Permian subduction of the Paleo-Pacific (Panthalassic) oceanic lithosphere beneath the Jiamusi Block: Geochronological and geochemical evidence from the Luobei mafic intrusions in Northeast China

Abstract

The late Paleozoic tectonic evolution of eastern northeast China and, in particular, the Jiamusi Block and its relationship with the Paleo-Pacific (Panthalassic) Ocean are controversial. Here we present petrological, zircon U-Pb age, whole-rock major and trace elements, and zircon Hf isotopic data for Permian (273–260 Ma) subduction-related mafic intrusions from the Luobei region along the western margin of the Jiamusi Block. These intrusions have mineral assemblages of pyroxene, Ca-rich plagioclase, and hornblende, indicating their crystallization from a water-saturated parental magma. The Luobei mafic rocks are calc-alkaline, enriched in large-ion lithophile and light rare earth elements, depleted in high-field-strength elements, and have variable zircon εHf(t) values (−7.7 to +5.0). These geochemical features suggest that their parental magma was probably derived by <10% melting of a spinel-facies mantle wedge metasomatized by variable amounts of sediment-derived melts and fluids from the subducted Paleo-Pacific (Panthalassic) oceanic slab. Crystal fractionation, mineral accumulation, and heterogeneous mantle sources were responsible for the geochemical and zircon Hf isotopic variations of the Luobei mafic intrusive rocks. Based on our results and regional geological data, a broadly N–S-trending magmatic arc that is 302–245 Ma in age and displays a westward younging trend is present in the Jiamusi Block. This suggests that persistent westward subduction of the Paleo-Pacific (Panthalassic) oceanic lithosphere occurred beneath the Jiamusi Block during the latest Carboniferous to earliest Triassic.