IBM-type forearc magmatism in the Qilian Orogen records evolution from a continental to an intra-oceanic arc system in the Proto-Tethyan Ocean

Wen, Tao; Song, Shuguang; Wang, Chao; Allen, Mark B.; Dong, Jie; Feng, Di; Su, Li

Abstract

Forearc magmatism in IBM-type intra-oceanic arc systems is crucial for understanding the processes of subduction initiation and subsequent evolution of the arc system. Here we report a forearc volcanic sequence exposed in the Danghe-Nanshan, at the westernmost end of the South Qilian Accretionary Belt of the Qilian Orogen, NW China. Based on petrography and bulk-rock composition, three types of volcanic rocks (boninite, high-Mg basaltic andesite and andesite) and arc-related felsic plutons are identified. These rocks show correlating trends of trace elements and isotopes. The high-Cr spinels and clustered Ti/V ratios indicate that the boninite and high-Mg basaltic andesite magmas were derived from highly refractory peridotite sources in a forearc setting, whereas the andesite and felsic plutons were formed by fractionation of clinopyroxene and plagioclase from primary magma. Notably, slab-derived components have limited involvement in magma generation. Variation of fluid mobile/immobile element contents and Sr-Nd isotopic mixing calculations suggest less than 1% incorporation of subducted slab-derived fluids into the boninite source, whereas ∼1–3% fluids and ∼3–7% partial melts from the slab contributed to the high-Mg basaltic andesite and andesite, respectively. Therefore, an abnormally high mantle potential temperature (>1429 °C) was required for boninite production because there were little aqueous fluids involved. Zircon U-Pb dating reveals these forearc magmas formed at ∼450 Ma. Combined with the regional tectonics and published data in this region, we suggest that these magmas record a tectonic process of arc system from active continental margin to newly formed intra-oceanic subduction zone during trench retreat of the Proto-Tethys Ocean.

• Journal Article Type: Article
• Acceptance Date: Jun 27, 2022
• Online Publication Date: Jun 30, 2022
• Publication Date: 2022-10
• Deposit Date: Feb 27, 2023
• Journal: Gondwana Research
• Print ISSN: 1342-937X
• Publisher: Elsevier
• Peer Reviewed: Peer Reviewed
• Volume: 110
• Pages: 197-213
• DOI: https://doi.org/10.1016/j.gr.2022.06.016
• Public URL: https://durham-repository.worktribe.com/output/1178329