Geologicheskii Vestnik

The article describes the geological structure of the Oseisk granitoid massif located in the northern part of the East Ural trough. Its structure involves both igneous rocks represented by plagiogranites and granodiorites, as well as metasomatic and rheomorphic varieties varying in composition from plagiogranites to tonalites and diorites. The host rocks from the east are intensely amphibolized Sargaza strata eff usive rocks (О1–2 sr), which are plagiogranitized in the contact zone and turned into amphibolites and amphibole-quartz-feldspar, biotite-quartz-feldspar schists; from the west — plagiogranites and granodiorites of the Plastovsk massif. The rocks of the considered intrusion consist of blue quartz (30–40%), plagioclase (about 55–60%) with rare checkerboard albite, muscovite, and sericite. In the transitional types there is an epidote, biotite, hornblende, and actinolite. According to chemical analyzes, they diff er from other granitoids in high acidity (74.7–78.7%), low potassium content (up to 1.3%) and the amount of alkalis (up to 5.2%). In terms of chemical composition, the granitoids under consideration correspond to normal alkaline granodiorites and plagiogranites with a predominantly sodium type of alkalinity. According to the Rb and Sr content, the rocks of the Oseisk massif are derivatives of orogenic andesite magma, clearly fi t into the fi eld of granite-migmatite formation and fall into the area near the fi eld of continental trondhjemites, quartz diorites, as well as continental granophyres and rhyolites of Iceland. The contents and distribution of REE in granitoids correspond to trondhjemites of continental margins. It was established that the crystallization of the granitoids of the Oseisk massif occurred at a temperature of 760–790°С. The presence of melt inclusions in quartz indicates their primary magmatic nature. The water pressure in the melt was 3480–3630 bar, which corresponds to depths of 12.0–12.5 km (abyssal zone). The high content of water (5.7–6.8 wt. %), chlorine (0.23–0.30 wt. %) and salts (6.6– 7.3 wt. %) in the melt inclusions of granitoids in quartz, which ensure the extraction and transfer of many metals, allow us to classify them as ore bearing granites of the fi nal plutonic series.

Oseisk massif, plagiogranites, tonalites, melt inclusions, petrogeochemistry, pressure, temperature

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