Late Devonian Back-Arc Extension at the Northern Margin of the North China Craton: Evidence From the Danaobao Alkaline Magmatism in the Shangyi Area

Abstract

Alkaline magmatic rocks from the Early-Middle Devonian are distributed linearly along the east–west trending Jining-Shangyi-Longhua Fault at the northern margin of the North China Craton (NCC). These rocks are associated with post-collision extension following the arc–continent collision between the Bainaimiao Island Arc and the NCC's northern margin. The affiliation of Late Devonian alkaline magmatic rocks with this tectonic system remains a subject for further investigation. A newly identified suite of Late Devonian alkaline intrusive rocks in the Xisai intrusive complex, Shangyi area, provides additional insights. Zircon U–Pb dating of the quartz monzonite yields an age of 380–377 Ma, confirming its Late Devonian age. Geochemically, these rocks are characterised by low silicon (SiO₂ = 60.21–67.19 wt.%), low Mg# values (9–18), Cr (2.08–26.70 ppm) Co (1.63–3.10 ppm) and Ni (1.40–11.00 ppm) content, high aluminium (Al₂O₃ = 16.28–21.03 wt.%), and elevated alkali content (K₂O + Na₂O = 8.54–10.17 wt.%). It is enriched in largeion lithophile elements such as Sr, Ba, Rb, La and Ce, and depleted in high-field-strength elements and heavy rare earth elements such as Nb, Ta, Zr, Hf, Y and Yb. The rock also displays a weak positive europium anomaly (δEu = 1.04–2.38). Zircon ε_Hf(t) values of the quartz monzonite range from −21.36 to −17.74, and the two-stage model age (t_DM2) spans 2701–2477 Ma. These values suggest protolith magma derived from ancient lower crust partial melting. Field observations reveal clear syn-intrusive extensional deformation. The research concludes that the Danaobao quartz monzonite formed at the Late Devonian back-arc extension associated with the subduction of the Paleo-Asian Ocean beneath the northern margin of the NCC.