苏鲁造山带超高压变质作用及其P-T-t轨迹

基于超高压变质岩的岩石学,特别是超高压矿物生长成分环带、扩散环带和蚀变作用研究,综合前人的岩石学和年代学研究成果,提出苏鲁造山带超高压变质作用峰期发生在1000-1100℃和6-7GPa条件下,俯冲深度相当于200km,形成年代为240-250Ma。在此基础上,重塑了一个包括八期变质作用的P-T-t轨迹,揭示出超高压变质岩经历了三个不同的折返阶段,即从200km到100km深度的快速折返阶段,抬升速率为5km/Ma,冷却速率为10℃/Ma;从100km到30km的快速折返,抬升速率为4km/Ma,或为近等温降压,或为缓慢降温的快速降压过程;从下地壳到近地表的缓慢折返阶段,抬升速率为1km/Ma,但为快速降温过程,冷却速率可达20℃/Ma。     

中国大陆科学钻探（CCSD）主孔地区岩石圈热结构

岩石圈热结构是指地球内部热量在壳幔的配分比例、温度以及热导率和生热率等热学参数在岩石圈中的分布特征。岩石圈的热结构直接影响着岩石的物理性质和流变学性质,同时还控制了化学反应的类型和速度,从而制约着岩石圈的发展和演化。本文在前人CCSD主孔岩石主、微量元素研究基础上,利用Rybach生热率公式计算了钻孔岩石的放射性生热率,并结合岩石热导率的测定研究了CCSD主孔100-2000m岩石的热结构和主孔榴辉岩在不同退变质程度下生热率、热导率的变化:钻孔中岩石的平均生热率为0.95μWm-3,平均热导率为2.96mWm-1K-1。,其中片麻岩生热率高迭1.01-1.7μWm-3,热导率为2.76-2.96mWm-1K-1;基性超基性岩石生热率最低(<0.21μWm-3),热导率则高达3.20mWm-1K-1以上;新鲜榴辉岩生热率、热导率居中,分剐为0.16-0.44μWm-3和3.31-3.85mWm-1K-1。钻孔中榴辉岩生热率、热导率变化主要受岩性控制:从新鲜榴辉岩到完全退变榴辉岩,热导率总体上降低,但从强退变榴辉岩到完全退变榴辉岩,岩石热导率升高;而在此过程中岩石生热率总体上升高,仅当从中等退变质榴辉岩退变为强退变质榴辉岩时,岩石生热率出现降低趋势。在综合研究的基础上预测CCSD主孔5000m深度处温度为139℃,温度范围为131-151℃。根据区域深部地球物理探测成果对CCSD主孔地区岩石圈热结构进行了研究:上地壳底部温度为256℃,中地壳底部温度为492℃,Moho面温度为683℃,岩石圈底部温度为1185℃,来自地幔的热流为44.1mWm-2,对地表热流的贡献率为58%。研究结果表明,由岩石物理方法获得的CCSD主孔地区岩石圈地温曲线与石榴石-二辉橄榄岩包体推断的中国东部地温曲线十分吻合,本文从实验岩石物理学角度为CCSD主孔地区岩石圈热结构研究提供了重要约束     

Continental subduction and exhumation of UHP rocks. Structural and geochronological insights from the Dabieshan (East China)

In the Dabieshan, the available models for exhumation of ultrahigh-pressure (UHP) rocks are poorly constrained by structural data. A comprehensive structural and kinematic map and a general cross-section of the Dabieshan including its foreland fold belt and the Northern Dabieshan Domain (Foziling and Luzenguang groups) are presented here. South Dabieshan consists from bottom to top of stacked allochtons: (1) an amphibolite facies gneissic unit, devoid of UHP rocks, interpreted here as the relative autochton; (2) an UHP allochton; (3) a HP rock unit (Susong group) mostly retrogressed into greenschist facies micaschists; (4) a weakly metamorphosed Proterozoic slate and sandstone unit; and (5) an unmetamorphosed Cambrian to Early Triassic sedimentary sequence unconformably covered by Jurassic sandstone. All these units exhibit a polyphase ductile deformation characterized by (i) a NW–SE lineation with a top-to-the-NW shearing, and (ii) a southward refolding of early ductile fabrics.

The Central Dabieshan is a 100-km scale migmatitic dome. Newly discovered eclogite xenoliths in a Cretaceous granitoid dated at 102 Ma by the U–Pb method on titanite demonstrate that migmatization post-dates HP–UHP metamorphism. Ductile faults formed in the subsolidus state coeval to migmatization allow us to characterize the structural pattern of doming. Along the dome margins, migmatite is gneissified under post-solidus conditions and mylonitic–ultramylonitic fabrics commonly develop. The north and west boundaries of the Central Dabieshan metamorphics, i.e. the Xiaotian–Mozitan and Macheng faults, are ductile normal faults formed before Late Jurassic–Early Cretaceous. A Cretaceous reworking is recorded by synkinematic plutons.

North of the Xiaotian–Mozitan fault, the North Dabieshan Domain consists of metasediments and orthogneiss (Foziling and Luzenguang groups) metamorphosed under greenschist to amphibolite facies which never experienced UHP metamorphism. A rare N–S-trending lineation with top-to-the-south shearing is dated at 260 Ma by the ⁴⁰Ar/³⁹Ar method on muscovite. This early structure related to compressional tectonics is reworked by top-to-the-north extensional shear bands.

The main deformation of the Dabieshan consists of a NW–SE-stretching lineation which wraps around the migmatitic dome but exhibits a consistently top-to-the-NW sense of shear. The Central Dabieshan is interpreted as an extensional migmatitic dome bounded by an arched, top-to-the-NW, detachment fault. This structure may account for a part of the UHP rock exhumation. However, the abundance of amphibolite restites in the Central Dabieshan migmatites and the scarcity of eclogites (found only in a few places) argue for an early stage of exhumation and retrogression of UHP rocks before migmatization. This event is coeval to the N–S extensional structures described in the North Dabieshan Domain. Recent radiometric dates suggest that early exhumation and subsequent migmatization occurred in Triassic–Liassic times. The main foliation is deformed by north-verging recumbent folds coeval to the south-verging folds of the South Dabieshan Domain. An intense Cretaceous magmatism accounts for thermal resetting of most of the ⁴⁰Ar/³⁹Ar dates.

A lithosphere-scale exhumation model, involving continental subduction, synconvergence extension with inversion of southward thrusts into NW-ward normal faults and crustal melting is presented.     

大别山超高压变质岩的变形特征及其地质意义

大别山超高压变质岩至少经过５期变形。第１期产生于榴辉岩相变质前；第２期大致与榴辉岩相变质作用同步．岩石产生紧密同斜褶皱及榴辉岩相糜棱岩；第３期产生于超高压变质岩向中地壳折返的过程中，以榴辉岩的布丁化及基质的透入性剪切为特征；第４期主要形成剪切条带及伸展沿劈理．是大别山碰撞后差异隆升，岩层向南滑脱的结果；最后一期为脆性变形，岩层沿北东向断层产生左行平移。通过超高压变质岩的变形分析，可以了解超高压变质岩形成和折返的构造运动过程。     

Prograde high- to ultrahigh-pressure metamorphism and exhumation of oceanic sediments at Lago di Cignana, Zermatt-Saas Zone, western Alps

Pelagic metasediments and MORB-type metabasalts of the former Tethyan oceanic crust at Cignana, Valtournanche, Italy, experienced UHP metamorphism and subsequent exhumation during the Early to Late Tertiary. Maximum PT conditions attained during UHP metamorphism were 600–630 °C, 2.7–2.9 GPa, which resulted in the formation of coesite-glaucophane-eclogites in the basaltic layer and of garnet-dolomite-aragonite-lawsonite-coesite-phengite-bearing calc-schists and garnet-phengite-coesite-schists with variable amounts of epidote, talc, dolomite, Na-pyroxene and Na-amphibole in the overlying metasediments. During subduction the rocks followed a prograde HP/UHP path which in correspondance with the Jurassic age of the Tethyan crust reflects the thermal influence of relatively old and cold lithosphere and of low to moderate shear heating. Inflections on the prograde metamorphic path may correspond to thermal effects that arise from a decrease in shear heating due to brittle-plastic transition in the quartz-aragonite-dominated rocks, induced convection in the asthenospheric mantle wedge and/or heat consumption by endothermic reactions over a restricted PT segment during subduction. After detachment from the downgoing slab some 50–70 Ma before present, the Cignana crustal slice was first exhumed to ca. 60 km and concomitantly cooled to ca. 550 °C, tracing back the UHP/HP prograde path displaced by 50–80 °C to higher temperatures. Exhumation at this stage is likely to have occurred in the Benioff zone, while the subduction of cool lithosphere was going on. Subsequently, the rocks were near-isothermally exhumed to ca. 30 km, followed by concomitant decompression and cooling to surface conditions (at < 500 °C, < 1 GPa). During this last stage the UHPM slice arrived at its present tectonic position with respect to the overlying greenschist-facies Combin zone. In contrast to the well-preserved HP/UHPM record of the coesite-glaucophane eclogites, the HP/UHP assemblages of the metasediments have been largely obliterated during exhumation. Relics from which the metamorphic evolution of the rocks during prograde HP metamorphism and the UHP stage can be retrieved are restricted to rigid low-diffusion minerals like garnet, dolomite, tourmaline and apatite.     

大别山高压-超高压岩石折返与扬子北缘构造变形的关系

大别山造山带及其"前陆"形成于三叠纪,是华北陆块与扬子地块经长期构造演化、最终碰撞的产物。研究表明,该"前陆"实际是扬子地块中部的九岭基底隆升带演化相关的褶皱-逆冲推覆构造带。综合横贯大别山造山带的大地电磁探测、宽角反射与折射地震探测、天然地震波(P波)层析成像研究、莫霍面地震反射图像,揭示大别山造山带及扬子地块北缘的地壳物性、速度结构、莫霍面错断、变形特征等,发现该区上、下地壳结构具有不一致性,总体表现为鱼骨刺状;并结合地表地质调查,推断扬子地块北缘深层向南逆冲的构造与大别山超高压变质岩的形成及折返过程密切相关,而浅部构造向北的逆冲推覆构造与大别山造山带向南的逆冲推覆构成对冲构造样式。最后,本文讨论了该区大地构造演化和背景,分析了其动力学机制。     

Microdiamonds — Frontier of ultrahigh-pressure metamorphism: A review

This is a comprehensive review paper devoted to microdiamonds from ultrahigh-pressure metamorphic (UHPM) terranes incorporated in orogenic belts formed at convergent plate boundaries in Paleozoic-Mesozoic-Alpine time. When in 1980 the first small diamonds were discovered within “amphibolite-granulate facies” metamorphic rocks, it came as a great surprise that buoyant continental crust could be subducted to depths of hundreds of kilometers and then subsequently exhumed. Since then, much progress has been made in understanding the mechanism of these diamonds' formation, and the number of new diamond-bearing UHPM terranes was significantly increased, especially within European orogenes. Moreover, new variations in tectonic settings in which UHP rocks can be formed and exhumed came to the attention of geologists simply due to the finding of diamonds in places previously “forbidden” for their formation—e.g., oceanic islands, ophiolites, and forearc environments. Over the past decade, the rapidly moving technological advancement has made it possible to examine microdiamonds in detail and to learn that part of them has a polycrystalline nature; that they contain nanometric, multiphase inclusions of crystalline and fluid phases; and that they keep a “crustal” signature of carbon isotopes. Scanning and transmission electron microscopy, focused-ion-beam techniques, synchrotron infrared spectroscopy, micro X-ray diffraction, and nano-secondary ion mass spectrometry studies of these diamonds provide evidence that they keep traces of fluid originated from both crustal and mantle reservoirs, and that they probably interacted with deep mantle plumes. Hypotheses proposed for diamond formation in subduction zones founded on both analytical and experimental studies are discussed. The paper also emphasizes that the discovery of these microdiamonds (as well as coesite) triggered a major revision in the understanding of deep subduction processes, leading to a clear realization of how continental materials can be recycled into the Earth's mantle and geochemically rejuvenate it.     

Tracing high-pressure metamorphism in marbles: Phase relations in high-grade aluminous calcite–dolomite marbles from the Greek Rhodope massif in the system CaO–MgO–Al2O3–SiO2–CO2 and indications of prior aragonite

Four different types of parageneses of the minerals calcite, dolomite, diopside, forsterite, spinel, amphibole (pargasite), (Ti–)clinohumite and phlogopite were observed in calcite–dolomite marbles collected in the Kimi-Complex of the Rhodope Metamorphic Province (RMP). The presence of former aragonite can be inferred from carbonate inclusions, which, in combination with an analysis of phase relations in the simplified system CaO–MgO–Al₂O₃–SiO₂–CO₂ (CMAS–CO₂) show that the mineral assemblages preserved in these marbles most likely equilibrated at the aragonite–calcite transition, slightly below the coesite stability field, at ca. 720 °C, 25 kbar and a_CO2 ~ 0.01. The thermodynamic model predicts that no matter what activity of CO₂, garnet has to be present in aluminous calcite–dolomite-marble at UHP conditions.     

中国大陆科学钻探工程主孔及周边地区花岗质片麻岩的地球化学性质和超高压变质作用标志的识别

花岗质片麻岩是中国大陆科学钻探工程主孔0-2000m深度范围内出露的主要岩石类型之一,集中分布于1113.14-1596.22m之间,而在0-1113.14m和1596.22-2000m深度范围内主要以“夹层”形式赋存于副片麻岩和(退变)榴辉岩中,夹层的厚度为0.54-5.82m左右。花岗质片麻岩累计厚度430.98m,占2000m岩心的21.55％左右。地球化学研究结果表明,主孔中花岗质片麻岩SiO2含量普遍偏高,为71.55％-77.18％之间,Al2O3含量为11.54％-13.57％。TiO2、Fe2O3、FeO、MnO和MgO含量则明显偏低,其中Fe2O3 FeO总量为1.05％-2.94％,MgO=0.06％-0.59％。CaO含量为0.30％-2.65％。Na2O和K2O含量变化相对较大,分别为0.29％-4.06％和2.90％-6.67％之间,且大多数样品K2O含量高于Na2O含量。稀土元素配分模式具有右倾式的特点,轻稀土相对富集,而重稀土相对亏损,具有强烈的负Eu异常,Eu/Eu=0.21-0.26之间。在标准化蛛网图上,则显示Ta、Nb、P和Ti的明显亏损以及中等-强烈的负Ba异常,Ba/Ba=0.25-O.64之间,平均值为0.45左右。上述主元素、稀土元素和微量元素特征与中国大陆科学钻探预先导孔CCSD-PP1岩心以及东海及其邻区地表露头的花岗质片麻岩的化学成分十分相似,具有A型花岗岩的地球化学特征。采用激光拉曼技术,配备阴极发光测试,确认中国大陆科学钻     

中国大陆科学钻探工程主孔2000～3000米正、副片麻岩的地球化学性质及其成因机制

中国大陆科学钻探工程主孔(CCSD-MH)2000.0-3000.0米深度范围内出露的岩心以正、副片麻岩为主,夹有薄层榴辉岩和斜长角闪岩等。地球化学研究结果表明,主孔20000.0-3000.0米之间的正片麻岩SiO2含量普遍偏高,为73.26%～78.17%之间,平均值76.40%;Al2O3含量为11.30%-13.66%。TiO2、Fe2O3、FeO、MnO和MgO含量则明显偏低,其中Fe2O3总量为0.39%-1.71%,FeO=0.20%-1.49%,MgO=0.01%-0.06%。CaO含量为0.19-1.41%,Na2O和K2O含量分别为3.38%-5.35%和1.31%-4.87%。正片麻岩的稀土元素和微量元素配分模式可分为三种类型。第一类表现出较强的轻、重稀土元素分馏,具有中等的负Eu异常,Eu/Eu*=0.39-0.64;洋脊玄武岩(MORB)标准化蛛网图表现出强烈富集大离子亲石元素(K、Rb、Ba、Th)的特点,显示明显的正Ba异常,Ba/Ba*=1.09-2.34,高场强元素Ti、Nb和Ta呈明显的负异常。第二类正片麻岩具有明显的负Eu异常,Eu/Eu*=0.39-0.41,稀土元素配分曲线具有明显右倾斜的特点,轻稀土元素明显富集,而重稀土元素明显亏损;洋脊玄武岩(MORB)标准化蛛网图与第一类正片麻岩比较相似,但却具有中等的负Ba异常,Ba/Ba*=0.57-0.67。第三类正片麻岩主要为含磁铁矿二长花岗质片麻岩,稀土元素球粒陨石标准化曲线呈“V”字型特点,具有异常强烈的负Eu异常,Eu/Eu*普遍低于0.11;洋脊玄武岩(MORB)标准化蛛网图显示出强烈富集大离子亲石元素(K、Rb、Th)的特点,具有异常强烈的负Ba异常,Ba/Ba*=0.03-0.21。2000.0-3000.O米深度范围内的正片麻岩具有多成因的特点,部分正片麻岩具有A型花岗岩的地球化学特征,反映它们有可能形成于板内的构造环境;而另一部分的原岩则可能形成于陆缘火山弧的构造环境。主孔2000.0-31000.0米深度范围内副片麻岩SiO2含量明显低于正片麻岩,Al2O3、Fe2O3 FeO、MgO和CaO含量则明显偏高,而Na2O和K2O含量则与正片麻岩大体相当。其中SiO2含量为64.21%-74.12%;Al2O3含量为13.06%-15.38%,Fe2O3 FeO含量为1.61%-4.92%;CaO含量为1.10%～3.27%,Na2O和K2O含量分别为3.68%-5.39%和2.46%-5.85%。副片麻岩稀土元素配分模式和洋脊玄武岩(MORB)标准化蛛网图与正片麻岩也存在明显差异。其中稀土元素配分模式表现出一定程度的轻、重稀土元素分馏,大多数样品具负Eu异常,Eu/Eu*=0.56-0.93之间,但远不及正片麻岩的明显;洋脊玄武岩(MORB)标准化蛛网图则显示出富集大离子亲石元素(K、Rb、Ba、Th)的特点,具有异常明显的正Ba异常,且变化范围较大,Ba/Ba*=1.02-4.83之间,高场强元素如Ti、Nb和Ta呈现负异常的特点。副片麻岩的原岩可能是形成于被动大陆边缘的一套典型的沉积岩或变沉积岩。SHRIMPU-Pb定年结果表明,主孔副片麻岩锆石微区记录了十分复杂的年代学信息。继承性碎屑锆石核部的年龄(206Pb/238U的年龄)为313-659Ma,表明原岩继承性碎屑锆石来源的复杂性,以及部分碎屑锆石在超高压变质过程中发生不完全重结晶,导致年龄变新;在含柯石英锆石微区记录的超高压变质年龄(206Pb/238U的年龄)为220-236Ma,加权平均值为227±5Ma;而锆石晶体边部所记录的退变质年龄(206Pb/238U的年龄)为209-219Ma,加权平均值为214±6Ma,上述含柯石英锆石微区和锆石边部的SHRIMPU-Pb定年结果分别与主孔CCSD-MH中的正片麻岩锆石微区获得的超高压变质年龄(227±2Ma)和角闪岩相退变质年龄(209±3Ma)十分接近,这进一步证明了中国大陆科学钻探工程主孔中的正、副片麻岩的原岩曾一起发生深俯冲,并经历了新三叠纪的超高压变质作用。