Late Paleozoic tectonic evolution of the North Tianshan Belt: New structural and geochronological constraints from meta-sedimentary rocks and migmatites in the Harlik Range (NW China)

摘要

The North Tianshan Belt (NTB) formed by the subduction and accretion of the Junggar Ocean is a key area for reconstructing the Paleozoic tectonic evolution of the southern Central Asian Orogenic Belt (CAOB). Despite numerous studies, the interpretation of the late Paleozoic tectonic evolution of the NTB meets no consensus yet. We conducted field investigations and LA-ICP-MS zircon U-Pb dating on metamorphic rocks from the Julideneng Metamorphic Complex (JMC) in the Harlik Range, which is located between the Turpan-Hami Basin to the south and the East Junggar Belt to the north. The metamorphic rocks are exposed in a NW-SE striking,-10 km-wide belt and mainly composed of migmatites, garnet-sillimanite mica schists, andalusite schists, and low-grade meta sandstones. Detrital zircons from the low-grade meta-sandstone yielded ages of 1400?1250, 1000?850,-780,-580,-490,-445 and-425 Ma. Three micaschists contain zircon populations of 2500?2175, 1800?1600, 1500?1100, 1000?850, 800?500,-475,-425, 420?380,-346 Ma, and a youngest age peak at-322 Ma. Two samples of leucocratic dykes in migmatites yielded comparable age populations with two major peaks at 322 Ma and 297 Ma, which are interpreted as two stages of successive partial melting and anatectic melts crystallization. On the basis of structural features, zircon textures and U-Pb ages, combined with already published data, we propose that: (1) the meta-sedimentary rocks of the JMC were deposited after 425 Ma and before 322 Ma (latest Silurian to late Carboniferous); (2) the Precambrian detrital zircons in the meta-sedimentary rocks were probably derived from the Central Tianshan Block, which was once connected with the NTB; and (3) the migmatization and coeval granitic plutonism occurred at-322?297 Ma (late Carboniferous), most likely associated with crustal thinning resulted from continent-based intra-arc or back-arc or post-orogenic extension.