Geochemical characteristics, isotopic ages and tectonic environment of Sangejing Formation in Beishan orogenic belt, Inner Mongolia

A set of island arc basalts is exposed in Sangejing Formation in the south of Baiyunshan ophiolite belt in the middle part of Beishan orogenic belt.It is mainly composed of pillow basalt and porous almond basalt, conformable contact with tuffaceous clastic rocks, thin-layered clayey siliceous rocks and thick-layered carbonate rocks.The weighted average age of basalts in Sangejing Formation is 396.6±3.3 Ma, suggesting Early Devonian.A study of the rock geochemistry shows that the major elements characteristics of the basalts of the Sangejing Formation indicate that the main body of the basalts belongs to the subalkaline tholeiite series.The main part of the normalized distribution curve of chondrite-normalized REE patterns of the basalts exhibit the right-oblique type of enrichment of LREE, with ∑REE being 35.88×10^-6~129.41×10^-6, LREE/HREE being 1.62~3.84, and (La/Yb)_N being 1.02~3.42.The bimodal magmatic assemblage consists of Sangejing basalts, and S-type and A-type granites are developed extensively in this region.It is inferred that the Sangejing Formation occurred after an orogenic extensional tectonic environment.Based on previous data and regional comparison, it is inferred that the Beishan Ocean in Beishan area was closed before the Early Devonian.A study of Lu-Hf Isotope of zircon indicates that the Sangejing basalt has the data ε_Hf(t)=(+4.67~+8.02), t_DM1=692~826 Ma and Dy/Yb=1.77~2.13, and that the Sangejing basalts were formed by partial melting of the depleted spinel-facies mantle peridotite to garnet-facies mantle peridotite in the Neoproterozoic mantle source area.