西藏安多陆块岩浆演化过程研究

发布时间：2018-03-18 16:26

  本文选题：安多陆块　切入点：花岗片麻岩　出处：《中国科学院大学(中国科学院广州地球化学研究所)》2017年硕士论文　论文类型：学位论文

【摘要】：青藏高原是自显生宙以来由一系列微陆块、增生带和岛弧连续地拼贴到欧亚大陆的南部边缘组成。作为青藏高原内部最古老的地体,安多陆块以透镜体形式夹持于羌塘地体和拉萨地体之间,发育大量新元古代至早中生代不同类型的岩浆岩,记录了不同时期的地质演化过程。在本研究中,我们对安多陆块新元古代、早古生代和早中生代三期主要的岩浆岩开展了详细的野外调查和室内岩相学研究,并进行了精确的锆石U-Pb年代学和Lu-Hf同位素研究、全岩主-微量元素地球化学和Sr-Nd同位素地球化学分析,旨在揭示安多陆块三期岩浆岩的组成、形成时代、岩石成因和构造环境,恢复地质演化过程,并探讨安多陆块大陆地壳的形成、改造和生长过程。我们对安多陆块一新元古代花岗质片麻岩体的锆石U-Pb同位素定年结果揭示其原岩侵位年龄为8~(87) Ma。该花岗质片麻岩体具有高的SiO2(65.56~79.82wt.%),A/CNK(1.39~1.68),CIPW刚玉含量(2.74~5.10 wt.%)。结合富铝矿物的出现(如白云母),推测其原岩为强过铝质S-型花岗岩。这些花岗质片麻岩具有低的CaO/(MgO+FeO~T)、Al2O3/TiO2、Rb/S和Rb/Ba比值,高的CaO/Na2O(0.3),富集的εNd(t)(-5.51~-2.76)和εHf(t)(-15.79~4.56)同位素特征,表明安多新元古代花岗片麻岩原岩很可能是由变泥质岩部分熔融形成。大量存在的继承锆石也支持这一解释。这些特点表明,安多新元古代花岗片麻岩原岩很可能是由变泥质岩部分熔融形成,样品高的MgO、Cr、Ni含量揭示其岩浆源区可能有幔源物质或新生地壳物质的加入。锆石U-Pb同位素定年揭示安多陆块早古生代花岗质片麻岩的侵位年龄介于484~500 Ma。这些花岗质片麻岩呈现出高且变化较大的SiO2(68.46~~(87).52%)含量,显示富K(Na2O/K2O1),低的P2O5(0.1%)和弱过铝质的地球化学特征。结合角闪石矿物的存在,其这些特征说明其原岩为I-型花岗岩。片麻岩具有富集的同位素(εNd(t)=-10.98~-4.54;εHf(t)=-10.38~1.31)特征,推测其为安多基底部分熔融形成。早古生代花岗质片麻岩具有比安多新元古代片麻岩高的εNd(t)和εHf(t)值,结合同期发育的镁铁质岩包体,揭示在其形成的过程中有地幔物质的加入。鉴于一些样品有很高的SiO2、FeO~T/MgO和(K2O+Na2O)/CaO,我们认为这些片麻岩的原岩在形成过程中发生了一定的岩浆分异作用。通过对安多陆块早中生代岩浆岩的研究,我们发现了morb型辉长岩,高镁闪长岩,钙碱性闪长岩和花岗闪长岩这四种不同类型的岩石。系统的锆石u-pb同位素定年显示这些岩浆岩的结晶年龄是一致的,主要集中在174-177ma。我们系统的岩石地球化学研究揭示这四种早中生代岩浆岩具有明显不同的成因:(1)辉长岩:辉长岩具有相对平坦的稀土配分模式((la/yb)n=0.70~1.13)和亏损的nd和hf同位素组成(εnd(t)=4.40~5.50;εhf(t)=10.33~15.55),其sr-nd同位素组成和ree配分模式类似于班公-怒江洋蛇绿岩,指示安多辉长岩可能来自亏损的软流圈地幔源区。安多辉长岩富集大离子亲石元素(lile:k、rb、pb)、亏损高场强元素(hfse:nb、ta、ti),高的ba/la(8~15),ba/th(29~150),u/th(0.57~2.73)和低的th/yb(0.05~0.33),显示含水流体的交代作用在其岩石形成过程中发挥了重要作用。这些地球化学特征和富水矿物角闪石的存在,说明了被含水流体交代的富集的岩石圈地幔也参与了安多辉长岩形成过程。故安多陆块辉长岩可能是由被富集岩石圈地幔交代的软流圈地幔部分熔融形成。辉长岩具有低的sm/yb和dy/yb,指示了辉长岩熔体熔融深度是在岩石圈地幔的尖晶石相二辉橄榄岩稳定区形成的,形成在较浅的深度(80km)。(2)高镁闪长岩:安多闪长岩有高的mgo(8.30~10.24wt.%)和mg#(70~74),高的cr(400~547ppm)和ni(120~152ppm),低的hree、sr/y和la/yb比值,类似于赞岐岩的地球化学特征。其高的k2o(k2o/na2o=3.19~5.04)和ba(1000ppm)含量,la/sm,th/yb和ba/la,低的ba/th和sr/nd比率以及富集的同位素nd和hf(εnd(t)=-10.80~-10.50;εhf(t)=-11.95~-6.47)特征,阐明了安多高镁闪长岩主要是由俯冲的大洋沉积物熔体和地幔楔橄榄岩熔体反应形成。(3)钙碱性闪长岩:相比于高镁闪长岩,安多钙碱性闪长岩有低的mgo、mg#、cr、ni和高的feot。其nd同位素组成(εnd(t)=-7.5~-7.3;εhf(t)=-0.04~-12.33)略高于安多地块的基底岩石,显示安多钙碱性闪长岩是由安多基底部分熔融且有地幔岩浆显著加入形成。(4)钙碱性花岗闪长岩:钙碱性闪长岩富集的nd同位素组成(εnd(t)=-10.80~-10.60)和初始的~(87)sr/86sr((86sr/~(87)sr)i=0.7303~0.7306)接近于安多基底的sr-nd同位素组成(εnd(t)=-7.49~-9.56,(86sr/~(87)sr)i=0.7268~0.7383),说明安多钙碱性闪长岩为安多基底部分熔融形成。结合前人研究成果,我们认为安多陆块三期岩浆岩都形成于大陆边缘弧构造环境,即俯冲-增生造山系统。安多大陆地壳形成于新元古代早期,此后在早古生和早中生代均经历了显著的大陆地壳生长与改造作用。
[Abstract]:The Qinghai Tibet Plateau since Phanerozoic consists of a series of micro continental blocks, accretion zone and island arc continuous collage to the southern edge of the Eurasian continent. As the interior of the Qinghai Tibet Plateau the oldest block block to form the lens body, sandwiched between the Qiangtang terrane and Lhasa terrane, the development of a large number of Neoproterozoic to different types of early Mesozoic magmatite, recorded the geological evolution in different periods. In this study, we on the Amdo landmass of Neoproterozoic, early Paleozoic and early Mesozoic period three main magmatite carried out detailed field investigation and indoor petrographic studies, and the study of zircon U-Pb geochronology and Lu-Hf isotope precise the analysis of whole rock major and trace element geochemistry and Sr-Nd isotope geochemistry, which aims to reveal the Amdo block three magmatite formation age, petrogenesis and tectonic environment, geological evolution recovery The formation process, and to explore the continental crust of the Amdo landmass, transformation and growth process of a continental block. We Amdo Neoproterozoic granitic gneissic rock zircon U-Pb isotopic dating results reveal the protolith emplacement age of 8~ (87) Ma. of the granitic gneiss body has a high SiO2 (65.56~79.82wt.%), A/CNK (1.39~1.68), CIPW (2.74~5.10 wt.%). The content of corundum based aluminum rich minerals (such as Bai Yunmu), the original rock is peraluminum S- type granite. The granitic gneiss with low CaO/ (MgO+FeO~T), Al2O3/TiO2, Rb/S and Rb/ Ba ratio, high CaO/Na2O (0.3), epsilon Nd enrichment (T) (-5.51~-2.76) and epsilon Hf (T) (-15.79~4.56) isotopic characteristics indicate that the original rocks of Amdo Neoproterozoic granite gneiss were probably formed by partial melting into muddy rocks. There are a lot of inherited zircons also support this interpretation. These features show that the Neoproterozoic granite The original rocks gneiss were probably formed by partial melting of varying argillaceous rock samples, high MgO, Cr, Ni content reveals the magma source region may have added mantle materials or new crustal material. The zircon U-Pb isotopic dating of Amdo landmass Early Paleozoic granitic gneiss emplacement ages of 484~500 Ma. granite gneiss exhibits high and large changes in the SiO2 (68.46~~ (87).52%) content, display K rich (Na2O/K2O1), low P2O5 (0.1%) and the weak geochemical characteristics of aluminum. Combined with amphibole, these characteristics show that the original rock is I- type granite gneiss is enriched. Isotope (Nd (T) =-10.98~-4.54 Hf (T) =-10.38~1.31; E) features that its partial melting of Amdo basement formation. Early Paleozoic granitic gneiss with ancient gneiss is higher than epsilon Nd (T) and Amdo Singapore epsilon Hf (T) value, combined with the earlier development of magnesium iron Rock xenoliths, reveal the process of the formation of the adding of mantle material. In view of some samples with high SiO2, FeO~T/MgO and /CaO (K2O+Na2O), we believe that the protolith of gneiss in the forming process of the occurrence of certain magmatic differentiation. Through the study of Amdo block early Mesozoic magmatite, we found MORB gabbro, high Mg diorite, calc alkaline diorite and granodiorite of the four types of rocks. The zircon U-Pb isotopic system dating indicates that the crystallization age of the magmatite is consistent, mainly concentrated in the 174-177ma. study of rock geochemistry of our system to reveal the four kinds of early Mesozoic magmatite has different causes: (1): gabbro gabbro has relatively flat REE pattern ((la/yb) n=0.70~1.13) nd and Hf isotopic composition and loss (epsilon Nd (T) =4.40~5.50; HF (T) =10.33~15. .55), the Sr-Nd REE isotopic composition and distribution patterns similar to the Bangong Lake Nu River ocean ophiolite, indicating possible loss from Amdo gabbro asthenospheric mantle source. Amdo gabbro enrichment of large ion lithophile elements (lile:k, Rb, Pb) and depleted in high field strength elements (hfse:nb, Ta, Ti), high ba/la (8~15), ba/th (29~150), u/th (0.57~2.73) and low th/yb (0.05~0.33), showed an aqueous fluid metasomatism formation plays an important role in the process of its rocks. These geochemical characteristics and water rich amphibole mineral exists, explain the process of forming an aqueous fluid metasomatism enrichment the lithospheric mantle in the continental block. Amdo gabbro gabbro Amdo may be formed by partial melting of the asthenospheric mantle is enriched lithospheric mantle metasomatism. Gabbro with low sm/yb and dy/yb, indicating that the gabbro melting depth is in the lithospheric mantle 灏栨櫠鐭崇浉浜岃緣姗勬�勫博绋冲畾鍖哄舰鎴愮殻�

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