Mineralogical features of ultramafits of the Agardag massif (South-Eastern Tyva)

The Agardag massif is composed mainly of restite ultramafic rocks of the dunite-harzburgite banded complex, which is the lower part of the ophiolite cover. The most depleted are dunites relative to harzburgites, which is reflected in changes in the olivine composition. From harzburgites to dunites in olivine, there is a decrease in the contents of silica and, accordingly, an increase in its magnesia. At the same time, the compositions of Cr-spinels from alumochromites to chromites also change from harzburgites to dunites. According to the composition of enstatite, it was established that the initial harzburgites were formed at pressures of 4-12 kbar and temperatures (1 050-1 170 °C). Dunites and harzburgites of the Agardag massif are cut through by gabbroid dyke bodies. Under their influence, as a result of high-temperature metasomatic processes, ultramafic rocks in the exocontact zones of dikes underwent uneven clinopyroxenization with the formation of wehrlites and clinopyroxenites. Such processes of clinopyroxenization of ultramafic rocks are quite common among meta-morphic peridotites of ophiolite complexes. Clinopyroxenization processes contributed to the change in the composition of minerals. It was found that the composition of olivine in wehrlites corresponds to the composition of dunites; however, with intense clinopyroxeni-zation in clinopyroxenites, a significant increase in the iron content of olivine and a decrease in its magnesia occur. The composition of Cr-spinels also changes significantly, and their alumina and iron content increases. Moreover, their composition corresponds to subferri-al alumochromites. Similar changes in the compositions of olivine and Cr-spinels were found in the Kyzyr-Burlyuk massif in the Western Sayan. The regressive stage of the massif evolution begins with serpentinization of ultramafic rocks, mainly with the replacement of olivine by looped chrysotile, which was accompanied by the "liberation" of iron and its separation into an independent phase in the form of finely dispersed magnetite. The resulting chrysotile is characterized by a minimal iron content. At the same time, enstatite in harzburgites was replaced by bastite. Serpentinized ultramafic rocks in tectonically active zones, contributing to their heating, often underwent progressive metamorphic transformations in the form of serpentine dehydration with the formation of regenerated olivine in the serpentinized olivine rock mass. Newly formed olivine differs in its composition from the original rock-forming one by a significant decrease in iron index. At this stage, the pseudomorphs of bastite after enstatite underwent progressive metamorphogenic transformations. The following sequence of their transformation is established. At the initial stages, the formation of acicular tremolite in basite along the initial cleavage of enstatite is noted. Then, as the temperature rises, diopside of a long-prismatic shape appears, also controlled by the initial cleavage. And the process ends with the formation of prismatic olivine along the initial cleavage. At the same time, newly formed minerals are characterized by distinctive features of their composition. Tremolite has a very high magnesia. The newly formed diopside is also characterized by a very high magnesia, high CaO contents, low Fe, and low SiO2 and Al2O₃ contents. In their composition, they differ markedly from the composition of diopside in rocks of the wehlite-clinopyroxenite association. It has been established that secondary olivine, based on bastite pseudomorphs, is characterized by the maximum magnesia in relation to secondary olivine in the groundmass of the rock. At the post-metamorphic stage, the ultramafic rocks of the massif, apparently under the influence of later intrusions of granitoids, underwent extensive serpentinization, contributing to the intensive formation of antigorite, often before the formation of serpentinites. At the same time, antigorite in its composition is distinguished by a higher iron content in relation to early chrysotile.

Keywords

Authors

References