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

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; εHf_(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 εHf_(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.

我们报道了中国东北兴安地块东缘火山岩的全新锆石U-Pb（zircon U–Pb）、Hf同位素（Hf isotopic）以及全岩地球化学（whole-rock geochemical）数据，以期深化对兴安地块与松嫩-张广才岭地块之间缝合带位置的认知。锆石二次离子质谱（secondary ion mass spectrometry）U-Pb定年结果显示，该套火山岩形成于奥陶纪早-中期（473~463 Ma）。与同时期莫古奇玄武岩（稀土元素[rare earth element, REE] = 171~183 ppm；εHf(t) = +0.3~+2.7；T_DM1 = 1074~977 Ma）相比，多宝山安山岩的稀土元素总丰度更低（109~131 ppm），但重稀土元素含量相对偏高，高场强元素（high-field-strength element）亏损更为显著，εHf(t)值更高（+13.0~+14.8），且T_DM1年龄更年轻（559~484 Ma）。这表明多宝山安山岩的原生岩浆来源于受俯冲相关流体（subduction-related fluids）交代的相对亏损地幔楔（depleted mantle wedge）的部分熔融（partial melting）；而莫古奇地区玄武岩的原生岩浆则可能源自同样受俯冲板片（subducted slab）流体改造的相对富集岩石圈地幔（lithospheric mantle）的部分熔融（partial melting）。上述认识表明，多宝山安山岩形成于新增生的岛弧环境（island arc setting），而同期莫古奇玄武岩则形成于活动大陆边缘（active continental margin）。据此，我们将兴安地块东缘的奥陶纪早-中期火山作用归因于嫩江-黑河洋板块向兴安地块下方的北西向俯冲。此外，结合兴安地块南部同期的岩浆活动（igneous activity），我们认为早古生代（early Palaeozoic）（490~420 Ma）兴安地块边缘存在一条岩浆弧（magmatic arc）。最终我们确定，兴安地块与松嫩-张广才岭地块之间的缝合带位置为：自阿日根苏木（Airgin Sum）经锡林浩特（Xilinhot）以南，至乌兰浩特（Ulanhot）、莫古奇、嫩江，最终延伸至黑河（Heihe）。