PETROMAGNETISM OF THE CONTINENTAL LITHOSPHERE AND THE ORIGIN OF REGIONAL MAGNETIC ANOMALIES: A REVIEW

Abstract

The paper discusses the current ideas of regional magnetic anomaly sources and experimental evidence for the formation and behavior of various magnetic minerals within a wide range of pressure, temperature and oxygen fugacity. Four thermodynamic zones of formation conditions of magnetic minerals are shown to exist; these are the hematite, magnetite, silicate and metal-Fe zones successively changing, as the oxygen fugacity decreases (from strongly oxidizing to strongly reducing conditions). The effects of pressure and diffusion processes on titanomagnetite alterations, as well as the oxygen fugacity and fluid composition implications for the composition and concentration of magnetic minerals, are considered. Experimental studies show that ferromagnetic minerals do not form from silicates under "dry'''' conditions or in the presence of water vapor. The paper presents results derived from our studies of petromagnetic characteristics of rocks that formed under near-surface conditions (basalts and gabbroids) and in the lowermost continental crust(xenoliths in igneous rocks of Afar, Mongolia, the Lesser Caucasus, Kurile Islands, and Yakutia), as well as rocks from Archean-Proterozoic metamorphic sequences (Aldan and Anabar shields and Voronezh crystalline massif). The implications of secondary processes, such as chloritization and amphibolization, for the alterations in the ferromagnetic fraction and magnetic properties of these rocks are considered. Our results and literature data reviewed indicate that, since the Archean, igneous rocks formed in extension zones under surface and near-surface conditions have made a major contribution to the crust magnetism and regional magnetic anomalies. This situation is presently preserved in spite of metamorphism and substantial recrystallization at various depths.