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Título

Reevaluation of the Amazonian Craton: A Contemporary Geodynamic Perspective

Texto do resumo

The seminal work of Cordani et al. (1979) defined the Amazonian Craton (AC) as consisting of two Archean cores surrounded by progressively younger orogenic belts (2.4 – 1.0 Ga), amalgamated through a succession of subduction/collision zones in both continental and oceanic settings. This hypothesis significantly influenced scientific thought in Brazil, coinciding with the consolidation of plate tectonics theory as the primary mechanism of Earth's evolution. For nearly half a century, the concept of AC evolution based on the plate tectonics model has been widely accepted. However, we contend that a reassessment is now necessary.
The original delineation of AC's geochronological provinces was primarily based on K/Ar, Rb/Sr and Sm/Nd dating. Despite the foundational value of these works, subsequent mapping and geophysical surveys, U-Pb geochronology, as well as numerous theses, have not provided new geological evidence supporting the hypothesis of terrane accretion to Archaean core, as observed in Phanerozoic orogens. Recent limited publications on the AC’s tectonic evolution have also raised questions about the original concept of terrane accretion.
Advances in our understanding of Archean/Paleoproterozoic geology, deep geophysics, experimental geodynamic models, and petrochronology justify a reevaluation of the AC geodynamic evolution. Over the past twenty years, numerous studies have highlighted the unique aspects of Archean and Paleoproterozoic geology. Notably, hallmarks of plate tectonics such as ophiolites, high dT/dP and low dT/dP metamorphic pairs, tectonic vergence, nappes, sutures, and ultrahigh-P rocks have not been found in the previously described "accretionary orogens" of the AC. Contrary to previous proposals, we identified that the belts surrounding the AC Archean nuclei exhibit ultra-high dT/dP, high-angle strain components of the convergence direction, and are affected by homogeneous deformation zones over large distances.
We propose that the AC to be composed of 1- Archaean TTG-greenstones (Carajás, Rio Maria); 2- Transamazonian TTG-greenstone belts (Maroni-Itacaiúnas), 3- intraplate plutono-volcanic sequences (Ventuari-Tapajós, Rondonia-Juruena). In the AC, the Paleoproterozoic belts surrounding the Archean core are often associated with bimodal LIPs. Such features may indicate higher temperatures than previously expected for the AC’s evolution, challenging the validity of this uniformitarian concept.
Recently obtained Hf isotopes in zircon from samples from the Rio Negro-Juruena and Ventuari-Tapajós provinces indicate the dominance of reworking of a pre-existing Archean-Paleoproterozoic crust. (e.g., TTG-greenstones) with minor participation of juvenile magmas. We propose the AC evolved from a TTG-greenstone cratonic keel (3.4-2.2 Ga), which was progressively destroyed by mantle plume impacts along its margins, forming ultra-hot orogens and facilitating peeling tectonics in an intraplate setting (stagnant lid).
The thermal conditions necessary for subduction of oceanic plates during the AC’s evolution likely only occurred during the development of the Paraguai-Araguaia and Grenvillean orogens. The concept of geochronological provinces, although partially accurate, is not supported by field geological evidence. Our geodynamic model proposition for the AC, through complementary geological updates, aligns with the pioneering intraplate evolution suggested in the early 1970s.

Palavras Chave

Amazonian Craton; geodynamic model; TTG-Greenstones; lithospheric keel