Geologicheskii Vestnik

The article provides a description of the geological structure of the Kisegach, Chashkov and Elanchik migmatite-gneiss granite massifs located in the southern and southeastern parts of the IlmenogorskSysert block. The petrographic, petrogeochemical characteristics of all varieties of their constituent rocks are given. It is shown that the Kisegach massif is composed mainly of gneissic biotite granites of phase I, much less granosyenites and porphyritic granites of phase II, which are intruded by dikes of phase III leucogranites. The Chashkov and Elanchik massifs are represented by granitoids, the formation of which occurred in two stages: migmatites and gneissic granites formed in the first stage, and biotite, muscovite, and two-mica granites formed in the second stage. Phase I granitoids belong to the potassium-sodium and potassium series and belong to the migmatite-granite formation. According to biotite-garnet and biotite-amphibole thermobarometers, it was found that the formation of gneissic granites and gneissic biotite granites of the Kisegach massif occurred at a temperature of 510–540°C as a result of metasomatic processes, and porphyritic granites crystallized from the melt at a temperature of 680–700°C in the abyssal zone on depth of about 23–24 km (Р=7.1–7.4 kbar). Subsequently, all its rocks, including monzogabbro and monzodiorite, experienced regional metamorphism under the conditions of the epidote-amphibolite facies (T=550–575°C, P=5.4–6.0 kbar). Based on the study of the garnet-biotite paragenesis, which is widely represented in the granitoids of the Elanchik and Chashkov massifs, the formation temperatures of gneissic granites (530–550°C and 510–530°C, respectively) and gneissic biotite granites (540–560°C and 520–540°C) were obtained, as well as an approximate estimate of the depth of rock formation was achieved, corresponding to the near-surface or hypabyssal zones (a few kilometers). The obtained data on the physicochemical conditions of formation of the rocks of the massifs made it possible to draw a conclusion about their metasomatic nature and exclude magmatic genesis. The obtained physicochemical characteristics repeat the parameters of metamorphism for the Precambrian stratigraphic units of the Ilmenogorsk-Sysert block, as well as the Kochkar anticlinorium (East Ural uplift). This indicates a single stage in the formation of migmatite-gneiss-granite domes within the entire Southern Urals, which apparently occurred of at the Upper Precambrian — Lower Paleozoic boundary.

Chashkov massif, Elanchik massif, Kisegach massif, Ilmenogorsky anticlinorium, granites, migmatites, biotite-garnet thermobarometer, amphibole-biotite thermometer

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