Double-layer structure of the crust beneath the Zhongdian arc, SW China: U–Pb geochronology and Hf isotope evidence

• Published in: Journal of Asian earth sciences 2016-01, Vol.115, p.455-467
• Main Authors: Cao, Kang, Xu, Ji-Feng, Chen, Jian-Lin, Huang, Xiao-Xiao, Ren, Jiang-Bo, Zhao, Xiang-Dong, Liu, Zhen-Xing
• Format: Article
• Language: English
• Subjects: Cretaceous granite; Double-layer crustal structure; Late Triassic porphyry; Metallogenesis; Zhongdian arc
• ISSN: 1367-9120; 1878-5786
• DOI: 10.1016/j.jseaes.2015.10.024

•The Late Triassic porphyries and Late Cretaceous granites were formed at ca. 216Ma and ca. 80Ma, respectively.•The Late Triassic porphyries were mainly derived from partial melting of a juvenile lower crust.•The Late Cretaceous granites probably resulted from partial melting of old middle–upper crustal materials.•The crust in the region of the Zhongdian arc has a double-layer structure. U–Pb ages and Hf isotopes of zircons in Late Triassic and Cretaceous intrusive rocks from the Zhongdian arc, SW China, are used to decipher the tectonic, magmatic, and metallogenic processes that occurred during this period. New U–Pb dating of zircons from Late Triassic porphyries yielded ages of ca. 216Ma and εHf(t) values of −2.1 to +6.1. Combined with previous results, the data indicate that these Late Triassic rocks were most likely derived from a juvenile mafic lower-crust with minor old crust material. However, the Cretaceous granites (∼80Ma) have lower εHf(t) values (−7.6 to −2.4) than the Late Triassic rocks, indicating that the former originated from old crust. Based on the new data and previous studies of Mesozoic magmatic activity, a plausible model for the tectono-magmatism and metallogenesis of the Zhongdian arc is proposed. The westwards subduction of the Ganzi–Litang oceanic crust began before ∼230Ma, resulting in the formation of a juvenile lower crust beneath the Zhongdian arc due to the underplating of mafic arc magmas during ca. 230–216Ma. At ca. 216Ma, break-off or slab-tearing of the west-dipping Ganzi–Litang oceanic slab led to partial melting of the juvenile lower crust, which gave rise to Cu-bearing porphyries. In the Late Cretaceous, the Zhongdian arc probably underwent post-collision extension, triggering the partial melting of the old middle–upper crustal materials and producing various granites and related Mo–Cu deposits. According to this model, the crust beneath the Zhongdian arc probably has a double-layer structure, with older crust at shallow levels and juvenile crust at deeper levels.