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-Khamard abanskaya volcano-tectonic structure (100 by 18 km), which is situated south of Lake Baikal. The majority of volcanic rocks in this structur e 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 m ade up by various rocks, from trachydolerite to syenite porphyry and c omendite, among which syenitoids are predominant. The rocks were dated at 167 - 150 Ma by the Rb-Sr and K-Ar methods. Mineralogical, geochem ical, and isotopic data suggest that fractionation was the dominating process responsible for the formation of the rock series: olivine-norm ative basalt --> quartz-normative basalt --> trachyte (syenite porphyr y) --> microgranosyenite. This conclusion is in complete agreement wit h the results of geochemical modeling. An abrupt increase of oxygen fu gacity in an intermediate magma chamber was probably the main reason f or the appearance of a gap in the differentiation sequence, which mani fests 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 crystalliz ing Fe-Ti oxides substantially. Their fractionation, along with plagio clase and clinopyroxene, resulted in a considerable increase of the ab undances of silica and alkalis in the residual melts, and the formatio n of trachyte magmas.