PHYSICOCHEMICAL CONDITIONS OF CRYSTALLIZATION OF INTRUSIVE COMPLEXES OF THE ULTRABASIC MAGMATIC PROVINCE ON THE SIBERIAN PLATFORM

Abstract

The widespread distribution of ultramafic complexes makes it possible to distinguish a large-scale ultrabasic magmatic province on the Siberian platform. In this regard, studies of the conditions of magmatic systems responsible for the formation of ultrabasic rocks situated in this region are of great importance. It is well known that the study of melt inclusions can be of great help in solving these problems. The article is based on the data obtained during the study of melt inclusions in Cr-spinels, as well as on the analysis of coex-isting olivines and chromites. As a result of processing this information with the help of modern computational programs, PT-parameters of crystallization of ultramafic complexes of the North-East of the Siberian platform were established. The original data on inclusions in Cr-spinels from ultramafic massifs (Guli, Konder, Inagli and Chad), as well as from Triassic deposits of the North-East of the Siberian platform, were used. The analysis of the inclusions showed that the minerals of the ultramafic complexes of the Siberian Platform crystallized from alkaline melts, initially corresponding to high-Mg ultrabasic magmas, the subsequent evolution of which occurred with a decrease in the magnesium content (from picrites to basalts) with the formation of olivine cumulates in the intrusive chambers. Representative data on the composition of melt inclusions in Cr-spinels indicate that the magmatic systems that formed ul-tramafic complexes in the north (Guli massif and Triassic deposits of the North-East) and south (Konder, Inagli, Chad massifs) of the Siberian platform differ. In the first case, inclusions form a single group, which is in a good agreement with the calculated trend of the magmatic systems of the Guli massif. In the second, inclusions contain significantly more alkalis and have relatively abnormal contents of SiO2, Al2O3 and CaO. Computational modeling based on data on the composition of melt inclusions in Cr-spinels from ultramafic complexes of the Siberian Platform using modern programs and mineralogical thermometers indicates that liquidus crystallization of olivines began at 1545–1460 °C and chromites at 1430–1370 °C. Subsequently, the processes of mineral formation in the intrusive chambers occurred during the evolution of magmatic systems with a decrease in the formation parameters of olivines to 1240–1160°C and Cr-spinels to 1315–1275 °C.