Evolution of physico-chemical conditions of crystallization of melts during formation of dunite-peridotite-gabbroic massifs of the Eastern Sayan

Abstract

© 2019 Tomsk Polytechnic University, Publishing House. All rights reserved. The relevance of the research. The study of melt inclusions in minerals is one of the most reliable ways of obtaining direct information on the nature of magmatic processes, allows building a model of formation of intrusions and tracing the evolution of physico-chemical conditions of crystallization of magmatic complexes. The main aim of the research is to prove the magmatic origin of rocks of this massif, which makes it possible to consider the distribution of noble metals in melts, based on the available experimental data. Objects: melt inclusions in Cr-spinel from dunites and wehrlites Medek basite-ultrabasite massifs. Methods. The search for melt inclusions was carried out in accessory Cr-spinel sample isolated from dunites and wehrlite, to determine the nature of the high temperature experiments. For this purpose, the samples were taken (about 50-100 grains of Cr-spinel fraction 0, 2-0, 5 mm) and placed in a graphite microcontainer with internal dimensions in the first millimeters. Micro containers were tightly closed with graphite covers. Thus, the Cr-spinel in these containers were at temperatures above 1000°C under reducing conditions as a result of the reaction of air oxygen with graphite. The experiments were carried out at high temperatures on the basis of existing methods of investigation of melt inclusions. For maximum transformation of the melt in inclusions into homogeneous glass, quenching into water was carried out. The choice of the temperature regime (1280-1300°C) was determined primarily by the fact that the main task of high-temperature experiments was to melt the contents of inclusions and to obtain glass during hardening, which was then analyzed on a microprobe and a scanning microscope. Result. Conducted research of homogenised melt inclusions glass showed that crystallization of Cr-spinel and containing wehrlite of the ultrabasic Medek massif (East Sayan) were involved in low potassium picrobasalt of melts of normal alkalinity, of the most similar petrochemical characteristics to the data on melt inclusions in Cr-spinel from oceanic complexes. The calculations were made for various mineralogical barometers and different models of crystallization on the basis of compositions of glasses of heated melt inclusions in Crspinel in the COMAGMAT program, which showed that the pressure in formation of massive rocks was about 4 kbar (2, 8-4, 7 kbar); the combined use of mineralogical thermometers and calculations on the basis of glass compositions of heated melt inclusions in Cr-spinel according to PETROLOG and COMAGMAT programs allows us to speak about the formation of dunites from picritoids (MgO 17, 85 wt. %) of the melt, since the crystallization of olivine at 1380°C, the formation of wehrlite occurred mainly from picrobasalt (MgO 13, 36 wt. %) of the melt with the crystallization of olivine starting from 1280-1275°C, but in much smaller quantities than clinopyroxene, the intensive formation of which was at lower temperatures (starting from 1235-1230°C); a comparative analysis of the results of computational modeling of melt and mineral compositions with data on melt inclusions and real olivines and clinopyroxenes indicates their consistency, as evidenced by the fact of the location of melt inclusions along the calculated trends of melt change, that is, experimental data on inclusions on the one hand confirm the reliability of the calculations, and on the other - this ratio shows that the inclusions are not an accident, but record the natural and theoretically justified evolution of the melt.