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At the latitude of the Lake Yalangaskul on the western limb of the Khudolaz syncline at the base of the Biyagoda formation are acidic volcaniclastic greywackes. The grains composing them are represented by quartz, plagioclase, and acid volcanics. Grains of medium and basic volcanics, quartzites and cherts are of primary importance. Three types of acidic greywacke are distinguished. The first type — greywackes together with charts make up an independent lentil-like body — the Yalangas lentil. Its thickness is 350 m, its length is about 20 km. The second type — single layers of acid greywackes are deposited among polymictic greywackes, which overlap the Yalangas lentil and are associated with layers of cherts. The third type — the block of acidic volcaniclastic greywackes 150×10–25 cm in size lies in a bed of polymictic greywackes. A petrographic study of acidic volcaniclastic greywackes allowed to describe and interpret very different penecontemporaneous  textures  developed  in  them. At  the  stage  of  diagenesis  and  early  catagenesis, regeneration  rims of quartz  and quartz polycrystalline  interstitial  cement were  formed. During  the  late catagenesis, conform and  incorporation  textures were  formed, quartz and plagioclase were  regenerated. At the metagenesis, deformation lamellae and undulatory extinction in quartz grains, cracks, blastic textures were formed. Nests and veins filled with polycrystalline quartz and rare crystals of authigenic plagioclase formed at the stage of retrograde epigenesis. The degree of transformation increases from the third-type acidic greywacke to the first. The revealed stages of epigenesis are linked to the geological history of the territory. The processes of dia and catagenesis  proceeded most  intensively  at  the  turn of  the Frasnian  and Famennian under conditions of high-speed sedimentation (about 1500 mm/thousand years). In the Famennian these processes probably gradually faded out. Metagenesis was caused by collisional processes in the Late Carboniferous and Early Permian. Studies have shown that a significant role in the epigenetic transformations of the studied rocks was played by  solutions  saturated with  silica.  It  is  assumed  that  their  source was predominantly  siliceous sediments/rocks abounding in the studied section.

Epigenetic transformations in acidic volcaniclastic greywackes from Biygoda Formation (the West-Magnitogorsk zone, the Southern Urals)