THE PALEOARCHEAN: A PIVOTAL TIME IN EARTH CRUSTAL EVOLUTION —THE VIEW FROM THE ZIMBABWE AND SÃO FRANCISCO CRATONS

Manuela Botero

doi:10.7273/000008419

The evolving Earth has been shaped by major geological processes that defined pivotal times in Earth’s history. Two of these periods in Earth’s history include the Paleoarchean, characterized by a significant increase in the production and preservation of continental crust and the likely transition from stagnant lid to mobile lid geodynamics, and the Neoarchean, marked by globally recorded age peaks and a probable shift towards modern-style plate tectonics (i.e., modern style subduction-zones). The geological record of these pivotal times in Earth’s history is preserved in rocks exposed within Precambrian terranes worldwide. The rock record of these terranes, however, is complex and often reflects multiple tectono-thermal events resulting in a mix of different ages and isotopic compositions, further complicated by disturbances that overprint original isotope compositions at both the mineral and bulk-rock scales. This complexity is evident in the contrasting Eo-Paleoarchean Hf and Nd isotope records obtained from bulk-rock samples and zircon, painting two different views of the early Earth. The highly heterogeneous Hf and Nd bulk-rock isotope records, with extreme positive and negative isotope compositions, have been used to support significant mantle differentiation with substantial crustal production in the first ~500 Ma of Earth’s evolution. In contrast, the homogeneous zircon Hf isotope record, showing chondritic to sub-chondritic isotope compositions, has been interpreted as evidence for limited mantle differentiation during the early Earth. These contrasting interpretations align with competing crustal growth models--one proposing progressive crustal growth with significant crustal production beginning after ~3.8 Ga, and the other suggesting that near-present-day crustal volumes formed early in the Hadean (often called the no-growth model). This doctoral research focuses on the northern São Francisco (NSFC) and Zimbabwe (ZC) cratons, two important Archean terranes that, despite their extensive Archean geological record, remain largely uncharacterized through high precision Hf and Nd isotope data. The São Francisco and Zimbabwe cratons are two of the best exposed ancient terranes with well-preserved archetypal Archean structures such as dome and keels. With protracted magmatic histories spanning from ~3.6 to ~2.0 Ga in the NSFC and between ~3.6 and ~2.6 Ga in the ZC, these ancient terranes preserve critical information on the evolution of the crust-mantle system during pivotal times of Earth’s history. An integrated geochronological (zircon and titanate U-Pb and REE-rich minerals Sm-Nd geochronology) and isotopic (bulk-rock and in-situ Lu-Hf and Sm-Nd) approach is used here to investigate the formation and evolution of Archean crust in the northern São Francisco and Zimbabwe cratons. The zircon U-Pb data from both cratons, as well as Sm-Nd geochronological data from the NSFC, define a main crust forming episode in the Paleoarchean at ~3.6-3.5 Ga followed by less voluminous magmatism accompanied by anatexis and high-grade metamorphism between 3.4 and 3.3 Ga. In the ZC, a craton-wide magmatic episode is also recorded in the Meso-Neoarchean between ~2.9 and 2.6 Ga through zircon U-Pb geochronology--likely associated with a transition in the geodynamics operating at the craton scale. Thermal perturbation--post-crystallization--events are additionally recorded in the NSFC by various REE-rich phases with Sm-Nd isochron ages at 2.6-2.4 Ga and 2.2-2.0 Ga and titanite U-Pb ages between 2.2-2.0 Ga. The Hf--and Nd for the NSFC--isotope compositions of Paleoarchean (3.6-3.4 Ga) components from both cratons are between ƐHf(i) +2 and –5. In the ZC, the Meso-Neoarchean (3.0-2.8 Ga) components have initial Hf isotope compositions ranging from ƐHf(i) +2 to –1. The Paleoarchean Hf and Nd isotope records of the NSFC and ZC indicate that crustal evolution likely began at ~3.8 Ga, with a broadly chondritic mantle reservoir persisting through the Eoarchean below these cratons. The absence of evidence for either highly evolved or strongly depleted reservoirs prior ~3.6-3.7 Ga in the NSFC and ZC, agrees with the global isotope record and aligns with models of progressive crustal growth following the development of a long-lived depleted mantle after ~3.6 Ga. Together, the data from this research fill key gaps in the global Archean isotope record and offer significant insights into early crust-mantle evolution and the geodynamic regimes that shaped the primitive Earth.

THE PALEOARCHEAN: A PIVOTAL TIME IN EARTH CRUSTAL EVOLUTION —THE VIEW FROM THE ZIMBABWE AND SÃO FRANCISCO CRATONS

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