An Early Cretaceous arc–back-arc system in western Yunnan, SW China: Constraints from U Pb zircon ages and geochemistry of volcanic rocks in the western Baoshan block

Abstract

Whether the voluminous Early Cretaceous magmatism in the Tengchong and Baoshan blocks, western Yunnan, China, was related to subduction of the Meso-Tethys or the Neo-Tethys is controversial. Here, we report SHRIMP and LA-ICP-MS UPb zircon ages, bulk-rock major and trace elements, and SrNd isotope data from volcanic rocks in the Lameng area of the Baoshan block, SW China, investigate their genesis, and discuss the evolution of the Tengchong and Baoshan blocks. These rocks yield UPb zircon ages of 120.3 ± 1.7 and 120.8 ± 1.4 Ma and constitute a bimodal volcanic suite (group 1 and group 2). Group 1 samples show alkaline basaltic signatures (SiO2 = 46.2–53.1 wt.%), enrichments in Zr, Hf, Ti, and light rare earth elements, and slight depletions in Nb, Ta, and heavy rare earth elements. They have εNd(t) = −1.0–+3.7 and initial 87Sr/86Sr = 0.70503–0.70709. Group 2 samples display higher silica contents (SiO2 = 61.7–65.0 wt.%) and evolved εNd(t) (−4.3 to −3.6) and initial 87Sr/86Sr (0.70653–0.70678) values. These features suggest that the group 1 melts were derived from an asthenospheric mantle source mixed with subcontinental lithospheric mantle components and that the group 2 samples were produced by partial melting of the lower crust in a continental back-arc setting. From an affinity with widespread coeval magmatic rocks in the Tengchong and Lhasa blocks, we suggest that the Bangong-Nujiang Ocean between the Tengchong and Baoshan blocks had closed by 120 Ma and that the Early Cretaceous magmatism in the Tengchong and Baoshan blocks formed in an arc–back-arc system associated with Neo-Tethyan slab rollback.