PECULIARITIES OF COMPOSITION EVOLUTION OF DEPLETED PERIDOTITES OF THE ANCIENT CRATON LITHOSPHERE ROOTS UNDERGONE OF SUBLITHOSPHERIC METASOMATIC AGENTS INFLUENCE.

N.P. Pokhilenko, A.M. Agashev, L.N. Pokhilenko

V.S. Sobolev Institute of Geology and Mineralogy, SB RAS, Novosibirsk

Corresponding author: N.P. Pokhilenko:  chief@igm.nsc.ru

 

Detail study of the most deep-seated peridotite xenoliths from the Udachnaya and Sytykanskaya kimberlites showed a sharp signs of influence upon these rocks of different type of metasomatic agents of sublithospheric origin. Initial stage of these rocks evolution was related with high and ultrahigh degrees (up to 70 %) of primitive mantle partial melting in the Middle Archean time. These processes were related with the period of intense increase of the lithosphere thickness reaching values of 150-160 km for the Siberian platform and Kapvaal craton to the Middle Archean time. Lithospheric mantle consisted mostly of refractory residua of the primitive mantle material partial melting – Cr-pyrope harzburgites having relatively low heat conductivity and heat capacity that  provided low values of heat flow inside the lithospheric mantle and its rapid cooling. After cooling of depleted peridotites by 300-400^oC at depth corresponding the diamond stability  field these rocks were undergone of influence of insignificant amount of infiltrating carbonattite melts. This process was responsible for formation of main part of diamonds in lithospheric mantle, and significantly changed some geochemical features (increase some incompatible elements, La/Yb ratio) but weakly influenced upon chemical composition of the rocks. Maximum degree of this type of influence resulted in definite increase of CaO potential in system and in wehrlitization of harzburgites: in conditions of SiO₂ deficit enstatite replaced by Cr-diopside and after full disappearance of enstatite started increase of Ca-component content in garnet (up to 75 mol. %).  Influence of alkaline basanitic melts upon the modified by carbonatitic metasomatism depleted peridotites was related with multistage interaction of upper parts of low mantle plumes with the lithosphere roots and was reflected in variable basification of the lithosphere roots substratum.