The Nenjiang oceanic slab rollback in late Devonian: Constraints from Heihe granite and monzonite in the Xing’an block, NE China

Abstract

Disputes regarding the subduction process, closure time, and the location of the Nenjiang Ocean have arisen due to limited exposure of late Paleozoic magmatic rocks in the Xing'an block. In this study, we present new zircon U-Pb age, zircon Hf isotope, zircon trace elements and whole rock geochemistry to reveal the evolution process of the Nenjiang Ocean in the late Paleozoic. The Heihe monzonite samples originated from the partial melting of a depleted mantle wedge metasomatized by subduction-related fluids and experienced fractional crystallization of plagioclase and amphibole. The Heihe granites show adakitic affinity, with low contents of compatible elements and U-shaped chondrite-normalized REEs patterns, indicating that the adakitic characteristics of the Heihe granites were formed by amphibole-dominated fractional crystallization. Geochemical modeling shows that the Heihe granite can be generated at a degree of 30–75 % amphibole-dominated fractional crystallization. The geochemical characteristics and genetic type of Heihe granites, associated with coeval sedimentary strata, imply extensional tectonic setting in late Devonian-early Carboniferous. By analyzing the spatiotemporal distribution and geochemical data of the 300–400 Ma magmatic rocks in the Xing'an block, and by conducting locally weighted regression fitting, we propose a geodynamic model. According to this model, the Nenjiang oceanic slab began to rollback at approximately 375 Ma, resulting in the production of the Heihe monzonite and granite. Subsequently, the closure of the Nenjiang Ocean, and collision between the Xing'an block and the Songnen block occurred at around 320 Ma.