From Diorite to Monzogranite: The Early–Middle Jurassic Arc Magmatic Sequence of Xifengshan and Its Constraints on the Southward Subduction of the Mongol–Okhotsk Ocean

The subduction polarity of the Mongol–Okhotsk Ocean (MOO) during the Mesozoic remains contentious, with competing models advocating for southward, northward, or bidirectional subduction. The Xifengshan area in the northern Great Xing’an Range, located south of the Mongol–Okhotsk suture, preserves Early–Middle Jurassic calc-alkaline intrusions, which provides important constraints on this debate. We present zircon U–Pb ages, whole-rock geochemistry, and Lu–Hf isotopes for diorite, granodiorite, and monzogranite from this area. Zircon U–Pb dating yields ages of 178–173 Ma, defining a short-lived magmatic pulse. The suite is calc-alkaline, enriched in LILE and depleted in Nb–Ta–Ti, typical of arc magmas. The diorite represents the most mantle-proximal preserved end-member of the system and records substantial mantle input from a slab-modified mantle wedge. Geochemical trends (increasing Rb/Th, decreasing Sr with differentiation) reflect plagioclase-dominated fractional crystallization with minor AFC. Local adakitic-like signatures are better interpreted as differentiation-related effects than as direct evidence for slab melting. Zircon εHf(t) values (+1.62 to +11.55) and T_DM1 ages (363–772 Ma) are greater than the crystallization ages, indicating substantial juvenile input together with the variable involvement of previously accreted crustal components. We suggest that mantle wedge-derived magmas modified by slab-related components triggered the partial melting of the arc crust, whereas subsequent intracrustal differentiation produced the observed intrusive sequence. The continental arc system provides robust evidence for the southeastward subduction of the MOO during the Early–Middle Jurassic, resolving the long-standing polarity controversy.