Journal of Petrology Advance Access originally published online on March 30, 2005
Journal of Petrology 2005 46(7):1489-1524; doi:10.1093/petrology/egi023
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Early–Middle Jurassic Dolerite Dykes from Western Dronning Maud Land (Antarctica): Identifying Mantle Sources in the Karoo Large Igneous Province
1 BRITISH ANTARCTIC SURVEY, NATURAL ENVIRONMENT RESEARCH COUNCIL, HIGH CROSS, MADINGLEY ROAD, CAMBRIDGE CB3 0ET, UK
2 NERC ISOTOPE GEOSCIENCES LABORATORY, KEYWORTH, NOTTINGHAM NG12 5GG, UK
3 COLLEGE OF OCEANIC AND ATMOSPHERIC SCIENCES, OREGON STATE UNIVERSITY, CORVALLIS, OR 97331-5503, USA
4 DEPARTMENT OF EARTH SCIENCES, WALTON HALL, OPEN UNIVERSITY, MILTON KEYNES MK7 6AA, UK
RECEIVED DECEMBER 22, 2003; ACCEPTED FEBRUARY 9, 2005
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ABSTRACT |
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TOP
ABSTRACT
INTRODUCTIONA 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 compositions 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 190 Ma. 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 190 Ma and have low TiO2 and Zr contents and a significant Archaean crustal component, but also evidence of hydrothermal alteration. Group 2 dykes were intruded at 178 Ma; 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 190 Ma 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, 
Ndi 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 interpreted 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 178 Ma; 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 190 Ma in response to the same regional stress field, which had changed by 178 Ma, 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
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INTRODUCTION |
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The role of mantle plumes in the generation of large igneous provinces (LIPs) has been, and remains, a subject of intense debate (e.g. Ernst & Buchan, 2001
; Foulger, 2002). The magmatism of the Karoo province of southern Africa has been reliably dated (40Ar/39Ar and U–Pb) at 179–184 Ma, with a significant peak of activity at 182–183 Ma (Duncan et al., 1997). Although a definite plume link has never been established for the Karoo igneous province (e.g. Hawkesworth et al., 1999), most workers (e.g. Cox, 1989; Ellam & Cox, 1991; Sweeney et al., 1994) have concluded that a significant thermal anomaly must have existed to generate the large volume of erupted magma (>2 x 106 km3; Elliot et al., 1999) over such a short period of time (Duncan et al., 1997). The geochemistry of Karoo igneous rocks has been interpreted by some workers to indicate either derivation of the magmas from an enriched lithospheric mantle source, or crustal contamination of partial melts of a lithospheric mantle source (e.g. Erlank, 1984). Other workers have proposed that the Karoo magmas are plume-derived, but contaminated by the lithospheric mantle en route to the surface (Cox, 1992; Ellam et al., 1992). Central to this debate is the need to establish that a plume source exists and to evaluate if its role is restricted to conductive heat transfer 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 of the Karoo large igneous province of southern Africa (Harris et al., 1991; Luttinen et al., 1998). The regional dolerite dykes of the Ahlmannryggen area (Fig. 1) have previously been described by Harris et al. (1991), who made comparisons with the composition of basalt lavas from the Kirwanveggen area further to the south (Harris et al., 1990), and also with the high-Ti basalts of the Karoo province (Duncan et al., 1984). This study extends the work of Harris et al. (1991) by providing a full geochemical and isotopic dataset over a broader geographical area, therefore allowing comparisons to be made with more recent data on the Karoo igneous province of southern Africa (e.g. Sweeney et al., 1994), and elsewhere in East Antarctica (e.g. Luttinen & Furnes, 2000).
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Where sub-lithospheric mantle sources for continental flood basalts can be identified there is mounting evidence that they are heterogeneous in composition, with both depleted and enriched source components (e.g. Kerr et al., 1995
; Fitton et al., 1997); the magmas derived from these components include ferropicrites (Gibson et al., 2000) and lamprophyric rock types (le Roex & Lanyon, 1998; Thompson et al., 2001). The data presented here allow us to evaluate, assess and model these components, with respect to the ‘Karoo plume’. |
GEOLOGICAL SETTING |
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Basaltic lavas and minor intrusions of Jurassic age crop out at several localities in western Dronning Maud Land, Antarctica (Fig. 1). Both flood basalts and dykes are exposed at Vestfjella, Heimefrontfjella and Kirwanveggen, whereas only dykes are exposed in the Ahlmannryggen, Mannefallknausane and H. U. Svedrupfjella areas (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 overlie clastic sediments of the Amelang Plateau Formation; these sedimentary rocks overlie Proterozoic gneisses basement of the Svedrupfjella Group. Locally, the sedimentary succession is absent and the lavas directly overlie the gneisses. Two of these lava flows have been dated by 40Ar/39Ar geochronology (Duncan et al., 1997), with Middle Jurassic ages of 180·6 ± 0·6 and 182·8 ± 0·6 Ma. In Vestfjella, the thickness of the lava pile exceeds 900 m in the north and 400 m in the south (Luttinen & Furnes, 2000). The lava pile is cut by dolerite dykes and sills and, at Muren and Utpostane (Fig. 1), by gabbro intrusions (Vuori & Luttinen, 2003), which have been dated at 177·0 ± 0·5 Ma (40Ar/39Ar on plagioclase; Zhang et al., 2003). The age of the Vestfjella lavas is poorly 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 to the age of the Kirwanveggen lavas (Duncan et al., 1997).
The basement of western Dronning Maud Land is divided into two major domains. Prior to the Mesozoic break-up of Gondwana, the Archaean Grunehogna craton (Fig. 1) is presumed to have been part of the Kaapvaal craton (Groenewald et al., 1995). The craton is bounded to the east and SE by the Mesoproterozoic Maud Belt, the Antarctic extension of the Natal Belt of Africa (Jacobs et al., 1993). The exact position of the Archaean–Proterozoic lithospheric terrane boundary is not firmly established, but, on the basis of gravity and aeromagnetic data, 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 area intrude Neoproterozoic age rocks of the Ritscherflya Supergroup, which cover the entire Ahlmannryggen and Borg Massivet regions (Fig. 1). The Ritscherflya Supergroup, which overlies Archaean (2·8–3·0 Ga) basement, consists of relatively undeformed sedimentary and volcanogenic rocks of the Ahlmannryggen and Jutulstraumen groups (Wolmarans & Kent, 1982), which have been intruded extensively by massive tholeiitic sills and dykes of the Borgmassivet Intrusions (Wolmarans & Kent, 1982). Wolmarans & Kent (1982) reported a Rb–Sr whole-rock isochron age of 1073 ± 40 Ma based on seven mafic sills from the Ahlmannryggen, therefore the Borgmassivet Intrusions could be coeval with the Umkondo large igneous province of southern Africa (1·1 Ga; Hanson et al., 1998). This date is close to the inferred lithification age of the Ritscherflya Supergroup sedimentary rocks (1085 ± 27 Ma; Moyes et al., 1995) and therefore supports the field observations of Krynauw et al. (1988) and Curtis & Riley (2003) that the Borgmassivet Intrusions were emplaced into wet, partially lithified sediments.
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SAMPLING STRATEGY |
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Over 90 dykes and sills were recorded from the Ahlmannryggen region of western Dronning Maud Land (Fig. 2). Each dyke or sill was sampled and its strike, dip, width and exact position were recorded. Based on petrography, whole-rock geochemistry and preliminary geochronology, a significant (
25%) subset of these dykes and sills were believed to be Proterozoic in age (1100 Ma). Forty-seven Mesozoic dykes (Fig. 2) were selected for this study, which were the freshest samples available and geochemically showed the least evidence of hydrothermal alteration.
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GEOCHRONOLOGY |
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Previous work
Very few reliable ages have been published for the minor intrusions of western Dronning Maud Land. Mesozoic ages have been reported by Wolmarans & Kent (1982)
, who dated an olivine-bearing dolerite dyke from Nils Jorgennutane (Fig. 1) at 192 ± 8 Ma (K–Ar whole rock), whereas Watters & Rex [K–Ar unpublished data cited by Harris et al. (1991)] have reported ages in the range 190–200 Ma. 40Ar/39Ar geochronology on plagioclase mineral separates by Brewer et al. (1996) indicated two episodes 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 older episode of magmatism has been confirmed by Duncan et al. (1997), who carried out an 40Ar/39Ar study on basaltic lavas from Kirwanveggen (Fig. 1), which yielded plateau ages of 180·4 ± 0·6 and 182·6 ± 0·6 Ma, coincident with the main Karoo volcanism of southern Africa (Riley & Knight, 2001). Zhang et al. (2003) have recently completed a detailed 40Ar/39Ar study on a variety of basaltic rocks from the Vestfjella region (Fig. 1) of western Dronning Maud Land, which display a broad range of Jurassic ages. They reported ages of 177·0 ± 0·5 Ma for the Utpostane gabbro and 176·6 ± 0·5 Ma for a dolerite dyke from the Kirwanveggen, whereas a dolerite dyke from Basen (Fig. 1) was dated at 193 Ma. 40Ar/39Ar geochronology data reported by Grantham (1996) for the Straumsvola and Tvora (Fig. 1) alkaline plutons (Harris & Grantham, 1993) from western Dronning Maud Land indicate intrusion ages in the range 178–182 Ma, and Harris et al. (2002) gave an age of 180 Ma for the nearby Sistefjell syenite (Fig. 1).
This study
Analytical methods
Whole-rock core samples of 5 mm diameter were packaged in evacuated quartz vials, and irradiated in the Oregon State University TRIGA Reactor for 6 h at 1 MW power. The neutron flux was measured using standard FCT-3 biotite, 28·03 Ma (Renne et al., 1994). Reactor temperatures can reach up to 270°C. Additionally, samples were baked at 195°C for 48 h during extraction line pump-down to 10–9 torr.
Depending on sample composition, the incremental heating experiment started in the range 400–600°C and was typically complete by 1400°C using a Heine low-blank resistance furnace with a Ta/Nb crucible and Mo liner. Each heating step was of 20 min duration with an additional 5 min cooling and continued removal of active gases with St101 Zr–Al and St172 Zr–V–Fe getters.
A MAP 215-50 rare gas mass spectrometer, source at 3000 V, and equipped with a Johnston MM1-1SG electron multiplier at 2050 V, was used for analysis. During the 15 min analysis time per mass peak height, data were collected for 10 cycles of masses 35–40 for baselines and peak-tops. Data were reduced and age calculations completed using ArArCALC v2.2 software for 40Ar/39Ar geochronology (Koppers, 2002).
Results
The 40Ar/39Ar data for five samples are presented in Table 1. The groundmass from sample Z.1801.1, an olivine 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 sufficiently disturbed that individual step ages are statistically different from the weighted mean age. There is a strong recoil 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’ and may be close to the crystallization age, although it is not completely reliable by the usual criteria.
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Whole-rock sample Z.1804.3 is an olivine basalt with a quenched glassy texture. It has a matrix partly replaced by clay minerals, but is also characterized by some fresh microphenocrysts. It has a classic recoil pattern with decreasing measured step age vs temperature. The seven-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 olivine dolerite, 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 strongly increasing age toward fusion step 3 is common in excess 40Ar profiles. An isochron calculated using steps 6–11 produces an age of 176·8 ± 9·4 Ma, which has a statistical measure of significance. The case for excess 40Ar is strong in this sample, hence the isochron age is preferred, albeit with relatively large uncertainty.
Whole-rock sample Z.1814.1 is an olivine basalt, which yielded a low-temperature four-step plateau of 190·7 ± 0·8 Ma (Fig. 3d), but comprising only 42% of the total gas released. A higher temperature, four-step plateau, comprising 54% of the gas released, yields an age of 176·2 ± 0·6 Ma. If excess 40Ar is involved, then the younger age would be preferred.
Whole-rock sample Z.1816.1 is an olivine basalt with abundant clay minerals replacing the poorly crystallized matrix. The four-step ‘error plateau’ yields an age of 187·3 ± 3·6 Ma (Fig. 3e) comprising 47% of the total gas released, which is considered to be acceptable. An isochron calculated using eight steps produced a statistically acceptable fit with an age of 176·4 ± 4·8 Ma.
40Ar/39Ar geochronology on a separate suite of Ahlmannryggen dykes and sills was undertaken by A. Fazel (unpublished data) at the Open University (UK) and reveals a similar age pattern to the data obtained from Oregon State University. The data, which are summarized in Fig. 4, indicate a prominent Mesozoic peak (65% of the intrusions yielded Mesozoic ages) composed of several small peaks at 178, 181, 188 and 198 Ma.
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Discussion
The minor intrusions from the Ahlmannryggen area proved very difficult to date, with both laboratories (Oregon State University and Open University) experiencing similar problems. In many cases, the criteria used to define age plateau [(1) each fraction of the plateau is internally concordant and overlaps with the plateau age within a 2
uncertainty; (2) the fractions containing the plateau are continuous and contain at least 50% of the total 39Ar released] were not met. There is little doubt concerning their affinity with the Karoo volcanic province, based on the age ranges obtained, but distinguishing age differences is difficult because of the effects of alteration, 39Ar recoil redistribution and excess 40Ar.
Two dates that are repeated throughout this study are a pre-Karoo volcanism (182 Ma) age of 190 Ma and a post-Karoo volcanism age of 178 Ma. The 178 Ma age is in close agreement with recent age data from the Okavango dyke swarm (Botswana), which yield 40Ar/39Ar (whole-rock and plagioclase) plateau ages in the 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, dykes and gabbros from Vestfjella and Kirwanveggen, western Dronning Maud Land (Fig. 1). The study by Zhang et al. (2003) highlighted similar problems to those of this study, and they noted that the majority of their plagioclase samples yielded discordant age spectra, which they interpreted as reflecting alteration, excess 40Ar and recoil redistribution.
The pre-Karoo ages of 190 Ma are problematic because dates of 10 Myr older than the main Karoo peak at 182 Ma have not been considered as viable crystallization ages before. Zhang et al. (2003) dated plagioclase from a Vestfjella dolerite dyke at 193·0 Ma, but discounted this date as discordant. Jourdan et al. (2004a) also published an integrated age of 191·5 ± 8·4 Ma for a dyke from the Okavango swarm using their ‘speedy’ step-heating experiments. Several of the Ahlmannryggen dykes yield ages of 190 Ma, although there is a scientific case for some of them also to be interpreted as 178 Ma if different steps at different temperatures are used for calculation, or if an isochron or errorchron age is adopted instead of a plateau age.
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WHOLE-ROCK MAJOR AND TRACE ELEMENT AND Sr–Nd ISOTOPE GEOCHEMISTRY |
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Analytical techniques
Powders for geochemical analysis were prepared from 2–3 kg of fresh rock. Samples were reduced to pass a 1700 µm sieve using a hardened steel fly press. The powders were produced using an agate Tema-mill. Sr and Nd isotope compositions were measured at the NERC Isotope Geosciences Laboratory (Keyworth, UK) on a Finnegan-MAT 262 mass-spectrometer. Rb–Sr and Sm–Nd analysis followed procedures described by Pankhurst & Rapela (1995)
. Sr isotope composition was determined in multidynamic peak-jumping mode. During the period of analysis, 32 analyses of the Sr isotope standard NBS987 gave a value of 0·710250 ± 0·000016 (2 errors). Nd-isotope composition was determined in static collection mode. Thirty-one analyses of the in-house J&M Nd isotope standard gave a value of 0·511199 ± 22 (2 errors); reported 143Nd/144Nd values were 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 by X-ray fluorescence (XRF) in Table 2] was by standard XRF techniques at the Department of Geology, University of Keele, with methods fully detailed by Floyd (1985). Higher precision trace element abundances were determined by inductively coupled plasma mass spectrometry (ICP-MS) at the University of Durham. The analytical methods, precision, and detection limits have been detailed by Ottley et al. (2003).
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Classification
Full major and trace element analyses of the Ahlmannryggen dykes are presented in Table 2. The data exhibit significant variations in SiO2, TiO2, K2O, Al2O3, Fe2O3 and MgO. The analysed samples are subalkaline and range 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 classified as quartz tholeiites, with two samples classifying as olivine tholeiites, which may be the result of olivine accumulation (Fig. 5 and Table 2). It is clear from major element variation diagrams (Fig. 6) that the rocks fall into two clear groupings and the main discriminant between the two groups is MgO content; one group has MgO contents typically <8 wt % and the other has MgO contents >8 wt %. The other key observations are the high TiO2 and Zr and the variation in Al2O3. Harris et al. (1991) also commented on the low Al2O3 at high MgO for some of the dykes of the Ahlmannryggen. Many of the major and trace elements exhibit compositional trends typical of tholeiites (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).
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In common with other studies of Mesozoic and older flood basalt provinces we primarily use the incompatible high field strength elements (HFSE; Ti, Zr, Y, Nb) as discriminants between magma types. These elements are considered largely immobile during low-temperature alteration processes (e.g. Peate, 1997
) and ratios between them are not significantly modified by moderate amounts of fractional crystallization or susceptible to variations in the degree of partial melting (e.g. Luttinen & Furnes, 2000). Zr can be used as an effective index of differentiation in magmas that do not crystallize zircon.
Zr vs TiO2, Nb and Y plots (Fig. 7a–c) for all the data from the Ahlmannryggen dykes (this study) allow us to differentiate a number of distinct dyke groups. Data from Harris et al. (1991) were not included in these HFSE plots because, based on the geochronology results of this study, there is considerable uncertainty regarding the age of the minor intrusions of the Ahlmannryggen and there is no guarantee that all of the Harris et al. (1991) data are from Mesozoic dykes.
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Based on the Zr vs TiO2 plot (Fig. 7a) three geochemical groups can be identified from the Ahlmannryggen dataset; 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–Zr group (
>4 wt % TiO2 and Zr >400 ppm). These three groups 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 ppm Nb). On the Zr vs Y plot (Fig. 7c) the three groups are again apparent; here the low Ti–Zr group has been split into 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 subgroup based on Nb contents (Fig. 7b). These four geochemical groups are subsequently referred 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 plotted against MgO (wt %) as an index of differentiation (Fig. 6) it is clear that samples from Groups 1 and 2 are typically the most differentiated, with MgO contents 6 wt %, whereas samples from Groups 3 and 4 have >7 wt % MgO. Ni is strongly correlated with MgO, suggesting olivine control during magmatic differentiation. Al2O3 increases sharply as MgO decreases, suggesting that plagioclase fractionation is not important until MgO contents fall below 6 wt %. Groups 1 and 2 are distinctive in showing wide ranges of variation in SiO2, Fe2O3, CaO, Al2O3 and Y at constant MgO contents.
Group 1
Only six of the minor intrusions analysed from the Ahlmannryggen 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 dykes overlap, 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 with the 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 = 100 x Mg/(Mg + Fe2+); FeO = Fe2O3/1·15]. Cr and Ni contents are varied, with Cr ranging from 75 to 708 ppm and Ni from 81 to 318 ppm. Group 1 rocks are light rare earth element (LREE) enriched with (La/Yb)N ranging from 0·5 to 4·9 and LREE contents up to 25 times chondrite (Fig. 8a). Almost all samples have relatively flat middle to heavy chondrite-normalized REE (MREE to HREE) patterns with (Sm/Lu)N 1·8. The mid-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 wide range of variation in 87Sr/86Sr180 (0·7064–0·7085) at fairly constant Ndi (–5·8 to –6·4) (Fig. 10). The variation in 87Sr/86Sr at fairly constant Nd is typical of post-magmatic hydrothermal alteration. A summary of the key characteristics of Group 1 rocks is provided in Table 3.
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Petrographic characteristics
Group 1 rocks are characterized by cracked and altered olivine phenocrysts, typically <0·5 mm, whereas the smaller olivine grains tend to be more altered and are often ophitically enclosed by clinopyroxene. Plagioclase laths are a major phenocryst and groundmass phase and are typically sericitized.
Group 2
Group 2 rocks are characterized by low to moderate TiO2 (2·17–2·62 wt %) and Zr (150–177 ppm) contents (Fig. 7a) and overlap with many of the samples analysed by Harris et al. (1991) from the Ahlmannryggen. MgO contents (5·18–6·10 wt %; Fig. 6) and mg-numbers (39·5–45·9) are very homogeneous. All samples are LREE enriched with (La/Yb)N of 2·6–2·8, La contents 20 times chondrite and have fairly smooth REE patterns (Fig. 8b). The MORB-normalized multi-element patterns 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 Ndi (0·7–1·7) at 180 Ma (Fig. 10). A summary of the key characteristics of Group 2 rocks is provided in Table 3.
Petrographic characteristics
The Group 2 rocks show some variation, but are typically feldspar–clinopyroxene-phyric with a groundmass of feldspar microphenocrysts and Fe–Ti oxides. The feldspar phenocrysts are euhedral and are occasionally ophitically enclosed by clinopyroxene. Olivine is absent.
Group 3
Fifteen samples have been identified in the Group 3 magma type and they are characterized by low SiO2 (45·22–48·55 wt %), high TiO2 (3·25–4·85 wt %), high Ti/Y (291–777) and high MgO (7·63–21·61 wt %). Several of the samples can be classified as picrites, using the classification scheme of Le Bas (2000), and three samples are ferropicrites (Fig. 11a), following the broad criteria (FeO > MgO >12 wt %; Al2O3 <10 wt %) used by Gibson et al. (2000). Harris et al. (1991) first identified the presence of picrites in western Dronning Maud Land and demonstrated that they were genuine high-MgO liquids. The ferropicrites are characterized by high contents of both MgO (12·00–13·45 wt %) and FeO (12·6–13·1 wt %), with mg-numbers in the range 63–65. The ferropicrites have low to moderate SiO2 contents (45·22–46·54 wt %), low Al2O3 (8·15–10·22 wt %) and low total alkalis (1·51–1·97 wt %). As would be anticipated given their high MgO contents, all Group 3 rocks have high concentrations of compatible trace elements (Cr 397–1006 ppm and Ni 251–769 ppm), with the ferropicrites having the highest values (Cr 683–1006 ppm and Ni 578–619 ppm).
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The REE patterns of Group 3 rocks are distinct from all other magma groups. They are characterized by ‘saddleback’ patterns (Fig. 8c) with (La/Sm)N <1, (La/Yb)N in the range 1·6–4·3, and a marked depletion in the HREE. The multi-element plots (normalized to N-MORB) are characterized by almost flat patterns with some variation in the more mobile elements (Rb, Ba, Th) between samples (Fig. 9c). Otherwise there is little variation from Nb to Ti.
Group 3 samples also have distinct isotope signatures (Fig. 10) with 87Sr/86Sri 0·7035–0·7062 and high Ndi (5·0–9·0). There appear to be two distinct sub-groups within Group 3, one with lower Ndi (5·0–5·5) and more radiogenic 87Sr/86Sri (0·7054–0·7062), and the other with higher Ndi (7·0–9·0) and less radiogenic 87Sr/86Sri (0·7035–0·7041). The three ferropicrites all fall into the high-Nd, low-87Sr/86Sr sub-group. The Sr–Nd isotope compositions of the more unradiogenic Sr sub-group compare closely with that of a single sample from Vestfjella (P27-AVL; Luttinen & Furnes, 2000), which was previously identified as the most ‘depleted’ rock type from the entire Karoo (South Africa and Antarctica) province. A summary of the key characteristics of Group 3 rocks is provided in Table 3.
Petrographic characteristics
Group 3 rocks are more porphyritic than any of the other magma groups. Olivine is the main phenocryst phase and is often up to 3–4 mm in diameter. It occurs in all samples, but is rarely unaltered and is typically replaced along cracks by green or yellow serpentine. Olivine compositions are Mg-rich (Fo70–86) and clinopyroxene is also present as a phenocryst phase, but is not as abundant as olivine. Plagioclase is not present as a phenocryst phase. The groundmass is dominated by clinopyroxene, plagioclase and Fe–Ti oxides.
Group 4
Nine samples from the Ahlmannryggen are identified as Group 4. Their defining characteristic is their very high TiO2 (3·87–5·28 wt %), high Zr (343–568 ppm) and very high Ti/Y (664–694). Five of the group are picrites (Fig. 11a), with MgO >12 wt % and Na2O + K2O <3 wt % (Fig. 5) and are characterized by high Cr (834–965 ppm) and Ni (666–885 ppm) contents. The REE patterns are the most enriched of the four geochemical groups with (La/Yb)N values of 7·2–19·3 and La contents 81–204 times chondrite (Fig. 8d). The MORB-normalized multi-element patterns are characterized by a shallow trough at Ta–Nb and a minor negative anomaly at Ti, but generally exhibit a smooth pattern (Fig. 9d). Samples from Group 4 show a range in 87Sr/86Sri of 0·7048–0·7059, and Ndi varies considerably from –4·6 to 2·4 (Fig. 10). A summary of the key characteristics of Group 4 rocks is provided in Table 3.
Petrographic characteristics
Group 4 rocks are the least altered of the four chemical groups. Olivine phenocrysts are typically euhedral and show only a minor amount of alteration along cracks. Some of the olivine phenocrysts occur in clusters of up to five grains; these typically have rounded grain boundaries. The Group 4 rocks are 30% porphyritic and the groundmass is very fine grained; feldspar phenocrysts are discernible only at the chilled margins of the dykes.
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COMPARISON WITH OTHER KAROO–ANTARCTIC MAGMA GROUPS |
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The Mesozoic intrusions of the Ahlmannryggen overlap with the main phase of volcanic and intrusive activity of the Karoo magmatic province of southern Africa and East Antarctica. Geochemical data from East Antarctica (Kirwanveggen and Vestfjella: Fig. 1) and southern Africa are plotted in Fig. 12 [Zr vs TiO2 plots for (a) Antarctica and (b) South Africa]. Figure 12a also includes the four geochemical groups of the Ahlmannryggen dykes for comparison.
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Data for the lavas of Vestfjella and the Kirwanveggen (Fig. 12a) form a much more restricted range relative to the Ahlmannryggen intrusions. The majority of the Vestfjella–Kirwanveggen samples fall into a low TiO2 (<2 wt %) and Zr <200 ppm group. One group of Vestfjella lavas, CT2 of Luttinen et al. (1998)
, has higher TiO2 (2·4–3·8 wt %), but these still have low Zr (<200 ppm).
The Karoo data from South Africa (sources in figure caption; Fig. 12b) also form a cluster at very low TiO2 (<1·5 wt %) and Zr (<120 ppm), although a significant number 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 Formation picrites of the Lebombo (Duncan et al., 1984). The Rooi Rand dyke swarm (RRDS) forms a broad spread and overlaps with the Ahlmannryggen Group 2. Group 4 overlap, in part, with the HTZ (high Fe) field of Sweeney et al. (1994) from the Lebombo part of the Karoo Province (Fig. 12b).
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STRUCTURAL GEOLOGY |
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Geometry and distribution of Ahlmannryggen dykes
Mafic dykes are widely distributed within the central Ahlmannryggen region, although they are found in greatest concentrations along the ridges of the Flårjuven and Grunehogna nunatak groups (Fig. 2). The vast majority of the dykes have intruded thick dioritic sills of the Borgmassivet intrusive suite, where they were emplaced along a pervasive suite of pre-existing, sub-vertical joints within the sills. The total dyke population displays a wide variety of orientations, although a frequency distribution plot of total dyke and joint population data reveals a dominant NNE–SSW trend to both datasets (Fig. 13). Subordinate joint sets oblique to the predominant dyke trend were also exploited during dyke emplacement, resulting in offsetting segments and en echelon geometries. Where limited lateral exposure of an offset segmented dyke prevented simple identification of the main dyke trend, the orientation of the widest dyke segment was taken as a proxy for the overall dyke trend.
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The geochemical characterization of the central Ahlmannryggen dyke suite has identified four distinct geochemical groups, each of which has a consistent orientation 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 in width. They occur at Grunehogna and west of Flårjuven, as well as at Neumayerskarvet, northern Kirwanveggen (Fig. 2). Group 2 dykes are distributed throughout the nunataks in the west of the Ahlmannryggen (Fig. 2). Dyke trends range from north–south to NE–SW (010° mean), sub-parallel to the Jutulstraumen ice stream and subglacial trough. Group 2 dykes are notable for their extreme range of widths, up to 80 m wide, with four dykes in excess of 5 m wide. Dykes with a thickness >5 m were sampled close to the wall rock margin. Group 3 dykes strike predominantly east–west to ENE–WSW (079° mean) and vary from 0·20 to 3·58 m in width, with a mean of 1·17 m. Group 3 dykes are geographically restricted to the Grunehogna nunataks group (east side of Kullen) and two localities along the general strike direction of the dykes to the west (Fig. 2). Group 3 dykes are oblique to the Jutulstraumen glacial trough (Fig. 2), but are broadly parallel to the Pencksökket ice stream 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. Their distribution is restricted to the Flårjuven nunatak group in the NW of the study area (Fig. 2). They are broadly parallel to the Group 2 dykes, which are sub-parallel to the Jutulstraumen subglacial trough.
Dilation direction
The orientation of dyke segments emplaced into pre-existing fractures is controlled by the ability of the fractures to dilate, which is the product of their orientation with respect to the minimum principal stress and the magma pressure at the time of emplacement (Delaney et al., 1986). Therefore, where pre-existing fractures exhibit a control over dyke segment orientation it is unlikely that the strike of the dykes will be a simple reflection of the original extension direction. Dykes following pre-existing fractures oriented obliquely to the direction of maximum extension will side-step, resulting in the development of bridges or a dyke offset. Application of simple stereographic analytical techniques to these dyke offsets (Bussell, 1989; Kretz, 1991) allows estimates to be made for the stress field acting on the fracture during dilation. The method of Bussell (1989) is employed here, which combines the line of intersection between the dyke wall and the offset dyke segment with the apparent extension direction to define the dilation plane for a particular dyke. The dilation plane contains the true dilation direction for an individual dyke, and the true dilation direction for the dyke swarm can be obtained from the best-fit great circle to the poles of the individual dilation planes derived from a number of dykes.
Unfortunately, given the generally restricted nature of dyke exposure encountered (e.g. cliff faces or escarpment edges), only a small number of dykes displaying the required structural characteristics were encountered. Only seven well-exposed dyke offsets were recorded in the four Mesozoic dyke groups, including examples from Groups 1, 2, and 4. Plotting the poles to the reconstructed dilation planes for the dyke offsets reveals two distinct orientations (Fig. 14). Five poles from dyke Groups 2 and 4 are distributed along an approximate NE–SW-trending girdle, while the remaining two poles from Group 1 dykes plot along an east–west girdle. We have calculated the mean girdles for both sets of data, the poles to which approximate the dilation direction for the dyke groups. Data from dykes of Groups 2 and 4 suggest that they were emplaced about a mean trend of 012° in response to an applied true dilation direction trending 307–127°. The best-fit girdle to the poles to dilation planes for Group 1 dykes, although limited, suggests that the dykes were emplaced in a stress field where the minimum principal stress direction was oriented 004–184°. Given the sub-parallel trend of the dykes within Groups 1 and 3 (mean trend 072°), we interpret them have all been emplaced in response to the same north–south-trending minimum principal stress direction. The calculated dilation direction is not perpendicular to the mean trend of the dykes, suggesting that dykes were probably emplaced along pre-existing planes of structural weakness (i.e. regional jointing) and exhibited oblique dilation.
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PETROGENESIS OF THE MAGMAS AND MANTLE SOURCES |
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In the following section we examine each geochemical dyke group and attempt to identify the mantle source of the magmas and make correlations with other Karoo magma types.
Interpretation
Group 1 (low Ti–Zr)
Group 1 rocks are all low-Ti tholeiites, which is the predominant rock type throughout the Karoo–Dronning Maud Land flood basalt province. Lava successions in Lesotho, Lebombo, Vestfjella and Kirwanveggen (Fig. 1) are all dominated by low-Ti basalts (Fig. 12). Luttinen et al. (1998) have made comparisons between the CT1, low-Nd basalts of Vestfjella and the low-Nd Sabie River Basalt Formation of southern Lebombo.
The Group 1 rocks have low MgO contents, with only two samples having MgO >8 wt % (but <9 wt %). In terms of their Sr–Nd isotope characteristics the Group 1 rocks are distinct from any other group of the Karoo–Antarctica volcanic province (Fig. 10). They are intermediate between the central Lebombo lavas and rocks of the broadly contemporaneous Ferrar magmatic province (Fig. 10). The Ahlmannryggen Group 1 dykes all have a characteristic Ta–Nb negative anomaly (Fig. 9a), suggesting crustal contamination. Evidence of crustal contamination is also well illustrated in Figs 15 and 16, where the Group 1 dykes have high values of Th/Ta and low values of Nb/Nb*, both reliable proxies for the involvement of crustal material. Although there is only a small sample set for Group 1, the variation in 87Sr/86Sr with SiO2 and mg-number (Fig. 17) could also suggest that magma chemistry was influenced by assimilation combined with fractional crystallization (AFC). To model AFC processes, a Group 3 sample (Z.1816.1) was used as the parent magma and the crustal contaminant was assumed to be local Archaean crust, as the Ahlmannryggen dykes were intruded into the Grunehogna Archaean craton. The Nd–Sr isotope composition of the Archaean crust is assumed to be 87Sr/86Sr = 0·710, Sr 500 ppm, Nd –52, Nd 11 ppm (Fig. 10), based on Luttinen & Furnes (2000), who used data from the Kaapvaal craton to model the potential crustal contaminant, as well as two granitic xenoliths from Vestfjella, all of which have Nd of –50. AFC at moderate rates of assimilation (r = 0·4) could account for the isotopic character of Group 1 rocks derived from a Group 3 parent magma. Both Group 1 and Group 3 dykes were intruded at 190 Ma and occupy the same structural trend.
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(ThN x LaN); normalized to primitive mantle] vs 87Sr/86Sr. The size of the Nb anomaly is used as a proxy for the extent of crustal contamination in mantle-derived magmas. Binary mixing curves are shown between a depleted end-member basalt (Z.1816.2; Group 3) and upper continental crust (Borgmassivet Intrusives) and SCLM partial melt (lamproite). The depleted end-member is a Group 3 basalt from the Ahlmannryggen; 87Sr/86Sr = 0·7035, Sr 300 ppm, Nb/Nb* = 1·65. The local upper crust is represented by Borgmassivet Intrusions: 87Sr/86Sr = 0·7240, Sr 130 ppm, Nb/Nb* = 0·19; the enriched subcontinental lithospheric mantle proxy partial melt is Gaussberg lamproite (Bergman, 1987
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Although the dataset is small (only four samples with isotope data; Fig. 10) an alternative explanation for isotope characteristics of the Group 1 dykes is that some of the variation in 87Sr/86Sr (0·7064–0·7085) at fairly constant
Nd (–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 3 dykes and partial melts of the subcontinental lithospheric mantle (SCLM) at 12% (Fig. 10), which is explained in the following section, where the petrogenesis of Group 2 and 4 dykes are discussed. Therefore the magmas of the Group 1 dykes could also represent mixtures of Group 3 melts plus 12% partial melts of the SCLM; any subsequent variation in their Sr isotope characteristics is probably the result of post-magmatic processes.
Group 2 (low Ti–Zr)
The Group 2 rocks of western Dronning Maud Land are all 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 South Africa (TiO2 2·20 wt %; Zr 156 ppm; Armstrong et al., 1984). They are all low-Mg rocks (mg-number <46), with accompanying low Cr and Ni. Isotopically, they are also relatively homogeneous (0·7034–0·7046; Fig. 10) and include a fairly depleted sub-group (87Sr/86Sri <0·7040 and Nd 1·5–2).
Dykes of Group 2 are all evolved, with an average mg-number of 42, but they show little evidence of crustal contamination. 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 interaction with continental crust is reinforced by a plot of Th/Ta vs Ti/Zr (Fig. 15), in which Group 2 rocks cluster close to the MORB field, suggesting that there was no significant interaction between the parent magmas and crust or lithospheric mantle. Crustal and lithospheric mantle values of Th/Ta are high and Ti/Zr very low, 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 by lower crust.
Binary mixing curves are shown in Figs 10 and 16 between the most isotopically depleted group of rocks (Group 3) and a partial melt of enriched SCLM. The Gaussberg 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 chemically homogeneous and show very little evidence for crustal contamination (Ewart et al., 2004). The mixing curves indicate that the Group 2 rocks could be interpreted as mixes of Group 3 magmas with <3% of an enriched component, akin to the partial melt of SCLM.
The Group 2 dykes are the only group from the Ahlmannryggen with consistently positive 
Nb (Fig. 18), which is a function used to express the excess or deficiency in Nb relative to the lower limits of the Iceland basalt array (Fitton et al., 1997) in a plot of Nb/Y vs Zr/Y. Samples with positive Nb plot above the lower line in Fig. 18, which implies derivation from an enriched mantle source, whereas those with negative Nb plot below the lower line, indicating a source in the depleted upper mantle. Assimilation of continental crust has little effect on the value of Nb because crustal rocks plot on or below the lower line of the Iceland array, therefore contamination will only lower Nb and cannot make a sample appear to be ‘Icelandic’ (Chambers & Fitton, 2000). Nb is also insensitive to the effects of variable degrees of mantle melting and source depletion following partial melting, and is, therefore, a characteristic of the mantle source (Fitton et al., 2003). The Group 2 dykes plot in a tight cluster and fall within the ‘Iceland array’ of Fitton et al. (1997). They overlap, in part, with the field of Vestfjella lavas and intrusions (Luttinen & Furnes, 2000). The Group 2 rocks have positive Nb and are clearly distinguishable from N-MORB magmas (Fig. 18), which have negative Nb and cannot have been derived from the same mantle source as Group 2.
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Similarities between the Group 2 dykes, the CT2 Vestfjella dykes of Luttinen & Furnes (2000)
and the Rooi Rand Dyke Swarm of Duncan et al. (1990) are apparent based on both geochemical and isotopic criteria, as well as their post-Karoo (182 Ma) age. Luttinen et al. (1998) noted the compositional similarity to recent South West Indian Ridge (SWIR) MORB associated with the Marion hotspot.
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 other three groups from the Ahlmannryggen, Group 3 are notably depleted in the LILE. They have ‘humpbacked’ REE (Fig. 8c) and N-MORB normalized multi-element patterns (Fig. 9c). They are characterized by high Ndi, and low 87Sr/86Sri (Fig. 10), consistent with derivation from a depleted mantle source. The Group 3 rocks form two distinct sub-groups, which are particularly evident on the Nd–Sr isotope diagram (3a and 3b in Fig. 10), but also on the multi-element diagram (Fig. 9c), where the low-Nd sub-group shows greater enrichment in Th. Superficially, the Group 3 rocks have many similarities to MORB (high Nd, low 87Sr/86Sr, LREE-depleted patterns) but their TiO2 contents are far too high (4 wt %) for them to derived from MORB-source mantle. The depth of melting is constrained by the Dy/Yb ratios (3: Fig. 19) and also by the shape of the multi-element patterns (Fig. 9c), which show that the HREE are being retained, presumably by garnet. Several workers (e.g. Fitton et al., 1997) have used the abundance of Nb relative to Zr and Y (Nb) to distinguish between mantle sources. Group 3 rocks all have negative Nb (Fig. 18), with the more depleted group of rocks (Group 3a) having Nb values 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 from MORB-mantle, but at lower degrees of partial melting. The Group 3 rocks include two distinct sub-groups in the Nd–Sr isotope diagram (Fig. 10), which can be explained by the addition of 20% upper crust to the uncontaminated melts. A mixing curve is plotted in Fig. 10 between a Group 3 magma and one of the Borgmassivet Intrusions (T. R. Riley, unpublished data), which represents the local upper crust at the time of intrusion. The model curve can reproduce the Nd–Sr isotope composition of the Group 3 dykes by bulk mixing, although realistically the actual process would involve assimilation plus fractional crystallization. The Group 3 samples with the lowest Nd (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 relative 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 on the Nb/Nb* vs 87Sr/86Sr diagram (Fig. 16) illustrate the bulk addition of local upper crust (Borgmassivet Intrusions) to an ‘uncontaminated’ Group 3 magma composition. The three samples that plot at higher 87Sr/86Sr, lower Nb/Nb* and lower Nd (5) can be explained by the addition of 20% of upper crust (akin to the mixing curve in Fig. 10). A separate subset of five samples (Fig. 16) that plot at slightly lower Nb/Nb* than the most primitive magmas at similar 87Sr/86Sr values could reflect mixing of a small (2–3%) lithospheric mantle partial melt component with the Group 3 uncontaminated end-member (Z.1816.2: 87Sr/86Sr = 0·7035; Nd = 8·8; Nb/Nb* = 1·65).
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The Group 3 dykes are clearly an unusual group of rocks, which were intruded at
190 Ma, have a strike direction parallel to the Pencksokket glacial trough, the future continental margin and the Explora escarpment (Fig. 1). They are E-MORB-like in many respects and include ferropicrites as well as picrites. However, the Ti contents are high (3–4 wt %) and in this respect they differ from E-MORB and more closely resemble ocean-island basalts (OIB), although they are more depleted in incompatible elements than OIB. The depleted source component in mantle plumes is interpreted to be distinct from the MORB-source mantle component beneath present-day mid-ocean ridges (Kerr et al., 1995). The MORB-like rocks associated with mantle plumes are believed to have been generated from a depleted source component that forms an intrinsic component of 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-Triassic Siberian Traps continental flood basalt (CFB) province by Wooden et al. (1993) and the Early Cretaceous Etendeka CFB province of Namibia by Gibson et al. (2000). These workers interpreted the ferropicrites as partial melts of Fe-rich streaks in mantle plume starting heads, combined with a significant melt contribution from the convecting mantle. It is predicted that such melts would be erupted early in the history of flood basalt provinces and generated at high pressure (35–45 kbar) and high temperature (Tp 1550°C). This is significant, as the available geochronology data for Group 3 ferropicrites indicate an age of 190 Ma, which reinforces the prediction of Gibson et 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 and are the most enriched of the four magma groups. Half of 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-like geochemistry of their CT4 subgroup and suggested that this may represent melts derived from a plume component in the Karoo volcanic province. The CT4 dykes have lower TiO2 than the Group 4 dykes of the Ahlmannryggen.
Sweeney et al. (1994) discussed the origins of the HTZ Karoo picrites and interpreted them as the likely parent of the abundant low-MgO, HTZ magmas. They also invoked a plume component in the petrogenesis of the HTZ 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 subglacial trough and also corresponds to the mean strike direction of the Group 2 dykes (Fig. 2). Both dyke groups were interpreted to have been emplaced at 178 Ma. This age is confirmed by the recent work of Zhang et al. (2003) who reported an age of 176·6 ± 0·6 Ma for a dyke (A309) from the southern Kirwanveggen (Fig. 1). The geochemistry of this dyke is reported in Table 2 (A. V. Luttinen, unpublished data); it corresponds closely to the Group 4 geochemical group (high Ti–Zr) and may actually be part of the same dyke as sample Z.1653.2 (Table 2), also from Petrel Peak (Fig. 1). The Zhang et al. (2003) age (177 Ma) for dyke A309 is in close agreement with the 178 Ma age for 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 of the Karoo volcanic province. They are the highest TiO2–Zr rocks of the Karoo, and with the exception of localized alkali–mafic intrusions and lavas, they also form some of the most incompatible element enriched rocks. Seven of the 10 samples identified in Group 4 have MgO contents >11·2 wt %, with mg-numbers up to 72, and are, therefore, picritic. Unlike the other three geochemical groups of the Ahlmannryggen, the Group 4 rocks display a wide range in Nd, varying from 2·4 to –4·6 (Fig. 10), which suggest a contribution from partial melts of enriched SCLM or contamination by continental crust. All Group 4 dykes shown in Fig. 9d have at least a minor negative Nb–Ta anomaly. The dykes with the weakest Nb–Ta anomalies are the samples with the highest Nd values (1·5), whereas those with lower Nd values (–4·5) have the most pronounced Nb–Ta anomaly. Using average lamproite as a proxy for a small degree partial melt of subcontinental lithospheric mantle, Group 4 magmas could represent mixes of a partial melt of a MORB source with significant (20–30%) partial melt of the lithospheric mantle (Fig. 16). The Th/Ta vs Ti/Zr plot (Fig. 15) also suggests involvement of an enriched component in the petrogenesis of the Group 4 magmas, which trend away from the MORB field toward a crustal or lithospheric mantle component. Dy/Yb values (3–3·5; Fig. 19) support melting in the presence of garnet. The steep slopes of the multi-element plots (Fig. 9d) also indicate that the HREE are being retained in the mantle source. The Group 4 dykes have negative Nb values, but are characterized by very high Zr/Y values (Fig. 18).
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CONCLUSIONS |
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Emplacement history and tectonics
Our new geochemical groupings, together with structural and stratigraphical observations and geochronology, allow us to reconstruct the emplacement history of the Mesozoic dykes of the central Ahlmannryggenn range. Available 40Ar/39Ar ages indicate that the picrites and ferropicrites of Group 3 are the oldest of the Mesozoic dyke suites, emplaced at
190 Ma. Group 3 dykes trend N 70 E and are restricted to a narrow, 8 km wide corridor, extending from the Grunehogna nunatak area to two isolated exposures along strike to the WSW (Fig. 2). Group 4 and Group 2 dykes represent the youngest, with an age peak at 178 Ma.
Dykes of Group 1 are sub-parallel to the Group 3 picrites and ferropicrites. Group 2 and Group 4 dykes are dominantly NNE–SSW striking. The parallel relationship between dykes from Groups 1 and 3 suggests that they were probably all emplaced in response to the same applied stress field, which, based on our geochronology, existed at 190 Ma. Although the exposures of Group 3 dykes did not yield data conducive for dyke swarm dilation estimates, limited data from Group 1 dykes suggest an approximately north–south dilation direction (004–184°), and, by association, a parallel oriented minimum principal stress. In contrast, the overwhelming majority of dykes 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).
A large number of dykes are present on the eastern flank of the Jutulstraumen ice stream, exposed at Straumsvola nunatak and in other nunataks within a 30 km radius (Harris & Grantham, 1993). Here two main dyke trends can be recognized, one NE–SW trending (Harris & Grantham, 1993), and a second more dominant trend of NNW–SSE, although no data regarding the relative or absolute chronology of these dykes are currently available, other than that some dykes postdate the Straumsvola nepheline syenite, which is dated between 180·9 ± 2·8 Ma (Grantham, 1996) and 178 ± 2 Ma (Grantham et al., 2001), nor are there any palaeostress or dilation direction data. It is possible that NE–SW-oriented dykes in the Straumsvola area were emplaced synchronously with similarly oriented dykes in the Ahlmannryggen. If such a correlation is correct the emplacement of these dykes implies the existence of a regional NW–SE-oriented minimum principal stress direction that was perpendicular to the crustal boundary between the Archaean Grunehogna craton and the 1 Ga mobile belt of the Maudheim Province (Fig. 1 inset).
The giant Okavango dyke swarm (ODS; Fig. 1) of southern Africa forms one arm of a giant radiating dyke swarm, which also includes the Sabie River (SRBF; Fig. 1) and Rooi Rand dyke swarms (RRDS; Fig. 1). Like the Groups 2 and 4 dykes of the Ahlmannryggen, the Okavango dyke swarm exploits a major crustal boundary. This swarm has recently been dated and shown to contain a significant component of 178 Ma dykes (Elburg & Goldberg, 2000; Le Gall et al., 2003; Jourdan et al., 2004a). It therefore seems likely that the NNE–SSW-oriented dykes of the Ahlmannryggen may have been a component of a similar radiating dyke swarm, although the small average width of the Ahlmannryggen dykes (1 m), which is considerably less than the mean width of the Okavango dykes (18 m), suggests that the potential Jutulstraumen arm of the radiating dyke swarm was subject to significantly reduced magmatism.
It is tempting to correlate the ENE–WSW-trending dykes (Groups 1 and 3) of the Ahlmannryggen with the Rooi Rand dyke swarm (RRDS) of southern Africa, based on their sub-parallel alignment in Gondwana reconstructions. However, both 40Ar/39Ar geochronology (RRDS is 173·9 ± 3·8 Ma; Jourdan et al., 2004b) and geochemistry suggest that the RRDS and Group 3 dykes were not emplaced as part of the same event.
Magma types
The minor intrusions of the Ahlmannryggen region of western Dronning Maud Land can be grouped into four distinct geochemical types (Groups 1–4) based on immobile incompatible elements (Ti, Zr, Y), LILE and Sr–Nd isotope composition. 40Ar/39Ar geochronology demonstrates 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, parallel to the Group 3 dykes (Fig. 2) and to the Pencksökket subglacial 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. They could be generated by mixing of 12% melt fraction of an enriched SCLM component with a partial melt of a depleted source. In this case the variation in 87Sr/86Sr at constant Nd might be the result of post-magmatic alteration. However, the variation in 87Sr/86Sr also correlates with SiO2 and mg-number, and an alternative interpretation is that the Group 1 dykes are the result of AFC processes from a Group 3 parental magma involving lower Archaean crust as the contaminant.
The Group 2 rocks show little or no chemical evidence of crustal contamination. However, trace element ratios and Nd–Sr isotopic data suggest the involvement of an enriched lithospheric mantle source component in their petrogenesis. If the primary mantle-derived magmas were similar in composition to the low-87Sr/86Sr Group 3 magmas then the Group 2 compositions could be generated by mixing of <10% melt fraction of an enriched SCLM component with a partial melt of a depleted source. The Group 2 rocks were intruded at 178 Ma, broadly parallel to the Jutulstraumen subglacial trough, which is interpreted as a continental rift and may be continuous with the Pencksökket trough (Fig. 1). The Jutulstraumen rift has associated alkaline magmatism along its eastern margin (Harris & Grantham, 1993). Group 2 rocks are also geochemically similar to the Rooi Rand dykes of the Lebombo rift, which were also emplaced late in the history of the province.
The Group 3 rocks of the Ahlmannryggen form the most unusual geochemical group of the entire Karoo–Antarctic magmatic province. They include high Ti–Zr picrites and ferropicrites, which have isotopic characteristics (Nd 8, 87Sr/86Sr 0·7035) consistent with derivation from a depleted mantle source. Their high TiO2 and Zr contents and MORB-normalized trace element patterns suggest that they are derived by small degrees of partial melting of a MORB-like source. The Group 3 dykes were intruded at 190 Ma, parallel to the Pencksökket glacial trough, and represent the first magmas of the Karoo–Antarctic province. They show evidence, in part, of both derivation from enriched mantle and crustal contamination. A subgroup with less depleted isotope ratios (Nd 5, 87Sr/86Sr 0·7055) is considered to be the result of 10% upper crustal contamination, whereas a secondary subgroup (Fig. 16) is the product of small amounts of mixing with partial melts of enriched lithospheric mantle. The ‘uncontaminated’ Group 3 (Nd 9; 87Sr/86Sr 0·7035) samples are considered to represent the closest composition to the primitive 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 of the 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 from the Ahlmannryggen to the southern Kirwanveggen (Fig. 1). They have a strike direction parallel to the Jutulstraumen glacial trough and to the 178 Ma Group 2 intrusions. They appear to be small-volume partial melts generated at depths similar to or greater than the Group 3 magmas. The Group 4 rocks fall into two subtypes: those with a clear contribution from subduction-modified lithospheric mantle (strongly negative Nd, Nb–Ta negative anomaly) and those with positive Nd and flatter multi-element patterns (Fig. 9d). Sweeney et al. (1994) suggested a key role for subcontinental lithospheric mantle and an asthenospheric plume in the generation of the Lebombo low-K picrites.
Role of a mantle plume
The geochemical characteristics of the Ahlmannryggen intrusions suggest complex mixing relationships between Ti-rich, small volume partial melts (Group 3) of a depleted mantle source and partial melts of enriched lithospheric mantle plus assimilation of local continental crust. The isotopically depleted end-member has both MORB- and OIB-like characteristics. It is tempting to invoke an asthenospheric mantle plume origin for the Group 3 magmas and, given their intrusion age (190 Ma), this lends support to the incubating asthenospheric plume model of Sweeney et al. (1994).
Where mantle plumes have been interpreted as being responsible for the magmatism of flood basalt provinces there is often continued debate as to whether the plume arrival at the base of the lithosphere and large volume melting occurred during a short time period (a few million years) or whether plume arrival was followed by a prolonged period (10 Myr) prior to the main episode of magmatism.
40Ar/39Ar geochronology by Duncan et al. (1997) on basic lavas from southern Africa and the Kirwanveggen suggests that the duration of magmatism was very short (1–2 Myr). However, the Duncan et al. (1997) study did not include any data on minor intrusions of the Karoo. The new 40Ar/39Ar geochronology data presented here indicate a long-lived magmatic event of >10 Myr (178–190 Ma). This extended time period is associated with diverse magma chemistry, in contrast to the dominantly low-Ti tholeiites associated with the 182–183 Ma flood basalt event. The style and chronology of the magmatism observed in the Karoo–Dronning Maud Land is akin to that described from the Etendeka province of NW Namibia (Thompson et al., 2001), although the Karoo event is considerably more prolonged and is associated with craton boundaries. The evidence presented here strongly suggests that a plume incubation model may be applicable for the Early Jurassic magmatism of the Karoo–Dronning Maud Land province.
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ACKNOWLEDGEMENTS |
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The field and air operations staff at Halley Base are thanked for their support. Graham Pearson (University of Durham) supplied the ICP-MS analyses, and Dave Emley (University of Keele) carried out the XRF analyses. This work has benefited greatly from the thorough and thoughtful reviews of Chris Harris, Andrew Kerr, Arto Luttinen and Marjorie Wilson. Adela Fazel acknowledges the receipt of an Antarctic Funding Initiative studentship.
* Corresponding author. Telephone: (+44) 1223 221423. Fax: (+44) 1223 362616. E-mail: t.riley{at}bas.ac.uk
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