The first significant buildup in atmospheric oxygen, the Great Oxidation Event (GOE), began in the early Paleoproterozoic in association with global glaciations and continued until the end of the Lomagundi carbon isotope excursion ca. 2,060 Ma. The exact timing of and relationships among these events are debated because of poor age constraints and contradictory stratigraphic correlations. Here, we show that the first Paleoproterozoic global glaciation and the onset of the GOE occurred between ca. 2,460 and 2,426 Ma, ∼100 My earlier than previously estimated, based on an age of 2,426 ± 3 Ma for Ongeluk Formation magmatism from the Kaapvaal Craton of southern Africa. This age helps define a key paleomagnetic pole that positions the Kaapvaal Craton at equatorial latitudes of 11° ± 6° at this time. Furthermore, the rise of atmospheric oxygen was not monotonic, but was instead characterized by oscillations, which together with climatic instabilities may have continued over the next ∼200 My until ≤2,250-2,240 Ma. Ongeluk Formation volcanism at ca. 2,426 Ma was part of a large igneous province (LIP) and represents a waning stage in the emplacement of several temporally discrete LIPs across a large low-latitude continental landmass. These LIPs played critical, albeit complex, roles in the rise of oxygen and in both initiating and terminating global glaciations. This series of events invites comparison with the Neoproterozoic oxygen increase and Sturtian Snowball Earth glaciation, which accompanied emplacement of LIPs across supercontinent Rodinia, also positioned at low latitude.

https://www.pnas.org/content/pnas/114/8/1811.full.pdf

Timing and tempo of the Great Oxidation Event

U^Pb and Lu^Hf isotope analyses, obtained by laser ablation-sector field-inductively coupled plasma-mass spectrometry on zircon grains from 37 granitoid samples indicate that the Kalahari Craton consists of at least five distinct terranescBarberton South (BS), Barberton North (BN), Murchison^Northern Kaapvaal (MNK), Limpopo Central Zone (LCZ), and Francistowncwhich underwent different crustal evolutions, and were successively accreted at c. 3� 23 Ga, 2� 9 Ga and 2� 65^2� 7 Ga. The investigated granitoids were emplaced over a period of c. 1.5 billion years, and are exposed along a c. 1000 km long traverse from the Barberton Mountain Land/ Swaziland to the Francistown arc complex, Botswana. The presented datasets reveal that most granitoids of the BS (3� 45^3� 10 Ga), MNK (2� 93^2� 67 Ga), Francistown (2� 70^2� 65 Ga) and LCZ terranes (3� 2^2� 03 Ga) show near-chondritic to subchondritic eHft (BS ¼ ^1� 7t oþ 0� 5; MNK ¼ ^3� 4t oþ 0� 7; Francistown ¼ ^0� 5t oþ 1� 1; LCZ ¼ ^12� 4t o ^1 � 8), indicating that crustal recyclingcperhaps by mixing of an older crust with a depleted mantle reservoircplayed an important role during their formation and growth. Higher, superchondritic eHft values, as indicative for an important depleted mantle influence, were obtained only from some granitoids of the BN terrane (eHf3� 23Ga ¼þ 2� 5 � 0� 8), the Gaborone Granite Suite (eHf2� 80Ga ¼þ 2� 0 � 1� 6), and from a few detrital zircons from the Mahalapye complex of the Limpopo Belt. In addition, the datasets show that the internal Hf isotope variation of magmatic zircon domains from most granitoids is commonly less than � 1� 5 e-units, and only in rare cases up to � 3� 1 e-units. The rare significant eHft variations may be ascribed to incomplete mixing of different sources during magma crystallization. It is also shown that the combined approach of cathodoluminescence imaging with U^Pb and Lu^Hf isotope analysis provides a powerful tool to distinguish zircon domains formed and/or altered at different times.

/pdf/archean-accretion-and-crustal-evolution-of-the-kalahari-4sb5kcsksb.pdf

Archean Accretion and Crustal Evolution of the Kalahari Craton—the Zircon Age and Hf Isotope Record of Granitic Rocks from Barberton/Swaziland to the Francistown Arc

This article reports a study of chromitites from the LG-1 to the UG-2/3 from the Bushveld Complex. Chromite from massive chromitite follows two compositional trends on the basis of cation ratios: trend A—decreasing Mg/(Mg + Fe) with increasing Cr/(Cr + Al); trend B—decreasing Mg/(Mg + Fe) with decreasing Cr/(Cr + Al). The chromitites are divided into five stages on the basis of which trend they follow and the data of Eales et al. (Chemical Geology 88:261–278, 1990) on the behaviour of the Mg/Fe ratio of the pyroxene and whole rock Sr isotope composition of the environment in which they occur. Following Eales et al. (Chemical Geology 88:261–278, 1990), the different characteristics of the stages are attributed to the rate at which new magma entered the chamber and the effect of this on aAl2O3 and, in the case of stage 5, the appearance of cumulus plagioclase buffering the aAl2O3. The similarity of PGE profiles across the MG-3 and MG-4 chromitites that are separated laterally by up to 300 km and the variation in V in the UG-2 argue that the chromitites have largely developed in situ. Modelling using the programme MELTS shows that increase in pressure, mixing of primitive and fractionated magma, felsic contamination of primitive magma or addition of H2O do not promote crystallization of spinel before orthopyroxene (in general they hinder it) and that the Cr2O3 content of the magma was of the order of 0.25 wt.%. Less than 20% of the chromite in the magma is removed before orthopyroxene joins chromite, which implies a >13-km thickness of magma for the Critical Zone. It is suggested that the large excess of magma has escaped up marginal structures such as the Platreef. The PGE profile of chromitites depends on whether sulphide accumulated or not along with chromite. Modelling shows that contamination of Critical Zone magma with a felsic melt will induce sulphide immiscibility, although not chromite precipitation. The LG-1 to LG-4 chromitites developed without sulphide, whilst those from the LG-5 upwards had associated liquid sulphide. Much of the sulphide originally in the LG-5 and above has been destroyed as a result of reaction with the chromite.

The origin of chromitites and related PGE mineralization in the Bushveld Complex: new mineralogical and petrological constraints

https://people.earth.yale.edu/sites/default/files/files/Evans/35_09g-deKock%2BVaalbara.pdf

Validating the existence of Vaalbara in the Neoarchean

https://www.eas.ualberta.ca/konhauser/Reprints/GCA-GS(2010).pdf

Deciphering formation processes of banded iron formations from the Transvaal and the Hamersley successions by combined Si and Fe isotope analysis using UV femtosecond laser ablation

Erosional remnants of Paleoproterozoic red bed successions such as the Olifantshoek, Soutpansberg, Waterberg and Palapye Groups cover an extensive area of the Kaapvaal Craton. Depositional ages of these successions have been ill-defined and their lateral correlation a subject of controversial debate. We report precise zircon SHRIMP U-Pb ages of 2054 ± 4 and 2051 ± 8 Ma for quartz porphyry lavas stratigraphically near the base of the Waterberg Group from the Lower Swaershoek and correlative Rust de Winter Formations. The Entabeni Granite, erosively overlain by the Soutpansberg Group, yields a precise zircon SHRIMP age of 2021 ± 5 Ma. These ages indicate that at least part of the Waterberg Group is older than the Soutpansberg Group. In addition, the lower part of the Waterberg Group was deposited immediately after intrusion of the Bushveld Complex and may be tectonically related to the complex.

/pdf/precise-shrimp-u-pb-zircon-age-constraints-on-the-lower-15f5b39ay4.pdf

Precise SHRIMP U-Pb zircon age constraints on the lower Waterberg and Soutpansberg Groups, South Africa

Paleomagnetic data from more than 250 samples of the lower part of the Waterberg Group (Nylstroom Subgroup) are reported in order to reevaluate the apparent polar wander path (APWP) of the Kaapvaal craton during the Paleoproterozoic Era. Our study broadly confirms the established APWP, but reveals some previously unidentified complexities in the time interval ~2.05 Ga to ~1.87 Ga. A primary remanence direction from the lower part of the Swaershoek Formation provides a paleomagnetic pole of (36.5° north, 051.3° east, K = 23.4, A95 = 10.9) for the Waterberg unconformity-bounded sequence I (WUBS-I) at ~2.05 Ga. A large shift in pole position takes place from the lower Swaershoek into the upper Swaershoek Formation and overlying Alma Formation of Waterberg unconformity-bounded sequence II (WUBS-II; pole at −10.5° north, 330.4° east, K = 25.0, A95 = 9.8), confirming the existence of a major unconformity and/or rapid continental motion within the span of Swaershoek deposition.

Paleomagnetism of the lower two unconformity-bounded sequences of the Waterberg Group, South Africa: Towards a better-defined apparent polar wander path for the Paleoproterozoic Kaapvaal Craton.

The processes responsible for the transformation of banded iron formations to hard high-grade hematite ore, and their timing, remain poorly understood despite many recent advances. The paleomagnetic method allows for the estimation of ore genesis timing as a complement to other techniques. The effectiveness of the paleomagnetic method at dating, and testing proposed models for, the genesis of hard high-grade hematite ore deposits is illustrated by two South African examples. A new dataset is reported for the Thabazimbi deposit that independently constrains the age of ore formation between 2054 and 1930 Ma, while previously published data from the Sishen-Beeshoek deposits highlight the association of those deposits with weathering preceding the development of a marked Paleoproterozoic-aged unconformity (older than 2060 Ma). Paleomagnetic results are in both cases consistent with proposed models of ore genesis (i.e., extensive carbonate metasomatism and meteoric fluid interaction at Thabazimbi and ancient supergene processes at Sishen-Beeshoek). The antiquity of these South African examples appears to reflect a common theme among other hard high-grade hematite deposits from around the world, as revealed by a review and re-evaluation of existing paleomagnetic literature. This review represents a first attempt at providing a synopsis of hard high-grade hematite deposits within a temporal framework. The apparent Paleoproterozoic to Mesozoic age distribution of deposits as discussed in this review, which must be tested and verified by both the expansion of the database and improvement of current available data, has important implications for proposed models of ore genesis, as well as for exploration. † Corresponding author: e-mail, michiel.dekock@yale.edu *Current address: Paleoproterozoic Mineralization Research Group, University of Johannesburg, P.O. Box 524, Auckland Park 2006, South Africa. ©2008 Society of Economic Geologists SEG Reviews vol. 15, p. 49–71 deKock et al 6/11/08 7:41 AM Page 49

H. C. Dorland

Papers

Precise SHRIMP U-Pb zircon age constraints on the lower Waterberg and Soutpansberg Groups, South Africa

Paleomagnetism of the lower two unconformity-bounded sequences of the Waterberg Group, South Africa: Towards a better-defined apparent polar wander path for the Paleoproterozoic Kaapvaal Craton.

Origin and Timing of Banded Iron Formation-Hosted High-Grade Hard Hematite Deposits—A Paleomagnetic Approach