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291.
Petrogenesis of the Swartruggens and Star Group II kimberlite dyke swarms,South Africa: constraints from whole rock geochemistry 总被引:1,自引:0,他引:1
Nancy Coe Anton le Roex John Gurney D. Graham Pearson Geoff Nowell 《Contributions to Mineralogy and Petrology》2008,156(5):627-652
Thirty-seven samples from the Swartruggens and Star Group II kimberlite dyke swarms, emplaced through the Kaapvaal craton,
have been analysed for their major and trace element and Sr, Nd and Hf isotope compositions. The samples are all MgO-rich
(~12–35 wt%) with high Mg# (0.72–0.90) and Ni (~610–2700 ppm) contents. The kimberlites are strongly enriched in incompatible
elements (Zr = 140–668 ppm; La = 124–300 ppm; Nb = 68–227 ppm; Ba = 1500–7000), and have high and variable chondrite normalised
La/Yb ratios (Swartruggens = 94 ± 21; Star = 202 ± 36). 87Sr/86Sr (0.70718–0.71050) ratios are elevated, whereas εNd (−11.95 to −7.84) and 176Hf/177Hf ratios (0.282160–0.282564) are low. Inter- and intra-dyke compositional variation is significant, and there are systematic
differences between the kimberlites found at the two localities. Intra-locality differences can largely be attributed to a
combination of the effects of alteration, crustal contamination, macrocryst entrainment and phenocryst fractionation. There
is some evidence for distinct parental magmas formed through variable and low degrees (0.5–2%) of partial melting, as illustrated
by crossing rare earth element patterns. The Star kimberlites have derived from a less radiogenic source, with higher LREE
enrichment than the Swartruggens kimberlites. Inferred primary magmas at each locality have high Mg# (~0.83), are Ni-rich
(850–1220 ppm) and are strongly enriched in incompatible elements. Calculated mantle source compositions are strongly enriched
in incompatible elements (La/Ybn ~ 10–50), but refractory in terms of Mg# and Ni contents. Incompatible element ratios such as Ba/Nb (>13.5), La/Nb (> 1.1)
and Ce/Pb (< 22) are unlike those characteristic of Group I kimberlites or ocean island basalts, but indistinguishable from
calc-alkaline magmas. Taken together with extremely low εNd and εHf, these compositional characteristics are used to argue
for derivation of these Group II kimberlite magmas from the deep subcontinental lithospheric mantle, metasomatised during
the Proterozoic by calc-alkaline fluids/melts. 相似文献