A new thinking on the process of ocean-continent transition in Beishan orogenic belt of Inner Mongolia: Evidence from the Devonian arc granite in the south of Dahong Mountain

This paper reports a set of Devonian arc granites firstly found on the south side of Dahong Mountain, the southern side of ophiolite tectonic mélange zone in Hongshi Mountain-Baihe Mountain.This set of arc granites are composed of tonalite, granodiorite and quartz monzonite.The rocks on the whole experienced intense thermodynamic metamorphism and developed permeable mylonitic texture.According to the geochemical characteristics of rocks, the arc granite assemblage can be divided into sodic T₁G₁ (tonalite+granodiorite) and potassium to potash QM(quartzite).The combination of T₁G₁ shows the characteristics of the high aluminum(the average content of Al₂O₃:15.22%, > 15%), and the QM is a set of high potassium calc-alkaline rocks which belong to Zheming type granite with low Sr and high Yb(the average content of Sr is 121.64×10^-6, and the average content of Yb is 2.95×10^-6).Both of them have the characteristics of obvious enrichment of light rare earth elements(LREE)and large ion lithophile elements(LILEs), obvious depletion of heavy rare earth elements(HREE)and high field strength elements(HFSEs)and obvious "TNT" anomaly(Nb, Ta, Ti depletion).The overall results show that the arc granite assemblage was formed in the background of island arc with high maturity.Besides, zircon U-Pb dating shows that this set of arc granites was emplaced in the Devonian period between 364 Ma and 399 Ma.Combined with the overall spatial and temporal configuration of the region, it is held that this set of arc granites may be the remnant of the Que'ershan island arc(D)in the northern belt of Beishan Mountain.This discovery provides new evidence and clues for the Paleozoic arc-basin evolution system in the north belt of northern Beishan Mountain.