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The short-lived 146 Sm-142 Nd isotope system is an important tool for tracing Hadean crustmantle differentiation processes and constraining their imprint on much younger rocks from Archean cratons. We report the first comprehensive set of high-precision 142 Nd analyses for granitoids and amphibolites of the Ancient Gneiss Complex (AGC; Swaziland) and the oldest metavolcanic units of the Barberton Greenstone Belt (BGB; South Africa). The investigated samples span an age range from 3.66 Ga to 3.22 Ga and are representative of major geological units of the AGC and the lower Onverwacht Group of the BGB. Measured samples yielded μ 142 Nd values in the range from -8 ppm to +3 ppm relative to the JNdi-1 terrestrial standard, with typical errors smaller than 4.4 ppm. The distribution of the μ 142 Nd values for these 17 measured samples is bimodal with ten samples showing a tendency towards slightly negative μ 142 Nd anomalies, whereas seven samples have 142 Nd similar to the terrestrial reference. The only confidently resolvable μ 142 Nd anomalies were found in a 3.44 Ga Ngwane Gneiss sample and in amphibolites of the ca. 3.45 Ga Dwalile Greenstone Remnant, revealing μ 142 Nd values ranging from -7.9 ± 4.4 to -6.1 ± 4.3 ppm. The μ 142 Nd deficits do not correlate with age, lithological unit, or sample locality. Instead, our results reveal that two distinct mantle domains were involved in the formation of the AGC crust. The two reservoirs can be distinguished by their µ 142 Nd signatures. Mantle-derived rocks tapped the enriched reservoir with negative μ 142 Nd at least until 3.46 Ga, whereas the granitoids preserved a negative μ 142 Nd signature that formed by incorporation of older AGC crust at least until 3.22 Ga. The oldest gneisses with no µ 142 Nd anomaly are up to 3.64 Ga in age, indicating that a modern terrestrial 142 Nd reservoir was already present by early Archean times. equivalents, as well as metasedimentary units. Often, only fragments of the original greenstone associations of various sizes are interlayered with the grey gneisses.Currently it is under debate in which tectonic environment the early continental crust on Earth formed and stabilized (e.g., van Kranendonk, et al., 2014; Hoffmann et al. 2016 and references therein). Models invoke an initial phase of TTG formation, followed by reworking caused by collision between crustal terranes and involvement of mantle-derived magmas by intrusion and underplating. This led to formation of various types of granitoids that were subsequently strongly deformed and migmatized during post-magmatic metamorphic events. The most prominent relicts of early crustal terranes are the Acasta Gneiss Complex, the Nuvvuagittuq region, the Hudson Bay terrane, and the

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The Pleistocene (2.2–1.5 Ma) Koloula Igneous Complex (KIC) on Guadalcanal in the Solomon island arc consists of a low-K calc-alkaline sequence of ultramafic to felsic plutonic rocks. We present whole-rock major and trace element and Sr–Nd-Pb isotope data, as well as mineral compositions that record the magmatic evolution of the complex. The intrusive sequence is grouped into two cycles, Cycle 1 and 2, comprising gabbroic or dioritic to granodioritic rocks. The major and trace element data of each cycle forms a single calc-alkaline fractional crystallisation trend. The distinct radiogenic isotope and incompatible element compositions of the Cycle 1 and 2 intrusions imply slightly different mantle sources. The KIC formed by shallow (0.1 GPa) fractional crystallisation of mantle-derived Al-rich basaltic parental magmas (6–8 wt.% MgO) that were formed by deeper-level (0.7 GPa) fractionation of olivine and pyroxene from Mg-rich (~ 11 wt.% MgO) primary magmas in the Solomon intra-oceanic island arc. Olivine, clinopyroxene, plagioclase, amphibole, biotite, apatite, and Fe–Ti oxides fractionated from the KIC’s high-Al basaltic parental magmas to form calc-alkaline magmas. Liquid line of descent trends calculated using mass balance calculations closely match major element trends observed in the KIC data. The KIC crystallised at shallow, upper crustal depths of ~ 2.0–3.0 km in ~ 20 km-thick island arc crust. This complex is typical of other Cenozoic calc-alkaline ultramafic to felsic plutons in Pacific intra-oceanic island arcs in terms of field relationships, petrology, mineral chemistry and whole-rock geochemistry. Hornblende fractionation played a significant role in the formation of the calc-alkaline felsic plutonic rocks in these Cenozoic arc plutons, causing an enrichment of SiO2 and light rare earth elements. These plutons represent the fossil magma systems of arc volcanoes; thus, the upper arc crust is probably generated by migration of magmatic centres.

Pb compositions requiring the presence of ancient components in the central European lower crust that are not sampled on the surface. Using energy-constrained assimilation-fractional crystallisation (EC-AFC) model calculations, differentiation of the SVF lithologies can be modelled by approximately 39-47 % fractional crystallisation and 6-15 % crustal assimilation. Notably, the transition from silica-undersaturated to silica-saturated compositions of many felsic lavas in the SVF that is difficult to account for in closed-system models is also well explained by such amounts of crustal assimilation.

Mesoarchean magmatism is widespread on the eastern margin of the Kaapvaal Craton, but its origin is still poorly understood, mainly because geochemical data is rare. To shed some light on the source of this Mesoarchean magmatism and to relate different Mesoarchean volcanic sequences to each other, we provide major and trace element data as well as Hf-Nd isotope compositions of amphibolites sampled close to the Kubuta Ranch in south-central Eswatini. These amphibolites, so far, were of unknown correlation to any volcanic sequence in Eswatini or South Africa. Hence, the aim of our study is to characterize the mantle source composition of these volcanic rocks and, furthermore, to constrain their genetic relation to other volcanic sequences in Eswatini and South Africa. Our findings reveal that, based on coherent trace element patterns and similar Nd isotope characteristics, the Kubuta volcanic rocks can be genetically linked to the ca. 3.0 Ga Usushwana Igneous Complex in West-Central Eswatini and the ca. 2.9 Ga Hlagothi Complex located in the KwaZulu-Natal province. In contrast, the coeval ca. 3.0 Ga Nsuze and ca. 2.9 Ga Mozaan Groups (Pongola Supergroup) of south-central Eswatini and northern KwaZulu-Natal province have slightly enriched compositions compared to the newly sampled Kubuta volcanic rocks. Our results suggest that the Nsuze and Mozaan Groups were sourced from a primitive mantle reservoir, whereas the Usushwana, Hlagothi, and Kubuta mafic rocks were derived by melting of a more depleted mantle source comparable to that of modern depleted MORB. Furthermore, our assimilation-fractional crystallization (AFC) calculations and Nd isotope constraints reveal that some samples were contaminated by the crust and that the crustal contaminants possibly represent felsic rocks related to the ca. 3.5 Ga crust-forming event in the Ancient Gneiss Complex. Alternatively, melting of a metasomatized mantle or plume-lithospheric mantle interaction may also produce the trace element and isotopic compositions observed in the samples. From a synthesis of our geochemical observations and age data from the literature, we propose a refined petrogenetic model, for a continental flood basalt setting in a Mesoarchean large igneous province on the eastern Kaapvaal Craton. Our petrogenetic model envisages two magma pulses sourced from a primitive mantle reservoir that led to the formation of the Nsuze (first) and Mozaan (second) lavas. Conductive heating of ambient depleted mantle by the mantle plumes caused partial melting that led to the formation of the Usushwana Igneous Complex associated with the first magmatic event (Nsuze) and the Hlagothi Igneous Complex associated with the second magmatic event (Mozaan). However, due to lacking age data of sufficient resolution, it is not possible to affiliate the Kubuta lavas to either the first or the second magmatic event.