STRUCTURE AND EVOLUTION OF A NEOARCHEAN CRUST REVEALED BY SEISMIC REFLECTION AND IGNEOUS PETROLOGY: THE EXAMPLE OF THE CHIBOUGAMAU AREA, ABITIBI GREENSTONE BELT

Understanding Neoarchean growth and cratonization of the continental crust requires insights into the tectonomagmatic evolution of greenstone belts such as the Abitibi Subprovince, southern Superior craton. This is addressed by imaging the crust in the Chibougamau area, north-eastern corner of the Abitibi greenstone belt, using seismic reflection data acquired as part of the Metal Earth project. Seismic reflection imaged shallowly south-dipping structures in the upper-crust and a northward-dipping mid-crust region. The upper part of the mid-crust zone is characterized by multiple reflectors that are likely faults superimposed on a major lithological boundary at ca. 2.70 Ga, during terrane accretion prior to cratonisation. Combining the seismic data with known stratigraphic, structural and magmatic records, we propose that the study area was initially a normal (i.e., thick) Archean oceanic crust that formed at or before 2.80 Ga and that evolved through terrane imbrication at 2.73 Ga or before. This caused rapid burial of mafic rocks followed by devolatilization and partial melting of hydrated mafic rocks to produce a ‘pulse’ (~ 20 Myr) of tonalite-dominated magmatism (TT; tonalite-trondhjemite) toward the end of the synvolcanic period. This magma may have mixed with mantle-derived melts to produce the TTD suite (TT-diorite) associated with ca. 2.715 Ga Cu-Au magmatic-hydrothermal porphyry-style mineralization. Then, during the syntectonic period (~2.70 Ga), magmatism is K-richer, has a higher oxidation state (zircon chemistry) and is related to polymetallic (Cu, Au, Ag, Mo, etc.) mineralization. The lithosphere of the Chibougamau area seems particularly favorable to magmatic-hydrothermal systems, for reasons under investigation. The lithosphere of the Chibougamau area thus consisted of a mafic that evolved into an intermediate-felsic crust and stabilized within ~100 Myr.