The Jiamusi, Khanka and Bureya blocks; a contiguous crustal entity accreted to the Central Asian Orogenic Belt in the Early Jurassic?

The evolution and tectonic setting of the eastern segment of the Central Asian Orogenic Belt (CAOB) is currently a matter for debate, especially the origin of the micro-continental blocks located between the Siberian and North China cratons. The Jiamusi, Khanka and Bureya blocks are the easternmost fragments and have variously been considered to represent separate units or else to form one contiguous block. They have traditionally been considered as units trapped during northward migration of the North China Craton towards Siberia, but, more recently it has been proposed that the Jiamusi Block was accreted to the CAOB from the east, after the main collision had taken place, in response to paleo-Pacific subduction. The Jiamusi Block is composed of a 500 Ma khondalite sequence (the Mashan Complex and associated granitoids) cut by Late Permian granites. Its western edge is marked by the Heilongjiang Complex, composed of tectonically interleaved granitic gneiss, marble, mafic-ultramafic rocks, blueschist, quartzite and mica schist. The granitic gneiss, marble and two-mica schist, were probably derived from the Late Permian granitoids and the Mashan Complex, respectively, whereas the other units most likely represent a subduction-accretionary complex. Biotite and phengite from granitic gneiss and mica schist record late Early Jurassic ⁴⁰Ar/³⁹Ar metamorphic ages of 184-174 Ma, considerably younger than the rest of the Jiamusi Block. The accretionary model is further supported by huge volumes of Jurassic granites in the Zhangguangcai Range to the west of the Jiamusi Block, possibly reflecting subduction beneath the eastern part of the CAOB. The southeast margin of the Jiamusi Block is marked by the Dunhua-Mishan Fault, viewed as defining a major terrane boundary with the Khanka Block. However, new SHRIMP and ICP-MS U-Pb zircon data from granitic gneisses in the latter show a cluster of ages around 500 Ma, suggesting a close affinity with similar rocks of the Mashan Complex in the Jiamusi Block. Our results therefore imply that the Khanka and Jiamusi blocks should be regarded as forming part of the same terrane. The Bureya Block, located to the north of the Jiamusi Block in Russia, has been considered to either represent a separate tectonic unit or to represent the northward extension of the Jiamusi Block. New SHRIMP U-Pb zircon data from Bureya indicate a range of ages from ~1000 Ma to ~200 Ma. Although this age range is greater than in the Jiamusi Block, it is significant that there are abundant rocks with ages of ~500 Ma and ~265 Ma, matching the Mashan Complex and Permian granitoids, respectively, that are the key components of the Jiamusi Block. It appears likely that the Bureya Block, at least in part, contains units in common with the Jiamusi Block and therefore that, instead of representing three separate crustal entities, the Jiamusi, Khanka and Bureya Blocks acted as a combined unit, possibly accreted to the CAOB in the Early Jurassic.