Preliminary geochemical characterization of the gneiss-migmatite complex rocks from the Dzirula crystalline massif (Caucasus)

The Caucasus is a complex mountain system formed at the convergence of the Eurasian and Africa-Arabian tectonic plates and includes the Greater and Lesser Caucasus folded belts with adjacent foredeeps and intermountain troughs. The Dzirula crystalline massif is a key exposure of the pre-Alpine crystalline basement of the Caucasus, situated in an intermountain area and composed of Precambrian rocks of a gneiss-migmatite complex, metabasites of different generations and ages, quartz-diorite orthogneisses, Paleozoic granitoids of the plagiogranite - granite series, and Late Variscan granite-gneisses and granites. The gneiss-migmatite complex comprises crystalline schists, amphibolites, plagiogneisses, and plagiomigmatites. The massif records at least two stages of regional metamorphism: an initial high-temperature progressive metamorphism followed by Late Variscan diaphtoresis, with peak mineral assemblages corresponding to amphibolite- and low-temperature granulite-facies conditions and accompanied by regional plagiomigmatization and formation of the plagiogranite-granite series. U-Pb LA-ICP-MS zircon dating constrains migmatite formation to 530-500 Ma. Biotite-, bi-mica-, andalusite-, sillimanite-, and cordierite-bearing varieties are recognized. Although the complex has been studied in petrological, mineralogical, geochronological, and geodynamic contexts, targeted geochemical work to constrain metamorphic processes and the original protolith has been lacking. Here we present preliminary geochemical data for gneiss-migmatite rocks sampled in the Dzirula massif (Kvirila, Dzirula, Qvadaura, Chkherimela, and Gezrula river valleys). The samples are dominated by quartz, K-feldspar, biotite, plagioclase, sillimanite, cordierite, garnet, and muscovite, with accessory zircon, monazite, and apatite. According to the chemical analyses, the gneiss-migmatites show substantial compositional variation: SiO₂ = 45.9-74.1 wt.%, Al₂O₃ = 11.7-24.9, Fe₂O₃(Total) = 3.35-17.16, CaO = 0.19-5.18, Na₂O = 1.06-3.07, K₂O = 1.77-5.83, and P₂O₅ = 0.04-0.77. Among the most informative trace-element indicators, we note ranges of Rb = 52-240 ppm, Sr = 82-362 ppm, Ba = 278-1454 ppm, Th = 13.7-32.3 ppm, U = 1.78-9.76 ppm, Zr = 140-636 ppm, and Hf = 3.8-16.3 ppm. The REE display wide variations (La = 36.3-87.1 ppm; Ce = 75.1-181 ppm; Nd = 31.5-76.1 ppm; Eu = 1.14-1.88 ppm) and particularly contrasting HREE behavior (Y = 12.2-241 ppm; Yb = 1.04-36.1 ppm; Lu = 0.172-5.66 ppm). On the SiO2-TiO2 diagram (Tarney, 1976), the gneisses plot in the igneous field. The ratios of Nb/Y vs. Zr/TiO2 (Whinchester, Floyd, 1977) were used to determine the type of protolith, and it suggests that they were rhyodacite-andesite. On the diagram Al vs Fe (Frost, 2008) all the rocks are metaluminous, which is consistent with an igneous protolith. Trace-element systematics indicate enrichment of LILEs (Rb, Ba) and Th-U relative to Nb-Ta (e.g., high Ba/Nb and Th/Nb ratios), suggesting relative Nb-Ta depletion. The rocks show a well-defined negative europium anomaly. Collectively, it suggests that the gneisses were meta-igneous rocks, and the protolith might be andesite/dacite. Based on the Th-Hf/3-Ta (Wood et al., 1979) discrimination diagrams, the rocks belong to the calc-alkaline type. The leucosome data on the Rb vs. (Yb+Ta) diagram (Pearce et al., 1984) plot within the field of volcanic arc granites.