Deciphering the Tectonics of the Caucasus from Post-Collisional Volcanism

Bewick, Samuel (2017). Deciphering the Tectonics of the Caucasus from Post-Collisional Volcanism. PhD thesis The Open University.



The southern margin of the Eurasian plate is a well-studied region of continental collision. The Caucasus mountains lie at the centre of the Alpine-Himalayan orogenic belt, and differs from their better studied neighbours by the presence of intense, largely mantle derived post-collisional magmatism. Volcanism began 5-15 Ma after the initial collision occurred, and after the crust had already thickened to 45-60 km. This thesis presents bulk rock and mineral element and isotope geochemistry to assess the role of asthenosphere, lithosphere and crust in generating contemporaneous volcanism across the region. Broad similarities are seen between volcanism in the Greater and Lesser Caucasus, dominated by calc-alkaline basalt-dacite (48-72 wt% SiO2) compositions. High Mg# cores (>85) of olivines suggest significant fractionation has occurred. Trace element patterns show a distinctive supra-subduction setting, suggesting the fluid-enriched source remained in place, and hydrated >10 Ma following collision. Isotope ratios require an asthenospheric component in the early Lesser Caucasus melts, whilst sediment-enrichment of the source is observed across the region. Little interaction with the thickened crust is observed. For the Lesser Caucasus, a model of Palaeo-Tethyan slab breakoff is proposed, resulting in asthenospheric upwelling and melting of an enriched lithosphere. Small-scale convection continues to produce drips, leading to episodic pulses of magmatism. Subduction of back-arc crust with slab breakoff is proposed beneath the Greater Caucasus, allowing melting of enriched lithosphere, with distinguishable fluid, and slab-melt components. Although contemporaneous, and volcanism in the Lesser and Greater Caucasus being geochemically similar in many respects, a shared source is ruled out because of tectonic constraints. Strike-slip faulting, the result of a tectonic regime shift at 5 Ma, determined the location of volcanoes. Although the style of post-collisional magmatism may not be characteristic of all continental collision zones, where present it has a high preservation potential. It may therefore play a significant role in the formation, and composition of continental crust that is largely unrecognised in models of continental crust evolution.

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