Compositional changes of granitoids from the Menglian Batholith in SW China at ca. 122 Ma: Implications for the origin of decoupled Nd-Hf isotopic compositions and crust generation in collision zones

Abstract

Determining the origin and related mantle dynamics responsible for the generation of giant granitic batholiths in convergent margins is critical to unravel the formation mechanism of the silica–rich continental crust. This paper reports the whole–rock major element, trace element, Sr-Nd isotopic, and zircon U-Pb age and Hf isotope data of samples from the Menglian Batholith in the eastern Tengchong Terrane of southwest Yunnan. These samples consist of two groups, including the Group I rocks that are mostly granitic with zircon U-Pb ages of 132–122 Ma and the Group II rocks that display varying compositions from gabbroic to granitic with zircon U-Pb ages of 122–112 Ma. The Group I samples have zircon εHf(t) of −11.7 to −4.2 and whole–rock εNd(t) of −10.1 to −8.9, whereas the Group II samples show elevated zircon εHf(t) of −7.8 to +7.1 and whole–rock εNd(t) of −9.0 to −6.9 and εHf(t) of −3.1 to −1.0. The Nd-Hf isotopic compositions of the Group I rocks are coupled, likely resulting from the partial melting of an ancient lower crust. However, the Nd-Hf isotopic compositions of the Group II rocks are decoupled, likely inheriting from the mantle–derived material with decoupled Nd-Hf isotopic compositions. Regional geological correlation and compositional transition documented by the Group I and Group II samples collectively indicate that the slab rollback and subsequent breakoff of the subducting Bangong–Nujiang ocean lithosphere during the Tengchong–Baoshan collision can be invoked to explain the generation of the Menglian Batholith. The similarity of average compositions between the Group II samples and the bulk continental crust suggests that the addition of mantle–derived material to the ancient crust–derived melts seems to be an efficient process for crustal growth during continental collisions.