Subduction and exhumation processes, interacting with each other, play a key role in crustal recycling. Downgoing oceanic lithosphere constitutes the dominant input at subduction margins, but subduction erosion, the removal of crustal material from the overriding plate, may add additional ingredients and complexity to the subduction factory. Different exhumation models have been proposed to explain how subducted materials are exhumed and therefore contribute to crustal recycling, e.g., exhumation up the subduction channel versus diapiric rise through the mantle wedge that overlies the subducted plate. The recently discovered Baqing eclogite- bearing high-pressure metamorphic complex, central Tibet, China, provides an excellent opportunity to decode the exhumation process, the origin of subduction-related magmatism, and the crustal structure of the North Qiangtang block, in addition to elucidating processes of crustal recycling. Pressure-temperature-time (P-T-t) paths and zircon U-Pb ages and trace-element compositions for Baqing high-pressure rocks were used to evaluate exhumation processes and to determine the geochemical and tectonic affinity of the Baqing metamorphic complex. The Baqing metamorphic complex is mainly composed of eclogite, gneiss, and schist. It is located between two geologically distinct terranes—the South Qiangtang block, which has early Paleozoic basement, and the North Qiangtang block, which has Proterozoic basement. In the schist, zircon cores with steep heavy rare earth element (HREE) slopes and oscillatory zoning yielded inherited ages that are similar to detrital zircon ages for the South Qiangtang block schist; in contrast, zircon rims with flat HREE slopes yielded metamorphic ages of 224 Ma that are similar to the metamorphic ages obtained for the Baqing eclogite. In contrast, zircons from the gneiss yielded an upper-intercept age of 1033 ± 32 Ma (interpreted as the crystallization age) and a lower-intercept metamorphic age of 198 ± 4 Ma. Field relations indicate that gneiss and eclogite/amphibolite were exhumed together, so the ∼20 m.y. gap between the gneiss and the metabasite metamorphism may indicate a long exhumation duration. In the region, Proterozoic ages of ca. 1000 Ma are known only from the North Qiangtang block; we thus propose that the Baqing gneiss originated from North Qiangtang block Proterozoic basement, which, along with North Qiangtang block Triassic arc magmatic rocks and the discrepancies between ancient and current arc-trench distances, results in estimates of ∼20–170 km of Triassic subduction erosion. Results of P-T analyses show that most eclogite, amphibolite, and schist shared a similar clockwise P-T path, different from that of the gneiss, which records a higher geothermal gradient. The clockwise P-T trajectory, long exhumation duration, lack of significant heating during exhumation, and the South Qiangtang block affinity of the schist (host rock of the Baqing eclogite) are consistent with subduction-channel exhumation rather than diapiric rise through the mantle wedge. Geochemical similarities between the North Qiangtang block Triassic subductionrelated rocks and the Baqing gneiss may signal the involvement of unexhumed Baqing metamorphic complex in the recycling of the Qiangtang crust.
Bibliographical noteFunding Information:
This research was financially supported by the Second Tibetan Plateau Scientific Expedition and Research (STEP) Program (grant 2019QZKK0703), the National Natural Science Foundation of China (grants 41472209 and 40802048), the University of Chinese Academy of Sciences, the China Geological Survey (grants DD2016008003, DD20160015, and 12120113033004), and the College of Science and Engineering (funding for work done at the University of Minnesota). We are grateful for the analytical support, helpful comments, and discussions from Anette von der Handt, Hannah Blatchford, Nan-Nan Cheng, Annia Fayon, Xiang-Kun Ge, Qi Guo, Quan-Lin Hou, Wen Jiang, Patricia Kang, Megan Korchinski, Jiao Li, Qiu-Huan Li, Qiu-Li Li, Hong-Wei Liu, Jia-Hui Liu, Jian-Ming Liu, Li Liu, Yu Liu, Qian Mao, Xiao-Hong Mao, Rui-Guang Pan, Tao Peng, Jia-Hui Qian, Meng-Yan Shi, Jin-Feng Sun, Jennifer Taylor, Zong-Yao Tai, Christian Teyssier, Hao Wang, Xin Wei, Chun-Ming Wu, Li-Long Yan, Quan-Ren Yan, Di Zhang, Ji-Heng Zhang, and Qian Zhang.
© 2020 Geological Society of America