| This thesis studies on the Mesozoic intrusive rocks in the Erguna Massif of NE China. LA-ICP-MS and SIMS zircon and titanite U-Pb dating results, and hornblende and biotite 40Ar/39 Ar dating results indicate that the Mesozoic magmatic activity in the Erguna Massif can be subdivided into seven stages at ～246 Ma, ～225 Ma, ～205 Ma, ～185 Ma, ～155 Ma, ～137 Ma, and ～125 Ma. In addition, the intrusive rocks located close to the Ergunahe Fault experienced late twice deformations, i.e., the early Early Cretaceous(corresponding to the ～137 Ma magmatism) and the late Early Cretaceous(corresponding to the ～125 Ma magmatism). Based on the whole-rock major and trace element data, and zircon Hf isotope data, together with their rock association, spatial-temporal extent, and regional tectonic events, this thesis reveals the petrogenesis and tectonic settings of different stages of the Mesozoic intrusive rocks, the Mesozoic tectonic evolution of the Mongol-Okhotsk suture zone, and its influence extent and metallogenic implications. 1. Late Permian-early Middle Triassic(～246 Ma) — an active continental margin settingThe ～246 Ma intrusive rocks comprise a suite of high-K calc-alkaline diorites, quartz diorites, granodiorites, monzogranites, and syenogranites, with mafic micro-granular enclaves(MMEs). The linear correlation between the major elements and SiO2 and their zircon εHf(t) values of –2.0～+6.6, suggest that the magma for ～246 Ma intrusive rocks could be derived from partial melting of juvenile crustal material, which underwent fractional crystallization, contamination, and magma mixing. The ～246 Ma granitoids are I-type granites and have a NE–SW orientation along the Mongol-Okhotsk suture zone, indicating that they formed within an active continental margin setting related to the southward subduction of the Mongol-Okhotsk oceanic plate beneath the Erguna Massif. 2. late Middle Triassic-early Late Triassic(～225 Ma) — a pause during the subduction processThe ～225 Ma intrusive rocks consist of gabbro–diorites, granodiorites, and syenogranites, displaying a bimodal igneous association. The gabbro–diorites are similar in chemistry to the primary magma. Moreover, they are enriched in light rare-earth elements(LREEs) and large ion lithophile elements(LILEs), and depleted in heavy rare-earth elements(HREEs) and high field-strength elements(HFSEs). Additionally, their zircon εHf(t) values range from +0.8 to +8.8. These characteristics indicate that their primary magma was derived from partial melting of a mantle wedge metasomatized by subduction-related fluids. The syenogranites are I-type granites and their zircon εHf(t) values range from –1.5 to +4.9, suggesting the magma was derived from partial melting of a juvenile lower crust. The granodiorites have an affinity to adakites with low K2 O contents, whose magma was derived from partial melting of the underplated basaltic crust. Combined with the weak magmatism, the ～225 Ma bimodal igneous rock association formed within an extensional environment in a pause during the subduction process of the Mongol-Okhotsk oceanic plate. 3. late Late Triassic-early Early Jurassic(～205 Ma) — an active continental margin settingThe ～205 Ma intrusive rocks in the Erguna Massif are dominated by granodiorites, monzogranites, and syenogranites, as well as subordinate intermediate–mafic rocks, comprising diorites, gabbro–diorites, and hornblende gabbros. The gabbro–diorites and hornblende gabbros are enriched in LREEs and LILEs, and depleted in HREEs and HFSEs. Moreover, their zircon εHf(t) values range from-2.9 to +5.9. These characteristics suggest that their magma was derived from partial melting of a mantle wedge metasomatized by subduction-related fluids. The coeval intermediate–felsic rocks are medium-K or high-K calc-alkaline I-type granitoids and have zircon εHf(t) values of-1.8～+6.1, indicating that their magma originated by partial melting of a depleted lower crust with minor contribution from ancient crustal material. Taken together, the ～205 Ma igneous rocks formed within an active continental margin setting related to the southward subduction of the MongolOkhotsk oceanic plate beneath the Erguna Massif. 4. Early Jurassic(～185 Ma) — an active continental margin settingThe ～185 Ma intermediate–felsic intrusive rocks comprise diorites, monzogranites, syenogranites, quartz porphyrys, and granite porphyrys. The monzogranites are I-type granites, whereas the syenogranites are A-type granites. Combined with their zircon εHf(t) values of +2.8～+11.6, the magma for these granitoids originated by partial melting of a depleted lower crust. These characteristics, together with the presence of the coeval calc-alkaline basalts, basaltic andesites, and andesites, indicate that the ～185 Ma igneous rocks formed under an active continental margin setting related to the southward subduction of the Mongol-Okhotsk oceanic plate beneath the Erguna Massif. 5. Late Jurassic(～155 Ma) — a post-orogenic extensionThe ～155 Ma intrusive rocks in the Erguna Massif consist of syenogranites with coeval MMEs, monzonites, and quartz monzonites. The syenogranites have an A-type granite affinity with zircon εHf(t) values of +0.7～+6.9, implying that their magma generated by partial melting of a depleted lower crust with magma mixing. The monzonites belong to alkaline series. Furthermore, they have high Mg# values, and high Cr, Co, and Ni contents. Combined with their zircon εHf(t) values of +0.4～+3.2, the monzonites formed from primary magmas generated by partial melting of a mantle wedge modified by subduction-related fluids. The quartz monzonites have an affinity to adakites with high K2 O contents and Mg# values, which was generated by partial melting of a delaminated lower crust and subsequently interacted with mantle material. The presence of ～170 Ma muscovite granite with adakitic characteristics, as well as a coeval regional unconformity in the northern part of Hebei Province and the western part of Laioning Province(NHWL), suggests that the Mongol-Okhotsk Ocean adjacent to the northwestern Erguna Massif closed during the Middle Jurassic. Therefore, ～155 Ma magmatism in the Erguna Massif occurred in an extensional environment related to the collapse or delamination of a thickened lithosphere after closure of the Mongol-Okhotsk Ocean. 6. early Early Cretaceous(～137 Ma) — an extensional environmentThe ～137 Ma syenogranites are A-type granites, and have zircon εHf(t) values of +5.6～+9.2, suggesting that the primary magma was derived from partial melting of a juvenile lower crust. This information, combined with the presence of A-type rhyolites of the coeval Jixiangfeng Formation in the Erguna Massif, suggests that the early Early Cretaceous magmatism and deformation in the study area occurred in an extensional environment. The regional unconformity between the Upper Jurassic strata and the overlying Lower Cretaceous strata in the NHWL and the basins located in the southeast of the Mongol-Okhotsk suture zone represents another thrust–nappe tectonic event. In summary, the ～137 Ma magmatism and deformation occurred in an extensional environment related to the collapse or delamination of a thickened continental crust after closure of the Mongol-Okhotsk Ocean. 7. late Early Cretaceous(～125 Ma) —an extensional environmentThe late Early Cretaceous rhyolites of the Shangkuli Formation in the Erguna Massif are A-type rhyolites. This fact, combined with the presence of coeval basalts of the Yiliekede Formation, indicates that late Early Cretaceous volcanic rocks in this area are bimodal, suggesting an extensional environment. This extension event was related to either subduction of the Paleo-Pacific Plate, delamination of the previously thickened crust related to the Mongol-Okhotsk tectonic regime, or the combined effects of these two tectonic regimes. 8. The spatial-temporal extent of influence of the Mongol-Okhotsk tectonic regime on NE China and its metallogenic implicationsAccording to the geochemical characteristics of the Late Paleozoic-Mesozoic igneous rocks in the Erguna Massif, lithostratigraphic relationship, and paleogeography reconstruction, the influence of the Mongol-Okhotsk tectonic regime on NE China began in the Late Permian and ended in the Early Cretaceous. Based on the spatial-temporal distribution of Mesozoic igneous rocks in NE China, it can be concluded that the influence of the Mongol–Okhotsk suture zone on NE China spatially includes to the west of the Songliao Basin(including the western slop of the basin), and to the northern margin of the North China Craton(NCC) and its north.Based on the discovery and distribution of the Mesozoic different types of deposits, it can be concluded that the formation of the porphyry Cu–Mo ore deposits and minor skarn Zn and Cu(Fe) deposits in the west of the Erguna Massif and the north of the Xing’an Massif is related to the early Mesozoic southward subduction of the Mongol-Okhotsk oceanic plate, whereas the formation of the epithermal Cu deposits and Ag-Pb-Zn-Mo deposits in the southern segment of the Great Xing’an Range and the northern margin of the NCC is related to the late Mesozoic extension after the closure of the Mongol-Okhotsk Ocean. |
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