Abstract
Ultramafic rocks can be divided into 5 principal geotectonic groups, as follows: (1) Layered gabbro-norite-peridotite masses, commonly associated with all ages of terrane, possibly connected with accretionary plate margins. (2) Concentrically zoned Alaska-type bodies, possibly associated with consuming plate margins. (3) Ultramafic lavas and associated intrusions, nearly exclusively in Archaean terranes, of uncertain tectonic position. (4) Alpine peridotites, in linear Phanerozoic and Late Precambrian deformed belts, indicative of activity both at accretionary and consuming plate margins. (5) Nodules and other ultramafic rocks associated with alkalic volcanic centers, which in some cases represent mid-plate activity, as well as direct samples of mantle rocks underlying the volcanic area. A key question in interpretation of the geotectonic significance of ultramafic rocks is the nature or even existence of plate-tectonic activity prior to that reflected in the present ocean basins.
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