Early-Middle Ordovician volcanism along the eastern margin of the Xing'an Massif, Northeast China: constraints on the suture location between the Xing'an and Songnen-Zhangguangcai Range massifs

摘要

We present new zircon U-Pb and Hf isotopic as well as whole-rock geochemical data for volcanic rocks from the eastern margin of the Xing'an Massif, Northeast China, in order to further our understanding of the suture location between the Xing'an and Songnen-Zhangguangcai Range massifs. Zircon secondary ion mass spectrometry U-Pb dating indicates that the volcanic rocks formed during the Early-Middle Ordovician (473-463 Ma). Compared with the coeval Moguqi basalts (rare earth element [REE] = 171-183 ppm; epsilon Hf-(t) = +0.3 to +2.7; T-DM1 = 1074-977 Ma), the Duobaoshan andesites exhibit lower overall REE abundances (109-131 ppm) with relatively high heavy REE contents, stronger high-field-strength element depletion, higher epsilon Hf-(t) values (+13.0 to +14.8), and much younger T-DM1 ages (559-484 Ma). This suggests that the primary magma for the andesites was generated by the partial melting of a relatively depleted mantle wedge that was metasomatized by subduction-related fluids. The primary magma for the basalts in the Moguqi area was probably derived from the partial melting of a relatively enriched lithospheric mantle that was also modified by fluids sourced from a subducted slab. These interpretations suggest that the andesites in Duobaoshan formed in a newly accreted island arc setting, whereas the coeval basalts in Moguqi formed along an active continental margin. We therefore attribute the Early-Middle Ordovician volcanism along the eastern margin of the Xing'an Massif to the northwestward subduction of the Nenjiang-Heihe oceanic plate beneath the Xing'an Massif. Furthermore, considering coeval igneous activity in the southern parts of the Xing'an Massif, we suggest that a magmatic arc existed along the margin of the Xing'an Massif in the early Palaeozoic (490-420 Ma). We conclude that the location of the suture between the Xing'an and Songnen-Zhangguangcai Range massifs runs from Airgin Sum, via south of Xilinhot, to Ulanhot, Moguqi, Nenjiang, and finally Heihe.