Metamorphic geology: progress and perspectives

Pierre Lanari, Silvio Ferrero, Philippe Goncalves, Eugene Grosch

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Pierre Lanari, Silvio Ferrero, Philippe Goncalves, Eugene Grosch. Metamorphic geology: progress and perspectives. The Geological Society, London, Special Publications, 2019, 478 (1), pp.1-12. 10.1144/SP478-2018-186 . hal-03009756

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Distributed under a Creative Commons Attribution 4.0 International License
Metamorphic geology: progress and perspectives

PIERRE LANARI¹*, SILVIO FERRERO²,³, PHILIPPE GONCALVES⁴ &
EUGENE G. GROSCH⁵

¹Institute of Geological Sciences, University of Bern, Baltzestrasse
1 + 3, 3012 Bern, Switzerland

²Institut für Erd- und Umweltwissenschaften, Universität Potsdam,
14476 Potsdam, Germany

³Museum für Naturkunde (MfN), Leibniz-Institut für Evolutions-
und Biodiversitätsforschung, 10115 Berlin, Germany

⁴Laboratoire Chrono-environnement, Université de Bourgogne-Franche-Comté,
16 route de Gray, 25030 Besançon, France

⁵Geology Department, Rhodes University, Grahamstown, Eastern Cape,
South Africa

<figure><img src="image_2.png" /></figure>

P.L., 0000-0001-8303-0771; P.G., 0000-0001-8069-7976

*Correspondence: pierre.lanari@geo.unibe.ch

<figure><img src="image_3.png" /></figure>

Our dynamic planet Earth has been in constant evolu-
tion since its accretion and formation about 4.56 bil-
lion years ago as a rocky planet in our solar system.
Since the onset of plate tectonic processes, migration
and collision of rigid tectonic plates, driven by mantle
convection, have promoted intense formation and
reworking of continental and oceanic crust. Oroogenic
belts, forming along convergent plate boundaries, are
unique natural laboratories for Earth scientists, as
they are the loci of interactions between tectonic,
magmatic and metamorphic processes. As metamor-
phic geologists, our main goal is to develop and apply
reliable methods to investigate the textural and min-
eralogical metamorphic rock records.

By definition, a metamorphic rock is the product
of the transformation of a protolith that can either
be sedimentary, magmatic or metamorphic due to
changes in physical and chemical conditions. There-
fore, a metamorphic rock may contain a wealth of
information related to its protolith and/or the preser-
vation of disparate rock records from several partial
re-equilibration stages of a single or multiple meta-
morphic cycles. Because of this complexity, only
snapshots of the cumulative processes are preserved
in the mineral assemblages, chemistry and micro-
structures. It is thus essential to study rock samples
that best preserve this fragmented metamorphic
record, and develop the best petrological tools
for analysing them. This naturalist and analytical
approach is supplemented by the input of experimen-
talists and modellers to quantify the physical and

chemical conditions of metamorphism and test
conceptual models.

This Special Publication reflects the variety of
novel techniques and approaches used to investigate
metamorphic processes. It encompasses a wide range
of metamorphic topics that include state-of-the-art
methods on deciphering the complex polyphase
nature of metamorphic geology. In this introduc-
tion, we have tried to describe each chapter in a
larger context, including for instance the evolution
of analytical and modelling techniques, new trends
in metamorphic petrology, and some brief comments
on future directions in the field.

Metamorphic geology: an evolving
discipline

The present book finds its predecessor in *Evolution of Metamorphic Belts* (Daly *et al.* 1989). Reading reveals some interesting aspects on the evolution of metamorphic geology in the past 30 years. Two main aspects are discussed below: the evolution of the petrological models and an incredible increase in the amount of geochemical data.

Thermobarometry: petrological modelling of
phase relationships

The chapters in Daly *et al.* (1989) describe the
modelling techniques that were used at that time to

From: FERRERO, S., LANARI, P., GONCALVES, P. & GROSCH, E. G. (eds) 2019. *Metamorphic Geology: Microscale to Mountain Belts*. Geological Society, London, Special Publications, **478**, 1–12.

First published online January 29, 2019, https://doi.org/10.1144/SP478-2018-186

© 2019 The Author(s). This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/). Published by The Geological Society of London.

Publishing disclaimer: www.geolsoc.org.uk/pub_ethics
decipher pressure-temperature-time (*P*–*T*–*t*) histo-
ries of metamorphic rocks and metamorphic belts.
Inverse modelling in the form of exchange thermom-
etry and net-transfer equilibria has been extensively
used since the late 1960s, although many of them
were either based on inaccurate thermodynamic
data or applied outside their validity range (Essene
1989). The widely accepted practice of estimating
*P*–*T* conditions from separate reactions was gener-
ally leading to errors and misinterpretation of *P*–*T*
paths, especially if post-peak modifications via
retrograde reactions are involved (e.g. Aranovich
& Podlesskii 1989). Forward modelling of phase
relationships via petrogenetic grids was restricted
to a few simple chemical systems and, as noticed
by Essene (1989, p. 12):

It [was] generally [considered to be] safer to select an
experimentally reversed univariant equilibrium in a
chemical subsystem that is buffered by mineral assem-
blages in the rock under study [...] than to rely quanti-
tatively on complex petrogenetic grids.

At the same time, Spear (1989) was introducing the
concept of relative thermobarometry – based on dif-
ferential forms of the equilibrium constants, a variant
of the Gibbs method (Spear 1988). This original
approach, if based on an internally consistent data-
base (e.g. Berman 1988; Holland & Powell 1988,
1998, 2011), permits the production of isopleth maps
of mineral compositions, moles and equilibrium
constants for a given bulk-rock composition, as well
as forward models along P–T trajectories predicting
the growth zoning of metamorphic minerals (see
Spear 1989, fig. 3c). Since that time, isopleth ther-
mobarometry has become a powerful aid in the inter-
pretation of complex reaction histories and mineral
zoning (Evans 2004; Tinkham & Ghent 2005; Gai-
dies et al. 2008; Moynihan & Pattison 2013; Lanari
et al. 2017). This major development is being used
routinely in modern petrological studies as isopleth
maps can be readily obtained within a couple of min-
utes (or hours) for simple (and complex) rock sys-
tems and any thermodynamic database using a
Gibbs energy minimizer such as Theriak-Domino
(de Capitani & Brown 1987; de Capitani & Petraka-
kis 2010) or Perple_X (Connolly & Kerrick 1987;
Connolly 2005), or using a phase-equilibrium calcu-
lator such as Thermocalc (Powell & Holland 1988,
2008) or gibbs (Spear 1988). In his paper, Tropper
(2018) uses an unconventional method to determine
the accuracy of the phase diagrams. Natural rocks
were used as the starting material in experiments
to forward model the stable mineral assemblage,
mineral modes and compositions at given pressure
and temperature conditions, with rather satisfacto-
rily results. Despite only partial equilibration being
achieved in the experimental products, as illustrated
by the presence of unreacted relicts (garnet, quartz),

the mineral assemblages and a few key isopleths,
such as the Na and Ca in plagioclase, Ti in biotite
or X<sub>Mg</sub> in cordierite observed in metapelites from
the South Alpine domain (northern South Tyrol,
Italy), are reasonably reproduced in the experiments.
This is not the case for the isopleths obtained using
thermodynamic models based on the bulk-rock com-
positions, suggesting that kinetics and/or fraction-
ation effects may play a significant role (Rubie
1998; Pattison et al. 2011; Carlson et al. 2015;
Lanari & Engi 2017; Spear & Pattison 2017; Lanari
& Duesterhoeft in press).

Geochemical data: from small to
large datasets

Another important change in metamorphic studies is
related to the amount and quality of geochemical
data available in present-day research. The number
of analytical instruments (scanning electron micro-
copy (SEM), electron probe microanalyser (EPMA),
Raman spectroscopy, laser ablation inductively cou-
pled plasma mass spectrometry (LA-ICP-MS), sec-
ondary ion mass spectrometry (SIMS/SHRIMP),
Fourier transform infrared spectroscopy (FTIR),
etc.) and their increasing precision fundamentally
changed the way petrologists and geochronologists
deal with the available geochemical information to
investigate rocks and their metamorphic history.
We estimate that the total number of EPMA analyses
used in the 55 chapters of the book from Daly *et al.*
(1989) to be around 2000, among which only Burton
*et al.* (1989) reported core to rim compositional zon-
ing profiles of two garnet grains. Between 40 and 50
Ar–Ar or K–Ar spectra, laser and spot analyses by
SIMS were reported in the entire book. Today, the
situation has changed dramatically in that much
larger datasets are used to support models and inter-
pretations. Many petrological studies include, for
instance, core to rim compositional zoning profiles
combined with semi-quantitative X-ray maps, pro-
viding a more detailed investigation of the composi-
tional variability of minerals. Nowadays, X-ray
maps can be quantified (calibrated like single-spot
analysis), allowing the compositional variability of
mineral phases to be analysed continuously in two
dimensions. In their paper, Lanari *et al.* (2018)
show how quantitative compositional mapping has
impacted mineralogical and petrological studies
over the past half-century. The authors present a
new calibration technique for electron probe micro-
analyser (EPMA) X-ray maps, which has been incor-
porated into the computer software XMapTools
(Lanari *et al.* 2014). Such data visualization tools
are important as a set of maps typically contains
between 200 000 and 1 000 000 spatially resolved
fully quantitative chemical analyses. This paper by
Lanari *et al.* (2018) illustrates how technological
evolution has caused an exponential increase in the
amount of data available and also shows how impor-
tant the tools facilitating data processing and data
handling are. It is not only the quantity of data that
is important, but also the quality of the reporting
and the analysis.

Are more high-resolution data improving our geological understanding of metamorphic rock evolution? Of course, it depends on the quality of the data and our capability to interpret this geological record. Two papers illustrate how large geochemical datasets are used to decipher the metamorphic evolution. The first example is presented in the article by Martin (2018), who uses quantitative compositional X-ray mapping to investigate the compositional variability of symplectite assemblages in eclogite from the Western Gneiss Region of Norway. This large geochemical dataset (corresponding to several million point analyses), used in conjunction with multi-equilibrium models, enabled the P–T conditions of symplectite formation to be unravelled and the influence of external fluid on the progressive retrogression of mafic eclogite to be discussed. A critical aspect concerns the evolution of the chemical composition of amphibole in each microstructural domain of a suite of samples showing different degrees of retrogression. The reconstruction of the crystallization sequence shows that the presence of a corona around garnet (kelyphite in Martin 2018) is indicative of interactions with external fluids during retrogression. The second example is presented in the paper by Hyppolito et al. (2018), who combine quantitative major element mapping by EPMA and trace element mapping by LA-ICP-MS to retrieve garnet growth and resorption pulses in amphibolitized eclogites from the Diego de Almagro Metamorphic Complex (Chile). These highly resolved microchemical investigations have led to a relatively detailed model for the partial re-equilibration of these eclogites under amphibolite-facies conditions, interpreted as a process that is enhanced by locally derived aqueous fluids and mainly controlled by short-scale dissolution-transport-precipitation. The study of Hyppolito et al. (2018) is a state-of-the-art example on how a large amount of data may contribute in a powerful way to our conceptual models; in this case, re-equilibration processes.

New strategies and trends in metamorphic
geology

In the Special Publication *The Earth Inside and Out: Some Major Contributions to Geology in the Twentieth Century* (Oldroyd 2002), Touret & Nijland (2002) defined three major trends in metamorphic petrology: (i) *mineralogical petrology* concerned with the determination of *P–T* conditions of

metamorphic minerals; (ii) *chemical (or geochemical) petrology* to determine the time factor; and (iii) *structural petrology* for the reconstruction of the kinematics of the rock evolution and the investigation of global tectonics. Although these trends are still valid, the papers compiled in this Special Publication demonstrate that some boundaries have shifted and new trends have emerged. The development and evolution of new analytical techniques, applied at high resolution, have triggered the imagination of Earth scientists and have impacted the way they practice metamorphic petrology and geochronology.

*Petrochronology: the full integration of*
*geochronology, geochemistry and*
*metamorphic petrology*

What does the age of a metamorphic rock mean? Could one say that a metamorphic rock has a single age? These questions might have been rather odd in the 1980s but the last couple of decades have seen the emergence of new high-resolution and *in situ* analytical and theoretical techniques to link ages with temperature, pressure and/or mineral reactions, and with petrological evolution (Kohn 2016; Kohn et al. 2017). This integrative approach referred to as 'petrochronology' focuses on the rate of the processes rather than on the determination of absolute ages (Engi et al. 2017).

A key aspect of petrochronology is to apply geo-
chemical correlation methods that link time (ages
and rates) obtained from accessory minerals (such
as monazite, zircon or rutile) to petrogenetic pro-
cesses (e.g. crystallization or re-equilibration) and
their conditions determined from major rock-
forming mineral assemblages largely involving gar-
net (Hermann & Rubatto 2003; Baxter et al. 2017).
If the partitioning relationships between the two
phases are known for diagnostic trace elements, it
is possible to test whether two specific growth
zones co-existed in equilibrium (or not) at any
stage of the *P*–*T* history. In their paper, Warren
et al. (2018) investigate monazite and garnet in six
amphibolite-facies metapelites from Bhutan, and
propose that the core of the matrix monazite may
have formed in equilibrium with the rim of garnet.
The obtained rare earth element (REE) partitioning
coefficients between monazite and garnet differ
partly from previous studies, up to one order of mag-
nitude lower for the heavy (H)REE (see Warren
et al. 2018, fig. 9). These results show that several
underestimated factors, such as the presence of
melt or different stable mineral assemblages, may
influence garnet–monazite partitioning behaviour.
Caution is therefore urged in applying garnet–
monazite REE trends to ‘prove’ equilibrium when
linking monazite ages to metamorphic stages.
Mineral chemical correlation is critical to en-
sure that two minerals, a chronometer and a geother-
mobarometer, formed in equilibrium – the textural
correlation methods alone can lead to erroneous
interpretations. In the paper by Pownall et al.
(2018), the authors investigate several major and
accessory minerals of garnet–silimanite granulites
exposed in the island of Seram (eastern Indonesia).
The large variety of analytical and theoretical tech-
niques employed have led to a fairly precise meta-
morphic history of these polycyclic rocks, which
were first deposited in the Late Triassic, metamor-
phosed under amphibolite-facies conditions at c.
200 Ma and, finally, involved in the ultrahigh-
pressure (UHP) event at 16 Ma caused by exten-
sional exhumation of hot mantle rocks behind the
rolling-back Banda Arc. A major finding of this
study is that zircon shielded within garnet was not
affected by the ultrahigh-temperature (UHT) epi-
sode. The age distribution between peak zircon and
retrograde monazite suggests that UHT conditions
have been very short lived and exhumation of the
granulite complex was very rapid.

*Combining physical and chemical modelling*
*at all scales*

In the mid-1980s, the discovery of coesite inclusions
in garnet porphyroblasts of continental rocks (Cho-
pin 1984; Smith 1984) demonstrated that the buoy-
ant continental crust can be subducted to mantle
depths. The P–T–t paths retrieved by the petrochro-
nological studies have challenged modellers who
tried to reproduce these trajectories in dynamic ther-
momechanical models. The remaining differences
may reflect the neglect of key processes such as
shear heating, the influence of phase transitions on
rock properties or the evolution and migration of flu-
ids (see Engi et al. 2017 and references therein).
Interestingly, these processes can be partly quanti-
fied by chemical modelling based on equilibrium
thermodynamics (Connolly 2005, 2009; Duester-
hoeft et al. 2014). This integration of physical and
chemical processes has been a hot topic in the last
decade (Fullea et al. 2009; Siret et al. 2009; Zunino
et al. 2011; Duesterhoeft & de Capitani 2013). In
their paper, Riel et al. (2018) present a modelling
tool, STyx, coupling melt (two-phase) flow, heat
flow and petrological calculations, and which aims
to link physical and chemical processes in partially
molten crust. Several models representing a mag-
matic event affecting an amphibolitic lower arc
crust were produced to quantify the relative contribu-
tion between partial melting of the pre-existing crust
and fractional crystallization from mantle-derived
hydrous magma. This study shows that mantle-
derived magmas are the main contributor to the
extracted products, whereas the effects of partial

melting in the pre-existing crust are rather limited.
This tool opens up new directions of research in
which to model the effects of complex fluid–rock
interactions at high-grade conditions.

Melt inclusions

An illustrative example of the kind of multidisciplin-
ary approach required to unravel the complexity of
natural metamorphic samples is provided in the
paper by *Ferrero et al.* (2018), who investigate melt-
ing processes via the study of melt inclusions. These
ancient droplets of melt are commonly preserved in
a major metamorphic mineral – garnet – as crystal-
lized aggregates of quartz + feldspar(s) + OH-bearing
phase(s) known as nanogranitoids (*Ferrero et al.*
2012; *Cesare et al.* 2015; *Ferrero & Angel* 2018).
The presence of melt inclusions provides qualitative
constraints, such as the presence of a melt phase dur-
ing metamorphism, as well as quantitative constraints
for the melt composition in both major and trace ele-
ments. In this paper, *Ferrero et al.* (2018) have com-
piled results from the literature and, in conjunction
with new data and observations, show that the pres-
ence of melt inclusions is more the rule than the
exception in high-grade metamorphic rocks from
the Bohemian Massif. Some inclusions were linked
to the UHP stage, demonstrating that these rocks
experienced partial melting during the early stage
of exhumation that followed continental subduction.
In addition, the experimental re-homogenization of
nanogranitoids provides microstructural criteria that
allow the conditions at which melt and host are mutu-
ally stable to be assessed.

*Exploring the lower limit of metamorphism*

The metamorphic phase transformations and the pro-
cesses driving these changes remain poorly under-
stood at low-grade conditions (<350°C) compared
to metamorphism at higher P–T conditions. One rea-
son for this lack of knowledge is that most metamor-
phic petrologists have traditionally investigated
changes taking place at high P–T conditions, where
equilibrium thermodynamics can be applied with a
higher level of confidence (Robinson & Merriman
2009). Investigating low-grade metamorphism is
challenging, requiring alternative strategies and
complex petrological models (Schiffman & Day
2009; Lanari et al. 2012; Cantarero et al. 2014,
2018; Scheffer et al. 2016; Vidal et al. 2016). In
their paper, Cárdenes et al. (2018) explore uncon-
ventional proxy methods to quantify the degree of
metamorphism in low-grade samples. The authors
have investigated the size distribution of framboidal
and euhedral microscopic crystals of pyrite in meta-
pelites that experienced different grades of low-
temperature metamorphism. The size distribution
was obtained using high-resolution X-ray tomogra-
phy and compared with the results of traditional ther-
mometers. Whereas new generations of pyrite form
along with increasing grade of metamorphism, the
maximum size of the crystals also increases. This
study demonstrates how complex the investigation
of samples is at the transition between diagenesis
and low-grade metamorphism.

The development of sophisticated techniques for the investigation of low-grade metamorphism opened up new avenues. In the paper by Grosch (2018), the author has combined a wide range of petrological and modelling techniques to unravel the metamorphic history of Archean rocks from the Barberton Greenstone Belt, recording very low-grade to upper-greenschist facies conditions. Such studies aim to identify evidence of metamorphism and metamorphism, processes providing critical information for assessing the tectonic settings, and crustal hydrothermal environments on the early Earth (e.g. Grosch et al. 2012). The author combined empirical chlorite thermometry with inverse thermodynamic models of chlorite and white mica, in conjunction with Raman thermometry on carbonaceous material and oxygen stable isotope analysis. Seawater hydrothermal alteration zones were successfully distinguished from tectonic and contact metamorphic processes. An important finding of this work is that an early form of plate tectonics was already active c. 3225 myr ago.

*The role of metamorphic fluids in the formation
of ore deposits*

Many ore deposits are hosted by metamorphic rocks or involve metamorphic fluids caused by the breakdown of volatile-bearing minerals. Metamorphic brines and fluids have a high ore-forming potential as they can carry a significant amount of metal in solution (Yardley & Cleverley 2013). In their paper, Ni et al. (2018) investigate a large Pb–Zn deposit in the West Qinling Orogen in central China that was previously classified as having a sedimentary or hydrothermal origin (SEDEX for sedimentary exhalative). This type of deposit is among the world’s most important source of Pb and Zn. Combining fluid-inclusion petrology, microthermometry, Raman and Pb isotope analysis, the authors showed that the Changba–Lijiagou ore deposit results from a multi-stage process: an early primary marine sedimentary mineralization and a late stage of metamorphic superimposition. Primary fluid inclusions in quartz were eliminated during the late Triassic episode of regional metamorphism and replaced by CO₂–CH₄-rich inclusions, which are symptomatic of a metamorphic fluid.

In the paper by Abu-Alam et al. (2018), the
authors demonstrate the link between active tecto-
nism and ore-carrying fluids. In their study, they

have performed a detailed investigation of orogenic
gold ores of the Arabian–Nubian Shield that are
linked to a large fault system. The analysis of fluid
inclusions from four ore deposits indicates that
the gold was precipitated from metamorphic fluids
at shallow- to medium-crustal levels. Petrological
modelling was applied to model the amount
and composition of fluid released by the metamorphic
rocks (meta-ophiolites and metasediments)
during prograde metamorphism. The largest amount
of fluid production is predicted to occur at the
greenschist–amphibolite-facies transition, allowing
effective gold mobilization from the deeply seated
host ophiolite in the host rock, and vertical transport
via the brittle fault system.

From mineral scale to tectonic processes

Research into the orogenic processes that shaped the
continental crust has a long-standing tradition in
metamorphic geology. Metamorphic data are critical
in this endeavour as they can be used to quantify tec-
tonic and geodynamic processes: their potential
application in tectonic and geodynamic reconstruc-
tions is clearly illustrated throughout the whole book.

In the paper by Waters et al. (2018), microscale
observations are used to characterize large-scale tec-
tonics. Here, Waters et al. (2018) have integrated
microscale observations with petrological and field
structural data to constrain the thermal structure
and large-scale behaviour of the South Tibetan
Detachment, a major shear zone in the Himalayan
belt. This study uses a series of samples collected
by L.R. Wager in 1933 during the British expedition
to Mt Everest. The authors characterize the evolution
of the metamorphism and microstructures along four
new profiles, each of them over a north–south dis-
tance of 35 km across the shear zone. Mineral assem-
blage and thermobarometry, as well as quartz
deformation regimes, determined through recrystal-
lization microstructures of quartz both suggest high
thermal gradients within the shear zone of up to
200°C km⁻¹. The authors explain the formation
and preservation of these high gradients by a combi-
nation of high strain rates with a component of
vertical shortening over a short period of time
(<18 myr) and a contribution of latent heat from
the emplacement of magmatic bodies. Interestingly,
viscous heating can generate anomalies of the
order of 12°C km⁻¹ in the shear zone, and is there-
fore not capable of generating the steep gradients
captured by the rock record.

Metamorphic isograds have, since the seminal
work of Barrow (1893) on the Scottish Highlands,
been a crucial concept that have helped generations
of metamorphic petrologists to decode the history
of metamorphic terranes. In his paper, Gervais
(2018) has revisited the isograd concept by investi-
gating the textural relationships in metapelites from
the west flank of the Frenchman Cap dome in the
Canadian Cordillera. In this area, four metamorphic
isograds (kyanite-in, sillimanite-in, muscovite-out
and kyanite-out), each of them corresponding to a
line on the map across which there is a change in
metamorphic mineralogy, were mapped along a dis-
tance of c. 5 km. The author shows that three main
modes of formation can be proposed for these iso-
grads: the prograde isograd (e.g. muscovite-out and
kyanite-in) corresponds to a prograde net-transfer
metamorphic reaction; the retrograde isograd (e.g.
sillimanite-in) is related to a new mineral forming
under exhumation and cooling; and the structural
isograd (e.g. kyanite-out) is marked by a structural
discontinuity. In this example, the four isograds
formed at different times via different petrological
and/or tectonic processes, thus providing a power-
ful caveat for petrologists: the main conclusion of
the author is that interpreting isograds according
to the classical definition – without a descriptive
definition – can lead to problematic and unrealistic
tectonic models.

One of the main tasks of metamorphic geology is to reconstruct detailed pressure-temperature-deformation paths. In their paper, *Plissart et al. (2018)* combine a detailed petrostructural study of the Alpine Upper Danubian-Balkan basement to unravel the fragmentary Variscan history of this region spreading across Romania, Serbia and Bulgaria. Two main deformation phases were documented and interpreted as reflecting two different tectonic regimes: a right-lateral-dominated shear thrust in the eastern part of the region linked to the obduction of an ophiolitic slice (D₁); and sinistral transpression recording the collisional event (D₂). One unit, the Corbu meta-sediment, was selected to forward model garnet growth and the corresponding evolution of the reactive bulk composition. Inverse thermometry allowed the *P*-*T* conditions of the retrograde conditions to be constrained. The detailed pressure-temperature-deformation (*P*-*T*-*D*) path shows that the D₂ phase includes a stage of prograde garnet growth, up to the peak conditions of 600°C and 5.2 kbar, as well as a re-heating event that took place at shallower levels and was likely to have been caused by Carboniferous syntectonic intrusions.

The Earth is a continuously evolving system, and
especially in Precambrian rocks. In their paper,
Wang *et al.* (2018) report on a new occurrence of
Neoarchean–early Paleoproterozoic high-pressure
(HP) granulites in Eastern Hebei, North China Cra-
ton. High-pressure granulites are important markers
for tectonic secular changes, as they require burial
either via crustal thickening and/or subduction.
The authors investigated garnet–clinopyroxene and
two-pyroxene granulites occurring as supra-crustal

lenses/slivers within the tonalite-trondhjemite-
granodiorite gneisses by combining petrography,
mineral chemistry, thermobarometric calculations,
bulk-rock elemental and Nd isotopic geochemistry
with *in situ* zircon U–Pb dating by ion probe. They
demonstrated that the magmatic protolith of the HP
granulites are island-arc andesites erupted during
the Neoarchean. Such magmatism coupled with col-
lisional metamorphism is analogous to modern plate
tectonic settings.

One of the next challenges for the geodynamists is the analysis of large metamorphic datasets. Integrating the large variety of petrochronological data into consistent tectonic and geodynamic models can strengthen our understanding of the formation of metamorphic belts. In the paper by *Oh & Lee* (2018), the authors have tracked the systematic changes in the metamorphic record and the distribution of the post-collisional igneous rocks along the Permo-Triassic Qinling–Sulu–Odesan belt located between the North China Craton and South China Craton. The authors have interpreted the variation in the peak-pressure conditions along the belt as being a result of the geometry of the continental margin. The amount of subducted oceanic slab increases westwards, resulting in lower peak-pressure conditions for continental subduction and a decrease in the depth at which slab break-off occurs. This study demonstrates that the geometry of the continental margin entering subduction strongly dictates the *P*-*T*-*t* trajectories followed by the subducted rocks. Similar conclusions were reached based on the petrochronological record of HP–UHP rocks for other metamorphic belts such as the Western Gneiss Region of Norway or the Himalayas (Hacker *et al.* 2010; Lanari *et al.* 2013).

Open questions and evolving perspectives

It is remarkable to see how some basic problems have
remained at the core of metamorphic petrology. In
the Special Publication entitled *What Drives Meta-
morphism and Metamorphic Reactions?* (Treloar &
O’Brien 1998a), Treloar & O’Brien (1998b, p. 4)
noted:

[T]hat all of our major findings of the last 25 or so years,
albeit based on analytical data and integrated into the
modern Plate Tectonic paradigm, do not necessarily
tell us more about ‘What Controls Metamorphism’
than our predecessors would have concluded.

This conclusion is, to some extent, still valid today
but our Special Publication also emphasizes the evo-
lution of our understanding of some critical meta-
morphic processes in the last couple of decades.
New ideas have been explored, notably to investigate
the role of seismic activity in initiating and facilitat-
ing metamorphism (e.g. Putnis et al. 2017 and
references therein) or the importance of aqueous
solutions in nanopores in driving metamorphism
(Plümper et al. 2017). In the following, a few open
questions that we think are important are briefly
discussed.

The increasing role of fluids

If fluid is not present, element transfer is very limited
in metamorphic rocks (e.g. Carlson 2010); being
insufficient in most cases to form new minerals
that are predicted to be stable by equilibrium thermo-
dynamics (Rubie 1998). How are the reactive fluids
transferred away from the fractures and veins? Is
microcracking and grain-boundary dilation the
main process, as suggested by Barker & Zhang
(1998) in a previous Special Publication (Treloar &
O'Brien 1998a), or is this transport more often per-
vasive through the entire rock (Putnis & John
2010)? Clearly, geochemical evidence can be used
to identify micro-chemical and microstructural evi-
dence of fluid-mineral reactions and to quantify
the extent of re-equilibration (Airaghi et al. 2017;
Hyppolito et al. 2018; Lanari et al. 2018). Then
the question of what initiates, drives and stops
re-equilibration can be addressed in a different way.

Element transfer during prograde
metamorphism

Is metamorphism largely isochemical as supposed in
most of the petrological models based on bulk-rock
compositions or does element transport via aqueous
fluids matter? This question remains largely
unsolved and divides the community. In the paper
by Likhанов (2018), the author investigated several
metapelite samples from the Garevka Complex in the
Yenisey Ridge (Siberia, Russia). Detailed chemical
analysis of garnet was combined with inverse ther-
mobarometry to reconstruct multiple stages of garnet
growth and to unravel the associated reaction
sequence. The mass balance of these mineral reac-
tions was then compared to the observed mineral
modes to test if the rock behaved as a closed or
open system for element exchanges. The author con-
cludes that prograde metamorphism was, in this
example, largely isochemical for the major elements,
in contrast to trace elements (especially HREE) that
behave in a more complex fashion.

Hydration and stress generation

What is the structural and chemical evidence for mass
and/or volume loss in metamorphic rocks? This
question, largely inspired by Vernon (1998), is still
a matter of debate today. An excellent illustration
of the problem is presented in the paper by Centrella
(2018). In the Lindås Nappe of the Bergen Arcs

(Norway), Neoproterozoic granulites were partially
transformed into eclogite and amphibolite during
the Caledonian Orogeny. The spatial relationships
between granulite, amphibolite and eclogite are com-
plex, preventing a relative sequence to be estab-
lished. Petrochronology does not help much in this
case as the ages of the eclogite and amphibolite stages
are coeval within uncertainty. The isochemical trans-
formations of granulite into amphibolite and eclogite
caused major density changes. The decrease in den-
sity from granulite to amphibolite requires either an
increase in volume, assuming a closed system, or a
significant loss of mass, assuming open-system
behaviour. Several lines of evidence reported by
Centrella (2018) point towards an unexpected sce-
nario. In a closed system, deviatoric stress is gener-
ated by the hydration of the granulite and this may
influence the local mineral assemblage that would
record higher pressures. These bring an additional
piece to the puzzle of possible grain-scale pressure
variations recorded in metamorphic rocks (Tajčma-
nová et al. 2014, 2015; Centrella et al. 2018).

Links between metamorphism and
short-term tectonics

It is now well recognized that earthquakes and
aftershocks, produced by frictional failure, are not
restricted to the seismogenic upper crust (c. <15 km
and T <450°C) but can occur in, or penetrate into,
the middle-lower crust of all collisional settings.
For instance, the 2015 M<sub>w</sub> = 7.8 Gorka earthquake
in India located at a depth of 10-15 km has induced
aftershocks down to 30 km (McNamara et al.
2017). In a similar way, the beginning of the twenty-
first century saw the major discovery of slow
earthquakes characterized by slow slip events accom-
panied by non-volcanic tremor and deep low- to very
low-frequency earthquakes in subduction zones
around the Pacific Ocean at depths of >30-45 km
(Cascadia, Alaska, Mexico, Costa Rica, Japan)
(Ohmi & Obara 2002; Rogers & Dragert 2003;
Nadeau & Dolenc 2005; Schwartz & Rokosky
2007) and along the deep extension of the strike-slip
plate boundary of the San Andreas Fault at depths of
>20 km (Nadeau & Dolenc 2005; Shelly 2010).

To better understand the mechanics of brittle fail-
ure in the ductile realm, field geologists (mostly
structural geologists) have looked for the rock record
of earthquakes in the deep metamorphic crust. But
because the main factors controlling the rock rheol-
ogy includes, in addition to mechanical parameters
(strain rate and deviatoric stress), temperature, chem-
ical composition, water activity and pore-fluid pres-
sure, metamorphic petrologists are becoming more
involved in the quest for the rock record of deep seis-
micity. Pseudotachylite is probably the most reliable
indicator of palaeo-earthquakes. To discuss the
deformation mechanisms that control pseudotachy-
lite development, it is first critical to establish via
thermodynamic modelling the *P* (depth)–*T* condi-
tions of formation (Hawemann *et al.* 2018). The
rock record and mechanics of non-volcanic tremors
are currently being investigated by studying
exhumed geological exposures of shear zones (e.g.
Fagereng & Sibson 2010; Fagereng *et al.* 2014; Hay-
man & Lavier 2014). It appears that shear zones are
characterized by the concomitant activation of duc-
tile and brittle deformation, as well as significant
metamorphic reactions due to synkinematic fluid
percolation. This observation raises the question of
the role of chemical processes in the mechanics of
deep seismicity.

Conclusion

In conclusion, metamorphic geology is an all-
encompassing discipline that borrows heavily the
approaches and techniques from a broad range of
disciplines – from material science to mathematics –
and in return delivers new insights based on the
investigation of natural materials; and sometimes
new problems. The discipline is rapidly evolving,
and this Special Publication aims to provide a snap-
shot of its state in the second decade of the 2000s.
The fundamental question remains: what drives
changes in nature? The tumultuous development of
technology and computational power will surely
bring new exciting ideas on the stage in the next
years. In the words of T. S. Elliott (from his poem Lit-
tle Gidding), as metamorphic geologists ‘We shall
not cease from exploration / And the end of all our
exploring / Will be to arrive where we started /
And know the place for the first time’.

**Acknowledgements** We would like to sincerely thank all of the authors for their hard work in writing and correcting the papers within the tight deadlines imposed by the editors. The success of this book weighs heavily on their shoulders and we are grateful for that. The authors were assisted by the constructive criticism and excellent recommendations of the reviewers; we acknowledge all of them for their work. We would like to thank Jo Armstrong, our production editor, who is responsible for the exceptional quality of the final papers. The inspiration of this book comes from the organization by the editors of four sessions beginning in 2014 during the General Assembly of the European Geoscience Union (EGU) in Vienna. We are grateful to the presidents of the GMPV Division, Nick Arndt and Mike Burton, for their guidance through these years. We would like to thank Angharad Hills from the Geological Society of London, who solicited us to edit this book; we wish you all the best in your next challenge. We also thank Bethan Phillips for all her help in the latest stages of this process. Finally, we would like to thank our colleagues, friends and family for their continued and unconditional support.

**Funding** This research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors.

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