内蒙古北山白云山蛇绿混杂岩带南部锡林柯博组碎屑岩地球化学特征、源区属性及构造意义

Geochemical characteristics,source area properties and tectonic significance of clastic rocks of Xilinkobo Formation in the south of Baiyunshan ophiolite melange belt,Beishan,Inner Mongolia

为揭示北山洋俯冲极性问题,对内蒙古北山造山带白云山地区的锡林柯博组砂岩进行了地球化学分析。结果表明,砂岩主要为长石石英砂岩和长石岩屑砂岩,少量为岩屑石英砂岩,且均表现为弱变质;矿物成分主要为石英、斜长石、钾长石等;主量元素平均含量SiO 2为80.70%,Al 2O 3为8.07%,MgO为1.08%,CaO为1.48%,TFe 2O 3为1.86%,K 2O为1.12%,Na 2O为1.75%;稀土元素配分模式曲线一致,LREE/HREE=11.32~16.54,平均为13.83,(La/Yb)N=7.38~12.39,平均10.43,负Eu异常,轻稀土元素相对重稀土元素明显富集,稀土元素分馏明显。锡林柯博组碎屑沉积岩CIA值反映研究区物源碎屑岩遭受了温暖、湿润条件下中等的化学风化作用,其母岩可能来源于酸性火山岩或花岗岩。结合锡林柯博组岩石组合特征、构造组合特征及地球化学特征,认为其源岩形成于被动陆缘构造环境,揭示了物源可能来自于塔里木地块,从侧面反映了北山洋向北俯冲的极性。

To reveal the polarity of the subduction of the Beishan ocean,the authors made a geochemical analysis of the sandstone of Xilinkebo Formation in Baiyunshan area of Beishan orogenic belt,Inner Mongolia.The results show that the rocks are mainly composed of feldspar quartz sandstone and feldspar lithic sandstone,with a small amount of lithic quartz sandstone,and all of them show weak metamorphism.The main mineral components are mainly quartz,plagioclase and K-feldspar.Average percentage content of major element oxides are SiO 280.70%,Al 2 O 38.07%,MgO 1.08%,CaO 1.48%,TFe 2 O 31.86%,K 2 O 1.12%and Na 2 O 1.75%.The REE patterns of curves are consistent,with LREE/HREE being 11.32~16.54,13.83 on average,and(La/Yb)N being 7.38~12.39,10.43 on average.Eu anomaly value is 0.57,LREE are obviously enriched relative to HREE,and REEs fractionate evidently.The CIA value of clastic sedimentary rocks in Xilinkebo Formation suggests that the clastic rocks of provenance in the study area were subjected to moderate chemical weathering under warm and wet conditions.Their parent rocks probably originated from acidic volcanic rocks or granites.Combined with the rock assemblage characteristics,tectonic combined characteristics and geochemical characteristics of Xilinkebo Formation,it is considered that the source rocks were formed in a tectonic environment of passive continental margin.It is revealed that the provenance might have come from the Tarim block,indirectly reflecting the polarity of northward subduction of Beishan Ocean.