CONDITIONS OF THE ULOR MASSIF OPHIOLITES FORMATION (SOUTH TUVA)

Geological and petrological studies have shown that the Ulor massif is an ophiolite association with an almost complete set of rocks characteristic of classical ophiolites: a tectonized dunite-harzburgite base complex + a layered dunite-wehrlitepyroxenite-gabbro series + a complex of upper gabbro and gabbro-dolerites. Petrochemical, geochemical and mineralogical studies indicate the formation of ophiolites of the Ulor massif in the ancient subduction zone with the development of marginal-marine basins with oceanic-type crust at the break of the paleocontinental granitoid crust. Data on mineralogy and thermobarogeochemistry indicate the presence in the ophiolites of the Ulor massif of dunites containing silicate multiphase inclusions in Cr-spinels, similar in a number of reliable features to melt inclusions, which is direct evidence of magmatogenic crystallization of ultrabasites. The analysis of the inclusions showed that high-Mg (MgO 20–30 wt.%) picrite magmas, which correspond in composition to the ultramafic rocks of layered ophiolite complexes, played an important role in the formation of dunites of the Ulor massif. Using an olivine-chromite thermometer [Coogan et al., 2014], it was found that dunites of the Ulor massif are dominated by fairly high crystallization parameters (1465–1300 °С), but moderate temperatures (1235–1070–980 °С) characteristic of subsolidus recrystallization are also determined ultramafic. The presence of clinopyroxene in the dunites of the Ulor massif made it possible to establish the PT conditions for the crystallization of this mineral using the WinPLtb program [Yavuz, Yıldırım, 2018]: 1250 °С, 6.5 kbar. Modeling of liquidus crystallization of melts with a picritic (20.61 wt %) MgO content using the PETROLOG program [Danyushevsky and Plechov, 2011] at a pressure of 6.5 kbar indicates that olivine was formed at temperatures of 1490–1455 °С, and chrome spinel, at 1360–1180 °С. Calculations of isobaric (6.5 kbar) equilibrium crystallization of a high magnesian (MgO – 20.61 wt %) melt using the COMAGMAT program [Ariskin, Barmina, 2004] show the massive formation of olivine starting from 1480°C, and clinopyroxene starting from 1320 °C. In general, based on information on the composition of minerals and melt inclusions in chrome spinels, the PT parameters of magmatic crystallization of minerals from dunites of the Ulor massif, which occurred under isobaric (6.5 kbar) conditions of the intrusive chamber, were established: olivine – 1490–1300 °С, chrome spinel – 1360–1180 °С, clinopyroxene – 1320–1185 °С. Magmatic processes were completed (1235–1070–980 °С) by subsolidus recrystallization of dunites. The further postmagmatic history of the development of the dunites of the Ulor massif was determined by plastic deformations and solid-phase flow, which led to a successive change of petrostructural types: protogranular → protogranular with porphyroclastesis → mesogranular → porphyroclastic.