内蒙西乌旗白音布拉格蛇绿岩地球化学特征

Geochemical characteristics of Baiyinbulage ophiolite in Xi U jimqin Banner,Inner Mongolia

内蒙古西乌旗白音布拉格蛇绿岩带是新发现的内蒙古西乌旗迪彦庙蛇绿岩带的北带,主要由蛇纹石化方辉橄榄岩、层状-块状辉长岩、斜长岩、枕状玄武岩、角斑岩-石英角斑岩及硅质岩等构造单元组成。白音布拉格蛇绿岩中的熔岩按照地球化学特征可以分为三组:第1组属于玻安岩系,以富Si(SiO2=52.71%、61.22%)、Mg(MgO=6.81%和10.88%)和贫Ti(TiO2=0.49%、0.51%)、HREE及HFSE为特征;第2组具有低Ti(TiO2=0.62%～0.78%)、高Mg(MgO=5.20%～11.30%)的特征,LREE弱亏损、类似N-MORB的稀土配分模式,但相对N-MORB,又具有富集LILE,亏损Nb、Ta等高场强元素的特征,类似岛弧拉斑玄武岩(IAT);第3组表现为:岩石具有高Ti(TiO2=1.86%、1.91%)、高Mg(MgO=5.25%和5.46%)及高P(P2O5=0.23%、0.27%),LREE和HREE分异较为明显((La/Yb)N=2.32、2.53)等特征,类似OIB。根据玻安岩与IAT的存在,推测白音布拉格蛇绿岩产于岛弧和弧前环境。

Baiyinbulage ophiolite lies in the northern part of Diyanmiao ophiolite in Xi U jimqin Banner,Inner Mongolia. Baiyinbulage ophiolite is mainly composed of augite peridotite,beded-massive gabbro,anorthosite,pillow basalt,keratophyre, baschtauite,and chert. According to geochemical characteristics,the lavas of Baiyinbulage ophiolite are divided into three groups,the first group belong to boninite series,which are characteristic of enrichment in Si（SiO2 = 52. 71% ,61. 22%）,Mg（MgO = 6. 81% , 10. 88%） and depletion of Ti（TiO2 = 0. 48%,0. 51%）,HREE,HFSE; the second are similar to island arc tholeiite （IAT）,with low Ti（TiO2 = 0. 62% ～ 0. 78%） ,high Mg（ MgO = 5. 20% ～ 11. 30%）,and the chondrite-normalized REE patterns of the second group are of characteristics of LREE depletion,similar to N-MORB,while the spider diagram shows the enrichment of the LILEs and depletion of HFSEs,the unique feature of the arc volcanic rocks; the third are similar to the ocean island basalt （OIB） ,characterized by high Ti（TiO2 = 1. 86% ,1. 91%） ,Mg（MgO = 5. 25% ,5. 46%） and P（P2O5 = 0. 23% ,0. 27%） ,and high Th/Ta radios （2. 32,2. 53）. According to the existence of boninite and IAT,we suggest that the Baiyinbulage ophiolite was likely formed in the island arc and fore-arc setting.