【作者】 张吉衡；

【导师】 高山；

【作者基本信息】 中国地质大学 ， 地球化学， 2009， 博士

【摘要】 大兴安岭地区以出露大面积中生代火山岩为特征。对于北部地区已有大量的年代学研究，结果表明北部地区中生代火山岩的形成时代集中在早白垩世，只有少量晚侏罗世火山岩；但是对于中南部地区，至今还没有系统的年代学研究。这严重制约了对于大兴安岭地区以及整个中国东部中生代期间地质构造演化过程的深入认识。本文重点对年代学研究缺乏的大兴安岭中南部进行了系统的定年工作。在此基础上，结合火山岩地球化学特征，探讨大兴安岭地区大规模中生代火山岩的岩浆源区、岩石成因以及岩石地球化学特征随时代的演化等问题，进而对岩浆作用的构造控制因素、地球动力学本质等问题进行了讨论，并重点讨论了中生代岩浆作用与中国东部中生代岩石圈减薄事件之间的关系。依据区域地层对比和岩石组合特征、沉积夹层中的化石资料及火山活动的旋回特征，大兴安岭中南部中生代火山岩自下到上依次划分为满克头鄂博组、玛尼吐组、白音高老组和梅勒图组。其中满克头鄂博组和白音高老组主要由酸性火山岩及相应的火山碎屑岩构成，岩石类型主要包括英安岩、流纹岩及凝灰质岩石；而玛尼吐组和梅勒图组则主要由中基性火山岩构成，包括玄武岩、安山岩及凝灰质岩石。已有的研究认为这些火山岩主体形成于晚侏罗世，但是缺乏高精度年龄数据的支持。只有很少高精度年龄数据报道，而且这些数据只局限在有限的范围内，因而不能限制大规模火山作用的时代，即其起止时间及主体形成时间。本文使用锆石U-Pb及全岩⁴⁰Ar／³⁹Ar定年方法对中南部火山岩进行了系统的年代学研究。对满克头鄂博组的17件样品定年结果表明其形成时代包括晚侏罗世和早白垩世两期，时代范围分别为160Ma-150Ma和136Ma-122Ma，峰值分别为150Ma与135Ma左右，但是有一个样品获得了较老的年龄为173Ma，其建组剖面上两个样品获得的年龄分别为152Ma和158Ma。而10件样品的定年结果表明玛尼吐组的年龄范围为158Ma-125Ma，同样可以分为两期：158Ma-150Ma与137Ma-125Ma，对应的火山作用峰期分别为155Ma-160Ma和125Ma，而建组剖面上获得的年龄为137Ma。满克头鄂博组和玛尼吐组具有相似的年代学格架，而区别在于满克头鄂博组以晚侏罗世火山岩为主体，而玛尼吐组则以早白垩世为主。相比较而言，白音高老组和梅勒图组具有较小的年龄范围，集中在141Ma到124Ma期间（白音高老组：141Ma-124Ma；梅勒图组：134-124Ma），全部为早白垩世，高峰期在130Ma左右，而建组剖面上获得的年龄分别为139Ma和131Ma。结合已有的年代学数据表明，大兴安岭中南段中生代火山岩主体形成于晚侏罗世-早白垩世期间，介于160Ma到122Ma之间，并且可以分为两个阶段：160Ma-150Ma与141Ma-122Ma，在这两个阶段之间有一个明显的间断（大约10Ma）。虽然都形成于晚侏罗世-早白垩世期间，并且火山作用的高峰期都出现在130Ma-120Ma期间，大兴安岭北部和中南部火山岩在年代学格架上仍具有明显的差异，具有不同的火山作用终止时间和不同的岩浆期次。这表明大兴安岭北部和中南部具有不同的岩浆作用过程，反映了深部动力学过程的不同。这些年龄结果也表明大兴安岭地区中生代火山岩地层划分对比需要重新考虑，本文对连续剖面的定年结果也表明了这一点。而以前划分的满克头鄂博组、玛尼吐组当中包含了不同时代的火山岩，因而将这些不同时代的火山岩划分为一个组是不合理的：同时这四个组在形成时代上具有明显的重叠，因而以前认为的地层上下叠置关系并不存在。对大兴安岭及其邻区中生代火成岩形成时代的统计表明存在侏罗纪和早白垩世两期岩浆作用，前者以侵入岩为主，后者则以火山岩为主。其中火山岩具有自西向东逐渐变新的特征，而侵入岩则具有相反的趋势，即自西向东逐渐变老。二者之间有一个明显的岩浆作用平静期，而这个平静期的持续时间从大陆边缘向大陆内部逐渐变小，在大兴安岭地区小于10Ma，而在日本列岛则大于60Ma。另外一个显著特征就是早白垩世岩浆作用在不同地区具有相似的起始时间，集中在135Ma左右，整体上不超过140Ma。大兴安岭北部中基性火山岩以碱性系列岩石为主，只有少量为亚碱性系列；而中南部火山岩则以亚碱性系列为主，只有少量晚侏罗世岩石为碱性系列。北部中基性岩石在稀土及微量元素上以富集轻稀土和大离子亲石元素而亏损高场强元素以及明显亏损Nb、Ta为特征，可以划分为高Ti和低Ti两种类型，其中高Ti岩类比低Ti岩类具有更高的轻稀土富集程度及较高的P、Ti丰度，富集Ba、Sr：而低Ti岩类以明显的Th富集和P、Ti亏损为特征。中南部中基性岩总体上具有较低但是变化较大的轻稀土富集程度，同样富集大离子亲石元素而亏损高场强元素且具有明显的Nb、Ta负异常。按照微量元素特征可以分为高K和低K两种类型，前者具有较高的轻稀土和Rb、Ba、Th、Sr丰度，具有明显的K正异常；后者则以较低的轻稀土丰度和明显的Ba正异常以及K、Zr、Hf负异常为特征。地球化学及同位素特征表明大兴安岭中生代中基性系列岩石显示地球化学双重性，既有富集特征又有亏损特征，其中北部以富集型地幔源区为主，而中南部同时出现富集型和亏损型地幔源区，表明其源区的不均一性。而Nd、Hf同位素年龄表明富集型地幔的形成与古亚洲洋闭合事件有密切关系。酸性岩类包含两种明显区别的类型：第一类具有较低的重稀土丰度及微弱的Eu异常，微量元素表现出较高的Ba、Sr丰度：第二类则具有较高的重稀土丰度，强烈的Eu及Ba、Sr负异常，这两类岩石被划分为高Ba-Sr和低Ba-Sr岩类。其中第一类岩石主要分布在北区，而第二类岩石则广泛分布在北部和中南部；此外在时代特征上，低Ba-Sr岩类主要出现在岩浆作用的晚期阶段。其中北部低Ba-Sr岩石具有明显偏高的形成温度，而中南部同类型岩石则具有明显偏低的形成温度，这表明北部在岩浆作用晚期等温面明显抬升，而中南部则相反。这也表明早白垩世期间北部和中南部具有不同的深部作用过程。部分晚侏罗世酸性岩石具有埃达克质岩石的特征，表明晚侏罗世期间大兴安岭地区存在明显加厚的地壳。大兴安岭早白垩世火山岩形成于伸展构造环境是众多研究者的共识，主要的证据包括区内广泛出露的同时代的A型花岗岩及其他碱性岩石，变质核杂岩和中基性-酸性脉岩群。而岩石组合、区域构造特征以及亚洲大陆边缘广泛发育的拼贴增生杂岩表明晚侏罗世处于挤压环境当中。这表明晚侏罗世-早白垩世期间，大兴安岭及其邻区经历了构造环境的转换过程，即由挤压转换为伸展。这个转换时期对应于岩浆作用的平静期。虽然中南部地区有大量火山岩形成于晚侏罗世期间，大兴安岭中生代火山作用的峰期为早白垩世，与中国东部早白垩世大火成岩事件具有一致的年代学格架。这表明大兴安岭中生代火山岩是中国东部早白垩世大火成岩事件的组成部分，即大兴安岭中生代火山岩是中国东部岩石圈减薄事件的浅部表现。侏罗纪-早白垩世火成岩的时空分布特征表明其形成受到古太平洋板块俯冲作用的控制。侏罗纪期间，受到古太平洋板块低角度俯冲作用的影响，形成了侏罗纪具有活动大陆边缘特征岩石组合，造成了明显的地壳加厚。早白垩世期间，由于加厚地壳重力失稳拆沉，拆沉作用造成软流圈地幔物质上涌，从而造成大规模岩浆作用。拆沉作用从西向东逐渐迁移，从而形成早白垩世岩浆作用自西向东逐渐变新的特征。拆沉作用的高峰时期，甚至出现软流圈与地壳直接接触的现象。而拆沉作用同时也造成了洋壳俯冲角度的改变，使其由早期的低角度俯冲转变为现今的高角度俯冲。尽管受到相同的构造因素的控制，大兴安岭北部和中南部具有不同的作用过程，北部表现出明显的岩石圈减薄过程，而中南部则主要表现为岩浆底侵作用，即减薄作用在空间上具有明显的不均一性。更多还原

【Abstract】 The Great Xing’an Range in NE China is characterized by the widespread Mesozoic volcanic rocks. For the volcanic rocks in the northern segment, large number of geochronological studies has been done, which indicate that most of these volcanic rocks formed in the the Early Cretaceous, with a few of the Late Jurassic time. But for the volcanics in the middle and southern segments, systematic geochronological study is still absent till now, which hampers further studying of the geological and tectonic evolution history of the Great Xing’an Range, as well as the eastern China. On the basis of the previous studies, a systematic geochronological study has been done focusing on the volcanic rocks in the middle and southern Great Xing’an Range. And then the obtained age date was combined with the geochemistry in order to determine the magma sources and the petrogenesis of the volcanic rocks, as well as the temporal evolution of the volcanics. Importantly, here I further discussed the tectonic controlling factors and geodynamic nature of the igneous events, and the relationship between the magmatism and the lithospheric thinning in the eastern China.Based on the field relationship of the volcano strata and the features of rocks assemblages, fossil information in the sedimentary layers, as well as the episodes of the volcanism, the volcanic strata were divided into the Manketouebo, Manitu, Baiyingaolao and Meiletu Formations from bottom to top. The Manketouebo and Baiyingaolao are composed of felsic volcanics and related volcano clastic rocks, including dacite, rhyolite, trachyte and taffaceous rocks, while the Manitu and Meiletu formations comprise a suite of mafic-intermediate rocks, including basalt, andesite and trachy basealt, basaltic trachy andesite, trachy andesite, and related tuffaceous rocks. These volcanics were previously thought to be formed mainly in the Late Jurassic, but lacking of high precise age evidence. Till now, there are only few precision data reported in restricted area, nevertheless, these data can not constrain the beginning time and duration of the voluminous volcanics in the region.Systematic dating on the volcanics in the southern segment was performed by zircon U-Pb and whole rock ⁴⁰Ar/³⁹Ar methods. 17 analyzed samples from the Manketouebo Formation indicates that this formation contains two episodes of magmatism, i.e., the Late Jurassic （160-150Ma） and Early Cretaceous （136-122Ma）, with the peak ages at 150Ma and 135Ma, respectively. But there is one sample have much older age of 173Ma. The obtained U-Pb ages of two samples from the location at where the Manketouebo Formation was defined are 152 and 158Ma. Dating on 10 samples from the Manitu Formation indicates that the Manitu Formation has the similer age frame to that of the Manketouebo Formation and also can be divided into two epidodes: 158Ma-150Ma and 137Ma-125Ma, with peak ages of 160Ma-155Ma and 125Ma, respectively. The only differenc is that the volcanis of the Early Cretaceous are dominant in the Manitu Formation, while the majority of the Manketouebo Formation is the Late Jurassic. In the contrast, the Baiyingaolao and Meiletu formations were formed in the Early Cretaceous with much shorter duration, during the period between 141Ma and 124Ma. LJ-Pb ages of the samples from the locations at where thses two formations were firstly founded are 139Ma and 131 Ma, respectively. All these data, combined with the previously published data, indicates that the volcanics in the middle-southern Great Xing’an Range mainly formed during the Late Jurassic-Early Cretaceous time, between 160Ma and 122Ma, with two episodes of 160Ma-150Ma and 141Ma-122Ma. Between them, there is a magmatic gap about 10Ma. Although the volcanic rocks in the both segments formed during the period of Late Jurassic-Early Cretaceous with identical peak time, the ending time and the episodes of the volcanism are different, reflecting the different magmatism sequence, which was probably caused by the different deep geodynamic procedures. These age data also suggest that the previous scheme of subdivision and regional correlation of the volcano strata need to be re-evaluated, which is also manifested by the age data obtained from the successive sections in this study. The former difined Manketouebo and Manitu formations actually contain volcanics of different time; while these four formations have obvious overlapping in age, which means that the previously defined stratigraphic sequence is not realistic.Age compilation of the igneous rocks in the Great Xing’an Range and adjacent area implies that there exist two episodes of magmatism in the Jurassic and Early Cretaceous time: the former mainly comprise intusive rocks, while the latter eruptive. Between them, the eruptive rocks have an age-decreasing trend from the west to the east; while the intrusive rocks have the oppsite trend, i.e., becoming young westward. Between them, there is a magmaic gap, which becomes shorter from the continental margin to the intracontinent, from about 60Ma in the Japanese island to less than lOMa in the Great Xing’an Range. Another important feature is that the initial time of the Early Cretaceous magmatism is similar in different region, at about 135Ma, not older than 140Ma.In the northern part of the Great Xing’an Range, the mafic-intermediate volcanics are mainly alkaline series, with minor of sub-alkaline; on the contray, those in the middle-southern segment are sub-alkaline series. In the northern part, the mafic-intermediate rocks are characterized by enrichment in LREE, LILE and depletion in HSFE, and can be divided into two types of high Ti and low Ti types. Compared to the low Ti rock, the high Ti rocks are relatively enriched in LREE and Ba-Sr, with high abundances of P and Ti; while the low Ti rocks have more enrichment in Th and depletion in P and Ti. In the middl-southern segment, the mafic-intermediate rocks have relatively low but variable degrees of LREE enrichment. They are enriched in the LILE and depleted in HFSE and 1Mb, Ta. Based on the features of trace elements, these rocks can be divided into the high K and low K types. The former is characterized by the relatively high LREE, Rb, Ba, Th, and Sr abundances and obvious enrichment of K; while the latter has low abundances of LREE, clear positive Ba and negative K, Zr and Hf anomalies. Geochemical and isotopic features of the mafic-intermediate volcanics in the Great Xing’an Range imply the heterogeneity and complexity of the magma sources, that is, the mantle sources include both enriched and depleted. The magma source of the bolcanic rocks in the northern segment is mainly enriched, while in the middle-southern segment includes both enriched and depleted mantle sources. The Nd and Hf isotopic ages show that the formation of the the enriched mantle was closely related to the subduction of the oceanic crust of the Paleo-Asian ocean.The felsic rocks include two distinct groups with different REE and trace elements patterns. Rocks of the first group have relatively low HREE abundances and high Ba-Sr abundances with weak Eu anomalies, while rocks of the second group have relatively high HREE abundances and obvious negative anomalies of Eu, Ba and Sr. They are nemed as high Ba-Sr rocks and low Ba-Sr rocks, respectively. The high Ba-Sr rocks mainly distribute in the northern segment, while the low Ba-Sr rocks in the whole range. Temporally, the felsic rocks of low Ba-Sr abundances mainly generated in late stage of the volcanism. The low Ba-Sr rocks in the northern segment have obviously high generation temperatures, indicating the high isothermal surface when they formed; while rocks of the same kind in the middle-southern segment have low temperatures. The different temperatures of the rocks probably reflect the different deep processes. Some of the felsic rocks formed in the Late Jurassic time have the geochemical features of adakitic rocks, indicating the existence of thichened crust at that time.For the tectonic environment of the Early Cretaceous, it is consensus that they formed in an extensional environment. The evidences supporting this conclusion include the widespread A-type granite and other kind of alkaline rocks, metamorphic core complexes and mafic-felsic dyke swarms of the same time. Whist, the rock assemblages, features of regional tectonics as well as the well developed accretionary complex along the continental margin indicate that the compressive environment during the Late Jurassic time. That is, during the period of Late Jurassic-Early Cretaceous, the tectonic environment of Great Xing’an Range and adjacent region transformed from compressive to extensional. And the transformation corresponds to the magmatic gap.Inspite of the Late Jurassic volcanic rocks in the Great Xing’an Range, the peak time of the magmatism is the Early Cretaceous time, which is identical to that of the Early Cretaceous giant igneous events. This means the huge voluminous volcanic rocks in the Great Xing’an Range is related to the lithospheric thinning event. The temporal-spatial distribution of the Mesozoic igneous rocks in the eastern margin of the continent indicates that they were controlled by the subduction of the Paleo-Pacific plate. During the Jurassic time, the flat subduction of the oceanic plate beneath the continental plate induced the igneous assemblages of active continental margin affinity and hickened the continental crust. Along the subduction, the igneous rocks become youngling westward from the continental margin to the intracontinent. In the Early Cretaceous time, the delamination of the thickened crust induced the upwelling of the asthenospheric mantle, and subsequent huge magmatism. The delamination migrated eastward from the intracontinent to the continental margin, and the corresponding migration of the volcanism. At the climax of the delamination, the asthenosphere contacted with the crust directly, and induced the voluminous felsic rocks. Delamination also induced the change of the subduction angle of the oceanic slabs, resulting in the present high angle subduction revealed by the geophysic data. Although controlled by the same tectonic background, the northern and middle-southern parts of the Great Xing’an Range had different deep processes. In the northern part, the lithospheric thinning was remarkable; while in the middle-southern part underplating of was dominant. That is, the lithospheric thinning is heterogeneous spatially.更多还原