Late Mesozoic Tectonic Transfer in the Southern Great Xing'an Range, NE China: Evidence From Geochronology, Geochemistry and Lu–Hf Isotopes of Intermediate-Acid Intrusions in Eastern Inner Mongolia

Abstract

The geodynamic background of late Mesozoic magmatism in eastern Inner Mongolia remains debated owing to multi-plate influences. Here, we present zircon U–Pb geochronology, geochemistry and Hf isotope data for Late Jurassic diorite and Early Cretaceous quartz monzonite porphyry and granite porphyry to investigate the petrogenesis and tectonic setting. Zircon U–Pb data suggest that multiphase magmatism extended from the Late Jurassic to the Early Cretaceous, that is, 161.0 ± 1.0 Ma (diorite), 129.9 ± 1.4 Ma (quartz monzonite porphyry), 125.8 ± 1.1 Ma and 126.8 ± 1.1 Ma (granite porphyry). The dioritic samples have lower SiO₂ (55.41–56.75 wt%), higher MgO (4.22–4.57 wt%) and Mg^# (52–55). The quartz monzonite shows mediate SiO₂ contents (63.17–64.89 wt%), belonging to the high-K, calc-alkaline series. The granite porphyry samples exhibit higher SiO₂ (74.49–77.03 wt%) and Na₂O + K₂O (6.37–7.38 wt%) contents, while lower Mg# (20–25). The diorites and quartz monzonite porphyry show slight negative Eu anomalies (δEu = 0.85–0.88 and 0.78–0.90, respectively), while Eu anomalies are intensely for the granite porphyry (δEu = 0.45–0.67). All these rocks show enrichments in LILEs (U and Pb) and depletions in HFSEs (Nb, Ta, P and Ti). The clinopyroxenes are classified as augite and diopside, and they show slightly inverted U-shaped patterns in the chondrite-normalised REE diagram. The plagioclases are mainly andesine from the diorites, while they are albite from the quartz monzonite porphyry and granite porphyry. The ε_Hf (t) values of zircon crystals range from +3.04 to +5.90, +2.04 to +4.10 and +1.93 to +4.35. These data indicate that the diorite formed by low degree partial melting of a depleted mantle wedge metasomatized by fluids. The quartz monzonite porphyry was generated by partial melting of the lower crust triggered by the underplating of depleted mantle-derived basaltic magma. The granite porphyry is classified as S-type granite and was sourced from greywacke. Combined with previous data, we conclude that Late Jurassic magmatism was related to the southward subduction of the Mongol-Okhotsk Ocean, while the Early Cretaceous magmatic rocks formed in a post-collision extensional setting.