Derivation of syenite by fractionation of alkali basalt magma: An example from the Oshurkovo pluton, transbaikalia

The Oshurkovo pluton is an igneous body, where the dominant role of crystal fractionation in the generation of syenite magma was recognized with confidence. It is composed of gabbro-syenite series rocks ranging from melanocratic alkali gabbro to syenite and alkali-feldspar syenite. Approximately 70 vol % of the pluton is alkali gabbro, and 30 vol % is syenite. An important mineralogical feature of the rocks of the massif is the occurrence of abundant ternary feldspar (Tfs) and high contents of apatite, up to 10 vol % in the melanocratic gabbro. The amphibole and ilmenite-magnetite geothermometers yielded liquidus temperatures for the basic magmas of approximately 1030-1050°C, which resulted from high water contents of the melt, about 4-6 wt %. The syenite magmas began to crystallize at T ∼ 900°C (Tfs thermometry). The rocks of the series crystallized under a range of oxygen fugacity, from a level corresponding to the buffer NNO to the buffer HM and higher; the value of -logf_O2 in the gabbro varied at 9-12. There are definite geological, mineralogical, and geochemical lines of evidence that the gabbro-syenite series formed through crystal fractionation of a basic magma, chemically similar to the mesocratic alkali gabbro. Mass balance calculations suggest that the leucocratic gabbro was formed by the fractionation of approximately 20% of Prg, Ap, and Ti-Mag. The syenite magmas were formed by the extraction of approximately 40% of Prg, Pl, Ap, and Ti-Mag from the melt chemically similar to the leucocratic gabbro. Crystallization differentiation occurred in magma-feeding reservoirs but not in the pluton.