Early–Middle Jurassic Dolerite Dykes from Western Dronning ... · Basaltic lavas and minor...

Early–Middle Jurassic Dolerite Dykes fromWestern Dronning Maud Land (Antarctica):Identifying Mantle Sources in the KarooLarge Igneous Province

TEAL R. RILEY1*, PHILIP T. LEAT1, MICHAEL L. CURTIS1,IAN. L. MILLAR1,2, ROBERT A. DUNCAN3 AND ADELA FAZEL4

1BRITISH ANTARCTIC SURVEY, NATURAL ENVIRONMENT RESEARCH COUNCIL, HIGH CROSS, MADINGLEY ROAD,

CAMBRIDGE CB3 0ET, UK

2NERC ISOTOPE GEOSCIENCES LABORATORY, KEYWORTH, NOTTINGHAM NG12 5GG, UK

3COLLEGE OF OCEANIC AND ATMOSPHERIC SCIENCES, OREGON STATE UNIVERSITY, CORVALLIS,

OR 97331-5503, USA

4DEPARTMENT OF EARTH SCIENCES, WALTON HALL, OPEN UNIVERSITY, MILTON KEYNES MK7 6AA, UK

RECEIVED DECEMBER 22, 2003; ACCEPTED FEBRUARY 9, 2005ADVANCE ACCESS PUBLICATION MARCH 30, 2005

A suite of dolerite dykes from the Ahlmannryggen region of western

Dronning Maud Land (Antarctica) forms part of the much more

extensive Karoo igneous province of southern Africa. The dyke com-

positions include both low- and high-Ti magma types, including

picrites and ferropicrites. New 40Ar/39Ar age determinations for

the Ahlmannryggen intrusions indicate two ages of emplacement at

�178 and �190Ma. Four geochemical groups of dykes have been

identified in the Ahlmannryggen region based on analyses of �60

dykes. The groups are defined on the basis of whole-rock TiO2 and

Zr contents, and reinforced by rare earth element (REE), 87Sr/86Sr

and 143Nd/144Nd isotope data. Group 1 were intruded at

�190Ma and have low TiO2 and Zr contents and a significant

Archaean crustal component, but also evidence of hydrothermal alter-

ation. Group 2 dykes were intruded at �178Ma; they have low to

moderate TiO2 and Zr contents and are interpreted to be the result of

mixing of melts derived from an isotopically depleted source with

small melt fractions of an enriched lithospheric mantle source. Group

3 dyke were intruded at �190Ma and form the most distinct

magma group; these are largely picritic with superficially mid-ocean

ridge basalt (MORB)-like chemistry ( flat REE patterns,87Sr/86Sri �0�7035, eNdi �9). However, they have very high

TiO2 (�4 wt %) and Zr (�500 ppm) contents, which is not

consistent with melting of MORB-source mantle. The Group 3

magmas are inferred to be derived by partial melting of a strongly

depleted mantle source in the garnet stability field. This group

includes several high Mg–Fe dykes ( ferropicrites), which are inter-

preted as high-temperature melts. Some Group 3 dykes also show

evidence of contamination by continental crust. Group 4 dykes are

low-K picrites intruded at �178Ma; they have very high TiO2–Zr

contents and are the most enriched magma group of the Karoo–

Antarctic province, with ocean-island basalt (OIB)-like chemistry.

Dykes of Group 1 and Group 3 are sub-parallel (ENE–WSW) and

both groups were emplaced at �190Ma in response to the same

regional stress field, which had changed by �178Ma, when

Group 2 and Group 4 dykes were intruded along a dominantly

NNE–SSW strike.

KEY WORDS: flood basalt; depleted mantle; enriched mantle;

Ahlmannryggen; Karoo dyke

INTRODUCTION

The role of mantle plumes in the generation of largeigneous provinces (LIPs) has been, and remains, a sub-ject of intense debate (e.g. Ernst & Buchan, 2001;Foulger, 2002). The magmatism of the Karoo province

*Corresponding author. Telephone: (þ44) 1223 221423. Fax: (þ44)

1223 362616. E-mail: [email protected]

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JJOURNAL OF PETROLOGY VOLUME 46 NUMBER 7 PAGES 1489–1524 2005 doi:10.1093/petrology/egi023

of southern Africa has been reliably dated (40Ar/39Ar andU–Pb) at 179–184 Ma, with a significant peak of activityat 182–183 Ma (Duncan et al., 1997). Although a definiteplume link has never been established for the Karooigneous province (e.g. Hawkesworth et al., 1999), mostworkers (e.g. Cox, 1989; Ellam & Cox, 1991; Sweeneyet al., 1994) have concluded that a significant thermalanomaly must have existed to generate the large volumeof erupted magma (>2 � 106 km3; Elliot et al., 1999) oversuch a short period of time (Duncan et al., 1997). Thegeochemistry of Karoo igneous rocks has been inter-preted by some workers to indicate either derivation ofthe magmas from an enriched lithospheric mantle source,or crustal contamination of partial melts of a lithosphericmantle source (e.g. Erlank, 1984). Other workers haveproposed that the Karoo magmas are plume-derived, butcontaminated by the lithospheric mantle en route to thesurface (Cox, 1992; Ellam et al., 1992). Central to thisdebate is the need to establish that a plume source existsand to evaluate if its role is restricted to conductive heattransfer to the base of the lithosphere, or if there is any

surface evidence of eruption of uncontaminated plume-derived magmas.

The lavas and dykes of western Dronning Maud Land,Antarctica, are generally considered as an extension ofthe Karoo large igneous province of southern Africa(Harris et al., 1991; Luttinen et al., 1998). The regionaldolerite dykes of the Ahlmannryggen area (Fig. 1) havepreviously been described by Harris et al. (1991), whomade comparisons with the composition of basalt lavasfrom the Kirwanveggen area further to the south (Harriset al., 1990), and also with the high-Ti basalts of theKaroo province (Duncan et al., 1984). This study extendsthe work of Harris et al. (1991) by providing a full geo-chemical and isotopic dataset over a broader geograph-ical area, therefore allowing comparisons to be made withmore recent data on the Karoo igneous province ofsouthern Africa (e.g. Sweeney et al., 1994), and elsewherein East Antarctica (e.g. Luttinen & Furnes, 2000).

Where sub-lithospheric mantle sources for continentalflood basalts can be identified there is mounting evidencethat they are heterogeneous in composition, with both

Fig. 1. Location map of rock outcrops in western Dronning Maud Land (Antarctica) from Vestfjella to H. U. Svedrupfjella. The inset is a pre-break-up Gondwana reconstruction of Africa and Antarctica showing the extent of the Kaapvaal–Grunehogna craton and the outcrop of Early–Middle Jurassic age Karoo igneous rocks (after Luttinen & Furnes, 2000). ODS, Okavango dyke swarm; SRBF, Sabie River Basalt Formation;RRDS, Rooi Rand dyke swarm.

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JOURNAL OF PETROLOGY VOLUME 46 NUMBER 7 JULY 2005

depleted and enriched source components (e.g. Kerr et al.,1995; Fitton et al., 1997); the magmas derived from thesecomponents include ferropicrites (Gibson et al., 2000) andlamprophyric rock types (le Roex & Lanyon, 1998;Thompson et al., 2001). The data presented here allowus to evaluate, assess and model these components, withrespect to the ‘Karoo plume’.

GEOLOGICAL SETTING

Basaltic lavas and minor intrusions of Jurassic age cropout at several localities in western Dronning Maud Land,Antarctica (Fig. 1). Both flood basalts and dykes areexposed at Vestfjella, Heimefrontfjella and Kirwanve-ggen, whereas only dykes are exposed in the Ahlman-nryggen, Mannefallknausane and H. U. Svedrupfjellaareas (Fig. 1). In the Kirwanveggen, the lavas are sub-horizontal and form a succession up to 300 m in thickness(Harris et al., 1990). The Kirwanveggen lavas overlieclastic sediments of the Amelang Plateau Formation;these sedimentary rocks overlie Proterozoic gneisses base-ment of the Svedrupfjella Group. Locally, the sediment-ary succession is absent and the lavas directly overlie thegneisses. Two of these lava flows have been dated by40Ar/39Ar geochronology (Duncan et al., 1997), withMiddle Jurassic ages of 180�6 � 0�6 and 182�8 �0�6 Ma. In Vestfjella, the thickness of the lava pile exceeds900 m in the north and 400 m in the south (Luttinen &Furnes, 2000). The lava pile is cut by dolerite dykes andsills and, at Muren and Utpostane (Fig. 1), by gabbrointrusions (Vuori & Luttinen, 2003), which have beendated at 177�0 � 0�5 Ma (40Ar/39Ar on plagioclase;Zhang et al., 2003). The age of the Vestfjella lavas ispoorly constrained, although plagioclase K–Ar ages of�180 Ma for the north Vestfjella lavas (Peters et al., 1991)provide the best age estimate available and correspond tothe age of the Kirwanveggen lavas (Duncan et al., 1997).

The basement of western Dronning Maud Land isdivided into two major domains. Prior to the Mesozoicbreak-up of Gondwana, the Archaean Grunehogna cra-ton (Fig. 1) is presumed to have been part of the Kaapvaalcraton (Groenewald et al., 1995). The craton is boundedto the east and SE by the Mesoproterozoic Maud Belt,the Antarctic extension of the Natal Belt of Africa ( Jacobset al., 1993). The exact position of the Archaean–Proterozoic lithospheric terrane boundary is not firmlyestablished, but, on the basis of gravity and aeromagneticdata, it has been interpreted to be located between 72�

and 73� S, i.e. close to north Vestfjella (Luttinen &Furnes, 2000).

The minor intrusions of the Ahlmannryggen areaintrude Neoproterozoic age rocks of the RitscherflyaSupergroup, which cover the entire Ahlmannryggenand Borg Massivet regions (Fig. 1). The RitscherflyaSupergroup, which overlies Archaean (2�8–3�0 Ga)

basement, consists of relatively undeformed sedimentaryand volcanogenic rocks of the Ahlmannryggen and Jutul-straumen groups (Wolmarans & Kent, 1982), which havebeen intruded extensively by massive tholeiitic sills anddykes of the Borgmassivet Intrusions (Wolmarans &Kent, 1982). Wolmarans & Kent (1982) reported a Rb–Sr whole-rock isochron age of 1073 � 40 Ma based onseven mafic sills from the Ahlmannryggen, therefore theBorgmassivet Intrusions could be coeval with theUmkondo large igneous province of southern Africa(1�1 Ga; Hanson et al., 1998). This date is close to theinferred lithification age of the Ritscherflya Supergroupsedimentary rocks (1085 � 27 Ma; Moyes et al., 1995) andtherefore supports the field observations of Krynauw et al.(1988) and Curtis & Riley (2003) that the BorgmassivetIntrusions were emplaced into wet, partially lithifiedsediments.

SAMPLING STRATEGY

Over 90 dykes and sills were recorded from the Ahlman-nryggen region of western Dronning Maud Land (Fig. 2).Each dyke or sill was sampled and its strike, dip, width andexact position were recorded. Based on petrography,whole-rock geochemistry and preliminary geochrono-logy, a significant (�25%) subset of these dykes and sillswere believed to be Proterozoic in age (�1100 Ma). Forty-seven Mesozoic dykes (Fig. 2) were selected for thisstudy, which were the freshest samples available and geo-chemically showed the least evidence of hydrothermalalteration.

GEOCHRONOLOGY

Previous work

Very few reliable ages have been published for the minorintrusions of western Dronning Maud Land. Mesozoicages have been reported by Wolmarans & Kent (1982),who dated an olivine-bearing dolerite dyke from NilsJorgennutane (Fig. 1) at 192 � 8 Ma (K–Ar whole rock),whereas Watters & Rex [K–Ar unpublished data cited byHarris et al. (1991)] have reported ages in the range 190–200 Ma. 40Ar/39Ar geochronology on plagioclase min-eral separates by Brewer et al. (1996) indicated two epis-odes of mafic magmatism, at 182�4 � 1�9 Ma (dolerite sill)and a younger episode at 172�4 � 2�1 Ma (basalt lava),from the nearby Heimefrontfjella area (Fig. 1). The olderepisode of magmatism has been confirmed by Duncanet al. (1997), who carried out an 40Ar/39Ar study onbasaltic lavas from Kirwanveggen (Fig. 1), which yieldedplateau ages of 180�4 � 0�6 and 182�6 � 0�6 Ma, coin-cident with the main Karoo volcanism of southern Africa(Riley & Knight, 2001). Zhang et al. (2003) have recentlycompleted a detailed 40Ar/39Ar study on a variety of

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RILEY et al. KAROO DYKES FROM ANTARCTICA

basaltic rocks from the Vestfjella region (Fig. 1) of westernDronning Maud Land, which display a broad range ofJurassic ages. They reported ages of 177�0 � 0�5 Ma forthe Utpostane gabbro and 176�6 � 0�5 Ma for a doleritedyke from the Kirwanveggen, whereas a dolerite dykefrom Basen (Fig. 1) was dated at �193 Ma. 40Ar/39Ar

geochronology data reported by Grantham (1996) for theStraumsvola and Tvora (Fig. 1) alkaline plutons (Harris &Grantham, 1993) from western Dronning Maud Landindicate intrusion ages in the range 178–182 Ma, andHarris et al. (2002) gave an age of �180 Ma for thenearby Sistefjell syenite (Fig. 1).

Fig. 2. Geographical distribution of the Ahlmannryggen minor intrusions by geochemical group. The four geochemical groups (1–4) are definedin the text. Two samples (Groups 1 and 4) from the Kirwanveggen are also shown in the inset map. The right-hand panel shows frequency strikeplots for geochemical Groups 1–4 and their mean strike directions (arrows).

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This study

Analytical methods

Whole-rock core samples of 5 mm diameter were pack-aged in evacuated quartz vials, and irradiated in theOregon State University TRIGA Reactor for 6 h at1 MW power. The neutron flux was measured usingstandard FCT-3 biotite, 28�03 Ma (Renne et al., 1994).Reactor temperatures can reach up to 270�C. Addition-ally, samples were baked at 195�C for 48 h duringextraction line pump-down to �10�9 torr.

Depending on sample composition, the incrementalheating experiment started in the range 400–600�C andwas typically complete by 1400�C using a Heine low-blank resistance furnace with a Ta/Nb crucible and Moliner. Each heating step was of 20 min duration with anadditional 5 min cooling and continued removal of activegases with St101 Zr–Al and St172 Zr–V–Fe getters.

A MAP 215-50 rare gas mass spectrometer, source at3000 V, and equipped with a Johnston MM1-1SG elec-tron multiplier at 2050 V, was used for analysis. Duringthe 15 min analysis time per mass peak height, data werecollected for 10 cycles of masses 35–40 for baselines andpeak-tops. Data were reduced and age calculations com-pleted using ArArCALC v2.2 software for 40Ar/39Argeochronology (Koppers, 2002).

Results

The 40Ar/39Ar data for five samples are presented inTable 1. The groundmass from sample Z.1801.1, anolivine basalt, yielded a six-step ‘error plateau’ (191�3 �3�2 Ma; Fig. 3a) comprising 72% of the total gas released.The ‘error plateau’ is close to a plateau age but is suffi-ciently disturbed that individual step ages are statisticallydifferent from the weighted mean age. There is a strongrecoil shape to the profile from step 5 to fusion (Fig. 3a).The corresponding errorchron age is 190�7 � 9�7 Ma,which is in close agreement with the ‘error plateau’ andmay be close to the crystallization age, although it is notcompletely reliable by the usual criteria.

Whole-rock sample Z.1804.3 is an olivine basalt with aquenched glassy texture. It has a matrix partly replacedby clay minerals, but is also characterized by some freshmicrophenocrysts. It has a classic recoil pattern withdecreasing measured step age vs temperature. Theseven-step ‘error plateau’ yields an age of 178�3 �3�7 Ma (Fig. 3b) comprising 57% of the total gas released.The corresponding total fusion age is 190�7 � 0�7 Ma.

The groundmass from sample Z.1812.1, an olivinedolerite, yielded a six-step ‘error plateau’ of 204�0 �3�0 Ma (Fig. 3c) comprising 74% of the total gas released.The ‘error plateau’ is clearly disturbed and the stronglyincreasing age toward fusion step 3 is common in excess40Ar profiles. An isochron calculated using steps 6–11produces an age of 176�8 � 9�4 Ma, which has a statist-ical measure of significance. The case for excess 40Ar isstrong in this sample, hence the isochron age is preferred,albeit with relatively large uncertainty.

Whole-rock sample Z.1814.1 is an olivine basalt, whichyielded a low-temperature four-step plateau of 190�7 �0�8 Ma (Fig. 3d), but comprising only 42% of the total gasreleased. A higher temperature, four-step plateau, com-prising 54% of the gas released, yields an age of 176�2 �0�6 Ma. If excess 40Ar is involved, then the younger agewould be preferred.

Whole-rock sample Z.1816.1 is an olivine basalt withabundant clay minerals replacing the poorly crystallizedmatrix. The four-step ‘error plateau’ yields an age of187�3 � 3�6 Ma (Fig. 3e) comprising 47% of the totalgas released, which is considered to be acceptable. Anisochron calculated using eight steps produced a statistic-ally acceptable fit with an age of 176�4 � 4�8 Ma.

40Ar/39Ar geochronology on a separate suite of Ahl-mannryggen dykes and sills was undertaken by A. Fazel(unpublished data) at the Open University (UK) andreveals a similar age pattern to the data obtained fromOregon State University. The data, which are summar-ized in Fig. 4, indicate a prominent Mesozoic peak(�65% of the intrusions yielded Mesozoic ages) composedof several small peaks at �178, 181, 188 and 198 Ma.

Table 1: 40Ar/39Ar age determinations (Ma)

Sample Material Total fusion Plateau age (steps, % 39Ar) Isochron age 40Ar/36Ar initial

Z.1801.1 groundmass 196.1 � 1.1 191.3 � 3.2 (6 of 11, 71.9) 190.7 � 9.7 302.3 � 169.3

Z.1804.3 whole rock 190.7 � 0.7 178.3 � 3.7 (7 of 13, 57.4) 177.0 � 7.7 295.7 � 2.3

Z.1812.1 groundmass 214.9 � 1.2 204.0 � 3.0 (6 of 13, 74.3) 176.8 � 9.4 978.1 � 201.5

Z.1814.1 whole rock 183.0 � 0.6 190.7 � 0.8 (4 of 12, 41.7) 190.7 � 1.6 295.1 � 20.2

176.2 � 0.6 (4 of 12, 54.0) 177.4 � 3.3 268.6 � 36.1

Z.1816.1 whole rock 224.9 � 1.7 187.3 � 3.6 (4 of 12, 46.6) 177.2 � 4.1 505.7 � 27.2

Samples irradiated at Oregon State University TRIGA reactor for 12 h at 1MW power. Neutron flux measured using FCT-3biotite monitor (28.04 � 0.16Ma, Renne et al., 1998). Samples in bold are the preferred ages.

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Fig. 3. 39Ar release spectra for samples Z.1801.1, Z.1804.3, Z.1812.1, Z.1814.1 and Z.1816.1. All samples generate plateaux but fail to satisfy thecriteria of three release steps comprising 50% of the total release. Also shown are the isochron diagrams for the five samples.

1494


Discussion

The minor intrusions from the Ahlmannryggen areaproved very difficult to date, with both laboratories(Oregon State University and Open University) experi-encing similar problems. In many cases, the criteria usedto define age plateau [(1) each fraction of the plateau isinternally concordant and overlaps with the plateau agewithin a 2s uncertainty; (2) the fractions containing theplateau are continuous and contain at least 50% of thetotal 39Ar released] were not met. There is little doubtconcerning their affinity with the Karoo volcanic prov-ince, based on the age ranges obtained, but distinguishingage differences is difficult because of the effects of altera-tion, 39Ar recoil redistribution and excess 40Ar.

Two dates that are repeated throughout this study area pre-Karoo volcanism (182 Ma) age of �190 Ma and apost-Karoo volcanism age of �178 Ma. The 178 Ma ageis in close agreement with recent age data from the

Okavango dyke swarm (Botswana), which yield40Ar/39Ar (whole-rock and plagioclase) plateau ages inthe range 178�4 � 1�1 to 180�9 � 1�3 Ma (Elburg &Goldberg, 2000; Le Gall et al., 2002; Jourdan et al.,2004a), with a magmatic peak at �178 Ma. Zhang et al.(2003) also reported ages of �177 Ma for lavas, dykesand gabbros from Vestfjella and Kirwanveggen, westernDronning Maud Land (Fig. 1). The study by Zhang et al.(2003) highlighted similar problems to those of thisstudy, and they noted that the majority of their plagio-clase samples yielded discordant age spectra, which theyinterpreted as reflecting alteration, excess 40Ar and recoilredistribution.

The pre-Karoo ages of �190 Ma are problematicbecause dates of �10 Myr older than the main Karoopeak at 182 Ma have not been considered as viablecrystallization ages before. Zhang et al. (2003) dated pla-gioclase from a Vestfjella dolerite dyke at 193�0 Ma, but

Fig. 3. Continued.

1495


discounted this date as discordant. Jourdan et al. (2004a)also published an integrated age of 191�5 � 8�4 Ma for adyke from the Okavango swarm using their ‘speedy’step-heating experiments. Several of the Ahlmannryggendykes yield ages of �190 Ma, although there is a scient-ific case for some of them also to be interpreted as�178 Ma if different steps at different temperatures areused for calculation, or if an isochron or errorchron age isadopted instead of a plateau age.

WHOLE-ROCK MAJOR AND TRACE

ELEMENT AND Sr–Nd ISOTOPE

GEOCHEMISTRY

Analytical techniques

Powders for geochemical analysis were prepared from2–3 kg of fresh rock. Samples were reduced to pass a1700mm sieve using a hardened steel fly press. Thepowders were produced using an agate Tema-mill. Srand Nd isotope compositions were measured at theNERC Isotope Geosciences Laboratory (Keyworth, UK)on a Finnegan-MAT 262 mass-spectrometer. Rb–Srand Sm–Nd analysis followed procedures describedby Pankhurst & Rapela (1995). Sr isotope compositionwas determined in multidynamic peak-jumping mode.During the period of analysis, 32 analyses of the Sr iso-tope standard NBS987 gave a value of 0�710250 �0�000016 (2s errors). Nd-isotope composition was deter-mined in static collection mode. Thirty-one analyses of

the in-house J&M Nd isotope standard gave a value of0�511199 � 22 (2s errors); reported 143Nd/144Nd valueswere normalized to a value of 0�511130 for this standard,equivalent to 0�511864 for La Jolla.

Major and trace element analysis [Cr, Ni, V, Zr byX-ray fluorescence (XRF) in Table 2] was by standardXRF techniques at the Department of Geology, Univer-sity of Keele, with methods fully detailed by Floyd (1985).Higher precision trace element abundances were deter-mined by inductively coupled plasma mass spectrometry(ICP-MS) at the University of Durham. The analyticalmethods, precision, and detection limits have beendetailed by Ottley et al. (2003).

Classification

Full major and trace element analyses of the Ahlman-nryggen dykes are presented in Table 2. The data exhibitsignificant variations in SiO2, TiO2, K2O, Al2O3, Fe2O3

and MgO. The analysed samples are subalkaline andrange in composition from basalt to basaltic andesite(Fig. 5). On the basis of their CIPW norms ( Yoder &Tilley, 1962) the majority of the samples can be classifiedas quartz tholeiites, with two samples classifying as olivinetholeiites, which may be the result of olivine accumula-tion (Fig. 5 and Table 2). It is clear from major elementvariation diagrams (Fig. 6) that the rocks fall into twoclear groupings and the main discriminant between thetwo groups is MgO content; one group has MgO con-tents typically <8 wt % and the other has MgO contents>8 wt %. The other key observations are the high TiO2

Fig. 4. Cumulative probability curve for the ages of basic minor intrusions of western Dronning Maud Land (A. Fazel, unpublished data). Themain peak includes two smaller peaks at �178 Ma and 188 Ma, which correspond closely to the proposed intrusive episodes at 178 and 190 Ma.

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Table2:Geochemicalandisotopiccompositionsofminorintrusivesfrom

theAhlmannryggen,western

DronningMaudLand(Antarctica)

Sam

ple:

Z.1801.1

Z.1801.2

Z.1814.1

Z.1814.2

Z.1814.3

Z.1814.4

Z.1805.1

Z.1808.1

Z.1810.1

Z.1822.1

Z.1823.1

Z.1828.1

CIPW:

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Group:

11

11

11

22

22

22

Latitude(S):

73. 1572

73. 1573

72. 0367

72. 0367

72. 0367

72. 0367

72. 2218

72. 1443

72. 2775

72. 0176

72. 0133

71. 9951

Longitude(W

):2.1363

2.1363

2.7986

2.7986

2.7986

2.7986

3.4170

3.1806

3.4239

3.3713

3.3656

3.3152

Altitude(m

):2020

2020

1202

1202

1202

1202

1304

1411

1631

1342

1354

1154

Dykestrike

(deg

.):

067

074

068

060

052

049

007

179

002

177

171

019

Dykewidth

(cm):

93112

96191

1110

409

2500

1500

387900

33

SiO

250. 69

50. 63

49. 96

50. 34

49. 32

55. 66

50. 68

47. 91

49. 57

48. 89

48. 57

49. 13

TiO

21.53

1.52

2.27

2.22

2.24

2.26

2.20

2.39

2.30

2.34

2.49

2.28

Al 2O3

11. 75

11. 73

13. 45

13. 45

13. 18

14. 62

14. 08

13. 44

13. 86

13. 66

13. 29

14. 08

Fe 2O3(T)

12. 17

12. 49

15. 47

15. 04

15. 17

11. 21

13. 44

15. 60

14. 81

15. 38

16. 26

14. 54

MnO

0.19

0.17

0.20

0.20

0.18

0.16

0.20

0.21

0.21

0.23

0.22

0.21

MgO

8.57

8.16

5.56

5.57

5.32

5.46

5.45

5.43

6.10

5.67

5.68

5.80

CaO

10. 00

10. 15

9.27

9.26

9.42

8.68

9.89

10. 25

9.78

9.80

9.41

10. 23

Na 2O

2.05

2.28

2.68

2.54

2.85

1.92

2.37

2.47

2.50

2.15

2.63

2.34

K2O

0.77

0.58

0.78

0.61

0.39

1.27

0.35

0.23

0.34

0.19

0.50

0.20

P2O5

0.18

0.18

0.22

0.21

0.21

0.12

0.24

0.24

0.24

0.25

0.25

0.24

LOI

1.67

2.62

0.64

0.97

1.10

0.56

1.30

1.23

0.50

1.74

0.24

1.42

Total

99. 59

100.52

100.49

100.40

99. 38

100.58

100.20

99. 40

100.23

100.30

99. 56

100.47

Sc

24. 4

24. 3

29. 6

30. 1

30. 8

30. 3

32. 0

33. 6

33. 1

45. 0

44. 7

35. 1

V258.2

260.1

424.8

418.6

420.4

424.8

327.2

341.6

336.0

423.6

430.2

343.4

Cr

668

708

7577

7876

8462

8472

7190

Co

52. 5

54. 9

51. 8

51. 0

51. 5

52. 2

44. 3

44. 8

46. 3

51. 6

51. 9

46. 4

Ni

294

318

8481

8683

6443

6662

6269

Cu

94. 9

98. 3

204.7

201.2

202.3

205.5

100.2

76. 6

97. 2

104.9

106.8

99. 1

Zn

92. 9

96. 2

124.4

123.9

123.8

124.4

118.9

131.9

118.0

149.0

141.2

110.4

Ga

17. 5

17. 4

22. 6

22. 6

22. 5

22. 7

19. 5

19. 4

19. 6

21. 4

22. 1

20. 1

Rb

14. 1

25. 5

12. 6

17. 5

3.3

12. 9

6.5

3.7

6.9

3.7

11. 9

3.4

Sr

248

340

288

281

299

356

181

196

213

208

225

200

Y27. 2

27. 4

30. 2

29. 3

29. 5

30. 1

34. 4

34. 2

34. 3

36. 0

37. 9

34. 6

Zr

122

123

164

163

160

163

159

155

153

159

167

154

Nb

7.2

7.2

9.6

9.5

9.3

9.4

11. 9

11. 7

11. 9

12. 1

12. 7

11. 7

Cs

0.3

1.1

0.2

0.4

1.3

7.8

0.6

0.4

0.4

2.9

0.6

1.9

Ba

310

313

267

208

405

314

115

91100

85129

109

La

14. 70

14. 83

17. 42

16. 50

17. 22

17. 54

12. 80

12. 08

12. 34

12. 56

13. 03

13. 13

1497


Table2:continued

Sam

ple:

Z.1801.1

Z.1801.2

Z.1814.1

Z.1814.2

Z.1814.3

Z.1814.4

Z.1805.1

Z.1808.1

Z.1810.1

Z.1822.1

Z.1823.1

Z.1828.1

CIPW:

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Group:

11

11

11

22

22

22

Latitude(S):

73. 1572

73. 1573

72. 0367

72. 0367

72. 0367

72. 0367

72. 2218

72. 1443

72. 2775

72. 0176

72. 0133

71. 9951

Longitude(W

):2.1363

2.1363

2.7986

2.7986

2.7986

2.7986

3.4170

3.1806

3.4239

3.3713

3.3656

3.3152

Altitude(m

):2020

2020

1202

1202

1202

1202

1304

1411

1631

1342

1354

1154

Dykestrike

(deg

.):

067

074

068

060

052

049

007

179

002

177

171

019

Dykewidth

(cm):

93112

96191

1110

409

2500

1500

387900

33

Ce

31. 79

32. 07

38. 73

36. 74

38. 07

38. 59

30. 62

29. 34

29. 87

30. 59

31. 92

31. 47

Pr

4.35

4.39

5.60

5.31

5.45

5.51

4.57

4.40

4.47

4.79

4.99

4.84

Nd

18. 87

19. 00

24. 64

23. 28

23. 78

24. 51

21. 35

20. 85

21. 03

22. 45

23. 44

22. 51

Sm

4.49

4.54

5.95

5.52

5.71

5.86

5.48

5.34

5.46

5.84

6.03

5.76

Eu

1.40

1.42

1.94

1.79

1.87

1.90

1.79

1.81

1.78

1.83

2.00

1.86

Gd

5.20

5.34

6.42

6.07

6.16

6.43

6.46

6.38

6.38

6.56

6.90

6.36

Tb

0.85

0.86

0.99

0.94

0.96

0.99

1.04

1.01

1.02

1.07

1.12

1.04

Dy

4.88

4.94

5.60

5.36

5.49

5.62

5.99

5.91

5.98

6.43

6.69

6.11

Ho

0.95

0.97

1.09

1.07

1.08

1.10

1.22

1.19

1.20

1.34

1.37

1.25

Er

2.46

2.51

2.78

2.64

2.76

2.76

3.25

3.19

3.20

3.50

3.60

3.35

Tm

0.39

0.40

0.43

0.41

0.42

0.43

0.53

0.52

0.52

0.56

0.57

0.53

Yb

2.21

2.25

2.45

2.29

2.35

2.41

3.08

3.02

3.00

3.25

3.34

3.12

Lu

0.34

0.35

0.38

0.36

0.37

0.37

0.49

0.49

0.48

0.52

0.54

0.50

Hf

3.12

3.17

4.32

4.24

4.17

4.37

4.05

3.91

3.92

4.17

4.49

4.04

Ta

0.45

0.44

0.60

0.59

0.59

0.60

0.77

0.75

0.77

0.79

0.84

0.75

Pb

4.18

4.13

4.45

4.66

4.54

4.22

11. 47

3.93

2.00

3.83

2.59

5.96

Th

1.86

1.86

2.47

2.38

2.38

2.42

1.35

1.22

1.22

1.10

1.17

1.26

U0.57

0.65

0.46

0.45

0.45

0.46

0.49

0.43

0.44

0.41

0.42

0.45

Nb/N

b*

0.46

0.47

0.49

0.51

0.49

0.49

0.97

1.03

1.04

1.10

1.10

0.98

DNb

�0.09

�0.09

�0.17

�0.18

�0.17

�0.17

0.00

0.02

0.03

0.03

0.03

0.02

87Rb/8

6Sr

0.1637

0.2168

0.1259

0.1047

0.1038

0.0547

0.0936

0.0508

0.1531

87Sr/

86Sr m

easured

0.707603

0.707885

0.706688

0.708805

0.704599

0.704049

0.704797

0.703554

0.704753

87Sr/

86Sr norm

alized

0.707608

0.70789

0.706693

0.70881

0.704604

0.704054

0.704802

0.703559

0.704758

87Sr/

86Sr 180

0.707189

0.707335

0.706371

0.708542

0.704338

0.703914

0.704563

0.703429

0.704366

147Sm/1

44Nd

0.1514

0.1511

0.1484

0.1479

0.1588

0.1592

0.1605

0.1607

0.1601

143Nd/1

44Ndmeasured

0.512339

0.512324

0.512338

0.512351

0.512699

0.512739

0.512751

0.512739

0.512743

143Nd/1

44Ndnorm

alized

0.512271

0.512256

0.51227

0.512283

0.512631

0.512671

0.512682

0.512671

0.512675

eNd180

�6.1

�6.4

�6.1

�5.8

0.7

1.5

1.7

1.5

1.6

Age(M

a)191.3

190.7

1498


Sam

ple:

Z.1828.3

Z.1828.4

Z.1828.5

Z.1830.1

Z.1835.2

Z.1835.3

Z.1835.4

Z.1839.1

Z.1839.2

Z.1803.1

Z.1803.2

Z.1803.3

CIPW:

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Group:

22

22

22

22

23

33

Latitude(S):

71. 9948

71. 9948

71. 9948

72. 0207

72. 0538

72. 0538

72. 0538

72. 0485

72. 0535

72. 1355

72. 1355

72. 1355

Longitude(W

):3.3113

3.3113

3.3111

3.3704

3.3940

3.3940

3.3940

3.3909

3.3927

3.3041

3.3041

3.3041

Altitude(m

):1147

1147

1153

1198

1348

1346

1348

1307

1321

1529

1529

1534

Dykestrike

(deg

.):

158

163

176

016

033

033

027

032

042

045

Dykewidth

(cm):

172

1330

3000

4143

161

43358

281

SiO

249. 06

49. 79

49. 08

49. 11

48. 85

47. 08

46. 70

49. 28

48. 68

48. 37

48. 46

48. 47

TiO

22.17

2.29

2.19

2.62

2.46

2.53

2.42

2.46

2.41

4.00

3.93

3.42

Al 2O3

13. 96

13. 86

13. 19

13. 20

13. 12

12. 48

13. 96

13. 22

12. 88

11. 83

11. 69

9.71

Fe 2O3(T)

14. 11

14. 04

14. 18

16. 27

15. 08

16. 52

16. 74

15. 74

15. 31

14. 17

13. 54

13. 94

MnO

0.20

0.17

0.20

0.23

0.21

0.24

0.19

0.22

0.25

0.17

0.16

0.17

MgO

5.83

5.69

5.65

5.18

5.66

5.73

5.66

5.53

5.44

8.52

9.35

11. 17

CaO

9.81

10. 16

10. 37

9.41

9.64

10. 46

10. 54

9.43

10. 11

10. 29

9.90

9.92

Na 2O

2.28

2.61

2.18

2.57

2.39

2.60

2.10

2.44

2.21

1.81

1.86

1.62

K2O

0.44

0.37

0.17

0.43

0.30

0.42

0.27

0.39

0.28

0.43

0.41

0.34

P2O5

0.23

0.24

0.23

0.26

0.24

0.23

0.25

0.24

0.24

0.24

0.24

0.21

LOI

2.06

1.57

1.90

0.38

1.34

1.12

1.76

1.02

1.57

0.69

0.66

1.07

Total

100.15

100.30

99. 53

99. 65

99. 29

99. 40

100.60

99. 97

99. 40

100.50

100.20

100.04

Sc

36. 2

35. 3

35. 2

38. 3

40. 1

40. 8

40. 2

40. 1

39. 8

29. 6

30. 0

29. 4

V355.7

349.3

350.7

404.5

406.8

415.8

410.0

415.5

404.9

344.7

344.7

311.7

Cr

9191

9050

7376

7564

70479

501

705

Co

47. 6

46. 5

47. 3

50. 0

52. 3

53. 2

52. 9

53. 1

52. 1

57. 9

59. 0

64. 7

Ni

7171

7053

6567

6963

64390

411

576

Cu

93. 9

94. 3

96. 2

110.6

107.7

103.0

98. 6

106.7

100.1

146.2

145.3

122.3

Zn

92. 6

85. 1

100.0

125.9

105.4

100.5

107.8

110.6

97. 1

143.7

138.7

127.0

Ga

20. 0

20. 3

20. 0

22. 1

21. 6

21. 9

22. 2

22. 4

21. 4

21. 0

20. 6

18. 3

Rb

13. 8

9.5

3.4

7.7

17. 5

19. 0

6.7

8.4

7.8

11. 6

11. 3

8.8

Sr

197

193

193

214

220

223

189

217

197

257

246

219

Y33. 8

34. 5

34. 7

39. 8

37. 3

37. 9

36. 7

38. 5

37. 0

43. 7

43. 4

37. 7

Zr

150

155

155

177

164

168

161

172

163

275

271

232

Nb

11. 4

11. 6

11. 6

13. 5

12. 3

12. 6

12. 1

12. 8

12. 2

10. 0

9.9

8.4

Cs

1.9

1.7

2.3

0.5

25. 7

21. 4

1.2

3.0

2.8

1.1

1.1

1.0

Ba

165

196

100

123

128

146

153

110

113

112

104

84

La

12. 56

12. 39

12. 80

14. 54

13. 10

13. 35

13. 28

13. 75

12. 99

10. 98

10. 97

9.26

Ce

30. 52

30. 80

31. 15

35. 66

32. 17

33. 02

32. 46

33. 93

32. 20

30. 82

30. 59

25. 85

1499


Table2:continued

Sam

ple:

Z.1828.3

Z.1828.4

Z.1828.5

Z.1830.1

Z.1835.2

Z.1835.3

Z.1835.4

Z.1839.1

Z.1839.2

Z.1803.1

Z.1803.2

Z.1803.3

CIPW:

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Group:

22

22

22

22

23

33

Latitude(S):

71. 9948

71. 9948

71. 9948

72. 0207

72. 0538

72. 0538

72. 0538

72. 0485

72. 0535

72. 1355

72. 1355

72. 1355

Longitude(W

):3.3113

3.3113

3.3111

3.3704

3.3940

3.3940

3.3940

3.3909

3.3927

3.3041

3.3041

3.3041

Altitude(m

):1147

1147

1153

1198

1348

1346

1348

1307

1321

1529

1529

1534

Dykestrike

(deg

.):

158

163

176

016

033

033

027

032

042

045

Dykewidth

(cm):

172

1330

3000

4143

161

43358

281

Pr

4.71

4.79

4.84

5.54

5.06

5.18

5.06

5.32

5.08

5.39

5.34

4.50

Nd

21. 98

22. 26

22. 48

26. 05

23. 83

24. 33

23. 62

25. 01

23. 92

29. 28

29. 00

24. 54

Sm

5.59

5.70

5.75

6.67

6.13

6.21

6.06

6.41

6.17

9.36

9.24

7.85

Eu

1.80

1.83

1.85

2.20

1.99

2.02

2.04

2.09

1.97

3.15

3.10

2.64

Gd

6.27

6.30

6.36

7.41

6.66

6.77

6.60

6.91

6.68

10. 98

10. 85

9.35

Tb

1.01

1.03

1.03

1.21

1.10

1.12

1.09

1.14

1.10

1.68

1.64

1.41

Dy

6.01

6.09

6.15

7.14

6.46

6.64

6.50

6.78

6.48

8.88

8.77

7.57

Ho

1.23

1.24

1.25

1.45

1.32

1.34

1.31

1.36

1.32

1.62

1.60

1.37

Er

3.25

3.26

3.33

3.86

3.49

3.57

3.48

3.62

3.49

3.85

3.81

3.25

Tm

0.52

0.53

0.54

0.62

0.56

0.58

0.56

0.59

0.55

0.56

0.55

0.47

Yb

3.04

3.07

3.16

3.59

3.25

3.28

3.22

3.35

3.24

3.04

3.03

2.58

Lu

0.49

0.49

0.50

0.58

0.52

0.53

0.51

0.54

0.51

0.44

0.44

0.38

Hf

4.00

4.02

4.08

4.75

4.23

4.34

4.23

4.45

4.17

7.37

7.28

6.16

Ta

0.74

0.75

0.76

0.87

0.79

0.81

0.79

0.83

0.78

0.68

0.68

0.57

Pb

3.34

2.98

4.73

2.21

2.49

2.60

3.04

1.90

2.37

2.13

2.08

1.81

Th

1.18

1.21

1.20

1.25

1.10

1.12

1.09

1.15

1.10

1.15

1.18

1.03

U0.43

0.44

0.43

0.45

0.39

0.40

0.39

0.41

0.39

0.34

0.34

0.30

Nb/N

b*

1.01

1.02

1.01

1.07

1.10

1.10

1.08

1.09

1.09

0.96

0.94

0.92

DNb

0.03

0.02

0.02

0.03

0.02

0.02

0.02

0.01

0.02

�0.43

�0.43

�0.43

87Rb/8

6Sr

0.051

0.2468

0.1146

0.1302

0.1165

87Sr/

86Sr m

easured

0.703966

0.705028

0.70403

0.705728

87Sr/

86Sr norm

alized

0.703971

0.705033

0.704035

0.705842

0.705733

87Sr/

86Sr 180

0.703840

0.704401

0.703742

0.705509

0.705435

147Sm/1

44Nd

0.1589

0.1603

0.1608

0.2019

0.2052

143Nd/1

44Ndmeasured

0.512748

0.512744

0.512728

0.512997

143Nd/1

44Ndnorm

alized

0.512679

0.512671

0.51266

0.512902

0.512928

eNd180

1.7

1.5

1.3

5.0

5.5

Age(M

a)

1500


Sam

ple:

Z.1803.4

Z.1803.5

Z.1812.1

Z.1812.2

Z.1812.3

Z.1812.5

Z.1813.1

Z.1816.1

Z.1816.2

Z.1816.3

Z.1817.2

Z.1834.3

CIPW:

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Oliv

Th

Qtz

Th

Qtz

Th

Group:

33

33

33

33

33

33

Latitude(S):

72. 1355

72. 1355

72. 0505

72. 0501

72. 0496

72. 0498

72. 0531

72. 0545

72. 0545

72. 0545

72. 0605

72. 0743

Longitude(W

):3.3041

3.3041

2.7160

2.7136

2.7077

2.7091

2.7399

2.7124

2.7124

2.7124

2.7133

3.4154

Altitude(m

):1529

1529

1071

1071

1098

1132

1257

1242

1245

1242

1306

1347

Dykestrike

(deg

.):

106

092

066

082

066

093

066

053

084

077

110

Dykewidth

(cm):

330

20193

110

8161

230

73143

180

100

SiO

248. 55

47. 83

45. 64

47. 62

48. 20

46. 96

45. 22

47. 35

45. 50

45. 78

46. 54

46. 74

TiO

24.19

3.53

3.99

4.21

4.04

4.60

3.50

3.25

3.76

3.25

3.86

4.85

Al 2O3

12. 60

10. 32

8.15

9.09

9.30

9.98

9.59

9.13

8.00

8.14

10. 22

11. 47

Fe 2O3(T)

13. 91

14. 19

15. 03

14. 61

14. 24

14. 71

14. 73

13. 92

14. 62

12. 27

14. 44

14. 03

MnO

0.17

0.17

0.19

0.17

0.17

0.17

0.17

0.16

0.17

0.18

0.17

0.17

MgO

7.63

11. 47

13. 45

11. 31

11. 68

9.96

12. 19

14. 33

14. 27

21. 61

12. 00

9.61

CaO

10. 42

9.69

9.62

10. 24

10. 10

10. 85

9.87

8.75

9.76

7.67

9.91

10. 57

Na 2O

1.87

1.64

1.31

1.32

1.31

1.44

1.65

1.39

1.22

1.25

1.57

1.79

K2O

0.47

0.27

0.20

0.19

0.18

0.20

0.32

0.20

0.14

0.55

0.27

0.28

P2O5

0.24

0.22

0.24

0.24

0.24

0.24

0.22

0.22

0.23

0.19

0.24

0.27

LOI

0.25

1.05

1.69

0.96

1.05

1.23

1.93

1.89

2.61

1.28

0.77

0.22

Total

100.30

100.40

99. 50

99. 97

100.50

100.34

99. 40

100.60

100.27

99. 50

100.00

99. 99

Sc

29. 3

28. 3

29. 7

30. 9

31. 4

32. 8

31. 5

37. 4

35. 0

32. 1

36. 0

33. 9

V351.5

311.8

302.0

327.8

327.3

358.6

338.1

374.7

333.4

315.6

368.5

412.1

Cr

397

676

1006

889

906

711

728

803

966

823

683

558

Co

50. 5

64. 8

67. 1

61. 6

64. 8

58. 8

67. 1

65. 7

74. 9

71. 3

68. 0

63. 2

Ni

251

578

619

432

467

336

597

500

769

727

578

291

Cu

157.3

128.2

121.8

134.6

132.3

150.1

140.6

145.5

130.5

128.2

154.4

134.3

Zn

140.4

131.6

132.9

137.6

150.7

146.8

128.6

146.5

149.7

127.2

139.9

125.8

Ga

21. 9

18. 4

16. 2

17. 8

17. 8

19. 7

18. 9

19. 5

16. 9

17. 6

20. 5

20. 6

Rb

8.2

11. 0

3.7

3.8

3.6

3.5

6.5

2.4

2.1

4.0

6.2

4.0

Sr

275

237

201

215

215

235

282

246

218

230

283

446

Y46. 0

38. 2

37. 7

41. 5

41. 1

45. 4

37. 4

43. 9

38. 0

35. 3

42. 0

37. 4

Zr

289

238

258

288

295

316

228

295

253

216

262

273

Nb

10. 6

8.7

7.4

8.2

8.2

9.0

9.6

8.4

7.3

3.2

10. 1

11. 8

Cs

1.8

4.6

0.3

0.3

0.4

0.2

1.6

0.4

0.6

4.3

0.5

2.0

Ba

120

102

3942

7666

7938

33132

65112

La

11. 58

9.61

6.64

7.44

7.33

8.12

9.96

7.47

6.32

7.82

9.26

11. 54

1501


Table2:continued

Sam

ple:

Z.1803.4

Z.1803.5

Z.1812.1

Z.1812.2

Z.1812.3

Z.1812.5

Z.1813.1

Z.1816.1

Z.1816.2

Z.1816.3

Z.1817.2

Z.1834.3

CIPW:

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Oliv

Th

Qtz

Th

Qtz

Th

Group:

33

33

33

33

33

33

Latitude(S):

72. 1355

72. 1355

72. 0505

72. 0501

72. 0496

72. 0498

72. 0531

72. 0545

72. 0545

72. 0545

72. 0605

72. 0743

Longitude(W

):3.3041

3.3041

2.7160

2.7136

2.7077

2.7091

2.7399

2.7124

2.7124

2.7124

2.7133

3.4154

Altitude(m

):1529

1529

1071

1071

1098

1132

1257

1242

1245

1242

1306

1347

Dykestrike

(deg

.):

106

092

066

082

066

093

066

053

084

077

110

Dykewidth

(cm):

330

20193

110

8161

230

73143

180

100

Ce

32. 26

26. 70

21. 83

24. 42

24. 05

26. 66

27. 54

24. 84

21. 00

16. 40

27. 48

33. 22

Pr

5.63

4.65

4.28

4.77

4.69

5.23

4.82

5.02

4.25

2.21

5.11

6.00

Nd

30. 80

25. 15

24. 90

27. 92

27. 47

30. 43

25. 30

29. 64

24. 97

9.26

27. 93

32. 13

Sm

9.78

7.97

8.20

9.21

9.02

10. 00

7.84

9.91

8.42

2.13

8.69

9.54

Eu

3.30

2.71

2.78

3.08

3.04

3.38

2.70

3.35

2.81

0.65

3.01

3.19

Gd

11. 53

9.47

9.83

10. 81

10. 75

11. 82

9.18

11. 36

9.62

2.35

10. 23

9.95

Tb

1.74

1.44

1.46

1.61

1.62

1.77

1.40

1.73

1.46

0.38

1.56

1.47

Dy

9.24

7.68

7.68

8.51

8.47

9.31

7.45

9.10

7.79

2.29

8.41

7.82

Ho

1.68

1.40

1.38

1.53

1.52

1.67

1.40

1.67

1.42

0.48

1.54

1.40

Er

3.99

3.34

3.26

3.60

3.54

3.94

3.29

3.88

3.34

1.25

3.58

3.29

Tm

0.58

0.49

0.47

0.52

0.51

0.57

0.48

0.57

0.49

0.20

0.52

0.47

Yb

3.17

2.64

2.51

2.78

2.76

3.03

2.57

3.01

2.58

1.22

2.81

2.48

Lu

0.46

0.39

0.36

0.41

0.40

0.44

0.38

0.43

0.37

0.21

0.42

0.37

Hf

7.68

6.30

6.76

7.56

7.68

8.23

6.19

8.05

6.80

1.51

6.93

7.28

Ta

0.72

0.59

0.51

0.57

0.58

0.64

0.66

0.59

0.51

0.21

0.69

0.83

Pb

2.17

1.90

0.87

0.83

1.15

1.51

1.33

0.79

1.03

2.92

1.10

1.37

Th

1.23

1.06

0.44

0.48

0.51

0.56

0.67

0.42

0.35

1.68

0.65

0.77

U0.36

0.31

0.16

0.18

0.19

0.20

0.23

0.15

0.14

1.38

0.22

0.23

Nb/N

b*

0.95

0.92

1.46

1.47

1.43

1.43

1.26

1.61

1.65

0.30

1.39

1.34

DNb

�0.43

�0.43

�0.58

�0.58

�0.60

�0.58

�0.36

�0.57

�0.56

�0.50

�0.41

�0.42

87Rb/8

6Sr

0.1344

0.0539

0.0511

0.0489

0.0432

0.0668

0.0284

0.0274

0.0502

0.0632

0.0258

87Sr/

86Sr m

easured

0.706492

0.703847

0.703678

0.70377

0.703903

0.704238

0.703642

0.703583

0.704051

0.703813

0.705382

87Sr/

86Sr norm

alized

0.706497

0.703852

0.703683

0.703775

0.703908

0.704243

0.703647

0.703588

0.704056

0.703818

0.705387

87Sr/

86Sr 180

0.706153

0.703714

0.703552

0.703650

0.703798

0.704072

0.703574

0.703518

0.703928

0.703656

0.705321

147Sm/1

44Nd

0.2023

0.2131

0.2095

0.2066

0.2077

0.2003

0.209

0.212

0.2069

0.2028

0.191

143Nd/1

44Ndmeasured

0.512983

0.513157

0.513162

0.513158

0.513154

0.513068

0.513181

0.513181

0.513084

0.513081

0.512976

143Nd/1

44Ndnorm

alized

0.512914

0.513089

0.513089

0.513089

0.513086

0.512999

0.513113

0.513108

0.513015

0.513008

0.512903

eNd180

5.3

8.4

8.5

8.6

8.5

7.0

9.0

8.8

7.1

7.1

5.3

Age(M

a)204

187.3

1502


Sam

ple:

Z.1804.3

Z.1825.1

Z.1825.3

Z.1826.1

Z.1826.2

Z.1831.5

Z.1833.1

Z.1833.2

Z.1838.1

Z.1653.2

A3091

CIPW:

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Oliv

Th

Qtz

Th

Qtz

Th

Group:

44

44

44

44

44

4

Latitude(S):

72. 2537

71. 9943

71. 9941

71. 9923

71. 9923

72. 0426

72. 0372

72. 0369

71. 9572

74. 0583

74. 0600

Longitude(W

):3.3770

3.3506

3.3515

3.3606

3.3606

3.5346

3.5064

3.5056

3.3229

6.3001

6.3000

Altitude(m

):1302

1227

1236

1296

1296

1210

1203

1185

1066

2248

2240

Dykestrike

(deg

.):

014

030

028

008

044

178

174

018

025

Dykewidth

(cm):

449

2632

14240

7333

225

46

SiO

249. 16

47. 86

48. 10

47. 97

47. 82

54. 81

45. 78

45. 23

46. 35

48. 31

50. 22

TiO

23.87

5.28

4.90

4.27

4.30

4.02

3.94

4.02

3.99

4.09

4.24

Al 2O3

9.38

9.42

9.37

8.57

8.39

9.97

8.65

7.98

7.51

14. 45

13. 64

Fe 2O3(T)

12. 71

12. 82

12. 78

12. 89

12. 91

10. 11

12. 60

13. 03

12. 75

14. 27

13. 09

MnO

0.17

0.15

0.15

0.15

0.15

0.14

0.15

0.16

0.15

0.18

0.18

MgO

11. 25

8.92

11. 55

14. 01

14. 33

5.77

14. 70

14. 44

15. 84

5.46

5.00

CaO

9.44

10. 11

9.12

8.50

8.41

7.70

8.13

8.37

7.74

8.99

9.52

Na 2O

1.66

2.03

1.68

1.43

1.29

1.79

1.73

1.81

1.82

2.55

2.62

K2O

0.89

0.83

0.55

0.42

0.41

3.60

0.69

0.71

0.80

0.97

1.00

P2O5

0.29

0.34

0.35

0.33

0.33

0.40

0.38

0.36

0.36

0.36

0.51

LOI

0.99

1.62

1.85

2.05

1.91

1.21

3.75

3.33

2.28

0.93

1.59

Total

99. 81

99. 40

100.40

100.60

100.25

99. 52

100.50

99. 45

99. 60

100.57

101.61

Sc

26. 5

38. 1

36. 1

34. 2

34. 8

25. 6

27. 3

28. 3

27. 0

28. 3

32. 90

V283.7

362.5

340.9

319.3

318.2

260.7

281.8

283.4

274.1

342.0

357

Cr

702

652

733

834

865

390

859

918

965

125

127

Co

53. 4

49. 3

56. 7

66. 4

66. 4

40. 0

71. 2

71. 1

72. 2

44. 1

Ni

419

226

425

666

675

160

787

789

885

8576

Cu

139.8

160.4

151.4

140.3

137.4

95. 9

133.1

132.7

122.4

268.0

266.0

Zn

120.5

150.7

149.0

140.6

137.2

91. 7

103.3

100.6

95. 7

145.0

125.0

Ga

16. 4

20. 0

18. 8

17. 0

16. 5

18. 7

17. 5

17. 3

15. 8

24. 5

21. 0

Rb

12. 0

30. 4

25. 5

21. 9

19. 8

59. 2

31. 1

33. 4

54. 0

41. 6

46. 9

Sr

509

817

774

584

569

557

983

1005

775

549

615

Y33. 4

46. 7

43. 4

38. 6

38. 2

35. 6

34. 9

34. 8

34. 6

44. 4

45. 9

Zr

343

517

481

413

408

568

479

477

444

373

344

Nb

22. 5

29. 5

27. 7

22. 0

21. 1

30. 4

27. 7

27. 5

32. 6

34. 4

31. 7

Cs

1.1

4.8

2.5

5.1

4.3

0.3

16. 0

14. 3

2.1

1.7

1.7

Ba

314

512

465

337

319

1352

713

702

561

797

947

La

26. 80

42. 19

38. 75

27. 68

27. 14

67. 00

60. 92

59. 79

47. 50

41. 76

43. 18

Ce

66. 01

103.43

94. 63

71. 15

70. 24

150.04

140.38

137.87

111.49

93. 21

92. 43

1503


Table2:continued

Sam

ple:

Z.1804.3

Z.1825.1

Z.1825.3

Z.1826.1

Z.1826.2

Z.1831.5

Z.1833.1

Z.1833.2

Z.1838.1

Z.1653.2

A3091

CIPW:

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Qtz

Th

Oliv

Th

Qtz

Th

Qtz

Th

Group:

44

44

44

44

44

4

Latitude(S):

72. 2537

71. 9943

71. 9941

71. 9923

71. 9923

72. 0426

72. 0372

72. 0369

71. 9572

74. 0583

74. 0600

Longitude(W

):3.3770

3.3506

3.3515

3.3606

3.3606

3.5346

3.5064

3.5056

3.3229

6.3001

6.3000

Altitude(m

):1302

1227

1236

1296

1296

1210

1203

1185

1066

2248

2240

Dykestrike

(deg

.):

014

030

028

008

044

178

174

018

025

Dykewidth

(cm):

449

2632

14240

7333

225

46

Pr

10. 15

16. 24

14. 94

11. 69

11. 57

22. 46

20. 64

20. 27

16. 69

13. 04

11. 72

Nd

47. 38

75. 70

68. 86

55. 57

55. 24

96. 61

85. 58

84. 50

72. 24

58. 09

52. 49

Sm

10. 89

16. 98

15. 57

12. 95

13. 11

17. 97

15. 65

15. 41

14. 55

12. 28

13. 01

Eu

3.27

5.02

4.58

3.94

3.95

4.63

4.26

4.18

3.98

3.68

3.94

Gd

10. 42

15. 30

14. 02

12. 09

11. 93

13. 26

11. 94

11. 69

11. 40

11. 80

11. 96

Tb

1.41

2.07

1.88

1.67

1.67

1.68

1.59

1.55

1.52

1.67

1.78

Dy

7.05

10. 40

9.45

8.40

8.38

8.07

7.77

7.64

7.53

8.83

9.91

Ho

1.23

1.82

1.67

1.47

1.49

1.36

1.32

1.30

1.29

1.62

1.79

Er

2.89

4.11

3.75

3.32

3.36

3.05

3.03

2.96

2.96

3.91

4.34

Tm

0.42

0.59

0.53

0.47

0.47

0.43

0.42

0.42

0.42

0.59

0.57

Yb

2.25

3.07

2.86

2.46

2.52

2.32

2.23

2.18

2.22

3.29

3.24

Lu

0.33

0.45

0.41

0.37

0.37

0.34

0.33

0.32

0.32

0.50

0.48

Hf

8.68

13. 77

12. 76

10. 82

10. 65

14. 41

12. 18

11. 98

11. 17

9.17

Ta

1.52

2.00

1.83

1.50

1.44

1.94

1.80

1.77

2.33

2.21

2.13

Pb

4.29

4.44

4.02

2.96

2.82

7.67

5.77

5.64

4.82

5.11

4.81

Th

2.55

3.58

3.28

2.08

2.04

4.86

4.58

4.50

3.73

4.11

3.80

U0.63

0.73

0.52

0.51

0.94

0.86

0.84

0.85

0.94

0.86

Nb/N

b*

0.92

0.81

0.83

0.98

0.96

0.57

0.56

0.57

0.83

0.89

0.84

DNb

�0.37

�0.46

�0.46

�0.48

�0.49

�0.64

�0.55

�0.55

�0.41

�0.15

�0.10

87Rb/8

6Sr

0.0683

0.1077

0.0953

0.1084

0.1008

0.3076

0.0916

0.0961

0.2016

87Sr/

86Sr m

easured

0.70492

0.705213

0.705007

0.705647

0.705051

0.70665

0.705645

0.705183

0.706012

87Sr/

86Sr norm

alized

0.704925

0.705218

0.705012

0.705652

0.705056

0.706655

0.70565

0.705188

0.706017

0.705385

87Sr/

86Sr 180

0.704750

0.704942

0.704768

0.705374

0.704798

0.705868

0.705416

0.704942

0.705501

0.704820

147Sm/1

44Nd

0.1459

0.1396

0.1499

0.1481

0.1509

0.1162

0.1162

0.1167

0.1257

0.1334

143Nd/1

44Ndmeasured

0.512726

0.512659

0.51267

0.512743

0.512779

0.512381

0.5124

0.512394

0.512577

143Nd/1

44Ndnorm

alized

0.512658

0.512591

0.512601

0.51266

0.512709

0.512308

0.512331

0.512321

0.512494

0.512514

eNd180

1.6

0.4

0.4

1.5

2.4

�4.6

�4.1

�4.3

�1.2

�1.0

Age(M

a)178.3

176.6

1A.V.Luttinen

(unpublished

data).

1504


and Zr and the variation in Al2O3. Harris et al. (1991) alsocommented on the low Al2O3 at high MgO for some ofthe dykes of the Ahlmannryggen. Many of the major andtrace elements exhibit compositional trends typical oftholeiites (Fig. 6), with negative correlations of Fe2O3

and TiO2 with MgO. Cr and Ni contents vary widely(Cr: 39–1006 ppm; Ni: 43–885 ppm).

In common with other studies of Mesozoic and olderflood basalt provinces we primarily use the incompatiblehigh field strength elements (HFSE; Ti, Zr, Y, Nb) asdiscriminants between magma types. These elementsare considered largely immobile during low-temperaturealteration processes (e.g. Peate, 1997) and ratios betweenthem are not significantly modified by moderate amountsof fractional crystallization or susceptible to variationsin the degree of partial melting (e.g. Luttinen & Furnes,2000). Zr can be used as an effective index of differenti-ation in magmas that do not crystallize zircon.

Zr vs TiO2, Nb and Y plots (Fig. 7a–c) for all the datafrom the Ahlmannryggen dykes (this study) allow us todifferentiate a number of distinct dyke groups. Data fromHarris et al. (1991) were not included in these HFSE plotsbecause, based on the geochronology results of this study,there is considerable uncertainty regarding the age of theminor intrusions of the Ahlmannryggen and there is noguarantee that all of the Harris et al. (1991) data are fromMesozoic dykes.

Based on the Zr vs TiO2 plot (Fig. 7a) three geochem-ical groups can be identified from the Ahlmannryggendataset; a low Ti–Zr group (<2�6 wt % TiO2 and<200 ppm Zr), a high Ti–Zr group (TiO2 in the range

2�6 5 wt % and Zr 200–400 ppm) and a very high Ti–Zrgroup (�>4 wt % TiO2 and Zr >400 ppm). These threegroups are replicated on the Zr vs Nb plot (Fig. 7b),although there is a clear split in the low Ti–Zr group,with a small subset with lower Nb contents (<10 ppmNb). On the Zr vs Y plot (Fig. 7c) the three groups areagain apparent; here the low Ti–Zr group has been splitinto two distinct subgroups, one with low Y (<30 ppm)and another with Y >35 ppm. The splitting of the low Ti–Zr group on the basis of Y is consistent with the subgroupbased on Nb contents (Fig. 7b).

These four geochemical groups are subsequentlyreferred to as Groups 1–4; Group 1: low Ti–Zr–Y(TiO2 <2�3 wt %, Zr <165 ppm, Nb <10 ppm and Y<30 ppm); Group 2: low Ti–Zr group (TiO2 <2�6 wt %,Zr <200 ppm, Nb >10 ppm and Y >33 ppm); Group 3:high Ti–Zr (TiO2 >3 wt % and Zr 200–400 ppm);Group 4: very high Ti–Zr group (TiO2 >4 wt % and Zr>400 ppm).

When the data for the four dyke groups are plottedagainst MgO (wt %) as an index of differentiation (Fig. 6)it is clear that samples from Groups 1 and 2 are typicallythe most differentiated, with MgO contents �6 wt %,whereas samples from Groups 3 and 4 have >7 wt %MgO. Ni is strongly correlated with MgO, suggestingolivine control during magmatic differentiation. Al2O3

increases sharply as MgO decreases, suggesting that pla-gioclase fractionation is not important until MgO con-tents fall below �6 wt %. Groups 1 and 2 are distinctivein showing wide ranges of variation in SiO2, Fe2O3,CaO, Al2O3 and Y at constant MgO contents.

0

1

2

3

4

5

6

7

8

40 45 50 55 60

Na

O+

KO

22

SiO2

Quartztholeiite

Olivinetholeiite

Basalt Basalticandesite

PicrobasaltFoidite

Alkaline

Sub-alkaline

Fig. 5. Total alkali vs SiO2 diagram (wt %) for the minor intrusions from the Ahlmannryggen. The samples are classified as quartz or olivinetholeiites based on their CIPW norms (see Table 2 for details). Classification boundaries are from Le Bas et al. (1986).

1505


10

0

20

0

30

0

40

0

50

0

60

0 00

0 0 0

05

55 5 5

10

10

10

10 10

15

15

15

15 15

20

20

20

20 20

25

25

25

25 25

ZrG

roup

1G

roup

1G

roup

2G

roup

2G

roup

3G

roup

3G

roup

4G

roup

4

00 0

0

55 5

5

10

10 10

10

15

15 15

15

20

20

20

20

25

25

25

25

Mg

OM

gO

Mg

O

Mg

OM

gO

Mg

O

Mg

O

Mg

OM

gO

Gro

up

1

Gro

up

1G

rou

p1

Gro

up

1

Gro

up

1

Gro

up

1G

rou

p1

Gro

up

2

Gro

up

2G

rou

p2

Gro

up

2

Gro

up

2

Gro

up

2G

rou

p2

Gro

up

3

Gro

up

3G

rou

p3

Gro

up

3

Gro

up

3

Gro

up

3G

rou

p3

Gro

up

4

Gro

up

4G

rou

p4

Gro

up

4

Gro

up

4

Gro

up

4G

rou

p4

6 5 4 3 2 1 0

TiO2

58

56 54

52

50

48

46 44

42

SiO2

20

0

40

0

60

0

80

0

10

00

12

00

Ni

15 14

13 12 11 10 9 8 7

AlO23

101112131415161718

FeO(total) 23

12 11 10 9 8 7

CaO 35 30

25 20

15 10 5 0

Nb 50

45 40

35 30

25

Y

Fig.6.

Vari

atio

ns

inZ

r,T

iO2,

SiO

2,

Ni,

Al 2

O3,

Fe 2

O3,

CaO

,N

ban

dY

vsM

gO

.T

he

fou

rgeo

chem

ical

gro

up

sare

def

ined

as

dis

cuss

edin

the

text.

1506


Fig. 7. Variations in (a) Zr vs TiO2, (b) Zr vs Nb and (c) Zr vs Y for Early–Middle Jurassic basic dykes from the Ahlmannryggen.

1507


Group 1

Only six of the minor intrusions analysed from theAhlmannryggen fall into the low TiO2–Zr Group 1.The six samples have TiO2 contents in the range 1�5–2�3 wt % and Zr contents of 122–164 ppm. These dykesoverlap, in part, with the field of Kirwanveggen lavas(Harris et al., 1990), which are Middle Jurassic in age(Duncan et al., 1997) and they also partially overlap withthe CT1 Vestfjella lavas of Luttinen & Furnes (2000).Group 1 rocks have SiO2 contents in the range 49�3–55�7 wt %. They typically have low mg-numbers [�50;mg-number ¼ 100 � Mg/(Mg þ Fe2þ); FeO ¼ Fe2O3/1�15]. Cr and Ni contents are varied, with Cr rangingfrom 75 to 708 ppm and Ni from 81 to 318 ppm. Group 1rocks are light rare earth element (LREE) enriched with(La/Yb)N ranging from 0�5 to 4�9 and LREE contents upto 25 times chondrite (Fig. 8a). Almost all samples haverelatively flat middle to heavy chondrite-normalized REE(MREE to HREE) patterns with (Sm/Lu)N �1�8. Themid-ocean ridge basalt (MORB)-normalized multi-element variation diagrams in Fig. 9a are characterized

by troughs at Ta–Nb and Ti. Group 1 rocks exhibit a widerange of variation in 87Sr/86Sr180 (0�7064–0�7085) atfairly constant eNdi (�5�8 to �6�4) (Fig. 10). The vari-ation in 87Sr/86Sr at fairly constant eNd is typical of post-magmatic hydrothermal alteration. A summary of the keycharacteristics of Group 1 rocks is provided in Table 3.

Petrographic characteristics

Group 1 rocks are characterized by cracked and alteredolivine phenocrysts, typically <0�5 mm, whereas thesmaller olivine grains tend to be more altered and areoften ophitically enclosed by clinopyroxene. Plagioclaselaths are a major phenocryst and groundmass phase andare typically sericitized.

Group 2

Group 2 rocks are characterized by low to moderateTiO2 (2�17–2�62 wt %) and Zr (150–177 ppm) contents(Fig. 7a) and overlap with many of the samples analysedby Harris et al. (1991) from the Ahlmannryggen. MgO

Fig. 8. Chondrite-normalized REE diagrams for (a) Group 1, (b) Group 2, (c) Group 3 and (d) Group 4 of the Ahlmannryggen dykes.Normalizing values are taken from Nakamura (1974). Data for Rooi Rand dykes (RRDS; dashed lines) are taken from Duncan et al. (1990)and P27-AVL (CT2) from Vestfjella from Luttinen & Furnes (2000).

1508


Fig. 9. N-MORB-normalized incompatible element diagrams for (a) Group 1, (b) Group 2, (c) Group 3 and (d) Group 4 of the Ahlmannryggen.Normalizing values are from Sun & McDonough (1989).

Table 3: Definition of geochemical Groups 1–4 Ahlmannryggen dykes

Group: 1 2 3 4

Number of samples: 6 14 15 9

SiO2 49.32�55.66 46.70�50.68 45.22�48.55 45.23�54.81

TiO2 1.52�2.27 2.17�2.62 3.25�4.85 3.87�5.28

MgO 5.32�8.57 5.18�6.1 7.63�21.61 5.77�15.84

Al2O3 11.73�14.62 12.48�14.08 8.00�12.60 7.51�9.97

Fe2O3(T) 11.21�15.47 13.44�16.74 12.27�15.03 10.11�13.03

Zr 122�164 150�177 216�316 343�568

Nb 7.2�15.2 11.4�13.5 3.2�11.8 21.1�32.6

Ni 81�318 43�71 251�769 160�885

(La/Yb)N 0.54�4.90 2.58�2.81 1.64�4.29 7.20�19.31

Ti/Y 333�455 378�420 291�777 664�694

87Sr/86Sri 0.7064�0.7085 0.7034�0.7046 0.7035�0.7062 0.7048�0.7059

eNdi �5.8 to �6.4 0.7 to 1.7 5.0 to 9.0 �4.6 to 2.4

Approx. strike (deg.) 062 010 079 017

Approx. age (Ma) 191 178 191 178

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contents (5�18–6�10 wt %; Fig. 6) and mg-numbers (39�5–45�9) are very homogeneous. All samples are LREEenriched with (La/Yb)N of 2�6–2�8, La contents�20 times chondrite and have fairly smooth REE pat-terns (Fig. 8b). The MORB-normalized multi-elementpatterns for Group 2 rocks (Fig. 9b) are also very smooth.Group 2 samples exhibit a small range in 87Sr/86Sri

(0�7034–0�7046) and eNdi (0�7–1�7) at 180 Ma (Fig. 10).A summary of the key characteristics of Group 2 rocks isprovided in Table 3.


The Group 2 rocks show some variation, but are typicallyfeldspar–clinopyroxene-phyric with a groundmass offeldspar microphenocrysts and Fe–Ti oxides. The feld-spar phenocrysts are euhedral and are occasionally ophit-ically enclosed by clinopyroxene. Olivine is absent.

Group 3

Fifteen samples have been identified in the Group 3magma type and they are characterized by low SiO2

(45�22–48�55 wt %), high TiO2 (3�25–4�85 wt %), highTi/Y (291–777) and high MgO (7�63–21�61 wt %). Sev-eral of the samples can be classified as picrites, using theclassification scheme of Le Bas (2000), and three samplesare ferropicrites (Fig. 11a), following the broad criteria(FeO > MgO >12 wt %; Al2O3 <10 wt %) used byGibson et al. (2000). Harris et al. (1991) first identified thepresence of picrites in western Dronning Maud Land anddemonstrated that they were genuine high-MgO liquids.The ferropicrites are characterized by high contentsof both MgO (12�00–13�45 wt %) and FeO (12�6–13�1 wt %), with mg-numbers in the range 63–65. Theferropicrites have low to moderate SiO2 contents (45�22–46�54 wt %), low Al2O3 (8�15–10�22 wt %) and low total

Group 1

Group 2

Group 3

Group 4

-25

-20

-15

-10

-5

0

5

10

0.702 0.704 0.706 0.708

0.704 0.706 0.708

0.710 0.712 0.714 0.716

Group 3

Group 2

Group 4 Group 1

87 86Sr/ Sr180

87 86Sr/ Sr180

εNd

18

0

εNd

18

0

VestfjellaCT1(low Nd)ε

Buheranephelinites

ODS

P27-AVL

FalklandIslands

CentralLebombo

Kirwanveggenlavas

South Lebombo(Karoo)

RooiRand

CentralLebombo

Ferrar

NorthernLebombo

60%

60%

40%

40%

20%

80%

80%

SCLMpartial melt

VestfjellaCT3

VestfjellaCT1(high Nd)ε

VestfjellaCT2 (dykes)

(r=0.4)

BorgmassivetIntrusives

3a

3b

AFC

-5

0

5

10

Group 3

Group 2

Group 4Group 1

P27-AVL

Kirwan lavas

Rooi Rand

20%

5%

1%

3%

10%

Fig. 10. Initial eNd and 87Sr/86Sr (T ¼ 180 Ma for all rocks shown) for Groups 1–4 from the Ahlmannryggen; the four main groups arehighlighted by continuous lines. Other rocks from the Karoo and Ferrar magmatic provinces are highlighted by dotted lines. Data sources:Duncan et al. (1990); Harris et al. (1990); Hergt et al. (1991); Sweeney et al. (1994); Fleming et al. (1995); Harmer et al. (1998); Mitchell et al. (1999);Elburg & Goldberg (2000); Luttinen & Furnes (2000). All data are age corrected to 180 Ma. ODS, Okavango dyke swarm, P27-AVL [mostdepleted composition of Luttinen & Furnes (2000)]. Binary bulk mixing curves are indicated between Group 3 and Borgmassivet Intrusions, andGroup 3 (Z.1816.2) and SCLM partial melt. Group 3: 87Sr/86Sr ¼ 0�7035, Sr 300 ppm, eNd ¼ 8�5, Nd 25 ppm; lamproite (SCLM partial melt):87Sr/86Sr ¼ 0�7096, Sr 1830 ppm, eNd ¼ �25, Nd 150 ppm. Borgmassivet Intrusions: 87Sr/86Sr ¼ 0�7240, Sr 130 ppm, eNd ¼ �11, Nd 13 ppm.AFC model curve: Archaean crust contaminant (Luttinen & Furnes, 2000): 87Sr/86Sr ¼ 0�710, Sr 500 ppm, eNd ¼ �52, Nd 11 ppm,r value ¼ 0�4.

1510


alkalis (1�51–1�97 wt %). As would be anticipated giventheir high MgO contents, all Group 3 rocks have highconcentrations of compatible trace elements (Cr 397–1006 ppm and Ni 251–769 ppm), with the ferropicriteshaving the highest values (Cr 683–1006 ppm and Ni 578–619 ppm).

The REE patterns of Group 3 rocks are distinct from allother magma groups. They are characterized by ‘saddle-back’ patterns (Fig. 8c) with (La/Sm)N <1, (La/Yb)N inthe range 1�6–4�3, and a marked depletion in the HREE.The multi-element plots (normalized to N-MORB) arecharacterized by almost flat patterns with some variation

18

17

16

15

14

13

12

11

100 5 10 15 20 25

MgO

Fe

O(T

)2

3F

eO

(T)

23

Picrites Olivine accumulation

Group 3

Group 4

Groups 1 & 2

>8wt% MgO

Ferropicrites

Group 1

Group 1

Group 2

Group 2

Group 3

Group 3

Group 4

Group 4

10

11

12

13

14

15

16

17

18

44 46 48 50 52 54 56

SiO2

(a)

(b)

Fig. 11. Variation in Fe2O3(T) vs (a) MgO and (b) SiO2 for Groups 1–4 from the Ahlmannryggen. (a) highlights the fields for ferropicrites (�)[MgO >12 wt % and Fe2O3 >13�8 wt % (>12 wt % FeO); Gibson et al., 2000] and picrites (MgO >12 wt %; Le Bas, 2000); (b) shows a generaltrend of decreasing Fe2O3 with increasing SiO2 for all four dyke groups.

1511


in the more mobile elements (Rb, Ba, Th) betweensamples (Fig. 9c). Otherwise there is little variation fromNb to Ti.

Group 3 samples also have distinct isotope signatures(Fig. 10) with 87Sr/86Sri 0�7035–0�7062 and high eNdi

(5�0–9�0). There appear to be two distinct sub-groupswithin Group 3, one with lower eNdi (5�0–5�5) and moreradiogenic 87Sr/86Sri (0�7054–0�7062), and the otherwith higher eNdi (7�0–9�0) and less radiogenic87Sr/86Sri (0�7035–0�7041). The three ferropicrites allfall into the high-eNd, low-87Sr/86Sr sub-group. TheSr–Nd isotope compositions of the more unradiogenicSr sub-group compare closely with that of a single samplefrom Vestfjella (P27-AVL; Luttinen & Furnes, 2000),which was previously identified as the most ‘depleted’rock type from the entire Karoo (South Africa andAntarctica) province. A summary of the key characterist-ics of Group 3 rocks is provided in Table 3.


Group 3 rocks are more porphyritic than any of the othermagma groups. Olivine is the main phenocryst phase andis often up to 3–4 mm in diameter. It occurs in all sam-ples, but is rarely unaltered and is typically replaced alongcracks by green or yellow serpentine. Olivine composi-tions are Mg-rich (Fo70–86) and clinopyroxene is alsopresent as a phenocryst phase, but is not as abundant asolivine. Plagioclase is not present as a phenocryst phase.The groundmass is dominated by clinopyroxene, plagio-clase and Fe–Ti oxides.

Group 4

Nine samples from the Ahlmannryggen are identifiedas Group 4. Their defining characteristic is their veryhigh TiO2 (3�87–5�28 wt %), high Zr (343–568 ppm)and very high Ti/Y (664–694). Five of the group arepicrites (Fig. 11a), with MgO >12 wt % and Na2O þK2O <3 wt % (Fig. 5) and are characterized by high Cr(834–965 ppm) and Ni (666–885 ppm) contents. TheREE patterns are the most enriched of the four geochem-ical groups with (La/Yb)N values of 7�2–19�3 and Lacontents 81–204 times chondrite (Fig. 8d). The MORB-normalized multi-element patterns are characterized bya shallow trough at Ta–Nb and a minor negative anom-aly at Ti, but generally exhibit a smooth pattern (Fig. 9d).Samples from Group 4 show a range in 87Sr/86Sri of0�7048–0�7059, and eNdi varies considerably from �4�6to 2�4 (Fig. 10). A summary of the key characteristics ofGroup 4 rocks is provided in Table 3.


Group 4 rocks are the least altered of the four chemicalgroups. Olivine phenocrysts are typically euhedral andshow only a minor amount of alteration along cracks.

Some of the olivine phenocrysts occur in clusters of up tofive grains; these typically have rounded grain boundar-ies. The Group 4 rocks are �30% porphyritic and thegroundmass is very fine grained; feldspar phenocrysts arediscernible only at the chilled margins of the dykes.

COMPARISON WITH OTHER

KAROO–ANTARCTIC MAGMA

GROUPS

The Mesozoic intrusions of the Ahlmannryggen overlapwith the main phase of volcanic and intrusive activityof the Karoo magmatic province of southern Africa andEast Antarctica. Geochemical data from East Antarctica(Kirwanveggen and Vestfjella: Fig. 1) and southern Africaare plotted in Fig. 12 [Zr vs TiO2 plots for (a) Antarcticaand (b) South Africa]. Figure 12a also includes the fourgeochemical groups of the Ahlmannryggen dykes forcomparison.

Data for the lavas of Vestfjella and the Kirwanveggen(Fig. 12a) form a much more restricted range relative tothe Ahlmannryggen intrusions. The majority of theVestfjella–Kirwanveggen samples fall into a low TiO2

(<2 wt %) and Zr <200 ppm group. One group ofVestfjella lavas, CT2 of Luttinen et al. (1998), has higherTiO2 (2�4–3�8 wt %), but these still have low Zr(<200 ppm).

The Karoo data from South Africa (sources in figurecaption; Fig. 12b) also form a cluster at very low TiO2

(<1�5 wt %) and Zr (<120 ppm), although a significantnumber of samples extend to higher TiO2 and Zr values.The high TiO2–Zr (HTZ) samples are from the HTZ(low-Fe) and HTZ (high-Fe) groups of Sweeney et al.(1994) from the central Lebombo, and the Letaba Forma-tion picrites of the Lebombo (Duncan et al., 1984). TheRooi Rand dyke swarm (RRDS) forms a broad spreadand overlaps with the Ahlmannryggen Group 2. Group 4overlap, in part, with the HTZ (high Fe) field of Sweeneyet al. (1994) from the Lebombo part of the Karoo Prov-ince (Fig. 12b).

STRUCTURAL GEOLOGY

Geometry and distribution ofAhlmannryggen dykes

Mafic dykes are widely distributed within the centralAhlmannryggen region, although they are found in great-est concentrations along the ridges of the Flarjuven andGrunehogna nunatak groups (Fig. 2). The vast majorityof the dykes have intruded thick dioritic sills of theBorgmassivet intrusive suite, where they were emplacedalong a pervasive suite of pre-existing, sub-vertical jointswithin the sills. The total dyke population displays a wide

1512


variety of orientations, although a frequency distributionplot of total dyke and joint population data reveals adominant NNE–SSW trend to both datasets (Fig. 13).Subordinate joint sets oblique to the predominant dyketrend were also exploited during dyke emplacement, res-ulting in offsetting segments and en echelon geometries.

Where limited lateral exposure of an offset segmenteddyke prevented simple identification of the main dyketrend, the orientation of the widest dyke segment wastaken as a proxy for the overall dyke trend.

The geochemical characterization of the centralAhlmannryggen dyke suite has identified four distinct

CT3

CT2

CT4

TiO

2

0

1

2

3

4

5

6

Zr

0 100 200 300 400 500 600

Kirwanveggenlavas field

(a) Antarctica(Vestfjella, Kirwanveggen,Ahlmannryggen)

Low Ti-Zr

High Ti-Zr

TiO

2

0

1

2

3

4

5

6

Zr

0 100 200 300 400 500 600

Rooi RandDyke Swarm

Letaba Formation(picrites)

Central LebomboHigh Ti-Zr basalts(high- & low-Fe)

Kirwanveggenlavas field

(b) Karoo(South Africa)

Low Ti-Zr

High Ti-Zr

Group 1

Group 2

Group 3

Group 4

Gp 3

Gp 4

Gp2

Gp 1

Fig. 12. Variation in TiO2 vs Zr for Early Jurassic basic igneous rocks from (a) Antarctica: Vestfjella and Kirwanveggen (Furnes et al., 1982, 1987;Harris et al., 1990; Luttinen et al., 1998; Luttinen & Furnes, 2000); (b) Karoo, South Africa (Sweeney et al., 1994; Mitchell et al., 1996, 1999; Reidet al., 1997; Harmer et al., 1998; Marsh & Mndaweni, 1998; De Bruiyn et al., 2000; Elburg & Goldberg, 2000). Also shown are the fields forGroups 1–4 from the Ahlmannryggen (this study).

1513


geochemical groups, each of which has a consistent ori-entation and/or a distinct geographical distribution(Fig. 2). Group 1 dykes display a fairly uniform ENE–WSW strike (062� mean) and vary from 0�1 to 1�9 m inwidth. They occur at Grunehogna and west of Flarjuven,as well as at Neumayerskarvet, northern Kirwanveggen(Fig. 2). Group 2 dykes are distributed throughout thenunataks in the west of the Ahlmannryggen (Fig. 2). Dyketrends range from north–south to NE–SW (010� mean),sub-parallel to the Jutulstraumen ice stream and subgla-cial trough. Group 2 dykes are notable for their extremerange of widths, up to 80 m wide, with four dykes inexcess of 5 m wide. Dykes with a thickness >5 m weresampled close to the wall rock margin. Group 3 dykesstrike predominantly east–west to ENE–WSW (079�

mean) and vary from 0�20 to 3�58 m in width, with amean of 1�17 m. Group 3 dykes are geographicallyrestricted to the Grunehogna nunataks group (east sideof Kullen) and two localities along the general strikedirection of the dykes to the west (Fig. 2). Group 3dykes are oblique to the Jutulstraumen glacial trough(Fig. 2), but are broadly parallel to the Pencksokket icestream and glacial trough and to Group 1 dykes (Fig. 2).Group 4 dykes form a NNE–SSW-trending swarm (017�

mean); they vary in thickness from 0�14 to 4�49 m. Theirdistribution is restricted to the Flarjuven nunatak groupin the NW of the study area (Fig. 2). They are broadlyparallel to the Group 2 dykes, which are sub-parallel tothe Jutulstraumen subglacial trough.

Dilation direction

The orientation of dyke segments emplaced into pre-existing fractures is controlled by the ability of the frac-tures to dilate, which is the product of their orientationwith respect to the minimum principal stress and themagma pressure at the time of emplacement (Delaneyet al., 1986). Therefore, where pre-existing fractures exhi-bit a control over dyke segment orientation it is unlikelythat the strike of the dykes will be a simple reflection of the

original extension direction. Dykes following pre-existingfractures oriented obliquely to the direction of maximumextension will side-step, resulting in the development ofbridges or a dyke offset. Application of simple stereo-graphic analytical techniques to these dyke offsets (Bus-sell, 1989; Kretz, 1991) allows estimates to be made forthe stress field acting on the fracture during dilation. Themethod of Bussell (1989) is employed here, which com-bines the line of intersection between the dyke wall andthe offset dyke segment with the apparent extension dir-ection to define the dilation plane for a particular dyke.The dilation plane contains the true dilation direction foran individual dyke, and the true dilation direction for thedyke swarm can be obtained from the best-fit great circleto the poles of the individual dilation planes derived froma number of dykes.

Unfortunately, given the generally restricted nature ofdyke exposure encountered (e.g. cliff faces or escarpmentedges), only a small number of dykes displaying therequired structural characteristics were encountered.Only seven well-exposed dyke offsets were recorded inthe four Mesozoic dyke groups, including examples fromGroups 1, 2, and 4. Plotting the poles to the reconstruc-ted dilation planes for the dyke offsets reveals two dis-tinct orientations (Fig. 14). Five poles from dyke Groups2 and 4 are distributed along an approximate NE–SW-trending girdle, while the remaining two poles fromGroup 1 dykes plot along an east–west girdle. We havecalculated the mean girdles for both sets of data, thepoles to which approximate the dilation direction for thedyke groups. Data from dykes of Groups 2 and 4 suggestthat they were emplaced about a mean trend of 012� inresponse to an applied true dilation direction trending307–127�. The best-fit girdle to the poles to dilationplanes for Group 1 dykes, although limited, suggeststhat the dykes were emplaced in a stress field wherethe minimum principal stress direction was oriented�004–184�. Given the sub-parallel trend of the dykeswithin Groups 1 and 3 (mean trend 072�), we interpretthem have all been emplaced in response to the samenorth–south-trending minimum principal stress direc-tion. The calculated dilation direction is not perpendic-ular to the mean trend of the dykes, suggesting thatdykes were probably emplaced along pre-existing planesof structural weakness (i.e. regional jointing) and exhib-ited oblique dilation.

PETROGENESIS OF THE MAGMAS

AND MANTLE SOURCES

In the following section we examine each geochemicaldyke group and attempt to identify the mantle source ofthe magmas and make correlations with other Karoomagma types.

Dyke segments Joints

Fig. 13. Frequency orientation plots for total dyke segments (left)and joints (right) within the Ahlmannryggen study area. Thedominant trend of the dyke segments is parallel to regional jointorientation.

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Interpretation

Group 1 (low Ti–Zr)

Group 1 rocks are all low-Ti tholeiites, which is the pre-dominant rock type throughout the Karoo–DronningMaud Land flood basalt province. Lava successions inLesotho, Lebombo, Vestfjella and Kirwanveggen (Fig. 1)are all dominated by low-Ti basalts (Fig. 12). Luttinenet al. (1998) have made comparisons between the CT1,low-eNd basalts of Vestfjella and the low-eNd SabieRiver Basalt Formation of southern Lebombo.

The Group 1 rocks have low MgO contents, with onlytwo samples having MgO >8 wt % (but <9 wt %). Interms of their Sr–Nd isotope characteristics the Group 1rocks are distinct from any other group of the Karoo–Antarctica volcanic province (Fig. 10). They are inter-mediate between the central Lebombo lavas and rocks ofthe broadly contemporaneous Ferrar magmatic province

(Fig. 10). The Ahlmannryggen Group 1 dykes all havea characteristic Ta–Nb negative anomaly (Fig. 9a),suggesting crustal contamination. Evidence of crustalcontamination is also well illustrated in Figs 15 and16, where the Group 1 dykes have high values of Th/Ta and low values of Nb/Nb*, both reliable proxies forthe involvement of crustal material. Although there isonly a small sample set for Group 1, the variation in87Sr/86Sr with SiO2 and mg-number (Fig. 17) couldalso suggest that magma chemistry was influenced byassimilation combined with fractional crystallization(AFC). To model AFC processes, a Group 3 sample(Z.1816.1) was used as the parent magma and the crustalcontaminant was assumed to be local Archaean crust, asthe Ahlmannryggen dykes were intruded into theGrunehogna Archaean craton. The Nd–Sr isotope com-position of the Archaean crust is assumed to be 87Sr/86Sr¼ 0�710, Sr 500 ppm, eNd �52, Nd 11 ppm (Fig. 10),based on Luttinen & Furnes (2000), who used data fromthe Kaapvaal craton to model the potential crustal con-taminant, as well as two granitic xenoliths from Vestfjella,all of which have eNd of ��50. AFC at moderate ratesof assimilation (r ¼ 0�4) could account for the isotopiccharacter of Group 1 rocks derived from a Group 3parent magma. Both Group 1 and Group 3 dykeswere intruded at �190 Ma and occupy the same struc-tural trend.

Although the dataset is small (only four samples withisotope data; Fig. 10) an alternative explanation for iso-tope characteristics of the Group 1 dykes is that some of thevariation in 87Sr/86Sr (0�7064–0�7085) at fairly constanteNd (��6) might have been caused by post-magmatic alteration. If the lowest 87Sr/86Sr value is used(0�7064), this lies close to a mixing curve between Group 3dykes and partial melts of the subcontinental lithosphericmantle (SCLM) at �12% (Fig. 10), which is explained inthe following section, where the petrogenesis of Group 2and 4 dykes are discussed. Therefore the magmas of theGroup 1 dykes could also represent mixtures of Group 3melts plus �12% partial melts of the SCLM; any sub-sequent variation in their Sr isotope characteristics isprobably the result of post-magmatic processes.

Group 2 (low Ti–Zr)

The Group 2 rocks of western Dronning Maud Land areall low to moderate Ti rock types and overlap, in part,with the chemical type 2 (CT2) of Luttinen et al. (1998)and also with the Rooi Rand dolerite dykes of SouthAfrica (TiO2 2�20 wt %; Zr 156 ppm; Armstrong et al.,1984). They are all low-Mg rocks (mg-number <46), withaccompanying low Cr and Ni. Isotopically, they arealso relatively homogeneous (0�7034–0�7046; Fig. 10)and include a fairly depleted sub-group (87Sr/86Sri

<0�7040 and eNd �1�5–2).

38 005

28 307

Group 1

Group 2

Group 4

Fig. 14. Stereograms showing the poles to constructed dilation planesfor dykes representing Groups 1, 2 and 4, using the method of Bussell(1989). Star represents the pole to the best-fit girdle to the poles ofdilation planes, and an approximation for the dilation vector of thedyke populations. The frequency orientation plots at the centre of thestereograms represent Mesozoic dykes of the relevant geochemicalgroups that make up the dyke populations related to two distinctdilation directions (see text for details).

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0

0.5

1

1.5

2

2.5

0.702 0.7100.706 0.708 0.712 0.714 0.7160.704 0.718 0.720 0.722 0.724

Nb

/Nb

*

87 86Sr/ Sri

Karoo-Depletedmantle

end member(Z.1816.2)

Group 3+SCLM

Group 3b dykeswith Nd ~5 &La/Nb >1

εGroup 2

CT1CT2 Ferrar 90%

80%70%

60%50%

40%40%

60%

30%20%

Gaussberglamproite

10%

Group 1

Group 2

Group 3

Group 4

CT4

20%

BorgmassivetIntrusives

10%

5%

Fig. 16. Magnitude of the Nb anomaly, measured as Nb/Nb* [¼NbN/H(ThN � LaN); normalized to primitive mantle] vs 87Sr/86Sr. The size ofthe Nb anomaly is used as a proxy for the extent of crustal contamination in mantle-derived magmas. Binary mixing curves are shown between adepleted end-member basalt (Z.1816.2; Group 3) and upper continental crust (Borgmassivet Intrusives) and SCLM partial melt (lamproite). Thedepleted end-member is a Group 3 basalt from the Ahlmannryggen; 87Sr/86Sr ¼ 0�7035, Sr 300 ppm, Nb/Nb* ¼ 1�65. The local upper crust isrepresented by Borgmassivet Intrusions: 87Sr/86Sr ¼ 0�7240, Sr 130 ppm, Nb/Nb* ¼ 0�19; the enriched subcontinental lithospheric mantle proxypartial melt is Gaussberg lamproite (Bergman, 1987): 87Sr/86Sr ¼ 0�7096, Sr 1830 ppm, Nb/Nb* ¼ 0�5. Fields for CT1, CT2 and CT4 lavasfrom Vestfjella are from Luttinen et al. (1998). Field of average Ferrar dolerite is taken from Molzahn et al. (1996).

0

2

4

6

8

10

0 50 100 150

Th

/Ta

Ti/Zr

Lamproite(SCLM

partial melt)

Ferrar

Lowercrust

Group 1

Group 2

Group 3

Group 4

Crust/SCLM partialmelt component MORB

Fig. 15. Variation in Th/Ta vs Ti/Zr for Ahlmannryggen minor intrusions, Groups 1–4. The field for MORB is from the GERM website(http://www.earthref.org/GERM); average lamproite is from Rock (1991); lower crust is from Rudnick & Fountain (1995); the Ferrar province isfrom Fleming et al. (1995).

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http://www.earthref.org/GERM

Dykes of Group 2 are all evolved, with an average mg-number of �42, but they show little evidence of crustalcontamination. They all have low Ba/Zr ratios (typically<1), indicating minimal crustal contamination (e.g.Kent & Fitton, 2000) and also have moderate Nb/Nb*

(�1�0; Fig. 16). The absence of any significant interac-tion with continental crust is reinforced by a plot ofTh/Ta vs Ti/Zr (Fig. 15), in which Group 2 rockscluster close to the MORB field, suggesting that therewas no significant interaction between the parent mag-mas and crust or lithospheric mantle. Crustal and litho-spheric mantle values of Th/Ta are high and Ti/Zr verylow, and are therefore distinct to those of MORB.Luttinen et al. (1998) also noted that his CT2 dykes and

sills showed little evidence of crustal contamination,although the CT2 lavas were variably contaminated bylower crust.

Binary mixing curves are shown in Figs 10 and 16between the most isotopically depleted group of rocks(Group 3) and a partial melt of enriched SCLM. TheGaussberg lamproites of Antarctica (Bergman, 1987)were used as a proxy for the partial melt of a hypothetical,enriched component in the SCLM, as they are chemic-ally homogeneous and show very little evidence for crus-tal contamination (Ewart et al., 2004). The mixing curvesindicate that the Group 2 rocks could be interpretedas mixes of Group 3 magmas with <3% of an enrichedcomponent, akin to the partial melt of SCLM.

40

45

50

55

60

49 50 51 52 53 54 55 56

0.7060

0.7065

0.7070

0.7075

0.7080

0.7085

0.7090

49 50 51 52 53 54 55 56SiO2

SiO2

mg-

num

ber

87

86

Sr/

Sr i

(a)

(b)

Fig. 17. Variations in SiO2 vs (a) mg-number and (b) 87Sr/86Sri for Group 1 dykes from the Ahlmannryggen. The trend is suggestive, in part, ofAFC proceeses.

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The Group 2 dykes are the only group from theAhlmannryggen with consistently positive DNb (Fig. 18),which is a function used to express the excess or defi-ciency in Nb relative to the lower limits of the Icelandbasalt array (Fitton et al., 1997) in a plot of Nb/Y vs Zr/Y.Samples with positive DNb plot above the lower line inFig. 18, which implies derivation from an enriched man-tle source, whereas those with negative DNb plot belowthe lower line, indicating a source in the depleted uppermantle. Assimilation of continental crust has little effecton the value of DNb because crustal rocks plot on orbelow the lower line of the Iceland array, therefore con-tamination will only lower DNb and cannot make asample appear to be ‘Icelandic’ (Chambers & Fitton,2000). DNb is also insensitive to the effects of variabledegrees of mantle melting and source depletion followingpartial melting, and is, therefore, a characteristic of themantle source (Fitton et al., 2003). The Group 2 dykesplot in a tight cluster and fall within the ‘Iceland array’ ofFitton et al. (1997). They overlap, in part, with the field ofVestfjella lavas and intrusions (Luttinen & Furnes, 2000).The Group 2 rocks have positive DNb and are clearlydistinguishable from N-MORB magmas (Fig. 18), whichhave negative DNb and cannot have been derived fromthe same mantle source as Group 2.

Similarities between the Group 2 dykes, the CT2Vestfjella dykes of Luttinen & Furnes (2000) and the RooiRand Dyke Swarm of Duncan et al. (1990) are apparent

based on both geochemical and isotopic criteria, as wellas their post-Karoo (182 Ma) age. Luttinen et al. (1998)noted the compositional similarity to recent South WestIndian Ridge (SWIR) MORB associated with the Marionhotspot.

Group 3 (high Ti–Zr)

The Group 3 rocks of the Ahlmannryggen are all high-Ti, high-Zr rocks types (Fig. 7a) and relative to the otherthree groups from the Ahlmannryggen, Group 3 arenotably depleted in the LILE. They have ‘humpbacked’REE (Fig. 8c) and N-MORB normalized multi-elementpatterns (Fig. 9c). They are characterized by high eNdi,and low 87Sr/86Sri (Fig. 10), consistent with derivationfrom a depleted mantle source. The Group 3 rocks formtwo distinct sub-groups, which are particularly evident onthe Nd–Sr isotope diagram (3a and 3b in Fig. 10), but alsoon the multi-element diagram (Fig. 9c), where the low-eNd sub-group shows greater enrichment in Th. Super-ficially, the Group 3 rocks have many similarities toMORB (high eNd, low 87Sr/86Sr, LREE-depleted pat-terns) but their TiO2 contents are far too high (�4 wt %)for them to derived from MORB-source mantle. Thedepth of melting is constrained by the Dy/Yb ratios(�3: Fig. 19) and also by the shape of the multi-elementpatterns (Fig. 9c), which show that the HREE are beingretained, presumably by garnet. Several workers (e.g.

Nb/Y

10

1

0.1

0.011 10

Zr/Y

High Ti-Zr

Low Ti-Zr

Iceland arrayDepleted

Enriched

PM

High degrees ofpartial melting

Low degrees ofpartial melting

+veNb∆

-veNb∆

Group 1

Group 2

Group 3

Group 4

Vestfjella field

N-MORB

Fig. 18. Nb/Y vs Zr/Y for Groups 1–4 from the Ahlmannryggen. The parallel lines represent the Iceland array of Fitton et al. (1997). PM,primitive mantle. Samples that plot below the lower line have negative DNb whereas those above this line have positive DNb [DNb ¼ 1�74 þlog (Nb/Y) – 1�92 log (Zr/Y); Fitton et al., 1997]. The high Zr/Y samples represent small degree partial melts (Groups 3 and 4) and those at lowZr/Y are larger degree partial melts.

1518


Fitton et al., 1997) have used the abundance of Nb relativeto Zr and Y (DNb) to distinguish between mantle sources.Group 3 rocks all have negative DNb (Fig. 18), with themore depleted group of rocks (Group 3a) having DNbvalues of �0�57 and Group 3b having values of �0�43.Although they are more enriched than N-MORB (Fig. 18)the Group 3 rocks could contain a component fromMORB-mantle, but at lower degrees of partial melting.The Group 3 rocks include two distinct sub-groups in theNd–Sr isotope diagram (Fig. 10), which can be explainedby the addition of �20% upper crust to the uncontam-inated melts. A mixing curve is plotted in Fig. 10 betweena Group 3 magma and one of the Borgmassivet Intru-sions (T. R. Riley, unpublished data), which representsthe local upper crust at the time of intrusion. The modelcurve can reproduce the Nd–Sr isotope composition ofthe Group 3 dykes by bulk mixing, although realisticallythe actual process would involve assimilation plus frac-tional crystallization. The Group 3 samples with thelowest eNd (�5) values also typically have lower Nb/Nb* values (�0�9 with one sample at �1�3; Fig. 16),which reflects the amount of crustal contamination relat-ive to the uncontaminated dykes, which have Nb/Nb*

values of 1�6–1�7 (close to depleted mantle, Nb/Nb*

�1�45; Hofmann, 1988). The mixing curves plotted onthe Nb/Nb* vs 87Sr/86Sr diagram (Fig. 16) illustrate thebulk addition of local upper crust (Borgmassivet Intru-sions) to an ‘uncontaminated’ Group 3 magma composi-tion. The three samples that plot at higher 87Sr/86Sr,lower Nb/Nb* and lower eNd (�5) can be explained bythe addition of �20% of upper crust (akin to the mixingcurve in Fig. 10). A separate subset of five samples

(Fig. 16) that plot at slightly lower Nb/Nb* than themost primitive magmas at similar 87Sr/86Sr values couldreflect mixing of a small (2–3%) lithospheric mantlepartial melt component with the Group 3 uncontami-nated end-member (Z.1816.2: 87Sr/86Sr ¼ 0�7035;eNd ¼ 8�8; Nb/Nb* ¼ 1�65).

The Group 3 dykes are clearly an unusual group ofrocks, which were intruded at �190 Ma, have a strikedirection parallel to the Pencksokket glacial trough, thefuture continental margin and the Explora escarpment(Fig. 1). They are E-MORB-like in many respects andinclude ferropicrites as well as picrites. However, the Ticontents are high (3–4 wt %) and in this respect theydiffer from E-MORB and more closely resemble ocean-island basalts (OIB), although they are more depleted inincompatible elements than OIB. The depleted sourcecomponent in mantle plumes is interpreted to be distinctfrom the MORB-source mantle component beneathpresent-day mid-ocean ridges (Kerr et al., 1995). TheMORB-like rocks associated with mantle plumes arebelieved to have been generated from a depletedsource component that forms an intrinsic componentof the plumes (e.g. Fitton et al., 1997).

Within Group 3, three samples are termed ferropicrites(Fig. 11); these have MgO and FeO contents >12 wt %(with FeO > MgO). Ferropicrites are rare worldwide,but have been identified from the Permo-TriassicSiberian Traps continental flood basalt (CFB) province byWooden et al. (1993) and the Early Cretaceous EtendekaCFB province of Namibia by Gibson et al. (2000). Theseworkers interpreted the ferropicrites as partial melts ofFe-rich streaks in mantle plume starting heads, combined

La

/Yb

Increasing depthof melting

Increasing% melting

Group 3

Group 4

Group 2

Group 1

5

10

15

20

25

30

1.5 2 2.5 3 3.5 4

Dy/Yb

0

Group 1

Group 2

Group 3

Group 4

Fig. 19. Variation in La/Yb vs Dy/Yb for Groups 1–4 from the Ahlmannryggen. The increase in Dy/Yb reflects the increased depth of meltingand decreasing La/Yb reflects an increase in the amount of partial melting.

1519


with a significant melt contribution from the convectingmantle. It is predicted that such melts would be eruptedearly in the history of flood basalt provinces and gener-ated at high pressure (35–45 kbar) and high temperature(Tp � 1550�C). This is significant, as the available geo-chronology data for Group 3 ferropicrites indicate an ageof �190 Ma, which reinforces the prediction of Gibsonet al. (2000) that ferropicrites would be amongst the first-formed melts from a plume head.

Group 4 (very high Ti–Zr)

Group 4 rocks are characterized by very high TiO2

(3�87–5�28 wt %) and Zr (343–568 ppm) contents andare the most enriched of the four magma groups. Halfof the Group 4 dykes are low-K picrites and overlap(Fig. 12b) the picritic HTZ group of Sweeney et al.(1994) and, in part, the CT4 picrites of Luttinen et al.(1998). Luttinen et al. (1998) emphasized the OIB-likegeochemistry of their CT4 subgroup and suggested thatthis may represent melts derived from a plume com-ponent in the Karoo volcanic province. The CT4dykes have lower TiO2 than the Group 4 dykes of theAhlmannryggen.

Sweeney et al. (1994) discussed the origins of the HTZKaroo picrites and interpreted them as the likely parentof the abundant low-MgO, HTZ magmas. They alsoinvoked a plume component in the petrogenesis of theHTZ group, which was supported by Ellam et al. (1992)using Re–Os isotopes.

Group 4 dykes have a narrow range in strike direction(�020�), which is parallel to the Jutulstraumen subglacialtrough and also corresponds to the mean strike directionof the Group 2 dykes (Fig. 2). Both dyke groups wereinterpreted to have been emplaced at �178 Ma. This ageis confirmed by the recent work of Zhang et al. (2003) whoreported an age of 176�6 � 0�6 Ma for a dyke (A309)from the southern Kirwanveggen (Fig. 1). The geochem-istry of this dyke is reported in Table 2 (A. V. Luttinen,unpublished data); it corresponds closely to the Group 4geochemical group (high Ti–Zr) and may actually be partof the same dyke as sample Z.1653.2 (Table 2), also fromPetrel Peak (Fig. 1). The Zhang et al. (2003) age (177 Ma)for dyke A309 is in close agreement with the 178 Ma agefor another Group 4 dyke dated as part of this study(Z.1804.3: 178�3 � 3�7 Ma; Fig. 3b).

Group 4 dykes are chemically unusual in the context ofthe Karoo volcanic province. They are the highest TiO2–Zr rocks of the Karoo, and with the exception of local-ized alkali–mafic intrusions and lavas, they also formsome of the most incompatible element enriched rocks.Seven of the 10 samples identified in Group 4 have MgOcontents >11�2 wt %, with mg-numbers up to 72, and are,therefore, picritic. Unlike the other three geochemicalgroups of the Ahlmannryggen, the Group 4 rocks display

a wide range in eNd, varying from 2�4 to �4�6 (Fig. 10),which suggest a contribution from partial melts ofenriched SCLM or contamination by continental crust.All Group 4 dykes shown in Fig. 9d have at least a minornegative Nb–Ta anomaly. The dykes with the weakestNb–Ta anomalies are the samples with the highest eNdvalues (�1�5), whereas those with lower eNd values(��4�5) have the most pronounced Nb–Ta anomaly.Using average lamproite as a proxy for a small degreepartial melt of subcontinental lithospheric mantle,Group 4 magmas could represent mixes of a partialmelt of a MORB source with significant (�20–30%)partial melt of the lithospheric mantle (Fig. 16). TheTh/Ta vs Ti/Zr plot (Fig. 15) also suggests involvementof an enriched component in the petrogenesis of theGroup 4 magmas, which trend away from the MORBfield toward a crustal or lithospheric mantle component.Dy/Yb values (3–3�5; Fig. 19) support melting in thepresence of garnet. The steep slopes of the multi-elementplots (Fig. 9d) also indicate that the HREE are beingretained in the mantle source. The Group 4 dykes havenegative DNb values, but are characterized by very highZr/Y values (Fig. 18).

CONCLUSIONS

Emplacement history and tectonics

Our new geochemical groupings, together with structuraland stratigraphical observations and geochronology,allow us to reconstruct the emplacement history of theMesozoic dykes of the central Ahlmannryggenn range.Available 40Ar/39Ar ages indicate that the picrites andferropicrites of Group 3 are the oldest of the Mesozoicdyke suites, emplaced at �190 Ma. Group 3 dykes trend�N 70 E and are restricted to a narrow, �8 km widecorridor, extending from the Grunehogna nunatak areato two isolated exposures along strike to the WSW (Fig. 2).Group 4 and Group 2 dykes represent the youngest, withan age peak at �178 Ma.

Dykes of Group 1 are sub-parallel to the Group 3picrites and ferropicrites. Group 2 and Group 4 dykesare dominantly NNE–SSW striking. The parallel relation-ship between dykes from Groups 1 and 3 suggests thatthey were probably all emplaced in response to the sameapplied stress field, which, based on our geochronology,existed at �190 Ma. Although the exposures of Group 3dykes did not yield data conducive for dyke swarm dilationestimates, limited data from Group 1 dykes suggest anapproximately north–south dilation direction (004–184�), and, by association, a parallel oriented minimumprincipal stress. In contrast, the overwhelming majority ofdykes in Groups 2 and 4 are almost exclusively north–south to NNE–SSW trending ( Jutulstraumen parallel)and were emplaced in response to NW–SE (307–127�)oriented dilation (minimum principal stress).

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A large number of dykes are present on the easternflank of the Jutulstraumen ice stream, exposed atStraumsvola nunatak and in other nunataks within a30 km radius (Harris & Grantham, 1993). Here twomain dyke trends can be recognized, one NE–SW trend-ing (Harris & Grantham, 1993), and a second moredominant trend of NNW–SSE, although no data regard-ing the relative or absolute chronology of these dykesare currently available, other than that some dykespostdate the Straumsvola nepheline syenite, which isdated between 180�9 � 2�8 Ma (Grantham, 1996) and178 � 2 Ma (Grantham et al., 2001), nor are there anypalaeostress or dilation direction data. It is possible thatNE–SW-oriented dykes in the Straumsvola area wereemplaced synchronously with similarly oriented dykes inthe Ahlmannryggen. If such a correlation is correct theemplacement of these dykes implies the existence of aregional NW–SE-oriented minimum principal stress dir-ection that was perpendicular to the crustal boundarybetween the Archaean Grunehogna craton and the 1 Gamobile belt of the Maudheim Province (Fig. 1 inset).

The giant Okavango dyke swarm (ODS; Fig. 1) ofsouthern Africa forms one arm of a giant radiating dykeswarm, which also includes the Sabie River (SRBF; Fig. 1)and Rooi Rand dyke swarms (RRDS; Fig. 1). Like theGroups 2 and 4 dykes of the Ahlmannryggen, theOkavango dyke swarm exploits a major crustal bound-ary. This swarm has recently been dated and shown tocontain a significant component of 178 Ma dykes(Elburg & Goldberg, 2000; Le Gall et al., 2003; Jourdanet al., 2004a). It therefore seems likely that the NNE–SSW-oriented dykes of the Ahlmannryggen may havebeen a component of a similar radiating dyke swarm,although the small average width of the Ahlmannryggendykes (�1 m), which is considerably less than the meanwidth of the Okavango dykes (18 m), suggests that thepotential Jutulstraumen arm of the radiating dyke swarmwas subject to significantly reduced magmatism.

It is tempting to correlate the ENE–WSW-trendingdykes (Groups 1 and 3) of the Ahlmannryggen with theRooi Rand dyke swarm (RRDS) of southern Africa,based on their sub-parallel alignment in Gondwanareconstructions. However, both 40Ar/39Ar geochrono-logy (RRDS is 173�9 � 3�8 Ma; Jourdan et al., 2004b)and geochemistry suggest that the RRDS and Group 3dykes were not emplaced as part of the same event.

Magma types

The minor intrusions of the Ahlmannryggen region ofwestern Dronning Maud Land can be grouped into fourdistinct geochemical types (Groups 1–4) based onimmobile incompatible elements (Ti, Zr, Y), LILE andSr–Nd isotope composition. 40Ar/39Ar geochronologydemonstrates two emplacement events at �190 Ma

(Groups 1 and 3) and �178 Ma (Groups 2 and 4),which bracket the main Karoo volcanic event at�182 Ma.

The Group 1 dykes were emplaced at �190 Ma, par-allel to the Group 3 dykes (Fig. 2) and to the Pencksokketsubglacial trough, which may represent a major graben-like structure that extends to the SW (Hungeling &Thyssen, 1991), parallel to the Heimefrontfjella (Fig. 1).The petrogenesis of the Group 1 dykes is uncertain. Theycould be generated by mixing of �12% melt fraction ofan enriched SCLM component with a partial melt of adepleted source. In this case the variation in 87Sr/86Sr atconstant eNd might be the result of post-magmaticalteration. However, the variation in 87Sr/86Sr also cor-relates with SiO2 and mg-number, and an alternativeinterpretation is that the Group 1 dykes are the result ofAFC processes from a Group 3 parental magma invol-ving lower Archaean crust as the contaminant.

The Group 2 rocks show little or no chemical evidenceof crustal contamination. However, trace element ratiosand Nd–Sr isotopic data suggest the involvement of anenriched lithospheric mantle source component in theirpetrogenesis. If the primary mantle-derived magmaswere similar in composition to the low-87Sr/86SrGroup 3 magmas then the Group 2 compositions couldbe generated by mixing of <10% melt fraction of anenriched SCLM component with a partial melt of adepleted source. The Group 2 rocks were intruded at�178 Ma, broadly parallel to the Jutulstraumen subgla-cial trough, which is interpreted as a continental rift andmay be continuous with the Pencksokket trough (Fig. 1).The Jutulstraumen rift has associated alkaline magmat-ism along its eastern margin (Harris & Grantham, 1993).Group 2 rocks are also geochemically similar to the RooiRand dykes of the Lebombo rift, which were alsoemplaced late in the history of the province.

The Group 3 rocks of the Ahlmannryggen form themost unusual geochemical group of the entire Karoo–Antarctic magmatic province. They include high Ti–Zrpicrites and ferropicrites, which have isotopic character-istics (eNd �8, 87Sr/86Sr �0�7035) consistent withderivation from a depleted mantle source. Their highTiO2 and Zr contents and MORB-normalized trace ele-ment patterns suggest that they are derived by smalldegrees of partial melting of a MORB-like source. TheGroup 3 dykes were intruded at �190 Ma, parallel to thePencksokket glacial trough, and represent the first mag-mas of the Karoo–Antarctic province. They show evid-ence, in part, of both derivation from enriched mantleand crustal contamination. A subgroup with less depletedisotope ratios (eNd �5, 87Sr/86Sr �0�7055) is con-sidered to be the result of �10% upper crustal contam-ination, whereas a secondary subgroup (Fig. 16) is theproduct of small amounts of mixing with partial meltsof enriched lithospheric mantle. The ‘uncontaminated’

1521


Group 3 (eNd �9; 87Sr/86Sr �0�7035) samples are con-sidered to represent the closest composition to the prim-itive sub-lithospheric magmas in the Ahlmannryggen.

The rocks of Group 4 are high Ti–Zr, low-K picrites,which overlap, in part, with the high Ti–Zr basalts ofthe central Lebombo (Sweeney et al., 1994) and the high-Ti CT4 Group from Vestfjella (Luttinen et al., 1998).They were intruded at �177–178 Ma and extend fromthe Ahlmannryggen to the southern Kirwanveggen(Fig. 1). They have a strike direction parallel to the Jutul-straumen glacial trough and to the 178 Ma Group 2intrusions. They appear to be small-volume partialmelts generated at depths similar to or greater than theGroup 3 magmas. The Group 4 rocks fall into two sub-types: those with a clear contribution from subduction-modified lithospheric mantle (strongly negative eNd, Nb–Ta negative anomaly) and those with positive eNd andflatter multi-element patterns (Fig. 9d). Sweeney et al.(1994) suggested a key role for subcontinental lithosphericmantle and an asthenospheric plume in the generation ofthe Lebombo low-K picrites.

Role of a mantle plume

The geochemical characteristics of the Ahlmannryggenintrusions suggest complex mixing relationships bet-ween Ti-rich, small volume partial melts (Group 3) of adepleted mantle source and partial melts of enrichedlithospheric mantle plus assimilation of local continentalcrust. The isotopically depleted end-member has bothMORB- and OIB-like characteristics. It is tempting toinvoke an asthenospheric mantle plume origin for theGroup 3 magmas and, given their intrusion age(�190 Ma), this lends support to the incubating astheno-spheric plume model of Sweeney et al. (1994).

Where mantle plumes have been interpreted as beingresponsible for the magmatism of flood basalt provincesthere is often continued debate as to whether the plumearrival at the base of the lithosphere and large volumemelting occurred during a short time period (a fewmillion years) or whether plume arrival was followed bya prolonged period (�10 Myr) prior to the main episodeof magmatism.

40Ar/39Ar geochronology by Duncan et al. (1997) onbasic lavas from southern Africa and the Kirwanveggensuggests that the duration of magmatism was very short(1–2 Myr). However, the Duncan et al. (1997) study didnot include any data on minor intrusions of the Karoo.The new 40Ar/39Ar geochronology data presentedhere indicate a long-lived magmatic event of >10 Myr(178–190 Ma). This extended time period is associatedwith diverse magma chemistry, in contrast to the domin-antly low-Ti tholeiites associated with the 182–183 Maflood basalt event. The style and chronology of the mag-matism observed in the Karoo–Dronning Maud Land isakin to that described from the Etendeka province of NW

Namibia (Thompson et al., 2001), although the Karooevent is considerably more prolonged and is associatedwith craton boundaries. The evidence presented herestrongly suggests that a plume incubation model may beapplicable for the Early Jurassic magmatism of theKaroo–Dronning Maud Land province.

ACKNOWLEDGEMENTS

The field and air operations staff at Halley Base arethanked for their support. Graham Pearson (Universityof Durham) supplied the ICP-MS analyses, and DaveEmley (University of Keele) carried out the XRF ana-lyses. This work has benefited greatly from the thoroughand thoughtful reviews of Chris Harris, Andrew Kerr,Arto Luttinen and Marjorie Wilson. Adela Fazel acknow-ledges the receipt of an Antarctic Funding Initiativestudentship.

REFERENCES

Armstrong, R. A., Bristow, J. W. & Cox, K. G. (1984). The Rooi Rand

dyke swarm, southern Lebombo. In: Erlank, A. J. (ed.) Petrogenesis of

the Volcanic Rocks of the Karoo Province. Geological Society of South Africa,

Special Publications 13, 77–86.

Bergman, S. C. (1987). Lamproites and other potassium-rich igneous

rocks: a review of their occurrence, mineralogy and geochemistry.

In: Fitton, J. G. & Upton, B. G. J. (eds) Alkaline Igneous Rocks. Geological

Society, London, Special Publications 30, 103–190.

Brewer, T. S., Rex, D., Guise, P. G. & Hawkesworth, C. J. (1996).

Geochronology of Mesozoic tholeiitic magmatism in Antarctica:

implications for the development of the failed Weddell Sea rift

system. In: Storey, B. C., King, E. C. & Livermore, R. A. (eds)

Weddell Sea Tectonics and Gondwana Break-up. Geological Society, London,

Special Publications 108, 45–61.

Bussell, M. A. (1989). A simple method for the determination of the

dilation direction of intrusive sheets. Journal of Structural Geology 11,

679–687.

Chambers, L. M. & Fitton, J. G. (2000). Geochemical transitions in the

ancestral Iceland plume: evidence from the Isle of Mull Tertiary

volcano, Scotland. Journal of the Geological Society, London 157, 261–263.

Cox, K. G. (1989). The role of mantle plumes in the development of

continental drainage patterns. Nature 342, 873–877.

Cox, K. G. (1992). Karoo igneous activity, and the early stages of the

break-up of Gondwanaland. In: Storey, B. C., Alabaster, T. &

Pankhurst, R. J. (eds) Magmatism and the Causes of Continental Break-up.

Geological Society, London, Special Publications 68, 137–148.

Curtis, M. L. & Riley, T. R. (2003). Mobilization of fluidised sediment

during sill emplacement, western Dronning Maud Land, East

Antarctica. Antarctic Science 15, 393–398.

De Bruiyn, H., Schoch, A. E., van der Westhuizen, W. A. & Myburgh,

C. A. (2000). Picrite from the Katse area, Lesotho: evidence for flow

differentiation. Journal of African Earth Sciences, 31, 657–668.

Delaney, P. T., Pollard, D. D., Ziony, J. I. & McKee, E. H. (1986).

Field relations between dikes and joints: emplacement processes and

palaeostress analysis. Journal of Geophysical Research 91, 4920–4938.

Duncan, A. R., Erlank, A. J. & Marsh, J. S. (1984). Geochemistry of the

Karoo igneous province. In: Erlank, A. J. (ed.) Petrogenesis of the

1522


Volcanic Rocks of the Karoo Province. Geological Society of South Africa, Special

Publications 13, 355–388.

Duncan, A. R., Armstrong, R. A., Erlank, A. J., Marsh, J. S. &

Watkins, R. T. (1990). MORB-related dolerites associated with the

final phases of Karoo flood basalt volcanism in southern Africa. In:

Parker, A. J., Rickwood, P. C. & Tucker, D. H. (eds) Mafic Dykes and

Emplacement Mechanisms. Rotterdam: Balkema, pp. 119–129.

Duncan, R. A., Hooper, P. R., Rehacek, J., Marsh, J. S. & Duncan, A. R.

(1997). The timing and duration of the Karoo igneous event, southern

Gondwana. Journal of Geophysical Research 102, 18127–18138.

Elburg, M. & Goldberg, A. (2000). Age and geochemistry of Karoo

dolerite dykes from northeast Botswana. Journal of African Earth

Sciences 31, 539–554.

Ellam, R. M. & Cox, K. G. (1991). An interpretation of Karoo picrite

basalts in terms of interaction between asthenospheric magmas and

the mantle lithosphere. Earth and Planetary Science Letters 105, 330–342.

Ellam, R. M., Carlson, R. W. & Shirley, S. B. (1992). Evidence from

Re–Os isotopes for plume–lithosphere mixing in Karoo flood basalt

genesis. Nature 359, 718–721.

Elliot, D. H., Fleming, T. H., Kyle, P. R. & Foland, K. A. (1999). Long-

distance transportation of magmas in the Jurassic Ferrar Large

Igneous Province, Antarctica. Earth and Planetary Science Letters 167,

89–104.

Erlank, A. J. (ed.) (1984). Petrogenesis of the Volcanic Rocks of the Karoo

Province. Geological Society of South Africa, Special Publications 13.

Ernst, R. E. & Buchan, K. L. (2001). The use of mafic dyke swarms

in identifying and locating mantle plumes. In: Ernst, R. E. &

Buchan, K. L. (eds) Mantle Plumes: their Identification through Time.

Geological Society of America, Special Papers 352, 247–265.

Ewart, A., Marsh, J. S., Milner, S. C., Duncan, A. R., Kamber, B. S. &

Armstrong, R. A. (2004). Petrology and geochemistry of Early

Cretaceous bimodal continental flood volcanism of the NW Etendeka,

Namibia. Part 1: Introduction, mafic lavas and re-evaluation of

mantle source components. Journal of Petrology 45, 59–105.

Fitton, J. G., Saunders, A. D., Norry, M. J., Hardarson, B. S. &

Taylor, R. N. (1997). Thermal and chemical structure of the Iceland

plume. Earth and Planetary Science Letters 153, 197–208.

Fitton, J. G., Saunders, A. D., Kempton, P. D. & Hardarson, B. S.

(2003). Does depleted mantle form an intrinsic part of the Iceland

plume? Geochemistry, Geophysics, Geosystems 4, 1029/2002GC000424.

Fleming, T. H., Foland, K. A. & Elliot, D. H. (1995). Isotopic and

chemical constraints on the crustal evolution and source signature

of Ferrar magmas, north Victoria Land, Antarctica. Contributions to

Mineralogy and Petrology 121, 217–236.

Floyd, P. A. (1985). Petrology and geochemistry of intraplate sheet-flow

basalts, Nauru Basin, Deep Sea Drilling Project leg 89. In:

Moberley, R. & Schlanger, S. O. (eds) Initial Reports of the Deep Sea

Drilling Project, 89. Washington, DC: US Government Printing

Office, pp. 471–497.

Foulger, G. R. (2002). Plumes, or plate tectonic processes? Astronomy and

Geophysics 43, 6.19–6.23.

Furnes, H., Neumann, E.-R. & Sundvoll, B. (1982). Petrology and

geochemistry of Jurassic basalt dikes from Vestfjella, Dronning

Maud Land, Antarctica. Lithos 15, 295–304.

Furnes, H., Vad, E., Austrheim, H., Mitchell, J. G. & Garmann, L. B.

(1987). Geochemistry of basalt lavas from Vestfjella and adjacent

areas, Dronning Maud Land, Antarctica. Lithos 20, 337–356.

Gibson, S. A., Thompson, R. N. & Dickin, A. P. (2000). Ferropicrites:

geochemical evidence for Fe-rich streaks in upwelling mantle

plumes. Earth and Planetary Science Letters 174, 355–374.

Grantham, G. H. (1996). Aspects of Jurassic magmatism and faulting

in western Dronning Maud Land, Antarctica: implications for

Gondwana break-up. In: Storey, B. C., King, E. C. &

Livermore, R. A. (eds) Weddell Sea Tectonics and Gondwana Break-up.

Geological Society, London, Special Publications 108, 63–73.

Grantham, G. H. & Hunter, D. R. (1991). The timing and nature of

faulting and jointing adjacent to the Pencksokket, western Dronning

Maud Land, Antarctica. In: Thomson, M. R. A., Crame, J. A. &

Thomson, J. W. (eds) Geological Evolution of Antarctica. Cambridge:

Cambridge University Press, pp. 47–51.

Grantham, G. H., Guise, P. G., Spell, T., Eglington, B. M. &

Kruger, F. J. (2001). The age, chemistry and structure of Jurassic

intrusions in H. U. Svedrupfjella, western Dronning Maud Land.

International Dyke Conference, Ithala, South Africa, 27 (abstract).

Groenewald, P. M., Moyes, A. B., Grantham, G. H. & Krynauw, J. R.

(1995). East Antarctic crustal evolution: geological constraints and

modelling in western Dronning Maud Land. Precambrian Research 75,

231–250.

Hanson, R. E., Martin, M. W., Bowring, S. A. & Munyanyiwa, H.

(1998). U–Pb zircon age for the Umkondo dolerites, eastern

Zimbabwe: 1�1 Ga large igneous province in southern

Africa—East Antarctica and possible Rodinia correlations. Geology

26, 1143–1146.

Harmer, R. E., Lee, C. A. & Eglington, B. M. (1998). A deep mantle

source for carbonatite magmatism: evidence from the nephelinites

and carbonatites of the Buhera district, SE Zimbabwe. Earth and

Planetary Science Letters 158, 131–142.

Harris, C. & Grantham, G. H. (1993). Geology and petrogenesis of the

Straumsvola nepheline syenite complex, Dronning Maud Land,

Antarctica. Geological Magazine 130, 513–532.

Harris, C., Marsh, J. S., Duncan, A. R. & Erlank, A. J. (1990). The

petrogenesis of the Kirwan Basalts of Dronning Maud Land,

Antarctica. Journal of Petrology 31, 341–369.

Harris, C., Watters, B. R. & Groenewald, P. B. (1991). Geochemistry of

the Mesozoic regional basic dykes of western Dronning Maud Land,

Antarctica. Contributions to Mineralogy and Petrology 107, 100–111.

Harris, C., Johnstone, W. P. & Phillips, D. (2002). Petrogenesis of

the Mesozoic Sistefjell syenite intrusion, Dronning Maud Land,

Antarctica and surrounding low-d18O lavas. South African Journal of

Geology 105, 205–226.

Hawkesworth, C. J., Kelley, S. P., Turner, S., le Roex, A. P. &

Storey, B. C. (1999). Mantle processes during Gondwana break-up

and dispersal. Journal of African Earth Sciences 28, 239–261.

Hergt, J. M., Peate, D. W. & Hawkesworth, C. J. (1991). The

petrogenesis of Mesozoic Gondwana low-Ti flood basalts. Earth and


Hofmann, A. W. (1988). Chemical differentation of the Earth: the

relationship between mantle, continental crust and oceanic crust.

Earth and Planetary Science Letters 90, 297–314.

Hungeling, A. & Thyssen, F. (1991). Reflection seismic measurements

in western Neuschwabenland. In: Thomson, M. R. A., Crame, J. A. &

Thomson, J. W. (eds) Geological Evolution of Antarctica. Cambridge:

Cambridge University Press, pp. 549–555.

Jacobs, J., Thomas, R. J. & Weber, K. (1993). Accretion and

indentation tectonics at the southern edge of the Kaapvaal craton

during Kibaran (Grenville) orogeny. Geology 21, 203–206.

Jourdan, F., F�eeraud, G., Bertrand, H., Kampunzu, A. B., Tshoso, G.,

Le Gall, B., Tiercelin, J. J. & Capiez, P. (2004a). The Karoo triple

junction questioned: evidence from Jurassic and Proterozoic40Ar/39Ar ages and geochemistry of the Okavango dyke swarm

(Botswana). Earth and Planetary Science Letters 222, 989–1006.

Jourdan, F., F�eeraud, G., Bertrand, H., Kampunzu, A. B.,

Watkeys, M. K., Le Gall, B. & Tshoso, G. (2004b). New age

constraints on the Karoo large igneous province: triple junction and

brevity questioned. Goldschmidt Geochemistry Conference, Copenhagen,

Program with Abstracts, A575.

1523


Kent, R. W. & Fitton, J. G. (2000). Mantle sources and melting

dynamics in the British Palaeogene igneous province. Journal of

Petrology 41, 1023–1040.

Kerr, A. C., Saunders, A. D., Tarney, J., Berry, N. H. & Hards, V. L.

(1995). Depleted mantle-plume geochemical signatures: no paradox

for plume theories. Geology 23, 843–846.

Koppers, A. A. P. (2002). ArArCALC—software for Ar-40/Ar-39 age

calculations. Computers and Geosciences 28, 605–619.

Kretz, R. (1991). The dilation direction of intrusive sheets. Journal of

Structural Geology 13, 97–99.

Krynauw, J. R., Hunter, D. R. & Wilson, A. H. (1988). Emplacement

of sills into wet sediments at Grunehogna, western Dronning Maud

Land, Antarctica. Journal of the Geological Society, London 145, 1019–

1032.

Le Bas, M. J. (2000). IUGS reclassification of the high-Mg and picritic

volcanic rocks. Journal of Petrology 41, 1467–1470.

Le Bas, M. J., Le Maitre, R. W., Streckeisen, A. & Zanettin, B. (1986).

A chemical classification of volcanic rocks based on the total alkali–

silica diagram. Journal of Petrology 27, 745–750.

Le Gall, B., Tshoso, G., Jourdan, F., F�eeraud, G., Bertrand, H.,

Tiercelin, J. J., Kampunzu, A. B., Modisi, M. P., Dyment, J. &

Maia, M. (2002). 40Ar/39Ar geochronology and structural data from

the giant Okavango and related mafic dyke swarms, Karoo igneous

province, northern Botswana. Earth and Planetary Science Letters 202,

595–606.

le Roex, A. P. & Lanyon, R. (1998). Isotope and trace element

geochemistry of Cretaceous Damaraland lamprophyres and carbon-

atites, northwestern Namibia: evidence for plume–lithosphere

interactions. Journal of Petrology 39, 1117–1146.

Luttinen, A. V. & Furnes, H. (2000). Flood basalts of Vestfjella: Jurassic

magmatism across an Archaean Proterozoic lithospheric boundary

in Dronning Maud Land, Antarctica. Journal of Petrology 41, 1271–

1305.

Luttinen, A. V., Ramo, O. T. & Huhma, H. (1998). Nd and Sr isotopic

and trace element composition of a Mesozoic CFB suite from

Dronning Maud Land, Antarctica: implications for lithosphere and

asthenosphere contributions to Karoo magmatism. Geochimica et

Cosmochimica Acta 62, 2701–2714.

Marsh, J. S. & Mndaweni, M. J. (1998). Geochemical variations in a

long Karoo dyke, eastern Cape. South African Journal of Geology 101,

119–122.

Mitchell, A. A., Rmaluckan, V. R., Dunlevey, J. N. & Eglington, B. M.

(1996). The basalt stratigraphy of the Sani Pass, Kwazulu/Natal

Drakensberg. South African Journal of Geology 99, 251–262.

Mitchell, C., Ellam, R. M. & Cox, K. G. (1999). Mesozoic

dolerite dykes of the Falkland Islands: petrology, petrogenesis and

implications for geochemical provinciality in Gondwanaland

low-Ti basaltic rocks. Journal of the Geological Society, London 156,

901–916.

Molzahn, M., Reisberg, L. & Worner, G. (1996). Os, Sr, Nd, Pb and O

isotope and trace element data from the Ferrar flood basalts,

Antarctica: evidence for an enriched subcontinental lithospheric

source. Earth and Planetary Science Letters 144, 529–546.

Moyes, A. B., Krynauw, J. R. & Barton, J. M. (1995). The age of the

Ritscherflya Supergroup and Borgmassivet Intrusions, Dronning

Maud Land, Antarctica. Antarctic Science 7, 87–97.

Nakamura, N. (1974). Determination of REE, Ba, Fe, Mg, Na and K in

carbonaceous and ordinary chondrites. Geochimica et Cosmochimica Acta

38, 757–773.

Ottley, C. J., Pearson, D. G & Irvine, G. J. (2003). A routine method

for the dissolution of geological samples for the analysis of REE and

trace elements via ICP-MS. In: Plasma Source Mass Spectrometry. Special

Publication, Royal Society of Chemistry 221–230.

Pankhurst, R. J. & Rapela, C. R. (1995). Production of Jurassic

rhyolites by anatexis of the lower crust of Patagonia. Earth and


Peate, D.W. (1997). The Paran�aa–Etendeka Province. In: Mahoney, J. &

Coffin, M. (eds) Large Igneous Provinces: Continental, Oceanic, and Planetary

Flood Volcanism. Geophysical Monograph, American Geophysical Union 100,

217–245.

Peters, M., Haverkamp, B., Emmermann, R., Kohnen, H. & Weber, K.

(1991). Palaeomagnetism, K–Ar dating and geodynamic setting of

igneous rocks in western and central Neuschwabenland, Antarctica.

In: Thomson, M. R. A., Crame, J. A. & Thomson, J. W. (eds)

Geological Evolution of Antarctica. Cambridge: Cambridge University

Press, pp. 549–555.

Reid, D. L., Rex, D. C. & Brandl, G. (1997). Karoo basalts in the

Ellisras sub-basin, northern Province. South African Journal of Geology

100, 151–156.

Renne, P. R., Swisher, C. C., Deino, A. L., Karner, D. B.,

Owens, T. L. & DePaolo, D. J. (1998). Intercalibration of standards,

absolute ages and uncertainties in Ar–Ar dating. Chemical Geology 145,

117–152.

Riley, T. R. & Knight, K. B. (2001). Age of pre-break-up Gondwana

magmatism: a review. Antarctic Science 13, 99–110.

Rock, N. M. S. (1991). Lamprophyres. Glasgow: Blackie.

Rollinson, H. (1993). Using Geochemical Data: Evaluation, Presentation,

Interpretation. Harlow: Longman, 352 pp.

Rudnick, R. L. & Fountain, D. M. (1995). Nature and composition of

the continental crust: a lower crustal perspective. Reviews of Geophysics

33, 267–309.

Sun, S. S. & McDonough, W. F. (1989). Chemical and isotopic

systematics of oceanic basalts: implications for mantle composition

and processes. In: Saunders, A. D. & Norry, M. J. (eds) Magmatism in

Ocean Basins. Geological Society, London, Special Publications 42, 313–345.

Sweeney, R. J., Duncan, A. R. & Erlank, A. J. (1994). Geochemistry

and petrogenesis of Central Lebombo basalts of the Karoo Igneous

Province. Journal of Petrology 35, 95–125.

Thompson, R. N., Gibson, S. A., Dickin, A. P. & Smith, P. M. (2001).

Early Cretaceous basalt and picrite dykes of the southern Etendeka

region, NW Namibia: windows into the role of the Tristan mantle

plume in Paran�aa–Etendeka magmatism. Journal of Petrology 42,

2049–2081.

Vuori, S. K. & Luttinen, A. V. (2003). The Jurassic gabbroic intrusions

of Utpostane and Muren: insights into Karoo-related plutonism in

Dronning Maud Land, Antarctica. Antarctic Science 15, 283–301.

Wolmarans, L. G. & Kent, L. E. (1982). Geological investigations in

western Dronning Maud Land, Antarctica—a synthesis. South African

Journal of Antarctic Research Supplement 2.

Wooden, J. L., Czamanske, G. K., Federenko, V. A., Arndt, N. T.,

Chauvel, C., Bouse, R. M., King, B. W., Knight, R. J. & Siems, D. F.

(1993). Isotopic and trace element constraints on mantle and crustal

contributions to Siberian continental flood basalts, Noril’sk area,

Siberia. Geochimica et Cosmochimica Acta 57, 3677–3704.

Yoder, H. S. & Tilley, C. E. (1962). Origin of basalt magmas: an

experimental study of natural and synthetic rock systems. Journal of

Petrology 3, 342–532.

Zhang, X., Luttinen, A. V., Elliot, D. H., Larsson, K. & Foland, K. A.

(2003). Early stages of Gondwana breakup: the 40Ar/39Ar geochro-

nology of Jurassic basaltic rocks from western Dronning Maud Land,

Antarctica, and implications for the timing of magmatic and

hydrothermal events. Journal of Geophysical Research 108, 2249.

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