Formation age, geochemical characteristics and petrogenesis of syenogranite in Chaihe area, central Daxingan Mountains: Constraints on Late Carboniferous evolution of the Xing'an and Songnen blocks

Authors: DONGXUE LI, CHANGQING ZHENG, CHENYUE LIANG, XIAO ZHOU

Abstract: The tectonic evolutionary history between the Xing'an Block (XB) and Songnen Block (SB) in the eastern Central Asia Orogenic Belt (CAOB) has been hotly debated. In this study, we present a series of new data to provide a better constraint on the magmatic process during plate subduction and its implications for the regional tectonic evolution of XB and SB, even the CAOB. The whole-rock geochemistry and zircon U-Pb chronology of syenogranite in the Chaihe area of the Great Xing'an Range have been carried out. The dating results show that the syenogranite was formed in the Late Carboniferous during the 303.1-316.1 Ma. The SiO₂ content of dated samples is around between 65.43%~75.88%, while the total alkali content (K₂O+Na₂O) is 7.23%~10.19%, the content of MgO (0.07%~0.63%) and the value of Mg# is 0.14~0.36. Moreover, they have right-inclined REE distribution patterns [(La/Yb)_N = 1.23- 15.61] with slight negative or inappreciable Eu anomalies (δEu = 0.06-0.49). All samples were enriched in LILEs (e.g., Rb and K) and depleted in HFSEs (e.g., Nb, Ta and Ti). Based on these data, combined with their trace element characteristics, we conclude that these rocks in our study area were likely derived from partial melting of the crust. Combining with regional tectonic evolution studies and our petrological and geochemical studies, we propose that they formed in a post-collisional extensional tectonic setting that developed after the amalgamation of the Xing'an and Songnen blocks and closure of the Nenjiang Ocean between them during the Late Carboniferous.

Keywords: Carboniferous syenogranite, Great Xing'an Range, Hegenshan?Heihe Suture Zone, central Asia Orogenic Belt

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