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This study presents new zircon LA-ICP-MS U–Pb ages, zircon Hf isotopes, and whole-rock geochemical compositions for Late Palaeozoic–Early Mesozoic intrusive rocks from the Erguna Massif, Northeast China. Zircon U–Pb dating results indicate that the intrusive rocks were emplaced during the Late Permian–Middle Triassic (ca. 256–234 Ma) and Late Triassic (ca. 210 Ma) in the western and eastern parts of the Erguna Massif, respectively. The Late Permian gabbros and quartz monzodiorites display enrichment in Rb and K, but are depleted in Nb, Ta, and Ti. Their εHf(t) values (+5.1 to +9.1 and −0.2 to +3.7, respectively) and one-stage Hf model ages (TDM1 = 544–704 Ma and 764–889 Ma, respectively) indicate that they might be derived from the reworking of the Neoproterozoic lithospheric mantle that was metasomatised by subduction-related fluids. The Late Permian–Early Middle Triassic quartz monzonites, monzogranites, and syenogranites display variable enrichment of rare earth elements (REEs) and large ion lithophile elements (LILEs). They have εHf(t) values varying from −5.7 to +2.7 with two-stage Hf model ages (TDM2) of 1.09–1.69 Ga, implying that they possibly originated from the Mesoproterozoic continental crust. The Late Triassic syenogranites show geochemical signatures of A2-type granites with various enrichment in LILEs and LREEs and depletion in Nb, Ta, and Ti. Combined with regional geological evidence, we conclude that Late Permian to Late Triassic igneous rocks on both sides of the Mongol–Okhotsk suture belt record an active continental margin environment associated with the ongoing bidirectional subduction of the Mongol–Okhotsk Ocean. The eastern Erguna Massif and Xing'an Massif were likely located in a back-arc setting at least since ca. 216 Ma, with long-term southward subduction of the Mongol–Okhotsk oceanic slab spanning from the Late Permian–Early Jurassic. [ABSTRACT FROM AUTHOR]

The Ordovician plutons in the Erguna Block, NE China, can be classified into two groups: Early Ordovician diorites with zircon U-Pb ages ranging from 486 to 485 Ma and Middle Ordovician gabbros and granites with zircon U-Pb ages ranging from 466 to 463 Ma. The diorites are calc-alkaline in nature and are characterized by weak to moderate enrichments of large ion lithophile elements (LILE) and light rare earth elements (LREE) relative to high field strength elements (HFSE) and heavy rare earth elements (HREE). The gabbros and granites have high total alkali contents, and all samples are enriched in LREE and LILE and depleted in HFSE such as Nb, Ta, and Ti. Isotopically, Early Ordovician diorites display values that are less radiogenic [εHf(t) = + 9.9-+16.8] compared to those of Middle Ordovician gabbros [εHf(t) = − 3.0-+5.0]. Middle Ordovician granites have positive εHf(t) values of +1.4 to +4.3 and two-stage Hf model ages (TDM2) of 1167 to 1356 Ma. These data indicate that the diorites may have been generated by the partial melting of a recently metasomatized mantle source, whereas the gabbros and granites may have been formed by the partial melting of enriched lithospheric mantle and Mesoproterozoic crust, respectively. Our results, combined with other regional results, suggest that Early Ordovician magmatism was likely associated with the northward subduction of the Heihe-Xilinhot oceanic plate beneath the Erguna-Xing'an Block, whereas the Middle Ordovician gabbros and granites were most likely formed in an extensional setting controlled by the rollback of this subducted oceanic plate. [ABSTRACT FROM AUTHOR]