The growth of continental crust

The petrological and geochemical composition of the mantle-derived igneous products that will eventually form the continental crust (protolith), the episodic nature, and the geodynamic sites of crustal growth are discussed. Models in which crustal growth takes place at converging boundaries from orogenic magmas contrast with those in which basaltic plume material is involved (underplating, loose-plate loading, oceanic plateaus). Because some chemical components of the crust are either preferentially returned to the mantle at subduction zones (Mg, Ca) or sequestered in the crust (Si, Al, Na, K), the composition of the crust and that of its protolith are probably very different. Continental crust may therefore form from basaltic magmas and not necessarily from intermediate (e.g., andesitic) magmas. Because subduction is a continuous process, the episodic pattern of crust formation ages is a strong argument against crustal growth at converging boundaries. The preferred model is based on major mantle instabilities (superplumes) and their surface expression, the oceanic plateaus where thick piles of plume basalts rapidly erupted on the ocean floor reach the buoyancy threshold that defines the status of continental crust. The plateaus are accreted against the continents, and the felsic magmas that stand out as the most conspicuous feature of continental crust chemistry, are produced subsequently upon subduction erosion and possibly by gravitational instability of thin hot young lithospheric plates. (C) 1998 Elsevier Science B.V. All rights reserved.