SPECIFIC STRUCTURE OF THE LOWERMOST EARTH'S CRUST IN THE ZAGADOCHNAYA KIMBERLITE PIPE (YAKUTIA)

Abstract

Study was given to xenoliths of the crustal rocks from the Zagadochnaya kimberlite pipe, in the Daldyn-Alakit diamondiferous district of the Yakutian kimberlite province. Most xenoliths are garnet granulites (60% of the bulk of xenoliths, of which paraganeses with amphibole amount to 20%), plagiogneisses (30%), and garnet-biotite schists (10%). The PT-conditions of metamorphism for garnet granulites are 700-780 °C and 8-11 kbar. The temperatures obtained forplagiogneisses are within 660-770°C, and for garnet-biotite schists, 500-600 °C. Densities and compressional wave velocities (Vρ) were calculated for the studied garnet granulites and plagiogneisses. Values of Vρ for garnet granulites, which, according to P and T, occur in the lower Earth's crust, are 7.7-8.0 km/s, i.e., much higher than the velocities recorded by geophysical methods (6.7-6.9 km/s). At the same time, the velocities for plagiogneisses, 6.7-6.8 km/s, are close to experimentally measured velocities. This discrepancy in values of Vρ may indicate the inhomogeneity of the medium between the boundaries of K2 and Moho. Given the composition of the lower crust has not changed since the time of intrusion of kimberlite pipes, we must admit that the lower crust is made up of plagiogneisses with lenses of garnet granulites. The section of the Earth's crust in the Zagadochnaya pipe is as follows: The lowermost crust is made up of plagiogneisses with interbeds (lenses) of garnet granulites, and upsection garnet-biotite schists occur.