Crustal evolution of the Central Tianshan Block: Insights from zircon U-Pb isotopic and structural data from meta-sedimentary and meta-igneous rocks along the Wulasitai - Wulanmoren shear zone

The Central Tianshan Block is one of the major continental constituents of the Central Asian Orogenic Belt (CAOB) and contains vital information for understanding the amalgamation of the Eurasia continent. Its tectonic affinity remains controversial hindering a better reconstruction of the paleogeographic evolution of the Paleo-Asian Ocean and geodynamic processes of the CAOB. In order to put more constraints on the Precambrian basement composition and crustal evolution of the Central Tianshan Block, we conducted field structural investigations and zircon U-Pb dating on the meta-sedimentary and meta-igneous rocks along the Wulasitai - Wulanmoren ductile shear zone (WWSZ), northern Baluntai area (NW China). The WWSZ is geometrically a SE-NW-striking high-strain deformation zone of ∼1-5 km wide. It is lithologically composed of greenschists, paragneiss, amphibolitic metabasalts, migmatitic granites and mylonitic granites. The greenschist to amphibolite facies meta-sandstones and meta-volcanic rocks belong to the Ahebulake Group, in which NE- or SW-dipping steep mylonitic foliations and sub-horizontal lineations are well developed. Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) U-Pb dating of detrital zircons from four representative meta-sedimentary rocks from the Ahebulake Group yielded several major age groups of 3130-2530 Ma, 2450-2150 Ma, 2000-1700 Ma, 1620-1433 Ma, 1170-655 Ma, ∼450 Ma, ∼425 Ma, ∼400 Ma, ∼360 Ma and ∼312 Ma. Three ductilely deformed and metamorphosed granitic samples show four major zircon U-Pb age peaks at ∼430 Ma, ∼400-390 Ma, ∼360 Ma and ∼330 Ma. On the basis of a careful analysis of the texture of the dated zircons and the structural features of their host rocks, and combined with the available isotopic ages and regional geological data, we propose that: (1) meta-sedimentary rocks from the Ahebulake Group were likely deposited later than about 400 Ma instead of in the Silurian, as previously assumed; (2) the distinct Neoproterozoic detrital zircon age population in these meta-sediments probably reflects the widespread occurrence of coeval in-situ basement rocks as source; (3) the Central Tianshan Block may have close tectonic affinities with both the Yili Block and the Tarim Craton during the Proterozoic; and (4) the ductile shearing along the WWSZ is likely later than ∼330 Ma, probably initiated at ∼312-299 Ma. Our new results provide further arguments for the paleogeographic linkage of the microcontinents within the Paleo-Asian Ocean and tectonic reconstruction of the southwestern CAOB.