Direct Signs of Eruptive Fluid-Rupture Breccias on the Ufa Plateau (Cis-Uralian Part of the East European Platform)

New data on eruptive fluid-rupture breccias of the Ufa Plateau, a geomorphological rise on the eastern (Cis-Urals) periphery of the East European Platform (EEP), bordering the Cis-Uralian Foredeep, are presented. The plateau is a 2.5–3 km thick carbonate platform (D2-P1) overlying a crystalline foundation of the AR–PR age. In the apical part of the plateau and at its foot there are strongly cavernous carbonate rocks, the so-called “foraminated brecciated limestones” (FBL), which sometimes contain debris and pebbles of siliceous rocks as well as metamorphic and igneous minerals. There are indications of an association between xenogenic material and eruptive fluid-rupture breccias, which became the focus of the authors’ special studies. FBL samples from 1 to 5 kg from various parts of the plateau were dissolved in a 10% HCl solution. The insoluble residues of several samples were found to contain microbreccias composed of debris (up to 1–2 mm) of amphiboles, pyroxenes, plagioclase, biotite, titanomagnetite, and serpentinite, immersed in finely dispersed material of the same composition. Up to 50% of the matrix by volume consists of magnetic and glass spherules and shapeless mass. The microbreccia matrix is very soft and easily crumbles when touched. Moissanite and plates of native metals and their alloys (Ni, Fe–Ni, Fe–Cr–Ni, and Fe) were noted in samples. The authors explain the presence of microbreccia fragments in the FBL by tectonomagmatic activation of the EEP margin during the Early Kungurian, crustal degassing, accompanied by the formation of eruptive fluid-rupture breccias, the removal of dispersed rock material of the crystalline basement, enriching poorly consolidated sedimentary rocks. As a result of thermal processes, magnetic and glass spherules were formed in the microbreccia.