Research on Craton Evolution and Stabilization

The South American and African continents contain ancient cratons, formed between 3.5 and 2.5 billion years ago, shielded from mantle convection by thick lithospheric roots. These cratons, composed of Archaean crustal segments, underwent tectonic accretion and partial melting in the Palaeo- and Mesoproterozoic, but the precise timing and processes that converted these early shields into solid cratons remain unclear.

Main Research Focus:

1. Determining magmatic and metamorphic ages of Archaean granitoids, gneisses, and volcanic rocks.
2. Geochemical modeling of Archaean rocks.
3. Dating detrital zircons from greenstone belt sequences.
4. Mapping the metamorphic evolution of greenstone belts through P-T-t modeling.
5. Linking magmatic and metamorphic events with craton stabilization.

Craton Stabilization and Potassic Magmatism:

• Key processes:
  • Crustal thickening and anatexis: During tectonic collisions, partial melting in the deep crust generates high-potassium (K) magmas, which rise to form granitoids.
  • Decompressional melting: After thickening, extensional tectonics reduce pressure, producing more potassic magmas.
• Tectonic implications:
  • Heat reduction: The withdrawal of K-rich magmas removes heat-producing elements, cooling and stabilizing the crust.
  • Rheological strengthening: The emplacement of felsic magma strengthens the crust, reducing susceptibility to future deformation.
  • Formation of stable continental blocks: These stable blocks form the core of cratons, preserving sedimentary basins and thick sedimentary covers.

What is a craton?

A craton is a large, stable block of the Earth’s crust that forms the foundation of continents and has survived the cycles of continental drift, rifting, and tectonic activity over billions of years. Cratons are some of the oldest and most stable geological formations on Earth, with portions of them often dating back to the Archean Eon (more than 2.5 billion years ago).

Key Characteristics of Cratons:

1. Stability:
   • Cratons are tectonically stable regions, meaning they have not been significantly affected by orogenic (mountain-building) processes or major tectonic activity for hundreds of millions to billions of years.
   • They usually show very little seismic activity and are largely unaffected by volcanic activity.
2. Age:
   • Cratons are ancient, typically containing rocks that are Archean (older than 2.5 billion years) and Proterozoic (up to about 541 million years old) in age.
   • These ancient rocks include granitoids, greenstone belts, and highly metamorphosed rocks.
3. Composition:
   • Cratons are made up of continental lithosphere, which is composed of a thick layer of low-density felsic (silica-rich) rock like granite in the crust, and a deeper lithospheric mantle that is cooler and more rigid compared to the surrounding mantle.
   • The crust in cratons can reach thicknesses of 35-40 km or more, while the underlying lithospheric mantle can extend down to 200-300 km depth.
4. Geological Layers:
   • Shield: The exposed portion of a craton at the surface, consisting of ancient metamorphic and igneous rocks. These areas tend to be low-relief landscapes.
   • Platform: In many cratons, ancient rocks are covered by younger sedimentary layers, often from Paleozoic to more recent periods. These are often referred to as cratonic platforms.

Evolving Cratons

Stabilisation of Archaean Cratons

The South American and African continents are riddled with a number continental blocks (or cratons) that have been shielded from mantle convection by thick roots of mantle lithosphere. A common feature of all these cratons is that they preserve Archaean crustal segments (or shields) that were formed between 3.5 and 2.5 billion years ago. These ancient shields assembled into broad, continental-scale cratons via tectonic accretion and subsequent partial melting in the Palaeo- and Mesoproterozoic.  However, little is known about the timing and the processes that converted the shields into solid cratonic units since the Archaean.

The main research activities are:

• Constrain magmatic/metamorphic ages of Archaean granitoids/gneisses and associated volcanic rocks within the shields 
• Geochemical modelling of Archaean gneisses and granitoids that form the bulk of the shields
• Detrital zircon age dating of greenstone belt sequences
• Construct the metamorphic evolution of greenstone belt pelites and amphibolites via P-T-t modelling – Theriak Domino and Thermocalc
• Link peaks of granitoids magmatism and high-grade metamorphic events within these cratonic shields

 Craton stabilization and magmatism

The withdrawal of large volumes of potassic (K-rich) magmas from the deep continental crust and their emplacement at higher structural levels is a key process that significantly influences the evolution of the lithosphere, especially in Archaean provinces. This type of magmatism typically occurs after the formation of older tonalite-trondhjemite-granodiorite (TTG) crust, which is common in the early stages of continental development. The emplacement of high-K, calc-alkaline granites and granodiorites marks a shift in tectonic and magmatic processes, often coinciding with or following the culmination of compressional deformation and crustal thickening.

Key Processes in Potassic Magmatism:

1. Crustal Thickening and Anatexis:
   • As continents undergo collision and thickening during tectonic events, the deep crust can experience partial melting (anatexis). This process generates high-K, felsic magmas, which are less dense than the surrounding material and tend to rise, eventually emplacing at higher structural levels. These magmas form the high-K granites and granodiorites often seen in later stages of Archaean tectonic cycles.
2. Decompression Melting:
   • After crustal thickening, the collapse of orogenic belts through extensional tectonics may lead to decompression of the deep crust. This sudden reduction in pressure can also trigger melting, producing large volumes of felsic, potassic magma.

Tectonic Implications:

• Impact on Heat Production: The emplacement of these granitoids removes heat-producing elements (HPEs), such as potassium, uranium, and thorium, from the deep crust, which reduces the long-term heat production in the lithosphere. This process cools and stabilizes the crust over geological timescales, making the lithosphere more rigid and less prone to future tectonic activity.
• Rheological Strengthening: The introduction of large volumes of felsic magma into the crust creates a strong rheological column by intruding and crosscutting pre-existing tectonic structures. This strengthening effect helps to “lock” the upper crust in place, reducing its susceptibility to deformation in future tectonic events.
• Formation of Stable Continental Blocks: As the crust becomes cold and rigid, these potassic-rich granitoid provinces can form the cores of long-lasting continental blocks. The strength and thermal stability of these blocks allow for the preservation of large sedimentary basins on their surface, facilitating the deposition of thick sedimentary covers in stable tectonic environments.

Long-Term Geological Consequences:

The production and emplacement of potassic granitoids play a critical role in stabilizing the continental lithosphere, contributing to the formation of cratonic blocks that persist for billions of years. These processes not only remove heat-producing elements but also create robust upper crustal structures that can support large sedimentary accumulations, which are key components in the long-term geological record of Earth’s continental evolution.

STUDIES IN THE SAO FRANCISCO CRATON

Fig 2 – Simplified geological map of the Sao Francisco Craton

The research on the São Francisco Craton in southeastern Brazil is addressing critical aspects of crustal evolution in one of the oldest Archaean continental shield areas of South America. The study of potassic granitoids in conjunction with the tonalite-trondhjemite-granodiorite (TTG) rocks and greenstone belts offers insights into significant tectonic and magmatic events that shaped this region during the late Archaean. The transition marked by the emplacement of potassic granitoids highlights changes in the tectonothermal regime, likely indicating crustal thickening and stabilization processes.

Key Research Questions:

1. Ages and Nature of the Source(s) Undergoing Melting:
   • Determining the ages of the potassic granitoids relative to the TTG rocks is crucial for understanding the timing of crustal thickening events and the overall growth of the craton. This requires precise geochronological techniques, such as U-Pb zircon dating, to pinpoint the crystallization ages of the granitoids.
   • Identifying the source(s) of the magmas is another major focus. This involves examining the geochemical signatures (e.g., trace elements, isotope compositions) to determine whether the potassic magmas were derived from the partial melting of the lower crust or involved input from mantle sources.
2. Processes Shaping the Geochemistry of the Magmas:
   • The geochemistry of the potassic granitoids can reveal much about the processes occurring during their formation. This includes looking at the differentiation of magma, contamination by surrounding rocks, or mixing with other magma sources.
   • High-K magmas are often associated with specific tectonic settings (e.g., post-collisional or extensional environments). The presence of such magmas may indicate either reworking of older crust or mantle-derived contributions that were affected by crustal processes.
3. Processes Leading to Heating and Melting of the Lower Crust:
   • The widespread distribution of undeformed potassic granitoids suggests a significant heating event in the lower crust, possibly driven by crustal thickening, decompression, or the intrusion of mantle-derived magmas.
   • Understanding whether this heating was due to conductive processes from below (mantle upwelling) or convective processes within the crust itself (e.g., through radiogenic heat production or magma intrusions) is key to unraveling the late-Archaean tectonic evolution.

The interplay between these factors—ages, source characteristics, and tectonic processes—will contribute to a more comprehensive understanding of the São Francisco Craton’s role in the broader context of Archaean continental development.

The Quadrilátero Ferrífero

The Quadrilátero Ferrífero (QF) province, in the Southern São Francisco Craton (SSFC), exposes a large segment of Mid- to Neoarchaean continental crust that served for many years as type locality for understanding early crustal evolution in South America. The Archaean history of this segment spans about 600 My, and can be subdivided into two fundamentally distinct stages. The first stage, lasting more than 400 My, involved the emplacement of voluminous TTG magmas and extrusion of mafic and ultramafic rocks between 3200-2770 Ma (Fig. 3). The TTG magmatism led to the construction of a regionally extensive TTG crust riddled with numerous greenstone belt occurrences. The second stage was marked by high-grade ductile deformation, partial melting of the TTG-greenstone crust, and concomitant emplacement of voluminous potassic granitoids between 2750-2600 Ma. This period of intra-crustal melting saw the emergence of a stable continental platform that was substantially rigid to sustain the deposition of thick Palaeoproterozoic sequences, including the extensive banded iron deposits of the Quadrilátero Ferrífero mining district (Fig. 3).

Recent work indicates that this sort of model where the K-granites are derived from TTG anatexis is very likely to be incorrect. TTGs are quite low in heat-producing elements, they also are not the source of the granites. Formation of the granites requires either K-rich mantle melts interacting with the lower crust during anatexis, maybe a combined heat source and major flux of mantle-derived material into the crust, or burial of K-rich sediments by an orogenic process. For BGB I favour the latter. Rocks like the volcaniclastic felsic units of the TheespruitFm and the Fig Tree group are the source of the 3.14 to 3.10 granites in BGB.

Fig. 3 – Schematic evolution of the Southern São Francisco Craton in the Late-Archaean. a) Undifferentiated crust recording concomitant deposition of the Rio das Velhas Group Lavas and emplacement of the younger TTG suites at ca. 2780-2770 Ma. b) Emplacement of potassic granitoids at 2760-2700 Ma. This process led to differentiation of the crust into a refractory lower crust and an upper crust enriched in heat producing elements HPE. c ) Subsequent redistribution of the HEP during erosion and peneplanation of the upper crust and accumulation of the Minas Supergroup clastic sediments

Objectives

The proposed study will pursue high-precision geochemical and isotopic analyses of the potassic granitoids in order provide detailed information about compositional diversity within them, the sources of their chemical diversity, details of the time span of crystallization of the plutons and, finally, modelling of geodynamic scenarios leading to the production of high volumes of potassic magmas.  We suggest the following goals within the context of the post-doctoral project:

• 1 – Characterize the geochemical diversity between the plutons through XRF and LA-ICP-MS analyses. Major and trace element analyses will be accompanied of detailed field and petrographic observations of granitoids exposed in several (8-10 would probably be sufficient) localities that cover most of the geographical range of the main batholiths in the region.
• 2- Constrain the nature of the primary source or sources of the potassic magmas via isotope studies. Analysis of the U/Pb ages and Hf-Lu isotopes will be carried out on a sufficiently large number of inherited zircons, titanites and/or monazites  (~ 200) from selected samples. A similar dataset should be acquired for the TTG rocks exposed in the interior of the granitoid-gneiss complexes.
• 3-Establish the duration of the magmatic process via detailed zircon geochronology of the magmatic zircons. This will be combined with Hf-Lu isotope analysis of the magmatic zircon fraction from different areas to define how homogenous was the magmatic source of these granitoids.
• 4-Define the U-Th and other REE element content of magmatic accessory phases and xenocrystals in other to understand the partitioning of these elements in the source and the phase melt.