Mineralogy and petrography of Chebarkul group dolerite sills (West Magnitogorsk Zone of the Southern Urals) in context of petrogenesis

The Chebarkul group of sills of Late Visean age (Early Carboniferous), localized in the vicinity of Lake Chebarkul located in the central part of the West Magnitogorsk zone, has been identified. It is part of a single extended sill belt formed on the periphery of the Magnitogorsk-Bogdanovskoe rift. A petrographic and mineralogical study of the rocks of two large sills (Davletovo and Yangi-Aul) has been carried out. The sills are composed of fairly homogeneous and uniform metasomatized hornblende dolerites. The main and one of the earliest minerals is plagioclase (the maximum content of anorthite mineral is 61.6 mol. %). Clinopyroxene is aged in composition and corresponds to high-calcium augite and salite. It is weakly concentric-zonal, so SiO2, FeO and MgO contents increase from the center to the edge, while contents TiO2 and Al2O3 decrease. Hornblende (edenite) crystallized in the temperature range of 968–839 °C at a pressure range of 1–3 kbar. The following crystallization order of magmatic minerals has been established: titanomagnetite-1 → Ca-Na plagioclase → clinopyroxene → titanomagnetite-2 + ilmenite → apatite + zircon (?) → hornblende → biotite → Na-Ca plagioclase + potassium feldspar. The chlorite corresponding to brunsvigite in the Davletovo sill and diabantine (+ ripidolite and pycnochlorite) in the Yangi-Aul is one of the main secondary minerals, which crystallized mainly in the temperature range of 200–150 °C. In all rocks, chlorite partially or completely replaced clinopyroxene and especially hornblende. In general, the petrographic and mineralogical features of dolerites indicate a single-act injection of the initial magmas and their weak differentiation. The studied sills are unlikely to have fed a large intrusive massif or volcanic edifice.