Unusual mineralogical features and origin of the mantle substratum Nakyn kimberlite fields (Yakutia)

Detail study kimberlites and mantle xenoliths from Nakyn field pipes has revealed their unusual, interesting and important mineralogical features. Absence of Megacrystic picroilmenites of is compensated by presence of large orange-red titanium pyropes of "megacryst" type, underlining the reduced character asthenospheric melts influences on the mantle lithosphere in Nakyn. Picroilmenite in Nakyn kimberlites present only in xenoliths eclogites, garnet peridotites and clinopyroxenites with directive structures attributed to zones of melt fluid interaction. The clinopyroxene composition referred to Cr-omfacite, c (instead of Cr-diopside) suggest the Na-Al oceanic spilitic metasomatism at subduction stage or later interaction of the mantle material with the subducted pelitic sediments which is in accord with the presence of Al-rich eclogites wide distribution of the wehrlitic associations may suggest carbonatitic metasomatism. Cr- diopsides occurred in the peridotites with primary magmatic textures.

Absence of picroilmenite megacrysts in Nakyn kimberlites is filled with presence of large orange-red titanious-pyropes of "megacryst" associations, underlining the reduced character astenospheric influences on the mantle substratum of area

Picroilmenites in Nakyn kimberlites are present only in xenoliths of eclogites, and garnet peridotites and clinopyroxenites with, directive structures related to the zones of the metasomatism or melt interaction. The picroilmenite compositions from these rock inclusions sharply differs from composition of picroilmenite typical "megacryst" associations the raised contents of the titanium and the lowest share hematite component. In the same types mantle rocks is unusual also the composition of clinopyroxene: omphacite, chrome-omphacite (but not chrome-diopside) suggesting the high activity of the Na-Al metasomatism probably related to the oceanic spilitic metasomatism. The important participation in their formation of subduction processes allows to assume the specific features of a structure, mineral composition and composition of minerals of these rock inclusions.

Th ALCREMITE and MARID associations probably refer to the interaction of the lamprophyric Al2O3, H2O rich melts with peridotites or interaction of mica bearing Al, alkali rich sediments with peridotites. . The Botuobinskaya and Mayskaya kimberlite pipes contain essential amount of color a green garnets of different shades and compositions, that are very rare in worldwide kimberlites. It specifies on intensive influence of processes "calcium" (“chrome-calcium” and the “titanium–chrome-calcium”) metasomatism in mantle lithosphere