Zhigang Song

The Shitoukoumen area in central Jilin Province, NE China, preserves a Permian blocks-in-matrix assemblage containing pre-Permian mafic and sedimentary blocks that provides an important record of the Paleozoic evolution of the eastern Paleo-Asian Ocean. We present petrographic, zircon U-Pb geochronological and Lu-Hf isotopic, and whole-rock geochemical and Sr-Nd isotopic data for newly discovered amphibolites. Zircons from the amphibolites show metamorphic characteristics, and zircon U-Pb dating yielded an age of 503 ± 4 Ma, recording Late Cambrian metamorphism. The amphibolite is tholeiitic, with low SiO₂ (48.46–52.22 wt.%) and K₂O (0.07–0.17 wt.%) and relatively high MgO (4.48–6.31 wt.%), Fe₂O₃^T (11.01–15.48 wt.%), and CaO (9.07–12.55 wt.%). It has low total REE abundances and slightly LREE-depleted patterns, with no negative Nb-Ta anomalies. Its ε_Hf(t) and ε_Nd(t) values are +14.9 to +20.4 and +7.2 to +8.4, respectively, while the initial ⁸⁷Sr/⁸⁶Sr ratios are slightly elevated, consistent with seawater alteration. These geochemical and isotopic characteristics are consistent with an N-MORB-like oceanic crustal protolith derived from partial melting of a depleted spinel-facies mantle source. Together with the spatial association with the Zhangjiatun–Toudaogou–Xiaosuihe SSZ ophiolite, the amphibolite may represent a component of a metamorphic sole related to Late Cambrian intra-oceanic subduction initiation of the Paleo-Asian Ocean, although this interpretation remains tentative.

The Early Cretaceous Weideshan granites are associated with large-scale Au and polymetallic Cu-Mo-Pb-Zn mineralization. To investigate the petrogenesis of the Weideshan granite and constrain its tectonic setting during the Early Cretaceous, we conducted a zircon U-Pb-Hf isotope and whole-rock geochemical and Sr-Nd isotopic study of the granite. In situ zircon U-Pb dating of three granite samples yielded Early Cretaceous ages of 112.83 ± 0.80, 112.64 ± 0.91, and 111.82 ± 0.78 Ma. The samples had high-K calc-alkaline compositions and were enriched in the light rare earth and large-ion lithophile elements (e.g., K, Rb, Ba, Th, and U) and depleted in high-field-strength elements (e.g., Nb, Ti, and P). The samples had small negative Eu anomalies and initial ⁸⁷Sr/⁸⁶Sr and ε_Nd(t) values of 0.70853–0.71029 and –18 to –14, respectively. The zircon ε_Hf(t) values varied between −16 and −12, with corresponding two-stage model ages (t_DM2) of 2180–2000 Ma. These characteristics indicated that the Weideshan pluton was formed in a back-arc extensional environment caused by subduction of the Paleo-Pacific Plate toward the Asian continent during the early Cretaceous. The magma was generated by crust–mantle interaction during lithospheric thinning. The diagenetic age of the Weideshan granites was the same as the formation age of gold and polymetallic ores in the Jiaodong area. The extensive fluid circulation induced by the magma emplacement may be the main source of ore-forming materials for the gold and polymetallic Cu-Mo-Pb-Zn deposits.

Mesozoic alkaline complexes associated with the rare earth element (REE) mineralization are developed in southeastern North China Craton (NCC), and they recorded some important information about the lithospheric thinning and destruction of the NCC. Zircon U–Pb dating results reveal that syenitic rocks from the Chishan and Longbaoshan alkaline complexes were emplaced at ca. 126.2–123.7 Ma. These syenitic rocks have an arc-like affinity with enriched LREEs and LILEs (e.g., Ba and Th) and depleted HFSE (e.g., Nb and Ta). However, they plot within the extensional alkaline–calc-alkaline field in the lg (CaO/(K₂O + Na₂O)) versus SiO₂ discrimination diagram and plot within the fields of the divergent plate and within-plate settings in the Nb_N versus Th_N discrimination diagram, implying an extensional tectonic setting. Thus, we suggest that the syenitic rocks should form in an extensional tectonic settings related to rollback and retreat of the subducting Paleo-Pacific plate. These syenitic rocks have similar Sr–Nd–Pb–Hf isotope compositions ((⁸⁷Sr/⁸⁶Sr)_i = 0.7062 to 0.7101, ε_Nd(t) = −8.2 to −15.0, ε_Hf(t) = −13.5 to −9.6), which are the same as the enriched subcontinental lithospheric mantle (SCLM) of the NCC. Taking into account not only our new data but also previously published data regarding the evolution of NCC in the Mesozoic, we conclude that the syenitic rocks from the Chishan and Longbaoshan complexes are derived from partial melting of the lithospheric mantle triggered by the Paleo-Pacific plate subduction. Moreover, the geochemical and Sr–Nd–Pb isotopic similarity between the Chishan REE ores, Longbaoshan altered rocks, and the syenitic rocks implied that they are homologous products of Early Cretaceous and that the mineralization shows inheritance to the magmatic hydrothermal evolution.

A suite of Paleogene mafic rocks was collected from boreholes in the Huimin Sag of the Jiyang Depression with the aim of investigating the petrogenesis and nature of mantle source for these rocks and further providing insights into the characteristic of related mantle plume. Whole-rock geochemical data indicate that the mafic rocks have relatively lower SiO₂ (42.93%–48.57%) contents and similar characteristics to alkaline basalt and belong to transitional calc-alkaline series. These samples were clearly enriched in LREEs and depleted in HREEs and were also characterized by the enrichment of LILEs, incompatible elements, and HFSEs, similar to those of the Ocean Island Basalt (OIB). In addition, they exhibited Pb enrichment; Y, Pr, and Yb depletion; absence of Nb-Ta anomalies; high Hf and low Zr; and Rb/Yb ratios exceeding 1.0, indicating characteristics of intraplate rift-type alkaline basalt. The samples exhibited (Th/Ta)_PM and (La/Nb)_PM ratios less than 1 and plotted within the OIB, EMI, and EMII fields, indicating that crustal components had no role in the generation of the rocks. With the exception of individual samples that have a distinctive range of values, the majority of samples have complex values of -1.15 to 5.56, indicating a mixture of different sources, which was also apparent in the δ¹⁸O-⁸⁷Sr/⁸⁶Sr diagram, in which the samples plot close to the downward nonlinear curve. Based on the isotopic and trace elemental analyses, these igneous rocks are intraplate rift-type alkaline basalt and are of mantle plume origin. The variations in ⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd, values, LREEs, and HFSEs were probably due to the different locations of the mantle plume for different samples. The primary magma of the rocks likely originated from the melting of a mantle plume and the further metasomatism of lithospheric mantle, continental, or oceanic crust.

The Permo–Triassic was a crucial interval for the Changchun–Yanji suture zone in NE China since it witnessed the late–stage tectonic evolution of the Paleo–Asian Ocean (PAO). Thus, provenance analysis of Triassic sedimentary rocks in this region can shed light on the eventual closure process of the PAO. In this study, we carried out petrological, geochemical, and detrital zircon U–Pb geochronological research on two (meta)–sedimentary rocks from the Yitong–Gongzhuling area located in the westernmost section of the Changchun–Yanji suture zone. LA–ICP–MS U–Pb ages of detrital zircons from the two samples indicate a middle Triassic maximum depositional age (~ 240 Ma). The two samples present a most significant Permo–Triassic (299–240 Ma) detrital zircon population that corresponds to the multi–phase subduction and collision–related magmatism along the Changchun–Yanji suture zone. In addition, a late Silurian–early Devonian (429–390 Ma) detrital zircon population is also present and presumably fed from a pre–existing accreted arc terrane along the northern margin of the North China Craton (NCC). In contrast to some previously reported Permo–Triassic sedimentary rocks in adjacent areas, our new results show an absence of ~ 2.5 and ~ 1.8 Ga ages characteristic of NCC magmatism or metamorphism. The results imply that the Yitong–Gongzhuling area probably has a higher relief than the adjacent area during the middle Triassic, preventing the deposition of materials from the NCC to the south. This is further evidenced by the proximal orogenic source, short–distance transportation, and rapid accumulation for the studied samples as indicated by the subhedral to anhedral and poor roundness morphology of detrital zircons. The inferred regional uplift was hypothesized to be caused by the continued transpression during the PAO final closure based on the widely distribute early–middle Triassic syn–collisional granitic rocks along the Changchun–Yanji suture zone.

This paper presents a new detailed study including zircon U-Pb-Hf isotopic, whole-rock geochemical and Sr-Nd isotopic analyses of conglomerate and granitic pebbles from the molasse deposits in central Jilin Province, NE China. These data are used to better constrain the Late Permian–Triassic tectonic evolution regarding particularly the final closure of the Paleo-Asian Ocean (PAO) along the Changhun-Yanji suture (CYS). Zircon U-Pb data indicate that the granitic pebbles formed in the end-Permian (254–253 Ma). The youngest detrital zircon age of 231 Ma from the conglomerate, and presence of the overlying Upper Triassic Sihetun Formation suggests that the molasse deposits on the Jin′gui Island formed during the Late Triassic. The end-Permian granitic rocks display high SiO₂ (66.07–74.77 wt %), with low MgO (0.55–2.05 wt %) and Mg# (31.61–43.64) values, together with depleted Hf and Nd isotopic values (ε_Hf(t) = +1.61 to +11.62; ε_Nd(t) = +3.3 to +4.2; (⁸⁷Sr/⁸⁶Sr)i = 0.706458–0.706842) and juvenile second-stage Hf model ages (1148–512 Ma), suggesting that they were probably generated by the partial melting of a Meso-Neoproterozoic juvenile metabasaltic lower crust. They are characterized by enrichments in large ion lithophile elements (LILEs) and depletions in high field strength elements (HFSEs), with affinities to igneous rocks forming in a subduction-related setting. This, combined with regional coeval subduction-related magmatic rocks, indicates that the PAO still existed along the CYS. In addition, the identification of Late Triassic molasse deposits on the Jin′gui Island in this study, coupled with occurrences of many Early Triassic syn-collisional granitoids along the CYS, indicates that the final closure of the PAO took place prior to the Late Triassic.

The Late Paleozoic–Early Mesozoic tectonic evolution of the Changchun-Yanji suture (CYS) was mainly associated with the Paleo-Asian and Mudanjiang tectonic regimes. However, the spatial and temporal overprinting and variations of these two regimes remains are still dispute. In order to evaluate this issue, in this contribution, we present new zircon U-Pb ages and a whole-rock geochemical and zircon Hf isotopic dataset on a suite of metamorphic rocks, including gneisses, actinolite schist, leptynites, and biotite schists, from tectonic mélanges in northern Liaoning and central Jilin provinces, NE China. Based on zircon LA-ICP-MS U-Pb dating results, protoliths show wide ranges of aging spectrum, including Paleoproterozoic (2441 Ma), Early Permian (281 Ma), Late Permian (254 Ma), and Late Triassic (230 Ma). The Permian protoliths of leptynites from the Hulan Tectonic Mélange (HLTM) and gneisses from the Kaiyuan Tectonic Mélange (KYTM) exhibit arc-related geochemical signatures, implying that the Paleo-Asian Ocean (PAO) did not close prior to the Late Permian. The Late Triassic protoliths of gneisses from the KYTM, in combination with previously reported coeval igneous rocks along the CYS, comprises a typical bimodal igneous suite in an E–W-trending belt, suggesting a post-orogenic extensional environment. Consequently, we infer that the final closure of the PAO took place during the Early–Middle Triassic. The Early Permian protoliths of biotite schists from the HLTM are alkali basaltic rocks and contain multiple older inherited zircons, which, in conjunction with the geochemical features of the rocks, indicate that they were generated in a continental rift related to the initial opening of the Mudanjiang Ocean (MO). Data from this contribution and previous studies lead us to conclude that the MO probably opened during the Middle Triassic, due to the north–south trending compression caused by the final closure of the PAO.