Acta Petrologica et Mineralogica

A series of Early Cretaceous A-type granites are developed in the central-western part of the Central Lhasa subterrane in Xizang, but no related A-type granites have been found in the eastern section, which constrains the understanding of the deep dynamics of the central Lhasa subterrane. In this paper, petrographic, zircon U-Pb chronology, zircon Hf isotope geochemistry and whole-rock geochemistry studies were carried out on the monzonitic granites in the Menba area based on detailed field geological investigations. The zircon ²⁰⁶Pb/²³⁸U weighted average age of this monzonitic granite measured by LA-ICP-MS dating is 120.9±0.8 Ma, indicating that the Menba monzonitic granite was formed in the Early Cretaceous. The Menba monzonitic granite has a high content of SiO₂ (69.69%~75.81%, average 73.84%), a high content of total alkali (Na₂O+K₂O) (8.22%~9.34%, average 8.92%), a medium content of Al₂O₃ (12.77%~15.60%, average 13.74%), and a low content of MgO (0.01%~ 0.12%, average 0.05%), values of 1.08~1.22 for A/CNK and 1.12~1.27 for A/NK, suggesting that the Menba monzonitic granite is a peraluminous, high-K calc-alkaline-potassium basaltic series rock. The Menba monzonitic granite is relatively enriched in trace elements such as Zr, Hf, Ce, and Y, and relatively deficient in trace elements such as Ba, Ti, Sr, and P. It has strong negative Eu anomalies (δEu=0.04~0.11, average 0.07), high 10 000 Ga/Al values (1.99~4.18, average 3.06), FeO^T/MgO values (18.45~ 69.96, average 47.05), Y/Nb (1.07~2.42, average 1.77), Rb/Nb (6.70~12.16, average 9.83), and a rightward gently dipping "V" rare earth element distribution curve, indicating that the Menba monzonitic granite is an A2-type granite produced in a post-collisional and extensional environment. In addition, the negative to positive εHf(t) values (-2.7~+2.6), relatively old zircon Hf crustal model ages (t_DM2=1 352.8~1 016.8 Ma), and Nb/Ta values close to those of the crust indicate that the Menba monzonitic granite originated from the ancient lower crust with a small amount of mantle material. Combining the results of previous research, this paper suggests that the Menba monzonitic granite was formed in the collisional orogenic background of the Early Cetaceous Lhasa subterrane and Qiangtang subterrane, and that its genesis was due to the upwelling of soft-fluidic material caused by the breakup of subducting plates, which induced partial crustal melting, and that it originated from magma that was partially melted by crustal melting with a small amount of mantle material, and underwent the segregation and crystallisation of predominantly K-feldspars.

Xizang; Central Lhasa subterrane; Menba area; A-type granite; post-collision