ABSTRACT
Recent evidence suggests that magmas are generated from both mantle and crustal material. Thus both basic and ultrabasic magmas are derived from ultra-basic mantle material, acid magmas are derived from crustal material, and magmas of intermediate composition may have a mixed mantle and crustal source. In recent years, two major trends have been outlined in the study of the petrology and geochemistry of subcrustai zones. The first is based on study of the composition and equilibrium P–T conditions of deep-seated rocks that are solid when they reach the surface. The basis of another concept is the generally accepted theory that the mantle is the source region for various types of magma. Peridotite type I is conventionally termed an undepleted undifferentiated (“chondritic”) peridotite. Although the present-day Earth’s lithosphere is heterogeneous, there are good reasons to believe that a large variety of subcrustal rocks were formed from roughly homogeneous primordial mantle material.
