Magmatic activity as the major process in the chemical evolution of the earth's crust and mantle

Abstract

Hypotheses of continental drift or plate tectonics require the formation of new oceanic crust at mid-oceanic ridges and imply the further modification or continued evolution towards continental type of crust in island arcs and orogenic belts. Volcanism is the main process by which chemical differentiation by partial melting of parental upper-mantle peridotite occurs to yield, firstly, basaltic oceanic crust and, in a second and more complex melting stage, “andesitic” continental crust. Further differentiation of the continental crust may occur with deep crustal melting leading to enrichment of the upper crust in silica-rich and alkali-rich intrusives. The evolutionary trend of the processes of magmatism is towards a gravitationally and thermally stable, layered earth. The geological record suggests that the modern patterns of volcanism and plate tectonics have persisted for more than a billion years, as the mechanism of earth differentiation. However, the oldest exposed parts of the crust (>2.5 b.y.) provide evidence for a characteristically different type of magmatic activity implying very high degrees of melting of the upper mantle at very shallow depths. It is suggested that the “greenstone belts” of the Archaean shields may be the folded and metamorphosed terrestrial equivalents of lunar maria and formed contemporaneously with the lunar maria. Major impacts are considered to have triggered partial melting at depths of 150–300 km and diapiric ascent from these depths produced characteristic ultramafic magmas. Further evolution of the “terrestrial maria” was controlled by endogenous processes producing andesitic volcanism and in-folding of the maria to yield the “greenstone belts”.