a rock composed chiefly of iron-magnesium silicates—olivine and pyroxene—with small admixtures of secondary minerals, such as chromite and magnesite. Chemically, ultrabasic rocks are relatively poor in Si02 (less than 45 percent) and rich in magnesium (more than 42 percent MgO). There are many different types of ultrabasic rocks. The most important types are the dunites, the olivinites (in which magnetite is present instead of chlorite), the peridotites, and the pyroxenites. Characteristic of ultrabasic rocks is the full or partial conversion of olivine and pyroxene into the serpentine minerals, such as chryso-tile, antigorite, and lizardite, with the formation of serpentinites. Ultrabasic rocks are widely distributed as massifs or tectonic detached masses in all regions of development of magmatic rocks and are also found in the region of the midoceanic ridges. They are often associated with gabbros, alkaline rocks, and carbona-tites. Lava flows of ultrabasic rocks were studied in Australia in the early 1970’s. Effusive ultrabasic rocks have been found in Siberia (maimechites) and on Kamchatka.
The conditions of the formation of ultrabasic rocks have not been fully established. Most tectonic geologists, for example, A. V. Peive, A. L. Knipper, and V. G. Kaz’min, believe that ultrabasic rocks are tectonic, detached masses of rock from the upper mantle, whereas many petrographers, in particular, V. N. Lodochnikov and the Americans H. Taylor and P. Wyllie, continue to develop the idea of the magmatic genesis of ultrabasic rocks.
Many types of mineral deposits are associated with ultrabasic rocks: asbestos and nephrite deposits, silicate deposits, and deposits of the ores of platinum, chromium, nickel, and alloyed iron.
REFERENCESPeive, A. V. “Okeanicheskaia kora geologicheskogo proshlogo.” Geotektonika, 1969, no. 4.
Wyllie, P. J. “The Origin of the Ultramafic and Ultrabasic Rocks.” Tectonophysics, 1969, vol. 7, nos. 5-6.
V. P. PETROV