Occurrence Of Eclogite Research Articles

Discovery of a >1,000 km Cambrian Eclogite‐Bearing High‐Pressure Metamorphic Belt in the Central Asian Orogenic Belt: Implications for the Final Closure of the Pan‐Rodinian Ocean

AbstractThe immense Central Asian Orogenic Belt (CAOB) records the transition from the Pan‐Rodinian Mirovoi Ocean to the Paleo‐Asian Ocean, but the final closure of the Mirovoi Ocean remains unexplored. Here we document two new eclogite occurrences, located between CAOB microcontinents and the Ediacaran–Cambrian island arc in Mongolia. Pseudosection modeling and geothermobarometry constrain peak conditions of ∼560°C and ∼2.2 GPa for the Urgamal eclogite and ∼715°C and ∼2.0 GPa for the Tsengel eclogite. Zircon U‐Pb dating results suggest the Urgamal eclogite‐facies metamorphism occurred at ∼522 Ma, and the granite gneiss enclosing the eclogite yielded ∼811 Ma crystallization age and ∼519 Ma metamorphic age. Detrital zircon age spectra of the paragneisses associated with the eclogite suggest a probable provenance from the adjacent Zavkhan microcontinent and the Neoproterozoic arc superimposed on the microcontinent. The Tsengel eclogite has a protolith age of ∼853 Ma and an eclogite‐facies metamorphic age of ∼522 Ma, and the associated migmatite gneiss record a ∼520 Ma migmatization event. The rock associations and geochemical affinities indicate that their protoliths represent mafic rocks formed in a continental rift setting and the Early Neoproterozoic subduction–accretion complex, respectively. Their similar tectonic position and metamorphic age suggest that these rocks are part of a single >1,000 km long Cambrian high‐pressure metamorphic belt. The rocks record closure of the Mirovoi Ocean within the CAOB and constrain this crucial event to ∼520 Ma along the western margin of the microcontinents in Mongolia.

Has the tectonic regime of the Baltic Shield always remained the same?

ABSTRACT Recent theories on the growth of continental crust stress its formation in pre-Archean and Archean times with minor additions at later times. The Earth has gone through a unidirectional evolution including surface changes, the genesis of life and an important loss of energy. The energy loss drives tectonic processes, but at a rate declining with available energy and thus with time. Semiquantitative modelling of energy-changes and transport indicates important differences between today’s Earth and the Paleoproterozoic Earth. Actualistic but non-uniformitarian arguments suggest that principal tectonic processes have changed over time. The Baltic Shield displays a fundamental difference in conditions between the c. 0.9 Ga high-pressure metamorphism including the occurrence of eclogite in the western Sveconorwegian Orogen and c. 2 Ga low-pressure metamorphism in the eastern Svecokarelian Orogen. This suggests that modern platetectonic processes were operative at c. 0.9 Ga but leaves the question whether it was active during the Svecokarelian orogeny open. The lack of eclogite in the Svecokarelian Orogen is explained with a change with time of the geotherm slope. The occurrence of two generations of granitic rocks of different compositions in the Svecokarelian is easily explained by an increased mantle heat flow in a stratified continental crust. In contrast to calc-alkaline platetectonic compositions, alkali-calcic rocks occur in a quarter-circle around the Svecokarelian core. The question arises whether modern plate-processes successively replaced proto-plate processes during Paleo- and Mesoproterozoic times.

Absence of evidence for Palaeoproterozoic eclogite-facies metamorphism in East Antarctica: no record of subduction orogenesis during Nuna development

The cratonic elements of proto-Australia, East Antarctica, and Laurentia constitute the nucleus of the Palaeo-Mesoproterozoic supercontinent Nuna, with the eastern margin of the Mawson Continent (South Australia and East Antarctica) positioned adjacent to the western margin of Laurentia. Such reconstructions of Nuna fundamentally rely on palaeomagnetic and geological evidence. In the geological record, eclogite-facies rocks are irrefutable indicators of subduction and collisional orogenesis, yet occurrences of eclogites in the ancient Earth (> 1.5 Ga) are rare. Models for Palaeoproterozoic amalgamation between Australia, East Antarctica, and Laurentia are based in part on an interpretation that eclogite-facies metamorphism and, therefore, collisional orogenesis, occurred in the Nimrod Complex of the central Transantarctic Mountains at c. 1.7 Ga. However, new zircon petrochronological data from relict eclogite preserved in the Nimrod Complex indicate that high-pressure metamorphism did not occur in the Palaeoproterozoic, but instead occurred during early Palaeozoic Ross orogenesis along the active convergent margin of East Gondwana. Relict c. 1.7 Ga zircons from the eclogites have trace-element characteristics reflecting the original igneous precursor, thereby casting doubt on evidence for a Palaeoproterozoic convergent plate boundary along the current eastern margin of the Mawson Continent. Therefore, rather than a Palaeoproterozoic (c. 1.7 Ga) history involving subduction-related continental collision, a pattern of crustal shortening, magmatism, and high thermal gradient metamorphism connected cratons in Australia, East Antarctica, and western Laurentia at that time, leading eventually to amalgamation of Nuna at c. 1.6 Ga.

The metamorphic PT history of Precambrian Belomorian eclogites (Shirokaya Salma), Russia

AbstractThis petrological study details representative well‐preserved and partly retrogressed eclogite samples from the Shirokaya Salma locality in the Belomorian Province (Russia) to resolve ambiguity in the PT evolution of one of the oldest eclogite occurrences on Earth. Based on microstructures, eclogite and granulite facies mineral assemblages comprise omphacite, garnet (interior), quartz, rutile±phengite and diopside, garnet (modified rim), plagioclase, orthopyroxene, amphibole, quartz, rutile/ilmenite±melt respectively. Omphacite and Na‐bearing diopside feature breakdown textures with multiple oriented quartz–amphibole and orthopyroxene lamellae, while diopside forms symplectite with plagioclase. Garnet often exhibits a particular compositional annulus in both major and trace elements (Y+REE). We interpret the high‐Ca and inclusion‐rich garnet (core) with outwards Grs decrease and Prp increase as relict of prograde growth. The mantle‐rim domain of garnet, containing omphacite, diopside, plagioclase and rutile formed during eclogitization. Compositionally homogeneous garnet porphyroblasts are most likely outcomes of high‐T modifications. A progressive zonation of Y+REE from core to rim in several garnets is related to resorption at high temperatures. Both geothermometric and pseudosection calculations reveal a typical orogenic clockwise trajectory for the studied eclogites, and their metamorphic PT path includes a possible pre‐eclogitic stage (M0) at <630°C, ~16–19 kbar, a peak eclogite facies stage (M1) at 19–21 kbar, 670–750°C, a granulite facies overprint (stage M2) from 14–16 kbar, 830–870°C (M2a) to 10–11 kbar, 880–910°C (M2b), and retrogression to <9–11 kbar, <650–680°C (stage M3). It agrees well with our previous estimates for other Salma eclogites with Palaeoproterozoic ages.

Constraints on retrograde metamorphism of UHP eclogites in North Qinling, Central China, from 40Ar/39Ar dating of amphibole and phengite

Coesite- and microdiamond- bearing ultra-high pressure (UHP) eclogites in the North Qinling terrane have been widely retrogressed to amphibolites. Previous geochronological studies on these UHP rocks mainly focused on the timing of peak eclogite facies metamorphism. The Kanfenggou UHP metamorphic domain is one of the best-preserved coesite-bearing eclogite occurrences in the North Qinling terrane. In this study, mafic amphibolites and host schists from this domain were collected for 40Ar/39Ar dating to constrain their retrograde evolution. Two generations of amphibole are recognized based on their mineral parageneses and 40Ar/39Ar ages. A first generation of amphibole from garnet amphibolites yielded irregularly-shaped age spectra with anomalously old apparent ages. Isochron ages of 484–473 Ma and initial 40Ar/36Ar ratios of 3695–774 are obtained from this generation of amphibole, indicating incorporation of excess argon. Second generation amphibole occurs in epidote amphibolites yielded flat age spectra with plateau ages of 464–462 Ma without evidence for excess argon. These ages suggest that the amphibolite-facies metamorphism has taken place as early as 484 Ma and lasted until 462 Ma for the North Qinling UHP metamorphic rocks. Phengite from the country-rock schists yielded 40Ar/39Ar plateau ages of 426–396 Ma, with higher phengite Si contents associated with the older the plateau ages. Based on our new 40Ar/39Ar ages and previous zircon UPb geochronological data, we construct a new detailed pressure-temperature-time (P-T-t) path illustrating the retrograde metamorphism and exhumation rate of the North Qinling eclogites and host schists. The P-T-t path suggests that these UHP metamorphic rocks experienced initial medium-to-high exhumation rates (ca. 8.7 mm/yr) during the Early Ordovician (489–484 Ma), which was mainly derived from buoyancy forces. Subsequently, the exhumation rate decreased gradually from ~0.8 to 0.3 mm/yr from 484 to 426 Ma, which was probably governed by extension and/or erosion.

Variscan eclogites from the Argentera–Mercantour Massif (External Crystalline Massifs, SW Alps): a dismembered cryptic suture zone

We document structural, geochemical, petrological and 40Ar/39Ar geochronological data performed on Variscan eclogites from the Argentera–Mercantour Massif, southwestern Alps. Based on the high-resolution field mapping, we present new eclogite occurrences and discuss the relationships between eclogites and surrounding migmatites. We recognized for the first time preserved eclogite facies assemblages. Trace elements and REE patterns establish that eclogites protoliths are MORBs contaminated by continental crust. Standard thermobarometry and thermodynamic modeling are consistent with P–T values of 640–740 °C for 1.5 ± 0.25 GPa, coherent with paleo-geotherms predicted for warm subduction of oceanic crust. We interpret these eclogites as a dismembered cryptic suture zone. 40Ar/39Ar dating on amphiboles yields an age of 339.7 ± 12 Ma for eclogite retrogression under amphibolite facies conditions. All these data are combined to link the Argentera–Mercantour Massif in the tectonic framework of both pre-Mesozoic Alpine basement and European southern Variscides.

Fluidizate‐Explosive Occurrences in Ophiolites as Indicator of the Subduction Zone Activity: The Urals Example

Fluidizate‐Explosive Occurrences in Ophiolites as Indicator of the Subduction Zone Activity: The Urals Example

P–T evolution of Precambrian eclogite in the Sveconorwegian orogen, SW Sweden

AbstractConditions of the prograde, peak‐pressure and part of the decompressional P–T path of two Precambrian eclogites in the eastern Sveconorwegian orogen have been determined using the pseudosection approach. Cores of garnet from a Fe–Ti‐rich eclogite record a first prograde and syn‐deformational stage along a Barrovian gradient from ~670 °C and 7 kbar to 710 °C and 8.5 kbar. Garnet rims grew during further burial to 16.5–19 kbar at ~850–900 °C, along a steep dP/dT gradient. The pseudosection model of a kyanite‐bearing eclogite sample of more magnesian bulk composition confirms the peak conditions. Matrix reequilibration associated with subsequent near‐isothermal decompression and partial exhumation produced plagioclase‐bearing symplectites replacing kyanite and clinopyroxene at an estimated 850–870 °C and 10–11 kbar. The validity of the pseudosections is discussed in detail. It is shown that in pseudosection modelling the fractionation of FeO in accessory sulphides may cause a significant shift of field boundaries (here displaced by up to 1.5 kbar and 70 °C) and must not be neglected. Fast burial, exhumation and subsequent cooling are supported by the steepness of both the prograde and the decompressional P–T paths as well as the preservation of garnet growth zoning and the symplectitic reaction textures. These features are compatible with deep tectonic burial of the eclogite‐bearing continental crust as part of the underthrusting plate (Eastern Segment, continent Baltica) in a collisional setting that led to an effectively doubled crustal thickness and subsequent exhumation of the eclogites through tectonic extrusion. Our results are in accordance with regional structural and petrological relationships, which demonstrate foreland‐vergent partial exhumation of the eclogite‐bearing nappe along a basal thrust zone and support a major collisional stage at c. 1 Ga. We argue that the similarities between Sveconorwegian and Himalayan eclogite occurrences emphasize the modern style of Grenvillian‐aged tectonics.

Geochemical constraints on petrogenesis of marble-hosted eclogites from the Sulu orogen in China

Marble-hosted eclogite is volumetrically minor in collisional orogens, but its geochemistry has great bearing on the origin of deeply subducted crustal rocks and the fluid mobility of subduction zones. This paper presents a combined study of whole-rock major-trace elements and SrNd isotopes, mineral O isotopes, carbonate C and O isotopes, and zircon UPb ages and LuHf isotopes for marble-hosted ultrahigh-pressure metamorphic eclogites from Rongcheng and Sanqingge in the Sulu orogen. The results provide insights into the protolith nature of eclogites and the fluid mobility of subduction zones. Zircon UPb dating yields consistent middle Triassic ages for the two occurrences of eclogites, indicating new growth of metamorphic zircon during continental collision. The Sanqingge eclogite shows LREE-enriched patterns and negative εNd(t) of −16.6 to −14.3 for whole-rock and negative εHf(t) of −27.1 to −15.2 for metamorphic zircon. A few relict zircon domains show middle Neoproterozoic UPb ages and negative εHf(t) of −35.2 to −15.5. Thus, the Sangqingge eclogite was metamorphosed from a mafic rock that was derived from partial melting of an anciently enriched mantle source. In contrast, the Rongcheng eclogite exhibits flat or even LREE-depleted patterns with negative εNd(t) values of −12.2 to −1.0 for whole-rock but positive εHf(t) values of 5.4 to 10.4 for zircon. The occurrence of interstitial and highly cuspate plagioclase along grain boundaries indicates the presence of partial melting in the eclogite. Thus, its positive zircon εHf(t) values are ascribed to the eclogite protolith of juvenile origin, whereas the LREE depletion is due to extraction of LREE-rich anatectic melt from the eclogite during the Triassic continental collision. As such, the Rongcheng eclogite was metamorphosed from a mafic rock that was derived from partial melting of a less enriched mantle source. All the eclogites from both areas show variably high δ18O values of 9.4‰ to 19.5‰. Oxygen isotope fractionations between mineral pairs mostly yield eclogite-facies temperature of 600 to 800°C, suggesting that the high δ18O signature was inherited from their protoliths before the Triassic subduction. In combination with the field relation between the eclogite and marble, it is inferred that the eclogite protolith is probably basaltic tuff and its high δ18O value would be acquired together with the marble protolith during their deposition from the surface water. Therefore, there would be the limited isotopic exchange between marble and eclogite during continental collision.

A new occurrence of eclogite in the Omi-area, Itoigawa-city, Niigata prefecture: P-T condition and tectonic implications

A new occurrence of eclogite in the Omi-area, Itoigawa-city, Niigata prefecture: P-T condition and tectonic implications

Metamorphic evolution of ultrahigh-pressure rocks from Chinese southwestern Tianshan and a possible indicator of UHP metamorphism using garnet composition in low-T eclogites

How to identify ultrahigh-pressure (UHP) metamorphism in the absence of coesite is a key problem to gain the correct P–T history for an orogenic belt. In this study, garnet composition combined with the pseudosection approach was used to identify ultrahigh-pressure metamorphism and to determine P–T paths for eclogites and metapelites from Chinese southwestern Tianshan. Porphyroblastic garnets from both eclogites and metapelites develop pronounced chemical core-rim textures: the relatively homogeneous core with low pyrope [Prp; Mg/(Ca+Mn+Mg+Fe2+)×100] and grossular [Grs; Ca/(Ca+Mn+Mg+Fe2+)×100] content is overgrown by a thin rim with sharply increased Prp and Grs. Phase equilibria modeling indicates that the ultrahigh-pressure rocks have undergone a clockwise P–T path characterized by heating during early exhumation with peak P–T at 31–33kbar and 490–520°C. The P–T pseudosections for eclogites show that isopleths of Prp and Grs strongly depend on temperature and pressure, respectively, especially in the stability fields of glaucophane–lawsonite-bearing eclogite facies assemblages. This indicates that garnet composition provides robust thermobarometric constraints. Consequently, we propose a Prp–Grs diagram which is subdivided into a high-pressure region and an ultrahigh-pressure region by the quartz–coesite–transition curve. Those garnet compositions which fall into the ultrahigh-pressure region are regarded to have experienced ultrahigh-pressure metamorphism. This approach is expected to be a useful tool to qualitatively identify ultrahigh-pressure metamorphism for glaucophane–lawsonite-bearing eclogites and its particular strength is the quick examination of large datasets comprising samples with similar bulk composition. Using this method, garnet compositions of eclogites and mafic blueschists from Chinese southwestern Tianshan and lawsonite eclogites worldwide are plotted in the Prp–Grs diagram and several possible ultrahigh-pressure eclogite occurrences are newly identified.

Late Cretaceous eclogitic high-pressure relics in the Bitlis Massif

A new occurrence of eclogites was found in the Kesandere valley in the eastern most part of the Bitlis complex, SE Anatolia. These high-pressure (HP) relics were preserved in calc-arenitic metasediments within the high-grade metamorphic basement of the Bitlis complex. The eclogitic parageneses were strongly overprinted during decompression and heating. These new eclogites locality complements the evidence of blueschist-facies metamorphism documented recently in the meta-sedimentary cover sequence of this part of the Bitlis complex. Thermodynamic calculations suggest peak conditions of ca. 480–540 °C/1.9–2.4 GPa. New U/Pb dates of 84.4 ± .9 and 82.4 ± .9 Ma were obtained on zircons from two Kesandere eclogite samples. On the basis of geochemical criteria, these dates are interpreted to represent zircon crystallization during the eclogitic peak stage. Kesandere eclogites differ from those previously described in the western Bitlis complex (Mt. Gablor locality) in terms of lithologic association, protolithic origin, and peak P–T conditions (600–650 °C/1.0–2.0 GPa, respectively). On the other hand, eclogitic metamorphism of Kesandere metasediments occurred shortly before blueschist-facies metamorphism of the sedimentary cover (79–74 Ma 40Ar/39Ar white mica). Therefore, the exhumation of Kesandere eclogites started between ca. 82 and 79 Ma, while the meta-sedimentary cover was being buried. During this short time span, Kesandere eclogite were likely uplifted from ~65 to 35 km depth, indicating a syn-subduction exhumation rate of ~4.3 mm/a. Subsequently, eclogite- and blueschist-facies rocks were likely retrogressed contemporarily during collision-type metamorphism (around 72–69 Ma). The Bitlis HP rocks thus sample a subduction zone that separated the Bitlis–Pütürge (Bistun?) block from the South-Armenian block, further north. To the south, Eocene metasediments of the Urse formation are imbricated below the Bitlis complex. They contain (post Eocene) blueschists, testifying separation from the Arabian plate and southward migration of the subduction zone. The HT overprint of Kesandere eclogites can be related to the asthenospheric flow provoked by subducting slab retreat or break off.

New occurrence of eclogite in the Belomorian mobile belt: Geology, metamorphic conditions, and isotope age

Finds of eclogitelike associations within the Belomorian mobile belt (BMB) in the areas of Gridino [1],Shirokaya and Uzkaya Salma bays, KuruVaara mine[2], and Krasnaya Guba [3] have different geologicalinterpretations in relation to their age and geologicalsetting, as well as geological models of the formation.The eclogitic mineral associations in basic and ultrabasic rocks of the Belomorian area have been knownfor more than 70 years. In the first part of the 20th century, they were registered in the BMB by the authors of[4–6]. Eclogites were repeatedly mentioned byK.A. Shurkin and colleagues (Institute of Precambrian Geology and Geochronology) in the 1950s. Wediscovered a number of new eclogite bodies on theislands of the Keretskii archipelago in the central partof the BMB; previously described [5–7] analogousobjects on Sidorov and Ileika islands have been studiedby the authors in detail as well.Two nappes with different rock compositions maybe distinguished in the geological structure of SidorovIsland. The upper nappe represented by strongly granitized biotite (rarely epidote) gneiss practically doesnot contain basic bodies. The lower nappe with athickness of <30 m is represented by gray granite–gneiss with numerous boudined bodies of basic rocks.Eclogitized bodies of basic rocks were registered in themost outcropped northern and southern parts of theisland. All of them have the character of boudins (up to30–40 m in diameter) surrounded by a granite–gneissmatrix (Fig. 1). Eclogitization is reflected in the formation of linear zones and veins composed of garnet,monoclinic pyroxene with a high Na content, andamphibole (Sample 202) in metabasites. Basic bodiesare usually altered with the formation of rims of intenseamphibolization along the perimeter (Sample 216) witha thickness up to 0.5 m and higher, and intersected bylate pegmatoid and carbonate–quartz veins(Sample 205). Sample 223 was investigated from thethin linear zone of eclogitization of the basic bodyfrom SW Ileika Island, which differs from metabasitesof Sidorov Island in the composition and form ofeclogitization. Metabasites of Sidorov Island correspond to the complex of gabbroanorthosites, whereasrocks of Ileika Island correspond to the complex ofmetaporphyrites–garnet gabbro [7].Eclogites have porphyroblastic and granoblastic,sometimes symplectitic texture. Garnet porphyroblasts are distributed in rock matrix represented bymonoclinic pyroxene with a dependent portion ofamphibole and plagioclase quite regularly. In additionto these minerals, biotite, quartz, magnetite, ilmenite,titanite, rutile, apatite, and pyrite (a total of <2–5% ofrock volume) were registered in eclogites. Garnet ischaracterized by poor zoning reflected in a decrease inthe grossular and pyrope contents from the center tothe margin of porphyroblast and an increase of almandine and spessartine contents. Inclusions of quartz,monoclinic pyroxene, rutile, amphibole, and chloriteare irregularly distributed in garnet. Monoclinicpyroxene of the matrix is represented by prismaticgrains and rarely symplectitic aggregates. According tothe composition, it corresponds to sodic (

Experimental and crystal chemical study of the basalt–eclogite transition in Mars and implications for martian magmatism

This paper represents the start of a campaign to identify possible source regions for pyroxene-phyric “enriched” and “depleted” martian basalts using high pressure studies on martian meteorite compositions that represent liquids or near liquids. Our first experiments focus upon mineralogical and crystal chemical aspects of the basalt–eclogite transformation on Mars. It is anticipated that like Earth, eclogites are not the dominant upper mantle assemblage. However, like Earth they may be important hosts for P, Cl, F, OH, Ti, REE, Sr, Y, high-field-strength elements, Hf, and Zr in the upper mantle. This initial experimental study evaluates the major and trace element crystal chemistry of potential martian eclogite assemblages using a martian melt composition (QUE 94201). This composition is appropriate for this study because it is evolved, so it is pyroxene-phyric, contains abundant phosphate, which is important for storage of REE, and is very well studied. In the high pressure experiments, garnet and omphacitic pyroxene are the dominant phases. The garnet has a compositional range from Al56.9Gr+An24.6Py18.5 to Al41.5Gr+An28.1Py30.4 to Al40.3Gr+An22.6Py37.1 (where Al = almandine, Gr+An = grossular+andradite, and Py = Pyrope) and the pyroxene has jadeite and acmite components of up to 26.0 and 9.9%, respectively. The garnet is enriched in REE over omphacite; this is especially true for the HREE which likely remain sequestered in residual garnet during melting. This observation suggests that garnet may play a key role in producing the relatively flat REE patterns of enriched martian basalts. The prime substitution couple that incorporates REE into garnet is: REE3+X Site+Mg2+Y Site↔R2+X Site+Al3+Y Site, where R2+=Ca, Mn2+, Fe2+, and Mg. This is a very effective couple that accounts for both charge balance and ionic size restrictions in individual sites. The prime substitution couple that incorporates REE into omphacite is: REE3+M2 Site+Na1+M2 Site for 2 Ca2+M2 Site. This is a less effective couple than that for garnet because Na is pulled in two directions in omphacite; it must balance the Al in the M1 site in the jadeite component and it also has to provide a charge deficiency for each REE3+ cation in the M2 site. The potential for the presence of eclogite in the martian mantle, has implications for bulk trace element partitioning differences between eclogite and peridotite assemblages that are closely linked to garnet and pyroxene compositions. Further, the occurrence of eclogites may have implications for the capability of the martian mantle to produce magmas, and the major element composition of those magmas.

Slow subduction and buoyant exhumation of the Sanbagawa eclogite

The Western Iratsu body of the Sanbagawa belt (SW Japan) represents a <2km thick garnet–epidote amphibolite slice with minor intercalated occurrences of eclogite. Equilibrium phase assemblage diagrams (pseudosections) for eclogite and garnet–epidote amphibolite from this body can be successfully used to reconstruct the pressure (P)–temperature (T) path and the evolution of the associated mineral assemblages. Microstructural observations combined with calculated pseudosections reveal two distinct stages of subduction-related metamorphism referred to as M1 and M2. P–T conditions of M1 evolved from 0.9GPa, 570°C to 1.2GPa, 660°C, implying relatively high thermal gradients possibly developed shortly after the initiation of subduction. The available age data and the characteristics of the P–T path suggest that the Western Iratsu body was detached from the slab at 1.2GPa (~40km depth) and the subsequent subduction occurred at a very slow rate due to weak mechanical coupling to the downgoing slab. The prograde metamorphism associated with the second subduction phase (M2) reached the eclogite facies (~1.8GPa, 510–560°C), but mafic rock compositions recrystallized into either eclogite or garnet–epidote amphibolite assemblages, depending on the bulk rock (CaO+Na2O)/(FeO+MgO) ratio. The thermal peak of M2 (~620°C at 1.6GPa) was attained during the early phase of exhumation. Further decompression and re-equilibration (M3) took place in the epidote-amphibolite facies. The M2 to M3 P–T evolution is associated with a progressive increase in thermal gradient (T/P ratio), which can be explained by thermal modeling as the result of the inflow of a very young hot section of the slab. The bulk density of the whole Western Iratsu body at the point of onset of exhumation—the return point—is estimated to be ~3180kgm−3, which is less than the density of mantle, implying that buoyant rise is a viable mechanism for its initial exhumation. Buoyant rise is in accordance with the widespread early-exhumation related deformation that has a dip-parallel stretching lineation. Exhumation of the buoyant discrete mafic slice along the decoupled slab–mantle interface could have been triggered by the heating that lead to fluid liberation and a resultant reduction of the rock strength.

Transmission electron microscopy of omphacite and other minerals in eclogites from the CCSD borehole, China: indications for their deformation and temperature history

Seven eclogite samples from 223 to 584 m depths in the Chinese Continental Scientific Drilling main borehole (CCSD-MH) in the Sulu ultrahigh-pressure metamorphic (UH P M) terrane, Eastern China, were studied by transmission electron microscopy (TEM), in order to characterise their microstructures. We observed, among others, omphacite, jadeitic diopside, garnet, amphibole, rutile, Na-rich plagioclase, quartz, K-feldspar, analcime, and diamond (contamination). Omphacite shows a well-known diffusion-controlled disorder-order phase transition which gives rise to antiphase domains (APDs). In our samples we observed a variation of the APDs’ size between ~5 nm and 2 μm which is correlated with the content in jadeite (Jd) component. The broad maximum of ~1–2 μm APDs’ size is centred on Jd50. This size drops to ~5–10 nm for Jd37 and Jd66. The size variation can be explained with the T -path of the respective omphacites. While the large APDs of the omphacites formed and coarsened during subduction and exhumation, the smallest APDs formed and grew during exhumation. In contrast to other eclogite occurrences, deformational defects in omphacites from the CCSD-eclogites are mostly very rare. Occasionally, stacking faults parallel to (010), free dislocations with the Burgers vector [001] and those with 1/2[110] connected with antiphase domain boundaries (APBs), deformation twins on (100) and small-angle grain boundaries were observed. The explanation is that most CCSD-omphacites were strongly affected by recrystallisation, which took place above 600 °C during subduction up to its maximum temperature of ~750 °C and continued during exhumation above 600 °C into the regime of amphibolite facies for several tens of Ma. Garnet shows very rarely crystal defects, while amphiboles usually displayed dislocations and chain multiplicity faults. Rutile of one sample, which contained a few percent Fe, showed fully coherent, nano-sized platelets (Guinier-Preston zones) parallel to (100) and (010). The existence of fluids during retrogression is documented by K-feldspar and analcime in quartz.

Eclogite from the Kumon range, Myanmar: Petrology and tectonic implications

This paper presents the first report on the occurrence of eclogite from the Kumon range of the Western province in Myanmar, which is in the southeastern extension of the Himalayan orogenic belt. The eclogite is mainly composed of omphacite, garnet, hornblende/edenite/katophorite/taramite, biotite, quartz, and rutile. The garnet grains in the eclogite usually show textures of barrier reef, atoll, and table reef types, and have a wide compositional range of Alm58–70Sps1–2Prp9–16Grs14–31. Omphacite grains that occur as garnet inclusions and as isolated crystals in the matrix have similar compositions of Jd34–45 and Jd37–44, respectively. Lesser amounts of jadeitic clinopyroxene (Jd21–38), phengite, biotite, albite, and quartz occur in the lagoon of barrier reef and atoll garnet grains. The matrix omphacite is partly replaced by symplectite of sodic clinopyroxene of Jd20–29 and albite. The lower limits of the pressure/temperature during the eclogite stage, which are defined by the assemblage of garnet, omphacite (Jd40–45), and quartz, are 1.2–1.3GPa/530–615°C. The finding of eclogite from Myanmar suggests the possibility of a wide occurrence of high-pressure metamorphic rocks in the ophiolite zone along the southeastern extension of the Indus-Yarlung Zangbo suture in Myanmar and Indochina.

Calculated phase equilibria for a morb composition in a P–T range, 450–650 °C and 18–28 kbar: the stability of eclogite

AbstractLower temperature eclogite (with T = 600 °C) represents a significant part of the occurrences of eclogite in orogenic belts. ‘True’ eclogite, with, for example, garnet + omphacite &gt;70%, is well represented in such an occurrence. Calculated phase equilibria in Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O (NCKFMASHTO), for just one rock composition – that of a representative mid‐ocean ridge basalt, morb– are used to see under what circumstances ‘true’ eclogite is predicted to occur. The variables considered are not only pressure (P) and temperature (T) but also water content and oxidation state. The latter two variables are known to exert a significant control on mineral assemblage but are difficult to establish retrospectively from the observed rocks themselves. It is found that whereas oxidation state does have a strong effect on mineral assemblage, the key control on developing ‘true’ eclogite is shown to be temperature and water content. If temperature is established to be &lt;600 °C, water content has to be low (less or much less than that for H2O saturation) in order for ‘true’ eclogite to form. Moreover, unless pressure is at the high end in the range considered, lawsonite eclogite and ‘true’ eclogite will tend to be mutually exclusive, with the former requiring high water content at the lower temperature where it occurs, but the latter requiring low water content.

Formation of high‐pressure metabasites in the southern Appalachian Blue Ridge via Taconic continental subduction beneath the Laurentian margin

Tectonic models that account for the occurrence of eclogite, retrograde eclogite, peridotite, and migmatitic basement gneisses in the southern Blue Ridge invoke closure of a small ocean basin between the eastern Blue Ridge (EBR) and western Blue Ridge (WBR). In this “obducted ophiolite” model, Franciscan‐type, eastward subduction of oceanic crust beneath the Piedmont arc occurred prior to obduction of the arc onto the Laurentian margin. The discovery of kilometer‐scale eclogite bodies near the EBR‐WBR boundary are cited as evidence for obducted ophiolite/accreted island arc models. However, thermobarometric estimates based on mineral chemistry from HP metabasites of the western, central, and eastern Blue Ridge indicate that eclogites and granulites are medium‐temperature type (550–900°C), inconsistent with the obducted ophiolite model. Mid‐ocean ridge basalt subducted beneath continental margins and island arcs rarely reaches temperatures in excess of 550°C. Thermobarometric comparisons of the present study and a reexamination of deep seismic profiles from the Blue Ridge allow for a model of westward subduction of continental material beneath Laurentia during the Taconic orogeny that produced a megamélange now preserved in the central and eastern Blue Ridge tectonic provinces. P‐T paths determined from Blue Ridge HP metabasites are similar to P‐T paths determined previously from Caledonian and Variscan eclogites. Thermobarometric similarities imply a common tectonic history of continental collision, subduction of continental crust to depths of 50–65 km, followed by rapid eduction.

The timing of eclogite facies metamorphism and migmatization in the Orlica–Śnieżnik complex, Bohemian Massif: constraints from a multimethod geochronological study

AbstractThe Orlica–Śnieżnik complex (OSC) is a key geological element of the eastern Variscides and mainly consists of amphibolite facies orthogneisses and metasedimentary rocks. Sporadic occurrences of eclogites and granulites record high‐pressure (HP) to ultrahigh‐pressure (UHP) metamorphic conditions. A multimethod geochronological approach (40Ar–39Ar, Rb–Sr, Sm–Nd, U–Pb) has been used to gain further insights into the polymetamorphic evolution of eclogites and associated country rocks. Special attention was given to the unresolved significance of a 370‐ to 360 Ma age group that was repeatedly described in previous studies. Efforts to verify the accuracy of c. 370 Ma K–Ar phengite and biotite dates reported for an eclogite and associated country‐rock gneiss from the location Nowa Wieś suggest that these dates are meaningless, due to contamination with extraneous Ar. Extraneous Ar is also considered to be responsible for a significantly older 40Ar–39Ar phengite date of c. 455 Ma for an eclogite from the location Wojtowka. Attempts to further substantiate the importance of 370–360 Ma zircon dates as an indicator for a melt‐forming high‐temperature (HT) episode did not provide evidence in support of anatectic processes at this time. Instead, SHRIMP U–Pb zircon dating of leucosomes and leucocratic veins within both orthogneisses and (U)HP granulites revealed two age populations (490–450 and 345–330 Ma respectively) that correspond to protolith ages of the magmatic precursors and late Variscan anatexis. The results of this study further underline the importance of Late Carboniferous metamorphic processes for the evolution of the OSC that comprise the waning stages of HP metamorphism and lower pressure HT overprinting with partial melting. Eclogites and their country rocks provided no chronometric evidence for an UHP and ultrahigh‐temperature episode at 387–360 Ma, as recently suggested for granulites from the OSC, based on Lu–Hf garnet ages (Anczkiewicz et al., 2007).