Physicochemical parameters of ultrabasic magmatic systems of the northeastern Siberian Platform (data on melt inclusions in chromian spinel)

Experimental studies and analysis of silicate inclusions testify to the magmatic genesis of part of chromian spinel from the Triassic deposits of the northeastern Siberian Platform. The compositions of melt inclusions in the chromian spinel show the participation of alkaline (potassic) magmas in their crystallization. Most data indicate the presence of magmatic systems similar to melts in the Guli ultrabasic massif in the northern Siberian Platform. Studies of the distribution of trace and rare-earth elements in the melt inclusions show the existence of several magmatic systems. These are, first of all, magmas that formed Guli-type ultrabasic massifs and gave rise to meimechites and picrites in the Maimecha-Kotui province. There are also plume-related magmatic systems with kimberlites, lamprophyres, and/or other continental "hot spots". The composition of the melt inclusions suggests the existence of several types of the primary sources of chromian spinel in the northeastern Siberian Platform, which confirms the earlier data on the heterogeneous composition of the deposits of the diamondiferous Carnian (Upper Triassic) Stage. Applying computer modeling with the well-known COMAGMAT, PETROLOG, and WinPLtb programs as well as the Ol-Sp geothermometers based on the melt inclusions in chromian spinel from the Triassic deposits of the northeastern Siberian Platform, we have determined the P-T conditions of crystallization of minerals in the igneous rocks being the sources of the examined chromites. The temperature of liquidus crystallization of chromian spinel is 1324-1275 degrees C. The P-T conditions of formation of olivine and clinopyroxene inclusions in it are estimated at ca. 4.5-4.1 kbar, 1510-1150 degrees C and 3.2-1.0 kbar, 1285-1200 degrees C, respectively.