneissic foliation. Coming from the centre ofthe dome, a gneissic fabric can be first observed about150m from the outer contact of the granite, as thephenocrysts start to loose their tabular shape (Johnson,1968). A pervasive foliation (S1) and lineation (L1) isdefined by the preferred orientation of biotite and K-feldspar phenocrysts. The K-feldspar augen werepartly subjected to brittle deformation, a featureindicated by fractures in the phenocrysts, while quartz isrecrystallized. The marginal foliation has a concentricstrike that parallels the walls of the dome and thefoliation of the hornblende schist that envelops the dome (Figure 4). At the western side of the dome the



293

foliation dips steeply towards the east-northeast,whereas at the eastern margin it dips shallowly in thesame direction (Figure 7B). The dome thus has anasymmetric geometry which resembles an elliptical conethat was overturned to the west-southwest (cf. Johnson,1968). Quartz veins, pegmatite and aplite dykes are littledeformed in the centre of the dome but become foldedand boudinaged towards the outer margin.

Two distinct lineations occur (termed L1a and L1b)and are defined by the preferred orientation of biotiteaggregates, feldspar phenocrysts and quartz rods. Themost common lineation (L1a) has a shallow to moderateplunge towards the east-northeast in the northern andeastern parts of the dome, and towards the east-

southeast in the southern part (Figure 4). This lineationin the granite is subparallel to a hornblende minerallineation and elongated pressure shadows around garnetdeveloped in the hornblende schist enclosing the dome.On average, L1a is shallowly plunging towards the east-northeast and is downdip to the foliation (Figure 7). It is thus similar to L1 described from the surroundinglithologies. S-C fabrics, asymmetric feldsparporphyroclasts and antithetic microfaults inporphyroclasts reveal a top-to-the-west sense of shear(dome-side-down kinematics along the eastern marginof the dome). A second subordinate lineation (L1b) islocally preserved within the granite rim and plungesradially away from the dome. In the hornblende schists

53/66

B, Chirwa dome granite, hornblende schist

n=72

n=76

F, xenolith-rich granodioritic gneiss

107/39

n=122 n=123

G, high-grade gneisses

60/61

n=128

C, metasediments north

n=155

137/44

n=28n=20

E, metasediments south

n=228

105/43 n=175

D, metasediments east

n=32 n=35

A, greenstones

A

B

C

E

D

F G

nodata

n=211 (granite)

n=52 (schist)

n=52 (schist)

n=170 (granite)

S1

L1

N

Figure 7. Lower hemisphere, equal area stereographic projections of foliation and lineation data of the study area. The study area has been

subdivided into structural domains A-G. The orientation of the penetrative foliation (S1) and penetrative lineation (L1) is shown for each

domain.

enveloping the dome a second generation ofhornblende occurs as unoriented crystals overgrowingthe earlier fabric. The unoriented growth seems to occurin a concentric band close to the dome, and can beattributed to contact metamorphism.

Anastomosing, small-scale cataclastic shear zones areabundant throughout the dome and are orientedsubparallel to the foliation. The deflection of thefoliation into the shear zones indicates a predominantlynormal sense of shear. The northern part of the domeand parts of the surrounding rocks have been transectedby an east-west striking, northerly dipping shear zonethat also yields a normal sense of shear.

Metasedimentary sequence/Nyamvu GneissThe metasediments envelop the dome, with the marble-schist suite forming a horseshoe pattern (Figure 4). Theyhave a penetrative foliation and compositional layering(S1) which is parallel to the axial planes of isoclinalfolds. A lineation (L1) is well defined by the preferredorientation of aluminosilicate minerals (sillimanite,kyanite, staurolite), biotite and muscovite in mica schist,a hornblende preferred orientation in amphibolite andstriations and quartz rodding in quartzite. Compositionallayering of marble is locally deformed into sheath foldswith fold axes parallel to the mineral lineation (Figure5B). Clasts in metaconglomerate are aligned parallel tothe main fabric. The pebbles have both prolate andtriaxially oblate shapes. Their long axes are parallel tothe mineral lineation. Decimetre- to metre-wide highstrain zones occur locally (Figure 5D). Asymmetricstructures such as sigma-clasts, pressure fringes aroundgarnet, snowball garnets (Figure 6A) and asymmetric

quartz lenses in biotite schist indicate a non-coaxialcomponent of deformation, related to top-to-the-westthrusting of the metasedimentary sequence.

In the northern triangular area, S1 defines a greatcircle with the pole plunging at moderate angles towardsthe northeast (Figure 7C); in the southern triangular areathe pole of the great circle plunges at moderate anglestowards the southeast (Figure 7E). S1 in the eastern areadefines a small circle with its pole plunging moderatelytowards the east (Figure 7D). L1 fabrics in the three areasare parallel to the poles of the great or small circles.

Small-scale folds are common in themetasedimentary sequence (Figure 6B). They occur inmica-rich lithologies throughout the study area,including schist-enclaves in the Ruenya gneiss terrain tothe east. The folds are cm-scale crenulationsoverprinting S1. On average, fold axes (L2) plunge atsubhorizontal to shallow angles towards the south-southeast; the axial planes (S2) dip at moderate angles tothe south-west (Figure 8). In the northern triangulararea, S2 is predominantly northeast-trending, whereas inthe eastern area and the southern triangular area it isnorthwest-trending.

Xenolith-rich granodioritic gneissThe xenolith-rich granodioritic gneiss has a pronouncedsouthwest-trending foliation (S1), as defined by thepreferred orientation of biotite. A lineation (L1) isdefined by the elongation of biotite and hornblende andplunges at shallow to moderate angles to the east(Figure 7F). Granodioritic to granitic dykes intruded thefoliation of the gneiss (Figure 5F) and weresubsequently deformed together with the host rock, asindicated by their gneissic fabric. The xenolith-richgneiss of the apophysis northwest of Chirwa dome ismore strongly deformed than elsewhere, as indicated bythe higher length to width ratio of deformed xenoliths.

High-grade gneiss/Ruenya GneissThe high-grade gneisses are characterized by a pervasivefoliation (S1) which is defined by the preferredorientation of hornblende and biotite and bycompositional layering. S1 is parallel to the axial planesof common isoclinal folds. A lineation (L1) is defined byrodding, a preferred orientation of phenocrysts ingranitoid gneiss and a preferred orientation ofhornblende and mineral aggregates of plagioclase andbiotite. Boudins and rootless folds forming foliation-parallel trails are common. Asymmetric structures suchas back-rotated boudins indicate a top-to-the-westtectonic transport direction. S1 in the high-grade gneissarea dips at moderate angles to the northeast orsoutheast. It defines a great circle with the pole plungingat a moderate angle to the east-southeast, identical to theorientation of L1 (Figure 7G). High-strain zones occurand are parallel to the general structural trend; granitoidrocks together with intrusive aplite dykes are transposedinto these zones giving rise to layered gneiss. Thelayering is locally deformed into sheath folds.



n=19n=27

n=101

n=18

N

C

E

DS2

L 2

C

E

D

Figure 8. Lower hemisphere, equal area stereographic projections

of crenulation cleavage (S2) and associated fold axes (L2) data of

the metasedimentary domains C-E.



295

There is discordance between the orientation ofstructural fabrics, such as pebble stretching lineation, inmetaconglomerate enclaves and the mineral lineation in surrounding intrusive granitoids, such as the xenolith-rich granodioritic gneiss. This relationship indicates thatmetaconglomerate was already deformed prior tointrusion of the gneiss precursor. Evidence for anatexisis provided by enclaves of migmatitic gneiss, comprisingcomplexly folded (during anatexis) intercalations ofleucosome and biotite-restite layers (Figure 5G).

Late stage structural features are narrow shear zones(<10 cm) that are oriented parallel to the main fabric.Foliation boudinage along late shears is common; theshears are partly filled with diffuse lensoid melt patchesof quartzo-feldspathic material.

GeochronologyIsotopic data from the study area are limited toimprecise Rb-Sr whole-rock and biotite mineral ages(Vail and Dodson, 1969) on which most of the previoustectonic interpretations have been based. In order toclarify the age of the main lithotectonic units and theirtemporal relationship to deformation, we haveperformed a Pb/Pb zircon evaporation study on fourrock samples from the Chirwa dome area. Thecharacteristics of the samples dated, and the samplinglocality are given in Table 2 and Figure 4. Themorphology of zircons analysed is presented in Table 3.

Analytical techniquesSamples selected for zircon evaporation dating weretypically 4 to 5 kg in weight. Zircon concentrates were produced by standard density and magneticseparation procedures as described by Jaeckel et al.(1997). Heavy minerals were separated from the non-magnetic fraction using heavy liquids, and single zirconswere hand picked from the remaining concentrate.

Laboratory procedures follow Kober (1986; 1987)with slight modifications (Kröner and Todt, 1988; Kröneret al., 1991; Kröner and Hegner, 1998). Isotopicmeasurements were carried out on a Finnigan-MAT 261mass spectrometer at the Max-Planck-Institut für Chemiein Mainz. Data acquisition was by magnetic peakswitching, using the Secondary Electron Multiplier. No

correction was made for mass fractionation of Pb.Common Pb corrections followed the two-stage modelof Stacey and Kramers (1975).

A zircon grain of tonalite sample KV-H from theKaapvalley Pluton, Barberton, South Africa, produced anage of 3227 ± 2 Ma, identical to a concordant U–Pb ageof 3227 ± 1.6 Ma determined conventionally on zirconsfrom the same sample (Kamo and Davis, 1994). Duringthe course of this study we have repeatedly analysedfragments of large zircon grains from the PhalaborwaComplex, South Africa. These zircons, used as aninternal standard, are euhedral, colourless to slightlypink and completely homogeneous when examinedunder cathodoluminescence. Conventional U–Pbanalyses of six separate grain fragments from this sampleyielded a 207Pb/206Pb age of 2052.2 ± 0.8 Ma (W. Todt,unpublished data), whereas the mean 207Pb/206Pb ratiofor 18 grains, evaporated individually over a period of 12 months, is 0.126634 ± 0.000027 (2� error of thepopulation), corresponding to an age of 2051.8 ± 0.4 Ma,identical to the U–Pb age. The above error is consideredthe best estimate for the reproducibility of ourevaporation data and approximately corresponds to the2� (mean) error reported for individual analyses in thisstudy (Table 3). In the case of pooled analyses the 2�(mean) error may become very low, and whenever thiserror was less than the reproducibility of the internalstandard we have used the latter value (i.e. an assumed2� error of 0.000027).

The calculated 207Pb/206Pb ratios and their 2� (mean)errors are based on the means of all measurementsevaluated and are presented in Table 3. Mean ages anderrors for several zircons from the same sample arepresented as weighted means of the entire population.The 207Pb/206Pb spectra are shown in histograms thatpermit visual assessment of the data distribution fromwhich the ages are derived.

Since the evaporation technique only provides Pbisotopic ratios, there is no a priori way to determinewhether a measured 207Pb/206Pb ratio reflects aconcordant age. Thus, principally, all 207Pb/206Pb agesdetermined by this method are necessarily minimumages. However, it has been shown in many studies thatthere is a very strong likelihood that these data represent

Table 2. Description of dated samples (refer to Figure 4 for sample location).

Sample Grid reference Sample description Field relation Mineralogy

Zim J1 VR830903 Light grey porphyritic granite Chirwa dome granite, Plagioclase, quartz,

moderately foliated micro-cline, biotite, ± muscovite

Zim 06 VR832846 Clasts (5-10cm across) Metaconglomerate, Plagioclase, quartz,

of whitish to orange, strongly foliated micro-cline, ±muscovite

m.-grained granite

Zim 46 VR815918 Medium grey, c.-grained Xenolith-rich Plagioclase, quartz, micro-cline,

granodioritic gneiss granodioritic gneiss biotite, ± hornblende

Zim 47 VR815918 Pale grey, m.-grained Dyke cross-cutting foliation Plagioclase, quartz, micro-cline,

granitic gneiss of xenolith-rich granodioritic biotite

gneiss

true zircon crystallization ages when: (1) the 207Pb/206Pbratio does not change with increasing temperature ofevaporation; and/or (2) repeated analyses of grains fromthe same sample at high evaporation temperatures yieldthe same isotopic ratios within error. The rationalebehind this is that it is highly unlikely that each grain ina zircon population has lost exactly the same amount ofPb, and that grains with Pb-loss appreciably prior to thePresent would, therefore, yield highly variable207Pb/206Pb ratios and ages. Comparative studies bysingle grain evaporation, conventional U–Pb dating andion-microprobe analysis have shown this to be correct(e.g. Kröner et al., 1991; Cocherie et al., 1992; Jaeckel et al., 1997; Karabinos, 1997).

Only long, idiomorphic zircons (100-200 µm) with ayellowish/brownish to grey colour were selected forevaporation (Table 3). These zircons are interpreted asmagmatic zircons (Hoppe, 1962; Pupin and Turco, 1972;Pupin, 1980; Kröner et al., 1998) and are easilyrecognized under the microscope because of theirdistinctive morphology. To overcome problems with lowlead contents and corresponding low beam intensity, two to six carefully selected zircon grains wereevaporated together.

Results of geochronologySample Zim 06 is represented by granite clasts from ametaconglomerate southeast of Chirwa dome,



Table 3. Analytical results of zircon evaporation.

Sample number No. Zircon colour and Number of Evaporation Mean Age (Ma) and

of morphology 207Pb/206Pb temperature 207Pb/206Pb 2� mean error

zircons ratios (°C) ratio and error1

Zim J1 (1) 6 yellow to light brown, 77 1599 0.171224±77 2569.6±0.8

long-prismatic,

diomorphic, ~120-150µm

Zim J1 (2) 4 as above 66 1604 0.171212±65 2569.5±0.6

Zim J1 (3) 6 as above 77 1601 0.171210±49 2569.5±0.5

Zim J1 (4) 6 yellow to brown, 88 1600 0.171219±28 2569.6±0.3

near idiomorphic,

~120-150µm

Zim J1 (5) 6 as above 99 1598 0.171217±22 2569.6±0.2

Zim J1 (1-5) ∑ 407 0.171217±27 2569.6±0.3

Zim 06 (1) 2 clear, idiomorphic, ~150µm 99 1600 0.175738±51 2613.1±0.4

Zim 06 (2) 2 short, stumpy, ~100µm 77 1600 0.175755±55 2613.2±0.5

Zim 06 (3) 2 yellow to light brown, 110 1602 0.175749±32 2613.2±0.3

stumpy, end rounded,

170-180µm

Zim 06 (4) 2 as above 110 1600 0.175740±43 2613.1±0.4

Zim 06 (5) 2 as above 143 1604 0.175742±20 2613.1±0.1

Zim 06 (1-5) ∑ 539 0.175744±27 2613.1±0.3

Zim 06 (6) 2 light brown, end rounded, 110 1603 0.177385±48 2628.6±0.5

~100-150µm

Zim 46 (1) 2 clear to light grey, 110 1596 0.174623±112 2602.5±1.1

long-prismatic,

idiomorphic to slightly

rounded, ~100-150µm

Zim 46 (2) 3 as above 99 1596 0.174321±85 2599.6±0.8

Zim 46 (3) 2 as above, 120-140µm 99 1598 0.174540±81 2601.6±0.8

Zim 46 (1-3) ∑ 308 0.174499±57 2601.3±0.5

Zim 47 (1) 2 grey to light brown, 88 1599 0.173671±73 2593.3±0.7

long-prismatic,

idiomorphic to slightly

120-160µm

Zim 47 (2) 2 grey to light brown, 132 1598 0.173682±47 2593.4±0.5

long-prismatic,

diomorphic, ~150µm

Zim 47 (3) 5 grey to light reddish brown, 110 1598 0.173655±36 2593.2±0.4

long-prismatic,

idiomorphic, ~100µm

Zim 47 (1-3) ∑ 330 0.173670±29 2593.3±0.31Observed mean ratio corrected for non-radiogenic Pb where necessary. Errors are 2� (mean), based on uncertainties in counting

statistics.



297

comprising predominantly granite pebbles and cobbles(refer to Table 2 and Figure 4 for description andlocation). The zircons are stumpy with rounded ends,yellowish brown in colour or idiomorphic and clear. Sixgrain fractions from different granite clasts wereevaporated of which five yielded identical 207Pb/206Pbisotopic ratios that combine to a mean date of 2613.1 ±0.3 Ma, while the remaining grain fraction produced aslightly older date of 2628.6 ± 0.5 Ma (Table 3, Figure 9A). We interpret this to mean that the fiveisotopically homogeneous zircons represent graniteclasts derived from the same granitic source, whereasthe sixth grain fraction reflects a different granite source.

Sample Zim 46 is from xenolith-rich granodioriticgneiss (Table 2, Figure 4). The zircons are long-prismatic, clear to light grey in colour and idiomorphicto slightly rounded. Three grain fractions wereevaporated and produced identical isotopic ratios thatyield a combined mean 207Pb/206Pb date of 2601.3 ± 0.5Ma (Table 3, Figure 9B). We interpret this date asapproximating the time of granodiorite emplacement.

Sample Zim 47 is from leucocratic, pale grey,medium-grained granitic gneiss cross-cutting the mainfabric of the xenolith-rich granodioritic gneiss (Table 2,Figures 4 and 5F). The zircons are long-prismatic, greyto light reddish brown in colour and idiomorphic. Threegrain fractions were evaporated and yielded a combinedmean 207Pb/206Pb date of 2593.3 ± 0.3 Ma (Table 3,Figure 9C). We interpret this date as approximating thetime of intrusion of the granite into the foliated xenolith-rich gneiss.

Lastly, we dated zircons from sample J1, foliatedgranite (Table 2, Figure 4) collected within Chirwadome. The zircons are long-prismatic, yellow to lightbrown in colour and idiomorphic to near idiomorphic.Five grain fractions were evaporated and producedidentical isotopic ratios yielding a mean 207Pb/206Pb dateof 2569.6 ± 0.3 Ma (Table 3, Figure 9D). Since thezircons of sample J1 do not reveal the presence ofinherited cores under cathodoluminescence, we areconfident that the above date reflects the time ofemplacement of the Chirwa granite.

Figure 9. Histograms illustrating the results of zircon evaporation.

0

50

100

150

200

250

0.17

49

0.17

53

0.17

57

0.17

61

0.17

65

0.17

69

0.17

73

0.17

77

0.17

81

Zim 06 (1-5)

Zim 06 (1)

Zim 06 (2)Zim 06 (3)Zim 06 (4)Zim 06 (5)

Zim 06 (6)

Num

ber

of 2

07P

b/20

6Pb

ratio

s

Zim 06 (6)

2613.1±0.3Ma

2628.6±0.5 Ma

A

0

10

20

30

40

50

60

70

0.17

27

0.17

31

0.17

35

0.17

39

0.17

43

0.17

47

0.17

51

0.17

55

0.17

59

0.17

63

Zim 46

Zim 46 (1)Zim 46 (2)Zim 46 (3)

2601.3±0.5Ma

B

0

10

20

30

40

50

60

70

0.17

26

0.17

28

0.17

3

0.17

32

0.17

34

0.17

36

0.17

38

0.17

4

0.17

42

0.17

44

0.17

46

Zim 47

Zim 47 (1)Zim 47 (2)Zim 47 (3)

Num

ber

of 2

07P

b/20

6Pb

ratio

s

207Pb/206Pb ratio

2593.3±0.3Ma

C

0

50

100

150

200

0.17

03

0.17

05

0.17

07

0.17

09

0.17

11

0.17

13

0.17

15

0.17

17

0.17

19

0.17

21

0.17

23

Zim J1

Zim J1 (1)Zim J1 (2)Zim J1 (3)Zim J1 (4)Zim J1 (5)

207Pb/206Pb ratio

2569.6±0.3Ma

D

DiscussionThe geochronological data presented above enable us toprovide a time frame for the depositional, intrusive anddeformational history of the study area. The datesderived from granite clasts indicate that ~2613 and ~2629Ma old granites formed a source terrain for themetaconglomerate and associated metasedimentaryunits. The xenolith-rich granodioritic gneiss intrudedalready deformed metaconglomerate. This constrains thetime of deposition and subsequent deformation of the metaconglomerate precursor to between ~2613 and~2601 Ma.

The nature of the sedimentary basin can no longerbe recognized because of the strong deformationexperienced by the sediments. Johnson (1968)correlated the sediments with the Shamvaian Group, theyoungest unit of the greenstone sequence in Zimbabwe.In contrast, Stocklmayer (1980) assumed that theyrepresent remnants of a Proterozoic sequence that wasdeposited in a basin along the edge of the Zimbabwecraton. Our data indicate a late Archaean age formetaconglomerate and associated sedimentary rocks.The late Archaean Shamvaian sediments in Zimbabwecommonly rest unconformably or with tectonic contactson mafic to felsic greenstones and contain coarse clasticmaterial derived from granitoid-greenstone terrains.Single zircon evaporation dates of metaconglomerate inthe Bindura-Shamva greenstone belt indicate a minimumage of ~2640 Ma for deposition of Shamvaian sediments(Hofmann and Kröner, unpubl. data), significantly olderthan the sediments described here, although depositionof the Shamvaian may have been diachronousthroughout the craton. The short time span betweendeposition and subsequent incorporation of thesedimentary sequence into thrust sheets (see below)may indicate a foreland-type basin setting. A passivecontinental margin setting, albeit short-lived, may alsobe envisaged (cf. Hoffman and Bowring, 1984).

The sedimentary pile was intruded, at ~2601 Ma, bythe xenolith-rich granodioritic gneiss precursor thatbecame strongly deformed after emplacement and wasitself intruded by a leucocratic gneiss precursor at ~2593Ma. The xenolith-rich granodioritic gneiss precursorintruded into already deformed conglomerates, i.e.deformation must have commenced earlier than ~2601Ma ago. The sheet-like geometry of the xenolith-richgneiss along the contact between two different gneisssuites may indicate that its precursor intrudedsyntectonically along a structural discontinuity. Since the2593 Ma granitoid phase is only slightly deformed andintrudes the more strongly deformed xenolith-richgranodioritic phase, we conclude that the maindeformation (D1) in the Chirwa dome area began priorto ~2593 Ma.

D1 was an event of colinear folding and shearing, asindicated by foliation and lineation data of the structuraldomains (Figure 7) and by the occurrence of sheathfolds in marbles and banded gneisses in the field. Themetamorphic grade increases gradually from greenschist

facies in the west to upper amphibolite facies in the east.Foliation planes and associated linear features generallydip/plunge to the east and sense-of-shear indicatorsconsistently indicate top-to-the-west tectonic transport.The westward-directed thrusting event affected all thelithotectonic units in the study area. Thrusting isparticularly evident in the juxtaposition ofmetasediments of the marble-schist suite againstgreenstones of the Makaha belt (Figure 4).

The ~2593 Ma leucocratic gneiss is slightly deformedbut shares a foliation with the same geometricorientation as the xenolith-rich host gneiss, suggestingthat deformation in the Chirwa dome area continuedafter emplacement of the leucocratic gneiss precursor.Continued deformation may have been interrupted bytime periods of tectonic quiscence until the Chirwadome granite was emplaced during a late stage ofthrusting ~2570 Ma ago. Compositionally similar, latetectonic granites (Chilimanzi Suite) occur throughout theZimbabwe craton and their intrusion heraldedstabilisation of the central part of the craton prior tointrusion of the Great Dyke at ~2575 Ma. Granites of thissuite have yielded U-Pb zircon dates of 2634 ± 17 in thecentral part of the craton (Horstwood, 1998) and of 2601± 14 Ma in the northern part (Jelsma et al., 1996).

The Chirwa dome granite contains an eastward-plunging lineation (L1a) as well as shear sense indicatorsrevealing top-to-the-west transport (dome side downalong the eastern contact), suggesting that it was alsoaffected by thrusting. The shape of the Chirwa domegranite (slightly elliptical cone overturned to the westwith the cone axis parallel to the main lineation)strongly resembles the geometry of a sheath fold.However, emplacement must have taken place duringthe late stages of deformation because the granite is onlymarginally deformed and much younger than thegranodioritic to granitic gneisses to the east. The radiallyarranged lineation (L1b) possibly formed during theemplacement of the Chirwa dome. The age relationshipbetween the two lineations could not be determined.The poor preservation of the L1b lineation may indicatethat it formed earlier and was overprinted by thrusting.However, heat emitted from the granite may have givenrise to unoriented hornblende growth in the vicinity ofthe dome, and this must have taken place after the mainphase of thrusting. Nevertheless, granite emplacementand cessation of thrusting were closely linked in time,and the two lineations may have formedcontemporaneously.

Emplacement of the Chirwa dome granite gave riseto the deformation of the surrounding rocks. This isindicated by (1) the deflection of the metasedimentarysequence around the dome, (2) the change in strike ofthe foliation in the greenstones from a southeast to anorth-northwest trend in the vicinity of Chirwa domeand (3) the higher strain intensities of rocks in theimmediate vicinity of the dome as, for example, shownby the geometry of xenoliths within the apophysis ofxenolith-rich granodioritic gneiss. Deformation





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associated with dome emplacement (D2) took placeduring the final phase of east-over-west thrusting. The folding of the marble-schist suite, resulting in thetriangular-shaped areas northwest and southwest of the dome, is attributed to D2. This folding resulted in thecrenulation cleavage S2 which is parallel to the axialsurfaces of the folds (as defined by the great circledistribution of S1, and L2 is identical to the pole of thegreat circle). Hence, S2 probably formed during domeemplacement. S2 developed in metasediments east of thedome dips at moderate angles southwest and may berelated to east-west compression during domeemplacement. The formation of cataclastic shear zoneswith normal sense of shear within the Chirwa granite isattributed to D3. Normal faulting may reflect adjustmentsto gravitational instabilities that formed shortly after, andas a consequence of, thrusting and granite emplacement.

We suggest that most of the deformation recorded inthe Chirwa dome area is a result of late Archaean ratherthan Pan-African tectonic processes. The area wasaffected by east-west compression (D1) for a minimumtime period of 30 Ma in the late Archaean. Ifdeformation resulted from a single tectonic event or aseries of similar events, giving rise to structural featureswith identical orientations, remains unclear to us. TheRb-Sr biotite date of 473 ± 8 Ma reported by Vail andDodson (1969) for the Chirwa dome granite represents acooling age below the ~450 °C isotherm (Villa, 1998),indicating a Pan-African event of thermal overprint. Noevidence has been observed that this overprint wasassociated with tectonic reworking of the study area.Cooling following thermal overprinting of Archaeanbasement is also indicated by 40Ar/39Ar hornblende agesof ~500 Ma from the MGT further west (Vinyu et al.,1999), consistent with previous studies which haveshown that Pan-African metamorphism wassuperimposed on Archaean metamorphic assemblageswithin the MGT (Barton et al., 1991; Dirks et al., 1998).

Recent work in several greenstone belts of theZimbabwe craton has shown that the supracrustalsequences represent tectonic amalgamations of variouslithotectonic units that were juxtaposed between ~2.70and ~2.64 Ga during an east-over-west thrusting event.This event is unrelated to, and took place prior to,granitoid diapirism (Jelsma and Dirks, 2000; Dirks et al.,2002). The west-directed thrusting event preserved inthe craton margin lithologies in northeast Zimbabwe anddescribed herein for the Chirwa dome area may be thefinal expression of the horizontal accretion processwhich ultimately resulted in the formation of theZimbabwe craton.

ConclusionsA major sedimentary sequence occurs along thenortheastern margin of the Zimbabwe craton. The sedimentary rocks were derived from a granitoidterrain of late Archaean age and were deposited andsubsequently deformed between ~2613 and ~2601 Maago.

Shortly after deposition, the sedimentary sequencewas thrust westward and juxtaposed against maficvolcanic rocks of the Makaha greenstone belt. The shorttime span between deposition and deformation mayindicate that sedimentation took place in a foreland-typebasin along the craton margin.

Thrusting gave rise to a penetrative foliation andlineation and was associated with a regionalmetamorphic event ranging from greenschist facies inthe west to upper amphibolite facies in the east. It wasaccompanied by the syntectonic intrusion of variousgranitoids, such as the xenolith-rich granodioritic gneissprecursor, between ~2601 and ~2593 Ma.

The end of the thrusting event is marked by the latesyntectonic emplacement of monzogranites, such as theChirwa dome granite at ~2570 Ma. Similar granites(Chilimanzi Suite) occur throughout the Zimbabwecraton and their intrusion heralded stabilisation of thecentral part of the craton prior to intrusion of the GreatDyke at ~2575 Ma. This relationship may indicate thatstabilisation of the continental crust in Zimbabwe was adiachronous event.

Our data indicate that emplacement of the Chirwadome granite took place in late Archaean times, and thatwest-directed thrusting took place prior to and duringgranite intrusion. This thrusting event in northeastZimbabwe may be the final expression of westward-directed horizontal accretion which gave rise to theformation of the Zimbabwe craton.

AcknowledgementsThis study was funded largely by the Volkswagen-Foundation. Financial support was further provided byDAAD, Universität Mainz and Freunde der UniversitätMainz to AH and Stichting Schürmannfonds (1997-2000/13) to PD and HJ. Many thanks to Maruza Tsaborafrom Chikwizo for his assistance during field work. Thispaper benefited from the constructive reviews of JanKramers and Maarten de Wit.

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Editorial handling: J. M. Barton Jr.



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