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Geng H.,University of Hong Kong | Brandl G.,Limpopo Unit | Sun M.,University of Hong Kong | Wong J.,University of Hong Kong | And 2 more authors.
Precambrian Research | Year: 2014

The precise age of the volcano-sedimentary Soutpansberg Group, which was deposited upon the Palala shear belt separating the Kaapvaal Craton from the Central Zone of the Limpopo Belt, has long been debated. The Soutpansberg Group is subdivided into a lower and upper succession, which are separated from each other by a prominent regional unconformity. Zircon grains from silicic pyroclastic rocks of both successions were investigated in order to constrain the timing of deposition of the Soutpansberg Group rocks. The zircon grains of the investigated samples from both successions yield a wide range of ages, spanning from 1831 to 3937. Ma. Most of the zircon grains have rounded shapes, however it is not clear whether they are mainly xenocrystic or detrital, or have been rounded by resorption in a silicic magma chamber. The youngest zircon grain ages obtained come from the lower succession and are 1832 ± 9 and 1831 ± 15. Ma. We interpret these youngest zircon grains as magmatic grains that have been rounded by resorption. This view is corroborated by the fact that no magmatic rocks of this particular age have been observed in the Kaapvaal Craton or the Central Zone of the Limpopo Belt, and that no apparent sedimentary admixtures are present in the well exposed pyroclastic rocks. We therefore conclude that deposition of the Soutpansberg volcano-sedimentary succession commenced around 1830. Ma. The Soutpansberg rocks were deposited apparently over a lengthy period of time (ca. 230. Ma), as provided by the published age of 1604. Ma for pyroclastic rocks of the upper succession in Botswana. Zircon grain age spectra of our Soutpansberg samples show prominent peaks at 2.0, 2.6 and 3.2. Ga, indicating the Central Zone of the Limpopo Belt as the source area, but excludes the adjacent northern part of the Kaapvaal Craton. The oldest zircon grain identified in the Soutpansberg samples has an age of 3937 ± 4. Ma, one of the oldest zircon grain ages yet reported from the African continent. © 2014 Elsevier B.V.

Mouri H.,University of Johannesburg | Maier W.D.,University of Oulu | Brandl G.,Limpopo Unit
South African Journal of Geology | Year: 2013

Komatiites occur in many Archaean and Proterozoic greenstone belts metamorphosed to greenschist or upper amphibolite facies conditions. However, komatiites have so far not been reported from high-grade metamorphic environments (upper granulite facies conditions). Here we report the occurrence of Mg-, Cr- and Ni-rich ultramafic rocks (26 to 31% MgO, 2800 to 3800 ppm Cr, 1400 to 1800 ppm Ni) with unfractionated PGE patterns (8.15 to 12.36 ppb Pt, 6.17 to 13.49 ppb Pd, Pd/Ir-2 to 6) in the high-grade polymetamorphic Central Zone of the Limpopo Belt, South Africa. The composition of the rocks overlaps with that of Al-undepleted (Munro-type) komatiites from elsewhere, except that the Central Zone samples are markedly enriched in highly incompatible trace elements and show negative Nb anomalies. Based on these data, we interpret the rocks to represent contaminated komatiites of Archaean age - the first such manifestation in the Limpopo Belt and in a high to ultra-high grade (≥900°C, 10 kbar) metamorphic environment. © 2013 June Geological Society of South Africa.

Clemens J.D.,Stellenbosch University | Belcher R.W.,Stellenbosch University | Belcher R.W.,Limpopo Unit | Kisters A.F.M.,Stellenbosch University
Journal of Petrology | Year: 2010

The Heerenveen batholith exemplifies an aerially extensive group of ~3·1 Ga felsic potassic batholiths that mark the final magmatic event associated with the cratonization of the central-eastern Kaapvaal Craton, and the transition from the earlier sodic tonalite-trondhjemite-granodiorite (TTG) to the more recent style of monzogranitic and granitic magmatism. As is typical of these bodies, it is geochemically and mineralogically heterogeneous, sheeted in structure, and was assembled incrementally, with eight main units in four assembly stages. Apart from a metaluminous quartz monzonite, all the Heerenveen rocks are peraluminous.There are few identifiable geochemical lineages among the rocks of the batholith.The geochemical heterogeneity is largely due to protolith internal heterogeneity, tapping of multiple sources, pulsed magma extraction related to sequential occurrence of discrete melting reactions, and varying degrees of entrainment of peritectic minerals formed during partial melting of the crustal protoliths, together with varying structural controls on magma extraction and ascent, which allowed distinct magma sources to be tapped, at different periods during batholith assembly. Fractional crystallization, magma mixing and wall-rock assimilation were not significant processes. TTG rocks are not suitable source materials for any of the Heerenveen magmas; nor are highly aluminous metasediments. For magmatic rocks with SiO2 <70 wt %, the likely source rocks were intermediate to mafic igneous rocks, with variable degrees of K-enrichment. For those rocks with SiO2>72 wt %, the sources were K-enriched intermediate to felsic rocks. It seems likely that the partial melting reactions that produced the Heerenveen magmas occurred at moderate temperatures and that hornblende was present in the residues. This suggests that initial melting may have occurred in the presence of an H2O-rich fluid. There is no evidence that the genesis of the potassic granitic magmas involved either recycling of TTG crust or the presence of aluminous metasediments in the deep crust. The K-enrichment in the protoliths was most probably due to their ultimate derivation from mantle rocks metasomatized by fluids evolved in subduction-like processes. The monzonite in the batholith may have been derived directly by partial melting of this metasomatized mantle. © The Author 2010.

Wu F.-Y.,CAS Institute of Geology and Geophysics | Yang Y.-H.,CAS Institute of Geology and Geophysics | Li Q.-L.,CAS Institute of Geology and Geophysics | Mitchell R.H.,Lakehead University | And 3 more authors.
Lithos | Year: 2011

The Phalaborwa carbonatite Complex, situated in the northeastern part of South Africa, is characterized by copper and zirconium mineralization, and is composed principally of pyroxenites, phoscorite and carbonatite (banded and transgressive). The complex is transected by mafic dykes, and is geographically associated with a satellite syenite and minor granite intrusions. Zircon and baddeleyite U. -Pb isotopic age determinations using CAMECA 1280 secondary ion mass spectrometry have shown that the outer pegmatitic pyroxenite at the Loolekop pipe was emplaced at 2060 ± 4 Ma, and the main phoscorite at 2062 ± 2 Ma. Both ages are identical to those of 2060 ± 2 and 2060 ± 1. Ma for the banded and transgressive carbonatites, respectively. The satellite syenite, which forms plug-like bodies outside of the border of the main complex, and the later mafic dyke have "similar" emplacement ages of 2068 ± 17 and 2062 ± 53 Ma, indicating that these intrusions were apparently near-synchronously emplaced. In contrast to other carbonatites, the Phalaborwa Complex is characterized by high initial Sr and low initial Nd and Hf isotopic compositions. In situ isotopic analyses of apatite, calcite, zircon and baddeleyite indicate that the primary magma was derived from an enriched mantle. As the complex was emplaced slightly earlier at ~ 2060 Ma than the nearby mafic phase of the Bushveld Complex (~ 2055 Ma), it is proposed that the Phalaborwa carbonatite magmatism was triggered by the same mantle plume activity, which partially melted the overlying lithospheric mantle. This contribution also highlights that isotopic studies used to constrain the genesis of ancient igneous complexes should concentrate on minerals with low parent/daughter elemental ratios, such as apatite and calcite for Sr isotopes, and zircon and baddeleyite for Hf isotopes. © 2011 Elsevier B.V.

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