Melt Sources and Formation Parameters of the Khokhol−Repyevka Granitoid Batholith in the Volga−Don Orogen, East European Craton

Abstract

The paper discusses the melt sources and formation parameters of the Khokhol-Repyevka granitoid batholith that compose the Don terrane of the Volga–Don orogen in the East European craton. The batholith consists of three granitoid types: Pavlovsk granitoids (quartz monzonites–granites, mostly without pyroxenes), Potudan granitoids (quartz monzogabbro–granodiorites containing pyroxene), and hybrid ones (quartz monzodiorites, monzonites, and quartz monzonites). These three types of rocks occur together and have a similar age of 2050–2080 Ma, similar geochemical characteristics (high contents of Ba, Sr, and highly fractionated REE patterns with Gd_N/Yb_N = 2–11), but differ in petrographic and isotopic geochemical parameters. The initial isotope characteristics of the sources of the Pavlovsk-type rocks are ε_Nd(t) = +0.2 to ‒3.7 and Sr_i = 0.70335, those of the Potudan type are ε_Nd(t) = –1.7 to –3.8, Sr_i = 0.70381–0.70910, and the hybrid rocks have ε_Nd(t) = –8.8, Sr_i = 0.70596. In addition to granitoids, the batholith was found out to host two types of leucogranite dikes. One of them is characterized by ε_Nd(t) = –3.8 and fractionated HREE patterns (Gd_N/Yb_N = 2.1–3.8) and could be formed as a result of the deep differentiation of Pavlovsk-type magma. The other type has ε_Nd(t) = –7.8 and less fractionated HREE patterns (Gd_N/Yb_N = 1.1–1.6), which likely resulted from the melting of a crustal source at shallow depths. The Rb–Sr isotope-geochemical characteristics of rocks of the Pavlovsk and Potudan types indicate that their melts were derived from different sources. Therefore the melts of the Khokhol–Repyevka batholith were derived from at least three sources: (1) lower (or buried oceanic) crust of predominantly mafic composition and/or enriched mantle, which was metasomatized in the Proterozoic, whose involvement is reflected in the composition of the Pavlovsk granitoids; (2) an enriched mantle source, which was likely subcontinental lithospheric mantle (SCLM) that had been metasomatized during an earlier stage of the geological development of the region, specific of the Potudan-type monzonitoids; and (3) Archean crust consisting mostly of TTG gneisses and metasediments, which underwent melting and participated in the formation of some of the leucogranite dikes and hybrid rocks. The results of thermodynamic modeling indicate that the mixing of two melts contrasting in composition (Potudan-type mafic and Pavlovsk-type intermediate–felsic) could form only some of the hybrid rocks. The others could be formed by mafic melt contaminated with anatectic melts derived from the Archean crust of the Kursk block.