Deformation Termination of the Kanggur Ductile Shear Zone in Eastern Tianshan, NW China: Insights from U-Pb Dating of Zircon and Apatite

Abstract

The Kanggur ductile shear zone (KDSZ), located in the south margin of the Central Asia Orogenic Belt (CAOB), plays a critical role in the tectonic evolution and mineralization in eastern Tianshan. Although different isotopic chronologies have been reported, the termination of the KDSZ deformation remains controversial. Here, we provide new data obtained by U-Pb dating of zircon and apatite from Huangshandong synkinematic granite (HSG) and Huludong deformed granite (HDG) to constrain the termination of the KDSZ deformation. The U-Pb age of apatite from HSG (249.1 ± 1.8 Ma) is identical to that of zircon (256.5 ± 2.1 Ma) within the error range. In contrast, the U-Pb age of apatite from HDG (248.1 ± 4.0 Ma) is significantly younger than that of zircon (347.3 ± 2.5 Ma). The HDG and HDG have the geochemical characteristics of I-type granites petrogenesis, including high SiO2 (up to 75.47%), high alkaline (K2O + Na2O = 6.39%–8.05%), low FeOT/MgO (2.4–3.4), and peraluminous (A/CNK = 1.01–1.08). Combined with previous Sr-Nd isotope compositions, the positive zircon εHf(t) values and TDM2 ages indicate that the ca. 347 Ma HDG originated from the re-melting of juvenile crust crustal-derived magma in a volcanic arc environment during the northward subduction of the Kanggur oceanic basin, and the ca. 257 Ma HSG originated from the partial melting of thickened juvenile crust in a post-collisional environment. Although trace elements of zircon show typical magmatic characteristics, apatite does not. With the presence of distinct major and trace elements in apatite, the apatite from HSG is characterized by high Mn (>2500 ppm), slight enrichment in the middle rare earth elements (MREEs), and obvious negative Eu anomalies (δEu = 0.09–0.21), indicating that it is related to magmatic apatite. In contrast, the apatite from HDG, with low Mn (<860 ppm), depleted light rare earth elements (LREEs), and variable Eu anomalies (δEu = 0.30–1.34), demonstrated fluid metasomatism with metamorphic overprinting. Combined with the regional geology and published geochronology data, the HSG is interpreted to be derived from the magma experiencing cooling crystallization in the plastic state from 256.5 to 249.1 Ma, while the HDG is considered to have experienced metamorphism and deformation between 347.3 and 248.1 Ma. Owing to the relatively low closure temperature of the U-Pb isotopic system, the apatite U-Pb ages are interpreted as Early Triassic tectono-magmatism events, corresponding to the end of deformation of the KDSZ. This is inferred to be related to the continuous evolution of the Paleo-Asian Ocean in the Late Permian to Early Triassic.