Evolution of ultrapotassic volcanism on the Kaapvaal craton: deepening the orangeite versus lamproite debate

Abstract

Orangeites are a significant source of diamonds, yet ambiguity surrounds their status among groups of mantle-derived potassic rocks. This study reports mineralogical and geochemical data for a ca. 140 Ma orangeite dyke swarm that intersects the Bushveld Complex on the Kaapvaal craton in South Africa. The dykes comprise distinctive petrographic varieties that are linked principally by olivine fractionation, with the most evolved members containing minor amounts of primary carbonate, sanidine and andradite garnet in the groundmass. Although abundant groundmass phlogopite and clinopyroxene have compositions that are similar to those of cratonic lamproites, these phases show notable Ti-depletion, which we consider a hallmark feature of type orangeites from the Kaapvaal craton. Ti-depletion is also characteristic for the bulk rock compositions and is associated with strongly depleted Th-U-Nb-Ta contents at high Cs-Rb-Ba-K concentrations. The resultant high LILE/HFSE ratios of orangeites suggest that mantle source enrichment occurred by metasomatic processes in the proximity of ancient subduction zones.The Bushveld-intersecting orangeite dykes have strongly enriched Sr-Nd-Hf isotopic compositions (initial 87Sr/86Sr = 0.70701-0.70741; εNd = −10.6 to −5.8; εHf = −14.4 to −2.5), similar to those of other orangeites from across South Africa. Combined with the strong Ti-Nb-Ta depletion, this ubiquitous isotopic feature points to the involvement of ancient metasomatized mantle lithosphere in the origin of Kaapvaal craton orangeites, where K-rich metasomes imparted a ‘fossil’ subduction geochemical signature. Previous geochronology studies identified ancient K-enrichment events within the Kaapvaal cratonic mantle lithosphere, possibly associated with collisional tectonics during the 1.2-1.1 Ga Namaqua-Natal orogeny of the Rodinia supercontinent cycle. It therefore seems permissible that the cratonic mantle root was preconditioned for ultrapotassic magma production by tectonomagmatic events that occurred along convergent plate margins during the Proterozoic. However, reactivation of the K-rich metasomes had to await establishment of an extensional tectonic regime, such as that during the Mesozoic breakup of Gondwana, which was accompanied by widespread (1000 × 750 km) small-volume orangeite volcanism between 200 and 110 Ma.Although similarities exist between orangeites and lamproites, these and other potassic rocks are sufficiently distinct in their compositions such that different magma formation processes must be considered. In addition to new investigations of the geodynamic triggers of K-rich ultramafic magmatism, future research should more stringently evaluate the relative roles of redox effects and volatile components such as H2O-CO2-F in the petrogeneses of these potentially diamondiferous alkaline rocks.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5440652

Uranium–lead geochronology applied to pyrope garnet with very low concentrations of uranium

“…However, such garnet is rare in kimberlite, more commonly occurring in ultramafic lamprophyres and orangeites (Mitchell, 1995). For information on the occurrence of similar rocks in South Africa see Tappe et al (2022). Rubidium–strontium and 40 Ar/ 39 Ar in phenocrystic phlogopite are also employed to date kimberlites and can yield precise determinations.…”

Emplacement of the Argyle diamond deposit into an ancient rift zone triggered by supercontinent breakup

“…Lithospheric extension in, or adjacent to, Archean cratons during continental rifting has previously been recognized as a mechanism for emplacement of kimberlites, lamproites and other related volatile-and incompatible element-enriched ultramafic diatremes [38][39][40][41][42] . Examples include the East European Platform during Nuna breakup (c. 1250-1230 Ma 43 ), the Kimberley Craton during attempted Rodinia breakup (c. 840-800 Ma 44 ) and the Kaapvaal Craton during Gondwana/Pangea breakup (c. 200-120 Ma 45,46 ), amongst many others. However, the role of (super)continental breakup has only recently begun to be significantly appreciated for kimberlite emplacement during periods of continental breakup 2,38,42 .…”

Emplacement ages of diamondiferous kimberlites in the Wafangdian District, North China Craton: New evidence from LA-ICP-MS U-Pb geochronology of andradite-rich garnet

“…To the best of our knowledge, there are surprisingly few welldocumented REE geochemical data of kimberlitic andradite available in the published literature up to date, despite of the sporadic but increasingly common occurrences of Ti-andradites discovered in worldwide kimberlite fields and related diamond deposits (e.g., Cheng et al, 2014;Dongre et al, 2016;Dongre and Tappe, 2019;Choudhary et al, 2020;Tappe et al, 2022). Normally, subcalcic garnet megacrysts from the Wafangdian and Mengyin diamondiferous kimberlites are extremely depleted in LREEs and have similar chondrite-normalized REE patterns to those kimberlite-borne peridotitic garnets from other Archean cratons in Siberia and South Africa, except for relatively high enrichment in MREEs and HREEs (Zhang et al, 2000;Zhu et al, 2019b).…”