CHEMISTRY OF THOLEIITIC MAGMATISM AS RELATED TO THE EVOLUTION OF MIDOCEANIC RIDGES

The idea of tectonically homogeneous zones is used to study the relati onship of variations in the composition of oceanic tholeiite basalts t o the evolution of mid-oceanic ridges. A major element in classificati on of tholeiite basalt glasses is presented, from which it follows tha t low-Ti basalts poor in alkalis and rich in calcium (Type I) are conf ined to areas formed during the initial 30 m.y. of the rift system, an d to tectonically complicated ridge zones. High-Ti basalts enriched in alkalis and depleted in calcium (Type III) occur in rift zones that h ave undergone more than 80 to 90 m.y. of evolution since the basalts w ere extruded. In terms of their composition and relationship to durati on of rift zone evolution, Type-II basalts are transitional.