Petrogenesis and geodynamic regime of monzonite and granite massifs of the Balbuk area (Southern Urals) according to whole-rock geochemistry, Sr-Nd isotopy, and Rb-Sr geochronology.

Research subject. In the area of the northern closure of the Magnitogorsk megazone and the Main Ural Fault bordering to the west, the state geological maps distinguish the Balbuk complex. This complex includes numerous monzonite-syenite-granite intrusions. Their isotope age, determined at different times and by different methods (K-Ar, Rb-Sr, Pb-Pb, U-Pb), varies from 363 ± 21 to 250 ± 5 Ma, which requires a deeper analysis. The composition of these granitoids has been poorly studied; as a result, the origin and geodynamic position of the Balbuk complex remain un clear. Methods. The chemical composition of rocks was studied by X-ray fluorescence analysis and mass spectrometry with inductively coupled plasma. The Sr-Nd isotopy of rocks was studied using thermal ionization mass spectrometry. Results and conclusions. The results of a mineralogical and geochemical study involving Sr-Nd isotopy of monzonites and granites of several medium-sized and small intrusions of the Balbuk area (Sharip group, Balbuk, Aushkul, and Kamatal) are presented. Geochemical features and Sr-Nd isotope data (ISr = 0.70355–0.70422, εNd t = +3.95) allow us to connect the source of monzonite magmas with the lithospheric mantle and crust of the Magnitogorsk island-arc terrane reworked by subduction fluids (including metabasites of ophiolite associations and continental margin). The main source of granites ((⁸⁷ Sr/ ⁸⁶ Sr) t = 0.70355– 0.70545, εNd t = +3.5…+4.8) is the metasedimentary rocks of the Magnitogorsk terrane. The distribution features of various elements in rocks reflect the complex fractionation of the parental melts. Two   Rb-Sr isochron dates were obtained: for monzonite porphyry from the small massif of the Sharip group (354.2 ± 1.4 Ma) and granite porphyry of the Kamatal massif (304 ± 29 Ma). The age data allows us to link them with the dating of other granitoids of the northern part of the Magnitogorsk megazone (Akhunovo-Petropavlovsk, Verkhneural’sk-Kassel areas) and to distinguish here discrete stages of monzonite-syenite (363–346 Ma) and subalkaline granite (307–294 Ma) magmatism. Monzonite-syenite magmatism is associated with the early destruction of the Late Devonian–Early Carboniferous accretion-collision orogen, and granite magmatism records the onset of the Ural collision orogeny. The data obtained showed that the combination of all types of granitoids into a single complex is incorrect and that the Balbuk area should be singled out as one of the centers of long-term mantle-crustal interaction.

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