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The Jiamusi Block is located in the easternmost Central Asian Orogenic Belt (CAOB) and its tectonic affinity and evolution is highly controversial. Here we describe newly discovered Neoproterozoic orthogneisses and early Paleozoic granitoids of the Mashan Complex from the Jiamusi Block. SIMS U-Pb dating for zircons from three high-grade orthogneisses yields protolith and metamorphic ages ranging from 757 to 751 Ma and from 530 to 476 Ma, respectively. LA-ICP-MS U-Pb dates for zircons from four gneissic granitoids define two stages of early Paleozoic magmatism (i.e., 530–502 Ma, and 490–476 Ma). These new data, together with recent published ages, indicate that the basement rocks of the Jiamusi Block record an important Neoproterozoic-early Paleozoic magmatic (ca. 895–476 Ma) and metamorphic (ca. 563–476) event, and we accordingly infer that the granulite facies metamorphism might have occurred before 541–476 Ma, and around 567–561 Ma. Geochemical and zircon Hf isotopic data suggest that the ca. 752 Ma rift-related gneisses were derived from dominantly juvenile crustal rocks with both the heat and material involvement of a depleted asthenosphere mantle at relatively low pressures, which then underwent an ancient wall-rock assimilation, whereas the ca. 490–476 Ma post-collisional granitoids originated from reworking of an ancient crustal source and subsequently experienced intensive crystal fractionation. The good correlation on the ca. 755–115 Ma geological events between the Jiamusi and Khanka blocks suggests they formed a contiguous crustal unit. The existing data indicate that this combined block cannot be directly derived from the Siberia, North China, South China, and Tarim cratons. Instead, it might be either a rifted portion of the Sayang-Baikal orogen or derived from a peri-Gondwana position. We evaluate the model that the Jiamusi-Khanka Block may have originated as part of the Late Neoproterozoic-Cambrian Kuunga orogen trapped between South India and East Antarctica-Australia. Sometime after ca. 470 Ma, it rifted away from East Gondwana and drifted northward to collide with the Songliao Block of NE China in the Early Jurassic. |
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