GEOCHEMICAL FEATURES OF DIKE AILLIKITES AND ALKALINE ROCKS OF THE BOLSHETAGNINSKY MASSIF (URIK-IYA GRABEN, EAST SAYAN REGION)

Abstract

The ICP-MS determinations have been made on microelement content of ~650–640 Ma ultramafic lamprophyre-aillikite dikes and alkaline silicate rocks and carbonatites from the Bolshetagninsky ijolite-syenite massif, spatially overlapped in the northern Urik-Iya graben on the southwestern margin of the Siberian craton. There have been identified two types of spectra of microelement distribution, typical of the Bolshetagninsky massif aillikites and alkaline silicate rocks, respectively; both types of spectra demonstrate significant (more than two orders of magnitude) enrichment in most incompatible elements relative to the primitive mantle. Aillikites have tilt-left distribution spectra of rare elements with Rb, К , Pb, Sr-P, Zr-Hf, ±U minimums and less-pronounced Y minimum. Multi-element spectra of ijolites-melteigites are characterized by Th, Та , Pb, Hf, ±Zr, ±Ti minimums which is also reflected in nepheline syenite and carbonatite spectra. The Bolshetagninsky massif aillikites and alkaline rocks differ also in Nb/Ta, Zr/Hf, Th/Nb, Th/U ratios. Geochemical differences imply that the parental melts of dike aillikites and alkaline rocks are derived from different mantle sources.