Bulk rock geochemistry of a swarm of orangeite dykes intersecting the Western Limb of the Bushveld Complex

The Western Limb of the Bushveld Complex hosts a vast, recently documented swarm of orangeite dykes that are significantly younger (177-132 Ma; Hughes et al., in prep.) than the c. 2.06 Ga Bushveld lithologies they intrude. Orangeite dykes are hybrid igneous rocks that form from very low-degree partial melting deep within the sub-cratonic lithospheric mantle (SCLM) and upon ascent entrain foreign material (primarily mantle xenocrysts). Thus, they can be used to probe the composition of and processes within the ancient lithospheric mantle. Whereas similar orangeite dyke swarms in South Africa typically span < 10 km, the considerable size of this swarm (> 50 km along strike and ~10 km wide) and number of closely-spaced dykes offers a unique opportunity to investigate the Kaapvaal SCLM on an unprecedented spatial scale. In this contribution we present the whole rock major and trace element abundances, and the radiogenic isotope compositions of the dykes.The Bushveld orangeites are mafic-ultramafic (whole rock Mg# of 65 to 88) and have overlapping major element abundances to other Kaapvaal orangeites, with significant similarity to the coeval Swartruggens orangeite dyke swarm (Coe et al., 2008). Trace element abundances of the Bushveld dykes are less consistent with Kaapvaal orangeite variability, displaying greater ranges in concentrations of certain elements (e.g. La, Th, Ba) despite being generally relatively depleted in these elements.Radiogenic isotope compositions of the orangeites typically confine to the global orangeite variability, with radiogenic Sr (87Sr/86Sriof 0.70642 to 0.70787) and unradiogenic Hf compositions (&#603;Hfi of -18.3 to -8.3). Initial Nd compositions are generally unradiogenic (&#603;Ndi of -11.6 to -9.0), conforming to values of global orangeites, however three samples display elevated initial Nd (&#603;Ndi of -5.4 to -0.4) and plot in a similar Sr-Nd compositional space to Kaapvaal transitional kimberlites.Using the trace element variations and radiogenic isotope compositions we aim to investigate the geochemistry of the mantle source regions tapped by the orangeites and whether we can identify changes in source characteristics on a swarm scale.References:Coe, N. et al. (2008) Cont. Min. Pet. 156(5). 627-652.Hughes, H.S.R. et al. (in prep).

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Trace-element abundances in the shallow lithospheric mantle of the North Atlantic Craton margin: Implications for melting and metasomatism beneath Northern Scotland

Mineralogical Magazine ◽

10.1180/minmag.2015.079.4.03 ◽

2015 ◽

Vol 79 (4) ◽

pp. 877-907 ◽

Cited By ~ 10

Author(s):

Hannah S. R. Hughes ◽

Iain McDonald ◽

John W. Faithfull ◽

Brian G. J. Upton ◽

Hilary Downes

Keyword(s):

Trace Element ◽

North Atlantic ◽

Lithospheric Mantle ◽

Geographical Location ◽

Mantle Xenoliths ◽

Dyke Swarm ◽

Element Abundances ◽

The North ◽

The North Atlantic ◽

North Atlantic Craton

AbstractBulk rock geochemistry and major- and trace-element compositions of clinopyroxene have been determined for three suites of peridotitic mantle xenoliths from the North Atlantic Craton (NAC) in northern Scotland, to establish the magmatic and metasomatic history of subcontinental lithospheric mantle (SCLM) below this region. Spinel lherzolites from the southernmost locality (Streap Com'laidh) have non-NAC mantle compositions, while the two northern xenolith suites (Loch Roag and Rinibar) are derived from the thinned NAC marginal keel. Clinopyroxene compositions have characteristic trace-element signatures which show both 'primary' and 'metasomatic' origins. We use Zr and Hf abundances to identify ancient cryptic refertilization in 'primary' clinopyroxenes. We suggest that Loch Roag and Rinibar peridotite xenoliths represent an ancient Archaean-Palaeoproterozoic SCLM with original depleted cratonic signatures which were overprinted by metasomatism around the time of intrusion of the Scourie Dyke Swarm (∼2.4 Ga). This SCLM keel was preserved during Caledonian orogenesis, although some addition of material and/or metasomatism probably also occurred, as recorded by Rinibar xenoliths. Rinibar and Streap xenoliths were entrained in Permo-Carboniferous magmas and thus were isolated from the SCLM ∼200 Ma before Loch Roag xenoliths (in an Eocene dyke). Crucially, despite their geographical location, lithospheric mantle peridotite samples from Loch Roag show no evidence of recent melting or refertilization during the Palaeogene opening of the Atlantic.

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Lithospheric Mantle Evolution beneath the Eifel (Germany): Constraints from Sr-Nd-Pb Isotopes and Trace Element Abundances in Spinel Peridotite and Pyroxenite Xenoliths

Journal of Petrology ◽

10.1093/petrology/44.6.1077 ◽

2003 ◽

Vol 44 (6) ◽

pp. 1077-1095 ◽

Cited By ~ 80

Author(s):

G. WITT-EICKSCHEN

Keyword(s):

Trace Element ◽

Lithospheric Mantle ◽

Pb Isotopes ◽

Spinel Peridotite ◽

Element Abundances ◽

Mantle Evolution

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Caveats in defining the magmas parental to the mafic rocks of the Bushveld Complex, and the manner of their emplacement: review and commentary

Mineralogical Magazine ◽

10.1180/0026461026660062 ◽

2002 ◽

Vol 66 (6) ◽

pp. 815-832 ◽

Cited By ~ 25

Author(s):

H. V. Eales

Keyword(s):

South Africa ◽

Trace Element ◽

Bushveld Complex ◽

Bulk Composition ◽

Trace Element Data ◽

Mafic Rocks ◽

Critical Zones ◽

Derivatives Of ◽

Western Limb ◽

Northern Sector

Abstract The composition of magmas proposed as parental to the layered suite of the Bushveld Complex, and some models for the manner of their emplacement, are reviewed briefly. Included are some contributions published in South Africa, with which overseas readers might be less familiar. Emphasis is given to the broader features of the cumulates, and the contradictions raised by whole-rock compositional, Sr-isotopic, and trace-element data that cloud their correlation with proposed parental magmas. It is concluded that the Lower, Critical and Main Zones are the derivatives of only two primary magmatic lineages, while a third was added to residual liquids from which the layered rocks above the Pyroxenite Marker were formed. Excessive amounts of olivine and chromium in the cumulates of the Lower and Lower Critical Zones in the northern sector of the Western Limb can seemingly not be accounted for by the composition and volume of the putative magmas. This is attributed to (1) this sector being a proximal facies located close to the original feeder, and/or (2) crystal-charged magma batches, expelled from a lower magma chamber, being periodically injected into and dispersed within the liquids already in place in the Bushveld chamber. Thus, ongoing changes in the bulk composition of the liquids within the chamber would not be reflected in the rinds of earlier, chilled-facies rocks. The expulsion of significant volumes of liquid residua from the chamber during cumulate deposition is not ruled out.

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U–Pb geochronology and geochemical variation within two Proterozoic mafic dyke swarms, Labrador

Canadian Journal of Earth Sciences ◽

10.1139/e93-128 ◽

1993 ◽

Vol 30 (7) ◽

pp. 1490-1504 ◽

Cited By ~ 66

Author(s):

Andrew C. Cadman ◽

Larry Heaman ◽

John Tarney ◽

Richard Wardle ◽

Thomas E. Krogh

Keyword(s):

North Atlantic ◽

Major Element ◽

Dyke Swarm ◽

Mafic Dyke ◽

Element Abundances ◽

Long Period ◽

The North ◽

Geochemical Variation ◽

Dyke Swarms ◽

North Atlantic Craton

An Early Proterozoic Kikkertavak mafic dyke intruding the Archaean Hopedale block, Labrador, gives an age of 2235 ± 2 Ma using U–Pb techniques on baddeleyite. A Harp mafic dyke in the same area gives an age of 1273 ± 1 Ma using U–Pb techniques on baddeleyite and zircon. The latter age is almost identical to that of the giant Mackenzie swarm and to the age of the BD0 dykes in South Greenland, and points to a major pulse of mafic magmatism over much of the North Atlantic craton at this time. The former age is a little older than available Rb–Sr ages for the extensive MD swarm in West Greenland, but there are possible correlatives.Geochemical data are presented to ascertain whether there are significant compositional differences between the Harp and Kikkertavak dyke swarms. In fact, two distinct chemical subgroups can be recognized within the Kikkertavak dykes, and three others are recognized within the Harp suite. These differences apply more to trace element patterns rather than major element abundances, but although there are compositional differences between the average Harp and average Kikkertavak dyke, it is unlikely that geochemistry could be used unequivocally to separate the two. The compositional differences probably reflect evolutionary processes in the lithosphere. The range of composition exemplified by the subgroups is most easily interpreted in terms of proportion of asthenosphere and lithosphere components, and does not necessarily imply that either dyke swarm was emplaced over a long period. The presence of subgroups within both swarms urges some caution in assuming all dykes correspond to one or other age.

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The extension of the western limb, Bushveld Complex (South Africa), at Cullinan Diamond Mine

Mineralogical Magazine ◽

10.1180/0026461067030328 ◽

2006 ◽

Vol 70 (3) ◽

pp. 241-256 ◽

Cited By ~ 7

Author(s):

R.G. Cawthorn ◽

N. McKenna

Keyword(s):

Bushveld Complex ◽

Incompatible Trace Element ◽

Critical Zone ◽

Alkaline Complex ◽

Rock Composition ◽

Mafic Rocks ◽

Element Abundances ◽

Metasedimentary Rocks ◽

Interstitial Component ◽

Western Limb

AbstractMafic rocks of the Bushveld Complex at the southeastern end of the western limb, intersected in bore core from the Cullinan Diamond Mine, are described. A 260 m thick ultramafic body of orthopyroxene and chromite cumulate rocks, with mg# – 100*Mg/(Mg+Fe) – values from 77 to 84 and 0.25 to 0.5% Cr2O3 in the pyroxene, is considered to have affinity to the Critical Zone. Such an interpretation considerably extends the eastern limit of Critical Zone rocks of the western limb of the Bushveld Complex. The whole-rock composition of the lower, chilled basal contact of this body has 10% MgO and 500 ppm Cr, and is comparable to magmas considered parental to the Bushveld Complex. Due to intrusion of a younger sill, the upper contact is not preserved in the bore core. The cumulate rocks have higher interstitial component, inferred from incompatible trace element abundances (Zr, Ti and K), than normal Critical Zone rocks, interpreted to be a result of more rapid cooling due to proximity to the basal contact. The near-constancy of mg# in the pyroxene in the entire succession suggests that large volumes of magma flowed through this conduit, with only the liquidus phases of orthopyroxene and chromite being precipitated.Five generations of sills, intruded into the underlying metasedimentary rocks, are identified. The oldest is tholeiitic, and was metamorphosed prior to the emplacement of the Bushveld Complex. The second equates to the magma proposed as being parental to the Bushveld Complex (2060 Ma). The third represents the products of differentiation of that magma. The fourth is syenitic, and related to the Pienaars River Alkaline Complex (1430–1300 Ma). The fifth is tholeiitic (1150 Ma), and cuts the Cullinan kimberlite.

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