A general model for the intrusion and evolution of 'mantle' garnet peridotites in high-pressure and ultra-high-pressure metamorphic terranes

Garnet‐bearing peridotite lenses are minor but significant components of most metamorphic terranes characterized by high‐temperature eclogite facies assemblages. Most peridotite intrudes when slabs of continental crust are subducted deeply (60–120 km) into the mantle, usually by following oceanic lithosphere down an established subduction zone. Peridotite is transferred from the resulting mantle wedge into the crustal footwall through brittle and/or ductile mechanisms. These ‘mantle’ peridotites vary petrographically, chemically, isotopically, chronologically and thermobarometrically from orogen to orogen, within orogens and even within individual terranes. The variations reflect: (1) derivation from different mantle sources (oceanic or continental lithosphere, asthenosphere); (2) perturbations while the mantle wedges were above subducting oceanic lithosphere; and (3) changes within the host crustal slabs during intrusion, subduction and exhumation. Peridotite caught within mantle wedges above oceanic subduction zones will tend to recrystallize and be contaminated by fluids derived from the subducting oceanic crust. These ‘subduction zone peridotites’ intrude during the subsequent subduction of continental crust. Low‐pressure protoliths introduced at shallow (serpentinite, plagioclase peridotite) and intermediate (spinel peridotite) mantle depths (20–50 km) may be carried to deeper levels within the host slab and undergo high‐pressure metamorphism along with the enclosing rocks. If subducted deeply enough, the peridotites will develop garnet‐bearing assemblages that are isofacial with, and give the same recrystallization ages as, the eclogite facies country rocks. Peridotites introduced at deeper levels (50–120 km) may already contain garnet when they intrude and will not necessarily be isofacial or isochronous with the enclosing crustal rocks. Some garnet peridotites recrystallize from spinel peridotite precursors at very high temperatures (c. 1200 °C) and may derive ultimately from the asthenosphere. Other peridotites are from old (>1 Ga), cold (c. 850 °C), subcontinental mantle (‘relict peridotites’) and seem to require the development of major intra‐cratonic faults to effect their intrusion.     

Detachment zones of Cordilleran metamorphic core complexes: thermal,fluid and metasomatic regimes

Metamorphic core complexes in Arizona are characterized by extensional tectonics, in which listrically faulted and unmetamorphosed volcanic rocks of an upper plate are tectonically superimposed on high-grade rocks of a lower plate along low angle detachment faults. The detachment faults are marked by microbreccias and chloritic breccias which developed by hydraulic fracturing and metasomatism that overprinted subjacent amphibolite facies mylonites. Oxygen and hydrogen isotope data indicate that the mylonites formed at 500 °C under conditions of low water/rock ratio. Chloritic breccias developed at 300–350 °C during incursion of hydrothermal fluids along the detachment fault. Hydrothermal alteration involved massive additions of Fe, Mn, and Mg, with concomitant depletions of K and Na, reflecting the hydrolysis of K-feldspar and plagioclase to chlorite.Upper plate volcanic rocks are sporadically altered to an assemblage of K-feldspar, calcite, and Fe, Mn-oxides. Geochemically, the volcanic rocks are characterized by enhanced abundances of alteration insensitive incompatible elements (Th, LREE, P₂O₅), fractionated REE distributions (La_cn/Yb_cn 18), and elevated levels of the mafic affiliated trace elements Cr, Co, Ni, Sc. Primary K₂O contents are estimated at 3–6 wt.%. A consistent feature is the presence of troughs at Ta-Nb and Ti, on chondrite normalized diagrams, indicative of magmas associated with convergent plate boundaries. Hydrothermal alteration involved massive additions of K, Fe, Ca, and CO₂, and Na and Mg were systematically depleted. Whole rock ¹⁸O values span 4 to 18 per mil, corroborating the extent of isotopic exchange with an aqueous reservoir. Alteration occurred at temperatures of 250 to 100 °C, fluid ¹⁸O of the thermal waters is estimated at limiting values of –11 to +11 per mil.Dual fluid regimes were operating during extensional tectonics. At lower structural levels, high temperature, reduced fluids of metamorphic and/or evolved meteoric origin, were focussed along the detachment faults under lithostatic pressure, and induced Fe, Mn, Mg-metasomatism. In contrast, cool, oxidized thermal waters, at hydrostatic conditions, and likely of variably evolved meteoric origin were involved at high structural levels.

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Zusammenfassung Die metamorphen Komplexe in Arizona sind charakterisiert durch Dehnungstektonik, in denen listrisch gestörte und unmetamorphe vulkanische Gesteine einer oberen Platte tektonisch auf hochgradig metamorphe Gesteine einer unteren Platte entlang einer flachwinkligen Abscherungsfläche verfrachtet wurden. Die Abscherungen werden markiert durch Mikrobrekzien und chloritische Brekzien, die sich durch hydraulisches Zerbrechen (»hydraulic fracturing«) und Metasomatose gebildet und damit die unterlagernden amphibolitfaziellen Mylonite überprägt haben. Sauerstoff- und Wasserstoff-Isotopendaten belegen, daß die Mylonite bei 500 °C unter einem niedrigen Wasser/Gesteins-Verhältnis gebildet wurden. Die chloritischen Brekzien entwickelten sich bei 300–305 °C während des Einbruchs hydrothermaler Fluide entlang der Abscherungsfläche. Die hydrothermalen Veränderungen hatten massive Zugabe von Fe, Mn und Mg zur Folge unter gleichzeitiger Wegnahme von K und Na, was sich in der Hydrolyse der K-Feldspäte und Plagioklase zu Chlorit widerspiegelt. In den vulkanischen Gesteinen des oberen Stockwerks ist es lokal zur Bildung von K-Feldspat, Kalzit und Fe-, Mn-Oxiden gekommen. Geochemisch sind die vulkanischen Gesteine charakterisiert durch eine gesteigerte Häufigkeit von Änderungen unempfindlicher, inkompatibler Elemente (Th, LREE, P₂O₅), fraktionierte REE-Verteilungen (La_cn/Yb_cn = 18) und eine Zunahme an »basischen« Spurenelementen wie Cr, Co, Ni, Sc. Primäre K₂O-Gehalte werden auf 3–6 wt.% geschätzt. Ein durchgehendes Merkmal ist in den Chondrit-normalisierten Diagrammen das Vorhandensein von Trögen für Ta-Nb und Ti. Dies ein Hinweis auf Magmen, die mit konvergierenden Plattengrenzen verknüpft sind. Hydrothermale Änderungen beinhalten massive Zugabe von K, Fe, Ca und CO₂, und eine systematische Abnahme von Na und Mg. -¹⁸O-Werte im Gesamtgestein reichen von 4 bis 18 pro mil. und bestätigen damit die Breite des Isotopenaustausches mit einem wässrigen Reservoir. Alterationen anden bei Temperaturen zwischen 250 und 100 °C statt. Die ¹⁸O-Werte der Fluide in den hydrothermalen Wässern liegen schätzungsweise mit ihren Grenzwerten zwischen –11 und +11 pro mil. Ein duales Fluidregime wirkt während der Dehnungstektonik. In den tiefen tektonischen Stockwerken mit hoher Temperatur konzentrieren sich reduzierende Fluidmengen von metamorphem oder meteoritischem Ursprung an Abscherungen unter lithostatischem Druck. Es tritt eine Fe, Mn, Mg-Metasomatose auf. Im Gegensatz dazu sind in den höheren tektonischen Stockwerken kühlere, oxidierende Thermalwässer unter hydrostatischen Bedingungen und meteorischen Ursprungs anzutreffen.

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Résumé Les complexes métamorphiques de l'Arizona sont caractérisés par une tectonique extensionnelle au cours de laquelle des roches volcaniques, listriquement faillées et non métamorphiques, appartenant à une plaque supérieure, ont été superposées aux formations très métamorphiques d'une plaque inférieure, par l'intermédiaire de failles plates de décollement. Ces failles sont accompagnées de microbrèches et de brèches chloriteuses engendrées par fracturation hydraulique et métasomatose, phénomènes qui ont affecté les mylonites de facies amphibolitique sous-jacentes. Les rapports isotopiques de l'oxygène et de l'hydrogène indiquent pour la genèse des mylonites une température d'environ 500° avec un faible rapport eau/roche. Les brèches chloriteuses se sont formées entre 300° et 350° à l'occasion de venues hydrothermales le long de la faille de décollement. L'altération hydrothermale s'est accompagnée d'un apport massif de Fe, Mn et Mg et d'un départ de K et Na, reflétant la transformation hydrolytique du feldspath potassique et du plagioclase en chlorite.Les roches volcaniques de la plaque supérieure sont altérées sporadiquement en une association de feldspaths, calcite et oxydes de Fe-Mn. Au point de vue géochimique, ces roches volcaniques sont caractérisées par un enrichissement en éléments incompatibles insensibles à l'altération (Th, LREE, P₂O₅), par des distributions fractionnées des terres rares (La_cn/Yb_cn = 18), et par une teneur élevée des éléments en traces associés aux ferro-magnésiens (Cr, Co, Ni, Sc). La teneur primaire en K₂O est estimée à 3 à 6%. Les diagrammes normalisés par rapport aux chondrites montrent de manière régulière un creux en Ta-Nb et en Ti, indicatif de magmas associés à des bordures de plaques convergentes. L'altération hydrothermale a été accompagnée d'un apport massif de K, Fe, Ca et CO₂ et d'une perte systématique en Na et Mg.Les valeurs de ¹⁸O sur roche totale s'échelonnent entre 4 et 18, ce qui confirme l'importance d'échanges isotopiques avec un réservoir aqueux. L'altération s'est déroulée à des températures qui vont de 250° à 100°; la valeur de ¹⁸O des fluides hydrothermaux devait être comprise entre –11 et + 11 %.Au cours de la tectonique d'extension, l'ensemble a été le siège d'un double régime de fluides. Aux niveaux structuraux inférieurs, des fluides réducteurs d'origine métamorphique et/ou météorique évoluée se sont concentrés de long des failles de décollement, sous haute pression lithostatique et ont provoqué la métasomatose en Fe, Mn et Mg. Par contre, dans les niveaux structuraux supérieurs, les fluides hydrothermaux étaient des eaux d'origine probablement météorique, modifiées à des degrés divers, moins chaudes et oxydantes.

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, , , , . , (hydraulic fracturing) ; . , 500 ° . 300–350 ° . , ; . , . (incompatible elements — Th, LREE, P₂O₅), (La_cn/Yb_cn = 18), «» , - r, , Ni, Sc. , 3–6%. — Nb Ti . , . , , . ¹⁸O 4 18%., . 250 100 °. ¹⁸ –11 +11 %.

     

高级变质地体中多期变质事件的甄别:以东南极埃默里地区为例

在多期高级变质地体中确定每期变质事件的性质、时代和P-T轨迹是一项非常困难的研究工作,也是变质地质学研究领域的前沿课题。将精细的地质年代学研究与详细的构造解析和岩石学观察相结合,这是建立同位素年龄与变质事件之间有机联系的最好方法。使用这种方法来甄别东南极埃默里地区格林维尔期和泛非期高级变质事件获得了初步的成功,基本上查明了两期变质事件的影响范围、变质时代和P-T演化,但也存在许多尚未解决的科学问题。一般而言,在同一变质地体中发育两期/多期具有不同构造指向的挤压-伸展变形、保存不同类型（如顺时针和逆时针）或者不连续的P-T演化轨迹以及含有两组/多组变质年龄等可视为存在多期变质事件的标志。当前对多期高级变质作用叠加的研究中还存在两个不清楚的岩石学问题,其一是早期变质作用对晚期变质重结晶施加了什么样的影响?其二是晚期变质作用如何叠加在早期变质矿物组合之上?所以,探索多期高级变质事件的叠加机理、变质行为及控制因素仍是未来岩石学家的艰巨任务。

     

变质岩中金红石研究进展及存在问题

随着原位微区分析技术的发展,金红石作为常见的副矿物,受到越来越多的关注.结合目前地学研究的热点科学问题,文中总结了金红石在变质岩石学中研究进展及存在的问题:(1)金红石在变质岩中的产状及形成过程;(2)金红石Zr含量温度计的进展及在低级、中级及高级变质岩中的应用;(3)金红石微量元素在弧岩浆过程和俯冲带变质作用的行为;...     

Tectonic evolution of high-grade metamorphic terranes in central Vietnam: Constraints from large-scale monazite geochronology

Several metamorphic complexes in Southeast Asia have been interpreted as Precambrian basement, characterized by amphibolite to granulite facies metamorphism. In this paper, we re-evaluate the timing of this thermal event based on the large-scale geochronology and compositional variation of monazites from amphibolite to granulite facies metamorphic terranes in central Vietnam. Most of the samples in this study are from metamorphic rocks (n = 38) and granitoids (n = 11) in the Kontum Massif. Gneisses (n = 6) and granitoids (n = 5) from the Hai Van Migmatite Complex and the Truong Son Belt, located to the north of the massif, were also studied. Two distinct thermal episodes (245–230 Ma and 460–430 Ma) affected Kontum Massif gneisses, while a single dominant event at 240–220 Ma is recorded in the gneisses from the Hai Van Complex and the Truong Son Belt. Monazites from granitoids commonly yield an age of 240–220 Ma. Mesoproterozoic ages (1530–1340 Ma) were obtained only from monazite cores that are surrounded by c. 440 Ma overgrowths. Thermobarometric results, combined with concentrations of Y₂O₃, Ce₂O₃, and heavy rare earth elements in monazite, and recently reported pressure–temperature paths suggest that Triassic ages correspond to retrograde metamorphism following decompression from high- to medium-pressure/temperature conditions. Ordovician–Silurian ages reflect low-pressure/temperature metamorphism accompanied by isobaric heating during prograde metamorphism. Some samples were affected by both metamorphic events. We conclude that high-grade metamorphism observed in so-called Precambrian basement terranes in central Vietnam occurred during both the Permian–Triassic and the Ordovician–Silurian, while peraluminous granitoid magmatism is Triassic. Additionally, our preliminary analyses for U–Pb zircon age and whole-rock chemistry of granitic gneisses from the Truong Song Belt suggests the presence of the Ordovician–Silurian volcanic arc magmatism in the region. Based on the pressure–temperature–time–protolith evolutions, metamorphic rocks from central Vietnam provide a continuous record of subduction–accretion–collision tectonics between the South China and Indochina blocks: in the Ordovician–Silurian, the region was characterized by active continental margin tectonics, followed by continental collision during the Late Permian to Early Triassic and subsequent exhumation during the Late Triassic. The results also suggest that the timing of metamorphism and protolith formation as well as the geochemical features in other Southeast Asian terranes should be verified to achieve a better understanding of the Precambrian to Early Mesozoic tectonic history in Asia.     

Low-temperature thermochronological record of exhumation of the Bitterroot metamorphic core complex, northern Cordilleran Orogen

The Bitterroot metamorphic core complex is an exhumed, mid-crustal, plutonic–metamorphic complex that formed during crustal thickening and subsequent extension in the hinterland of the North American Cordilleran Orogen, in the northern Idaho batholith region. Extension was accommodated mainly on the Bitterroot mylonite zone, a 500–1500-m-thick shear zone that deforms granitic intrusive rocks as young as 53–52 Ma, as well as older high-grade metamorphic rocks and plutons. Exhumation of the core complex, in Eocene time, is marked in the shear zone by the transition from amphibolite-facies mylonitization, to greenschist-facies mylonitization, chloritic brecciation, to brittle faulting that progressed from shallower crustal levels in the west to deeper crustal levels in the east from ca. 53 –30 Ma based on U–Pb, Ar–Ar, and fission-track data. Apatite and zircon fission-track data record the lower-temperature part of the exhumation history and help define when the shear zone became inactive, as well as the transition from rapid, core complex-style extension to slower basin-and-range-style extension. They indicate that the western part of the complex was exhumed to within 1–2 km of the surface by 48–45 Ma, while the eastern part of the complex was still at amphibolite-facies conditions and that the eastern part of the complex was not exhumed below 60 °C until after 30 Ma. Younger apatite fission-track ages (≤26 Ma) on the eastern range front of the Bitterroot Mountains suggest that the present topographic expression of the mylonite front was due to Miocene high-angle faulting and widening of the Bitterroot Valley.     

Pumpellyites in two low grade metamorphic terranes North of Newcastle,NSW Australia

Microprobe analyses of pumpellyites from rocks of variable chemistry formed under similar metamorphic conditions in two Palaeozoic, low grade metamorphic terranes show that they have an extreme range in composition (FeO^*=0.9–22.96) and that Fe²⁺Mg²⁺ and Fe³⁺Al³⁺ are the dominant substitutions. A less extreme variation in composition of pumpellyites has been noted in samples taken from a metamorphosed differentiated metadolerite. On an A1-Fe^*-Mg diagram, these pumpellyites extend through the fields of high pressure to low pressure terranes, indicating that pumpellyite compositions should be used with caution when determining metamorphic conditions.Bulk chemical composition of the host rock does not appear to be a controlling factor in determining pumpellyite compositions. Rather, intensity of alteration, particularly of opaque mineral phases, fluid chemistry and variation in oxidation potential are considered to be more important variables. Coexisting epidote and composition of the precursor mineral also appear to be important in some rocks.     

Albite vein formation during exhumation of high-pressure terranes: a case study from alpine Corsica

The formation of late‐stage veins can yield valuable information about the movement and composition of fluids during uplift and exhumation of high‐pressure terranes. Albite veins are especially suited to this purpose because they are ubiquitously associated with the greenschist facies overprint in high‐pressure rocks. Albite veins in retrogressed metabasic rocks from high‐pressure ophiolitic units of Alpine Corsica (France) are nearly monomineralic, and have distinct alteration haloes composed of actinolite + epidote + chlorite + albite. Estimated P–T conditions of albite vein formation are 478 ± 31 °C and 0.37 ± 0.14 GPa. The P–T estimates and petrographic constraints indicate that the albite veins formed after the regional greenschist facies retrogression, in response to continued decompression and exhumation of the terrane. Stable isotope geochemistry of the albite veins, their associated alteration haloes and unaltered hostrocks indicates that the vein‐forming fluid was derived from the ophiolite units and probably from the metabasalts within each ophiolite slice. That the vein‐forming fluid was locally derived means that a viable source of fluid to form the veins was retained in the rocks during high‐pressure metamorphism, indicating that the rocks did not completely dehydrate. This conclusion is supported by the observation of abundant lawsonite at the highest metamorphic grades. Fluids were liberated during retrogression via decompression dehydration reactions such as those that break down hydrous high‐pressure minerals like lawsonite. Albite precipitation into veins is sensitive to the solubility and speciation of Al, which is more pressure sensitive than other factors which might influence albite vein formation such as silica saturation or Na:K fluid ratios. Hydraulic fracturing in response to fluid generation during decompression was probably the main mechanism of vein formation. The associated pressure decrease with fracturing and fluid decompression may also have been sufficient to change the solubility of Al and drive albite precipitation in fracture systems.     

青藏高原主要地体地壳短缩作用研究现状及存在的问题

在对喜马拉雅、拉萨和羌塘3个地体已有的有关地壳短缩研究成果系统分析的基础上,对3个地体进行了平衡剖面恢复:北羌塘侏罗系短缩率为25.18%.南羌塘短缩率为33.57%;对拉萨地体南段(措勤盆地南部坳褶带)上白垩统恢复得出其短缩率为20.68%北段中部坳褶带到班公湖一怒江缝合带南缘短缩率为25.3%;地处特提斯喜马拉雅地体东段的郎杰学地体三叠系短缩率达75%.大于前人研究的特提斯喜马拉雅56%~6O%的短缩率.通过对比,对3个地体短缩变形的规律进行了分析,认为各地体内部短缩作用并不是一个连续均匀的过程,陆内变形主要是通过稳定地体边界和大型逆冲构造带来吸收的;拉萨地体和羌塘地体新生代内部变形较小.     

青藏高原主要地体地壳短缩作用研究现状及存在的问题

在对喜马拉雅、拉萨和羌塘3个地体已有的有关地壳短缩研究成果系统分析的基础上,对3个地体进行了平衡剖面恢复：北羌塘侏罗系短缩率为25.18%,南羌塘短缩率为33.57%;对拉萨地体南段(措勤盆地南部坳褶带)上白垩统恢复得出其短缩率为20.68%,北段中部坳褶带到班公湖-怒江缝合带南缘短缩率为25.3%;地处特提斯喜马拉雅地体东段的郎杰学地体三叠系短缩率达75%,大于前人研究的特提斯喜马拉雅56%~60%的短缩率。通过对比,对3个地体短缩变形的规律进行了分析,认为各地体内部短缩作用并不是一个连续均匀的过程,陆内变形主要是通过稳定地体边界和大型逆冲构造带来吸收的;拉萨地体和羌塘地体新生代内部变形较小。     

大别山中生代钾质岩浆作用与超高压变质地体的剥露机理

在已知的超高压变质地体中, 大别山是碰撞后花岗岩类侵入作用最为强烈的唯一地区。这里, 中生代的钾玄岩系列和高钾钙碱性系列的侵入岩, 可以依其年龄和组成特征划分为３ 组。第 Ｉ组, 主要由晚三叠世 （约２１０ Ｍａ） 二长闪长岩 辉长岩体组成, 它可能是在板片断离过程中富集的大陆岩石圈地幔部分熔融的产物 （板片断离型）; 第 Ｉ Ｉ组, 由中侏罗世－ 早白垩世 （１６０～１２０ Ｍａ） 的角闪石英二长岩、黑云母二长花岗岩和正长花岗岩组成, 主要是由幔源岩浆的分离结晶与地壳混染共同作用的产物 （岩浆底垫型）; 第 Ｉ Ｉ Ｉ组, 以白垩纪 （１２５～９５ Ｍａ） 的花岗岩和花岗斑岩为代表, 是在热穹窿作用过程中大别杂岩深熔作用和高度演化的产物 （穹窿型）。大别山超高压变质岩通过地幔的剥露可能与晚三叠世—早侏罗世的板片断离有关, 而大规模的岩浆侵位与超高压变质岩的快速剥露相伴出现说明, 岩浆底垫和岩浆漂浮作用可能在大别山中侏罗世—早白垩世的快速剥露中发挥了重要作用。     

吉林-黑龙江高压变质带的初步厘定:证据和意义

本文定义的吉林-黑龙江高压变质带是指我国东北地区佳木斯-兴凯地块西缘和南缘共同发育的呈弧形展布的高压变质带,具体包括佳木斯-兴凯地块西缘增生杂岩带(黑龙江蓝片岩带和张广才-小兴安岭增生杂岩带)和佳木斯-兴凯地块南缘的长春-延吉增生杂岩带.其中佳木斯-兴凯地块西缘增生杂岩带形成于晚三叠-早侏罗世(180 ～ 210Ma),为佳木斯-兴凯地块向西冲增生而形成的高压变质带;而长春-延吉增生杂岩带由一系列特征性俯冲-增生杂岩组成,包括石头口门-烟筒山红帘石片岩带、呼兰群变质杂岩、色洛河群变质杂岩、青龙村群变质杂岩和开山屯变质杂岩等,形成时代为187～230Ma,峰期为220～230Ma.长春-延吉增生杂岩带曾被认为是西拉木伦河断裂带的东延部分,但是区域构造分析表明,它们形成的动力学背景与佳木斯-兴凯地块西缘增生杂岩带相同,均为太平洋板块三叠纪-早侏罗世的西向俯冲导致佳木斯-兴凯地块自东向西的“剪刀式”闭合过程.我们将佳木斯-兴凯地块西缘和南缘发育的三叠纪-早侏罗世增生杂岩带作为统一的构造单元来考虑,结合该区发育有典型的高压变质带,因此命名为“吉林-黑龙江高压变质带,简称吉黑高压带”.吉黑高压带形成于太平洋板块三叠纪-早侏罗世的西向俯冲导致佳木斯-兴凯地块自东向西的“剪刀式”闭合的过程,同时该带记录了古亚洲构造域的结束和太平洋俯冲开始的关键时期,为两大构造域叠加与转换的关键性地质证据.     

变质地质学研究中的一些困难问题

变质地质学研究中,首先需要准确判明变质过程是属于多期或单期、多峰或单峰变质作用。研究中应该对同一研究区不同类型的变质岩石开展综合研究,查明各种变质反应结构,以期对变质反应结构的解释尽可能准确,并对变质反应进行定量论证。同一变质岩内前、后两个世代变质矿物组合之间,并不存在所谓的热力学局部平衡。在应用方面,矿物温度计与压力计属于反演方法,热力学视剖面图模拟属于正演方法。本文指出了它们之间的相同之处和各种区别。众所周知,目前反演出的变质作用压力并无方向性,差异应力对总应力的贡献尚不明朗。最好根据同一视域或同一块变质岩石,来反演变质作用P-T-t轨迹。实际上,P-T-t轨迹一般不应该是圆滑的曲线,其构造意义的解释也还存在不确定性。同一地区变质作用P-T条件的突变、P-T-t轨迹的明显差异,表明地质体之间存在断层接触关系。变形作用与变质作用的耦合研究需要加强,变质作用的定年也不能仅仅局限于变质副矿物。特别值得注意的是,同一造山带内范围有限的地域,也未必属于同一变质相系。

     

叠层石研究现状及进展

在地球早期生命演化过程中,叠层石-微生物席生态系统主宰地球海洋近30亿年,叠层石作为微生物岩,是一种特殊的生物沉积构造,记录了大量的微生物、环境、地球化学和地球物理等方面的信息。叠层石研究经历了迂回曲折的道路,本文在综述国内外叠层石研究现状的基础上,重点阐述了中国叠层石研究取得的主要成绩和存在的不足,并从多学科交叉融合的角度,结合上下地层岩相变化,提出叠层石研究的新方法和新思路,为我国今后叠层石的研究和古环境的重建提供参考。     

青藏高原主要地体地壳短缩作用研究现状及存在的问题

在对喜马拉雅、拉萨和羌塘3个地体已有的有关地壳短缩研究成果系统分析的基础上,对3个地体进行了平衡剖面恢复：北羌塘侏罗系短缩率为25.18%,南羌塘短缩率为33.57%;对拉萨地体南段(措勤盆地南部坳褶带)上白垩统恢复得出其短缩率为20.68%,北段中部坳褶带到班公湖-怒江缝合带南缘短缩率为25.3%;地处特提斯喜马拉雅地体东段的郎杰学地体三叠系短缩率达75%,大于前人研究的特提斯喜马拉雅56%~60%的短缩率。通过对比,对3个地体短缩变形的规律进行了分析,认为各地体内部短缩作用并不是一个连续均匀的过程,陆内变形主要是通过稳定地体边界和大型逆冲构造带来吸收的;拉萨地体和羌塘地体新生代内部变形较小。     

Constraints on the application of carbon isotope thermometry in high- to ultrahigh-temperature metamorphic terranes

Nine marble horizons from the granulite facies terrane of southern India were examined in detail for stable carbon and oxygen isotopes in calcite and carbon isotopes in graphite. The marbles in Trivandrum Block show coupled lowering of δ¹³C and δ¹⁸O values in calcite and heterogeneous single crystal δ¹³C values (? 1 to ? 10‰) for graphite indicating varying carbon isotope fractionation between calcite and graphite, despite the granulite facies regional metamorphic conditions. The stable isotope patterns suggest alteration of δ¹³C and δ¹⁸O values in marbles by infiltration of low δ¹³C–δ¹⁸O‐bearing fluids, the extent of alteration being a direct function of the fluid‐rock ratio. The carbon isotope zonation preserved in graphite suggests that the graphite crystals precipitated/recrystallized in the presence of an externally derived CO₂‐rich fluid, and that the infiltration had occurred under high temperature and low f_O2 conditions during metamorphism. The onset of graphite precipitation resulted in a depletion of the carbon isotope values of the remaining fluid+calcite carbon reservoir, following a Rayleigh‐type distillation process within fluid‐rich pockets/pathways in marbles resulting in the observed zonation. The results suggest that calcite–graphite thermometry cannot be applied in marbles that are affected by external carbonic fluid infiltration. However, marble horizons in the Madurai Block, where the effect of fluid infiltration is not detected, record clear imprints of ultrahigh temperature metamorphism (800–1000 °C), with fractionations reaching <2‰. Zonation studies on graphite show a nominal rimward lowering δ¹³C on the order of 1 to 2‰. The zonation carries the imprint of fluid deficient/absent UHT metamorphism. Commonly, calculated core temperatures are > 1000 °C and would be consistent with UHT metamorphism.     

Graphitization in a high-pressure,low-temperature metamorphic gradient: a Raman microspectroscopy and HRTEM study

The graphitization of carbonaceous material (CM) in a high-pressure metamorphic gradient is characterized along a cross section in the Schistes Lustrés formation, Western Alps. Along this 25-km cross section, both the CM precursor and the host-rock lithology are homogeneous, and the prograde evolution of the pressure-temperature metamorphic conditions from the lower blueschist-facies (13 kbar, 330 °C) to the eclogite-facies (20 kbar, 500 °C) is tightly constrained by literature data. Raman microspectroscopy shows that at the micrometre scale, this process is progressive and continuous with increasing metamorphic grade, and that the structure of CM is very sensitive to temperature variations. At the nanometre scale (HRTEM), the CM is composed of a mixture of a microporous phase and an onion-ring like phase, both known as non-graphitizing under the effect of temperature at ambient pressure. The HP-LT graphitization produces structurally and microtexturally heterogeneous CM. With increasing metamorphic grade, the graphitization of the two types of CM proceeds up to the triperiodic graphite stage because of microtextural and structural changes that are specific to each type of CM. The microporous material is progressively transformed into graphite through a macroporous transitional stage. In this case, graphitization mainly occurs on the pore walls as a result of pore growth. In the case of concentric onion-ring like material, graphitization occurs in the regions with the largest radius of curvature, i.e. on the outer part of the ring. In comparison with 1-bar experiments, pressure seems to induce microtextural changes, which allows the subsequent structural modifications of the starting material.     

Lu-Hf and PbSL geochronology of apatites from Proterozoic terranes: A first look at Lu-Hf isotopic closure in metamorphic apatite

The mineral apatite is characterized by elevated and highly variable Lu/Hf ratios that, in some cases, allow for single-crystal dating by the Lu-Hf isotopic system. Apatites from the Adirondack Lowlands and Otter Lake area in the Grenville Province, and from the Black Hills, South Dakota, yield Lu-Hf ages that are consistently older than their respective Pb step leaching ages. Isotopic closure for the Lu-Hf system, therefore, occurs before U-Pb system closure in this mineral. In the Adirondack Lowlands, where H₂O activity was low, Lu-Hf systematics of cm-sized apatite crystals remained undisturbed during upper amphibolite facies metamorphism (∼700 to 675 °C) at 1170-1130 Ma. The relatively old Lu-Hf ages of 1270 and 1230 Ma observed for these apatites correlate with decreasing crystal size. In contrast, apatite from the fluid-rich Otter Lake area and Black Hills yields unrealistically low apparent Lu-Hf closure temperatures, implying that in these apatites, fluids facilitated late exchange. The Lu-Hf ages for the metamorphic apatites were thus controlled either by the prevailing temperature and grain size, or by fluid activity.