Formation and compositional variation in igneous garnets from the Tezhsar Alkaline Complex (Lesser Caucasus, Armenia)

Garnet in alkaline igneous rocks is of interest due to its compositional and textural variability that provides insights into magmatic and hydrothermal processes. This study investigates textures and mineral chemistry of garnets from the Tezhsar Alkaline Complex (Armenia) to constrain their petrogenetic origin by determining whether the garnets are a primary magmatic liquidus phase or whether they have a secondary, subsolidus origin. Element mobility during garnet formation is evaluated, focusing on rare earth elements (REE), for which alkaline igneous rocks are a globally important resource and which are a valuable geochemical tracer to understand the evolution of rock-melt-fluid systems.

In the Tezhsar Alkaline Complex, K-rich plutonic and volcanic rocks occur in concentric units, representing the remnants of a palaeocaldera (Sokół et al., 2018). Garnet occurs in euhedral to subhedral clusters in pegmatitic nepheline syenite and more rarely as phenocrysts in syenites. The calcic garnets have a high Ti content (c. 2-4 wt.% TiO₂), which is typical for garnet in alkaline igneous rocks. Garnet in the pegmatitic nepheline syenite is devoid of inclusions and shows only limited chemical variability, interpreted to reflect crystallization from a melt. In the syenite, garnet is rich in mineral inclusions and is interpreted to reflect a metasomatic origin during late/post-magmatic growth. Trace element data is being acquired to constrain the physicochemical conditions of garnet growth and evaluate REE incorporation into garnet.

Reference: Sokół, K. et al., 2018. Lithos 320-321, 172-191.