Sr-Yb classification and genesis of Late Carboniferous granites in Arenshaobu area of Erenhot, Inner Mongolia

According to the content of Sr and Yb, Late Carboniferous granites distributed in northern Erenhot area can be divided into three types: low-Sr and high-Y type, very low-Sr and high-Yb type and low-Sr and Y type. Low-Sr and high-Yb granites have the following characteristics: relatively low Si, high Al, Na2O>K2O, negative or no Eu anomaly, Sr=183×10-6, Ba=585×10-6, Y=30.06×10-6 and Rb/Sr=0.97. Very low Sr and high Yb granites have high content of Si and REE, low Al, Sr and Ba, high Rb/Sr ratio, and obvious negative Eu anomaly. Low Sr and Yb granites have high Si, low Al, Ca, Mg and HREE, Y=(7.26-10.6)×10-6, Yb=(1.04-1.89)×10-6, δEu=0.64-0.94, high Ba and low Rb/Sr. A/CNK ratios of the three types of granites are lower than 1.0, suggesting partial melt of igneous rocks. The differences in petrology and geochemistry might result from the difference in melting depth and melt residual phase. Very low Sr and high Yb granites might have been formed in a relatively shallow depth with the melt residual phase being mainly plagioclase. Low-Sr and high-Yb granites were probably formed in middle-low crust with the melt residual phases being mainly pyroxene and plagioclase. Low Sr and Yb granites might have been formed in the depth of thickened low crust with the melt residual phases being mainly garnet and amphibole.