The geochemistry and geochronology of Quaternary collision-related volcanism in the southern Lesser Caucasus: from development of the mantle source to magma generation and eruption

The Arabia-Eurasia continent-continent collision zone is unique on Earth for hosting widespread Quaternary post-collisional volcanism. This thesis explores the geochemistry of understudied volcanic rocks from the southern Lesser Caucasus mountains. It builds a holistic model of post-collisional magmatism: from how the magmatic source evolved before and after collision, the mechanism by which this source partially melted, and how magmas were able to ascend through the crust. The thesis also explores how aspects of this model might have changed over the duration of Quaternary volcanism. Major and trace element concentrations, as well as Sr-Nd isotopes are used to build a petrogenetic model for magmatism. Boron isotopes provide further information on the evolution of the mantle source. Ar-Ar ages show how the processes of magma ascent and eruption may have evolved over time. Amphibole and clinopyroxene geothermobarometry establish the pressure-temperature conditions of crystal fractionation.
Southern Lesser Caucasus magmas are not contaminated by assimilation of continental crust. The ubiquitous arc-like geochemistry reflects an inherited subduction component, which is dominated by sediment melts. Metasomatic amphibole stores the component in the lithosphere after collision, and then initiates melting following its heating-induced breakdown. Heating results from small-scale convective removal at the lithosphere-asthenosphere boundary and/or relaxation of non-linear geothermal gradients within the lithosphere. The parental magma produced is a 1% partial melt formed at depths close to the garnet-spinel transition (~80 km). Initial crystal fractionation in mafic magmas occurs over a narrow depth interval, probably in the mid-crust, at temperatures of ~1000˚C. Magmas then ascend from this reservoir, forming polygenetic (> 1 Ma) and monogenetic (< 1 Ma) volcanoes. The polygenetic to monogenetic transition occurred despite a consistent supply of magma, requiring an increased rate of local extension. Magmas ascend vertically, or along normal faults, with no dyke coalescence, preventing the formation of a stable magma plumbing system.