【作者】 王敏；

【导师】 郑建平； 戴宏坤；

【作者基本信息】 中国地质大学 ， 矿物学、岩石学、矿床学， 2024， 博士

【摘要】 大陆岩石圈不仅蕴藏了丰富的资源,还记录了地球长期演化的信息。相比大陆内部,大陆边缘的岩石圈更易受到俯冲和碰撞等地质过程的改造,主要包括俯冲板片释放熔/流体的交代作用、熔体抽取及地幔再交代作用,这将导致岩石圈的岩性组合、化学组成及结构特征发生显著变化。扬子克拉通西缘为经历复杂改造的典型地区,构造演化过程主要包括新元古代大洋俯冲、特提斯大洋俯冲及峨眉山地幔柱活动的叠加改造;新生代以来,该区域还受到青藏高原侧向生长的影响。由于缺乏深源捕虏体的直接限定,扬子西缘深部岩石圈的物质组成和演化过程仍不清楚。本论文选择扬子克拉通西缘滇西六合地区新生代正长斑岩中的榴辉岩捕虏体,以及大理地区新生代超钾质岩中的橄榄岩捕虏体和橄榄石、铬铁矿、金云母等捕虏晶作为研究对象,进行了野外地质特征、岩石及矿物显微结构、矿物化学、全岩化学和年代学特征的系统分析,确定了榴辉岩和橄榄岩捕虏体及多种矿物捕虏晶的成因,揭示了区域地质演化的过程。结合已有的地球物理等研究成果,本研究构建了该区域深部岩石圈（包括下地壳和岩石圈地幔）的结构和组成,并探讨了岩石圈的形成和改造过程。主要研究内容和取得的认识包括:1.榴辉岩捕虏体的成因与加厚下地壳扬子西缘新生代富碱斑岩中含有丰富的地壳捕虏体。本文对云南六合地区正长斑岩（约35 Ma）中相对新鲜的榴辉岩和退变榴辉岩进行了系统研究。新鲜榴辉岩由粗粒绿辉石、石榴石组成。退变榴辉岩中,绿辉石分解为“筛网状”细粒透辉石,次生的细粒他形角闪石和斜长石充填在透辉石解理和裂隙中;石榴石发育不透明后成合晶和角闪石冠状边,仅有少量绿辉石作为石榴石的包裹体出现。六合榴辉岩与地幔榴辉岩在全岩主量和微量元素特征上显著不同。结合其平衡压力为0.7-2.2 GPa,这些证据表明,六合榴辉岩起源于至少60 km的加厚下地壳。所研究的榴辉岩全岩显示大离子亲石元素（LILE）富集和高场强元素（HFSE）亏损的特征,并且全岩及矿物地球化学特征与天然和实验堆晶辉石岩非常相似。结合同区域已报道的石榴石角闪岩（退变榴辉岩）的形成与改造年龄,六合榴辉岩的原岩可能是新元古代（约800 Ma）由地幔楔部分熔融产生的含水弧岩浆,在弧根地壳中通过堆晶作用形成的石榴辉石岩。这些堆晶岩的原岩在形成后经历了榴辉岩相变质作用（约736 Ma）,计算的密度为3.50-3.63 g/cm3,显著高于地幔橄榄岩。这些致密的榴辉岩在深部地壳中长期保存（>700 Ma）,直到约35 Ma时被寄主岩浆携带至地表。普遍认为致密榴辉岩会在重力作用下迅速拆沉并返回深部地幔。然而,本文研究显示致密榴辉岩可以长期保存,这可能与下伏难熔岩石圈地幔的漂浮性质有关,即研究区的岩石圈地幔在长期演化过程中经历了复杂的熔体抽取和地幔柱与岩石圈相互作用,保持了低密度、高浮力和高度难熔的特征,这为致密榴辉岩提供了向上的浮力支持。2.地幔捕虏体、捕虏晶的成因及地幔改造过程新生代超钾质岩在扬子西缘广泛发育,已有研究多集中于岩浆斑晶和全岩化学反演岩石圈地幔属性,但地幔捕虏体的直接研究非常有限。本文在大理地区的幔源超钾质岩中发现了毫米级橄榄岩捕虏体及橄榄石、铬铁矿和金云母等捕虏晶。仔细分析了它们的成因,并进一步探讨了其记录的岩石圈地幔改造历史。橄榄岩捕虏体为具有粒状镶嵌结构的纯橄岩（橄榄石含量>98%）,其粒间及内部含有大量铬铁矿包裹体。捕虏体中的橄榄石显示出高Fo（89-92）和低Ca O（平均0.09 wt.%）特征,铬铁矿则具有高Cr^#（75-90）、低Ti O₂（<0.6 wt.%）及高的氧逸度（△FMQ=1.26-2.53）特征。这些证据表明,含铬铁矿的纯橄岩是在俯冲环境下,由硅不饱和熔体与难熔地幔楔发生反应形成。橄榄石捕虏晶常见肯克带,并具有明显的核边结构,矿物化学特征与地幔橄榄石相似。部分橄榄石捕虏晶和橄榄岩捕虏体相比普通地幔橄榄石具有更高的Li含量（5-50 ppm）及已报道的高δ¹⁸O（5.07-7.25‰）特征,表明交代熔体中含有少量再循环的沉积碳酸盐组分。金云母捕虏晶表现出扭折变形特征和核边结构,核部具有与典型地幔金云母相似的特征,如高Mg^#（90-92）、Cr（平均6936 ppm）,低Fe O^T（3.52-4.53 wt.%）和Ti O₂（0.38-1.18 wt.%）含量。边部发生溶蚀,形成港湾状反应边或增生边,具有明显低Mg^#和高Ti O₂特征。金云母微量元素显示出LILE富集和HFSE亏损的弧岩浆特征,相比克拉通内部地幔金云母,其具有更高的Al₂O₃和重稀土含量。这些证据共同表明,研究区经历了由俯冲板片释放的富水流体与浅部尖晶石相地幔发生显性交代作用的过程。3.深部岩石圈复杂壳幔结构及演化过程本文结合捕虏体/捕虏晶、侵入体和地球物理等资料,尝试恢复滇西地区新生代的深部地壳结构,并总结了主要的地壳生长再造过程。地震学莫霍面（约50 km）与本文榴辉岩记录的岩石学莫霍面（约60 km）之间的差异表明,研究区可能存在超过10 km的壳幔过渡带。该壳幔过渡带中岩性复杂,不同岩石类型的形成时代及地球化学特征与该地区多次地壳增生和再造的岩浆事件密切相关。地壳捕虏体中的古老锆石表明,研究区与扬子克拉通其他区域相似,存在未暴露的太古宙古老地壳组分。扬子西缘滇西地区深部岩石圈主要经历了多阶段形成及改造过程:（1）在新元古代（约900-700 Ma）大洋俯冲背景下,弧下地幔楔受到俯冲板片交代改造,软流圈地幔发生部分熔融形成硅不饱和熔体与浅部难熔方辉橄榄岩发生熔岩反应形成纯橄岩。随后,俯冲板片来源的富钾、富水流体与岩石圈地幔发生交代作用形成含金云母的富集区域。同时,硅不饱和熔体底侵至厚的弧下地壳,通过堆晶作用形成榴辉岩的原岩。（2）在晚二叠世（约260 Ma）,扬子西缘受到峨眉山地幔柱活动的影响,深部岩石圈受到强烈改造,高温作用可能移除了大量富集组分,但部分含金云母的地幔仍部分保留。此外,地幔柱相关的岩浆底侵至下地壳,对先存的新元古代地壳进行改造,并引起地壳增生。（3）在晚三叠世（约230-210 Ma）,扬子西缘与印支-思茅地块发生碰撞拼合,在俯冲大洋板片断离后的伸展背景下,软流圈上涌减压熔融,底侵熔体与下地壳中先存的古老富集地壳混合,并形成新生地壳物质。（4）新生代以来,印度与欧亚大陆碰撞,随着印度板块持续向东俯冲,上涌的软流圈促使未被地幔柱完全消耗的含金云母交代地幔发生部分熔融,形成超钾质岩浆。这些超钾质岩浆底侵并加热下地壳,促使早期新生地壳部分熔融,形成长英质富碱斑岩。更多还原

【Abstract】 Continental lithosphere contains abundant resources and archives Earth’s long-term evolution.Compared to the interiors,continental marginal lithosphere is more vulnerable to modification by subduction and collision.The associated melt extraction and metasomatism can significantly modify the lithology,geochemical composition,and structural characteristics of the lithosphere.The western margin of the Yangtze Craton is a representative continental margin with a complex evolutionary history,including Neoproterozoic oceanic subduction,Tethyan oceanic subduction,and the influence of the Emeishan mantle plume.In the Cenozoic,this region was affected by the lateral growth of the Tibet Plateau.However,due to the lack of direct constraints from deep-seated xenoliths,the composition,structure,and evolutionary processes of the deep lithosphere beneath the western margin of the Yangtze Craton remain unclear.This study focuses on eclogite xenoliths from Cenozoic syenite in the Liuhe area of western Yunnan beneath the western Yangtze Craton,as well as peridotite xenoliths and xenocrysts such as olivine,chromite,and phlogopite,from Cenozoic ultrapotassic rocks in the Dali area.Through a systematic analysis of field geology,petrographic and mineral microstructures,mineral geochemistry,whole-rock geochemistry,and geochronological data,this research identifies the origins of the eclogite and peridotite xenoliths as well as the various xenocrysts,shedding light on the regional geological evolution.In combination with reported geophysical and other research,this study constructed a model of the deep lithospheric structure and composition in this region and explores the evolutionary processes.The details are as follows:1.Origin of eclogite xenoliths and the thickened lower crust.The Cenozoic alkaline-rich porphyry in the western margin of the Yangtze Craton contains abundant crustal xenoliths.In this study,relatively fresh eclogites and retrograde eclogites from syenite（ca.35 Ma）in the Liuhe are systematically analyzed.Fresh eclogites consist predominantly of coarse-grained omphacite and garnet.Most of the retrograded eclogites exhibit secondary fine-grained amphibole and plagioclase filling the“mesh-like”cracks and fractures of omphacite formed by decomposition,with garnet showing opaque symplectites and amphibole coronae.Only minor omphacite remains as inclusions within the garnets.The Liuhe eclogites exhibit significant differences from mantle eclogites in terms of whole-rock major and trace compositions.Combined with the equilibrium pressure range of 0.7-2.2 GPa,the evidence suggests that the Liuhe eclogites originated from a thickened lower crust at least 60 km.Whole-rock geochemistry of the eclogites show enrichment in large-ion lithophile elements（LILE）and depletion in high-field-strength elements（HFSE）,with geochemical characteristics of both whole-rock and minerals resembling those of natural and experimental cumulated pyroxenites.Combined with the formation and modification ages of garnet amphibolites（retrograded eclogites）reported in the same region,the protolith of eclogites likely originated as hydrous arc magmas generated by partial melting of the mantle wedge during the Neoproterozoic（ca.800 Ma）,forming cumulated garnet pyroxenites in the root of the arc crust.These pyroxenites underwent eclogite-facies metamorphism（736Ma）,with a calculated density of 3.50-3.63 g/cm3,significantly higher than that of mantle peridotites.These dense eclogites were preserved in the deep crust for over 700 Ma until they were brought to the surface in Cenozoic（ca.35 Ma）.It is widely believed that dense eclogites delaminate rapidly back into the deep mantle under gravitational forces.However,this study reveals that dense eclogites can be preserved for extended periods,possibly due to the buoyant nature of the underlying refractory lithospheric mantle.The lithospheric mantle beneath the study area underwent complex melt extraction and mantle plume-lithosphere interaction during its long-term evolution,maintaining low density,high buoyancy,and a highly refractory character,which provided buoyant support for the dense eclogites.2.Origin of mantle xenoliths/xenocrysts and mantle modification.The Cenozoic ultrapotassic magmatism in the western margin of the Yangtze Craton widely occurs,and previous studies mainly focusing on the magmatic phenocrysts and whole-rock chemistry to explore the properties of the lithospheric mantle.However,direct studies of mantle xenoliths remain limited.In this study,millimeter-sized peridotite xenoliths,along with xenocrysts such as olivine,chromite,and phlogopite,were discovered in mantle-derived ultrapotassic rocks from the Dali area.Their origins were carefully analyzed,and the lithospheric mantle metasomatic history they record was further investigated.The peridotite xenoliths belong to olivine-rich dunite（>98%olivine）with a granular embedded structure,containing abundant chromite inclusions.The olivine exhibits high Fo（89-92）and low Ca O（average 0.09 wt.%）,while chromite exhibits high Cr^#（75-90）,low Ti O₂（<0.6 wt.%）,and high oxygen fugacity（ΔFMQ=1.26-2.53）.These features suggest that the chromite-bearing dunites formed through reactions between a silica-undersaturated melt and a refractory mantle wedge in a subduction setting.The xenocrystic olivine display kink bands and distinct zoned,with mineral geochemistry similar to that of mantle olivine.Some xenocrystic olivine show higher Li contents（5-50ppm）and previously reported highδ¹⁸O values（5.07-7.25‰）compared to typical mantle olivine,indicating that the silica-undersaturated melt contained minor recycled sedimentary components.The phlogopite xenocrysts exhibit kink-band deformation and core-rim structures.The core shares similar with mantle phlogopite,including high Mg^#（90-92）,Cr（average 6936 ppm）,low Fe O^T（3.52-4.53 wt.%）and Ti O₂（0.38-1.18 wt.%）.The rims,however,show dissolution reactions with the host magma,forming embayed or overgrowth rims with significantly lower Mg^#and higher Ti O₂.The trace element composition of the phlogopite shows enrichment in LILE and depletion in HFSE,showing arc magma characteristics.Compared to mantle phlogopite from cratonic interiors,it has higher Al₂O₃ and heavy rare earth element contents.Together,these pieces of evidence suggest that the lithospheric mantle beneath the study area underwent significant metasomatism,induced by hydrous fluids released from subducted slabs that interacted with the shallow spinel-facies mantle wedge.3.Complex crust-mantle structure and evolution of the deep lithosphere.This article attempts to reconstruct the deep crustal structure of the Cenozoic in western Yunnan by combining data on xenoliths/xenocrysts,intrusive bodies,and geophysics,and summarizes the main processes of crustal growth and reworking.The difference between the seismological Moho discontinuity（ca.50 km）and the petrological Moho discontinuity recorded in the eclogite in this study（ca.60 km）suggests the possibility of a crust mantle transition zone exceeding 10 km in the study area.The lithology of this transition zone is complex,with the formation ages and geochemical characteristics of its rock types closely linked to multiple magmatic events of crustal accretion and reworking.Ancient zircons found in the crustal xenoliths suggest that similar to other areas of the Yangtze Craton,the study region exists unexposed Archean crustal components.The deep lithosphere beneath western Yunnan in the western Yangtze Carton has undergone a multi-stage formation and modification process:（1）Neoproterozoic（900-700 Ma）oceanic subduction.The sub-arc mantle wedge was metasomatized and modified by fluids from the subducted slab,while partial melting of the asthenospheric mantle produced silica-undersaturated melts that reacted with shallow refractory harzburgite,forming dunite.Subsequently,potassium-and water-rich fluids derived from the subducted slab metasomatized the lithospheric mantle,creating regions enriched in phlogopite.Meanwhile,the silica-undersaturated melts from the mantle underplated the thick sub-arc crust,forming the cumulated protoliths of eclogites.（2）Late Permian（ca.260 Ma）Emeishan mantle plume activity.The western margin of the Yangtze Craton was affected by the activity of the Emeishan mantle plume.The high-temperature plume activity removed large amounts of enriched components,although some phlogopite-bearing mantle remained preserved.Additionally,magmatic underplating associated with the mantle plume modified the pre-existing Neoproterozoic crust and contributed to the accretion of juvenile crustal material.（3）Late Triassic（ca.230-210 Ma）collision with the Indochina-Simao Block.As the Paleo-Tethys branch closed,the western margin of the Yangtze Craton collided and accreted with the Indochina-Simao block.In the extensional setting following the detachment of the subducted oceanic slab,upwelling asthenosphere underwent decompression melting,and the underplated melts mixed with the pre-existing ancient enriched lower crust,forming juvenile crustal material.（4）Cenozoic post-India-Eurasia collision.Following the collision between India and Eurasia,continued eastward subduction of the Indian plate caused asthenospheric upwelling,which induced partial melting in the phlogopite-metasomatized mantle that had not been completely depleted by earlier mantle plume,producing ultrapotassic magmas.These ultrapotassic magmas underplated and heated the lower crust,triggering partial melting of the early-formed juvenile crust and generating felsic and alkali-rich porphyry rocks.更多还原