Petrofabric and geochemical features of ultramafic rocks on the example of restite metamorphites of the Kuznetsk Alatau (Western Siberia), olivine cumulates of the Yoko-Dovyren layered massif (Northern Cisbaikalia) and their analogues from ultrabasic xenoliths of the Canary Islands (Spain)

Abstract

The article presents the results of petrostructural and mineralogical studies of olivine grains from ultrabasic and basic rocks of different genesis. In particular, they correspond to cumulative dunites of the Yoko-Dovyren layered massif (Northern Cisbaikalia), restite hyperbasites of the Velvet massif (Kuznetskiy Alatau), and xenoliths of peridotites from basalts of the Canary Islands (Spain). The relationship between the petrostructural and mineralogical features of ultrabasic rocks is shown, which makes it possible to identify their cumulative and restite varieties. An important element of the petrostructure of ultrabasites is the orientation of olivine, which reflects either the conditions for the formation of primary crystals in the magmatic melt, or the conditions for its recrystallization as a result of plastic deformations during exhumation to the Earth's surface. The mineral composition of rocks is an additional feature that reflects the real conditions of formation. In the presence of basic plagioclase, it is already quite difficult to speak about the restitic nature of these ultrabasic rocks. On the other hand, plastic deformations of olivine are characteristic of restitic hyperbasites, in which plagioclase is absent. The results of mineralogical studies in ultrabasic xenoliths of the Canary Islands showed the presence of basic plagioclase (labradorite), as well as chrysolite-type olivine (12-16 Fa), which corresponds to the attributes of ultrabasic layered intrusions such as the Yoko-Dovyren dunite-troctolite-gabbro pluton. For restite hyperbasites (by the example of the Kuznetsk Alatau), the iron content of olivine does not exceed 9-10% of the fayalite end, while plagioclase and clinopyroxene are absent. A characteristic feature of the xenoliths of the Canary Islands is the presence of endiopside, which is present in layered intrusions, but is absent in the restrained fragments of the upper and lower mantle. It is assumed that the xenoliths in the basalts of the Canary Islands are not of a mantle nature, but are fragments of a deep magma chamber. The studies of the optical orientation of olivine in xenoliths of the rocks of Lanzarote Island confirm these conclusions. The geochemical parameters of ultramafic xenoliths on Tenerife Island may well correspond to deeper formations.