Composition of Phlogopite from Ultramafic Lamprophyres as an Indicator of Formation Conditions (Zima Alkaline Ultramafic Carbonatite Complex, Southern Siberian Craton)

Abstract

Abstract ––The paper presents data on the age and composition of phlogopite from dikes of ultramafic lamprophyres (aillikites) of the Zima Complex located within the Urik–Iya graben in the East Sayan region (southern margin of the Siberian craton). Samples for study were taken from postcarbonatite dikes of the Bol’shaya Tagna intrusion, Bushkanai dike, and Yuzhnaya volcanic pipe. The studied samples consist of olivine macrocrysts and fully crystallized groundmass composed mainly of phlogopite, perovskite, minerals of spinel and apatite groups, carbonates, and other minerals. Phlogopite is a typical groundmass mineral in the ultramafic lamprophyres of the Zima alkaline ultramafic carbonatite complex. It was also found in polycrystalline inclusions in olivines and in polyphase inclusions in chromites. In the studied samples, the groundmass phlogopite has tabular zoned crystals of greatly varying size and composition. The phlogopite grains show mainly a reverse zoning, with the cores composed of ferruginous high-alumina phlogopite. The intricate zoning probably resulted from the mixing of two magmas. The cores of phlogopites of the Zima Complex aillikites (with Al2O3 = 14–18 wt.%, FeO = 7–18 wt.%, and TiO2 = 3–6 wt.%) formed from early portions of a more evolved aillikite melt. Phlogopite with Al2O3 = 10–14 wt.%, FeO = 4–10 wt.%, and TiO2 = 1–2 wt.% crystallized from the parental aillikite magma. The phlogopite age of the Bol’shaya Tagna aillikites is 635 ± 7 Ma (TGK 3). The age of phlogopite from the Yuzhnaya pipe aillikites is 647 ± 7 Ma (BZT 4/21). The two ages are consistent with the time of formation of Neoproterozoic alkaline ultramafic carbonatite complexes of the Siberian craton and of other occurrences of aillikites resulted from the extension of the Rodinia lithosphere.