MAGNETIC INTENSITY AND CHEMICAL COMPOSITION OF NONCONTEMPORANEOUS BASIC EFFUSIVES IN CERTAIN MOBILE BELTS AND PLATFORMS

Results of a comparative study of magnetic properties and the petrochemical composition of noncontemporaneous effusives may be applied in differentiating their primary and diagenetic varieties, epicontinental (platform) and geosynclinal formations, and ore-contact alteration of basic effusives, associated with propylitization. In addition, it becomes clear that the mineral and chemical composition of mobile belt effusives, especially pre-Cenozoic, depends less on the overall chemical composition of their original magma than on elements added and removed from them during the nascent metamorphism. This point must be taken into consideration in determining the specific material composition of magmas involved, as well as of their evolution within orogenic zones. In the instance of the so-called "fresh" rocks of mobile belts, their content of magnesium and iron oxides, and alkalis, with all their ratios, are not the geochemical correlatives of their parent magmas. On the other hand, some of such oxides and their ratios, such as MgO and Fe₂O₃/FeO, may constitute, along with magnetic characteristics, objective indicators of nascent metamorphism in basic effusives — and of their age, as long as the two are usually proportionate.--Author's summary.