THE GROWTH OF CONTINENTAL-CRUST

The petrological and geochemical composition of the mantle-derived ign eous products that will eventually form the continental crust (protoli th), the episodic nature, and the geodynamic sites of crustal growth a re 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, oceani c 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 cr ust may therefore form from basaltic magmas and not necessarily from i ntermediate (e.g., andesitic) magmas. Because subduction is a continuo us process, the episodic pattern of crust formation ages is a strong a rgument 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 ba salts rapidly erupted on the ocean floor reach the buoyancy threshold that defines the status of continental crust. The plateaus are accrete d 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 ins tability of thin hot young lithospheric plates. (C) 1998 Elsevier Scie nce B.V. All rights reserved.