GEOCHEMICAL THERMOMETRY OF ROCKS OF THE TALNAKH INTRUSION: ASSESSMENT OF THE MELT COMPOSITION AND THE CRYSTALLINITY OF THE PARENTAL MAGMA

Abstract

Techniques of geochemical thermometry with the use of the COMAGMAT computer program were utilized to assay the phase and chemical composition of the parental magma of the Talnakh intrusion and the melt–crystal mixtures that produced the taxitic and picritic gabbro-dolerites of the lower zone of the massif. These results point to the cotectic (Ol + Pl) composition of the Talnakh magma, which was intruded at a temperature of approximately 1200°C and contained 10–15% intratelluric phenocrysts, predominantly Ol (7–11%). The composition of the parental magmatic melt (the liquid constituent of the magma) corresponded to tholeiitic ferrobasalt with somewhat higher concentrations of MgO (~8 wt %) and K O (~0.7 wt %) than in the “normal” flood basalts in the Siberian Platform. This composition was determined to be analogous to the composition of the residual liquids, which are preserved as intercumulus material in the rocks of the taxitic and picritic units. The compositional variations of rocks in the lower part of the Talnakh intrusion are explained as resulting from variations in the proportions of intratelluric Ol and Pl crystals and the intercumulus liquid. This validates the conclusion that the taxitic and picritic gabbro-dolerites were produced by a single parental magma of the tholeiitic type. The composition of its liquid can be quite well approximated by that of the contact gabbro-dolerites. The calculated melt composition is close to that of the tholeiitic basalts of the late stage of flood-basalt magmatism in the Siberian Platform (Mokulaevskaya Formation).