The role of fractional crystallization in the formation of a bimodal trachybasalt-trachyte series: Malo-Khamardabanskaya volcano-tectonic structure, Transbaikalia

The origin of subalkaline bimodal series are discussed on the basis of the data analysis on the geology and geochemistry of the Malo-Khamardabanskaya volcano-tectonic structure (100 by 18 km), which is situated south of Lake Baikal. The majority of volcanic rocks in this structure are trachybasalts and trachytes, with the approximate proportion of 10:1. Basaltic trachyandesites comprise no more than 2% of the bulk volume of the volcanic sequence. The numerous subvolcanic bodies are made up by various rocks, from trachydolerite to syenite porphyry and comendite, among which syenitoids are predominant. The rocks were dated at 167 - 150 Ma by the Rb-Sr and K-Ar methods. Mineralogical, geochemical, and isotopic data suggest that fractionation was the dominating process responsible for the formation of the rock series: olivine-normative basalt → quartz-normative basalt → trachyte (syenite porphyry) → microgranosyenite. This conclusion is in complete agreement with the results of geochemical modeling. An abrupt increase of oxygen fugacity in an intermediate magma chamber was probably the main reason for the appearance of a gap in the differentiation sequence, which manifests itself by the formation of trachyte residual melts directly from the basic magma. The value of -logf_O2 changed by two units, from 8.7 - 9.8 to 6.3 - 7.9, probably increasing the amount of the crystallizing Fe-Ti oxides substantially. Their fractionation, along with plagioclase and clinopyroxene, resulted in a considerable increase of the abundances of silica and alkalis in the residual melts, and the formation of trachyte magmas.