Formation of the trachybasalt–trachyte bimodal series of the Malo-Khamardaban volcanotectonic complex, southwestern Transbaikalia: Role of fractional crystallization and magma mixing

Abstract

The Malo-Khamardaban volcanogenic association is one of the largest and typomorphic structures for the Jurassic–Cretaceous rift magmatism of the Western Transbaikalia. It is mainly made up of bimodal volcanogenic trachybasalt–trachyte series with subordinate amount of trachyandesites, trachyrhyodacites, and trachyrhyolites. Geochemical modeling gives grounds to conclude that the formation of trachytic melts was controlled by crystallization fractionation of trachybasaltic magma. Mineralogical–petrographic, mineralogical–geochemical, and geochemical features of trachyandesites and some syenite porphyries (subvolcanic analogues of trachytes) indicate their hybrid nature. The trachyandesites were formed by mixing of trachybasaltic magma with trachytic magmas (endohybridisation) and trachybasaltic magma with contaminated trachytic magma. Hybrid syenite porphyries were derived by contamination of trachytic magma with host rocks (Late Paleozoic granitoid rocks of the Zaza complex). It is suggested that the trachybasaltic magma prior to generation of daughter trachytic melt and contamination of the latter by host granitoids evolved in a zoned magma chamber, where differentiated salic melt of the apical part interacted with the framework rocks. The hybrid trachyandesite magma was produced by mixing of basaltic and trachytic magmas during their simultaneous injection in one magma chamber.