Early early-Cretaceous post-collisional tectonic setting of the southern segment of the Great Xing'an Range: Evidence from the Lanjiayingzi gabbro-diorite in Linxi area

Based on the systematic petrology, zircon chronology and geochemistry analysis of the Lanjiayingzi gabiodiorite pluton in the north of Linxi County in southern Great Khingan Range, genesis of the pluton and the Early Cretaceous tectonic setting are discussed.Zircon U-Pb dating reveals that the pluton was formed in Early Cretaceous (145.6±0.6 Ma).Geochemistry of the pluton is characterized by 53.38%~54.45% of SiO₂ contents, 1.34%~1.43% of K₂O contents and 3.85%~4.05 % of Na₂O contents, with 0.8~0.9 of A/CNK, which indicates that the pluton belongs to a calc-alkaline metaluminous magmatic rock.Its MgO and TFe₂O₃ contents vary from 5.44%~5.73% and 7.53%~8.33% respectively, with 36.96 to 38.24 of Mg^# values.Combined with the girdle of plagioclase and the poikilitic texture of olivine in clinopyroxene, it is suggested that the pluton was the product of the fractional crystallization of mantle-derived primary magma.In addition, the pluton is relatively rich in LILEs such as Rb and Ba, and depletion in HFSEs such as Nb and Ta, with +5.4 to +9.0 of zircon ε_Hf(t) values, which indicates that the pluton was derived from a depleted lithospheric mantle that experienced metasomatism from the subduction fluid.The Lanjiayingzi gabbrodiorite and coeval A-type granites in Linxi area constitute the bimodal magmatic assemblage, which suggests that they were formed in an extensional setting.Combined with regional geology, it is suggested that the formation of the Early Cretaceous magmatic rocks in southern Great Khingan Range has relation with the post-collision extensional setting resulting from closure of the Mongol-Okhotsk Ocean.