内蒙古东部博克图地区石炭纪埃达克质花岗岩地质特征及构造意义

Geological characteristics and tectonic significance of Carboniferous adakite-like granite in the Boketu area of eastern Inner Mongolia

报道了大兴安岭东部博克图地区二支沟岩体的岩相学、岩石地球化学以及地质年代学特征,探讨了它们的形成时代、岩石成因以及成岩构造背景。根据LA-ICP-MS U-Pb定年结果,二支沟岩体的侵位年龄为(317.2±2.2)Ma与(319.2±2.3)Ma。二支沟岩体岩性为花岗闪长岩,具高硅、富碱,低MgO、CaO的特征,属于准铝质过铝质,高钾钙碱性I型花岗岩。二支沟岩体的Ba/Nb=70.44~98.20,Ba/La=17.85~28.12,δEu具负异常,暗示源区可能为壳源,存在少量地幔物质与强氧化流体混入。此外,二支沟岩体具有高Sr和低Y、Yb的特征(Sr含量>400×10^-6,Y含量<18×10^-6,Yb含量<1.9×10^-6),与埃达克岩类似,但其更为富钾,表明二支沟岩体具有C型埃达克岩特征。二支沟岩体富集大离子亲石元素(LILE)Rb、K、Ba、Th,相对亏损Ta、Nb、P、Hf、Zr等高强场元素(HFSE),其(La/Yb)N=12.05~18.03,轻稀土元素(LREE)富集,重稀土元素(HREE)亏损。其Rb、Ba含量和Rb/Yb(51.49~148.93)、Ba/Yb(430.17~698.83)比值明显高于MORB,与海拉尔盆地和西藏冈底斯埃达克岩相似,表明其成因为底侵作用新生的增厚的玄武质下地壳的熔融。研究区晚石炭世岩浆形成于造山阶段挤压环境。额尔古纳兴安地块与松嫩地块于晚石炭世((317.2±2.2)Ma之后)构造背景由碰撞造山晚期的挤压、地壳加厚向造山后期的伸展、垮塌的环境转换,并向更加稳定的板内构造环境演变。

In this paper,we introduced petrographical,petrogeochemical and geochronological characteristics of the Erzhigou rock mass in the Boketu area of Eastern Greater Khingan Mountains.We studied its formation age,petrogenesis and tectonic setting.Based on LA-ICP-MS U-Pb dating,we found that the emplacement age of the Erzhigou rock mass ranged from 317.2±2.2 to 319.2±2.3 Ma,consistent with a Late Carboniferous magmatism event in the study area.The lithology of Erzhigou rock mass included granodiorite,with petrogeochemical characteristics of high silicon content,rich alkaline and quasi-aluminous to peraluminous and low MgO,CaO.The 10000 Ga/Al series diagram,as well as P2O5-SiO2 correlation,showed that the Erzhigou rock mass was high-K calc-alkaline I-type granite.Some results were obtained as follows:Ba/Nb=70.44-98.20;Ba/La=17.85-28.12;δEu appeared to be negative anomaly,implying crustal source region,where a small amount of mantle substances mixed with strongly oxidative fluids.Besides,since the contents of SiO2 exceeded 56%,with MgO lower than 3%,these rocks belonged to high-K calc-alkaline series,with Al2O3 content higher than 14%.For trace REE,Sr concentration was higher than 400×10^-6,with Y and Yb concentrations lower than 18×10^-6 and 1.9×10^-6,respectively.These rocks were rich in large ion lithophile elements(LILE)(e.g.,Rb,K,Ba,Th),and relatively depleted in high field-strength elements(HFSE)(e.g.,Ta,Nb,P,Hf,Zr).δEu appeared to be negative anomaly.LREE/HREE=4.14-6.19 and(La/Yb)N=12.05-18.03,showing enrichment of LREE and depletion of HREE.These rocks had the characteristics of adakite,but they were much richer in potassium than typical adakite,indicating C-type adakite characteristics of the Erzhigou rock mass.The Rb and Ba contents,and the ratios Rb/Yb(51.49-148.93)and Ba/Yb(430.17-698.83)in these rocks,were obviously higher than that in MORB,but similar with those in adakite of the Hailar Basin and Kailas in Tibet,suggesting a petrogenesis mechanism of granite generation through fusion of lower basaltic crust generated and thickened by underplating.Late Carboniferous magmas in the study area were formed in the compression environment during the orogenic stage.The tectonic setting of the Honkirnaur-Xing'an and Songnen massifs in the Late Carboniferous period(younger than 317.2±2.2 Ma),is that compression and crustal thickening in the late collision orogenic stage is transformed to extension-collapse environment in the post orogenic stage,followed by more stable intraplate tectonic environment through evolution.