EVOLUTION OF THE ARCHEAN CONTINENTAL CRUST

Abstract

Some 20 years of plate tectonic theory, combined with new insights into the fine structure of the lithosphere, the application of multielement geo­ chemical and isotopic studies, paleomagnetism, and geophysical modeling of mantle processes, have profoundly influenced present thinking on the origin and evolution of the Earth's early continental crust; previously, our knowledge of the continental crust was based almost exclusively on field geological observations. Although there is now general agreement on how the Earth worked for the last 200 m.y. because of observable evidence in the oceans and continents (e.g. Bird 1980, Condie 1982), it has proved difficult to extend this history into more ancient times in view of the lost oceanic record and the ambiguity and complexity of the pre-Mesozoic rock relationships in the continents (Dewey 1982). However, preserved characteristic rock as­ semblages uniquely identifying modern-type Wilson-cycle processes (i.e. opening and closure of oceans underlain by oceanic crust) have now been recognized in continental terranes as old as �900 m.y., and these assemblages provide strong evidence for the conclusion that the present global tectonic regime has governed the evolution of the lithosphere at least since the late Precambrian (Kroner 1977, 1981a,b; Goodwin 1981). Profound disagreement on the older crustal history, however, prevails to the present day, since typical features characterizing Phanerozoic accretionary terranes (such as obducted ophiolites, blueschists, and Franciscan-type melanges) have not been found in more ancient regions. Thus, two types of evolutionary models have been developed. One type postulates uniformitarian development back to the earliest Archean (Burke