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

In recent years, the melange 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 melange model has not been applied to any significant extent in paleo-subduction zones. The lack of evidence for bulk AOC and serpentinite components in melange sources also hinders our understanding of any melange origin for arc-type magmas. Here, we report MgBa-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 delta Ba-138/134 values of -0.38 to +0.10 parts per thousand, similar to those of sediments (-0.2 to +0.1 parts per thousand) but lower than those of MORBs (+0.03 to +0.14 parts per thousand), indicating that these low delta Ba-138/134 values are most likely derived from sediment components. One Fushui sample has a high delta Ba-138/134 of +0.31 parts per thousand, similar to that of AOC (up to +0.4 parts per thousand). This high delta Ba-138/134 is not correlated with fluid input; instead, it results from the contribution of bulk AOC. The Fushui rocks exhibit variable delta Mg-26 values (-0.23 to -0.11 parts per thousand), 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 melange process. This study therefore provides solid evidence for the generation of arc magmas by melange processes in paleo-subduction zones, which confirms an important role for the melange model in slab material transport. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

The study in central China's Qinling orogen of the early Paleozoic Fushui mafic rocks indicates the likely contribution of subducted sediments and possibly also of subducted altered oceanic crust (AOC) and serpentinite based on their Mg-Ba-Sr-Nd isotope compositions.