Sources of melts and genesis conditions of the Khokhol-Repyevka batholith granitoids in the Volga-Don orogen, Eastern European craton

The paper discusses the possible conditions and involvement of sources in genesis of the Khokhol-Repyevka batholith granitoids, that build up the Don terrane in the Volga-Don orogen of the East European Craton. In the batholith, three types of granitoids are distinguished – pavlovsk (quartz monzodiorite–granites, mainly pyroxene-free), potudan (quartz monzogabbro–granodiorites containing pyroxene) and hybrid (quartz monzodiorites, monzonites, quartz monzonites). These three types of rocks are spaсely co-located and have a similar age of formation 2050–2080 Ma, similar geochemical characteristics (high contents of Ba, Sr, highly fractionated REE patterns (GdN/YbN= 2–11)), however, they differ in petrographic and isotopic geochemical parameters. Primary isotope characteristics of sources for rocks of the pavlovsk type εNd(t) = +0.2…–3.7, Sri= 0.70335, for potudan εNd(t) = – 1.7 ... –3.8, Sri= 0.70381–0.70910, for hybrid εNd(t) = – 8.8, Sri= 0.70596. Apart from granitoids, two types of leucogranite dikes were found in the batholith. The first type is characterized by εNd(t) = –3.8 and fractionated HREE patterns (GdN/YbN= 2.1–3.8) and could have formed as a result of deep differentiation of pavlovsk-type magma. The second type is with εNd(t) = –7.8 and less fractionated HREE patterns (GdN/YbN = 1.1–1.6), which presumably appeared as a result of melting of a crustal source at shallow depths. Rb-Sr isotope-geochemical characteristics of rocks of the pavlovsk and potudan types indicate their formation from different sources. In total, at least three sources took part in the formation of the Khokhol-Repyevka batholith: 1) lower (or buried oceanic) crust, predominantly of mafic composition and/or enriched mantle, metasomatized in the Proterozoic, the participation of which is reflected in the composition of the Pavlovsk granitoids; 2) an enriched mantle source, probably represented by subcontinental lithospheric mantle (SCLM), possibly metasomatized during the previous stage of geological development of the region, specific for Potudan-type monzonitoids; 3) Archean crust, consisting mainly of TTG gneisses and metasediments, which underwent melting and participated in the formation of part of the leucogranite dikes and hybrid rocks. The results of thermodynamic modeling indicate that the mixing of two melts contrasting in composition – mafic (potudan-type) and intermediate-felsic (pavlovsk-type) can lead to the formation of only part of the composition of hybrid rocks. The formation of the rest was influenced by the contamination of mafic melt by anatectic melts from the Archean crust of the Kursk block.