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2011
doi:10.1093/petrology/egr011
Journal of Petrology Advance Access published April 11, 2011
Multistage Evolution of Dolerites in the
Karoo Large Igneous Province,
Central South Africa
ELSE-RAGNHILD NEUMANN1*, HENRIK SVENSEN1,
CHRISTOPHE Y. GALERNE1,2 AND SVERRE PLANKE1,3
PGPçPHYSICS OF GEOLOGICAL PROCESSES, UNIVERSITY OF OSLO, PO BOX 1048 BLINDERN, 0316 OSLO, NORWAY
2
GEODYNAMIK, STEINMANN-INSTITUT, UNIVERSITA«T BONN, NUSSALLEE 8, 53115 BONN, GERMANY
3
VBPRçVOLCANIC BASIN PETROLEUM RESEARCH, OSLO INNOVATION CENTER, 0349 OSLO, NORWAY
RECEIVED JANUARY 15, 2010; ACCEPTED FEBRUARY 25, 2011
Early Jurassic sheet-like intrusions (sills and dykes) are abundant
in the Karoo Basin in South Africa, and were emplaced as a part of
the Karoo Large Igneous Province. Here we discuss the evolutionary
history of dolerite sills and dykes in different parts of the basin on
the basis of new major and trace element analyses of dolerite samples
collected from drill-cores (five sites spanning 1700 m of basin stratigraphy) and previously published data on sills and dykes in the
Golden Valley Sill Complex (GVSC). In addition, we present Sr^
Nd isotope data for selected samples. The dolerites are subalkaline
tholeiitic basalts and basaltic andesites characterized by enriched
trace element patterns, variable degrees of depletion in Nb^Ta relative to light rare earth elements, negative to positive Pb anomalies,
and mild to moderate enrichment in initial Sr^Nd isotopic ratios.
The aim of this study is to unravel the evolutionary history of the
melts that gave rise to the dolerites. We propose that the primary
melts were derived from sub-lithospheric mid-ocean ridge basalt (or
ocean island basalt) source mantle and had acquired a weak subduction signature (relative depletion in Nb^Ta, mildly enriched
Sr^Nd isotopic ratios) through interaction with metasomatized
lithospheric mantle. In the deep crust the magmas underwent assimilation and fractional crystallization (AFC) processes involving up
to 10% assimilation of granulites with strong arc-type geochemical
signatures. The AFC processes may alternatively have taken place
in the uppermost mantle. Distinct geochemical characteristics
among the GVSC and drill-core units reflect different amounts of
AFC. During and/or after intrusion into the sedimentary rocks in
the Karoo Basin the magmas underwent a second stage of fractional
crystallization (50^60%) and local contamination by their
Continental flood basalts (CFB) occur world-wide and
represent an important style of intra-plate magmatism in
the continents. CFBs are characterized by the emplacement of large volumes of magma (of the order of several
million km3) within time periods of only a few million
years or less (e.g. Cox, 1988; White & McKenzie, 1989;
Duncan et al., 1997; Jourdan et al., 2007). Extrusion of lavas
is accompanied by extensive intrusion of sills and dykes
into contemporary sedimentary basins (e.g. Symonds
et al., 1998; Chevallier & Woodford, 1999; Planke et al.,
*Corresponding author. E-mail: e.r.neumann@geo.uio.no
ß The Author 2011. Published by Oxford University Press. All
rights reserved. For Permissions, please e-mail: journals.permissions@
oup.com
sedimentary wall-rocks. High U concentrations and U/Th ratios
in some dolerites in the southwestern part of the Karoo Basin
were probably caused by fluids released from shales rich in organic
material (e.g. Ecca Group shales) during devolatilization and contact metamorphism. Contamination in a GVSC unit may reflect
interaction withTa^Th^U-rich minerals of the type found in stratiform uranium ore bodies in the Karoo Basin, or fluids that have
interacted with such rocks. Considering that continental flood basalts
are emplaced through continental crust and sedimentary basins, it
is likely that other LIPs have similar evolutionary histories to that
proposed for the Karoo Basin.
Gondwana; geochemistry; Nd and Sr isotopes; AFC
processes; Karoo
KEY WORDS:
I N T RO D U C T I O N
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et al., 2007, 2009). The Karoo CFB is thus eminently suited
for further studies of the evolutionary history of flood
basalts from their mantle source to solidification in the
upper crust.
Galerne et al. (2008) recently presented major and trace
element data on 327 samples from sills and dykes exposed
within a restricted area in the Karoo Basin, the Golden
Valley Sill Complex (GVSC; Figs 1 and 2). The aim
of that paper was to test geochemical similarities or differences between sills and dykes in the Golden Valley
Sill Complex to throw light on their emplacement mechanism. Forward Stepwise Discriminant Function Analysis
(FS-DFA) showed that all samples from the same unit
(sill or dyke) have the same geochemical signature, whereas different units generally have different signatures.
These results imply that most sills and dykes in the GVSC
formed from different magma batches, although one batch
gave rise to two large sills and one small sill. The emplacement mechanism is thus a combination of the following
models: (1) the sills are fed by chemically distinct batches
of magma through separate feeders; (2) single sill complexes are generated from a single magma batch, suggesting a mechanism of sill feeding sill, thereby forming an
interconnected network.
The aim of this study is to use the large dataset on the
Golden Valley Sill Complex to throw light on the evolutionary processes that led to the different geochemical
characteristics of the various GVSC units. In addition, we
Fig. 1. Simplified geological map of the Karoo Basin showing the locations of the studied boreholes. The blue dashed lines mark the boundaries
between basement terranes or domains. Based on maps by De Wit et al. (1992), Cole (1998) and Schmitz (2002), and references therein.
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2005; Polteau et al., 2008). Many CFBs are believed to be
associated with the opening of new oceans (e.g. White &
McKenzie, 1989; Coffin & Eldholm, 1990; Klausen, 2009).
Among the geochemical features common to many CFBs
are negative Nb^Ta anomalies and enriched Nd^Sr isotope ratios (e.g. Gallagher & Hawkesworth, 1992; Puffer,
2001). The origin of these features has been discussed extensively over the last decades; proposed models include (1)
partial melting of heterogeneous sub-continental lithospheric mantle (SCLM; e.g. Gallagher & Hawkesworth,
1992; Jourdan et al., 2007, 2009); (2) contamination during
passage through the SCLM of magmas derived from
sub-lithospheric plume or mid-ocean ridge basalt
(MORB) sources, or mixing between sub-lithospheric and
lithospheric mantle melts (e.g. Marsh & Eales, 1984;
Arndt & Christensen, 1992; Ellam et al., 1992; Riley et al.,
2005; Jourdan et al., 2007; Heinonen & Luttinen, 2008,
2010; Heinonen et al., 2010); (3) assimilation of sediments
(Elburg & Goldberg, 2000); (4) polybaric fractional crystallization (Marsh & Eales, 1984); (5) contamination by
sedimentary country rocks combined with fractional crystallization in addition to deep processes (e.g. Riley et al.,
2005, 2006; Jourdan et al., 2007).
All of these models have been proposed for the Karoo
Large Igneous Province (LIP) in South Africa (Fig. 1; e.g.
Duncan et al., 1984; Marsh & Eales, 1984; Ellam & Cox,
1989, 1991; Ellam et al., 1992; Harmer et al., 1998; Elburg &
Goldberg, 2000; Ellam, 2006; Riley et al., 2006; Jourdan
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EVOLUTION OF KAROO DOLERITES
Fig. 2. Simplified map showing the main sills and dykes in the
Golden Valley Sill Complex (after Galerne et al., 2008). Different
colors distinguish the geochemical signatures established by Galerne
et al. (2008) by forward stepwise-discriminant function analysis; sills
not included in this study are shown in grey. GVS, Golden Valley Sill;
GS, Glen Sill; HS, Harmony Sill; MS, Morning Sun Sill; L1, L1 sill;
GVD, Golden Valley Dyke. The position of the Golden Valley Sill
Complex is indicated in Fig. 1. The c. 100 km long, up to 30 m wide
Cradock dyke (Marsh & Mndaweni, 1998) is located 70 km west of
the Golden Valley Sill Complex.
have collected a new set of samples of sills from drill-cores
at various stratigraphic levels in the Karoo Basin, as well
as Sr^Nd isotope data on representative samples from
both groups. We believe that the results throw additional
light on magmatic differentiation processes in crust and
mantle in the Karoo LIP, and on magmatic processes in
CFBs in general. Furthermore, we show that the detailed
geochemical data collected on the GVSC, with numerous
samples from each of a limited number of magmatic units,
provide unique information that cannot be obtained without such extensive sampling campaigns. The results also
have implications for our understanding of the formation
of ore complexes associated with LIPs [e.g. Norilsk-type
Ni^PGE (platinum group element) complexes such as the
Enziswa formation; Lightfoot et al., 1984; Marsh et al.,
2003] and the processes that may lead to LIP-related
climatic changes.
GEOLOG IC A L S ET T I NG
The Early Jurassic Karoo magmatism occurred over the
entire southern African subcontinent and extended into
adjacent areas in Antarctica (e.g. Eales et al., 1984). The
total volume of magma emplaced amounts to several million km3, making this event one of the largest CFBs in the
world (e.g. White & McKenzie, 1989; Storey & Kyle, 1997;
White, 1997). 40Ar/39Ar age determinations suggest that
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emplacement of the Karoo CFB lasted from 184 to 176 Ma
(e.g. Riley & Knight, 2001; Riley et al. 2005; Jourdan et al.
2007, 2008). However, U/Pb zircon ages suggest that in
some parts of the Karoo Basin, including the GVSC, volcanism and sill emplacement may have occurred as a
short pulse at about 182·5 Ma (Encarnacio¤n et al., 1996;
Svensen et al., 2007). The 1·8 km thick Drakensberg lava sequence in Lesotho (Fig. 1) is the remnant of a much wider
lava cover (e.g. Duncan et al., 1997). The total area affected
by the Karoo magmatism is demonstrated by numerous
sill intrusions and dykes (some are hundreds of kilometres
long; e.g. Chevallier & Woodford, 1999) which cover
nearly two-thirds of southern Africa (Fig. 1). Locally the
sills form up to 70% of the Karoo Basin stratigraphy
(Roswell & De Swardt, 1976).
The Karoo Basin sills have intruded essentially undeformed sedimentary rocks of the Karoo Supergroup,
which were deposited from about 310 Ma until the initiation of volcanism (Visser et al., 1997; Catuneanu et al.,
1998). The majority of the sills are strata bound and concentrated in the Ecca and Beaufort groups (Fig. 1), which
essentially consist of shales, siltstones, mudstones and sandstones (e.g. Cole, 1992; Veevers et al., 1994; Johnson et al.,
1997). Sill intrusions commonly follow lithological boundaries. There are virtually no sills in the basement complexes west and east of the Karoo Basin; it is therefore
possible that the most distal sills (and dykes) in the Karoo
Basin were fed laterally from feeder channels in the central
parts of the basin (J. Marsh, personal communication,
2009). Sill emplacement resulted in widespread contact
metamorphism and the formation of hundreds of vertical
pipe-like structures that released thermogenic carbon
gases into the atmosphere (Jamtveit et al., 2004; Svensen
et al., 2004, 2006, 2007, 2008; Aarnes et al., 2010).
The Karoo basaltic lavas have been divided into low-Ti
and high-Ti basalts (e.g. Cox et al., 1967). Both groups are
characterized by negative Nb^Ta anomalies and enriched
Sr^Nd^Pb isotopic compositions (e.g. Hawkesworth et al.,
1984; Ellam & Cox, 1991; Marsh et al., 1997; Elburg &
Goldberg, 2000), but also show significant geochemical
variations (e.g. Pemberton, 1978; Marsh et al., 1997;
Jourdan et al., 2007; Galerne et al., 2008). The Golden
Valley Sill Complex and the drill-core sills discussed in
this study belong to the low-Ti group.
The GVSC comprises four major saucer-shaped sills,
two large dykes and some minor sills and dykes (Fig. 2)
intruded into the upper part of a sequence of sandstones
and shales of the Beaufort Group. The drill-core samples
were collected in various parts of the Karoo Basin (Fig. 1).
One drill-core (LA1/68, here called LA1), located NW of
the Lesotho lava area, comprises sills emplaced in the
upper Beaufort and Ecca groups. The QU1/65 borehole
(referred to as QU1) in the western part of the Karoo
Basin intersects sills emplaced into the Dwyka, Ecca, and
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Fig. 3. Borehole logs of the studied drill-cores showing sill thicknesses, positions of analyzed samples, and variations in host-rock lithology. The
approximate intrusion level of the Golden Valley Sill Complex is also shown. The three leftmost logs are based on those of Svensen et al. (2007).
Beaufort groups. Three boreholes, G39856, G39974 and
G39980 (referred to as G56, G74 and G80, respectively),
in the westernmost part of the Karoo Basin cut sills
emplaced into the Ecca Group (Fig. 3). The borehole sills
have cumulative thicknesses varying from550 m to several
hundred meters.
The basement beneath the Karoo lavas and sills comprises the Kaapvaal Craton in the northern, and the
Proterozoic Namaqua^Natal (PNN) mobile belt in the
western and southern parts of the Karoo Basin (Fig. 1; e.g.
De Wit et al., 1992; Schmitz, 2002; and references therein).
The PNN is bounded to the south and west by the Cape
Foldbelt. The exposed boundary between these terranes indicates that borehole LA1 is located on Archaean basement, whereas the other locations lie within the PNN
mobile belt.
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EVOLUTION OF KAROO DOLERITES
Samples from the GVSC were collected during field work
in 2004 and 2005. The borehole samples were collected in
2004 at the Core Library of the Council for Geosciences
in Pretoria, South Africa. The cores were logged for lithology during sampling; those presented in Fig. 3 represent
combinations of this logging and available logs from
the Council for Geosciences. The boreholes were drilled
in the 1960s by Soekor (Suid Afrikaanse Olie Explorasie
Korporasie) for stratigraphic purposes (QU1, LA1), and in
the 1990s by the Council for Geosciences for ground
water research (G56, G74 and G80). The boreholes we
have used were fully cored and represent a unique possibility to study sill geochemistry throughout the basin. In addition to sill samples, we also collected a number of
samples from the intruded sedimentary sequences for reference. Moreover, a sample of a pseudo-coal (solid bitumen:
borehole PC2; Fig. 1) was included in this study as reference for the composition of bitumen in the Karoo Basin.
P E T RO G R A P H Y
The petrographic description of dolerites in the GVSC presented below is a summary of that given by Galerne et al.
(2008, 2010) with some additions. The sills generally show
a shift from fine-grained textures at their margins (upper
and lower 5 m) to medium- or coarse-grained in their central regions. Most dolerites consist of about 50 vol. %
plagioclase (pl), 5% olivine (ol), 40% pyroxene and 55%
Fe^Ti oxides. Minor amounts (50·5%) of apatite, pyrite
and very rare fluorite and titanite are present in some of
the most evolved rocks. Olivine (euhedral to subhedral)
occurs partly as single grains (0·4^2 mm in diameter) and
partly in clusters. Plagioclase is generally zoned and forms
elongated laths (typically 1^2 mm long, but may reach
6 mm) partly enclosed by pyroxene and Fe^Ti oxides.
Plagioclase also forms aggregates of zoned, large, subhedral grains (2^4 mm). Such aggregates are found both in
the chilled margins and in central parts of the sills. The
main pyroxene species is augite (cpx), but orthopyroxene
(opx) and pigeonite (pig) have also been observed. The
pyroxenes occur mainly as oikocrysts (up to 3 mm in diameter) enclosing plagioclase and olivine; some oikocrysts enclose euhedral to subhedral cores. Most samples also
contain magnetite (sp) and ilmenite and small amounts of
sulfide. Minor alteration has caused local growth of iddingsite, carbonate and serpentine at the expense of olivine, and biotite and chlorite at the expense of pyroxene.
Biotite and chlorite seem to occur exclusively as secondary
phases. The textures of the dolerites indicate the following
in situ crystallization sequence for the main phases: olivine,
plagioclase, pyroxenes, Fe^Ti oxides.
The drill-core dolerites have similar textures to those
described for the GVSC. Also in the drill-cores most samples show evidence of minor alteration with growth of
Geochemistry
The samples were analyzed for major and trace elements
by inductively coupled plasma atomic emission spectrometry (ICP-AES) and inductively coupled plasma mass
spectrometry (ICP-MS) at the University of London,
Royal Holloway. Blocks of fresh rock, at least 3 cm in all
directions, were cut and crushed. Powders for geochemical
analyses were prepared from 50 g of crushed rock using a
tungsten tema-mill; 0·2 g of powdered sample was weighed
into a graphite crucible and 1·0 g of LiBO2 added. The
powders were carefully mixed and fused at 9008C for
20 min, then dissolved in 200 ml of cold 5% nitric acid.
Ga was added to the flux to act as an internal standard.
This solution was then analysed for Si, Al and Zr by
ICP-AES using a Perkin Elmer Optima 3300 R. The instrument was calibrated with natural and synthetic standards. The solution was also used to analyse for Cs, Nb,
Rb, Ta, Th, Tl, U, Y, and rare earth elements (REE) by
ICP-MS on a Perkin Elmer Elan 5000 calibrated with natural and synthetic standards. Another batch of 0·2 g of
powdered sample was dissolved in 6 ml of HF and HClO4
(2:1 mixture), evaporated to dryness, cooled and dissolved
in 20 ml of 10% HNO3. This solution was analysed by
ICP-AES for Fe, Mg, Ca, Na, K, Ti, P, Mn, Ba, Co, Cr,
Cu, Li, Ni, Pb, Sc, Sr, V, and Zn.
Representative samples were also analyzed for Sr and
Nd isotopes at the Geoanalytical Facility, University of
Bergen. As Pb appears to have been disturbed by late-stage
contamination processes (see below) we chose not to analyze for Pb isotopes. The chemical processing was carried
out in a clean-room environment with HEPA-filtered air
supply and positive pressure, using reagents purified in
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two-bottle Teflon stills. Samples were dissolved in a mixture of HF and HNO3. Sr was separated by specific extraction chromatography using the method described by Pin
et al. (1994). Sr was loaded on a double Re-filament and
analysed in static mode using a Finnigan 262 mass spectrometer. Sr isotopic ratios were corrected for mass fractionation using an 88Sr/86Sr value of 8·375209. Repeated
measurements of the NBS 987 Sr standard yielded an average of 0·710223 0·000007 (2s; n ¼ 9; accepted value is
0·710240). The REE were separated by specific extraction
chromatography using the method described by Pin
et al. (1994), and Nd was subsequently separated using a
modified version of the method described by Richard
et al. (1976). The separated Nd was loaded on a double
Re-filament and analysed in dynamic mode. The
Nd-isotopic ratios were corrected for mass fractionation
using a 146Nd/144Nd ratio of 0·7219. Repeated measurements of the La Jolla standard yielded an average
143
Nd/144Nd ratio of 0·511842 0·000006 (2s) (n ¼ 8).
M E T H O D S A N D A N A LY T I C A L
P RO C E D U R E S
Sampling
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Electronic Appendix 1. Many of the drill-core sills are relatively thin (a few meters to a few tens of meters) and are
represented by only one or a few samples (Fig. 3). Sills
within the same drill-core are therefore shown with the
same symbol in the figures; altered samples are shown by
open symbols.
Most of the dolerites classify as basalts to basaltic andesites with tholeiitic, subalkaline affinity; a few evolved samples classify as dacites [using the classification systems of
Kuno (1968) and Le Bas et al. (1986)]. With the exception
of two evolved samples with 2·1 and 2·2 wt % TiO2, the
dolerites contain 0·4^2·0 wt % TiO2 (Fig. 4) and thus
biotite, chlorite and Fe^Ti oxides at the expense of primary
silicates. A few samples from boreholes G74 and LA1
(Figs 1 and 3; Electronic Appendix 1) are strongly
altered with biotite, chlorite/chamosite, smectite, quartz
and zeolite (possibly laumonite) as the main alteration
products.
W H O L E - RO C K C H E M I S T RY
Major and trace element data on samples from the GVSC
were reported by Galerne et al. (2008). Major and trace
element data on the drill-core samples are given in
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Fig. 4. Whole-rock major element variations (wt %) among the main sills and dykes in (a) the Golden Valley Sill Complex (GVSC; data from
Galerne et al., 2008) and (b) the drill-core sills. Arrows indicate trends defined by the Glen Sill and Harmony Sill samples. GVS, Golden
Valley Sill; GS, Glen Sill; HS, Harmony Sill; MS, Morning Sun Sill; GV dyke, Golden Valley Dyke; Cr dyke, Cradock Dyke. Locations of the
GVSC units are shown in Fig. 2, drill-core locations in Fig. 1, and drill-core logs in Fig. 3. Glen Sill samples that, on the basis of trace elements,
are believed to have undergone upper crustal contamination (*cont, contaminated; open circles) fall within the field of other Glen Sill samples.
Altered drill-core samples show addition of SiO2 and K2O, and removal of CaO and Na2O. (See text for further information.)
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EVOLUTION OF KAROO DOLERITES
Fig. 4. Continued.
belong to the low-Ti series. Total ranges in MgO and SiO2
are 2·1^16·8 and 48^65 wt %, respectively, but most of the
samples lie within the range 5^9 wt % MgO and 48^
53 wt % SiO2. The GVSC samples define clear trends of
increasing concentrations of TiO2, Fe2O3, K2O, P2O5,
and decreasing Al2O3, CaO and Cr with decreasing
MgO (Figs 4 and 5). Most units also show trends of strongly decreasing Ni with decreasing MgO (Fig. 5). The
Harmony Sill forms separate trends in most diagrams.
Also, the Cradock Dyke appears to define a low CaO^Ni
trend.
Among the drill-cores the LA1 and G80 sills have relatively uniform compositions whereas the others show considerable variations (Electronic Appendix 1; Fig. 4b); the
widest range is found in QU1 (e.g. 2·7^8·1wt % MgO,
0·8^2·7 wt % TiO2). The majority of the drill-core samples
fall within the GVSC ranges. The largest scatter is found
among the altered samples from holes G56 and QU1, and
among samples with MgO 410 wt %, which also show
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very high Ni and Cr contents (Fig. 5). The MgO-rich
rocks differ petrographically from the other rocks by
having a high proportion of euhedral to subhedral olivine
surrounded by near-equal porportions of clinopyroxene
and plagioclase, plus some magnetite. These samples are
generally located at the base of sills (G74-1-3, G56-2-3,
QU1-1-7; Electronic Appendix 1; Fig. 6) and are probably
cumulates. Very high Pb, K2O and U contents, suggesting
contamination, are found in sample QU1-1-8 near the
roof of a sill, but also in sample QU1-1-13 in the middle of
a sill (c. 190 and 240 m, respectively, in Fig. 6). The lowermost QU1 sill shows an upwards development towards
more differentiated rocks. Otherwise there are no systematic variations.
The majority of the dolerites have uniform, essentially
parallel PM-normalized [where PM is Primitive Mantle
as defined by McDonough & Sun (1995)] trace element
patterns showing mild enrichment in the most incompatible elements relative to heavy REE (HREE; e.g.
LaN ¼ 8^33; YbN ¼ 3·9^8·3; the subscript N indicates normalization to PM; Fig. 7a and b). These samples are
referred to as Group 1. Group 1 patterns also show weak
to strong negative Nb^Ta anomalies, weak negative
anomalies for P and Ti, and enrichment in K relative to
La. Pb shows mildly negative to strongly positive anomalies. Sr shifts from mildly positive anomalies in the least
evolved samples to negative anomalies in the most evolved
ones which, together with depletion in Eu relative to neighboring REE in the most enriched samples is compatible
with the removal of plagioclase. The majority of the
drill-core samples have trace element patterns that are essentially identical to those of the GVSC Group 1 dolerites
and thus belong to Group 1 (Fig. 7b). The main difference
is that a few REE-rich drill-core dolerites lack the marked
enrichment in K relative to La shown by all GVSC
Group 1 samples. This may be an alteration effect.
Both in the GVSC and in the drill-cores a few samples
have trace element patterns that deviate from those of
Group 1 (Fig. 7c and d); these are referred to as Group 2.
In the GVSC Group 2 samples are restricted to a single
profile across the Glen Sill (Fig. 2). These samples have
major element compositions similar to Group 1 (Fig. 4),
but are strongly enriched in Th, U, Nb and Ta, and have
high Ta/Nb ratios (Fig. 7c). In the drill-cores Group 2 is
represented by the moderately to strongly altered samples
in drill-cores G56 and QU1 (G56-3-1, G56-3-2, G56-3-4,
QU1-1-5, QU1-1-7, QU1-1-8 and QU1-1-13; Electronic
Appendix 1; Fig. 3). These rocks tend towards higher SiO2
and K2O, and lower TiO2 and CaO than Group 1 samples
with similar MgO contents (Fig. 4b), and show much
stronger enrichment in most of the strongly incompatible
elements (Fig. 7d).
In GVSC Group 1 dolerites from the same unit,
pairs of incompatible elements define trends through
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the origin, whereas samples from different units may
have contrasting ratios (Fig. 8). Group 2 samples fall
off the Group 1 trends. Drill-core Group 1 samples
fall within, or close to, the trends defined by GVSC
Group 1 (Fig. 9), whereas Group 2 samples are mildly
to strongly enriched in Rb, Ba, Th, La and Pb relative
to Group 1 samples with similar Y contents. Pb differs
from other incompatible elements by showing significant variations that are unrelated to concentrations in
other incompatible elements (e.g. Y) and wide ranges
in concentration within each unit. Also, Sr shows some
scatter unrelated to Y, and a weak increase with
increasing loss on ignition (LOI) among the GVSC
samples (not shown).
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Fig. 5. Variations in Ni and Cr (ppm) relative to MgO (wt %) among dolerites from the GoldenValley Sill Complex (data from Galerne et al.,
2008) and drill-cores in the Karoo Basin (Electronic Appendix 1). The field termed ‘basalts þ dykes’ shows the range covered by low-Ti dykes
in eastern Lesotho (data from Riley et al., 2006) and low-Ti basalts and dykes in Botswana^Zimbabwe (data from Jourdan et al., 2007).
Decreasing Ni and Cr with decreasing MgO are the typical consequences of fractionation of olivine and clinopyroxene. With a few exceptions
among the drill-cores, the dolerites of this study have significantly lower Ni and Cr contents than the most mafic ‘basalts þ dykes’, implying
that they are more evolved. Abbreviations as in Fig. 4.
NEUMANN et al.
EVOLUTION OF KAROO DOLERITES
C O M PA R I S O N W I T H O T H E R
K A RO O -T Y P E M A G M AT I C
RO C K S
Fig. 6. Variations in selected chemical parameters in the sill
intrusions from boreholes LA1 and QU1. The geochemical data are
plotted against cumulative sill thickness in the boreholes; that is,
by excluding the sedimentary rocks from the logs in Fig. 3. The
individual sill intrusions are highlighted as alternating grey and
white bands.
SR ^ N D I SOTOP E S
The dolerites also show a significant range in Nd^Sr isotopic composition [initial ratios 87Sr/86Sr183: 0·7047^0·7080
and eNd183: 0·72 to 4·3, calculated assuming an age of
183 Ma determined by Svensen et al. (2007) for a sill in
G74; Electronic Appendixes 1 and 2; Fig. 10]. Such Sr^Nd
isotopic compositions strongly suggest contamination
by crustal rocks (e.g. Carlson et al., 1981; DePaolo, 1981;
DePaolo et al., 1982).
Within each GVSC unit (sill or dyke) Group 1 samples
show uniform initial Sr^Nd isotope ratios, whereas there
are significant differences between units. Exceptions are the
GoldenValley Sill and the adjacent Glen Sill (Fig.10), which
have similar ratios. In the Glen Sill Group 1 and 2 samples
show identical initial Sr^Nd isotope ratios. The isotope data
thus mimic the geochemical variations among the GVSC
units that were obtained through statistical analysis of major
and trace elements (Galerne et al., 2008). With respect to
87
Sr/86Sr183^eNd183 the GVSC units define a trend from
about (0·705, 1) to (0·708, 4·5) with the L1 sill, G74 and
G80 at the depletedend, andthe Harmony Sill at the most enriched end (Fig. 10). Most Group 1 drill-core dolerites fall
within the GVSC field, but samples from three sills in
drill-core LA1 have slightly higher eNd183 at any given
87
Sr/86Sr183. The Group 2 drill-core samples with the most
deviating trace element compositions (Fig. 4b) scatter to low
eNd183 and high 87Sr/86Sr183 (Fig.10).
DISCUSSION
The observed variations in major and trace element compositions and in initial Sr^Nd isotope ratios (Figs 4^10)
among the GVSC and drill-core dolerites show systematics
that make it possible to reconstruct (at least parts of) the
evolutionary history of the melts that gave rise to these
dolerites. We will show that the GVSC units and
the drill-core sills have a complex post-magma generation
history that involves fractional crystallization and contamination at different depths. These processes are discussed
below.
Fractional crystallization
Significant variations in major and trace element concentrations (e.g. 3·6^8·3 wt % MgO), similar ratios between
pairs of incompatible trace elements (Figs 7 and 8) and
similar Sr^Nd isotopic ratios (Fig. 10a), as shown by each
Group 1 GVSC unit, are typical of fractional
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Karoo-type lavas and dykes cover a wide range in trace
element and Sr^Nd isotope compositions. However, most
low-Ti basaltic rocks (lavas and dolerites) in the Karoo
Basin (Fig. 11; data from Riley et al., 2006; Jourdan et al.,
2007; J. Marsh, unpublished data) fall within a narrow
87
Sr/86Sr183^eNd183 range from about (0·7045, þ1) to
(0·7065, 4), which overlaps the GVSC drill-core field.
Karoo-type lavas and dykes in South Africa and
Antarctica have been divided into the chemical types
CT1^CT4 (Luttinen et al., 1998; Luttinen & Furnes, 2000),
Groups 1^4 (Riley et al., 2005) and D-FP and E-FP
(depleted and enriched ferropicrites, respectively;
Heinonen & Luttinen, 2008). The key discriminant parameters are listed in Table 1. The Group 1 dolerites of this
study show similarities to many of the chemical groups
listed in Table 1, but no overall match. Compared with the
other groups, the most significant feature of the Group 1
dolerites is their high average (Pb/Ce)N ratio (Table 1), although Group 2 dykes show a similar range in Pb anomalies. However, the dolerites differ from Group 2 by their
negative Nb^Ta and positive K anomalies (Fig. 7, Table 1).
With respect to the Drakensberg lavas the dolerites show
clear similarities to the Lesotho Formation and the southern part of the Barkley East Formation (south of 308S).
The latter is, like the GVSC and drill-cores other than
LA1 (Fig. 1), located in the Proteozoic Namaqua^Natal
Mobile Belt. Many of the lavas in the northern part of
the Barkley East Formation, located on the Archaean
Kaapvaal Craton, have significantly more enriched Sr^
Nd isotopic signatures and significantly higher Ba/Nb
ratios (Table 1).
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crystallization. We tried to mimic the trends formed by the
various sample groups using the MELTS model of
Ghiorso & Sack (1995) and Smith & Asimow (2005). The
results for the groups showing the largest compositional
ranges (Group 1 samples from the Harmony Sill and
drill-core QU1) are shown in Fig. 12. Although QU1 represents several sills, the two groups show similar trends. The
systematics of the MELTS curves shown in Fig. 12 are representative of all the modelling results, irrespective of starting composition. The MELTS curves show a series of
evolutionary stages: QU1: ol þ l, cpx þ l, pl cpx ol þ l,
pl ol opx pig þ l, pl þ opx þ sp þ l; Harmony Sill:
ol þ l, ol þ pl þ l, pl ol cpx þ l, pl þ cpx þ sp þ l. The
first stages, which are semi-parallel to the dolerite trends,
are compatible with the crystallization sequence indicated
by the dolerite textures and trace element variations
(Figs 5, 7 and 8). The last stage, involving removal of
spinel/magnetite, is not seen in the dolerite trends,
indicating that spinel/magnetite was never a fractionating
phase. The MELTS results imply that the stage of strongly
decreasing Al2O3 and CaO, and increasing Fe2O3 in the
dolerite trends requires the combined removal of pl þ cpx
( ol). This is compatible with decreasing Ni and Cr with
decreasing MgO (Fig. 5), and increasingly negative Sr
anomalies with increasing REE contents (Fig. 7) in the
dolerites.
We were, however, unable to find a set of physicochemical conditions and starting compositions (based on the
analyzed samples) that exactly reproduced the dolerite
trends. The main difference is that the MELTS model predicts olivine as the only crystallizing phase to significantly
lower MgO contents than we observe in the dolerite
trends. This problem showed up in the MELTS results irrespective of starting composition. Lower oxygen fugacity,
addition of H2O (Fig. 12) and higher pressures (not
shown) enhance this problem. Under more oxidizing
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Fig. 7. Trace element concentrations normalized to Primordial Mantle (PM; McDonough & Sun, 1995) for uncontaminated (Group 1) and
contaminated samples (Group 2) in the Golden Valley Sill Complex (a and c, respectively) and drill-cores (b and d). (See text for further
explanation.)
NEUMANN et al.
EVOLUTION OF KAROO DOLERITES
Fig. 8. Variations in incompatible trace element abundances (ppm)
between units in the Golden Valley Sill Complex. GVS, Golden
Valley Sill; GS, Glen Sill; HS, Harmony Sill; MS, Morning Sun Sill;
GV dyke, Golden Valley Dyke; Cr dyke, Cradock Dyke. Thin dashed
lines indicate trends of fractional crystallization; thick dotted grey
lines marked ‘cont’ indicate the effects of contamination. (See text for
further discussion.)
11
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conditions spinel enters the liquidus at higher MgO concentrations; this is not compatible with the dolerite trends.
Another important difference between the observed
and modelled trends lies in the Na2O^MgO relationship
(Fig. 12). The dolerites show increasing Na2O with decreasing MgO, whereas in the MELTS model removal of
plagioclase causes Na2O to decrease. A possible explanation of the discrepancy between the dolerite trends and
the MELTS results may be that flow of melt through a
large cooling sill involves more complex processes than
simple fractional crystallization. This is reflected in different compositional profiles. Proposed explanations include
prolonged continuous magma influx (Gorring & Naslund,
1995), convective flux of refractory components within the
crystal^liquid mush in the boundary layer during in situ
differentiation or compositional convection (e.g. Tait &
Jaupart, 1996), gravitational settling (e.g. Frenkel et al.,
1989), Soret fractionation (e.g. Latypov, 2003; Latypov
et al., 2007) and post-emplacement melt flow (Aarnes
et al., 2008; Galerne et al., 2010). These complex processes
are expressed in a variety of compositional profiles in the
GVSC (Galerne et al., 2010). The discrepancy between
the data and the model may also be due to problems in
the MELTS model. Putirka (2005) reported that in a comparison with experimental melts that all had plagioclase
on the liquidus, the MELTS or pMELTS model failed to
place plagioclase on the liquidus for more than half the
test melts.
Despite some differences between the MELTS and the
dolerite trends the results obtained from MELTS provide
important information. The results imply that the compositional variations among the Group 1 dolerites involve extensive fractional crystallization at low pressure (in situ),
relatively oxidizing conditions (c. FMQ þ1, where FMQ
is the fayalite^magnetite^quartz buffer) and dry conditions (or negligible water contents). Furthermore, under
these conditions, the MELTS results suggest that a MgO
reduction from 7·5 to 3·5% requires about 50^60%
crystallization (F ¼ 0·40·5; Fig. 12). An estimate based
on the range of thorium contents in the Harmony Sill,
1·5^3·4 ppm (Electronic Appendix 1), gives a similar
result, 40^45% crystallization (F ¼ 0·550·60) assuming
a bulk mineral/melt distribution coefficient for Th in the
range 0·01^0·1 [based on data from Francalanci (1989)
and Green et al. (2000)].
However, the parent melts of the dolerites must also
have experienced some differentiation processes before
intruding the upper crust (pre-intrusion processes). This is
reflected in the evolved compositions of all the dolerites of
this study. Their Ni and Cr contents (Fig. 5) are, with two
exceptions among the high-MgO^Ni^Cr drill-core samples, significantly lower than those found in the most
mafic low-Ti basalts and dykes in other parts of the Karoo
Basin (e.g. Riley et al., 2006; Jourdan et al., 2007). The
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Fig. 9. Variations in incompatible trace element abundances (ppm) vs Y between drill-cores. Dashed lines indicating fractional crystallization
are from Fig. 8. Dotted vertical arrows labelled ‘cont’ indicate the effects of contamination. (See text for further discussion.)
12
NEUMANN et al.
EVOLUTION OF KAROO DOLERITES
In situ contamination
As shown above, the altered and contaminated Group 2
samples fall off the trends defined by incompatible elements in Group 1 samples, and they tend towards higher
87
Sr/86Sr183 (Figs 7^10). These differences are interpreted
as the results of local crustal contamination that has affected a few samples in different ways, implying contrasting contaminants.
Group 2 samples in drill-cores G56 and QU1 in the western part of the Karoo Basin (Fig. 1) are found in sills that
have intruded Ecca shales (Fig. 3). Ecca shales and hornfelses are markedly enriched in Th, U, K, light rare earth
element (LREE) and Pb relative to heavy REE (HREE),
have high U/Th ratios and show strongly negative Ba,
Nb^Ta, P and Ti anomalies (Electronic Appendix 3;
Figs 13a and 14). Also, pseudo-coal from borehole PC2
(Fig. 1) is highly enriched with a high U/Th ratio but it differs from other sedimentary rocks and from Group 2
drill-core samples by having a strong positive Sr anomaly
(Fig. 13b). Compared with Group 1 the Group 2 drill-core
dolerites are more enriched in strongly incompatible elements (Fig. 13b).
Sedimentary rock samples collected in drill-core G74
at different distances from the sill contact (Figs 1 and 3)
show dissimilar variations in the concentrations of trace
elements (Electronic Appendix 3; Fig. 13a). This diversity
suggests a complex redistribution of elements between
restite minerals, new metamorphic phases and fluids
during contact metamorphism and devolatilization of the
wall-rock black shales. It is beyond the aim of this study
to go into detail about these processes. The explanation
favored at present is that during contact metamorphism
and devolatilization a number of elements, including U,
Th and LREE, are preferentially partitioned into the
Fig. 10. Variations in initial Sr^Nd isotope composition, assuming
an age of 183 Ma determined for sills in the Karoo Basin (Svensen
et al., 2007; Svensen & Corfu, personal communication, 2009), among
(a) GVSC units and (b) drill-cores (note the different scales).
Abbreviations as in Fig. 8. The crustal contamination trend is estimated on the basis of the EC-RAFC model of Spera & Bohrson
(2004) using dyke SA.20.1 from southern KwaZulu^Natal, South
Africa (Riley et al., 2006) as the model initial melt and a combination
of the Proterozoic mafic granulite xenoliths HCA28 and HCA35
from the northern Lesotho^central Cape province (Huang et al.,
1995) as the contaminant. EC-RAFC parameters are given in
Table 2. (See text for discussion.) Tick marks on the model curve are
5% increments of crustal contamination.
highest Fo content in olivine is 82, and the highest
mg-number in pyroxenes is similar (Galerne et al, 2008,
2010; E.-R. Neumann, unpublished data). Data on ultramafic Karoo-type rocks and mantle xenoliths in the
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Karoo Basin imply that the primary melts should have significantly more Mg-rich compositions. Olivine cores in picritic dykes in Vestfjella (Dronning Maud Land,
Antarctica) have Fo contents up to 92 (Heininen &
Luttinen, 2010). Fo contents reported for olivine in mantle
xenoliths from various South African localities fall in the
range 88^94 (e.g. Boyd et al., 1999; Konzett et al., 2000;
Gre¤goire, et al., 2003, 2005; Simon et al., 2007). The lowest
Fo values are observed in MARID-type xenoliths in
which metasomatism involving the addition of iron appears to be associated with kimberlite magmatism at
c. 80^140 Ma (e.g. Konzett et al., 1998, 2000), thus
post-dating the Karoo event at c. 183 Ma. Fe-rich dunite
xenoliths (Fo87^89) in the Kimberley kimberlite have been
interpreted by Rehfeldt et al. (2007) to be recrystallized cumulates related to fractional crystallization of low-Ti type
Karoo flood basalt magmas. These data confirm that the
primary low-Ti Karoo magmas must have been significantly more mafic than the dolerites of this study.
Pre-intrusion processes are discussed below.
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fluid. Very high concentrations closest to the sill contact
strongly suggest that enriched fluids not only ascended towards shallower levels, but also moved towards (and into)
the sill. The latter effect may be caused by the development
of a negative pressure gradient at the sill margins owing
to volume reduction during cooling and crystallization
(Aarnes et al., 2008; Galerne et al., 2010). The chemical differences between Group 2 dolerites in the drill-cores and
Group 1 dolerites are compatible with contamination by
enriched fluids. Influx of fluids into the sills must have continued after crystallization of the dolerites and caused
alteration.
In the GVSC contamination is seen only in a single profile across the Glen Sill (Group 2). Contamination in this
profile has caused different degrees of enrichment in Th,
U, Ta, and less extensively in Nb, relative to HREE as
compared with the Glen Sill Group 1 samples (Figs 7 and
8); Sr^Nd isotopic ratios and major elements appear unchanged (Figs 4 and 10). The trace element patterns of
these Group 2 samples differ significantly from those of
the sedimentary wall-rocks (Figs 3 and 13c), comprising
sandstones and siltstones of the Beaufort Group (e.g. Cole,
1992; Veevers et al., 1994; Johnson et al., 1997) that have
uncommonly high Ta/Nb ratios up to 0·23. Lavas and
dykes from the Botswana^Zimbabwe province also tend towards high Ta/Nb ratios (0·060^0·35; Jourdan et al., 2007).
Lavas and dykes in the Karoo Basin and Vestfjella
(Antarctica) in general, including Group 1 GVSC and
drill-core dolerites, fall within the range Ta/Nb ¼ 0·05^
0·07, similar to MORB, ocean island basalt (OIB) and
their mantle sources (0·055^0·056; e.g. data from
Hofmann, 1988; Sun & McDonough, 1989; McDonough
& Sun, 1995). Sedimentary rocks in the Karoo Basin have
Ta/Nb ratios in the range 0·070^0·16 (Electronic
Appendix 3); averages for the upper, middle and lower
crust are 0·075, 0·060, and 0·12, respectively (Rudnick &
Gao, 2003). The high Ta/Nb ratios in the Group 2 dolerites
imply that the contaminant is not an ordinary rock type.
One possibility is that it comprises fluids that have interacted with U^Ta^Th-rich minerals. The Beaufort Group
includes stratiform uranium ore bodies believed to have
formed during the Cape orogeny (e.g. le Roux, 1993, and
references therein). In addition to U, these bodies show enrichment in Pb, As, Co and Hg and very high U/Th
ratios (e.g. le Roux, 1993); unfortunately, data on Ta and
Nb are not available. No ore body is reported from the
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Fig. 11. Initial Sr^Nd isotope compositions at 183 Ma for the GVSC and drill-cores of this study (dark grey field marked GVSC), compared
with data for low-Ti type basalts, dykes and sills from other parts of the Karoo Basin. BAS1 and BAS2, Lesotho lavas (J. Marsh, personal communication, 2008), Botswana^Zimbabwe lavas (Jourdan et al., 2007); KwaZulu^Natal dykes (Riley et al., 2006); Effingham intrusive rocks (J.
Marsh, personal communication, 2004); Rooi Rand dykes (RRd; high- and low-Ti; J. Marsh, personal communication, 2010). The model
curves are calculated using the EC-RAFC model of Spera & Bohrson (2004) using the starting melt, assimilant and EC-RAFC parameters
listed in Table 2. (See text for detailed discussion.)
NEUMANN et al.
EVOLUTION OF KAROO DOLERITES
Table 1: Comparison between selected chemical parameters in GVSC and drill-core Group 1 dolerites and basalts in the
Drakensberg Group, Lesotho, South Africa (Fig. 1) and chemical types CT1-CT4, Groups 1^4, and depleted and enriched
ferrobasalts (D-FP and E-FP, respectively) in Vestfjella, Dronning Maud Land, Antarctica
Ba/Nb
range
(Nb/La)N
av.
range
La/Yb
av.
range
av.
range
87
eNd183
range
range
Sr/86Sr183
(Pb/Ce)N
av.
Low-Ti
GVSC1
14–67
27
0·41–1·0
0·72
2·4–6·0
4·4
0·3–4·9
2·7
0·7049–0·7080
0·7 to 4·3
Drill-cores
10–51
29
0·48–0·86
0·66
3·6–6·7
4·4
0·4–5·2
3·2
0·7053–0·7086
0·9 to 3·8
Lesotho Fm2
28–36
0·47–0·63
0·7048–0·7069
Barkley East (S)2
17–43
0·56–1·0
0·7049–0·7075
Barkley East (N)2
31–83
7·1–254
CT2 sill3
CT33
CT44
Group 15
Group 25
0·24–0·65
95
0·37–0·56
0·44
73
0·48–0·85
0·56
22
25–152
8·6–9·4
21–43
7·8–17
4·1–9·1
6·2
2·6–4·5
3·8
0·62
12
1·9 to 14·4
2·5 to 11·1
1·4
0·9
0·7033
þ7·6
0·8–1·6
1·2
0·7037–0·7053
þ2·0 to 1·3
þ0·7
2·9
12–13
0·7056–0·7165
0·7070–0·7096
1·0–2·1
9
0·95–1·1
0·98
1·1–1·2
1·1
0·70434
36
0·48–0·57
0·52
6·6–7·3
7·0
1·2–1·5
1·4
0·7034–0·7085
5·8 to 6·4
10
0·88–0·95
0·92
3·9–4·2
4·1
0·6–4·2
1·3
0·7034–0·7046
þ1·7 – þ0·7
High-Ti
CT2 lavas3
Group 35
Group 45
E-FP6
26–51
4·5–41
14–44
35
0·39–0·62
0·42
4·5–8·0
5·7
1·0–1·4
1·2
0·7058–0·7082
0·2 to 7·5
10
0·40–1·1
0·97
2·5–6·4
3·5
0·4–2·0
0·7
0·7035–0·7062
þ9·0 – þ5·0
0·5–0·7
0·5
17
0·45–0·83
0·67
0·7048–0·7059
þ2·4 to 4·6
7·2–58
20
0·39–1·1
0·82
11–29
1·8–9·1
17
6·0
0·7033–0·7070
þ7·6 to 10·7
3·2–49
13
0·62–0·84
0·73
3·9–8·7
5·4
0·7030–0·7036
þ8·3 – þ4·8
Variable Ti
D-FP6
1
Galerne
2
et al. (2008).
Range in averages of single lava flows; data from J. Marsh (personal communication, 2008).
3
Data from Luttinen & Furnes (2000).
4
Data from Luttinen et al. (1998).
5
Riley et al., (2005).
6
Data from Heinonen & Luttinen (2008) and Heinonen et al. (2010).
vicinity of the GVSC, but that does not exclude the possible
presence in the area of layers rich in U^Ta^Th-rich minerals. Interaction with such minerals or with fluids
formed through contact metamorphism might lead to
local strong U^Ta^Nb-enrichment. Although important
information is lacking, we believe that this is the best explanation for the contamination in the Glen Sill.
In addition to the contamination shown by the Group 2
dolerites, the wide ranges in Pb within each unit shown
by Group 1 rocks strongly suggest addition through
local contamination. Local contamination has also caused
a weak increase in Sr with increasing LOI. We found
no correlation between sill thickness and degree of
contamination.
Nd isotope signatures exhibited by the different units
(Figs 7^10 and 15) suggest that pre-intrusion processes
comprise a combination of fractional crystallization and
assimilation^contamination (AFC). We have used the
EC-RAFC model of Spera & Bohrson (2004) to test this
possibility.
Primitive-melt compositions for use in the EC-RAFC
modeling were sought among those Karoo-type lavas and
dolerites that show the most depleted Sr^Nd isotope and
trace element compositions and the least evidence of crustal contamination, based on data from Luttinen et al.
(1998), Luttinen & Furnes (2000), Riley et al. (2005, 2006),
Jourdan et al. (2007, 2008), Heinonen & Luttinen (2008,
2010), Heinonen et al. (2010) and unpublished data (J. S.
Marsh, personal communication, 2009). The primitive
melt must be at least as depleted as the least enriched
GVSC dolerite (87Sr/86Sr183 0·7049; eNd183 ^0·72;
Nd 9·5 ppm; Figs 10 and 16). The Sr^Nd isotope
Pre-intrusion differentiation processes
The evolved character of all the Group 1 GVSC and
drill-core samples and the distinct trace element and Sr^
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CT13
0·14 to 7·0
1·0 to 3·6
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Fig. 12. Compositional variations among Group 1 dolerites from the Harmony Sill and drill-core QU1 (recalculated to a total of 100%) compared with trends of fractional crystallization estimated by the MELTS model (Ghiorso & Sack, 1995; Smith & Asimow, 2005). Blue continuous
line and numbers: 1 kbar, dry, fO2 buffer curve FMQ þ1, sample QU1-1-9 as starting composition; blue dashed line: same, but using the FMQ
buffer curve; red continuous line: 0·5 kbar, dry, fO2 buffer curve FMQ þ1, sample K05-196 as starting composition; red dashed line: same, but
assuming 0·3% H2O in the starting melt; vertical lines indicate changes in the phase assemblage. Because all Fe is assumed to be trivalent in
the starting composition, the MELTS compositions, with both FeO and Fe2O3, are shifted to slightly higher concentrations in SiO2, Al2O3,
etc. The MELTS curves do not reproduce the dolerite trends in detail, but they clearly indicate that the stage of strongly decreasing Al2O3
and CaO, and increasing Fe2O3 with decreasing MgO requires the removal of both plagioclase and clinopyroxene ( olivine). Numbers indicate proportion of melt remaining (F). Small irregularities along the MELTS curves reflect variations in the liquidus assemblages (e.g. pl,
pl þ ol and pl þ cpx in the pl ol cpx field). ol, olivine; pl, plagioclase; cpx, clinopyroxene; opx, orthopyroxene; pig, pigeonite; sp, spinel.
(See text for discussion.)
16
NEUMANN et al.
EVOLUTION OF KAROO DOLERITES
relationships of representative candidates are shown in
Fig. 17. Karoo basalts or dolerites that meet these restrictions are relatively rare. In general crustal rocks in the
Karoo are markedly depleted in Nb and Ta relative to
other strongly incompatible elements (Eglington et al.,
1989; Huang et al., 1995; Cornell et al., 1996; Eglington &
Fig. 13. Concentrations of incompatible trace elements normalized
to Primordial Mantle (PM; McDonough & Sun, 1995) for (a) representative sedimentary rocks in the Karoo Basin (Electronic
Appendix 3), (b) sedimentary rocks (grey shaded field) and altered
or contaminated drill-core samples (Group 2), and (c) sedimentary
rocks (grey shaded field) and contaminated (Group 2) dolerites in
the Glen Sill. (See text for discussion.)
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Fig. 14. U/Th^U relationships among the drill-core dolerites compared with black shales (U/Th ¼ 0·67, U ¼ 23·0 ppm) and pseudocoal (PC2) (U/Th ¼ 2·2, U ¼ 29·4 ppm): (a) overview; (b) details.
Concentrations in black shale hornfels at different distances from the
contact (red stars) were measured in drill-core G74 (Fig. 3;
Appendix 3). The U/Th^U relationships in altered samples
(Group 2) in drill-cores G74 and QU1 are compatible with contamination by fluids released from black shales and/or pseudo-coal.
Somewhat elevated U contents also suggest that some of the QU1 samples assigned to Group1 may have undergone minor contamination.
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Fig. 16. Relationships between initial Sr^Nd isotopic compositions
and Nd concentrations compared to EC-RAFC model curves (continuous lines) using starting materials as for Fig. 15, and the parameters given in Table 2. Dotted lines are EC-RAFC model curves using
a starting melt with Ndmelt ¼ 3 ppm, Ts ¼ 7108C, and other parameters as in Table 2. Abbreviations as in Fig. 15.
87
Sr/86Sr183 (0·708^0·738). The assimilant used in the modelling must be at least as enriched as the most enriched
dolerite (87Sr/86Sr183 0·7086; eNd183 4·4).
EC-RAFC parameters are listed in Table 2. The low Nd
concentrations in the dolerites (Fig. 15) pose important restrictions on the starting conditions used in the modelling.
If the solidus temperature (Ts) is higher than the initial
temperature of the assimilant (Tao) the isotope ratios in
the melt will stay constant until the assimilant reaches Ts
and starts melting; at the same time cooling and fractional
crystallization of the melt will increase its content of incompatible elements (dotted lines in Fig. 16). To mimic the
isotope^trace element trend of the GVSC Group 1 units
Armstrong, 2003; Kampunzu et al., 2003; Lana et al., 2004).
Sr^Nd isotope ratios are available for Archaean and
Proterozoic mafic granulite xenoliths (Huang et al., 1995).
These rocks cover a significant range in eNd183 (þ14 to ^
19) and relatively low 87Sr/86Sr183 (0·7035^0·7110; Fig. 17),
which is typical of the lower to middle crust. Sedimentary
rocks retrieved from drill-cores (Eglington & Armstrong,
2003) are enriched in terms of eNd183 (4·4 to 21·2) and
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Fig. 15. Relationships between initial Sr^Nd isotopic compositions
and size of Nb^Ta anomalies (expressed by the ratio NbN/LaN)
among Group 1 GVSC and drill-core dolerites. The dolerites show a
clear decrease in NbN/LaN with decreasing 87Sr/86Sr183 and decreasing eNd183, indicating that these parameters are closely related. GVS,
Golden Valley Sill; GS, Glen Sill; HS, Harmony Sill; MS, Morning
Sun Sill; L1, L1 sill; GV dyke, Golden Valley Dyke; Cr dyke, Cradock
Dyke. The model curve is derived using the EC-RAFC model of
Spera & Bohrson (2004), with mafic dyke SA.20.1, KwaZulu^Natal,
South Africa (Riley et al., 2006) as the starting melt and a combination of Proterozoic granulite xenoliths HCA35 and HCA28 (Huang
et al., 1995) as the assimilant. EC-RAFC parameters are listed in
Table 2. (See text for discussion.)
NUMBER 0
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EVOLUTION OF KAROO DOLERITES
Fig. 17. Sr^Nd isotope compositions of primitive Karoo-type flood
basalts and crustal rocks in the Karoo Basin, recalculated to an age
of 183 Ma. Karoo-type mafic rocks (Karoo Basin and Vestfjella,
Antarctica): data from Luttinen et al. (1998), Luttinen & Furnes
(2000), Riley et al., (2005, 2006) and Heinonen et al. (2010). Archaean
and Proterozoic mafic granulite xenoliths from northern Lesotho
and central Cape Province, South Africa (Huang et al., 1995). Upper
crust: granite, granodiorite, gneiss, hornfels retrieved from 2500^
6000 m depths in drill-cores in the Karoo Basin (Eglington &
Armstrong, 2003). (See text for discussion.) Sample numbers indicate
rock compositions used in the EC-RAFC modelling.
Ts^Tao must be very small and/or the Ndmelt very low. The
models shown in Figs 10, 11, 15 and 16 are based on
Ndmelt ¼ 9·5 ppm and Tao ¼Ts (Table 2). For Tao ¼Ts the
initial assimilant temperature must be several hundred
degree centigrades; we chose 7008C, the exact value is not
important. Such a highTao means that the EC-RAFC processes must have taken place at elevated pressures, in the
lower crust or upper mantle. Riley et al. (2006) proposed
that enrichment in the KwaZulu^Natal dykes was caused
by contamination by Ecca Group hornfels (Electronic
Appendix 3). However, in addition to the high temperature
and pressure required, EC-RAFC runs show that the high
Nd content (43 ppm) in the Ecca hornfels makes it an impossible candidate for an assimilant in the dolerites of this
study.
The most primitive Karoo-type rocks have been
found in Vestfjella (Antarctica; Luttinen & Furnes, 2005;
Heinonen et al., 2010). A high-Ti, high-Nd sample, Z.1816.2,
was used to test the possibility that the GVSC dolerites originated from an isotopically depleted mantle source
(Table 2; Figs 10 and 11). However, independent of the
assimilant, bulk partition coefficient or temperature settings used in the modelling, this sample failed to reproduce
the dolerite trend. Similarly, Riley et al. (2006) discarded
the Z.1816.2 sample as a model primitive melt composition
Mantle^crust plumbing system
The questions regarding the composition of the primitive
melt(s) and location of AFC processes require a review of
the composition of the mantle beneath the PNN and adjacent part of the Kaapvaal Craton, and the SCLM.
There has been extensive discussion about the source of
the primary Karoo magmas, involving partial melting
of heterogeneous SCLM (e.g. Gallagher & Hawkesworth,
1992; Jourdan et al., 2007, 2009), or derivation from
sub-lithospheric mantle plume or MORB-source mantle
(e.g. Marsh & Eales, 1984; Arndt & Christensen, 1992;
Ellam et al., 1992; Riley et al., 2005; Jourdan et al., 2007;
Heinonen & Luttinen, 2008, 2010; Heinonen et al., 2010).
Strongly depleted Nd^Sr isotope compositions and
MORB- or OIB-type trace element patterns, indicating a
‘normal’ sub-lithospheric mantle source have been mainly
reported for high-Ti and variable-Ti Karoo groups
(Group 3, E-FP and D-FP; Table 1). However, there are
also a few examples of depleted Sr^Nd isotope compositions among low-Ti groups (CT2 sill and Rooi Rand
dykes; Table 1). In a recent study, Heinonen et al. (2010) suggested that heterogeneous sub-lithospheric, long-term
19
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for AFC processes in dykes in the KwaZulu^Natal area.
Tests with different primitive-melt^assimilant combinations showed that the best fit was obtained using mafic
dyke SA.20.1 (KwaZulu^Natal; Riley et al., 2006) as the
model primitive-melt composition, and a combination of
the Proterozoic mafic granulites HCA28 (trace elements)
and HCA35 (Sr^Nd isotopes) as assimilant model composition (HCA28/35; data from Huang et al., 1995).
Although the modelling results (Figs 10, 11, 15 and 16)
do not fit the GVSC trend perfectly, they show that it is
possible to develop the trace element^Nd^Sr isotope characteristics of the different GVSC and drill-core units by
EC-RAFC processes at elevated pressures, involving
10% assimilation of crustal rocks. The results imply that
the initial melt had mildly enriched Sr^Nd isotope ratios
and was, most probably, somewhat more depleted in Nb^
Ta relative to other strongly incompatible elements than
sample SA.20.1 (Fig. 15).
We cannot exclude the possibility that AFC processes
took place in the upper SCLM. Interaction with the lithospheric mantle has been proposed by several workers (e.g.
Marsh & Eales, 1984; Arndt & Christensen, 1992; Ellam
et al., 1992; Gallagher & Hawkesworth, 1992; Luttinen &
Furnes, 2000; Riley et al., 2005, 2006; Jourdan et al., 2007,
2009; Heinonen et al., 2010). However, the relatively flat
HREE patterns ([Dy/Yb]N ¼1·0^1·2; Fig. 7) of the dolerites preclude interaction with garnet-bearing materials.
The weak subduction-like signature indicated for the
primitive melt(s) may either have been inherited from the
sub-lithospheric mantle source of the primary magmas, or
been acquired during ascent through the SCLM. These alternatives are discussed below.
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Table 2: EC-RAFC parameters
Tlm
liquidus of magma
13008C
Tmo
initial temperature of magma
13008C
Tla
liquidus of assimilant
11008C
Tao
initial temperature of assimilant
7008C
Ts
solidus (melt and assimilant)
7008C
Cpm
specific heat of magma
1495 J kg1 K1
Cpa
specific heat of assimilant
1400 J kg1 K1
Hcry
heat of crystallization
395000 J kg1
Hfus
heat of fusion
354000 J kg1
Nd (ppm)
Z.18.16.21
218
25
A1292
150
Nb (ppm)
La (ppm)
87
Sr/86Sr
eNd
Magmas
9·6
SA.20.13
256
9·5
SA.20.1-mod
256
9·5
5·9
0·703518
8·75
0·70385
4·12
12·8
0·704607
0·83
12·8
0·704607
0·83
7·0
0·711026
13·9
0·708423
21·2
Assimilants
HCA35/284
481
4·9
QU1/65 (2993 m)5
658
19·9
2·0
Bulk D0 magma
0·2
0·15
0·15
0·15
Enthalpy magma
0
0
0
0
Bulk D0 assimilant
0·2
0·15
0·15
0·15
Enthalpy assimilant
0
0
0
0
1
Dolerite, Dronning Maud Land, Antarctica (Riley et al., 2005).
2
Dyke, Rooi Rand, South Africa (J. Marsh, personal communication,
3
Mafic dyke, KwaZulu–Natal, South Africa (Riley et al., 2006).
4
Proterozoic granulite xenolith (Huang et al., 1995).
5
2010).
Granodiorite, borehole (Eglington & Armstrong, 2003).
on mantle xenoliths from off-craton kimberlites, Janney
et al. (2010) concluded that the PNN mantle lithosphere
became strongly depleted through moderate degrees of
partial melting during the earliest Proterozoic, and additional moderate to high degrees of partial melting at
1·3^1·0 Ga, and was undisturbed from about 1 Ga until
the time of the Karoo event. They found no mantle
wedge signature in the PGE patterns of the PNN peridotites. Unfortunately, Janney et al. (2010) did not present
REE, high field strength element (HFSE) or Sr ^Nd isotope data that might be directly compared with the
GVSC and drill-core data.
There is consensus of opinion that the SCLM beneath
the Kaapvaal Craton has been subjected to repeated enrichment processes (e.g. Simon et al., 2003, 2007; Hopp
et al., 2008), but the resulting geochemical signatures are
unclear. Hopp et al. (2008) proposed subduction-related
metasomatism. Simon et al. (2007) reconstructed the
depleted, peridotitic mantle with enriched components is
the main source for the primary Karoo melts.
An important observation is that although the Group 1
dolerites were sampled partly on the Archaean Kaapvaal
Craton (LA1 drill-core; Fig. 1) and partly on different
parts of the PNN mobile belt (all others), they show very
similar trace element and Sr^Nd isotope characteristics
(Figs 7^11). There is some controversy with regard to the
evolution and composition of the mantle lithosphere beneath these two crustal domains. Hopp et al. (2008) reported 40Ar/39Ar ages of 1·0^1·2 Ga for phlogopites in
kimberlite-hosted mantle xenoliths in the southern
Kaapvaal Craton. These ages coincide with the Kibaran
orogenic cycle that formed the PNN, and Hopp et al.
(2008) proposed that during this period melts and fluids
caused significant metasomatism of the PNN mantle lithosphere, and that the effects reached far into the Kaapvaal
Craton. However, based on PGE and Re^Os isotope data
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Sr (ppm)
NEUMANN et al.
EVOLUTION OF KAROO DOLERITES
Table 3: Schematic outline of possible scenarios for the evolution of dolerites in the Golden Valley Sill Complex
and drill-cores
Scenario 1
Upper crust
Lower crust
Scenario 2
Scenario 3
FC
FC
FC
FC
Local contamination
Local contamination
Local contamination
Local contamination
W-r: strong arc sign.
????
W-r: strong arc sign.
W-r: strong arc sign.
AFC
Upper SCLM
Lower SCLM
Scenario 4
AFC
AFC
W-r: depleted
W-r: mild arc sign.
W-r: strong arc sign.
Melt interaction
AFC
No melt interaction
W-r: mild arc sign.
W-r: mild arc sign.
W-r: depleted
Melt interaction
Melt interaction
No melt interaction
Sublithospheric
MORB type (or OIB)
MORB type (or OIB)
Mild arc signature
source
Partial melting
Partial melting
Partial melting
Partial melting
W-r, wall-rocks; arc sign., arc signature.
pre-kimberlite compositions of kimberlite-hosted xenoliths
in the Kaapvaal Craton and found that before the kimberlite metasomatism typical mantle compositions had
concave-upward trace element patterns with weakly negative to positive Nb^Ta anomalies. According to Schmitz
& Bowring (2004) the effects of the Late Proterozoic
Namaquan orogeny is manifested in the formation of
lower crustal granulites, both in the PNN mobile belt and
along the southern margin of the Kaapvaal Craton.
Other parts of the cratonic crust in southern Africa show
strong arc-like trace element signatures.
In reality, any combination of the compositional characteristics proposed for the Kaapvaal and PNN mantle domains may be possible. The chemical similarity of sills
emplaced in cratonic (LA1; Fig. 1) and PNN terranes
(all others) may thus be explained by ascent through
SCLM domains with similar geochemical signatures. An
alternative explanation, which allows for the possibility
that the two mantle domains had different geochemical
signatures, is that the primary magmas were transported
over long distances (e.g. Encarnacio¤n et al., 1996; Elliot
et al.,1999), so that the magmas giving rise to all the sills ascended through the same SCLM domain, which had an
arc-like signature. A third possibility is that all the
magmas ascended through depleted PNN lithospheric
mantle without interaction with the mantle wall-rocks.
Luttinen & Furnes (2000) found evidence among Karootype flood basalts in Antarctica that melts extruded on
Proterozoic crust were not contaminated by lithospheric
mantle material, whereas melts extruded on cratonic crust
were contaminated by cratonic mantle material. If the
PNN mantle was depleted and the melts ascended without
interacting with the wall-rocks, the sublithospheric source
must have had a mild arc-like signature.
Assuming different combinations of the evolutionary
processes outlined above and of compositions in lithospheric and sublithospheric mantle domains, four main scenarios are possible. These are schematically outlined in
Table 3. We consider Scenario 1 most likely.
S U M M A RY M O D E L
Assuming Scenario 1 in Table 3 the evolutionary history of
the dolerites in the sills and dykes in the GVSC and in
drill-cores in various parts of the Karoo Basin, South
Africa, is summarized below, and shown schematically in
Fig. 18.
(1) The primary magmas originated in sub-lithospheric
mantle with MORB (or OIB) source composition.
(2) We accept the results of Hopp et al. (2008) that
the lithospheric mantle beneath the Proterozoic
Namaqua^Natal mobile belt and the adjacent
Kaapvaal Craton had acquired an arc-like signature
(enriched Sr^Nd isotopic compositions and negative
Nb^Ta anomalies) owing to metasomatism during
the Kibaran orogenic cycle at 1·2^1·0 Ga.
(3) The magmas that gave rise to the dolerites of this
study interacted with lithospheric mantle wall-rocks
during ascent to the near-surface and thus acquired a
weak arc-like signature (Stage 1).
(4) The magmas intruded the deep crust, where they
underwent different degrees of contamination by
crustal rocks with a strong arc-type geochemical signature, accompanied by fractional crystallization
(AFC processes; Stage 2). Assimilation of 10%
lower crustal rocks led to different degrees of enrichment in strongly incompatible elements and Sr^Nd
isotopic ratios, and relative deletion in Nb and Ta.
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W-r: mild arc sign.
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(5)
(6)
(7)
(8)
(9)
High-grade metamorphic rocks with strong arc-type
signatures are found in the PNN as well as in the
Kaapvaal lower crust.
Different degrees of contamination gave rise to
dissimilar geochemical characteristics in the melt
batches that formed the various GVSC and drill-core
units.
Contaminated and somewhat evolved melts ascended
to the upper crust where they were intruded to form
sills and dykes in the layered sedimentary sequence.
During and/or after emplacement into the upper crust
the melts were subjected to a new stage of fractional
crystallization (up to 60%: Stage 3).
Locally the magmas also underwent a new type of
contamination (Stage 3), possibly caused by fluids
released from the sedimentary wall-rocks during contact metamorphism and devolatilization. Different
types of wall-rocks gave rise to fluids with contrasting
chemical characteristics.
In drill-cores G56 and QU1 that intruded black shales
rich in organic material within the Ecca Group, some
samples were contaminated by fluids released from
the shales, resulting in marked addition of Si, K, Cs,
Rb, Ba, Th, Pb and LREE, increased U/Th ratios
and more enriched Sr^Nd isotopic ratios.
(10) In the GVSC contamination in one of the sills (the
Glen Sill) led to strong enrichment in Th, U and Ta,
and less extensively in Nb, whereas Sr^Nd isotopic
ratios and major elements appear unchanged. This
contamination may have been caused by interaction
with ore minerals of the type found in stratiform uranium ore bodies in the Karoo Basin, or with fluids
that had interacted with such minerals.
AC K N O W L E D G E M E N T S
We are indebted to Julian S. Marsh for letting us use his unpublished Sr^Nd isotope data on magmatic rocks from
the Karoo Basin, for helpful discussions based on his
vast knowledge of the geology and petrology of the Karoo
magmatism, and for feedback on an earlier version of this
paper. Many thanks are also due to N. S. C. Simon for enlightening discussions, and to P. E. Janney, who gave us
access to a paper on the evolution of the lithospheric
mantle in the Proterozoic Namaqua^Natal mobile belt
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Fig. 18. Schematic presentation of the ascent through the lithosphere of the melts that gave rise to the dolerites in the GVSC and drill-cores,
and the processes believed to have taken place at different depths and in different areas. The figure is not to scale and the possibility of significant
lateral magma transport (e.g. Encarnacio¤n et al., 1996; Elliot et al., 1999) is not considered. Color variations between the intrusions in the deep
crust indicate different degrees of contamination (imposing an arc signature) and fractional crystallization (Stage 2); vertical color variations
in the shallow sills indicate a new stage of fractional crystallization locally combined with contamination that reflects the local sedimentary
country-rocks (Stage 3). Information about variations in the thickness of the crust and lithospheric mantle from the southern Kaapvaal
Craton, across the Namaqua^Natal mobile belt into the Cape Foldbelt is taken from Nguuri et al. (2001), James et al. (2003) and Li & Burke
(2006). Alternative scenarios are presented in Table 3. (See text for further information and discussion.) E, Ecca Group; B, Beaufort Group; S,
Stormberg Group.
NEUMANN et al.
EVOLUTION OF KAROO DOLERITES
before publication. We would also like to thank Doug Cole
for kindly providing the PC2 pseudo-coal sample, and
Luc Chevallier for discussions about the Karoo drill-cores.
The paper was improved significantly through constructive criticism by the reviewers Fred Jourdan, Arto Luttinen
and Teal R. Riley.
FUNDING
The project was made possible through financial support
from the Norwegian Research Council (NFR) for the project 159824/V30 ‘Emplacement mechanisms and magma
flow in sheet intrusions in sedimentary basins’, including
a doctoral fellowship to C.Y.G. The work has also been
supported by a Centre of Excellence grant from the
Norwegian Research Council to Physics of Geological
Processes.
S U P P L E M E N TA RY DATA
Supplementary data for this paper are available at Journal
of Petrology online.
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