Paleozoic magmatism in the Nenjiang area, NE China: implications for the tectonic evolution of the Heihe-Hegenshan Suture

The timing and geodynamic processes of the transition from oceanic subduction to continental collision along the Heihe–Hegenshan Suture remain debated, particularly the relationship between initial collision and final amalgamation of the Xing’an and Songliao-Xilinhot blocks. This study presents new zircon U-Pb age determinations and comprehensive whole-rock geochemical analyses for Late Paleozoic magmatic and metamorphic rocks from the Xinkailing Complex, situated on the northeastern margin of the Xing’an Block in the Nenjiang area. These findings provide new constraints on the Late Paleozoic tectonic evolution of the Heihe-Hegenshan Suture. Our new U-Pb data indicate a maximum depositional age of 311 Ma for the sedimentary protolith of biotite plagioclase gneiss. The magmatic rocks, including quartz diorites, gneissic granodiorite and the magmatic protolith of amphibolite, were emplaced from the Late Carboniferous to the Early Permian, at 318–319 Ma, 288 Ma, and 278 Ma, respectively. Metamorphic zircon populations record subsequent metamorphism during the Middle Jurassic, at approximately 175–169 Ma, and 164 Ma. Geochemical evidence suggests that the protolith of the biotite-plagioclase gneiss was a Late Carboniferous ferruginous-aluminous sedimentary deposit formed in a continental island arc setting. The Late Carboniferous quartz diorites are characterized by high Sr/Y ratios (30–113), elevated Mg# values (43–54) and high MgO contents (1.80–5.67 wt%), coupled with depleted heavy rare earth elements (HREE), indicative of adakitic affinities. The Early Permian gneissic granodiorite also shows elevated Sr/Y ratios (71–93) and depleted HREE, suggesting adakitic affinities, but with lower MgO contents (1.41–1.65 wt%) than the quartz diorites. The protolith of the amphibolite is interpreted as basaltic volcanic rock formed during the Early Permian. The geochemical compositions of the quartz diorites point to derivation from magmas generated by partial melting of a subducted oceanic slab. In contrast, the low-MgO gneissic granodiorite is likely derived from magmas produced by partial melting of a thickened mafic lower continental crust. The basaltic protolith of the amphibolite is interpreted to have formed from the mantle wedge in a subduction setting. By integrating the new data with previous regional research, it is concluded that the initial collision between the Xing’an Block and the Songliao-Xilinhot Block occurred during the Early Permian, whereas the final amalgamation of the two blocks did not take place until after the Early Permian.