New geochemical and Sr-Nd-Hf isotopic constraints on the genesis of kimberlites and ultramafic lamprophyres from the Adelaide Fold Belt and Gawler Craton, South Australia

Abstract

This study investigates the geochemical and isotopic characteristics of Jurassic kimberlites and ultramafic lamprophyres (UMLs) from four clusters within the Adelaide Fold Belt (AFB) and two within the Gawler Craton in South Australia. Petrographic analysis, including the occurrence of magmatic clinopyroxene in the groundmass, supported by a review of available mica and spinel compositional data, indicates that many previously classified kimberlites (Eurelia, Angaston and Terowie) are, in fact, ultramafic lamprophyres. New whole-rock major-, trace element and Sr-Nd-Hf isotopic results, augmented by in-situ perovskite and carbonate Sr isotopes, reveal that this sample suite exhibits extensive geochemical variability. These new data highlight the significant role of crustal contamination in modifying not only bulk-rock major, trace element and Sr isotope systematics, the latter being pristine exclusively in low-SiO₂ samples, but also Nd and Hf isotopic signatures. This is most evident for the Mount Hope (εNd_(i) = -5.1 to -1.5; εHf_(i) = -8.6 to -2.7) and Cleve (εNd_(i) = -3.7 to -0.2; εHf_(i) = -6.6 to +0.3) kimberlites of the Gawler Craton which display geochemically enriched compositions that are rarely seen in kimberlitic rocks. In contrast, the AFB samples exhibit less inter-sample isotopic variability and have compositions that are more typical of kimberlites and UMLs globally (εNd_(i) = +0.3 to +3.9; εHf_(i) = +0.7 to +6.6). There is no clear lithospheric thickness control governing the absence of UMLs on the Gawler Craton and their presence within the AFB. Similarly, there are no systematic differences in Sr–Nd-Hf isotopes between uncontaminated kimberlites and UMLs, arguing against obvious differences in their asthenospheric sources. We tentatively suggest that contribution by more pervasively metasomatised lithospheric mantle beneath the AFB compared to the Gawler craton (based on existing garnet xenocryst data) may facilitate the formation of ultramafic lamprophyres in this region. While subduction along the southern palaeo-margin of Pangea likely did not directly trigger magmatism, it may have facilitated deep mantle upwelling linked to the contemporaneous Karoo-Ferrar Large Igneous Province, with related formation of kimberlites and UMLs in South Australia.