Eclogite-facies Event Research Articles

Provenance, protolith and metamorphic ages of jadeite-bearing orthogneiss and host paragneiss at Tavagnasco, the Sesia Zone, Lower Aosta Valley, Italy

Abstract. An eclogite-facies orthogneiss and host paragneiss from a quarry near Tavagnasco in the Lower Aosta Valley were studied in order to refine the protolith, provenance and metamorphic ages of the Eclogitic Micaschist Complex of the Sesia Zone. The orthogneiss contains jadeite with quartz + phengite + K-feldspar ± garnet + rutile + zircon, whereas the paragneiss hosts garnet + jadeite + phengite ± glaucophane + epidote + rutile + quartz. Phase diagram modeling of two representative samples yields minimum equilibration conditions of 550 ± 50 ∘C and 18 ± 2 kbar. Cathodoluminescence images of zircon from the orthogneiss show oscillatory-zoned cores that are embayed and overgrown by complex, oscillatory-zoned rims. Four concordant secondary ion mass spectrometry analyses from the cores give a weighted mean 206Pb / 238U age of 457 ± 5 Ma. The cores have Th/U = 0.1 and negative Eu anomalies indicative of an igneous protolith, which we interpret to have crystallized in the Ordovician at 780 ∘C, based on Ti-in-zircon measurements. Zircon rims yield a range of 206Pb / 238U dates from 74 to 86 Ma, and four concordant analyses define a weighted mean 206Pb / 238U age of 78 ± 2 Ma. The rims are interpreted to have grown in the eclogite facies based on their lower Th/U (0.01), less negative Eu anomalies and steeper heavy rare earth element (HREE) patterns at <600 ∘C. The paragneiss yielded a detrital zircon population with major peaks at 575–600, 655 and 765 Ma; minor older components; and a maximum depositional age of approximately 570 Ma. The prominent Neoproterozoic zircon population and Ediacaran depositional age suggest derivation from the Gondwana margin. The metamorphic zircon is consistent with the oldest eclogite-facies event in the Sesia Zone; it does not show evidence of multiple periods of rim growth or any pre-Alpine (e.g., Variscan) metamorphism.

Constraining the P–T path of (U)HP rocks with reaction overstepping during subduction; example from the Western Gneiss Region (Norway)

AbstractThis paper presents the results of petrological observations and diffusion modelling on garnet from high‐pressure to ultrahigh‐pressure ((U)HP) metamorphic rocks of the Western Gneiss Region in the Nordfjord. Garnet from kyanite‐bearing micaschist preserves two generations of garnet growth that are related to the Pre‐Caledonian granulite facies and Caledonian eclogite facies metamorphic events. Mafic eclogite, forming lenses in the micaschist, contains only eclogite facies assemblages with partial recrystallization under amphibolite facies conditions. Caledonian garnet in both the micaschist and hosting eclogite indicates reaction overstepping and nucleation near or above 550°C/2.0 GPa. Maximum pressure and temperature, calculated using pseudosection modelling for the eclogite facies event, were ~2.6 GPa and 650°C. The interface between the Pre‐Caledonian and Caledonian garnet in the micaschist shows a strong compositional gradient or possibly a compositional jump. The preservation of such a gradient together with the hummocky‐shaped composition profiles in the Caledonian garnet from the eclogite indicates either no relaxation or a short‐time of relaxation of the rocks at their peak temperature conditions, as well as their exhumation by cooling. Possibly, heating or exhuming of the rocks by isothermal decompression could have easily modified such compositional irregularities along the garnet profiles. A cooling rate of ~187°C/Ma and exhumation rate in the vertical direction of ~2.5 cm/year for the HP rocks were obtained by considering that the temperature and transport distance changes from their maximum depth and peak temperature to the surface were proportional to the time (3.5 Ma) calculated by modelling for the garnet.

Multistage Metamorphic Evolution of Retrograded Eclogites from the Songshugou Complex, Qinling Orogenic Belt, China

Abstract The Qinling Orogenic Belt is one of the major collisional orogens in eastern Asia and marks the boundary between the North China Craton and South China Craton. The Songshugou complex is the largest basic to ultrabasic body to be found in the North Qinling Belt, and was emplaced as a lens-shaped body at the southern margin of the Qinling Group. A detailed petrological investigation of garnet amphibolite, augen amphibolite and well-foliated amphibolite together with garnet zoning patterns of major and trace elements, inclusions in garnet, and thermodynamic modelling indicate a multistage metamorphic history. Garnets clearly show characteristics of discontinuous growth, as they display optically light-colored snowball-textured cores surrounded by a darker mantle with few inclusions as well as chemically a sudden increase in grossular and decrease in almandine components. A partly resorbed rim is not recognized optically but mineral inclusions and a discontinuous chemical composition of garnet are proof of this third garnet growth stage. Rare earth element distribution patterns of garnet also show clear evidence for discontinuous growth and allow us to identify the reactions responsible for garnet growth. Garnet core compositions as well as amphibole inclusions allow us to constrain a P–T window where this rock equilibrated in a first stage. Calculated pseudosections and the application of the garnet–amphibole thermometer indicate an upper amphibolite- to lower granulite-facies metamorphic episode at 630–740 °C and 0·7–0·9 GPa. The presence of relict omphacite as well as a discontinuously grown garnet mantle with rutile inclusions clearly places the peak metamorphic stage in the eclogite facies. Garnet (XGrs, XAlm, XPrp) and omphacite isopleths (XMg, XNa) constrain this event at 1·7–2·1 GPa and 570–650 °C. Consistent temperatures of 500–650 °C were also determined by clinopyroxene–garnet geothermobarometry for this event. Growth of an outermost rim as well as different stages of garnet breakdown to plagioclase + amphibole coronae and the nearly complete replacement of former omphacite by a variety of symplectites point to an intricate retrograde P–T path. In more strongly retrograded samples plagioclase + amphibole ± quartz pseudomorphs entirely replace former garnet grains. Certain coronae around garnets and symplectites also contain prehnite and pumpellyite, which formed during a late retrograde stage or during a different event at very low P–T conditions (250–350 °C). Based on the detailed petrological study, we favour a multistage metamorphic history of the Songshugou metabasic rocks. The age of the eclogite-facies metamorphic event must be related to the deep subduction of the Songshugou complex during the early Paleozoic, although the age of garnet core growth remains enigmatic. The development of garnet cores indicates an upper amphibolite-facies regional metamorphic overprint succeeded by an eclogite-facies event around 500 Ma and subsequent retrogression seen in replacement of garnet and formation of symplectite. The latest imprint evidenced by prehnite and pumpellyite may be the result of fluid infiltration during the fading orogenic phase or represents a low-temperature overprint by a later process, probably related to the uplift of the North Qinling terrane at around 420 Ma.

Evidence of cumulate crystallization and local development of the eclogite-facies metamorphism in the olivine gabbro of the Marun-Keu complex, Polar Urals, Russia

The paper presents the results of a detailed petrological study of olivine gabbro transformed to different extent by eclogite-facies metamorphism. Textural features of cumulate crystallization in the rock are presented. Plagioclases underwent the greatest alteration and replaced by finegrained aggregate during eclogite-facies metamorphism. Corona textures are developed along the boundaries of the plagioclase (felscic) domains at the contacts with (Fe,Mg)-minerals during eclogite facies event. The mineral thermobarometry and phase equilibrium modeling provide P-T metamorphic conditions in relatively narrow range of temperature 680±60 ᵒC and pressure Р=2,2±0.4 GPa. This is in a good agreement with the previously established conditions for the formation of eclogites and garnet-amphibole peridotite from the same complex.

Metamorphic record of the Asemi‐gawa eclogite unit in the Sanbagawa belt, southwest Japan: Constraints from inclusions study in garnet porphyroblasts

AbstractThe Sanbagawa belt is one of the famous subduction‐related high‐pressure (HP) metamorphic belts in the world. However, spatial distributions of eclogite units in the belt have not yet satisfactorily established, except within the Besshi region, central Shikoku, southwest Japan because most eclogitic rocks were affected by lower pressure overprinting during exhumation. In order to better determine the areal distribution of the eclogite units and their metamorphic features, inclusion petrography of garnet porphyroblasts using a combination of electron probe microanalyser and Raman spectroscopy was applied to pelitic and mafic schists from the Asemi‐gawa region, central Shikoku. All pelitic schist samples are highly retrogressed, and include no index HP minerals such as jadeite, omphacite, paragonite, or glaucophane in the matrix. Garnet porphyroblasts in pelitic schists occur as subhedral or anhedral crystals, and show compositional zoning with irregular‐shaped inner segments and overgrown outer segments, the boundary of which is marked by discontinuous changes in spessartine. This feature suggests that a resorption process of the inner segment occurred prior to the formation of the outer segment, indicating discontinuous crystallization between the two segments. The inner segment of some composite‐zoned garnet grains displays Mn oscillations, implying infiltration of metamorphic fluid during the initial exhumation stage. Evidence for an early eclogite facies event was determined from mineral inclusions (e.g., jadeite, paragonite, glaucophane) in the garnet inner segments. Mafic schists include no index HP minerals in the matrix as with pelitic schists. Garnet grains in mafic schists show simple normal zoning, recording no discontinuous growth during crystal formation. There are no index HP mineral inclusions in the garnet, and thus no evidence suggesting eclogite facies conditions. Quartz inclusions in garnet of the pelitic and mafic schists show residual pressure values (∆ω1) of >8.5 cm−1 and <8.5 cm−1 respectively. The combination of Raman geobarometry and conventional thermodynamic calculations gives peak P–T conditions of 1.6–2.1 GPa at 460–520°C for the pelitic schists. The ∆ω1 values of quartz inclusions in mafic schists are converted to a metamorphic pressure of 1.2–1.4 GPa at 466–549°C based on Raman geothermometry results. These results indicate that a pressure gap definitely exists between the mafic schists and the almost adjacent pelitic schists, which have experienced a different metamorphic history. Furthermore, the peak P–T values of the Asemi‐gawa eclogite unit are compatible with those of Sanbagawa eclogite unit in the Besshi region of central Shikoku, suggesting that these eclogite units share a similar P–T trajectory. The Asemi‐gawa eclogite unit exists in a limited area and is composed of mostly pelitic schists. We infer that these abundant pelitic schists played a key role in buoyancy‐driven exhumation by reducing bulk rock density and strength.

Timing and duration of Variscan high-pressure metamorphism in the French Massif Central: A multimethod geochronological study from the Najac Massif

Accurate dating of eclogite-facies metamorphism is of paramount importance in order to understand the tectonic evolution of an orogen. An eclogite sample from the Najac Massif (French Massif Central, Variscan belt) displays a zircon-bearing garnet-omphacite-amphibole-rutile-quartz peak assemblage. Pseudosection modeling suggests peak pressure conditions of 15–20 kbar, 560–630 °C. Eclogite-facies garnet displays Lu-enriched cores and Sm-rich rims and yields a Lu-Hf age of 382.8 ± 1.0 Ma and a Sm-Nd age of 376.7 ± 3.3 Ma. The ages are interpreted as marking the beginning of the prograde garnet growth during the initial stages of the eclogite-facies metamorphism, and the high-pressure (and temperature) peak reached by the rock, respectively. Zircon grains display chondrite-normalized REE spectra with variably negative, positive or no Eu anomalies and are characterized by either enriched or flat HREE patterns. However, they yield a well constrained in situ LA-ICP-MS U-Pb age of 385.5 ± 2.3 Ma, despite this REE pattern variability. Zr zonation in garnet, Y content in zircon and the diversity of zircon HREE spectra may suggest that zircon crystallized prior to and during incipient garnet growth on the prograde P-T path, recording the initial stages of the eclogite-facies conditions. Consequently, the zircon age of 385.5 ± 2.3 Ma, comparable within error with the Lu-Hf age obtained on garnet, is interpreted as dating the beginning of the eclogite-facies metamorphism. Accordingly, the duration of the prograde part of the eclogite-facies event is estimated at 6.1 ± 4.3 Myr. Subsequent exhumation is constrained by an apatite U-Pb age at 369 ± 13 Ma.

Neoarchean-Paleoproterozoic granulite-facies metamorphism in Uzkaya Salma eclogite-bearing mélange, Belomorian Province (Russia)

A TTG gneiss of Uzkaya Salma, Belomorian Province and retrogressed eclogites embedded in it have been thoroughly analyzed. The gneiss is made of plagioclase, K-feldspar, biotite, and quartz with accessory epidote, chlorite, titanite and zircon. The retrogressed eclogite is featured by garnet, clinopyroxene, plagioclase, amphibole, and quartz with subordinate orthopyroxene, muscovite, epidote, and melt phase exhibiting a typical symplectic texture. Diversity of compositional zonation in garnets depends on different garnet diameters and is related to ion diffusion between garnet and clinopyroxene/amphibole during retrogression. Clinopyroxene is mostly diopside, although a few Jd-enriched varieties (Na-rich clinopyroxene) have been found at some sites, indicating an omphacite precursor. Plagioclase formed in both symplectites and aggregates displays a wide compositional range, revealing alteration at certain PT conditions. K-bearing ternary feldspar casually occurs in association with muscovite, zoisite/epidote, and melt phase, denoting a partial melting product. Phase equilibria modeling, combined with mineralogical constrains yielded a clock-wise PT path with peak conditions of P≥17–18kbar, T=700–750°C, followed by granulite-facies at P=13–15kbar, T=800–850°C, and, eventually into the (upper) amphibolite-facies of T≤700–750°C at lower pressures. A geothermal gradient of ∼12–13°C/km is inferred accordingly, rather common for the Belomorian eclogites and some other eclogite/high-pressure granulite belts (EHPG) on Earth. Zircon U-Pb dating shows that the TTG gneiss formed at 2.73–2.74Ga, while the protolith of the eclogites may have experienced magmatism/anatexis at ≥2.77Ga. There is no implication of any Archean eclogite-facies metamorphism in this work. Nevertheless, a granulite-facies metamorphism, featured by a diopside+plagioclase+amphibole±garnet, orthopyroxene assemblage, has been determined at 2.7Ga that also superimposed the host gneiss. Afterwards, due to regional plume activities, repeated thermal events of granulite-facies metamorphism, occurred at least locally at 2.4–2.5Ga. An eclogite-facies metamorphism at 1.9–2.0Ga was dated only that corresponds to the Lapland-Kola orogeny. Further retrogression into the (upper) amphibolite-facies was documented at 1.8Ga. The proposed metamorphic PT path corresponds to the late Paleoproterozoic eclogite-facies event, while before that, only (repeated) granulite-facies metamorphism has been recorded by zircons.

High-pressure polymetamorphic garnet growth in eclogites from the Mariánské Lázně Complex (Bohemian Massif)

The Marianske-Lazně Complex is a Cambro-Ordovician terrane with metabasites of oceanic-crust affinity and tectonically emplaced between the Saxothuringian Zone and the Tepla-Barrandian Unit (western part of the Bohemian Massif). It is formed by amphibolite (retrogressed after eclogite), serpentinized peridotite, coronitic metagabbro with paragneiss, and felsic orthogneiss. Available geochronological data support Variscan ages for the eclogite-facies metamorphism and subsequent amphibolite-facies reequilibration. Garnet crystals with multi-stage growth features in eclogite were studied to analyse the relation of their chemical zoning with pressure–temperature ( P – T ) changes and/or reactions among coexisting phases during metamorphism. The core garnet has highly contrasted compositions with high Mg and low Ca compared to the surrounding host-garnet crystal. Compositional zoning and crystallographic orientation of the garnet showed that the cores and rims were formed during two different metamorphic events. In addition to conventional geothermobarometry, pseudosection and garnet isopleths were used to estimate P T conditions for both metamorphic events. The older core (garnet I) indicates high-pressure amphibolite facies and the younger rim (garnet II) indicates eclogite-facies conditions. Garnet (II) contains inclusions of Na–Ca amphibole and omphacite, suggesting a prograde metamorphism from blueschist- to eclogite-facies conditions. Full multicomponent-diffusion modelling of compositional zoning at the interface of amphibolite-/eclogite-facies garnets in conjunction with the retrieved P – T paths were used to evaluate the average heating/cooling rates during the eclogite-facies event. These two metamorphic events deciphered from different garnet generations are consistent with available geochronological data and bring new insight into the subduction history of the Saxothoringian and/or Rehic oceanic basins during the Variscan orogeny in the Bohemian Massif.

Timing of eclogite-facies metamorphism of the Chuacús complex, Central Guatemala: Record of Late Cretaceous continental subduction of North America's sialic basement

A Late Cretaceous collision of the southernmost portion of the North American continental margin with an undetermined southern block was first established based on the sedimentation history of the plate's supracrustal cover, which is overthrust by harzburgite-dominated nappes of the Guatemala Suture Complex. The collision is also well registered in the metamorphic evolution of continental eclogites of the Chuacús complex, a geologic unit that represents Mesoproterozoic–Triassic sialic basement of North America at the boundary with the Caribbean plate. Garnet–clinopyroxene–phengite thermobarometry of eclogites hosted in Chuacús gneisses indicates near ultra-high-pressure conditions to ~700°C and ~2.1–2.4GPa. SHRIMP-RG U–Pb dating of eclogite metamorphic zircon yielded a 75.5±2Ma age (95% confidence level). Chondrite-normalized rare-earth element patterns of zircon lack Eu anomalies and show depletions in heavy rare earths, consistent with zircon growing in a plagioclase-free, garnet-rich, eclogite-facies assemblage. Additionally, a Sm–Nd clinopyroxene-two garnet–whole rock isochron from an eclogite band yielded a less precise but consistent age of 77±13Ma. The above features imply subduction to >60km depth of at least a portion of the North American sialic basement during Late Cretaceous collision.The Chuacús complex was overprinted by an amphibolite-facies event. For instance, mafic high-pressure paragneiss contains symplectite, resorbed garnet, and amphibole+plagioclase poikiloblasts. Zircon rims from the paragneiss sample show rare-earth patterns consistent with plagioclase growth and garnet breakdown. Their 74.5±3.5Ma SHRIMP-RG U–Pb age is therefore interpreted as the time of retrogression, which is consistent with previously published results.Within error, the ages of the eclogite-facies event and the amphibolite-facies retrogression are equivalent. Thus exhumation of the Chuacús slab from mantle to mid-crustal depth was quick, taking few million years. During exhumation, partial melting of Chuacús gneisses generated ubiquitous pegmatites. One of the pegmatites intruded the North Motagua mélange, which is a serpentinite-rich subduction complex of the Guatemala Suture Complex containing Early Cretaceous oceanic eclogites. U–Pb, Rb–Sr, and K–Ar ages of the pegmatite range ~76–66Ma. Thus initial juxtaposition of continental and oceanic high-pressure belts of the Guatemala Suture Complex predates Tertiary–present strike-slip faulting between the North-American and Caribbean plates.

Geochronological Evidence of Pan-African Eclogites from the Central Menderes Massif, Turkey

The Menderes Massif in western Anatolia documents a complex geodynamic history from Precambrian to recent. Eclogitic relics found in metagabbros in the Precambrian basement were dated by the U/Pb method. The zircon age data from granulitic (coronitic) and eclogitic metagabbros is consistent with geological constraints as well as relative and radiometric ages of the host granulitic gneisses and augen gneisses. A Pan-African intrusion age (540 Ma) of the metagabbros and a shortly later eclogite facies event (530 Ma) are inferred. This scenario fits with the geodynamic evolution deduced form the acidic country rocks of the eclogitic metagabbros. Direct links between the Menderes Massif and Mozambique belt are obliterated by Alpine deformation. Nevertheless, the tectonic setting and age of the Menderes eclogites support a terminal collision of East and West Gondwana and the final suturing of the Mozambique Ocean during the Early Cambrian.

Eclogite relics in the Variscan orogenic belt of Bulgaria (SE Europe)

Eclogitic rocks were sampled from two zones in the basement of the Sredna Gora terrane (central western Bulgaria): (1) partially retrogressed eclogites and amphibolites embedded in sillimanite-bearing garnet-micaschists with kyanite relics and migmatites and (2) banded amphibolites associated with muscovite-bearing metagranites within two-mica paragneisses. Rutile relics and oligoclase + green hornblende + epidote ± biotite pseudomorphs after garnet suggest an eclogite facies event. A tholeiitic, transitional affinity was determined for the protoliths, suggesting a continental rift environment, consistent with several eclogite-bearing complexes in the eastern segments of the Variscan belt that arose from the Cambro-Ordovician Gondwana break-up. Decreasing pressure after the eclogite overprint was demonstrated by (a) diopside-albite symplectite, and (b) plagioclase + red–brown to green amphibole kelyphite. The early static re-equilibration, dated to 398 ± 5.2 Ma by 40Ar–39Ar technique, was followed by an amphibolite facies foliation, which was pervasive in amphibolites, gneisses and micaschists, and poorly developed in eclogites. The lithospheric PT paths corresponding to higher and lower metamorphic gradients reflect the juxtaposition of crustal and lithospheric mantle units, respectively. In the build-up of the basement of the Balkan orogen, the physical properties of the lithological complexes might have influenced the collisional pattern of involved microplates.

Stress-induced redistribution of yttrium and heavy rare-earth elements (HREE) in garnet during high-grade polymetamorphism

Almandine garnet (Alm62.6Pyr11.4Sps8.4And6.5Grs4.0) exceptionally rich in Y2O3 (1.8–2.3 wt%), Sc2O3 (0.11–0.23 wt%), HREE2O3 (1.9–2.3 wt%), and Na2O (~0.3 wt%) occurs in Precambrian quartzofeld-spatic granulites ( T ~ 860 °C) on Hisaroya, Western Gneiss Region, Norway. The granulites were partially reequilibrated to eclogite-facies ( P = 14.9 ± 1.3 kbar, T = 649 ± 67 °C) and amphibolite-facies assemblages during the Caledonian Orogeny. The granulite-facies garnet is reequilibrated in an outer zone, typically ~4 μm thick, where both Y2O3 and HREE2O3 concentrations decrease to 1.2 wt%, and overgrown by Caledonian garnet with Y2O3 and HREE2O3 both below 0.1 wt%. The granulite-facies cores are also reequilibrated to lower Y2O3 and HREE2O3 (both ~1.8 wt%) along deformation structures including microfaults with horsetails, en-echelon bands, and splaying features around the tip of fractures. Locally, these reequilibrated zones, typically 10–20 μm across, have shoulders with higher Y and HREE (~2.9 wt%) than in the original garnet, suggesting closed-system behavior for these elements. In addition, the garnet locally displays a complex mesh-like pattern with high and low HREE and Y bands, possibly related to strain. Two charge-balancing mechanisms account for Y and HREE incorporation: (1) the yttrium aluminum garnet (Y2Al2O6, YAG) substitution involving incorporation of Al into the tetrahedral site is dominant in the granulite-facies garnet, and (2) coupled substitution with Na into the dodecahedral site is increasingly important during high- P reequilibration. The internal closed-system reequilibration of the granulite-facies garnet may be related to stress concentrations build up in garnet during the eclogite-facies event. Local stress concentrations during propagation of fractures may be the driving force behind the redistribution of elements seen within cores of granulite-facies garnet.

Phase diagram sections applied to amphibolites: a case study from the Ossa-Morena/Central Iberian Variscan suture (Southwestern Iberian Massif)

Amphibolites included in the Ossa-Morena/Central Iberian suture of SW Iberia offer the opportunity of investigating the subduction and collision events, followed by exhumation, which affected this major boundary during the Variscan orogeny. This suture contact is marked by a band of highly deformed and metamorphosed rocks, namely, the Central Unit. The main deformation affecting this unit is a ductile shearing that produced an intense planar–linear fabric. This shearing has two components: a dominant left lateral one and a subordinate dip-slip normal one, which can be taken to be responsible for the exhumation of this unit in Devonian–Carboniferous times. The rocks studied in the present paper are garnet-free amphibolites and garnet-bearing amphibolites intercalated between gneisses. They appear as lens-, dike- or elongated-shaped bodies, their thickness ranging between a metre and several hectometres. The investigation of their metamorphic evolution has been carried out by computing phase diagram sections (pseudosections) for specified bulk rock compositions. This method enables us to achieve a twofold goal: (i) to model the P– T path undergone by the rocks during the metamorphism in question, and (ii) to assess the influence of the fluid phase abundance on the formation and the preservation of the mineral assemblages as well as on the density decrease of the rocks during exhumation. The metabasites of the Central Unit underwent a metamorphic evolution, characterized by an initial eclogite facies event with peak pressures around 1.9 GPa and temperatures about 550 °C, during which grossular-rich garnet and jadeite-rich clinopyroxene coexisted with chlorite, glaucophane, and paragonite. The former two hydrous minerals disappeared during the subsequent stage, characterised by a slight decompression with strong heating. At the peak temperature (≈725 °C and 1.6 GPa), important amounts of water were released owing to paragonite breakdown. This water produced (i) the formation of large amounts of amphibole coexisting with jadeite-rich clinopyroxene and sodium-rich plagioclase, and (ii) a sharp density decrease of the rocks, which must have contributed to their exhumation. Fast uplift is consistent with the deduced sharp isothermal decompression (1.18 GPa). This was, in turn, followed by simultaneous strong cooling and decompression (≈455 °C and 0.3 GPa), which produced the calcium enrichment observed in plagioclase and the jadeite decrease and diopside enrichment in clinopyroxene. This final retrograde evolution occurred at fluid absent conditions, thus allowing the preservation of previously formed high-pressure assemblages.

Metabasites and Charnockites as Bimodal Occurrence Around Narsipatnam Area in the Eastern Ghats Granulite Belt, India

Amphibolites included in the Ossa-Morena/Central Iberian suture of SW Iberia offer the opportunity of investigating the subduction and collision events, followed by exhumation, which affected this major boundary during the Variscan orogeny. This suture contact is marked by a band of highly deformed and metamorphosed rocks, namely, the Central Unit. The main deformation affecting this unit is a ductile shearing that produced an intense planar–linear fabric. This shearing has two components: a dominant left lateral one and a subordinate dip-slip normal one, which can be taken to be responsible for the exhumation of this unit in Devonian–Carboniferous times.The rocks studied in the present paper are garnet-free amphibolites and garnet-bearing amphibolites intercalated between gneisses. They appear as lens-, dike- or elongated-shaped bodies, their thickness ranging between a metre and several hectometres. The investigation of their metamorphic evolution has been carried out by computing phase diagram sections (pseudosections) for specified bulk rock compositions. This method enables us to achieve a twofold goal: (i) to model the P–T path undergone by the rocks during the metamorphism in question, and (ii) to assess the influence of the fluid phase abundance on the formation and the preservation of the mineral assemblages as well as on the density decrease of the rocks during exhumation.The metabasites of the Central Unit underwent a metamorphic evolution, characterized by an initial eclogite facies event with peak pressures around 1.9 GPa and temperatures about 550 °C, during which grossular-rich garnet and jadeite-rich clinopyroxene coexisted with chlorite, glaucophane, and paragonite. The former two hydrous minerals disappeared during the subsequent stage, characterised by a slight decompression with strong heating. At the peak temperature (≈725 °C and 1.6 GPa), important amounts of water were released owing to paragonite breakdown. This water produced (i) the formation of large amounts of amphibole coexisting with jadeite-rich clinopyroxene and sodium-rich plagioclase, and (ii) a sharp density decrease of the rocks, which must have contributed to their exhumation. Fast uplift is consistent with the deduced sharp isothermal decompression (1.18 GPa). This was, in turn, followed by simultaneous strong cooling and decompression (≈455 °C and 0.3 GPa), which produced the calcium enrichment observed in plagioclase and the jadeite decrease and diopside enrichment in clinopyroxene. This final retrograde evolution occurred at fluid absent conditions, thus allowing the preservation of previously formed high-pressure assemblages.

Separate episodes of eclogite and blueschist facies metamorphism in the Aegean metamorphic core complex of Ios, Cyclades, Greece

Abstract Blueschists and eclogites outcrop together throughout the Cyclades, Aegean Sea, Greece. Their coexistence has been explained by compositional variation between different lithologies and a single metamorphic episode under transitional eclogite—blueschist facies conditions has been proposed. However, a detailed study of the fabrics and microstructures in an exceptionally well-preserved eclogite boudin reveals a distinct sequence of several different mineral growth events, all under conditions of high-pressure metamorphism. The earliest metamorphic mineral paragenesis recognized is a blueschist facies, glaucophane-bearing assemblage. Eclogite facies metamorphism followed, which is suggested here as being pervasive throughout the rock mass. This period of metamorphic mineral growth was overprinted by widespread blueschist facies retrogression, with hydration ± metasomatism of earlier formed (eclogite facies) assemblages. These hydration reactions were responsible for a period of porphyroblastic mineral growth in the blueschist facies (prior to c. 40 Ma) which largely obliterated the earlier formed eclogite facies assemblages. Omphacite + garnet assemblages relict from the eclogite facies event are preserved only within the cores of the mafic boudins because fluid was not able to penetrate them. Evidence for the early blueschist assemblage is now preserved only as inclusions within the eclogitic garnets in the core of the boudin. This pattern of metamorphic mineral growth may have significant tectonic implications, as it is consistent with the pattern of metamorphic evolution across the high-pressure metamorphic belt of the Aegean region.

RbSr and 40Ar 39Ar laser probe dating of high-pressure phengites from the Sesia zone (Western Alps): underscoring of excess argon and new age constraints on the high-pressure metamorphism

The combined use of the RbSr and 40Ar 39Ar laser probe methods allows confirmation of the existence of excess argon in phengites from eclogites from the Sesia zone (Western Alps) despite systematic 40Ar 39Ar plateau ages. 40Ar 39Ar phengite ages from two different areas show integrated age spreads between 65.4 ± 0.3 Ma and 109.5 ± 0.4 Ma. The youngest 40Ar 39Ar plateau age (65.9 ± 0.4 Ma) is concordant with two RbSr phengite-whole-rock isochron ages from the same outcrop which display a mean age at 64.2 ± 2.5 Ma and older than a Rb-Sr biotite-whole-rock isochron age at 53.0 ± 1.0 Ma. This age concordance, being incompatible with the concept of isotopic closure temperature, is probably fortuitous and probably results from an excess argon contamination. However, the concordance of the RbSr phengite isotopic closure temperature with the temperature reached during the eclogite-facies event suggests that the age at 64.2 ± 2.5 Ma could be a crystallization age probably contemporaneous with the eclogite-facies event in the Sesia zone. When compared to recent age data from the Piemont zone and the Dora Maira Massif, the entire data set (which needs to be confirmed by extra measurements) support diachronous high-pressure (HP) events in the Western Alps.

Contrasting Eoalpine P-T evolutions in the southern Koralpe, Eastern Alps

The Koralpe crystalline complex and the Plankogel unit represent two lithologically distinct units within the Koralpe region of the southeastemmost Austroalpine crystalline basement. The Eoalpine P-T evolution of these two units is derived from new petrographical data. The Plankogel unit and the Koralpe crystalline complex show markedly different P-T evolutions during the early stages of the Eoalpine event. The rocks of the Koralpe crystalline complex experienced eclogite facies conditions with minimum pressures in the range of 15–16 kbar and temperatures in excess of 700°C. At the same time the Plankogel unit resided in a shallower environment at pressures of 10–11 kbar and temperatures of less than 600°C. The tectonic emplacement of the Plankogel unit into its present position on top of the Koralpe crystalline complex took place after the eclogite facies event in a relatively shallow crustal level. After their juxtaposition the Koralpe crystalline complex and the Plankogel unit were affected by a common amphibolite facies metamorphic overprint. The distinctly different P-T evolution during the early stages of the Eoalpine event and a common history at later stages imply that major tectonic processes were operative in this part of the Austroalpine crystalline basement during the Cretaceous. Such processes may have involved subduction of oceanic and continental lithosphere which may have lead to significant crustal shortening within the Austroalpine basement.

Fluid evolution during high-pressure partial melting in the Austroalpine Ulten Zone, northern Italy

Fluid inclusions in high-grade metamorphic rocks of the Ulten Zone (northern Italy) have been investigated by microthermometry and Raman microspectrometry. Two different types of fluid inclusions can be distinguished: (1) high- to low density CO2±N2-CH4 inclusions in kyanite and quartz of coarse grained garnet-kyanite felses and (2) aqueous inclusions of high salinity in quartz of metapelites, paragneisses and amphibolites as well as migmatitic orthogneisses. The migmatites are thought to have formed on the prograde path to the eclogite-facies event (M1) under fluid oversaturation. Kyanite and quartz of the garnet-kyanite felses contain a CO2-dominated fluid, which may result from a higher solubility of H2O in the melt. A saline, H2O-rich fluid occurs as early inclusions forming small clusters in quartz of the leucosome from migmatitic orthogneisses and as late trail-bound inclusions of lower density in quartz of metasediments and amphibolites. During uplift and crystallization of the melt, this fluid was released and together with external highly saline aqueous fluid added to the rocks, re-equilibration of paragneisses, metapelites and metabasites under amphibolite-facies conditions (M2) was enabled.

The effect of fluid and deformation on zoning and inclusion patterns in poly-metamorphic garnets

Within the Bergen Arcs of W Norway, Caledonian eclogite facies assemblages (T≥650°C, P≥15 kbar) have formed from Grenvillian granulites (T= 800–900°C, P≥10 kbar) along shear zones and fluid pathways. Garnets in the granulites (grtI: Pyr56–40 Alm45–25Gro19–14) are unzoned or display a weak (ca. 1 wt% FeO over 1000μm) zoning. The eclogite facies rocks contain garnets inherited from their granulite facies protoliths. These relict garnets have certain areas with compositions identical to the garnets in their nearby granulite, but can be crosscut by bands of a more Almrich composition (grtII: Pyr31–41Alm40–47Gro17–21) formed during the eclogite facies event. These bands, orientated preferentially parallel or perpendicular to the eclogite foliation, may contain mineral filled veins or trails of eclogite-facies minerals (omphacite, amphibole, white mica, kyanite, quartz and dolomite). Steep compositional gradients (up to 9 wt% FeO over 40 μm) separate the two generations of garnets, indicating limited volume diffusion. The bands are interpreted as fluid rich channels where element mobility must have been infinitely greater than it was for the temperature controlled volume diffusion at mineral interfaces in the granulites. The re-equilibration of granulite facies garnets during the eclogite facies event must, therefore, be a function of fracture density (deformation) and fluid availability. The results cast doubts on modern petrological and geochronological methods that assume pure temperature controlled chemical re-equilibration of garnets.

Structural and metamorphic history of a segment of the Sesia-Lanzo zone, and its bearing on the kinematics of Alpine deformation in the western Alps

Summary The Sesia-Lanzo zone of the western Alps is a zone of heterogeneous crystalline basement that was reworked and remobilised during the Alpine orogeny, partly under eclogite-facies metamorphic conditions. Reworking took place in three separable phases of deformation, each under different metamorphic conditions and each localised predominantly within a particular zone in the crust. The structures and fabrics produced by the phase of deformation post-dating the early eclogite-facies event, and associated with glaucophane-albite parageneses, suggest a strike-slip tectonic setting. Within the Sesia-Lanzo zone, up to a few tens of kilometres of N-S strike-slip movement is inferred for the late Cretaceous-early Tertiary. Movement within a strike-slip system may have been responsible for much of the uplift of the eclogitic rocks from 60 km depth.