ABSTRACT
The primary magmas responsible for the alkaline rocks normally equilibrate at greater depths than the magmas that yield the more common subalkaline rocks so the alkaline rocks and their xenoliths are more likely to provide novel data about mantle materials and mantle processes. The atephrites are ultrabasic to basic volcanic rocks that are essentially composed of calcic plagioclase, clinopyroxene and one or more feldspathoids. The term melanephelinite was originally used to describe volcanic rocks that contained abundant pyroxene and some nepheline. Geophysical investigations have revealed that the exposed intrusive rocks are part of a laccolith that continues downwards for more than 7 km. The characteristic rock emplaced during the second intrusive episode is an olivine melilitolite, or kugdite. Assorted midalkaline and alkaline magmatic rocks crop out on volcanic islands and seamounts in all the oceans. Melilitic rocks never contain primary quartz, seldom contain feldspars, but often contain feldspathoids, like nepheline and leucite.
