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Mg-Ba-Sr-Nd isotopic evidence for a mélange origin of early Paleozoic arc magmatism

Hao, Lu-Lu, Nan, Xiao-Yun, Kerr, Andrew C. ORCID:, Li, Si-Qi, Wu, Yuan-Bao, Wang, Hao and Huang, Fang 2022. Mg-Ba-Sr-Nd isotopic evidence for a mélange origin of early Paleozoic arc magmatism. Earth and Planetary Science Letters 577 , 117263. 10.1016/j.epsl.2021.117263

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In recent years, the mélange model has been increasingly considered as an important way to transfer slab components to arc sources in modern subduction zones. This model differs from the classic slab fluid/melt metasomatism model in that it invokes physical mixing of bulk sediment, altered oceanic crust (AOC), serpentinite, and mantle wedge at the slab-mantle interface. However, due to the lack of subducted sediment compositions, the mélange model has not been applied to any significant extent in paleo-subduction zones. The lack of evidence for bulk AOC and serpentinite components in mélange sources also hinders our understanding of any mélange origin for arc-type magmas. Here, we report Mg-Ba-Sr-Nd isotope compositions of the early Paleozoic Fushui mafic rocks in the Qinling orogen of central China, to trace their origin and constrain slab component transfer. The Fushui mafic rocks show typical arc-type geochemical features and enriched Sr-Nd isotope compositions, indicating the likely contribution of subducted sediments. They have low Rb/Ba ratios (<0.1), and most samples show Ba values of −0.38 to +0.10‰, similar to those of sediments (−0.2 to +0.1‰) but lower than those of MORBs (+0.03 to +0.14‰), indicating that these low Ba values are most likely derived from sediment components. One Fushui sample has a high Ba of +0.31‰, similar to that of AOC (up to +0.4‰). This high Ba is not correlated with fluid input; instead, it results from the contribution of bulk AOC. The Fushui rocks exhibit variable Mg values (−0.23 to −0.11‰), slightly higher than those of MORBs. This most likely reflects 26Mg-enriched subducted serpentinite components in their source. Our results not only identify the variable slab components (sediment, AOC, and serpentinite) in the arc source, but also suggest that these slab components may be transferred to their arc source by the mélange process. This study therefore provides solid evidence for the generation of arc magmas by mélange processes in paleo-subduction zones, which confirms an important role for the mélange model in slab material transport.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Earth and Environmental Sciences
Publisher: Elsevier
ISSN: 0012-821X
Date of First Compliant Deposit: 29 October 2021
Date of Acceptance: 22 October 2021
Last Modified: 07 Nov 2023 01:23

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