БЛОК ЭДИАКАРСКИХ ВУЛКАНИЧЕСКИХ ПОРОД В СТРУКТУРЕ ЮЖНО-МОНГОЛЬСКО–ХИНГАНСКОГО ОРОГЕННОГО ПОЯСА

Abstract

The publication presents the results of studies on chemical composition of rocks from the volcanogenic part of the Shirokopadinskaya Formation in the Bereya River basin of the Amur fragment of the northeastern flank of the South Mongolia–Khingan orogenic belt, as well as the results of geochronological (U-Th-Pb, LA-ICP-MS) and isotopic (Lu-Hf) studies on zircons from crystalloclastic tuff. Metaandesite-basalts, their tuffs and tuffites dominate among the volcanic and volcano-sedimentary rocks from the Shirokopadinskaya Formation. The geochemical features of the rocks and Hf-isotopic composition of zircons from volcanic rocks indicate their suprasubduction origin. The concordant age of the youngest zircon population from crystal tuff is 589 ± 5 Ma (MSWD = 0.60, concordance probability = 0.90 %), which corresponds to the Ediacaran. These zircons are characterized by well-preserved crystal faceting without traces of roundness. In our opinion, the age of the tuffs reflects the age of volcanism and the studied volcanic rocks. The data are rather unexpected, because the most ancient rocks of the oceanic crust of the South Mongolia–Khingan orogenic belt are Ordovician in age, which means the studied volcanic rocks are more than 100 million years older and cannot be part of the South Mongolia–Khingan belt. Ediacaran volcanic rocks, considering their structural position, are located at the boundary between the South Mongolia–Khingan orogenic belt and the Mamyn (Xing’an) terrane. They are a fragment of the Mamyn (Xing’an) terrane, which was tectonically placed into the modern structure of the South Mongolia–Khingan orogenic belt. This conclusion is supported by the occurrence of formations in the Mamyn (Xing’an) terrane that are similar in age and geochemical features.