The First Data on the Age and Sources of the Protolith of Gneisses of the Dichun Formation in the Northeastern Bureya Continental Massif, Central Asian Orogenic Belt

The results of geochemical studies of biotite gneisses of the Dichun Formation of the Bureya continental massif, as well as U–Th–Pb (LA-ICP-MS) geochronological and Lu–Hf isotope studies of zircon, are presented. Metamorphic rocks of the Dichun Formation are traditionally considered as part of the Early Precambrian basement of the Bureya continental massif. As a result of the reconstruction of the origin of the protolith, it was established that biotite gneisses of the Dichun Formation in the Niman River basin of the northeastern part of the Bureya massif are of primary magmatic origin and correspond to dacites. It is assumed that biotite gneisses are a fragment of the volcano-sedimentary sequence. As a result of U–Th–Pb geochronological studies of zircons, it was revealed that the age of the protolith of these orthogneisses corresponds to the Late Tonian (771 ± 2 Ma), and not to the Early Precambrian, as was previously believed. The obtained age estimate is the first for metamorphic rocks of the Dichun Formation within the Bureya massif. The geochemical features of metamorphic rocks of the Dichun Formation indicate their similarity to within-plate felsic magmatic rocks. The obtained results of Lu–Hf in situ isotopic studies indicate a significant role of juvenile material during the formation of the initial melts of their protolith and do not contradict this conclusion. The new and previously published geochronologic data point out widespread Neoproterozoic magmatic processes in the geologic history of the Bureya massif. The coeval magmatic impulses are also noted within the entire ensemble of continental massifs of the eastern part of the Central Asian Orogenic Belt (CAOB) and are probably related to the breakup of the Rodinia supercontinent.