Zircon U-Pb dating and tectonic significance of Shichang-shan pluton in central Da Hinggan Mountains

In this paper, the authors conducted geochronologic and geochemical studies of the Shichangshan pluton in central Da Hinggan Mountains. Zircon U-Pb dating demonstrates that the weighted mean ²⁰⁶Pb/²³⁸U ages for the zircons from the monzonitic granite are 201.69±0.72Ma (MSWD=0.04) and 202.09 ±0.69Ma (MSWD=0.12), suggesting late Triassic emplacement. Petrology and geochemistry show that it is of the alkaline and peraluminous series. The monzonitic granite is characterized by high SiO₂ (73.77~77.69), high alkali (Na₂O+K₂O=6.7%~8.81%), high TFeO/MgO (7.24~45.57, 15.87 on average), low CaO (0.13%~1.01%), low MgO (0.08%~0.32%) and TiO2 (0.08%~0.31%). The monzonitic granite is characterized by a "sea-gull" REE pattern, significant negative Eu anomaly (δEu=0.04~0.61, 0.23 on average), depletion of Sr (18.31×10^-6~67.91×10^-6), Ba(50.07×10^-6~265.5×10^-6) and enrichment of Yb (5.09×10^-6~17.33×10^-6), Zr (91.34×10^-6~192.26×10^-6) and Y(35.44×10^-6~113.07×10^-6), suggesting that it is A-type granite. The high Rb/Sr ratios (ranging from 3.92 to 21.69, 10.14 on average) and Rb/Nb ratios (ranging from 13.9 to 54.8, 22.1 on average) of the rock indicate a crustal origin. A comprehensive analysis shows that the monzonitic granite was formed by low pressure felsic crust partial melting. In Na₂O-K₂O diagrams, it belongs to A-type granite. The Rb-(Y+Nb), Rb-(Yb+Ta), Nb-YCe diagrams in combination with regional tectonic evolution suggest that the monzonitic granite was formed in a post-orogenic setting.