内蒙古二连浩特北部阿仁绍布地区晚石炭世花岗岩Sr-Yb分类及其成因

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

对分布在内蒙古二连浩特北部阿仁绍布地区的晚石炭世花岗岩类,依据Sr、Yb含量,划分为低Sr高Yb型、极低Sr高Yb型和低Sr低Yb型花岗岩3种类型。低Sr高Yb型花岗岩类相对低Si、富Al,Na2O>K2O,稀土元素分馏中等,有或无负Eu异常,Sr含量低,平均为183×10-6,Ba含量较高,平均585×10-6,Y含量高,平均30.06×10-6,Rb/Sr比值较低,平均0.97;极低Sr高Yb型花岗岩富Si、REE,低Al、Sr、Ba,高的Rb/Sr比值(平均为7.47),具明显的负Eu异常等;低Sr低Yb型花岗岩富Si,贫Al、Ca、Mg,重稀土元素(Y、Yb)含量低,Y含量在(7.26～10.6)×10-6之间,平均9.76×10-6,Yb含量在(1.04～1.89)×10-6之间,平均1.44×10-6,δEu=0.64～0.94,具弱负Eu异常,微量元素Ba含量高,Rb/Sr比值低。3种类型的花岗岩类过铝指数(A/CNK)多小于1.0,说明它们均源自变质火成岩的部分熔融。由于源区的深度不同(pT条件不同)和残留的主要矿物相不同,它们的岩石地球化学特征存在差异。极低Sr高Yb型花岗岩形成深度最浅(中上地壳),熔融残留相以斜长石为主;低Sr高Yb型花岗岩类形成于中下地壳,熔融残留相为斜长石和辉石;低Sr低Yb型花岗岩形成深度最深,推测可能形成于加厚下地壳(>40km)底部,熔融残留相为石榴子石、斜长石和角闪石。

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.