Geochronological, geochemical, and Sr-Nd-Hf isotopic characteristics of Neoproterozoic granites in the Tugeman area of Altyn Tagh (NW China): Implication for Neoproterozoic tectonic evolution

The Altyn Orogenic Belt (AOB) is widely recognized as a key constituent of the Rodinia supercontinent and hosts a well-preserved geological archive essential for reconstructing Rodinia’s tectonic history. Although the Central Altyn Block (CAB) plays a potentially critical role in the Neoproterozoic assembly of Rodinia, its tectonic history during this period remains highly debated. This research provides an integrated geochemical and isotopic characterization of the Tugeman granites, including major and trace element geochemistry, Nd isotopic signatures, and U-Pb of zircon and Lu-Hf isotopic measurements. Zircon U-Pb geochronology indicates crystallization ages ranging from approximately 987 to 878 Ma, representing the first identification of Neoproterozoic granitoids in the region. From a geochemical perspective, the samples exhibit high concentrations of total alkalis (～8 wt%), SiO₂ (69-83 wt%), and Al₂O₃ (～12 wt%), coupled with notably low CaO (～0.8 wt%). The presence of Al-rich minerals, combined with these geochemical characteristics, suggests a strong peraluminous S-type affinity for the Tugeman granites. The Tugeman granites exhibit enrichments in K, Pb, Rb, Th, Nd, Sm, and Dy, depletions in Ba, Sr, Nb, Ce, P, Zr, and Ti. These samples exhibit REE patterns are right-inclined, accompanied by pronounced negative Eu anomalies (Eu/Eu* = 0.04-0.55). Correlations among major and trace elements indicate that fractional crystallization was a dominant process, with the fractionation of plagioclase, K-feldspar, and biotite being critical to the generation of these granitoids from the parental magma. Zircons exhibit ε_Hf(t) values between +1.29 and +8.82, corresponding to two-stage Hf model ages of 1.22 to 1.70 Ga. Meanwhile, whole-rock ε_Nd(t) values fall within a negative range (-1.6 to -0.7), reflecting Nd model ages of approximately 1.64 to 1.68 Ga. These contrasting Hf-Nd isotopic characteristics suggest derivation from juvenile crustal materials. Integrating these findings with regional geological evidence, we infer that the Tugeman granites formed in response to the closure of the Mirovoi Ocean and the subsequent collision between the Paleo-Altyn (P-Al) and Paleo-Qaidam (P-Qai) blocks, a tectonic episode marking the northern segment of Rodinia’s amalgamation. Furthermore, this study refines the location of the Neoproterozoic collisional belt within the P-Al block and provides new constraints on its paleogeographic position during both the assembly and fragmentation stages of Rodinia.