Source of trachydacite-rhyolite pebbles of Middle Jurassic conglomerates on the northwestern shore of Lake Baikal: comparison of the pebbles with rocks of Upper Paleozoic and Mesozoic igneous complexes from Transbaikalia

From paleontological data, conglomerates exposed on the northwestern shore of Lake Baikal, in the Bolshiye Koty and Listvyanka areas, belongs to the upper part of the section of the southeastern part of the Irkutsk coal-bearing basin (Angara-Koty depression) and is considered as part of the lower subformation of the Koty Formation dated back to the upper part of the Aalenian Stage of the Middle Jurassic (174–170 Ma). To identify a source of clastic material of the conglomerates, petrogenic oxides and trace elements of pebbles of volcanic rocks are determined and thin sections are examined. Defined are five groups of trachydacite-rhyolite pebbles of plumasite composition in the former outcrop and a group similar in composition to the main Bolshiye Koty group in the second one. The difference between the pebbles of conglomerates from the upper part of the section in the Angara-Koty depression and those of alkaline effusives of acidic and intermediate composition from the lower and middle parts of the section is indicated. It is assumed that the composition of the pebbles of the Upper Koty subformation is similar to that of the Middle Jurassic volcanic and subvolcanic rocks from Transbaikalia. Their origin from three sources is possible: 1) from nearest one – an unknown Middle Jurassic volcano buried under Cenozoic sedimentary stratum of the South Baikal basin, in the suture zone of the Siberian paleocontinent, 2) from the middle-remote one (distance over 250–500 km) – Middle Jurassic volcano-plutonic complexes of the Dzhida-Vitim fault zone, and 3) from the more remote one – the Mongolian-Okhotsk fault zone.