【作者】 徐旺春；

【导师】 张宏飞；

【作者基本信息】 中国地质大学 ， 地球化学， 2010， 博士

【摘要】 青藏高原的形成演化过程一直是国际地质研究中的热点问题之一。青藏高原所包含的广大地区经历过原特提斯、古特提斯、新特提斯和印度-亚洲大陆碰撞等多个阶段的地质事件,强烈的岩浆活动出现在新特提斯洋俯冲和印度-亚洲大陆碰撞过程中。拉萨地体位于欧亚板块的最南缘,主体由面积大致相等的花岗岩和火山岩组成,花岗岩出露面积约占西藏花岗岩的80%,约11万平方公里,其中,尤以其南部的冈底斯花岗岩类最为发育。冈底斯花岗岩类的形成与新特提斯洋板片俯冲消减、印度-欧亚大陆的碰撞和后碰撞等事件密切相关,对于了解新特提斯洋演化、青藏高原隆升和巨厚地壳形成具有重要的科学意义。另外,冈底斯带是我国重要的斑岩铜(钼、金)成矿带,它们或是岩浆作用的产物,或与岩浆作用期后的构造-热液活动密切相关。因此,冈底斯岩浆作用的研究,对查明该地区大规模成矿作用的区域地质背景,寻找矿产资源,发展国民经济建设也有重要的意义。本文选择雅鲁藏布江缝合带以北的冈底斯花岗岩类为研究对象,对它们进行了野外地质观察、岩相矿物学、锆石U-Pb年龄、主量元素和微量元素、Sr-Nd同位素和锆石Hf同位素组成的综合研究,结合前人在冈底斯带有关花岗岩的研究成果,探讨了冈底斯带花岗岩类的岩石成因以及冈底斯带的构造演化历史,并进一步与西部科希斯坦-拉达克-喀喇昆仑地区和东部波密-八宿-然乌-察隅地区花岗岩类进行了对比研究,探讨了冈底斯带与它们的联系及区别。本文获得了以下几点主要认识：1.获得了中东部冈底斯带及中部拉萨地体42个花岗岩类LA-ICPMS锆石U-Pb年龄数据,年龄分布于205-12 Ma之间。根据锆石U-Pb年龄的间断、锆石Hf同位素组成的演化特征和早期文献的划分方案,这些年龄所标示的岩浆事件可以划分为以下5个时间段：(1)205-202 Ma,(2)～178 Ma,(3)94-87 Ma,(4)68-40 Ma和(5)25-13 Ma。其中,晚三叠世花岗岩类位于中部拉萨地体的南缘,其他花岗岩类均位于冈底斯带内。2.中部拉萨地体南缘出露一个晚三叠世二云母花岗岩和一个晚三叠世花岗闪长岩。二云母花岗岩属于强过铝质岩石(A/CNK=1.16-1.20),富集Rb、Th和U等元素,Eu/Eu*=0.29-0.41,Rb/Sr=2.6～5.5,Rb/Ba=1.1～1.3,锆石εHf(t)值为-12.4～-1.8。二云母花岗岩地球化学特征类似于喜马拉雅中新世淡色花岗岩,反映它们应该具有相似的岩石成因机制,即二云母花岗岩的岩浆产生于地壳中泥质岩类在无外来流体加入的情况下云母类矿物脱水反应所诱发的部分熔融作用。花岗闪长岩属于准铝质岩石(A/CNK=0.96-0.98),K2O/Na2O=1.42-1.77,Eu/Eu*=0.54-0.65,(La/Yb)N=6.76-13.35,锆石εHf(t)值为-8.2～-5.5。地球化学特征表明,花岗闪长岩的岩浆来自于地壳中基性岩类的部分熔融。拉萨地体印支期强过铝质花岗岩的确定,表明了拉萨地体在印支晚期以前曾发生地壳的缩短与加厚作用,从而进一步明确了拉萨地体印支早期的造山事件及拉萨地体经历了多期造山作用。3.产于冈底斯带南缘的变形花岗岩定年结果表明,其形成于早侏罗世(～178 Ma)。变形花岗岩为高硅(SiO2=73.38-76.06%)钙碱性岩系,弱过铝质岩石(A/CNK=1.03～1.07),贫大离子亲石元素和Nb、Ta等高场强元素,具有岛弧型花岗岩的地球化学特征。锆石εHf(t)值变化于+17.7～+14.1之间,表明变形花岗岩岩浆来自初生地壳的部分熔融。结合最近在冈底斯带获得的晚三叠世-早侏罗世岩浆岩的锆石年龄和岩石成因信息,推测新特提斯洋发生俯冲消减作用的开始时代应不晚于早侏罗世,说明新特提斯洋经历了较长时间的演化。4.古新世-始新世花岗岩类(68-40 Ma)是冈底斯岩基的主体,岩石组合多样,包括花岗岩,花岗闪长岩,石英二长岩,闪长岩,二长岩和辉长闪长岩等。岩石主体为高钾钙碱性岩系列,它们的A/CNK=0.80-1.06,表明它们为准铝质岩石或弱过铝质岩石。在微量元素组成上,该期花岗岩类总体上富集Rb、Th、U和K等大离子亲石元素和轻稀土元素,亏损Nb、Ta、P和Ti等高场强元素,具有岛弧型花岗岩的地球化学特征。花岗岩类总体具有低的初始87Sr/86Sr值(ISr=0.70369-0.70585),全岩εNd(t)值变化于+5.6～-4.1之间,锆石εHf(t)值变化于+14.7～-6.4之间。分析表明它们的岩浆主要来源于初生地壳物质,但在岩浆演化过程中混入了拉萨地体古老地壳物质。与新特提斯洋俯冲消减有关的花岗岩类在中生代仅分布于冈底斯带内,而在古新世-始新世时,向北延伸到中部拉萨地体的南部,这可能是轻和热的残留新特提斯洋板片在印度-亚洲大陆碰撞背景下向上运移,以低角度俯冲于拉萨地体之下,脱水并加热亚洲大陆使其部分熔融所致。5.尼木和谢通门地区发现了两个较大规模的埃达克质花岗岩体,它们均被小体积的埃达克质斑岩所侵入。LA-ICPMS锆石U-Pb定年结果表明,埃达克质花岗岩的岩浆结晶年龄为14.0-14.4 Ma,而侵入其中的斑岩也具有相似的年龄(14.2-14.6 Ma)。这反映花岗岩浆在中地壳深度结晶后快速隆升至上地壳深度,表明在中新世时拉萨地体经历了快速的地壳隆升。埃达克质花岗岩和斑岩的Sr-Nd同位素和锆石Hf同位素组成表明,它们均来自于地壳物质的部分熔融。冈底斯埃达克质岩石的锆石εHf(14Ma)值在-2.3～+6.1之间,这与东喜马拉雅构造结基性麻粒岩(印度基性下地壳)锆石的εHf(14Ma)值(-2.5～+4.8)基本一致。冈底斯埃达克质岩石的Sr-Nd同位素组成落入由印度基性下地壳和拉萨下地壳/超钾质火山岩构成的二端元混合线之间。这些证据表明渐新世-中新世埃达克质岩石可能来源于印度基性下地壳的部分熔融,并在上升就位过程中混染了超钾质岩浆和拉萨下地壳物质。这表明在早-中中新世时,印度大陆已经俯冲于拉萨地体之下。6.结合已有的研究成果和本文资料,冈底斯带的形成和演化及岩浆过程可总结为：在新特提斯洋盆打开之前并不存在冈底斯带。晚三叠世-早侏罗世时,随着新特提斯洋向北的俯冲,在俯冲带之上开始出现岩浆作用(即晚三叠世-早侏罗世花岗岩和早侏罗世叶巴组火山岩),但这期岩浆作用是发生在洋壳内的。晚侏罗世-白垩纪时,新特提斯洋板片持续向北俯冲,产生了早白垩世桑日群火山岩和晚白垩世花岗岩类。古新世-始新世时,新特提斯洋盆逐渐关闭,印度-亚洲大陆开始发生碰撞,由于碰撞作用使俯冲速度变慢,轻和热的残留新特提斯洋板片向上运移,以相对较低角度继续俯冲于拉萨块体之下,导致冈底斯岩浆作用向北扩展,达到中部拉萨地体的南部地区；在此之前,中部拉萨地体的古老地壳物质已经剥蚀进入南边的冈底斯带内,并通过混染等方式进入古新世-始新世花岗岩浆中,使冈底斯带内该期岩浆锆石εHf(t)值显著地降低。晚渐新世-早中新世时,印度大陆已俯冲至亚洲大陆之下,其前端因变质作用导致密度增大,由于重力不稳定,从印度大陆主体断离,致使已深俯冲的印度中上地壳较轻部分折返形成高喜马拉雅结晶岩系,而残留的印度基性下地壳与热的软流圈充分接触,加热并部分熔融产生了埃达克质岩浆,在该岩浆上升过程中混染了超钾质岩浆和拉萨下地壳物质。7.冈底斯带花岗岩类锆石总体具有高的176Hf/177Hf比值、正的εHf(t)值和年轻的Hf同位素模式年龄(0.1～1.1 Ga；峰值为～0.3 Ga)；其中,中生代花岗岩类锆石具有更为均一的高176Hf/177Hf比值,对应的εHf(t)值为+17.7～+9.5,但新生代花岗岩类的Hf同位素组成显示了大的变化范围(εHf(t)=+14.7～-6.4),εHf(t)值开始向低值方向延伸,并出现少量负值。中部拉萨地体岩浆岩锆石主要显示负的εHf(t)值(+6.0～-14.2),但在-110 Ma时也出现了部分正的锆石εHf(t)值,指示了地幔组分的加入。中部拉萨地体花岗岩类锆石对应的地壳模式年龄主要为古元古代-中元古代早期(0.8～2.1 Ga),峰值大约出现在1.6 Ga。由此可见,中部拉萨地体和冈底斯带花岗岩类锆石Hf同位素组成存在着明显的差异,反映了它们具有不同的岩浆源区。此外,中部拉萨地体以相对发育早白垩世花岗岩类而区别于冈底斯带。西部的科希斯坦-拉达克岛弧地体和喀喇昆仑地体花岗岩类无论在年龄结构上,还是锆石Hf同位素组成上都分别与冈底斯带和中部拉萨地体可以进行很好的对比,暗示科希斯坦-拉达克岛弧地体是冈底斯带的西延,而喀喇昆仑地体是中部拉萨地体的西延。冈底斯带东部从波密-八宿-然乌-察隅地区一直向南延伸到高黎贡-腾冲-盈江地区,出露的花岗岩类锆石U-Pb年龄结构类似于中部拉萨地体,而明显不同于冈底斯带；东部中生代花岗岩类锆石εHf(t)值大多数分布在+5.0～-15.0之间,主体为负的εHf(t)值,具有老的Hf同位素模式年龄(1.0～2.2 Ga),峰值大约出现在1.7 Ga,与中部拉萨地体可以进行很好的对比,表明东部八宿-然乌-察隅等地区是中部拉萨地体的东延。但东部地区还出现有少量新生代花岗岩类,它们皆临近雅鲁藏布江缝合带一侧分布(如波密地区)。这些新生代花岗岩类锆石既具有负的εHf(t)值,同时也具有正的εHf(t)值,甚至可达+10.0左右,大部分与冈底斯带花岗岩类锆石Hf同位素组成类似。这可能意味着东部地区也存在着类似的冈底斯带,但也有可能仅代表中部拉萨地体东延部分的南缘。更多还原

【Abstract】 The Himalayan-Tibetan orogenic system is the most distinctive landform on our planet. However, the geological evolution of the Tibetan plateau still hotly debated. The Tibetan Plateau has experienced the subduction of Tethyan oceanic lithosphere (involving the Proto-Tethys, Paleo-Tethys, Neo-Tethys) and the India-Asia continental collision, in the processes of which widespread magmatic activities occurred. In the Tibetan Plateau, the Lhasa terrane locating in the most south of the Asian continent is characterized by the widespread granitoids, which occupy 80% area of granitoid within Tibet. Especially, the granitoid magmatism is the most widesrpread in the Gangdese belt, the southern portion of the Lhasa terrane. The Gangdese belt formed as the result of subduction of the Neo-Tethyan oceanic slab and the India-Asia continental collision as well as post-collisional convergence, and thus can provide constraints on the evolution of the Neo-Tethys and the Tibetan uplift. In addition, the Gangdese belt is an important porphyry copper (molybdenum, gold) ore belt, which is either the product of magmatism, or closely related to tectonic-hydrothermal activity after magmatism. Therefore, the investigation of Gangdese magmatism could provide important constraint for the geological background of a large-scale mineralization, looking for mineral resources and the development of the national economy.In the paper, we present field geology, petrography, LA-ICPMS zircon U-Pb geochronology, geochemistry and Sr-Nd-Hf isotopic compositons for the Gangdese granitoids, with the aim of constraining the petrogenesis and origin of these rocks and the tectonic evolution of the Gangdese belt. Combining with published data, we compared the Gangdese granitoids with the the western Kohistan-Ladakh-Karakorum granitoids and the eastern Bomi-Basu-rawu-Chayu Granitoids, and discussed their affinity. The main research results are as follows:1. Forty-two granitoid samples from the Gangdese belt and the Middle Lhasa terrane were selected for in situ zircon U-Pb analyses. These ages range from～205 Ma to～12 Ma, with a peak age at～50 Ma. According to the zircon U-Pb and Hf isotopic data as well as previous study, the magmatic activities are divided into five intrusive stages:(1)205-202 Ma, (2)-178 Ma, (3)94-87 Ma, (4)68-40 Ma and (5)25-13 Ma. The most granitoids were collected from the Gangdese belt, except for two Late-Triassic granitoids, which were obtained from the Middle Lhasa terrane.2. The paper reports geochemistry and zircon Hf isotopic compositions for two Late-Triassic plutons (a two-mica granite and a granodiorite) from the Middle Lhasa terrane. The two-mica granite is strongly peraluminous, with A/CNK=1.16-1.20. The two-mica granite is characterized by enrichments of Rb, Th and U etc. They have Eu/Eu*=0.29-0.41, Rb/Sr=2.6～5.5 and Rb/Ba=1.1～1.3. Dated zircon Hf isotopic compositions exhibitεHf(t) values ranging from-12.4 to-1.8. The geochemistry of the two-mica granite is comparable to the Himalayan Tertiary leucogranites, suggesting that the magma of the two-mica granite was dominantly derived from partial melting of argillaceous rocks in crust. The granodiorite is metaluminous, with A/CNK=0.96-0.98. They display K2O/Na2O=1.42-1.77, Eu/Eu*=0.54-0.65 and (La/Yb)N=6.76-13.35.εHf(t) values from the dated zircons range from -8.2 to -5.5. The geochemical signatures and zircon Hf isotopic compositions suggest that the magma of granodiorite formed by partial melting of basaltic rocks in crust. The occurring of the strongly peralumineous granite reveals Lhasa crustal thickening prior to Late Indosinian, and gives an impelling evidence that the Lhasa terrane took place an Early Indosinian orogenic event.3. LA-ICPMS zircon U-Pb dating results show that the deformed granite in the southern edge of the Gangdese belt yield a magma crystallization age of～178 Ma. The deformed granites are high silicon calc-alkline series, with SiO2=73.38-76.06% and A/CNK=1.03～1.07. The deformed granite is characterized by low large ion lithophile element (LILE) contents and low high field strength element (HFSE) contents, indicating that the granite has an island-arc-type geochemical affinity. Zircon Hf isotopic compositions from the granite displayεHf(t) values ranging from +14.1 to +17.7, suggesting that the magma was derived from partial melting of juvenile crust. Based on the study of the Late-Triassic and Early-Jurassic igneous rocks and their petrogenesis in the Gangdese belt, the beginning time of the Neo-Tethyan oceanic slab subduction is not later than Early Jurassic. The Neo-Tethyan Ocean has long lasting tectonic evolution.4. The Gangdese batholith are composed mainly of the Paleocene to Eocene (68-40 Ma) granitoids, which include granite, granodiorite, quartz-monzonite, diorite, monzonite and gabbric diorite etc. These granitoids are dominantly high-potassic calc-alkaline series, and are metaluminous or weakly peraluminous with A/CNK=0.80-1.06. These granitoids are relatively enriched in Rb, Th, U and K, and show negative Nb, Ta, P and Ti anomalies, indicating that the granitoids have an island-arc-type geochemical affinity. They have ISr values ranging from 0.70369 to 0.70585 andεHf(t) values ranging from +5.6 to -4.1. Zircon Hf isotopic compositions from these granitoids displayεHf(t) values ranging from +14.7 to -6.4, suggesting that the magmas were mainly derived from partial melting of juvenile crust with incorporation of the ancient crustal material deriving from the Middle Lhasa terrane. While the Neo-Tethys subduction-related granitoids in the Mesozoic distributed limitedly in the Gangdese belt, the granitoid magmatism during the Paleocene-Eocene has extended into the Middle Lhasa terrane, suggesting that a light and hot residual Neo-Tethyan ocean slab was underthrust beneath the Lhasa terrane with a low angle. Its dehydration and thermal effect caused the extensive magmatism in both the Gangdese belt and the Middle Lhasa terrane.5. The large-volume Pagu and Nanmuqie granitoids are firstly identified to be adakitic rocks, which are intruded by three adakitic porphyries in the Lhasa. LA-ICPMS zircon U-Pb dating for the Pagu granodiorite and Nanmuqie granite yielded identical magma crystallization ages of 14.0～14.4 Ma, which is indistinguishable from their associated adakitic porphyries (14.2～14.6 Ma). The granitoid was intruded at middle-crust depth, whereas the porphyry was intruded at upper-crust depth, indicating that the Lhasa terrane has experienced a rapid crustal uplift during the magma emplacement. Zircon Hf isotopic and whole-rock Sr-Nd isotopic compositions for these granitoids and porphyries suggest that their magmas were dominantly derived from partial melting of crustal materials. The granitoids and porphyries haveεHf(t) values overlapping with the mafic granulites in the Himalayan terrane (Indian plate). Their Sr-Nd isotopic compositions show two-endmember mixing between the Himalayan mafic lower crust and the ultrapotassic lavas/the Lhasa lower crust. We suggest that the adakitic magmas in the Lhasa terrane could be derived from partial melting of subducted Indian mafic lower crust with incorporation of the ultrapotassic lava and/or the Lhasa lower crust components. Our study suggests a new model for the adakitic magma generation in the Lhasa terrane and provides a line of geochemical evidence that the Indian continental crust was subducted beneath the southern Lhasa terrane in the Early-Middle Miocene.6. The evolutional history of the Gangdese belt is inferred from the granitoid magmatism, based on this study and published data. The Gangdese belt did not exist before opening of the Neo-Tethyan oceanic basin. The Late-Triassic to Early-Jurassic Neo-Tethyan oceanic subduction beneath the Lhasa micro-continental block (i.e., the Middle Lhasa terrane) yielded Yeba volcanic rocks and the contemporaneous granitoids in oceanic crust along south of the Lhasa micro-continental block. From Late-Jurassic to Cretaceous, the lasting oceanic subduction produced the Sangri volcanics and Late-Cretaceous granitoids in the Gangdese belt. Along with closing of the Neo-Tethyan oceanic basin during the Paleocene-Eocene, the India-Asia continental collision initiated. During the continental collision, light and hot residual Neo-Tethyan ocean slab was underthrust beneath the Lhasa terrane with a low angle, which induced the widespread magmatism in both the Gangdese belt and the Middle Lhasa terrane. Then the Indian continental margin began to be subducted beneath the Lhasa terrane with a slow subduction velocity and low subduction angle. By～25 Ma, the subducted felsic part was detached from the subjacent mafic part and lithosphere mantle of the Indian plate. The above felsic part was exhumed due to the buoyancy. This felsic part is equivalent to the Great Himalayan Sequence in the Himalayan terrane. The residual mafic part and lithosphere mantle became steeper from low-angle underthrusting to high-angle underthrusting, even forming a subvertical lithosphere slab. A consequence of this downwelling would be a deficit of asthenosphere, which should be balanced by an upwelling counterflow, and thus warm both the Asian mantle and the Indian mafic lower crust. It would induce partial melting of the Indian mafic lower crust to produce the pristine adakitic melt. During the magma migration and emplacement, the magma may incorporate the ultrapotassic magmas and/or the Lhasa lower crust materials.7. Zircons from granitoids in the Gangdese belt have dominantly high 176Hf/177Hf ratios, corresponding to positiveεHf(t) values and young Hf model age of 0.1 to 1.1 Ga, with a peak at～0.3 Ga. Among them, the Mesozoic zircons have homogeneous Hf isotopic compositions, withεHf(t)=+17.7～+9.5. However, the Cenozoic zircons have heterogeneous Hf isotopic compositions (εHf(t)=+14.7～-6.4). By comparison, zircons from granitoids in the Middle Lhasa terrane display dominantly negativeεHf(t) values (-14.2～+6.0). Their Hf model ages are mainly Paleoproterozoic and early Mesoproterozoic (0.8～2.1 Ga), with a peak at～1.6 Ga. Therefore the Hf isotopic compositions of zircons for granitoids in the Gangdese belt are significantly different from those in the Middle Lhasa terrane, indicating distinct sources. In addition, the Middle Lhasa terrane has widespread magmatism during Early Cretaceous, which is also different from the Gangdese belt. Either zircon U-Pb ages or Hf isotopic compositions of granitoids in the Kohistan-Ladakh island arc terrane and the Karakorum terrane are comparable to the Gangdese belt and the Middle Lhasa terrane, respectively. It suggests that the western Kohistan-Ladakh island arc terrane could be correlated with the Gangdese belt, and the Karakorum terrane could be correlated with the Middle Lhasa terrane. In the east, the framework of zircon U-Pb ages for granitoids in the Bomi-Basu-ranwu-Chayu area and Gaoligong-Tengchong-Yingjiang area is indistinguishable from that of the Middle Lhasa terrane. The Mesozoic zircons display dominantly negativeεHf(t) values (+5.0～-15.0), corresponding to old Hf model ages (1.0～2.2 Ga) with a peak at～1.7 Ga, which are also consistent with that of the Middle Lhasa terrane. Both the U-Pb ages and the Hf isotopic compositions suggest that the eatern area could be correlated with the Middle Lhasa terrane. However, there are also a small amount of Cenozoic granitoids along the Indus-Yarlung suture. They have zirconεHf(t) values overlapping largely with the granitoids in the Gangdese belt. It may imply that the eastern area either appear the Gangdese belt, or only present the southern margin of the Middle Lhasa terrane.更多还原