Bulk rock geochemistry of a swarm of orangeite dykes intersecting the Western Limb of the Bushveld Complex
- 1Camborne School of Mines, University of Exeter, Penryn, United Kingdom (cc715@exeter.ac.uk)
- 2School of Geosciences, University of the Witwatersrand, Johannesburg, South Africa
- 3School of Earth and Ocean Sciences, Cardiff University, Cardiff, United Kingdom
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 (ɛHfi of -18.3 to -8.3). Initial Nd compositions are generally unradiogenic (ɛNdi of -11.6 to -9.0), conforming to values of global orangeites, however three samples display elevated initial Nd (ɛNdi 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).
How to cite: Compton-Jones, C., Hughes, H., McDonald, I., Bybee, G., Kinnaird, J., and Andersen, J.: Bulk rock geochemistry of a swarm of orangeite dykes intersecting the Western Limb of the Bushveld Complex, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12414, https://doi.org/10.5194/egusphere-egu21-12414, 2021.
