Subduction-modified mantle-derived Triassic high-Mg andesites in the Sanjiang Tethys, eastern Tibet

• Published in: Journal of Asian earth sciences 2020-04, Vol.191, p.104216, Article 104216
• Main Authors: Fan, Hong-Peng, Li, Bo, Zhou, Jia-Xi, Du, Li-Juan, Sun, Hai-Rui, Huang, Zhi-Long, Wu, Tao
• Format: Article
• Language: English
• Subjects: High-Mg andesites; Sanjiang Tethys; Subduction-modified mantle; The Yangla Cu ore district
• ISSN: 1367-9120; 1878-5786
• DOI: 10.1016/j.jseaes.2019.104216

[Display omitted] •High-Mg andesites in the Jinshajiang suture zone formed at ~230 Ma.•The andesites were derived from a previously subduction-modified mantle wedge.•Regional tectonic collapse and thermal relaxation occurred before Late Triassic. Andesites in the Yangla district of the Sanjiang Tethys, eastern Tibet, are characterized by relatively high MgO and with rich large-ion lithophile elements (LILEs), indicating similarities to typical high-Mg andesites formed in continental collisional zones. Zircon U-Pb age of 232 ± 1 Ma for these andesites shows that they occurred synchronously with the regionally widespread late Middle Triassic granitoids and bimodal volcanic rocks. Primitive mantle-normalized spidergrams for these andesites show depletion in high field-strength elements (HFSEs), with Sr-Nd-Pb isotopic compositions that are comparable to both arc- and MORB-like high-Mg rocks generated from subduction-modified sources. Modelling results suggest that an input of ~20% sediment-derived melts into mantle-derived melts could be possibly responsible for the Sr-Nd isotopic characteristic of these andesites. MORB- and arc-like compositions likely indicate a metasomatized mantle wedge modified by subduction processes during the closure of the Paleo-Tethys Ocean and subsequent continental collision. It is speculated that the late Middle to Late Triassic intra-continental re-activation associated with regional tectonic collapse and thermal relaxation triggered the melting of this previously subduction-modified mantle, leading to the formation of the high-Mg andesites in the Yangla district.