u-pb sensitive high-resolution ion microprobe (shrimp) zircon...

U-Pb sensitive high-resolution ion microprobe (SHRIMP) zircon

geochronology of granitoid rocks in eastern Zambia:

Terrane subdivision of the Mesoproterozoic Southern Irumide Belt

S. P. Johnson,1,2 B. De Waele,3,4 and K. A. Liyungu5

Received 7 April 2006; revised 11 August 2006; accepted 11 September 2006; published 21 November 2006.

[1] The Southern Irumide Belt (SIB) is a structurallyand metamorphically complex region of mainlyMesoproterozoic igneous rocks in southern and easternZambia, northern Mozambique and northern Malawithat was strongly overprinted in the Neoproterozoicto Cambrian Damara-Lufilian-Zambezi (DLZ)orogeny. Because of the scarcity of geological datafrom this region, little is known about the timingof tectonomagmatic events; however, this belt hastraditionally been considered to be a southerlycontinuation of the adjacent Irumide Belt (IB). Herewe provide 27 new U-Pb sensitive high-resolution ionmicroprobe (SHRIMP) zircon ages that constrain thePaleoproterozoic to Cambrian tectonomagmatic historyof this belt and which, for the first time, allow fordirect comparison with the adjoining IB. The SIB isfloored by a predominantly late Paleoproterozoicbasement, which was intruded by voluminouscontinental margin arc-related magmas between 1.09and 1.04 Ga and accompanied by high-temperature/low-pressure metamorphism. In contrast, the IB isfloored by a late Paleoproterozoic basement that isgenerally older than 2.0 Ga, contains significant mid-Mesoproterozoic plutonic rocks that are not presentwithin the SIB, and underwent moderate-pressure/moderate-temperature compressional metamorphismand S-type granitoid magmatism at circa 1.02 Ga.These data indicate that the crust underlying the SIBis not a continuation of that underlying the IBbut represents an allocthonous continental margin arcterrane juxtaposed against the Congo-Tanzania-Bangweulu Craton during the late MesoproterozoicIrumide orogeny. Reworking and shearing of theSIB occurred during the DLZ orogen, resulting in thepresent-day architecture as a series of stacked terranes

which have been exploited by voluminous posttectonicgranitoid batholiths. Citation: Johnson, S. P., B. De Waele,

and K. A. Liyungu (2006), U-Pb sensitive high-resolution ion

microprobe (SHRIMP) zircon geochronology of granitoid rocks in

eastern Zambia: Terrane subdivision of the Mesoproterozoic

Southern Irumide Belt, Tectonics, 25, TC6004, doi:10.1029/

2006TC001977.

1. Introduction

[2] The Irumide Belt and Southern Irumide Belt ofZambia comprise a series of Mesoproterozoic structuralterranes of high-grade gneisses and supracrustal units alongthe southern margin of the central African Congo-Tanzania-Bangweulu Craton (hereafter the CTB Craton), but specif-ically, the Paleoproterozoic Bangweulu Block (Figure 1a).The presence of Permo-Triassic ‘‘Karoo’’ graben betweenthese two tectonic provinces precludes direct correlationsbetween them, and it is entirely possible that these youngerrifts conceal an important suture along this margin of theCTB Craton (Figure 1). To the east and west, Neoproter-ozoic tectonism of the East African and Damara-Lufilian-Zambezi (DLZ) orogens respectively (Figure 1a), havethoroughly affected the region, largely obliterating anypre-Pan African fabrics and this deformation was followedby the intrusion of numerous late Neoproterozoic-Cambrianigneous complexes [Drysdall et al., 1972; Haslam et al.,1986; Johns et al., 1989].[3] The Irumide Belt (IB) is a NE-SW trending belt

composed of Paleoproterozoic to Meosproterozoic rocks.The mid-Paleoproterozoic granitoid basement known as theBangweulu Block [Anderson and Unrug, 1984] is overlainby a thick sequence of late Paleoproterozoic supracrustaland volcanic units termed the Muva Supergroup [Dalyand Unrug, 1982]. During both the middle and lateMesoproterozoic these basement units were intruded by aseries of S-type granitoids [De Waele et al., 2006] andduring the late Mesoproterozoic event, magmatism wasaccompanied by compressional tectonometamorphism, i.e.,the Irumide orogeny [De Waele, 2005]. Throughout theentire Paleoproterozoic and Mesoproterozoic, it is evidentthat this margin of the CTB Craton, i.e., the IB andBangweulu Block, was never an active margin [De Waeleet al., 2006].[4] The Southern Irumide Belt (SIB) is a relatively new

term introduced by Johnson et al. [2005] to describe a widevariety of variably metamorphosed igneous and metasedi-mentary lithologies which are distinct from the monotonous

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1Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan.

2Now at Geological Survey of Western Australia, East Perth, WesternAustralia, Australia.

3Tectonics Special Research Centre, School of Earth and GeographicalSciences, University of Western Australia, Crawley, Australia.

4Now at British Geological Survey, Keyworth, UK.5Geological Survey Department of Zambia, Lusaka, Zambia.

Copyright 2006 by the American Geophysical Union.0278-7407/06/2006TC001977$12.00

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http://dx.doi.org/10.1029/2006TC001977

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TC6004 JOHNSON ET AL.: MESOPROTEROZOIC SOUTHERN IRUMIDE BELT

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Irumide Belt (in the strictest sense) granitoids (Figure 1).However, geological knowledge of the Southern IrumideBelt remains fragmentary. The belt, which is currently bestdefined in southern and eastern Zambia, passes into thepoorly known regions of Mozambique and northern Malawi(Figure 1) where various Rb-Sr and Sm-Nd whole rock–mineral isochrons and zircon Pb evaporation dates revealthe presence of Mesoproterozoic protoliths [Kroner et al.,1997; Evans et al., 1999]. On the basis of a reconnaissancestudy, Mapani et al. [2001, 2004; B. Mapani et al., Terranesubdivision of the Irumide orogen in Zambia: A testabletectonic hypothesis, submitted to Tectonophysics, 2006,hereinafter referred to as Mapani et al., submitted manu-script, 2006] divided the SIB in Zambia into several terraneson the basis of lithotype, geochemical characteristics, lim-ited laser ablation inductively coupled mass spectrometer(LA-ICPMS) U-Pb zircon ages and the presence of bound-ing shear zones. The orientation of these terrane boundariesare generally transverse to the predominant fabrics withinthe IB. Based mainly on the geochemical composition ofigneous lithologies, these terranes are interpreted to repre-sent island arcs, associated sediments and one or moreexotic continental block(s) that accreted to the CTB Cratonmargin during the late Mesoproterozoic Irumide orogeny[Mapani et al., 2001, 2004]. In this paper we test the terranesubdivisions of Mapani et al. [2001, 2004, also submittedmanuscript, 2006] and provide supporting evidence fromgeochronological data. The sample locations were carefullyselected in an effort to characterize each terrane and testcorrelations between the various segments of the belt. Thedata also allow, for the first time, a comparison between theSouthern Irumide Belt and the Irumide Belt to the north.

2. Terrane Subdivision

2.1. Chewore-Rufunsa Terrane

[5] The Chewore-Rufunsa Terrane (indicated as C-R inFigure 1) is composed of a wide variety of calc-alkalinemafic to felsic gneisses, metavolcanic rocks and associatedmetasediments. In the northern part of the terrane, aroundthe town of Rufunsa (R on Figure 1), greenschist-faciesvolcanic rocks [Barr, 1974] with intercalated quartzite andpelite can be traced southward into progressively highermetamorphic grades that culminate in the upper amphiboliteto locally granulite-facies along the Zambezi escarpmentand in basement horsts of the Chewore Inliers (CI on

Figure 1). The superposition of Neoproterozoic fabrics onthe Mesoproterozoic rocks makes interpretation of the pre-Pan African structural metamorphic history especially cryp-tic. However, a wealth of recent geochemical, isotopic andgeochronological data from the Chewore Inliers [Goscombeet al., 2000; Johnson and Oliver, 2004; Johnson et al.,2006a, S. P. Johnson et al., Geochemistry, geochronologyand isotopic evolution of the Chewore-Rufunsa Terrane,Southern Irumide Belt: A Mesoproterozoic continental-margin-arc, submitted to Journal of Petrology, 2006, here-inafter referred to as Johnson et al., submitted manuscript,2006] and the Zambian Zambezi Escarpment [Johnson etal., 2006a, also submitted manuscript, 2006] indicate thatthese volcano-plutonic lithologies formed in a continentalmargin arc setting between circa 1090–1040 Ma. Further-more, the presence of a circa 1393 Ma [Oliver et al., 1998]marginal basin ophiolite in the Chewore Inliers points tothe possibility of long-lived suprasubduction zone magma-tism in this region. Whole rock isotopic model ages[Johnson et al., 2006a, submitted manuscript, 2006] andabundant inherited zircon extracted from the arc rocks[Goscombe et al., 2000; Johnson et al., 2005] indicate thatthe continental arc was built predominantly on juvenilePaleoproterozoic crust with a minor Archaean age compo-nent. Low Th/U zircon overgrowths, interpreted to haveformed during the peak of low-pressure/high-temperaturegranulite-facies metamorphism [Goscombe et al., 2000],have ages similar to the igneous cores which they overgrow(1.08 Ga) [Goscombe et al., 2000] suggesting that localizedhigh-temperature metamorphism was the result of intensivemagma loading of the crust. Neoproterozoic to Paleozoichigh-pressure metamorphism and structural overprinting isrecorded by metamorphic zircon and monazite growthbetween circa 570–515 Ma (data summarized by Johnsonet al. [2005]). Since there is a wealth of high-precisionigneous crystallization and metamorphic geochronologicaldata from this terrane, we have not dated any additionallithologies in this study.

2.2. Luangwa-Nyimba Terrane

[6] The Luangwa-Nyimba Terrane (L-N on Figure 1; seealso Figure 2) is dominated by supracrustal rocks includingpelitic migmatite (Figure 3a) and quartzite with minor maficto intermediate volcanic horizons [Agar and Ray, 1983].Plutonic igneous rocks are restricted to a few isolatedoccurrences of posttectonic granite and basement inliers

Figure 1. (a) Simplified tectonic map of Africa after Hanson [2003]. 1, Phanerozoic belts; 2, Neoproterozoic-Cambrianbelts; 3, Mesoproterozoic belts; 4, Paleoproterozoic belts; 5, Archaean Cratons. Abbreviations are BB, BangweuluBlock; CFB, Cape Fold Belt; D, Damara Belt; EAO, East African Orogen; I, Irumide Belt; K, Kibaran Belt; LUF,Lufilian Belt; L, Limpopo Belt; M. S. Z, Mwembeshi Shear Zone; NA, Namaqua Belt; NL, Natal Belt; SIB, SouthernIrumide Belt; TC, Tanzania Craton; U, Ubendian Belt-Usagaran Belt; ZC, Zimbabwe Craton. (b) Explanation anddistribution of political borders shown in main geological map. DRC, Democratic Republic of Congo; MAL, Malawi;MOZ, Mozambique; TAN, Tanzania; ZIM, Zimbabwe. (c) Simplified lithological geological map of the central, southernAfrican region, showing the location of the Southern Irumide belt and Irumide Belt. The SIB subdivisions are based onthose by Mapani et al. [2001, 2004] and are abbreviated as follows: C-R, Chewore-Rufunsa Terrane; L-N, Luangwa-Nyimba Terrane; P-S, Petauke-Sinda Terrane; Chp, Chipata Terrane. Other abbreviations are C, Chipata Town; CI, CheworeInliers; LTN, Lake Tanganyika; N, Nyimba Town; P, Petauke Twon; R, Rufunsa Town.


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(Figure 2). Both upper and lower contacts of the Luangwa-Nyimba Terrane are interpreted to be ductile thrusts andstretching lineations on the lower thrust, the LuangwaThrust (Figure 2), suggest that the Luangwa-Nyimba Ter-rane was thrust over the Chewore-Rufunsa Terrane fromESE to WNW (Mapani et al., submitted manuscript, 2006);however, the age of this thrusting event is unknown.Igneous zircon extracted from dioritic to granodioriticgneiss, interpreted to be basement to the metasediments,have been dated by LA-ICPMS at circa 2.6 Ga (sample i-8 inFigure 2) while thin, low Th/U metamorphic zircon rimsaround these igneous zircon have an age of circa 1.04 Ga[Cox et al., 2002; Mapani et al., submitted manuscript,2006]. Detrital zircon extracted from nearby quartzite(sample i-9 in Figure 2) reveal age peaks at circa 2.6 Gaand 2.2 Ga [Cox et al., 2002; Mapani et al., submittedmanuscript, 2006] suggesting that they were derived locallyfrom the underlying basement. Currently there are few ageconstraints for the timing of deposition of these metasedi-ments, but the youngest detrital grain dated at circa 2.2 Gaprovides a maximum age of deposition. In general themetamorphic grade increases from lower amphibolite faciesin the east to upper amphibolite facies in the west, but wehave noted migmatized pelitic assemblages at the easternmargin (structural top) of the terrane suggesting thatyounger metamorphic overprinting resulted in a complexregional metamorphic pattern.

2.3. Petauke-Sinda Terrane

[7] The western portion of the Petauke-Sinda Terrane isdominated by carbonate, calc-silicate, quartzofeldspathicparagneiss and granitic orthogneiss (Figure 2) all of whichare moderately to strongly deformed. The central andeastern parts comprise granitoid batholiths that invariablyshow a strong magmatic banding (Figure 3b) and areintruded by equigranular syenitic stocks such as theLusandwa Syenite (Figure 2). One of these batholiths, theSinda Granite, yielded a poorly constrained Rb-Sr age of489 ± 42 Ma [Haslam et al., 1986] suggesting they may beposttectonic with respect to the Pan African deformation.The bedrock around the town of Petauke (P on Figure 2) ispoorly exposed but rare outcrops are comprised predomi-nantly of very coarse-grained quartz-feldspar pegmatitewith rare 20 to 30 cm patches of equigranular, fine- tomedium-grained granite, presumably the host into which thepegmatites intruded. In this region, Phillips [1960] alsodescribed sparse outcrops of highly deformed basementorthogneisses (Nyanji Gneiss), one of which (sample i-26in Figure 2) has been dated by LA-ICPMS at 1.13 Ga [Coxet al., 2002], and at the same outcrop, a folded cross cuttinggranite sheet was dated at circa 1.12 Ga [Cox et al., 2002].

The northern part of the Petauke-Sinda Terrane is composedof porphyritic granitoids that are unconformably overlain bya thin sequence of intercalated quartzite and andesiticvolcanics known as the Sasare volcanics [Barr and Drysdall,1972]. The timing of volcanic activity and age of thebasement remain unresolved at present. In the west, thecontact with the underlying Luangwa-Nyimba Terrane haslong been recognized as a thrust, the Nyimba Thrust (or theMchimadzi lineament), and the lithologies in this regionare strongly deformed, carry a penetrative planar and linearfabric (Mapani et al., submitted manuscript, 2006), andhave been metamorphosed to at least midamphibolitefacies. Similar metamorphic conditions were attained inthe central parts of the terrane, but the granitoid batholithsand syenitic stocks show no evidence for metamorphismor brittle/ductile deformation. Again the age of thevarious tectonometamorphic/magmatic event(s) are cur-rently unconstrained.

2.4. Nyamadzi Shear Zone

[8] The eastern margin of the Petauke-Sinda Terrane andwestern margin of the Chipata Terrane are marked bylithologies with intense ductile planar and linear fabrics.This zone is some 30km wide and forms a predominantfeature on the aeromagnetic maps of southern and easternAfrica (SADC data) and Zambia (SADC data) (Figures 4aand 4b) where it can be traced for at least 100 km beforebeing truncated by Karoo grabens. We have named thispreviously unrecognized shear zone the Nyamadzi ShearZone (NSZ) after the Nyamadzi River along which the partsof the shear zone are well exposed. The planar fabrics in thezone are generally northeast-southwest oriented with near-vertical inclinations, with shallow, north or south plunginglineations; however, shear sense indicators are rare and thedominant movement direction has yet to be identified. Thezone contains a wide variety of lithologies ranging frompelitic migmatite, hornblende gneiss, porphyritic to ultra-mylonitic granite and highly deformed quartzite to kilome-ter-scale, isolated mafic igneous gabbro and amphibolite(Figures 2 and 3c–3h) of varying metamorphic grade;however, there are currently no age constraints for any ofthese lithologies, for the age of metamorphism nor the ageof shearing. The NSZ has been stitched by undeformedsyenite stocks of the Lusandwa Syenite, one of which(sample i-30 in Figure 2) has yielded a zircon and monaziteLA-ICP-MS age of �510 Ma (R. A. Cox et al., personalcommunication, 2002).

2.5. Chipata Terrane

[9] The Chipata Terrane is by far the most distinct andlithologically complex of all the terranes. It consists pre-

Figure 2. Compilation of the Zambia Geological Survey Department 1:250,000 regional maps [Vavrda, 1974; Agarand Ray, 1983]. The geological map shows the distribution and age of samples dated in this study and those dated byLA-ICPMS [Cox et al., 2002; Mapani et al., submitted manuscript, 2006]. In addition to latitude and longitude, thelocal UTM grid is also shown. These grid reference numbers are prefixed by 36L and are in the ARC1950 datum.Town abbreviations are the same as in Figure 1 except LS, Lusandwa Syenite. Terrane boundary thrusts and thrustmovement directions are taken from Mapani et al. [2001, 2004, also submitted manuscript, 2006].


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dominantly of variably retrogressed mafic, felsic and peliticgranulite, with subordinate hornblende-biotite gneiss, vari-ably deformed granitoid and undeformed syenite. Hyper-sthene-bearing granitoids (charnockites) with abundantgarnet-pyroxene-bearing mafic boudins (Figures 5a–5g)predominate, while garnet- and cordierite-bearing peliticgranulites indicate that these lithologies underwent high-

temperature/moderate-pressure tectonometamorphism[Schenk and Appel, 2001, 2002]. Metamorphic monazitefrom pelitic granulite in the Chipata Terrane (unknownlocality) has been dated at circa 1046 Ma [Schenk andAppel, 2001, 2002]. In places, the granulites are crosscutand retrogressed along high-strain amphibolite-facies shearzones. The contact zones between the granulite and lower-

Figure 3. Field photographs of samples dated from the Nyimba, Petauke-Sinda Terrane, and theNyamadzi Shear Zone. For detailed rock descriptions, sample locations, and rock ages refer to text,Table 1, and Text S1.


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Figure 4


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grade hornblende and biotite gneisses are not exposed andthe relationship between them is unclear. In general thelower-grade gneisses were derived from highly deformedK-feldspar-bearing porphyritic augen granite (Figures 5aand 5b), but in places it is clear that some of the gneisses havea sedimentary parentage. The augen gneisses are locallyintruded by decimeter-scale, undeformed amphibolite dikesand coarse-grained, syenite (Figure 5a). There is evidence formagma mingling between the mafic and syenitic melts(Figures 5a–5c) indicating that they were intruded contem-poraneously. Limited geochemical data indicate that theaugen gneisses are calc-alkaline whereas the mafic amphib-olites suggest formation in a continental extensional setting[Tembo et al., 2002; Mapani et al., submitted manuscript,2006]. Occasionally there are isolated outcrops of unde-formed K-feldspar-bearing porphyritic granite (Figure 5f)and garnet-bearing pelitic migmatite (Figure 5g) but theirrelation to the surrounding granulite/gneiss and timing ofintrusion/migmatization are not known.[10] The 1:250,000 geological map of the Chipata area

(Figure 2) [Vavrda, 1974] indicates that the granulites andgneisses are intruded by a series of 20- to 50-km-widebiotite granites (e.g., the Mboza, Msosoka, and Ntimbagranites) and elongate syenite bodies (e.g., the LunkwakwaSyenite). The biotite granites are generally coarse-grainedand equigranular (Figure 5e) and locally contain a mildfabric, presumably of magmatic origin. A finer apliticbiotite granite is occasionally observed to intrude the coarsegrained phase. Igneous zircon from one of these leucogran-ites (sample i-36 in Figure 2) has been dated by LA-ICPMSat circa 0.94 Ga (R. A. Cox et al., personal communication).The syenites appear to be similar to those that form part ofthe Petauke-Sinda Terrane, but confirmation of their similarage awaits further work.

3. Sample Locations and Descriptions


[11] One sample was collected from the Luangwa-Nyimba Terrane (NY82). It is a pelitic migmatite from aroad cutting along the Great East Road (Table 1, Figures 2and 3a). The sample is a coarse-grained (1–5 mm) biotitedominant (50–60%) quartz-feldspar gneiss that containslocal 10- to 20-cm-long, 5- to 8-cm-thick quartz (40%)and feldspar (60%) melt pockets (leucosomes) (Figure 3a).Small, 0.5 mm diameter garnet porphyroblasts are randomlydistributed throughout the biotite gneiss and are concen-trated (10–20%) in biotite selvages surrounding the meltpockets. The rock carries a strong southwest dipping

foliation defined by aligned biotite and quartz-feldsparaggregates, and the elongate melt pockets.

3.2. Petauke-Sinda Terrane and Its Eastern Shear Zone

[12] Nine samples were collected from the Petauke-SindaTerrane, including eight granitoids (PS17, PS18, PS19,PS65, PS71b PS73, PS76 and PS78) and one syenite(PS28). Five samples of various character, metamorphicgrade and deformation state were collected from the 30-km-wide shear zone that affects the eastern margin of thisterrane (SZ16, SZ23, SZ25c, SZ26 and SZ27).3.2.1. Samples PS17, 18, 19, 28, and 76[13] Sample PS17 was collected from the Sinda Granite,

which is an unfoliated, coarse-grained, K-feldspar-phyricgranite with a strong primary magmatic layering defined bythe alignment of microcline phenocrysts (Table 1 andFigures 2 and 3b). The matrix is composed of coarse-grained (2–4 mm) plagioclase and quartz with minor,randomly oriented biotite (<5%). The K-feldspar phenoc-rysts comprise 30–40% of the rock and, although stronglyaligned, do not display any evidence for postmagmaticrecrystallization or deformation suggesting that the fabricis of magmatic origin. Sample PS18 is a coarse-grained(2–4 mm) equigranular granite (Table 1 and Figure 2) thatis compositionally similar to the Sinda Granite, but lacksK-feldspar phenocrysts. Sample PS19, the LusandwaSyenite, is much finer grained (<1 mm) but composition-ally similar to granitoid PS18 (Table 1 and Figure 2) andis equigranular. Sample PS28 is a representative sampleof the Pule Syenite and is an equigranular, coarse-grained(2–4 mm), undeformed hornblende-biotite-K-feldsparsyenite (Table 1 and Figure 2). Sample PS73 was collectedfrom Petauke Town (Figure 2) and is representative of a 20–30 cm patch of equigranular, medium-grained (1–2 mm)biotite-feldspar-quartz granitoid that forms the basement intowhich numerous pegmatites have intruded (Table 1 andFigure 2). Sample PS76 is also a medium-grained (1–2 mm)equigranular biotite-feldspar-quartz granitoid similar toPS73 (Table 1 and Figure 2).3.2.2. Sample PS78[14] The Hofmeir Gneiss is a strongly deformed coarse-

grained quartzofeldspathic orthogneiss collected close to theboundary with the Nyimba Terrane (Table 1 and Figure 2).The dominant foliation is oriented northwest-southeast andsteeply dips toward the northeast. This fabric is essentiallyparallel to the Nyimba (Mchimadzi) terrane boundingthrust and may have attained its fabric during thrustmovement. The gneiss is composed of elongate aggregatesof quartz (30%) and plagioclase (40%) with aligned biotite

Figure 4. (a) Aeromagnetic data of the central southern African region (equivalent to that shown in Figure 1) (SADCdata). Transverse Mercator map projection. White shaded areas correspond to the Permo-Triassic Karoo rift basins.(b) Part of the aeromagnetic map of Zambia which has a higher pixel resolution to the regional aeromagnetic map shown inFigure 4a. However, this map lacks coverage in some regions (i.e., the black region centered on the Petauke-SindaTerrane). The Nyamadzi Shear Zone is clearly defined by very strong magnetic lineaments. Also note the strong east-westtrending lineament which may correspond with the Sasare volcanics (Figure 2) which have previously been interpreted as arift-related volcanic feature [Barr and Drysdall, 1972]. Our data suggest that this rifting event may have occurred at circa720 Ma. This lineament is truncated by the Nyamadzi Shear Zone, dated here between 535 and 510 Ma.


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Figure 5. Field photographs of samples dated from the Chipata Terrane. For detailed rock descriptions,sample locations, and rock ages refer to text, Table 1, and Text S1.


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Table 1. Sample Location, Description, and Calculated Age

Sample Sample Name Sample DescriptionGrid Reference,UTM, meters

Crystallization,Ma

Metamorphism,Ma

Chipata TerraneCHP2a Augen Gneiss deformed K-feldspar augen gneiss

(Figures 4a and 4b)0462972–8504316 1046 ± 4

CHP2c Post-tectonic syenite undeformed syenite that intrudesCHP2a (Figures 4a and 4c)

0462972–8504316 1050 ± 7

CHP3 Lunkwakwa Syenite moderately deformedcoarse-grained syenite

0455181–8504376 543 ± 6

CHP4a Madzimoyo Granulite hypersthene-bearing granulite from mainMadzimoyo quarry

0445403–8483854 1076 ± 6

CHP4b Madzimoyo xenolith garnet-hypersthene-bearingmafic layer mainMadzimoyo quarry

0445403–8483854 1977±11

CHP5 Madzimoyo Granulite hypersthene-bearing granulite from roadsideMadzimoyo quarry

0444751–8484886 1047 ± 20

CHP6a Leopard Granite coarse-grained hornblende-biotiteequigranular granite(Figure 4e)

0475157–8482902 1038 ± 6

CHP8 Porphyritic granite coarse-grained K-feldsparporphyritic granite

0415934–8469084 1061 ± 13

CHP10 Knob Hill Granite coarse-grained K-feldsparporphyritic granite (Figure 4f)

0414197–8462182 1076 ± 14

CHP11a Mpangwe HillsMigmatite

strongly foliated/bandedqtz-feld migmatites.Leucocratic portion (Figure 4g)

0409876–8456182 1950 ± 67

CHP12 Porphyritic Granite K-feldspar porphyritic granite 0457726–8475452 1038 ± 9CHP13 Porphyritic Granite K-feldspar porphyritic granite 0476703–8505790 1058 ± 34

Petauke-Sinda TerranePS17 Sinda Granite magmatically banded k-spar

porphyritic granite(Figure 3b)

0361349–8427596 479 ± 9

PS18 Equigranular Granite coarse-grained bt-poorqtz-plag granite

0359904–8444482 510 ± 6

PS19 Lusandwa Syenite fine-grained magmaticallybanded syenite

0349640–8453272 494 ± 5

PS28 Pule Syenite medium-grained, equigranularqtz-plag syenite

0361964–8435682 495 ± 10

PS65 Katanga ResourcesGranite

undeformed equigranular coarsegrained qtz-Kfs-bt granite

0317310–8467094 1043 ± 14

PS71b Porphyritic granite undeformed K-sparporphyritic granite

0325327–8460298 720 ± 12

PS73 Petauke Granite 20–30 cm patch of equigranulargranite included withincoarse pegmatites

0315051–8424740 504 ± 7

PS76 Equigranular Granite equigranular medium grainedqtz-plag-bt granite

0311030–8405486 474 ± 8

PS78 Hofmeir Gneiss strongly deformedquartzofelspathic gneiss

0266944–8379082 742 ± 13 536 ± 10

SZ16 Migmatite garnet-bearing peliticmigmatite (Figure 3c)

0366870–8450938 1984 ± 21 1942 ± 5

SZ23 Mtanzi Gneiss strongly deformed qtz-feldgneiss with thin amphibolitelayers (Figure 3d)

0377025–8446308 1008 ± 17

SZ25c UltramylonitisedGranite

progressively mylonitised porphyriticgranite (Figures 3f–3h)

0376035–8449088 1023 ± 12

SZ26 Nyamadzi Gneiss strongly deformedquartzofeldspathicgneiss (Figure 3e)

0372106–8451452 1961 ± 31 1065 ± 13

SZ27 Wutepo Gneiss strongly deformedhornblende-biotite gneiss

0366530–8438568 647 ± 11 555 ± 11

Nyimba TerraneNY82 Great East Road Migmatite mildly deformed stromatic

pelitic migmatite0260459–8373383 2000–1800 1057 ± 5


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(30%) blades that define the strong foliation. A dominantlineation is defined by the long axes of the quartz-feldsparaggregates.3.2.3. Samples PS65 and 71b[15] Many samples were collected from the Sasare Vol-

canic region including numerous examples of the andesiticvolcanics; however, the andesitic and low silica rocks didnot contain zircon, thus a direct age for the volcanic activitycould not be obtained. However, we did manage to separatezircon from a coarse-grained (1–5 mm) equigranular, por-phyritic K-feldspar-dominant (�70%), biotite (20%) andquartz (10%) bearing granitoid (PS65), which is interpretedto form the basement to the thin volcano-sedimentarysuccession. The granite was collected close to the currentKatanga Resources mining camp (Table 1 and Figure 2).Zircon was also extracted from a strongly deformed, por-phyritic K-feldspar-bearing granite gneiss (PS71b) (Table 1and Figure 2). The gneiss is composed of aligned, 4- to5-mm-long, K-feldspar porphyroclasts (30% of the rockvolume) that are set within a fine-grained (<1 mm) matrixof biotite (60%), quartz (30%), and plagioclase (10%).The biotite is strongly aligned and defines the tectonicfabric, which wraps around the aligned and variablydeformed K-feldspar porphyroclasts.3.2.4. Samples SZ16, 23, 25c, 26, and 27[16] Many traverses were conducted through the Nya-

madzi Shear Zone, but it is generally poorly exposed. Wemanaged to separate zircon from five distinctly differentsamples. Sample SZ16 is a layered garnet-bearing peliticmigmatite (Table 1 and Figures 2 and 3c). This migmatite islayered on the 1–5 cm scale with the leucosome (60%)being composed of quartz (40%), plagioclase (55%) andgarnet (5%) and the melanosome (40%) comprised predom-inantly of biotite (90–95%) with small (0.5 mm) randomlydistributed garnet porphyroblasts. Although the unit isstrongly banded/layered there is little evidence for highshear strains, i.e., the biotite blades are only weakly aligned.Sample SZ23, the Mtanzi Gneiss, is a strongly deformedleucocratic plagioclase (60%), quartz (35%) and biotite(<5%) bearing orthogneiss that contains thin (1–10 cmthick) amphibolite bands (Table 1 and Figures 2 and 3d).The unit carries both a foliation, defined by aligned aggre-gates of plagioclase and quartz and aligned biotite, andshallow south plunging lineation defined also by the quartz-plagioclase aggregates. The amphibolite layers are parallelto the dominant foliation. In one traverse a porphyriticgranitoid with randomly oriented K-feldspar augen up to8 cm in length (Figure 3f) can be traced into the high strainzone where upon the augens are progressively flattened,stretched out and ductily deformed (Figure 3g) until the unitbecomes an ultramylonite (Figure 3h). This ultramylonite isthe predominant rock type in this region and demonstratesthat the regional shear strains are extremely high. SampleSZ25c was collected from an ultramylonite portion of thegneiss (Table 1 and Figure 2). Sample SZ26, the NyamadziGneiss, is an intensely deformed garnetiferous quartz-biotiteorthogneiss (Figure 3e) that was collected from along theNyamadzi River. The rock is comprised predominantly ofbiotite (85%), quartz (10%) and garnet (5%) but contains

<0.5 mm to 1.0 cm, discontinuous bands of quartz (30% ofthe rock volume), which along with aligned biotite in themain unit, define a composite foliation/banding. The rock isintruded by various amphibolite, aplitic and pegmatiticdikes (Table 1 and Figure 2). Sample SZ27, the WutepoGneiss, is fine-grained (<1 mm) cordierite-bearing quartz-ofeldspathic gneiss that contains a mild tectonic foliationdefined by aligned biotite (Table 1 and Figure 2).


[17] A total of twelve samples of differing composition,metamorphic grade and deformation state were selected fordating from this terrane.3.3.1. Samples CHP2a and 2c[18] Samples CHP2a and c were collected from a small

roadside quarry near the Zambia-Malawi border (Table 1and Figure 2). Sample CHP2a is a strongly deformed K-feldspar-phyric augen gneiss (Figures 5a and 5b) into whicha contemporaneous, equigranular mafic-syenitic body wasintruded (Figures 5a and 5c). The augen gneiss (CHP2a) iscomposed of now elongate (1–15 cm) and flattened (5–15 mm) K-feldspar phenocrysts (30%) set within a feldspar,quartz and biotite matrix. The K-feldspar augen have beenductilely deformed, and along with aligned biotite in thematrix, define a composite foliation/banding. This gneiss isintruded by a 10-m-wide, apparently undeformed, maficamphibolite dike (Figure 5a), the core of which contains amedium grained (1–2 mm) equigranular, K-feldspar-rich(80–90%) syenite (CHP2c) (Figure 5c). The boundariesbetween the amphibolite and the syenite are diffuse andthere is evidence for mixing/mingling of these two magmassuggesting that they are contemporaneous. The amphibolitedike has a millimeter-sharp contact with the augen gneissand the alignment of hornblende and biotite in the dikemargins suggest that this contact has been intensely sheared.3.3.2. Sample CHP3[19] The Lunkwakwa Syenite was collected at an army

post some 5 km to the west of the previous samples (Table 1and Figure 2) and is a medium- to coarse-grained (1–5 mm)K-feldspar (50%)–biotite (30%)–hornblende (20%) bear-ing syenite. The biotite and hornblende form elongate clotsand aggregates that define a penetrative fabric.3.3.3. Samples CHP4a, 4b and 5[20] These samples were collected from Madzimoyo

Quarry (Table 1 and Figure 2). Samples CHP4a and b werecollected from the main (western) quarry and are represen-tative of charnockite and a mafic garnet-granulite enclave,respectively (Figure 5d). The charnockite (CHP4a) is alayered gneiss, comprising pink layers of quartz, perthite,microcline, plagioclase, biotite, garnet and hypersthene, anddiscontinuous dark layers composed mainly of biotite,magnetite and plagioclase. The mafic enclave (CHP4b)consists of biotite, garnet and hypersthene, and is in sharpcontact with the gneiss. Sample CHP5 was collected fromthe eastern quarry, located next to the roadside, and is also acharnockite, consisting of a pink and gray layered hyper-sthene bearing K-feldspar gneiss. All samples from bothMadzimoyo Quarries carry an intense planar fabric definedby the alignment of the mafic minerals such as biotite and


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pyroxene and in the felsic units, additionally by clots, trains,and aggregates of quartz-feldspar and garnet.3.3.4. Samples CHP6a, 12, and 13[21] The Leopard Granite (CHP6a) is weakly layered

biotite granite (Figure 5e) collected close to the Zambia-Malawi border (Table 1 and Figure 2) and comprisesplagioclase (50%), quartz (10%) with clots and aggregatesof biotite (40%) that are locally vertically elongated; how-ever, the biotite is randomly oriented within these clotssuggesting that the banding is of magmatic rather thantectonic origin. The main Leopard Granite contains afiner-grained (�1 mm) phase of similar composition thatforms subvertical dikes. The biotite is randomly orientedwithin these aplitic units and the margins with the maincoarse phase are diffuse suggesting a magmatic rather thantectonic origin. Another two biotite granites (CHP12 andCHP13) were sampled some 50 km to the east and north,respectively (Table 1 and Figure 2). Similar to the LeopardGranite, both these units are composed of plagioclase (50%)and quartz (10%) with clots and trains of randomly orientedbiotite (40%).3.3.5. Samples CHP8 and 10[22] Two undeformed porphyritic granites were collected

from two localities. CHP8 was collected close to the contactbetween the Mtetezi Granulite and the surrounding augengneisses (Table 1 and Figure 2). This granite is composed ofa coarsely grained (2–5 mm) quartz (40%) feldspar (50%)and biotite (10%), equigranular matrix containing randomlyoriented, 2- to 5-cm-long, K-feldspar phenocrysts (20%).CHP10 (Table 1 and Figures 2 and 5f), a K-feldsparporphyritic granite, was collected some 10 km to thesoutheast. This granitoid contains up to 60%, 2- to 4-cm-long, randomly oriented, K-feldspar phenocrysts set withina coarse-grained (2–3 mm), equigranular matrix of quartz(40%), plagioclase (50%) and biotite (10%).3.3.6. Sample CHP11a[23] Zircon were also extracted from a leucocratic por-

tion of the Mpangwe Migmatites (CHP11a), which aresituated some 10kms to the south of CHP10 (Table 1 andFigure 2). These migmatites are strongly foliated and layered(Figure 5g), the leucosomes are composed of deformedquartz (40%) and plagioclase (60%) and the melanosomesfrom aligned biotite (70%), plagioclase (10%) and 1–2 mmdiameter garnet porphyroblasts (20%). The leucosomes,which are between 1 cm and 20 cm in thickness, compriseroughly 30% of the rock volume and are parallel to thetectonic fabric defined by the aligned minerals in themelanosome. This migmatite is intruded by numerous10- to 30-cm-thick aplites and pegmatites that truncatethe main gneissic layering.

4. Zircon U-Pb Sensitive High-Resolution Ion

Microprobe Geochronology

[24] Considering the large number of samples analyzed,only a brief summary of the U-Pb data will be presentedhere. Detailed descriptions of the sensitive high-resolutionion microprobe (SHRIMP) analytical methods and condi-tions, zircon morphology, cathodoluminescence (CL) imag-

ing, U, Th and common Pb contents and a detailed analysesof the age regressions are presented in Text S1 in theauxiliary material.1


[25] Sample NY82 (pelitic migmatite) yielded abundantzircon ranging in size from 100 to 300mm and theirmorphology suggests they are of detrital origin. All of thegrains appear to have been derived from igneous protoliths(Figure 6 and Text S1) although some grains contain wide,sector-zoned, low Th/U rims that may be of metamorphicorigin [Rubatto and Gebauer, 2000]. Seventeen analyseswere conducted on 15 grains. The core regions of the zircongrains have a range of variably discordant Archaean andPaleoproterozoic ages. Two Archaean grains define a co-herent set of analyses that correspond to a concordia age of2617 ± 20 Ma (Figure 7 and Table 2) while the remainingArchaean grains have 207Pb/206Pb ages that range betweencirca 2641–2554 Ma. Nine Paleoproterozoic grains looselydefine a regression toward an upper intercept age of 1967 ±13 Ma (Figure 7 and Table 2) suggesting derivation from arelatively homogeneous igneous source. Two low Th/Urims (Figure 7 and Table 2) are dated at circa 1843 ±15 Ma (analyses 10r) and 1057 ± 5 Ma (analyses 7r),suggesting metamorphic events at these times; however,more data are needed to confirm this.

4.2. Petauke-Sinda Terrane

4.2.1. Sample PS17 (Sinda Granite)[26] All the separated zircon grains have typical igneous

features showing at least two stages of magmatic growth(Figure 6 and Text S1). Seven of the eight data points fromall zircon zones yield a coherent 206Pb/238U ratio that allowsthe calculation of a concordia age of 479 ± 9 Ma (meansquare weighted deviation (MSWD) = 0.15) (Figure 7)which we interpret to be the age of crystallization of thegranite.4.2.2. Sample PS18 (Equigranular Granite)[27] Similar to PS17, all the zircon appear to be of

igneous origin with at least two phases of igneous growth(Figure 6 and Text S1). All five analyzed grains define aconcordia age of 510 ± 6 Ma (MSWD = 1.5) (Figure 7)which we interpret to be the age of crystallization of thegranite.4.2.3. Sample PS19 (Lusandwa Syenite)[28] All the zircon are of igneous origin (Figure 6 and

Text S1) with all four analyzed grains defining a concordiaage of 494 ± 5 Ma (MSWD = 0.004) (Figure 7) whichrecords the timing of crystallization of the syenite.4.2.4. Sample PS28 (Pule Syenite)[29] Again all of the zircon have an igneous origin

(Figure 6 and Text S1) with all five analyzed grainsproducing a concordia age of 495 ± 10 Ma (MSWD =5.4) (Figure 7) and nearly identical weighted mean206Pb/238U age of 497 ± 9 Ma (MSWD = 0.3) (Figure 7).

Auxiliary materials are available in the HTML. doi:10.1029/2006TC001977.


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Figure 6. Cathodoluminescence images of the main zircon populations dated in this study. For detaileddescriptions, refer to Text S1.


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Table 2. Zircon U-Pb SHRIMP Data for Igneous Lithologies in the Southern Irumide Belta

Spotf206,%

U,ppm

Th,ppm Th/U

238U/206Pb±1s abs err

207Pb/206Pb±1s abs err

206Pb/238U Age±1s abs err,

Ma

207Pb/206Pb Age±1s abs err,

Ma Sessionb

Sample NY82, Great East Road MigmatiteNY82-1 0.09 879 172 0.20 2.80913 ± 0.03979 0.12029 ± 0.00027 1963 ± 24 1960 ± 4 ANY82-2c 0.15 311 60 0.20 1.98122 ± 0.02846 0.17602 ± 0.00161 2634 ± 31 2616 ± 15 ANY82-3c 0.48 116 24 0.22 2.39052 ± 0.03641 0.17873 ± 0.00155 2253 ± 29 2641 ± 14 ANY82-3r 0.14 387 44 0.12 2.88047 ± 0.04126 0.12005 ± 0.00056 1921 ± 24 1957 ± 8 ANY82-4 0.42 305 47 0.16 3.33007 ± 0.05226 0.11577 ± 0.00077 1693 ± 23 1892 ± 12 ANY82-5c 1.27 73 46 0.66 2.02385 ± 0.03221 0.17659 ± 0.00196 2588 ± 34 2621 ± 18 ANY82-6c 0.36 307 88 0.30 2.84949 ± 0.04100 0.12171 ± 0.00151 1939 ± 24 1981 ± 22 ANY82-7r 0.08 2312 7 0.00 5.47347 ± 0.07846 0.07456 ± 0.00020 1082 ± 14 1057 ± 5 ANY82-8r 0.50 971 28 0.03 3.41366 ± 0.04808 0.11473 ± 0.00082 1656 ± 21 1876 ± 13 ANY82-9 0.13 785 446 0.59 2.78177 ± 0.03919 0.12143 ± 0.00030 1980 ± 24 1977 ± 4 ANY82-10c 0.34 585 332 0.59 3.49422 ± 0.04950 0.11223 ± 0.00048 1622 ± 20 1836 ± 8 ANY82-10r 0.45 280 42 0.16 3.13099 ± 0.04575 0.11266 ± 0.00095 1787 ± 23 1843 ± 15 ANY82-11 0.44 337 51 0.16 3.05409 ± 0.04391 0.11722 ± 0.00071 1826 ± 23 1914 ± 11 ANY82-12c 0.16 726 501 0.71 2.90499 ± 0.04105 0.14075 ± 0.00059 1907 ± 23 2236 ± 7 ANY82-13c 0.15 566 329 0.60 2.16655 ± 0.03121 0.16962 ± 0.00060 2446 ± 29 2554 ± 6 ANY82-14c 0.58 324 151 0.48 2.71963 ± 0.03925 0.15541 ± 0.00097 2019 ± 25 2406 ± 11 ANY82-15c 0.37 460 21 0.05 2.91493 ± 0.04183 0.11819 ± 0.00075 1901 ± 24 1929 ± 11 A

Sample PS17, Sinda GranitePS17-1 0.20 1567 324 0.21 13.37794 ± 0.28040 0.05625 ± 0.00037 465 ± 9 462 ± 14 BPS17-2 0.45 471 1965 4.31 12.48337 ± 0.26551 0.05592 ± 0.00096 497 ± 10 449 ± 38 BPS17-3 0.15 841 139 0.17 12.75565 ± 0.26864 0.05777 ± 0.00054 487 ± 10 521 ± 20 BPS17-4 0.59 1123 1874 1.72 13.08852 ± 0.28368 0.05350 ± 0.00303 475 ± 10 350 ± 128 BPS17-5 0.14 1103 403 0.38 12.74870 ± 0.26781 0.05697 ± 0.00046 487 ± 10 490 ± 18 BPS17-6 0.16 796 353 0.46 12.92114 ± 0.27458 0.05687 ± 0.00053 481 ± 10 487 ± 20 BPS17-7 0.35 693 315 0.47 13.43433 ± 0.28369 0.05688 ± 0.00076 463 ± 9 487 ± 29 BPS17-8 1.18 809 188 0.24 16.60886 ± 0.35121 0.05592 ± 0.00144 377 ± 8 449 ± 57 B

Sample PS18, Equigranular granitePS18-1 0.23 693 104 0.15 12.29449 ± 0.25951 0.05804 ± 0.00067 504 ± 10 531 ± 25 BPS18-2 3.54 376 88 0.24 12.21558 ± 0.26356 0.05589 ± 0.00296 507 ± 11 448 ± 118 BPS18-3 0.37 485 70 0.15 12.16350 ± 0.25804 0.05786 ± 0.00086 509 ± 10 525 ± 33 BPS18-4 0.39 333 46 0.14 12.19019 ± 0.26089 0.05871 ± 0.00112 508 ± 10 556 ± 42 BPS18-5 1.06 591 79 0.14 12.10049 ± 0.26025 0.05759 ± 0.00121 512 ± 11 514 ± 46 B

Sample PS19, Lusandwa SyenitePS19-1 0.68 322 785 2.52 12.64732 ± 0.27139 0.05717 ± 0.00148 491 ± 10 498 ± 57 BPS19-2 3.22 153 374 2.51 12.30708 ± 0.27748 0.05771 ± 0.00423 504 ± 11 519 ± 161 BPS19-3 1.80 129 288 2.31 12.54978 ± 0.28430 0.05794 ± 0.00364 494 ± 11 528 ± 138 BPS19-4 1.16 196 410 2.16 12.67732 ± 0.27891 0.05638 ± 0.00273 489 ± 10 467 ± 107 B

Sample PS28, Pule SyenitePS28-1 0.37 280 116 0.43 12.57452 ± 0.26999 0.05585 ± 0.00113 493 ± 10 446 ± 45 BPS28-2 0.64 256 714 2.88 12.51635 ± 0.26958 0.05551 ± 0.00145 495 ± 10 433 ± 58 BPS28-3 0.69 267 636 2.46 12.60268 ± 0.28304 0.05502 ± 0.00134 492 ± 11 413 ± 54 BPS28-4 0.51 169 322 1.96 12.22875 ± 0.26772 0.05714 ± 0.00181 507 ± 11 497 ± 70 BPS28-5 0.82 157 303 1.99 12.43660 ± 0.27324 0.05424 ± 0.00214 499 ± 11 381 ± 89 B

Sample PS71, Porphyritic GranitePS71-1 0.03 812 538 0.69 8.61313 ± 0.18096 0.06399 ± 0.00032 708 ± 14 741 ± 11 BPS71-2 0.18 436 180 0.43 8.48575 ± 0.17958 0.06208 ± 0.00067 718 ± 14 677 ± 23 BPS71-3 0.15 831 404 0.50 8.50424 ± 0.17867 0.06319 ± 0.00042 717 ± 14 714 ± 14 BPS71-4 0.14 376 179 0.49 8.54418 ± 0.18413 0.06337 ± 0.00065 713 ± 15 721 ± 22 BPS71-5 0.03 682 437 0.66 8.57380 ± 0.18042 0.06402 ± 0.00051 711 ± 14 742 ± 17 B

Sample PS73, Petauke GranitePS73-1 0.30 725 588 0.84 12.26651 ± 0.25881 0.05691 ± 0.00071 505 ± 10 488 ± 28 BPS73-2 0.34 498 339 0.70 12.05191 ± 0.25555 0.05723 ± 0.00083 514 ± 10 500 ± 32 BPS73-3 0.44 542 382 0.73 12.29010 ± 0.26035 0.05577 ± 0.00093 504 ± 10 443 ± 37 BPS73-4 0.34 595 424 0.74 12.38140 ± 0.26194 0.05667 ± 0.00083 501 ± 10 479 ± 32 BPS73-5 0.52 605 446 0.76 12.28649 ± 0.25975 0.05614 ± 0.00079 504 ± 10 458 ± 31 BPS73-6 0.88 648 405 0.65 12.13561 ± 0.25750 0.05686 ± 0.00226 510 ± 10 486 ± 88 B


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Table 2. (continued)

Spotf206,%

U,ppm

Th,ppm Th/U




Ma


Ma Sessionb

Sample PS76, Equigranular GranitePS76-1 0.21 1154 833 0.75 13.36074 ± 0.28053 0.05708 ± 0.00048 465 ± 9 495 ± 19 BPS76-2 0.16 2104 1732 0.85 13.19503 ± 0.27616 0.05647 ± 0.00032 471 ± 10 471 ± 13 BPS76-3c 0.03 458 819 1.85 12.93000 ± 0.27437 0.05760 ± 0.00055 480 ± 10 515 ± 21 BPS76-3r 0.57 2581 465 0.19 12.69064 ± 0.26547 0.05612 ± 0.00052 489 ± 10 457 ± 20 BPS76-4 0.24 907 1221 1.39 13.19795 ± 0.27782 0.05570 ± 0.00058 471 ± 10 441 ± 23 BPS76-5 0.20 1366 1432 1.08 13.21481 ± 0.27718 0.05625 ± 0.00039 470 ± 10 462 ± 15 B

Sample PS78, Hofmeier GneissPS78-1 2.26 89 88 1.02 8.17223 ± 0.13725 0.06792 ± 0.00378 744 ± 12 866 ± 116 APS78-2 1.44 92 98 1.09 8.06114 ± 0.13416 0.07030 ± 0.00331 754 ± 12 937 ± 97 APS78-3c 0.80 341 465 1.41 8.69693 ± 0.12676 0.06458 ± 0.00122 702 ± 10 761 ± 40 APS78-3r 0.61 1486 212 0.15 11.58243 ± 0.16345 0.05861 ± 0.00062 534 ± 7 553 ± 23 APS78-4 2.22 78 97 1.29 8.16691 ± 0.14062 0.06491 ± 0.00386 745 ± 12 771 ± 125 APS78-5 2.44 96 101 1.08 8.48894 ± 0.14117 0.05982 ± 0.00381 718 ± 11 597 ± 138 APS78-6r 0.27 890 26 0.03 11.44237 ± 0.16585 0.05748 ± 0.00061 540 ± 8 510 ± 23 APS78-7 1.51 208 147 0.73 9.03891 ± 0.14086 0.06251 ± 0.00236 676 ± 10 692 ± 81 APS78-8 2.62 95 128 1.39 8.16411 ± 0.13769 0.06546 ± 0.00378 745 ± 12 789 ± 121 A

Sample SAS65, Katanga Resources GraniteSAS65-1 0.46 138 133 0.99 5.65515 ± 0.12303 0.07172 ± 0.00139 1050 ± 21 978 ± 40 BSAS65-2 0.37 122 118 1.00 5.58655 ± 0.12231 0.07348 ± 0.00182 1062 ± 21 1027 ± 50 BSAS65-3 0.51 131 125 0.98 5.63697 ± 0.12291 0.07281 ± 0.00153 1053 ± 21 1009 ± 43 BSAS65-4 0.50 121 111 0.94 5.61329 ± 0.12258 0.07364 ± 0.00140 1057 ± 21 1032 ± 38 BSAS65-5 0.52 175 170 1.00 5.66752 ± 0.12219 0.07225 ± 0.00120 1048 ± 21 993 ± 34 BSAS65-6 0.54 126 131 1.08 5.66633 ± 0.12373 0.07404 ± 0.00166 1048 ± 21 1043 ± 45 B

Sample SZ16, MigmatiteSZ16-1r 0.05 759 5 0.01 2.86292 ± 0.04052 0.11906 ± 0.00030 1931 ± 24 1942 ± 5 ASZ16-1c 0.55 421 116 0.29 3.00995 ± 0.04995 0.12190 ± 0.00146 1849 ± 27 1984 ± 21 A

Sample SZ23, Mtanzi GneissSZ23-1 0.78 100 85 0.88 6.09792 ± 0.13494 0.07048 ± 0.00197 979 ± 20 942 ± 57 BSZ23-2 0.81 137 141 1.06 5.78369 ± 0.12592 0.07045 ± 0.00143 1028 ± 21 941 ± 42 BSZ23-3 2.92 22 20 0.93 5.98998 ± 0.16625 0.07020 ± 0.00861 995 ± 26 934 ± 252 BSZ23-4 1.20 51 35 0.71 5.72577 ± 0.13430 0.07451 ± 0.00368 1038 ± 22 1055 ± 99 BSZ23-5 0.83 73 51 0.72 5.78492 ± 0.13101 0.07173 ± 0.00240 1028 ± 22 978 ± 68 BSZ23-6 0.23 279 83 0.31 6.49583 ± 0.13850 0.07141 ± 0.00074 923 ± 18 969 ± 21 BSZ23-7 0.30 179 100 0.58 5.90109 ± 0.12722 0.07317 ± 0.00124 1009 ± 20 1019 ± 34 B

Sample SZ25c, Ultramylonitised GraniteSZ25c-1 1.83 116 72 0.64 5.85668 ± 0.09472 0.07041 ± 0.00272 1016 ± 15 940 ± 79 ASZ25c-2 1.31 74 47 0.65 5.74373 ± 0.09561 0.07580 ± 0.00267 1035 ± 16 1090 ± 70 ASZ25c-3 1.00 184 137 0.77 5.87793 ± 0.08969 0.07181 ± 0.00155 1013 ± 14 981 ± 44 ASZ25c-4 1.34 149 117 0.81 5.71760 ± 0.08742 0.07043 ± 0.00182 1039 ± 15 941 ± 53 ASZ25c-5 1.07 144 87 0.63 5.93447 ± 0.09095 0.07429 ± 0.00194 1004 ± 14 1049 ± 53 ASZ25c-6 1.69 103 73 0.74 5.73041 ± 0.09105 0.07468 ± 0.00242 1037 ± 15 1060 ± 65 ASZ25c-7 1.62 110 77 0.72 5.97986 ± 0.09667 0.07555 ± 0.00263 997 ± 15 1083 ± 70 ASZ25c-8 2.13 71 55 0.80 5.68746 ± 0.09609 0.07744 ± 0.00346 1044 ± 16 1132 ± 89 A

Sample SZ26, Nyamadzi GneissSZ26-1r 0.06 454 55 0.13 2.82460 ± 0.04082 0.12547 ± 0.00249 1954 ± 24 2035 ± 35 ASZ26-1c 0.14 191 111 0.60 3.36871 ± 0.04959 0.11219 ± 0.00121 1676 ± 22 1835 ± 20 ASZ26-2c 0.17 197 120 0.63 2.67845 ± 0.04033 0.12100 ± 0.00109 2045 ± 26 1971 ± 16 ASZ26-2r 0.16 484 55 0.12 5.51770 ± 0.07984 0.07488 ± 0.00049 1074 ± 14 1065 ± 13 ASZ26-3c 0.28 143 67 0.49 2.83201 ± 0.04248 0.12063 ± 0.00101 1949 ± 25 1966 ± 15 ASZ26-3r 0.06 983 39 0.04 3.16871 ± 0.05079 0.11378 ± 0.00049 1768 ± 25 1861 ± 8 ASZ26-4c 0.21 654 311 0.49 2.92758 ± 0.04278 0.14649 ± 0.00070 1894 ± 24 2305 ± 8 ASZ26-4r 0.08 990 30 0.03 2.85876 ± 0.04018 0.12011 ± 0.00040 1934 ± 23 1958 ± 6 ASZ26-5r 0.04 1373 28 0.02 2.77998 ± 0.04106 0.11977 ± 0.00069 1981 ± 25 1953 ± 10 ASZ26-6r 0.25 971 48 0.05 3.20401 ± 0.04527 0.11753 ± 0.00070 1751 ± 22 1919 ± 11 ASZ26-7r 0.04 1126 39 0.04 2.67039 ± 0.03751 0.12466 ± 0.00130 2050 ± 25 2024 ± 18 ASZ26-8r 0.05 1145 45 0.04 3.14896 ± 0.04444 0.11520 ± 0.00043 1778 ± 22 1883 ± 7 ASZ26-9r 0.04 1108 26 0.02 2.76028 ± 0.04026 0.11823 ± 0.00115 1993 ± 25 1930 ± 17 ASZ26-9c 0.14 439 220 0.52 3.16108 ± 0.04565 0.11499 ± 0.00047 1772 ± 22 1880 ± 7 ASZ26-10c 0.29 182 94 0.53 2.32215 ± 0.03431 0.13749 ± 0.00098 2309 ± 29 2196 ± 12 ASZ26-11r 0.20 781 119 0.16 4.43906 ± 0.06249 0.09510 ± 0.00057 1310 ± 17 1530 ± 11 ASZ26-12r 0.25 408 55 0.14 3.43153 ± 0.05160 0.11321 ± 0.00095 1649 ± 22 1852 ± 15 A


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Spotf206,%

U,ppm

Th,ppm Th/U




Ma


Ma Sessionb

Sample SZ27, Wutepo GneissSZ27-1 0.49 264 21 0.08 11.02998 ± 0.24220 0.06169 ± 0.00143 559 ± 12 664 ± 50 BSZ27-2 3.21 204 3 0.01 10.96673 ± 0.24237 0.05666 ± 0.00348 563 ± 12 478 ± 136 BSZ27-3c 0.97 166 16 0.10 11.30201 ± 0.24808 0.06029 ± 0.00230 547 ± 12 614 ± 82 BSZ27-3r 67.23 17 3 0.21 28.03961 ± 6.34912 0.00000 ± 0.00000 226 ± 50SZ27-4 1.10 177 5 0.03 11.00594 ± 0.24074 0.05807 ± 0.00215 561 ± 12 532 ± 81 BSZ27-5 1.19 172 14 0.08 11.36277 ± 0.24902 0.05994 ± 0.00214 544 ± 11 601 ± 77 BSZ27-6 0.91 172 66 0.39 9.28909 ± 0.21886 0.06239 ± 0.00202 659 ± 15 688 ± 69 BSZ27-7 0.87 152 68 0.46 9.43115 ± 0.20651 0.06204 ± 0.00202 650 ± 14 676 ± 69 BSZ27-8 1.43 127 51 0.42 9.60279 ± 0.21274 0.05810 ± 0.00235 639 ± 13 534 ± 89 BSZ27-9 0.62 226 97 0.44 9.43086 ± 0.20305 0.06188 ± 0.00113 650 ± 13 670 ± 39 BSZ27-10 0.67 237 121 0.53 9.28105 ± 0.20564 0.06004 ± 0.00122 660 ± 14 605 ± 44 BSZ27-11 0.83 209 122 0.60 9.76311 ± 0.21093 0.05878 ± 0.00154 629 ± 13 559 ± 57 BSZ27-12 5.09 112 62 0.57 9.81357 ± 0.22678 0.05774 ± 0.00559 626 ± 14 520 ± 213 BSZ27-13 0.71 251 128 0.53 9.02581 ± 0.19394 0.06097 ± 0.00139 677 ± 14 638 ± 49 B

Sample CHP2a, Augen GneissCHP2a-1r 0.65 101 49 0.50 5.94510 ± 0.05878 0.07038 ± 0.00223 1002 ± 9 940 ± 65 CCHP2a-2c 0.05 467 247 0.55 5.63800 ± 0.02467 0.07511 ± 0.00053 1053 ± 4 1072 ± 14 CCHP2a-3c 0.09 607 289 0.49 6.01863 ± 0.02334 0.07358 ± 0.00049 991 ± 4 1030 ± 13 CCHP2a-4c 0.23 286 186 0.67 5.67629 ± 0.03393 0.07488 ± 0.00103 1046 ± 6 1065 ± 28 CCHP2a-4r 0.06 1019 386 0.39 5.50870 ± 0.01669 0.07463 ± 0.00033 1075 ± 3 1058 ± 9 CCHP2a-5c 0.12 512 300 0.61 5.72143 ± 0.02307 0.07452 ± 0.00058 1038 ± 4 1056 ± 16 CCHP2a-5r 0.41 251 68 0.28 6.64257 ± 0.04135 0.07467 ± 0.00123 904 ± 5 1060 ± 33 CCHP2a-6c 0.28 196 125 0.66 5.93568 ± 0.04032 0.07173 ± 0.00107 1004 ± 6 978 ± 30 CCHP2a-7c 0.45 426 190 0.46 5.65583 ± 0.02580 0.07447 ± 0.00083 1050 ± 4 1054 ± 22 CCHP2a-8c 0.03 718 352 0.51 5.68184 ± 0.01990 0.07379 ± 0.00046 1045 ± 3 1036 ± 13 CCHP2a-9c 0.10 269 138 0.53 5.69540 ± 0.03260 0.07409 ± 0.00076 1043 ± 6 1044 ± 21 C

Sample CHP2c, SyeniteCHP2c-1 0.09 316 175 0.57 6.52125 ± 0.03509 0.07538 ± 0.00095 920 ± 5 1079 ± 25 CCHP2c-2 0.07 296 151 0.53 5.73510 ± 0.03114 0.07421 ± 0.00084 1036 ± 5 1047 ± 23 CCHP2c-3 0.04 959 400 0.43 5.57514 ± 0.01688 0.07440 ± 0.00044 1064 ± 3 1052 ± 12 CCHP2c-4 0.35 610 619 1.05 5.85979 ± 0.02158 0.07481 ± 0.00065 1016 ± 3 1063 ± 18 CCHP2c-5 0.00 8128 1586 0.20 4.80300 ± 0.00557 0.07436 ± 0.00010 1219 ± 1 1051 ± 3 CCHP2c-6 0.06 714 322 0.47 5.68741 ± 0.01965 0.07409 ± 0.00040 1044 ± 3 1044 ± 11 CCHP2c-7 0.67 1621 512 0.33 8.84682 ± 0.02240 0.07388 ± 0.00063 690 ± 2 1038 ± 17 C

Sample CHP3, Lunkwakwa SyeniteCHP3-1 0.59 292 612 2.16 11.41396 ± 0.07299 0.05727 ± 0.00148 541 ± 3 502 ± 57 CCHP3-2 0.48 542 1509 2.88 11.33271 ± 0.05327 0.05760 ± 0.00105 545 ± 2 515 ± 40 CCHP3-3 1.32 338 627 1.92 11.31699 ± 0.06906 0.05525 ± 0.00192 546 ± 3 422 ± 77 CCHP3-4 0.00 258 464 1.86 11.58433 ± 0.07832 0.06064 ± 0.00076 534 ± 3 626 ± 27 C

Sample CHP4a, Garnet-Hypersthene Bearing Madzimoyo Charnockitic Gneiss (Western Part of the Quarry)CHP4a-1 0.10 176 51 0.30 2.87299 ± 0.02872 0.11954 ± 0.00080 1925 ± 17 1949 ± 12 ECHP4a-2 0.05 716 683 0.99 5.52031 ± 0.04757 0.07531 ± 0.00036 1073 ± 9 1077 ± 10 ECHP4a-3 0.01 720 638 0.92 5.46281 ± 0.04726 0.07464 ± 0.00045 1084 ± 9 1059 ± 12 ECHP4a-4 0.01 866 646 0.77 5.51598 ± 0.06819 0.07561 ± 0.00032 1074 ± 12 1085 ± 8 ECHP4a-5 0.10 704 606 0.89 5.51087 ± 0.06610 0.07482 ± 0.00041 1075 ± 12 1064 ± 11 ECHP4a-6 0.03 643 522 0.84 5.43111 ± 0.06540 0.07510 ± 0.00038 1089 ± 12 1071 ± 10 E

Sample CHP4b, Garnet-Hypersthene Bearing Xenolith in the Madzimoyo Charnockitic Gneiss (Western Part of the Quarry)CHP4b-1 0.07 624 83 0.14 2.75591 ± 0.02562 0.12121 ± 0.00050 1996 ± 16 1974 ± 7 DCHP4b-2 0.47 266 91 0.35 2.81582 ± 0.02818 0.12157 ± 0.00121 1959 ± 17 1979 ± 18 DCHP4b-3 0.09 149 142 0.99 2.99997 ± 0.04989 0.11902 ± 0.00072 1855 ± 27 1942 ± 11 FCHP4b-4 0.14 136 140 1.06 2.68487 ± 0.04723 0.11738 ± 0.00088 2041 ± 31 1917 ± 13 FCHP4b-5 0.37 102 75 0.76 3.57143 ± 0.06408 0.11091 ± 0.00275 1591 ± 25 1814 ± 45 FCHP4b-6 0.10 98 85 0.89 3.18123 ± 0.04504 0.11961 ± 0.00272 1762 ± 22 1950 ± 41 ECHP4b-7 0.10 390 169 0.45 3.09520 ± 0.03626 0.11800 ± 0.00121 1805 ± 18 1926 ± 18 ECHP4b-8 0.32 47 42 0.94 2.77943 ± 0.04151 0.12246 ± 0.00152 1981 ± 25 1992 ± 22 E

Sample CHP5, Garnet-Hypersthene Bearing Madzimoyo Charnockitic Gneiss (Eastern Part of the Quarry)CHP5-1 3.92 113 82 0.75 4.22235 ± 0.05680 0.11085 ± 0.00632 1370 ± 17 1813 ± 104 DCHP5-2 0.15 2512 16 0.01 5.31119 ± 0.06420 0.07858 ± 0.00084 1112 ± 12 1162 ± 21 ECHP5-3 0.29 471 14 0.03 5.62634 ± 0.06859 0.07422 ± 0.00074 1055 ± 12 1047 ± 20 ECHP5-4 1.71 1252 11 0.01 5.94662 ± 0.07694 0.08320 ± 0.00271 1002 ± 12 1274 ± 63 E


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Spotf206,%

U,ppm

Th,ppm Th/U




Ma


Ma Sessionb

Sample CHP6a, Leopard GraniteCHP6a-1 0.14 188 60 0.33 5.65731 ± 0.03823 0.07312 ± 0.00086 1049 ± 7 1017 ± 24 CCHP6a-2 2.07 218 68 0.32 5.79624 ± 0.03882 0.07101 ± 0.00230 1026 ± 6 958 ± 66 CCHP6a-3 0.36 307 80 0.27 5.75279 ± 0.03059 0.07405 ± 0.00091 1033 ± 5 1043 ± 25 CCHP6a-4 1.71 39 11 0.29 5.86178 ± 0.09221 0.06095 ± 0.00505 1015 ± 15 638 ± 178 CCHP6a-5 0.17 135 40 0.30 5.61474 ± 0.04564 0.07372 ± 0.00110 1057 ± 8 1034 ± 30 CCHP6a-6 0.07 415 70 0.17 5.73027 ± 0.02618 0.07399 ± 0.00058 1037 ± 4 1041 ± 16 CCHP6a-7 0.37 182 51 0.29 5.70533 ± 0.04103 0.07236 ± 0.00122 1041 ± 7 996 ± 34 C

Sample CHP8, Porphyritic GraniteCHP8-1 0.08 418 29 0.07 5.58420 ± 0.02526 0.07461 ± 0.00056 1062 ± 4 1058 ± 15 CCHP8-2 0.63 97 93 0.99 5.53285 ± 0.05276 0.07212 ± 0.00217 1071 ± 9 989 ± 61 CCHP8-3 0.08 435 280 0.67 5.65617 ± 0.02602 0.07538 ± 0.00054 1049 ± 4 1079 ± 14 CCHP8-4 0.06 414 209 0.52 2.84347 ± 0.01288 0.12100 ± 0.00065 1943 ± 8 1971 ± 10 CCHP8-5 0.14 242 93 0.40 5.41859 ± 0.03187 0.07524 ± 0.00086 1092 ± 6 1075 ± 23 CCHP8-6 0.03 303 51 0.17 5.10600 ± 0.02681 0.08004 ± 0.00228 1153 ± 6 1198 ± 56 CCHP8-7 0.16 186 170 0.94 5.82391 ± 0.03931 0.07397 ± 0.00111 1022 ± 6 1041 ± 30 CCHP8-8 0.04 852 65 0.08 5.48800 ± 0.01734 0.07457 ± 0.00034 1079 ± 3 1057 ± 9 C

Sample CHP10, Knob Hill GraniteCHP10-1c 0.87 698 139 0.21 5.68144 ± 0.02045 0.07507 ± 0.00091 1045 ± 3 1070 ± 24 CCHP10-1r 1.58 614 312 0.53 6.88553 ± 0.02785 0.07601 ± 0.00160 874 ± 3 1095 ± 42 CCHP10-2 0.20 796 506 0.66 5.57216 ± 0.01886 0.07554 ± 0.00051 1064 ± 3 1083 ± 14 CCHP10-3c 0.04 394 101 0.27 2.98275 ± 0.01321 0.11763 ± 0.00049 1864 ± 7 1921 ± 8 CCHP10-3r 0.22 814 144 0.18 3.12377 ± 0.00938 0.11725 ± 0.00042 1790 ± 5 1915 ± 6 CCHP10-4r 0.09 1122 124 0.11 5.64714 ± 0.01586 0.07545 ± 0.00038 1051 ± 3 1080 ± 10 CCHP10-5c 1.56 335 354 1.09 6.06737 ± 0.03337 0.07300 ± 0.00169 983 ± 5 1014 ± 47 CCHP10-5r 0.50 880 152 0.18 5.75692 ± 0.01978 0.07464 ± 0.00071 1032 ± 3 1059 ± 19 CCHP10-6c 0.40 74 108 1.50 5.74155 ± 0.06529 0.07478 ± 0.00191 1035 ± 11 1063 ± 51 C

Sample CHP11a, Mpangwe Hills MigmatiteCHP11a-1c 0.41 439 185 0.44 3.45813 ± 0.01405 0.11022 ± 0.00073 1637 ± 6 1803 ± 12 CCHP11a-2c 0.93 336 221 0.68 4.11464 ± 0.02076 0.10491 ± 0.00127 1402 ± 6 1713 ± 22 CCHP11a-3 0.24 489 575 1.21 3.26659 ± 0.01392 0.11449 ± 0.00057 1722 ± 6 1872 ± 9 CCHP11a-4 0.69 577 1080 1.93 3.67007 ± 0.01381 0.10782 ± 0.00086 1553 ± 5 1763 ± 15 CCHP11a-5 1.16 189 264 1.44 4.54430 ± 0.03142 0.09919 ± 0.00206 1282 ± 8 1609 ± 39 CCHP11a-6 0.82 227 569 2.58 3.25955 ± 0.01885 0.12077 ± 0.00117 1725 ± 9 1968 ± 17 CCHP11a-7 0.18 145 168 1.19 2.97474 ± 0.02444 0.11916 ± 0.00106 1868 ± 13 1944 ± 16 C

Sample CHP12, Porphyritic GraniteCHP12-1 0.13 491 243 0.51 5.76386 ± 0.05101 0.07326 ± 0.00053 1031 ± 8 1021 ± 15 FCHP12-2 0.54 392 92 0.24 5.77357 ± 0.05277 0.07289 ± 0.00093 1030 ± 9 1011 ± 26 FCHP12-1c 0.17 604 472 0.81 5.84953 ± 0.06338 0.07383 ± 0.00051 1017 ± 10 1037 ± 14 FCHP12-3 0.41 212 192 0.94 5.73054 ± 0.06051 0.07013 ± 0.00114 1037 ± 10 932 ± 33 FCHP12-10 0.08 363 49 0.14 5.47830 ± 0.08935 0.07398 ± 0.00057 1081 ± 16 1041 ± 16 FCHP12-11 0.09 221 88 0.41 5.55793 ± 0.09191 0.07375 ± 0.00095 1067 ± 16 1035 ± 26 FCHP12-12 0.02 298 224 0.78 5.61190 ± 0.09109 0.07444 ± 0.00053 1057 ± 16 1053 ± 14 FCHP12-13 0.04 507 269 0.55 5.67781 ± 0.09052 0.07406 ± 0.00037 1046 ± 15 1043 ± 10 FCHP12-14 0.13 311 235 0.78 5.65099 ± 0.09281 0.07353 ± 0.00065 1050 ± 16 1029 ± 18 F

Sample CHP13, Porphyritic GraniteCHP13-1 0.04 558 291 0.54 6.66557 ± 0.07392 0.07523 ± 0.00048 901 ± 9 1075 ± 13 GCHP13-2 - 361 348 1.00 5.70462 ± 0.06647 0.07437 ± 0.00055 1041 ± 11 1051 ± 15 GCHP13-3 0.52 179 94 0.54 5.61019 ± 0.07444 0.09188 ± 0.00205 1058 ± 13 1466 ± 42 GCHP13-4 0.03 999 334 0.35 6.24191 ± 0.10061 0.07433 ± 0.00038 958 ± 14 1050 ± 10 G

aThe parameter f206 is the proportion of common 206Pb in the total 206Pb; Th/U is 232Th/238U; all ratios and ages are corrected for common Pb usingmeasured 204Pb and composition appropriate to the age of the zircon [Stacey and Kramers, 1975].

bAnalyses were conducted during six different sessions as follows: A, 16 TEM2 standard analyses yielded a 2s error of the mean of 0.86%; B, 18 TEM2standard analyses yielded a 2s error of the mean of 1.07%; C, 15 TEM2 standard analyses yielded a 2s error of the mean of 0.52%; D, 17 CZ3 standardanalyses yielded a 2s error of the mean of 0.51%; E, 14 CZ3 standard analyses yielded a 2s error of the mean of 0.61%; F, 19 CZ3 standard analysesyielded a 2s error of the mean of 0.78%; and G, 11 CZ3 standard analyses yielded a 2s error of the mean of 0.79%.


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Figure 7. Tera-Wasserburg U-Pb Concordia diagrams for the Nyimba and Petauke-Sinda Terranesamples. A detailed description of zircon morphology and data regression can be found in Text S1.


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The lower MSWD on the weighted mean age attests to thecoherence of the data and we interpret this age to representthe crystallization age of the granitoid.4.2.5. Sample PS65 (Katanga Resources Granite)[30] All six analyzed grains have igneous features

(Figure 6 and Text S1) and define a tight cluster onconcordia (Figure 7) corresponding to a concordia age of1043 ± 14 Ma (MSWD = 5.2) which is the best estimatefor the timing of crystallization of the granitoid.4.2.6. Sample PS71b (Porphyritic Granite)[31] Again all five analyzed zircon grains are consistent

with crystallization from an igneous protolith (Figure 6 andText S1) at 720 ± 12 Ma (concordia age with an MSWD =2.4, Figure 7).4.2.7. Sample PS73 (Petauke Granite)[32] Six igneous (Figure 6 and Text S1) zircons were

analyzed and provided a concordant age of 504 ± 7 Ma(MSWD = 4.5) (Figure 8).4.2.8. Sample PS76 (Equigranular Granite)[33] Most zircon grains have typical igneous character-

istics but some appear to be sector zoned (Figure 6 andText S1) while other grains are surrounded by unzoned rims.Six analyses were conducted on five grains, including onecore-rim pair; however, the core and rim analyses show noage difference, with all the data defining a tight concordantcluster with a concordia age of 474 ± 8 Ma (MSWD = 0.02,Figure 8) which we take to be the crystallization age of thegranitoid.4.2.9. Sample PS78 (Hofmeir Gneiss)[34] Most zircon grains are complex comprising an

igneous core surrounded by a dark CL rim (Figure 6 andText S1). Nine analyses were conducted on eight zircon,including one core-rim pair and one analysis on a rim. Theigneous cores have a concordia age of 742 ± 13 Ma(MSWD = 1.4, Figure 8) whereas the two rims define ayounger age group with concordia age of 536 ± 10 Ma(MSWD = 0.112). We interpret the age of 742 ± 13 Ma asthe crystallization age of the precursor granite to theHofmeir Gneiss, while the two younger rims of 536 ±10 Ma document the thermal/metamorphic event respon-sible for the formation of the gneiss.

4.3. Nyamadzi Shear Zone

4.3.1. Sample SZ16 (Pelitic Migmatite)[35] Very few zircon were recovered from the migmatitic

paragneiss. One zircon was analyzed, which consists of asmall igneous core, surrounded by a thin, dark CL, low Th/Urim (Figure 6 and Text S1 and Table 2). The core yieldeda slightly discordant 207Pb/206Pb age of 1984 ± 21 Ma(Figure 8) and the low Th/U rim, possibly of metamorphicorigin, yielded a concordant 207Pb/206Pb age of 1942 ± 5Ma.With only two analyses on a single zircon available for thissample, it is not reasonable to assign a geological context tothese ages.4.3.2. Sample SZ23 (Mtanzi Gneiss)[36] All zircon in this sample has igneous features

(Figure 6 and Text S1) and form a concordant age clusterwith a concordia age of 1008 ± 17 Ma (MSWD = 1.7)

(Table 2 and Figure 8). We interpret this age to be the bestestimate of the crystallization age of gneiss protolith.4.3.3. Sample SZ25c (Ultramylonitic PorphyriticGranite)[37] All of the zircon grains have igneous characteristics

(Figure 6 and Text S1), and despite the intense mylonitiza-tion experienced by this sample, all eight analyzed zirconare concordant and have not experienced Pb loss. The datadefine a concordia age of 1023 ± 12 Ma (MSWD = 0.087;Figure 8 and Table 2), which we interpret to be the age ofcrystallization of the granitoid.4.3.4. Sample SZ26 (Nyamadzi Gneiss)[38] Most zircon in this sample are complex (Figure 6

and Text S1), showing core and rim relationships. The coreregions can generally be interpreted to have formed from anigneous precursor, but their irregular shape suggest they aredetrital grains having been overgrown by metamorphiczircon rims during a later metamorphic episode (Text S1).Seventeen analyses were conducted, including core-rimpairs (Table 2). The detrital igneous cores, plus mostsector-zoned rims, provide a range of variably discordantPaleoproterozoic ages that can be regressed to an upperintercept of 1961 ± 31 Ma suggesting that the grains weresourced from a relatively homogeneous igneous source.Three low Th/U rims plot in a concordant clustercorresponding to a concordia age of 1956 ± 14 Ma(MSWD = 0.71) (Figure 8 and Table 2) and which mayrepresent an early metamorphic event experienced by thisgneiss or that these grains were sourced from rocks whichhad experienced a circa 1956 Ma metamorphic event. Theage of these potential metamorphic zircon rims are withinerror of low Th/U zircon rims recorded in sample SZ16. Thelower intercept of 940 ± 220 Ma (MSWD = 2.6, Figure 8) iswithin error of a single low Th/U rim analysis (Table 2) thatyields a concordant 207Pb/206Pb age of 1065 ± 13 Ma.4.3.5. Sample SZ27 (Wutepo Gneiss)[39] All zircon are interpreted to be of igneous origin

(Figure 6 and Text S1) but the age data clearly define twogroups with tightly constrained and different 206Pb/238Uratios (Figure 8 and Table 2). One group has a concordia ageof 647 ± 11 Ma (MSWD = 0.95) and the other a concordiaage of 555 ± 11 Ma (MSWD = 2.6) (Figure 8). There is noclear systematic relationship between age or zircon charac-ter and thus it is possible that the younger group representzircon grains that have undergone Pb loss during a meta-morphic event at circa 555 Ma. The older circa 647 Ma agegroup most likely represent the age of crystallization of thegneiss protolith.


4.4.1. Sample CHP2a (K-Feldspar Augen Gneiss)[40] Most grains display concentrically zoned igneous

cores overgrown by high-CL rims (Figure 6 and Text S1).The data from core and rim do not define statisticallydistinctive age groups, and all data define a weighted mean207Pb/206Pb age of 1049 ± 14 Ma (MSWD = 1.6) (Figure 9and Table 2). Considering that the rims do not have lowTh/U contents that are considered characteristic of growthduring metamorphism we suggest they grew during a late


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Figure 8. Tera-Wasserburg U-Pb Concordia diagrams for the Petauke-Sinda Terrane and shear zonesamples. A detailed description of zircon morphology and data regression can be found in Text S1.


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Figure 9. Tera-Wasserburg U-Pb Concordia diagrams for the Chipata Terrane samples. A detaileddescription of zircon morphology and data regression can be found in Text S1.


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stage of igneous crystallization. The core analyses define amore precise concordia age of 1046 ± 4 Ma (MSWD =0.90) which we interpret to be the age of crystallization ofthe granitoid.4.4.2. Sample CHP2c (Syenite)[41] All zircon grains are interpreted to be of magmatic

origin (Figure 6 and Text S1) and have coherent 207Pb/206Pbratios. All seven grains regress to an upper intercept of1050 ± 7 Ma with a lower intercept demonstrating recentPb loss (MSWD = 0.50; Figure 9 and Table 2). We takethis upper intercept age as the best estimate for thecrystallization of the syenite.4.4.3. Sample CHP3 (Lunkwakwa Syenite)[42] All grains are interpreted to be igneous in origin

(Figure 6 and Text S1) and all four analyzed zircon define aconcordant cluster (Figure 9 and Table 2) corresponding to aconcordia age of 543 ± 6 Ma (MSWD = 1.11), taken torepresent the age of crystallization of the syenite.4.4.4. Sample CHP4a (Madzimoyo Charnockite)[43] All grains are interpreted to be of igneous origin

(Text S1). One of six analyzed zircon gives a concordant207Pb/206Pb age of 1949 ± 12 Ma, whereas the five otheranalyses define a concordant age group with a concordiaage 1076 ± 6 Ma (MSWD = 0.80; Figure 9 and Table 2). Weconsider the latter age to represent the time of crystallizationof protolith of the charnockite, whereas the older zircon isinterpreted as a xenocryst.4.4.5. Sample CHP4b (Madzimoyo Xenolith)[44] All zircon grains in this sample have igneous fea-

tures (Text S1). Seven of the grains define a linear Pb losstrend that has an upper intercept at 1974 ± 18 Ma and alower intercept at 749 ± 270 Ma (MSWD = 0.50; Figure 9and Table 2). Three of the most concordant grains provide aconcordia age of 1977 ± 11 Ma (MSWD = 0.036) (Figure 9and Table 2) that we take as the best estimate for the igneouscrystallization age of the xenolith.4.4.6. Sample CHP 5 (Madzimoyo Charnockite)[45] The zircon grains extracted from this sample display

complex zonation patterns indicative of igneous coressurrounded by low Th/U metamorphic rims (Text S1).Many grains contained very high common Pb (204Pb) andso the analysis of these grains was aborted. One core region,presumably of magmatic origin, provided an extremelydiscordant 207Pb/206Pb age of 1813 ± 104 Ma which canbe taken as a minimum age estimate for the crystallizationof this core (Figure 9). Three low Th/U rims are alsovariably discordant but the most concordant point definesa 207Pb/206Pb age of 1047 ± 20 Ma (Figure 9 and Table 2),which can be taken as a minimum estimate of crystallizationof the low Th/U rims during a metamorphic event affectingthe granulite. Given the paucity of zircon in this sample, amore detailed determination of the age of metamorphiczircon growth awaits further work.4.4.7. Sample CHP6a (Leopard Granite)[46] All zircon in this sample are interpreted to be of

igneous origin (Figure 6 and Text S1) with six of the sevenzircon analyzed defining a concordia age of 1038 ± 7 Ma(MSWD = 0.92; Figure 9 and Table 2) which we interpret tobe the age of crystallization of the granite.

4.4.8. Sample CHP8 (Porphyritic Granite)[47] All zircon are interpreted to be of igneous parent-

age (Figure 6 and Text S1). Two of the eight analyzedzircon provide ages outside the main age population andare interpreted to be of xenocrystic origin (Table 2 andFigure 9). These two grains have near concordant207Pb/206Pb ages of 1971 ± 10 and 1198 ± 56 Ma, whereasthe main population, defined by six grains, defines aweighted mean 207Pb/206Pb age of 1061 ± 13 Ma (MSWD =0.78). The latter age is interpreted to be the age of crystal-lization of the granite.4.4.9. Sample CHP10 (Knob Hill Granite)[48] Although most zircon consist of cores and rims

(Figure 6 and Text S1), there is no apparent age differencebetween them (Table 2). One core-rim analyses provide aweighted mean 207Pb/206Pb age of 1917 ± 10 Ma (MSWD =0.34, Figure 10 and Table 2) and is interpreted as being ofxenocrystic origin. The remaining seven analyses yield aweighted mean 207Pb/206Pb age of 1076 ± 14 Ma (MSWD =0.55) (Figure 10 and Table 2), which we take to representthe age of crystallization of the granitoid.4.4.10. Sample CHP11a (Mpangwe Hill Migmatite)[49] Zircon from this sample also show complex rela-

tionships consistent with recrystallization of the protolithigneous grains (Figure 6 and Text S1). The data for sevenanalyzed grains range from near concordant to verydiscordant, but six data points define a regression line withupper intercept of 1950 ± 67 Ma and lower intercept at 851 ±140 Ma (MSWD = 2.3, Figure 10). We take this upperintercept as the best age estimate for crystallization of theigneous protolith, while the lower intercept of 851 ± 140 Maprovides an imprecise estimate for the timing of the migma-tization event.4.4.11. Sample CHP12 (Porphyritic Granite)[50] All zircon from this sample are interpreted to be

igneous in origin (Text S1) and nine of the ten zirconanalyzed define a concordia age of 1038 ± 9 Ma (MSWD =0.04; Figure 10 and Table 2), which we take to representthe age of crystallization of the granite.4.4.12. CHP13 (Porphyritic Granite)[51] The zircon from this sample are interpreted to be of

igneous origin (Text S1) but only five grains were analyzeddue to high common Pb contents. Four grains define a tightset of 207Pb/206Pb ratios and have a weighted mean age of1058 ± 34 Ma (MSWD = 1.3; Figure 10 and Table 2) andone grain of xenocrystic origin has a highly discordant ageof 1466 ± 42 Ma, but little can be attached to this age.

5. Discussion

[52] Twenty-seven new U-Pb zircon SHRIMP crystalli-zation ages, six metamorphic ages and a host of inheritedzircon ages provide a wealth of data that for the first timeallow the definition of a coherent tectonomagmatic historyfor the Southern Irumide Belt and permit direct comparisonwith the Irumide Belt to the north. Figure 11 shows therelative temporal and spatial distributions of these rocks,including those dated by LA-ICP-MS (Mapani et al.,submitted manuscript, 2006) and those within the Che-


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wore-Rufunsa Terrane [Oliver et al., 1998; Goscombe et al.,2000; Johnson and Oliver, 2004; Johnson et al., 2006a, alsosubmitted manuscript, 2006; Mapani et al., submitted man-uscript, 2006] and the Irumide Belt [De Waele, 2005].

5.1. Tectonomagmatic Evolution of the SouthernIrumide Belt

5.1.1. Archaean and Paleoproterozoic Development[53] The pre-Mesoproterozoic history of the Southern

Irumide Belt remains essentially cryptic since the majorityof grains from this period occur as xenocrysts or inheritedcores in younger magmatic rocks or as detrital grains inmetasediments. However, we have identified three Paleo-proterozoic rocks, a mafic xenolith (CHP4b) and a felsicmigmatite (CHP11a) from the Chipata Terrane and anintermediate-felsic orthogneiss (SZ26, Nyamadzi Gneiss)from the Nyamadzi Shear Zone, all of which have a narrow

age range of between circa 1970–1950 Ma (Table 1).Without geochemical or tracer isotopic analyses, it isinappropriate to propose a tectonic origin, but a comparisonof these ages with the age distribution of inherited anddetrital zircon from all of the SIB lithologies (Figure 12c)indicates that 1970–1950 Ma lithologies characterize theSIB basement in all three terranes. In the Chewore-RufunsaTerrane, where a larger number of inherited grains has beenanalyzed, 1970–1950 Ma zircon predominate, but a minorArchaean component has also been identified at circa 2.6 Gaand 2.9 Ga (Figures 11 and 12c) [Goscombe et al., 2000;Johnson et al., 2006a, also submitted manuscript, 2006;Mapani et al., submitted manuscript, 2006]; three grainswith circa 2.6 Ga ages have also been dated from theLuangwa-Nyimba Terrane metasediments in this study andfrom the study of Mapani et al. (submitted manuscript,2006) and from zircon cores extracted from basement

Figure 10. Tera-Wasserburg U-Pb Concordia diagrams for the Chipata Terrane samples. A detaileddescription of zircon morphology and data regression can be found in Text S1.


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Figure 11. Time versus space diagram showing the distribution and tectonomagmatic events associatedwith the various SIB terranes and the IB. Igneous crystallization, detrital-inherited, and metamorphiczircon ages are from this study, the LA-ICPMS study of Mapani et al. (submitted manuscript, 2006) andCox et al. [2002], and other sources that are summarized by Johnson et al. [2005]. Data for the IB arefrom De Waele [2005].


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gneisses (sample i-8 on Figure 2) [Cox et al., 2002].Furthermore, in situ Lu-Hf isotopic measurements of fiveinherited zircon grains from the Chewore-Rufunsa Terrane,ranging in age from 2178 Ma to 1938 Ma, have predom-

inantly juvenile isotopic signatures (eHf(t) = +2.9 to –3.7)indicating that crust formation at this time was dominatedby juvenile mantle-derived magmatic processes, i.e., arcmagmatism rather than crustal recycling [Johnson et al.,

Figure 12. Probability density distributions (PPD) and histograms [after Sircombe, 2004] of all high-precision U-Pb zircon ages from the SIB and IB. Light gray PPDs represent the crystallization ages forrocks, whereas the dark gray PPDs represent the ages of xenocrystic or detrital zircon included eitherwithin the igneous rocks or from metasedimentary lithologies. Histograms are only shown for the igneousrock crystallization ages and were calculated with a bin size of 50 million years. Only data within 10% ofconcordia (i.e., 90–110%) have been used to plot the PPDs. Data for the SIB are taken from this study,Goscombe et al. [2000], and Johnson et al. [2006a, also submitted manuscript, 2006]; these data are alsosummarized by Johnson et al. [2005]. Data for the Irumide Belt are from De Waele [2005].


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2006a, also submitted manuscript, 2006]. The whole rockSm-Nd isotopic signature of late Mesoproterozoic mag-matic rocks in the Chewore-Rufunsa Terrane also indicatethe predominance of a juvenile, isotopically homogeneous,mid-Paleoproterozoic basement [Johnson et al., 2006a,also submitted manuscript, 2006]. All the SIB terranesappear to be underlain by basement with a restricted agedistribution between 1970 and 1950 Ma, suggesting thatthe entire SIB formed, and remained, as a coherenttectonic unit throughout the Paleoproterozoic and Meso-proterozoic. This is contrary to the conclusions of Mapaniet al. [2001, 2004], who suggested that the terranes formedas independent entities that were juxtaposed during lateMesoproterozoic accretionary orogenesis. A limited num-ber of Paleoproterozoic low Th/U metamorphic rimsidentified in the Nyamadzi Shear Zone in samples ofNyamadzi Gneiss (SZ26) and surrounding a single detritalgrain in a pelitic migmatite (SZ16), suggest that some ofthese basement lithologies were subjected to a thermalevent at circa 1950–1940 Ma, but more data are needed toconfirm this. Within the metasedimentary lithologies, theyoungest dated detrital zircon are close to the age of thethree Paleoproterozoic rocks identified in this study and ofthe main age peak defined by the inherited zircon, sug-gesting that the sediments were derived locally from theunderlying basement; however, their age of depositionremains poorly constrained, between the age of the youn-gest detrital zircon of circa 1950 Ma and the age ofmigmatization at circa 1060 Ma.5.1.2. Mesoproterozoic Magmatism and Metamorphism[54] Apart from the circa 1393 Ma Chewore Ophiolite

[Oliver et al., 1998] within the Chewore-Rufunsa Terrane,there appears to have been little if any magmatic activity inthe SIB from circa 1730 Ma, the age of the youngestinherited zircon within the Chewore-Rufunsa Terrane[Goscombe et al., 2000], until the onset of voluminousmagmatism at circa 1090 Ma (Figure 11) [Goscombe et al.,2000; Johnson and Oliver, 2004; Johnson et al., 2006a, alsosubmitted manuscript, 2006; Mapani et al., submitted man-uscript, 2006]. In the C-R Terrane, this period of magma-tism is constrained between circa 1090 Ma and 1040 Ma[Goscombe et al., 2000; Johnson and Oliver, 2004; Johnsonet al., 2006a, also submitted manuscript, 2006]. An almostidentical age range is observed from the Petauke-Sinda andChipata terranes, 1076–1038 Ma (Figures 11 and 12c),although in the Nyamadzi Shear Zone two slightly youngergranitoids, with ages of circa 1023 Ma (SZ25c) and circa1008 Ma (SZ23) have been identified. Detailed geochemicaland isotopic data from the Chewore-Rufunsa Terrane onthese late Mesoproterozoic calc-alkaline volcano-plutonicrocks reveal trace and REE patterns similar to present-daysuprasubduction magmas, but their isotopic compositionsindicate that they have been heavily contaminated by arelatively isotopically uniform, mid-Paleoproterozoic conti-nental crust [Johnson et al., 2006a, also submitted manu-script, 2006]. These lithologies most likely formed in acontinental margin arc setting [Johnson and Oliver, 2000,2004; Johnson et al., 2006a, also submitted manuscript,2006]. The synchronicity of magmatism throughout the SIB

suggests that it was linked to a common tectonomagmaticprocess, although without detailed geochemical and isotopicdata from the Petauke-Sinda and Chipata Terrane rocks, thisassumption remains speculative. The possibility that theChewore-Rufunsa Terrane, or even the entire SIB, faced anopen ocean throughout the Mesoproterozoic is supported bythe presence of the circa 1393 Ma Chewore Ophiolite;however, the apparent lack of any magmatic activity inthe SIB throughout the entire early to mid-Mesoproterozoic,suggests that most oceanic subduction must have occurredwithin intraoceanic arcs.[55] Even though most of these Mesoproterozoic rocks

have been extensively reworked during the Neoproterozoic-Cambrian it is evident that some lithologies have locallyexperienced a period of pre-Pan African (>570 Ma) gran-ulite-facies metamorphsim. Petrological work [Goscombeet al., 1998; Schenk and Appel, 2001] has demonstratedthat metamorphism developed under high-temperature(>800�C)/low-pressure (<4.5 kbar) conditions, and datingof metamorphic zircon [Goscombe et al., 2000] andmonazite [Schenk and Appel, 2001], suggests that thismetamorphic episode was synchronous with magmatism atcirca 1050 Ma. Our new metamorphic age data obtainedfrom similar high-temperature migmatites and granulitesconfirm this observation and indicate metamorphism be-tween 1065 Ma and 1047 Ma (NY82, SZ26 and CHP5)(Table 1 and Figures 11 and 12d). The synchronicity of high-temperature/low-pressure metamorphism with the intrusionof large voluminous plutonic bodies suggests a scenarioinvolving magma loading in the upper to midcrustal levels ofa continental margin arc [Schenk and Appel, 2001].5.1.3. Neoproterozoic Pre- and Post-Pan AfricanMagmatism and Metamorphism[56] Three rocks from the Petauke-Sinda Terrane and the

Nyamadzi Shear Zone have yielded mid-Neoproterozoiccrystallization ages, the Hofmeir Gneiss (PS78) at 742 ±13 Ma, a deformed porphyritic granite (PS71b) at 720 ±12 Ma and the Wutepo Gneiss (SZ27) at 647 ± 11 Ma(Table 1 and Figures 2, 7, and 8). All three ages are wellconstrained and the zircon morphology and Th/U ratios areconsistent with a magmatic origin. Currently these are theonly known magmatic rocks of this age in this region, andtheir volume, geographic extent and tectonomagmaticorigin remain uncertain, especially in the absence of anygeochemical or isotopic information. However, it is inter-esting to note that the deformed porphyritic granitoid(PS71) occurs in close association with the potentiallyrift-related andesitic Sasare volcanics and sediments [Barrand Drysdall, 1972] for which there are no robust ageconstraints, except that they are younger than circa 1043 Ma,the age of the porphyritic granitoid (PS65) which theyunconformably overlie. There is increasing evidence tosuggest the presence of a wide oceanic tract, the ZambeziOcean, between the CTB and Kalahari Cratons duringNeoproterozoic time [John et al., 2003, 2004; Johnson etal., 2005] but little evidence to suggest when this oceanmay have formed. Various rift sequences, containing eitherbimodal or mafic volcanics occur in the Zambezi, Lufilian,and Damara belts. In the Zambezi Belt this magmatism has


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been dated at circa 880 Ma [Johnson et al., 2006b, 2006]while in the Lufilian Belt, mafic lavas which form the baseof the Mwashya-Nguba-Kundelungu strata have been datedat circa 765 Ma [Key et al., 2001]. Without resorting to wildextrpolation, it is possible that the mid-Neoproterozoicmagmatic activity recorded here may be related to localizedor even far-field extensional stresses associated with suchrifting events.[57] Both the Hofmeir and Wutepo gneisses (PS78 and

SZ27) contain late Neoproterozoic metamorphic zircon(Table 1 and Figures 6 and 8) that are indistinguishable inage from those in the Zambezi and Lufilian belts (datasummarized by Johnson et al. [2005]). This event, knownas the Damara-Lufilian-Zambezi orogeny, is interpreted torecord the collision between the CTB and Kalahari Cratonswith the subsequent closure of the Zambezi Ocean [Johnand Schenk, 2003; John et al., 2003, 2004; Johnson et al.,2005]. These new metamorphic ages demonstrate that someparts of the SIB were reworked during this orogenic event atmetamorphic grades high enough to allow the growth ofnew, metamorphic zircon. Without direct geochronologicalconstraints (i.e., dating of metamorphic zircon rims), the ageof intense shearing and development of the Nyamadzi ShearZone is best constrained between circa 543 Ma, the youngestrock that it affects (CHP3), and the age of the oldestundeformed posttectonic syentic stock that seals the shear-dominated deformation, PS18 at circa 510 Ma (Table 1). It isinteresting to note that nearly all of the undeformed posttec-tonic syenites (510–474 Ma) lie within the bounds or alongthe margins of the shear zone itself, suggesting that theshear zone may have acted as a conduit for these magmas.Lithologies on either side of this shear zone have similarages and basement sources and have been affected by thesame Mesoproterozoic tectonomagmatic events suggestingthat this shear zone does not represent a Mesoproterozoicor Neoproterozoic suture zone, but is best interpreted as azone of intense, ductile reworking. This also leads to theconclusion that the SIB terranes represent different thrustnappes derived from a formally coherent SIB crustal block.

5.2. A Cryptic Suture Between the Irumide Belt(in the Strictest Sense) and the Southern Irumide Belt?

[58] Figures 11 and 12 summarize the tectonic, magmaticand sedimentary history for the Irumide and SouthernIrumide belts. Both belts apparently display broadly similartectonic histories, i.e., Mesoproterozoic magmatism derivedfrom a predominantly Paleoproterozoic basement; however,a critical examination of these data indicates major anddistinct differences between the two belts:[59] 1. The age of basement terranes, defined by the age

of exposed basement and inherited zircon grains is different.The Irumide Belt is characterized by a basement that isgenerally older than 2.0 Ga (based on 17 inherited zirconand 5 rocks) [De Waele, 2005] whereas the SIB has arelatively restricted age range that is younger than 2.0 Ga(40 inherited zircon and 3 rocks). The Irumide Belt containsa considerable number of middle to late Paleoproterozoicplutonic rocks (circa 1.8–1.55 Ga) [De Waele, 2005; DeWaele et al., 2006] that are not present in the SIB. The

isotopic characteristics of these basement terranes are alsodissimilar. The Paleoproterozoic rocks of the SIB havepredominantly juvenile isotopic compositions, whereas allof the Paleoproterozoic and Mesoproterozoic magmaticintrusions in the IB have highly enriched isotopic compo-sitions, having formed by the direct recycling of a uniformArchaean basement [De Waele et al., 2006].[60] 2. The style of Mesoproterozoic magmatism and

metamorphism between the two belts is also distinctlydifferent. The SIB is characterized by continental marginarc magmatism (at least in the Chewore-Rufunsa Terrane)between circa 1090–1040 Ma and was accompanied byhigh-temperature (>850�C)/low-pressure (<4 kbar) meta-morphism [Goscombe et al., 1998; Schenk and Appel,2001] (Figure 12d). Metamorphism does not appear to havebeen accompanied by any significant compressional tecto-nism and is most likely related to magma loading in theupper crustal portions of the continental margin arc. There-fore this metamorphic event is not part of the collision-related Irumide orogeny. Conversely, magmatism in the IBis characterized by granitoids with crustally recycled geo-chemical and isotopic signatures (S-type magmas) [DeWaele et al., 2006]. Magmatism was also accompanied bymoderate-temperature (800�C)/moderate-pressure (8 kbar)compressional tectonometamorphism [De Waele, 2005](Figure 12b).[61] 3. The timing of tectonometamorphism and magma-

tism is also very distinct. Continuous subduction-relatedmagmatism in the SIB until circa 1040 Ma suggests thatsubduction of oceanic crust occurred until that time. Theapparent switching off of subduction magmatism in theSIB and the initiation of compression, and S-type magma-tism in the IB (i.e., the Irumide orogeny) between circa1040–1020 Ma (with peak metamorphism and magmatismoccurring at 1020 Ma) suggests that ocean closure and theinitiation of collision began during this time period.[62] These data indicate that the tectonomagmatic char-

acter of the two belts is not identical and permit theinterpretation that they may have formed independently ofeach other before juxtaposition during the Mesoproterozoic.The contact between the two belts is occupied by Permo-Triassic rift basins and so the identification of potentialsuture zone rocks such as ophiolites and exotic fragments isunfortunately not possible. The data presented here, andreported for the Irumide Belt [De Waele et al., 2003; DeWaele, 2005; De Waele et al., 2006] and the Chewore-Rufunsa Terrane [Oliver et al., 1998; Goscombe et al.,2000; Johnson and Oliver, 2000, 2004; Johnson et al.,2005, 2006a, also submitted manuscript, 2006], are consis-tent with a scenario in which arc magmatism in the SIB wasrelated to the subduction of oceanic crust under the northernmargin (present-day coordinates) of an unknown continentor continental fragment. Arc magmatism ceased with theclosure of this ocean and collision between the SIB conti-nental margin with the southern margin of the CTB craton,i.e., the Bangweulu Block, heralding the start of the Irumideorogeny between circa 1040–1020 Ma. We suggest that theformation of the Neoproterozoic Zambezi Ocean in theinterval circa 880–720 Ma caused the rifting of this orogen


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leaving behind a thin vestige (i.e., the SIB) of the unknowncraton. Collisional orogenesis during the Zambezi Orogenycaused significant reworking, thrusting and shearing of theSIB generating its current architecture, possibly in part byreactivation of preexisting faults and shear zones. Remotesensing and aeromagnetic data suggest that the MwembeshiShear Zone (MSZ; Figures 1 and 4), also a late Neoproter-ozoic to Cambrian structure located between the Zambeziand Lufilian belts, may have exploited the former suturebetween the SIB and IB.

6. Conclusions

[63] New U-Pb SHRIMP zircon ages for the SIB support,and also redefine, the original terrane subdivisions proposedby Mapani et al. [2001, 2004, also Mapani et al., submittedmanuscript, 2006]. The data provide a robust tectonomag-matic history for this belt and which, for the first time, allowfor direct comparison with the adjacent IB. The data suggestthat (1) all terranes in the SIB formed on a late Paleoproter-ozoic basement (circa 1970–1950 Ma), (2) the main phaseof Mesoproterozoic magmatism occurred within a relativelyshort time interval between circa 1090–1040 Ma, and(3) high-temperature, low-pressure metamorphism wassynchronous with this late Mesoproterozoic magmaticevent. These data are in contrast to those from the IrumideBelt, which has a predominantly late Paleoproterozoicbasement characterized by ages older than 2.0 Ga with acryptic Archaean component. Late Paleoproterozoic toearly Mesorpoterozoic magmatic intrusions (circa 1.8–1.55 Ga) within the IB have not been identified in the

SIB, and synmagmatic compressional metamorphismoccurred after the cessation of magmatism in the SIB(i.e., at circa 1020 Ma). Taken together, these data indicatethat the SIB is not the southerly continuation or directcorrelative of the IB. Our preferred interpretation is thatMesoproterozoic magmatism in the SIB formed as part of acontinental margin arc on the northern margin (present-daycoordinates) of an unknown continent that, upon closure ofthe intervening ocean, collided with the CTB Craton margin(Bangweulu Block in our area) causing cessation of circa1090–1040 Ma arc magmatism in the SIB and initiatingcollisional deformation and magmatism in the IB (i.e., theIrumide orogeny).[64] Mid-Neoproterozoic intrusions in the SIB may po-

tentially record rifting processes related to the breakup ofthis orogen, and numerous metamorphic zircon indicate thatparts of the SIB were deformed and metamorphosed atmoderate to high metamorphic grades during the Damara-Lufilian-Zambezi orogeny, i.e., during the assembly ofGondwana at circa 550–520 Ma. These events were mostlikely partitioned along preexisting planar zones of weak-ness (shears and thrust boundaries) resulting in the present-day architecture of the SIB as a series of stacked terranes,one of which, the Nyamadzi Shear Zone, has been exploitedby voluminous posttectonic granitoid batholiths.

[65] Acknowledgments. We would like to thank Willy Nundwe forhis excellent driving skills, ability with a hammer, good humor, and adetailed knowledge of the Eastern Province. Thanks go to Veronika Tenczerand Toby Rivers for their excellent reviews and critical comments that havegreatly improved this manuscript. This is a contribution to IGCP 440 andTSRC manuscript 373.

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u-pb sensitive high-resolution ion microprobe (shrimp) zircon...

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