Southward subduction of the Mongol–Okhotsk Ocean: constraints from Late Triassic granites in the Hailar Basin, NE China

The southward subduction of the Mongol–Okhotsk Ocean played a crucial role in the tectonic evolution of East Asia during late Paleozoic to Mesozoic. However, the details of the subduction process remain unclear. To better constrain the southward subduction of the Mongol–Okhotsk Ocean, we conducted zircon dating, whole-rock geochemical analysis, and Lu–Hf isotopic studies on Late Triassic granites from the Hailar Basin, NE China. The Late Triassic granites include granodiorites and monzogranites. Zircon U–Pb dating results indicate that the granodiorites and monzogranites were emplaced at 240 Ma and 232 Ma, respectively. The monzogranites are weakly peraluminous, with low concentrations of heavy rare earth elements and high field strength elements, and have low zircon saturation temperatures (average 786℃), indicating an I-type granite affinity. The granodiorites display high Sr/Y values and low concentrations of Y and HREEs, indicating adakitic features. Combining these characteristics with the depleted Hf isotopic compositions suggests that the monzogranites and granodiorites formed through partial melting of juvenile crustal materials and thickened basaltic lower crust, respectively. Based on our data, along with the temporal-spatial distribution of Mesozoic magmatism and episodic variations in crustal thickness along the southern margin of the Mongol–Okhotsk Ocean, we propose that periodic pulses of magmatism in the Erguna and Xing’an blocks are correlated with repeated steepening and shallowing of the Mongol–Okhotsk slab dip. We divided the subduction into five stages: (1) 260 to 225 Ma, the Mongol–Okhotsk Ocean subducted at a shallow angle beneath the Erguna and Xing’an blocks, (2) 225 to 215 Ma, flat-slab subduction occurred, (3) 215 to 205 Ma, the Mongol–Okhotsk Ocean transitioned from flat-slab subduction to slab rollback, (4) 205 to 180 Ma, the Mongol–Okhotsk Ocean experienced a renewed shallow-angle subduction, and subsequently, (5) 180 to 160 Ma, the final ocean closure and continental collision.