TECTONIC MODELS FOR THE GEOLOGICAL EVOLUTION OF CRUST, CRATONS AND CONTINENTS IN THE ARCHAEAN

BRIAN F. WINDLEY

Resumo


The earliest sialic crust was probably destroyed by meteorite impacts, leaving only detrital 4.2-3.9 Ga zircons in later sediments. The oldest rocks are 3.9-3.8 Ga high-grade gneisses commonly associated with lavas and sediments which were transported into the deep crust. They indicate that mantle recycling was extensive, and that large volumes of surprisingly mature continental crust had formed by 3.7-3.8 Ga. Decreasing radiogenic heat production of the Earth is consistent with increasing indication of appreciable Archaean oceanic lithosphere and plume-generated oceanic plateaux, which in turn require extensive subduction, which accounts for the current evidence for many island arcs in the Archaean. Accretion of collages of arcs led to formation of the first protocratons bordered by the first identifiable active continental margins. Sections of upper Archaean crust, seen in many greenstone belts, and of deep Archaean crust, represented by granulite-gneiss belts, indicate massive thrust-generated imbrication of diverse rock units and tectonic belts presumably in collisional events. The Archaean was a period of high crustal growth and the eventual formation of stable cratons of mature continental crust with thick keels of sub-continental lithosphere. Sedimentary successions in greenstone belts are comparable to those in Phanerozoic depositional basins, and preserve a record of sedimentation adjacent to oceanic islands, in calc-alkaline island arcs, in syn- to post-rift stable shelves, in foreland basins, and strike-slip pull-apart basins. The presence of several late Archaean sedimentary basins many kilometres deep indicates the local stability of continental crust. Towards the end of the Archaean impingement of mantle plumes beneath mature continental lithosphere in the centre of some cratons led to reheating, diapirism, and generation of diverse magma types. This heralded the beginning of the Proterozoic, by which time large continents or even supercontinents had formed.

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