Early Devonian alkaline intrusive complex from the northern North China craton: a petrological monitor of post-collisional tectonics

Laser ablation inductively coupled plasma mass spectrometry zircon U–Pb dating and geochemical data document the Early Devonian Sandaogou alkaline complex (409 Ma) from the northern margin of the North China craton. The rock suite includes pyroxene syenite, quartz syenite and monzonite. These rocks exhibit high contents of K2O (5–13 wt%), strong enrichments in large ion lithophile elements and light REE, slightly negative Eu anomalies, and pronounced depletions in high field strength elements. They are characterized by moderate 87Sr/86Sri ratios of 0.7052–0.7071, low εNd(t) values of −12.7 to −17.9 and zircon εHf(t) values from −27.8 to −32.3. These geochemical features and quantitative isotopic modelling suggest that they might have been formed through simultaneous fractional crystallization and lower crustal assimilation of a metasomatized mantle-derived alkali basaltic magma. These trachytic rocks, together with the Middle Devonian alkaline rocks and mafic–ultramafic complex from neighbouring regions, constitute a linear post-collisional magmatic belt along the northern North China craton, possibly formed under a tectonic regime of slab breakoff. They serve not only as a magmatic milestone for marking the termination of Early Palaeozoic orogenic cycle around the northern North China craton, but also as a spatial tracer for locating the position of the potential ancient slab breakoff at the surface.

激光剥蚀电感耦合等离子体质谱（Laser ablation inductively coupled plasma mass spectrometry）锆石U-Pb定年与地球化学数据，记录了华北克拉通（North China craton）北缘早泥盆世三道沟碱性杂岩体（形成年龄409 Ma）的相关信息。该套岩石组合包括辉石正长岩、石英正长岩及二长岩。这些岩石具有较高的K₂O含量（5~13 wt%），显著富集大离子亲石元素（large ion lithophile elements）与轻稀土元素（light REE），表现出轻微的铕负异常，且高场强元素（high field strength elements）明显亏损。其初始锶同位素比值（⁸⁷Sr/⁸⁶Srᵢ）介于0.7052~0.7071之间，εNd(t)值较低，为-12.7~-17.9，锆石εHf(t)值则处于-27.8~-32.3的区间内。上述地球化学特征与定量同位素模拟结果表明，这类岩石可能由交代富集的幔源碱性玄武质岩浆经分离结晶（fractional crystallization）与下地壳同化（lower crustal assimilation）共同作用形成。该套粗面质岩石，连同邻区产出的中泥盆世碱性岩石与镁铁质-超镁铁质杂岩体（mafic–ultramafic complex），共同构成了沿华北克拉通北缘展布的线性碰撞后岩浆带（post-collisional magmatic belt），其形成可能处于板片断离（slab breakoff）的构造背景之下。它们不仅可作为标识华北克拉通北缘早古生代造山旋回（Early Palaeozoic orogenic cycle）终结的岩浆里程碑，同时也可作为潜在古板片断离地表位置的空间示踪剂。