The first discovery of Paleoproterozoic highly fractionated granites in the south of the Vorontsovka terrane of the Volga-Don Orogen

Abstract

Introduction: granites make up a significant part of the continental crust and contain information about the geodynamic regimes of the upper part of the lithosphere. According to their characteristics, they are divided into unfractionated, which are usually considered in terms of alphabetical classification, and fractionated, which are formed from magmas that have undergone significant fractional crystallization processes. Such magmas are usually high-temperature and fluid-saturated, which is reflected in the absence of mafic minerals and the presence of minerals from rare earths. They are enriched in LILE, have low Nb/Ta and Zr/Hf ratios, negative Eu* anomalies, and a tetrad effect in the REE distribution. The article is devoted to mineralogy, geochemistry and petrogenesis of a new type of highly fractionated granites of the Vorontsovka terrane.

Methodology: the chemical composition of the samples was determined using an S8 Tiger X-ray fluorescence spectrometer (Bruker AXS GmbH, Germany) at Voronezh State University. Small and rare elements were determined by the induction-coupled plasma method with mass spectrometric termination of analysis (ICP-MS) in the laboratory of Physical and Chemical Methods of Mineral Matter Research at the Institute of Geology and Geochemistry of the Ural Branch of the Russian Academy of Sciences. Local analyses of minerals were performed using a Jeol 6380LV scanning electron microscope with an INCA 250 quantitative energy dispersion analysis system at Voronezh State University and a Tescan Mira IV scanning electron microscope with an X-Max 80 energy dispersion (EDX) spectrometer at Schmidt Institute of Physics of the Earth of the Russian Academy of Sciences.

Results and discussion: granitoids are massive medium-grained rocks with hypidiomorphic and poikillitic structures. They are composed of plagioclase (35−40 vol. %), microcline (up to 30 vol. %), quartz (20−25 vol. %), muscovy (up to 10 vol. %) and biotite (up to 10 vol. %). They are characterized by high SiO₂ contents (71−75 wt. %) and alkalis (K₂O+Na2O = 9−14.2 wt. %) with a predominance of potassium over sodium (K₂O/Na₂O = 1.1–5.6), low concentrations of Al₂O₃ (9.4−12.8 wt%). They are depleted of siderophilic, highly charged, and lithophilic elements, with the exception of Li (4−130 ppm) and Rb (120−140 ppm). They contain low concentrations of REE (∑REE = 11−36 ppm), with a moderate enrichment of LREE relative to HREE (La/Yb_N = 2.87–13.48) and a negative Eu anomaly (Eu/Eu* = 0.4–0.56). They are ferruginous, alkaline and metalominous rocks, which does not allow them to be attributed to any of the types of alphabetical classification, and suggests that they belong to a highly fractionated type. This is indicated by the presence of fluorite and xenotime, high concentrations of Li and Rb, and low ratios of Nb/Ta and Zr/Hf. The compositions of the studied granites and S-type granites of the Vorontsovka terrane form a single trend of fractional crystallization and suggest that Paleoproterozoic (2050–2070 Ma) S-type granites were fractionated. The temperature of biotite crystallization and saturation of the melt with zirconium indicates the formation of melts within the middle crust. The formation of S-type granites occurred as a result of melting of immature Vorontsovka series meta-sediments as a result of increased heat flow on the flank of the collisional orogen.

Conclusions: in the southern part of the Voronotsovka terrane, alkaline and metalominous ferroan granitoids have been identified for the first time. The distribution of petrogenic and REE indicates that they belong to a highly fractionated type, while petrographic characteristics and features of geochemical affinity indicate that S-type melts were fractionated.