Reworking and maturation of continental crust in collision zones: Insights from Early Cretaceous compositionally diverse magmatic rocks in central Tibet.

The reworking of continental crust in collision zones is a key process in the chemical maturation of crustal rocks. However, the mechanisms that cause crustal reworking remain highly controversial. The compositionally diverse magmatic rocks along the Lhasa–Qiangtang collision zone provide a favorable opportunity for studying the mechanism of crustal reworking and maturation in continental collision zones. Here we report zircon U Pb ages and Hf isotopes, mineral major elements, and whole-rock geochemistry and Sr Nd isotopes for the Momu intrusive complex in the Lhasa–Qiangtang collision zone, central Tibet. Zircon U Pb ages reveal that the intrusive complex formed during the late Early Cretaceous (115–113 Ma). Diorites from the intrusive complex have high Mg# values (49.7–51.2) and initial 87Sr/86Sr ratios (0.7082 to 0.7083), and low ε Nd (t) values (−7.3 to −6.9), with negative to slight positive zircon ε Hf (t) values (−3.8 to +0.8), suggesting that these rocks were likely generated by the partial melting of the enriched lithospheric mantle. Granodiorites from the intrusive complex have high SiO 2 (64.4–67.9 wt%) and Al 2 O 3 (14.8–16.1 wt%) and low MgO (1.4–1.8 wt%) contents. These rocks are also characterized by high initial 87Sr/86Sr ratios (0.7091–0.7098) and whole-rock negative ε Nd (t) (−9.9 to −8.4) and zircon ε Hf (t) (−7.9 to −2.2) values. The associated granites have the highest SiO 2 (68.7–71.5 wt%) contents among the intrusive complex rocks, high initial 87Sr/86Sr (0.7089–0.7093) ratios, and negative whole-rock ε Nd (t) (−10.1 to −7.7) and zircon ε Hf (t) (−5.3 to −0.2) values. These geochemical characteristics indicate that the granodiorites were most likely derived from the partial melting of ancient continental crust and that the coeval granites were formed by the fractional crystallization of granodioritic magmas. Integrating our new results with data for the coeval magmatic rocks of the Lhasa–Qiangtang collision zone, we suggest that the Momu intrusive complex formed in a post-collisional slab breakoff setting. Asthenosphere upwelling caused by slab breakoff could have provided a substantial source of heat for crustal reworking, which gradually contributed to the maturation of continental crust in the Lhasa–Qiangtang collision zone. • The Momu intrusive complex of central Tibet formed at 115–113 Ma. • The diorites were derived by partial melting of modified lithospheric mantle. • The granodiorites and granites formed via partial melting of ancient crust. • Crustal maturation occurred by collision of the Lhasa and Qiangtang terranes. [ABSTRACT FROM AUTHOR]