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ARTICLE IN PRESSG ModelRECAM-4048; No. of Pages 24

Precambrian Research xxx (2014) xxx–xxx

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Precambrian Research

jo ur nal homep ag e: www.elsev ier .com/ locate /precamres

eneration of early Archaean grey gneisses through melting of olderrust in the eastern Kaapvaal craton, southern Africa

lfred Krönera,b,∗, J. Elis Hoffmannc, Hangqiang Xiea, Carsten Münkerc, Ernst Hegnerd,usheng Wana, Axel Hofmanne, Dunyi Liua, Jinhui Yangf

Beijing SHRIMP Centre, Institute of Geology, Chinese Academy of Geological Sciences, Baiwanzhuang Road 26, Beijing 100037, ChinaInstitut für Geowissenschaften, Universität Mainz, 55099 Mainz, GermanyInstitut für Geologie und Mineralogie, Universität zu Köln, Zülpicher Strasse 49b, 50674 Köln, GermanyDepartment für Geo- und Umweltwissenschaften and GeoBio-Center LMU, Universität München, Theresienstrasse 47, 80333 München, GermanyDepartment of Geology, University of Johannesburg, P.O. Box 524, Auckland Park, Johannesburg, South AfricaState Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, China

r t i c l e i n f o

rticle history:eceived 23 December 2013eceived in revised form 4 July 2014ccepted 26 July 2014vailable online xxx

eywords:rey gneissalaeoarchaeanaapvaal cratonf–Nd isotopesircon agewaziland

a b s t r a c t

We report zircon ages, Hf-in-zircon isotopes as well as whole-rock geochemistry and Hf–Nd isotopicsystematics for Palaeoarchaean grey gneisses of the Ancient Gneiss Complex of Swaziland, the oldestcomponents of the Kaapvaal craton, southern Africa. The Hf-in-zircon isotopic compositions in thesecompositionally heterogeneous, multicomponent, migmatitic gneisses are highly variable, even in theoldest zircons dating back to 3.66 Ga, suggesting growth of zircon from melts representing a mix ofjuvenile and anatectic material derived from differentiated continental crust of Eoarchean to late Hadeanage. In contrast, the initial Nd and Hf whole-rock isotopic compositions are frequently not in agreementwith the Hf-in-zircon data that mostly show approximately chondritic initial values for Nd and stronglyradiogenic initial values for Hf. We consider it likely that both the Lu–Hf and Sm–Nd whole-rock isotopicsystems were disturbed and partly reset during later episodes of partial melting and crustal reworking,most likely during a pervasive 3.2 Ga tectono-metamorphic event.

Primitive mantle-normalized trace element patterns show the variable influence of residual plagioclaseand garnet in the sources as well as high contents of strongly incompatible elements. In conjunctionwith the Hf-in-zircon isotopic data the trace element contents are best explained by the incorporation
of older continental crustal material into the sources of the grey gneisses. Our data support evidencefrom other Palaeoarchean terranes that crustal recycling, as seen in even the oldest crustal components,played an important role in early continental evolution. Rocks previously classified as a subduction-related tonalite–trondhjemite–granodiorite suite are complex, and their chemistry alone cannot be usedto reconstruct tectonic settings.
© 2014 Elsevier B.V. All rights reserved.

. Introduction

There has been much debate on the origin of the tonalite–rondhjemite–granodiorite (TTG) suite of rocks in the earlyrchaean record worldwide (Martin, 1994; Condie, 2005; Moyen,011; Hoffmann et al., 2011a). These have almost universally been

Please cite this article in press as: Kröner, A., et al., Generation of eareastern Kaapvaal craton, southern Africa. Precambrian Res. (2014), ht

elated to magmatic arc formation as a result of subduction (Foleyt al., 2003; Condie, 2005) or to plume-related processes (Smithiest al., 2007, 2009), and many seem to be genetically related to

∗ Corresponding author at: Institut für Geowissenschaften, Universität Mainz,5099 Mainz, Germany. Tel.: +49 6131 2922163; fax: +49 6131 3924769.

E-mail address: [email protected] (A. Kröner).

ttp://dx.doi.org/10.1016/j.precamres.2014.07.017301-9268/© 2014 Elsevier B.V. All rights reserved.

greenstone belt evolution (e.g., De Wit et al., 1987). In most geneticmodels a juvenile origin is implied, and this has led to geodynamicscenarios in which virtually all felsic early Archaean crust was gen-erated through subduction-induced mantle-melting events (e.g.,Polat, 2012; Mueller et al., 2010). Although some geochemical datasuggested involvement of early continental crust in the formationof these felsic melts (e.g., Smithies et al., 2007; Iizuka et al., 2006),these were considered minor, and the available Nd whole-rock iso-topic data with overwhelmingly positive εNd(t)-values supportedthe above view (e.g., Carlson et al., 1983; Schoene et al., 2008).

ly Archaean grey gneisses through melting of older crust in thetp://dx.doi.org/10.1016/j.precamres.2014.07.017

This interpretation became questionable with the discovery ofxenocrystic zircons in ancient grey gneisses (Kröner et al., 1996;Nelson et al., 2000; Iizuka et al., 2006; Zeh et al., 2011) and theappearance of Hf-in-zircon isotopic data for very old granitoid rocks

dx.doi.org/10.1016/j.precamres.2014.07.017


http://www.sciencedirect.com/science/journal/03019268

http://www.elsevier.com/locate/precamres

mailto:[email protected]


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ARTICLERECAM-4048; No. of Pages 24

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uch as the Acasta Gneiss of Canada (Iizuka et al., 2009), gneissssemblages in West Greenland (Næraa et al., 2012), the Min-esota River Valley gneisses (Satkoski et al., 2013), early Archaeanssemblages in eastern China (Geng et al., 2012), and parts of thencient Gneiss Complex of Swaziland (Zeh et al., 2011; Krönert al., 2012), all suggesting considerable involvement of earlier con-inental material in their genesis, mainly on the basis of isotopicharacteristics.

In a recent review of a large data base for early Archaean gran-toid gneisses that occur in virtually all ancient cratons Moyen2011) concluded that many rocks loosely described as TTG doot fulfil the chemical criteria for this suite and are of diverserigin. Moyen (2011) suggested the general term “grey gneiss”or these rocks and estimated that nearly 50% of compositions inis database represent recycled crustal material, including anate-tic melts of earlier TTGs. From this Moyen (2011) concluded thatrustal recycling was already an important process in the earlyrchaean and that different components of the TTG and grey gneisseries probably formed in variable tectonic settings.

We have undertaken Lu–Hf and Sm–Nd whole-rock as wells Hf-in-zircon isotopic analyses on SHRIMP-dated zircons fromalaeoarchaean tonalitic–trondhjemitic gneisses of the Ancientneiss Complex in Swaziland, southern Africa. These data argue

or derivation of many zircons from very diverse and, in severalases, significantly older crustal sources dating back to the earliestrchaean and even Hadean. From these data we follow the aboveuthors in confirming that crustal reworking processes beganery early in Earth history. Consequently, many early Archaeanranitoid gneisses are not juvenile but inherited their geochemicalignatures from older crustal source(s). In view of increasingvidence for recycling, we question the previously reported highrust-production rates in the early Archaean which are mostlynferred from zircon ages, geochemistry and whole-rock Ndsotopic data. We also challenge the view of Hawkesworth et al.2013) that crustal reworking was not an important process inrustal evolution prior to about 3.2 Ga.

. Brief geology of the Ancient Gneiss Complex

Southern Africa preserves one of the most complete and detailedecords of early Precambrian crustal evolution (De Wit et al., 1992;cCarthy and Rubridge, 2005), and the Ancient Gneiss Complex

f Swaziland (AGC, Hunter, 1970; Kröner, 2007) and related rockslong the southern margin of the 3.5–3.2 Ga Barberton Green-tone Belt (Fig. 1) have played a prominent role in models for thearly evolution of continental crust (e.g., Anhaeusser, 1973; Kröner,985; Hunter and Wilson, 1988; De Wit et al., 1992).

The AGC is a typical early to Palaeo- to Mesoarchaean ter-ain comprising multiply deformed granitoid gneisses of theonalite–trondhjemite–granodiorite (TTG) suite (Hunter et al.,978) and interlayered amphibolites, most of which are proba-ly derived from mafic dykes (Hunter et al., 1984; Jackson, 1984).his suite of layered gneisses was renamed as Ngwane Gneiss (NG,ilson, 1982) and is the oldest part of the AGC. This is suggested

y its structural evolution (Jackson, 1984; Jackson et al., 1987) andeochronology (see Fig. 2a), yielding ages from ca. 3.66 Ga to 3.45 Gae.g., Compston and Kröner, 1988; Kröner et al., 1989; Kröner, 2007;choene et al., 2008; Zeh et al., 2011). However, the NG as mappedn published sheets of the Swaziland Geological Survey, is alsonterlayered with younger banded gneisses, about 3.2 Ga in agee.g., Kröner et al., 1989) which, in the field, are often indistin-uishable from the older rocks, though generally more granitic in


omposition. Strong ductile deformation has obliterated all orig-nal contacts, and it is thus very difficult, or impossible, in theeld to distinguish between the ca. 3.45–3.66 and 3.2 Ga gneisses.e provide examples below and discuss the relationship between

PRESSsearch xxx (2014) xxx–xxx

these rocks. The NG complex also includes remnants of greenstonebelt supracrustal assemblages that vary in size from xenoliths afew centimetres long to inliers several kilometres across (Jackson,1984; Kröner and Tegtmeyer, 1994; see Fig. 1). Originally the gran-itoid gneisses of the AGC were interpreted to have formed in situby migmatization and granitization of older sedimentary and/orvolcanic rocks (Hunter, 1970), but this view is no longer accepted.

The AGC has its widest distribution in a broad belt throughSwaziland (Fig. 1), and the area so far studied in most detail cen-tres around the town of Mankayane (Hunter, 1970; Hunter et al.,1978; Jackson, 1984; Kröner et al., 1989; Kröner and Tegtmeyer,1994). The preserved metamorphic grade of most Ngwane gneissesis upper amphibolite-facies, but at least part of the suite has beenmetamorphosed to granulite-facies as shown by rare charnockiticremnants (Hunter, 1970) and inliers of >3.1 Ga metasedimentaryrocks with high-grade mineral assemblages indicating tempera-tures of 700–900 ◦C and pressures of 6–7.5 kbar (Kröner et al., 1993;Condie et al., 1996; Taylor et al., 2012). In southwestern Swazi-land the Ngwane gneisses and greenstone remnants are intrudedby the Tsawela Gneiss (TG, see Fig. 1), a distinct, weakly to well-foliated, but not compositionally layered, tonalitic to trondhjemiticrock, defining a 3.40–3.46 Ga pluton exposed as several distinctbodies within the Ngwane gneiss. Field evidence shows that theTsawela Gneiss that was emplaced after the oldest Ngwane gneissesand greenstones had already been deformed at least once (Jackson,1984; Kröner, 2007).

The AGC is separated from the Barberton Greenstone Belt (BGB)by a large granitoid sheet-like pluton some 3.1 Ga in age and knownas the Mpuluzi and Piggs Peak Batholiths (Hunter, 1973; Bartonet al., 1983; see Fig. 1). In northwest Swaziland, however, a smallinlier of AGC gneisses occurs in faulted and sheared contact withBGB rocks (Phophonyane Inlier near Piggs Peak, see Fig. 1), and thisrelationship may suggest that the two units were in direct contactprior to about 3.1 Ga.

The BGB occurs in the eastern part of South Africa and north-western Swaziland (Fig. 1) and is a complex association of variablydeformed volcanic and sedimentary rocks that contains a magmaticand depositional record of more than 300 million years from 3.53to 3.2 Ga (Kröner et al., 1991, 1996; Lowe and Byerly, 2007; VanKranendonk et al., 2009; Furnes et al., 2013). The greenstones andrelated deeper-level plutonic and metamorphic units constitute theBarberton Granite-Greenstone Terrain (Lowe and Byerly, 2007).

The tectonic environments in which the supracrustal sequencesand surrounding time-equivalent TTG plutons of the Barberton ter-rain and adjacent AGC were generated and deformed has been amatter of speculation and debate for more than 30 years. Earlymodels favoured continental rift-settings (Anhaeusser et al., 1969)followed by intra-oceanic models (Anhaeusser, 1975; De Witet al., 1987, 2011; Furnes et al., 2013) and those favouring evo-lution along a continental margin (Eriksson, 1980; Kröner, 1985)or in plume-related environments (Smithies, 2000; Chavagnac,2004; Smithies et al., 2009). Schoene and Bowring (2011) inter-preted the Barberton-Swaziland terrane in terms of a doublyverging subduction system between 3.28 and 3.22 Ga, whereas VanKranendonk (2011a,b) suggested that much of the evolution ofearly Archaean greenstone and TTG assemblages may be due tosinking of the thick, dense greenstones into partially molten andthus reworked granitoid middle crust during partial convectiveoverturn at 3.26–3.22 Ga.

3. Previous geochronology of the Ancient Gneiss Complexand evolutionary models


The oldest types of Ngwane gneiss so far dated are tonalitic–trondhjemitic gneisses in the Phophonyane Inlier north of PiggsPeak (Fig. 1). Compston and Kröner (1988) SHRIMP-analyzed single


ARTICLE IN PRESSG ModelPRECAM-4048; No. of Pages 24

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zhAaeate(P

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Fig. 1. Simplified geological map of Swazilanodified from Kröner et al. (1989).

ircons from a compositionally heterogeneous and finely bandedornblende- and biotite-bearing trondhjemitic gneiss (sampleGC150) that exhibited four distinct age groups between 3644nd 3433 Ma, interpreted as reflecting igneous and metamorphicpisodes. Schoene et al. (2008) confirmed this ancient age through

conventional single grain U–Pb age of 3662.8 ± 0.5 Ma for anotheronalitic gneiss from the same inlier but a different locality. Thextent of ca. 3660 Ma crust was further documented by Zeh et al.2011), who dated zircons from a migmatitic gneiss within thehophonyane shear zone.

Skjerlie and Johnston (1993) undertook fluid-absent meltingxperiments on a sample of AGC150 at 6–14 kbar and 875–975 ◦Cnd produced F-rich granitic liquids with compositions withinhe range of A-types granites and leaving behind a granuliticesidue dominated by orthopyroxene, quartz, and plagioclase. Theyoncluded that, under fluid-absent conditions, intrusion of hot,antle-derived magma into the lower crust is necessary to initi-

te widespread dehydration-melting in rocks with compositionsimilar to those represented by AGC150.

Further indications for the antiquity of the Ngwane gneissesome from zircon ages of 3550–3200 Ma, summarized in Kröner2007). There is also a zircon age of 3570 ± 6 Ma for a high-K gran-te clast in a conglomerate of the BGB (Kröner and Compston, 1988),elieved to be derived from erosion of the AGC terrain during green-tone basin evolution (Eriksson, 1980).


The oldest date so far reported from the supracrustal assemblagef the BGB is a SHRIMP mean 207Pb/206Pb zircon age of 3552 ± 1a for a felsic metavolcanic rock of the Theespruit Formation, the

owest stratigraphic unit of the BGB (Kröner et al., 2013), and two

wing major rock units and sample locations.

samples of metasedimentary rock from the small Dwalile green-stone remnant in SW Swaziland that contain detrital zircons withconcordant SHRIMP mean 207Pb/206Pb zircon ages between 3544and 3563 Ma (Kröner and Tegtmeyer, 1994).

The available isotopic data therefore indicate that at least partsof the NG are older than the oldest parts of the BGB and Dwalilegreenstone sequences. The presence of zircon xenocrysts up to3.7 Ga in age in samples of both the felsic volcanic rocks and gran-itoids in the southern Barberton terrane suggests that older crustwas involved in their formation (Kröner et al., 1996).

Apart from the 3.64 to 3.66 Ga zircon ages for tonalitic gneissesin northwestern Swaziland there are several other pre-3.5 Ga agesfor similar rocks occurring in the NE and central parts of the coun-try, and these are detailed in Kröner et al. (1989) and Kröner andTegtmeyer (1994). Material of similar age as exposed in northwest-ern Swaziland was probably involved in the formation of several ofthe 3.55–3–2 Ga grey gneisses, as suggested by whole-rock Nd iso-topic systematics which provide depleted mantle Nd mean crustalresidence ages of 3.6–3.7 Ga (Kröner et al., 1993; Kröner, 2007). Ournew Lu–Hf whole-rock and Hf-in-zircon isotopic data reported herefor some of the above and additional samples confirms this view.

4. Zircon ages, Hf-in-zircon data and whole-rockLu–Hf/Sm–Nd isotopic compositions


We have investigated whole-rock samples and magmatic zir-cons, previously and newly dated, from several Palaeoarchaeangrey gneisses in Swaziland, using in situ Lu–Hf isotopic analysis byLA-ICP-MS. Most analytical spots for Lu–Hf isotopes were placed



4 A. Kröner et al. / Precambrian Research xxx (2014) xxx–xxx

0.40

0.50

0.60

0.70

0.80

0.90

1.00

1.10

20 25 30 35 40 45

32003300

34003500

36003700

3800

Pb/

U206

238

Pb/ U207 235

AGC 150 - Heterogeneoustrondhjemitic gneiss,Phophonyane Inlier, NWSwaziland (data ofCompston & Kröner, 1988)

Oldest component,17 grains, mean207Pb/206Pb age:

3644±2 Ma(a)

0.58

0.62

0.66

0.70

0.74

0.78

0.82

27 28 29 30 31 32 33 34 35 36 37 38

3450

3500

3550

3600

3650

3700

Pb/ U207 235

AGC 150B - Homogeneous,banded tonalitic gneiss, road-cut in Phophonyane Inlier,northwest Swaziland

(b)

0.58

0.62

0.66

0.70

0.74

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24 26 28 30 32 34 36 38

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3400

3450

3500

3550

3600

3650

3700

AGC01-5 - Homogeneoustonalitic gneiss, Phophonyaneshear zone, NW Swaziland

Oldest component,16 grains, mean207Pb/206Pb age:

3645±5 Ma

Pb/

U206

238

Pb/ U207 235

(c)0.55

0.60

0.65

0.70

0.75

0.80

25 26 27 28 29 30 31 32 33 34 35 36

3350

3400

3450

3500

3550

3600

3650

Pb/ U207 235

9 grains, mean207Pb/206Pb age:3620.4±1.7 Ma

3352±4 Ma

AGC 207 - Homogeneouslayered trondhjemitic gneiss,Phophonyane shear zone,NS Swaziland

(d)

0.60

0.62

0.64

0.66

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0.74

0.76

0.78

26 27 28 29 30 31 32 33 34

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3550

3600

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AGC 370 - Tonalitic gneiss,roadcut N of 3.66 Ga locality,Phophonyane Inlier, north-western Swaziland

εNd(t) = 1.5±0.4εHf(t) = 2.1±0.4

(e)0.64

0.66

0.68

0.70

0.72

0.74

0.76

0.78

28 29 30 31 32 33

34403460

34803500

35203540

3560

Pb/ U207 235

AGC 138 - Well layeredtonalitic Ngwane gneiss,Njoli Dam quarry,northeastern Swazland


(f)

0.78



Fig. 2. Concordia diagrams showing SHRIMP zircon analyses of granitoid gneisses of the Ancient Gneiss complex of Swaziland. Data boxes are defined by standard errors( AGC1l SHRIS Swaz

omtCwUtsCz

1 − �) in 206Pb/238U, 207Pb/235U and 207Pb/206Pb. (a) Reproduction of data for sampleocality as AGC150. (c) Trondhjemitic gneiss of Schoene et al. (2008), reanalyzed onwaziland. (e) Tonalitic gneiss sample AGC370, roadcut in Phophonyane Inlier, NW

n zircon pits previously analyzed on SHRIMP I or II, whereasost whole-rock Lu–Hf and Sm–Nd analyses were performed on

he same powders as used for chemistry, using MC-ICP-MS inologne/Bonn following the protocols of Münker et al. (2001). Somehole-rock Sm–Nd determinations were performed by TIMS at theniversity of Munich. For a detailed description of the analytical


echniques see the Supplementary Material. The new U–Pb analy-es were made on a SHRIMP II instrument in the Beijing SHRIMPentre, Chinese Academy of Geological Sciences, whereas the Hf-in-ircon isotopic date were obtained by LA-ICP-MS in the Institute of

50 (Compston and Kröner, 1988). (b) Homogeneous gneiss sample AGC150b, sameMP II. (d) Mylonitic tonalitic gneiss sample AGC207, Phophonyane shear zone, NWiland. (f) Layered tonalitic gneiss sample AGC138, Njoli Dam quarry, NE Swaziland.

Geology and Geophysics, Chinese Academy of Sciences, Beijing. Theisotopic ratios recommended by Bouvier et al. (2008) and the 176Ludecay constant of Scherer et al. (2001) and Söderlund et al. (2004)were used in the calculation of εHf(t)- and εNd(t)-values. As detailedin Kröner et al. (2013, 2014) and shown by the data presented here,a ratio of 0.01 most realistically seems to reflect the potential pro-

176 177


toliths Lu/ Hf ratio of our samples. This was therefore usedin the calculation of Hf crustal model ages Hfc for the analyzedzircons. However, there is an inherent potential error in the esti-mation of this ratio (see Zeh et al., 2011 for detailed discussion),




Table 1Summary of locations and rock types of dated granitoid gneisses, Ancient Gneiss Complex of Swaziland.

Sample no. South Lat. EastLong.

Locality, area Rock type

AGC150 25◦54′29.9′′

31◦18′40.4′′Roadcut 1 km NE of The Falls, Phophonyane Inlier, NW Swaziland Heterogeneous layered grey gneiss

AGC150b 25◦54′29.9′′

31◦18′40.4′′Roadcut 1 km NE of The Falls, Phophonyane Inlier, NW Swaziland Homogeneous part of layered grey gneiss

AGC01-5 25◦53′46.3′′

31◦18′39.0′′Phophonyane River at bridge of main asphalt road, NW Swaziland Porphyritic tonalitic gneiss

AGC207 25◦53′57.1′′

31◦17′42.0′′Roadcut 1.5 km N of The Falls, Phophonyane Inlier, NW Swaziland Trondhjemitic mylonitic gneiss

AGC370 25◦54′32.0′′

31◦18′37.5′′Roadcut 1.8 km N of The Falls, Phophonyane Inlier, NW Swaziland Homogeneous tonalitic gneiss

AGC185 26◦07′20.6′′

31◦42′15.8′′Causeway over Mbuluzi River, south of Tshaneni, NE Swaziland Layered homogeneous tonalitic gneiss

AGC138 26◦08′40.1′′

31◦39′55.1′′Abandoned Njoli Dam Quarry, NE Swaziland Layered homogeneous tonalitic gneiss

AGC200 26◦41′04.5′′

31◦23′54.5′′Causeway over tributary of Ngwempisi River S of Sidvokodvo, central Swaziland Non-layered homogeneous tonalitic gneiss

AGC216 26◦41′04.5′′

31◦23′54.5′′Causeway over tributary of Ngwempisi River S of Sidvokodvo, central Swaziland Non-layered homogeneous tonalitic gneiss

AGC351 26◦39′50.2′′◦ ′ ′′

Upper Mtimane River E of Mankayane, west-central Swaziland Garnetiferous granitic gneiss

d

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4

gKmt1ifKiaFtcs31a1doo

dTnc

31 07 41.9AGC228 26◦31′34.2′′

31◦07′41.9′′Roadcut NW of Malkerns, central Swazilan

articularly if the zircons crystallized from a melt more mafic thanonalitic in composition. There is also an analytical uncertainty inhe determination of the 176Hf/177Hf ratio of about 0.4 εHf-units (seeable 2), so the model ages given here have likely errors of between0 and 100 Ma and should therefore not be taken literally. The Hf

nitial ratios εHf(t) for individual zircon analyses are plotted in thef-evolution diagrams of Figs. 4 and 6 in which the depleted man-

le evolution lines are based on 176Hf/177Hf (4.56 Ga) = 0.279799εHf(t) = 0) and 176Hf/177Hf (present) = 0.283250 (εHf(t) = 16.5). Seeupplementary File for details. The analytical procedures are sum-arized in the Supplementary File, and the analytical data are

hown in Tables 1–5.

.1. Phophonyane Inlier, NW Swaziland

Our first three samples represent the 3.64–3.66 Ga tonaliticneisses from NW Swaziland previously dated by Compston andröner (1988) and Schoene et al. (2008). Sample AGC150 is theedium-grey part of a finely banded gneiss intruded by veins of

he 3.1 Ga Piggs Peak granite (see Fig. 2 in Compston and Kröner,988) in a roadcut some 7 km N of Piggs Peak (Table 1). Its chem-

stry is trondhjemitic, and Compston and Kröner (1988) suggestedrom strong LREE enrichment, coupled with high Rb/Sr but low2O/Na2O, that this rock was generated through a multistage

ntracrustal melting process. The U–Pb analytical data of Compstonnd Kröner (1988) for the oldest zircons are reproduced here inig. 2a, leaving out the most discordant data. It is evident fromhis pattern of ages hat the gneiss consists of several magmaticomponents that could not be separated in the field because oftrong ductile deformation. There is also a minor component of.2 Ga zircons in this sample (see Fig. 5 in Compston and Kröner,988), probably representing a separate granitoid phase that waslso recognized elsewhere in the Phophonyane Inlier (Kröner et al.,989). Several zircons with ages around 3.1 Ga most likely formeduring emplacement of the Piggs Peak Batholith that intrudes theld gneisses and of which several pegmatitic phases traverse theutcrop (see Fig. 2 in Compston and Kröner, 1988).

We have analyzed Lu–Hf isotopes on SHRIMP pits only 0.5 �m


eep, using concordant or near-concordant ca. 3644 Ga zircons.hese zircons are mostly long and prismatic with rounded termi-ations and reveal well-preserved oscillatory or striped zoning inathodoluminescence (CL) images (Fig. 3a). The Lu–Hf analytical

Fine-grained tonalitic gneiss

data are shown in Table 3. Five spots were analyzed by LA-ICP-MSand yielded negative εHf(t)-values between −1.2 and −3.8 (Table 3and Fig. 4a). The whole-rock εHf(t)-value for this composite sam-ple, based on an age of 3.64 Ga, is −3.3 (Table 4). However, thisvalue may be misleading because the original sample of AGC150contained zircons ranging in age between 3.4 and 3.64 Ga, suggest-ing that the banded gneiss contains multiple age components, andthus a proper assessment of its whole-rock isotopic compositionis difficult. The 176Lu/177Hf ratio is 0.02107, significantly higherthan average crustal values of 0.010–0.015. The εNd(t)-value for thiswhole-rock composite sample is +0.4 (Bonn laboratory, see Table 5).

A carefully selected homogeneous, light grey part of the bandedgneiss outcrop from which AGC150 was taken (AGC150b) con-tained a single population of zircon that yielded a much simplerage spectrum than the composite sample AGC150 (Table 2). Twelveoscillatory-zoned magmatic zircons (Fig. 3b) are concordant ornear-concordant and yielded virtually identical 207Pb/206Pb ratioswith a mean age of 3660.8 ± 0.6 Ma (Fig. 2b). Two further grainsfrom this population yielded grossly discordant results (analysesAGC150b-1 and 150b-10 in Table 2) and were not used in the agecalculation. Two further igneous grains, morphologically and interms of their CL images indistinguishable from the above popula-tion, yielded a significantly younger concordant age of 3482.7 ± 1.1Ma that is similar to some ages found in sample AGC150 (seeFig. 2a). We interpret this to reflect a younger magmatic event rep-resented by thin layers in the gneiss that could not be separatedduring processing. These results support the view of Compston andKröner (1988), namely that the roadcut sampled here represents alayered gneiss consisting of a variety of granitoid intrusions thatwere tectonized and brought into structural parallelism by duc-tile deformation. The tectonic process resulting in such bandedgneisses was described by Myers (1978) and Passchier et al. (1990).

The Hf and Nd isotopic data of this single-component partof the gneiss (AGC150b) reiterate the conclusions reached fromAGC150. 14 grain domains were analyzed by LA-ICP-MS, yielding awide spread in εHf(t)-values between −5.1 and +1.2 (Table 3 andFig. 4a). However, two whole-rock Lu–Hf analyses yielded pos-itive εHf(t)-values of +2.5 and +4.5 (Table 4) with more realistic


crustal 176Lu/177Hf ratios of 0.01316 and 0.01239 respectively. TheεNd(t)-value for this whole-rock sample (duplicate analyses) is +0.9and +0.6 (Bonn) and +0.1 (Munich) (Table 5). A dark, hornblende-rich tonalitic layer in the gneiss (AGC150m) yielded a whole-rock


Please cite this article in press as: Kröner, A., et al., Generation of early Archaean grey gneisses through melting of older crust in theeastern Kaapvaal craton, southern Africa. Precambrian Res. (2014), http://dx.doi.org/10.1016/j.precamres.2014.07.017



Table 2SHRIMP II analytical data for spot analyses of single zircons from granitoid gneisses of the Ancient Gneiss Complex, Swaziland.

Sample no. U (ppm) Th (ppm) 206Pb/204Pb 208Pb/206Pb 207Pb/206Pb 206Pb/238U 207Pb/235U 206/238age ± 1�

207/235age ± 1�

207/206age ± 1�

AGC015-1 958 25 20,408 0.0111 ± 3 0.2559 ± 17 0.2200 ± 44 7.762 ± 163 1080 ± 20 2204 ± 19 3222 ± 10AGC015-2 709 28 23810 0.0182 ± 7 0.3063 ± 9 0.5139 ± 93 21.701 ± 390 2319 ± 42 3170 ± 18 3502 ± 4AGC015-3 1094 21 1333 0.0692 ± 11 0.2063 ± 23 0.2755 ± 50 7.830 ± 164 1405 ± 24 2212 ± 18 2876 ± 18AGC015-4 917 11 5002 0.0079 ± 2 0.2664 ± 8 0.3293 ± 59 12.089 ± 218 1569 ± 27 2611 ± 17 3285 ± 5AGC015-5 1098 32 5163 0.0172 ± 4 0.2236 ± 9 0.2366 ± 43 7.292 ± 131 1195 ± 20 2149 ± 15 3007 ± 7AGC015-6 1089 81 2003 0.0369 ± 6 0.2078 ± 11 0.1418 ± 26 4.062 ± 77 742 ± 13 1647 ± 9 2888 ± 9AGC015-7 1399 21 5263 0.0371 ± 6 0.1185 ± 9 0.1065 ± 19 1.739 ± 33 613 ± 11 1023 ± 12 1933 ± 14AGC015-8 1273 25 2000 0.0253 ± 5 0.1482 ± 27 0.1360 ± 29 2.778 ± 78 755 ± 16 1350 ± 20 2325 ± 31AGC015-9 208 137 20,833 0.1732 ± 14 0.3364 ± 13 0.7381 ± 140 34.231 ± 650 3415 ± 120 3616 ± 19 3646 ± 6AGC015-10 281 211 7692 0.2168 ± 22 0.3398 ± 13 0.6952 ± 132 32.572 ± 619 3104 ± 77 3568 ± 18 3662 ± 6AGC015-11 274 201 55,556 0.1920 ± 13 0.3362 ± 12 0.7439 ± 141 34.509 ± 656 3471 ± 130 3625 ± 17 3646 ± 6AGC015-12 182 106 135,135 0.1594 ± 14 0.3360 ± 16 0.7551 ± 151 34.972 ± 699 3578 ± 170 3638 ± 19 3645 ± 7AGC015-13 152 80 16,393 0.1482 ± 18 0.3360 ± 16 0.7590 ± 152 35.178 ± 739 3633 ± 210 3644 ± 20 3645 ± 7AGC015-14 451 305 19,231 0.1821 ± 11 0.3378 ± 11 0.6998 ± 133 32.622 ± 620 3144 ± 80 3570 ± 17 3653 ± 5AGC015-15 827 4 9091 0.0211 ± 7 0.2482 ± 13 0.1817 ± 33 6.220 ± 118 906 ± 16 2009 ± 15 3173 ± 8AGC015-16 236 147 8333 0.1793 ± 20 0.3364 ± 16 0.6802 ± 136 31.562 ± 631 3034 ± 76 3535 ± 21 3647 ± 7AGC015-17 377 262 9094 0.1852 ± 13 0.3369 ± 10 0.7492 ± 135 34.788 ± 661 3507 ± 130 3632 ± 18 3649 ± 5AGC015-18 208 71 9098 0.1089 ± 13 0.3282 ± 16 0.7438 ± 164 33.681 ± 674 3538 ± 150 3600 ± 20 3609 ± 7AGC015-19 872 29 20,833 0.0124 ± 4 0.2306 ± 12 0.1618 ± 29 5.145 ± 98 825 ± 14 1850 ± 10 3056 ± 9AGC015-20 378 315 12,987 0.2252 ± 14 0.3359 ± 11 0.7230 ± 137 33.512 ± 637 3306 ± 96 3596 ± 18 3644 ± 5AGC015-21 359 238 100054 0.1740 ± 12 0.3374 ± 11 0.7333 ± 139 34.132 ± 649 3371 ± 110 3615 ± 18 3651 ± 5AGC015-22 76 26 667 0.1847 ± 24 0.3354 ± 31 0.7403 ± 163 34.211 ± 821 3435æ140 3616 ± 24 3642 ± 14AGC015-23 729 17 4167 0.0189 ± 22 0.2482 ± 14 0.2331 ± 42 7.980 ± 152 1150 ± 20 2229 ± 17 3173 ± 9AGC015-24 751 4 30303 0.0022 ± 1 0.2915 ± 38 0.4422 ± 124 17.769 ± 551 2031 ± 57 2977 ± 30 3426 ± 20AGC015-25 192 88 5556 0.1402 ± 13 03248 ± 14 0.7003 ± 133 31.342 ± 627 3208 ± 89 3530 ± 19 3593 ± 7AGC015-26 163 99 10,417 0.1659 ± 15 0.3368 ± 15 0.7768 ± 155 36.081 ± 722 3706 ± 44 3668 ± 20 3648 ± 7AGC015-27 245 136 23,256 0.1598 ± 13 0.3346 ± 12 0.7002 ± 133 32.278 ± 613 3156 ± 82 3558 ± 19 3638 ± 6AGC015-28 370 92 3704 0.0951 ± 8 0.3272 ± 11 0.6732 ± 128 30.361 ± 577 3033 ± 72 3499 ± 18 3604 ± 5AGC015-29 167 101 4545 0.1653 ± 15 03334 ± 15 0.7320 ± 146 33.642 ± 673 3385 ± 110 3600 ± 19 3633 ± 7AGC015-30 1133 43 9091 0.0182 ± 6 0.1654 ± 10 0.1190 ± 21 2.713 ± 52 653 ± 11 1332 ± 14 2512 ± 10AGC015-31 288 160 21,739 0.1731 ± 6 0.3302 ± 13 0.6051 ± 115 27.560 ± 524 2673 ± 55 3405 ± 17 3618 ± 6AGC015-32 360 211 14,706 0.1750 ± 12 0.3368 ± 12 0.6899 ± 131 32.052 ± 617 3088 ± 75 3551 ± 20 3649 ± 6AGC138-1 463 267 21,124 0.1519 ± 10 0.3126 ± 10 0.6708 ± 111 28.908 ± 498 3309 ± 43 3450 ± 17 3534 ± 5AGC138-2 283 147 30130 0.1403 ± 13 0.3136 ± 13 0.7147 ± 121 30.906 ± 554 3476 ± 45 3516 ± 18 3539 ± 6AGC138-3 460 261 44,803 0.1453 ± 9 0.3133 ± 9 0.7446 ± 124 32.158 ± 553 3587 ± 46 3555 ± 17 3537 ± 4AGC138-4 451 249 3103 0.1067 ± 23 0.2329 ± 15 0.2105 ± 25 6.761 ± 96 1232 ± 13 2081 ± 13 3072 ± 10AGC138-5 268 106 20,437 0.0543 ± 9 0.3138 ± 13 0.7365 ± 97 31.869 ± 456 3558 ± 36 3546 ± 14 3540 ± 6AGC138-6 230 137 7442 0.0855 ± 14 0.3138 ± 14 0.6789 ± 87 29.371 ± 414 3340 ± 33 3466 ± 14 3540 ± 7AGC138-7 559 29 1472 0.0141 ± 26 0.2033 ± 15 0.2068 ± 24 5.796 ± 85 1212 ± 13 1946 ± 13 2853 ± 12AGC138-8 231 127 16,189 0.0984 ± 13 0.3136 ± 14 0.7375 ± 10 31.892 ± 458 3561 ± 36 3547 ± 14 3539 ± 7AGC150b-1 350 232 9817 0.1764 ± 10 0.3396 ± 9 0.6079 ± 76 28.467 + 374 3061 ± 31 3435 ± 13 3661 ± 4AGC150b-2 156 29 53,135 0.0453 ± 8 0.3396 ± 12 0.7594 ± 100 35.564 ± 497 3642 ± 37 3654 ± 14 3661 ± 5AGC150b-3 275 177 3783 0.2120 ± 13 0.3396 ± 10 0.7327 ± 93 34.308 ± 459 3543 ± 35 3619 ± 13 3661 ± 4AGC150b-5 746 12 7962 0.0043 ± 2 0.3024 ± 5 0.7171 ± 88 29.901 ± 377 3485 ± 33 3484 ± 12 3483 ± 3AGC150b-6 93 23 27,964 0.0631 ± 13 0.3399 ± 16 0.7283 ± 102 34.128 ± 522 3527 ± 38 3614 ± 15 3662 ± 7AGC150b-7 160 104 21432 0.1705 ± 13 0.3391 ± 12 0.7148 ± 96 33.418 ± 473 3476 ± 36 3593 ± 14 3659 ± 5AGC150b-8 235 3 7993 0.0038 ± 5 0.3396 ± 10 0.7469 ± 97 34.970 ± 476 3596 ± 36 3638 ± 13 3661 ± 4AGC150b-9 75 51 6727 0.1767 ± 20 0.3397 ± 18 0.7518 ± 109 35.209 ± 564 3614 ± 40 3645 ± 16 3661 ± 8AGC150b-10 543 254 2832 0.1328 ± 8 0.3397 ± 7 0.6002 ± 74 28.115 ± 361 3031 ± 30 3423 ± 13 3662 ± 3AGC150b-11 77 41 1429 0.1306 ± 26 0.3031 ± 19 0.7224 ± 103 30.189 ± 488 3505 ± 38 3493 ± 16 3486 ± 10AGC150b-12 69 38 4881 0.2555 ± 38 0.3397 ± 22 0.7296 ± 107 34.170 ± 573 3532 ± 40 3615 ± 17 3661 ± 10AGC150b-13 85 49 2850 0.1637 ± 20 0.3397 ± 16 0.7628 ± 108 35.724 ± 556 3654 ± 40 3659 ± 15 3661 ± 7AGC150b-14 45 25 14,784 0.1439 ± 32 0.3397 ± 25 0.7433 ± 119. 34.809 ± 640 3583 ± 44 3633 ± 18 3661 ± 11AGC150b-15 73 51 11,815 0.1870 ± 21 0.3396 ± 17 0.7639 ± 99 35.771 ± 522 3658 ± 36 3660 ± 14 3661 ± 8AGC150b-16 177 117 9229 0.2404 ± 13 0.3396 ± 10 0.7882 ± 90 36.907 ± 448 3747 ± 32 3691 ± 12 3661 ± 5AGC150b-17 99 45 9229 0.1259 ± 16 0.3394 ± 15 0.6915 ± 84 32.359 ± 438 3389 ± 32 3561 ± 13 3660 ± 7AGC200-1 177 135 21,739 0.2122 ± 17 0.3133 ± 14 0.7321 ± 139 31.631 ± 633 3542 ± 53 3539 ± 20 3537 ± 7AGC200-2 108 86 10,101 0.2234 ± 23 0.3107 ± 19 0.7188 ± 151 30.800 ± 678 3492 ± 56 3513 ± 21 3524 ± 10AGC200-3 1126 66 8333 0.0290 ± 8 0.1028 ± 11 0.0645 ± 12 0.914 ± 19 403 ± 7 659 ± 10 1675 ± 20AGC200-4 109 89 43,478 0.2249 ± 22 0.3144 ± 17 0.7462 ± 179 32.362 ± 809 3594 ± 66 3561 ± 24 3543 ± 8AGC200-5 124 67 8342 0.1482 ± 18 0.3118 ± 18 0.7021 ± 190 30.182 ± 845 3429 ± 72 3492 ± 28 3530 ± 9AGC200.6 170 125 11,905 0.2068 ± 17 0.2560 ± 12 0.6458 ± 123 22.802 ± 456 3212 ± 49 3218 ± 19 3222 ± 8AGC200-7 131 113 58,824 0.2341 ± 21 0.3123 ± 20 0.7443 ± 179 32.058 ± 801 3587 ± 65 3551 ± 17 3532 ± 10AGC200-8 206 126 5556 0.2023 ± 26 0.2531 ± 18 0.5211 ± 99 18.172 ± 382 2702 ± 43 2999 ± 19 3204 ± 11AGC200-9 1633 285 8333 0.0679 ± 8 0.1150 ± 9 0.0861 ± 15 1.365 ± 26 532 ± 9 874 ± 11 1880 ± 14AGC200-10 1123 19 10,003 0.0192 ± 6 0.1058 ± 9 0.0640 ± 12 0.934 ± 19 400 ± 7 670 ± 10 1729 ± 16AGC200-11 597 85 3226 0.2248 ± 16 0.2260 ± 12 0.2381 ± 45 7.421 ± 148 1377 ± 24 2164 ± 17 3023 ± 8AGC200-12 165 151 47,619 0.2461 ± 22 0.3136 ± 13 0.7372 ± 140 31.872 ± 637 3560 ± 53 3547 ± 20 3539 ± 7AGC200-13 906 63 35,714 0.0261 ± 34 0.2345 ± 7 0.3537 ± 67 11.431 ± 217 1952 ± 32 2559 ± 17 3083 ± 5AGC200-14 504 18 26,316 0.0093 ± 25 0.2513 ± 15 0.4048 ± 73 14.023 ± 266 2191 ± 34 2754 ± 15 3193 ± 10AGC200-15 124 104 6667 0.2358 ± 24 0.3104 ± 18 0.7021 ± 176 30.058 ± 751 3430 ± 65 3488 ± 24 3523 ± 9AGC200-16 112 46 18,182 0.1079 ± 18 0.2729 ± 19 0.6160 ± 129 23.188 ± 510 3095 ± 51 3234 ± 23 3323 ± 11AGC200-17 207 180 126,582 0.2452 ± 17 0.3116 ± 12 0.7322 ± 139 31.469 ± 629 3543 ± 53 3534 ± 19 3529 ± 6AGC200-18 132 77 13,158 0.1791 ± 32 0.3144 ± 15 0.7388 ± 148 32.024 ± 640 3565 ± 54 3551 ± 20 3543 ± 8AGC207-1 208 112 11,463 0.1676 ± 16 0.3300 ± 14 0.6741 ± 117 30.667 ± 563 3322 ± 45 3508 ± 18 3617 ± 7




Table 2 (Continued)

Sample no. U (ppm) Th (ppm) 206Pb/204Pb 208Pb/206Pb 207Pb/206Pb 206Pb/238U 207Pb/235U 206/238age ± 1�

207/235age ± 1�

207/206age ± 1�

AGC207-2 487 117 13,019 0.0843 ± 11 0.2850 ± 12 0.3324 ± 55 13.064 ± 229 1850 ± 27 2684 ± 17 3391 ± 7AGC207-3 245 124 4071 0.2685 ± 22 0.3311 ± 15 0.6177 ± 105 28.201 ± 511 3101 ± 42 3426 ± 18 3622 ± 7AGC207-4 410 29 20,986 0.0165 ± 6 0.3303 ± 11 0.7502 ± 126 34.166 ± 596 3608 ± 46 3615 ± 17 3618 ± 5AGC207-5 196 134 16,485 0.1584 ± 16 0.3312 ± 16 0.6967 ± 123 31.813 ± 599 3408 ± 47 3545 ± 19 3623 ± 7AGC207-6 126 48 16,215 0.0909 ± 15 0.3301 ± 18 0.7141 ± 131 32.50o ± 643 3474 ± 49 3566 ± 19 3617 ± 8AGC207-7 607 287 21,182 0.1260 ± 9 0.2891 ± 9 0.4710 ± 77 18.773 ± 323 2488 ± 34 3030 ± 17 3413 ± 5AGC207-8 516 47 9248 0.0259 ± 8 0.2967 ± 11 0.5509 ± 92 22.539 ± 394 2829 ± 38 3207 ± 17 3453 ± 6AGC207-9 235 64 10,844 0.0653 ± 11 0.3304 ± 13 0.6524 ± 111 29.725 ± 536 3238 ± 43 3478 ± 18 3619 ± 6AGC207-10 399 143 29,542 0.1316 ± 11 0.2956 ± 11 0.4945 ± 82 20.153 ± 353 2590 ± 36 3099 ± 17 3447 ± 6AGC207-11 247 150 29,727 0.1546 ± 12 0.3310 ± 12 0.7314 ± 126 33.386 ± 601 3539 ± 47 3592 ± 18 3622 ± 6AGC207-12 683 180 5558 0.2212 ± 16 0.2982 ± 11 0.2628 ± 43 10.804 ± 185 1504 ± 22 2506 ± 16 3461 ± 6AGC207-13 153 84 6611 0.1614 ± 22 0.3316 ± 18 0.6108 ± 108 27.926 ± 537 3073 ± 43 3417 ± 19 3624 ± 8AGC207-14 924 79 13,701 0.0322 ± 10 0.2164 ± 11 0.1319 ± 21 3.936 ± 69 799 ± 12 1621 ± 14 2954 ± 8AGC207-15 196 138 49,334 0.1068 ± 8 0.3308 ± 10 0.7495 ± 92 34.179 ± 443 3606 ± 34 3615 ± 13 3621 ± 5AGC207-16 496 9 119,190 0.0018 ± 2 0.2781 ± 7 0.6830 ± 78 26.193 ± 316 3356 ± 30 3354 ± 12 3352 ± 4AGC216-1 205 173 25,183 0.2332 ± 18 0.3156 ± 15 0.6517 ± 119 28.354 ± 549 3235 ± 46 3431 ± 19 3548 ± 7AGC216-2 86 46 17,590 0.1371 ± 21 0.3165 ± 21 0.7277 ± 143 31.753 ± 681 3525 ± 53 3543 ± 21 3553 ± 10AGC216-3 190 184 25,628 0.2462 ± 16 0.3179 ± 13 0.7527 ± 137 32.996 ± 630 3618 ± 50 3580 ± 19 3560 ± 6AGC216-4 147 131 6803 0.2566 ± 23 0.3160 ± 16 0.7285 ± 135 31.741 ± 628 3528 ± 50 3542 ± 19 3550 ± 8AGC216-5 156 143 19,708 0.2371 ± 19 0.3168 ± 15 0.7081 ± 131 30.933 ± 609 3451 ± 49 3517 ± 19 3555 ± 7AGC216-6 272 49 51,230 0.0466 ± 7 0.2627 ± 10 0.6595 ± 117 23.885 ± 447 3265 ± 46 3264 ± 18 3263 ± 6AGC216-7 112 73 16,173 0.1774 ± 20 0.3154 ± 18 0.7321 ± 137 31.839 ± 644 3541 ± 51 3545 ± 20 3548 ± 9AGC216-8 169 147 30,012 0.2280 ± 17 0.3162 ± 14 0.7345 ± 134 32.018 ± 621 3550 ± 50 3551 ± 19 3551 ± 7AGC216-9 385 146 16,748 0.1483 ± 12 0.2737 ± 11 0.4042 ± 71 15.251 ± 280 2188 ± 32 2831 ± 18 3327 ± 6AGC216-10 135 99 22,109 0.1970 ± 18 0.3180 ± 16 0.7356 ± 136 32.252 ± 637 3554 ± 51 3558 ± 19 3560 ± 8AGC216-11 136 84 11,182 0.1645 ± 13 0.3169 ± 12 0.7373 ± 123 32.216 ± 565 3560 ± 46 3557 ± 17 3555 ± 6AGC216-12 171 117 23,552 0.1804 ± 10 0.3166 ± 9 0.6678 ± 110 29.154 ± 497 3297 ± 42 3459 ± 17 3554 ± 4AGC216-13 158 115 15,326 0.1967 ± 12 0.3165 ± 10 0.7032 ± 116 30.689 ± 528 3433 ± 44 3509 ± 17 3553 ± 5AGC216-14 135 87 12,705 0.1641 ± 11 0.3166 ± 10 0.7241 ± 120 31.611 ± 548 3512 ± 45 3538 ± 17 3553 ± 5AGC216-15 150 105 6472 0.1887 ± 14 0.3165 ± 11 0.5916 ± 98 25.820 ± 447 2996 ± 40 3340 ± 17 3553 ± 5AGC216-16 81 48 8728 0.1563 ± 17 0.3165 ± 14 0.6956 ± 119 30.350 ± 551 3404 ± 45 3498 ± 18 3553 ± 7AGC228-1 374 109 11,346 0.0851 ± 12 0.2982 ± 13 0.4865 ± 86 20.005 ± 377 2556 ± 37 3092 ± 18 3461 ± 7AGC228-2 136 74 5976 0.1368 ± 20 0.2984 ± 16 0.7444 ± 138 30.630 ± 613 3587 ± 51 3507 ± 20 3462 ± 8AGC228-3 247 146 26853 0.1485 ± 12 0.2990 ± 12 0.7112 ± 128 29.317 ± 552 3463 ± 48 3464 ± 18 3465 ± 6AGC228-4 808 417 8553 0.1343 ± 9 0.3001 ± 8 0.3883 ± 67 16.064 ± 285 2115 ± 31 2881 ± 17 3471 ± 4AGC228-5 117 53 41,339 0.1181 ± 20 0.3008 ± 18 0.6957 ± 132 28.855 ± 597 3404 ± 50 3449 ± 20 3474 ± 9AGC228-6 192 119 17,612 0.1585 ± 14 0.2990 ± 13 0.7128 ± 130 29.387 ± 565 3469 ± 49 3467 ± 19 3465 ± 7AGC228-7 223 104 29,002 0.1178 ± 12 0.2990 ± 13 0.6207 ± 111 25.588 ± 483 3113 ± 44 3331 ± 18 3465 ± 7AGC228-8 201 105 21,227 0.1325 ± 13 0.2989 ± 13 0.7245 ± 131 29.864 ± 572 3513 ± 49 3482 ± 19 3465 ± 7AGC228-9 398 230 44,326 0.1519 ± 10 0.2997 ± 10 0.6211 ± 109 25.774 ± 468 3114 ± 43 3334 ± 18 3469 ± 5AGC228-10 242 143 37,216 0.1710 ± 14 0.2994 ± 12 0.7159 ± 128 29.556 ± 557 3481 ± 48 3472 ± 19 3467 ± 6AGC351-1 111 81 10,584 0.1891 ± 16 0.2970 ± 13 0.6732 ± 72 27.564 ± 332 3318 ± 28 3404 ± 12 3455 ± 7AGC351-2 67 44 8145 0.1673 ± 20 0.2979 ± 16 0.7087 ± 87 29.113 ± 407 3453 ± 33 3457 ± 14 3460 ± 8AGC351-3 52 28 10,782 0.1466 ± 20 0.2966 ± 18 0.6765 ± 88 27.666 ± 415 3331 ± 34 3407 ± 15 3453 ± 9AGC351-4 82 55 13,475 0.1758 ± 17 0.2969 ± 14 0.6878 ± 80 28.152 ± 371 3374 ± 31 3424 ± 13 3454 ± 8AGC351-5 73 57 7704 0.1673 ± 20 0.2967 ± 16 0.6606 ± 79 27.023 ± 371 3270 ± 31 3384 ± 13 3453 ± 8AGC351-6 76 48 9566 0.1631 ± 18 0.2970 ± 15 0.6999 ± 84 28.661 ± 390 3420 ± 32 3442 ± 13 3455 ± 8AGC351-7 100 70 12,773 0.1837 ± 17 0.2970 ± 14 0.6701 ± 76 27.440 ± 349 3306 ± 29 3399 ± 12 3455 ± 7AGC351-8 81 54 7896 0.1701 ± 20 0.2972 ± 16 0.6983 ± 83 28.612 ± 392 3414 ± 32 3440 ± 13 3456 ± 8AGC351-9 61 39 4692 0.1632 ± 23 0.2979 ± 18 0.6824 ± 86 28.027 ± 409 3353 ± 33 3420 ± 14 3459 ± 9AGC351-10 54 30 6729 0.1452 ± 22 0.2966 + 18 0.6820 ± 88 27.891 ± 417 3352 ± 34 3415 ± 15 3453 ± 10AGC351-11 46 27 6748 0.1537 ± 24 0.2976 ± 20 0.7027 ± 97 28.835 ± 463 3431 ± 37 3448 ± 16 3458 ± 10AGC351-12 62 41 57,040 0.1657 ± 22 0.2970 ± 17 0.6801 ± 87 27.852 ± 409 3345 ± 33 3414 ± 14 3455 ± 9AGC370-1 675 248 1058 0.1386 ± 22 0.2933 ± 13 0.4786 ± 47 19.353 ± 219 2521 ± 21 3060 ± 11 3435 ± 7AGC370-2 298 94 159 0.1244 ± 65 0.2657 ± 30 0.6739 ± 76 24.691 ± 419 3321 ± 29 3296 ± 17 3281 ± 18AGC370-3 780 551 15,413 0.1816 ± 8 0.2921 ± 7 0.5107 ± 49 20.565 ± 210 2659 ± 21 3118 ± 10 3429 ± 4AGC370-4 602 281 12,809 0.1164 ± 11 0.2208 ± 9 0.2523 ± 24 7.680 ± 82 1450 ± 12 2194 ± 10 2986 ± 6AGC370-5 401 300 3699 0.1891 ± 14 0.2847 ± 10 0.5174 ± 52 20.306 ± 221 2688 ± 22 3106 ± 11 3389 ± 5AGC370-6 398 292 400 0.2423 ± 35 0.3162 ± 18 0.6231 ± 65 27.164 ± 339 3122 ± 26 3389 ± 12 3551 ± 9AGC370-7 79 23 4995 0.0749 ± 20 0.3165 ± 19 0.7118 ± 97 31.068 ± 483 3465 ± 36 3521 ± 15 3553 ± 9AGC370-8 482 292 1185 0.1607 ± 20 0.2698 ± 11 0.4457 ± 44 16.582 ± 188 2376 ± 20 2911 ± 11 3305 ± 7AGC370-9 445 189 21,887 0.1118 ± 7 0.3166 ± 8 0.6981 ± 70 30.470 ± 324 3413 ± 27 3502 ± 10 3553 ± 4AGC370-10 397 168 4057 0.1129 ± 11 0.3166 ± 9 0.7083 ± 71 30.918 ± 333 3452 ± 27 3516 ± 11 3553 ± 4AGC370-11 136 43 4681 0.0858 ± 18 0.3163 ± 16 0.7172 ± 86 31.277 ± 424 3485 ± 32 3528 ± 13 3552 ± 8AGC370-12 234 105 1700 0.1340 ± 20 0.3164 ± 13 0.7534 ± 82 32.873 ± 399 3620 ± 30 3577 ± 12 3553 ± 7AGC370-13 197 87 17,467 0.1156 ± 11 0.3163 ± 12 0.7280 ± 82 31.745 ± 390 3526 ± 31 3542 ± 12 3552 ± 6AGC370-14 177 104 9980 0.1489 ± 14 0.3164 ± 13 0.6716 ± 77 29.300 ± 370 3312 ± 30 3464 ± 12 3553 ± 6

0.706

A

εbto

AGC370-15 123 33 7237 0.0745 ± 15 0.3167 ± 15

GC01-5-1 is spot on grain 1, AGC01-5-2 is spot on grain 2, etc.


Nd(t)-value of −1.6 (Munich, Table 5). Furthermore, a gabbroicoudin (AGC150c), interlayered with the heterogeneous gneiss athe locality of AGC150, contained no zircon but, assuming an agef 3.64 Ga, the εNd(t)-values for duplicate analyses are −1.9 and

4 ± 84 30.843 ± 414 3445 ± 32 3514 ± 13 3554 ± 7


−2.0 (Munich, Table 5). The lightest part of the layered gneiss atthis locality (sample AGC150l) is a horrnblende-free, biotite poor,quartzo-feldspathic rock of which only the whole-rock Sm–Nd iso-topes were determined (Table 5). Assuming an age of 3644 Ma as for





Table 3Lu–Hf isotopic data for zircons from granitoid gneisses of the Ancient Gneiss Complex, Swaziland.

Sample and spot U–Pb age 176Yb/177Hf 2� 176Lu/177Hf 2� 176Hf/177Hf 2� εHf(0) 2� εHf(t) 2� tDM tc fLu/Hf

AGC138 01 3537 0.03498 0.00247 0.001419 0.000093 0.280550 0.000014 −78.57 0.48 −1.9 0.5 3.77 3.84 −0.96AGC138 02 3537 0.03937 0.00062 0.001501 0.000021 0.280535 0.000011 −79.12 0.38 −2.6 0.4 3.80 3.88 −0.95AGC138 03 3537 0.01921 0.00031 0.000724 0.000015 0.280498 0.000012 −80.40 0.42 −2.0 0.4 3.78 3.85 −0.98AGC138 04 3537 0.03138 0.00091 0.001267 0.000035 0.280576 0.000018 −77.66 0.64 −0.6 0.6 3.72 3.78 −0.96AGC138 05 3537 0.03702 0.00217 0.001497 0.000087 0.280649 0.000022 −75.08 0.78 1.5 0.8 3.65 3.68 −0.95AGC138 06 3537 0.03261 0.00076 0.001329 0.000029 0.280528 0.000013 −79.35 0.45 −2.4 0.4 3.79 3.87 −0.96AGC138 07 3537 0.02975 0.00112 0.001229 0.000047 0.280522 0.000012 −79.56 0.43 −2.4 0.4 3.79 3.87 −0.96AGC138 08 3537 0.05222 0.00198 0.002093 0.000076 0.280587 0.000012 −77.26 0.44 −2.2 0.4 3.79 3.86 −0.94AGC138 09 3537 0.03765 0.00093 0.001551 0.000039 0.280550 0.000012 −78.59 0.44 −2.2 0.4 3.79 3.86 −0.95Median −2.2

AGC150 01 3644 0.06744 0.00219 0.002290 0.000075 0.280561 0.000020 −78.18 0.73 −1.2 0.7 3.85 3.90 −0.93AGC150 02 3644 0.04036 0.00066 0.001397 0.000022 0.280487 0.000026 −80.82 0.91 −1.7 0.9 3.86 3.92 −0.96AGC150 03 3644 0.04388 0.00045 0.001554 0.000013 0.280705 0.000024 −73.08 0.85 5.8 0.9 3.58 3.56 −0.95AGC150 04 3644 0.05777 0.00089 0.002087 0.000033 0.280799 0.000029 −69.77 1.05 7.8 1.0 3.50 3.46 −0.94AGC150 05 3644 0.11851 0.00245 0.003858 0.000074 0.280608 0.000043 −76.53 1.54 −3.5 1.5 3.95 4.01 −0.88AGC150 06 3644 0.03523 0.00076 0.001267 0.000028 0.280445 0.000035 −82.30 1.27 −2.8 1.3 3.90 3.98 −0.96AGC150 07 3644 0.04371 0.00026 0.001609 0.000011 0.280443 0.000022 −82.38 0.78 −3.8 0.8 3.94 4.03 −0.95AGC150 08 3644 0.06315 0.00017 0.002233 0.000002 0.280733 0.000021 −72.09 0.76 5.0 0.8 3.60 3.59 −0.93AGC150 09 3644 0.05238 0.00078 0.001833 0.000027 0.280554 0.000025 −78.45 0.87 −0.4 0.9 3.81 3.86 −0.94AGC150 10 3644 0.06039 0.00039 0.002195 0.000010 0.280532 0.000023 −79.21 0.81 −2.0 0.8 3.88 3.94 −0.93Median −1.4

AGC150b 01 3661 0.03420 0.00080 0.001345 0.000033 0.280456 0.000044 −81.92 1.55 −2.3 1.6 3.89 3.97 −0.96AGC150b 02 3661 0.06620 0.00234 0.002545 0.000089 0.280614 0.000045 −76.31 1.61 0.4 1.6 3.80 3.84 −0.92AGC15bb 03 3661 0.02362 0.00043 0.000941 0.000016 0.280410 0.000031 −83.53 1.12 −2.9 1.1 3.91 3.99 −0.97AGC150b 04 3661 0.03446 0.00118 0.001364 0.000038 0.280553 0.000030 −78.46 1.07 1.2 1.1 3.76 3.80 −0.96AGC150b 05 3661 0.05416 0.00106 0.002092 0.000032 0.280508 0.000027 −80.07 0.98 −2.3 1.0 3.90 3.97 −0.94AGC150b 06 3661 0.05637 0.00096 0.002413 0.000032 0.280600 0.000029 −76.82 1.03 0.2 1.0 3.81 3.84 −0.93AGC150b 07 3661 0.04578 0.00132 0.001761 0.000050 0.280470 0.000033 −81.42 1.18 −2.8 1.2 3.92 3.99 −0.95AGC150b 08 3661 0.04029 0.00201 0.001516 0.000052 0.280474 0.000034 −81.27 1.20 −2.0 1.2 3.89 3.95 −0.95AGC150b 09 3661 0.03847 0.00075 0.001477 0.000028 0.280384 0.000035 −84.46 1.23 −5.1 1.2 4.00 4.11 −0.96AGC150b 10 3661 0.05764 0.00135 0.002164 0.000048 0.280568 0.000036 −77.94 1.29 −0.3 1.3 3.82 3.87 −0.93AGC150b 11 3661 0.03786 0.00031 0.001485 0.000012 0.280443 0.000030 −82.37 1.09 −3.1 1.1 3.93 4.00 −0.96AGC150b 12 3661 0.02616 0.00032 0.001011 0.000012 0.280454 0.000034 −81.96 1.23 −1.4 1.2 3.86 3.93 −0.97AGC150b 13 3661 0.04059 0.00045 0.001585 0.000019 0.280473 0.000034 −81.29 1.22 −2.2 1.2 3.90 3.96 −0.95AGC150b 14 3661 0.08100 0.00121 0.003195 0.000051 0.280640 0.000031 −75.39 1.10 −0.3 1.1 3.83 3.87 −0.90Median −2.1

AGC-01-5L 01 3647 0.04851 0.00036 0.001817 0.000005 0.280513 0.000012 −79.87 0.42 −1.7 0.4 3.87 3.93 −0.95AGC-01-5L 02 3647 0.03820 0.00017 0.001268 0.000006 0.280461 0.000013 −81.73 0.46 −2.2 0.5 3.88 3.96 −0.96AGC-01-5L 03 3647 0.04061 0.00028 0.001532 0.000009 0.280545 0.000010 −78.74 0.34 0.2 0.3 3.80 3.84 −0.95AGC-01-5L 04 3647 0.06545 0.00081 0.002410 0.000032 0.280712 0.000016 −72.84 0.55 3.9 0.6 3.65 3.66 −0.93AGC-01-5L 05 3647 0.04774 0.00028 0.001795 0.000008 0.280743 0.000015 −71.75 0.53 6.6 0.5 3.55 3.52 −0.95AGC-01-5L 06 3647 0.05229 0.00128 0.001798 0.000039 0.280695 0.000012 −73.44 0.44 4.8 0.4 3.62 3.61 −0.95AGC-01-5L 07 3647 0.04015 0.00027 0.001414 0.000009 0.280543 0.000011 −78.82 0.40 0.4 0.4 3.79 3.83 −0.96AGC-01-5L 08 3647 0.04930 0.00140 0.001689 0.000045 0.280525 0.000013 −79.47 0.46 −1.0 0.5 3.84 3.90 −0.95AGC-01-5L 09 3647 0.07535 0.00097 0.002546 0.000040 0.280735 0.000016 −72.02 0.56 4.4 0.6 3.63 3.63 −0.92AGC01-55L 10 3647 0.05345 0.00093 0.001713 0.000028 0.280590 0.000013 −77.16 0.46 1.3 0.5 3.75 3.78 −0.95Median 0.8

AGC-01-5D 01 3647 0.06553 0.00053 0.002120 0.000013 0.280489 0.000016 −80.72 0.59 −3.3 0.6 3.93 4.01 −0.94AGC-01-5D 02 3647 0.08689 0.00103 0.002788 0.000028 0.280548 0.000016 −78.66 0.57 −2.9 0.6 3.92 3.99 −0.92AGC-01-5D 03 3647 0.09886 0.00064 0.003172 0.000019 0.280610 0.000016 −76.47 0.57 −1.7 0.6 3.88 3.93 −0.90AGC-01-5D 04 3647 0.04322 0.00013 0.001453 0.000003 0.280454 0.000015 −81.97 0.54 −2.9 0.5 3.91 3.99 −0.96AGC-01-5D 05 3647 0.03783 0.00045 0.001258 0.000012 0.280518 0.000014 −79.71 0.50 −0.1 0.5 3.81 3.85 −0.96AGC-01-5D 06 3647 0.08848 0.00069 0.002829 0.000020 0.280597 0.000015 −76.90 0.53 −1.2 0.5 3.86 3.91 −0.91AGC-01-5D 07 3647 0.06736 0.00153 0.002315 0.000044 0.280568 0.000014 −77.95 0.49 −1.0 0.5 3.84 3.90 −0.93AGC-01-5D 08 3647 0.10488 0.00547 0.003036 0.000126 0.280598 0.000017 −76.88 0.62 −1.7 0.6 3.88 3.93 −0.91AGC-01-5D 09 3647 0.05738 0.00060 0.001779 0.000014 0.280474 0.000014 −81.26 0.50 −3.0 0.5 3.92 4.00 −0.95AGC-01-5D 10 3647 0.07970 0.00383 0.002583 0.000117 0.280607 0.000017 −76.56 0.61 −0.3 0.6 3.82 3.86 −0.92AGC-01-5D 11 3647 0.05091 0.00071 0.001999 0.000025 0.280570 0.000013 −77.87 0.48 −0.1 0.5 3.81 3.85 −0.94AGC01-55D 12 3647 0.06001 0.00047 0.002189 0.000016 0.280493 0.000013 −80.59 0.45 −3.3 0.5 3.93 4.01 −0.93AGC-01-5D 13 3647 0.01790 0.00011 0.000647 0.000004 0.280379 0.000013 −84.64 0.48 −3.6 0.5 3.93 4.02 −0.98AGC-01-5D 14 3647 0.03675 0.00035 0.001275 0.000015 0.280435 0.000013 −82.65 0.45 −3.1 0.5 3.92 4.00 −0.96AGC-01-5D 15 3647 0.07352 0.00078 0.002357 0.000025 0.280541 0.000015 −78.91 0.54 −2.1 0.5 3.89 3.95 −0.93AGC-01-5D 16 3647 0.11151 0.00337 0.003469 0.000088 0.280673 0.000019 −74.23 0.69 −0.1 0.7 3.82 3.86 −0.90AGC-01-5D 17 3647 0.06701 0.00132 0.002306 0.000042 0.280606 0.000016 −76.61 0.57 0.4 0.6 3.79 3.83 −0.93AGC-01-5D 18 3647 0.04818 0.00025 0.001647 0.000010 0.280498 0.000013 −80.43 0.46 −1.8 0.5 3.87 3.94 −0.95AGC-01-5D 19 3647 0.07169 0.00323 0.002397 0.000096 0.280743 0.000015 −71.77 0.53 5.0 0.5 3.61 3.60 −0.93AGC-01-5D 20 3647 0.08972 0.00153 0.002897 0.000047 0.280576 0.000015 −77.66 0.53 −2.2 0.5 3.89 3.95 −0.91AGC-01-5D 21 3647 0.09377 0.00039 0.003044 0.000015 0.280549 0.000014 −78.63 0.51 −3.5 0.5 3.95 4.02 −0.91AGC-01-5D 22 3647 0.05745 0.00093 0.002016 0.000028 0.280511 0.000013 −79.97 0.46 −2.3 0.5 3.89 3.96 −0.94AGC-01-5D 23 3647 0.02082 0.00025 0.000668 0.000012 0.280456 0.000011 −81.92 0.41 −0.9 0.4 3.83 3.89 −0.98AGC-01-5D 24 3647 0.04913 0.00048 0.001553 0.000017 0.280478 0.000013 −81.14 0.48 −2.3 0.5 3.89 3.96 −0.95AGC-01-5D 25 3647 0.09572 0.00198 0.003071 0.000059 0.280608 0.000017 −76.54 0.61 −1.5 0.6 3.87 3.92 −0.91AGC-01-5D 26 3647 0.10471 0.00127 0.003436 0.000037 0.280597 0.000017 −76.90 0.59 −2.8 0.6 3.92 3.98 −0.90AGC-01-5D 27 3647 0.08616 0.00104 0.002734 0.000032 0.280512 0.000016 −79.94 0.58 −4.1 0.6 3.97 4.05 −0.92





Table 3 (Continued)


AGC-01-5D 28 3647 0.07492 0.00101 0.002600 0.000031 0.280604 0.000014 −76.67 0.51 −0.4 0.5 3.82 3.87 −0.92AGC-01-5D 29 3647 0.07080 0.00056 0.002249 0.000020 0.280587 0.000013 −77.26 0.48 −0.1 0.5 3.81 3.85 −0.93AGC-01-5D 30 3647 0.03906 0.00066 0.001335 0.000026 0.280498 0.000015 −80.43 0.54 −1.0 0.5 3.84 3.90 −0.96Median −1.8

AGC185 01 3505 0.03187 0.00177 0.001189 0.000061 0.280568 0.000022 −77.95 0.79 −1.4 0.8 3.73 3.80 −0.96AGC185 02 3505 0.02268 0.00014 0.000922 0.000005 0.280529 0.000015 −79.34 0.54 −2.2 0.5 3.75 3.83 −0.97AGC185 03 3505 0.05315 0.00046 0.002040 0.000019 0.280623 0.000024 −76.01 0.87 −1.5 0.9 3.74 3.80 −0.94AGC185 05 3505 0.07032 0.00134 0.002450 0.000045 0.280659 0.000022 −74.73 0.77 −1.2 0.8 3.73 3.79 −0.93AGC185 07 3505 0.05243 0.00289 0.001775 0.000093 0.280586 0.000035 −77.30 1.25 −2.2 1.3 3.76 3.83 −0.95AGC185 08 3505 0.04203 0.00185 0.001425 0.000055 0.280530 0.000024 −79.29 0.85 −3.3 0.8 3.80 3.89 −0.96AGC185 09 3505 0.05121 0.00078 0.001930 0.000030 0.280615 0.000021 −76.28 0.76 −1.5 0.8 3.74 3.80 −0.94Median −1.9

AGC200 01 3534 0.03792 0.00036 0.001328 0.000013 0.280560 0.000017 −78.23 0.62 −1.4 0.6 3.76 3.82 −0.96AGC200 02 3534 0.02082 0.00055 0.000755 0.000016 0.280570 0.000015 −77.85 0.53 0.4 0.5 3.69 3.74 −0.98AGC200 03 3534 0.03396 0.00190 0.001175 0.000063 0.280654 0.000018 −74.89 0.64 2.4 0.6 3.61 3.64 −0.96AGC200 04 3534 0.02360 0.00022 0.000851 0.000008 0.280567 0.000016 −77.98 0.56 0.0 0.6 3.70 3.75 −0.97AGC200 05 3534 0.01842 0.00047 0.000681 0.000015 0.280607 0.000015 −76.58 0.54 1.9 0.5 3.63 3.66 −0.98AGC200 06 3534 0.02275 0.00054 0.000793 0.000016 0.280561 0.000014 −78.19 0.49 0.0 0.5 3.70 3.76 −0.98AGC200 07 3534 0.02993 0.00138 0.001075 0.000046 0.280576 0.000016 −77.67 0.58 −0.2 0.6 3.71 3.77 −0.97AGC200 08 3534 0.01516 0.00005 0.000602 0.000001 0.280609 0.000012 −76.51 0.44 2.1 0.4 3.62 3.65 −0.98AGC200 09 3534 0.01546 0.00006 0.000621 0.000003 0.280579 0.000012 −77.56 0.44 1.0 0.4 3.66 3.71 −0.98AGC200 10 3534 0.03060 0.00061 0.001085 0.000019 0.280580 0.000016 −77.53 0.57 −0.1 0.6 3.71 3.76 −0.97AGC200 11 3534 0.02766 0.00028 0.000989 0.000009 0.280606 0.000017 −76.60 0.61 1.1 0.6 3.66 3.70 −0.97AGC200 12 3534 0.01641 0.00095 0.000612 0.000033 0.280601 0.000014 −76.76 0.50 1.9 0.5 3.63 3.66 −0.98AGC200 13 3534 0.02864 0.00098 0.000999 0.000031 0.280609 0.000017 −76.51 0.60 1.2 0.6 3.66 3.70 −0.97AGC200 14 3534 0.02418 0.00060 0.000849 0.000020 0.280562 0.000015 −78.15 0.53 −0.1 0.5 3.71 3.76 −0.97AGC200 15 3534 0.02473 0.00070 0.000877 0.000021 0.280547 0.000015 −78.70 0.53 −0.7 0.5 3.73 3.79 −0.97AGC200 16 3534 0.03531 0.00030 0.001213 0.000011 0.280586 0.000017 −77.32 0.59 −0.2 0.6 3.71 3.76 −0.96AGC200 17 3534 0.03897 0.00050 0.001319 0.000015 0.280582 0.000016 −77.45 0.57 −0.6 0.6 3.73 3.78 −0.96AGC200 18 3534 0.02329 0.00025 0.000808 0.000008 0.280585 0.000014 −77.34 0.51 0.8 0.5 3.67 3.72 −0.98AGC200 19 3534 0.01771 0.00010 0.000659 0.000004 0.280596 0.000011 −76.95 0.41 1.5 0.4 3.64 3.68 −0.98AGC200 20 3534 0.01472 0.00035 0.000547 0.000013 0.280614 0.000013 −76.30 0.46 2.5 0.5 3.61 3.63 −0.98AGC200 21 3534 0.02197 0.00021 0.000771 0.000008 0.280601 0.000015 −76.77 0.53 1.5 0.5 3.65 3.68 −0.98AGC200 22 3534 0.02336 0.00017 0.000834 0.000008 0.280565 0.000016 −78.05 0.57 0.0 0.6 3.70 3.76 −0.97AGC200 23 3534 0.01984 0.00024 0.000733 0.000010 0.280620 0.000014 −76.11 0.51 2.2 0.5 3.62 3.65 −0.98AGC200 24 3534 0.09719 0.00798 0.003153 0.000248 0.280683 0.000025 −73.87 0.89 −1.4 0.9 3.77 3.83 −0.91AGC200 25 3534 0.04391 0.00024 0.001489 0.000009 0.280611 0.000016 −76.43 0.58 0.1 0.6 3.70 3.75 −0.96Median 0.4

AGC207 01 3601 0.06877 0.00456 0.002169 0.000128 0.280546 0.000015 −78.73 0.52 −2.4 0.5 3.86 3.93 −0.93AGC207 02 3601 0.07836 0.00145 0.002776 0.000048 0.280549 0.000013 −78.61 0.45 −3.8 0.4 3.91 3.99 −0.92AGC207 03 3601 0.04985 0.00074 0.002166 0.000051 0.280494 0.000013 −80.56 0.45 −4.3 0.5 3.93 4.02 −0.93AGC207 04 3601 0.04733 0.00161 0.001650 0.000051 0.280484 0.000013 −80.93 0.46 −3.4 0.5 3.89 3.97 −0.95AGC207 05 3601 0.06694 0.00125 0.002383 0.000043 0.280559 0.000013 −78.28 0.46 −2.5 0.5 3.86 3.93 −0.93AGC207 06 3601 0.03947 0.00062 0.001400 0.000019 0.280545 0.000014 −78.75 0.51 −0.6 0.5 3.78 3.83 −0.96AGC207 07 3601 0.08497 0.00492 0.002982 0.000169 0.280568 0.000018 −77.96 0.63 −3.7 0.6 3.91 3.99 −0.91AGC207 08 3601 0.02633 0.00106 0.000963 0.000032 0.280517 0.000014 −79.74 0.50 −0.5 0.5 3.77 3.83 −0.97AGC207 09 3601 0.03077 0.00066 0.001120 0.000019 0.280550 0.000012 −78.59 0.42 0.3 0.4 3.75 3.79 −0.97AGC207 10 3601 0.04421 0.00048 0.001562 0.000014 0.280549 0.000011 −78.61 0.40 −0.8 0.4 3.79 3.85 −0.95Median −2.5

AGC216 01 3553 0.04749 0.00237 0.001726 0.000083 0.280628 0.000019 −75.81 0.66 0.5 0.7 3.70 3.74 −0.95AGC216 02 3553 0.01760 0.00024 0.000744 0.000008 0.280598 0.000010 −76.86 0.35 1.9 0.3 3.64 3.67 −0.98AGC216 03 3553 0.02635 0.00189 0.000997 0.000064 0.280579 0.000017 −77.57 0.62 0.5 0.6 3.70 3.74 −0.97AGC216 04 3553 0.03313 0.00051 0.001264 0.000020 0.280559 0.000012 −78.27 0.41 −0.8 0.4 3.75 3.81 −0.96AGC216 05 3553 0.01780 0.00116 0.000703 0.000040 0.280563 0.000013 −78.14 0.45 0.7 0.4 3.69 3.73 −0.98AGC216 06 3553 0.02207 0.00057 0.000958 0.000024 0.280607 0.000009 −76.56 0.32 1.7 0.3 3.65 3.68 −0.97AGC216 07 3553 0.01668 0.00043 0.000662 0.000015 0.280603 0.000013 −76.70 0.48 2.2 0.5 3.63 3.66 −0.98AGC216 08 3553 0.01996 0.00041 0.000850 0.000014 0.280599 0.000010 −76.85 0.36 1.6 0.4 3.65 3.69 −0.97AGC216 09 3553 0.02420 0.00033 0.001064 0.000014 0.280617 0.000009 −76.20 0.32 1.8 0.3 3.65 3.68 −0.97AGC216 10 3553 0.02414 0.00032 0.000923 0.000012 0.280558 0.000013 −78.28 0.45 0.0 0.4 3.72 3.77 −0.97AGC216 11 3553 0.01969 0.00124 0.000776 0.000040 0.280547 0.000012 −78.68 0.42 0.0 0.4 3.72 3.77 −0.98AGC216 12 3553 0.02165 0.00046 0.000888 0.000019 0.280632 0.000012 −75.68 0.42 2.7 0.4 3.61 3.63 −0.97Median 1.2

AGC228 01 3466 0.04443 0.00091 0.001800 0.000034 0.280670 0.000014 −74.32 0.49 −0.1 0.5 3.65 3.70 −0.95AGC228 02 3466 0.03571 0.00091 0.001466 0.000034 0.280656 0.000012 −74.85 0.44 0.2 0.4 3.64 3.69 −0.96AGC228 03 3466 0.03337 0.00105 0.001331 0.000040 0.280634 0.000013 −75.60 0.46 −0.3 0.5 3.65 3.71 −0.96AGC228 04 3466 0.03089 0.00066 0.001261 0.000022 0.280652 0.000013 −74.97 0.47 0.5 0.5 3.62 3.67 −0.96AGC228 05 3466 0.03341 0.00107 0.001408 0.000045 0.280647 0.000015 −75.15 0.52 0.0 0.5 3.64 3.69 −0.96AGC228 06 3466 0.02563 0.00082 0.001068 0.000032 0.280632 0.000014 −75.69 0.50 0.3 0.5 3.63 3.68 −0.97AGC228 07 3466 0.02904 0.00179 0.001166 0.000068 0.280646 0.000013 −75.19 0.46 0.5 0.5 3.62 3.67 −0.96AGC228 08 3466 0.03992 0.00024 0.001594 0.000009 0.280650 0.000013 −75.05 0.47 −0.3 0.5 3.66 3.71 −0.95AGC228 09 3466 0.04530 0.00100 0.001873 0.000032 0.280664 0.000012 −74.53 0.42 −0.5 0.4 3.66 3.72 −0.94AGC228 10 3466 0.04976 0.00223 0.001920 0.000085 0.280677 0.000013 −74.10 0.48 −0.2 0.5 3.65 3.70 −0.94AGC228 11 3466 0.03635 0.00103 0.001537 0.000044 0.280614 0.000025 −76.32 0.89 −1.5 0.9 3.70 3.77 −0.95AGC228 12 3466 0.03189 0.00050 0.001364 0.000018 0.280636 0.000029 −75.54 1.03 −0.3 1.0 3.65 3.71 −0.96




Table 3 (Continued)


AGC228 13 3466 0.02656 0.00087 0.001138 0.000037 0.280722 0.000028 −72.50 1.00 3.3 1.0 3.52 3.53 −0.97AGC228 14 3466 0.02665 0.00039 0.001218 0.000017 0.280625 0.000023 −75.93 0.82 −0.3 0.8 3.65 3.71 −0.96AGC228 15 3466 0.02579 0.00022 0.001149 0.000007 0.280650 0.000020 −75.04 0.71 0.7 0.7 3.61 3.66 −0.97AGC228 16 3466 0.03396 0.00268 0.001434 0.000111 0.280600 0.000023 −76.81 0.82 −1.7 0.8 3.71 3.78 −0.96AGC228 17 3466 0.29125 0.00014 0.001290 0.000007 0.280604 0.000034 −76.67 1.21 −1.2 1.2 3.69 3.76 −0.96AGC228 18 3466 0.01965 0.00092 0.000898 0.000040 0.280608 0.000022 −76.53 0.78 −0.2 0.8 3.65 3.70 −0.97AGC228 19 3466 0.02476 0.00028 0.001146 0.000011 0.280562 0.000029 −78.15 1.03 −2.4 1.0 3.73 3.81 −0.97AGC228 20 3466 0.03729 0.00032 0.001645 0.000013 0.280608 0.000022 −76.53 0.78 −1.9 0.8 3.72 3.79 −0.95Median 0.2

AGC351 01 3455 0.07055 0.00040 0.002618 0.000016 0.280773 0.000012 −70.7 0.44 1.4 0.4 3.58 3.62 −0.92AGC351 02 3455 0.08299 0.00055 0.003048 0.000018 0.280793 0.000012 −70.0 0.41 1.1 0.4 3.60 3.63 −0.91AGC351 03 3455 0.10231 0.00189 0.003773 0.000067 0.280822 0.000014 −69.0 0.52 0.4 0.5 3.63 3.67 −0.89AGC351 04 3455 0.04887 0.00064 0.001833 0.000021 0.280705 0.000013 −73.1 0.46 0.8 0.5 3.60 3.65 −0.94AGC351 05 3455 0.07453 0.00380 0.002681 0.000103 0.280763 0.000013 −71.0 0.46 0.9 0.5 3.60 3.64 −0.92AGC351 06 3455 0.06940 0.00067 0.002582 0.000022 0.280786 0.000013 −70.2 0.47 1.9 0.5 3.56 3.59 −0.92AGC351 07 3455 0.08043 0.00171 0.002990 0.000059 0.280806 0.000015 −69.5 0.55 1.7 0.5 3.57 3.60 −0.91AGC351 08 3455 0.05376 0.00106 0.002007 0.000036 0.280705 0.000012 −73.1 0.42 0.4 0.4 3.62 3.67 −0.94AGC351 09 3455 0.07818 0.00101 0.002884 0.000032 0.280776 0.000013 −70.6 0.45 0.9 0.4 3.61 3.64 −0.91Median 1.0

AGC370 01 3553 0.02853 0.00098 0.001163 0.000037 0.280578 0.000011 −77.59 0.41 0.1 0.4 3.71 3.76 −0.96AGC370 02 3553 0.06008 0.00030 0.002263 0.000007 0.280590 0.000012 −77.16 0.42 −2.1 0.4 3.80 3.87 −0.93AGC370 03 3553 0.08701 0.00135 0.003269 0.000042 0.280646 0.000014 −75.18 0.51 −2.6 0.5 3.83 3.89 −0.90AGC370 04 3553 0.04606 0.00213 0.001793 0.000084 0.280542 0.000013 −78.86 0.48 −2.7 0.5 3.82 3.90 −0.95AGC370 05 3553 0.02968 0.00161 0.001148 0.000059 0.280508 0.000012 −80.07 0.44 −2.3 0.4 3.80 3.88 −0.97AGC370 06 3553 0.07477 0.00413 0.002817 0.000146 0.280659 0.000016 −74.73 0.56 −1.0 0.6 3.76 3.82 −0.92AGC370 07 3553 0.05952 0.00067 0.002156 0.000019 0.280612 0.000012 −76.39 0.43 −1.1 0.4 3.76 3.82 −0.94AGC370 08 3553 0.05357 0.00119 0.002091 0.000045 0.280601 0.000014 −76.76 0.50 −1.3 0.5 3.77 3.83 −0.94AGC370 09 3553 0.01097 0.00065 0.000504 0.000028 0.280503 0.000011 −80.26 0.40 −1.0 0.4 3.75 3.81 −0.98

8 0.

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AGC370 10 3553 0.04984 0.00292 0.001903 0.00010

CG138-1 is spot on grain 1, AGC138-2 is spot on grain 2, etc. AGC01-5L is light bro

he original sample of AGC150, the isotopic systematics are similaro the other samples from this exposure.

Guitreau et al. (2012), in a review and compilation of a largearly Archaean data base for TTGs, analyzed a trondhjemitic whole-ock sample from the same outcrop where AGC150 and 150b wereollected, but they provide no detailed sample description, so it isifficult to assess whether their sample reflects the heterogeneityf the exposure or is from a homogeneous gneiss. They obtained ahole-rock εHf(t)-value of +2.8, similar to our result for AGC150b,

ut their 176Lu/177Hf ratio is 0.0044, significantly lower than ouralues for either AGC150 or 150b. In view of the uncertainty ofheir exact sample location, the data of Guitreau et al. (2012) areifficult to assess.

Sample AGC01-05 is a homogeneous, porphyritic tonalitic


neiss from the Phophonyane shear zone in the Phophonyane River,ome 800 m north-northwest of samples AGC150 and 150b andas described and dated by Schoene et al. (2008). We analyzedagmatic zircons of this sample (Fig. 3c) on SHRIMP II in Beijing

able 4hole-rock Lu–Hf isotopic data for granitoid gneisses of the Ancient Gneiss Complex, Sw

Sample Rock type Age (Ma) Lu (ppm)

AGC138 Layered homogeneous tonalitic gneiss 3537 0.3755

AGC150 Heterogeneous layered grey gneiss 3644 0.5746

AGC150b Homogeneous part of layered grey gneiss 3661 0.7784

AGC150b Homogeneous part of layered grey gneiss 3661 0.9472

AGC01-5c Porphyritic tonalitic gneiss 3645 0.5020

AGC185 Layered homogeneous tonalitic gneiss 3505 0.1492

AGC200 Non-layered tonalitic gneiss 3534 0.3227

AGC207 Mylonitic tonalitic gneiss 3620 0.03763

AGC351 Garnitiferous granitic gneiss 3455 1.381

AGC370 Homogneous tonalitic gneiss 3553 0.09877

a External reproducibility of 176Lu/177Hf is 0.2% and 1.2 × 10−5 for 176Hf/176Hf. CHUR vahe 176Lu decay constant of 1.867 × 10−11 of Scherer et al. (2001) and Söderlund et al. (2ee Hoffmann et al. (2011).

b Initial value above depleted mantle evolution line, thus no meaningful model-age canc Whole-rock sample AGC01-5 was supplied by Blair Schoene.

280562 0.000016 −78.17 0.55 −2.3 0.6 3.81 3.88 −0.94−1.6

transparent zircon phase; AGC01-5D is dark brown zircon phase.

(Table 2) and then determined the Lu–Hf isotopic composition ofconcordant or near-concordant spots. The U–Pb data are shownin Fig. 2c and display a similar pattern as reported in Schoeneet al. (2008). Our mean 207Pb/206Pb age for the 16 grains of theoldest component is 3647 ± 4 Ma, identical to the zircon age ofAGC150 but younger than the oldest age of Schoene et al. (2008) of3662.8 ± 0.5 Ma that was based on 3 abraided grains. Note that ourgrain 10 has a 207Pb/206Pb age of 3662 ± 6 Ma (Table 2).

Twenty-one spots on zircons were analyzed from this sample forLu–Hf by LA-ICP-MS and yielded a large spread in mostly negativeεHf(t)-values between −3.7 and +1.3 (Table 3 and Fig. 4a). However,similar to AGC150b, duplicate analyses for the whole-rock εHf(t)-value yielded +2.0 and +4.0 respectively, and the 176Lu/177Hf ratiois 0.01071, similar to AGC150b (Table 4). The εNd(t)-value for this


whole-rock sample is +0.6 (Table 5).A combined Hf evolution diagram for all Hf-in-zircon data of

AGC150, 150b and 01-5 is shown in Fig. 4a. These data demon-strate that the gneisses, although multi-component in origin, are

aziland.

Hf (ppm) 176Lu/177Hfa 176Hf/177Hfa εHf(t) tDM (Ga)

4.804 0.01109 0.281277 ± 5 +1.0 ± 0.4 3.703.870 0.02107 0.281826 ± 5 −2.7 ± 0.4 4.138.394 0.01316 0.281410 ± 7 +2.5 ± 0.4 3.73

10.85 0.01239 0.281412 ± 6 +4.6 ± 0.4 b

6.652 0.01071 0.281293 ± 5 +4.3 ± 0.4 b

5.873 0.003604 0.280755 ± 5 −0.1 ± 0.4 3.695.172 0.008855 0.281118 ± 5 +0.7 ± 0.4 3.706.559 0.003763 0.280752 ± 7 +1.9 ± 0.4 3.70

16.86 0.01163 0.281427 ± 7 +3.8 ± 0.4 3.494.465 0.003138 0.280755 ± 7 +2.1 ± 0.4 3.63

lues are 0.0336 for 176Lu/177Hf and 0.282785 for 176Hf/177Hf (Bouvier et al., 2008).004) was used for the calculation of initial epsilon values. For analytical procedure

be calculated.





Fig. 3. Cathodoluminescence images of zircons analyzed in this study. SHRIMP analytical spots with corresponding numbers from Table 2 are shown in (a–j), whereasLA-ICP-MS analytical spot for Hf analysis is shown in (k). (a) AGC150, (b) AGC150b, (c) AGC01-5, (d) AGC207, (e) AGC370, (f) AGC185, (g) AGC138, (h) AGC200, (i) AGC216,(j) AGC351, and (k) AGC228.





Fig. 4. Hf evolution diagrams showing analytical data and Hf crustal model ages (Hfc) for zircons of early Archaean granitoid gneisses in Swaziland. For analytical data seeTable 3. The broken Hf evolution lines are based on 176Lu/177Hf = 0.01. (a) Composite diagram for data from heterogeneous gneiss sample AGC150, homogeneous gneisssample AGC150b and porphyritic gneiss sample AGC01-5; (b) Trondhjemitic gneiss sample AGC207; (c) Tonalitic gneiss sample AGC370. (d) Trondhjemitic gneiss sampleAGC185. (e) Banded tonalitic gneiss sample AGC138; (f) Tonalitic gneiss sample AGC200.




Table 5Whole-rock Sm–Nd isotopic data for granitoid gneisses of the Ancient Gneiss Complex, Swaziland.

Sample Rock type Age (Ma) Sm (ppm) Nd (ppm) 147Sm/144Ndb 143Nd/144Ndb εNd(t) tDM (Ga)

AGC138 Layered homogeneous tonalitic gneiss 3537 5.24 25.35 0.125 0.510919 ± 4 −1.0 3.79AGC150 Heterogeneous layered grey gneiss 3644 5.84 23.43 0.141 0.511305 ± 9 0.0 3.82AGC150b Homogeneous part of layered grey gneiss 3661 9.01 42.53 0.128 0.511019 ± 14 +0.7 3.75Repeat Homogeneous part of layered grey gneiss 3661 8.66 40.57 0.129 0.511028 ± 8 +0.4 3.78AGC150ba Homogeneous part of layered grey gneiss 3661 9.195 43.71 0.1272 0.510963 ± 4 0.0 3.81AGC150ca Gabbroic boudin 3644(?) 3.741 14.13 0.1600 0.511658 ± 10 −1.9 c

Repeata Gabbroic boudin 3644(?) 3.610 13.69 0.1595 0.511643 ± 6 −2.0 c

AGC150ma Hornblende-rich tonalitic layer 3644 10.27 52.59 0.1191 0.510758 ± 6 −0.1 3.81AGC150la Trondhjemitic layer 3644 6.363 34.46 0.1116 0.510658 ± 6 +1.5 3.68AGC01-5d Porphyritic tonalitic gneiss 3645 7.30 36.86 0.120 0.510818 ± 9 +0.4 3.75AGC185 Layered homogeneous tonalitic gneiss 3505 5.56 33.71 0.0997 0.510348 ± 8 −1.0 3.71AGC185a Layered homogeneous tonalitic gneiss 3505 5.68 34.30 0.1000 0.510346 ± 5 −1.1 3.72AGC200 Non-layered tonalitic gneiss 3534 5.33 31.12 0.104 0.510429 ± 8 −1.0 3.74AGC200a Non-layered tonalitic gneiss 3534 5.323 30.54 0.1054 0.510485 ± 6 −0.5 3.71AGC207 Mylonitic tonalitic gneiss 3620 1.97 13.74 0.0923 0.510178 ± 10 +0.6 3.69AGC351 Garnitiferous granitic gneiss 3455 14.58 65.20 0.135 0.511314 ± 15 +1.5 3.52AGC351a Garnitiferous granitic gneiss 3455 12.39 55.78 0.1343 0.511258 ± 10 +0.7 3.59AGC370 Homogneous tonalitic gneiss 3553 1.97 13.74 0.0868 0.510137 ± 13 +1.5 3.58AGC370a Homogneous tonalitic gneiss 3553 1.859 12.88 0.08725 0.510096 ± 9 +0.4 3.64

a Data measured in Munich by TIMS.b External reproducibility for 147Sm/144Nd is 0.15% and 1.2 × 10−5 for the 143Nd/144Nd ratio. External error on initial εNd is <0.3 ε-units

External reproducibility of the Bonn ICP-MS data for 147Sm/144Nd is 0.2% and 2.0 × 10−5 for the 143Nd/144Nd ratio. External error on initial εNd is <0.4 ε-units. The εNd values werecalculated with the parameter of Bouvier et al. (2008) with present-day values for the chondritic uniform reservoir (CHUR) of 147Sm/144Nd = 0.1960 and 143Nd/144Nd = 0.512630.t calculated with a linear isotope evolution model for the N-MORB reservoir of Goldstein and Jacobsen (1988).

960 f

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DMc Sm/Nd ratio too high for meaningful model age calculation. CHUR values are 0.1d Whole-rock sample AGC01-5 was supplied by Blair Schoene.

ot juvenile but are derived from melting, at least in part, of aREE-enriched source, most likely older continental material. Theact that the oldest zircons have predominantly negative εHf(t)-alues strongly argues for an older crustal history of the oldestomponents of the gneiss protoliths, possibly dating back to theate Hadean. However, positive whole-rock εHf(t)-values for twof these samples may either imply input from a juvenile compo-ent or are due to post-crystallization modification during lateretamorphism and migmatization. Thus it may be possible that

he original source of these gneisses was a mixture of crustal andantle-derived material.Alternatively, the positive whole-rock initial Hf values for

neisses of the Phophonyane Inlier may have resulted from aixture of different age components in these rocks. If younger

omponents were homogenized during migmatization, the back-alculated initial values would be unreliable and too high. As aesult of the variable Hf isotopic compositions in the zircons, whichre the main host mineral for Hf, the whole-rock samples also dis-lay this heterogeneity as shown for sample AGC150b.

Interlayering between Palaeoarchaean and ca. 3.2 Ga layeredneisses is particularly well preserved in a large exposure in thehophonyane River just above a waterfall. The gneisses are stronglyeformed, in part mylonitic in character, but tight isoclinal foldingan be recognized at many localities. Our sample AGC207 was col-ected from this locality and is a mylonitic tonalitic gneiss within

major shear zone that is tens of metres wide. The zircons areell-preserved, near-euhedral with slight rounding at their ter-inations and oscillatory zoning (Fig. 3d). Eight grains yielded

oncordant or slightly discordant and reversely discordant results,nd the 207Pb/206Pb ratios are virtually identical (Table 2), result-ng in a mean age of 3620.4 ± 1.7 Ma (Fig. 2d). Several additionalnalyses produced strongly discordant results that are not furtheronsidered. We interpret the above age as approximating the timef emplacement of the original tonalite that most likely belongs tohe same generation as the previous samples.


Ten spots analyzed for Lu–Hf by LA-ICP-MS produced consid-rable spread in mostly negative εHf(t)-values between −4.3 and0.3 (Table 3 and Fig. 4b). The whole-rock εHf(t)-value is +1.4,nd the 176Lu/177Hf ratio is very low at 0.00376 (Table 4). The

or 147Sm/144Nd and 0.512630 for 143Nd/144Nd (Bouvier et al., 2008).

εNd(t)-value for this whole-rock sample is +0.6 (Table 5). Thesedata are similar to those of the previous samples and suggestthat the gneiss was predominantly derived from an older crustalsource.

Another grey, layered trondhjemitic gneiss (AGC370) was sam-pled in a roadcut ca. 500 m to the north of the AGC150 locality.The zircons are reddish to dark brown in colour, mostly long andprismatic with slight rounding at their terminations, and displayexcellent striped or oscillatory zoning (Fig. 3e). Eight grains areconcordant or near-concordant and yielded a 207Pb/206Pb age of3552.7 ± 0.6 Ma (Table 2 and Fig. 2e). One strongly discordant grainhas the same 207Pb/206Pb ratio but was not considered in the agecalculation.

Ten spots were analyzed for Lu–Hf by LA-ICP-MS and yieldedsome spread in mostly negative εHf(t)-values between −2.7 and +0.1(Table 3 and Fig. 4c). The whole-rock εHf(t)-value is +2.1, and the176Lu/177Hf ratio is very low at 0.003138 (Table 4). The εNd(t)-valuesfor this whole-rock sample are +1.5 (Bonn) and +0.6 (Munich)(Table 5).

The Hf-in-zircon data are almost identical to those in the pre-vious samples, thus confirming the conclusion that these ancientgneisses are predominantly derived from melting of still olderLREE-enriched material. However, the slightly positive whole-rockεHf(t)- and εNd(t)-values either imply some juvenile component orreflect isotopic disturbance during post-crystallization metamor-phism and migmatization. The very low 176Lu/177Hf ratios of ca.0.003 in AGC207 and AGC370 are probably caused by the presenceof garnet during melting of the protoliths which were most likelyof mafic composition.

The above samples demonstrate that the oldest AGC gneisses inthe Phophonyane Inlier range in age from 3.66 to about 3.55 Ga andconsist of a strongly deformed suite of rocks ranging in composi-tion from tonalite to granite. The Hf and Nd isotopic data suggestthat these rocks originated through melting of older LREE-enrichedmaterial, most likely continental crust of mostly felsic composition,


with variable but relatively minor additions of juvenile material.However, the low 176Lu/177Hf of some samples (e.g., AGC207 and370) suggest that at least part of these gneisses were derived frommafic crust in the presence of restitic garnet.


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.2. Njoli Dam area, NE Swaziland

The Njoli Dam area in NE Swaziland exposes similar bandedneisses as in the Phophonyane Inlier, but outcrops are sparseecause this lowland area is largely soil-covered, and exposuresre only found along rivers.

A layered and strongly deformed tonalitic to granodioritic gneisss exposed next to a causeway across the Mbuluzi River south ofshaneni (Fig. 1 and Table 1), and a homogeneous trondhjemiticample (AGC185) was previously described and dated by Krönert al. (1989) with the oldest phase providing a mean concordantHRIMP zircon age of 3505 ± 24 Ma. The Lu–Hf compositions wereeasured by LA-ICP-MS on zircon pits providing concordant ages.

even analyses on oscillatory-zoned zircons (Fig. 3f) yielded neg-tive εHf(t)-values of −3.3 to −1.2 (Table 3 and Fig. 4d), and thehole-rock εHf(t)-values of the Bonn laboratory is −0.6 with a cor-

esponding 176Lu/177Hf ratio of 0.003604 (Table 4). The whole-rockNd(t)-values are −0.9 (Bonn) and −1.0 (Munich) (Table 5).

Another banded tonalitic gneiss was exposed in a quarry nexto Njoli Dam in NE Swaziland, interlayered with a similar gneissated at 3211 ± 11 Ma by Kröner et al. (1989). The quarry no longerxists, but we have investigated sample AGC138 (Fig. 1 and Table 1)hat is macroscopically indistinguishable from the gneiss datedarlier. The zircons are long and prismatic with well rounded ter-inations, and many grains show striped zonation in CL images

Fig. 3g) but also variable recrystallization. Six SHRIMP-spots onrains with striped zonation yielded concordant to moderatelyiscordant results, but all 207Pb/206Pb ratios are similar, suggest-

ng recent Pb-loss, and the mean age is 3537.3 ± 2.6 Ma (Fig. 2f).e interpret this to reflect the time of tonalite emplacement and

his result, in combination with the data reported by Kröner et al.1989), demonstrates that the Njoli Dam area also consists of inter-ayered TTG gneisses of variable age.

Nine spots were analyzed by LA-ICP-MS on zoned zircons, 6 ofhese on previous SHRIMP-pits, and most yielded negative εHf(t)-alues between −2.6 and −1.9 with one analysis at +1.5 (Table 3 andig. 4e). The whole-rock εHf(t)-value for this sample is +0.5 (Table 4),nd the corresponding 176Lu/177Hf ratio is 0.01109. The εNd(t)-valueor this whole-rock sample is +0.4 (Table 5).

The two gneisses from NE Swaziland, although slightly youngerhan those of the Phophonyane Inlier, display similar isotopic char-cteristics in that the Hf-in-zircon values strongly point towardsrustal recycling and generation of these gneisses predominantlyrom older crustal precursors, whereas the whole-rock data suggestome involvement of sources with short-lived crustal residenceimes or later disturbance.

.3. Central and SW Swaziland

The largest exposure of the Ancient Gneiss Complex is in cen-ral and southwestern Swaziland, and the oldest ages in this regionere reported from the Ngwempisi River southwest of Sidvokodvo

nd from the area around Mankayane (Fig. 1). Kröner et al. (1989)ad previously described and dated a hornblende-rich tonaliticneiss from a small river bank close to the confluence with thegwempisi River (AGC200, Fig. 1 and Table 1). The rock is well foli-ted but not compositionally layered, and the chemistry (Table 6)s similar to other siliceous tonalitic gneisses described by Huntert al. (1984) from the AGC. The REE concentrations reported byhese authors are uniformly low and define a gently sloping pat-ern with a slight negative Eu anomaly (see Fig. 4 in Hunter et al.,984). Most of the SHRIMP I zircon analyses reported by Kröner


t al. (1989) are near-concordant and define a mean 207Pb/206Pbge of 3563 ± 3 Ma. Our additional SHRIMP II analyses from aewly prepared mount and on oscillatory-zoned, long-prismaticrains with rounded terminations (Fig. 3h) yielded 9 concordant


results with a mean 207Pb/206Pb age of 3534 ± 5 Ma (Fig. 5a). Twodiscordant grains were not considered, and one further concord-ant magmatic zircon at 3222 ± 8 Ma (Fig. 5a) seems to reflect thepervasive 3.2 Ga event. The Hf-in-zircon isotopic analyses on theabove 9 SHRIMP pits and 16 additional spots provided a range ofεHf(t)-values between −1.41 and +2.48 (Table 3 and Fig. 3f). Thewhole-rock εHf(t)-value for this sample is +0.3, and the correspond-ing 176Lu/177Hf ratio is 0.008855 (Table 4). Two εNd(t)-values for thiswhole-rock sample agree well at −0.9 (Bonn) and −1.0 (Munich)(Table 5).

Another sample of foliated, homogeneous tonalitic gneiss(AGC216) was collected a little farther north, some 200 m awayfrom AGC200 (Fig. 1 and Table 1), and containing very thin, rareleucocratic veins. The zircons are stubby to long and prismaticand have well rounded terminations, and concentric oscillatoryzoning is well preserved in many grains (Fig. 3i). 16 SHRIMPanalyses yielded 11 concordant and 2 slightly discordant and 2results with similar 207Pb/206Pb ratios that provided a mean ageof 3553.4 ± 1.8 Ma (Table 2 and Fig. 5b) that is slightly higher thanthat of sample AGC200. Two discordant grains of the same popu-lation were not used for the age calculation. One grain has a muchyounger concordant age of 3263 ± 6 (Table 2 and Fig. 5b), and weinterpret this to represent one of the thin leucocratic granite veins,possibly reflecting melt injection during the ca. 3.2 Ga event. As inthe Phophonyane Inlier, the fact that the thin veins are parallel tothe foliation in this gneiss signifies that the deformation is youngerthan ca. 3.26 Ga.

Hf-in-zircon isotopic analyses on 8 SHRIMP pits and 4 additionalspots provided mostly positive εHf(t)-values between −0.8 and +2.7(Table 3 and Fig. 3f). There are no whole-rock Lu–Hf and Sm–Ndisotopic data for this sample, but the above Hf-in-zircon pattern isvery similar to that of zircons in sample AGC200 and supports theinterpretation that these gneisses were derived from a distinctlydifferent source than the gneisses discussed so far, with much inputfrom material with a short crustal residence time, possibly a maficunderplate.

An extensive area along the Mtimane River in the Mankayanearea is underlain by banded Ngwane gneisses where these rocks areintruded by the ca. 3450–3470 Ma Tsawela tonalitic–trondhjemiticgneiss that forms distinct plutons (Jackson, 1984; Kröner, 2007).Our sample AGC351 comes from the same locality as sampleAGC201 dated by Kröner et al. (1989), namely a well banded,siliceous garnetiferous granitic gneiss exposed in the upper Mti-mane River near a rural school (Fig. 1 and Table 1). According toHunter (1993) this is a high-silica, high-potassium rock, probablyproduced by partial melting of an older TTG source, and our sampleis of granitic composition (SiO2 = 78.48 weight %) and has a K2O-content of 2.4 wt%. Kröner et al. (1989) identified two magmaticzircon generations in this rock, one with a mean 207Pb/206Pb ageof 3455 ± 11 Ma, the other dated at 3239 ± 39 Ma. They interpretedthe older age as reflecting emplacement of the gneiss precursor,whereas the younger age was linked to a magmatic-metamorphicevent.

Our sample AGC351 was carefully selected not to contain thinleucocratic granite veins that are ubiquitous in the outcrop, andthe zircons are long and prismatic to stubby with excellent mag-matic oscillatory zoning and well-rounded terminations (Fig. 3j),resulting from extensive “metamorphic corrosion”, a typical fea-ture of high-grade rocks (Kröner et al., 1994). Twelve grains weredated on SHRIMP II and are concordant to slightly discordant,with similar 207Pb/206Pb isotopic ratios that provide a mean ageof 3455 ± 1 Ma (Table 2 and Fig. 5c) identical to the age previ-


ously determined by Kröner et al. (1989). No grains of the 3.2 Gageneration were found in this sample. We follow Kröner et al.(1989) in interpreting this age to reflect emplacement of the gneissprotolith.


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15

Table 6Chemical composition of granitoid gneisses of the Ancient Gneiss Complex dated in this study.

Sample AGC138 AGC150 AGC150 AGC150b AGC150b AGC150c AGC150l AGC150m AGC01-5 AGC185 AGC185 AGC200 AGC200 AGC207 AGC351 AGC351 AGC351 AGC370 AGC370 AGC370Lab/ref. Mainz/Kiel Compston and

Kröner (1988)Cologne Xian Mainz Durban Durban Durban Cologne Kröner et al.

(1989)Cologne Cologne Kröner et al.

(1989)Xian Xian Durban Mainz Durban Xian Mainz

Major elements (wt.%)SiO2 66.99 69.93 69.65 69.76 54.4 73.88 69.31 73.01 66.78 68.88 78.48 80.87 78.55 78.05 75.86 75.15TiO2 0.52 0.27 0.39 0.4 0.78 0.16 0.52 0.24 0.64 0.4 0.19 0.21 0.2 0.16 0.18 0.18Al2O3 14.67 15.38 12.55 12.66 16.54 13.86 13.76 14.24 14.48 15.77 10.96 9.7 11.05 12.39 13.13 13.13Fe2O3T 4.81 3 5.69 5.71 9.19 2.02 6.13 2.22 5.03 3.71 2 1.34 1.99 1.47 2.18 2.18MnO 0.07 0.04 0.07 0.08 0.17 0.02 0.07 0.04 0.15 0.06 0.01 0.03 0.01 0.04 0.03 0.03MgO 1.72 0.82 2.43 2.32 4.13 0.58 1.41 0.49 1.52 0.97 0.63 0.64 0.53 0.45 0.42 0.32CaO 3.54 2.71 1.41 1.42 9.76 1.97 2.62 2.35 4.39 3.25 0.75 0.38 0.78 1.22 1.91 1.99Na2O 3.64 5.98 1.71 1.58 4.58 3.39 3.65 4.5 4.43 4.73 4.01 3.73 3.94 4.78 4.34 4.25K2O 2.66 1.48 2.54 2.52 0.67 3.75 2.25 2.45 1.2 1.36 2.16 2.38 2.16 1.64 1.6 1.58P2O5 0.07 0.07 0.09 0.07 0.15 0.04 0.14 0.1 0.22 0.12 0.02 0 0.01 0.04 0.05 0.03LOI 1.24 0.6 3.03 3.23 1.14 0.9 0.83 0.73 0.47 0.64 0.4 0.53 0.52 0.43 0.38 0.5

Total 99.93 100.0 99.56 99.75 100.4 99.67 99.9 100.23 99.95 99.89 99.61 99.27 99.78 100.24 100.08 99.4

Trace elements (ppm)Li 48 241 131 11 11.8 109Sc 7.02 40 8.96 4.82 5.00 30.0 1.17 6.03 0.362 1.59 5.19 3.63 2.79 2.39 4.87 1.72 2.22 2V 47.3 21.6 25 9.5 58 28.1 0.4 0.61 8.7 11Cr 49.9 38 7.67 17.5 22 14.3 15.3 26.9 11 11 11 26 56 16.1 1.9 3.9 6.91 6.7 5.6 12Co 76.3 11.1 13. 0 39 4.04 11.4 4.1 14 7.75 26.7 0.99 31.9 4.32 23.2 27Ni 46.4 11 9.34 12.2 13 167 5.34 11.4 8 4 23 8.08 1.19 4.72 4.18 18.3 12.1 11Cu 65.4 53.2 109 93 27 3 9 2.88 1.06 6.48 2.55 3Zn 92 140.4 164 159 137 49 141 66.7 37.4 18.3 38.3 40Ga 21.9 23.5 22 21 19.1 18.2 16.5 17Rb 46.1 350 148 229 242 18.7 67 33.9 34.2 71 22.5 15.2 23 56.9 39.4 43.6 38.7 61.5 76.5 79Sr 155 114 217 121 122 145 239 155 145 261 134 163 221 321 57 48 53.9 215 254 261Y 34.5 81 37.0 59.4 59 32 12.1 38.1 31.4 13 9.8 21.1 30 15.2 119 50.7 91.8 4.5 5.3 6Zr 152 352 149 340 262 59.3 145 230 233 188 219 230 212 265 528 564 382 134 126 125Nb 21.4 40 20.7 33.8 32 14.7 9.93 26.3 24.1 13 10.1 12.6 13 9.09 31.7 32.3 33.9 6.57 6.38 7Sn 1.08Sb 0.0678Cs 4.5 40 48.3 21.2 10.5 13 0.92 0.28 13 7.75 0.67 0.63 14.18Ba 668 353 537 627 89.1 3171 487 400 565 263 522 332 411 333 633 580 602La 32.6 24.0 51.2 45 11.5 41.7 62.3 35 70 34 44 48.1 54.1 43.3 56.3 20.8 28.9 20Ce 58.9 67 51.4 93.8 92 25.2 82.4 106 64 95 62 71 44 78.7 120 111 143 40.8 48.5 44Pr 6.8 117 6.01 11.2 11 3.18 8.98 13.4 7.96 6.55 8.70 99 8.09 15.9 12.4 17.4 3.49 4.91 8Nd 25.1 24.5 41.8 39 14 33.7 50.4 31.67 24.0 31.6 26.8 66.9 53.3 77.5 12.2 15.9 17Sm 5.39 54 5.70 8.88 10 3.58 4.85 9.39 6.43 4.31 5.43 30 4.31 15.8 11.7 17.9 1.55 2.35 4Eu 1.11 1.41 1.37 1.51 1.26 0.977 1.54 1.18 0.63 1.32 5.8 1.19 3.77 3.21 3.79 0.76 0.89 4Gd 5.97 6.04 9.15 11.7 4.42 4.36 8.64 6.17 3.04 4.45 1.52 3.91 17.8 11.8 17.8 1.22 1.92Tb 0.98 1.02 1.55 0.787 0.572 1.31 0.951 0.40 0.66 4.9 0.490 3.06 1.67 18.1 0.15 0.22Dy 5.95 6.46 9.88 12.9 5.36 2.96 7.98 5.75 2.00 3.89 2.64 20.1 10 0.88 1.07Ho 1.12 1.33 2.04 2.45 1.14 0.49 1.52 1.17 0.36 0.77 4.4 0.500 4.31 2.06 3.69 0.16 0.18Er 2.92 3.79 5.76 6.67 3.45 1.15 3.98 3.30 0.94 2.16 0.86 1.37 12.4 6.01 0.44 0.48Tm 0.42 0.57 0.83 0.51 0.16 0.563 0.481 0.126 0.312 2.5 0.190 1.80 0.92 11.1 0.07 0.07Yb 2.64 3.77 5.24 5.75 3.41 0.93 3.36 2.97 0.798 2.08 1.23 11.6 6.34 10.4 0.46 0.45Lu 0.39 0.62 0.76 0.8 0.496 0.146 0.487 0.532 0.183 0.379 0.180 1.75 1.03 1.59 0.08 0.08Hf 3.99 4.25 9.65 9.47 1.14 3.73 6.16 7.10 6.14 5.29 0.33 5.93 14.7 15.3 10.6 3.39 3.23Ta 3.04 1.97 2.54 2.76 2.47 0.796 1.39 1.67 0.402 0.673 0.470 1.37 1.02 1.40 0.42 0.52W 413 1 122Tl 0.667Pb 42.2 9.61 9.6 11 7.84 18.6 10.9 9.28 9.14 8.01 12.4 2.9 2.5 1 10.6 11.6 14Th 13.2 8.89 14.5 15 2.39 17.3 17.7 10.6 13.9 14.5 19.5 4.7 4 6.53 3 5.2 4.8U 3.13 12.2 3.73 4.6 5.06 1.42 3.89 2.92 0.74 0.37 2.87 0.94 0.78 0.76 0.98 0.85 2.2




Pb/ U207 235

0.50

0.54

0.58

0.62

0.66

0.70

0.74

0.78

17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

3000

3100

3200

3300

3400

3500

3600AGC 200 - Tonalitic gneiss atbank of Ngwempisi River SWof Sidvokodvo, centralSwaziland

9 grains, mean207Pb/206Pb age:3534±5.5 Ma

(a)

Pb/

U206

238

0.64

0.66

0.68

0.70

0.72

0.74

26 27 28 29 30

3360

3380

3400

3420

3440

3460

3480

Pb/

U206

238

Pb/ U207 235

AGC 351 - Trondhjemiticgneiss, Mtimane River,Mankayane District,central Swaziland


εNd(t) = 1.5±0.4εHf(t) = 3.8±0.4

(c)

Pb/ U207 235

AGC 228 - Fine-grainedtonalitic Ngwane gneisson road north of Malkerns,west-central Swaziland


3232±6 Ma0.60

0.62

0.64

0.66

0.68

0.70

0.72

0.74

22 23 24 25 26 27 28 29 30

3250

3300

3350

3400

3450

(d)

Pb/ U207 235


AGC 216 - Tonalitic Ngwanegneiss, drift over small riversouth of Sidvokodvo, centralSwaziland

3263±6 Ma

0.58

0.60

0.62

0.64

0.66

0.68

0.70

0.72

0.74

0.76

0.78

23 24 25 26 27 28 29 30 31 32 33 34 35

3300

3350

3400

3450

3500

3550

3600

3222±8 Ma

(b)

F ncienA GC216R lkern

tas((tars

gepetida3thi

19ps

ig. 5. Concordia diagrams showing SHRIMP zircon analyses of gneisses from the AGC200 (Kröner et al., 1989), reanalyzed on SHRIMP II. (b) Tonalitic gneiss sample Aiver, southwest Swaziland. (d) Tonalitic gneiss sample AGC228, roadcut NW of Ma

Hf-in-zircon isotopic analyses on 8 SHRIMP pits and 2 addi-ional spots yielded exclusively positive εHf(t)-values between +0.4nd +1.4 (Table 3 and Fig. 6b). The whole-rock εHf(t)-value for thisample is +3.3, and the corresponding 176Lu/177Hf ratio is 0.01163Table 4). Two εNd(t)-values for this whole-rock sample are +1.6Bonn) and +0.9 (Munich l) (Table 5). The above data are similar tohose found in the gneisses on the Ngwempisi River farther eastnd suggest derivation of the Mtimane gneiss from a source withelatively short crustal residence time. The granitic compositionuggests that this rock is a result of crustal melting.

Our last sample is a coarse-grained and homogeneous tonaliticneiss (AGC228) from a roadcut northwest of the town of Malk-rns in west-central Swaziland (Table 1 and Fig. 1). The zircons areredominantly long-prismatic with well rounded terminations andxcellent concentric oscillatory zoning in CL images (Fig. 3k). Thir-een grains were dated on SHRIMP of which 12 define a linear arrayn the concordia diagram with 8 quasi-concordant analyses and 4iscordant results. As in the previous cases, the 207Pb/206Pb ratiosre virtually identical, suggesting recent Pb-loss and a mean age of438.9 ± 0.9 Ma (Table 2 and Fig. 5d). We interpret this to reflecthe time of tonalite emplacement. One grain, identical to all others,as a concordant age of 3232 ± 6 Ma (Table 2 and Fig. 5d) and is

nterpreted to reflect a younger magmatic event.Hf-in-zircon isotopic analyses were performed in Beijing on


0 SHRIMP pits and at Hong Kong University on an additional spots, providing a relatively wide range of both negative andositive εHf(t)-values between −2.4 and +0.5 (Table 3 and Fig. 6c),uggesting that the gneiss protolith originated from a fairly

t Gneiss complex of Swaziland. Data boxes as in Fig. 2. (a) Tonalitic gneiss sample, Ngwempisi River, central Swaziland. (c) Granitic gneiss sample AGC351, Mtimane

s, west-central Swaziland.

heterogeneous source consisting of old crust as well as materialwith short crustal residence time. There are no whole-rock Lu–Hfand Sm–Nd isotopic data for this sample, but the above Hf-in-zircon pattern is somewhat similar to that of samples AGC200 and216, favouring substantial input from an isotopically primitivesource.

5. Major and trace element chemistry

Eight samples were analyzed for their major and trace elementcompositions using XRF and ICP-MS at different laboratories andfollowing the techniques outlined in the Supplementary Material.For three samples duplicate analyses were obtained on differentsample powders ground by agate mill from the same materialpreviously reduced by jaw-crusher. Therefore, slight differencesin the major and trace element compositions reflect inhomogen-ities in the analyzed materials. We also point out that the samplesanalyzed here partly consist of different magmatic phases ofdifferent ages although sampled as homogeneously as possible,which is also underlined by some of the heterogeneous zirconpopulations discussed before. The major and trace element dataare presented in Table 6, and Fig. 7 classifies the samples inthe albite–anorthite–orthoclase diagram (after O’Connor, 1965;Barker, 1979) as trondhjemite, two as tonalite, two as granodiorite


and two as granite.Our samples are in part different from those described as the

typical tonalite–trondhjemite–granodiorite (TTG) suite and shouldbetter be classified as grey gneisses (Moyen, 2011). The silica




F r zircoT alitic

cca(

(sHTgh

fpRhtbbd

ig. 6. Hf evolution diagrams showing analytical data and Hf crustal model ages foonalitic gneiss sample AGS216; (b) High-K granitic gneiss sample AGC351; (c) Ton

ontents range from ca. 67 to more than 80 wt.%, and aluminaontents are also variable from ca. 10 to 15.8 wt.%. MgO contentsre low (0.8–2.4 wt.%), and some samples have high K2O contentsup to 2.66 wt.%).

In chondrite-normalized rare earth element (REE) patternsFig. 8A) our samples are similar to the REE compositions ofiliceous grey gneisses and migmatites of the AGC as reported byunter et al. (1978). Compared with the average composition ofTGs older than 3.5 Ga as compiled by Martin et al. (2005), the greyneisses studied here have lower LaN/YbN ratios, and most samplesave an overall higher content of total REE.

They have strongly variable LaN/YbN ratios of 5–63 and exceptor sample AGC370 that has a positive Eu-anomaly all other sam-les exhibit negative Eu-anomalies. AGC370 also has the steepestEE pattern with slightly higher Lu-contents compared to the othereavy REE (HREE). Primitive mantle-normalized trace element pat-


erns are shown in Fig. 8B where the AGC gneisses are characterizedy negative Nb–Ta–Ti anomalies and, except for sample AGC138,y positive Zr and Hf anomalies. The Nb–Ta–Ti anomalies are likelyue to Ti-oxide phases such as rutile, ilmenite or titanite in the

ns of early Archaean granitoid gneisses in Swaziland. Explanations as in Fig. 4. (a)gneiss sample AGC228.

residue during partial melting (e.g., Martin, 1987; John et al., 2011;Hoffmann et al., 2011a,b, 2014; Nagel et al., 2012).

6. Discussion

6.1. Major and trace element constraints on the sources ofPalaeoarchaean grey gneisses

Major and trace element contents of the AGC gneisses can beused to place constraints on the source composition. Gneisses ofnear granitic composition (e.g., AGC351) and with strongly ele-vated silica contents clearly resulted from melting of older felsiccrust or from the involvement of fractional crystallization pro-cesses. Hoffmann et al. (2014) showed that fractional crystallizationof a ca. 70 wt.% SiO2 tonalite magma, involving plagioclase andclinopyroxene/amphibole at pressures of 10–5 kbar, may result in


the formation of such SiO2-rich granitoid compositions. Elevatedcontents of SiO2 compared to TTGs of clearly juvenile origin (e.g.,Nutman et al., 1999; Hoffmann et al., 2011a) may be related tothe incorporation of older crustal material in the sources of the


ARTICLE ING ModelPRECAM-4048; No. of Pages 24

18 A. Kröner et al. / Precambrian Re

Albite Orthoclase

Anorthite

Granite

Granodiorite

Tonalite

Trondhjemite

AGC370

AGC351

AGC150

AGC150b

AGC138

AGC150m

AGC207

AGC185

AGC200

Fs

Nm

AtprmEspfmm2NbeTaiwc

wstimtd

6t

oPfa(tr

ig. 7. Feldspar diagram after O’Connor (1965) and Barker (1979) showing compo-itional range of samples from the Ancient Gneiss Complex.

gwane gneisses or, alternatively, due to element mobility duringigmatization.Except for the positive Eu-anomalies in samples AGC370 and

GC200, all other analyzed samples exhibit pronounced nega-ive Eu-anomalies which is best explained by the presence oflagioclase in the source or the involvement of plagioclase as a seg-egating mineral phase during fractional crystallization. AGC370ost likely contains cumulate plagioclase that causes the positive

u-anomaly. The variability in La/Yb and Sr/Y ratios of the sampleuite is likely due to different proportions of garnet as a cumulatehase during partial melting. Garnet has high partition coefficientsor the HREE and Y and causes depletion of these elements in the

elt (e.g., Jenner et al., 1994). Compared to juvenile TTGs thatelted from mafic crust (e.g., Nutman et al., 1999; Hoffmann et al.,

011a), the AGC grey gneisses show surprisingly high contents ofb (7–40 ppm) and Zr (125–564 ppm). This may either be explainedy a mafic precursor that was strongly enriched in incompatiblelements and HFSE such as (underplated?) OIB-like gabbro or olderTG crust. Condie and Kröner (2013) used the ratios Th/Yb vs. Nb/Ybnd La/YbN vs. Sr/Y to discriminate between felsic rocks originat-ng from different geodynamic settings. AGC samples of this study,

hen plotted in these diagrams (Fig. 9) show a large variability,overing the entire range of geodynamic settings.

We feel that these and similar diagrams must be interpretedith caution because in migmatitic gneisses, such as several of our

amples, Th and Nb may be depleted and enriched due to par-ial melting and fluid overprint (e.g., Hoffmann et al., 2011a). Its also likely that element mobility during migmatization and/or

ixing and differentiation processes obscure the picture. Alterna-ively, it can be argued that modern analogues are not suitable toiscriminate between Archaean tectonic settings.

.2. Hf-in-zircon and Hf–Nd whole-rock isotopic constraints onhe sources of the oldest components of the Ngwane gneisses

In many ways our Hf-in-zircon data reiterate the conclusionsf Zeh et al. (2011) that the oldest grey gneisses of the AGC in thehophonyane Inlier resulted from reworking of still older mafic orelsic continental crust, possibly dating back to the latest Hadaean,


s reflected by our oldest Hf crustal model ages just beyond 4 Gasee Fig. 4a and b). In contrast to the findings of Zeh et al. (2011),he slightly younger gneiss generation at about 3.55–3.5 Ga alsoeflects substantial involvement of older crust (see Fig. 4c–e). Older


xenocrysts are extremely rare but were reported at 3.68 Ga byKröner and Tegtmeyer (1994). Significant input of material derivedfrom a depleted mantle source becomes increasingly evident in theprotoliths of these and younger gneisses (e.g., Fig. 4f and 6a, b).A surprising aspect is the relatively juvenile nature of the K-richgranitic gneiss of the Mtimane River (Fig. 6b) that is likely derivedfrom an older granitoid source such as exposed in the PhophonyaneInlier. The Hf-in-zircon data thus support extensive Palaeoarchaeancrustal reworking in the Ancient Gneiss Complex of Swazilandwhich has also been suggested for 3.54–3.45 Ga felsic volcanic rocksin the nearby Barberton Greenstone Belt (Kröner et al., 2013). Thelarge variation in initial εHf values for zircons in most of our sam-ples most likely reflects heterogeneous sources but may also implythe involvement of juvenile, depleted mantle-derived melts in theformation mechanism. Tang et al. (2014) showed that such largevariations in initial Hf isotopic values can also be the result of crustalmelting in the presence of zircon. However, the high SiO2 contentsof several of the analyzed samples require fractional crystalliza-tion processes or melting. Melting of older felsic crust with juvenilemafic magma additions alone cannot explain the whole-rock Hf iso-topic compositions because mafic melts in most cases have less Hfthan already differentiated felsic magmas.

Because no older crust than the granitoid gneisses in thePhophonyane Inlier of the Piggs Peak area is so far known from theKaapvaal craton, the type and origin of the source rocks are difficultto evaluate. Some gneisses have granitic compositions and, there-fore, require felsic crustal precursors (e.g., AGC150b, AGC150l).

The strongly negative εHf(t) values of some of the Palaeoarcheansamples may have been produced by either melting of a Hadeanmafic protocrust, developing since 4.42 Ga (the age of the least dis-turbed Jack Hills zircon with juvenile composition; Kemp et al.,2010) and with a 176Lu/177Hf ratio of 0.020–0.029 (compositionssuggested by Blichert-Toft and Albarède, 2008, for average MORBand plateau basalt). This would be similar to the model for the Cana-dian Nuvvuagittuq TTGs recently proposed by O’Neil et al. (2013).Such a model is not compatible with the Sm–Nd isotopic compo-sitions because mafic crust that formed at 4.43 Ga would yield farmore negative εNd(t) values at ca. 3.65 Ga than observed in the oldestsamples from Piggs Peak. We cannot exclude the possibility that theSm–Nd isotopic system may have been disturbed in some samplesduring migmatization and later metamorphic events as seem to bethe case for some of the whole-rock Lu–Hf isotopic compositions.

Coherence with the εNd(t) data for the younger suites that areon a similar crustal evolution trend argues against this scenario(Fig. 10). Therefore, it is likely that the precursors of the oldestgneisses of the AGC were derived, between 4.2 and 3.8 Ga ago, froma depleted mantle source in accordance with the interpretationof Zeh et al. (2011). Using a 176Lu/177Hf ratio of 0.01 for averagecontinental crust, the DM crustal model ages calculated from theHf-in-zircon compositions of the grey gneisses are between 3.56 Gaand 4.04 Ga, in accordance with model ages calculated from the Ndand Hf whole-rock isotopic compositions.

There has been much debate about the average degree of deple-tion of the early Archaean mantle as monitored by Hf–Nd isotopes(e.g., Bennett et al., 1993; Gruau et al., 1996; Vervoort et al., 1996;Moorbath et al., 1997; Vervoort and Blichert-Toft, 1999; Hoffmannet al., 2010). Some authors found highly depleted Hf–Nd isotopesignatures in Eo- to Mesoarchaean rocks (e.g., Collerson et al.,1991; Bennett et al., 1993; Blichert-Toft and Arndt, 1999; Hoffmannet al., 2010; Nebel et al., 2013, 2014), whereas others reportedEoarchean initial εNd and εHf values which were essentially chon-dritic (Hoffmann et al., 2011b; Hiess et al., 2009; Næraa et al., 2012).


This imprints large uncertainties on the validity of Hf model agesfor Palaeo- and Eoarchean rocks.

Another way of interpreting our Hf-in-zircon data is using themedian of the variable initial εHf values obtained from all analyses





Fig. 8. (A) Chondrite-normalized REE diagram, (B) primitive mantle-normalized trace element diagram. Chondrite and primitive mantle compositions from McDonough andSun (1995).




Fig. 9. (A) Nb/Yb vs. Th/Yb diagram after Pearce (2008) for AGC granitoids withcompiled fields from Condie and Kröner (2013) for felsic rocks from different geo-dtw

oatlzcveGaeacTrtrpt

6

wtA

M

rocks

3

Fig. 10. Nd isotopic evolution diagram showing analytical data compared to liter-ature data for AGC and felsic Barberton rocks from Kröner and Tegtmeyer (1994),Kröner et al. (1996), Clemens et al. (2006), Schoene et al. (2008) and Kröner et al.(2013). CHUR values are from Bouvier et al. (2008). The blue field is defined bygrowth lines for the bulk continental crust (Nägler and Kramers, 1998). (For inter-

ynamic settings. For comparison the modern MORB-OIB and ARC arrays as well ashe modern ARC array are shown. (B) La/YbN vs. Sr/Y diagram for AGC granitoidsith fields compiled by Condie and Kröner (2013) for comparison.

f one zircon population. The median is preferred over the weightedverage compositions because some samples show positive ini-ial εHf values amongst predominantly negative ones. This wouldead to meaningless weighted averages. The median values of theircons vary from −1.9 to +1.4 (Table 3), still offset from thehondritic value of Bouvier et al. (2008) and in agreement with pre-ious crustal differentiation events (Fig. 11). This is different fromarly Archaean rocks of the Itsaq Gneiss Complex (southern Westreenland) where near-chondritic Hf isotopic compositions werelso reported (e.g., Hiess et al., 2009; Amelin et al., 2011; Næraat al., 2012). A chondrite-like Hf isotopic throughout the Archaeans suggested by Guitreau et al. (2012). Guitreau et al. (2012) con-luded from a large data base that virtually all global early ArchaeanTG crust formed by melting of juvenile rocks. For most of the AGCocks presented here, the whole-rock data does not overlap withhe median values of the Hf-in-zircon analyses. Thus, the whole-ock data may have been disturbed by Mesoarchaean leucosomesroduced during the widespread ca. 3.2 Ga melting and migmatiza-ion event, leading to erroneous initial εHf(t) values that are too high.

.3. Implications for the earliest history of the Kaapvaal craton


The granitoid rocks of the Ancient Gneiss Complex of Swazilandere long considered to represent one of the typical TTG terranes

hat characterize Archaean cratons (Hunter, 1991; De Wit, 1998;nhaeusser, 2006). The use of discrimination diagrams to deduce

pretation of the references to colour in this figure legend, the reader is referred tothe web version of the article.)

tectonic settings as well as apparent chemical similarities withmodern subduction-related granitoids led to tectonic models thatviewed these rocks as predominantly juvenile additions producedduring long-lasting subduction–accretion processes (e.g., De Witet al., 1992; De Wit and Hart, 1993). However, the presence of zir-con xenocrysts in some of these gneisses and the complexity oftheir evolution as documented by field relationships, zircon ages(e.g., Compston and Kröner, 1988) and Hf–Nd isotopic compositionspresented here do not support this simplistic interpretation.

Hunter (1991), based on extensive mapping of the AGC and eval-uation of geochemical data for rocks of his Bimodal Suite, concludedthat the Palaeoarchaean (pre-3.4 Ga) sialic crust in Swaziland andadjacent areas was generated as a consequence of hot spot activ-ity developed on a presumed primordial mafic crust. He followedBarker and Arth (1976) in generating the various granitoid gneissesfrom melting of amphibolite and/or hbl-bearing gabbro. In contrast,De Wit and Hart (1993) speculated that these rocks were gener-ated by plate tectonic processes occurring in modern-type oceanicbasins. Rollinson (2010) favoured generation of Archaean TTGs bymelting of subducted oceanic crust, whereas others such as Kerrichand Polat (2006), Polat (2012) and Nagel et al. (2012) proposedmelting of basalt or gabbro at the base of oceanic arcs.

In contrast, models favouring derivation of the TTG suite frommafic underplated reservoirs suggest melting of (dis?)continuouslyupwelling, hot mantle in the absence of arc-accretion tectonics to


form a thick mafic (gabbroic) crust. Partial melting of this maficcrust led to the formation of multiple generations of granitoidrocks and the development of first stable continental lithosphere,


ARTICLE ING ModelPRECAM-4048; No. of Pages 24

A. Kröner et al. / Precambrian Res

Fig. 11. Hf isotopic evolution diagrams showing analytical data and Hfc model agesfor zircons from early Archaean granitoid gneisses in Swaziland and elsewhere inthe eastern Kaapvaal craton. The 176Lu/177Hf ratio for MORB and OIB-like mafic crustis from Blichert-Toft and Albarède (2008). The mafic evolution lines begin at ca.4450 Ma, which is the age of the oldest juvenile Jack Hills zircons (Kemp et al.,2010), representative of an early mafic crust. Felsic crustal evolution is modelledwith a 176Lu/177Hf ratio of 0.01 covering the range of calculated model ages. CHURvac

uKratiof(ooif

ptgatapa3tG

alues are from Bouvier et al. (2008). The decay constant of Scherer et al. (2001)nd Söderlund et al. (2004) of 1.867 × 10−11 a was used to calculate initial isotopicompositions. See Ref. Zeh et al. (2009).

nderlain by a melt-depleted, buoyant residual mantle (e.g., Vanranendonk, 2010). The large volumes of TTG-type granitoid mate-ial in early Archaean terranes such as Swaziland, the Slave Provincend SW Greenland may be compatible with this model, but all theseerranes show ubiquitous evidence of intense horizontal shorten-ng and ductile deformation, suggesting an accretion-collision typef tectonic scenario. Nagel et al. (2012) ruled out derivation of TTGsrom subducting slab melts, so the most plausible protoliths remainunderplated?) gabbroic arc type crust in a plate-tectonic scenarior magmatic underplating in a plume-driven environment. The lackf andesitic rocks in Palaeoarchaean greenstone assemblages mayndicate that modern-type oceanic arcs may not be good analoguesor the early Earth.

Our data for Palaeoarchaean granitoid gneisses of the AGC sup-ort the contention of Moyen (2011) and Moyen and Martin (2012)hat many Archaean continental terranes are dominated by greyneiss complexes reflecting a range of components of which only

portion are true TTGs sensu Moyen and Martin (2012) and thathese rocks may have formed within a variety of tectonic settingss suggested by Van Kranendonk (2011a,b). Even the oldest com-onents of the AGC show evidence of intense horizontal shortening


nd ductile deformation, as documented by gneiss xenoliths in the.45 Ga Tsawela gneissic tonalite (Jackson, 1984; Kröner, 2007), andhis favours a terrane amalgamation model as proposed for SWreenland by Nutman and Friend (2009). We find the structural

PRESSearch xxx (2014) xxx–xxx 21

field evidence from Palaeoarchaean grey gneiss terranes, includ-ing the AGC of Swaziland, difficult to reconcile with a non-platetectonic scenario dominated by vertical tectonics, as suggestedby Shirey and Richardson (2011), Van Kranendonk (2011a,b) andGerya (2014). A model envisioning coalescence of protocratonsconsisting of TTG-dominated continental arc terranes (Condie andKröner, 2013), and with roots of already depleted subcrustal man-tle lithosphere, is more compatible with the field evidence andthe complexity of Palaeoarchaean grey gneiss terranes. Such a sce-nario is also supported by seismic data suggesting that cratoniclithosphere may have formed via thrust stacking of protocratoniclithosphere (Cooper et al., 2006).

Likely derivation of several of our 3.66–3.44 Ga gneisses frommelting of still older felsic crust, as already recognized by Zeh et al.(2011), reflects the importance of reworking processes since theearliest Archaean (Hoffmann et al., 2011a, 2014), perhaps eventhe late Hadean, as suggested by data for the Jack Hills zirconsin Western Australia (Harrison et al., 2008), the Acasta Gneissof Canada (Iizuka et al., 2006, 2007), several granitoid gneissesin West Greenland (Horie et al., 2010) and some components ofthe Wyoming Province in the northwestern US (Mueller et al.,1996, 1998). This is in contradiction to Cawood et al. (2013) andHawkesworth et al. (2013) who proposed that crustal reworkingonly began shortly before ca. 3 Ga, possibly associated with theonset of subduction.

7. Conclusions

Studying the oldest components of the Kaapvaal craton in theAncient Gneiss Complex of Swaziland using combined geochrono-logical and geochemical methods leads to the following conclusionsabout the evolution of these rocks:

The Hf-in-zircon isotopic compositions are strongly heteroge-neous, even in the oldest zircons dating back to 3.66 Ga. This is inagreement with mixtures of juvenile melts interacting with alreadydifferentiated and compositionally variable continental crust ofEoarchean to late Hadean age.

The initial Nd and Hf whole-rock isotopic compositions are inmost cases not in agreement with the median values of the Hf-in-zircon compositions and mostly reflect approximately chondriticinitial values for Nd and radiogenic initial values for Hf. However, ifweight is based on the more reliable Hf-in-zircon compositions insuch highly heterogeneous, multicomponent migmatitic gneisses,it is likely that, at least in some instances, extrapolation to the esti-mated magmatic age is not valid for the whole-rock compositions.

The primitive mantle-normalized trace element patterns showthe variable influence of residual plagioclase and garnet in thesources as well as high contents of strongly incompatible elements.In conjunction with the Hf-in-zircon isotopic data and the occur-rence of rare zircon xenocrysts the trace element contents are bestexplained by the incorporation of older continental crustal materialinto the sources of the grey gneisses.

Acknowledgements

We thank Martin Van Kranendonk and members of an inter-national field workshop at the University of Johannesburg in 2011for discussions. Guangshen Ni and Baoying Zheng of the BeijingSHRIMP Centre prepared most of the zircon concentrates, LiqinZhou and Xiao-Chao Che provided the zircon CL images, Chun Yangprepared perfect zircon mounts, and Jianhui Liu and Zhiqing Yang


made sure that SHRIMP II was in excellent operating conditions.Fuyuan Wu, Institute of Geology & Geophysics, Chinese Academy ofSciences, kindly arranged for Hf-in-zircon analyses in his laboratoryand calculated most of the data in Table 3. Jianqi Wang and Ye Liu


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f Northwest University, Xian, provided major and trace elementnalyses of several samples. Blair Schoene of Princeton Universityindly made zircons and a whole-rock aliquot available from hisample AGC01-5 for this study. Jianfeng Gao of the Geologicalurvey of Canada, Ottawa, provided an unpublished Excel-basedrogramme to construct the Hf evolution diagrams of Figs. 4 and 6..-U. Kasper kindly provided ICP-MS whole-rock chemical anal-ses in Cologne and D. Garbe-Schönberg in Kiel. The constructiveomments of two reviewers are gratefully acknowledged. Thistudy was supported by Deutsche Forschungsgemeinschaft (DFG)rants KR590/94-1 to AK and HO4794/1-1 to JEH and Nationalatural Science Foundation of China Project No. 41221002 to JY.

ppendix A. Supplementary data

Supplementary material related to this article can be found,n the online version, at http://dx.doi.org/10.1016/j.precamres.014.07.017.

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