Metamorphic Rocks of the Dichun Formation of the Northwestern Bureya Massif: Results of Geochemical, U-Th-Pb (LA-ICP-MS) Geochronological, and Lu-Hf Isotope Studies

The paper presents the results of geochemical analyses of metamorphic rocks of the Dichun Formation in the NW Bureya massif, as well as the results of U–Th–Pb (LA–ICP–MS) geochronological and Lu–Hf isotope studies of zircon from them. It was established that amphibole–biotite gneisses of the Dichun Formation were formed after magmatic rocks and correspond in composition to andesites and trachyandesites. Geochronological studies revealed that the formation of the magmatic protolith of the studied gneisses is associated with the late Paleozoic stage of igneous activity ~299 Ma (late Carboniferous–early Permian boundary) rather than with the early Precambrian, as has been previously believed. The similarity of age values obtained for unaltered/weakly altered crystals and for crystals with complex types of zoning and low Th/U values < 0.1 indicates that the protoliths of the considered orthogneisses underwent high-temperature structural-metamorphic transformations soon after their crystallization. Hf isotopic characteristics of a zircon from orthogneisses of the Dichun Formation – εHf(t) +5.2 to +9.5, tHf(C) = 0.8–0.6 Ga point to a juvenile mantle source. A zircon from late Paleozoic granitoids of the northwestern Bureya Massif exhibits similar isotope characteristics. At the same time, zircon isotope characteristics from the late Paleozoic igneous rocks in the central part of the massif –εHf(t) from –11.5 to –0.4, tHf(C) = 2.0–0.9 Ga – indicate the involvement of the older crustal sources during the formation of their primary melts. This fact suggests the heterogeneity of the Bureya massif basement. Analysis of geochemical and regional geological data showed that the formation of igneous protoliths of amphibole–biotite gneisses of the Dichun Formation in the NW Bureya massif is associated with the late Paleozoic stage of the Mongol–Okhotsk Ocean plate subduction beneath the margin of the Amur block.