Zircon U-Pb dating and geochemistry of the syenogranite from the Bianjiadayuan Pb-Zn-Ag deposit of Inner Mongolia and its tectonic implications

In this study, a series of analyses such as LA-ICP-MS zircon U-Pb isotopic dating and major elements, trace elements and Sr-Nd isotope composition investigation were performed for the syenogranite located in the deep layer of the Bianjiadayuan Pb-Zn-Ag polymetallic deposit, Inner Mongolia. Formed during the magmatic concentration period of early Cretaceous in southern Da Hinggan Mountains, the syenogranite in this deposit has age of (140.31±0.34) Ma. There were at least two periods of magmatic activity in the study area:Acid magma invaded in the early period, whereas intermediate magma and basic magma invaded about 10 Ma later. Ore-forming and rock-forming activities occurred over the same period. Geochemistry of major elements in the syenogranite is characterized by high SiO₂ and K₂O and low MgO, CaO and TiO₂ with A/CNK ratio between 0.98 and 1.19, suggesting metaluminous-weakly peraluminous series. The syenogranite is enriched in LILE such as Rb, Th, U and K and depleted in HFSE such as Sr, P and Ti. The ΣREE values are slightly high. The δEu lies between 0.12 and 0.14, exhibiting significant negative Eu anomalies. The initial ratio of (⁸⁷Sr/⁸⁶Sr)_i is between 0.7066 and 0.7077, while the initial ratio of (¹⁴³Nd/¹⁴⁴Nd)_i is between 0.5121 and 0.5122 (t=140 Ma); ε_Nd(t) values vary in the range of-5.0 to-6.6. Therefore, the petrogenetic materials were the products of partial melting of mafic-ultramafic source rock in middle Proterozoic lower crust. The analyses reveal that the syenogranite in the Bianjiadayuan deposit is A-type granite formed in the environment of high temperature and low pressure with the impact of the post-orogenic extension of Mongolia-Okhotsk scissor-type closed orogeny and lithospheric thinning in early Cretaceous. The high temperature and low pressure environment was probably related to the regional lithosphere demolition effect.