【作者】 金玲；

【导师】 杨德明；

【作者基本信息】 吉林大学 ， 第四纪地质学， 2014， 硕士

【摘要】 兴蒙造山带东段的大兴安岭地区发育大规模中生代火山岩，其复杂的岩石类型构成了中国东部中生代火山岩带的主体部分。内蒙古科右中旗地区梅勒图组安山质岩石作为大兴安岭地区主要的火山岩类型之一，为全面了解大兴安岭火山岩的成因和所属大地构造背景提供了线索。本文通过对内蒙古科右中旗地区梅勒图组火山岩详细的野外地质调查，确定区内梅勒图组火山岩岩石组合以安山岩、玄武安山岩和安山质角砾凝灰熔岩为主，夹安山质角砾凝灰岩、熔结角砾凝灰岩及英安质凝灰熔岩、角砾凝灰岩等，局部夹有玄武岩。梅勒图组是区内出露最新、产出层位最高的一套岩层，底部以角度不整合覆盖其它地层之上。其产状很缓，倾角多在10°左右，地貌多组成高陡的山体，明显不同于区内其他地层单元。通过对代表性的梅勒图组安山岩进行锆石LA-ICP-MS U-Pb年代学研究发现，其形成年龄为123~125Ma，是早白垩世中期岩浆喷发的产物。本文对梅勒图组安山岩锆石Hf同位素和岩石地球化学的研究结果表明，梅勒图组安山岩属于亚碱性系列，主要为弱过铝质，部分为准铝质。研究区安山岩的Ta/Hf、Nb/Ta、Ta/Yb、Sm/Nd平均值近似于球粒陨石、原始地幔值，其SiO2含量不高，Mg#值中等，具有幔源的特征。安山岩轻重稀土分异较强，轻稀土富集程度高，Rb、K等大离子亲石元素明显富集，高场强元素Nb、Ta、Ti等明显亏损，其微量元素含量类似大陆岛弧安山岩的特征；安山岩的SiO2、Na2O、K2O含量和K2O/Na2O比值分别落入钾质火成岩相应含量范围内，类似葛文春等（1999）报道的亚碱性系列高钾岩类，表明岩石主要起源于有来自消减带的水和大离子亲石元素加入的富集地幔。梅勒图组安山岩具有较高的εHf(t)正值（+7.79±0.26），显示了Hf同位素亏损，结合安山岩大离子亲石元素富集的特征反映出梅勒图组安山岩的源区为新形成的富集地幔。梅勒图组安山岩TiO2含量平均为0.92%， Ba/La的平均比值为41，La/Nb的平均比值为2.4，Th/Ta的平均比值为7.07，Ta/Hf的平均比值为0.15，显示安山岩形成于与俯冲作用有关的拉张环境。综合大兴安岭地区中生代火成岩的展布方向、年代学和地球物理资料以及亚洲大陆东部边缘发育侏罗纪-白垩纪拼贴增生杂岩等表明，研究区早白垩世梅勒图组安山岩的形成可能与太平洋板块俯冲作用后的陆内伸展环境有关。更多还原

【Abstract】 Immense Mesozoic volcanic rocks are found in the eastern part of XingmengOrogenic Belt in Da-Hinggan Mountains with complicated rock types, which form themain part of Mesozoic volcanic rock belt of East China. As one of the major types ofigneous rocks in Da-Hinggan Mountains, Meiletu Formation andesitic rocks give away to better understand the petrogenesis and tectonic background of Da-HingganMountains igneous rocks.According to the detailed field investigation in Keyouzhongqi area, it can beconfirmed that the rock association of Meiletu Formation is andesite, basalt-andesiteand andesitic breccia tuff lava, interbedded with andesitic breccia tuff, andesiticbreccia ignimbrite, dacitic breccia tuff lava, dacitic breccia tuff and basalt. MeiletuFormation is the youngest stratum in the studying area covering other strata with anangular unconformity. Its occurence is flat,with the dip angles, below10°. Fortopography and geomorphology, Meiletu Formation always constitute the high andsteep hills which is significantly different from other formation units. According tothe LA-ICP-MS U-Pb isotopic dating of zircons, the forming age of MeiletuFormation is123-125Ma, and thus it is the product of volcanic eruption in the middleof Early Cretaceous.The studies of zircon Hf isotope and geochemistry of Meiletu Formationandesites indicate that they belong to the sub-alkaline series. Most of the samples areweakly peraluminous and the rest are peraluminous. The average values of Ta/Hf, Nb/Ta, Ta/Yb and Sm/Nd are similar to the chondrites and primitive mantle. Thesamples are characterized by relatively low SiO2and moderate Mg#, which are closeto the characteristics of the mantle. And the strong fractionation of LREE and HREE,apparently enrichment in LILE such as Rb, K, and depletion in HFSE such as Nb, Taand Ti all show similar characteristics to continental island andesite. The values ofSiO2, Na2O and K2O and the ratio of K2O and Na2O of andesites in the studying areaare within the range of potassic igneous analogous to subalkalic series high-K rocksas Ge reported (1999), which suggest that the magma of the andesites was derivedfrom the enriched mantle added the water and LILE from the subduction zone. Thehigh positive value of εHf(t)(+7.79±0.26) implying the depletion of176/177Hf isotope,combined with the characteristic of enrichment in LILE, it implies that the sourceregion of Meiletu Formation andesites is newly generated enriched-mantle.The average content of TiO2of Meiletu Formation andesites is0.92%, and theaverage ratio of Ba/La, La/Nb, Th/Ta, Ta/Hf are4,2.4,7.07, and0.15, respectively,which suggest that the andesties was formed in the extensional environment related tothe subduction. Taking several fractors into consideration, such as the distributiondirection of Mesozoic volcanic rocks, chronology results, geophysical data andamalgamated accretionary complexes well-developed in the eastern part of Asiancontinent in Jurassic to Cretaceous, the tectonic environment of Meiletu Formationandesites in Keyouzhongqi area is the intra-continental extension related to thesubduction of Pacific Plate.更多还原