Petrographic and petrogeochemical features of harzburgites and dunites of the Agardag and Ulor massifs (South-Eastern Tyva)

Abstract

The objects of study are the Agardag and Ulor ultramafic massifs, which are part of the South Tuva ophiolite belt. The aim of this work is to study the petrographic, petrochemical and geochemical features of harzburgites and dunites of the studied massifs. As a result of petrographic study, the main types of rocks that make up the studied massifs were identified. Harzburgites predominate in the Agardag massif, while dunites are more common in the Ulor massif. In the Ulor massif, dunites and harzburgites are often plastically deformed by syntectonic recrystallization with the formation of porphyroclastic structures. The chemical compositions of olivine and Cr-spinels in harzburgites and dunites in both massifs are similar. The composition of olivine corresponds to forsterite, while the ultramafic rocks of the Ulor massif show large variations in Fa and a tendency towards a decrease in NiO. Chromium spinels in ultramafic rocks of the Ulor massif have wider compositional variations and are represented by chromites, alumochromite and subferrial alumochromite; they are completely free of V2O5. According to the composition of Cr-spinels, the degree of melting of ultramafic rocks was determined, which turned out to be similar for both massifs, ~40–50%. A higher chromium content of the ultramafic rocks of the Agardag massif is noted, which is probably responsible for its increased potential chromite content. Ultramafic rocks of the Agardag and Ulor massifs are intensely depleted formations that arose in similar conditions, which determines the closeness of their rock and material compositions. The revealed differences are due to variations in the degree of depletion and the conditions of their consolidation in the earth’s crust, determined by the structural position, as well as metamorphism of the enclosing frame and the introduction of later intrusions.