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Fluid is a key agent for the mass transfer between the subducting slab and the mantle wedge, which greatly affects the evolution of the crust-mantle system at convergent plate boundaries. A geochemical study was carried out for coesite-bearing jadeite quartzite and its country rock granitic gneiss from the Dora Maira Massif in the Western Alps. The results provide new insights into the composition of metamorphic fluids and fluid-rock interaction processes in the continental subduction zone. Coesite inclusions are found for the first time in metamorphic zircons of the granitic gneiss. The ultrahigh-pressure (UHP) metamorphism was dated to occur at 34.7 ± 0.3 Ma, confirming that the country rock experienced the UHP metamorphism synchronously with the whiteschist and jadeite quartzite. The jadeite quartzite occurs as layers and boudins within the coesite-bearing whiteschist, and shows similar whole-rock REE distribution patterns to both granitic gneiss and whiteschist. In addition, relict domains of magmatic zircon in the three types of UHP metamorphic rocks exhibit similar U-Pb ages and δ18O values, indicating that they have the same protolith of granites. However, metamorphic zircons in the jadeite quartzite show significantly lower δ18O values of 6.1–7.3‰ than the relict magmatic domains of 9.4–10.6‰. Furthermore, the jadeite quartzite has whole-rock δ18O values of 7.9–8.7‰, significantly lower than those of 9.5–10.9‰ for the granitic gneiss but higher than 6.5–7.9‰ for the whiteschist. These differences suggest that the jadeite quartzite was probably formed through metasomatism of metagranite by external fluids with relatively low δ18O values. The jadeite quartzite shows high δ56Fe values of 0.69–0.87‰, considerably higher than those of 0.18–0.38‰ for the granitic gneiss, but falling within the range of 0.32–1.03‰ for the whiteschist from the same outcrop. The Fe isotope modeling results suggest that the metasomatic fluids responsible for the formation of jadeite quartzite were possibly derived from the host whiteschist under UHP metamorphic conditions. Such fluids are enriched in Si, Al and Mg, and have high δ56Fe but low δ18O values. Therefore, the jadeite quartzite records the action of metamorphic supercritical fluids at the subarc depth. If the metasomatized rocks in the continental subduction zone would dehydrate to generate fluids with variable geochemical compositions due to multistage fluid-rock interactions, the fluids could further infiltrate the surrounding rock and possibly the mantle wedge to result in further geochemical transfer from the subducting slab to the mantle wedge.

International audience; The tectono-metamorphic evolution of the European Alps is still contentious. The Monte Rosa tectonic unit is a prominent nappe in the Central European Alps and estimates of its peak Alpine pressure (P) and temperature (T) conditions are essential for reconstructing its tectonometamorphic evolution. However, the reported peak Alpine P and T estimates vary considerably between 1.2 and 2.7 GPa and 490 and 640 °C for a variety of lithologies. Here, we show petrology and pseudosection modelling of metapelitic assemblages from the western portions of the Monte Rosa nappe (upper Ayas valley, Italy). We present newly discovered staurolite-chloritoid bearing metapelitic assemblages. These assemblages exhibit an Alpine high-P metamorphic overprint of a former contact-metamorphic mineral assemblage generated by post-Variscan granitic intrusions. Staurolite contains major amounts of Zn (up to 1.0 atoms per formula units), which is currently, in contrast to Fe-and Mg-staurolite end-members, not considered in any thermodynamic database. We employ two end-member mixing models for Zn in staurolite, site mixing and molecular mixing. Both models enlarge the P and T stability range for the observed assemblage, where site mixing has the largest influence of  0.2 GPa and  20 C. Our results for three metapelite assemblages, with and without staurolite, indicate peak Alpine P of 1.6 ± 0.2 GPa and peak T of 585  20 C. These peak P estimates agree with previously published estimates for metagranites in the nappe, and are in stark contrast with peak P obtained from talc, chloritoid, phengite and quartz bearing lithologies termed 'whiteschists' (>2.2 GPa). Our results confirm a variation of peak Alpine P of 0.6 ± 0.2 GPa between metagranite/metapelite lithologies and a nearby whiteschist lens (>2.2 GPa) within the metagranite. Field observations indicate that the studied region is structurally coherent and that the whiteschist is not a tectonic slice formed by tectonic mélange. We suggest that the consistent peak P for metapelite and metagranite assemblages represents the regional peak P and that the higher pressure recorded in the whiteschist lens is likely due to dynamic pressure, possibly resulting from tectonic and/or reaction-induced stresses. If the calculated P of 1.6 ± 0.2 GPa represents regional peak Alpine conditions, then the Monte Rosa nappe was exhumed from a significantly shallower depth than previously assumed, based on peak P estimates >2.2 GPa for whiteschist lithologies.

The geological toolbox for the analysis of orogenic processes has seen substantial additions over the recent decades. Major advances have been made in the ability to simulate geological conditions and processes by computer modeling, we have a much improved knowledge of geochemical processes (trace elements and isotopes in particular), and, last but not least, the range and versatility of micro-analytical methods and instrumentation available to us has been expanded dramatically. The latter aspect has had a particular impact on metamorphic petrology as the conventional tools to determine pressure-temperature-time paths via rock and mineral analysis combined with isotope geochronology have generally gained in precision and accuracy through a much improved resolution. Geochronology has moved from whole-grain analysis to spot analyses on zoned single grains while analyzing trace elements and isotopes at the same time. It is therefore possible to recognize complex multi-stage orogenic events in single samples or single grains. Diffusion profiles in minerals provide us with time-scales of metamorphic and magmatic processes. Major and trace elements as well as isotope profiles of mineral grains give us insight into changing ambient conditions during mineral growth. This special issue focuses on pressure-temperature conditions and geochronology of metamorphic rocks, isotope studies of magmatic rocks, partial melting and metasomatic processes, all with a view at tectonic implications and orogeny. Much of the evidence has been gained from small-scale observation and analysis. Diligent field work and detailed microscopic examination remain indispensable, though, as demonstrated clearly by the “expansion” of the P-T space as applicable to crustal metamorphic processes towards extreme pressures and temperatures. As a consequence, much interest has thus focused on UHP and UHT rocks and the orogenic processes that generate them. There is petrological evidence for subduction to, and subsequent exhumation from, depths in excess of 250 km. While this impacts substantially on the large-scale geodynamic models of orogenic processes, the evidence is gathered primarily from small-scale petrological observation. The same applies in principle to UHT granulites. To generate temperatures in the 900 to 1 100 °C range within the sillimanite stability field in orogenic heat budget models remains a serious challenge, however. The large overlap of solid-state metamorphism, partial anatexis and magmatic crystallization in P-T space suggests close interrelationships, and the role and contribution of the diverse magmatic activities in forming orogenic belts, from the onset of subduction to late-orogenic delamination or slab-break-off is crucial for our understanding of geodynamics, with focus on geochemistry and geochronology. Aspects of metasomatism in the framework of orogeny are given some special consideration in this issue. The topics include the formation of sagvandite, pyrope quartzite (or whiteschist), jadeitite and rodingite. One of the key questions addressed is the source and the nature of the fluids that triggered the different types of metasomatic events. Finally, it is shown how small-scale methods contribute to a better understanding of the formation of deep-seated mantle xenoliths.

A series of tourmaline reference materials are developed for in situ oxygen isotope analysis by secondary ion mass spectrometry (SIMS), which allow study of the tourmaline compositions found in most igneous and metamorphic rocks. The new reference material was applied to measure oxygen isotope composition of tourmaline from metagranite, meta-leucogranite, and whiteschist from the Monte Rosa nappe (Western Alps). The protolith and genesis of whiteschist are highly debated in the literature. Whiteschists occur as 10 to 50 m tube-like bodies within the Permian Monte Rosa granite. They consist of chloritoid, talc, phengite, and quartz, with local kyanite, garnet, tourmaline, and carbonates. Whiteschist tourmaline is characterized by an igneous core and a dravitic overgrowth (XMg > 0.9). The core reveals similar chemical composition and zonation as meta-leucogranitic tourmaline (XMg = 0.25, δ18O = 11.3–11.5‰), proving their common origin. Dravitic overgrowths in whiteschists have lower oxygen isotope compositions (8.9–9.5‰). Tourmaline in metagranite is an intermediate schorl-dravite with XMg of 0.50. Oxygen isotope data reveal homogeneous composition for metagranite and meta-leucogranite tourmalines of 10.4–11.3‰ and 11.0–11.9‰, respectively. Quartz inclusions in both meta-igneous rocks show the same oxygen isotopic composition as the quartz in the matrix (13.6–13.9‰). In whiteschist the oxygen isotope composition of quartz included in tourmaline cores lost their igneous signature, having the same values as quartz in the matrix (11.4–11.7‰). A network of small fractures filled with dravitic tourmaline can be observed in the igneous core and suggested to serve as a connection between included quartz and matrix, and lead to recrystallization of the inclusion. In contrast, the igneous core of the whiteschist tourmaline fully retained its magmatic oxygen isotope signature, indicating oxygen diffusion is extremely slow in tourmaline. Tourmaline included in high-pressure chloritoid shows the characteristic dravitic overgrowth, demonstrating that chloritoid grew after the metasomatism responsible for the whiteschist formation, but continued to grow during the Alpine metamorphism. Our data on tourmaline and quartz show that tourmaline-bearing white-schists originated from the related meta-leucogranites, which were locally altered by late magmatic hydrothermal fluids prior to Alpine high-pressure metamorphism.

The Kokchetav ultrahigh-pressure ( UHP ) metamorphic belt, located in the western part of the Central Asia Orogenic Belt (CAOB), consists of a well-studied diamond-bearing western domain and a less well-known coesite-bearing domain. To better understand the time of subduction and exhumation of the eastern domain and the tectonic evolution of the entire Kokchetav microcontinent, we conducted SIMS U-Pb dating of zircon from eclogite and rutile from whiteschist of the eastern domain. We identified two types of zircon from the Kulet eclogite: type-I zircon grains possess a large, low-cathodoluminescence (CL) magmatic core with high Th/U ratios of 1.0–1.5, yielding a Proterozoic protolith age (1437 ± 29 Ma), and a thin rim of intermediate or high-CL. Type-II zircon crystals are rounded to ovoid, and have very low Th/U ratios ( UHP rocks from both the eastern and western domains represent Proterozoic protoliths, and that they were subjected to nearly coeval UHP metamorphism at 527–537 Ma, even though the two domains formed at different peak P-T conditions. Exhumation took place during 526–498 Ma for the eastern domain, comparable to the western domain (526–507 Ma). The UHP slabs were exhumed from depths of 100–200 km to a mid-crustal level over 20–25 My. The final suturing of the uplifted Kokchetav microcontinent against the Stepnyak paleo-island arc occurred at ≤500 Ma. Our new SIMS U-Pb age data and previous petrological and structural analyses provide new constraints on the metamorphism and tectonic evolution of the Kokchetav microcontinent, and on the continental history in the western part of the CAOB.

Whole rock lithium (Li) and Li isotopic compositions of coesite-bearing whiteschists and their country rock metagranites from the Dora-Maira Massif in the Western Alps were investigated to understand their behavior during fluid-rock interaction in mantle depths of continental subduction zones. The whiteschists reveal relatively low δ7Li values (−6.4‰ to +2.4‰, with an average value of −2.1‰) in comparison to the country rock metagranites (δ7Li = −1.2‰ to +3.6‰, with an average value of +1.4‰). The δ7Li values of whiteschists do not show any correlations with major elements (e.g. Mg, Fe), Li, or isotopes (i.e., δ26Mg and δ18O values) and cannot be explained by kinetic fractionation. Hence, the low δ7Li values of whiteschists result from interactions with metasomatic fluids containing isotopically light Li that occurred in deep portions of a continental subduction zone. Fluid-rock interaction modeling suggests that the metasomatic fluids contain low δ7Li values (down to −4‰) and moderate Li concentrations (50 to 150 ppm), which were probably derived from the dehydration of serpentinites in the subduction channel. The presence of such low δ7Li fluids in deep subduction zones could cause Li isotopic heterogeneity in the earth's interior.

Rare earth element (REE) concentrations of ultrahigh-pressure (UH P ) metamorphic rocks from the Dora-Maira Massif as well as of their granitic country rocks were investigated to evaluate their primary relationships. Additionally, retrograde derivatives of these rocks and mineral separates of pyrope, phengite, and monazite from ‘pyrope quartzite’ were analyzed to study the REE fractionation behaviour and mobilization conditions. Comparative REE studies were performed on leucophyllite from the Eastern Alps that had formed along shear zones in granite gneiss, and whiteschist of evaporitic origin from Afghanistan. While the ‘pyrope quartzite’ rocks typically show flat chondrite-normalized REE patterns, their retrograde products are distinctly depleted in heavy REEs (HREEs). The REE distribution patterns of all granitic country-rock gneiss, phengite schist- and garnet-jadeite-quartz-layer intercalations within ‘pyrope quartzite’ are nearly identical, whereas the ‘pyrope quartzite’ shows variations in total REE contents. Nevertheless, all ‘pyrope quartzite’ spectra are parallel to each other and follow those of the surrounding orthogneiss. The whole suite of rock types is characterized by a distinct negative europium anomaly typical for differentiated material of the continental crust. Pyrope shows an enrichment of the HREEs. During the retrograde breakdown of pyrope to a chlorite-bearing assemblage, these elements became mobilized and were partly removed. As a result, the HREEs in these altered rocks are depleted. Monazite shows an enrichment of all, especially light, REEs, whereas in phengite only the light REEs (LREEs) La, Ce, and Nd were detected. The metasomatically formed ‘pyrope quartzite’ still preserves its original crustal REE pattern in spite of the strong major- and minor-element changes during the prograde stage. The REEs, which were incorporated into garnet and monazite, behaved immobile. However, during slight retrograde fluid infiltration the REEs may become partly mobilized. This contrasting mobilization behaviour is related to the fractionation of the REEs into newly growing phases. The original crustal REE distribution spectrum of ‘pyrope quartzite’ maintained during Mg-metasomatism is not diagnostic for a specific protolith, thus fitting the REE distributions of both, the Afghanistan whiteschists and the leucophyllite samples as well.

A complete set of geochemical data for “pyrope quartzite”, various phengite-schist and jadeite-bearing rock intercalations, as well as for enclosing orthogneiss from different localities within the southern Dora-Maira Massif is presented, in order to establish the nature of the different protoliths and their primary relationships. For comparison, Mg-rich rocks from the Eastern Alps occurring along shear zones within granitic gneiss, as well as metaevaporitic rocks from Sar e Sang, Afghanistan, were also studied. Typically, the “pyrope quartzite” and its retrograde derivative are enriched in Mg and strongly depleted in Na, Ca, Fe, Cu, P, Rb, Ba, and Sr with respect to the enclosing orthogneiss of granitic bulk composition. This orthogneiss is usually peraluminous, has normative corundum, and may be best related to S-type granite protoliths. Trace-element contents of the different phengite schists included in the “pyrope quartzite” confirm their close overall kinship to the enclosing orthogneiss. Jadeite-bearing rocks, which occur as conformable layers and boudins within “pyrope quartzite”, in general differ by having lower contents in K, Mg, Rb, and higher contents in Na, Fe, Ca, Mn, P, and Zn. The present study indicates that the “pyrope quartzite” has a metasomatic origin, whereas an evaporitic nature of the protolith can be ruled out. Discrimination plots demonstrate that the “pyrope quartzite” can clearly be distinguished chemically from metasedimentary whiteschist derived from former evaporite environments. The field relations of trails of “pyrope quartzite”, now forming lensoid inclusions within an orthogneiss (former granite), would also make a metasedimentary origin rather unlikely. On the other hand, consistent genetic relationships between “pyrope quartzite” and its enclosing orthogneiss were obtained that virtually match those of Mg-metasomatic leucophyllites from the Eastern Alps occurring along shear zones within granitic gneisses. Similar Mg-rich rocks within the Tauern Window such as within the Monte Rosa and Gran Paradiso Massifs of the Western Alps indicate that local processes of Mg metasomatism occurred throughout the entire range of the Alps.

Keywords High-pressure fluids · Whiteschists · U–Pb dating · Oxygen isotopes · Ion microprobe · Metasomatism Introduction The subduction of crustal material to mantle depths and its chemical modification during burial and exhumation contribute to element recycling in the mantle and the formation of new crust through arc magmatism. Crustal rocks that Abstract The Dora-Maira whiteschists derive from metasomatically altered granites that experienced ultrahighpressure metamorphism at ~750 °C and 40 kbar during the Alpine orogeny. In order to investigate the P–T–time–fluid evolution of the whiteschists, we obtained U–Pb ages from zircon and monazite and combined those with trace element composition and oxygen isotopes of the accessory minerals and coexisting garnet. Zircon cores are the only remnants of the granitic protolith and still preserve a Permian age, magmatic trace element compositions and δ18O of ~10 ‰. Thermodynamic modelling of Si-rich and Si-poor whiteschist compositions shows that there are two main fluid pulses during prograde subduction between 20 and 40 kbar. In Si-poor samples, the breakdown of chlorite to garnet + fluid occurs at ~22 kbar. A first zircon rim directly overgrowing the cores has inclusions of prograde phlogopite and HREE-enriched patterns indicating zircon growth at the onset of garnet formation. A second main fluid pulse is documented close to peak metamorphic conditions in both Si-rich and Si-poor whiteschist when talc + kyanite react to garnet + coesite + fluid. A second metamorphic overgrowth on zircon with HREE depletion was observed in the Si-poor whiteschists, whereas a single metamorphic overgrowth capturing phengite and talc inclusions was observed in the Si-rich whiteschists. Garnet rims, zircon rims and monazite are in chemical and isotopic equilibrium for oxygen, demonstrating that they all formed at peak metamorphism at 35 Ma as constrained by the age of monazite (34.7 ± 0.4 Ma) and zircon rims (35.1 ± 0.8 Ma). The prograde zircon rim in Si-poor whiteschists has an age that is within error indistinguishable from the age of peak metamorphic conditions, consistent with a minimum rate of subduction of 2 cm/year for the Dora-Maira unit. Oxygen isotope values for zircon rims, monazite and garnet are equal within error at 6.4 ± 0.4 ‰, which is in line with closed-system equilibrium fractionation during prograde to peak temperatures. The resulting equilibrium Δ18Ozircon-monazite at 700 ± 20 °C is 0.1 ± 0.7 ‰. The in situ oxygen isotope data argue against an externally derived input of fluids into the whiteschists. Instead, fluidassisted zircon and monazite recrystallisation can be linked to internal dehydration reactions during prograde subduction. We propose that the major metasomatic event affecting the granite protolith was related to hydrothermal seafloor alteration post-dating Jurassic rifting, well before the onset of Alpine subduction.

Microstructures in minerals from ultrahigh-pressure metamorphic (UHPM) terranes are keys to understanding the rheological properties and the exhumation mechanisms of rocks from subduction zones. Kyanite-bearing whiteschist, associated with eclogite lenses, is part of UHPM unit II located south-west of Lake Zheltau in the Kulet region of the Kokchetav Massif. The equilibrium assemblage is kyanite + garnet + talc + phengite + coesite/quartz. Previously reported peak pressure-temperature (P-T) conditions are � 3.5 GPa at 750 � C. A strong foliation is defined by the talc and phengite, with a corresponding weak shape preferred alignment of kyanite. Crystallographic orientation maps and analysis of kyanite blades were performed using electron backscatter diffraction methods. The data are consistent with a (100)(001) slip system for the formation of undulose extinction and kink bands in kyanite. Rotations measured across individual kink bands are 10-50� about , and rotations along kyanite with undulose extinction are up to 50� about with variations between adjacent points typically

Mautia Hill, situated in the Mozambique Belt of East Africa, is a locality famous for its unique high‐pressure whiteschist mineral assemblages. The rocks are located in a complex regional tectonic setting that is critical to understanding the amalgamation history of Gondwana.The timing of the Pan‐African Gondwana collision in this region is still a topic of considerable debate, especially because the age of the Mautia high‐pressure metamorphism/metasomatism has yet to be established. We have extracted detrital zircons with extensive low‐U overgrowths and dated them using the U‐Pb SHRIMP method. Detrital zircon cores indicate that the sediments came from a region identical to the adjacent Tanzania Craton and adjacent Western Mozambique Belt. One detrital grain has a 207Pb/206Pb age of ca. 3.5 Ga and is the oldest dated zircon in the region. The zircon rims provide 207‐corrected 206Pb/238U and 208Pb/232Th weighted mean ages of \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepac...

Akdalaite (5Al2O3·H2O), as well as hogbomite, kyanite, Zn-bearing staurolite, Mg-chlorite, gahnite, hercynite, quartz, apatite, and zircon, are present as submicrometer-size minerals associated with rutile aggregates occurring as inclusions in garnet porphyroblasts of whiteschist from the Kulet Kol region, Kokchetav Massif, northern Kazakhstan. The akdalaite crystals, ∼0.5 to 1 μm in size, are subhedral to euhedral, and are bounded by (0001), (0001), {1100}, {1101} facets. EDX analyses show minor Si, Ti, Cr, Fe, Mg, Zn, and Ga, in addition to the major component Al. Electron diffraction data yield the following crystal structural information: space group P63mc, and a = 5.58(1) A, c = 8.86(2) A, which are similar to those of synthetic tohdite, but are distinctly differentfrom those reported for the type specimen of akdalaite (P612 or P61 , a = 14.97 A, c = 2.87 A). The akdalaite type specimen, stored in the museum of the All-Russian Institute of Mineral Resources (VIMS) in Moscow, had also been re-examined...

Pressure–temperature grids in the system Na2O–CaO–K2O–FeO– MgO– Al2O3–SiO2–H2O and its subsystems have been calculated in the range 15–45 kbar and 550–900 C, using an internally consistent thermodynamic dataset and new thermodynamic models for amphibole, white mica, and clinopyroxene, with the software THERMOCALC. Minerals considered for the grids include garnet, omphacite, diopside, jadeite, hornblende, actinolite, glaucophane, zoisite, lawsonite, kyanite, coesite, quartz, talc, muscovite, paragonite, biotite, chlorite, and plagioclase. Compatibility diagrams are used to illustrate the phase relationships in the grids. Coesite-bearing eclogites and a whiteschist from China are used to demonstrate the ability of pseudosections to model phase relationships in natural ultrahigh-pressure metamorphic rocks. Under water-saturated conditions, chlorite-bearing assemblages in Mgand Al-rich eclogites are stable at lower temperatures than in Fe-rich eclogites. The relative temperature stability of the three amphiboles is hornblende > actinolite > glaucophane (amphibole names used sensu lato). Talc-bearing assemblages are stable only at low temperature and high pressure in Mgand Al-rich eclogites. For most eclogite compositions, talc coexists with lawsonite, but not zoisite, in the stability field of coesite. Water content contouring of pressure–temperature pseudosections, along with appropriate geotherms, provides new constraints concerning dehydration of such rocks in subducting slabs. Chlorite and lawsonite are two important H2O-carriers in subducting slabs. Depending on bulk composition and pressure–temperature path, amphibole may or may not be a major H2O-carrier to depth. In most cases, dehydration to make ultrahigh-pressure eclogites takes place gradually, with H2O content controlled by divariant or higher variance assemblages. Therefore, fluid fluxes in subduction zones are likely to be continuous, with the rate of dehydration changing with changing pressure and temperature. Further, eclogites of different bulk compositions dehydrate differently. Dehydration of Fe-rich eclogite is nearly complete at relatively shallow depth, whereas Mgand Al-rich eclogites dehydrate continuously down to greater depth.

New SHRIMP U–Pb age data on metamorphic monazite, as well as step-heating 40Ar–39Ar ages on metamorphic biotite, muscovite and microcline, from Katangan metasedimentary rocks of the Central African Copperbelt are presented. These rocks were deformed and metamorphosed during the Pan-African Lufilian Orogeny. Three samples of metamorphic monazite from the Chambishi structural basin give ages of 592 ± 22 Ma, 531 ± 12 Ma and 512 ± 17 Ma, which correspond respectively to the ages of eclogite facies metamorphism, high-pressure talc-kyanite whiteschist metamorphism, and of a regional metamorphism/mineralisation pulse elsewhere within the Lufilian Orogen. A biotite population from Luanshya gives a 40Ar/39Ar plateau age of 586.1 ± 1.7 Ma, coinciding with the oldest monazite age. Several samples from the Chambishi basin and the Konkola area give 40Ar/39Ar biotite plateau ages in the range of 496.6 ± 0.6–485.2 ± 0.9 Ma, and a muscovite plateau age of 483.6 ± 1.1 Ma, which are a manifestation of regional uplift and cooling that affected the whole Katangan basin. The youngest apparent 40Ar/39Ar ages obtained are from microcline at Musoshi and range from 467.0 ± 2.7 Ma to 405.8 ± 3.8 Ma, reflecting continued slow cooling of the Lufilian Orogen.

Dissolved cation and anion species were measured in the water-soluble fraction of massive seafloor serpentinites leached with distilled H 2 O. Soluble chloride contents ranged from 0.09 to 0.51 wt.% (average=0.26 wt.%), SO 4 2− to 0.09 wt.%, Mg 2+ to 0.16 wt.% and Na + to 0.09 wt.%. The concentration of evolved ions was the same in samples equilibrated with water at 25 and 230 °C, demonstrating that a significant amount of Cl present in serpentinites occurs as water-soluble salts. A nearly equal amount of Cl exists in the leached serpentinite, likely sited in serpentinite itself. The formation of intergranular salts in the serpentinite can be explained by seawater hydrothermal alteration of peridotite. Hydration reactions and H 2 O-incorporation in the serpentine leave the residual fluid with ever-increasing salinity and ultimately lead to precipitation of water-soluble chlorides. Chlorine isotope ratios in the water-soluble fraction average −1.49‰ (vs. standard mean ocean chlorine, SMOC) from a core at the Iberian Abyssal Plain, and +0.16‰ from a much younger core at Hess Deep. The δ 37 Cl value of insoluble Cl is ∼0.33‰ higher than the soluble fraction. The nonzero δ 37 Cl values demonstrate that the Cl in the serpentinites is not an artifact of seawater incorporation during drilling. The following conclusions are presented: (1) Serpentinite may be the major conduit for surficial chlorine transfer to mantle depths and an important part of the global chlorine cycle. The Cl flux in serpentinites into the mantle is larger than all other previously identified fluxes. (2) Cl is sited in both a water-soluble and -insoluble component. (3) A high-salinity fluid ‘plume’ is evolved during serpentinite dehydration of a subducting slab. The high-salinity fluid affects metamorphic reactions and melting properties above the slab and can cause intense metasomatism. The unusual bulk composition of the whiteschist from Dora Maira can be explained by channeled passage of a high-salinity fluid. (4) High-salinity fluids reduce the stability field of serpentine, causing its dehydration to occur over a broad interval, which may partly explain the broad locus of intermediate-depth earthquakes.

One of the world’s most extensive occurrences of whiteschists (talc‐kyanite schists) is located in south central Africa with several exposures along a ca. 700 km northwest‐southeast striking zone. The metamorphic evolution and age relations of whiteschists and associated rocks from four localities, three in the Lufilian Arc and one in the Zambezi Belt, were investigated. In the Lufilian Arc whiteschists, associated garnet‐amphibolites and biotite‐kyanite‐garnet gneisses record peak metamorphic conditions of about \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{...

One of the important aims of metamorphic petrology is to unveil the physicochemical conditions of the Earth's interior. Decoding the record of metamorphism is a case of the inverse problem based on observations of stable equilibrium mineral assemblages and mineral compositions. Mineralogical forward-modeling is an alternative approach to estimate the metamorphic conditions of a rock. It enables a prediction of equilibrium composition of zoned minerals or mineral inclusions with the matrix phase assemblage. Thus, the forwardapproach tests the assumption of an equilibrium mineral assemblage, which is critical for inversion analysis.In this paper, we introduce thermodynamic forward-modeling in the field of metamorphic petrology. Then, we show an example of its application in estimating a prograde metamorphic P-T path of a whiteschist from the Kokchetav ultrahigh-pressure terrane in Kazakhstan. Garnet in the whiteschist shows prograde compositional zonation and contains mineral inclusions. We carried out geothermobarometric estimates using ilmenite-rutile composite inclusions in garnet, combined with an evaluation of the equilibrium mineral assemblage in a P-T pseudosection of the whiteschist in the K2O-CaO-MgO-FeO-Al2O3-SiO2-H2O system. The result yielded a counter-clockwise prograde P-T path for the rock. The amount and equilibrium composition of garnet were sequentially calculated for model P-T conditions along the P-T path, and the results were compared with a line-profile observation of the garnet. A consistency between the model and the observations was confirmed for XCa in garnet, however XFe and XMg had a large inconsistency. Both uncertainties in the equilibrium model and nonequilibrium effects during the crystal growth are possible reasons for such a discrepancy. Hence, it must be noticed that there are complexities in estimating the equilibrium composition of a zoned garnet and matrix minerals.

The peak metamorphic conditions of subducted continental crust in the Dora-Maira massif (Western Alps) have been revised by combining experimental results in the KCMASH system with petrologic information from whiteschists. Textural observations in whiteschists suggest that the peak metamorphic assemblage garnet+phengite+kyanite+coesite±talc originates from the reaction kyanite+talc↔garnet+coesite+liquid. In the experimentally determined petrogenetic grid, this reaction occurs above 45 kbar at ∼730 °C. At lower pressures, talc reacts either to orthopyroxene and coesite or, together with phengite, to biotite, coesite and kyanite. The liberated liquid contains probably similar amounts of H2O and dissolved granitic components. The composition of the liquid in the whiteschists at peak metamorphic conditions, a major unknown in earlier studies, was probably very similar to the liquid composition produced in the experiments. Therefore, the experimentally determined petrogenetic grid represents a good model for the estimation of the peak metamorphic conditions in whiteschists. Experimentally determined Si-isopleths for phengite further constrain peak pressures to ∼43 kbar for the measured Si=3.60 of phengite in the natural whiteschists. All these data provide evidence that the whiteschists reached diamond-facies conditions. The fluid-absent equilibrium 4 kyanite+3 celadonite=4 coesite+3 muscovite+pyrope has been calibrated on the basis of garnet and phengite compositions in the experiments and serves as a geothermobarometer for ultra-high-pressure (UHP) metapelites. For graphite-bearing metapelites and kyanite–phengite eclogites, forming the country rocks of the whiteschists, peak metamorphic pressures of about 44±3 kbar were calculated from this barometer for temperatures of ∼750 °C estimated from garnet–phengite thermometry. Therefore, the whole ultra-high-pressure unit of the Dora-Maira massif most likely experienced peak metamorphic conditions in the diamond stability field. While graphite is common in the metapelites, diamond has not been found so far. The absence of metamorphic microdiamonds might be explained by the low temperature of metamorphism, the absence of a free fluid phase in the metapelites and a short residence time in diamond-facies conditions resulting in kinetic problems in the conversion of graphite to diamond.

Using a previously published, internally consistent thermodynamic dataset and updated models of activity–composition relations for solid solutions, petrogenetic grids in the model system KFMASH (K2O–FeO–MgO–Al2O3–SiO2–H2O) and the subsystems KMASH and KFASH have been calculated with the software THERMOCALC 3.1 in the P–T range 5–36 kbar and 400–810 °C, involving garnet, chloritoid, biotite, carpholite, talc, chlorite, staurolite and kyanite/sillimanite with phengite, quartz/coesite and H2O in excess. These grids, together with calculated AFM compatibility diagrams and pseudosections, are shown to be powerful tools for delineating the phase equilibria and P–T conditions of pelitic high-P assemblages for a variety of bulk compositions. The calculated equilibria and mineral compositions are in good agreement with petrological observation. The calculation indicates that the typical whiteschist assemblage kyanite–talc is restricted to the rocks with extremely high XMg values, decreasing XMg in a bulk composition favoring the stability of chloritoid and garnet. Also, the chloritoid–talc paragenesis is stable over 19–20 kbar in a temperature range of ca. 520–620 °C, being more petrologically important than the previously highlighted assemblage talc–phengite. Moreover, contours of the calculated Si isopleths in phengite in P–T and P–X pseudosections for different bulk compositions extend the experimentally derived phengite geobarometers to various KFMASH assemblages.

Hualiangting whiteschist in the Dabie Shan ultrahigh-pressure eclogite complex is a specific Mg-Al- rich metamorphic rock. Its protolith was a felsic dyke, which suffered replacement by high-pressure fluids during eclogite-facies metamorphism. Based on fluid inclusion results in whiteschist, primary high-pressure fluids are CO2-bearing aqueous solutions. Those prove that there exists CO2-H2O fluid during the ultrahigh-pressure metamorphism. Pseudosecondary H2O inclusions were trapped during ultrahigh-pressure exhumation, whereas secondary H2O inclusions were related to the latest evolutionary stage of the ultrahigh-pressure exhumation.

Rare centimeter-sized superzoned garnets (SZGs) were discovered in two coesite-bearing whiteschists of the Brossasco-Isasca Unit (BIU), southern Dora-Maira massif (DMM), Western Alps. The superzoned garnet consists of a reddish-brown almandine core crowded with inclusions of staurolite, chloritoid, kyanite, chlorite and paragonite, and of a pinkish pyrope rim with sporadic inclusions of kyanite, and magnesian chlorite. The core–rim contact is relatively sharp and marks the termination of the inclusion-rich portion. The core composition of the superzoned garnet is almost identical to, or slightly richer in Mg, than that of the rim of porphyroblastic garnet in metapelites from the same unit. In the rim of the superzoned garnet, Mg–Fe ratio increases abruptly towards the outermost rim, whose composition is identical to that of the common pyrope in the whiteschist. At the core–rim boundary, there is no chemical gap. Chloritoid and staurolite are common inclusions in the core of the superzoned garnet in the whiteschist and in the porphyroblastic garnet in the metapelite. The staurolite composition (Si=2.00 and total R 2+ Grt>Cld>Chl. The resulting petrogenetic grid suggests that the core of the superzoned garnet contains incompatible assemblages, such as St–Cld–Chl vs. Cld–Chl–Ky. New and literature data and results of experiments in the KFASH system suggest that: (1) the superzoned garnet was formed under a single prograde high-pressure/ultra high-pressure (HP/UHP) Alpine metamorphism, (2) the almandine inclusion-rich core of the superzoned garnet crystallized at disequilibrium in a pelitic composition system at around 600°C and less than 16 kbar, probably from a former metapelite xenolith included in a Variscan granitoid, and (3) the chemical environment of the host rock suddenly changed from the normal pelite to the whiteschist composition by a metasomatic process during the rim growth, i.e., at a stage close to the UHP climax.

To investigate the regional thermobaric structure of the diamondiferous Kokchetav ultrahigh-pressure and high-pressure (UHP–HP) massif and adjacent units, eclogite and other metabasites in the Kulet and Saldat–Kol regions, northern Kazakhstan, were examined. The UHP–HP massif is subdivided into four units, bounded by subhorizontal faults. Unit I is situated at the lowest level of the massif and consists of garnet–amphibolite and acidic gneiss with minor pelitic schist and orthogneiss. Unit II, which structurally overlies Unit I, is composed mainly of pelitic schist and gneiss, and whiteschist locally with abundant eclogite blocks. The primary minerals observed in Kulet and Saldat–Kol eclogites are omphacite, sodic augite, garnet, quartz, rutile and minor barroisite, hornblende, zoisite, clinozoisite and phengite. Rare kyanite occurs as inclusions in garnet. Coesite inclusions occur in garnet porphyroblasts in whiteschist from Kulet, which are closely associated with eclogite masses. Unit III consists of alternating orthogneiss and amphibolite with local eclogite masses. The structurally highest unit, Unit IV, is composed of quartzitic schist with minor pelitic, calcareous, and basic schist intercalations. Mineral assemblages and compositions, and occurrences of polymorphs of SiO2 (quartz or coesite) in metabasites and associated rocks in the Kulet and Saldat–Kol regions indicate that the metamorphic grades correspond to epidote–amphibolite, through high-pressure amphibolite and quartz–eclogite, to coesite–eclogite facies conditions. Based on estimations by several geothermobarometers, eclogite from Unit II yielded the highest peak pressure and temperature conditions in the UHP–HP massif, with metamorphic pressure and temperature decreasing towards the upper and lower structural units. The observed thermobaric structure is subhorizontal. The UHP–HP massif is overlain by a weakly metamorphosed unit to the north and is underlain by the low-pressure Daulet Suite to the south; boundaries are subhorizontal faults. There is a distinct pressure gap across these boundaries. These suggest that the highest grade unit, Unit II, has been selectively extruded from the greatest depths within the UHP–HP unit during the exhumation process, and that all of the UHP–HP unit has been tectonically intruded and juxtaposed into the adjacent lower grade units at shallower depths of about 10 km.

The central part of the Kokchetav Massif is exposed in the Chaglinka–Kulet area, northern Kazakhstan. The ultrahigh-pressure–high-pressure (UHP–HP) metamorphic belt in this area is composed of four subhorizontal lithological units (Unit I–IV) metamorphosed under different pressure–temperature (P–T) conditions. The coesite- and diamond-bearing Unit II, which consists mainly of whiteschist and eclogite blocks, is tectonically sandwiched between the amphibolite-dominant Unit I on the bottom and the orthogneiss-dominant Unit III on the top. Total combined thickness of these units is less than 2 km. The rocks of the UHP–HP metamorphic belt are affected by at least four deformational events post-dating peak metamorphism: (i) The earliest penetrative deformation is characterized by non-coaxial ductile flow in a NW–SE direction. The shear sense indicators in oriented samples from Unit I provide consistent top-to-the-northwest motions and those from Unit III provide top-to-the-southeast, south or south-west motions; (ii) Upright folds with subhorizontal enveloping surface refold earlier foliations including shear-indicators throughout the metamorphic belt; (iii) The third stage of deformation is denoted by large-scale bending around a subvertical axis; and (iv) Late localized fault (or shear) zones cut all earlier structures. The fault zones have subvertical shear planes and their displacements are essentially strike-slip in manner. The subhorizontal structure and opposite shear directions between Unit I and Unit III during the earlier deformation stage suggest north-westward extrusion of UHP Unit II.

High- to ultrahigh-pressure metamorphic (HP–UHPM) rocks crop out over 150 km along an east–west axis in the Kokchetav Massif of northern Kazakhstan. They are disposed within the Massif as a 2 km thick, subhorizontal pile of sheet-like nappes, predominantly composed of interlayered pelitic and psammitic schists and gneisses, amphibolite and orthogneiss, with discontinuous boudins and lenses of eclogite, dolomitic marble, whiteschist and garnet pyroxenite. On the basis of predominating lithologies, we subdivided the nappe group into four north-dipping, fault-bounded orogen-parallel units (I–IV, from base to top). Constituent metabasic rocks exhibit a systematic progression of metamorphic grades, from high-pressure amphibolite through quartz–eclogite and coesite–eclogite to diamond–eclogite facies. Coesite, diamond and other mineral inclusions within zircon offer the best means by which to clarify the regional extent of UHPM, as they are effectively sequestered from the effects of fluids during retrogression. Inclusion distribution and conventional geothermobarometric determinations demonstrate that the highest grade metamorphic rocks (Unit II: T = 780–1000°C, P = 37–60 kbar) are restricted to a medial position within the nappe group, and metamorphic grade decreases towards both the top (Unit III: T = 730–750°C, P = 11–14 kbar; Unit IV: T = 530°C, P = 7.5–9 kbar) and bottom (Unit I: T = 570–680°C; P = 7–13.5 kbar). Metamorphic zonal boundaries and internal structural fabrics are subhorizontal, and the latter exhibit opposing senses of shear at the bottom (top-to-the-north) and top (top-to-the-south) of the pile. The orogen-scale architecture of the massif is sandwich-like, with the HP–UHPM nappe group juxtaposed across large-scale subhorizontal faults, against underlying low P–T metapelites (Daulet Suite) at the base, and overlying feebly metamorphosed clastic and carbonate rocks (Unit V). The available structural and petrologic data strongly suggest that the HP–UHPM rocks were extruded as a sequence of thin sheets, from a root zone in the south toward the foreland in the north, and juxtaposed into the adjacent lower-grade units at shallow crustal levels of around 10 km. The nappe pile suffered considerable differential internal displacements, as the 2 km thick sequence contains rocks exhumed from depths of up to 200 km in the core, and around 30–40 km at the margins. Consequently, wedge extrusion, perhaps triggered by slab-breakoff, is the most likely tectonic mechanism to exhume the Kokchetav HP–UHPM rocks.

Coarse-grained whiteschist, containing the assemblage: garnet+kyanite+phengite+talc+quartz/coesite, is an abundant constituent of the ultrahigh-pressure metamorphic (UHPM) belt in the Kulet region of the Kokchetav massif of Kazakhstan. Garnet displays prograde compositional zonation, with decreasing spessartine and increasing pyrope components, from core to rim. Cores were recrystallized at T=380°C (inner) to 580°C (outer) at P

New results acquired in the coesite-bearing terrane of the Dora-Maira massif, Italian Western Alps, indicate that the UHP unit extends over less than 45 km2 and is bounded by two lower-grade continental units. The contact to the footwall unit is a late, lowangle, top-to-the-SW extensional one, which probably cuts across the original thrust contact. The protolith of the pyrope “whiteschist” of the UHP unit is shown to be the metasomatic transformation product of granitic rock. Nearend member magnesiochloritoid and magnesiostaurolite occur in pyrope megablasts. Ordering in mag-nesiostaurolite leads to cross-hatched twinning and superstructures with either a doubled a or doubled c parameter. The Si3.5 phengite-3T from the same whiteschist contains nano- to microscale quartz platelets and talc layers parallel to (001) of the host mica, interpreted as in situ decompression products of the mica. Some of the pyrope megablasts contain late, crack-related granulite-facies mineral assemblages, which reflect very lo...

A second example of yoderite has been discovered in whiteschists from the Southern Chewore Hills of northern Zimbabwe. The mineral is pale green in colour and occurs in an equilibrium assemblage with talc+chlorite+kyanite+dravite+hematite. There is no quartz present. Recalculated microprobe analyses give a structural formula of Mg2Al5.7Fe0.3Si4O18(OH)2, similar to that obtained for the type locality at Mautia Hill, Tanzania, i.e. Mg2Al5.6Fe0.4Si4O18(OH)2. Textural relationships and relative proportions of minerals suggest that the yoderite was formed by reaction between talc, chlorite, kyanite and hematite. Experimental evidence suggests high-water-pressure metamorphic conditions at temperatures exceeding a reaction curve that extends between 13 kbar at 590 °C and 21 kbar at 650 °C. The yoderite-bearing whiteschist is associated with a 1.4 Ga dismembered ophiolite. It is proposed that this yoderite occurrence is associated with a relict subduction/suture zone.

Ion microprobe (SHRIMP) data on zircons from various rock types of an ultrahigh-pressure (UHP) metamorphic whiteschist-type pyrope quartzite lens of the Dora Maira Massif (DMM) consistently show domains giving a Late Eocene age of 35.4 ± 1.0 Ma which is taken as the age of UHP metamorphism. These domains partially replace the older oscillatory zoning pattern of the zircons formed during primary magmatic crystallization at about 275 Ma. Zircons of a metagranitic country rock next to the UHP metamorphic lens have these primary features best preserved. All zircons measured also yield intermediate ‘ages’ between 275 and 35 Ma with a statistical concentration between 260 and 210 Ma. Thus the uniformity of the initial zircon population both in the lens and the country rock evidences a common protolith, that is a granite intruded during the Late Herynian. While the intermediate ages are at least partly due to incomplete resetting of the zircons during UHP metamorphism, those in the 260–210 Ma range may be related to rifting episodes in the Permotriassic. The Mg-rich chemistry of the whiteschist lenses is due to local metasomatic alterations of the granite, perhaps by fluids derived from evaporitic sediments dating as early as the Permotriassic as well. The more pervasive resetting of zircon ages during UHP metamorphism in the pyrope quartzite lenses is explained by the ubiquity of fluids and/or melts produced during subduction by a series of dehydration reactions that occurred only in the more hydrous Mg-rich protoliths and not in their drier granitic neighborhood. Fission track ages determined partly on the same zircon samples yielding 29.9 ± 1.4 Ma mark the time when the UHPM unit had reached about 290°C at a shallow location within the crust. Thus exhumation over a vertical distance of about 120 km must have occurred within about 5–6 Ma indicating an average uplift rate of about 20–24 km/Ma and an average cooling rate of about 85–100°C/Ma. The radiometric data obtained do not lend any support to an Eo-Alpine Cretaceous subduction event so that deep subduction and immediately following exhumation all took place during Early to Mid Tertiary time. This scenario seems to apply to large portions of the Western and Central Alps as well calling for drastic geodynamic reinterpretations of these parts of the Alps.

The lower Austroalpine orthogneiss-micachist complex of the Sopron-Fertorakos area of W. Hungary contains Mg-chlorite-muscovite-quartzphyllites (leuco- phyllite) and Mg-chlorite-bearing kyanite quartzites whose chemical compositions differ greatly from their surrounding rocks. Formation of leucophyllites took place in shear zones and was associated with depletion in alkalies and iron and enrichment of magnesium and H2O. Mg-zonation of relict igneous muscovites of leucophyllites and changes in the whole rock chemical compositions suggest Mg-metasomatism. Material gains and losses have been assessed using the composition-volume relationship approach. Proceeding from metagranite through transition rocks to leucophyllites, MgO, H2O, FeO, and alkalies show continuously increasing dispersion in isocon plots with Mg-enrichment even in sheared gneiss not in contact with leucophyllite. The metasomatic processes that formed the Mg-rich rocks may be similar to those responsible for the formation of high pressure whiteschists in the Central and Western Alps. The geochemical characteristics of the Dora Maira whiteschists (Italy) and their country gneisses are very similar to those of the Sopron leucophyllites, supporting the theory that Mg-metasomatism produced the whiteschist chemistry. On the basis of oxygen isotope compositions of relict igneous muscovites, the precursor granitic rock had a δ18O value around 13‰ proving its crustal anatectic origin. The leucophyllites have whole rock oxygen isotope compositions around 8.5‰ which is in conflict with the theory of an Mg-rich sedimentary protolith. Rather, the low δ18O values reflect fluid/rock interaction with a low δ18O fluid. Quartz-mineral oxygen isotope fractionations yield a metamorphic temperature of 560 ± 30 °C which agrees with earlier estimates from mineral stabilities. Silicon contents of phengites correspond to a metamorphic pressure of ∼13 GPa at this temperature indicating eclogite facies metamorphism. The fluids in equilibrium with leucophyllites had oxygen isotope compositions around 7.9‰, similar to those calculated for the ultrahigh pressure Dora Maira whiteschists (7.6‰), further supporting the genetic link between the leucophyllites and whiteschists. Hydrogen isotope compositions of mixed white mica + chlorite samples from leucophyllites range from −40 to −35‰, correlating with chlorite contents. The calculated endmember chlorite and white mica have δD values of −30 and −40‰, respectively. The similar δD values of the white micas in leucophyllites, gneisses and metagranites suggest an overall equilibration with respect to H isotopes. The calculated δD value of the fluid is approximately 0‰, suggesting a seawater origin. This conclusion was also reached for the Dora Maira whiteschists. A possible fluid source that satisfies both metasomatic and isotopic data is dehydration of hydrothermally altered oceanic crust. The mafic–ultramafic complex of the Alpine Penninic unit underlying the Austroalpine nappes is a likely candidate. The subduction and subsequent dehydration of the ophiolite series would supply the Mg-rich fluids whose migration brought about the metasomatic alteration of the overlying gneiss-micaschist complexes.

Representative diamond-bearing gneisses and dolomitic marble, eclogite and Ti-clinohumite-bearing garnet peridotite from Unit I at Kumdy Kol and whiteschist from Unit II at Kulet, eastern Kokchetav Massif, northern Kazakhstan, were studied. Diamond-bearing gneisses contain variable assemblages, including Grt+Bt+Qtz±Pl±Kfs±Zo±Chl±Tur±Cal and minor Ap, Rt and Zrn; abundant inclusions of diamond, graphite+chlorite (or calcite), phengite, clinopyroxene, K-feldspar, biotite, rutile, titanite, calcite and zircon occur in garnet. Diamond-bearing dolomitic marbles consist of Dol+Di±Grt+Phl; inclusions of diamond, dolomite±graphite, biotite, and clinopyroxene were identified in garnet. Whiteschists carry the assemblage Ky+Tlc+Grt+Rt; garnet shows compositional zoning, and contains abundant inclusions of talc, kyanite and rutile with minor phlogopite, chlorite, margarite and zoisite. Inclusions and zoning patterns of garnet delineate the prograde P–T path. Inclusions of quartz pseudomorphs after coesite were identified in garnet from both eclogite and gneiss. Other ultrahigh-pressure (UHP) indicators include Na-bearing garnet (up to 0.14 wt% Na2O) with omphacitic Cpx in eclogite, occurrence of high-K diopside (up to 1.56 wt% K2O) and phlogopite in diamond-bearing dolomitic marble, and Cr-bearing kyanite in whiteschist. These UHP rocks exhibit at least three stages of metamorphic recrystallization. The Fe-Mg partitioning between clinopyroxene and garnet yields a peak temperature of 800–1000 °C at P >40 kbar for diamond-bearing rocks, and about 740–780 °C at >28–35 kbar for eclogite, whiteschist and Ti-bearing garnet peridotite. The formation of symplectitic plagioclase+amphibole after clinopyroxene, and replacement of garnet by biotite, amphibole, or plagioclase mark retrograde amphibolite facies recrystallization at 650–680 °C and pressure less than about 10 kbar. The exsolution of calcite from dolomite, and development of matrix chlorite and actinolite imply an even lower grade greenschist facies overprint at c. 420 °C and 2–3 kbar. A clockwise P–T path suggests that supracrustal sediments together with basaltic and ultramafic lenses apparently were subjected to UHP subduction-zone metamorphism within the diamond stability field. Tectonic mixing may have occurred prior to UHP metamorphism at mantle depths. During subsequent exhumation and juxtaposition of many other tectonic units, intense deformation chaotically mixed and mylonitized these lithotectonic assemblages.