Identification of High δ 18 O Adakite‐Like Granites in SE Tibet: Implication for Diapiric Relamination of Subducted Sediments

• Published in: Geophysical research letters 2022-05, Vol.49 (9)
• Main Authors: Xu, Jian, Xia, Xiao‐Ping, Spencer, Christopher J., Wang, Qiang, Yin, Chang‐Qing
• Format: Article
• Language: English
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2022GL098541

Sediment relamination in subduction zones is posited to be important for the compositional evolution, physical structure and material recycling of the Earth. Despite support from numerical experimental modeling and geophysical observation, magmatic evidence for relamination is rare and difficult to identify in ancient and modern convergent systems. Here we report newly identified ca.32–27 Ma adakite‐like granites with high δ 18 O values (up to 10.38‰) from the Ailaoshan‐Red River (ASRR) shear zone in western Yunnan (SE Tibet). Their geochronological and compositional data strongly exhibits significant contributions from Permian‐Eocene sedimentary rocks in the lower crust (30–45 km). Our study provides further evidence that the lower crust may be partially composed of more felsic lithologies likely originating from rapid supracrustal material recycling via diapiric relamination of (meta)sedimentary material during Cenozoic‐age continental subduction. The ASRR adakite‐like granites thus represent clear magmatic evidence for sediment relamination in a subduction system. Subducted sediments may detach from the downgoing slab due to the large density contrast with surrounding mantle, and ascend as diapirs to become emplaced into the base of the overriding plate. Sediment relamination would help building a more felsic continental lower crust, and be important to Earth material recycling. However, identifying and verifying sediment relamination is difficult. Here, we carry out an integrated in situ analysis of zircon U‐Pb and Hf‐O isotopes, along with whole‐rock geochemistry for the Cenozoic granites from the Ailaoshan‐Red River (ASRR) shear zone in western Yunnan (SE Tibet). Our data reveal that the newly identified ca. 32–27 Ma adakite‐like granites are characterized by high δ 18 O values (up to 10.38‰), and other geochemical compositions similar to sediment‐derived melts formed under the pressure of 1.0–1.5 Gpa corresponding to depths of 30–45 km. Occurrence of Paleocene‐Eocene inherited zircon cores (ca. 67–41 Ma) further imply that the (meta)sedimentary rocks in the lower crustal depth may be resulted from rapid supracrustal material recycling via diapiric relamination during Cenozoic‐age continental subduction. We propose that the high δ 18 O adakite‐like granites identified in the ASRR represent clear magmatic evidence for sediment relamination in a subduction system. The newly identified ca. 32–27 Ma adakite‐like granites were formed in the Cenozoic conti