Abstract
The Bailinchuan alkaline syenite (BAS) is located in the easternmost part of the Triassic alkaline magmatic belt along the northern North China Craton (NCC). Based on a detailed study of the zircon U–Pb age, petrological, and geochemical data of the complex, the characteristics of the magmas system, petrogenesis and the nature of mantle source provide new constraints on the origin and tectonic setting of the Triassic alkaline belt. The BAS is composed of alkaline syenite and/or aegirine-nepheline syenite, with zircon U–Pb age of 226–229 Ma. Aegirine, Na-rich augite, biotite, orthoclase, and nepheline are the major minerals. Most of the zircons selected for the analysis show fine-scale to weak oscillatory growth zoning in CL images, suggesting a magmatic origin. Mineralogy, petrology and geochemical studies show that the parental magma of the BAS is SiO2-undersaturated, potassic, and is characterized by high contents of CaO, Fe2O3, K2O, Na2O. The BAS originated from a phlogopite-rich, enriched lithospheric mantle source in a garnet-stable area. The occurrence of the BAS, together with many other alkaline complexes of similar ages (235–209 Ma) in the northern NCC during the Late Triassic implies that the lithospheric mantle beneath the northern NCC was previously metasomatized by melts/fluids. Bailinchuan Late Triassic syenites were formed in a post-collisional extensional setting, which provides time constraints on the major geodynamic setting at the northern NCC.
Highlights
Syenites are commonly intimately associated with alkaline mafic rocks, especially alkali to transitional basalts [1,2]
To date the emplacement ages of these alkaline rocks, two alkaline syenites samples were chosen for zircon U–Pb dating by LA-ICP-MS
Based on some of the above some aspects, we propose that the Late Triassic magmatism in the Bailinchuan district is genetically associated with a post-collisional extension setting
Summary
Syenites are commonly intimately associated with alkaline mafic rocks, especially alkali to transitional basalts [1,2]. Geochemical and experimental studies have suggested that the lithosphere mantle [3,4] and crust [5,6] are possible sources for syenites. Liaodong Peninsula, located in the northeastern part of the NCC, is one of the important areas where the Mesozoic granitoid rocks are widely distributed. Bailinchuan alkaline syenite (BAS) is a representative pluton and located in the easternmost part of the belt [10,11,12]. This alkaline rock, contains valuable information concerning geodynamic processes. Tectonic environments of BAS still remain a controversial issue, three models for tectonic setting
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