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The ca. 3.07 Ga volcanic rocks of the Dominion Group, South Africa, represent the oldest example of intracontinental, rift-related volcanism on the Archaean Kaapvaal craton. The volcanic assemblage comprises a >2 km-thick succession of mafic-intermediate lavas interlayered with felsic lavas and pyroclastic rocks. Textural and geological features indicate emplacement in a subaerial environment probably in an incipient intracontinental rift. We report SHRIMP U Pb zircon ages, elemental and Nd-isotope bulk-rock analyses of drill core samples and interpret their petrogenesis in the context of a Mesoarchaean continental setting. The U Pb zircon ages of four felsic samples from different stratigraphic levels yielded the same dates, resulting in a pooled 207Pb/206Pb age of 3074 ± 5 Ma. Primitive mantle-normalised incompatible trace element concentrations show enriched patterns with fractionated rare earth elements over high field-strength elements and negative anomalies of Nb and Ta relative to La. Initial ε Nd values for mafic and felsic rocks from −1.0 to −0.2 indicate melting of sources comprising time-integrated incompatible element-enriched mantle. The combined trace element and Sm Nd isotopic data suggest that the enrichment of incompatible elements and the low ε Nd values in the most primitive basalt samples (Mg# of 65–67) can be explained with contamination of asthenosphere-derived melts with crustal material or melting of an incompatible element-enriched upper mantle. The chemical compositions of the Dominion Group and Pongola Supergroup represent a significant petrogenetic departure from earlier Archaean (>3.6–3.1 Ga) magmatism as recorded in the Kaapvaal craton, which was dominated by komatiite-basalt volcanism and tonalite-trondhjemite-granodiorite intrusions. This change reflects the transition from a "greenstone belt type" tectonic setting to a failed intracontinental rift setting shortly after stabilisation of the Kaapvaal craton. Unlabelled Image • Onset of bimodal mafic-felsic Mesoarchaean intracontinental volcanism in the Kaapvaal craton • U Pb zircon ages define a short magmatic event at ca. 3074 Ma. • Transition from "granite-greenstone belt" regime to intracontinental magmatism • Changing tectonic regime reflected in magma characteristics • Comparison of the Dominion Group with the Pongola Supergroup [ABSTRACT FROM AUTHOR]

Abstract: We report SHRIMP zircon ages for felsic volcanic rocks of the early Archaean Nondweni greenstone belt (NGB) located in northern KwaZulu-Natal Province, South Africa. The NGB is part of a chain of greenstone belt remnants that occurs south of the well-known Barberton greenstone belt (BGB). Two samples of felsic volcanic rocks of the basal Toggekry Formation yielded zircon ages of ~3.53Ga, whereas significantly younger ages of ~3.41Ga were obtained for two samples of felsic rocks from near the top of the Witkop Formation. The latter date now constrains the age of stromatolite-like structures in cherts associated with the felsic rocks, which probably represent some of the oldest preserved evidence of life on Earth. The zircon ages also indicate that the NGB correlates well chronologically with the BGB and is one of the oldest volcano-sedimentary successions in the southeastern Kaapvaal Craton. Geochemical data for the felsic NGB rocks suggest formation by partial melting or crystal fractionation of a mafic source that was contaminated by older crust, possibly in a back-arc environment. Initial εNd values for NGB rocks range from −1.2 to +0.3 and suggest involvement of pre-greenstone continental crust that was possibly extensive in the southeastern part of the Kaapvaal Craton during Palaeoarchaean times. [Copyright &y& Elsevier]

The Nhlangano granite-gneiss in south-west Swaziland, dated at 2.98 Ga, forms an elliptical dome of remobilised, migmatitic crust. Strongly deformed supracrustal rocks wrap around or form synformal keels infolded with subdomes of Nhlangano Gneiss and represent Mesoarchaean Pongola Supergroup strata at amphibolite- to granulite-facies metamorphic grade. Amphibolites represent the metamorphosed equivalents of Nsuze Group basalts and andesites on the basis of similar trace element and Nd isotope data. Garnetiferous Mahamba Gneiss, dated at c. 2.96 Ga, represents the metamorphosed equivalents of felsic to intermediate volcanic rocks and minor pelitic interflow sedimentary units of the Nsuze Group. Quartzites and associated metamorphosed iron formations are part of the lower Mozaan Group (Sinceni Formation) as indicated by the presence of detrital zircons as young as 3.07 Ga and matching geochemical provenance characteristics. Ar-Ar analysis of hornblende separates dates amphibolite-facies metamorphism and associated dome emplacement at c. 2.72 Ga. Exhumation took place during NW-SE crustal extension and was facilitated by partial melting of the mid-crustal rocks. Exhumation took place below a southeastward-dipping detachment fault, giving rise to NW-SE oriented elliptical gneiss domes due to vertical and lateral flow of partially molten rocks. The Nhlangano gneiss dome has features in common with migmatite-cored metamorphic core complexes. Its emplacement points to tectonic instability of the eastern part of the Kaapvaal Craton in the Neoarchaean. [ABSTRACT FROM AUTHOR]

We report zircon ages, Hf-in-zircon isotopes as well as whole-rock geochemistry and Hf–Nd isotopic systematics for Palaeoarchaean grey gneisses of the Ancient Gneiss Complex of Swaziland, the oldest components of the Kaapvaal craton, southern Africa. The Hf-in-zircon isotopic compositions in these compositionally heterogeneous, multicomponent, migmatitic gneisses are highly variable, even in the oldest zircons dating back to 3.66 Ga, suggesting growth of zircon from melts representing a mix of juvenile and anatectic material derived from differentiated continental crust of Eoarchean to late Hadean age. In contrast, the initial Nd and Hf whole-rock isotopic compositions are frequently not in agreement with the Hf-in-zircon data that mostly show approximately chondritic initial values for Nd and strongly radiogenic initial values for Hf. We consider it likely that both the Lu–Hf and Sm–Nd whole-rock isotopic systems were disturbed and partly reset during later episodes of partial melting and crustal reworking, most likely during a pervasive 3.2 Ga tectono-metamorphic event. Primitive mantle-normalized trace element patterns show the variable influence of residual plagioclase and garnet in the sources as well as high contents of strongly incompatible elements. In conjunction with the Hf-in-zircon isotopic data the trace element contents are best explained by the incorporation of older continental crustal material into the sources of the grey gneisses. Our data support evidence from other Palaeoarchean terranes that crustal recycling, as seen in even the oldest crustal components, played an important role in early continental evolution. Rocks previously classified as a subduction-related tonalite–trondhjemite–granodiorite suite are complex, and their chemistry alone cannot be used to reconstruct tectonic settings. [ABSTRACT FROM AUTHOR]

The Singhbhum Craton of East India has a remarkably well-preserved Archaean geological record that rivals those of the Kaapvaal and Pilbara cratons. Based on a critical literature review, and aided by new field and geochronological data, a consistent framework of craton evolution during the Archaean is proposed that aims to put to rest some of the uncertainties on its geological history. The oldest rock assemblage so far identified in the Singhbhum Craton consists of ~3.5 to 3.3 Ga old greenstones of the Badampahar Group that are in tectonic contact with, or intruded by, broadly contemporaneous tonalite-trondhjemite-granodiorite of the Champua Suite, representing the precursor of grey gneisses of the Singhbhum Granitoid Complex. A reported Eoarchaean to Hadean detrital and xenocrystic zircon record suggests an even older onset of craton evolution, the details of which remain to be deciphered. Following >200 Ma of submarine volcanism and sedimentation, concomitant with granitoid emplacement of the Singhbhum Suite in the evolving craton, a regional event of deformation and metamorphism took place at ~3.3 Ga ago. It gave rise to syn -tectonic deposition of coarse siliciclastic successions of the Dhosrapahar Group, tight to isoclinal upright folding of the supracrustal rocks, and a greenschist to amphibolite facies metamorphic overprint. Starting at around 3.35 Ga, but overlapping with this regional event, was a broad change in the composition of felsic plutons towards more K-rich compositions, which culminated in the emplacement of granites of the Mayurbhanj Suite at 3.1 Ga that led to stabilization of the craton. Following tectonic uplift, erosion, and peneplanation, the intracontinental to continental margin volcano-sedimentary successions of the Koira Group and the correlative Simlipal Group were deposited on granitoid-greenstone basement between 3.0 and 2.75 Ga. Subsidence may have been related to extension of the craton that gave rise to the extrusion of largely subaerial basalts that may have been fed by a prominent mafic dyke swarm at ~2.80 Ga. At the same time, deformation and high-grade metamorphism affected the southwestern part of the craton. This event gave rise to widespread emplacement of granites that heralded the onset of tectonic reworking of the craton along its margins, which continued well into the Proterozoic. [ABSTRACT FROM AUTHOR]

Abstract: High-potassium felsic volcanic rocks interlayered with basalt and komatiite in early Archaean greenstone sequences in the Barberton Greenstone Belt of South Africa and Swaziland, previously considered to be derived from melting of mafic precursors, are shown to be the result of melting of significantly older felsic crust. This is documented by a combination of SHRIMP zircon dating with Hf-in-zircon and whole-rock Lu–Hf and Sm–Nd isotopic data. Zircons from felsic rocks of the oldest Barberton unit, the 3.53 Ga Theespruit Formation, yielded predominantly negative -values, whereas whole-rock - and -data are slightly negative to slightly positive. Similar results were obtained for ca. 3.45 Ga felsic rocks in the Hoeggenoog and Noisy Formations higher up in the greenstone stratigraphy. These data rule out derivation of the felsic units from melting of basaltic precursors and favor a crustal source, most likely of TTG composition. The isotopic data are not compatible with an entirely oceanic origin of the Barberton greenstone sequences and favor a pre-greenstone basement beneath the volcanic rocks. Crustal melting of Eo- to Paleoarchaean lower crust probably generated the felsic volcanic rocks and is likely to have been responsible for gradual stabilization of the Kaapvaal craton. [Copyright &y& Elsevier]

The Central Zone of Limpopo Belt is a polymetamorphic and complexly deformed Precambrian terrane whose tectonic evolution is still uncertain because details on the timing of its evolution are not well established. We report zircon ages for newly discovered rocks in the Central Zone which document three distinct high-grade tectono-metamorphic events at 3.22, 2.62 and 2.02 Ga. The youngest of these has been well established, but the older two events were so far largely based only on rare zircon overgrowth rims. Most of our samples were collected from a large enclave in the 2.61 Ga Bulai pluton and constitute a strongly deformed, brecciated high-grade assemblage of supracrustal rocks, meta-anorthosite and granitoid gneisses, some of which are most likely equivalents of the pre-3.33 Ga Beit Bridge Complex. Magmatic zircon ages of 3.34–3.33 Ga for anorthositic and tonalitic gneisses in the enclave breccia support this correlation. Ca. 3.22 Ga metamorphic zircons, possibly reflecting the D 1 /M 1 event, were only found in one felsic granulite sample, and their regional significance remains uncertain. Abundant metamorphic zircons grew at ca. 2.62 Ga under granulite-facies conditions (D 2 /M 2 ), and some of these experienced variable degrees of recrystallization and/or Pb-loss during the third granulite-facies event at ca. 2.0 Ga (D 3 /M 3 ). Rocks outside the Bulai enclave contain abundant newly-formed ca. 2.02 Ga metamorphic zircons but only rarely preserve evidence for the ca. 2.62 Ga event since these late Archaean zircons show strong recrystallization and/or Pb-loss at ca. 2.0 Ga. We suggest that the Bulai pluton “protected” the rocks inside the enclave from severe overprinting at ca. 2.0 Ga because these rocks remained dry, whereas the assemblages outside the Bulai pluton experienced fluid-assisted metamorphic overprinting at ca. 2.0 Ga. Our new data sugggest that the tectonic evolution of the Central Zone was more complex than previously thought. We favour a scenario where collision involving the Limpopo Belt occurred in the Neoarchaean, whereas the high-grade Palaeoproterozoic event was most likely intracontinental and records far-field effects of plate movements elsewhere. The tectonic setting controlling the early Archaean D 1 /M 1 event remains unknown, largely because of insufficient data and extensive overprinting during later times. [ABSTRACT FROM AUTHOR]

Mafic granulite xenoliths are hosted by garnetiferous charnockites in the Archaean to Palaeoproterozoic Bumbuger Complex of northwestern Mongolia, one of the exotic basement terranes in the Central Asian Orogenic Belt. These rocks crystallized at ca. 1850 Ma under granulite-facies conditions (800 ± 27 °C, 6.8 ± 0.6 kbar) in the lower crust and were partly retrogressed to amphibolite-facies during ascent to higher crustal levels as a result of strong deformation resulting in northwest-trending isoclinal folds. The mafic xenoliths are likely derived from gabbroic protoliths, and geochemical, Hf-in-zircon and Nd whole-rock isotopic data suggest these rocks to have originated from parental melts of an enriched mantle source or from a depleted mantle with minor contamination by crustal material. The host garnetiferous charnockites crystallized from a strongly inhomogeneous melt predominantly derived from an Archaean to early Palaeoproterozoic lower crustal source, as also evidenced by partly recrystallized xenocrystic zircons, and minor contributions of mantle-derived material. The above data are compatible with a model whereby mantle-derived gabbroic melts under- and intraplate the lower crust, causing high-grade metamorphism, migmatization and anatexis, and the resulting ascending melts are mixtures of mantle and crustal sources. We speculate that the late Palaeoproterozoic events in the Baidarik Block may be the result of ca. 1.90–1.85 Ma accretion and collision processes that led to formation of the Columbia supercontinent. In the Neoproterozoic this block drifted toward Siberia and was incorporated into the Central Asian Orogenic Belt. [ABSTRACT FROM AUTHOR]

• New geochronological data of the Mesoarchaean Dominion Group. • ~ 2.96 Ga old Dominion-Nsuze Igneous Province. • Oldest large igneous province of the Kaapvaal Craton. • ~2.96 Ga as start of the deposition of the Witwatersrand Supergroup. The Kaapvaal Craton records a long Archaean to Phanerozoic history of intracratonic magmatism that reflects contrasting regimes of melt generation. The correct interpretation of this rich geological record relies critically on field evidence and ever refined chronological and isotopic data. Here, we present new U – Pb zircon, apatite and titanite ages, and Lu-Hf isotope results for volcanic and sedimentary rocks of the Dominion Group, its granitoid basement, and overlying strata of the Witwatersrand Supergroup. The data indicate an emplacement age of ca. 2.96 Ga for the Dominion Group, approximately 0.11 Ga later than previous estimates. These new ages, combined with lithological and geochemical similarities, allow the comparison and correlation between the Dominion Group and the Nsuze Group of the Pongola Supergroup. Together, the two successions constitute a single igneous province, here termed the Dominion-Nsuze Igneous Province, which was emplaced shortly after craton consolidation, spanning approximately 500 km across the central and eastern Kaapvaal Craton. This province constitutes the first intracratonic igneous province of the Kaapvaal Craton and one of the oldest in the world. The new age determinations also place limits on the onset of sedimentation of the overlying Witwatersrand Supergroup, which can now be constrained at < 2960 Ma. [ABSTRACT FROM AUTHOR]