COMPOSITION OF PHLOGOPITE FROM ULTRAMAFIC LAMPROPHYRES AS AN INDICATOR OF FORMATION CONDITIONS (ZIMA ALKALINE-ULTRAMAFIC CARBONATITE COMPLEX, SOUTH OF THE SIBERIAN CRATON)
Yazgul Nugumanova 1, Anna Doroshkevich 1,2, Anastasiya Starikova 1, Anton Ponomarchuk1
1V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences Novosibirsk, Russia
2Geological Institute, Siberian Branch of the Russian Academy of Sciences Ulan-Ude,В Russia
Keywords: Aillikites, 40Ar/39Ar age of phlogopite, Reverse zoning, Polyphase inclusions
Abstract
The paper presents data on the age and composition of phlogopite from dykes of ultramafic lamprophyres (aillikites) of the Zima complex, located within the Urik-Iya graben, Eastern Sayan region, southern margin of the Siberian craton. Samples for study were selected from post-carbonatite dykes of the Bolshaya Tagna massif, Bushkanay dyke and Southern pipe. The studied samples consist of olivine macrocrysts and fully crystallized groundmass. The groundmass consists mainly of phlogopite, perovskite, spinel and apatite group minerals, carbonates and other minerals. Phlogopite is a typical groundmass mineral in the ultramafic lamprophyres of the Zima alkaline ultramafic carbonatite complex. Phlogopite was also found in polycrystalline inclusions of olivines and in polyphase inclusions of chromites. In the studied samples, phlogopite has tabular, zoned crystals; the size and composition of the phlogopite grains vary greatly. Phlogopite is mainly represented by anticrystals, with cores composed of ferruginous high-alumina phlogopite. Phlogopite cores of the Zima complex aillikites containing Al2O3 (14-18 wt.%), FeO (7-18 wt.%), TiO2 (3-6 wt.%) were formed from early portions of a more evolved aillikite melt. Phlogopite containing Al2O3 (10-14 wt.%), FeO (4-10 wt.%), TiO2 (1-2 wt.%) was crystallized from the parental magma. The age of formation of the Bolshaya Tagna aillikites, obtained from phlogopite, is 635±7 Ma (TGK 3). The age of phlogopite from the Southern pipe aillikites is 647±7 Ma (BZT 4/21). The obtained ages are consistent with the age of formation of Neoproterozoic alkaline-ultramafic carbonatite complexes of the Siberian craton and other occurrences of aillikites formed as a result of extension of the Rodinia lithosphere.
|
