Age and Petrogenesis of Metamorphic Rocks of the Tulovchikha Formation, Northeastern Bureya Massif, Central Asian Orogenic Belt

ABSTRACT

We present zircon U–Pb age and Lu–Hf isotope data, as well as biotite ⁴⁰ Ar/ ³⁹ Ar age, for metamorphic rocks from the Tulovchikha Formation in the northeastern Bureya Massif, Central Asian Orogenic Belt. Our results show that this formation contains metamorphic rocks of various ages: amphibolites and gneisses with protolith ages of 767 ± 2 and 469 ± 1 Ma, respectively. The amphibolites formed as a result of metamorphism of Neoproterozoic intermediate rocks with suprasubduction signatures and positive ɛ _Hf(t) values (+3.0…7.7), indicative of a mantle source. We suggest that the formation of these intermediate rocks occurred in a subduction setting at the margin of Rodinia. The age of the superimposed metamorphic overprint on the amphibolite protoliths is constrained by U–Pb dating of zircon grains exhibiting unambiguous metamorphic textures, yielding an age of 529 ± 26 Ma. We suggest that this metamorphic event is related to the final amalgamation of Gondwana, linked to subsequent deformation or the initial stages of breakup of this continent. The gneisses were formed from Ordovician acidic igneous rocks. Their zircon ɛ _Hf(t) values −4.7…+3.4, which yield t _Hf(C) model ages of 1.5–1.0 Ga, indicating they formed by melting of heterogeneous Mesoproterozoic crustal sources. This event was likely related to interactions between the Bureya–Songnen and Jiamusi–Khanka massifs in a subduction–collision–post‐collisional extensional setting. In addition, there were localised thermal overprints at 258 ± 4 and 225 ± 4 Ma, based on biotite ⁴⁰ Ar/ ³⁹ Ar dating. These thermal events might have been related to either deformation of the Bureya Massif during the late stages of closure of the Paleo‐Asian Ocean or back‐arc extension resulting from subduction of the Mongol–Okhotsk oceanic plate beneath the margin of this massif.