New Insights on Partial Melting and Migmatization in the Greater Caucasus Main Range Zone

The Caucasus is located at the convergence of the Eurasian and African-Arabian tectonic plates and represents a link between the European and Asian components of the Mediterranean (Alpine-Himalayan) collisional mobile belt. This region offers valuable insights into understanding collision tectonics and high-grade metamorphism. The presented study aims at investigating the high-grade metamorphism and crustal melting processes in the Greater Caucasian Main Range zone, which is subdivided into the Pass and the Elbrus sub-zones. They differ from each other in terms of geological structures, lithologies, metamorphism and magmatism. Migmatization and partial melting in the Caucasus refers to Cadomian (626±2 Ma) and Caledonian (461±5.3, 468±5, 471.7±4.6 Ma) stages of high-temperature regional metamorphism of Elbrus subzone infrastructure. The final, low-temperature stage of regional metamorphism occurred during the Variscan orogeny. The Elbrus subzone represents a migmatitic complex and its study is a key for understanding high-temperature metamorphic and anatectic processes in this region. This study combines geochronological data with detail mineralogical and textural analyses of migmatites from the Elbrus sub-zone to determine the conditions and mechanisms of partial melting and to shed light on the relationships between crustal anatexis and tectonics. The migmatites collected in the River Nenskra valley (Upper Svaneti region) are mostly fine-to medium-grained stromatic metatexites, characterized by light bands composed predominantly of quartz, potassium feldspar and plagioclase, and dark bands with biotite, garnet and sillimanite. Leucosomes contain euhedral feldspar crystals and thin quartz-feldspar films along grain boundaries, which can be interpreted as microstructures indicating the presence of former melt. Garnet crystals are often characterized by numerous tiny inclusions, giving them a cloudy appearance. MicroRaman investigation reveals the presence of cristobalite, graphite, phlogopite, biotite, chamosite, carbonate, CH₄ and N₂ in the vast majority of these inclusions, which are interpreted as former fluid inclusions and not as nanogranitoids. SEM analysis results show enrichments of Mn at the rims of some garnet crystals, which are related to local resorptions of the garnet crystals and replacement by biotite. In the highest grade samples muscovite is rare, displays a skeletal shape, is not in contact with quartz and is adjacent to crystals of K-feldspar and sillimanite. These observations suggest that partial melting conditions exceeded the stability of Ms+Qz and reached the stability of Kfs+Sil. In addition, inclusions of green spinel have been observed in some sillimanite clots, which represent the product of staurolite decomposition. These are the only microstructures that provide constraints on the pre-anatectic history of these migmatites. The proposed conditions of regional metamorphism suggest temperatures and pressures corresponding to upper amphibolite and up to granulite facies. These conditions support the occurrence of partial melting processes, followed by slow cooling. To further refine the understanding, new microstructural, microchemical, and geochronological data will be presented, providing deeper insights into the petrogenesis of migmatites in the Greater Caucasus. This work will also contribute to understanding the thermal regime of regional high-grade metamorphism and melting in other metamorphic structural zones of the Caucasus.

ACKNOWLEDGEMENTS: This work was supported by Shota Rustaveli National Science Foundation of Georgia (SRNSFG) [FR-22-11295].