The Petrogenesis of the Earliest Early Cretaceous Subvolcanic Granitic Intrusions in the Huoluotai Area, Northern Great Xing’an Range: Implications for the Regional Compression–Extension Tectonic Transition in the Eastern Mongol–Okhotsk Tectonic Doma

Late Mesozoic magmatic rocks are widely exposed in the northern Great Xing’an Range, a key region recording Phanerozoic crustal growth along the eastern Central Asian Orogenic Belt (CAOB). This study integrates petrology, LA–ICP–MS zircon U–Pb geochronology, zircon Hf isotopes, and whole-rock geochemistry for Late Mesozoic granitic subvolcanic intrusions in the Huoluotai area, northern Great Xing’an Range. Zircon U–Pb dating yields crystallization ages of 143.4 ± 1.2 Ma for the felsite and 138.8 ± 2.0 Ma for the rhyolite porphyry, documenting early Early Cretaceous emplacement. Geochemical data reveals that the investigated felsite and rhyolite porphyry exhibit high SiO2, K2O and total alkali (K2O + Na2O) contents, coupled with low MgO, Mg#, Ni, Sr and Yb values, and thus belong to shoshonitic I-type granites. The samples are enriched in large-ion lithophile elements (LILIs; e.g., Rb, K, U) and light rare earth elements (LREEs), with depletion of high-field-strength elements (HFSEs; e.g., Nb, Ta, P, Ti) and heavy rare earth elements (HREEs), accompanied by pronounced negative Eu anomalies, typical of crust-derived magmas. Zircon εHf(t) values range from −2.25 to +1.57 for the felsite (TDM2 = 1092–1334 Ma) and −0.65 to +3.58 for the rhyolite porphyry (TDM2 = 960–1229 Ma). Zircon Hf isotopic features demonstrate that the primary magmas were mainly derived from Mesoproterozoic juvenile crust, accompanied by variable mixing with ancient sialic crustal components. The felsite and rhyolite porphyry formed in a post-collisional extensional setting, constraining the final closure of the eastern Mongol–Okhotsk Ocean to earlier than ~144 Ma. By contrast, Late Jurassic (150–146 Ma) Mo-mineralization-related granitoids in this area are Na-rich adakites with high Sr and low Yb, generated during oceanic subduction. The remarkable geochemical transition from 150–146 Ma subduction-related Na-rich adakites to 143–139 Ma post-collisional K-rich granites restricts a tectonic transition from compression to extension at ~145 Ma in the northern Great Xing’an Range.