内蒙古莫合尔图中—晚侏罗世火山岩年代学、地球化学研究及其意义

Geochronology and Geochemistry of Middle-Late Jurassic Volcanic Rocks in the Mohe'ertu Area , Inner Mongolia and Their Geological Significance

内蒙古莫合尔图地区发育大面积的中—晚侏罗世酸性火山岩,是大兴安岭中生代火山岩的重要组成部分,锆石U-Pb定年结果显示,火山岩形成于174.1±2.2～157.5±2.3Ma,属中—晚侏罗世。主量元素分析表明,火山岩具高硅(SiO2=69.12%～80.66%)、富碱(K2O+Na2O=7.26%～8.99%)、富钾(K2O/Na2O多数大于1)、贫钙镁(CaO=0.09%～0.71%;MgO=0.07%～0.47%,Mg#多<45)和高FeOT/MgO(1.50～12.28,多数大于3)值的特征,属过铝质(A/CNK=1.11～3.07)钙碱性—高钾钙碱性—钾玄岩系列岩石。微量元素数据显示:稀土元素总量∑REE为95.92×10-6～205.70×10-6,轻、重稀土元素分馏较明显[(La/Yb)N=5.17～14.69],轻稀土元素(LREE)较为富集,重稀土分布相对较为平坦,球粒陨石标准化的配分模式图显示典型的右倾型,中等—弱负铕异常(δEu=0.23～0.87),微量元素富集大离子亲石元素(LILE)Rb、Ba、Th、K、贫Sr和亏损高场强元素(HFSE)Nb、Ta、P、Ti;岩石具低Sr(31×10-6～187×10-6),高Yb(1.23×10-6～3.37×10-6)含量,具造山晚期—造山后花岗岩的特征。综合最新区域资料及本文研究成果,笔者认为研究区中—晚侏罗世火山岩为造山晚期向造山后伸展转换阶段的产物,转换时期介于174.1±2.2～157.5±2.3Ma,与蒙古-鄂霍次克造山后伸展作用有关。

Mid-Late Jurassic persilicic volcanic rocks in the Mohe＇ertu area , Inner Mongolia , is one of the most important part of Mesozoic volcanic rocks in Da Hinggan Mountains.The zircon U-Pbdating by LA-MC-ICP-MS and SHRIMP techniques , indicates that the crystallizing age of the volcanic rocks , ranges from 174.1±2.2Ma to 157.5±2.3Ma ,which belongs to Middle-Late Jurassic epoch.Geochemical researches indicate that the volcanic rocks from the Mohe＇ertu area are rich in silica （ SiO2 =69.12%～80.66% ）, alkali （ K20＋Na20=7.26% ～8.99% ）, K （ K20 / Na201.0 ）, poor in calcium （ CaO=0.09% ～0.71% ）, magnesium （ MgO=0.07%～0.47% , Mg # 45 ） and have high FeO T / MgO （ FeO T / MgO=1.50～12.28 , average=4.97 ） ratio ,which is a typical high-K calc-alkaline to shoshonitic series with the character of metaluminous （ A / CNK=1.11～3.07 ） .The rocks are poor in rare earth elementswith ∑REE=95.92× 10 -6 ～205.70×10 -6 （ average=151.90×10 -6 ）, enriched in LREE [（ La / Yb ） N =5.17～14.69 ], LILEs （ Rb , Ba , Th , K ） anddepleted in HFSE , especially Nb , Ta , Sr , P and Ti.TheδEu values vary from 0.23to 0.87 （ average=0.45 ） and most rocks have moderate negative Eu anomalies.Similar REE is tribution pattern implies that the volcanic rocks come from the same magma source.Most of the volcanic rocks with the characteristics of post-collision granites d, have lower Sr （ Sr=31～187×10 -6 ）, high Yb （ Yb=1.23×10 -6 ～3.37×10 -6 , generality2 ） and well geochemical correlation.Combining with previous studies ,we suggest that the rhyolitic magma ,which has related with Mongolia okhotsk orogenesis , formed between late organic and post-collision ,with ages from 174.1±2.2Ma to 157.5±2.3 Ma.The conclusions have most important significance for studying the volcanic action and tectonic settings of Mid-Late Jurassic volcanic rocks.