Coesite Stability Field Research Articles

Retrograde isochemical phase transformations of majoritic garnets included in diamonds: A case study of subcalcic Cr-rich majoritic pyrope from a Snap Lake diamond, Canada

Homogeneity of a peridotitic garnet inclusion in diamond demonstrating excess in Si concentration (i.e. presence of majorite component) was investigated by TEM using FIB prepared foils. The host diamond is a low-nitrogen brown stone, which can be related to type IIa with features of strong plastic deformation. The studied sample is represented by Ca-poor Cr-pyrope of harzburgitic (H) paragenesis from Snap Lake dyke, Canada The garnet had been previously reported to contain Si=3.16 apfu. The revised examination of the sample, resulted in detection of extremely fine-grained symplectite consisting of low Ca-orthopyroxene, clinopyroxene, Cr-spinel and coesite completely located and isolated in the inner part of the garnet crystal, which forms a sharp interface with the surrounding homogeneous garnet. XRD study confirmed the presence of the minerals constituting the symplectite. EPMA showed an identical bulk chemistry of the nanometer-sized symplectite and garnet. Further polishing of the garnet inclusion on the same surface with diamond removed the symplectite, which possibly was present as a thin lens within garnet. The remaining garnet is completely homogeneous as checked by two profiles, and contains unusually high Ni (118.2ppm) and depleted REE patterns. Estimated PT formation conditions of this garnet are 10.8GPa and 1450°C within asthenosphere. Symplectite testifies partial retrograde isochemical phase transformation of the examined garnet which is suggested to be caused by decompression along with plastic deformation of diamond within the coesite stability field at T>1000°C and depth no less than 100km. Because previously published studies of rare majoritic garnets composition were performed by EPMA only, it is possible that the traces of partial phase transformation (symplectite formation) could have been overlooked without additional XRD and/or TEM/AEM studies.

Blueschist- to eclogite-facies rocks: from HP to UHP

Impressed by the beauty of a metamorphic rock from the Saualpe, Austria, Rene-Just Hauy (1743–1822) created the name “eclogite” (the chosen rock), which he derived from the Greek word eκλογή (choice). The name was used for the first time in 1822 in the second edition of Hauy’s “Traite de mineralogie”. However, the first “scientific” description dates back to the second half of the 18th century, when Horace-Benedict de Saussure (1740–1799) mentioned a “beautiful rock that is not described yet” from the Rhone Valley in his famous book “Voyages dans les Alpes”. Eclogite is a metabasic rock dominated by the minerals omphacite and garnet, and in the 20th century eclogite was recognized as a high-pressure (HP) metamorphic rock. Concurrently, it was discovered that metabasic blueschist, dominated by blue sodium-rich amphiboles, represented another HP metamorphic rock formed at lower metamorphic grade than eclogite. Then, in the mid nineteen-eighties, metamorphic coesite (and somewhat later even diamond) was discovered in eclogite-facies rocks, indicating pressures even higher than had been considered possible up to that time. The term “ultrahigh-pressure (UHP) metamorphism” was born. The term describes metamorphism at depths corresponding to pressure–temperature conditions exceeding the lower limit of the coesite stability field. The significance of UHP metamorphism is most striking for rocks that initially formed in the crust and were then subducted to UHP conditions, but our understanding of mantle processes has been enriched through study of mantle-derived garnet peridotites at UHP conditions as well. These findings generated numerous new research projects with enormous implications for petrology, geodynamics, global tectonics, seismology, and geochemical recycling. This special issue contains a selection of contributions which were presented at the 11th International Eclogite Conference (IEC), in accordance with the theme “Blueschist-to eclogite-facies rocks: from HP to UHP”. The meeting took place near Rio San Juan …

An (in‐)coherent metamorphic evolution of high‐P eclogites and their host rocks in the Chinese southwest Tianshan?

AbstractIn the (ultra‐)high‐P–low‐T metamorphic terrane of the Chinese South Tianshan, discontinuous mafic blocks and boudins (former upper oceanic crust) are now embedded in voluminous (mainly metasedimentary) host rocks. Two different models were proposed and relate the occurrence of both high‐P and ultra‐high‐P mafic and metasedimentary rocks to either (i) a tectonic mélange style exhumation, with no exhumation of coherent units, but different lithologies derived from different depths juxtaposed and intermingled during exhumation in the subduction channel, or (ii) the evolution of two coherent metamorphic belts: one with high‐P and the other with ultra‐high‐P conditions. In contrast to most previous studies in the Chinese South Tianshan which focused either on single eclogites or metasedimentary rocks (assumed as representative), this study concentrates on the systematic investigation of both mafic boudins and their immediate sedimentary host rocks, because the investigation of both lithologies and the comparison of their metamorphic evolution is crucial to reconstruct the geodynamical context of the whole (ultra‐)high‐P–low‐T metamorphic complex. Several sample pairs consisting of both lithologies were geochemically investigated and their respective metamorphic evolution was reconstructed using geothermobarometry and thermodynamic modelling. The latter approach considers changes in the mineral assemblage during the metamorphic evolution, as well as changes in mineral composition, which may help to determine the metamorphic history of a rock despite the preservation of critical mineral assemblages. All samples experienced a clockwise P–T path with overall maximum P–T conditions of 540–550 °C and 1.9–2.25 GPa for the host rocks, and 555–575 °C and 2.2–2.5 GPa for the eclogites. Peak‐metamorphic temperatures of ~525–540 °C of the metasedimentary host rocks were also confirmed by Raman spectroscopy of carbonaceous material. Results from thermobarometry and thermodynamic modelling are consistent with the observation that none of the samples contains mineral relicts indicating UHP conditions (like coesite in garnet) and neither conventional thermobarometry, nor thermodynamical modelling resulted in P–T conditions in the stability field of coesite. Thus, no evidence of ultra‐high‐P conditions was found. Given that the whole sampled river valley lies within the proposed ‘ultra‐high‐P sub‐belt’ and considering former studies, which showed that at other places within this ‘unit’ both ultra‐high‐P and high‐P rocks are now juxtaposed on a small scale, the formation of the whole (ultra‐)high‐P–low‐T metamorphic belt in the Chinese South Tianshan as a tectonic mélange style exhumation is more convincing than the formation and juxtaposition of two coherent metamorphic units with high‐P and ultra‐high‐P conditions respectively.

High-pressure granulites of the Podolsko complex, Bohemian Massif: An example of crustal rocks that were subducted to mantle depths and survived a pervasive mid-crustal high-temperature overprint

The Podolsko complex, Bohemian Massif, is a mid-crustal migmatite–granite dome exposed along a tectonic boundary separating the upper crust from the deeply eroded interior of the Variscan orogen, referred to as the Moldanubian Zone. This study examines metamorphic history of mafic and felsic granulites that occur in this complex as minor lenses or layers hosted in pervasively anatectic rocks. The mafic granulite contains garnet with preserved high-Ca cores, which based on pseudosection modelling indicates pressure conditions near the coesite stability field at temperatures of ca. 550–600°C. The relicts of an earlier eclogite-facies stage have been overprinted by a later granulite-facies assemblage consisting of ternary feldspar, orthopyroxene, and spinel in the mafic granulite and sillimanite and spinel in the felsic granulite. Composition of younger garnet (in rims and as smaller grains) in both granulites suggests that a near isothermal decompression of these rocks was followed by heating that reached temperature of ca. 900°C at pressure of ca. 0.5GPa. It is thus concluded that the granulites underwent at least two temporally separate tectonometamorphic events: they were first subducted to mantle depths and exhumed rapidly at relatively low temperatures and then near isobarically heated at mid-crustal levels. The preservation of earlier eclogite-facies garnet in the mafic granulite indicates that the latter event was short-lived and was followed by near isobaric cooling. The geologically brief granulite-facies metamorphism was previously explained as a result of slab break-off and mantle upwelling after the main phase of microplate convergence in the Bohemian Massif. To put the Podolsko complex into a broader tectonic context, we synthesize the available petrologic and structural data from the correlative (U)HP assemblages of the Moldanubian Zone to suggest that they typically do not preserve structural record of the subduction stage, only rarely preserve an early exhumation phase to the middle crust, and most, if not all, have been extensively overprinted by late-stage processes at shallower crustal levels. This synthesis also reveals that kinematics and thus tectonic processes driving the final exhumation of the (U)HP rocks were diverse and may vary from one location to another. These local settings include exhumation in the footwall of major extensional detachments, due to folding and subsequent lateral extrusion, and along crustal-scale strike-slip shear zones. The extensive late-stage overprint masking the earlier (pre-340Ma) subduction history is greatly exemplified in the Podolsko complex where the granulites are only small relics in pervasively anatectic rocks melted at mid-crustal conditions and then rapidly exhumed along extensional detachments to upper-crustal levels.

Preserved near ultrahigh-pressure melt from continental crust subducted to mantle depths

Remnants of hydrous melt formed at mantle depths have been identified and characterized within high-pressure leucogranulites of the Orlica-Snieznik Dome (Bohemian Massif, central Europe). They occur as nanogranites in garnet formed via partial melting of granitoids during the Variscan orogeny. Melt composition and H2O content have been investigated in situ after experimental re-homogenization of the nanogranites, and are consistent with melts produced experimentally from crustal lithologies at mantle depths. This is the first geochemical study of melt inclusions from natural crustal rocks equilibrated close to the stability field of coesite, shedding light on how continental crust melts during deep subduction. Whereas decompressional melting is commonly invoked for deeply subducted crustal lithologies, melting occurred near or at the metamorphic peak pressure in the Orlica-Snieznik granulites. Melting of deeply subducted crustal rocks significantly modifies the rheology and thus promotes fast exhumation: this process has a critical influence on the geodynamic evolution of subduction-collision orogens as well as crustal differentiation at depth.

Experimental constraints on the genesis of Jadeite quartzite from Shuanghe, Dabie Mountain ultra-high pressure metamorphic terrane

Jadeite quartzite, essentially a two-phase rock made up of jadeite and quartz, is one of the most important UHP lithologies occurring in the Dabie Mountain ultrahigh pressure metamorphic belt and forms layers in biotite-plagioclase gneiss. High pressure-high temperature studies on natural albite from the country rock gneiss were undertaken to reveal the—in parts—complex mineralogical changes that occur in the jadeite quartzite during prograde metamorphism. Experiments were conducted at 800-1200℃, in the pressure range of 2.0-3.5 GPa. One of the most intriguing results shows that the low pressure boundary of the jadeite+coesite stability field is located between about 3.2 GPa at 1000℃ and 3.4 GPa at 1200℃, thus about (0.2-0.3)±0.1 GPa higher than the quartz-coesite transition curve, given the uncertainty in the present study. Minor amounts of sodium and aluminum entering the structure of quartz and the intimate intergrowth texture of the run products may contribute to the observed pressure shift. Combined petrological and mineralogical studies on the run products and the natural rocks yield the following prograde reaction sequence to have occurred: The protolith of the jadeite-quartzite from Dabie Mountain is an albitized siltstone/greywacke characterized by an albite+quartz assemblage. During prograde metamorphism albite breaks down to form jadeite+quartz and thus at this stage two types of quartz can be distinguished whereas type-I-quartz already existed in the protolith, type-Ⅱ-quartz represents a newly formed reaction product of albite. During further P - T -increase the pure type-I-quartz was transformed to coesite, whereas type-Ⅱ-quartz (together with jadeite) was still present as a stable phase because of its impurities of Na and Al. At a later stage during further subduction, type-Ⅱ-quartz also decomposes to form coesite. These studies represent an important puzzlement for a better understanding of the evolution of jadeite-quartzite from the Dabie Mountain during continental crust subduction and thus contribute to a more complete knowledge of the formation of the Dabie Mountain UHP orogenic belt in general.

Pressure–temperature estimates on the Tjeliken eclogite: new insights into the (ultra)-high-pressure evolution of the Seve Nappe Complex in the Scandinavian Caledonides

Abstract The metamorphic evolution of the Tjeliken eclogite, occurring within the Seve Nappe Complex of northern Jämtland (Swedish Caledonides), is presented here. The prograde part of the pressure and temperature ( P–T ) path is inferred from the mineral inclusions (pargasitic amphibole) in garnet and intracrystalline garnet exsolutions in omphacite. Peak metamorphic conditions of 25–26 kbar at 650–700 °C are constrained from geothermobarometry for the peak-pressure assemblage garnet+omphacite+phengite+quartz+rutile, using the garnet–clinopyroxene Fe–Mg exchange thermometer in combination with the net-transfer reaction (6 diopside+3 muscovite=3 celadonite +2 grossular+pyrope) geobarometer, the average P–T method of Thermocalc and pseudosection modelling. Quartz inclusions with well-developed radial cracks were identified within omphacite, which suggest that the studied rock could have been buried down to the coesite stability field. Post-peak P–T evolution is inferred from diopside–plagioclase symplectites and amphibole coronas around garnet. Previous studies in northern Jämtland suggest a substantial gap between the P–T conditions of the Lower and Middle Seve nappes: 14–16 kbar and 550–680 °C and 20–30 kbar and 700–800 °C, respectively. The Tjeliken eclogite has been considered previously to be a part of Lower Seve by most authors, but the new P–T data suggest that it may be an isolated klippe of Middle Seve.

The Realm of Ultrahigh-Pressure Metamorphism

The discovery of diamond and coesite in crustal rocks is compelling evidence that continental material has experienced pressures that can only be achieved at mantle depths. At least 20 terranes of unequivocal continental crust containing diamond or coesite are now recognized around the globe; their study constitutes a new field in petrology called ultrahigh-pressure metamorphism. The idea that continents do not subduct has given way to the notion that Earth has been sufficiently cool since the Cryogenian (~850 Ma) to allow density changes to drive continental crust into the mantle during collision. Some of this crust is exhumed to the surface, some pools at the Moho, and the rest sinks into the mantle. In this issue, microscopic observations, phase-equilibrium modeling, geochronology, and geodynamic modeling track the journey of crustal rocks to the mantle and back to Earth's surface. * Buoyancy : the upward force exerted on the crust by the mantle; the term also refers to the difference in density between the mantle and the oceanic or continental crust because density determines the amount of displacement (floating or sinking) of the crust with respect to the mantle. Coesite : a high-pressure polymorph of SiO2. Coesite is more dense than quartz and is stable at depths greater than 90 km ( P > 2.6 GPa) at 600 °C. Coesite is the main index mineral of UHP metamorphism, but it can also form by shock from a meteorite impact. Delamination : a process whereby rocks at the base of the crust become dense, perhaps by overthickening or gravitational instability, and sink into the mantle. These so-called mantle drips have been imaged seismically and produced in geodynamic models. Diamond : a dense, high-pressure polymorph of carbon, in which each carbon atom is bonded to four other carbon atoms in a rigid tetrahedron, resulting in its cubic symmetry. Diamond is also an index mineral of UHP metamorphism. Eclogite : a metamorphosed mafic rock that contains garnet and omphacite, but not plagioclase. Other common minerals in eclogite are kyanite, quartz/coesite, phengite, epidote-group minerals, magnetite, rutile, and zircon. Exhumation : the movement of a buried rock towards Earth's surface. Exhumation is distinct from uplift, which is simply the increase in elevation of the surface. Garnet peridotite : an ultramafic metamorphic rock composed of garnet + olivine ± pyroxene, sometimes called orogenic peridotite because such rocks are found in collisional orogens. Garnet peridotites are commonly derived from the sublithospheric mantle, but they can also represent minor ultramafic cumulates associated with crustal mafic intrusions. High-pressure (HP) metamorphism : metamorphism at pressures above the calcite to aragonite transition. Na pyroxenes are also stable in this field. HP rocks occupy the blueschist, eclogite, and HP granulite facies. Metamorphic facies : the collection of mineral assemblages from rocks of all bulk compositions that crystallize at the same pressure and temperature conditions. The greenschist, blueschist, amphibolite, granulite, and eclogite facies boundaries are established based on the mineral assemblages in metamorphosed mafic rocks. Omphacite : a dense, Na-rich clinopyroxene, with a minimum of 20% jadeite component. Jadeite is the pure Na pyroxene (NaAlSi2O6) end-member. Pseudomorph : the result of the replacement of one mineral by another, where the new mineral assumes the form of the original. Relamination : a process whereby continental rocks that have reached mantle depths—or their melt products—are returned to the base of the crust. Stishovite : the second high-pressure polymorph of SiO2, formed by the conversion of coesite at mantle depths in excess of 200 km (~7 GPa) at 600 °C or 300 km (~10 GPa) at 1400 °C. Natural stishovite has only been observed in meteorite impact structures. Terrane : a crustal block with a distinct stratigraphy, structure, and geologic history. Exotic terranes are fault-bounded blocks that have a foreign origin compared to the surrounding rocks. Most UHP terranes are of known provenance and belong to a craton, although exotic UHP terranes that originated as continental arcs are also known. A terrain can be thought of as any physical landmass, and the term sometimes refers to topography, as in “rough terrain.” Ultrahigh-pressure (UHP) metamorphism : metamorphism of crustal rocks that occurs at pressures in the coesite stability field. UHP metamorphism is identified by the presence of coesite or diamond, or by the equivalent pressure–temperature conditions calculated from thermodynamic models of mineral compositions.

Two fresh types of eclogites in the Dabie-Sulu UHP metamorphic belt, China: Implications for the deep subduction and earliest stages of exhumation of the continental crust

Two fresh types of eclogites, namely the massive eclogite and foliated eclogite, are discernible in large eclogite bodies surrounded by country rock gneisses from the Dabie (大别)-Sulu (苏鲁) UHP metamorphic zone. They are different in mineral assemblage, texture and structure at various scales. The massive eclogite has a massive appearance with a metamorphic inequigranular and granoblastic texture, which consists mainly of nominally anhydrous minerals such as garnet, omphacite, rutile with inclusions of coesite and rare microdiamond. Massive eclogites which formed at the peak UHP metamorphic conditions (∼3.1–4.0 GPa, 800±50 °C) within the coesite to diamond stability field recorded the deep continental subduction to mantle depths greater than 100 km during the Triassic (∼250–230 Ma). The diagnostic UHP minerals, mineral assemblages and absence of notable macroscopic deformation indicate the peak metamorphic’ forbidden-zone’ P-T conditions, an extremely low geothermal gradient (≤7 °C·km−1) and low differential stress. The foliated eclogite is composed of garnet+omphacite+rutile+phengite+kyanite+zoisite+talc+nyboite±coesite/quartz pseudomorphs after coesite. It is quite clear that the foliated eclogite bears relatively abundant hydrous mineral, and shows well-developed penetrative foliation carrying mineral and stretching lineation reflecting intense plastic deformation or flow of eclogite minerals. The foliated eclogite occurred at mantle levels and recorded the earliest stages of exhumation of UHP metamorphic rocks. At a map scale, the foliated eclogites define UHP eclogite-facies shear zones or high-strain zones. Asymmetric structures are abundant in the zones, implying bulk plane strain or dominant non-coaxial deformation within the coesite stability field. The earliest stages of exhumation, from mantle depths to the Moho or mantle-crust boundary layering, were characterized by a sub-vertical tectonic wedge extrusion, which occurred around 230-210 Ma. The three-dimensional relationship between the massive and foliated eclogites is well displayed a typical ‘block-in-matrix’ rheological fabric pattern indicating the partitioning of deformation and metamorphism in the UHP petrotectonic unit. The existing data support the now widely accepted concept of deep continental subduction/collision and subsequent exhumation between the Yangtze and Sino-Korean cratons. The pressure is a constitutive geological variable. The influence of tectonic overpresure on UHP metamorphism is rather limited.

The Habutengsu metapelites and metagreywackes in western Tianshan, China: metamorphic evolution and tectonic implications

AbstractThe HP‐UHP metamorphic belt of western Tianshan in northwestern China is a rarely preserved oceanic UHP terrane which consists predominantly of meta‐siliciclastic rocks, occasionally accompanied by lens‐shaped metabasites. The metapelites and metagreywackes from the Habutengsu Valley and adjacent area within this belt contain quartz, albite, garnet, white mica, chlorite and rutile/titanite, with or without minor amounts of barroisite, glaucophane, clinozoisite, allanite, graphite, carbonate and tourmaline. Included in coarse‐grained garnet, pseudomorphs of clinozoisite + paragonite after lawsonite are common, seldom also together with inclusions of chloritoid, jadeite and glaucophane. In the northern Habutengsu area, garnet is compositionally characterized by similar cores with consistently low‐Ca content. Similar garnet armouring coesite has been reported in UHP schists from the same area. Deduced P–T conditions during formation of these Ca‐poor garnet cores are 25–31 kbar and 430–510 °C, which are consistent with the computed stability of the observed assemblage Grt + Gln + Lws ± Jd ± Cld in the coesite stability field. Thus, the occurrences of the UHP metapelites and metagreywackes define an internally coherent UHP unit in the north of the Habutengsu area, the spatial extension of which is much larger than previously known. Compared with the northern ones, the southern metapelites and metagreywackes in the Habutengsu area consist of similar minerals and have similar bulk rock compositions but significantly different garnet chemistry, indicating an abrupt variation in P–T conditions during garnet growth. The derived conditions initiating the garnet growth for the southern rocks in a similar range (18–21 kbar and 450–500 °C) and thus constrain a coherent HP unit in the south of the Habutengsu area. The juxtaposition of two exhumed slices of contrasting metamorphic grades probably indicates the change of subduction dynamics of the palaeo‐Tianshan oceanic crust, the subduction polarity (from south to north) of which accounts for the spatial relationship between these two units.

UHP metamorphism recorded by kyanite-bearing eclogite in the Seve Nappe Complex of northern Jämtland, Swedish Caledonides

The first evidence for ultrahigh-pressure (UHP) metamorphism in the Seve Nappe Complex of the Scandinavian Caledonides is recorded by kyanite-bearing eclogite, found in a basic dyke within a garnet peridotite body exposed close to the lake Friningen in northern Jämtland (central Sweden). UHP metamorphic conditions of ~3GPa and 800°C, within the stability field of coesite, are constrained from geothermobarometry and calculated phase equilibria for the peak-pressure assemblage garnet+omphacite+kyanite+phengite. A prograde metamorphic evolution from a lower P–T (1.5–1.7GPa and 700–750°C) stage during subduction is inferred from inclusions of pargasitic amphibole, zoisite and kyanite in garnet cores. The post-UHP evolution is constrained from breakdown textures, such as exsolutions of kyanite and silica from the Ca-Eskola clinopyroxene. Near isothermal decompression of eclogite to lower crustal levels (~0.8–1.0GPa ) led to formation of sapphirine, spinel, orthopyroxene and diopside at granulite facies conditions. Published age data suggest a Late Ordovician (460–445Ma) age of the UHP metamorphism, interpreted to be related to subduction of Baltoscandian continental margin underneath an outboard terrane, possibly outermost Laurentia, during the final stages of closure of the Iapetus Ocean. The UHP rocks were emplaced from the hinterland collision zone during Scandian thrusting of the nappes onto the Baltoscandian foreland basin and platform. The record of P–T conditions and geochonological data from UHP rocks occurring within the allochthonous units of the Scandinavian Caledonides indicate that Ordovician UHP events may have affected much wider parts of the orogen than previously thought, involving deep subduction of the continental crust prior to final Scandian collision between Baltica and Laurentia.

Phase relations during peak metamorphism and decompression of the UHP kyanite eclogites, Pohorje Mountains (Eastern Alps, Slovenia)

Phase relations among the mineral assemblages of UHP kyanite eclogite were investigated in the Pohorje Mountains of the Eastern Alps. Ultrahigh-pressure metamorphism resulted from intracontinental subduction during the Cretaceous (ca. 92Ma). Kyanite-bearing eclogites are associated with meta-ultramafic rocks including UHP garnet peridotites and are embedded in metapelitic gneisses and micaschists. The kyanite eclogites contain a peak metamorphic assemblage of garnet, omphacite, kyanite and phengite. Pyrope-rich garnet is unzoned and almost free of inclusions. The non-stoichiometric supersilicic omphacite contains up to 5mol% of Ca-Eskola molecule. Breakdown of omphacite during decompression resulted in exsolution of oriented rods of silica. Phengite contains up to 3.5 Si a.p.f.u. Polycrystalline quartz inclusions in peak-pressure minerals – garnet, omphacite and kyanite – are surrounded by radial fractures diagnostic of the former presence of coesite. Peak-pressure minerals are replaced by symplectites of diopside+plagioclase+amphibole after omphacite, plagioclase+biotite after phengite and sapphirine+corundum+spinel+anorthite after kyanite. Sapphirine has composition close to (Mg, Fe)12.4 Al38.9 Si4.5 O80 in average, which is amongst the most aluminous yet reported. Peak metamorphic conditions were constrained from calculated phase equilibria in the NKCFMASH system with the fixed bulk-rock composition, and conventional geothermobarometry. This approach led to consistent results, the calculated peak P–T conditions of 3.0–3.7GPa and 710–940°C, in the stability field of coesite and the same range as metamorphic conditions recorded by the associated garnet peridotites. This implies that eclogites and their host rocks were subducted to depths of about 100km. The relatively high temperature at peak pressure, compared to UHP rocks of Tertiary age in the Western Alps where mostly oceanic crust was subducted, probably resulted from radiogenic heat production by subducting continental crust, in the intra-continental setting of the Cretaceous subduction zone in the Eastern Alps.

P-T evolution of kyanite eclogite from the Pirin Mountains (SW Bulgaria): implications for the Rhodope UHP Metamorphic Complex

A new occurrence of kyanite eclogite in the Pirin Mountains of southwestern Bulgaria within the rocks belonging to the Obidim Unit of the Rhodope Metamorphic Complex is presented. This eclogite provides important information about the peak-pressure conditions despite strong thermal overprint at low pressure. Textural relationships, phase equilibrium modelling and conventional geothermobarom- etry were used to constrain the metamorphic evolution. Garnet porphyroblasts with inclusions of omphacite (up to 43 mol.% Jd), phengite (up to 3.5 Si p.f.u.), kyanite, polycrystalline quartz, pargasitic amphibole, zoisite and rutile in the Mg-rich cores (XMg = 0.44-0.46) record a prograde increase in P-T conditions from ! 2.5 GPa and 650 ! C to ! 3 GPa and 700-750 ! C. Maximum pressure values fall within the stability field of coesite. During exhumation, the peak-pressure assemblage garnet + omphacite + phengite + kyanite was variably overprinted by a lower pressure one forming symplect- itic textures, such as diopside + plagioclase after omphacite and biotite + plagioclase after phengite. The development of spinel (XMg = 0.4-0.45) + corundum + anorthite assemblage in the kyanite- bearing domains at ! 1.1 GPa and 800-850 ! C suggests a thermal overprint in the high-pressure granulite facies stability field. This thermal event was followed by cooling at ! 0.8 GPa under amphibolite facies conditions; retrograde kelyphite texture involving plagioclase and amphibole was developed around garnet. Our results add to the already existing evidence for ultra high pressure (UHP) metamorphism in the Upper Allochthon of the Rhodope Metamorphic Complex as in the Kimi Unit and show that it is more widespread than previously known. Published age data and field structural relations suggest that the Obidim Unit represents Variscan continental crust involved into the Alpine nappe edifice of the Rhodopes and that eclogite facies metamorphism was Palaeozoic, in contrast to the Kimi Unit where age determinations suggest a Jurassic or Cretaceous age for UHP metamorphism. This implies that UHP metamorphism in the Upper Allochthon of the Rhodopes may have occurred twice, during Alpine and pre-Alpine orogenic events, and that two independent HP⁄UHP provinces of different age overlap in this area.

Coesite and quartz characteristic of crystallization from shock-produced silica melt in the Xiuyan crater

Abundant coesite and quartz coexist with the shock-produced silica glasses that retain the morphology of primary quartz in the gneiss clasts of polymict breccia from the Xiuyan crater. Quartz occurs as idiomorphic, needle-like and spherulitic crystals, and polycrystalline aggregates. Coesite occurs as granular or idiomorphic, dendritic and needle-like crystals, and polycrystalline aggregates of stringers. The occurrence and morphology of coesite and quartz characterize crystallization from an undercooled dense silica melt. During decompression, coesite firstly crystallizes from shock-produced dense silica melt at high pressure. As pressure decreases to the stability field of quartz, quartz crystallizes from the melt, whereas coesite remains metastable state. A growth rate of 10 − 3 m/s is evaluated for the crystallization of both coesite and quartz based on an estimation of the pressure duration of 10 ms in the coesite stability field for the crater. Higher pressure in silica melt could play a key role for promoting the rapid growth of coesite and quartz by reducing the viscosity of silica melt and the glass transition temperatures substantially. The occurrence of both coesite and quartz embedded in silica glass provides unambiguous evidence for an impact origin of the crater.

Twenty-five years of ultrahigh-pressure metamorphism Preface

This special issue of the European Journal of Mineralogy is dedicated to ultrahigh-pressure (UH P ) metamorphism and its increasing role in the geosciences. The term UH P metamorphism refers to those rock types that were formed or embedded in shallow levels of the continental or oceanic lithosphere and subsequently experienced PT -conditions within or above the lower limits of the coesite stability field. It was 25 years ago that Christian Chopin discovered metamorphic coesite in “pyrope-quartzites” from the Dora-Maira Massif, Italian Western Alps (Chopin, 1984). In the same year, Dave Smith found coesite in eclogites from the Western Gneiss Region, Norway (Smith, 1984). At that time, UH P metamorphism was thought to represent a rather “exotic” type of metamorphic grade. However, the worldwide importance of these rocks is now documented in more than 25 recognized UH P terranes. Thus, UH P metamorphism is far from an “exotic” process, and reflects its significance regarding global endogenic forces. Only through the application of fundamental mineralogical and petrological methodologies was it possible to decode burial and subsequent exhumation of UH P metamorphic rocks. It is clear that all other disciplines of solid earth sciences are affected as a …

Aragonite and magnesite in eclogites from the Jæren nappe, SW Norway: disequilibrium in the system CaCO3–MgCO3 and petrological implications

AbstractEclogites from the Jæren nappe in the Caledonian orogenic belt of SW Norway contain aragonite, magnesite and dolomite in quartz‐rich layers. The carbonates comprise composite grains that occur interstitially between phases of the eclogite facies assemblage: garnet + omphacite + zoisite + clinozoisite + quartz + apatite + rutile ± dolomite ± kyanite ± phengite. Pressure and temperature conditions for the main eclogite stage are estimated to be 2.3–2.8 GPa and 585–655 °C. Published ultrahigh pressure (UHP) experiments on CaO‐, MgO‐ and CO2‐bearing systems have shown that equilibrium assemblages of aragonite and magnesite form as a result of dolomite breakdown at pressures >5 GPa. As a result, recognition of magnesite and aragonite in eclogite facies rocks has been used as an indicator for UHP conditions. However, petrological testing showed that the samples studied here have not experienced such conditions. Aragonite and magnesite show disequilibrium textures that indicate replacement of magnesite by aragonite. This process is inferred to have occurred via a coupled dissolution–precipitation reaction. The formation of aragonite is constrained to eclogite facies conditions, which implies that the studied rocks have experienced metasomatic, reactive fluid flow during their residence at high pressure (HP) conditions. During decompression, the bimineralic carbonate aggregates were overgrown by rims of dolomite, which partially reacted with aragonite to form Mg‐calcite. The well‐preserved carbonate assemblages and textures observed in the studied samples provide a detailed record of the reaction series that affected the rocks during and after their residence at P–T conditions near the coesite stability field. Recognition of the HP mechanism of magnesite replacement by aragonite provides new insight into metasomatic processes that occur in subduction zones and illustrates how fluids facilitate HP carbonate reactions that do not occur in dry systems at otherwise identical physiochemical conditions. This study documents that caution is warranted in interpreting aragonite‐magnesite associations in eclogite facies rocks as evidence for UHP metamorphic conditions.

Ultrahigh-pressure minerals and metamorphic terranes – The view from China

Ultrahigh-pressure (UHP) metamorphism refers to mineralogical modifications of continental and oceanic crustal protoliths ± associated mafic-ultramafic rocks initially formed or emplaced in shallow levels of the lithosphere, but which subsequently have experienced P– T conditions within or above the coesite stability field (>∼2.7 GPa, ∼700 °C). Typical products include eclogite, garnet peridotite, and UHP varieties of metapelite, quartzite, marble, paragneiss, and orthogneiss. UHP metamorphic assemblages require relatively cold lithospheric subduction to mantle depths; some recrystallization even occurs under “forbidden” P– T conditions, characterized by a geotherm of <5 °C/km. In appropriate bulk compositions, UHP metamorphism produces coesite, microdiamond and other indicator phases such as majoritic garnet, TiO 2 with α-PbO 2 structure, supersilicic clinopyroxene, high- P clinoenstatite, K-cymrite and stishovite. Globally, at least 20 coesite-bearing eclogitic, eight diamond-bearing, and five majoritic garnet-bearing UHP regions have been documented thus far; they are mostly of Phanerozoic ages. The presence of majoritic garnet, and even apparent stishovite pseudomorph in supracrustal rocks suggests continental subduction to mantle depths exceeding 300 km; such UHP metamorphic terranes should be distinguished from deep-seated mantle xenoliths that contain UHP minerals. Cold subduction zones may be sites of major recycling of H 2O back into the mantle; high- P experiments on mafic-ultramafic bulk compositions reveal that many important hydrous and formally anhydrous phases are stable under such UHP conditions. The current explosion of research on continental UHP terranes reflects their significance for mantle dynamics and the tectonics of continental subduction, collision, exhumation, mantle–slab interactions, and geochemical recycling. A further characterization of UHP phases and positive identification of UHP minerals requires new experimental studies coupled with state-of-the-art analyses. For example, the very rare occurrence of microdiamond inclusions in zircons from Dabie–Sulu UHP rocks may reflect high f O 2 attending recrystallization inasmuch as epidote is rather common. Rutile needles within garnets from Sulu UHP eclogitic rocks may not be the result of exsolution, so in such cases the apparent UHP pressure may have been over estimated. Hadean igneous microdiamond inclusions in Jack Hills detrital zircons could have originated from mantle xenoliths whereas abundant detrital Phanerozoic diamonds containing inclusions of coesite and other eclogitic minerals from New South Wales might have been derived from unexposed UHP metamorphic terranes. Micro-mineral intergrowth and nano-size minerals may hold important key to deciphering the actual P– T paths of subduction and mantle return flow. Although most exhumed terranes have returned surfaceward relatively rapidly after short time of UHP condition, the long duration of storage at great depth and slow exhumation for the largest UHP terranes remain as major problems.

Preservation of coesite in exhumed eclogite: insights from Raman mapping

Abundant coesite relics have been identified in a single thin section from a carbonate-bearing eclogite from the Saidenbach reservoir, Erzgebirge, German Variscides. The rock is only very slightly retrogressed despite having experienced granulite facies conditions during exhumation. Coesite, mostly 0.1–0.2 mm in long dimension, occurs mainly in garnet but has also been identified in omphacite. Partial transformation of coesite to palisade quartz, the relief contrast between these two phases, and radial cracks around the inclusions allowed an easy optical identification of coesite. Micro-Raman mapping of specific phases from grids of thousands of individual Raman spectra helped show that: (1) a visible palisade quartz corona between coesite and host is in places lacking, and (2) some inclusions contain aragonite. Based on published results from modelling of elastic behaviour of minerals along a pressure–temperature path, a scarcity of palisade quartz suggests that capturing of coesite during garnet growth occurred well into the coesite stability field such that quartz growth at the coesite = quartz univariant curve during decompression was only minimal in unfractured, overpressured inclusions. Another consequence of a high pressure for inclusion capture is that final fracturing of host garnet occurs at lower temperatures. This would explain the presence of aragonite relics in some inclusions. Based on extrapolations from kinetic experiments, aragonite should invert to calcite even faster than coesite to quartz and thus suggests temperatures of 300 °C or less for final fracturing.

Lawsonite eclogites from the Pinchi Lake area, British Columbia—new P– T estimates and interpretation

Lawsonite eclogites from Pinchi Lake, British Columbia, occur as tectonic blocks associated with lawsonite, aragonite, and jadeite-bearing blueschists. One tectonic block from Tezzeron Lake shows extensive lower-temperature retrogression. The block from Beaver Lake Road (BLR) shows less retrogression and the garnet crystals contain abundant inclusions of lawsonite and omphacite suggesting these minerals were stable throughout most of the crystallization history. Compositional profiles and element maps of garnets show: (1) Mn enrichment in cores with decrease toward rims; (2) little change in Ca from core to rim; (3) general decrease in Fe/Mg from core to rim. Phase diagram sections for sample BLR-3 and garnet-core compositions suggest garnet-core growth at 505 °C and 24.3 kbar (1 kbar = 0.1 GPa). Temperature estimates based upon garnet–clinopyroxene Fe–Mg exchange geothermometry are, for inclusions in garnet at 25 kbar, 450 ± 30 °C (4 spots); 435 ± 15 (10); and 445 ± 25 (6). For garnet rims and matrix clinopyroxene the results are: 515 ± 50 °C (2); 440 ± 25 (5); 475 ± 20 (2). These results suggest very little if any statistically significant changes in temperature during crystal growth. Estimates of pressure are based upon several different calibrations of the garnet–clinopyroxene–phengite equilibria. Using the calibration of Krogh-Ravna we obtain pressures of: 24.2–26.8 kbar, compatible with that estimated for garnet-core growth in BLR-3. For the glaucophane = talc + jadeite geobarometer, the activity relations for glaucophane are not well constrained. At 450 °C a pressure of 26 kbar is calculated (one sample). A pressure of 25 kbar at 450 °C suggests depths of ~ 90 km, leading to a P– T gradient of ~ 5 °C km − 1 . This gradient passes through the inferred stability field of lawsonite eclogites and the calculated stability fields of lawsonite for Pinchi Lake samples. This is very near the P– T stability field of coesite, which is characteristic of Ultra-High-Pressure metamorphism.

Tracing high-pressure metamorphism in marbles: Phase relations in high-grade aluminous calcite–dolomite marbles from the Greek Rhodope massif in the system CaO–MgO–Al 2O 3–SiO 2–CO 2 and indications of prior aragonite

Four different types of parageneses of the minerals calcite, dolomite, diopside, forsterite, spinel, amphibole (pargasite), (Ti–)clinohumite and phlogopite were observed in calcite–dolomite marbles collected in the Kimi-Complex of the Rhodope Metamorphic Province (RMP). The presence of former aragonite can be inferred from carbonate inclusions, which, in combination with an analysis of phase relations in the simplified system CaO–MgO–Al 2O 3–SiO 2–CO 2 (CMAS–CO 2) show that the mineral assemblages preserved in these marbles most likely equilibrated at the aragonite–calcite transition, slightly below the coesite stability field, at ca. 720 °C, 25 kbar and a CO 2 ~ 0.01. The thermodynamic model predicts that no matter what activity of CO 2, garnet has to be present in aluminous calcite–dolomite-marble at UHP conditions.