First data on U-Pb isotope age of zircons from the harzburgites and chromitites of the Agardaga ultramafite massive (South Tuva)

For the first time, for the territory of Tuva, relict and epigenetic zircons from restitogenic harzburgites and chromitites of the Agardag chromite-bearing ultramafic massif were discovered and sold by U-Pb by the isotope method. An array located in the southeastern part of Tuva near Lake Shara-Nur, located on the southwestern flank of the ophiolite South Tuva mafic-ultramafic area. The lens-shaped body of the massif has a length of about 20 km with a maximum width of 4 km. Its long axis is oriented in a northeast direction. The massif has steep (75-80° in NW) tectonic contacts with host metaterrigenous-volcanic Riphean-Lower Cambrian (?) Formations and is considered by us as a protrusion. The massif is composed to varying degrees with serpentinized harzburgites and dunites, as well as antigorite and more rare antigorite-chrysotile serpentinites. In the near-contact zones of the massif, serpentinites are intensively schistose. The massif is intruded with gabbro rods, gabbro-diorites, gabbro-diabases, diabase and basalt porphyrites stocks and dikes. Zircons isolated from large-sized (~ 20 kg) samples of harzburgites and chromitites are represented by relict and epigenetic genetic varieties. Their short-prismatic crystals sometimes have rounded edges due to resorption, a relatively low intensity of the cathodoluminescent glow to its complete absence, and also often disturbed oscillatory zoning. The sub-concordant and concordant values of the U-Pb isotope age of relict zircons range from 885-392 Ma. It is assumed that these age variations are associated with a partial diffusion loss of radiogenic lead, which caused the uneven “rejuvenation” of the U-Pb isotopic systems of very ancient juvenile zircons located in the upper mantle protolite during its partial melting with the formation of harzburgite and dunite restites, as well as chromite deposits associated with them . The observed close values of the isotopic age of zircons from harzburgites and from chromitites suggest that both formed approximately at the same time. A few zircon grains from harzburgites, which showed concordant age values in the range of 293-276 Ma, are considered by us as an epigenetic phase. Their formation, as expected, was caused by the infiltration of fluids, which were released by melts, which formed later granitoid intrusions, cutting the ultramafic massif, and also framing it with metaterigenic-volcanogenic formations. Evidence of the infiltration of such fluids is found in the chromitites of the mass of Uvarovite - Kemmerite veins, the formation of which requires the addition of silica.

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