Geochemical thermometry of rocks of the Talnakh intrusion: Assessment of the melt composition and the crystallinity of the parental magma

Techniques of geochemical thermometry with the use of the COMAGMAT computer program were utilized to assay the phase and chemical composition of the p arental magma of the Talnakh intrusion and the melt-crystal mixtures that p roduced the taxitic and picritic gabbro-dolerites of the lower zone of the massif. These results point to the cotectic (Ol + Pl) composition of the Ta lnakh magma, which was intruded at a temperature of approximately 1200 degr eesC and contained 10-15% intratelluric phenocrysts, predominantly Ol (7-11 %). The composition of the parental magmatic melt (the liquid constituent o f the magma) corresponded to tholeiitic ferrobasalt with somewhat higher co ncentrations of MgO (similar to8 wt %) and K2O (similar to0.7 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, whic h are preserved as intercumulus material in the rocks of the taxitic and pi critic units. The compositional variations of rocks in the lower part of th e Talnakh intrusion are explained as resulting from variations in the propo rtions of intratelluric Ol and PI crystals and the intercumulus liquid. Thi s validates the conclusion that the taxitic and picritic gabbro-dolerites w ere produced by a single parental magma of the tholeiitic type. The composi tion of its liquid can be quite well approximated by that of the contact ga bbro-dolerites. The calculated melt composition is close to that of the tho leiitic basalts of the late stage of flood-basalt magmatism in the Siberian Platform (Mokulaevskaya Formation).