Zircon U-Pb age and geochemistry of TTG rocks in Xiaohongshan area of the Beishan orogenic belt, Inner Mongolia, and their constraints on the properties of the Baihe Mountain tectonic belt

The Late Carboniferous granites distributed in the south of Xiaohong Mountain in Beishan area of Inner Mongolia are a set of TTG rock combination composed of tonalites(T₁), granodiorites(G₁)and trondhjemites(T₂).This set of TTG combination has the geochemical characteristics of rich silicon, rich sodium, and low aluminum.Meanwhile, the TTG has low total content of REEs(∑REE=27.86×10^-6~104.56×10^-6), and is enriched in LREEs(LREE/HREE=4.71~22.76, (La/Yb)_N=4.78~43.18), with δEu being 0.62~1.72, showing the right-inclined type in chondrite-normalized REE patterns.In addition, the rocks which have the obvious negative anomalies of Nb-P-Ti and the low content of Y-Yb mainly show the enrichment of large ion lithophile elements(LILEs)and the loss of high field-strength elements(HFSEs). LA-ICP-MS zircon U-P dating shows that the ²⁰⁶Pb/²³⁸U weighted average age of zircons of tonalites is 315.6±1.6 Ma(MSWD=0.33, n=18), that of granodiorites is 310.8±1.4 Ma(MSWD=0.47, n=25), and that of trondhjemites is 305±1.9 Ma (MSWD=0.79, n=11), suggesting that the rock was formed in Late Carboniferous. Research suggests that the Xiaohong Mountain TTG which has the genetic characteristics of continental marginal arc granite was formed in the partial melting of the subduction plates of the ocean crust in the process of the subduction of Baihe Mountain inter-arc ocean basin to the south continental margin during the Carboniferous period. The development of this set of continental margin arcs which were formed in the same period with Baihe Mountain ophiolite indicates that the Baihe Mountain tectonic belt was formed at the stage of arc-basin system evolution.