Roles of Subducted Pelagic and Terrigenous Sediments in Early Jurassic Mafic Magmatism in NE China: Constraints on the Architecture of Paleo-Pacific Subduction Zone

The role of subducted sediments in arc magmatism has been widely documented. However, identifying the sedimentary provenance (e.g., pelagic vs. terrigenous) input in subduction systems is difficult because of the wide compositional range of sedimentary components and the complex magmatic evolution of arcs. Here we report zircon U-Pb-Hf-O isotopes and whole-rock elemental and Sr-Nd-Hf isotopic compositions of four Early Jurassic subduction-related mafic intrusions from the Yanbian area, NE China. These rocks show typical trace element and isotopic features of arc magmas. In combination with the published data, we discover two distinct elemental-isotopic arrays of the Early Jurassic mafic rocks across the arc magmatic belt. Such geochemical variations are mainly attributed to variable subducted sediment input into the mantle wedge instead of crustal contamination or assimilation during magmatic evolution. The mantle source for the southern Yanbian mafic rocks was enriched by addition of a crustal component dominated by terrigenous sediments, whereas that for the northern Lesser Hinggan-Zhangguangcai Range, mafic rocks was modified by another crustal component comprising mainly pelagic sediments. The results can be best interpreted if the southern part was a continental arc as opposed to an oceanic arc in the north, which is analogous to the modern Kamchatka-Honshu-Izu-Bonin-Mariana arc systems in the western Pacific. Our reconstructed architecture, based mainly on the geochemical data, further suggests that the Khanka-Jiamusi-Buleya Massif was probably sinistrally and northwardly displaced to the present position after Mesozoic subduction of the Paleo-Pacific Ocean. Plain Language Summary Subducted sediments play an important role in generation of arc magmatism. However, the identification of the source (pelagic vs. terrigenous) of the sediments in subduction systems is difficult because of the wide compositional range of sedimentary components and the complex magmatic evolution of arcs. Here we carry out an integrated analysis of Sr-Nd-Hf-O isotopic and elemental compositions of subduction-related mafic intrusions across the Early Jurassic Paleo-Pacific subduction zone in NE China. The combined geochemical data from our and previous studies suggest that the southern Yanbian mafic rocks were generated by melting of a depleted mantle with input of a terrigenous sediment component, whereas the mafic rocks from the northern Lesser Hinggan-Zhangguangcai Range were derived from a depleted mantle modified by a pelagic sediment component. We thus propose that the Mesozoic subduction zone in NE China consisted of an intraoceanic arc in the northern segment and a continental arc in the southern part, as observed from the modern Kamchatka-Honshu-Izu-Bonin-Mariana arc systems in the western Pacific. Our study therefore provides a potential method to identify the provenance of the subducted sediments added into the subarc mantle, which may allow to reconstruct the architecture of paleo-subduction zones.