Sm–Nd and Rb–Sr isotope systems in migmatites of the Main Range Zone, Greater Caucasus

The Caucasus belongs to the Mediterranean mobile belt and is located at the junction of the Eurasian and Afro-Arabian plates. In the Elbrus subzone of the Main Range zone of the Greater Caucasus, pre-Alpine metamorphic rocks of a gneiss–migmatite complex are widely exposed. These rocks experienced high-temperature regional metamorphism during the Caledonian orogeny, under P-T conditions of ~650–732°C and 3.37 to 4.17 kbar. This study focuses on migmatites from the Elbrus subzone (Upper Svaneti segment, Nenskra River valley), represented by quartz–feldspar leucosomes and paleosomes containing garnet, biotite, sillimanite, muscovite, plagioclase, and quartz; cordierite is rare, and accessory minerals include zircon, monazite, and apatite. This contribution reports whole-rock isotopic results for the Rb–Sr and Sm–Nd systems of these rocks. Rb–Sr analyses were performed by TIMS, and Nd isotopic compositions were determined by MC-ICP-MS. The Rb–Sr system yielded Rb contents of 127–310 ppm and Sr contents of 59–81 ppm, with a wide range of ⁸⁷Rb/⁸⁶Sr ratios (4.555–15.324) and radiogenic ⁸⁷Sr/⁸⁶Sr values of 0.746614–0.789257. In the Sm–Nd system, Sm contents range from 5.654 to 9.579 ppm and Nd contents from 29.43 to 51.80 ppm, with a narrow range of ¹⁴⁷Sm/¹⁴⁴Nd ratios (0.1111–0.1161). The measured ¹⁴³Nd/¹⁴⁴Nd ratios vary from 0.511843 to 0.511898, and the calculated present-day values εNd(0) = −15.5 to −14.4 are consistently negative for all samples. The Sm–Nd data show closely similar Nd isotopic compositions and uniformly negative εNd(0) values, indicating an evolved continental-crustal isotopic signature with no significant juvenile mantle contribution. In contrast, the large spread in Rb–Sr parameters reflects substantial variations in Rb/Sr ratios among samples and likely redistribution of Rb and Sr during high-temperature processes at the whole-rock scale (i.e., at the level of bulk-rock composition). The combined Sm–Nd and Rb–Sr data are consistent with the formation of migmatites as a result of anatexis of an ancient crustal protolith without significant involvement of juvenile mantle material. The isotopic characteristics correspond to reworked, predominantly metasedimentary sources and reflect crustal recycling under high-temperature conditions.

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