【作者】 施彬；

【导师】 钟增球；

【作者基本信息】 中国地质大学 ， 矿物学、岩石学、矿床学， 2014， 博士

【摘要】 花岗岩作为大陆地壳的重要组成部分,是区别于大洋地壳的标志之一,它的形成与大陆地壳的生长演化密切相关。作为大陆地壳最为活跃的位置—造山带往往是地壳活动最为强烈的场所,同样也是花岗岩最为发育的区域。东昆仑造山带作为分割中国两大单元的中央造山系的一部分,具有漫长复杂的地质演化史,发生过多期构造—岩浆作用,构成了—巨型构造—岩浆岩带,以加里东期和海西—印支期为主要的岩浆作用时期,发育丰富的幔源岩浆底侵作用和岩浆混合作用,使得东昆仑岩浆带成为又—研究壳幔作用及地壳生长的理想研究场所。研究区地处格尔木市西南黑海地区,大地构造位置属于东昆仑南部地质单元—东昆南俯冲增生杂岩楔。通过野外地质调查发现研究区内分布有大量的过铝质花岗岩体,本文应用岩相学、主量元素和微量元素分析、电子探针(EPMA)分析和锆石LA-ICP-MS U-Pb定年、同位素地球化学等多种分析手段对黑海地区过铝质花岗岩开展综合研究,主要解决其岩石组合类型、源区性质、形成时代和形成环境及机制等,结合区域地质资料,探讨东昆南俯冲增生杂岩楔早古生代形成与演化,进而为东昆仑造山带早古生代地壳生长方式和的构造演化研究提供相关的地质依据。通过详细研究得出以下结论：(1)黑海地区加里东期过铝质花岗岩主要由黑云母英云闪长岩、黑云母花岗闪长岩、黑云母花岗岩、二云母花岗岩和白云母花岗岩组成,野外露头特征显示白云母花岗岩为最晚期侵入体。岩体侵入的围岩为志留纪赛什腾组,岩性由变砂岩、含堇青石变砂岩和堇青石片岩组成,受岩体侵入影响广泛发育接触变质作用。过铝质花岗岩矿物组成简单,主要由斜长石、钾长石、云母和石英组成,差别在于云母类型和副矿物种类有所不同,黑云母英云闪长岩云母主要为黑云母且含量较高,不含白云母,黑云母普遍发育受后期热影响形成的针状金红石；黑云母花岗闪长岩云母主要为黑云母,同样发育针状金红石,白云母少量且往往发育于黑云母边缘,颗粒呈细小鳞片状；黑云母花岗岩云母由黑云母和白云母共同组成,黑云母含量略高于白云母,晶形较白云母好；二云母花岗岩云母为几乎等量的黑云母和白云母组成,二者颗粒大小和自形程度相近；白云母花岗岩云母为白云母,几乎不含黑云母,此外,白云母花岗岩中发育特征的富铝质矿物—石榴子石,石榴子石按颗粒大小和自形程度可分为两类,一类自形程度高且颗粒较小,一类自形程度差、裂纹发育且颗粒较大。(2)在主量元素组成上,黑海地区加里东期过铝质花岗岩与西部构造区花岗岩相比,英云闪长岩和花岗闪长岩具有相对低硅、高钛镁铁锰、高铝、高钙磷、低钾钠的特征,黑云母花岗岩、二云母花岗岩和白云母花岗岩则以富硅铝、低铁锰镁钙、低钠、高钾磷为特征；所有岩类均属钙碱性—高钾钙碱性系列,绝大多数样品A/CNK大于1.1,计算标准矿物刚玉(C)含量大于1%,为强过铝质花岗岩,属S型花岗岩类,其中白云母花岗岩为淡色花岗岩,主量元素组成特征暗示源区形成的岩浆为富铝质的。在稀土元素组成上,从英云闪长岩至白云母花岗岩,稀土总量逐渐减少,具有相似的球粒陨石标准化配分曲线,为稀土相对富集而重稀土相对亏损的右倾曲线,轻稀土分馏明显,重稀土分馏不明显,普遍具有中等负铕异常。在微量元素组成方面,原始地幔标准化曲线图为“齿状,,右倾型曲线,具有相对富集Cs, Rb、Th、K和相对亏损Ba, Sr的选择性富集LILE特征,Nb和Ti等HFS亏损明显,Rb/Sr比值表现为升高的趋势,低Sr/Y比值与淡色花岗岩相似,Nb/Ta比值接近于壳源岩石。(3)电子探针分析结果显示：斜长石以Ab(67.74%--97.27%)为主,An(2.46%—31.96%)次之,Or(0.16%—1.56%)最少,主要种属为钠长石—奥长石,为酸性斜长石,其中,黑云母花岗岩和二云母花岗岩的斜长石牌号略高于白云母花岗岩和英云闪长岩：钾长石以Or端元组份(91.901%—97.916%)最高,Ab端元组份(2.37%—7.829%)次之,An端元组份极低,黑云母花岗岩中的条纹长石为正长条纹长石；黑云母为高铁黑云母,A1203和TiO2含量由花岗闪长岩至二云母花岗岩的变化趋势反映成岩温度逐渐降低的特征,多数TiO2/MnO比值<11和Mg/(Mg+Fe)比值和全岩的Mg/(Mg+Fe)比值接近的特征表明黑云母为岩浆成因；白云母具有高铝、富钾铁、低镁钛和贫钠的特征；两类石榴子石均为铁铝榴石(Alm)+锰铝榴石(Spess),具有成分环带的特征,由核部至边部,一类显示铁铝榴石(Alm)含量降低而锰铝榴石(Spess)含量增高,二类呈现振荡方式的成分变化,但总体上呈现铁铝榴石(Aim)含量升高而锰铝榴石(Spess)含量降低的特征,富锰且寄主岩富铝的特征表明可能是在中低压条件下从过铝质岩浆中结晶形成的岩浆型石榴子石,而且结晶部位较浅。(4)白云母花岗岩岩浆锆石LA-ICP-MSU-Pb年龄为420.4±3.5Ma (MSWD=0.025),二云母花岗岩岩浆锆石LA-ICP-MSU-Pb年龄为424.0+2.6M a(MSWD=2.0),421.5±4.4Ma (MSWD=2.6),测年结果表明黑海地区加里东期过铝质花岗岩形成于晚志留世,白云母花岗岩的年龄(420.4±3.5Ma)代表了此次岩浆作用事件的上限。(5)锆石Hf同位素特征表明在晚志留世研究发生了地幔物质底侵下地壳事件,不仅提供了热源,而且参与了新生地壳的形成。(6)黑云母英云闪长岩和黑云母花岗闪长岩TZr分别为827—861℃和836℃,略高于黑云母花岗岩TZr(772--826℃)和二云母花岗岩TZr(755--835℃),白云母花岗岩TZr最低,为647-724℃,在形成温度上呈现温度逐渐降低的特征,这可能与源区热源的提供程度有关,由英云闪长岩至白云母花岗岩,源区熔融热量的供给逐渐减少,暗示构造—岩浆作用强度逐渐减弱。二长石和石英共结压力较低,为0.1—0.2Gpa,与白云母花岗岩富锰石榴子石的形成压力条件相符,而且微量元素特征表明其形成压力为中等—低压环境,代表了较厚—正常地壳向伸展减薄地壳过渡的状态。主量元素组成显示源区可能是一个由变杂砂岩和变泥质岩组成的混合源区,源区部分熔融的诱发因素主要是白云母在无水条件下的脱水熔融。(7)构造环境判别分析显示黑海地区过铝质花岗岩兼具火山弧和同碰撞花岗岩特征,可能形成于活动大陆边缘环境。结合区域地质演化资料推断其可能的形成机制为：在南部洋壳向南板退的环境下,地幔物质底侵东昆南俯冲增生杂岩楔地体,促使其下部富硅、铝的地壳物质发生不同程度的部分熔融所形成,随着温度的降低而发生不同程度的部分熔融,形成了黑云母英云闪长岩—白云母花岗岩系列,黑云母英云闪长岩和黑云母花岗闪长岩可能是伸展初始阶段的产物,而黑云母花岗岩和二云母花岗岩则可能代表了此次板退引起的伸展达到了最大限度,而白云母花岗岩作为最晚期形成的岩体可能代表此次板退趋于结束而进入下一个洋壳俯冲阶段。总之,研究区在晚志留世可能处于由挤压向伸展转换的关键构造机制转换时期,为研究东昆仑造山带早古生代地壳生长方式和构造演化提供了新的地质依据。更多还原

【Abstract】 As an important part of the continental crust, granite is one of the symbols which is different from the ocean crust. Its formation is closely related to the growth of the continental crust. As the most active location of the continental crust, the activity of the orogenic belt is violent. It is also a good area of granite development. As the segmentation unit, the Eastern Kunlun orogenic belt is the part of central orogenic system. It has a long complicated geologic evolution history. Multiphase tectonic magmatism make it to constitute a huge tectonic-magmatic belt. The Caledonian and hercynian are main periods of magmatism. It has rich mantle magma underplating and migmatization. The Eastern Kunlun magma belt is a good study place for crustal mantle action and crustal growth.The study area is located in Heihai region, the southwest of Geermu. It belongs to the southern subduction complex wedge. Through field geological survey, we found abundant peraluminous granites in this area. Many methods will be carried out to finish the comprehensive research, such as petrography, major elements and trace elements analysis, electron microprobe analysis(EPMA) and zircons LA-ICP-MS U-Pb analysis. With the regional geological data, This paper will discuss the early Paleozoic formation and evolution of the southern subduction complex wedge and provide some related geological basis to study the early Paleozoic crust growth and Tectonic evolution.(1)The Caledonian peraluminous granites is mainly composed of biotite tonalite, biotite granodiorite and biotite granite, two-mica granite and muscovite granite, intruded into the Silurian Saishiteng group. Outcrop characteristics showed white muscovite granite is the latest intrusions. The Silurian Saishiteng group contain metasandstone, cordierite Sandstone and cordierite schist. Peraluminous granites mineral composition are simple, mainly composed of plagioclase, potassium feldspar, mica and quartz. The mica types and accessory mineral species are different between them. The micas of Biotitetonalite are biotite. Mainly, the micas of Biotite granodiorite are biotite.They also have needle rutile. Muscovites are small and often occur in edge of biotite. Biotite granite’s mica are composed of biotite and muscovite, the content of biotite is slightly higher than muscovite. Biotite’s crystal shape is good than muscovite. In two mica granites, the contents of biotite and muscovite are similar, with similar size and shape. Muscovite granites have muscovite. In addition, muscovites have rich aluminum garnet. According to the degree of particle size and the shape, they can be divided into two classes. One has good shape and small particles, another has poor shape, crack and bigger grain.(2)On the composition of major elements, tonalite and granodiorite have relatively low Si, high Ti,Mg,Fe,Mn, high Al, high Ca,P, low K,Na Biotite granite, two mica granite and muscovite granite have relatively rich silicon, aluminum, low iron, manganese, low sodium and high potassium phosphate. All rocks belong to calc-alkaline-high potassium calc-alkalineseries. M ost A/CN K are greater than 1.1. Cal cul ati on standard mi neral corundum content isgreater than 1%. They are strong peraluminous granites, belongs to S type granite. Muscovite granite is leucogranite. The main elements characteristics suggest that source magma is aluminum. On the composition of rare earth elements, from tonalite to white mica granite, ∑ REE gradually reduce. They have similar chondrite standardized distribution curves, relative enrichment in LREE and loss of HREE. Heavy rare earth fractionation is not obvious Generally, they have moderate Eu negative abnormality. On trace element compositions, Standardization of the primitive mantle curves are right deviation, with relative enrichment of Cs, Rb, Th, K and relative loss of Ba, Sr. Nb and Ti losses significantly, Rb/Sr ratios show increased trend, low Sr/Y ratio are similar to leucogranite, Nb/Ta ratios are close to the shell source rocks.(3)Electron microprobe analysis results show that:Plagioclases have Ab=67.74%-97.27%, An=2.46%-31.96%, Or=0.16%-1.56%, the main species are albite-oligodase. They are acidic plagioclase. Among them, An of biotite granites and some two-mica granites plagioclase are slightly higher than that of muscovite granites and tonalites. Potassium feldspars have Or =91.901%-97.916%, Ab=2.37%-7.829% and extremely low An. The perthites of biotite granites are orthoclase-perthites. Biotites have high iron. The trend of Al2O3 and TiO2 contents from granodiorites to two-mica granites reflects diagenetic temperature decreases. The characteristics of TiO2/MnO ratios and Mg/(Mg+Fe) ratios indicate that biotites are magmatogene. Muscovites have high Al, K, Fe and low Mg, Ti, Na garnets are almandine (Alm)+spessartine (Spess), They all have Compositional zonation. From core to edge, One has decreased almandine (Alm) and increased spessartine, Another has oscillatory zone. The characteristics of rich Mn and high Al in host rocks showed that garnet may crystallize from peraluminous magma, under the low pressure.(4)The magmatic zircon LA-icp-ms U-Pb age of Muscovite granite is 420.4±3.5 Ma (MSWD=0.025), mica granite magmatic zircon LA-icp-ms U-Pb age was 424.0±2.6 Ma (MSWD=2.0),421.5±4.4 Ma (MSWD=2.6). Dating results show that peraluminous granites in Heihai region formed in the late Caledonian. The age of Muscovite granite (420.4±3.5 Ma) represents the upper limit of the magmatism event.(5)The characteristics of Hf isotope of zircon indicate that It had occurred mantle upplating during Late Silurian Epoch. Not only mantle had supplied the heat source, but also it had participated in the formation of juvenile crust.(6)The TZr of Biotite tonalite and biotite granodiorite are 827-861℃ and 836℃, respectively, slightly higher than the biotite granite’s TZr (772-826℃) and two-mica granite’s TZr(755-835℃), muscovite granite’sTZr(647-724℃) are the lowest. The temperatures gradually reduce, which may be associated with degree of source heat, From tonal ite to muscovite granite, source area melting heat supply gradually decreased, suggesting that tectonic-magmatic activity intensity gradually weakened. Two feldspar and quartz eutectic pressure(0.1-0.2 Gpa) is low. The characteristics of trace element show that the formation pressure is medium-low. It represents the thicker-normal crust became thinning. Source is likely a mixed source with graywacke and argillaceous rock. Partial melting of the source area are mainly induced by muscovite dehydration, under the condition of no water.(7)Discriminant analysis show that the peraluminous granites are similar with volcanic arc granites and collision granites, probably formed in the active continental margin environment. Combination with regional geological evolution data, this paper conclude the possible mechanism:in the environment of the southern oceanic plate retreat, with the mantle underplating, the lower crust produced different degrees of partial melting. With the decreased temperature, partial melting of the crust formed the biotite tonal ite-white mica granite series. Biotite tonalite and biotite granodiorite might formed in the initial stage. Two-mica granite and biotite granite may represent the the maximum extension. As the latest intrusion, Muscovite granite represents that the block retreat might stop, and the next oceanic crust subduction might start. In conclusion, during the late Silurian, the study area might be in the stage of the tectonic switch setting which changed from compression to extension. This study may provide new geological basisesabout crustal growth and tectonic evolution.更多还原