In this study, we present zircon U-Pb ages, Hf-O isotopes, and whole-rock geochemical data for the newly identified Duobaoshan appinite-granite suite in the eastern Central Asian Orogenic Belt (CAOB). Field and petrographic investigations show that this suite of rocks is composed of hornblende gabbro, quartz monzonite, and granite porphyry. The appinites are abnormally hornblende-rich, and contain spectacular hornblende phenocrysts, pegmatitic textures, and miarolitic cavity structures that indicate an anomalous water-rich origin. The quartz monzonite and granite porphyry show intrusive contact relations with the appinites. Secondary ion mass spectrometry (SIMS) zircon dating reveals that the Duobaoshan appinite-granite suite was emplaced in a very short time interval of 224–223 Ma. These samples show typical subduction related arc magmatic geochemical characteristics that are enriched in large-ion lithophile elements (LILEs) and light rare earth elements (REEs), depleted in high field strength elements (HFSEs) and middle and heavy REEs. All samples exhibit comparative total REEs and no significant evolutionary relationships. The appinites show high Sr/Y ratios (reaching ~100) and positive or zero Eu anomalies, which reveal amphibole fractionation promotion and plagioclase fractionation suppression in a water-rich magma system. The quartz monzonite possesses high Sr/Y and La/Yb ratios that exhibit adakite-like properties and were derived from thickened lower continental crust. The granite porphyry is silica-rich and non-adakitic. All samples have juvenile whole-rock initial 87Sr/86Sr ratios (~0.7038) and ɛNd(t) values. Zircons have depleted mantle-like ɛHf (t) (to +12.0) and δ18O (~+5.65‰) values. Combining these geochemical features with observations of the regional geology and tectonic setting, we propose that the Duobaoshan appinite-granite suite was generated under a Triassic back-arc extensional setting related to the southward subduction of the Mongol-Okhotsk Ocean. The appinites were derived from an unusual water-rich mantle source, which was previously metasomatized by slab-derived fluids. The quartz monzonite and granite porphyry were generated from partial melting of the juvenile lower continental crust triggered by coeval appinitic magma underplating.

This paper presents new zircon U-Pb ages and whole-rock geochemical data for Triassic granitoids in the Duobaoshan ore field, in the eastern Central Asian Orogenic Belt (CAOB). Dating results reveal that the Triassic granitoids in the Duobaoshan area were emplaced ca. 244–223 Ma. These samples are typical subduction-related arc magmatic rocks that are enriched with large-ion lithophile elements (LILEs), depleted in high field strength elements (HFSEs), and have relatively high Sr (average 1,036 ppm) and low Y (average 10 ppm) and Yb (average 1 ppm) contents that exhibit adakite-like geochemical properties. The samples feature a juvenile whole rock average initial 87Sr/86Sr ratio of 0.70394 and an average ɛNd(t), zircon ɛHf(t), and two-stage Hf model age of 4.62, 10.29, and 766 Ma respectively. These geochemical features reveal a thickened lower crust source. Comprehensive analyses of the early Paleozoic, Triassic, and early Jurassic granitoids in the Duobaoshan area indicate that the magmatism of these three stages was related to the melting of the juvenile lower continental crust materials. The early Paleozoic was the primary period of vertical crust growth, during which time the juvenile lower continental crust was formed. The Triassic granitoids derived from magma were generated by partial melting of the thickened juvenile lower crust and the Jurassic granitoids derived from the partial melting of the delaminated juvenile lower crust. Geological observation and tectonic setting analyses reveal that the Paleo-Asian Ocean tectonic regime in the Duobaoshan area switched to the Mongol-Okhotsk Ocean in the Mesozoic after its closure. The early Paleozoic granitoids were formed in a subduction setting related to the northwestward subducted Paleo-Asian Oceanic slab underlying the Erguna-Xing’an composite block. The Triassic granitoids were formed under a supra-subduction extensional regime related to the southward subduction of the Mongol-Okhotsk Ocean, and the Jurassic granitoids were generated under a slab rollback setting related to the late stage subduction of the Mongol-Okhotsk Ocean.