Aylangar Fm

The absolute age of amphibolites of the A. Formation, determined by the potassium–argon method, is 1063 ± 62 million years; garnet amphibolites, 1034 ± 49 million years (350); apo-andesitic tonalite gneisses (from zircons by the alpha lead method), 680 ± 20 and 1200 ± 80 million years; from amphiboles (potassium–argon method), 558–668 million years (307); (rubidium–strontium method), 1386 million years (Askarov, Saigdyganiev, 1985).In rocks that have undergone intense metamorphism, the biostratigraphic method is practically ineffective; paleontological determinations, as a rule, raise doubts and disputes. Therefore, the age of the formation is provisionally assigned to the Early Proterozoic, taking into account radiometric data indicating the time of early metamorphism. A.K. Bukharin and others (122) believe that the available radiometric data for the subdivisions of the Baysuntau Series, including the A. Formation, indicate an older age of the rocks and consider the formation as part of the Lower Paleozoic (Lower Silurian). A.V. Pokrovsky (349) assigns the A. Formation to the Lower Paleozoic based on correlation with metamorphic units of the Surkhantau Range and the northern part of the Baysuntau Range, where acritarchs and microfossils of the Lower–Middle Cambrian were found (92). In Str. el. (442) it is tentatively assigned to the Archean; in the monograph (165) — tentatively assigned to the Archean–Lower Proterozoic. On the State Geological Map of Uzbekistan at 1:500,000 scale (460), it is tentatively assigned to the Lower Proterozoic.