The Shangdan Ocean represents the eastern branch of the Proto-Tethys Ocean, which is important to decipher the evolution of the Central China Orogenic Belt. However, its evolution is still in debated. In this contribution, we carried out an integrated study of zircon UPb ages, whole-rock geochemistry, as well as zircon HfO isotope compositions for the Lajimiao mafic complex in the Shangdan Suture Zone in the North Qinling orogen. The Lajimiao mafic complex is composed of coarse-grained gabbros to norite-gabbros (Group 1) in the inner and fine-grained diorites (Group 2) in the outer part. The Group 1 samples are comprised predominantly of salite to augite (Mg^# = 75–80), magnesi-hornblende (Mg^# = 65–67) and Ca-rich plagioclase (An = 45–67); the Group 2 samples are comprised predominantly of magnesi-hornblende (Mg^# = 60–66) and plagioclase (An = 28–53). New SIMS and LA-ICP-MS UPb dating of zircon grains from a gabbro and a diorite sample yielded similar weighted mean ages of 412 ± 4 Ma and 414 ± 2 Ma, indicating that they were formed simultaneously. The Group 1 samples have relatively low SiO₂ (49.56–50.78 wt%), high MgO (6.67–7.70 wt%) and Fe₂O₃T (7.46–9.61 wt%) contents, while the Group 2 samples have relatively high SiO₂ (57.74–59.53 wt%), low MgO (3.28–3.42 wt%) and Fe₂O₃T (5.76–7.01 wt%) contents. They both belong to calc-alkalic series and enriched in large ion lithophile elements (LILEs), light rare earth element (LREEs) and depleted in high field strength elements (HFSEs), which are typical of arc-related rocks. The Group 2 diorites exhibit relative enrichment in light rare earth element (LREE) ((La/Yb)_N = 10.9–13.1) with slightly negative Eu anomalies (Eu/Eu^⁎ = 0.82–0.93), compared to the Group 1 samples ((La/Yb)_N = 5.2–9.6; Eu/Eu^⁎ = 0.91–1.43). Both of them have enriched initial Sr (0.7049–0.7054), and chondritic to slightly depleted Nd (Ɛ_Nd(t) = −0.09 ~ +1.50), but depleted whole-rock and zircon Hf isotopic compositions (Ɛ_Hf(t) = 7.81–8.10 and 8.2–9.1). The HfNd isotopic systems are decoupled with positive △Ɛ_Hf values (5.22 to 5.92) (△Ɛ_Hf = Ɛ_Hf(t) – (1.59 × Ɛ_Nd(t) + 1.28)). The zircon grains display higher δ¹⁸O values of 6.3 to 6.5‰ than those of the normal mantle zircon. Whole-rock Pb isotopic compositions of the studied samples are of 15.610–15.624 for ²⁰⁷Pb/²⁰⁴Pb, 38.027–38.178 for ²⁰⁸Pb/²⁰⁴Pb, and 18.100–18.269 for ²⁰⁶Pb/²⁰⁴Pb. These geochemical features indicate that the Lajimiao mafic complex was derived from relatively depleted mantle that had been metasomatized by 2–5% subducted zircon-barren oceanic sediments. Combining with regional geological, geochronological, mineralogical, and geochemical data, the Lajimiao mafic complex was suggested to be generated by partial melting of a phlogopite-bearing mantle source at relatively low pressure in an arc setting. We suggested that the generation of the Lajimiao mafic complex was induced by the subduction of Shangdan Ocean during the early Devonian and thus the eastern branch of Proto-Tethys Ocean might have not been closed until that time.

上丹海洋代表着原始特提斯海洋的东部分支，这对于解密中部造山带的演化很重要。但是，其演变仍在争论中。在这项贡献中，我们对北秦岭造山带尚吉缝合带的拉吉庙铁镁质复合物进行了锆石U Pb年龄，全岩地球化学以及锆石Hf O同位素组成的综合研究。拉吉庙铁镁质复合物是由内部的粗粒辉长岩到黑土辉长岩（第1组）和外部的细粒闪长岩（第2组）组成。第1组样品主要包括唾液石到 闪锌矿（Mg ^＃ = 75–80），镁砂-角闪石（Mg ^＃ = 65–67）和富含Ca的斜长石（An = 45–67）；第2组样品主要由菱镁矿（Mg ^＃  = 60-66）和斜长石（An = 28-53）组成。来自长辉岩和闪长岩样品的锆石晶粒的新SIMS和LA-ICP-MS U Pb测年产生了相似的加权平均年龄412±4 Ma和414±2 Ma，表明它们是同时形成的。第一组样品的SiO ₂（49.56–50.78 wt％）相对较低，MgO（6.67–7.70 wt％）和Fe ₂ O ₃ T（7.46–9.61 wt％）含量高，而第二组样品的SiO 2相对较高。₂（57.74–59.53 wt％），低MgO（3.28–3.42 wt％）和Fe ₂ O ₃T（5.76-7.01 wt％）含量。它们都属于钙-碱系列，富含大离子的锂亲石元素（LILE），轻稀土元素（LREE）和贫化的高场强元素（HFSE），这是与弧有关的岩石的典型特征。 与第1组样品相比，第2组 硅藻土在轻稀土元素（LREE）（（La / Yb）_N = 10.9-13.1）中表现出相对富集，且Eu异常稍为负（Eu / ^Eu⁎ = 0.82-0.93）。 （LA / Yb）的_ñ  = 5.2-9.6;欧洲/欧洲^⁎  = 0.91-1.43）。两者都富集初期SR（0.7049-0.7054），和球粒稍微贫化钕（ɛ_钕（T）= -0.09〜1.50），但耗尽全岩和锆石Hf同位素组合物（ɛ_铪（t）= 7.81–8.10和8.2–9.1）。Hf的Nd同位素系统被解耦具有正△ɛ_的Hf的值（5.22至5.92）（△ɛ_铪 =ɛ_的Hf（T） - （1.59×ɛ_钕（T）+ 1.28））。所述锆石颗粒显示更高δ ^18个的高于正常地幔锆石6.3至6.5‰O值。所研究的样品的全岩Pb同位素组合物的15.610-15.624为²⁰⁷的Pb / ²⁰⁴铅，38.027-38.178为²⁰⁸的Pb / ²⁰⁴ Pb和18.100-18.269为²⁰⁶的Pb / ²⁰⁴铅 这些地球化学特征表明，拉吉庙铁镁质复合物来自相对贫乏的地幔，该地幔已被2％至5％俯冲的锆石-贫瘠的海洋沉积物交代。结合区域地质，地质年代，矿物学和地球化学数据，拉吉庙铁镁质复合物被认为是由含金云母的地幔源在相对较低的压力下在弧形环境中部分熔融而产生的。我们认为，拉格庙黑铁质复合体的产生是由泥盆纪早期上丹洋俯冲作用引起的，因此原特提斯海洋的东部分支可能直到那个时候才被关闭。