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.

Abstract Pelitic gneisses and schists from the Garevka Complex (Yenisey Ridge) at the western margin of the Siberian craton contain zoned garnet porphyroblasts that show clear evidence of multistage growth. The three discrete stages define a counter-clockwise pressure–temperature ( P–T ) path involving initial prograde low pressure heating followed by near-isothermal medium-pressure compression and post-peak retrograde synexhumation decompression and cooling. The combined study of the variations of in-situ major and trace element mineral compositions of coexisting minerals and mineral modes with changing P and T conditions and metamorphic reactions in rocks allowed detailed investigation of metamorphic processes. A significant increase in pressure during prograde collision-related metamorphism correlates with an abrupt increase of Ca in garnets, which in turn is accompanied by a significant decrease in heavy rare earth elements (HREE) and Y contents. Decrease in temperature and pressure during retrograde metamorphism led to an increase in the HREE and Y content in garnets with concomitant decrease of grossular component of garnet. A pronounced systematic negative correlation between HREE including Y content and Ca content in garnet can be attributed to substitution of trivalent rare earth elements (REE) and Y for divalent Ca cations on eightfold sites. The main reasons for the sharp increase in Ca content in garnets during collisional metamorphism are either (1) the redistribution between garnet and plagioclase, which led to less calcium in composition, or (2) consequence of epidote breakdown. A mass balance of major and trace elements between the reactants and products of metamorphic reactions yield very good matches between the measured and reconstructed modal abundances in all cases, indicating that collisional metamorphism was essentially isochemical with respect to most elements, with the exception of HREE. The minimum equilibration volume for the major elements was c. 1 mm 3 . The total HREE balance requires a rather greater reaction volume involved in the redistribution of HREEs, of the order of 3–8 mm 3 which provides evidence for their relatively higher migration mobility during metamorphism in comparison with other REE.

Abstract Experimental investigations on metamorphic processes can be viewed as an alternative forward modelling approach of the pressure–temperature–composition ( P–T–X ) evolution of a rock. To obtain results as close as possible to natural observations one can use natural rocks as starting materials. The disadvantage of this method is the complex chemical compositions of the rocks and therefore these whole-rock experiments need to be evaluated not only (1) in terms of their ability to reproduce the natural observations but also (2) in their ability to reproduce theoretical calculations. In this contribution high- T low- P experiments (550–780°C and 0.15–0.6 GPa) simulating contact metamorphism of metapelites at the rims of the Permian Brixen Granite and Klausen Diorite are evaluated with respect to the points discussed above. The agreement between the experimental results and the observed mineral assemblages and mineral compositions (plagioclase, biotite) from the contact aureoles is very good. Thermodynamic testing of the experiments showed, however, a variable match between observed and calculated assemblages, ranging from satisfactory to rather poor. Finally, observed and calculated mineral compositions showed a very poor match. Overall there is good agreement between the experiments and the natural observations, but theoretical calculations are still hampered by the complex nature of the starting materials.

Abstract Size distribution and evolution of framboidal and euhedral microscopic crystals of pyrite (micropyrites, MPy) have been used for the last thirty years to deduce palaeo-redox conditions. The analysis of the MPy distributions can give valuable information about these palaeo-redox conditions. However, other information can also be retrieved from this type of analysis. In this work, we propose that the formation of new populations of MPy is a proxy of the transition from the anchizone to the epizone. High-resolution X-ray tomography (micro-CT) was used to determine the size distributions of MPy hosted in pelitic rocks subjected to different grades of low temperature metamorphism. These data were filtered and statistically analysed, which allowed us to find a statistical representative size distribution of the MPy present in the samples. The metamorphic grade was determined using the Kübler Index in combination with petrological and scanning electron microscopy (SEM) examination. The results show a relationship between metamorphic grade and MPy size distributions, and that new populations of MPy formed due to the effects of metamorphism. This new methodology for MPy size distribution has different potential applications in some fields of Earth sciences, such as palaeoenvironment reconstruction, ore mining or metamorphic petrology.

Abstract A key aim of modern metamorphic geochronology is to constrain precise and accurate rates and timescales of tectonic processes. One promising approach in amphibolite and granulite-facies rocks links the geochronological information recorded in zoned accessory phases such as monazite to the pressure–temperature information recorded in zoned major rock-forming minerals such as garnet. Both phases incorporate rare earth elements (REE) as they crystallize and their equilibrium partitioning behaviour potentially provides a useful way of linking time to temperature. We report REE data from sub-solidus amphibolite-facies metapelites from Bhutan, where overlapping ages, inclusion relationships and Gd/Lu ratios suggest that garnet and monazite co-crystallized. The garnet–monazite REE relationships in these samples show a steeper pattern across the heavy (H)REE than previously reported. The difference between our dataset and the previously reported data may be due to a temperature-dependence on the partition coefficients, disequilibrium in either dataset, differences in monazite chemistry or the presence or absence of a third phase that competed for the available REE during growth. We urge caution against using empirically-derived partition coefficients from natural samples as evidence for, or against, equilibrium of REE-bearing phases until monazite–garnet partitioning behaviour is better constrained.

Abstract The island of Seram, eastern Indonesia, incorporates Miocene ultrahigh-temperature (UHT; >900°C) garnet–sillimanite granulites that formed by extensional exhumation of hot mantle rocks behind the rolling-back Banda Arc. UHT metamorphic conditions are supported by new Zr-in-rutile thermometry results and the Miocene age of the UHT event is confirmed by closely-matched heavy rare earth element (HREE) abundances between garnet and c. 16 Ma zircon. Monazites also record identical U–Pb ages, within uncertainty. However, these geochronometers do not date peak UHT metamorphism; instead, they date retrograde, garnet-consuming (Zr- and rare earth element (REE)-liberating) reactions that produced the granulites’ post-peak cordierite + spinel reaction microstructures. Zircons shielded within garnet did not crystallize c. 16 Ma rims and so were unaffected by the entire UHT event. Miocene UHT metamorphism overprinted a Late Triassic–Early Jurassic upper-amphibolite facies event that grew garnet cores and 216–173 Ma zircon. In the Miocene, these garnet cores were overgrown by peritectic garnet rims during UHT metamorphism, with some rutiles recording c. 900°C Zr-in-rutile temperatures. Garnet Lu–Hf ages of 138 Ma – produced by core–rim mixing – demonstrate that a component of Hf 4+ produced since c. 200 Ma was retained through the c. 16 Ma UHT event. Accordingly, UHT conditions must have been very short-lived and exhumation of the granulite complex very rapid.

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.