Polymict breccia xenoliths: Evidence for the complex character of kimberlite formation

Very rare xenoliths of polymict breccia provide important information for understanding the nature, mechanism and conditions of kimberlite-forming processes. The purposes of this study are: a) to detail the petrology and mineralogy of the first find of a polymict breccia xenolith from the Precambrian Premier Mine, South Africa, and the same type of xenolith from the Middle Paleozoic Sytykanskaya pipe, Yakutia; and b) to estimate the possible relationship between the polymict breccias and kimberlite formation. A comparative analysis of extreme disequilibrium between the mineral phases leads to the conclusion that upper mantle material from a depth interval from 120 to 220 km is present in the studied xenoliths. A combination of compositional and petrographic features shows that initially the most deep-seated material from the base of the lithospheric mantle was intensely deformed under conditions of significant deviatoric stress, and then the first stage of partial melting produced a liquid fraction which served as a lubricant that enabled movement of the mixture of dominantly solid phases plus minor liquid from the base of the lithosphere to a shallower level in the lithospheric mantle. The absence of material of crustal origin or from the upper levels of lithospheric mantle in the studied polymict breccias leads to the conclusion that, before the transportation to the Earth surface, this complex mixture halted for a very short time (but enough for its consolidation) at a depth near 120 km. So, most probably the formation of polymict breccias was directly related to the initial phases of kimberlite formation and polymict breccias are a type of “underdeveloped embryo” of ordinary kimberlite.