GD7.2

The term craton has a complex and confused etymology. Despite originally specifying only strength and stability – of the crust – the term craton has seen widespread use as referring to a region characterised by crustal basement older than 2.5 Ga, despite the fact that some such “cratons” no longer possess their deep lithospheric root and have geological histories that contnue well beyond the Archean/Proterozoic boundary.  Viscous, buoyant lithospheric mantle roots are key to the survival and stability of continental crust. Here we use a revised craton definition (Pearson et al., 2021, Nature), that includes the requirement of a deep (~150 km or greater) and intact lithospheric root, to re-examine extent and character of regions defined as crtons. The revised definition has a nominal requirement for tectonic stability since ~ 1 Ga and recognises that some regions are “modified cratons” – having lost their deep roots, i.e., they may have behaved like cratons for an extended period but subsequently lost much of their stabilising mantle roots during major tectono-thermal events. In other words, despite being long-lived features, cratons are not all permanent. The 150 km lithospheric thickness cut-off provides an optimal match to crustal terranes with 1 Ga timescale stability.

We examine the processes involved in craton ormation and growth. Seismology can help to define the lateral extent of today’s cratons, but a detailed understanding of the regional geological history, kimberlite eruption ages and geothermal conditions is required to evaluate periods of past diamond potential, no-longer evident today. 

How to cite: Pearson, G.: What are cratons?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6819, https://doi.org/10.5194/egusphere-egu22-6819, 2022.

Most interpretations of the Archean rocks in the Central Congo Craton have only focused on data from Cameroon and Gabon, few of them have included data from the Ivindo region in northwest Republic of Congo. This study presents for the first time a regional interpretation of the Archean rocks of the Congo craton from data on granitoids of the Ivindo region. Modal compositions vary between quartz-rich granitoids or pegmatite, granodiorites, granites and tonalites. These rocks are metaluminous and peraluminous (~0.8≤A/CNK≤~1.3) and define magmatic lineages that are predominantly calc-alkaline, tholeiitic, and rarely highly potassic calc-alkaline. REE diagrams show that these rocks are rich in rare earth elements (LREE) and large ionic lithophile (LILE), while exhibiting significant negative anomalies in Nb-Ta, and in Ti. Such geochemical signatures indicate that these granitoids formed possibly in a subduction tectonic setting. These geochemical signatures are comparable with the Dharwar, North China, and Pilbara cratons, also in similar Archean cratons.

The U-Pb ages based on zircon indicate that tonalites were amplaced at (2891.2 ± 10.6 and 2820.37 ± 6.23 Ma), pegmatite were amplaced at (2878.2 ± 13.6 and 2891.0 ± 12.6 Ma), granodiorite were ampleced at (2828. 98 ± 6.23 Ma) and granite were ampleced at (2430.19 ± 8.11 Ma). Thesse periods of magmatisme describe here revels the magmatic history of the Archean granitoids of the Congo craton in the Ivindo Bassement from 3085 ± 21.6 and 2430.19 ± 8.11 Ma.

Keywords: Archean, Crustal growth, Granitoids, Ivindo region, Congo craton, Republic of Congo.

How to cite: Loemba, R. P. A., Ntsiele, L. J. E. P., Opo, U. F., Bazebizonza Tchiguina, C., Nkodia, H. M. D.-V., and Boudzoumou, F.: Crustal growth of Archean and early Proterozoic granitoids of the Ivindo region in the Souanké and Bomalinga areas from Congo Craton (North-West Republic of Congo), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5438, https://doi.org/10.5194/egusphere-egu22-5438, 2022.

We present results of cluster analysis and geophysical modelling of the West and Central African rift system, where we integrate seismological and satellite data. For a description of lithospheric domains, two different methods based on seismic tomography and satellite gravity data have been used. First, the terracing method using the shape index, has been applied to the gravity field in order to enhance the signal of the large-scale tectonic units. In addition, the K-means cluster method (which is an unsupervised machine learning algorithm) has been applied to a seismic tomography model over the area.

Both models are compared and interpreted towards similarities and differences. The preliminary analysis based on K-means clustering of seismic tomography shows that the West and Central African rift system and its surroundings can be divided into at least three clearly distinct tectonic domains: The Northern part of the Congo craton, the Eastern part of the West African craton and an area in between. In addition, the preliminary analysis of the terracing of satellite gravity data, confirms the location of both the Congo and the West African craton, but also splits the area in between into two known tectonic units, the Southern part of the Saharan meta-craton and the West and Central African rift system in the center.

The cluster analysis is also pointing to differences at crustal and upper mantle level and is the first step towards the evolution of a lithospheric scale model. In the model, we integrate our tectonic domain analysis with the existing seismic Moho depths estimate and other information.

How to cite: Fosso Teguia M, E. E., Ebbing, J., and Haas, P.: Lithospheric domains of the West and Central African rift system based on Terracing and Cluster analysis, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7310, https://doi.org/10.5194/egusphere-egu22-7310, 2022.

South America presents a diverse tectonic set-up, with an active subduction margin on the western border and a stable continental interior to the east. In the ancient stable part, two main cratonic domains can be separated. The Amazonian, consolidated in Archean-Paleoproterozoic times, and the Brasiliano, marked by Neoproterozoic events related to the West Gondwana assembly. In each domain, geology and geophysical methods separate different cratonic nuclei. However, some nuclei's detailed lateral and vertical extent and even existence are debated.

In seismic tomography, we can define regions of cratonic lithosphere due to the shear wave sensitivity to temperature and composition. However, until recently, seismic data sampling in South America was highly scarce and uneven. Here, we assembled all freely available seismic data globally, with the addition of the FAPESP "3-Basins Thematic Project" temporary network. After selecting all paths crossing the hemisphere centred at South America and performing an automatic outlier rejection, we obtain a massive dataset of ~1 million waveform fits, constraining our final model.

We compute a new S-velocity tomographic model of the upper mantle of South America and surrounding oceans using the Automated Multimode Inversion of surface, S- and multiple S-waves. The increase in the data coverage of the model combined with the optimized tuning of the inversion parameters on the continent allows us to identify for the first time the fine details present in the cratonic structure. We observe that regions of thinner lithosphere inside cratons correspond to areas of rifting in previous tectonic cycles. Inside the boundaries of the Amazon craton, we image two cratonic blocks, separated by the Amazon basin. In this area, an aborted rift system preceded the formation of the Amazon basin in the Neoproterozoic, and rift reactivation occurred with the break-up of Pangea in the Mesozoic. Similarly, in the São Francisco Craton, we image a significantly thinner lithosphere in the Paramirim Aulacogen area, a Paleoproterozoic intracontinental rift system. These observations show that the continental lithospheric topography is closely related to upper mantle dynamic processes. We also image high-velocity lithospheric blocks under sedimentary basins. East of the Amazon craton, we image a high-velocity anomaly beneath the Parnaíba block, and under the Paraná basin the fragmented Paranapanema block lithosphere. Finally, by imaging the boundary of the cratonic units in detail, we can observe that magmatic events and large igneous provinces are distributed around the thick roots of the cratons, where the lithosphere is thinner.

How to cite: Chagas de Melo, B., Lebedev, S., Celli, N., and Assumpção, M.: Detailed Structure of the South American Cratons Using Waveform Tomography, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8441, https://doi.org/10.5194/egusphere-egu22-8441, 2022.

Despite the influence of several extrinsic parameters that inhibits the use of trace element composition of detrital zircon grains in inferring their host rocks, workers had overcome many related problems and particularly constrained zircon/bulk rock partition co-efficient at least for different granitoids, for example. Based on these kind of progress and few other fundamental works, we have tried to apply trace element composition of detrital zircon grains retrieved from some basal quartz pebble conglomerate units and orthoquartzites of Dharwar craton in studying the crustal evolution pattern of this craton, specifically in terms of its changing crustal thickness with time. In this study, after categorising the pristine zircon grains identified by their La>1, Pr>1 and LREE-I<30 values, the values of their LREE/HREE ratio (measured by their Lu/Nd ratio) are used to infer the temporal variation of crustal thickness within this craton. Here, the zircon grains show depressed values of LREE/HREE ratio manifested in their higher Lu/Nd ratio which possibly attests the absence of thicker continental crust in Dharwar craton between 3.4-3.1 Ga. We would also try to establish our observation regarding the secular evolution of crustal thickness of Dharwar craton with the help of other bivariate plots using the other trace elemental proxies. Our result stand in contradiction with the finding of other workers who, with the help of geophysical parameters, inferred the greater thickness of continental crust attested in WDC within the said time frame  

How to cite: Mitra, A. and Dey, S.: Tale of crustal evolution of western Dharwar craton in Paleo-to- Meso Archean time: Insights from trace elemental composition of detrital zircons of some selected quartzite units., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-609, https://doi.org/10.5194/egusphere-egu22-609, 2022.

Petrogenetic processes of the Archean (>2500 Ma) andesitic rocks are strongly debated because of their distinct geochemical similarities to the modern subduction zone andesites contrast with sparse evidence for Archean lithospheric subduction. Therefore, processes responsible for generation of the andesitic rocks preserved in an Archean craton would potentially place constraints on the Archean geodynamic process. The Western Iron Ore Group (W-IOG) volcano-sedimentary succession in Singhbhum craton is overlain by unmetamorphosed Jagannathpur amygdular volcanics (basaltic andesite – andesite). The W-IOG preserves deformed lower greenschist-facies tholeiitic basalt and calc-alkaline basaltic andesite interlayered with BIF and Fe-Mn-rich phyllite and shale. Previously, petrogenesis of the basaltic andesite in W-IOG was interpreted as having formed in a subduction zone whereas the origin of Jagannathpur volcanics has remained unclear. Therefore, geochemical modelling using trace elements and Sm–Nd geochronology of these basaltic-andesitic rocks were performed to constrain the petrogenetic process and timing of volcanic eruption of these metavolcanic rocks.

Primitive mantle-normalized trace element patterns, chondrite-normalized REE patterns and Nb/Th, Zr/Th ratios of the W-IOG and Jagannathpur basaltic andesite – andesite show enrichment in large ion lithophile elements (LILE), light rare earth elements (LREE), Zr and Th indicating incompatible trace element enrichment in their petrogenesis. The W-IOG tholeiitic basalt is depleted in LILE, LREE, Zr and Th and an absence of Nb-Ta-Ti anomalies that imply a depleted mantle source. The W-IOG basaltic andesite yield an isochron age of 3041±94 Ma (2SD) with Nd_i = 0.50875±0.00009, MSWD = 0.62 (n=10) and ε_Nd(T) = +1.1±1.6; whereas the tholeiitic basalt yielded an isochron age of 3050±71 Ma (2SD) with Nd_i = 0.50885±0.00010, MSWD = 0.17 (n=10) and ε_Nd(T) = +3.3±1.6. Geochemical modelling indicates that the W-IOG basaltic andesite could have been generated by 20-40% assimilation-fractional crystallization (AFC) (r=0.2, ratio of rate of assimilation to the rate of fractional crystallization) of primitive tholeiitic magma that is derived by 14% partial melting of depleted MORB-type mantle (DMM) under spinel lherzolite depth in an extensional setting. The Jagannathpur basaltic andesite – andesite yielded an Sm-Nd isochron age of 2799±67 Ma (2SD) with Nd_i = 0.50895±0.00006, MSWD = 0.36 (n=16) and ε_Nd(T) = -1.1±0.5 and represents one of the oldest Neoarchean intracratonic flood basaltic volcanism. These basaltic andesite – andesite could have been produced by 20-60% AFC (r=0.2) of hybrid magma during lithospheric extension. Generation of the hybrid magma has been modelled by two end member components involving ~18% partial melt of enriched-DMM that interacted with low degree (~5%) partial melt of metasomatised subcontinental lithospheric mantle (SCLM). In addition, our geochemical model results suggest that Meso- to Neoarchean basaltic andesite – andesite rocks in Singhbhum craton were not generated by 1) assimilation of crustal material with primitive tholeiitic magma without fractional crystallization, 2) direct partial melting of different enriched mantle reservoirs (enriched-DMM, EM I, EM II) and mantle wedge peridotite in a subduction environment and 3) partial melting of solely metasomatised SCLM.

How to cite: Adhikari, A., Nandi, A., Mukherjee, S., and Vadlamani, R.: Modelling petrogenesis of Meso- and Neoarchean andesitic rocks: an example from Singhbhum craton, India, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9048, https://doi.org/10.5194/egusphere-egu22-9048, 2022.

In a geodynamic, geological and geophysical review of global Archean cratons, we find that the survival of Archean cratons depends on the initial conditions of their formation, as well as the tectonic processes to which they were exposed.  In a sense, we must consider both their nature and how they were nurtured.  In a review of existing literature and models, we use stability regime diagrams to understand the factors that contribute to the intrinsic strength of a craton: buoyancy, viscosity, and relative integrated yield strength. We find that cratons formed in the Archean when thermal conditions enhanced extraction of large melt fractions and early cratonization promoted the formation of stable Archean cratonic lithosphere.  In terms of the cratons' nurturing, processes that may have modified and weaken cratonic lithosphere include subduction and slab rollback, rifting, and mantle plumes, as these processes introduced materials and conditions that warmed and metasomatized the lithosphere.  Examining four Archean cratons that are more stable, and four that are categorized as modified or destroyed, we note that continental lithosphere that was cratonized prior to the end of the Archean has more potential to survive deformation during the last 500 My. Although, the survivability of these cratons is highly dependent on their unique positions relative to larger scale tectonic processes, such as subduction.   We also observe that once an Archean craton begins to undergo even a small amount of modification, it is more likely to continue to be modified, as it loses the preservation advantage that it had upon birth.

How to cite: Bedle, H., Cooper, C., and Frost, C.: Nature vs. Nurture: Understanding the survival of Archean cratons, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6102, https://doi.org/10.5194/egusphere-egu22-6102, 2022.

A number of geological, geochemical and seismological studies suggest that cratonic lithospheres may be associated with thinning and destruction. For such unique plate configurations, the most well-known example is the North China craton. Geological studies suggest that during the Mesozoic era (120-80 Ma), a surge of magmatism occurred across the North China Craton as a response to the removal of the portions of the lithosphere beneath it. However, the question of which processes control lithospheric thinning/removal is yet to be understood. The one that is the subject of this study is the deformation controlled by gravitational instabilities (convective removal), that develop because of density variations between the lithosphere and the underlying sub-lithospheric (asthenospheric) mantle.

In accordance with numerical model predictions conceptual geological hypotheses are inferred to invoke the phase transitions in the lower crust and densification of this layer through the transformation of the basalt to eclogite during late Jurassic where Pacific flat-slab subduction led to shortening in the continental back arc (e.g Andean type tectonics). The removal event possibly occurred following the plate shortening during Early Cretaceous and various surface geological features, for instance, normal faulting/extension and pull apart basins and are interpreted in the context of coupled crust-mantle dynamics. This research aims to facilitate new 3D modelling strategies to further explain how large-scale plate geodynamics may account for the geological-geophysical fingerprints of destruction at North China Craton. The problem of deformation of the North China Craton will be approached on a much broader aspect including the extensional events that took place in Cretaceous. The overarching goal of this work is to explain the first order geodynamic mechanism that possibly constrain the craton destructions not only under North China but also other areas where such mechanism has been postulated (e.g North America, South Africa). 

How to cite: Ballı, A., Göğüş, O., and van Hunen, J.: 3D Modeling of Crust-Mantle Dynamics on Cratonic Regions: Implications for the Deformation of North China Craton, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-540, https://doi.org/10.5194/egusphere-egu22-540, 2022.

The concealed basement of the Mid-Lithuanian domain (MLD) is considered to be part of a larger Precambrian unit within the western East European Craton (EEC), the Mid-Baltic belt (MBB), established by Bogdanova et al. (2015). New data on rock chemistry, U-Pb ages, and the Sm-Nd and Rb-Sr isotopic systems allow to subdivide the MLD into distinct parts, discuss their origin and correlate them with similar units on the Swedish side.

The MLD can be subdivided into two parts: NW and SE. The NW MLD magmatic rocks crystallized from 1.86 to 1.83 Ga and were subsequently intruded by 1.81-1.80 Ga granitoids and charnockitoids. The NW MLD samples have SiO₂ contents between 48 and 71 wt.% but have similar initial ε_Nd values at -1 to -2, while their initial Sr isotope ratios scatter. Nd isotope data suggest either an enriched mantle source, or a mantle magma that was mixed with older crustal material.

The SE MLD magmatic rocks originated from a slightly depleted mantle source from 1.87 to 1.82 Ga. At 1792±9 Ma, they were intruded by gabbronorites which in turn were crosscut by thin veinlets of microgabbronorite at 1758±11 Ma. The SE MLD rocks have positive ε_Nd (+1 to +3) and undisturbed Rb/Sr systems suggesting mantle-derivation, with the variation in composition (mafic to felsic) due to fractionation rather than crustal contributions.

The SE MLD magmatic series with oceanic island arc affinity correlate well with the ca 1.85 Ga Fröderyd metavolcanics of the Vetlanda-Oskarshamn belt (Salin et al., 2021) in SE Sweden, while the NW MLD rocks are similar to the TIB-0 (1.86-1.85 Ga) Askersund granitoids (cf. Salin et al., 2021) in the southern Bergslagen area. The younger (1.81-1.79 Ga) intrusives in both areas are time-equivalents of the TIB-1 magmatism on the Swedish side. Thus, the MLD as well as its counterparts on the Swedish side of the Baltic Sea, the TIB-0 magmatism in the southern Bergslagen area and the Vetlanda-Oskarshamn belt, may be assigned to the same Mid-Baltic Belt, representing an active, south-facing continental margin established at ca. 1.86 Ga. The shape and outline of the Belt was affected by the Fennoscandia-Sarmatia collision at ca. 1.82-1.80 Ga, the 1.81-1.76 Ga TIB-1 magmatism, as well as by later Mesoproterozoic intraplate magmatism.

Bogdanova, S. et al., 2015. Precambrian Research 259, 5–33.

Salin, E. et al., 2019. Precambrian Research 328, 287–308.

Salin, E. et al., 2021. Precambrian Research 356, 106134

How to cite: Skridlaite, G., Siliauskas, L., Whitehouse, M., Johansson, Å., and Rimsa, A.: Evidence for a ca 1.86 Ga continental margin in the Baltic Sea region: rock chemistry, U-Pb ages, and Nd and Sr isotopic data, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2524, https://doi.org/10.5194/egusphere-egu22-2524, 2022.

A 3000 m deep hole is being drilled in the Archean Karelian Craton in northeastern Finland in an area where the granitoids dominating the surface have yielded Neoarchean ages (2.8–2.7 Ga). Archean greenstones and Paleoproterozoic dolerites are exposed within the domain as well. The drilling site lies between ca. 2.44 Ga Koillismaa and Näränkävaara mafic layered intrusions. This site was chosen based on gravimetric, magnetic, magnetotelluric and reflection seismic studies, which have revealed a deep anomaly that seems to connect the two mafic layered intrusions. Based on modelling of the geophysical data, the upper boundary of this ca. 60 km long, roughly E-W oriented anomaly lies at approximately 1.5 km depth.

We sampled various rock types from depths of ~40–1600 m for zircon U-Pb dating. The lithologies include leucogranites, tonalite gneiss, hornblende diabase, quartz diorite and granodiorite. Based on observations from the drill core extracted so far, the source of the anomaly is likely to be ultramafic cumulates. Also, presence of Paleoproterozoic granitoids is likely. We will perform the U-Pb analyses during the winter of 2022. The results are expected to confirm the interconnection of the two layered intrusions, clarify the age distribution of the granitoids in the region, and help to decipher the detailed tectonic evolution of the Archean Koillismaa area. 

How to cite: Kurhila, M., Anttilainen, T., Karinen, T., and Mikkola, P.: Geochronology of the unexposed crust within the Finnish Archean – insights from the Koillismaa Deep Hole in Kuusamo, northeastern Finland, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6975, https://doi.org/10.5194/egusphere-egu22-6975, 2022.