The northern North China, tectonically composed of a Phanerozoic orogen and a Precambrian craton, witnesses a panoramic scenario of continental crustal accretion and reworking in the model of plate tectonicsduring the Paleozoic times. It is driven by rifting-spreading-extinction cycle of paleo-oceans and subduction- collision-extension processes at convergent plate margin, with a series of the associated magmatic events produced. Documentation of multiple orogenic cycles around the northern North China craton has been essential for the ongoing reconstruction of Asia. Given their association with extensional tectonic settings, the recognition of syenitic rocks in ancient orogenic belts can present a diagnostic tracer of the geodynamic environment, and thus provide significant information on post-collisional or intraplate extensional processes within the continental lithosphere.
More recently, Associate Prof. Xiaohui Zhang from the Institute of Geology and Geophysics, Chinese Academy of Sciences and other investigators present a systematic field, geochronological and geochemical study on the Early Devonian 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 element 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 to 0.7071, low eNd(t) values of -12.7 to -17.9 and zircon eHf(t) values from -27.8 to -32.3. These geochemical features and quantitative isotopic modeling 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 neighboring regions, constitute a linear post-collisional magmatic belt along the northern North China craton, possibly under a tectonic regime of slab breakoff. They not only serve as an insightful magmatic milestone for marking the termination of Early Palaeozoic orogenic cycle around the northern North China craton, but also present a spatial tracer for locating the position of the potential ancient slab breakoff at the surface.
This study has recently been published by Journal of the Geological Society (London) （Zhang XH, Zhang HF, Jiang N, Zhai MG and Zhang YB. 2010. Early Devonian alkaline intrusive complex from the northern North China Craton: a petrologic monitor of post-collisional tectonics. Journal of the Geological Society (London), 167: 717-730）
