Search

This study presents multiple sulphur isotope (32S, 33S, 34S, 36S) data on pyrites from silicified volcano‐sedimentary rocks of the Paleoarchean Onverwacht Group of the Barberton greenstone belt, South Africa. These rocks include seafloor cherts and felsic conglomerates that were deposited in shallow marine environments preserving a record of atmospheric and biogeochemical conditions on the early Earth. A strong variation in mass independent sulphur isotope fractionation (MIF‐S) anomalies is found in the cherts, with Δ33S ranging between −0.26‰ and 3.42‰. We explore possible depositional and preservational factors that could explain some of this variation seen in MIF‐S. Evidence for microbial activity is recorded by the c. 3.45 Ga Hooggenoeg Formation Chert (HC4) preserving a contribution of microbial sulphate reduction (−Δ33S and –δ34S), and a c. 3.33 Ga Kromberg Formation Chert (KC5) recording a possible contribution of microbial elemental sulphur disproportionation (+Δ33S and –δ34S). Pyrites from a rhyo‐dacitic conglomerate of the Noisy Formation do not plot along a previously proposed global Felsic Volcanic Array, and this excludes short‐lived pulses of intense felsic volcanic gas emissions as the dominant control on Archean MIF‐S. Rather, we suggest that the MIF‐S signals measured reflect dilution during marine deposition, early diagenetic modification, and mixing with volcanic/hydrothermal S sources. Given the expanded stratigraphic interval (3.47–3.22 Ga) now sampled from across the Barberton Supergroup, we conclude that large MIF‐S exceeding >4‰ is atypical of Paleoarchean near‐surface environments on the Kaapvaal Craton. [ABSTRACT FROM AUTHOR]

On the basis of new bulk major and trace element (including REE) as well as Sm-Nd and Rb-Sr isotope data, used in conjunction with available geochronological data, a post-tectonic mafic igneous province and four groups of pre- to syntectonic amphibolite are distinguished in the polymetamorphic Maud Belt of western Dronning Maud Land, East Antarctica. Protoliths of the Group 1 amphibolites are interpreted as volcanic arc mafic intrusions with Archaean to Palaeoproterozoic Nd model ages and depletion in Nb and Ta. Isotopic and lithogeochemical characteristics of this earliest group of amphibolite indicate that the Maud Belt was once an active continental volcanic arc. The most likely position of this arc, for which a late Mesoproterozoic age (c. 1140 Ma) is indicated by available U-Pb single-zircon age data, was on the southeastern margin of the Kaapvaal-Grunehogna Craton. The protoliths of Group 2 amphibolites are attributed to the 1110 Ma Borgmassivet-Umkondo thermal event on the basis of comparable Nd model ages and trace element distributions. Group 3 amphibolite protoliths are characterized by mid-ocean ridge basalttype REE patterns and low Th/Yb ratios, and they are related to Neoproterozoic extension. Group 4 amphibolite protoliths are distinguished by high Dy/Yb ratios and are attributed to a phase of syntectonic Pan-African magmatism as indicated by Rb-Sr isotope data.

Recognition of oceanic crust in Archean greenstone belts has remained a controversial and unresolved issue, with implications for understanding early Earth geodynamics. In the search for early Archean oceanic crust, we present a multi-pronged approach to test for the presence of an ophiolite-type sequence preserved in the Paleoarchean Barberton greenstone belt (BGB) of South Africa. New field observations are combined with detrital U-Pb zircon geochronology and geochemistry on fresh drill-core material from the Kromberg type-section sequence of mafic-ultramafic rocks in the 3.56-3.33 Ga Onverwacht Group of the BGB. Trace element geochemistry indicates that the Kromberg metabasalts were derived from the primitive mantle. The εNd values and Nd model ages of the metabasalts record a depleted Archean mantle source similar to CHUR (chondritic uniform reservoir) with no continental (tonalite-trondhjemitegranodiorite [TTG]) crustal contamination. U-Pb geochronology by laser ablation-inductively coupled plasma-mass spectrometry on detrital zircons from an uppermost chert unit indicate a homogeneous age distribution and a gabbroic source in the greenstone belt, in direct contrast to zircons from felsic conglomerates structurally underlying the Kromberg sequence. Collectively, the new data and field observations indicate that the 3.33 Ga Kromberg mafic-ultramafic sequence formed in a juvenile oceanic setting and represents a remnant of tectonically accreted oceanic crust. The identification of oceanic crust preserved as dismembered allochthons in the BGB suggests that horizontal plate tectonic processes were operating on the Archean Earth as early as Mesoarchean times. [ABSTRACT FROM AUTHOR]