Petrogenesis and tectonic implications of the Neoproterozoic A-type granite in the Quanji Massif, northeastern of the Tibetan Plateau

Abstract

A-type granites have unique geochemical characteristics, indicating an extensional tectonic environment, thus providing critical constraints on magma sources and tectonic mechanisms, and playing a crucial role in reconstructing orogenic events. In this contribution, we conducted a systematic study on zircon U-Pb geochronology, whole-rock geochemistry, and zircon Hf isotope of the new identified A-type granites in the Quanji Massif in the northeastern Tibetan Plateau to discuss its petrogenesis and the tectonic implications. The zircon U-Pb dating results show that the crystallization ages of granites and granite porphyries are 797 ± 8 Ma and 799 ± 7 Ma, respectively, indicating that they were formed in the Neoproterozoic. Geochemically, these rocks have high SiO₂ (67.47–75.74 wt%) and total alkali (Na₂O + K₂O = 6.89–8.84 wt%) contents, low MgO (0.11–1.37 wt%), CaO (0.76–1.29 wt%), and P₂O₅ (0.05–0.18 wt%) contents, with negative anomalies of Nb, Ta, Ti, Sr, and Eu elements. They have high FeO^T /(MgO + FeO^T) values, Zr + Nb + Ce + Y contents (343–562 ppm), 10000 × Ga/Al ratio (2.12–3.20), and zircon saturation temperature (804–860 ℃), which are consistent with the characteristics of typical A-type granites. Their high K₂O/Na₂O ratios, low Sr/Y and La/Yb ratios, combined with the zircon ε_Hf(t) values (–4.04 to 3.25) and the whole-rock ε_Nd(t) values (–8.28 to –4.91), indicate that the A-type granites were derived from partial melting of the calc-alkaline rocks in the ancient crust with a moderate contribution of juvenile crust under high temperature and low-pressure conditions. Based on the tectonic setting discrimination diagram combined with regional data, it is comprehensively determined that the A-type granites were formed in the post-collisional extensional environment, which is a response to the breakup of the Rodinia supercontinent. According to the comprehensive data of the Quanji Massif and adjacent areas show that the Quanji Massif, the Qaidam Block and the Qilian Block were combined into a unified block during the Late Mesoproterozoic-Neoproterozoic and participated in the evolution of the Rodinia supercontinent.