【作者】 郭亮；

【导师】 张宏飞；

【作者基本信息】 中国地质大学 ， 地球化学， 2012， 博士

【摘要】 南拉萨地块(狭义上的冈底斯带)出露的花岗岩类和火山岩形成于新特提斯洋板片向北俯冲以及随后印度-欧亚板块的碰撞阶段。近年来,随着测试技术的提高,大批高质量的锆石U-Pb年代学资料陆续报道,基本建立了南拉萨地块内岩浆岩的年代学格架。这些研究主要集中在南拉萨地块的中部,然而对于这些岩浆岩在东西走向上变化研究相对较少,并且很少有关于南拉萨地块变质作用的研究。本文以东喜马拉雅构造结西缘(南拉萨地块东段)出露的花岗岩类和高级变质基底林芝杂岩为研究对象,通过对其进行详细的野外地质观察、岩相学研究、全岩主量-微量元素、Sr-Nd同位素、锆石U-Pb定年和Hf同位素组成研究,讨论了东喜马拉雅构造结西缘出露的花岗岩类和变质岩的岩石成因,通过与南拉萨地块中部对比,进一步揭示了南拉萨地块东段的构造热演化历史。本研究获得的主要认识如下：1. LA-ICP MS锆石U-Pb定年结果显示,东喜马拉雅构造结西缘出露花岗岩类形成于～165Ma、90～80Ma、66～48Ma和26～22Ma四个阶段,与南拉萨地块中部的冈底斯岩基岩浆活动时间基本一致。2.中侏罗世(～165Ma)片麻状花岗岩的锆石εHf(t)值为+1.4～+3.5,低于南拉萨地块中部同时期的冈底斯岩基,反映其岩浆主要来自于地壳物质的部分熔融,但不排除有少量地幔物质的贡献,与新特提斯洋板片向北俯冲有关。晚白垩世(90～80Ma)花岗岩类岩浆源区多样,其中～83Ma花岗闪长质片麻岩的锆石εHf(t)值为+7.3～+10.7,反映其原岩岩浆来自新生地壳物质的部分熔融。～81Ma的含榴黑云母花岗岩中的锆石含大量继承锆石,且锆石Hf同位素组成变化较大,εHf(t)值为-0.9～+6.2,可能为源自新生地壳和古老地壳物质部分熔融产生的岩浆发生岩浆混合作用的产物。～80Ma的卧龙岩体具有埃达克质岩石的特征,Sr-Nd-Hf同位素组成显示其岩浆主要源自新生地壳物质的部分熔融,同时混染了少量古老地壳物质(林芝杂岩)。结合区域上存在同时期的高温变质作用,该埃达克质岩石可能为新特提斯洋中脊俯冲导致增厚下地壳发生熔融的产物。晚白垩世-始新世(66～48Ma)花岗岩类岩石成因复杂,其中～66Ma的花岗岩具有埃达克岩的特征,锆石εHf(t)值为-3.8-1.3,为新特提斯洋板片回转导致增厚下地壳发生部分熔融的结果。古新世(～61Ma)花岗岩经历强烈的构造变形和变质作用,锆石ε种)值为+5.4～+8.0,反映其岩浆来自新生地壳物质的部分熔融。两河口岩体(～49Ma)和始新世花岗岩捕虏体(～50Ma)都具有埃达克质岩石的特征,它们的初始(87Sr/86Sr)i=0.706939～0.708162,εNd(t)为-6.7～-4.3,锆石εHf(t)值为-11.8～-0.2。Sr-Nd-Hf同位素组成显示,它们主要源自林芝杂岩的部分熔融,同时有新生地壳物质的加入。天坡龙岩体(～53Ma)不具有埃达克质岩石的特征,锆石εHf(t)值为+5.3～+7.7,反映其岩浆主要源自新生地壳物质的部分熔融。始新世花岗岩类的形成可能与新特提斯洋板片的断离作用有关。板片断离导致软流圈地幔上涌并加热上覆地壳,使地壳不同深度的岩石发生部分熔融,由加厚的下地壳发生部分熔融形成埃达克质岩石,由较浅部地壳物质发生部分熔融形成非埃达克质岩石。渐-中新世(26～22Ma)花岗岩类都具有埃达克质岩石的特征,Sr-Nd-Hf同位素组成反映其主要源自林芝杂岩的部分熔融,同时混染了少量的新生地壳物质,为印度板块的断离作用导致增厚的拉萨地块下地壳发生部分熔融的结果。3.对林芝杂岩麻粒岩相变质单元中的含榴斜长角闪岩和不纯大理岩进行了详细研究。含榴斜长角闪岩的峰期变质矿物组合为石榴子石+斜方辉石+高Ti角闪石+斜长石+石英+金红石,退变质矿物组合为斜长石+低Ti角闪石+石英+金红石。峰期矿物组合中的石榴子石、石英和角闪石中含有大量的针状金红石出溶体,指示其经历了高温变质作用。利用石英中Ti含量(TitaniQ)温度计获得含榴斜长角闪岩峰期变质温度为803-924℃。全岩主量-微量元素、Sr-Nd同位素和锆石Hf同位素组成显示含榴斜长角闪岩的原岩为亚碱性岛弧玄武岩。锆石U-Pb定年结果显示其原岩岩浆结晶年龄为89.3±0.6Ma,变质年龄为81.1±0.8Ma。不纯大理岩中的碎屑岩浆锆石的年龄为86～167Ma,变质年龄为81.4±0.5Ma。碎屑岩浆锆石的年龄分布和Hf同位素组成与南拉萨地块侏罗纪-白垩纪花岗岩类的年龄分布和Hf同位素组成相似,反映其原岩碎屑物源主要来自冈底斯岩浆弧,沉积环境为弧前盆地。南拉萨地块东部的弧前盆地和岛弧岩浆岩同时在～81Ma发生高温麻粒岩相变质,说明弧前盆地有异常高的热量输入。结合区域上存在同期埃达克质岩石,本文认为晚白垩世高温麻粒岩相变质作用与新特提斯洋中脊俯冲有关。4.林芝杂岩中地壳深熔作用发育,多期长英质岩脉间的穿插关系反映林芝杂岩经历了多期地壳深熔作用。对四个代表性的露头中的混合岩的淡色体和长英质岩脉进行了锆石U-Pb定年和Hf同位素组成研究。结果显示林芝杂岩主要经历了65～63Ma、50～48Ma和30～25Ma三期地壳深熔事件。脉体中的继承锆石的年龄分布和锆石Hf同位素组成反映其母岩主要为林芝杂岩中的变沉积岩和片麻状花岗岩,少部分为古老的基性地壳物质。同时区域上还发育与三期深熔事件相对应的变质事件。这三期变质-深熔事件与南拉萨地块古-中新世冈底斯岩浆活动在时间上是耦合的。本文认为65～63Ma、50～48Ma和30-25Ma深熔-变质事件分别与新特提斯洋板片的回转、断离和印度板块断离所诱发的地幔热扰动有关。5.对林芝杂岩中不同层位的变沉积岩类进行了碎屑锆石U-Pb年代学研究。在林芝杂岩下段,石英云母片岩和变质粉砂岩互层产出,并被变流纹岩所覆盖。石英云母片岩和变质粉砂岩中的碎屑锆石主要存在1000～1250Ma和1400～1800Ma两个年龄群,其中最年轻的碎屑锆石的谐和年龄为1006±51Ma,变流纹岩的原岩年龄为507±4Ma,因此林芝杂岩下段变沉积岩的原岩沉积年龄为1006～507Ma之间。这些变沉积岩明显不同于同时期的特提斯喜马拉雅和高喜马拉雅地层中的碎屑锆石年龄分布特征,而与澳大利亚板块西部碎屑岩类的碎屑锆石年龄分布特征相似,因此支持拉萨地块在古生代时位于澳大利亚北缘的观点。林芝杂岩中段和上段变沉积岩形成于234～165Ma之间,碎屑锆石主要存在330～370Ma、450～650Ma、1000～1250Ma和1400～1800Ma四个年龄群,其物源主要来自拉萨地块本身,其中大量330-370Ma碎屑锆石反映拉萨地块内存在强烈的泥盆纪-石炭纪岩浆活动。6.全岩Sr-Nd同位素和锆石Hf同位素组成显示,东构造结西缘出露的花岗岩类的岩浆主要源自古老(0.9～1.5Ga)地壳物质的部分熔融,明显不同于南拉萨地块中部的花岗岩类,反映东构造结西缘南拉萨地块存在中元古代地壳基底。南拉萨地块中部与东部花岗岩类在Sr-Nd-Hf同位素组成上的差异,反映了在印度-欧亚板块汇聚过程中,南拉萨地块中部以地壳生长为主,而东部则以地壳物质再循环为主。7.南拉萨地块中部和东部具有相似的构造热演化历史。中侏罗世-晚白垩世(165～80Ma)时,新特提斯洋板片向北俯冲于南拉萨地块之下,在俯冲带之上发育岛弧岩浆作用。-80Ma时,新特提斯洋中脊发生俯冲,并导致弧前盆地和岛弧地区都发生了高温麻粒岩相变质作用,同时导致底侵的新生玄武质下地壳发生部分熔融形成埃达克质岩浆。由于俯冲的洋壳是年轻的,且具有高的温度和低的密度,因此具有较大的浮力,俯冲板片将从正常角度俯冲转为平板俯冲。68～40Ma时,俯冲板片发生回转作用,岛弧岩浆作用重新开始启动。同时在板片回转的拖拽力下,软流圈地幔顺着板块回转的方向注入,导致上覆地壳发生变质和深熔作用。在～50Ma时,俯冲的特提斯洋壳发生断离,导致地壳不同尺度的岩石发生了变质和深熔作用,增厚下地壳发生部分熔融形成埃达克质岩石。渐-中新世(26～22Ma)时,持续俯冲的印度板块发生断离,导致早期加厚的南拉萨地块下地壳发生部分熔融形成埃达克质岩。更多还原

【Abstract】 The granitoid batholiths and volcanic rocks, distributed in the southern Lhasa terrane (Gangdese belt), resulted from the northward subduction of the Neo-Tethys oceanic slab under the southern Lhasa terrane, and the collision between Indian and Asian continents. A large number of high-quality zircon U-Pb and Hf isotope data for these rocks have been published in recent years. The geochronological framework of the granitoids and volcanic rocks has been established. However, the previous work mainly focused on the central part of the southern Lhasa terrane. How the Gangdese magmatism evolved along the strike of Himalayan-Tibet collisional belt remains poorly known. In addition, the metamorphism of the southern Lhasa terrane is still not clear. In order to better understand the geological evolution of the southern Lhasa terrane, this study focuses on the granitoids and metamorphic rocks of the Nyingchi Complex in the western margin of the eastern Himalayan syntaxis (EHS). We present an integrated study of detiailed field geology, petrography, whole rock geochemistry (including major elements, trace elements and Sr-Nd isotope), zircon U-Pb dating and Hf isotope composition for the granitoids and metamorphic rocks, and further discuss their petrogenesis and geodynamic implications. The main results related to this study are given as follows.1. The zircon U-Pb dating results reveal that the granitoids in the western margin of the EHS formed during at～164Ma,90～80Ma,66～48Ma and26-22Ma, which are consistent with those in the central part of the southern Lhasa terrane.2. The middle Jurassic granitic gneiss (165Ma) has εHf(t) values of+1.4to+3.5, which is lower than coeval granitoids in the central part of southern Lhasa terrane, suggesting that they mainly sourced from partial melting of crustal materials. We attribute the petrogenesis to the northward subduction of the Neo-Tethyan oceanic slab. The late Cretaceous (90-80Ma) granitoids have diversity magma source.The～-83Ma granodioritic gneiss is characterized by positive εHf(t) values of+7.3to+10.7, indicating a derivation primarily from a depleted-mantle or juvenile crustal source. The～81Ma granitic gneiss shows variablei(t) values from-0.9to+6.2, indicating a binary mixing between juvenile and old crustal materials. The～80Ma Wolong pluton displays adakitic characteristics. The Sr-Nd-Hf isotopic compositions indicate that it can be generated by melting of juvenile lower crust, accompanied by small degrees of contamination by older crustal materials. Combined with the coeval HT granulite facies metamorphism, we suggest that the～80Ma adakitic rocks resulted from Neo-Tethyan ocean ridge subduction. The-66Ma granite also shows adakitic characteristics, and has εHf(t) values of-3.8to-1.3, indicating it sourced from partial melting of old crustal materials. The Paleocene (61Ma) granodioritic gneiss hasεHf(t) values of+5.4to+8.0, suggesting that it originated from partial melting of a juvenile crustal material. Both the Comfluence granite (-49Ma) and the granite enclave (-50Ma) show adakitic characteristics, and have (87Sr/86Sr)i of0.706939to0.708162, εNd(t) of-6.7to-4.3and zircon εHf(t) of-11.8to-0.2, suggesting that they mainly derived from partial melting of old crustal materials. The Tianpolong pluton (-53Ma) hasεHf(t) values of+5.3to+7.7, and does not show adakitic characteristics, suggesting it formed under relatively upper crustal level. We attribute the petrogenesis of Eocene granitoids to the break-off of the Neo-Tethyan oceanic slab, which resulted in the upwelling of asthenospheric mantle. The asthenospheric upwelling heated the base of lower crust and resulted in partial melting of the lower crust to generated adakitic magma (lower crust) and non-adakitic magma (relatively upper crust). All the Oligocene-Miocene granitoids (26-22Ma) shows adakitic geochemical characteristics. The Sr-Nd-Hf isotopic compositions suggest that they sourced from partial melting of the Nyingchi Complex. The late Oligocene adakitic rocks resulted from the break-off of the subducted Indian continental crust starting at-25Ma.3. The garnet-bearing amphibolite and impure marble from the granulite fancies unit of Nyingchi Complex have been studied. Petrographic study indicates that the garnet-bearing amphibolite underwent two stages of peak granulite-fancies and retrograde amphibole-facies metamorphism. The peak mineral assemblage is characterized by Grt+high-Ti Amp+Opx+Pl+Qtz+Rt, and the retrograde amphibole-facies assemblage is characterized by low-Ti Amp+Pl+Czo+Qtz+Rt. There are a large number of rutile exsolutions in the garnet, quartz and amphiboles, suggesting that the primary composition of these minerals had high Ti contents and formed under high temperature conditions. Based on the Ti-in-quartz (TitaniQ) thermometer, the peak metamorphic temperatures is803～924℃. The whole rock geochemical characteristics indicate that the protolith of the garnet-bearing amphibolites are sub-alkaline island arc basalt. The zircons U-Pb dating results show that the crystallization age for the protolith of the garnet-bearing amphibolite is89.3±0.6Ma, and the peak metamorphic age is81.1±0.8Ma. Detrital zircons from the marble show core-rim structure in the CL images. The cores yielded206pb/238U ages ranging from86.3t0167Ma, and the metamorphic rims yielded206Pb/238U age of81.4±0.5Ma. The age distribution and Hf isotopic compositions of zircon cores match well with the age spectra of the Jurassic-Cretaceous Gangdese batholiths, suggesting that the protolith of the impure marble deposited in the fore-arc basin of the Gangdese arc. The above results indicate that both the arc magmatic rocks and the fore-arc sedimentary rocks undergone HT granulite facies metamorphism at～81Ma, suggesting a significant heat input into the forearc area. Combined with the presence of coeval adakitic rocks near studied area, we suggest that the～81Ma high-temperature metamorphism resulted from the upwelling of asthenosphere through the slab window opened as a result of ridge subduction.4. The Nyingchi Complex underwent strongly migmatization. Multiphase felsic melts can be recognized based on the intercalating relationships in the field. This study concentrates on four localities where typical migmatite and leucogranite vein crop out. The zircon U-Pb dating results reveal three stages of crustal anatectic events:65～63Ma,50～48Ma and30-25Ma. The inherited zircons and Hf isotope compositions in these felsic veins indicate that they mainly sourced from partial melting of the metasedimentary rocks and granitic gneisses of the Nyingchi Complex, and some derived from partial melting of the old mafic crustal materials. In addition, there are three stages of metamorphic event corresponding to the crustal anatexis. All the metamorphism and crustal anatexis are consistent with the Paleocene-Miocene magmatism in the central part of the southern Lhasa terrane. The65～63Ma,50～48Ma and30-25Ma crustal anatexis and metamorphism in the western margin of the EHS were related to the thermal perturbations caused by the roll-back, break-off of Neo-Tethyan oceanic slab and the break-off of the Indian plate, respectively.5. Detrital zircons from the metasedimentary rocks in lower, middle and upper Nyingchi Complex have been studied in this study. The age distributions of two detrital zircon samples from the lower Nyingchi Complex are dominated by1000-1250Ma and1400～1800Ma. The youngest detrital zircon age is1006±51Ma, which provides a maximum sedimentary age for these metasedimentary rocks. A sample from the volcaniclastic rock overling the metesedimentary rocks yielded an age of507±4Ma, which provides a minimum sedimentary age for these metasedimentary rocks. The age distribution of these detrital zircons is distinctly different from those in the Paleozoic metasedimentary rocks from Tethyan and Great Himalayan sequences, but similar to these in the metasedimentary rocks from Western Australia. This supports the viewpoint that the Lhasa terrane should be placed at the northwestern margin of Australia during the late Precambrian-early Paleozoic. Detrital zircons from the metasedimentary rocks of middle and upper Nyingchi Complex have four major age populations of330～370Ma,450～650Ma,1000～1250Ma and1400-1800Ma. The depositional ages of the protoliths of these metasedimentary rocks are between234and165Ma. The detrital materials mainly derived from the Lhasa terrane. The presence of aboundant330～370Ma detrital zircons indicates a significant magmatism during Devonian and Carboniferous in the Lhasa terrane.6. The whole rock Sr-Nd and zircon Hf isotopic compositions of the granitoids from the western margin of the EHS show that they mainly sourced from partial melting of the old crustal (0.9～1.5Ga) materials, which is different from those in the central part of the southern Lhasa terrane, indicating existence of an Middle-Proterozoic cruatal basement under the southern Lhasa terrane in the western margin of the EHS. The differences between the central and eastern parts of the southern Lhasa terrane probably result from different processes responsible for forming the continental crust during the convergence between India and Asia. Crustal growth mainly occurred in the central part of the southern Lhasa terrane. By contrast, crustal reworking mainly occurred in the eastern part of the southern Lhasa terrane.7. Our results show that the eastern and central parts of the southern Lhasa terrane have a similar tectono-magma evolution history. The northward subduction of the Neo-Tethyan oceanic slab beneath the southern Lhasa terrane resulted in the arc magmatism during Middle Jurassic-Late Cretaceous (165-80Ma). The Neo-Tethyan ocean ridge subduction occurred at-80Ma and resulted in a formation of slab window. The slab window placed the sub-slab asthenospheric mantle against the base of the overlying plate, which resulted in HT metamorphism in the roots of the arc and fore-arc. High heat flow through the slab window further induced partial melting of overlying lower crust to form adakitic magma. Because the ridge is young, hot and thus buoyant, so there is resistance to subduction, leading to flat subduction of the Neo-Tethyan oceanic slab. After a period of shallow-dip subduction, Gangdese magmatism was rejuvenated at-68Ma possibly due to steepening of the subduction angle. At the same time, the asthenospheric mantle filled into the space of previously occupied by oceanic slab, and heated the lower crust, resulting in crustal anatexis and formation of adakitic magma. The break-off of the subducted Neo-Tethyan oceanic slab from the adherent but more buoyant Indian plate occurred at-50Ma. The asthenospheric upwelling heated the base of lower crust and resulted in partial melting of the lower crust to generated adakitic magma (lower crust) and non-adakitic magma (relatively upper crust). The break-off of the subducted Indian continental crust starting at-25Ma, which induced partial melting of the thickened Asian lower crust by thermal advection resulting from asthenospheric mantle upwelling.更多还原