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The circum-Atlantic Precambrian provinces in Africa and South America have recorded the progressive growth of the continental crust from Archaean to Neoproterozoic times related to episodic models for continental growth. Three worldwide superevents occured at 2.7, 1.9, 1.2 Ga in relation with episodic collapse of subducted slabs (Condie, 2000). An increase production rate of oceanic plateaus and of subduction-related crust (Albarede, 1998) leads to production increase of juvenile CC in arcs systems. Due to their crustal to lithospheric scale, the phenomena constraining individual models such as such as diapirism, oceanic plateau, subduction complexes and collisional orogenic belts are generally difficult to assess on a local to regional surface level.
Airborne geophysical surveys cover large regions in South America and Africa including radiometry and gamma-ray spectrometry. Window channels are processed to obtain the ground concentrations of K (%), Th (ppm) and U (ppm). Further gridding and interpolation of these spatial data allow production of geochemical maps showing element concentrations and ratios or ternary images. In the present study, Hierarchic Ascendant Classification is applied and based on the similarity of geochemical signatures attributed to known geological provinces. Maps provide information that can be used to monitor the progressive evolution of the continental crust from primitive TTG and greenstone belts, typically depleted in radioelements, to the development of subduction systems, resulting in the significant input of U, K and Th into the crust. Subsequent erosion and deposition of these terranes into large detrital basins reflect the radioelement characteristics of the source areas. The Gabon craton (3.2 - 2 Ga) and the Paleoproterozoic province of the Guyana shield (2.2 - 2 Ga) illustrate such trends. Although covered by rain forests, gamma-ray response reveals contrasted geological units and structures that are used to evaluate this evolution.
A similar evolution is recorded at the north-western margin of the Kalahari craton. Here, the evolution of a juvenile Late Paleoproterozoic (∼1.9 - 1.7 Ga) greenstone belt, depleted in radioelements, is followed by a Mesoproterozoic (∼ 1.4 - 1.1 Ga) arc-related volcanic-plutonic system, resulting again in the crustal enrichment of U, K, and Th. This older crust is finally eroded and redeposited in the Neoproterozoic (∼ 0.6 - 0.5 Ga) shallow marine Nama basin that developed in a foreland position south of the Damara orogenic belt.
In conclusion, superevents marked by a significant change in the composition of radioelements in the crust are recorded at 2.7 Ga in Gabon and 2.2 Ga in French Guiana and West Africa, at 1.8 Ga in Namibia. Radioelement enriched crustal signature is globally related to calkalcaline magmatism and structure-controlled basins and an increase in strength in the lithosphere is marked by the development of continental scale strike-slip and thrust faults.
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