Evidence from Dwyka tillite cobbles of Archaean basement beneath the Kalahari sands of southern...

Lithos 125 (2011) 482–502

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Evidence from Dwyka tillite cobbles of Archaean basement beneath the Kalaharisands of southern Africa

David H. Cornell a,⁎, Valby van Schijndel a, Olafur Ingolfsson b, Anders Scherstén c,1, Linn Karlsson a,Joanna Wojtyla a, Kristin Karlsson a

a Department of Earth Sciences, University of Gothenburg, SE40530, Swedenb Institute of Earth Sciences, University of Iceland, Ís-101 Reykjavík, Icelandc Geological Survey of Denmark and Greenland, Ø. Voldgade 10, DK-1350 Cph. K, Copenhagen, Denmark

⁎ Corresponding author. Tel.: +46 31 786 2805; fax:E-mail address: [email protected] (D.H. Cornell).

1 Now at: Department of Earth and Ecosystem ScienUniversity, Sölvegatan 13, SE-22362, Sweden.

0024-4937/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.lithos.2011.03.006

a b s t r a c t
a r t i c l e i n f o
Article history:Received 2 November 2010Accepted 3 March 2011Available online 10 March 2011

Keywords:KalahariDwyka FormationRehoboth ProvinceKaapvaal CratonIon probe zircon datingZircon oxygen isotope

We have found a new source of information about what lies beneath the Kalahari sands. The Kheis andRehoboth Provinces of southern Africa were thought to be underlain by either an ~1800 Ma orogenic belt, or anorthern branch of the ~1200 Ma Namaqua–Natal Province. Glacial diamictites of the PermocarboniferousDwyka Group exposed at Rietfontein west of the Kalahari sands carry cobbles plucked from the bedrock by theice sheet which covered the Gondwana supercontinent about 300 Ma ago. Despite altered mineralogy, thecobbles are not severely altered geochemically. Their normative mineral compositions give classifications astrondhjemites and granites, supported by rare earth element diagrams. Microbeam U–Pb zircon dating of thegranitic cobbles shows that they contain no evidence of crustal growth or orogeny at either 1800 or 1200 Ma.Rather they testify to the presence of 2500 to 2900 Ma Archaean trondhjemitic and granitic crust beneath theKalahari, with a lesser ~2050 Ma granite component. The pebble assemblages from the diamictites wesampled lack the diagnostic banded iron formation (BIF), stromatolitic limestone and other supracrustalpebbles which characterise diamictites derived from the Kaapvaal Craton, thus we envisage shorter transportdistances and derivation from the region now beneath the Kalahari sands. Three of the Archaean granitecobbles have unusual less-than-mantle zircon oxygen isotope values around +3 (δ18O VSMOW), which mayreflect interaction of their source with high-temperature, originally meteoric water before melting to producethe granites. Themafic cobbles described in a companion paper aremuch younger and are related to intrusionsof the 1.1 Ga Umkondo Large Igneous Province, probably located on the Kalahari Line or Rehoboth Province.Five trondhjemitic granites from the westernmost outcrops of the Kaapvaal Craton were dated, the oldestbeing 3061±9 Ma and four others between 2882±7 Ma and 2854±7 Ma, reflecting the cratonisation of theKimberley Terrane. Four of the Archaean Dwyka cobbles we dated are younger than the 2.7 Ga Kaapvaal coversequence and are thus too young to be derived from the craton.All the Dwyka cobbles described here are most likely derived from either the Rehoboth Province or theKalahari Line with origins from the Kheis Province, Kaapvaal Craton, or further afield considered unlikely. Weenvisage the Rehoboth Province to consist of an Archaean core supplemented by Palaeoproterozoicgranitoids, which was joined to the Kaapvaal Craton at an early stage of crustal development and played animportant role during later tectonic events. This has important implications not only for the tectonicframework and assembly of Southern Africa, but also for exploration for diamonds and other ore deposits.

+46 31 786 19 86.

ces, Division of Geology, Lund

ll rights reserved.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

The Precambrian framework of southern Africa is well known, asshown in Fig. 1a and reviewed by Johnson et al. (2006), with thenotable exception of the Kheis and Rehoboth Provinces which arelargely covered by Cretaceous to Recent Kalahari Desert sands.

According to published work, the Kheis Province could either be an~1800 Ma orogenic belt (Cornell et al., 1998), or a northern branch ofthe ~1200 Ma Namaqua–Natal Province (Moen, 1999; Moen andArmstrong, 2008). The Rehoboth Province also contains 1850–1720 Ma and 1.37–1.1 Ga components (Becker et al., 2006). In thiswork we investigate and date a group of sixteen granitoid cobblesplucked from the basement east of Rietfontein by the ~300 MaPermocarboniferous Dwyka ice sheet, now exposed in tillite on theborder between Namibia and South Africa (Visser, 1997). Surprising-ly, these cobbles contain no evidence of crustal growth or orogeny ateither 1800 or 1200 Ma, but rather suggest the presence of Archaean

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KAAPVAALCRATON

3.1 Ga

Kalahari Basin

b

**Border

Rietfontein

*KoopanSuid10 km

N A

MIB

IA

BOTS-WANA

SOUTH AFRICA

*

27 S

20E

2030

E

26 45 S

27 15 S

Mafic (sub)-outcrops1.1 Ga

International BorderTectonic Province BoundaryInferred extent of UmkondoLarge Igneous ProvinceRoad River

ICE MOVEMENT DIRECTIONSMoore and Moore (2004)Visser (1987, 1997)

Sample locations

ZiMBN

SA

B: BotswanaM: MozambiqueN: Namibia

SA: South AfricaZi: Zimbabwe

200 km

Pretoria

Swaziland

Lesotho

Windhoek

Cape Town

a

Gaborone

N

*KarnaldamSannaspoort

Rehoboth

Rehoboth Province

Vioolsdrif

OKWA

Upington

Figure 1b

KO

*

*

Kh

eis

Pro

vin

ce

ZIMBABWE CRATON 3.2 Ga

*Kgokgole

Koppiespan**

Olifantshoek

Motition

Xadecx

Tshane cxTsetseng cx

Mabuasehube

Ditshipeng

LIMPOPO BELT2.7 Ga

SALDANHA

Groblershoop

Fig. 1. a. Crustal framework of southern Africa showing structural provinces modified after Cornell et al. (1998) and Corner (2003), large mafic intrusions and Dwyka ice movementvectors after Visser (1987, 1997, solid arrows) and Moore and Moore (2004, dashed arrows). The area covered by the Kalahari sand is shaded, after Haddon and McCarthy (2005),KO: Koras Group. b: Detail of sampling sites.

483D.H. Cornell et al. / Lithos 125 (2011) 482–502

granite beneath the Kalahari, with a younger 2.0 Ga graniticcomponent. In a companion paper (Van Schijndel et al., submittedfor publication) we dealt with the 1.1 Ga mafic cobbles and in thiswork we discuss the broader implications of both datasets.

1.1. Glacial clasts as provenance indicators

Glacial cobbles and boulders have been used in many studies toinvestigate the ice movement directions, the provenance of bouldersand the distance that they have been transported. Bouchard andSalonen (1990) report typical transport distances in basal tills of 4 to25 km in Finland and Canada. They note that local stratigraphy tendsto be dominant in till stone counts. By contrast, erratic boulders foundin Germany and Poland have been traced to their Scandinaviansources and testify to Quaternary glacial transport distances up to1000 km. In southern Africa, Visser et al. (1986) identified character-istic stromatolitic limestone, chert and banded iron formation clasts inDwyka diamictites some 300 km south of their provenance in the

Kaapvaal Craton cover sequence. It is significant that these threedistinctive rock types are clearly able to survive large transportdistances. Such huge transport distances probably involved thecombined effect of several glacial cycles.

2. Crustal framework of southern Africa

Fig. 1a depicts the structural provinces of southern Africa, withtheir ages, following Cornell et al. (1998) and modified using regionalmagnetic maps (Becker et al., 2006; Corner, 2003, 2008). These first-order subdivisions are important in understanding how and when thecontinents were assembled and for the planning of mineral prospect-ing. The continuity and boundaries of provinces such as theNamaqua–Natal Province have been established by geophysics, deepdrilling and radiometric dating despite the thick Phanerozoic coverwhich separates the Namaqua and Natal sectors (Cornell et al., 2006).However, the regions labelled Rehoboth and Kheis in Fig. 1a, arealmost entirely buried below the Late Cretaceous to Recent Kalahari

484 D.H. Cornell et al. / Lithos 125 (2011) 482–502

sands and are poorly understood. The descriptions, age and integrityof these tectonostratigraphic units are based on geophysical studies,combined with limited information from outcrops and from rocksexposed in adjacent terranes.

2.1. The Kheis Province

The Kheis Province is a region characterised by a thin-skinned, east-verging thrust belt comprising Palaeoproterozoic quartz-richmetasedi-ments and probably underlain by granitic rocks (Schlegel, 1988; Stowe,1986). It is bounded to the east along the Black Ridge Thrust (Altermannand Hälbich, 1991; Beukes and Smit, 1987) by the Kaapvaal Craton,which was not strongly deformed after 2.5 Ga. The Kaapvaal Cratonconsists of Archaean granite basement with a cover sequencedominated by Ventersdorp lavas, followed by dolomites, banded iron

24oS

20oE

Nam

ibia

So

uth

Afr

ica

B

ots

wan

a

REHOBOTH

Rehoboth Province

UPINGT

200 km KOOP

Namaqua

Natal

Province

Rehoboth Inlier

KAAPVA

Molopo Fa

Dolomite

Banded Ir

GriqualaSupergr

Intrusive

Koras Group; Volcanic and Intrusive rocks

Mafic Complexes; Tshane, Tsetseng and Xade

Hydrothermally altered magnetite- rich granites ; According to Corner (2008)

Mafic Complexes; According to Meixner (1984)

Quartzite Inliers

REHOBOTH and KHEIS PROVINCE

Granite Su

1870- 1720 Ma

Kimberlite

Gibeon Kimberlite Field

28oS

RIETFONTEIN

Fig. 2. Map showing the Rehoboth and Kheis provinces in the area marked in Fig. 1, with re2006; Lock and Carney, 1991; Miller, 2008) or interpreted from geomagnetic data (Corner,Moore (2004, solid arrows) and Visser (1987, solid arrows; 1997, dashed arrows) are show1999); BRF: Black Ridge Fault. BF: Blaauwbospan Fault (Stowe, 1986).

formation and jaspilites of the Griqualand West (greater Transvaal)Supergroup. Tinker et al. (2002) published a seismic reflection profile(A–B in Fig. 2) showing that the Kaapvaal supracrustal formations dipwestwards at the craton margin and are thickened by major thrustingtowards the east,which they thoughthappened soon after the extrusionof the 1928 MaHartley Basalt. However, there is some evidence that theKheis Province owes its structural pattern to the MesoproterozoicNamaqua–Natal orogeny. Moen (1999) documented the similarity ofstructural histories where the two provinces adjoin around Groble-rshoop (Fig. 1a). The 1290 MaWilgenhoutsdrif Group forms part of theKheis thrust belt, in which Rb–Sr muscovite that has ages from 1183 to1144 Ma byMoen andArmstrong (2008) shows that the Kheis Provincefabric is likely to correspond to the early Namaqua collisions. This idea issupported by a fewargon and zircondates, notably from theOkwa Inlierto the north (Fig. 1a, Key and Rundle, 1981; Singletary et al., 2003).

Xade cx

Tsetseng cx

Tshane cx 1105±11 Ma

ON

KA

LAH

AR

I LIN

E

K

hei

s

P

rovi

nce

AN SUID

24oE

MOLOPO FARMS CX

2050 Ma

AL CRATON

rm Complex

on Formation

nd West oup

s

Visser (1989,1997)

Moore and Moore (2004)

ICE MOVEMENT DIRECTIONS

DF

OLIFANTSHOEK

International Boundary

Tectonic Province Boundary

GANYESE DOME

A

B

GRANITE SUB-OUTCROP

b-outcrop

1090 Ma

BF

1101±1 Ma

Morokweng Impact Kelso

BR

T

MIDDELPUTS

levant details of basement geology known from outcrop and boreholes (Johnson et al.,2003, 2008; Meixner and Peart, 1984). Dwyka ice transport directions from Moore andn. Line AB indicates a seismic profile by Tinker et al. (2002). DP: Dabep Fault (Moen,


HoweverMapeoet al. (2006) reportedprecise zircondates for theoriginof all fourmajor gneiss, granite and rhyolite units of the Okwa complex,with indistinguishable ages at 2056±2 Ma. Van Niekerk (2006)proposes that the Kalahari Line represents an extension of the~1280 Ma volcanic arc-related Areachap Group from the Namaqua–Natal Province into Botswana, based on an aeromagnetic mapinterpretation.

The western boundary of the Kheis Province is the Kalahari Line.Meixner and Peart (1984), described the Kalahari Line as a crustalsuture between the Archaean Kaapvaal Craton and a younger crustalblock to the west. They interpreted the Kalahari Line's magneticsignature in termsof a group ofmafic intrusions, ofwhich theXade andTshane intrusionshave beendrilled anddated at 1.1 Ga, correspondingto the Umkondo Large Igneous Province (Hanson et al., 2006;Pouliquen et al., 2008). However Corner (2008) modelled the KalahariLine as a zone of hydrothermally altered magnetite-rich granites,similar to the Colesburg Lineament on the Kaapvaal Craton (Corner,1998). He shows only three mafic bodies on the Kalahari Line, arguingthat most of the magnetic highs lack the positive gravity anomalies,which large mafic intrusions should show. Brett et al. (2000)confirmed the presence of both types of geophysical anomaly anddocumented the presence of large mafic intrusives, hydrothermallyaltered diorites and magnetic granites along their Kalahari SutureZone. The Kalahari Line seems to extend southwards into fault-boundedexposures of Koras Group bimodal lavas and intrusions, including theBetadamGabbronorite, the 1093±10Ma Rooiputs Granophyre and the1093±11Ma Blauwbosch Granite (Pettersson et al., 2007).

A foreland thrust complex developed in the 1.92 Ga OlifantshoekSequence redbeds west of Olifantshoek (Fig. 1a) was postulated byCornell et al. (1998), based on the work of Stowe (1986), to reflect apost-Bushveld (2.05 Ga) Wilson cycle, which culminated in the Kheiscollisional orogeny and ended by about 1.75 Ga. Several authors suchas Singletary et al. (2003), Jacobs et al. (2008), andMuller et al. (2008)distinguish the Kheis and Rehoboth Provinces or terranes and in thiswork we distinguish the Kheis Province from the Rehoboth Provinceas shown in Fig. 1a. (Note that Moen's (1999) and Moen andArmstrong's (2008) boundary is different, following the Dabep Faultso that his Kheis Province excludes the Olifantshoek Group to the eastand includes part of our Rehoboth Province. Van Niekerk (2006) alsoshows different boundaries for his Kheis Terrane.

2.2. The Rehoboth Province

This is a magnetically quiet region which extends from theKalahari Line to the Damara foreland in the northwest and to theNamaqua–Natal Province in the west. It corresponds to Corner's(2008) Nama Terrane, but the name Rehoboth has historical priority(Hartnady et al., 1985). Reeves (1978) reported surprisingly large'depth to magnetic basement' contours up to 15 km in the sand-covered region in Botswana west of the Kalahari Line, which wereinterpreted by Meixner and Peart (1984) to reflect a thick sedimen-tary sequence overlying oceanic basement. The oldest dated rocksexposed in the Rehoboth Basement Inlier comprise 1.87–1.76 Gamagmatic intrusions (Becker et al., 2004; Van Schijndel et al., 2011,Fig. 2). Samples from the Rehoboth Basement Inlier gave TDM Ndmodel ages from 2.37 to 1.66 Ga, (Ziegler and Stoessel, 1993). Redepletion model ages by Hoal and Heaman (1995) for peridotitexenoliths from the Gibeon kimberlites are 2.2 to 2.0 Ga, suggestingcrustal growth at that time. Detrital zircons aged 2749±42 Ma and2892±27 Ma in a b1750 Ma Billstein Formation quartzite from theRehoboth Basement Inlier (Van Schijndel et al., 2011) show that anArchaean crustal component is present in the Rehoboth Province.

The Rehoboth Province seems to have joined the Kaapvaal andZimbabwe Cratons and Limpopo Belt to form the Kalahari Craton beforeor during the Namaqua–Natal Orogeny, in which exotic terranescollided with, but only deformed the margins of the Proto–Kalahari

Craton (comprising Kaapvaal Craton together with Zimbabwe Craton,Limpopo, Kheis and Rehoboth Provinces) at about 1.2 Ga. (Jacobs et al.,2008). In the Rehoboth Province the 1.37–1.1 Ga Sinclair Sequencevolcanic and plutonic rocks exposed in the Damara Foreland area havebeen interpreted as representing a magmatic arc (Becker et al., 2006;Watters, 1976). This suggests that the Rehoboth Province experiencedan Andean-type crust-forming event at the end of the NamaquaWilsonCycle. The younger parts of the Sinclair Sequence are undeformed incontrast with the high-grade gneisses in the Namaqua–Natal Provinceto thewest, consistentwith the idea that theyoverlie RehobothProvincebasement which was not deformed by the Namaqua orogeny.

2.3. The Kaapvaal Craton

The Kaapvaal Craton is a huge crustal block comprising mainly ofArchaean granites with some greenstone belts. It has been subdividedinto the Kimberley, Witwatersrand, Pietersburg and SwazilandTerranes (Eglington and Armstrong, 2004; Schmitz et al., 2004).These are thought to have been assembled to form the craton beforeextrusion of the Ventersdorp lavas at 2700 Ma. The Kimberley Terraneoccupies the western part of the craton and comprises granites in therange of 3.2 to 2.7 Ga with subordinate greenstone belts such as theAmalia, Stella and Kraaipan belts, whereas the Witwatersrand blockhas components as old as 3.55 Ga in the Barberton GreenstoneTerrane (Johnson et al., 2006). The Kimberley Terrane is commonlyshown as extending to the Kalahari Line, including the Kheis Provincebecause it is thought to overlie Kaapvaal granitic basement. Thus thearea with high diamond prospecting potential includes the KheisProvince. However, a craton should not by most definitions includeareas deformed after the craton formed, so we distinguish the KheisProvince in Figs.1a and 2. The age of the westernmost exposures ofgranite in the Kimberley Terrane is not well documented and in thiswork we investigated five outcrop samples. Sample DC0912 is fromKoppiespan, just south of the Kraaipan Greenstone Belt and the othersare from basement outcrops at Kgokgole and Ditshiping (Table 1 andFig. 1a) in the ~160 km Ganyesa dome (Hunter and Hamilton, 1978,Fig. 2). Diamond drilling and regional gravity and magnetic data haveshown that the Ganyesa dome is centred on a 800 m-thick meteoriteimpact melt sheet dated at ~144 Ma (Andreoli et al., 1999; Reimoldet al., 2002). In view of these finds, including the discovery of asignificant central uplift beneath the Morokweng impact melt sheet,we think that the granites now exposed in the Ganyesa dome wereprobably buried beneath the Transvaal Sequence at the time ofglaciation.

3. Glacial diamictite exposures

At Rietfontein on the western margin of the Kalahari basin (Figs. 1band 2), glacial diamictites of the late Carboniferous to early PermianDwyka Group are exposed in dry river beds, unconformably overlyingthe Cambrian Nama Group siltstones (Fig. 3). The diamictites arematrix-supported, massive to stratified, and contain abundant cobble-sized (64–256 mm) clasts of crystalline bedrock, some of which havestriated and polished surfaces confirming the glacial origin. Fourlocalities were sampled as shown in Fig. 1b, called Border, Sannaspoort,Karnaldam (within a radius of 6 km from Rietfontein) and Koopan Suid(70 km to the south) and listed in Table 1. The cobbleswere taken out ofthematrix (in-situ) at the first two localities, whereas at Karnaldam andKoopan Suid they lay loosely on the diamictite. The Border andSannaspoort exposures are close to the base of the Dwyka Formation,whereas Karnaldam is midway up the Formation (b100 m thick in thisarea, Visser, 1987) and Koopan Suid close to the top, judging from thedistances between the base (Nama Group siltstone) and top (EccaGroup shale) on 1:250 000 SA Geological Survey maps. Structural andtextural observations suggest that the diamictite was deposited in aglaciolacustrine or possibly glaciomarine environment, in which the

Table 1Sample localities, hand specimen descriptions and ages for granitoid cobbles and outcrop samples.

Sample Hand specimen description and grain size Locality Latitude Longitude Age Error In-situ?

Dwyka cobbles from the western border of the Kalahari Basin Deg. Min. Deg. Min. Ma 2σ

DC0505 Pale brown granite, 10 mm Karnaldam 26 43.431 20 4.988 2047 14 NoDC0506 Pink granite, 3–10 mm Karnaldam 26 43.431 20 4.988 2017 15 NoDC0706 Dark brown weathered granite 20 mm, mafic xenoliths Sannaspoort 26 43.239 20 0.637 2797 6 YesDC0710 Pale brown biotite granite–gneiss, fine-10 mm Sannaspoort 26 43.239 20 0.637 2525 20 YesDC0712 Rapakivi granite, 15 mm grains Sannaspoort 26 43.239 20 0.637 2778 10 YesDC0716 Brown banded felsic biotite schist, fine-5 mm Sannaspoort 26 43.239 20 0.637 3079 15 YesDC0722 Pink 2-feldspar granite, 10 mm Karnaldam 26 43.431 20 4.988 2061 14 NoDC0723 Pale brown foliated biotite granite, 10 mm Border 26 45.072 20 0.841 2499 24 YesDC0724 Pale brown weathered biotite granite, 5 mm Border 26 45.072 20 0.841 2623 16 YesDC0727 Migmatite biotite granite gneiss, 3 mm Border 26 45.072 20 0.841 2621 13 YesDC0731 Migmatitic gte gneiss badly weathered Border 26 45.072 20 0.841 2013 10 YesDC0740 Brown granite, 20 mm Koopan S 27 20.399 20 18.79 2889 11 NoDC0743 Biotite granite, weathered, 10 mm Koopan S 27 20.399 20 18.79 2877 11 NoDC0745 Dark brown biotite granite, 5 mm Koopan S 27 20.399 20 18.79 2879 11 NoDC0747 Pale brown quartz feldspar porphyry, fine and 4 mm Koopan S 27 20.399 20 18.79 2661 24 NoDC07101 Gray rapakivi granite, 10 mm Sannaspoort 26 43.242 20 0.637 2912 13 Yes

Western Kaapvaal granites from outcrop Locality Latitude Longitude Age Error

DC0912 Pink foliated granite 5 mm Koppiespan 26 46.185 25 17.05 3061 9 OutcropDC0914 Pink massive granite, 5 mm, weathering orange Motition 26 57.116 23 52.79 2867 7 OutcropDC0915 Gray granodiorite, 2 mm Ditshiping 26 56.62 23 49.88 2882 7 OutcropDC0916 Pink massive granite, 5 mm, intrudes DC0915 Ditshiping 26 56.648 23 49.86 2854 7 OutcropDC0917 Pink massive granite, 4 mm, cut by 20 cm pegmatites Kgokgole 26 43.331 23 56.47 2857 6 Outcrop


outsized clastsweredeposited fromafloating ice shelf or icebergs. Somelarge clasts were observed to have bent and penetrated the underlyinglaminae, which conformswith the clasts being deposited as dropstones.Lobate beds of massive matrix-supported diamictite, typically a fewdecimetres thick and a meter to few meters in lateral extent, withslightly scoured bases, are interpreted to be plug zone sediments ofcohesive debris flows into subaquaeous environment. At Border andKarnaldam, fluvoglacial beds of cross-bedded conglomerate and coarsesandstone are intercalated in the diamictite (Fig. 4), implying that theice-frontwas close by during their deposition. In the veld away from therivers, the surface is covered with pebbles representing erosion lagdeposits, in which the finer material is transported away leaving theclasts behind (Fig. 5).

Bearing in mind the generally westward transport directions forthe Dwyka ice sheet (Visser, 1997, Figs. 1a, 2) that provided the icerafted clasts from icebergs or an ice shelf, the exposures were

Fig. 3. Dwyka diamictite overlying Nama Group siltstone at Sannaspoort. The visiblepart of the pole is 600 mm long.

investigated and sampled to see if they contained information aboutwhat lies below the Kalahari sands.

4. Sampling and methods

Sample localities are given in Table 1. Cobbles were broken fromthe diamictite matrix and whole rock samples from granite outcropsin the western Kaapvaal Craton by hammer. Pebble counts onfluvoglacial conglomerates at Border and in the Molopo River atBogogobo (near Middelputs, Fig. 2), are given in Table 2 and theresults from Border agree approximately with the rock typeabundances of the larger cobbles.

The mafic and granitic pebbles were investigated separately and inthis work only the granitic pebbles are documented in detail. Themafic pebbles are described in a companion paper by Van Schijndelet al. (submitted for publication).

Fig. 4. Coarse-grained (~10 mm) cross-bedded sandstone forms channel-fills in DwykaGroup diamictites at Border. The book is 150 mm wide.

image of Fig.�4

Fig. 5. Pebble lag surface in the field derived from erosion of Dwyka diamictite.


Zircons were extracted from the granitic samples after arepresentative whole rock sample had been collected, by crushingto pass a 400-micron sieve, then washed and manually panned toconcentrate the heavy minerals. Zircon grains were hand-picked andmounted in epoxy pucks for SEM imaging (Raith et al., 2003). Wholerock analyses of most of the samples were made by standard X-rayand ICPMS methods, with results given in supplementary data TableA1.

4.1. U–Pb microbeam dating and oxygen isotope analysis

U–Pb dating was done using the NordSIM Cameca ims1270/80 ionprobe on grains mounted in epoxy pucks, as described byWhitehouseand Kamber (2005), and by laser ablation ICPMS at GEUS, as describedby Frei and Gerdes (2008). The U–Pb analytical data for the Dwykacobbles and for the western Kaapvaal granite outcrop samples aregiven in supplementary data Tables A3 and A4 respectively, withdiscordance calculations following Eglington and Harmer (1999).

Oxygen isotope analyses of zircons were made using the NordSIMion probewith a Cs primary beam andmulticollector on the same type

Table 2Pebble counts in Dwyka glacial material at two localities west of the Kaapvaal Craton.The Bogobo data is by Elin Jansson (2010), supervised by D. Cornell.

Bordernr.Rietfontein

Bogogobonr. Middelputs

Latitude 26°45.072′S 26°40.435′SLongitude 20°00.841′E 21°55.879′EPosition due west of Kaapvaal Craton 280 km 90 kmMafic lava (with feldspar at Border) 3.9 16.2Red sandstone 26.2 0.6Green sandstone 24.1 –

Granite 13.1 –

Dolerite or gabbro 21.4 –

Quartz 5.1 –

Dark hornfels 6.2 –

Banded iron formation (BIF) – 26.3Chert and jasper – 37.4Felsic lava – 12.3Quartz porphyry – 2.8Shale – 1.7Unidentified – 2.8Total 100 100Number of pebbles (N5 mm) 157 179Minimum size of pebbles counted (mm) 5.0 10

of epoxy mounts as for U–Pb dating, as described by Whitehouse andNemchin (2009). The oxygen isotope results are given in Table 3 andthe complete data in supplementary data A5. Some samples gavevalues with a larger spread than expected from the individual errors,assuming that the magmatic zircon of each sample had a single value,reflecting the magma oxygen isotope composition, which is unlikelyto have changed during zircon crystallisation. Mean values werecalculated for each sample using the weighted mean calculation ofISOPLOT v3.0 by Ludwig (2003) and only results with probabilitygreater than 0.15were accepted. The ion probe pits for each data pointwere examined by SEM and outlying points with obvious cracks orinclusions were omitted as SEM rejects, as suggested by Cavosiea et al.(2005). In the remaining data, rejection of one or two outlying pointswas allowed for groups of more than ten data. This is justified by thefact that even the homogeneous and crack-free 91500 standard zircongave some spurious values (see supplementary data A5).

5. Results

5.1. Petrology and geochemistry

In thin section the cobbles are seen to be extensively altered due todiagenesis or weathering, so that modal classification was notpossible. This alteration may be reflected geochemically in somesamples by variations in alkali elements, leading to diverse totalalkali–silica (TAS, supplementary data A2.2) classifications in Table 4such as syenodiorite for DC0710 and DC0745, although most cobblesamples do classify as granites.

Themobility of calciumduringDwykadiagenesis is evidencedby thepresence of ovoid calcite concretions in the diamictite at Sannaspoort.This may have given rise to variable catanorm anorthite contents(Table 4). Normative corundum N2% in three of the granitoids cobblesamples may also reflect alkali element or calcium mobility, whereasfelsic biotite schist DC0716, which is interpreted as a metasediment,consistent with 4% normative corundum. Nevertheless, all but onesample (DC0723, Na-enriched) plot within the igneous spectrum ofHughes (1973, supplementary data A2.1), suggesting that geochemicalalteration is not severe.

The rare earth profiles shown in Fig. 6 were probably not greatlyaffected by alteration, apart from samples DC0723 and DC0745 whichshow small Ce anomalies, reflecting oxidation during near-surfacealteration.

The ab-or-an catanorm-based granite classification of Barker(1979, Table 4 and supplementary data A2.3) identifies five cobblesamples as trondhjemites, which is consistent with their heavy rareearth-depleted rare earth profiles shown in Fig. 6a, and described inTable 4. Trondhjemites usually show heavy rare earth-depletion,which is interpreted to reflect the influence of garnet in the meltingresidue (discussed by e.g. Frost et al., 2006; Van Tonder and Mouri,2010).

Two other genetic groups are apparent from the rare earth profiles.Normal granites in Fig. 6b show regularly decreasing chondrite-normalised levels from La to Lu, but not significantly depleted inheavy rare earths and with less steep slopes compared to thetrondhjemites. Two granites (Fig. 6c) show light rare earth enrich-ment from La to Gd, but flat heavy rare earth profiles. This mightreflect hydrothermal alteration of these rocks or their source.

The west Kaapvaal outcrop samples are thought to be unaltered.They are all granites according to TAS and catanorm classifications,however the REE profiles in Fig. 6d show trondhjemitic, heavy rareearth depleted character for three of them. The other two sampleshave parallel profiles but with about four times higher concentrations,possibly reflecting their origin by crystal fractionation of trondhjemi-tic magmas. Thus the west Kaapvaal samples are all geochemicallyrather similar, whereas the Dwyka cobbles belong to three distinctgenetic types.

image of Fig.�5

Table 3Results of ion microprobe oxygen isotope analyses of zircon in permil relative toVSMOW. The complete data is given in supplementary data A3. The procedure forcalculating weighted mean values for each sample is described in Section 4.1. SEM rej.shave cracks or inclusions, outliers look ok. Data in bold were used to calculate meanvalues.

Comments Grain δ18O ±1σ

DC0505 n3500 Karnaldam cbl. 2047 MaMain gp. 201 7.20 0.41Main gp. 203 7.43 0.49Main gp. 204 6.98 0.43Main gp. 205 7.68 0.45Main gp. 207 7.48 0.43Main gp. 209 7.62 0.47Mean of 6 Prob. 0.88 7.38 0.18

DC0506 Karnaldam cobble 2017 MaOutlier 214 5.34 0.40SEM rej. 348a 5.86 0.33SEM rej. 314b 6.24 0.36Main gp. 329a 6.40 0.31Main gp. 212 6.46 0.42Main gp. 337b 6.47 0.37Main gp. 317a 6.54 0.35Main gp. 314a 6.54 0.29Main gp. 311a 6.56 0.29Main gp. 328a 6.66 0.36Main gp. 306a 6.71 0.29Main gp. 202 6.80 0.40Main gp. 310a 6.80 0.32Main gp. 328b 6.80 0.30Main gp. 332a 6.84 0.31Main gp. 324a 6.92 0.33Main gp. 207 6.94 0.42Main gp. 337a 7.13 0.30Main gp. 317b 7.39 0.36SEM rej. 201 7.87 0.43SEM rej. 320b 7.87 0.53Outlier 320a 8.03 0.30SEM rej. 210 9.24 0.41SEM rej. 318b 9.32 0.32SEM rej. 318a 10.37 0.36Mean (16) Prob. 0.88 6.74 0.09

DC0706 n2875 Sannaspoort cobble 2797 MaOutlier 204 1.83 0.48

212 2.56 0.42219 2.67 0.44201 2.82 0.43210 3.44 0.42207 3.59 0.43

Mean of 5 Prob. 0.31 3.02 0.19

DC0710 n2879 Sannaspoort cbl. 2525 MaOutlier 322a 2.02 0.35SEMrej. 204b 2.68 0.31Outlier 849a 2.85 0.31SEMrej. 838b 3.01 0.34Outlier 216b 3.01 0.34SEMrej. 830a 3.08 0.37SEMrej. 839a 3.18 0.40Outlier 323c 3.26 0.38SEMrej. 305b 3.53 0.31Main gp. 323b 4.03 0.56Main gp. 841a 4.12 0.33Main gp. 305a 4.23 0.32Main gp. 309a 4.26 0.35Main gp. 820a 4.29 0.32Main gp. 323a 4.32 0.31Main gp. 827b 4.37 0.31Main gp. 827a 4.73 0.35Main gp. 209d 4.77 0.32Main gp. 312a 5.07 0.34SEMrej. 838a 5.40 0.32SEMrej. 318a 5.62 0.40SEMrej. 316a 5.74 0.30Outlier 305c 5.91 0.30Outlier 816a 5.93 0.32

Table 3 (continued)


DC0710 n2879 Sannaspoort cbl. 2525 MaSEMrej. 819a 5.93 0.32SEMrej. 217b 7.06 0.31SEMrej. 827c 7.24 0.34SEMrej. 835a 7.55 0.53Heterogeneous spl. may have main gp.

Main gp.10 Prob. 0.54 4.44 0.1127 of 28 Prob. 0.0 4.49 0.26

DC0712 n2876 Sannaspoort cbl. 2778 Ma203 2.18 0.47213 2.25 0.45303a 2.31 0.33312c 2.33 0.38304a 2.68 0.32203b 2.74 0.44311b 2.86 0.37312b 2.96 0.31312a 2.98 0.34311a 3.13 0.37303b 3.14 0.32213b 3.18 0.44

SEM rej. 309a 4.66 0.33Mean (12) Prob. 0.51 2.76 0.12

DC0716 n3502 Sannaspoort cbl. 3079 MaSEM rej. 213b 3.24 0.29gp. of 5 202 3.55 0.45gp. of 5 208 4.11 0.45gp. of 5 218a 4.13 0.26gp. of 5 213 4.22 0.45gp. of 5 215 4.46 0.41SEM rej. 216a 4.71 0.27SEM rej. 211a 5.13 0.29SEM rej. 202b 5.19 0.28Gp. of 9 204a 5.20 0.27Gp. of 9 219a 5.37 0.29Gp. of 9 212 5.38 0.42Gp. of 9 222a 5.79 0.29Gp. of 9 206a 5.84 0.28Gp. of 9 201a 5.84 0.31Gp. of 9 217a 5.95 0.29Gp. of 9 222b 6.08 0.29Gp. of 9 205a 6.14 0.30SEM rej. 209 6.32 0.42SEM rej. 214a 6.78 0.29Two gp.s distinguished in metasediment

Gp. of 9 Prob. 0.25 5.74 0.20Gp. of 5 Prob. 0.67 4.11 0.17

DC0722 Karnaldam cobble 2061 Ma202 7.69 0.45203 6.13 0.48204 7.31 0.41207 6.72 0.47209 7.24 0.47211 7.31 0.51

Mean of 6 prob. 0.22 7.09 0.19

DC0723 n2880 Border cobble 2499 MaSEM rej. 304a 1.73 0.34Low gp. 207 1.99 0.48SEM rej. 306a 2.08 0.33SEM rej. 315a 2.39 0.36SEM rej. 328c 2.40 0.35low gp. 342b 2.49 0.32SEM rej. 306b 2.50 0.33SEM rej. 342a 3.11 0.39SEM rej. 328a 3.22 0.34Main gp. 218 4.79 0.47Main gp. 201 4.79 0.44Main gp. 203 4.91 0.42Main gp. 213 5.17 0.45Main gp. 328b 5.24 0.33SEM rej. 220 6.26 0.46


Table 3 (continued)


Heterogeneous sample, poss. main gp.Main gp. 5 Prob. 0.89 5.02 0.19

DC0724 n2883 Border cobble 2623 MaSEM rej. 315a 3.21 0.33Main gp. 322a 4.25 0.33Main gp. 303a 4.74 0.36Main gp. 315b 4.92 0.33Main gp. 304a 4.97 0.34Main gp. 302a 5.07 0.30Main gp. 307a 5.11 0.32Main gp. 337a 5.26 0.42Main gp. 322b 5.72 0.37SEM rej. 336a 6.88 0.32Mean of 8 Prob. 0.19 4.98 0.12

DC0727 n3503 Border cobble 2621 MaMain gp. 204 3.59 0.42Main gp. 206 3.39 0.42Main gp. 208 3.47 0.47Main gp. 210 3.41 0.47Main gp. 211 3.77 0.46Main gp. 212 2.41 0.44Mean of 6 Prob. 0.34 3.33 0.18

DC0731 n2884 Border cobble 2013 MaSEM rej. 211b 1.84 0.27Low gp. 211 2.18 0.42Low gp. 217a 2.34 0.30Low gp. 222a 3.38 0.29Low gp. 213a 3.46 0.30Low gp. 222b 3.90 0.29SEM rej. 201a 4.53 0.28SEM rej. 220a 4.68 0.28SEM rej. 219a 5.01 0.29Main gp. 215b 6.88 0.28Main gp. 215 7.00 0.41Main gp. 210b 7.21 0.34Main gp. 204 7.51 0.53Main gp. 206 7.76 0.42Main gp. 210 7.78 0.43SEM rej. 212a 8.82 0.28Outlier 208 8.84 0.42SEM rej. 208b 9.35 0.28A coherent main gp. and a broad low gp.

Low gp. 6 Prob. 0.0 3.16 0.45Main gp. 6 Prob. 0.37 7.25 0.15

DC0740 n2885 Koopan S. cbl. 2889 MaLow gp. 216b 5.00 0.30Low gp. 215 5.30 0.44Low gp. 208a 5.31 0.32Low gp. 201 5.50 0.41Low gp. 204a 5.60 0.36High gp. 7 211 5.97 0.45High gp. 7 219a 6.27 0.28High gp. 7 222a 6.42 0.35High gp. 7 214b 6.59 0.33High gp. 7 210a 6.82 0.42High gp. 7 202 6.90 0.46High gp. 7 202b 6.91 0.33SEM rej. 214 7.25 0.43SEM rej. 213 7.50 0.46SEM rej. 216a 7.57 0.33SEM rej. 217a 7.70 0.35Inhomogeneous sample with two possible gps.

Low gp. 5 Prob. 0.76 5.31 0.16High gp. 7 Prob. 0.54 6.54 0.14

DC0743 n2881 Koopan S. cbl. 2877 MaOutlier 204 4.17 0.35Outlier 203b 4.19 0.34Gp. of 11 323a 4.41 0.30Gp. of 11 315a 4.66 0.32Gp. of 11 319b 5.03 0.36Gp. of 11 201 5.06 0.31Gp. of 11 308b 5.10 0.33

(continued on next page)

Table 3 (continued)


DC0743 n2881 Koopan S. cbl. 2877 MaGp. of 11 315b 5.20 0.31Gp. of 11 323b 5.27 0.35Gp. of 11 308a 5.31 0.35Gp. of 11 302a 5.31 0.31Gp. of 11 319a 5.48 0.30Gp. of 11 302b 5.61 0.38SEM rej. 206b 6.53 0.34Mean of 11 Prob. 0.3 5.11 0.10

DC07101 n2886 Sannasprt. cbl. 2912 MaSEM rej. 333a 4.46 0.34SEM rej. 335a 4.54 0.31SEM rej. 311b 4.68 0.31SEM rej. 330a 4.70 0.34SEM rej. 306a 4.76 0.40Gp. of 9 316a 4.86 0.38Gp. of 9 345a 4.95 0.38Gp. of 9 304b 5.27 0.39Gp. of 9 315c 5.40 0.41Gp. of 9 304a 5.47 0.32Gp. of 9 315a 5.61 0.35Gp. of 9 325a 5.64 0.33Gp. of 9 315b 5.90 0.40Gp. of 9 325b 5.92 0.45SEM rej. 311a 6.55 0.38Mean of 9 Prob. 0.50 5.44 0.13

DC0912 n3506 Koppiespan W. Kaapvaal Gte. 3046 MaOutlier 17 2.86 0.85

27 4.80 0.9628 4.65 0.8345 5.17 0.9348 5.32 0.8950 5.42 0.8558 4.61 0.80

Mean of 6 Prob. 0.97 4.98 0.35

DC0914 n3508 Motition W. Kaapvaal Gte. 2868 Ma24a 6.98 0.8824b 6.73 0.9032 6.36 0.8641 6.85 0.8552 7.34 0.91

Mean of 5 Prob. 0.96 6.84 0.39

DC0915 n3510 Ditshiping W. Kaapvaal Gte. 2882 MaSEM rej. 304c 4.00 0.36SEM rej. 342a 4.25 0.32Outlier 342b 5.26 0.28Gp. of 10 327a 5.34 0.36Gp. of 10 323c 5.39 0.34Gp. of 10 339a 5.74 0.36Gp. of 10 342c 6.03 0.37Gp. of 10 328a 6.17 0.36Gp. of 10 329a 6.19 0.33Gp. of 10 304b 6.25 0.35Gp. of 10 344a 6.40 0.32Gp. of 10 323b 6.45 0.44Gp. of 10 323a 6.53 0.40Outlier 304a 6.75 0.32Mean (10) Prob. 0.18 6.03 0.12

DC0916 n3509 Ditshiping W. Kaapvaal Gte. 2856 MaSEM rej. 339b 4.94 0.33

320a 5.29 0.33338a 5.35 0.36312b 5.44 0.34326b 5.54 0.31326a 5.56 0.29314a 5.63 0.30302a 5.69 0.30312c 5.71 0.31320b 5.73 0.35326c 5.88 0.31334a 5.92 0.31

(continued on next page)


Table 3 (continued)


335a 5.93 0.30312a 6.05 0.44

SEM rej. 323a 6.08 0.34339a 6.11 0.35

SEM rej. 338b 6.81 0.33Mean (15) Prob. 0.87 5.72 0.09

DC0917 n3507 Kgokgole W Kaapvaal Gte. 2830 Ma318a 5.59 0.37329a 5.65 0.33321a 5.91 0.35323a 5.96 0.34326b 6.19 0.31326a 6.49 0.35

Mean of 6 Prob. 0.44 5.97 0.14

DC0916 n3509 Ditshiping W. Kaapvaal Gte. 2856 Ma


5.2. Age calculations

Sixteen of the granitic cobbles and five Kaapvaal outcrop graniteswere dated, with regression results using ISOPLOT v3.0 by Ludwig(2003) given in Table 5 and concordia diagrams in Fig. 7. Many of thezircon data were discordant due to complex lead-loss histories andmost datasets had to be culled as documented in Fig. 7 and Table 5.Concordia calculations were preferred where feasible (for fivesamples), because they do not involve any of the assumptions madewhen proposing lead loss models. Discordia regressions generally

Table 4Catanormmineral compositions and rock classifications. Catanormswere calculated from theTAS diagram and Barker (1979) catanorm AnAbOr classification diagram are given in supp

Dwyka cobbles TAS Class

Q C Or Plag Ab An Hy Hm Il Tn Ru Ap Cox et al.(1979)

DC0505 32.6 1.4 28.9 36.3 35.2 1.1 0.5 0.0 0.0 0.0 0.1 0.3 GraniteDC0506 35.2 1.8 32.0 29.7 29.2 0.5 0.8 0.0 0.0 0.0 0.1 0.4 GraniteDC0706 27.5 0.6 30.1 37.6 28.4 9.3 3.2 0.0 0.1 0.0 0.5 0.4 Granite

DC0710 15.4 3.1 31.6 42.9 38.1 4.7 5.3 0.0 0.2 0.0 0.8 0.9 Syenodio

DC0712 32.6 0.0 35.2 31.2 28.1 3.2 0.6 0.0 0.1 0.2 0.1 0.1 Granite

DC0716 35.3 4.4 11.4 38.5 32.2 6.4 10.0 0.0 0.0 0.0 0.3 0.0 Quartz diDC0722 31.0 0.2 36.2 31.6 26.9 4.7 0.6 0.0 0.1 0.0 0.2 0.2 Granite

DC0723 37.1 1.2 5.6 54.7 40.8 13.9 1.3 0.0 0.0 0.0 0.1 0.0 Granite

DC0724 29.3 1.1 20.9 45.9 39.0 6.9 1.5 1.0 0.0 0.2 0.0 0.0 GraniteDC0727 31.1 1.9 13.3 51.4 43.3 8.2 1.7 0.0 0.1 0.0 0.2 0.4 Granite

DC0731 30.1 1.0 31.9 35.1 28.9 6.2 1.1 0.0 0.1 0.0 0.3 0.5 GraniteDC0740 27.8 1.8 20.7 47.0 43.0 4.0 1.3 0.9 0.0 0.3 0.0 0.1 GraniteDC0743 24.7 1.8 18.4 51.6 45.1 6.5 2.5 0.0 0.1 0.0 0.2 0.7 GraniteDC0745 14.9 0.0 18.5 56.9 40.8 16.1 4.8 0.0 0.4 1.3 1.7 1.5 Syenodio

DC0747 34.7 2.5 15.6 45.0 37.0 8.0 1.9 0.0 0.0 0.0 0.2 0.0 GraniteDC07101 24.8 3.1 10.2 55.6 46.7 8.9 4.9 0.0 0.1 0.0 0.5 0.8 Quartz di

Western Kaapvaal Granites from outcropDC0912 30.0 0.9 24.7 42.6 38.9 3.6 0.7 0.8 0.0 0.0 0.1 0.3 Granite

DC0914 28.1 1.0 30.3 38.3 34.2 4.1 0.9 1.0 0.0 0.0 0.1 0.3 GraniteDC0915 32.3 0.9 25.2 40.3 36.8 3.5 0.5 0.6 0.0 0.0 0.1 0.1 Granite

DC0916 30.6 1.2 26.5 39.4 35.5 3.9 0.8 1.0 0.0 0.0 0.1 0.3 Granite

DC0917 28.6 1.1 19.9 47.4 41.5 5.9 1.3 1.2 0.0 0.0 0.2 0.3 Granite

have large lower intercept errors and only about half of the samplesconform to the recent lead loss model within error. The lowerintercepts for seven of the cobble samples overlap the 302 Ma age ofthe Dwyka Group (Bangert et al., 2000), reflecting ancient lead lossduring or after the glaciation and in such cases weighted mean Pb–Pbages on discordant samples are only minimum ages. Each sample thushas its own Pb-loss history due to different responses to glaciation,diagenetic and weathering events. Felsic biotite schist cobble sampleDC0716 was interpreted as a metasediment because of its modalbanding, with alternating felsic and biotite-rich bands. Half of thedata conformed to a 3079±15 Ma discordia line which is inter-preted as the age of the main provenance and all but three of theother grains were older, up to 3320 Ma, reflecting a mixedArchaean provenance. Some cobble samples like DC0723 are highlydiscordant but can nevertheless be interpreted: a group of datapoints with consistent Pb–Pb ages scatters about a discordia linesuggesting a lead-loss model with intercepts at 2499±24 and 77±38 Ma. The lower intercept is not far from Recent and is probablyrelated to weathering and groundwater movement in the near-surface environment. The two oldest points give a weighted meanminimum Pb–Pb age of 2484±9 Ma, within error of the discordiaupper intercept age. All the other points plot between the discordialine and concordia, consistent with ancient lead loss at differenttimes varying from 800 to 300 Ma. Thus we conclude that the upperintercept age probably reflects the Archaean magmatic origin of thesample.

The west Kaapvaal granite outcrop samples generally gave gooddiscordia regressions with Archaean upper intercept ages (Fig. 7t–v,

whole rock analyses in Table A1, using the GCDkit package of Janoušek et al. (2006). Thelementary A2.2 and A2.3.

Catanormclass

Classification according to rare earth element profile (Fig. 6)

Barker (1979)

Granite No dataGranite No dataQuartzMonzonite

Granite with light REE enrichment but flat heavy REE profileand -ve Eu anomaly

rite Granite Granite with consistent light vs heavy REE enrichment andminor+ve Eu anomaly

Granite Granite with light REE enrichment but flat heavy REE profileand -ve Eu anomaly

orite Trondhjemite Granitic and Trondhjemitic (metasediment).Granite Granite with consistent light vs heavy REE enrichment and

-ve Eu anomalyTrondhjemite Trondhjemite with low heavy REE and+ve Eu and -ve Ce

anomaliesGranite No dataTrondhjemite Granite with consistent light vs heavy REE enrichment and

small -ve Eu anomalyGranite No dataGranite No dataTrondhjemite No data

rite Granodiorite Granite with consistent light vs heavy REE enrichment butsmall positive Ce anomaly

Trondhjemite Trondhjemite with low heavy REE and small -ve Eu anomalyorite Trondhjemite Trondhjemite with low heavy REE and minor -ve Eu anomaly

Granite Trondhjemite with low, curved heavy REE profile and small-ve Eu anomaly

Granite Trondhjemite with low heavy REE and small -ve Eu anomalyGranite Granite with profile similar to trondhjemite but four times

higher concentrationsGranite Granite with profile similar to trondhjemite but four times

higher concentrationsGranite Trondhjemite with low heavy REE and small -ve Eu anomaly

1

10

100

1000

Sa

mp

le/C

ho

nd

rite

DC0722 DC0710

DC0727 DC0745

Granite cobbles with regular

light-REE enrichment

1

10

100

1000

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Sam

ple/

Cho

ndrit

eDC0723

DC0747

DC07101

DC0716

Trondhjemite cobbles

1

10

100

1000

Sa

mp

le/C

ho

nd

rite

DC0712 DC0706

Granite cobbles with flatheavy-REE profiles

1

10

100

1000

Sa

mp

le/C

ho

nd

rite

DC0912 DC0914DC0914 DC0915DC0916 DC0917

West Kaapvaal outcrop

trondhjemites & granites

a b

c d

Fig. 6. Rare earth element profiles for granitoids cobbles and west Kaapvaal granite samples.


Table 5). Sample DC0912 from Koppiespan gives 3061±9 Ma, with alower intercept of 1132±220 Ma. Similar lower intercept ages werealso found in granites of the region by Mapeo et al. (2004) and

Reimold et al. (2002) and probably reflect the Namaqua tectogenesis.The 3061±9 Ma age is the oldest reported for the early trondhjemiticbasement of the Kraaipan Greenstone belt, other ages reviewed by

image of Fig.�6

Table 5Calculated results for U–Pb zircon dating of granitoid cobbles from the Dwyka diamictite on the western border of the Kalahari Basin and granite outcrop samples from the westernKaapvaal Craton. Dating was by ion probe except for three samples marked # which are by laser ablation ICPMS.

Sample Rock type Age of originMa

Error2σ Ma

Lower interceptMa

Error2σ Ma

Probability(MSWD)

No.pointsused

No. pts.excluded

Reason for exclusion and comments

1 DC0505 Granite 2047 14 Concordia 0.48 (0.51) 3 2 N3.5% discordant, concordia agrees with discordiaand wt. mean Pb–Pb age

2 DC0506 Granite 2017 15 Concordia 0.24 (1.4) 3 2 N4% discordant concordia agrees with discordiaand wt. mean Pb–Pb age

3 DC0706 Granite 2797 6 282 73 0.68 (0.57) 6 3 22a, 8a (give too old lower intercept for Dwyka),32b2 (excluded duplicate)

4 DC0710 Biot. Gte. Gneissxenoxts.

2687 16 1 7 Minimum Pb–Pb age of oldest xenocryst domain,complex Pb-loss

5 DC0710 Biot. Gte. Gneissmain

2525 20 1 5 Minimum Pb–Pb age of oldest spot in maindomain, complex Pb-loss

6 DC0710 Biot. Gte. Gneissrims

1395 34 5 6 0.93 (0.22) 6 6 Scatter reflects complex Pb loss history, some lowTh/U reflect metamorphism

7 DC0712 Rapakivi granite 2778 10 54 240 0.21 (1.4) 8 4 High initial lead, discordia agrees with wt. meanPb–Pb age

8 DC0716# Biotite schistmetased.

3079 15 238 54 0 (5.5) 12 11 Main detrital population in metasediment, oldergrains up to 3320 Ma

9 DC0722 Granite 2061 14 249 93 0.18 (1.4) 12 2 Main domain, scatter on 2 of 3 spots in grain 1810 DC0723 Foliated granite 2499 24 77 38 0.002 (3.7) 7 9 Selection of scattered data, agrees with Pb–Pb age

of 2 oldest spots11 DC0724 Biotite granite

gneiss2623 16 concordia 0.05 (4.0) 3 none Concordia age agrees with all-spot discordia

(poor fit) and selected (good fit)12 DC0727# Migmatite gneiss 2621 13 190 9 0.85 (0.44) 8 17 Complex Pb loss, selected discordia agrees with

least-discordant Pb–Pb age13 DC0731 Migmatite

granite gneiss2013 10 112 34 0.53 (0.8) 7 6 Scatter in main domain, agrees with wt. mean

near-concordant Pb–Pb age14 DC0740 Granite 2889 11 Concordia 0.13 (2.3) 5 none Spots b3% discordant, agrees with discordia

intercept15 DC0743 Biotite granite 2877 11 Concordia 0.15 (2.1) 1 none Discordia plot has excess scatter but agrees:

2891±42 Ma16 DC0745 Granite 2879 11 27 39 0.2 (1.5) 6 3 All data gives excess scatter, but same age:

2886±1617 DC0747# Quartz porphyry 2661 24 397 100 0.33 (1.1) 9 1 One spot is 400 Ma younger, xenocryst concordia

age is 2753±19 Ma18 DC07101 Rapakivi granite 2912 13 210 28 0.02 (3.2) 5 2 Main domain, one spot has high initial Pb19 DC07101 Rapakivi granite 2912 26 397 110 0.77 (0.1) 3 1 Rims, 1 has high common Pb, upper intercept

agrees with main domain

Western Kaapvaal Granites from outcrop20 DC0912 Koppiespan

Granite3061 9 1132 220 0.52 (0.88) 9 6 Four discordant points high common Pb, 1

reverse disc., one 140 Ma younger21 DC0914 Motition Granite 2867 7 353 100 0.54 (0.84) 8 0 Concordia age on 4 points agrees: 2868±7 Ma22 DC0915 Ditshiping

Granite2882 7 276 88 0.37 (1.0) 4 0 Concordia age on one point agrees: 2887±9 Ma

23 DC0916 Ditshipingyounger Granite

2854 7 -1 110 0.23 (1.3) 9 0 This intrudes the granite sampled as DC0915 andis 30 Ma younger

24 DC0917 KgokgoleGranite

2857 6 39 190 0.27 (1.3) 8 2 Point 17b is 30 Ma younger, 9b has high commonPb


Mapeo et al. (2004) ranging from 3008±4 to 2940±4 Ma andascribed by them to the accretion of the Kimberley and other Kaapvaalterranes. The hydrothermally altered granite dated by Kamo et al.(1995) 2928±4 Ma at Mabuasehube in central Botswana (Fig. 1a)also belongs to the Kimberley Terrane and probably also reflects thisaccretion event.

The four samples from the Ganyesa dome gave upper interceptages from 2882±4 to 2854 ±7 Ma. The Ditshiping younger granitedated at 2854±7 Ma intrudes an older 2882±7 Ma granite, both ofwhich are interpreted as granites evolved from trondhjemites byfractionation. The 2878±4 Ma granite age from the Kelso borehole~50 km to the west (Fig. 1a) by Reimold et al. (2002) confirms thatthis 2.88–2.85 Ga magmatic event was important event in theKimberley Terrane, about 100 Ma older than the 2780 Ma intrusion

Fig. 7. Concordia diagrams for granitic cobbles from the Dwyka diamictites west of the Kalahand age calculation results are all shown with 2σ errors disregarding decay constant errors

of the Gaborone Granite in the Pietersburg Terrane and the Kraaipanred granite dated by Mapeo et al. (2004).

The lower intercepts of two samples support the recent lead lossmodel, whereas two samples from Motition and Ditshiping reflectancient lead loss events probably related to Karoo basin developmentaround 300 Ma.

Fig. 8 shows the results for the age of zircons and their cobble hostrocks as a histogram. Eleven of the cobbles have Archaean ages between2500 and 2900 Ma and a few xenocrysts and detrital grains are asold as 3.3 Ga. The other four are Palaeoproterozoic with ages between2061±14 and 2013±10Ma. Three of the Palaeoproterozoic samplesare from Karnaldam and one from Border. One gneiss sample hadyounger zircon rims dated at 1395±34Ma. Otherwise none of thecobbles had Mesoproterozoic or Neoproterozoic ages.

ari and outcrop granite samples from the western Kaapvaal Craton. Discordia regression, with MSWD shown where probability is less than 0.15.

2200

1800

1400

1000

600

24

10

9

8

29

0.0

0.1

0.2

0.3

0.4

0.5

0.0

0.1

0.2

0.3

0.4

0.50 2 4 6 8 10

0 2 4 6 8 10

207Pb/235U

All 5 pointsintercepts 2041 ± 17 & 197 ± 470 Maprobability 0.94

a DC0505 granite

Concordia Age 3 spots: 2047 ±14 Maprobability 0.48

Wt. Mean Pb-Pb all 52044 ± 13 probability 0.90

3000

2600

2200

1800

1400

1000

32b32a

22b

8c

1c1a

32b2

22a

8a

0.0

0.2

0.4

0.6

207Pb/235U

DC0706 granite

6 points (ellipses), excluding 3 (circles)intercepts 2797 ± 6 & 282 ± 73 Maprobability 0.68

600

1000

1400

2200

2600

0.0

0.1

0.2

0.3

0.4

0.5

0 2 4 6 8 10 12 14

207Pb/235U

xenocryst spots: crossed ellipses, minimum age of oldest: 2687 Ma

main spots: open ellipses minimum age of oldest: 2525 Ma

d DC0710 biotite granite gneissrim points (see figure e)intercepts 1395 ± 34 & 4.7 ± 6.3 Ma

complex

Pb-loss

1300

1100

900

700

500

300

1005b

36b34b

36b1

4b236b2

48b

16b

40b1

4b3

40b2

40b3

0.00

0.04

0.08

0.12

0.16

0.20

0.24

0.0 0.4 0.8 1.2 1.6 2.0 2.4 2.8

207Pb/235U

6 rim points (open ellipses)excl 6 (filled diamonds) intercepts 1395 ± 34 & 4.7 ± 6.3 Ma probability 0.93

DC0710 biot. gte. gneiss rim points

200

600

1000

1400

1800

2200

3a

9a1

9a218a2 25a1

25a2

25a3

51a

52a152a2

52a3

excluded 18a1&3

0.0

0.1

0.2

0.3

0.4

0 2 4 6 8

207Pb/235U

12 of 14 main points (ellipses)excluding two (circles)intercepts 2061 ± 14 & 249 ± 93 Maprobability 0.18

h DC0722 granite2000

50c xenocryst

b2 rim

b1 rim

3000

2600

2200

1800

1400

1000

36b 36a236a32a

31a

6a5a

1a

27c

27a

19c

19a

0.0

0.2

0.4

0.6

0 4 8 12 16 20 24

0 4 8 12 16 20 24

207Pb/235U

3 points with high common Pb

DC0712 rapakivi granite

wt. mean Pb-Pb 8 points 2776 ± 3 probability 0.28

8 points (ellipses) excluding four (circles)intercepts 2778 ± 10 & 54 ± 240 Ma probability 0.21

3400

3000

2600

2200

1800

1400

1000

81

80

79

78

70

69

6564

59

48

44

43

82b76b

76a

74

75b

75a

72

62 60

58

0.0

0.2

0.4

0.6

0.8

0 4 8 12 16 20 24 28

207Pb/235U

12 points in main detrital grain population (ellipses)excluding 11 points (filled squares, 52 reverse discordant, not shown)intercepts 3079 ± 15 & 238 ± 54

DC0716 felsic biotite schist, metasediment

older grains up to 3320 Ma

ancient Pb-loss, thus discordant Pb-Pb ages are invalid

2200

1800

1400

1000

600

17

15

13

9

14

207Pb/235U

b DC0506 granite

All 5 points, intercepts 2029 ± 34 Ma& 57 ± 170 Ma, MSWD 5.9, probabilty 0.001

Concordia Age 3 points 2017 ±15 Ma probability 0.24

Wt. mean Pb-Pb all 5 points2026 ± 16 Ma, probabilty 0.20

206 P

b/23

8 U20

6 Pb

/238 U

206 P

b/23

8 U20

6 Pb

/238 U

206 P

b/23

8 U20

6 Pb

/238 U

206 P

b/23

8 U20

6 Pb

/238 U

c

e

f

g



5.3. Oxygen isotope analyses

Oxygen isotope analyses of magmatic zircons have been shown tofaithfully reflect the isotopic composition of the magmas from whichthey crystallised and they are much less prone to post-magmaticalteration than most other minerals (Valley, 2003). The zircon oxygenisotope data for zircons from granitic samples from Table 3 is shown inFig. 9, plotted against the U–Pb age of each sample. For two sampleswhich show a large spread of data, the results for probable maingroups are shown without error ellipses. The samples plot in threegroups. Three of the four ~2 Ga cobbles cluster with δ18O values from6.7 to 7.4‰ and one is heterogeneous. The five outcrop samples fromthe western Kaapvaal Craton Kimberley Terrane Archaean granitoidsshow a larger range from 5 to 6.8‰. The seven Archaean cobbles withhomogeneous values vary from 2.8 to 5.1‰, three of them remarkablylower than the small range ofmantle zircon (5.3±0.3‰, Cavosiea et al.,2005). Sample DC0716, interpreted as a metasediment, has groupsabove and below the mantle field.

6. Discussion

6.1. Transport distance from the ice-front

One question which arises is whether the cobbles were depositedessentially at the ice-front, or had been transported some distance inicebergs before deposition. We argue that the presence of fluvoglacialsandstone and conglomerate beds in the diamictite at Border andKarnaldam shows that deposition at those localities was close to theice-front. These coarsely cross-bedded layers do not show muchcontinuity and can only have deposited from glacial outflow close tothe ice front. All three granite samples from Karnaldam beingPalaeoproterozoic with a small spread of ages from 2061 to 2017 Ma(Table 1) might suggest a change of provenance up the stratigraphy,however one cobble with similar age of 2013 Ma was also taken fromBorder, suggesting that the provenance did not change much.

6.2. The mafic cobbles

The mafic cobbles were investigated geochemically and dated at1116.4±8.5 Ma in the companion paper by Van Schijndel et al.(submitted for publication). This overlaps with the 1099±9Ma to1112±0.5 MaUmkondo Large Igneous Province, awidespreadmagmaticevent represented by dykes and mafic intrusions on the Kalahari Craton,reviewedbyHanson et al. (2004). The cobbles also overlap in agewith the1090±12Ma upper Koras Group bimodal volcanic rocks and intrusionsin theKaaienTerraneof theNamaqua–Natal Province, datedbyPetterssonet al. (2007). Several mafic intrusions have been identified geophysically(Fig. 2), along the Kalahari Line, including the 1105±11Ma TshaneIntrusion and the 1109±1.3 Ma Xade Complex (reviewed by Hanson etal., 2006), The Koras intrusions exposed along the southern extension ofthe Kalahari Line around Upington include the undated Betadam Gabbroand the 1093±10Ma Rooiputs Granophyre and 1093±11Ma Blauw-bosch Granite (Pettersson et al., 2007). Thus the mafic cobbles mostprobably come from mafic intrusions along the Kalahari Line which arebroadly coevalwith theUmkondo Large Igneous Province (Figs. 1a and8).Surprisingly, the Dwyka cobbles we sampled did not include anygranitoids of Koras age. This allows us to speculate that the maficintrusions sampled by the Dwyka ice sheet intrude are not accompaniedby coeval granites and possibly intrude Archaean granitoids.

6.3. Source of the granitoid cobbles

Assuming transport broadly from the east, the cobbles could havecome from (1) the Kaapvaal Craton, (2) the Kheis Province, (3) theKalahari Line or (4) the Rehoboth Province. These possibilities will beconsidered, after discussion of the oxygen isotope data.

6.3.1. Oxygen isotope dataThe zircon oxygen isotope data was culled as explained in

Section 4.1, to produce a single value for the magmatic domain ofeach sample. The values for three Archaean granitic cobble samplesand one zircon group in a cobble interpreted as a metasediment areremarkably lower than mantle values, unique for Archaean zirconaccording to Valley et al. (2005). Do the sub-mantle values reflectalteration of zircons within the Dwyka Formation? The consistent andmuch higher values shown by three of the four ~2.0 Ga cobbles, whichhave been through the same diagenetic and weathering history in theDwyka Formation, make selective alteration of zircon in the Archaeancobbles seem unlikely. We conclude that although the ArchaeanWestern Kaapvaal outcrop and Dwyka cobble sample groups overlapslightly in age and oxygen isotope data, they cannot be considered asgenetically equivalent.

Oxygen isotope values for magmatic zircon are considered toclosely mimic those of the magma and its source (i.e. protolith),because at magmatic temperatures little fractionation can occur. Thewest Kaapvaal granite outcrop samples can be explained as havingformed by melting of unaltered protoliths derived from the mantlesuch as komatiites, or by melting of protoliths containing somesedimentary material, which usually have higher oxygen isotopevalues. The three Dwyka cobble samples with low values which do notoverlap the mantle range require a more exotic explanation. Onepossibility is that before melting to produce the granites, theirprotoliths had been altered by interaction with low-δ18O originallymeteoric water (possible composition −8 to −10 δ18O) at hightemperature (N300 °C to limit rock-water fractionation to about+10‰, with possible further fractionation of +2 during melting,Bottinga and Javoy, 1975). This is the scenario favoured for theSkye granites which also have anomalously low zircon oxygenisotope values, as discussed by Valley (2003). Samples DC0706 andDC0712 are granites which show flat heavy REE profiles andDC0727 has a profile which flattens out in the heavy REE part,features which may reflect hydrothermal alteration (e.g. Schade andCornell, 1989).

The most extreme case known is in the Dabie–Sulu orogenic belt ofChina, where Tang et al. (2008) reported δ18O values in zircons as low as−10‰. They interpreted this in terms of crystallisation from magmasformed by the melting of previously meteoric-hydrothermally alterednegative δ18O rocks in an active rift setting at about 780 Ma. Such aprocess should have affected the source region of at least three of ourgranitic cobble samples before the granitic magmas actually formed,that is before 2.8 Ga. A speculative setting for such a process is anArchaean subduction or terrane suture zone, in which low-δ18O waterswere rapidly transported down into an environment where high-temperature alteration could occur.

6.3.2. Could the granitoid cobbles come from the Kaapvaal Craton?The Archaean cobbles could have been transported more than

450 km from the granitic basement of the Kaapvaal Craton to the eastof the Kheis Province, assuming that the Ganyese Dome granites didnot crop out before the meteorite impact at 145 Ma (Fig. 2). However,in that case, cobbles derived from the Kaapvaal cover sequence,particularly the distinctive banded iron formations, jaspers anddolomites of the Griqualand West (greater Transvaal) Supergroupshould also be abundant in the diamictites sampled aroundRietfontein, which they are not, as shown in Table 2.

At Bogogobo (near Middelputs, Fig. 2) close to the Kalahari Line,the pebble counts are dominated by chert, jasper and banded ironformation (BIF, 65%), with 16%mafic lava, no granite and only one redsandstone (pebble counts by E. Jansson, 2010). This pebble assem-blage represents mainly the Griquatown BIFs and Ongeluk lavas of theupper Griqualand West Supergroup in the Kaapvaal cover sequence,which presently crop out ~100 km to the east (Fig. 2). Limestones anddolomites, now exposed on the circumference of the Ganyese Dome

1000

1400

1800

2200

2600

3000

16b9b

35a1

69

50

6

95b1

93

35b2

35a2

51b86b

95a95b276b9a 86a

51a74b331b2

31a116a74b2

74b1

23b207Pb/235U

8 points (ellipse & +)excluding 17 (circles)intercepts 2621 ± 13 Ma& 190 ± 9 Ma, probability 0.85

gte. gneisspossible migmatization

2350 Ma?

wt. mean Pb-Pb age of3 least discordant2602 ± 30 probability 0.99

complex

Pb-loss

++

1000

1400

1800

2200

2600

3000

25a29a

31a2

31a3

31a4

31a5

1a

11a

20a

31a

31a6

207Pb/235U

granite gneissConcordia Age 3 points2623 ±16 Maprobability 0.05

6 points (ellipses) excluding five (circles)intercepts: 2613 ± 13 & 580 ± 49 Maprobability 0.15all points give 2611 ± 31, poor fit

3000

2600

2200

1800

1400

100083a

82a

81a4

81a281a1

207Pb/235U

granite

81a3 Concordiaage 2877 ±11 Maprobability 0.15

All 6 points, intercepts 2891 ± 42 & 171 ± 180 MaMSWD 10.2, Probability 0.0omitting points does not improve fit

81a3

3000

2600

2200

1800

1400

1000

2a5

2a4

2a3

2a1

1a5

1a2

1a4

1a3

1a1

207Pb/235U

6 points (ellipses)excluding three (circles)intercepts 2879 ± 11 Ma& 27 ± 39 Ma, probability 0.20

all 9 points intercepts 2878 ± 42 & 146 ± 230 MaMSWD 14, probability 0.0

1000

1400

1800

2200

2600

3000

7a

7b1

7b2

7b3

12a

17a40a

1a1

1a2

10b1

17b

18b

34b1&2

36a

38a243a

207Pb/235U

206 P

b/23

8 U

foliated granite

7 points (ellipses) excluding 10 (circles) intercepts 2499 ± 24 & 77 ±38 Ma MSWD 3.7, probability 0.002

wt. mean Pb-Pb age of 2 oldest spots(valid in view of recent Pb-loss)7b1 & 7b3: 2484 ± 9 Ma probability 0.97

complex

Pb-loss

3000

2600

2200

1800

1400

1000

20a2

19a

18a

15a1- 4

4c

4b

207Pb/235U

206 P

b/23

8 U

8 points excluding one (circle)intercepts 2888 ± 10 & 75 ± 72 Maprobability 0.14

Concordia Age on 5 spots <3% discordant 2889 ±11 Ma probability 0.13

200

600

1000

1400

1800

220024a

24a2

24a3

24a4

36a

43a1

12a

34a

36a2

40a

48a1

48a2

207Pb/235U

206 P

b/23

8 U

11c metami ct core included

7 main points (ellipses)excluding 6 main (circles), and 4 rims (diamonds)intercepts 2013 ± 10 & 112 ± 34 Maprobability 0.53

granite gneiss3000

2600

2200

1800

1400

1000

121a2

121a1100

80a

71b

40b2

40a

38

1a

207Pb/235U

206 P

b/23

8 U

83b

34a xenocryst

9 main points excluding 83b (circle) intercepts 2661 ± 24 & 397 ± 100 Maprobability 0.33

12a xenocrystConcordia Age 2753 ±19 MaMSWD 4.5probability 0.0312b

k DC0727 migmatite

j DC0724 biotite n DC0743 biotite

o DC0745 granite

i DC0723 m DC0740 granite

l DC0731 migmatitic p DC0747 quartz porphyry

0 4 8 12 16 20 0 4 8 12 16 20

0 4 8 12 16 20 0 4 8 12 16 20

0 4 8 12 16 20 0 4 8 12 16 20

0 4 8 12 16 20

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.0

0.1

0.2

0.3

0.4

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.0

0.2

0.4

0.6

0 2 4 6 8

206 P

b/23

8 U20

6 Pb

/238 U

206 P

b/23

8 U20

6 Pb

/238 U

wt. mean Pb-Pb age3 spots 2014 ± 15 probability 0.67

Fig. 7 (continued).


3000

2600

2200

1800

1400

1000

207Pb/235U

all 8 points, intercepts 2867 ± 7 Ma& 353 ± 100 Ma probability 0.54

s DC0914 Motition W. Kaapvaal Granite

6 points <2% discordant Concordia Age 2865 ±6 MaProbability 0.25

3000

2600

2200

1800

1400

100061a

50a

37a2

35a2

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0 4 8 12 16 20 0 4 8 12 16 20

0 4 8 12 16 20

207Pb/235U

37a1

35a1

45b

22c xenoxt.

q DC07101 rapakivi granite

9b

22b

25b

4 main points (ellipses) excl. 2 (circles)intercepts 2910 ± 16 & 197 ± 17 Maprobability 0.71

3 rim points (diamonds) excl. 1 (filled)intercepts 2912 ± 26 & 397 ± 110 Maprobability 0.77

3000

2600

2200

1800

1400

1000

0.0

0.2

0.4

0.6

207Pb/235U

206 P

b/23

8 U

206 P

b/23

8 U

all 4 points, intercepts 2882 ± 7 Ma & 276 ± 88 Ma probability 0.3 7

t DC0915 Di tshiping older W. Kaapvaal Granite

Concordia age 18b 2887 ± 9 Maprobability 0.90

1400

1800

2200

2600

3000

0.0

0.2

0.4

0.6

0.0

0.2

0.4

0.6

0 4 8 12 16 20 24

207Pb/235U

9 points (ellipses)intercepts: 3061 ± 9 & 1132 ± 220 MaProbability 0.52

40 31

48

45c

r DC0912 Koppiespan W. Kaapvaal Gte. 45

28

6 excluded (diamonds)4 discordant points with high common Pb

2300

2500

2700

290012a

6a

1a10a

10c

0.25

0.35

0.45

0.55

0.65

0.25

0.35

0.45

0.55

0.65

7 9 11 13 15 17

7 9 11 13 15 17

207Pb/235U

All 9 points, intercepts at 2854 ± 7 & -1 ± 110 Ma

Probability 0.23

Wt. mean Pb-Pb all 9 points: 2854 ± 7 Ma

Probability 0.32

Concordia Age on 2 points, 1a & 10a:

2865 ±11 MaProbability 0.26

u DC0916 Ditshiping younger W. Kaapvaal Granite

2900

2700

2500

2300

207Pb/235U

8 points (ellipses) excluding 2 (diamonds)intercepts: 2857 ± 6 Ma& 39 ± 190 MaProbability 0.27

9b high common Pb

17b

v DC0917 Kgokgole W. Kaapvaal Granite

206 P

b/23

8 U

206 P

b/23

8 U20

6 Pb

/238 U

206 P

b/23

8 U

Fig. 7 (continued).


150 km to the east (Fig. 2) are not found in the pebble assemblageconfirming the suggestion that they were not exposed before theMorokweng meteorite impact 145 Ma ago. The nearest ArchaeanKaapvaal granites likely to have cropped out during Dwyka glaciationlie 300 km east of Bogogobo, and did not make it to the Bogogobooutcrops we investigated. By contrast, at Rietfontein 280 km west ofthe craton, the pebbles are predominantly red and green sandstones(37%), with 25% gabbro and basalt and 13% granite, with no BIF ordolomite (which are known to survive long glacial transport, seeSection 1.1 and Visser, 1986). No banded iron formation, jasper ordolomite clasts were found in-situ in the Dwyka outcrops nearRietfontein and only two iron formation and no jasper or dolomiteclasts were found amongst thousands of cobbles surveyed lying on theadjoining ablation lag surface, accumulated as the diamictite matrixwas eroded.

The abundant Nama sandstone pebbles and cobbles in thediamictites at Rietfontein were clearly derived locally and it thusseems likely that the granite and mafic cobbles had not beentransported from as far as the craton. The oxygen isotope and agedata shown in Figs. 8 and 9 also show that the granite cobbles did notcome from granites like those we sampled in the western KaapvaalCraton.

All eight mafic cobbles from Rietfontein Dwyka investigated byVan Schijndel et al. (submitted for publication) are geochemicallysimilar and thus probably belong to a single magmatic suite about1116 Ma old. If the cobbles were derived from the Kaapvaal Craton wewould expect to find mafic cobbles derived from the ~2.7 GaVentersdorp Supergroup, the 2222 Ma Ongeluk Formation or the1920 Ma old Hartley Basalt (Cornell et al., 1996, 1998) which crop outin the western Kaapvaal Craton. We found none.

0

1

2

3

4

0 500 1000 1500 2000 2500 3000 3500 4000

Nu

mb

er o

f ag

es

DC0710rims

Bushveld Complex

Archaean granitoid cobbles

DC0710 xenocryts

Pan- Africanorogenies

Namaqua-Natalorogeny

Kheisorogeny?

U-Pb Age Ma

Umkondo LIP &MAFIC

COBBLES

OldestRehoboth

basement

Kaap- vaal Craton

Zimbabwe Craton

Witwatersrand terrane

W. Kimberley terr. this work

Dwyka glaciation DC0716

detritalgrains

Kimberley terrane

Fig. 8. Histogram showing the distribution of microbeam U–Pb ages for Dwyka granite cobbles and the known ages of some regionally important geological features.


Another aspect is that thewestern Kaapvaal craton granites dated inthe literature range in age from 3.20 down to 2.70 Ga, (Schmitz et al.,2004) and the youngest Archaean intrusion events in the KimberleyTerrane are trondhjemites at 2.88–2.85 Ga and granites at 2.78 Ga,whereas four of the thirteen Archaean granitoid cobbles we dated areyounger than 2.7 Ga. After 2.7 Ga Ventersdorp volcanic rocks coveredmost of the Kaapvaal Craton and in the interval between 2.67 and 2.50,during which time four of the dated Dwyka granitoid cobbles wereformed, Transvaal Sequence dolomites and banded iron formationswere being deposited in a quiet shelf environment on the entireKaapvaal Craton. These chemical sediments do contain tuffaceous

Fig. 9. Oxygen isotope data for zircons in granitic cobbles from the Dwyka Formation plottedKimberley Terrane shown for comparison. The error ellipses show 1σ uncertainties for oxygstatistically homogeneous for oxygen isotopes. Mantle-derived zircons plot within the narr

layers, reflecting minor magmatic activity, commonly assumed to bewest of the known craton, e.g. Altermann and Nelson (1998) and Knolland Beukes (2009) who identify volcanic episodes at 2.64, 2.59–2.50and 2.43 Ga.

The four Palaeoproterozoic cobbles we dated overlap in age withthe 2.05 Ga granites of the Bushveld Complex, and could possibly havecome from similar rocks associated with the 2044±24 Ma MolopoFarms Complex (Fig. 2), a sub-outcropping mafic intrusion regardedas a Bushveld extension, (Coetzee and Kruger, 1989) howeverassociated large granite plutons of the same age have not yet beendescribed. We conclude that the granite cobbles in the Dwyka at

against U–Pb age, with five outcrop granite samples from the Kaapvaal Craton westernen isotopes and 2σ uncertainties for ages. Samples shown without error bars were notow limits shown.


Rietfontein are very unlikely to have originated from as far east as theKaapvaal Craton. Bearing in mind that the cobbles are interpreted asbeing deposited from floating ice, we cannot completely exclude anorigin from further afield such as the Zimbabwe or Congo Cratons.However we think that this is unlikely in view of the large transportdistances required.

6.3.3. Did the cobbles originate from the Kheis Province?The N2.7 Ga granite cobbles might have originated from tectonic

slices of Kaapvaal basement in the Kheis Province thrust complex westof the craton, east of the Kalahari Line. It is generally accepted fromgeophysical evidence that the Kaapvaal basement extends to theKalahari Line and several workers include the Kheis Province in theKaapvaal Craton. However the Kheis Province is a Proterozoic thrustcomplex which cannot be a true part of the Archaean Kaapvaal Cratonbecause of the severe post-craton deformation. The seismic sectionshown asA–B in Fig. 2 described by Tinker et al. (2002) shows the entireKaapvaal supracrustal stratigraphy dipping westwards into the KheisProvince. Thus it seems likely that thrusts which exposed Kaapvaalbasement granites would also expose the Transvaal Group ironformations, jaspers and dolomites as well as Ongeluk and HartleyFormation mafic lavas. Metamorphic rocks should also be exposed inthrusts, for example mafic greenschists, because Rb–Srmuscovite datesaround 1170 reported by Moen and Armstrong (2008) for the KheisProvince quartzites reflect at least greenschist facies metamorphicconditions. None of these rock types were found in the cobbles fromRietfontein. Theonly evidenceof a Proterozoicmetamorphic event is the1395±34Madate for zircon rims in cobble sampleDC0710,whichdoesnot correspond to any known or envisaged event.

Thus we conclude that the granite cobbles probably did not comefrom as far east as the Kheis Province. A possible source for thePalaeoproterozoic cobbles is the 2056±2 Ma Okwa Inlier (Mapeo etal., 2006; Fig. 1a) which may lie in the northern extension of the KheisProvince, however this requires a transport distance of 550 km.

6.3.4. Did the cobbles originate in the Kalahari Line?The Kalahari Line does seem to reflect a major crustal discontinuity

or suture as postulated by Meixner and Peart (1984), because of theabrupt change in magnetic and outcrop fabric across it. South of theKalahari sand cover, the Kalahari Line is manifested as a number ofnorth-trending faults, between which undeformed 1173–1090 MaKoras Group lavas are exposed in preservational grabens, andthe extension of the Kalahari Line also represents a significantlithostratigraphic discontinuity in the supracrustal stratigraphy. TheBlaauwbospan Fault,which corresponds to the southernKalahari Line inFig. 2, is thewestern boundary of the thrust sheets of the Kheis Provinceaccording to Stowe (1986). The 1290 MaWilgenhoutsdrif Group formsthe uppermost sheet, thus its deformation and greenschist faciesmetamorphism cannot be older than 1.3 Ga and most likely reflectsthe 1.20 Ga collision of the Namaqua–Natal orogeny (Pettersson et al.,2007).

The extensive magnetic signature of the Kalahari Line seems to bepartly related to a zone of hydrothermally altered granites, assuggested by Corner (2008) and their alteration is probably relatedto the crustal suture. If the suture formed during the Archaean, thenthe hydrothermal alteration scenario envisaged to explain the cobbleoxygen isotope data could speculatively be related to the Archaeansubduction zone. Thus some of the Rietfontein Dwyka granitic cobblescould come from the Kalahari Line, although a broad area would seemmore likely than an essentially linear source.

The extent of mafic intrusions shown along the line in Fig. 2 afterMeixner and Peart (1984) is probably an exaggeration because ofthe lack of gravity anomalies associated with most magneticanomalies, according to Corner (2008). However, there are somewell-documented 1100 Ma mafic intrusions along the Kalahari Linesuch as the Tshane Complex, the hydrothermally altered Mabua

Diorite (Brett et al., 2000) and the Betadam gabbronorite which cropsout north of Upington. The Rietfontein Dwykamafic cobbles representmainly intrusive rocks with minor basalts, they are undeformed andare the same age as the known Kalahari Line mafic intrusions(Van Schijndel et al., submitted for publication). Thus they seem likelyto be derived frommafic intrusions either on the Kalahari Line or westof it in the Rehoboth Province. The granitic cobbles could well have asimilar source region.

6.3.5. Did the cobbles originate in the Rehoboth Province?Much of the Rehoboth Province crust in Namibia seems to have

originated in a prolonged subduction process between about 2.0 and1.7 Ga, according toBecker et al. (2005)andMiller (2008).However, ourmicrobeam U–Pb detrital zircon data for a b1.77 Ga Billstein Formationmetasediment in the Rehoboth Inlier (Van Schijndel et al., submitted forpublication), shows that Archaean components aged 2.75 and 2.90 Ga(concordant zircon) were present in the provenance, supporting ourspeculation based on the cobble data that the PalaeoproterozoicRehoboth crust (represented in that sample by 2.2 to 1.9 detritalzircons) was added to an Archaean core, not exposed at present. TheRehoboth Province was clearly a rigid cratonic block which must havebeen accreted to the Kalahari Craton before or during the NamaquaWilson cycle. Its western and northern boundaries were activecontinental margins which enjoyed subduction-related magmatism ofthe Sinclair Supergroup during the closure of Namaqua Wilson Cycleoceans (reviewed by Miller, 2008; Watters, 1976). The RehobothProvince then withstood collisions from the southwest along theNamaqua Front at about 1.2 Ga, developing a significant deformationfabric only along its margin.

Rietfontein lies in the Rehoboth Province. Below the Dwykadiamictites are the siltstones and sandstones of the Cambrian FishRiver Subgroup, Nama Group and the contact is exposed atSannaspoort (Fig. 3). The Nama siltstones probably thin westwards,exposing the Rehoboth Province basement beneath the Dwykadiamictites, which could well be the source of the granite cobbles.

Zircon xenocrysts in the range of 2.1 to 1.7 Ga found in the 1.17–1.09 Ga Koras bimodal intrusions and volcanic rocks near Upington(Pettersson et al., 2007), show that the south-western corner of theRehoboth Province has a Paleoproterozoic basement. The four 2061 to2013 Ma Dwyka granite cobbles also fit in this age bracket, thus theycould clearly come from Rehoboth Province basement west of theKalahari Line. Could the Archaean granite cobbles also come fromRehoboth Province basement? This seems more likely than theirorigin in the more distant Kaapvaal Craton or Kheis Province, asdiscussed above. The Archaean component in the Rehoboth Provincenow identified in the Rehoboth Inlier strengthens this idea. Howevermore borehole information about the Rehoboth Province basementeast of Rietfontein is needed to test the idea of an Archaeanhydrothermal event and explain the oxygen isotope data. The extreme'depth tomagnetic basement' that contours up to 15 km for the area inBotswana northeast of Rietfontein given by Reeves (1978) suggeststhat the sub-Kalahari basement in western Botswana is a thicksedimentary sequence, possibly equivalent to the Nama Group.However this probably thins southward. Thus the Archaean cobbleswe dated could definitely come from the zone of magnetite-enrichedgranites interpreted from magnetic data by Corner (2003), whichlargely follows the Kalahari Line, but swings westwards into theRehoboth Province at the Botswana border as shown in Fig. 2.

6.3.6. The sandstone cobblesThe red and green sandstone pebbles and cobbles which predom-

inate at Rietfontein probably comemainly from the directly underlyingCambrian Nama Group, reflecting short transport distances for most ofthe cobbles. Similar sandstones also occur in the ~1.8 Ga Brulpan (Moenand Armstrong, 2008) and 1.20 Ga Olifantshoek Groups further to the

KAAPVAAL

RICHTERSVELD

BUSHMANLAND -

GARIEP

C 1300 Ma

TUGELA OCEAN

UMZUMBE ARC

AREACHAPARCKAKAMAS BLOCK

HO

UT

S

REHOBOTH

SINCLAIRARC

DRIF

WIL

GE

N

KAAPVAAL

REHOBOTH

F 1170 Ma

KH

EIS

LOWERKORAS

BURIED TERRANES

NAMAQUA NATAL

KAAPVAAL REHOBOTH

RICHTERSVELD

BUSHMANLAND -

GARIEP

AREACHAP- KAKAMAS

UMZUMBE ARC

MARGATE ARC

D 1220 Ma

KH

EIS

B 2050 -1750 Ma

BUSHVELD MAGMATISMREHOBOTH

MAGMATISM

LIMPOPO METAMORPHISM

ZIMBABWE

OKWAGRANITES

WATERBERG BASINS

OL

IFAN

TSH

OE

K

BASIN

KAAPVAAL

REHOBOTH

BUSHMANLAND -

GARIEP

AREACHAP&KAKAMAS

TUGELA

UMZUMBE

MARGATE

E 1190 Ma

KH

EIS

RICHTERSVELD

ZIMBABWE

A 2500 Ma

REHOBOTHArchaean core?

LIMPOPO

KAAPVAAL CRATON

KIM

BE

RL

EY

TE

RR

AN

E

WITWATERSRAND

TERRANE

ZIMBABWE

PIETERSBURGTERRANE

SWAZILAND

TERRANE

BO

RG

AN

TAR

TIC

A

Fig. 10. Tectonic block diagrams A to F showing one possible sequence of events in the accretion of the Kheis, Rehoboth and Namaqua Provinces to the Kaapvaal Craton (thenattached to the Antarctic Borg Massif) during the Palaeoproterozoic and Mesoproterozoic eras.


west. These rock types might prove to be useful markers when theKalahari basement is drilled.

6.4. Tectonic relationship between Rehoboth Province and KaapvaalCraton

We envisage two alternative scenarios which fit the current dataregarding the Archaean to Palaeoproterozoic relationship between theRehoboth Province and Kaapvaal Craton. The first one is that theKaapvaal Craton and an Archaean core of the Rehoboth Provinces werejoined during the late Archaean, about 2500 Ma ago and had a commonhistory from that time. This scenario is shown in tectonic blockdiagramsFig. 10A andB, according towhich theKalahari Line represents

the Archaean suture (with hydrothermal alteration in a subductionzone) betweenWestern Kaapvaal and Rehoboth crust, the latter with along history of Archaean plutonism, starting with trondhjemites from3.3 Ga (oldest detrital grains in DC0716) through 2.9 Ga, then joined bygranites until 2.6 or 2.5 Ga (the least reliable ages). This crustal suturewas then reactivated four times. The regional thermal and magmatic(superplume-related?) events associated with the 2.05 Ga bimodalintrusions of the Bushveld Complex, thermal metamorphism of theLimpopo Belt and entire Kaapvaal Craton (Cornell, 1978) should alsohave given rise to crustal growth in the Rehoboth Province, noting thatBushveld magmatism extended westwards as far as the Molopo FarmsComplex and Okwa Inlier (Fig. 2). Olifantshoek Group redbeds werelater deposited in a basin developed along the Kalahari Line at about

image of Fig.�10


1.9 Ga, with the Rehoboth crustal surface at a lower elevation. At thesame time the Waterberg redbeds formed north of the BushveldComplex (Fig. 10B). Later, during the Namaqua–Natal Wilson Cycle, amajor ocean developed south of the assembled blocks (Fig. 10C). TheKalahari Line rifted to form a restricted Red Sea-like basin at about1300 Ma (Fig. 10C), inwhich theWilgenhoutsdrif Groupwas deposited.At the same time the Tugela and Areachap oceans began to close, withthe developmentof oceanic arcs to the south. TheWilgenhoutsdrif Basinclosed at 1200 Ma, forming the Kheis thrust complex, thrustingWilgenhoutsdrif, Brulpan and Olifantshoek metasediments onto theCraton. This was a response to the Namaqua collisions to the southduring which the arc terranes were accreted, sandwiched betweenthe Archaean Kaapvaal–Rehoboth block and the PalaeoproterozoicBushmanland, Richtersveld and Kakamas Terranes. (Fig. 10D and E, seeNamaqua–Natal review by Cornell et al., 2006). After assembly of theNamaqua–Natal terranes by 1190, pull-apart basins or rift valleysformed along the Kalahari Line in response to right-lateral movementsalong the Doornberg Lineament (Jacobs et al., 2008; Van Bever Donker,1991). In these small basins, the lower Koras volcanosedimentarysequencewasdeposited at 1170 Ma (Fig. 10F), and later theupperKorasvolcanic sequence and coevalUmkondomafic intrusions originating in asuperplume event from 1110 to 1090Ma (Fig. 11).

The second scenario is that the Rehoboth Province developed as astable crustal block with Archaean core far from the Kaapvaal Cratonuntil they joined during the Namaqua Wilson cycle. In this model(requiring modifications to Fig. 10A to C), the Areachap and Kakamasterranes were probably accreted onto the (present-day) south-westernmargin of the Rehoboth Province at an early stage of the 1200 Macollision. This was followed by an oblique collision of the RehobothProvincewith the Kaapvaal Cratonwith the suture corresponding to theKalahari Line, and joining the sequence of events described above fromFig. 10D. In thismodel the Kheis thrust sheets developed at ~1220Ma inthe sediments of the Kaapvaal Craton's passive continental margin. Wemarginally prefer the first scenario, because detrital zircons in theWilgenhoutsdrif Group correspond to a Kaapvaal Craton Provenance(Pettersson et al., 2007), suggesting that it formed in a restricted basinrather than a large ocean basin. These scenarios for tectonic evolutionare similar to, but differ in several details from that proposed by VanNiekerk (2006).

6.5. Further geological and economic implications

Theprobable occurrenceofArchaeangranitic crust alongandwest ofthe Kalahari Line implies that a large area of the Rehoboth Province

Fig. 11. Tectonic assembly of southern Africa during the formation of the UmkondoLarge Igneous Province at 1100 Ma. The Kalahari Line, an ancient zone of weakness, isreactivated due to transpressional strike-slip faulting in the Namaqua Front.

beneath the Kalahari sands could contain diamondiferous kimberlites. Itis also possible that Archaean basement extends into Namibia, wherehitherto only Proterozoic basement has been known. Muller et al.(2008) reviewed the diamond prospectivity of the Rehoboth Province(his Terrane) from several viewpoints. The mantle geotherm deter-mined from xenoliths at Gibeon is hotter than that of Kimberley on theKaapvaal Craton. Lower electrical resisitivities and shearwave velocitiesin the mantle beneath the Rehoboth and Kheis Provinces also suggesthotter, thinner mantle lithosphere than the Kaapvaal Craton, implyingpoor diamond prospectivity. However Appleyard et al. (2007) founda cool, Kaapvaal-like mantle geotherm in orthopyroxene eclogitexenoliths from the Rietfontein kimberlite, which they think originatedat 85–115 km depths in the graphite stability field. Xenoliths andxenocrysts from greater depths should thus be diamondiferous.Prospecting in the Rietfontein kimberlite did yield a small number ofdiamonds, although their legitimate origin is questioned by Gurney(1984).

Another aspect is the discovery of four granite cobbles close in ageto the 2050 Ma Bushveld Complex of the Kaapvaal Craton, whichcomprises both granite and mafic intrusions, and is coeval with theunexposed mafic Molopo Farms Complex (Fig. 2). This suggests thepossibility that the granite intrusions might be accompanied byBushveld-like layered mafic intrusions with similar mineral deposits(Cr, V, Ti, Platinum Group). Several other possible mineral depositscould be related to the Kalahari Line, according to Brett et al. (2010).These possibilities should be tested by drilling with geophysicalcontrol.

7. Conclusions

7.1 Dwyka diamictites which crop out around Rietfontein andKoopan Suid, west of the Kalahari Desert sands contain cobbleseroded and transported from the sub-Kalahari bedrock by thePermocarboniferous Dwyka ice sheet.

7.2 Geochemistry and zircon dating of sixteen granitoid cobblesreveal the presence of only two bedrock granite age groupsunder the Kalahari, an Archaean group comprising trondhje-mites and granites with ages 2500 to 3100 Ma and aPalaeoproterozoic granite group with ages between 2050 and2020 Ma.

7.3 The mafic cobbles (described in a companion paper by VanSchijndel et al., submitted for publication) are geochemicallycoherent and are dated at 1116.4±8.5 Ma. They probably allcome from intrusions related to the Umkondo Large IgneousProvince, either along the Kalahari Line or in the RehobothProvince west of the Line. The lack of Kaapvaal-derived maficcobbles supports a short transport distance for the graniticcobbles.

7.4 The granitoid cobbles contain no evidence to support previoustheories of crustal growth or orogeny in the region at either1800 or 1200 Ma.

7.5 The Archaean and Palaeoproterozoic granite cobbles areunlikely to have come from the Kaapvaal Craton or KheisProvince in view of the lack of characteristic Kaapvaalsupracrustal rock types in the cobble suite, the younger agesof some cobbles (2.7–2.5 Ga) compared to the known 3.0 to2.7 Ga age limits of western Kaapvaal basement and theanomalously low oxygen isotope ratios found in some of thecobbles, which are not matched by our data for the western-most Kaapvaal granite outcrops. Bearing in mind that thecobbles are interpreted as being deposited from floating ice, wecannot exclude an origin from further afield such as theZimbabwe or Congo Cratons. However we think that this isunlikely.

7.6 The granitic cobbles weremost likely derived from the RehobothProvince basement directly east of Rietfontein. Some of them


could also come from the Kalahari Line, which might be anArchaean crustal suture, thus providing an explanation for thelow δ18O isotope data.

7.7 The possibility that the Rehoboth Province has an Archaeanbasement has important implications for mineral explorationand to the crustal evolution of southern Africa.

Supplementarymaterials related to this article can be found onlineat doi:10.1016/j.lithos.2011.03.006.

Acknowledgements

This paper is dedicated to the memory of Gawie Moen, a gentlegiant who contributed greatly to the Kheis Province debate. DHCacknowledges a Swedish National Research Council grant no. 2006–2402. OI received a travel grant from the University of IcelandResearch Fund. The careful reviews by R. Mapeo and W. Altermannimproved the quality of this paper. Martin Whitehouse and theNordSIM team are gratefully acknowledged for their help with ionprobe dating and important contributions regarding the method andquality control of oxygen isotope analysis of zircon. The NordSIMfacility is supported by the research councils in Denmark Norway andSweden and the Geological Survey of Finland, together with theSwedish Museum of Natural History. This is Nordsim contribution no.287 and a contribution to IGCP project 509, PalaeoproterozoicSupercontinents and Global Evolution.

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Evidence from Dwyka tillite cobbles of Archaean basement beneath the Kalahari sands of southern...

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