The origin of ultramafic and ultrabasic rocks

Abstract

Ultramafic rocks are classified in terms of their field associations and tectonic environment. Eleven associations are distinguished, some with subdivisions. The major features of each association are described in turn, and it is emphasized that the variety of field and petrographic associations indicates that a variety of processes is involved in the origin and emplacement of these rocks. For petrogenetic discussion the-associations are considered in three groups: (1) layered, stratiform and other intrusions involving gabbro or diabase together with accumulations or concentrations of mafic minerals; (2) the alkalic rocks of stable continental regions; including kimberlites, mica peridotite, members of ring complexes, and ultrabasic lava flows; (3) the several peridotite-serpentinite associations of the orogenic belts that have been classified together as alpine-type intrusions. The petrogenesis of ultramafic nodules in alkali basalts and kimberlites is also discussed. The petrogenesis of these rocks is concerned with the source of the material, its variation in physical state and temperature from source to position of intrusion, and its post-intrusion history. The study of ultramafic rocks and nodules is one approach towards determination of the composition and mineralogy of the upper mantle.