内蒙古中东部玛尼吐组火山岩形成时代及其大地构造环境

Zircon U–Pb Age and Tectonic Setting of the Manitu Formation in the Middle–East Inner Mongolia,China

玛尼吐组广泛分布在内蒙古中东部地区,长期以来与满克头鄂博组和白音高老组的地质时代被定为晚侏罗世,然而前人在玛尼吐组中发现Nestoriapissovi化石,并认为是早白垩世冀北阶的带化石,同位素年代学结果差异也较大,而且形成的大地构造环境仍然存在争论。因此,笔者对玛尼吐组火山岩进行了同位素年代学分析,并利用全岩成分探讨其形成的大地构造环境。玛尼吐组采集的3个安山岩和粗安岩的锆石U–Pb(LA–ICP–MS)测年结果分别为(142.4±1.4)Ma、(130.0±1.4)Ma和(145.0±1.5)Ma,显示该地区玛尼吐组形成于早白垩世。样品SiO2含量为59.43%~64.73%,Al_(2)O_(3)含量为15.66%~17.19%,K_(2)O含量为2.56%~5.03%,Na_(2)O含量为3.59%~4.53%,全碱含量为6.36%~9.56%;微量元素以富集大离子亲石元素和LREE,亏损Nb、Sr、P和Ti为特征;稀土元素ΣREE=118.71×10^(–6)~189.99×10^(–6),轻重稀土分馏系数(La/Yb)N为8.88~9.58,δEu值为0.66~0.95,Eu负异常,与岛弧火山岩地球化学特征相似。对比大兴安岭西部玛尼吐组西部、中部(~140 Ma)和东部火山岩年龄,笔者认为其玛尼图组火山岩类最老形成于晚侏罗世(~158 Ma),向东逐渐变新为早白垩世(140~130 Ma),这一变化与蒙古–鄂霍次克洋自西向东的闭合及陆块的后碰撞伸展有关。

The Manitu Formation widely occurs in the middle–east Inner Mongolia Autonomous region,and was thought to be formed in the late Jurassic together with the regional Manketouebo Formation and the Baiyingaolao Formation.However,the Nestoriapissovi fossils in the andesite interlayer of the Manitu Formation indicate this formation was formed during the Jibei Stage of the early cretaceous by previous studies.In addition,the zircon U–Pb ages for the volcanic rocks in the Manitu Formation have a wide range,and the tectonic setting is still under debate.Therefore,this study carried out the analyses of zircon U–Pb dating and whole–rock compositions to examine the age and tectonic setting of the Manitu Formation.The zircon U–Pb ages for three samples collected from Zarutqi–Bahrainyouqi are 142.4±1.4Ma,130.0±1.4Ma and 145.0±1.5Ma,respectively,suggesting the Manitu Formation in the Zarutqi–Bahrainyouqi area were formed in the early Cretaceous.Rocks from the Manitu Formation have SiO2,Al_(2)O_(3),K_(2)O and Na_(2)O of 59.43%~64.73%,15.66%~17.19%,2.56%~5.03%,3.59%~4.53%,respectively,with ALK of 6.36%~9.56%.They have high concentration of LILE and LREE,and depletion of Nb,Sr,P and Ti.Total rare earth element(ΣREE)concentration ranges from 118.71×10^(–6)to 189.99×10^(–6),and LREE and HREE are highly fractionated with(La/Yb)N of 8.88~9.58,with negative Eu anomalies(δEu=0.66~0.95),showing an island arc–like geochemistry.We propose that the formation age becomes younger from the west to east of the Great Xing’an Mountain based on the comparison of ages of rocks from the west,middle and east segment,and this may ascribed to the closure of the Mongol–Okhotsk ocean and the subsequent post–collision extension.