Zhaoxun Cheng

Late Early Jurassic continental arc magmatism in the northern Greater Khingan Range enables the investigation of complicated tectonic processes associated with the subduction and closure of the Mongol–Okhotsk Ocean. To further clarify the timing, genesis, and geodynamic mechanisms driving the magmatic activity during this period, the present study addresses these critical questions by integrating zircon U–Pb geochronological, geochemical, and isotopic analyses of a wide variety of igneous rocks, including gabbro, gabbro-diorite, granodiorite, porphyritic monzogranite, and biotite-bearing monzogranite from the Fushan region. Zircon U–Pb geochronology constrains the timing of magmatic activity to 184–179 Ma, coinciding with active subduction phases. Geochemical data reveal arc-like signatures characterized by enrichment in light rare-earth elements (LREEs) and large-ion lithophile elements (LILEs), together with pronounced depletion in high field strength elements (HFSEs). A comprehensive analysis of geochemical and Sr–Nd–Hf isotopic signatures suggests that the mafic rocks originated from an enriched lithospheric mantle modified by subduction-related fluids and sediment-derived melts. By contrast, the granodiorite and porphyritic monzonite exhibit adakitic characteristics, indicating partial melting of the thickened Mesoproterozoic lower crust with contributions from mantle-derived or newly formed crustal material. The biotite-bearing monzogranite, with its pronounced Eu anomaly and lower zircon saturation temperatures, reflects advanced magmatic differentiation from a shallower source. These findings indicate extensive crust–mantle interactions during the southward subduction of the Mongol–Okhotsk Ocean, driven by high-angle subduction and slab rollback.

Fe-Mn carbonate is the dominant mineral in the Erdaokan Ag deposit, which represents the first large independent silver deposit during the Late Triassic Period in the Duobaoshan Cu-Mo-Au mineralization concentrated area of Heilongjiang Province, NE China. The Fe-Mn carbonates in the deposit frequently co-exist with Ag minerals. Thus, the presence of Fe-Mn carbonates plays a crucial role in the ore-formation process, making their analysis essential for obtaining valuable metallogenic information about the Erdaokan deposit. Through microexamination, SEM and EDS analysis, a clear relationship between Fe-Mn carbonate minerals and Ag minerals was established. Furthermore, electron probe microanalysis, LA-ICP-MS, and Sr-Nd isotope tests were conducted to analyze Fe-Mn carbonates for significant metallogenic insights. The distribution pattern of trace elements and rare-earth elements in Fe-Mn carbonates is similar, characterized by Zr depletion (below 0.131 ppm), enrichment of light rare-earth elements, a noticeable deficit of Eu (δEu = 0.06–0.63), and an average Y/Ho value of 34.29, indicating the involvement of upper mantle-derived deep magma in the formation of ore-forming materials. The samples had a Sm-Nd isochron age of 233.7 ± 1.2 Ma, suggesting that the Erdaokan Ag deposit was formed during the Late Triassic Period. This study highlights the significance of Fe-Mn carbonate as a valuable mineral indicator for regional silver prospecting purposes, and confirms the Late Triassic Period as another important metallogenic stage in the Duobaoshan Cu-Mo-Au mineralization concentrated area.