ABSTRACT LXXIII, August 2004 n. 1:

Attilio Boriani and Evelina Giobbi - Does the basement of western southern Alps display a tilted section through the continental crust? A review and discussion

Abstract - One of the most intriguing geo-petrological problems of Central Alps is the meaning of the presence side by side in the southalpine basement of a lower crustal unit, the Ivrea-Verbano Zone and an upper crustal unit, the Serie dei Laghi separated by a ductile shear zone, the Cossato-Mergozzo-Brissago (CMB) Line. This situation has been interpreted as an «exposed continental section». In this paper a great part of the data produced in the last thirty years are examined and discussed critically. However the authors are well aware that their work is unfortunately not exhaustive especially for what concerns the Ivrea-Verbano Zone on which an enormous amount of papers has been produced. The idea of a tilted section of the lower Paleozoic crust is criticized in favor of a model of a trans-tensional emplacement of the Ivrea Verbano Zone.

Francesco Paolo Sassi, Bernardo Cesare, Claudio Mazzoli, Luca Peruzzo, Raffaele Sassi and Richard Spiess - The crystalline basements of the Italian eastern Alps: a review of the metamorphic features

Abstract - We briefly review the state of the art on the metamorphic evolution of the crystalline basements of the Eastern Alps, with emphasis on the Italian sector. For each of the main structural units, Penninic, Austroalpine and Southalpine, we present a short outline followed by the most recent results obtained by the research team of metamorphic petrology operating at the University of Padova.

Marcello Franceschelli, Giovanni Gianelli, Enrico Pandeli and Mariano Puxeddu - Variscan and Alpine metamorphic events in the northern Apennines (Italy): a review

Abstract - The Northern Apennines consist of oceanic and continental Mesozoic-Tertiary units thrust over the deepest Tuscan metamorphic units, including Paleozoic successions. Tuscan metamorphic units underwent two Alpine tectono-metamorphic deformation phases, and only pre-Carboniferous ones were subjected to polyphase Variscan metamorphism. For the first Variscan phase with a Sudetian age of 328 ±5.3 Ma, the estimated P-T values are ~0.8 GPa and ~670°C; for the second Variscan phase, P~0.4-0.5 GPa, T~530°C. These P-T conditions were attained only in the Gneiss and Micaschist Complexes. In Larderello Micaschist C. and Gneiss C., pre-Alpine andalusite and muscovite (285±11Ma) might be related to a Late Variscan thermal event (third Variscan phase) with P=0.15-0.25 GPa and T >550 °C. During the first Alpine compressional phase (27-20 Ma), Mesozoic-Tertiary sediments were buried to a maximum depth of 40-55 km. Peak pressure values ranged from 0.8 to 1.2 GPa for Verrucano metasediments (Tuscan metamorphic units), with the exception of those on Giglio Island (1.0-1.5 GPa). The Schistes Lustrés of Gorgona show peak pressure values of 1.3-1.6 GPa, while the same rocks on the Argentario Promontory and Giglio Island are in the range 0.6-0.85 GPa. For both units, temperatures varied from 300° to 420° C (450°- 480°C in the Massa Unit). Extensional tectonics began in the early to late Miocene according to different models, caused very rapid exhumation (rough estimates: 2.0-2.5 mm/yr for the Apuan units and 3.9-4.0 mm/yr for the Schistes Lustrés of Gorgona) up to 10-15 km depth and produced a second main folding metamorphic phase (14-12 Ma) with P=0.3-0.5 GPa and T=300°-370°C. The distribution pattern of Al-silicates reveals that Tuscany is divided into NW-SE trending narrow parallel zones. Metamorphic grade increases from an eastern kaolinite-bearing zone to a central kyanite-bearing zone corresponding to the Massa Unit-Larderello alignment and then shows a westward decrease in the Punta Bianca-Elba areas. Similarly, non-metamorphic rocks show a rough decrease in illite crystallinity (°D2Q increase) from west to east and from the lower to upper tectonic units. In Southern Tuscany and in the Tuscan Archipelago, Tortonian-Quaternary magmatism produced contact metamorphism and hydrothermal alterations.

Vincenzo Festa, Antonia Messina, Antonio Paglionico, Giuseppe Piccarreta and Alessandro Rottura - Pre-Triassic history recorded in the Calabria-Peloritani segment of the Alpine chain, southern Italy. An overview

Abstract - Clockwise P-T-(t) trajectories are generally recorded in the pre-Triassic basement rocks of the Calabria-Peloritani segment of the Alpine chain. Geochronological data point to a synchronism of T-peak and emplacement of huge masses of granitoids at about 300 Ma. Decompression and heat supply from the granitic intrusions mark the transition from Barrovian to low P/high T metamorphism. When the preserved characters allow to trace prograde trajectories, a strong P increase and a moderate T increase, sometimes with T peak delayed relatively to the P peak.

These features are consistent with orogenic evolution in which crustal thickening in early- middle-Hercynian contractional stages was followed by crustal thinning in late-Hercynian extensional stages.

Moreover, in the Calabria-Peloritani segment of the Alpine chain, pre-Hercynian metamorphic and magmatic evidence also occurs.

Carlo Alberto Ricci, Rodolfo Carosi, Gianfranco Di Vincenzo, Marcello Franceschelli and Rosaria Palmeri - Unravelling the tectono-metamorphic evolution of medium-pressure rocks from collision to exhumation of the Variscan basement of NE Sardinia (Italy): a review

Abstract - The basement of NE Sardinia consists of magmatic-sedimentary sequences affected by a polyphase deformation (D₁, D₂, D₃ and D₄₎) and an increase in the metamorphic grade northwards, toward the inner zone of belt. D₁ is mainly detected in the low-grade zone, while D₂ is widespread in the basement but partitioned into domains with prevailing folding in the southernmost areas and shearing in the north-central part. D₂ structures were later overprinted by two main systems of open to tight folds. On the basis of the mineral assemblages of pelitic and quartz-feldspathic rocks, six metamorphic zones have been distinguished. From south to north they are: biotite, garnet, staurolite + biotite, kyanite + biotite, sillimanite + muscovite and sillimanite + K-feldspar. The P-T path of different metamorphic zones is a typical clockwise one due to thermal relaxation after a homogeneous thickening stage. The metamorphic peak is pre- to-syn S₂ in the garnet and staurolite zones, whereas in the sillimanite zones it is essentially syn-S₂. In biotite and garnet zones, both the D₁ and D₂ stages of tectono-metamorphic evolution can be documented. In the garnet zone, petrological evidence indicates that from the S₁ to the S₂ mineralogical assemblage there was an increase in temperature (up to 30°C), along with a decrease in pressure (0.2-0.6 GPa). From the staurolite + biotite zone to the sillimanite + K-feldspar-zone, the D₁ phase is not clearly documented, and samples recorded only the retrogressive path after the thermal climax. The field gradient is nearly concave towards the temperature axis. This is a consequence of the diachronous development of the main parageneses and fabrics in the six metamorphic zones during the exhumation stage. An age of about 340 Ma, found on syn-S₁ celadonite-rich white mica, is interpreted as the time of maximum thickening stage in the garnet zone; ages of 315-320 Ma for syn-S₂ muscovites are attributed to the end of chemical re-equilibration of white mica during the D₂ stage. The emplacement of granitoids in the axial zone has had no influence on the early exhumation of the lower crust, but the different exhumation rate may be related to the transpressional tectonic regime which affected NE Sardinia during Variscan orogeny.

Laura Gaggero, Luciano Cortesogno and Jean Michel Bertrand - The pre-Namurian basement of the Ligurian Alps: a review of the lithostratigraphy, pre-Alpine metamorphic evolution, and regional comparisons

Abstract - The Pre-Namurian basement of the Ligurian Alps (southern External Pennides) records evidence of poly-orogenic pre-Variscan (Ordovician), Variscan and Alpine evolution. In the Briançonnais domain, bimodal volcanism (tholeiitic to transitional basalts and acidic calc-alkaline volcanites) of Cambro-Ordovician age (~500 Ma) possibly suggesting a continental rift phase is associated with pelitic and psammitic sedimentary rocks and to large volumes of turbiditic (?) metarenites. The sequence was intruded by Early Ordovician, S-type granitoids and later metamorphosed from high-pressure amphibolite up to eclogite-facies conditions. The cogenetic or exotic nature of the eclogites, which are associated with migmatites and evolved through granulite facies conditions, is doubtful. Later, the metamorphic sequence was intruded by large granitic bodies (from monzogranites and granites to granodiorites, in the 470-460 Ma time span), and by rare gabbros and basic to intermediate dykes. Large metarhyolite bodies are likely coeval with the intrusive events. Finally, medium to low P, amphibolite facies, Variscan (327 Ma) metamorphism and with km-scale megafolding affected all the metamorphic and magmatic sequences. A small basement slice, considered of pre-Piedmont provenance, with Alpine high-pressure overprint, preserves sequences of metarhyolites, metaconglomerates, metapelites (garnet micaschists), garnet–bearing marbles with minor metabasalts and serpentine schists. The geodynamic pattern is compared with other neighbouring Palaeozoic terranes.

Marco Bruno, Bruno Messiga, Gisella Rebay and Marco Rubbo - High pressure coronites in the western Alps: a record of reaction pathways

Abstract - This paper, after a short review on the historical development of the use of the term «corona texture» to indicate a layered mineral structure developed at the contact between two reacting minerals, describes methods and problems encountered in the study of coronitic rocks. Four well-known examples of high-and ultrahigh-pressure coronitic rocks from the western Alps (Monviso and Zermatt-Saas concerning eclogitized ophiolites and Monte Mucrone and Dora Maira concerning eclogitized continental crust) are presented and discussed as case studies.

It is stressed out that the study of coronitic rocks by means of classic microstructural and determination of chemical composition of reactants and products coupled with more recent thermodynamic modelling and mass-balance calculation enhance our understanding of reaction mechanisms, our knowledge of the role played by fluids and our ability to determine accurate P-T paths followed by rocks, involved in major tectonic processes.

Valeria Caironi, Annita Colombo and Annalisa Tunesi - Geochemical approach to characterization and source identification of the protoliths of metasedimentary rocks: an example from the southern Alps

Abstract - We compare here the geochemical characters of metasediments from two main units of the Southern Alps (Serie dei Laghi and Orobic basement). Concerning the protoliths of the paragneisses from Serie dei Laghi, the Th/Sc vs Zr/Sc correlations indicate that they are mainly controlled by the composition of their sources (likely diorites to granodiorites); their REE patterns are well comparable to the average composition of continental arc wackes. The suggested environment is a dissected continental arc on an active margin. For the Serie dei Laghi metapelites (Scisti dei Laghi) the large compositional variations and the wide spread in some parameters (Th 5-15 ppm, Th/U 1.4-4.8, Th/Sc 0.35-0.92, La/Th 3.3-7.8, REE_tot 149-248) suggest mixing in different proportions of sediments from geochemically «undifferentiated» and from differentiated igneous sources, with a variable contribution of recycled sediments enriched in heavy minerals. The metasediments from the Orobic basement have a more pelitic character and show the influence of old upper crustal materials (Th/U= 3.6-6.7). Their main geochemical characters are compatible with a passive margin environment.

Supplemental Material - Caironi et al. (Fig.1).pdf; Caironi et al. (Tab. 1).pdf

Lauro Morten, Paolo Nimis and Elisabetta Rampone - Records of mantle–crust exchange processes during continental subduction–exhumation in the Nonsberg–Ultental garnet peridotites (eastern Alps). A review

Abstract - In the Nonsberg-Ultental area (Tonale nappe), Grt-bearing peridotites occur in high-P migmatitic gneisses enclosing relics of eclogites. The peridotites record the transformation of high-T Spl-lherzolites to coronitic Grt+Spl-lherzolites to fine-grained Amph±Grt-peridotites. The transformation is preceeded by LILE enrichment and intrusion of hot melts, and accompanied by deformation, hydration, LILE enrichment and LILE/HFSE fractionation. High-P metamorphism of peridotites and eclogites and early migmatization of the country gneisses were virtually coeval and possibly isofacial. The peridotites may represent former mantle-wedge material that was subducted and cooled due to incorporation in a crustal slab and then metasomatized by hydrous fluids left after crystallization of leucosomes. Possible directions for further work on this and other sectors of the Variscan belt where broadly similar rock associations have been found are proposed.

Raffaele Sassi, Corrado Venturini and Péter Árkai - The boundary between the metamorphic and non- to anchi-metamorphic domains in the Southalpine basement s.l. of the eastern southern Alps: a review

Abstract - The Southalpine basement in the Eastern Alps consists of thick metamorphic mainly phyllitic sequences which formed within greenschist facies conditions, and unmetamorphosed to very-low grade Paleozoic sequences in its easternmost part (Palaeocarnic Chain).

Many authors tried to localize the boundary between the metamorphic and non-metamorphic domains. Recent studies demonstrated, by means of KI (Kübler Index) data, that the locations of this boundary given in the maps are uncorrect, and that the boundary occurs more eastwards than previously suggested, mainly in correspondence of the Val Bordaglia Line.

The present review critically summarizes all data existing in the literature (Kübler Index, Árkai Index, vitrinite reflectance, Colour Alteration Index of Conodonts, fluid inclusions) concerning the location of the eastern boundary of the Hercynian metamorphism, and discusses the occurrence and possible significance of a local Alpine overprint to the east of the Val Bordaglia Line.

Annamaria Fornelli, Giuseppe Piccarreta, Pasquale Acquafredda, Francesca Micheletti and Antonio Paglionico - Geochemical fractionation in migmatitic rocks from Serre granulitic terrane (Calabria, southern Italy)

Abstract - The Hercynian crustal section in the Serre (southern Italy) is about 20 km thick and includes sub-greenschist to granulite facies metamorphic rocks which have been used to study chemical modifications during metamorphism and migmatization. The rocks studied include: (1) Stilo phyllites and Mammola paragneisses from the upper crustal metasediments; (2) migmatitic paragneisses from the lower crustal segment that contain evidence for multi-stage melting and different degrees of partial melting and extraction of melts. The upper crustal rocks and the uppermost migmatites retain the composition of common shales. Descending to the lower levels the migmatitic metapelites show modified bulk compositions relative to common shales. The degree of chemical modification increases towards the base of the investigated section. The bulk-migmatites and the melanosomes show an enrichment in MgO, FeO*, TiO₂, Al₂O₃, MnO, Sc, Y, V, Cr and REE and a depletion in SiO₂, K₂O, Na₂O, CaO, Rb, P, U. In the lowermost migmatitic metapelites enrichment of Th and Zr is also evident. Bulk migmatites have negative Eu anomaly which is larger in the residua. The migmatites reflect: i) diverse protolith bulk compositions, ii) the effects of hydrous melting and mica dehydration melting producing both melts crystallizing in situ and melts which were extracted, iii) a different degree of melting and of separation melt-restite, iv) different effects of rehydration on cooling. The overall geochemical features suggest that the lower migmatites were derived from a source similar to the Stilo phyllites, and that the upper migmatites from protoliths similar to the uppermost unmodified Stefanaconi paragneisses. With these assumptions the major element geochemical fractionation between restites, bulk migmatites and melts formed during the multi-stage partial melting has been modelled.

Supplemental Material - Fornelli et al. (Table 1).pdf; Fornelli et al. (Table 2).pdf

Eugenio Piluso and Lauro Morten - Hercynian high temperature granulites and migmatites from the Catena Costiera, northern Calabria, southern Italy

Abstract - The deepest portion of a Hercynian lithospheric section is exposed in the northern sector of the Catena Costiera, Calabrian-Peloritan Arc. The section consists of migmatites and granulites derived from pelitic-arenaceous sequences, of small volumes of granitoid rocks, of Spl-harzburgites and pyroxenites intruded by metagabbros. The Hercynian P-T evolution of the migmatites and granulites follows a clockwise trajectory. The prograde path, in the stability field of sillimanite, reaches the metamorphic peak condition at 750-800°C and 0.9-1.0 GPa. The retrograde path is characterized by a decompression up to 0.5 GPa and 600°C through two intermediate steps at 0.8 Gpa and about 800°C, and 0.6-0.7 Gpa at 650-700°C, respectively accompanied by a general retrogression under lower amphibolite facies and successively under lower greenschist facies conditions. The exhumation of deepest portion of the Hercynian lithospheric section to middle crustal levels took place along HT extensional shear zones. The following stages of exhumation under amphibolitic and greenschist facies conditions were accommodated by LT shear zones, at the ductile-brittle transition. The crustal sequence of the Catena Costiera is crosscut by unmetamorphosed basaltic dyke dated at 120 ± 1.3 Ma. Therefore, the Alpine orogenesis did not overprint the Hercynian lithospheric section of the Catena Costiera, except for the brittle deformation event.

Franco Rolfo and Simona Ferrando - Selected case studies in the high and ultrahigh pressure terranes of Dabie Shan and Sulu (China)

Abstract - A unique metamorphic terrane, the Dabie Shan and Sulu regions in east-central China, is well known worldwide for the widespread occurrence of rocks with ultrahigh pressure metamorphic (UHPM) overprint. Among the considered case studies, the Zhujiachong quartz-eclogites and related metamorphic veins followed a clockwise P-T path. The veins are considered to have formed before metamorphic peak by prograde breakdown of lawsonite. The eclogitic peak, estimated at ca. 2.4 GPa and 700 °C, is followed by a two-stage retrograde evolution. UHPM talc-kyanite quartzite with the mineral association typical of «whiteschist» has been found in Dabie Shan at Hualiangting. The geologic setting and trace element geochemistry suggest that the whiteschist derived from a felsic dyke. A minimum P of 2.6-2.7 GPa and a T of ca. 710 °C have been estimated for the metamorphic peak. The identification in Dabie Shan of a E-W trending narrow belt ca. 40 km long and 1-2 km wide, the Wuhe-Pailou Unit (WPU), supports the «in-situ» origin of the UHPM, and constrains the real size of the exhumed tectonic units. A clockwise P-T path was inferred for the WPU, which consists of an early prograde part up to P ~ 3.3 GPa and T ~ 720 °C and a two-stage retrograde evolution. In southern Sulu, at Donghai, kyanite-phengite-epidote eclogite and kyanite – OH-rich topaz quartzite were subjected to UHPM at P ~ 3.5 GPa and T ~ 850 °C. The growth of hydrous and/or zoned minerals under eclogite facies conditions is promoted by brines with variable salinity and type of dissolved cations ± CO₂. The presence of high-density brines at the coesite to quartz transition is also suggested by the primary fluid inclusions within OH-rich topaz and by the related isochore that indicates P = 2.8 GPa at 800 °C. Considerable differences in peak metamorphic P have been recorded by quartz- and coesite eclogites in Dabie Shan and Sulu, but the shape of post-peak P-T paths, characterized by a significant decompression coupled with minor cooling, seems to be peculiar of all studied units.

Franco M. Talarico, Rosaria Palmeri and Carlo Alberto Ricci - Regional metamorphism and P-T evolution of the Ross Orogen in northern Victoria Land (Antarctica): a review

Abstract - The main focus of this paper is on the petrological evolution of medium- to high-grade metamorphic units in the Wilson Terrane, the westernmost lithotectonic unit of the Ross Orogen in northern Victoria Land. The petrological data set is reviewed for all areas where P-T-t paths have been reconstructed and geochronological data are sufficiently complete to provide an overview of the regional metamorphic evolution of a ca. 600 km long segment of the Ross Orogen, from its termination along the Pacific coast to the Eisenhower Range near the Ross Sea coast.

Petrological evidence reveals that different lithological units of the Wilson Terrane equate with distinct lithotectonic metamorphic complexes with partly independent P-T-t histories. In spite of the wide range of estimated peak metamorphic conditions, and variability in both shape of the P-T path (clockwise or counter-clockwise) and type of retrograde evolution (isobaric cooling or cooling/unloading), the reviewed P-T-t trajectories consistently support a setting of evolving subduction and accretion in the context of a Palaeozoic cordilleran-type active margin.

Patrizia Fumagalli and Stefano Poli - Modelling metamorphic rocks in complex systems: present-day developments in high pressure experimental petrology

Abstract - Metamorphic processes at subduction zones are strictly related to the transfer and release of water via dehydration reactions from the subducting slab toward the mantle wedge. Results and present-day developments in high pressure experimental petrology in the laboratory of Milano are presented here, dealing with lithologies which are expected to be subducted, i.e. mainly mafics and ultramafics. Particular emphasis will be given to hydrate-bearing assemblages.

Experiments performed in mafic rocks at pressures of up to 5 GPa show that phase relationships are controlled by amphibole s.s. to 2.5 GPa and by epidote group minerals and lawsonite at higher pressure. Phase relationships point out to strong sensitivity of assemblages from continuous reactions and therefore from bulk composition considered.

In ultramafics, at temperatures between 680 and 800°C, amphibole and chlorite are the major hydrous phases able to transport water deep into the Earth’s Mantle. At pressure > 4.8 GPa, a hydrous silicate with a 10Å phase structure forms at the expense of chlorite, providing a «carrier» by which water might be transported down to 200 km depth. The interaction between alkali-rich fluids and mantle peridotites lead to the occurrence of potassic hydrous phases such as phlogopite. Experiments on K-peridotite suggest a pressure dependent relevance of a «talc» component in phlogopites.

Maria Luce Frezzotti, Bernardo Cesare and Marco Scambelluri - Fluids at extreme P-T metamorphic conditions: the message from high-grade rocks

Abstract - The study of fluid inclusions in rocks which experienced extreme P-T metamorphic conditions can provide valuable information on the composition of peak fluids, to some extent on their origin, and on fluid-rock interactions which take place in the lithosphere. A review is given, on the basis of the authors’ experience, of the major results obtained by fluid inclusions studies in the fields of high pressure and ultrahigh pressure metamorphic rocks (eclogites), high temperature and ultrahigh temperature metamorphic rocks (migmatites and granulites), and high temperature and pressures mantle rocks.

Marco Scambelluri and Pascal Philippot - Volatile and mobile element recycling during subduction of the oceanic lithosphere. Insights from metasediments and serpentinites of the Alps

Abstract - We discuss some aspects of the release of light elements and volatile components during subduction of serpentinite and metasedimentary systems. Enrichment in B, Cl, and Sr during serpentinization of the oceanic mantle is followed by their progressive release in the subduction fluids. Olivine–orthopyroxene rocks formed after significant serpentinite dehydration still contain bulk B, Cl and Li considerably higher than in mantle reservoirs and may introduce light element anomalies into the upper mantle. The high to ultrahigh pressure metasediments underwent little or no devolatilization, nor N release, down to at least 90 km, acting as closed systems to the infiltration of externally derived fluids. They therefore reflux to the mantle appreciable amounts of exogenic N. This behaviour is supported by the oxygen isotope geochemistry of high and ultrahigh pressure rocks, showing closed system behaviour and limited fluid mobility at these depths. Many high pressure rocks thus do not represent the levels of fluid extraction from slabs; we suggest that the antigorite breakdown may represent a dehydration reaction creating a mobile, trace element-enriched, subduction fluid.

Alessandro Borghi and Richard Spiess - Studying metamorphic microstructures: a brief insight on modern methodological approaches

Abstract - Microstructures evolve in response to processes that rocks undergo during their residence time in the Earth crust and mantle. For a thorough understanding of microstructures all their critical aspects need to be analysed. Much of the recent advances in the comprehension of microstructures and the interrelated processes is linked to the advent of modern instrumental equipments as electron and proton microprobes (EMPA, mPIXE) and to novel analytical techniques as electron backscatter diffraction (EBSD) and orientation contrast imaging (OC). In this review we highlight the basic concepts and the potential applications of such modern analytical tools in microstructural analysis of metamorphic rocks.

Maria Iole Spalla and Michele Zucali - Deformation vs. metamorphic re-equilibration heterogeneities in polymetamorphic rocks: a key to infer quality P-T-d-t path

Abstract - The interaction between fabric gradients and reaction rate as a tool for individuating volumes carrying the longest «rock memory» is discussed through some examples from continental units of the Alpine chain. Here quality P-T-d-t paths have been inferred using a sampling strategy based on reconstruction of the metamorphic evolution, supported by a regionally valid deformation history and on the choice of sites for investigations on compositional variations, where mineral growth and sequences of overprinting fabrics are known. The examples show that correlation between degree of fabric evolution and progress of metamorphic transformation is positive and influence of strain partitioning on tectono-thermal rock memory must be taken into account during P-T-d-t reconstruction to avoid errors in determining the sequence of P-T re-equilibration steps and to obtain clustered P-T estimates relative to each step.

Supplemental Material - Spalla_and_Zucali(Tab.01).pdf; Spalla_and_Zucali(Fig.01).pdf; Spalla_and_Zucali(Fig.06).pdf;
Spalla_and_Zucali(Fig.07).pdf;

Igor M. Villa - Geochronology of metamorphic rocks

Abstract - Metamorphic minerals are formed at moderate to high depth in the Earth’s interior. Those that have been subsequently exhumed to the Earth’s surface are those that survived metastably outside their stability fields. The majority of studied metamorphic rocks did not survive unscathed, and contains diachronic mineral assemblages that record a plurality of P-T-A-X conditions along the exhumation path, rather than a single P-T-A-X point that includes all minerals in that rock. Such mixed assemblages are in mutual disequilibrium. Dating a metamorphic event therefore requires great interpretive caution. While an apparent age always results from mathematical equations, translating a number into a geological interpretation can be ambiguous and requires establishing a context among different isotope systems, and even more importantly, between the isotope record and mineralogy, microtextures, and microchemistry. Electron microscopy reveals that many minerals consist of mixtures at the sub-µm scale, as they are intergrown with their retrogression products, and/or contain unequilibrated relics (e.g. zircons, monazites, amphiboles, micas). Lack of chemical homogeneity means that diffusivity under anhydrous conditions is extremely low. Modern, direct determinations of dry diffusion in minerals yield rates that are orders of magnitude slower than what was thought twenty years ago. If recrystallization is incomplete, petrographic relics ensure isotopic inheritance. By contrast, water accelerates recrystallization and so enhances element and isotope reequilibration, both in experiments and in natural systems. Availability of water, not temperature, is the principal factor controlling isotope transport. The isotope record of a mineral is thus a geohygrometer and can be used, but only in connection with petrological constraints, to reconstruct a rock’s petrogenetic path.