Oxygen isotope diversity in the anorogenic Koegel Fontein Complex of South Africa: a case for...

Oxygen Isotope Diversity in the AnorogenicKoegel Fontein Complex of South Africa: a Casefor Basement Control and Selective Melting forthe Production of Low-d18OMagmas

CATHERINEG.CURTIS1, CHRISHARRIS1*, ROBERTB.TRUMBULL2,COENRAADDEBEER3ANDLUSANIMUDZANANI1

1DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF CAPE TOWN, RONDEBOSCH 7701, SOUTH AFRICA2COUNCIL FOR GEOSCIENCE, BELLVILLE, SOUTH AFRICA3GFZ GERMAN RESEARCH CENTRE FOR GEOSCIENCES, POTSDAM, GERMANY

RECEIVED DECEMBER 19, 2011; ACCEPTEDJANUARY 30, 2013ADVANCE ACCESS PUBLICATION MARCH 13, 2013

Koegel Fontein, about 350 km north of CapeTown, is the only known

early Cretaceous anorogenic igneous complex along the volcanic

rifted margin of South Africa. The oldest rocks of the complex are

minor granite and syenite intrusions at 144 Ma, which were followed

by tholeiitic and alkaline basalt dykes, then by microsyenite and

quartz porphyry dykes.The youngest and largest igneous unit is the

135 Ma Rietpoort Granite, with an exposed diameter of about

20 km. The country rocks are Mesoproterozoic gneisses of the

Namaqua^Natal Province, which in many places were deformed

and retrogressed by Pan-African tectonism.Whole-rock d18O values

from the Rietpoort Granite and smaller plutonic units (syenite,

granite) are in the range 6^9% (outliers 4% and 17%). Quartz

d18O values from all units are in a narrow range and indicate

magma d18O values between 6 and 8%. In contrast to the syenites

and granites, most mafic and silicic dyke units have d18O56%, as

low as ^4·1%. Quartz porphyry dykes that are compositionally simi-

lar to the Rietpoort Granite have a bimodal distribution of d18Ovalues in both whole-rock and quartz phenocrysts.The magma d18Ovalues estimated from the phenocryst data define a ‘normal group’

identical to the Rietpoort Granite (6^8%) and a ‘low-d18O group’

(0 to ^2%). The microsyenite and mafic dykes also yield negative

d18O values, but the strong hydrothermal alteration of these rocks

and lack of fresh phenocrysts make a primary origin of the low

d18O values unlikely and untestable.Whole-rock dD values of igne-

ous units and basement rocks average ^99%, which corresponds to

a palaeo-meteoric water with d18O as low as ^9%. This is much

lower than the expected value for meteoric water at the time of em-

placement, given the low latitude (30^408S). Quartz veins cuttingthe mafic dykes have d18O values as low as ^2%, which attest to

hydrothermal fluids having low d18O values. Country rocks in the

study area have a large range of d18O (^3 to 10%), with the majority

below the mantle value of 6%.The low d18O values of the country

rocks, although prevalent in the roof pendant of the Rietpoort

Granite, do not appear to have originated from a meteoric^hydrother-

mal system established by the intrusions.We suggest instead that the

Koegel Fontein complex was emplaced into a structurally controlled

zone in the Namaqualand basement whose d18O values had been

lowered by interaction with meteoric fluid during reactivation along

Pan-African shear zones. Initial emplacement of the magmas

caused thermal dehydration of the country rocks and expulsion of

low-d18O fluids.This was followed by local partial melting of the

altered crust with formation of low-d18O crustal magmas. The O

isotope data provide new constraints on the crustal vs mantle source

of the Koegel Fontein magmas.The Rietpoort Granite and ‘normal

d18O’ quartz-porphyry dykes crystallized from magmas with d18Ovalues of 7^8%, "Nd of ^5 to ^7, and initial 87Sr/ 86Sr of

0·716^0·732, which fit a model for 30^50% meta-igneous crust

similar to the local Namaqua gneisses, with a minor component of

low-d18O crust. The ‘low-d18O’ quartz porphyry magma had an

identical Nd isotope composition, but lower initial 87Sr/86Sr

*Corresponding author. E-mail: [email protected]

� The Author 2013. Published by Oxford University Press. Allrights reserved. For Permissions, please e-mail: [email protected]

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(0·709^0·725) and d18O (0 to ^2%), which we attribute to melting

or assimilation of hydrothermally altered basement rocks with Rb

and 18O depletion.

KEY WORDS: low-d18O magma; hydrothermal alteration; anorogenic

granite; crustal contamination

I NTRODUCTIONThe breakup of Western Gondwana in the earlyCretaceous was preceded by the formation of thewell-known Parana¤ Êtendeka Large Igneous Province inNW Namibia and Brazil (e.g. Peate, 1997), and by smallerand less well-studied occurrences along the South Africanmargin as far as CapeTown (Fig. 1). The latter include nu-merous dolerite dykes of early Cretaceous age (Reid, 1990;Trumbull et al., 2007; Backeberg et al., 2011), and theKoegel Fontein silicic intrusive complex, which is the sub-ject of this study. Koegel Fontein is located near theAtlantic coast about 350 km north of Cape Town (308590S,178590E). The complex consists mainly of granite and syen-ite intrusive units with numerous felsic and mafic dykesand plugs. An early Cretaceous age for the complex wasdemonstrated by single-zircon U^Pb dating (de Beer &Armstrong 1998), which confirmed suggestions of a youngage by Jansen (1960) and McIver (1981). The U^Pb ages of144�2 Ma for syenite and 133·9�1·3 Ma for theRietpoort Granite and quartz porphyry dykes put themain intrusive phase at Koegel Fontein in the same timeframe as the compositionally similar Damaraland intru-sive complexes in NW Namibia (Harris, 1995; Trumbullet al., 2004). A plume model is favoured by many workersfor initiation of magmatism in the Parana¤ Êtendeka prov-ince (e.g. O’Connor & Duncan, 1990; Ewart et al., 1998;Trumbull et al., 2003), but the Koegel Fontein complexand associated mafic dykes are more than 1000 km farthersouth, so the reason for initiation of this magmatism is un-clear. Other potential mantle plumes in the SouthAtlantic, namely those at present associated with BouvetIsland and the Discovery and Shona seamounts, weremuch closer to Koegel Fontein at 135 Ma than was theTristan plume (O’Connor & Duncan, 1990). In any case,the location of the complex may be more strongly con-trolled by crustal structures than by a thermal anomaly inthe mantle. De Beer (2010) pointed out that KoegelFontein is located at the intersection of coast-parallelextensional faulting andWNWÊSE faults.Curtis et al. (2011) published the first detailed geochem-

ical information on the Koegel Fontein Complex, with anemphasis on the origin of the silicic magmas. In a prelim-inary O-isotope study, Curtis (2010) discovered that manysamples have d18O values well below the composition ofmantle-derived magmas (5·7%, e.g. Eiler, 2001), and in sev-eral cases negative. Furthermore, quartz phenocrysts in

some of the samples also have low d18O values, indicatingthat these represent low-d18O magmas (e.g. Taylor &Sheppard,1986). Low-d18O magmas are uncommon world-wide (e.g. Bindeman, 2008), and none are known in thecontemporary Damaraland complexes in Namibia(Harris, 1995; Trumbull et al., 2004).The present study was undertaken to document the oc-

currence of low-d18O magmas at Koegel Fontein and to de-termine their origin in the context of magma genesis inthe complex. The work involved systematic sampling andanalysis of d18O values from the igneous components ofthe complex, as well as from the basement rocks andhydrothermal veins in the vicinity. In addition to O iso-topes we also determined H isotope compositions ofwhole-rock and mineral separates to constrain the originof the fluid(s) responsible for hydrothermal alteration inthe complex.

GEOLOGICAL SETT INGThe Koegel Fontein complex was emplaced at the intersec-tion of two shear zones, one trending north^south and theother NE^SW. The north^south lineament appears to bean ancient, large-scale zone of crustal weakness, whichhas been the focus of intrusion of many igneous rocks inthe area surrounding Koegel Fontein, including the com-plex itself (de Beer et al., 2002; de Beer, 2010).The host-rocks of the Koegel Fontein complex are felsic

gneisses of the Mesoproterozic (1000^1200 Ma)Namaqua^Natal Province.What appears to be a roof pen-dant within the Rietpoort Granite is preserved in thecentre of the complex (Fig. 1); this consists mainly ofaugen gneiss termed the Jakkalshoek Granite Gneiss byde Beer (2010) (Figs 1 and 2b). The roof pendant rocks aresurrounded on the eastern and western margins byRietpoort Granite that forms topographically higher hills,which suggests that the granite intrusion is funnel- orsaucer-shaped. From the western margin of the complexto the Atlantic coast, the basement rocks are more vari-able, consisting of Neoproterozoic marbles in the south ofthe area, and schists and grey gneisses of the Namaqua^Natal Province further north. An important feature of theNamaqua^Natal basement in the region is the presence ofductile shear zones associated with retrogressive quartz^mica schists that formed during Pan-African deformation(de Beer et al., 2002; de Beer, 2010). The Jakkalshoek augengneiss in the roof pendant is variably sheared (Fig. 2g),and intersected by a narrow mylonitized zone with north^south orientation (Fig. 1). In thin section, the roof pendantrocks appear highly altered, with chloritization of themafic minerals, turbid feldspar and granoblastic quartz.The country rocks throughout the region are cut by a var-iety of quartz veins. Most of the veins are older than thecomplex as they are cut by the intrusions, but a minority

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KoegelFontein

CapeTown

Messum

Brandberg

Cape CrossSpitzkoppe

Erongo

Etaneno

Paresis

Okorusu

80 km

18oE

31oS

Atlantic Ocean

10 km

Sandkopsdrif Complex

RietpoortKotzesrus Roof pendant

CCK20-21Zout

Rivierbasalt

f

f

Rietpoort Granite

Precambrian country rock

Quartz porphyry dyke

Minor intrusions

Sample site

Shear zone

Sandkop Syenite

Rooivleitjie GraniteCCK1

CCK2

CCK3

CCK4

CCK5CCK6-9

CCK10-11CCK12

CCK13CCK14

CCK15-17

CCK18

CCK19

CCK22

CCK23

CCK23CCK25

CCK26-29CCK30-32

CCK33CCK34

CCK35

CCK36

CCK37-38

CCK39

CCK40-41

CCK42-45 CCK46-48

CCK49CCK50

CCK51

CCK52

CCK53

CCK54

CCK55

KRL1-4KRL5-9

KRL10

KRL14-15

KRL11-12

CDB383CDB388

CDB541

CDB564

CDB572

CN495

CDB580

CDB588

CDB594

CDB601/4

CDB639

CDB650

CDB678

CDB681

CDB683

CDB687

CDB753

CDB703

CDB825

Etendeka lavasIntrusions

Damaraland

Low- 18O plug

(a)(b)

(b)

E

H

B

V

M

L

45’

18oE45’

31oS

15’15’

N

Fig. 1. Simplified geological map of the Koegel Fontein Complex fromVerwoerd & de Beer (2006).The upper inset map (a) shows the locationof Koegel Fontein in Southern Africa, with other Mesozoic extrusive rocks of the region (E, the 135 Ma Etendeka province; remnants of the180 Ma Karoo Igneous Province include: H, Hardap Dam; V, Victoria Falls; B, Botswana; L, Lesotho; M, Mwanezi, Lebombo and Tuli).The lower inset map (b) shows the location of the 135 Ma extrusive and intrusive igneous rocks of the Damaraland region of NW Namibia.The Sandkopsdrif Carbonatite Complex in the northern part of the main map is c. 50 Ma in age and is, therefore, not part of the KoegelFontein Complex. Sample locations and numbers are shown; the traverse through the roof pendant runs from sample KRL1 to KRL10. Thelocation of the mafic plug with the lowest d18O rocks is indicated.

CURTIS et al. LOW-d18OMAGMAS, KOEGEL FONTEIN

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Fig. 2. (a) Outcrop photograph of low-d18O breccia plug, samples CCK21 and KRL11. (b) Outcrop photograph of Jakkalshoek augen gneissin the roof pendant of Rietpoort Granite. (c) Photomicrograph in plane-polarized light (PPL) of Rietpoort Granite sample WP019.(d) Photomicrograph (PPL) of quartz porphyry sample WP006. (e) Photomicrograph (PPL) of Rietpoort Granite sample CCK19.(f) Photomicrograph (PPL) of glomeroporphyritic quartz phenocrysts, altered feldspar and chloritized amphibole from quartz porphyry dykesample WP006. (g) Photomicrograph in cross-polarized light (CPL) of sheared augen gneiss sample CCK22 from the roof pendant ofRietpoort Granite, with recrystallized mosaic of quartz and altered mafic minerals. (h) Photomicrograph (CPL) of mafic breccia plugCCK21 showing fine-grained dark matrix and crustal xenoliths with granoblastic quartz.


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of thin quartz veins exposed along the coast cross-cutdykes related to the complex and are therefore younger.

The Koegel Fontein ComplexDetailed descriptions of the Koegel Fontein igneous unitsand their geochemical characteristics have been given byCurtis et al. (2011). Briefly, two phases of felsic magmatismare recognized. The first was the intrusion of the 144·4�2Ma Sandkop Syenite, which according to field relationsand compositional affinities was probably accompaniedby microsyenite (Kerskloof dykes) and the RooivleitjieGranite plug (de Beer, 2010). The second intrusive phasebegan with NW^SE-trending quartz porphyry dykesdated at 133·9�1·3 Ma, closely followed by emplacementof the Rietpoort Granite. The zircon ages of granite andquartz porphyry dykes are indistinguishable, but thequartz porphyry dykes do not cut the Rietpoort Granite(de Beer, 2010; Fig. 1). Tholeiitic and alkaline basalticdykes (‘mafic dykes’ hereafter) have not been dated, butfield relations show that they intruded between the twofelsic phases.

Petrographic descriptions

The Rietpoort Granite (Fig. 1) is a homogeneous medium-to coarse-grained pluton with occasional local miaroliticcavities, consisting of 35^40 vol. % quartz and about60 vol. % feldspar (orthoclase, microperthite and albite^oligoclase), with minor biotite, hornblende, opaquephases, zircon and apatite (Fig. 2c and e). The SandkopSyenite is medium- to coarse-grained and consists of ortho-clase, oligoclase, microperthite, hornblende, biotite, apatiteand zircon. The characteristically red Rooivleitjie Graniteis coarse-grained and consists of about 70% alkali feldsparand 20% quartz, with minor biotite and hornblende andaccessory zircon and opaque minerals.The Kerskloof dykes (Kerskloof Bostonites of de Beer

et al., 2002) are of microsyenite to granite, typically east^west to WSW^ENE striking with near-vertical dips andabout 1m thick. The Kruisvlei Quartz Porphyries of deBeer et al. (2002), termed quartz porphyry dykes here, gen-erally strike NW^SE and are wide, mostly 3^10m, withrare examples reaching 20m in width. The dominantphenocryst phases are subequal amounts of alkali feldsparand quartz, the latter generally being bipyramidal. Thephenocrysts range in size from 0·5 to 10mm in diameterand the proportion of phenocrysts ranges from about 10 to50%.Tholeiitic mafic dykes are distinguished from geochemi-

cally similar regional dolerites by their smaller width(52m), and dominant north^south to NW^SE strike direc-tion. Most occurrences are near the Zout Rivier Basaltplug, with which they may be related (Fig. 1). Alkalinemafic dykes are thinner (c. 1m thick) and have NE^SWstrike directions (de Beer et al., 2002). Curtis et al. (2011)demonstrated compositional differences between the two

series of mafic dykes but their relationships are obscuredby the high degrees of alteration, and in this study theyare not distinguished.

Geochemical and radiogenic isotope characteristics

Curtis et al. (2011) described the geochemical diversity ofunits in the Koegel Fontein Complex in detail. The sam-ples included in the present study (Table 1, Fig. 3) coverthe compositional range, but our focus is on the silicicunits. The mafic rocks are particularly affected by alter-ation [loss on ignition (LOI) values¼1·4^5·0 wt %, mean2·6wt %,Table 1], and they lack fresh phenocrysts suitablefor O-isotope analysis. Of the major rock types analysedin this study, the Rietpoort Granite and the quartz por-phyry dykes (de Beer, 2010) plot in the rhyolite field on thetotal alkalis^silica (TAS) diagram (Fig. 3), whereas theSandkop Syenite and most of the Kerskloof dykes plot inthe trachyte field. The felsic samples analysed generallyhave less than 2wt % LOI (Table 1) and are reasonablyfresh petrographically. The intermediate (Kerskloofdykes) and mafic rocks have higher LOI values and exhibitstronger petrographic evidence for alteration (Fig. 2c ande), including turbid feldspar, chloritized biotite and horn-blende, and secondary epidote and calcite. The presenceof epidote suggests that the alteration is not simply theresult of low-temperature interaction with groundwater,but resulted from hydrothermal alteration.On the basis of radiogenic isotopes, the felsic units from

Koegel Fontein belong to two compositional series, whichcorrespond to the two phases of intrusion (Curtis et al.,2011). The earlier units, Sandkop syenite, Kerskloof dykesand the Rooivleitjie Granite, have a narrow range of eNdvalues from ^1·8 to ^0·4 (Table 1), which is consistent witha dominance of mantle-derived components in themagma source. The younger, more silica-rich series com-prising the Rietpoort Granite and quartz-porphyry dykeshave lower values of eNd from ^6·9 to ^4·8 consistentwith a greater crustal input. The Sr isotope compositionof the plutonic units can similarly be divided into groupswith more and less radiogenic Nd isotope signatures. Thedykes have more variable Sr isotope ratios and overlapboth groups.A small, 10m diameter, mafic breccia plug (Figs 1 and

2a) intrusive into the Rietpoort Granite roof pendant issignificant in the context of this study because it has thelowest d18O value of all the igneous units analysed (twosamples gave ^4·1 and ^2·9%), and it contains crustalxenoliths with similar low d18O values. The mafic plug hasa dark and fine-grained matrix with abundant xenolithsof various size and shape (Fig. 2h). In thin section, it is dif-ficult to identify any primary igneous minerals in thematrix, but many small (55mm) rounded to angularxenoliths of granoblastic quartzite are present. The plug iscompositionally variable, based on two samples with SiO2

of 56·5 and 53·5wt %, Al2O3 of 15·6 and 20·7wt % and


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Table 1: Nature, location, SiO2, LOI and Sr and Nd isotope composition of the Koegel Fontein Complex and surrounding

country rocks

Sample Type Longitude Latitude SiO2 LOI 87Sr/86Sri eNd

(S) (E)

Igneous rocks

CCK19 RG 31·09275 17·95692 75·05 0·45

CCK51 RG 31·09681 17·96439 75·45 0·48

CCK52 RG 31·08322 17·94544 76·55 0·29 0·727190 �6·24

CCK53 RG 31·05264 17·87306 75·14 0·59 0·725696 �6·35

CCK55 RG 30·98486 17·98646 77·52 0·28 0·724595 �6·05

CDB825 RG 30·96111 17·89444 76·04 0·66

CCK4 QP 31·08500 18·04197 74·33 1·21

CCK10 QP 31·10358 18·08211 70·73 1·62 0·715504 �5·58

CCK12 QP 31·10283 18·06333 72·82 0·99

CCK23 QP 31·02375 17·92667 75·27 0·62 0·709175 �4·94

CCK31 QP 31·19169 17·79322 72·11 2·96

CCK32 QP 31·18664 17·78869 69·61 2·02

CCK56 QP

CDB383 QP 31·16944 18·14278 75·11 1·70

CDB388 QP 31·17417 18·15361 75·81 1·49 0·731879 �5·42

CDB541 QP 31·13056 18·01111 73·25 1·08

CDB564 QP 31·08972 18·06528

CDB572 QP 31·03333 17·95583 74·83 0·59

CDB580 QP 31·00750 17·94583 76·04 0·66 0·712783 �4·98

CDB588 QP 30·98417 17·91556 75·71 0·74 0·715800 �5·26

CDB594 QP 31·01306 17·89444 77·43 0·61 0·716063 �5·30

CDB601 QP 31·04250 17·95222 72·38 0·69

CDB604 QP 31·04222 17·92306

CDB639 QP 31·07000 17·88611 72·57 0·82

CDB650 QP 31·19000 17·78917 70·71 1·89 0·725212 �6·90

CN495 QP 31·10361 18·06139 72·00 1·40 0·722665 �4·76

CCK3 QP 31·10986 18·00986 75·63 1·24

CCK7 QP 31·10181 18·06233 66·06 1·70

CCK15 Kerskloof dyke 31·15181 17·93517 74·89 0·74



CDB336 Kerskloof dyke 31·11333 18·26472 62·44 5·19 0·720337 �6·20

CDB552 Kerskloof dyke 64·71 1·59 0·736734 �0·96

CDB678 Kerskloof dyke 31·09833 17·93333 69·64 1·13



CDB704 Kerskloof dyke 65·98 1·17 0·708044 �0·35

CCK18 S Syenite 31·14953 17·89903 60·71 0·42 0·706017 �0·47

CDB703 S Syenite 31·15000 17·90833 60·12 1·28 0·704785 �1·82

CDB753 S Syenite 31·14306 17·91056 61·00 1·02 0·704510 �0·83

CCK17 RVG 31·15217 17·92467 69·93 0·45 0·702348 �1·23

(continued)


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Table 1: Continued


(S) (E)

CDB687 RVG 31·15778 17·92500 56·17 3·02

CCK6 MD 31·10139 18·06233 48·78 2·26

CCK11 MD 31·10156 18·08644 49·73 4·98

CCK33 MD 31·16400 17·76831 51·20 2·09

CCK35 MD 31·13581 17·74453 50·35 1·45

CCK36 MD 31·19169 17·79322 51·54 3·04

CCK38 MD 31·07250 17·71292 42·61 4·57

CCK39 MD 31·19169 17·79322 50·16 1·59

CCK47 MD 31·08156 17·88472 51·98 1·36

CDB1491 MD 47·06 1·57

CCK21 Breccia plug rock 31·05042 17·91431 55·93 0·41

CCK21 Xenolith in CCK21 31·05042 17·91431 53·49 1·23

KRL11 Breccia plug rock 31·05052 17·91455 58·64 0·87

CCK41 KQV 31·07572 17·71319

CCK42 KQV 31·08058 17·71667

CCK43 KQV 31·08058 17·71667

CCK44 KQV 31·08058 17·71667

CCK48 KQV 31·08156 17·88472

Country rock

KRL12 Gneiss 90m from KRL11 31·05071 17·91520 68·81 0·77




CCK9 Augen gneiss 31·10181 18·06233 70·70 1·12

CCK20 Augen gneiss 31·05042 17·91431 69·61 1·46

CCK22 Augen gneiss 31·13336 17·91469 70·88 0·72

CCK27 Augen gneiss 31·19803 17·79456 71·87 0·52 0·909869 �8·08

CCK30 Augen gneiss 31·19169 17·79322 71·90 1·92

CCK45 Augen gneiss 31·08058 17·71667 75·39 0·72

CCK46 Augen gneiss 31·08156 17·88472 71·15 0·68

CCK54 Augen gneiss 31·02681 17·95067 63·43 16·28

CCK2 Banded gneiss 31·15408 18·04583 68·81 0·80 0·726315 �9·90

CCK8 Banded gneiss 31·10181 18·06233 87·88 0·47

CCK13 Shear zone mylonite 31·08467 18·04558 91·01 1·08

CCK1 QV 31·15408 18·04583

CCK5 QV 31·10144 18·06219

CCK16 QV 31·15214 17·93581

CCK29 QV 31·19803 17·79456

CCK40 QV 31·07461 17·71278

CCK49 QV 31·08317 17·71950

CCK50 QV 31·08314 17·72008

CCK14 Bi schist 31·09342 18·01342 57·31 0·79

CCK34 Bi schist 31·15681 17·76306 50·85 0·77

CCK25 Bi schist 31·23578 17·83903 67·45 1·92

CCK28 Musc schist 31·19803 17·79456 80·98 1·55

(continued)


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MgO of 3·01and 3·58wt % (Curtis et al., 2011; Mudzanani,2011). The range in composition presumably reflects thevariation in proportion and identity of xenolith material.

SAMPL ING AND ANALYTICALMETHODSSampling strategyWhole-rock d18O values were determined on 78 samplesrepresenting the major intrusive bodies and basementrocks, and hydrothermal quartz veins from the regionaround the complex and westwards to the Atlantic coast(Fig. 1). In addition to whole-rock samples, d18O valueswere determined on mineral separates from a subset ofsamples to better constrain the d18O values of the originalmagmas. This was unfortunately not possible for themafic dykes and Kerskloof dykes. Minerals analysed in-clude quartz, feldspar, biotite, amphibole, and zircon.Pyramidal quartz phenocrysts were selected where pos-sible, as opposed to morphologically ambiguous anhedralgrains, which could include crushed vein material. Wherenecessary, quartz grains were cleaned in warm 10% HFfor 15min to remove adhering material. The feldspar sep-arates in many cases consist of a mixture of plagioclaseand alkali feldspar. Fresh amphibole and biotite are rare,and were separated from only the Sandkop Syenite. Forsome quartz porphyry dykes, fine-grained groundmassmaterial was separated along with the phenocrysts to in-vestigate O isotope homogeneity. Where quartz separatedfrom the quartz porphyries was found to have low d18Ovalues, it was analysed at least in duplicate. The Rietpoort

Granite roof pendant was given special attention becauseall of the low-d18O quartz porphyry dykes, as well as thelow-d18O mafic breccia plug, are restricted to this part ofthe complex. We collected a set of samples along a 10 kmeast^west traverse to assess the interaction of the graniteand its country rocks and the possible role of a regionalshear zone through the roof pendant (Fig. 1).

Table 1: Continued


(S) (E)

Roof pendant traverse

KRL01 Gneiss 0 km 31·0206 17·94586 68·26 0·95

KRL02 Gneiss 1·5 km 31·02162 17·93314 75·94 0·67

KRL03 Gneiss 2·1 km 31·02330 17·92749 70·04 0·62

KRL04 Gneiss 3·3 km 31·02233 17·91772 70·77 0·61

KRL05 Gneiss 5·4 km 31·02150 17·89866 78·20 0·61

KRL06 Sheared gneiss 6·0 km 31·02057 17·89270 71·54 0·64

KRL07 Augen gneiss 7·1 km 31·01520 17·88465 70·37 0·40

KRL08 Mylonite zone 7·1 km 31·01531 17·88423 74·00 0·40

KRL09 Gneiss 7·7 km 31·00940 17·87888 70·01 0·44

KRL10 Gneiss 9·8 km 30·99870 17·86281 69·83 3·39

RG, Rietpoort Granite; QP, quartz porphyry dyke; RVG, Rooivleitjie Granite; S Syenite, Sandkop Syenite; MD, maficregional dyke; KQV, quartz vein associated with intrusion of complex; QV, quartz veining in country rock not associatedwith complex.

(a)

40 50 60 70 802

4

6

8

10

12

SiO2

Na 2

O+

K2O

T R

B

TA

BT

TB

BAA

D

Rietpoort Granite

Quartz porphyry

Kerskloof dyke

Sandkop Syenite

Rooivleitjie Granite

Breccia plug

Mafic dyke

Fig. 3. Total alkalis vs silica diagram for samples of the KoegelFontein from this study (large symbols) and other samples from thecomplex; all data are from Curtis et al. (2011) and are normalized to100% volatile-free. The classification fields from Le Maitre (1986)]are shown: TB, trachybasalt; BT, basaltic trachyandesite; TA, tra-chyandesite; T, trachyte; R, rhyolite; D, dacite; A, andesite; BA, bas-altic andesite; B, basalt.


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Stable isotope analysisAll stable isotope data presented in this study were pro-duced at the University of Cape Town (UCT). For oxygenisotopes, both conventional and laser fluorination methodswere used. All whole-rock samples and about half of themineral separates were analysed using a conventional sili-cate line (described by Harris & Ashwal, 2002).Approximately 10mg of sample was reacted with ClF3,and the liberated O2 was converted to CO2 using a hotplatinized carbon rod. Some of the quartz separates, bothzircon samples, and xenolith material from a mafic plugwere analysed by laser fluorination using methodsdescribed by Harris & Vogeli (2010). Each 1^3mg samplewas reacted in the presence of c. 10 kPa BrF5 and the puri-fied O2 was collected onto a 5— molecular sieve containedin a glass storage bottle. Hydrogen isotope ratios weremeasured on whole-rock, biotite and amphibole separates.The hydrogen was extracted from c. 100mg of whole-rockand amphibole, and 50mg of biotite using the method ofVennemann & O’Neil (1993), with reduction by ‘low-blankIndiana Zinc’ (Schimmelmann & DeNiro, 1993).Isotope ratios were measured off-line using a Finnigan

Delta XP mass spectrometer in dual-inlet mode. All dataare reported in d notation, where d¼ (Rsample/Rstandard�1)� 1000, and R is the measured ratio (i.e.18O/16O or D/H). Duplicate splits of a quartz standard(either NBS28 or our internal standard MQ) were runwith each batch of eight samples to monitor analytical pre-cision and convert the raw data to the SMOW scale usingthe d18O value of 9·64% for NBS28 (Coplen et al., 1983),or 10·1% for our internal standard (MQ). During thecourse of this work, eight analyses of NBS28 gave a 2serror of 0·16%. The O-isotope ratios of samples analysedby laser fluorination were measured on O2 gas. Measuredvalues of our internal standard MON GT (MonasteryGarnet, Harris et al., 2000) were used to normalize rawdata and correct for drift in the reference gas. Thelong-term average difference in d18O values of duplicatesof MON GT analysed during this study was 0·13%(n¼ 87), which corresponds to a 2s value of 0·16%. MONGTwas calibrated against the UWG-2 garnet standard ofValley et al. (1995) using the current laser system, and itsrevised d18O value is 5·38%, assuming 5·80% for UWG2.The d18O value of zircon standard 91500 analysed duringthis work was 9·9% (accepted value 10·1%,Valley, 2003).For D/H determination, an internal biotite standard

(CGbi, dD¼ ^59%, H2O¼ 3·70wt %) was analysed induplicate with each batch of samples, and internal waterstandards (CTMP3, dD¼ ^7%; Evian, dD¼ ^70%) wereused to convert raw data to the SMOW scale and correctfor scale compression. Water concentrations (H2O

þ) weredetermined from the voltage measured on the mass 2 col-lector of the mass spectrometer, using identical sampleinlet volume (Vennemann & O’Neil, 1993). The expected

voltage per mg water was determined from accuratelymeasured amounts of our standard water. The precisionfor dD and H2O values are typically of the order of 2%(1s) and 0·10wt % (1s), respectively.

RESULTSOxygen isotope variations in the KoegelFontein complexWhole-rock compositions

The plutonic units, Rietpoort Granite and SandkopSyenite, have similar whole-rock d18O values in the rangeof 6^9%, excepting one outlier of Rietpoort Granite at16% and another at 5% (Table 2, Fig. 4). These values canbe considered ‘normal’ as opposed to ‘low d18O’, as theyare above the mid-ocean ridge basalt (MORB) value of5·7% (e.g. Ito et al., 1987). The Rooivleitjie Granite has sig-nificantly lower d18O (4·1 and 5·8%), which overlaps theMORB value. Compared with the plutonic units, thequartz porphyry dykes, Kerskloof dykes, and mafic dykesall have a much wider range in d18O value and, apartfrom a few exceptions, the dykes have very low d18Ovalues (Fig. 4). Negative values were found in each dyketype. The lowest values from the Koegel Fontein complexare from two samples of the mafic breccia plug that cutsthe Rietpoort Granite roof pendant (CCK21 and KRL10,with d18O values of ^4·1 and ^2·9%, respectively, Table 3).Despite the large range of whole-rock d18O values, there

are no obvious correlations of O isotope composition withany major or trace element features in the sample set, al-though it can be said that the average d18O value of rockswith SiO2 460wt % is higher than that of rocks withSiO2560wt % (Fig. 5a). There is also no correlation be-tween whole-rock d18O value and LOI (Fig. 5b), indicatingthat the whole-rock d18O variations are not simply relatedto hydrothermal alteration. This is supported by results ofseparate analyses of groundmass in some of the dykerocks. In these examples (Table 2) the groundmass separ-ates had d18O values (þ6·4 to ^1·1%) similar to those ofthe whole-rock. The groundmass separate from the maficbreccia plug sample CCK21 yielded a d18O value of ^3·5%, similar to quartz-rich xenoliths from this rock, with^5·1 to ^1·6% (mean¼ ^3·3%�1·5, 1s, n¼ 5). The d18Ovalue of adjacent country rock gneiss (KRL12) is also ^3·2%, and many more country rock samples from the roofpendant have low d18O values (see below).

Mineral compositions

The total range of d18O values for quartz separated fromthe Koegel Fontein igneous rocks is from 0·5 to 9·4%(Table 2). As noted for the whole-rock values, the range ofd18O values is much smaller for samples from the plutonicunits relative to the dykes. The quartz d18O values fromall three plutonic units overlap, those of the Rietpoort


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Table 2: O and H isotope composition and water content of Koegel Fontein and surrounding country rocks

Sample Type d18O dD H2Oþ

WR Gdm Qz Qz P Fsp Amph Zir magma WR Amp Bi (wt %)

Igneous rocks

CCK19 RG 8·8 8·2, 8·9, 8·3 8·2 7·4 �108 0·2

CCK51 RG 7·9 8·4 8·0 7·0 �117 0·3

CCK52 RG 5·7 8·0, 7·7, 7·7 5·8 6·6 �134 0·2

CCK53 RG 7·4 8·2 6·8 7·1 �104 0·3

CCK55 RG 7·2 8·7, 8·5 7·9 7·4 �115 0·2

CDB825 RG 15·3 8·7 8·3 7·6 �95 0·3

CCK4 QP 5·2 8·0 7·6 �101 0·7

CCK7 QP 3·5 7·2, 7·7 5·7 7·1 �102 0·1

CCK10 QP 6·8 �111 0·8

CCK12 QP 6·6 8,2, 8·0 3·2 7·7 �184 0·1

CCK23 QP 1·0 1·2, 1·3 0·4 1·2 �110 0·5

CCK31 QP 4·2 �94 2·3

CCK32 QP 2·6 �95 0·9

CCK56 QP 8·1 7·7

CDB383 QP 7·1 8·8 9·1 4·3 8·5 �80 0·8

CDB388 QP 6·1 8·2 7·8 �67 0·7

CDB541 QP 5·1 �83 0·5

CDB564 QP 4·2 8·8 1·5 8·4 �91 0·6

CDB572 QP 0·3 0·5 1·8, 0·4 �0·7 0·5 �123 0·1

CDB580 QP 3·1 4·5, 4·7 2·1 4·2 �105 0·2

CDB588 QP �1·0 1·4, 1·6 �1·5 1·1 �104 0·3

CDB594 QP �0·3 �1·1 3·2 �0·2, 0·2, 1·7 �2·6 1·1 �124 0·2

CDB601 QP 1·2 �111 0·2

CDB604 QP �1·0 �0·9, �0·2 �1·0 �81 0·4

CDB639 QP �0·3 �91 0·5

CDB650 QP 1·4 1·2 8·5 8·3 0·6 8·0 �107 1·6

CDB678 QP 4·1 �68 0·3

CN495 QP 4·7 4·7 8·5 8·8 3·3 8·3 �96 0·6

CCK3 Kerskloof dyke 3·3 �65 0·7

CCK7 Kerskloof dyke 3·5 5·7 5·3 �102 0·1



CCK37 Kerskloof dyke �2·2 �103 2·1

CDB336 Kerskloof dyke 6·4 6·4 9·4 �91 0·9

CDB681 Kerskloof dyke �2·9 �90 2·0

CDB683 Kerskloof dyke 2·5 �100 0·6

CCK18 S Syenite 6·5 �113 0·4

CDB703 S Syenite 8·5 8·0 7·1 5·6 5·4, 5·4 6·9 �107 �114 0·5

CDB753 S Syenite 7·6 8·5 7·5 6·3 7·4 �81 �90 �94 0·4

CCK17 RVG 4·1 7·9, 8·3 1·8 7·0 �95 0·2

CDB687 RVG 5·8 7·8 3·7 6·7 �86 0·2

CCK6 MD 2·6 �101 2·5

CCK11 MD 1·3 �97 2·6

CCK33 MD 3·9 �103 1·7

CCK35 MD �1·7 �97 2·1

(continued)


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Table 2: Continued

Sample Type d18O dD H2Oþ

WR Gdm Qz Qz P Fsp Amph Zir magma WR Amp Bi (wt %)

CCK36 MD �3·1 8·0 (xenolith) �1·5 �94 2·5

CCK38 MD �2·9 �106 �114 4·6

CCK39 MD �1·2 �111 2·1

CCK47 MD �2·2 �94 2·0

CDB1491 MD 5·9 �82 1·2

CCK41 KQV �1·5 �1·5 �107 0·5

CCK42 KQV 2·2 �118 0·5

CCK43 KQV �1·4 3·8, 1·6, 3·0 �111 0·3

CCK44 KQV �1·7 �3·5 �85 0·7

CCK48 KQV �0·9 �2·5 �101 0·3

Country rock

CCK9 Augen gneiss 5·6 �101 0·6

CCK20 Augen gneiss �2·8 6·5 �112

CCK22 Augen gneiss 0·2 �105




CCK46 Augen gneiss 1·1 9·3, 9·4 �106 0·5

CCK54 Augen gneiss 2·6 6·1 �95 0·3

CCK2 Banded gneiss 8·8 �75 0·6

CCK8 Banded gneiss 9·6 �173 0·1

CCK13 Shear zone mylonite 10·1 �97 0·7

CCK1 QV 10·7

CCK5 QV 10·2

CCK16 QV 9·9

CCK29 QV 10·1

CCK40 QV 9·6

CCK49 QV 9·2 9·6

CCK50 QV 5·6

CCK14 Bi schist 4·6 �122 1·4

CCK34 Bi schist �3·0 �120 1·0

CCK25 Bi schist 17·4 �84 0·8

CCK28 Musc schist 8·1 �93 1·4

Roof pendant traverse

KRL01 Gneiss 0 km 0·5 7·5, 7·9, 7·2, 6·7, 6·9 �1·9 �65 0·7

KRL02 Gneiss 1·5 km 1·1 3·6 0·1 �90 0·3

KRL03 Gneiss 2·1 km 1·9 5·9 2·1 �104 0·6

KRL04 Gneiss 3·3 km 5·7 7·5 5·6 �105 0·5

KRL05 Gneiss 5·4 km 5·9 �68 0·2

KRL06 Sheared gneiss 6·0 km 5·6 7·4 5·0 �99 0·3

KRL07 Augen gneiss 7·1 km �0·2 0·4 �0·4

KRL08 Mylonite zone 7·1 km 2·5

KRL09 Gneiss 7·7 km 3·0 5·8 5·2

KRL10 Gneiss 9·8 km 7·5 7·6 �99 0·4

Abbreviations for rock types as for Table 1. WR, whole-rock; Gdm, groundmass; Qz, quartz; Qz P, quartz phenocryst;Fsp, feldspar; amph, amphibole; Zir, zircon. Magma d18O value calculated from quartz d18O assuming �quartz–magma¼1·1% (granite and syenite) and 0·4% (quartz porphyry). H2Oþ refers to whole-rock. Italic indicates samples were analysedby laser fluorination (zircon and some quartz).


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Granite being 8·3%�0·8 (2s, n¼ 6), two Sandkop Syenitesamples being 8·0 and 8·5%, and two samples from theRooivleijtie Granite being 7·8 and 8·1% (Table 2). Thequartz and whole-rock d18O values in samples of the Riet-poort Granite and Sandkop Syenite are similar. However,quartz from the Rooivleitjie Granite has significantlyhigher d18O values than the whole-rock, the d18O valuesof which are at or below the MORB value. In this case, asdiscussed in a later section, the quartz probably retainsthe magmatic composition and the lower d18O of theRooivleitjie whole-rock reflects later bulk-rock alteration.Quartz separates from the quartz porphyry dyke rocks

define a bimodal distribution of d18O (Fig. 6a), with ahigh-d18O group similar to the plutonic units (8·3%�0·92s, n¼ 9), and a second group of much lower d18O values(0·9�1·6 2s, n¼ 6). Only one of 15 samples (CDB580)has a quartz d18O value between the two groups (4·6%).To confirm the significance of the low d18O values in theserocks, quartz from the low-d18O group of quartz porphyrydykes was analyzed in duplicate. In some samples, the du-plicate analyses of quartz agreed within expected error(e.g. CDB580 and 588, both within 0·2%) whereas inothers (e.g. CDB572 and 594), the difference was41·0%and thus significant, but still small in relation to the gapbetween the high- and low-d18O groups. In some samplesit was possible to separate two morphological types ofquartz, one of clearly phenocrystic origin (euhedral, bipyr-amidal crystals) and one of anhedral shape, of unknownorigin (fragmented phenocryst or secondary quartz). Thetwo types of quartz have the same d18O value in thehigh-d18O group but somewhat different d18O values inthe two low-d18O group samples (Fig. 6a). It appears,therefore, that quartz in the low-d18O quartz porphyries

Table 3: O isotope composition of low-d18O plug rock

Sample Type d18O d18O fsp d18O qz dD H2Oþ

CCK21 Bulk-rock (plug) �4·1 �112

CCK21 Groundmass �3·5

CCK21 Xenolith �5·1*





KRL11 Bulk-rock (plug) �2·9 �105 1·5

KRL12 Country rock (90m from plug) �3·2

KRL13 Country rock (100m from plug) 3·7 4·9

KRL14 Country rock (200m from plug) 6·2 4·4 8·6

KRL15 Country rock (300m from plug) 1·0 1·2 5·1

O isotope analysis by conventional method and * by laser fluorination.

-5 0 5 10 15 20

18O Whole rock

Rietpoort Granite

Quartz porphyry

Kerskloof dykes

Sandkop Syenite


Breccia plug

Mafic dykes

Cretaceous qz veins

Pan African (?) qz veins

Country rock

Roof pendant

Mantle-derived magmaK

oege

lFon

tein

Com

plex

igne

ous

rock

sC

ount

ryro

ckan

dqu

artz

vein

s

(a)

(b)

Fig. 4. Compilation of all whole-rock d18O data from this study, sepa-rated into igneous units (a) and country rocks and veins (b). TheJakkelshoek Granite Gneiss forms the roof pendant to the RietpoortGranite.


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has variable d18O at the scale of single grains. This may bedue to O isotope zoning, possibly as a result of interactionwith external fluids. The small difference in d18O value be-tween quartz types is consistent with the absence of sec-ondary quartz.Feldspar d18O values from the Koegel Fontein units

(Fig. 6a) range from ^2·6 to þ8·3%, with an average of4·5% (n¼12). Feldspar from the low-d18O group samplesalso has low d18O (^2·6 to þ0·4%), whereas feldspar fromthe high-d18O group spans the full range in d18O value(0·6^5·7%). Feldspar from both high- and low-d18Ogroups shows a greater variation in O isotope compositionthan quartz from the same samples, which reflects thegreater susceptibility of the former to exchange withhydrothermal fluids.The O isotope variations of quartz, feldspar and

whole-rock are shown together in Fig. 6a. The key featureis the bimodal variation in quartz d18O values in con-trast to the more continuous range in feldspar d18Ovalues. The equilibrium quartz^feldspar fractionationfactor (�quartz^feldspar) varies from 1·1% at 7008C to 0·8%at 8508C (Clayton et al., 1989; feldspar assumed to bealkali feldspar), so at magmatic temperatures the two

minerals from an igneous rock should give values of�quartz^feldspar near unity. For Koegel Fontein, quartz andfeldspar in all three plutonic units (Rietpoort andRooivleitjie granites and Sandkop Syenite) are consistentwith O-isotope equilibrium at magmatic temperatures. Incontrast, most of the quartz porphyry dyke samples havehigh �quartz^feldspar values beyond the equilibrium range,and the most likely explanation is that feldspar d18Ovalues in these rocks were lowered by exchange with anexternal fluid.A small number of amphibole and zircon separates were

analysed from samples of the Sandkop Syenite.The amphi-bole gave similar d18O values of 5·6 and 6·3% from twosamples, and duplicate analyses of zircon from syenite

= zero

Rietpoort Granite

Quartz porphyry

Sandkop Syenite


0 2 4 6 8 10

0

5

10

0 2 4 6 8 10

0

5

10

18O quartz

18O quartz

18O

min

eral

18O

feld

spar

qz-fsp = 1.14 (700

o C)

qz-fsp = 0.66 (1000

o C)

qz-mineral= 1.76

(1000oC hbl;952oC zirc)

qz-mineral = 2.94 (700

o C hbl; 674o C zir

c)

Zircon

Quartz phenocryst

Amphibole

Low- 18O group

High- 18O group

(a)

(b)

Fig. 6. (a) Variation of feldspar d18O vs quartz d18O for KoegelFontein magmatic units. Quartz^feldspar isotherms for 700 and10008C were calculated using the equations of Clayton et al. (1989).(b) Zircon, amphibole, and quartz phenocryst d18O vs quartz d18O(see text for distinction between two types of quartz). Equilibrium iso-therms were calculated using the equations of Valley (2003) forquartz^zircon, and Zheng (1993) for quartz^hornblende.

40 50 60 70 80-5

0

5

10

15

20

SiO2

18O

who

lero

ck

0 1 2 3 4 5 6-5

0

5

10

15

20

LOI

18O

who

lero

ck

Rietpoort Granite

Quartz porphyries

Kerskloof dykes

Sandkop Syenite

Rooivjetjjie Granite

Mafic dykes

CCK21

CCK21

(a)

(b)

Fig. 5. Variation of whole-rock d18O vs (a) SiO2, and (b) LOI forKoegel Fontein igneous rocks. Sample CCK21 from the mafic plughas the lowest d18O value in the dataset. LOI, loss on ignition.


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CDB703 yielded a d18O value of 5·4% (Table 2). Com-pared with mineral^quartz fractionation factors (Fig. 6b),these values for amphibole and zircon d18O are consistentwith O isotope equilibrium at magmatic temperature.

Oxygen isotope variations in country rocksand quartz veinsThe country rocks around the Koegel Fontein complex,and in coastal exposures a few tens of kilometres to thewest, have a similar wide range in whole-rock d18O valuesas the igneous rocks (Fig. 4), including a large number oflow-d18O samples (17 of 31 with56%). All of the samplesalong the traverse across the roof pendant JakkalshoekGneiss (Fig. 1) have low whole-rock d18O values, from ^0·2to þ5·9%, with an average of 2·5% (Fig. 7). It may be sig-nificant that the lowest d18O value among the roof pendantsamples (KRL12, Table 3) was collected adjacent to thelow-d18O mafic plug. However, there is a clear correlationof low d18O values with the position of a north^southshear zone that runs across the roof pendant (Fig. 7b). Tointerpret the whole-rock d18O values of the country-rockgneisses more effectively, we also analyzed quartz and feld-spar separates from selected samples of the roof pendantgneiss. The results (Fig. 7a) show a strong positive correl-ation (r¼ 0·71, n¼11) and, with the exception of KRL01,the quartz and feldspar separates show a good approachto isotopic equilibrium (Fig. 7b). The exceptional samplehas �quartz^feldspar �9%, indicating disequilibrium. Singlequartz grains from this sample range in d18O value from6·7 to 7·9% (n¼ 5), whereas the feldspar composition, at^2%, is the lowest of any sample on the traverse.Hydrothermal quartz veins in the study area provide

direct evidence for the composition of the hydrothermalfluids. Furthermore, cross-cutting relations of quartz veinswith igneous units of the Koegel Fontein Complex make itpossible to distinguish and sample veins that are earlierand later than the intrusion (labelled ‘Cretaceous veins’ inFig. 4). The important distinction is that the quartz veinsthat cut mafic dykes associated with the complex have lowd18O values (four samples with negative values), whereassix out of seven of the pre-Cretaceous quartz veins haved18O values47%. This observation implies that circulationof low-d18O fluids must have occurred at some stageduring, or after, the emplacement of the complex.

Hydrogen isotope variationsThe whole-rock dD values of the Koegel Fontein igneousrocks define a wide overall range from ^65 to ^184%(Table 2) with an average of ^102�4% (n¼ 69). However,all but two samples have dD values between ^134 and^65%, and the outliers have unusually low water contents,suggesting that their extremely low dD values may be ananalytical effect (owing to the ‘blank’). Ignoring the twoquestionable values, the average whole-rock dD compos-ition is ^99%. The H isotope data show no relationship

with the degree of alteration based on petrography. Themeasured water concentration in these samples variesfrom less than 0·1 to 4·6wt %, and there is also no correl-ation of dD values with water content (Fig. 8a). Impor-tantly, there is no systematic difference in dD valuesbetween the ‘normal’ and ‘low-d18O’ groups of igneousrocks from Koegel Fontein (Fig. 8b); indeed, the O and Hisotope compositions show no correlation in the dataset.Fresh biotite and amphibole separates from the igneousunits have the same range of dD values as the whole-rocks(dD biotite¼ ^114 and ^94% and dD amphibole¼ ^90%).The whole-rock dD values for the country rocks in the

roof pendant and surrounding the complex vary from^124 to ^78%, which is in the same range as that of theKoegel Fontein igneous rocks (Table 2). As in the igneous

Whole-rock

Feldspar

-5 0 5 10-5

0

5

10

18O quartz

18O

who

le-r

ock

orfe

ldsp

ar

= 1.14 (700o C)

qz-fsp

= 0.6(1000

o C)

= 3 (288o C)

17.86 17.9 17.94-2

0

2

4

6

8

Longtitude

18O

who

le-r

ock,

quar

tzor

feld

spar

mylonite

Whole-rock

Quartz

EW

(a)

(b)

KRL01

KRL01

Fig. 7. (a) Variation of whole-rock and feldspar d18O vs quartz d18Ofor the roof pendant of the Rietpoort Granite. (b) Whole-rock,quartz and feldspar[Q12] d18O values vs longitude for the traverseacross the roof pendant. The grey arrow shows the position of anorth^south-trending shear zone (see Fig. 1 for location of profile).


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units, the O and H isotope variations in the country-rocksamples are not correlated (Fig. 8b).

DISCUSSIONMagmatic d18O valuesThe whole-rock d18O values of most Koegel Fontein igne-ous rocks do not give reliable estimates of the magmaticd18O composition, as demonstrated by the lack of internalO isotope equilibrium between quartz and feldspar atmagmatic temperatures. The lack of correlation betweenquartz and feldspar d18O, and the fact that quartz from

both high- and low-d18O samples has much less variabled18O values than feldspar (Fig. 6a), indicates that whateverprocess lowered the d18O value of the feldspar did not sig-nificantly affect the quartz. Quartz is considered to be re-sistant to O isotope exchange with hydrothermal fluids attemperatures below 3508C (e.g. Taylor, 1978; Criss &Taylor, 1986), and among the rock-forming minerals it ismost likely to retain magmatic d18O values. Zircon is gen-erally regarded to have the highest resistance to O isotopeexchange (e.g. Valley, 2003). Zircon and quartz d18O ana-lyses were made on sample CDB703 of the SandkopSyenite. The results yield a value for �quartz^zircon of 2·6%,which corresponds to an equilibrium temperature of7408C based on Lackey et al. (2008).These data are consist-ent with internal O isotope equilibrium at magmatic con-ditions and confirm (for this sample, at least) that thequartz d18O values retain the magmatic signature.Therefore, we feel confident in using quartz data toestimate magma d18O values.Estimates of magmatic d18O values from quartz data

depend on knowing the quartz^magma fractionationfactor (�quartz^magma). Published fractionation factors be-tween quartz and rhyolite magma vary from 0·77%(Zhao & Zheng, 2003) to 0·4% (Bindeman & Valley,2001). These may be appropriate for the rapidly cooledquartz porphyry dykes, but not for the Rietpoort Granite,where quartz and the other minerals will continue to ex-change oxygen by diffusion down to the closure tempera-ture. For example, in their study of the Nebo Granite ofthe Bushveld Complex, Fourie & Harris (2011) estimatedthat the d18O value of quartz is 1·1% higher than that ofthe original magma. The mineralogy and grain size of theNebo Granite and the Rietpoort Granite are similar, sothe same �quartz^magma value is used here. A value of�quartz^magma of 0·4% from Bindeman & Valley (2001) isused for the quartz porphyry dykes. The corresponding es-timates of magma d18O (Table 2, Fig. 9) are in the‘normal’ range for igneous rocks; that is, from 6 to 10%(Fig. 6b). This applies to all of the plutonic units (SandkopSyenite: mean 7·2%, n¼ 2; Rooivleitjie granite: mean6·9%, n¼ 2; Rietpoort Granite: 7·2%�0·8 2s, n¼ 6), aswell as the majority (nine of 15) of the quartz porphyrydykes (7·9%,�0·9 2s, n¼ 9). However, five quartz por-phyry samples have much lower magma d18O values(0·5%,�1·6 2s, n¼ 5), and one is intermediate (4·2%).The magma d18O estimates for the quartz porphyry dykesare among the lowest yet described for magmas worldwide(e.g. Bindeman, 2008, fig. 1). All samples of quartz por-phyry dykes with low d18O values are located within theroof pendant of the Rietpoort Granite (Fig. 10).The quartz phenocryst data proved crucial for interpret-

ing the low d18O values of quartz porphyry dykes as mag-matic. Unfortunately, the approach cannot be applied totest the significance of low d18O in the Kerskloof dykes

0 1 2 3 4 5-200

-150

-100

-50

Wt.% H2O+

Dw

hole

-roc

k

-20 -10 0 10 20-150

-100

-50

0

18O whole-rock

Dw

hole

-roc

k

Glo

bal M

WL(b)

(a)

Rietpoort Granite

Quartz porphyry

Kerskloof dykes

Sandkop Syenite


CCK21 plug

Mafic dyke

Country rock

rock

-wat

er=

-300 /

00

ave. whole-rock D

W Cape hot springs

Cape Town rain

Sutherland groundwater

Fig. 8. Variation of dD vs (a) wt % water and (b) d18O whole-rockfor Koegel Fontein Complex rocks. The global meteoric water line(MWL) is from Craig (1961). The average whole-rock dD value ofthe Koegel Fontein units is ^99%, similar to the country rocks (seetext). Assuming drock^water¼�30%, the average fluid dD value is ^69%. This corresponds to a d18O value of c. ^10%, which is signifi-cantly lower than the fields shown for modern Cape Town rainfall(Harris et al., 2010), Western Cape hot springs (Diamond & Harris,2000) and Sutherland groundwater (Adams et al., 2001).


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and mafic dykes, which are aphyric and/or stronglyaltered. The mafic dykes in particular show the strongestpetrographic effects of alteration, and it is unlikely thattheir magmatic O isotope composition is preserved.

Implications for genesis of the KoegelFontein magmasBelow, we evaluate new constraints on the origin of theRietpoort Granite and other ‘normal’-d18O felsic unitsfrom the Koegel Fontein complex, based on a combinationof new O isotope results with existing radiogenic isotopedata from Curtis et al. (2011). The origin and significanceof magmas and hydrothermal rocks with low d18O valuesat Koegel Fontein is discussed subsequently.In their isotope study of the Damaraland igneous com-

plexes from NW Namibia, Trumbull et al. (2004) found astrong positive correlation between Nd and O isotopecompositions that could be interpreted in simple terms bya mixing of mantle-derived magmas and crustal material.The Koegel Fontein units do not show any such simple re-lationship between the magma d18O and the Nd (or Sr)isotope composition. The Sandkop Syenite and RooivleitjeGranite, which plot close to Bulk Earth in Fig. 11, have

d18O values around 7%, consistent with these rockshaving formed from mantle-derived parent magmas(Curtis et al., 2011). The Rietpoort Granite has significantlylower eNd values of around ^6, but similar d18O values tothe other plutonic units. The quartz porphyry dykes arehighly variable in d18O, but all have a narrow range ineNd (Fig. 12). The Sr isotope compositions of quartz por-phyry samples from the low-d18O and normal-d18O groupsoverlap (0·7092^0·7252 and 0·7155^0·7319, respectively),but the tendency is for lower initial Sr isotope ratios in thelow-d18O group.The weak correlation of Sr with O isotopecomposition and the lack of any such relationship witheNd are consistent with the greater mobility of Sr and Rbvs Nd and Sm during fluid^rock interaction. We arguebelow that the low-d18O magmas derive from melting ofcrust affected by an earlier episode of hydrothermal alter-ation, possibly related to Pan-African reworking (e.g.Porada, 1989). If, as is reasonable, the alteration loweredthe bulk Rb/Sr ratio, the 400 Myr time span betweenPan-African reworking and intrusion of the KoegelFontein would result in lower initial Sr isotope ratio in thelow-d18O magmas but leave the Nd isotope ratiounaffected.The resistance of Nd isotope ratios to change as a result

of hydrothermal alteration makes them more suitable formodelling magma genesis. Curtis et al. (2011) noted thatthe initial Nd isotope ratios discriminate two groups offelsic units at Koegel Fontein with no overlap betweenthem. The first group comprises the Sandkop Syenite,Rooivleitjie Granite and Kerskloof dykes, with initial eNdvalues near zero (^1·8 to ^0·3); that is, near the BulkEarth composition and close to the proposed compositionof the Tristan mantle plume at 135 Ma (le Roex &Lanyon, 1998; Trumbull et al., 2003). The second group,including the Rietpoort Granite and quartz-porphyrydykes, has much lower eNd values (^6·9 to ^4·8) suggest-ing a stronger crustal contribution (Fig. 11). Curtis et al.(2011) also determined the Nd isotope ratios from two sam-ples of the local Namaqua basement gneisses, whichyielded eNd135Ma of ^8·1 to ^9·9. From the combination ofthese Nd isotope data with the new O isotope results forthese units we can estimate better the proportion of crustalmaterial in the Koegel Fontein magmas. Following Curtiset al. (2001), we assume for the mixing model two distinctcomponents in the mantle in terms of Nd isotope compos-ition (eNd¼ ^1 and þ4). The first is plume-related, with anear Bulk Earth composition, and the other is a depletedmantle component related to shallow mantle astheno-sphere as typified by the Horingbaai dykes of NW Na-mibia. Both mantle components are assigned the d18Ovalue of 5·7%. Mixing models for the Rietpoort Graniteare illustrated in Fig. 13. The crustal component isbased on analyses of two Koegel Fontein country-rocksamples (banded gneiss CCK2 and augen gneiss CCK27).

-2 0 2 4 6 8 100

2

4

6

8

Fre

quen

cy

Quartz porphyry dykes

Reitpoort Granite

Sandkop Syenite


18O magma

Fig. 9. Histogram of magma d18O calculated from quartz data usingquartz^magma fractionation factors of 0·66% (quartz porphyry)and 1·11% (plutonic rocks). Samples plotted, for example, in the classinterval 4^5 have values between 4·0 and 4·9%.


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These have similar d18O and eNd values (8·8 and 8·2%,and ^8·1 and ^10, respectively) whereas their Nd concen-trations are very different (181 and 34 ppm, respectively).In Fig. 13, the crustal end-member KG (Koegel FonteinGneiss) represents these compositions. A different crustalcomposition was used in the study of the Damaralandcomplexes by Trumbull et al. (2004). Their end-membercrust (NWN in Fig. 13) has the same eNd but a muchhigher d18O of 11·1%, as it represents metapelite-dominated country rocks in the Damara Belt. The Ndconcentrations of the two mantle magma end-membersare set at 46 ppm Nd (the average Nd content ofKoegel Fontein alkaline mafic dykes; Curtis et al., 2011).Mixing curves were calculated for both high and lowNd concentrations in the crustal end-members. It is evi-dent (Fig. 13) that the Rietpoort Granite could representmixtures of either mantle end-member with KoegelFontein gneiss, provided the Nd content of the contamin-ant is high. The amount of crust is between 30 and50%, depending on the eNd composition of the mantleend-member.

The Rooivleitjie Granite and Sandkop Syenite do notplot on the same mixing curves as the Rietpoort Granite.Furthermore, their Nd^O isotope compositions are incon-sistent with a mixing model involving aTristan plume-likemantle component, which is already questionable becauseof the41000 km distance from the Parana¤ ^Etendeka prov-ince where the plume influence is evident. Assuming adepleted mantle composition, the Rooivleitjie andSandkop data suggest 20^30% Namaqua-type crust forthe high- and low-Nd models, respectively. The RietpoortGranite combines relatively radiogenic Nd and Sr isotoperatios with much more mantle-like magma d18O values(average 7·2%). The Rietpoort Granite has an averaged18O value that is only 0·9% higher than expected in arhyolitic magma produced from a mantle-derived maficmagma by closed-system fractional crystallization (d18Ovalue of 6·3%; Bindeman, 2008). If the Rietpoort Graniteplotted on a mixing line (Fig. 13) between Koegel Fonteingneiss and a mantle-derived magma, which passedthrough the Sandkop Syenite and Rooivleitjie Granite, itsd18O value would be about 8·0%, for an eNd of ^6·2

Normal 18O

Low 18O

Rietpoort Granite

Roof pendant

CDB650 (8.0)

CDB388 (7.8)

CDB383 (8.6)

CN495 (8.3)CCK7 (7.1)

CCK4 (7.6)

CDB588 (1.1)

CDB580 (4.2)

CDB572 (0.5)

CDB604 (-1.0)

CDB594 (1.1)CCK23 (0.8) f

f

CCK12 (7.7)

CDB564 (8.4)

Fig. 10. Location of quartz porphyry dykes for which magma d18O values were determined (magma d18O value in parentheses after the samplenumber). Outline of the geology is based on Fig. 1.


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(Table 1). This would represent a magma that comprisesapproximately 85% crust, 15% mantle. The RietpoortGranite samples actually have magma d18O values thatrange from 6·6 to 7·6%, 0·4^1·4% lower than expected.This probably results from the incorporation of a smallproportion of low-d18O altered crust by the magmasystem. If it is assumed that the mafic breccia plug is repre-sentative of the lowest d18O country rock (^4%), then therange in d18O values of the Rietpoort Granite could beexplained by incorporation of between 5 and 13% of thismaterial in the magma.

Origin of low d18O values in the countryrockThere are no particular petrographic features of country-rock samples with low d18O values relative to the others.For example, augen gneiss samples CCK20 (^2·8%) andCCK9 (þ5·6%) show similar features of post-magmatic al-teration, such as granoblastic recrystallization of quartzand chloritization of mafic minerals, but their d18O valuesare very different. Other examples are the biotite schistsamples CCK25 and CCK34, which are petrographicallysimilar and not highly altered, but show an extreme con-trast in d18O values (17·4 and ^3·0%, respectively). Thelack of a relationship between the petrographic evidenceof alteration and whole-rock d18O value may be related tothe different sensitivity of these phenonema to the tem-perature and water/rock ratio of alteration. Significant

18O depletion requires high-temperature interaction(44008C) and a relatively high water/rock ratio, whereaspetrographic signs of alteration can develop at lower tem-peratures and at relatively low water/rock ratios.Alternatively, the lowering of d18O value took place priorto peak metamorphic conditions.All samples of quartz porphyry dykes with low d18O

values are located within the roof pendant of theRietpoort Granite (Fig. 10). The augen gneisses from theroof pendant are also low in d18O (^0·2 to þ5·9%). It canbe ruled out that this resulted from exchange with mag-matic^hydrothermal fluids emanating from the RietpoortGranite because fluid in equilibrium with the magmawould have d18O values similar to the granite (i.e. about

-10 -8 -6 -4 -2 00

2

4

6

8

10

Nd

18O

mag

ma

0.7 0.705 0.71 0.715 0.72 0.7250

2

4

6

8

10

Initial 87Sr/86Sr

18O

mag

ma

Rietpoort Granite

Sandkop Syenite

Quartz porphyry


Mantle-derived

Mantle-derived

Altered crust

Altered crust

unaltered crust

unaltered crust

0.73

(a)

(b)

Fig. 12. Variation of magma d18O values for Koegel Fontein igneousunits (calculated from quartz data; see Fig. 9) vs (a) whole-rock eNdand (b) initial Sr isotope ratio.The light shaded field encloses sampleswith mantle-dominated isotope ratios (see text and Fig. 12). The greyfield encloses samples with radiogenic isotope compositions that indi-cate a stronger crustal involvement in the magma source. Thelow-d18O samples of quartz porphyry dykes are attributed to a crustalsource that was previously altered. It should be noted that these sam-ples also have lower initial Sr ratios (see text).

Rietpoort Granite

Quartz porphyry

Kerskloof dyke

Sandkop Syenite


Gneiss

0.7 0.71 0.72 0.73 0.74

-10

-5

0

Initial 87Sr/ 86Sr

Nd

Namaqualand metamorphic rocks

Erongo granites

Spitzkoppe

Brandberg

Tristan da Cunha

CDB552

Fig. 11. Variation of eNd vs initial Sr isotope ratio for Koegel Fonteinigneous rocks (data from Curtis et al., 2011). Ratios were age-correctedto 144 Ma for the Sandkop Syenite, Rooivleitjie granite andKerskloof dykes, and to 134 Ma for Rietpoort Granite and thequartz-porphyry dykes. The data field for the Namaqua metamorphicrocks is from Reid (1979, 1997). The fields for the Damaraland com-plexes Brandberg, Spitzkoppe, Cape Cross, and Erongo are fromTrumbull et al. (2004); Tristan da Cunha at 132 Ma from LeRoex &Lanyon (1998).


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7·5%). Although lowering of d18O values in the gneisscould be related to an early phase of intrusion, the evi-dence favours a pre-magmatic origin because the feldsparand whole-rock d18O values in the roof pendant samples(except KRL01) show a strong positive correlation withquartz (r¼ 0·86 and 0·84, respectively; see Fig. 7a and b).Hydrothermal alteration generally affects feldspar andwhole-rock values more than quartz and produces disequi-librium arrays such as those in Fig. 6a. Sample KRL01shows this kind of d18O disequilibrium, but the otherroof-pendant samples do not. It is possible that fluid^rockinteraction was sufficiently intense and prolonged thatd18O values in all phases approached equilibrium, but it ismore likely that the alteration preceded a metamorphicevent that re-equilibrated quartz and feldspar d18Ovalues. The timing of this is speculative, but theMesoproterozoic gneisses are locally sheared and retro-gressed to schist along mylonitic shear zones like the onethat cuts the roof pendant (de Beer, 2010) and has a cleareffect on whole-rock d18O (Fig. 7b). Whatever the exactage and origin of the low-d18O character of the countryrocks, the arguments that it was a pre-magmatic eventpermit the interpretation that the low-d18O magmas ofthe Koegel Fontein complex were derived from partialmelting or assimilation of the basement, and not from ahydrothermally altered earlier unit of the complex.

Isotope composition of the hydrothermalfluidQuartz veins that are contemporaneous with or youngerthan the Koegel Fontein magmatism (i.e. cutting maficdykes) have low d18O values between þ2·2 and ^1·7%(mean ^0·7%). For a reasonable temperature range forvein formation of 300^5008C, the fractionation factor�quartz^water is between 6·9% and 2·3% (Matsuhishaet al., 1979). Therefore, at 3008C, the hydrothermal fluidwould have had a d18O value of ^7·6% at 3008C or ^3%at 5008C. A similar range of fluid compositions can be esti-mated from the d18O values of feldspar separates affectedby hydrothermal fluid exchange. At 3008C, the fraction-ation factor �albite^water is 4·8% (Matsuhisa et al., 1979), sofor feldspar d18O values less than zero, the fluid wouldhave had d18O values less than ^4·8% or a higher tempera-ture. This constraint on the fluid composition holds forboth post-magmatic hydrothermal alteration and for ear-lier fluid^rock interactions that produced low-d18O rocksin the basement.The d18O value of the alteration fluid(s) can be con-

strained further with the combination of H isotope data.The interpretation assumes that the fluid was of meteoricorigin and would, therefore, plot along the GlobalMeteoric Water Line (Craig, 1961) before the onset offluid^rock exchange.The average dD value for whole-rocksof the Koegel Fontein complex is ^99% (n¼ 66). Thewhole-rock dD value represents the combination of theconstituent hydrous minerals, which formed at differenttemperatures (amphibole and biotite at magmatic tem-perature, chlorite at much lower, sub-solidus, tempera-tures) and potentially from different fluids. However, thedifferences in mineral^water fractionation factors forhigh- and low-temperature hydrous phases are such thatthey produce the same effective rock^water fractionationfactor. For example, the mineral^water fractionationfactor for hydrogen isotopes in biotite at 7008C is similarto that of chlorite at 3008C (both around �30%; Suzuoki& Epstein, 1976; O’Neil, 1986). Thus we assume a value of^30% for the H isotope exchange of the bulk-rock withfluid, and this yields an estimate of ^69% for dD of thehydrothermal fluid. Turning now to the meteoric waterline (Fig. 8b), this dD value equates to a d18O value of^9·9% for the hydrothermal fluid. As a rough check onthis estimate, we can calculate a model O isotope exchangetemperature for this fluid value and the average d18Ovalue for Cretaceous vein quartz (^0·7%). The result is2448C, which is reasonable for quartz vein formation (e.g.Sharp et al., 2005), but higher temperatures would berequired to explain the lowering of d18O values in therocks. The vein formation temperature is underestimatedby this method as it assumes a pristine meteoric watercomposition whereas the fluid from which the quartzveins precipitated would have exchanged oxygen with the

-10 -5 0 50

2

4

6

8

10

12

Nd whole-rock

18O

mag

ma

Mantle Fra

ctio

nal

crys

talli

zatio

n

BT

NWN

KG

T = Tristan (enriched)

B = Bouvet (normal)

KG = Koegel Fontein gneiss

NWN = North west Namibia

Erongo/Spitzkoppe

High Nd = 181 ppmLow Nd = 34 ppm A

ssim

ilatio

nof

low

-18

Ocr

ust

Fig. 13. Variation of magma d18O value vs whole-rock eNd for the 135Ma Damaraland complexes of NW Namibia (Trumbull et al., 2004)and the Koegel Fontein magmatic rocks. Simple mixing curves areshown between a Tristan-type (T) mantle end-member (eNd¼�1)and a more depleted mantle-end member (eNd þ4), which may rep-resent the ancestral Bouvet (B) plume or ambient asthenosphericmantle. Two crustal end-members are shown. Koegel Fontein Gneiss(KG) is based on analyses of country rocks by Curtis et al. (2011), andNW Namibian crust (NWN) is based on Damara Belt metapelitesand granites referenced by Trumbull et al. (2004). Mixing curves areshown for high (181ppm) and low (34 ppm) Nd concentrations. Tickmarks on each curve correspond to 20% intervals.


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surrounding rocks to some extent, thus raising its d18Ovalue and resulting in a higher calculated �quartz^water

temperature.The estimated dD and d18O values of the hydrothermal

fluids are significantly lower than those of present-day rain-fall and groundwater from Southern Africa (Fig. 8a), andthis is inconsistent with the latitude of the Koegel Fonteinregion in the Cretaceous. Such low dD and d18O values ofmeteoric water are typical for high latitudes, high altitudesor locations far inland from the oceans (e.g. Dansgaard,1964). The latitude effect can be ruled out for the earlyCretaceous because plate reconstructions show that the pos-ition of Southern Africa has remained relatively constant(30^408S) over the last 130 Myr (e.g. Scotese et al. 1999).The continental effect of Dansgaard (1964) may have con-tributed to lower dD and d18O values of ambient meteoricwater, as the Koegel Fontein formed in a continental rift set-ting before opening of the South Atlantic began. However,the magnitude of the continental effect is impossible toquantify. The elevation effect is possibly significant, and anexample is Mt. Kenya (4200^5200m elevation), whose gla-cial melt-water and frost have d18O values from ^6·6 toþ1·9% (Rietti-Shati et al., 2000). The palaeo-elevation ofKoegel Fontein is unknown, but fission-track data indicatea maximum of 3 km of erosion along the west coast ofSouth Africa since the Cretaceous (Gallagher & Brown,1999; Kounov et al., 2008). It has been suggested that felsicvolcanic material found in offshore wells of the OrangeBasin originated from the Koegel Fontein Complex(Verwoerd & de Beer, 2006), which would imply a volcanicedifice significantly higher than at present. However, apalaeo-elevation effect of the order of Mt. Kenya seems un-likely given that highly negative meteoric water is character-istic of mountain belts (Sheppard,1986).Finally, it is noted that all of the effects mentioned above

are factors of the palaeo-environment, setting and originalelevation of the complex and thus they would applyequally well to the anorogenic igneous complexes of NWNamibia (e.g. Brandberg, Erongo). The latter have similarsize, age and rock compositions to those of KoegelFontein, yet none of them has similarly low d18O values(Harris, 1995; Trumbull et al., 2004). Therefore, the explan-ation for low-d18O fluid in the Koegel Fontein rocksshould be independent of palaeogeography effects onmeteoric water composition at the time of intrusion. Theimplications of this are developed further below in thediscussion of the origin of the low-d18O magmas.

Origin of low-d18O magmas at KoegelFonteinThe proportion of igneous rocks at Koegel Fontein thatformed from low-d18O magmas is small, but, as mentionedabove, the quartz porphyry dykes have some of the lowestd18O values described worldwide, and understanding theirorigin is of more than regional significance. Two different

explanations for the origin of low-d18O magmas havebeen proposed (e.g. Boroughs et al., 2012): (1) partial melt-ing or assimilation of hydrothermally altered materialfrom an earlier stage of volcanism (e.g. Yellowstone,Hildreth et al., 1984; Taylor & Sheppard, 1986; Bindeman& Valley, 2001; Bindeman et al., 2008); (2) partial meltingor assimilation of crust that already had a low-d18O com-position before the onset of magmatism (e.g. Snake River,USA, Boroughs et al., 2005; eastern China, Zheng et al.,1998; Rumble et al., 2002; Wei et al., 2008). Mechanism (1)is related to areas of long-lived magmatism in rifts orlarge caldera systems (Taylor, 1986), and it also appear tobe restricted to regions where the ambient meteoric waterhas highly negative d18O values (i.e. high latitudes or highelevation). Mechanism (2) places no conditions for a par-ticular magmatic system or setting, but it is limited inpractice by the scarcity of low-d18O crustal rocks with theappropriate compositions and P^T^time position to bepartially melted or assimilated. These two end-membermodels for the formation of low-d18O magmas, to para-phrase Bindeman et al. (2007), can be abbreviated as ‘self-induced’ (remelting of hydrothermally altered precursorunits) or ‘fluke location’ (presence of low-d18O countryrocks below the site of intrusion). Here, we explore inmore detail the more plausible mechanisms for generatinglow-d18O magmas at Koegel Fontein.Typical for many intrusion-related hydrothermal sys-

tems producing low-d18O rocks is a ‘bulls-eye’ pattern ofd18O contours (e.g. the Tertiary complexes of NWScotland, Taylor & Forester, 1971; the Boulder Batholith,Gregory & Criss, 1986), with a decrease in d18O value to-wards the central intrusion that drove the hydrothermalsystem. The d18O values around the Koegel Fontein intru-sion do not show a concentric pattern of variation.Instead, samples with low d18O values occur in two areas:(1) associated with the roof pendant in the centre of thecomplex; (2) along the present coastline. Furthermore, thesampling traverse of the roof pendant revealed a concen-tration of low d18O values associated with a north^south-trending mylonite zone. This association couldsimply reflect the passage of fluid along a structural weak-ness but, as explained above, the development of internalO isotope equilibrium within the roof pendant rocks(Fig. 7a) is inconsistent with a shallow intrusion-relatedhydrothermal system, and the low d18O values cannothave resulted from interaction with magmatic fluids fromthe Rietpoort Granite because the latter would have beenclose to 7% (i.e. �magma^water near zero at magmatictemperatures).The only evidence in favour of the low d18O values at

Koegel Fontein being temporally related to the igneouscomplex is that many of the mafic and Kerskloof dykes,and the xenolith-filled mafic plug especially (d18O ^4·1%),have extremely low whole-rock d18O values, as do the late


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quartz veins that cut the mafic dykes. It is inherently un-likely that the Koegel Fontein mafic magmas intruded withlow d18O values because such magmas have been describedonly from Iceland (e.g. Muehlenbachs et al., 1974), and be-cause the strong alteration of the mafic dykes makes preser-vation of a magmatic signature unlikely. There are no freshphenocrysts present in the rocks to test this supposition. Inany case, the late quartz veins indicate that low-d18O hydro-thermal fluids accompanied the magmatic development ofthe complex. We argue below that these fluids were meta-morphic, rather than meteoric, in origin.The following evidence suggests that the Koegel Fontein

Complex intruded into an area of crust that already hadlow d18O values.

(1) The estimated d18O value of the alteration fluid ismuch lower than that expected for meteoric water inthis region at the time of intrusion (144^135 Ma).Possible factors of environment or palaeogeographythat would lower the ambient meteoric water d18Oappear insufficient, as argued above.

(2) There is no relationship between the spatial distribu-tion of low-d18O country rocks and the intrusivebodies. Instead, the low d18O values in the roof pen-dant to the Rietpoort Granite appear to be related toa pre-existing shear zone.

(3) Quartz and feldspar are in approximate O isotopeequilibrium with the whole-rock in the low-d18Oroof pendant samples. This is not expected in hydro-thermally altered rocks associated with shallow-levelvolcanic systems. It is, however, consistent withdeeper level exchange followed by slow cooling,or fluid^rock exchange followed by regionalmetamorphism.

(4) The low-d18O felsic igneous rocks also have lower ini-tial Sr isotope ratios than the ‘normal’-d18O felsicrocks. This is best explained if the Rb^Sr isotopesystem was disturbed long enough before the intrusionof Koegel Fontein to allow differences in 87Sr/86Srratios to develop.

(5) There are no low-d18O values in rocks associated withthe petrographically similar and contemporary intru-sive complexes from NW Namibia.

Possible sequence of events at KoegelFonteinSignificant volumes of water from hydrous phases in thecountry rocks would have been driven off by dehydrationreactions during magma ascent and emplacement of theKoegel Fontein complex. Although ‘metamorphic water’ isnormally thought to have relatively high d18O values (e.g.Sheppard, 1986), this is because of the generally high d18Ovalues of typical metasedimentary protoliths. Given thepresence at Koegel Fontein of reworked, retrogressed, and18O-depleted shear zones in the Namaqua basement

gneisses with d18O values as low as ^5% (xenoliths in themafic plug), some of the ‘metamorphic fluids’ in this local-ity would have had low d18O values. It is suggested thatthe early mafic dykes intruded preferentially along suchreactivated shear zones and related fractures, and thesealso provided pathways for migration of low-d18O fluids(see Fig. 7). A similar type of process, where internallyderived fluids emanated outwards, has been documentedfor the Tertiary Lilloise Intrusion, East Greenland(Sheppard et al., 1977). Further addition of heat in themagma system below Koegel Fontein would have eventu-ally resulted in partial melting of fertile rocks in the crust,giving rise to hybrid crust^mantle magmas. The d18Ovalue of the magmas produced in this way will depend onthe proportion of crust and its d18O value, which we haveshown to be highly variable. The volume of low-d18Omagma produced was small (a subset of the quartz por-phyry dykes) so selective melting, and not large volumesof crust with lowered d18O values �0%, is required. Therelatively mantle-like d18O value of the Rietpoort Granitemagma (7·1%) can be reconciled with the negative eNdvalue if the granite magma incorporated a small compo-nent (5^13 %) of low-d18O crust.The proposed sequence of events at Koegel Fontein is

summarized in Fig. 14. The timing of pre-intrusive fluid^rock interaction responsible for lowering the d18O value ofthe country rocks locally is poorly constrained. It is likelythat this occurred during the Pan-African events, which af-fected the region from about 750 to 500 Ma (e.g. Gresseet al., 2006). Pan-African reworking has been recognizedin the Koegel Fontein area (de Beer, 2010) in the form ofshear zones, reoriented foliation and retrogressed meta-morphic assemblages. One can speculate why the fluidinvolved in this early alteration event had a low d18Ovalue as inferred from the O^H isotope data (Fig. 8b).Palaeolatitude reconstructions of the Kalahari Cratonvary between about 608 and 208 between 600 and 525Ma (e.g. Tohver et al., 2006) so it is possible that fluid^rockinteraction occurred while the region was at high latitudesand ambient meteoric water would be highly negative. Itis also significant that at least two global glaciations arethought to have occurred during the late Proterozoic from1000 to 542 Ma (e.g. Maruyama & Santosh, 2008).Extreme 18O depletion (d18O as low as ^27%) in theBelomorian Belt in Russia was attributed by Bindeman &Serebryakov (2011) to fluid^rock interaction during the2·4 Ga global glaciation. Low-d18O crust could have beenproduced in the Koegel Fontein area by a similar, process,while the area was at relatively low latitudes.

CONCLUSIONS

(1) At Koegel Fontein the majority of the various igneousrock types and some of the immediate country rocks


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have d18O values 56%. These low d18O values areattributed to water^rock interaction at high tem-perature. A small number of quartz porphyry dykescrystallized from magmas that had d18O values ofabout 1·1%.

(2) The low-d18O dykes are found only intruding the roofpendant in the centre of the Rietpoort Granite.These roof rocks themselves have low d18O valuesand are also intruded by the mafic plug that containscrustal xenoliths with d18O values as low as ^5%.These data indicate the presence of low-d18O rocks atdepth beneath this part of the complex, probablyrelated to the presence of a much older shear zone.

(3) Meteoric^hydrothermal fluids were originally respon-sible for the high-temperature alteration at KoegelFontein. The H isotope composition of the hydrother-mally altered rocks suggests a dD value of ^69% forthe alteration fluid. If this fluid is of meteoric originthe d18O of the fluid would have been ^9·9%.Cretaceous quartz veins have d18O values as low as^1·7%, consistent with formation from fluids withlow d18O value. Ambient meteoric water with d18Ovalues this low is unlikely to have existed in theKoegel Fontein area at the time of intrusion. Thecomplex was at a relatively low latitude, 51000 kmfrom the nearest ocean, and there is no evidence of

SSSSSSSSSS

SSSSSSSSSS

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Fluid-rock interaction along shearzones during Pan-African (?) Regional

metamorphism

Ear

lyqu

artz

vein

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Circulation ofmeteoric fluid

(a) (b)

(c) (d)

Brittlezone

Ductilezone

She

arzo

ne

Early syenite

Maf

icdy

kes

Mantle

Mafic magma

SSSSSSSSSS

SSSSSSSSSS

SSSSSSSSSS

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Ker

sklo

ofS

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Mafic magmaintruded intomiddle crust

Crustal melting_+ input of mafic

magma

Efflux offluid from

dehydrationmelting

Por

phyr

ydy

kes

Low

-18

Oqz

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Fractionalcrystallization+ assimilation

Intrusion ofRietpoortGranite

Roof pendantPresent erosion level

Fig. 14. Schematic illustration to show the proposed stages of development of the Koegel Fontein Complex. (a) Generation of low-d18O crust byfluid^rock interaction along shear zones during Pan-African(?) regional metamorphism. (b) Emplacement of early igneous units: SandkopSyenite, Kerskloof dykes and mafic dykes. (c) Growth of magma system at depth with generation of low-d18O crustal melts and intrusion oflow-d18O quartz porphyry dykes at shallow levels. Thermal metamorphism causes efflux of dehydration fluids from altered crust to formlow-d18O quartz veins. (d) Generation of the hybrid magmas and intrusion of the Rietpoort Granite. The quartz porphyry dykes with‘normal’ d18O values may be precursor intrusions of the Rietpoort magmas or were generated in stage (c) from unaltered crustal rocks.


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high elevation around the complex. The factors thatresult in highly negative d18O values of meteoricwater were, therefore, not present when the KoegelFontein complex was emplaced.

(4) The magmas with low d18O values have similar eNdvalues but much lower initial 87Sr/86Sr ratios than thecontaminated magmas with ‘normal’ d18O values.This is best explained if the alteration event that low-ered the d18O value of the local crust also lowered itsRb/Sr ratio. This would have to have happened mil-lions of years prior to Koegel Fontein magmatism togenerate differences in 87Sr/86Sr ratio in altered andunaltered crust at 135 Ma.

(5) Based on Sr and Nd isotope data, the earlyRooivleitjie Granite and the Sandkop Syenite are es-sentially mantle-derived, whereas the RietpoortGranite and the quartz porphyry dykes contain ahigh component of assimilated crust (Curtis et al.,2011). The d18O value of the contaminated magmaswas variable, with a bimodal distribution. Themagma d18O value reflects the d18O value of the crustthat was melted (or assimilated), and this in turn wascontrolled by the degree of prior fluid^rock inter-action. In this way the mantle-like d18O value of theRietpoort Granite can be reconciled with the negativeeNd values that are consistent with derivation fromthe crust.

(6) Mafic dykes with low d18O whole-rock values were af-fected by hydrothermal alteration synchronouswith the emplacement of the complex. They are cutby the low-d18O quartz veins. It is suggested that thefluid was metamorphic in origin, derived by dehy-dration of altered low-d18O country rock that washeated by the initial intrusion of the Koegel Fonteinmagmas.

ACKNOWLEDGEMENTSWe are grateful to Fayrooza Rawoot for assistance in theStable Isotope Laboratory, to David Reid for provision ofsome samples, and to Paul Macey for assistance in thefield. David Reid, Peter Larson, Anton le Roex andValentin Troll are thanked for constructive comments onthe thesis of C.G.C. Reviews by Scott Boroughs,Kurt Knesel, an anonymous reviewer, and editorial com-ments by Jim Beard led to significant improvements to thepaper.

FUNDINGWe are grateful to the Inkaba y Africa and the NationalResearch Foundation (South Africa) Incentive fundingprogrammes for financial support.

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