THE SOUTH TIEN SHAN BELT OF DIAMONDIFEROUS ALKALINE BASIC ROCKS

Abstract

The study of alkaline (subalkaline) basic rocks in the central southern Tien Shan (Tajikistan) revealed some specific features of their diamond potential. The region incorporates approximately 90 diatremes of various Lower Mesozoic alkaline basic rocks with mantle nodules. Mantle inclusions are mainly composed of rocks of the spinel and garnet–spinel facies (16–22 kbar, 1000–1170°C). The rocks are characterized by a high rate of depletion and abundance of mantle-related metasomatism (fluid-rich magmatism) with the formation of potassic ultrabasic rocks (pyroxenites, glimmerites, anorthoclasites, and others) and high-pressure metacrystals of phlogopite, K-kaersutite, sanidine, Al-augite, chrome diopside, apatite, spinel, and other minerals, which are products of the disintegration of mantle pegmatites. Some rock bodies include chromites (Cr2O3 up to 58–60%), moissanite, native metals, and sulfides (pyrrhotite and others). The magnetite content is low. Some xenoliths contain graphite. The frequency of its occurrence in concentrates of host basic rocks (43–49%) indicates their crystallization in a carbon-saturated reducing environment.