Subduction zone decoupling and rapid forearc spreading at an active continental margin: Evidence from an SSZ-type ophiolite in Qilian Orogen, NW China

Qiao, Jinran; Dong, Jie; Allen, Mark B.; Su, Li; Song, Shuguang

Abstract

The forearc supra-subduction zone (SSZ) ophiolites are of great significance for understanding the geological processes of subduction systems. They are generally considered to mark the initiation of oceanic subduction. Here we present a well-preserved forearc sequence in the Ebo area, North Qilian Accretionary Belt, NW China. Three rock types are identified from bottom to top, including the forearc basalts (FABs), boninites, and calc-alkaline arc rocks, which are similar to those from the Izu-Bonin-Mariana (IBM) area. The FABs display similar compositions to N-MORB but with lower Ti/V ratios (17.16–20.92) and they were derived from 5 to 10 % melting of depleted MORB mantle in the spinel-stability field at P–T conditions of 1.5–2.3 GPa/1442 ± 56 °C after < 2 % melt extraction in the garnet-stability field. The boninites are characterized by significant geochemical characteristics of subduction-zone fluid/melt involvement, which were the products of flux melting of refractory harzburgitic mantle that interacted with slab-derived fluids/ carbonate melt under the P–T conditions of 0.2–1.2 GPa /1316 ± 38 °C. The calc-alkaline rocks show typical arc geochemistry, suggesting the partial melting of the well-established enriched subarc mantle. Zircon U-Pb dating of the boninite and the arc gabbro yielded ages of 507 ± 10 Ma and 483 ± 4 Ma, respectively. Combined with regional data, we infer that the Ebo FABs and boninites are not the products of subduction initiation but formed during subduction-zone decoupling-retreat and rapid forearc spreading following earlier subduction at the North Qilian continental margin, which is subsequently followed by typical continental arc development.

Citation

Qiao, J., Dong, J., Allen, M. B., Su, L., & Song, S. (2024). Subduction zone decoupling and rapid forearc spreading at an active continental margin: Evidence from an SSZ-type ophiolite in Qilian Orogen, NW China. Journal of Asian Earth Sciences, 266, https://doi.org/10.1016/j.jseaes.2024.106132

• Journal Article Type: Article
• Acceptance Date: Mar 26, 2024
• Online Publication Date: Apr 3, 2024
• Publication Date: May 1, 2024
• Deposit Date: Feb 27, 2025
• Journal: Journal of Asian Earth Sciences
• Print ISSN: 1367-9120
• Electronic ISSN: 1878-5786
• Publisher: Elsevier
• Peer Reviewed: Peer Reviewed
• Volume: 266
• DOI: https://doi.org/10.1016/j.jseaes.2024.106132
• Public URL: https://durham-repository.worktribe.com/output/3549501