Abstract Symplectite, defined as plagioclase + Ca-pyroxene (±amphibole) intergrowths after omphacite, and kelyphite, defined as amphibole + plagioclase coronas around garnet, are common features of retrogressed eclogites. These textures are related to exhumation under (ultra) high pressure towards the surface, but the estimation of the pressure–temperature ( P – T ) of symplectite formation is difficult because of the narrowness of pyroxene and plagioclase lamellas, and the compositional variability of the phases. Retrogressed eclogites from Norwegian localities with different eclogite peak conditions have been chosen to investigate the formation of symplectite and associated kelyphite. Thermobarometry calculations show that symplectite crystallizes as soon as the rocks enter the stability field of plagioclase and continues crystallizing until they have reached amphibolite facies. Symplectite yields a pressure range from 18 to 10 kbar, and a temperature range from 700 to 550°C. Amphibole found in the symplectite assemblage crystallizes later, at lower pressures and temperatures (10–4 kbar, 680–420°C). Kelyphite is always associated with well-developed symplectite, when the former omphacite is totally transformed into symplectite. These features likely testify to the influence of an external fluid during retrogression. Samples with limited symplectite and no kelyphite are likely retrogressed with an internal fluid.

Abstract EPMA and LA-ICP-MS trace-element maps have been acquired from amphibolitized eclogites from the Diego de Almagro Metamorphic Complex (Chile). Several garnet growth pulses and garnet resorption stages are revealed by major elements chemical zoning and by heterogeneous Y and rare earth element (REE) behaviour, associated with subduction and exhumation of these rocks. Distribution of REE in prograde garnet is texturally and chemically coupled with the breakdown of REE-bearing minerals while formation of epidote and titanite generations during amphibolitization is recorded by complex textures involving new garnet generation and overprinting phases. The latest overprint stage is characterized by fine-grained intergrowth between garnet and epidote micro-veins, phengite, hornblende, albite and titanite. Garnet cracks have been gradually re-equilibrated during this event witnessing short-scale dissolution–transport–precipitation. Pseudosection modelling shows that local variability in water content during amphibolitization controls garnet stability at the expense of epidote. Overprinting microstructures are explained by the effect of locally-derived aqueous fluids that trigger the ‘unlocking’ of elements from the reacting eclogite-facies paragenesis. These findings highlight the microscopic characteristics of amphibolitization processes documented in exhumed eclogite-facies terranes and shed light on the importance of thorough micro-chemical investigations while undertaking pressure–temperature (PT) estimates on rocks with strong textural disequilibrium.

Abstract This study is focused on a specific outcrop in the Bergen Arcs, Norway where the transition between dry granulite and the hydrated eclogite and amphibolite is exposed. In this outcrop the foliation in the granulite is continuous as it passes through eclogite- and amphibolite-facies rocks, presenting a challenge to understanding the nature of these spatial relationships. Although there is no major change in the bulk chemical composition of all three metamorphic-facies rocks, the loss of ignition (LOI) content increases from granulite to the eclogite and to the amphibolite. During hydration and metamorphism, the density changes from c. 3 g cm −3 for the anorthositic granulite to 3.2 g cm −3 for the eclogite, and 2.75 g cm −3 for the amphibolite. Based on the mass balance equation, eclogitization of the granulite shows a reduction of volume of c. 3% whereas amphibolitization of the granulite gains c. 5% in volume. By assuming equilibrium, modelling the phase equilibria provides estimates of the amount of fluid necessary to form the eclogite and the amphibolite assemblages. Results show that both assemblages can be stable at similar temperature and a similar fluid composition but differ in pressure by c. 10 kbar. This study suggests that the stress generated during hydration of the granulite may influence the local mineral assemblage equilibrium.

Abstract Several large to giant Pb–Zn deposits in the West Qinling Orogen in central China are argued to be of SEDEX (sedimentary exhalative) type or of epigenetic hydrothermal type. Additionally, the nature of the mineralizing fluids is poorly known. Our observations suggest that early stage primary marine sedimentary mineralization is characterized by laminated or disseminated fine-grained massive sulphide ores, and late stage metamorphic superimposition is represented by coarser equigranular annealed textures and the disruption of thinly laminated structures. Three coexisting types of fluid inclusions were recognized: H 2 O–NaCl (type I); H 2 O–NaCl–CH 4 –CO 2 (type II); and CH 4 –CO 2 (type III). The coexisting type I and II inclusions show similar homogenization temperature values but different salinities, indicating that fluid immiscibility occurred. Formation pressures calculated using type III inclusions are high (72.5–174.5 MPa). The lead isotopes of the sulphides and calcites show a narrow range. The primary sedimentary ore textures plus the similar lead isotopes between the ores and the wall rocks suggest a SEDEX origin, but the annealed recrystallization textures, the immiscible carbonic fluid inclusion assemblages and higher formation pressures suggest a strong late-stage metamorphic superimposition on the original SEDEX-type ores.

Abstract Orogenic gold ores of the Arabian–Nubian Shield are structurally controlled by the Najd Fault System. The Najd Fault System controlled the exhumation of the metamorphic complexes and, as such, there is a genetic relationship between the metamorphism and the formation of the orogenic gold ores. In order to constrain this genetic relationship, field observations, petrography, geochemistry and fluid inclusions of four mines from the Egyptian side of the shield are presented. The studied gold-bearing dykes and veins are structurally controlled where the gold-bearing fluid precipitated in pre-existing second-order and third-order extensional faults of the major NW–SE Najd Fault System. Fluid inclusions indicate that the gold was precipitated at shallow- to medium-crustal levels, equivalent to a temperature range of 250–350°C, and from low salinity metamorphic fluids, possibly mixed with magmatic/meteoric water. Thermodynamic modelling suggests that gold-bearing fluids were generated due to metamorphic devolatilization processes across the greenschist–amphibolite-facies transition of ophiolitic and metasedimentary source rocks. The Najd Fault System enables the vertical transport of gold-bearing fluids from the source region to the depositional sites. Decreasing the temperature of the fluid is required to precipitate the gold. However, the gold precipitation process needs to be buffered by Fe-bearing wall rocks.

Abstract The Palaeoarchean supracrustal rocks of the Barberton Greenstone Belt (BGB) consist of hydrothermally altered volcanic and silicified sedimentary rocks that form the dramatic topography in the Barberton Mountain Land of Mphumalanga Province in South Africa and Swaziland. Previous metamorphic studies are limited and used mainly empirical chlorite geothermometry in the BGB sequence regionally, or Raman thermometry, both proposing a narrow range in metamorphic conditions of around 320°C regional for the BGB. This study demonstrates that a range of petrological and thermodynamic modelling techniques are required in an integrated approach to unravel the very low- to medium-grade metamorphic conditions preserved in the oldest part of the BGB. The study investigates low-temperature metamorphic processes in the Komati, Hooggenoeg, Kromberg and Mendon formations of the c. 3530–3298 Ma Onverwacht Group. Chlorite thermodynamic modelling indicates metamorphic conditions of between 250 and 445°C, in the c. 3482 Ma Komati Formation, but uncertainties on these conditions are large ( c. 80 and 100°C) due to the protoliths being mostly ultramafic in composition with uncertainty in XFe 3+ estimates. Raman thermometry on carbonaceous material in the c. 3472 Ma volcano-sedimentary Middle Marker chert (HC1) at the base of the Hooggenoeg Formation indicates metamorphic conditions of between 281 and 301°C, lower than in previous studies. Raman thermometry on a carbonaceous chert (HC5) in the 3472–3432 Ma Hooggenoeg Formation in conjunction with δ 18 O oxygen isotope values on underlying metabasalts, indicate metamorphic temperature conditions of less than 340°C, preserved in the Hooggenoeg Formation. Application and critical evaluation of chlorite thermodynamic modelling results confirm the presence of an inverted metamorphic field gradient beneath a fuchsite–chlorite–quartz shear zone in the c. 3334 Ma Kromberg Formation. Thermodynamic modelling using a di-octahedral mica model indicates that retrograde metamorphism in the Kromberg ultramafic shear zone occurred along a kyanite-type geothermal gradient, providing evidence in support of tectonic thrusting at c. 3227–3223 Ma. In the c. 3298 Ma Mendon Formation seawater alteration involving metasomatism occurred at conditions of T = 150°C with log a CO 2 (aq) of −2.74. This study sheds new light on the metamorphic history of the BGB supracrustal sequence and early Earth metamorphic processes. It also demonstrates that combining different petrological and thermodynamic methods across the BGB supracrustal stratigraphy allows a robust critical assessment of metamorphic conditions for the first time, with implications for the early Archean geodynamic evolution of the BGB.

Abstract Microstructural and petrological data from >60 samples, collected by L.R. Wager in 1933, have been used alongside existing data to investigate temperature gradients and deformational style in four profiles across the South Tibetan Detachment shear zone, over a north–south distance of 35 km in the Mt Everest area, east-central Himalaya. The ductile shear zone, defined on petrographic criteria, extends for c. 900 m beneath the brittle Qomolangma Detachment (QD). New thermobarometry from the north flank of Mt Everest reveals a gradient from 440°C at the QD down to samples recording peak conditions around 650°C, 5.5 kbar. The upper limit of leucogranite sheets forms an approximately isothermal surface at 600–650°C within the developing shear zone. The recrystallized grain size of quartz shows a systematic increase down-section in four transects. Profiles of deformation temperature reveal gradients of up to 200°C km −1 whose formation and preservation required a combination of processes: a shear zone active for a short period (≤18–15.5 Ma) at high strain rates, with a component of vertical shortening, and a contribution of latent heat from emplacement of sheeted granites. The likely horizontal displacement was >40 km, with up to 10 km of vertical exhumation.

Abstract This contribution re-examines textural relationships in metapelitic schist from five metamorphic zones separated by four isograds along the west flank of the Frenchman Cap dome in the southeastern Canadian Cordillera. There is a muscovite-out isograd formed by muscovite-dehydration melting in the kyanite-field. Two others, including one coinciding with the muscovite-out isograd, mark the appearance of sillimanite, which was produced along with biotite by garnet breakdown in the presence of melt. Two others mark the disappearance of kyanite: one formed during retrogression, the other coinciding with a shear zone (the Monashee décollement). Only the muscovite-out isograd, therefore, corresponds with the classical definition of an intersection of the topography with a prograde, isogradic-surface. This contribution exemplifies the need to use a descriptive definition of isograd as the trace of a surface across which a specific change in metamorphic mineralogy takes place. It is further proposed to specify their origin whenever possible with terms such as prograde-, retrograde- and structural-isograds. This revised terminology for isograds will improve our understanding of metamorphic terrains by helping to localize cryptic shear zones, gain a better understanding of the retrograde path and put firmer constraints on viable tectonic models.

Abstract The basement of the Alpine Upper Danubian/Balkan nappe, dismembered between Romania, Serbia and Bulgaria, contains evidence of the Variscan orogenic evolution (Lower Devonian Balkan–Carpathian ophiolite, Carboniferous granites). Our study presents a new tectono-metamorphic interpretation of this basement and documents two main deformation phases, D 1 and D 2 . D 1 is a right-lateral thrust recorded in the metagabbroic rocks at the base of the ophiolitic nappe that also affects the underlying units. This phase is related to the ophiolite emplacement on a northerly margin with a top-to-the-palaeo-WNW (Variscan coordinates) at c. 360 Ma. D 2 records a collisional event in a sinistral transpressive regime. In a zone of sheared folds, it juxtaposed low- (Eşelniţa metasediments) to high-grade metamorphic rocks (Corbu rocks: 600°C/ 5.2 kbar). Syntectonic granitic intrusions later heated these rocks locally before their final cooling, still during the D 2 phase, and with localized circulation of fluids. The sinistral transpressive regime would prevail after the docking of the Balkans and the Sredna Gora terranes separating the Balkan–Carpathian oceanic basin and could correspond to left-lateral escape due to large-scale readjustments between both terranes during the Carboniferous. The Upper Danubian/Balkan basement appears to be located more northerly in the Variscan Belt than the other intra-Alpine basements (Getic, Western Carpathians, Eastern Alps, Western Alps).

Abstract Precambrian high-pressure (HP) granulites provide important information for reconstructing ancient continental nuclei. Here we report granulites in Eastern Hebei, North China Craton (NCC). They experienced three metamorphic events related to Neoarchean–early Paleoproterozoic orogenesis. The garnet–clinopyroxene granulite defines the M 1 event at P–T conditions of 11–13 kbar and 780–830°C, while the two-pyroxene granulite was produced during the M 2 event at 7–9 kbar and 850–950°C. Both the garnet–clinopyroxene and two-pyroxene granulites experienced amphibolite retrogression during the M 3 event at 5–7 kbar and 710–730°C. Geochemical compositions of these granulites exhibit affinity to island-arc andesites. Zircon U–Pb dating shows that their magmatic precursors were erupted at c. 2538 Ma, and have experienced two-stage growth of zircon rims at c. 2458 and c. 2285 Ma, respectively. The c. 2458 Ma age may represent orogenic events during the amalgamation of micro-continental blocks of the eastern NCC, and the c. 2285 Ma age may be interpreted as the effect of late Paleoproterozoic magmatism. We suggest that the Neoarchean andesitic protoliths of the granulites were metamorphosed at HP granulite-facies conditions during collision of micro-continental blocks, and then exhumed to shallow levels. These early Paleoproterozoic HP granulites recorded the amalgamation of micro-continental blocks to reach the cratonization of the eastern NCC.

Abstract Along the Qinling–Sulu–Odesan collisional belt, the peak metamorphic conditions reflect systematically changing metamorphic regimes from east to west: ultra-high temperature (UHT) granulite-facies metamorphism in the Odesan area; high pressure (HP) eclogite-facies metamorphism in the Hongseong area; ultra-high pressure (UHP) eclogite-facies metamorphism in the Sulu, Dabie and Hong'an areas; HP eclogite-facies metamorphism in the Tonbai area; epidote–amphibolite- and blueschist facies metamorphism in the Wudang area; and HP granulite-facies metamorphism in the western Qinling area. Additionally, the Triassic post-collisional igneous rocks regionally intruded at both ends of the collision belt, in the west Qinling and Hongseong–Odesan areas. These observations indicate that the following collisional processes occurred along the Qinling–Dabie–Odesan collisional belt. The North China Craton and South China Craton first collided at their eastern margins (the Odesan area) with an angle of approximately 60° between their long axes. Consequently, the amount of oceanic slab subducted before the collision increased from the Odesan area to the Sulu area, resulting in an increase in the depth of slab break-off. After slab break-off occurred in the Sulu area, the lateral tearing force strengthened, decreasing the depth of slab break-off from the Sulu area to the western Qinling area as collision propagated westward.

Abstract In Earth evolution, mountain belts are the loci of crustal growth, reworking and recycling. These crustal-scale processes are unravelled through microscale investigations of textures and mineral assemblages of metamorphic rocks. Multiple episodes of metamorphism, re-equilibration and deformation, however, generally produce a complex and tightly interwoven pattern of microstructures and assemblages. Over the last two decades, the combination of advanced computing and technological capabilities with new concepts has provided a vast array of novel petrological tools and high-resolution/high-sensitivity techniques for microanalysis and imaging. Such novel approaches are proving fundamental to untangling the enigma represented by metamorphism with an unprecedented level of detail and confidence. As a result, the first decade and a half of this century has already seen the tumultuous development of new research avenues in metamorphic petrology. This book aims to provide a timely overview of the state of the art of this field, of newly developed petrological techniques, future advancements and significant new case studies.

Abstract The central European Bohemian Massif has undergone over two centuries of scientific investigation which has made it a pivotal area for the development and testing of modern geological theories. The discovery of melt inclusions in high-grade rocks, either crystallized as nanogranitoids or as glassy inclusions, prompted the re-evaluation of the area with an ‘inclusionist’ eye. Melt inclusions have been identified in a wide range of rocks, including felsic/perpotassic granulites, migmatites, eclogites and garnet clinopyroxenites, all the result of melting events albeit over a wide range of pressure/temperature conditions (800–1000°C/0.5–5 GPa). This contribution provides an overview of such inclusions and discusses the qualitative and quantitative constraints they provide for melting processes, and the nature of melts and fluids involved in these processes. In particular, data on trace-element signatures of melt inclusions trapped at mantle depths are presented and discussed. Moreover, experimental re-homogenization of nanogranitoids provided microstructural criteria allowing assessment of the conditions at which melt and host are mutually stable during melting. Overall this work aims to provide guidelines and suggestions for petrologists wishing to explore the fascinating field of melt inclusions in metamorphic terranes worldwide, based on the newest discoveries from the still-enigmatic Bohemian Massif.

Abstract Compositional mapping has greatly impacted mineralogical and petrological studies over the past half-century with increasing use of the electron probe micro-analyser. Many technical and analytical developments have benefited from the synergies of physicists and geologists and they have greatly contributed to the success of this analytical technique. Large-area compositional mapping has become routine practice in many laboratories worldwide, improving our ability to measure the compositional variability of minerals in natural geological samples and reducing the operator bias as to where to locate single spot analyses. This chapter aims to provide an overview of existing quantitative techniques for the evaluation of rock and mineral compositions and to present various examples of applications. A new advanced method for compositional map standardization that relies on internal standards and accurately corrects the X-ray intensities for continuum background is also presented. This technique has been implemented into the computer software XMapTools. The improved workflow defines the appropriate practice of accurate standardization and provides data-reporting standards to help improve petrological interpretations.

Abstract The magmatic processes occurring in the lowermost arc crust play a major role in the evolution of mantle-wedge-derived melt. Geological evidence indicates that mantle-derived magmas and in-situ products of lower crust partial melting are reacting in a pervasive melt system and are eventually extracted towards higher levels of the crust. Resolving the relative contribution of mantle-derived magma and partial melting products of pre-existing crust is essential to: (1) quantify crustal growth rate; (2) better understand the compositional range of arc magmatic series; and (3) constrain the chemical differentiation of the lower crust. In this study, we present STyx, a new modelling tool, coupling melt and heat flow with petrology to explore the dynamics of storage, transfer and hybridization of melts in complex liquid/rock systems. We perform three models representing a magmatic event affecting an amphibolitic lower arc crust in order to quantify the relative contribution between partial melting of the pre-existing crust and fractional crystallization from mantle-derived hydrous-magma. Our models demonstrate that most of the differentiated arc crust is juvenile, deriving from the differentiation of mantle melts, and that pre-existing crust does not significantly contribute to the total thickness of magmatic products.