COMPOSITION OF SPINEL GROUP MINERALS FROM LATE PROTEROZOIC ULTRAMAFIC LAMPROPHYRES OF THE BOLSHETAGNINSKII ALKALINE ULTRAMAFIC CARBONATITE COMPLEX LOCATED AT THE URIK-IYA GRABEN, EASTERN SAYAN REGION

Abstract

This paper provides the results on spinel sampled from ailikites of the Bolshetagninskii massif of the East Sayan region. The study aimed to determine the history of formation of ultramafic lamprophyres of the Ziminsky complex. Spinel from three dikes of the Bolshetagninskii massif with different mineral compositions of the groundmass but similar textural and structural features (BTG 2/21, TGK 3, BTG 6/21) was analyzed. The content of the main elements and microcomponents in spinel was determined using electron microscope and microprobe. All spinel crystals are zonal, the following types have been identified with regard to the composition: chromites (Chr), aluminum-chromites (Al-Chr), chromium magnetites (Cr-Mgt), titanomagnetites (Ti-Mgt) and magnetites (Mgt). The composition of spinel from sample BTG 2/21 of ultramafic lamprophyres of the Bolshetagninskii massif shows the following trend: chromite→ aluminum-chromite→ chromium magnetite→ titanomagnetite→ magnetite. Sample TGK 3 contains spinel with compositional change from chromite→ chromium magnetite. Chromite cores from sample BTG 6/21 are overgrown with chromium magnetite, as well as titanomagnetite.We assume the primary melt, from which the primary chromite crystallized, to be the same for all samples studied. After chromite formed from the same melt and possibly in the same chamber, each melt portion experiences its own history of formation as evidenced by spinel study.