Metamorphism and metasomatism of carbonate rocks in the frame of the Suunduk granite-gneiss massif (Southern Urals)

Research subject. Carbonate rocks in the frame of the Suunduk granite-gneiss massif of the East Ural uplift (Southern Urals). Aim. Determination of possible petrological and geochemical zonation in carbonate rocks of the metamorphic frame of the granite-gneiss massif. Identification of structural regularities of distribution and the character of relationships between carbonate rocks of different types. Materials and Methods. Expedition works were carried out using the methods of field structural geology and mineralogy. Samples of carbonate rocks were selected along the profile transverse to the strike of rocks. The contents of petrogenic elements were determined by X-ray fluorescence analysis using CPM-35 and EDX-8000 spectrometers; rare and trace elements were determined by ICP-MS using an inductively coupled plasma ELAN 9000 mass spectrometer. Results. Carbonate rocks are represented by calcite marbles, Mg-calcite marbles, dolomite marbles, calcite-dolomite marbles, and organogenic marmorized limestones (C1–2). Zonality in the carbonate framing of the southern part of the massif was identified, apart from metamorphic zonality: marbles and marmorized limestones in the distance. At the early collision stage, a terrigenous-carbonate sequence was formed. At the end of C2, due to regional metamorphism and formation of a granite-gneiss dome structure, in the areas of movement of metamorphogenic fluids, limestones underwent Mg-metasomatosis with the formation of pre-metamorphic dolomites with relics of fauna of C1–2 age. As a result of progressive dynamothermal metamorphism, under stress conditions, metasomatic dolomites in places experienced dedolomitization, with the formation of calcite marble with a polygonal-grained structure; this marble was also formed on limestones. Under stress, calcite marble was squeezed out along weakened zones in the direction of pressure drop, i.e. upward. This process was associated with plastic and quasi-plastic deformations, metamorphic recrystallization, metasomatic alterations. Mg-calcite marble with accessory red corundum, phlogopite, Cr-tourmaline, and Cr-muscovite was formed in the areas of movement of metamorphogenic fluids. At the early regressive stage, Mg-metasomatosis reappeared in the fluid movement areas with the formation of dolomite-calcite marble with accessory graphite, tremolite, Cr-tourmaline, and pyrite. The lack of a pronounced zonality of marbles in the frame of the granite-gneiss massif can be explained by the duration and variety of marmorization processes. Conclusion. The granite-gneiss massif was the center of zonal metamorphism. At the progressive and regressive stages of metamorphism, the earlier- middle-carboniferous limestones in the frame of the massif underwent metamorphic and metasomatic transformations with the formation of marbles of various compositions; there were plastic and quasi-plastic deformations, rheomorphic displacements of calcite marble. There is no obvious zonality in the marbles.

marble, limestone, metamorphism, metasomatism, granite-gneiss, Ural