长江中下游地区晚中生代火山岩地球化学研究

发布时间：2018-05-05 21:47

  本文选题：安山质岩浆作用 + 双峰式火山作用　； 参考：《中国科学技术大学》2016年博士论文

【摘要】：板块构造理论可以很好地解释现代汇聚板块边缘弧岩浆作用的发生机制,但是对于大陆弧镁铁质岩浆岩的地幔源区性质、俯冲板片物质加入到地幔楔的具体机制以及大陆弧岩浆从源区提取之后在上覆板块发生的分异演化过程等问题仍不是很清楚。在大陆内部,具有弧型地球化学特征的镁铁质岩浆岩也很常见,这些岩浆岩在形成时远离现今活动大陆边缘,它们的成因是受古代消亡板块板块俯冲构造控制还是受现今活动板块构造控制,已经成为一个亟待解决的问题。本博士学位论文选取华南长江中下游地区晚中生代火山岩作为对象进行系统的岩石地球化学研究,结果不仅对长江中下游地区晚中生代火山岩的成因机制提供了新的系统的认识,而且建立了古大洋俯冲带壳幔相互作用与现今陆内弧型岩浆岩之间的成因联系本文第一章到第四章分别系统介绍了选题研究背景、区域地质背景、样品采集和描述以及所采用的一系列分析方法。第五章以长江中下游地区庐枞盆地晚中生代安山岩为例,通过系统的全岩主微量、Sr-Nd-Pb-Hf同位素、锆石U-Pb年龄以及Hf-O同位素研究。探讨了古安第斯型俯冲带富集地幔源区的形成过程及其对大陆弧安山岩成因的指示意义。在第六章中,通过对宁芜火山岩进行锆石U-Pb年龄和Hf-O同位素分析、全岩主微量元素和Sr-Nd-Hf同位素分析,验证了庐枞安山质火山岩的成因机制是否适用于宁芜盆地安山岩。第七章对宁芜火山岩中的单斜辉石斑晶进行了岩石学和地球化学分析,以期对宁芜火山岩的岩浆演化过程有系统的认识,从而进一步制约源区组成和岩浆过程对宁芜安山质火山岩地球化学成分的相对贡献。在第八章中对繁昌盆地晚中生代双峰式火山岩进行了系统的U-Pb定年和Hf-O同位素分析,以期确定它们的岩石成因。第九章中通过编辑文献发表的长江中下游地区火山岩地球化学数据,对比了该地区玄武岩、安山岩以及英安岩-流纹岩的地球化学组成,对该地区晚中生代火山岩的成因给出了系统的解释,同时也对这些火山岩形成的构造背景进行了限定。论文根据江南造山带北部在长江中下游地区的构造出露,提出在新元古代时期华夏洋向扬子克拉通南缘的北向俯冲过程中发生了俯冲洋壳衍生熔体与地幔楔-橄榄岩之间的化学反应。因此,长江中下游地区晚中生代岩浆作用本质上就是古老安第斯型俯冲带形成的富集富沃岩石圈地幔在晚中生代太平洋板块西向俯冲引发华南陆块岩石圈伸展背景下再活化的产物。庐枞盆地火山岩主要由玄武质粗面安山岩和粗面安山岩组成,伴随有少量的粗面玄武岩和粗面岩。这些岩石具有变化的SiO2(48.66-63.43 wt.%), MgO (0.39-4.85 wt.%), Na2O (1.22-6.07 wt.%)和K2O (2.53-10.10 wt.%)含量,其同时具有高的变化的K2O/Na2O比值(0.45-7.39).庐枞火山岩具有弧型徽量元素分布特征,表现为富集LILE、Pb和LREE,亏损HFSE.它们还具有富集的Sr-Nd-Pb-Hf同位素组成,其87Sr/86Sr初始比值介于0.7050和0.7066之间,εNd(t)值介于-8.0和-3.1之间,εHf(t)值介于-11和-1.1之间；在给定的206Pb/204Pb比值下.具有显著高出北半球参考线的207Pb/204Pb和207Pb/204Pb比值。 锆石U-Pb定年给出了127：1：2至137±1 Ma的岩浆结晶年龄,这些锆石具有高的δδ18O值(5.3-7.6%以及高度变化的ε:Hf(t)值从-13.1变化到2.6。这些地球化学特征表明,庐枞晚中生代火山岩源自富集富沃地幔源区的部分熔融,这一地幔源区形成于古安第斯型俯冲带沉积物来源熔体与上覆地幔楔橄榄岩之间的相互作用,类似现今大陆弧之下的壳幔相互作用过程。因此,该研究结果表明,大量沉积物来源的熔体加入到地幔楔橄榄岩中形成富集富沃的地幔源区可能是安山岩形成的重要前提条件。结合大洋弧玄武岩的成因机制,我们认为大陆弧安山岩与大洋弧玄武岩的成分差别本质上受控于交代介质的化学成分,前者以沉积物来源含水熔体为主,后者以蚀变洋壳来源的富水流体为主。宁芜火山岩主要由粗面安山岩和粗面岩组成,包含少量的玄武质粗面安山岩。这些火山岩具有低的变化的MgO含量(0.11-3.73wt.%),低的Mg#值(1.60-51.18),高的K20含量(1.99-9.88wt.%)。它们具有弧型微量元素配分模式,表现为富集LILE,Pb和LREE,亏损HFSE,它们还具有富集的Sr-Nd-Pb-Hf同位素特征,其初始87Sr/86Sr比值介于0.7050和0.7082之间,εNd(t)值介于-8.1和-1.8之间,εHf(t)值介于-6.7和0.39之间；在给定的206Pb/204pb比值(17.76-18.28)下,具有显著高出北半球参考线的207pb/204pb(15.47-15.61)和208Pb/204Pb(37.75-38.44)比值。 锫石U-Pb定年给出了128±2到133±2 Ma的岩浆结晶年龄,这些锆石具有高的8190值(5.7-7.8‰)以及变化的εHf(t)值从-5.8变化到0.7。根据其截然不同的成分特征,可以将这些安山岩划分为两组。相对组Ⅱ样品,组1样品具有更为富集的Sr-Nd-Pb-Hf同位素组成和相对低的不相容元素含量。对这些地球化学特征的综合分析表明,组Ⅰ火山岩形成于富集程度较高、含有大量金云母地慢源区较高程度的部分熔融；相反,组Ⅱ火山岩形成于富集程度相对较低、金云母含量较少的地幔源区较低程度的部分熔融。由于沉积物来源的长英质熔体组分的加入,这两种地幔源区在岩石化学上比正常橄榄岩地幔都相对富沃。对宁芜火山岩中的单斜辉石斑晶进行了系统岩石学和地球化学分析,结果对宁芜火山岩的岩浆演化过程有了系统的认识,从而进一步认识到源区成分和岩浆过程对宁芜安山质火山岩地球化学成分的相对贡献。分析发现,这些斑晶具有复杂的结构和成分环带,且不同的环带对应于不同的REE配分形式。根据其截然不同的REE配分模式,可以将这些单斜辉石分为三类。第一类单斜辉石具有平坦的REE配分模式和强烈的Eu负异常,第二类单斜辉石具有LREE-MREE富集的右倾的REE配分模式以及中等程度的Eu负异常,第三类单斜辉石具有MREE亏损的REE配分模式和强烈的Eu负异常。这三类辉石呈现出两种截然不同的成分演化趋势,指示它们结晶自两种截然不同的熔体。其中,第一类单斜辉石结晶自由REE富集程度较弱的初始熔体演化形成的熔体,第二类和第三类辉石则结晶自由REE富集程度较高的初始熔体演化形成的熔体。第二类辉石和第三类辉石的成分差异表明,在二者结晶的间隙存在大量的斜长石和角闪石的结晶。然而,这些辉石的寄主岩石均具有微弱的Eu负异常和MREE鍕不亏损的REE配分模式,表明其成分没有受到斜长石和角闪石分离结晶的影响。因此,第三类辉石所记录的斜长石和角闪石结晶改变了第三类辉石结晶时的熔体的成分特征,但是这些斜长石和角闪石在结晶之后直至喷发之前并没有从熔体中分离出去,因此对宁芜安山质火山岩的最终成分没有显著的影响。辉石斑晶广泛发育的补丁状环带和反环带记录了贯穿整个宁芜火山作用的岩浆注入过程,这一长期存在的岩浆注入过程导致了火山作用晚期的岩石相对早期的岩石具有更为狭窄的成分变化范围,也导致了宁芜火山岩整体上相对长江中下游地区其它火山岩具有更为狭窄的成分变化范围。在繁昌盆地还发育一套晚中生代双峰式火山岩,其地球化学成分与同时代安山岩有很多相似性。因此.对这些双峰式火山岩的研究不仅有助于理解整个长江中下游地区晚中生代岩浆作用的实质,而且能够对该地区安山质火山岩的成因做更进一步的限定。繁昌盆地镁铁质火山岩主要由玄武岩、粗面玄武岩和碱性玄武岩组成,它们均具有玄武质成分特征,其长英质火山岩主要由粗面岩、流纹岩以及少量的粗面安山岩组成,它们具有英安质或者流纹质成分特征。本文锆石U-Pb定年结果表明,繁昌盆地双峰式火山作用发生在129到125 Ma之间。镁铁质火山岩具有如下特征：(1)弧型微量元素分布模式,高度富集流体活动性元素：(2)中等程度富集的Sr-Nd-Hf同位素组成,具有非常均一的(87Sr/86Sr)i值0.7065-0.7067,低的εNd(t)值-5.61至-3.5 1以及低的εHf(t)值-3.01至-2.73：(3)变化范围极大的锆石εHf(t)值-20.5至7.9。上述这些特征表明,这些镁铁质岩来自含角闪石的富集地幔源区的部分熔融,该源区是以流体为主、熔体为辅的俯冲板片来源物质与地幔楔橄榄岩反应形成的富集地幔。另一方面,该地区长英质岩石具有与镁铁质岩类似但变化范围稍大的(87Sr/86Sr)i比值0.7060至0.7076,比玄武岩更负的εNd(t)值-8.26至-6.62,更负的εHf(t)值-8.01至-7.77。其钻石同样具有高度变化的εHf(t)值-17.9至-1.8,这些锆石还具有相对高的81s0值6.3至9.2%o。这些地球化学特征表明,长英质火山岩来自陆源沉积物的部分熔融。镁铁质和长英质火山岩在某些地球化学组成上具有亲缘性,表明二者具有成因上的联系,这一点在锆石Hf同位素上体现的尤为明显,表明二者源区含有同一来源的沉积物组分。因此,繁昌盆地双峰式火山岩的成因可以解释为古俯冲隧道中的交代地幔和俯冲洋壳沉积物在碰撞后环境下的分别再活化。尽管本文实测样品研究对长江中下游地区安山岩和双峰式火山岩的成因有了一定的认识,但是它们之间的相互联系以及对整个长江中下游地区晚中生代火山岩成因的含义有待探索。为此,通过统计编辑文献发表的相关数据,可以对比该地区玄武岩、安山岩以及英安岩-流纹岩的地球化学组成,对该地区晚中生代火山岩的成因给出了系统的解释,同时也对长江中下游晚中生代火山岩形成的构造背景进行进一步的限定。结果发现,长江中下游地区晚中生代火山岩均具有弧型微量元素分布特征,富集的Sr-Nd-Pb-Hf同位素特征。总体上,安山岩具有显著高于玄武岩的Th和U的含量,安山岩与玄武岩具有类似的Sr和Pb同位素组成,但其Nd和Hf同位素组成比玄武岩显著富集。同时,安山岩具有低的Ba/Th比值和高的(La/Sm)N比值,玄武岩则刚好相反。这些特征表明,安山岩的地幔源区受到了沉积物来源的含水熔体为主、富水流体为辅的俯冲地壳来源物质的交代,而玄武岩地幔源区则恰恰相反。英安岩-流纹岩在Sr-Nd-Hf同位素组成上比玄武岩和安山岩更为富集,但缺乏同位素与分异指标之间的相关性,表明该特征不能用镁铁质岩浆的AFC过程来解释。其富集且高度变化的锆石Hf同位素以及高的锆石氧同位素组成表明,这些英安岩-流纹岩来自陆源沉积物的部分熔融。至此,长江中下游晚中生代火山岩的形成可以概括为古大洋俯冲隧道内不同介质交代形成的富集地幔源区和俯冲洋壳沉积物共同熔融的结果。因此,长江中下游地区晚中生代弧型玄武质-安山质岩浆作用实质上是新元古代古安第斯型俯冲隧道内形成的地幔源区在晚中生代太平洋板块西向俯冲诱发华南区域性岩石圈拉张及软流圈上涌背景下的再活化,这个地幔源区的形成机制与现代活动大陆边缘类似,但是再活化时间滞后了700百万年以上。从这个角度来说,长江中下游地区晚中生代火山岩成因模型对现代活动大陆边缘玄武岩-安山岩的成因具有借鉴意义。
[Abstract]:The theory of plate tectonics can explain the mechanism of the magmatism of the marginal arc of the modern converging plate, but for the nature of the mantle source area of the continental arc mafic magmatism, the specific mechanism of the subduction plate material added to the mantle wedge and the differentiation and evolution process of the overlying plate after the extraction of the continental arc magma from the source area. It is still not clear that the mafic magmatic rocks with the characteristics of arc geochemistry are also common in the continent. These magma rocks are far from the active continental margin at present, and their genesis has become an urgent problem to be controlled by the subduction of the plate plate subduction of the ancient disappearing plate or the structure of the present active plate. In this thesis, the late Mesozoic volcanic rocks in the middle and lower reaches of the Yangtze River, Southern China, were selected as the object of systematic rock geochemical study. The results not only provided a new systematic understanding of the genetic mechanism of the late Mesozoic volcanic rocks in the middle and lower reaches of the Yangtze River, but also established the crust mantle interaction in the paleo oceanic subduction zone and the present land. In the first chapter to the fourth chapter, the research background, the regional geological background, the sample collection and description and the series of analysis methods are introduced. The fifth chapter takes the late Mesozoic andesite in the middle and lower reaches of the Yangtze River in the middle and lower Yangtze River as an example, through the system of the main trace of the whole rock, Sr-Nd-Pb-Hf Isotopes, zircon U-Pb age and Hf-O isotopes study. The formation process of the source area of the enriched mantle in the ancient Andean subduction zone and its directive significance to the genesis of the continental arc andesite are discussed. In the sixth chapter, the analysis of U-Pb age and Hf-O isotopes of zircon from Ningwu volcanic rocks, the analysis of the trace elements and Sr-Nd-Hf isotopes of the whole rock It is verified whether the genetic mechanism of the luzongan mountain quality fire rock is applicable to andesite in the Ningwu basin. In the seventh chapter, the petrography and geochemical analysis of the monoclinic pyrophyla in the Ningwu volcanic rocks are carried out to make a systematic understanding of the magma evolution process in Ningwu volcanic rocks and further restrict the source area composition and the magma process to Ningwu. The relative contribution of the geochemical components of the Anshan volcanic rocks. In the eighth chapter, a systematic U-Pb dating and Hf-O isotopic analysis of the late Mesozoic volcanic rocks in the Fanchang basin were carried out to determine their petrogenesis. In the ninth chapter, the geochemical data of the volcanic rocks in the middle and lower reaches of the Yangtze River, published in the edited literature, were compared. The geochemical composition of the basalt, andesite and the anganite rhyolite is made to explain the formation of the late Mesozoic volcanic rocks in this area, and the tectonic setting of these volcanic rocks is limited. During the northern subduction of the Chinese ocean to the southern margin of the Yangtze craton, the chemical reaction between the subducted oceanic crust derived melt and the mantle wedge peridotite occurred during the North subduction of the southern margin of the Yangtze craton, so the late Mesozoic magmatism in the middle and lower reaches of the Yangtze River was essentially the rich fertile lithosphere mantle formed by the ancient Andean subduction zone in the late Mesozoic Pacific plate. The block west subduction leads to the reactivation of the Southern China continental block lithosphere extension background. The Luzong basin is mainly composed of basaltic andesite and rough andesite, accompanied by a small amount of rough basalt and rough rock. These rocks have changed SiO2 (48.66-63.43 wt.%), MgO (0.39-4.85 wt.%), Na2O (1.22-6.07 wt.%) and K. The content of 2O (2.53-10.10 wt.%) has a high variation of K2O/Na2O ratio (0.45-7.39). The Lujiang Fong volcanic rocks are characterized by the distribution of the arc emblem elements, showing the enrichment of LILE, Pb and LREE, and they have a loss HFSE. they also have a enriched Sr-Nd-Pb-Hf isotopes. The initial ratio of 87Sr/ 86Sr is between 0.7050 and 0.7066. Between -3.1 and -3.1, the value of epsilon Hf (T) is between -11 and -1.1; at a given 206Pb/204Pb ratio, the ratio of 207Pb/204Pb and 207Pb/204Pb to the northern hemisphere reference line is significantly higher. The zircon U-Pb year gives a 127:1:2 to 137 + 1 Ma magma crystallization age. These zircon has a high Delta 18O value. The geochemical characteristics of the change from -13.1 to 2.6. indicate that the late Mesozoic volcanic rocks of the Lujiang Fong were derived from the partial melting of the rich fertile mantle source area. This mantle source region was formed in the interaction between the sediments source of the paleandean subduction zone and the overlying mantle wedge peridotite, like the crust mantle interaction under the present continental arc. Therefore, the result of the study shows that the addition of a large amount of sediment source melt into the mantle wedge peridotite is an important precondition for the formation of andesite in the source area of rich fertile soil. Combining the genesis mechanism of the oceanic arc basalt, we think the difference in composition between the continental arc andesite and the oceanic arc basalt is essentially controlled by the intersection. The former is mainly composed of water - bearing melts from sediment sources, and the latter is dominated by water rich fluids derived from altered oceanic shells. The volcanic rocks of Ningwu are mainly composed of rough andesite and rough rock, containing a small amount of basaltic rough andesite. These volcanic rocks have low changing MgO content (0.11-3.73wt.%) and low Mg# value (1.60-51.18 ), high K20 content (1.99-9.88wt.%). They have arc type trace element distribution patterns, which are enriched in LILE, Pb and LREE, and have a loss of HFSE. They also have rich Sr-Nd-Pb-Hf isotopes, whose initial 87Sr/86Sr ratio is between 0.7050 and 0.7082, and the value of E Nd (T) is somewhere between -8.1 and 0.39; Under the 206Pb/204pb ratio (17.76-18.28), the ratio of 207pb/204pb (15.47-15.61) and 208Pb/204Pb (37.75-38.44) to the northern hemisphere reference line is significantly higher than that of the northern hemisphere. U-Pb gives the magma crystallization age of 128 + 2 to 133 + 2 Ma. These zircon has a high 8190 value (5.7-7.8 per 1000) and the change of the value of the Hf (T) from -5.8. These andesite can be divided into two groups. The relative group II samples, the group 1 samples have more enriched Sr-Nd-Pb-Hf isotopes and relatively low incompatible elements. The comprehensive analysis of these geochemical characteristics shows that the group I volcanic rock formed at a high degree of enrichment and contains a large number of gold mica. On the contrary, the group II volcanic rocks are formed in a relatively low degree of partial melting in the mantle source with relatively low enrichment and less gold mica content. The two mantle source regions are relatively rich in rock chemistry than the normal peridotite mantle because of the addition of the long angled melt components from the sediments. A systematic petrographic and geochemical analysis of the monoclinic pyrophyla in the Wuhuo yam rock has been carried out, and the result of the systematic understanding of the magma evolution process of the Ningwu volcanic rocks has further recognized the contribution of the source region composition and the magmatic process to the phase pairs of the geochemical composition of the ningwuan volcanic rocks. The structure and composition ring are corresponding to different REE distribution forms. According to their distinct REE distribution patterns, these monoclinopyroxene can be divided into three types. The first type of clinopyroxene has a flat REE distribution pattern and a strong negative Eu anomaly, and the second type of clinopyroxene has a LREE-MREE enrichment right REE partition model. The third type of clinopyroxene has a MREE loss REE distribution pattern and a strong Eu negative anomaly. The three types of pyroxene exhibit two distinct evolutionary trends, indicating that they crystallize from two distinct melts. Among them, the first type of mono clinopyroxene crystallized at the initial concentration of free REE. The melts formed by the melt evolution, the second and third types of pyroxene crystallized the melt formed by the initial melt evolution of the free REE enrichment. The differences in the composition of the second types of pyroxene and third types of pyroxene indicate that there are a large number of plagioclase and amphibolite in the two crystallized spaces. However, the host rocks of these pyroxene are all micro. The weak Eu negative anomaly and the REE distribution pattern of MREE without loss indicate that their components are not affected by the separation and crystallization of plagioclase and hornblende. Therefore, the crystallization of plagioclase and hornblende recorded by the third types of pyroxene changes the composition characteristics of the melts in the crystallization of the third types of pyroxene, but these plagioclase and hornblende are crystallized until after crystallization. It has not been separated from the melt before the eruption, and therefore has no significant influence on the final composition of the ningwuan volcanic rocks. The patched and reverse belt of the pyroxene phenocrysts recorded the magma injection through the whole Ningwu volcano. This long existence of magma injection resulted in the late volcanism. The rock has a more narrower range of composition than the early rocks, and it also leads to a more narrower variation in the composition of the other volcanic rocks in the middle and lower reaches of the Yangtze River. In the Fanchang basin, a set of Late Mesozoic Shuangfeng volcanic rocks was developed, and the geochemical composition of the volcanic rocks is very similar to that of the andesite of the same age. Therefore, the study of these Shuangfeng volcanic rocks is not only helpful to understand the essence of the late Mesozoic magmatism in the middle and lower reaches of the Yangtze River, but also to further restrict the genesis of the Anamic volcanic rocks in this area. The mafic volcanic rocks in the Fanchang basin are mainly composed of Xuan Wuyan, rough Xuan Wuyan and alkaline Xuan Wuyan. All of them are characterized by basaltic composition. Their felsic volcanic rocks are mainly composed of rough rocks, rhyolite and a small amount of rough andesite. They have the characteristics of the Anglo Amy or rhyolite. The zircon U-Pb dating results showed that the Shuangfeng type volcanism in the Fanchang basin occurred between 129 and 125 Ma. The following characteristics: (1) the distribution pattern of arc trace elements, highly enriched fluid active elements: (2) Sr-Nd-Hf isotopes of moderate enrichment, with a very homogeneous (87Sr/86Sr) I value 0.7065-0.7067, low Nd (T) value -5.61 to -3.5 1 and low I Hf (T) value -3.01 to -2.73: (3) These characteristics indicate that the mafic rocks are partially melted from the rich mantle source area containing amphibole, the source area is a rich mantle derived from the fluid dominated subduction plate source and the mantle wedge peridotite. On the other hand, the felsic rocks in this area are similar to the mafic rock but have a slight variation in range. The larger (87Sr/86Sr) I ratio is 0.7060 to 0.7076, the negative epsilon Nd (T) value is -8.26 to -6.62 more than the basalt, and the negative e Hf (T) value -8.01 to -7.77. its diamond also has a highly variable epsilon Hf (T) value. These zircon also has a relatively high value of 6.3 to the geochemical characteristics indicating that the felsic volcanic rocks come from the terrestrial sediments. The mafic and feldsic volcanic rocks are related to some geochemical compositions, indicating the genetic relationship between the two, which is particularly evident in the zircon Hf isotopes, indicating that the two source areas contain the same source of sediment components. Therefore, the genesis of the Shuangfeng type volcanic rocks in the Fanchang basin can be interpreted as a source of origin. The metasomatic mantle and subducted oceanic sediments in the subducted tunnel are reactivated respectively in the environment after the collision. Although the measured samples have a certain understanding of the genesis of andesite and Shuangfeng type volcanic rocks in the middle and lower reaches of the Yangtze River, the interrelation between them and the late Mesozoic volcanism in the middle and lower reaches of the Yangtze River For this reason, the geochemical composition of the Xuan Wuyan, andesite and the anganite rhyolite in this area can be compared by the relevant data published by the statistical editing literature, and the causes of the late Mesozoic volcanic rocks in this area are explained systematically, and the formation of Late Mesozoic volcanic rocks in the middle and lower reaches of the Yangtze River is also formed. The results show that the late Mesozoic volcanic rocks in the middle and lower reaches of the Yangtze River arc arc trace elements distribution.

【学位授予单位】：中国科学技术大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：P588.14
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本文编号：1849427