Miocene–Pliocene Volcanism of Central Armenia: Geochronology and the Role of AFC Processes in Magma Petrogenesis

This paper presents the results from an isotope-geochronological and geochemical-petrological study of young volcanic rocks sampled in central Armenia, which were formed during the first stage of the Late Cenozoic magmatism in the Lesser Caucasus at the Miocene/Pliocene boundary. We have found the boundary of the area where the Miocene to Pliocene volcanism was evolving, determined its scale and total duration (~1 Ma, between 5.7 and 4.7 Ma), and identified phases of magmatic activity: I during 5.56 ± 0.18, II during 5.39 ± 0.04, and III during 4.86 ± 0.20 Ma. Petrologic data show that the volcanic rocks of central Armenia studied here compose a continuous compositional series from potassium trachybasalt to mugearite to (trachy)andesite to trachyte to rhyolite, while the leading process in the petrogenesis of their parent magmas had been fractional crystallization during the entire period of magmatic activity from the Late Miocene to the Early Pliocene. Crustal assimilation was of limited importance: some small influence of this process was only found for some late lavas that were erupted during phase III. The geochemical evolution of lavas during the earlier phases of activity was occurring unidirectionally from basic to acid compositions; the resumed eruptions of trachyandesites and andesites during phase III after a long intermission was caused by replenishment of mantle melts into magma chambers beneath the region. The entire data set for the area of central Armenia was analyzed to identify one volcanic unit that includes all Miocene to Pliocene magmatic rocks that are found there. The mantle source that was responsible for melt generation beneath the better known area of the Lesser Caucasus during the time period of interest was an OIB-type asthenospheric mantle that had been metasomatized by the subduction of the oceanic lithosphere from the Neotethys Basin.