Eclogite Formation Research Articles

The Diverse Habitats of Eclogite Formation: Insights From the Mesoproterozoic Glenelg Inlier, Scotland

ABSTRACTEclogite‐facies rocks provide important constraints on the behaviour of convergent plate boundaries and the geometries of tectonic reconstructions due to the high to ultrahigh pressure conditions at which they form. Many eclogite occurrences are documented near the suture zone of active collisional settings where they are interpreted to mark the approximate location of former ocean basin subduction. Such observations influence tectonic interpretations for older eclogites within more deeply eroded and/or less well‐exposed terranes. The eclogitic Glenelg inlier in northwest Scotland is one such example, with c. 1 Ga eclogites having previously been interpreted as marking the trace of a Grenville‐aged collisional suture zone that defines a third ‘arm’ to the Grenville orogen alongside well‐defined sutures in North America and Scandinavia. Here we use a combination of geochronology, phase equilibrium modelling and accessory‐phase thermometry to show that the eclogite‐facies assemblages were produced at 18–19 kbar and 700°C–750°C from c. 1.1 to 1.0 Ga. Accounting for the foreland basin setting of equivalent‐aged sedimentary rocks in the region and demonstrating the thermal viability of this setting, we show that eclogite formation occurred in deforming foreland crust adjacent to the Grenville orogen, in a setting broadly analogous to fault‐bounded basement uplifts in the forelands of active deformation belts, such as the Himalayas and Andes. Our results demonstrate that eclogite‐facies rocks can form in a greater range of tectonic settings than are sometimes considered, with implications for tectonic reconstructions of collisional zones. In this instance, our results remove the need for a third ‘arm’ of the Grenville orogen by placing Glenelg in a foreland setting, reconciling the absence of plentiful Grenville‐aged metamorphic rocks in northwest Scotland, the sedimentological record and paleomagnetic data in the wider region.

Stress drops of intermediate-depth intraslab earthquakes beneath Tohoku, northern Japan

We calculated stress drops for 2875 small intraslab earthquakes at intermediate depths beneath Tohoku, Japan. We applied an S-coda-wave spectral ratio method to almost 900,000 event pairs. Detailed velocity values for the oceanic crust (OC) were adopted from previous observational studies. The median stress drops in the OC are about half those in the oceanic mantle (OM). The median stress drop for earthquakes in the OC decreases from depths of 70 to 120 km and increases from 120 to 170 km. Our preferred interpretation is that the rigidity in the OC decreases and then increases with depth due to combined effects of the dehydration associated with the eclogite formation and the increasing temperature with depth. These depth variations are consistent with results of a similar study beneath Hokkaido. The median stress drops in the oceanic plate beneath Tohoku are generally smaller than those beneath Hokkaido. Previous studies imaging the seismic structure at shallow depths and b-value analyses of intraslab earthquakes indicate that the near-trench region of the oceanic plate off Tohoku is more hydrated than that off Hokkaido. Taken together, these results suggest that differences in the degree of hydration of the oceanic plate in the near-trench regions could produce the different behaviors of stress drops of intermediate-depth earthquakes observed in Tohoku and Hokkaido.Graphical

P-T pattern of eclogite formation of the Shafarud Metamorphic Complex in the Talesh subzone, NW Iran using zoned garnets

P-T pattern of eclogite formation of the Shafarud Metamorphic Complex in the Talesh subzone, NW Iran using zoned garnets

Eclogite with biotite porphyroblasts—Which conditions are responsible for their formation? An example from the northern Fleur‐de‐Lys Supergroup, Newfoundland, Canada

AbstractAn eclogite from the Early Palaeozoic Fleur‐de‐Lys Supergroup in Newfoundland was studied because of its biotite porphyroblasts, which very rarely occur in this rock type. Thermodynamic modelling suggests that eclogitic biotite in common metabasite (former basalt–gabbro) is limited to (1) bulk‐rock compositions, which are relatively rich in Fe2+ and K and poor in Fe3+, and (2) the low‐pressure range of the eclogite facies. The latter reason is supported by the determination of the pressure–temperature (P–T) path of the Newfoundland eclogite. Chemical zonation of garnet, presence of phengite with Si contents of ~3.4 per formula unit, Zr contents in rutile and petrographic observations resulted in a P–T trajectory starting at medium‐pressure conditions. Nearly isothermal burial led to a peak pressure of 18–19 kbar at ~575°C, followed by exhumation and slight heating. Deformation occurred at or close to the peak pressure. Subsequent introduction of hydrous fluids caused the formation of porphyroblasts of biotite and Ca–amphibole in the pressure range of 12–17 kbar at peak temperatures of 625–640°C. Retrogression led to very fine‐grained symplectites around omphacite and phengite and marginal replacement of biotite porphyroblasts by plagioclase and titanite. Geodynamic scenarios invoking either a flat subduction of oceanic crust followed by continent–continent collision or intracontinental subduction along a transpressional fault system might best explain the formation of eclogite with biotite porphyroblasts in general. For the Newfoundland eclogite, the latter scenario is preferred.

Origin of Erzgebirge ultrahigh‐pressure garnetite: Formation from a basaltic protolith by serpentinization‐assisted metasomatism?

AbstractErzgebirge ultrahigh‐pressure (UHP) garnet peridotite includes scarce layers of garnet pyroxenite, nodules of garnetite and, very rarely, of eclogite. Peridotite‐hosted eclogite shows the same subalkali‐basaltic bulk rock composition, mineral assemblage and peak conditions as gneiss‐hosted eclogite present in the same UHP unit. Garnetite has considerably more Mg, moderately enhanced Ca and Fe and significantly lower contents of Na, Ti, P, K and Si than eclogite, whereas Al is very similar. In addition, the compatible trace elements (Ni, Co, Cr, V) are elevated and most incompatible elements (Zr, Hf, Y, Sr, Rb and rare Earth elements [REE]) are depleted in garnetite relative to eclogite. In contrast to other large ion lithophile elements (LILEs), Pb (+121%) and Ba (+83%) are strongly enriched. The REE patterns of garnetite are characterized by depletion of light and heavy REE and a medium REE hump indicative of metasomatism, features being absent in eclogite. An exceptional garnetite sample shows an REE distribution similar to that of eclogite. Garnetite is interpreted to have formed from the same, but metasomatically altered, igneous protolith as eclogite. Except for Ba and Pb, the chemical signature of garnetite is explained best by metasomatic changes of its basaltic protolith caused by serpentinization of the host peridotite. Garnetite is chemically similar to basaltic rodingite/metarodingite. Although rodingite is commonly more enriched in Ca, there are also examples with moderately enhanced Ca matching the composition of Erzgebirge garnetite. Limited Ca metasomatism is attributed to the preservation of Ca in peridotite during hydrous alteration. This can be explained by incomplete serpentinization favouring metastable survival of the original clinopyroxene. In this case, most Ca is retained in peridotite and not available for infiltration and metasomatism of the garnetite protolith. This inescapable consequence is supported by the fact that clinopyroxene is part of the garnet peridotite UHP assemblage, which would not be the case if Ca had been removed from the protolith prior to high‐pressure metamorphism. The enrichment of compatible elements in garnetite is attributed to decomposition of peridotitic olivine (Ni, Co) and spinel (Cr, V) during serpentinization. Enrichment of Ba and Pb contrasts the behaviour of other LILEs and is ascribed to dehydration of the serpentinized peridotite (deserpentinization). This requires two separate stages of metasomatism: (1) intense chemical alteration of the basaltic garnetite precursor, together with serpentinization of peridotite at the ocean floor or during incipient subduction; and (2) prograde metamorphism and dehydration of serpentinite during continued subduction, thereby releasing Pb–Ba‐rich fluids that reacted with associated metabasalt. Finally, subduction to >100 km and UHP metamorphism of all lithologies led to formation of garnetite, eclogite and garnet pyroxenite hosted by co‐facial garnet peridotite as observed in the Erzgebirge.

Eclogite in the East Kunlun Orogen, northwestern China: A record of the Neoproterozoic breakup of Rodinia and early Paleozoic continental subduction

An early Paleozoic eclogite belt extends discontinuously for ∼500 km in the East Kunlun Orogen, northwestern China, which provides insight into the evolution of the Proto-Tethys Ocean. However, the nature and age of the eclogite protoliths and subsequent metamorphism are unclear. Here, we report mineral chemical, zircon, and titanite geochronological; whole-rock major and trace elemental; and Sr-Nd isotopic data for eclogites from the Xiarihamu area in the western East Kunlun Orogen. At a minimum, these eclogites underwent peak eclogite-facies metamorphism at 27−28 kbar and 630−650 °C, post-peak decompression, and retrograde amphibolite-facies overprinting. Geochemically, eclogites have characteristics typical of enriched mid-oceanic-ridge basalts and oceanic-island basalts. Inherited magmatic zircon cores yielded a protolith age of 828 Ma, and metamorphic zircons yielded peak and post-peak decompression ages of 443 Ma and 422 Ma, respectively. A titanite U-Pb age of 413 Ma constrains the timing of later amphibolite-facies overprinting. The protoliths of the Xiarihamu eclogites were the products of continental rift-related basaltic magmatism, which was generated during the initial breakup of Rodinia. The protoliths experienced eclogite-facies metamorphism in a continental subduction/collisional setting during the early Paleozoic. The formation of the Xiarihamu eclogites suggests that the Proto-Tethys Ocean had closed by 443 Ma. Given the spatio-temporal relationship between the Xiarihamu magmatic Cu-Ni sulfide deposit and the eclogites studied, the former may have formed in a post-collisional extensional setting. The eclogites and magmatic Cu-Ni sulfide deposit from the Xiarihamu area jointly reveal the evolution of the East Kunlun Orogen from Neoproterozoic supercontinent breakup to early Paleozoic tectonic transitions from collisional convergence to post-collisional extension.

Discovery of Permian–Triassic eclogite in northern Tibet establishes coeval subduction erosion along an ~3000-km-long arc

Abstract Eclogite bodies exposed across Tibet record a history of subduction-collision events that preceded growth of the Tibetan Plateau. Deciphering the time-space patterns of eclogite generation improves our knowledge of the preconditions for Cenozoic orogeny in Tibet and broader eclogite formation and/or exhumation processes. Here we report the discovery of Permo-Triassic eclogite in northern Tibet. U-Pb zircon dating and thermobarometry suggest eclogite-facies metamorphism at ca. 262–240 Ma at peak pressures of ~2.5 GPa. Inherited zircons and geochemistry show the eclogite was derived from an upper-plate continental protolith, which must have experienced subduction erosion to transport the protolith mafic bodies to eclogite-forming conditions. The Dabie eclogites to the east experienced a similar history, and we interpret that these two coeval eclogite exposures formed by subduction erosion of the upper plate and deep trench burial along the same ~3000-km-long north-dipping Permo-Triassic subduction complex. We interpret the synchroneity of eclogitization along the strike length of the subduction zone to have been driven by accelerated plate convergence due to ca. 260 Ma Emeishan plume impingement.

Moho depth and crustal density structure in the Tibetan Plateau from gravity data modelling

Although a large number of geophysical experiments have been performed in the Tibetan plateau and its surroundings, our knowledge and understanding of the uplift and deformation in the Tibetan Plateau caused by the India-Asia collision is still incomplete. Gravity method is inevitable and indispensable tool to investigate the evolution of the Tibetan Plateau due to the environmental complexity. In this study, images of the Moho depth and density disturbances in the Tibetan plateau are derived from the separated regional and local gravity anomalies, respectively. The results show that the Moho depth and density present distinct change from north to south in the central and western Tibetan Plateau, but both are relatively featureless in the eastern region. Moho depth sublinearly increases from the south to north and exceed 70 km at the Bangong-Nujiang suture, interpreted as the downward bending of the Tibetan crust due to the subductions of the Indian plate to the south and Asian plate to the north. The significant low density occurs in the middle and lower crust beneath Himalayas, suggesting the absence of the eclogites under the fast subducting effect of the cold Indian plate. In contrast, the high density may suggest the probable presence of eclogites underneath the Lhasa and Qiangtang terranes, where the mantle convection and thickened crust would provide environment of the high temperatures and pressures for eclogite formation. Low density presents underneath the Jinsha suture, indicating that the partial melting occurs due to the upwelling of asthenospheric material.

Numerical study on the style of delamination

Delamination of the lower crust or lithospheric mantle is one explanation for the surface uplift observed in areas of mountain building. This process describes the removal of the lower part of the tectonic plate and can occur in various ways. Different styles of delamination typically have in common that the upper material (e.g., lowermost crust or lithospheric mantle) is denser than the underlying material (e.g., asthenosphere) and therefore sinks. It has been proposed that the higher density can be caused by the formation of eclogite. In this study we apply a thermomechanical model featuring a density increase within the lithosphere by a phase transition. The model setup is designed to investigate surface uplift and mountain building in an intracontinental setting. Specifically, the model is arranged to closely resemble central Mongolia. The models give insights into the dynamically evolving flow field with respect to the style of removal, therefore the general outcome is also applicable to other orogenic regions. In addition to a systematic study on the phase transition, we also investigate the influence of convergent motion and of the rheology of the crust. Our results reveal that for the absence of a dense (eclogite) layer, delamination initially occurs as a stationary Rayleigh-Taylor instability which appears as a late and short-lived event. In comparison, for a strong density contrast an early, long-lived peeling-off removal style with a stationary slab results. The subsequent asthenospheric upwelling causes further peeling-off events for all density contrasts. For this removal style a retreating slab is observed that occasionally breaks off giving way to a periodic behaviour. The findings confirm that a strong convergence and low viscosity of the crust promote delamination. In addition, the asthenospheric upwelling yields a wide and flat surface uplift. Such dome-like features are observed to be more pronounced for high density contrasts (i.e., strong eclogitisation).

Zinc isotope evidence for carbonate alteration of oceanic crustal protoliths of cratonic eclogites

Zinc isotopic compositions (δ66ZnJMC-Lyon) of low-MgO (<13 wt.%) and high-MgO (>16 wt.%) eclogites from the Koidu kimberlite complex, Sierra Leone, West African Craton, help constrain the origins of cratonic eclogites. The δ66Zn of low-MgO eclogites range from MORB-like to significantly higher values (0.21‰ to 0.75‰), and correlate inversely with Zn concentrations. Since marine carbonates are characterized by higher δ66Zn and lower Zn concentration than basaltic rocks, the low-MgO eclogites are suggested to originate from altered oceanic crustal protoliths that underwent isotopic exchange with carbonates within the crust during subduction. Compared to low-MgO eclogites, all but one of the high-MgO eclogites also have high δ66Zn (0.35‰ to 0.95‰), but they have lower Zn concentrations and Zn/Fe ratios, both of which are negatively correlated with MgO contents. These features point to formation of high-MgO eclogites via metasomatic overprinting of low-MgO eclogites through addition of secondary clinopyroxenes crystallized from infiltrating ultramafic melts. Thus, both low-MgO and high-MgO eclogites bear the imprint of subducted carbonate-bearing oceanic crust. Our study shows that the distinctively high-δ66Zn signatures of marine carbonates can be retained in deeply subducted oceanic crust that may contribute to mantle sources of intraplate alkali basalts with elevated δ66Zn and Zn/Fe. Therefore, Zn isotopes provide a viable means to trace carbonate recycling in the mantle.

Evolution of fluid pathways during eclogitization and their impact on formation and deformation of eclogite: A microstructural and petrological investigation at the type locality (Koralpe, Eastern Alps, Austria)

Eclogites representing three strain stages from the type locality (Saualpe-Koralpe Complex, Eastern Alps, Austria) have been analyzed with regard to their microstructural, petrological and mechanical evolution during the gabbro-eclogite transformation and subsequent deformation. Thermodynamic forward models suggest the presence of a H2O dominated fluid phase during eclogitization and following deformation. While all three eclogite types show the same mineral paragenesis composed of garnet, omphacitic clinopyroxene, quartz and a fine grained polyphase aggregate of kyanite, clinozoisite, quartz and retrograde plagioclase, we do observe a microstructural and mechanical evolution with strain.Eclogitization is controlled by dissolution of metastable gabbro and precipitation of the stable finer grained eclogitic mineral paragenesis. This induces a micro-porosity which allows fluids to migrate in an unchannelized way. Distributed eclogitization results in strain weakening, where both transformational and volumetric strain are accommodated by dissolution-precipitation creep. Subsequent deformation of eclogite results in the development of pronounced eclogitic foliation accompanied by grain coarsening and reduction of porosity. The latter results in a reduced efficiency of dissolution-reprecipitation possibly causing apparent strain hardening. Based on our observations we suggest a model for the rheological evolution during the transformation of a dry mafic rock into an eclogite where the combination of volumetric and transformational strain at the onset of eclogitization assisted by fluid induces rapid weakening followed by apparent hardening once eclogitization is completed and steady state deformation of the eclogite mineral paragenesis sets in.

COMMENTS ON THE ARTICLE AUTHORED BY M.V. MINTS AND K.A. DOKUKINA – THE BELOMORIAN ECLOGITE PROVINCE (EASTERN FENNOSCANDIAN SHIELD, RUSSIA): MESO-NEOARCHEAN OR LATE PALEOPROTEROZOIC?

The comments are given on the article authored by M.V. Mints and K.A. Dokukina – The Belomorian Eclogite Province (Eastern Fennoscandian Shield, Russia): Meso-Neoarchean or Late Paleoproterozoic? (Geodynamics &amp; Tectonophysics 2020, 11 (1), 151–200). The Belomorian (White Sea) province of the Fennoscandia Shield is a key site for studying the tectonics of the early periods because numerous Precambrian eclogites have been found there. It was not anticipated, but the problem of age determinations of the eclogite metamorphism of gabbroids in the White Sea mobile belt has turned out to be extremely relevant not only for this region, but also for the Precambrian geology in general. The reason is that a number of authors determine the age of eclogites as Archean (2.7–2.8 Ga), which makes the White Sea mobile belt the only example of the Archean eclogite metamorphism in the world and, therefore, the only dated evidence in support of the plate tectonic model of the evolution of the Earth’s crust at the earliest stage of its formation. The article consistently provides a critical analysis of the arguments put forward by the supporters of the Archean age of the eclogites of the White Sea mobile belt. Special emphasis is made on the isotope geochronological and geochemical features of the composition of zircons from eclogite samples, as well as on the phase and chemical compositions and distribution patterns of mineral inclusions. Considering the age of eclogite metamorphism that led to the formation of eclogites in the White Sea mobile belt, we propose our interpretation based on a set of independent isotope geochemical dating methods, including the local U- Pb method for heterogeneous zircons with magmatic cores and eclogite rims, the Lu-Hf and Sm-Nd methods for the minerals of eclogite paragenesis (garnet and omphacite). And this age interpretation is fundamentally different from the one described in the commented article: all the three methods independently determine the eclogite metamorphism as Paleoproterozoic and yield the same age of circa 1.9 Ga. According to our data, the eclogites of the White Sea mobile belt are among the most ancient high-pressure rocks, their reliably established age of metamorphism is circa 1.9 Ga, and the age of the magmatic protolith is the range of 2.2–2.9 Ga.

Scaling laws for stagnant-lid convection with a buoyant crust

SUMMARY Stagnant-lid convection, where subduction and surface plate motion is absent, is common among the rocky planets and moons in our solar system, and likely among rocky exoplanets as well. How stagnant-lid planets thermally evolve is an important issue, dictating not just their interior evolution but also the evolution of their atmospheres via volcanic degassing. On stagnant-lid planets, the crust is not recycled by subduction and can potentially grow thick enough to significantly impact convection beneath the stagnant lid. We perform numerical models of stagnant-lid convection to determine new scaling laws for convective heat flux that specifically account for the presence of a buoyant crustal layer. We systematically vary the crustal layer thickness, crustal layer density, Rayleigh number and Frank–Kamenetskii parameter for viscosity to map out system behaviour and determine the new scaling laws. We find two end-member regimes of behaviour: a ‘thin crust limit’, where convection is largely unaffected by the presence of the crust, and the thickness of the lithosphere is approximately the same as it would be if the crust were absent; and a ‘thick crust limit’, where the crustal thickness itself determines the lithospheric thickness and heat flux. Scaling laws for both limits are developed and fit the numerical model results well. Applying these scaling laws to rocky stagnant-lid planets, we find that the crustal thickness needed for convection to enter the thick crust limit decreases with increasing mantle temperature and decreasing mantle reference viscosity. Moreover, if crustal thickness is limited by the formation of dense eclogite, and foundering of this dense lower crust, then smaller planets are more likely to enter the thick crust limit because their crusts can grow thicker before reaching the pressure where eclogite forms. When convection is in the thick crust limit, mantle heat flux is suppressed. As a result, mantle temperatures can be elevated by 100 s of degrees K for up to a few Gyr in comparison to a planet with a thin crust. Whether convection enters the thick crust limit during a planet’s thermal evolution also depends on the initial mantle temperature, so a thick, buoyant crust additionally acts to preserve the influence of initial conditions on stagnant-lid planets for far longer than previous thermal evolution models, which ignore the effects of a thick crust, have found.

Two hundred years eclogites and one hundred years of eclogite discussion

The first significant publication on eclogites (Escola, 1921) marked the start of the longest-discussed problem in petrology – the genesis and the place of eclogite formation and eclogite facies on the Earth. The mineral paragenesis of garnet, omphacite, rutile with rare inclusion of microdiamond in garnet requires conditions of T=800–1000 °C and P=0–60 kbar. According to the geothermal gradient and lithostatic pressure calculations, such conditions should exist at a depth of 60–250 km. The dominant nowadays “subduction-exhumation” hypothesis does not offer a satisfactory explanation of the idea of deep subcrustal crystallization and the actual finding of eclogites in the middle parts of the Earthʼs crust among the amphibolites. Contradictions disappear if it is assumed that they are formed in situ in the geotribological zones of friction within the crust, where kinetic energy is generated, providing the necessary high temperature and pressure.

Bulk inclusion micro‐zircon U–Pb geochronology: A new tool to date low‐grade metamorphism

AbstractDating low‐grade metamorphism is challenging since such rocks commonly lack suitable target minerals for acquiring pressure–temperature–time–deformation (P–T–t–d) data. Herein a new geochronological method termed ‘bulk inclusion dating’ is applied to a chloritoid‐bearing schist from the Staufen‐Höllengebirge Nappe (SHN, Austroalpine Unit, Eastern Alps, Austria) for which Cretaceous metamorphism is imprecisely constrained. Thermodynamic modelling of the phase relations and mineral chemistry predicts the stability of the equilibrium assemblage in a P–T field between 450–490℃ and 0.5–0.7 GPa, which agrees with peak temperature constraints ~490℃ derived from Raman spectroscopy of carbonaceous material. Chemical zoning of, and the zonation of inclusions within, chloritoid confirm porphyroblast growth at these conditions. High‐resolution imaging reveals thousands of minute (length: 0.1–3 µm), euhedral micro‐zircon crystals included in chloritoid porphyroblasts and in the matrix. The morphological and microstructural characteristics of micro‐zircon as well as the crystal size distributions indicate that it nucleated and grew at greenschist facies conditions most likely from a Zr‐saturated fluid. In situ laser ablation inductively coupled plasma mass spectroscopy bulk inclusion dating of metamorphic zircon in the chloritoid rim using a laser spot diameter of 120 μm yields a U–Pb age of 116.7 ± 9.1 Ma (MSWD: 1.5, n: 79). We interpret zircon precipitation and progressive coarsening coeval with chloritoid growth during prograde metamorphism and thus link the age to the late prograde part of the P–T evolution. The contribution of other U‐bearing phases (apatite, epidote, rutile) does not significantly disturb the U–Pb age. The data provide clear evidence for Early Cretaceous metamorphism in the SHN and indicates that metamorphism started at least 20 m.y. before the formation of eclogites in the Austroalpine Unit. The method introduced here allows integration between metamorphic conditions and age constraints in low‐grade metamorphic rocks and opens up new potential applications in petrochronology.

How Himalayan collision stems from subduction

AbstractThe mechanisms and processes active during the transition from continental subduction to continental collision at the plate interface are largely unknown. Rock records of this transition are scarce, either not exposed or obliterated during subsequent events. We examine the tectono-metamorphic history of Barrovian metamorphic rocks from the western Himalayan orogenic wedge. We demonstrate that these rocks were buried to amphibolite-facies conditions from ≤47 Ma to 39 ± 1 Ma, synchronously with the formation (46 Ma) and partial exhumation (45–40 Ma) of the ultrahigh-pressure eclogites. This association indicates that convergence during continental subduction was accommodated via development of a deep orogenic wedge built through successive underplating of continental material, including the partially exhumed eclogites, likely in response to an increase in interplate coupling. This process resulted in the heating of the subduction interface (from ~7 to ~20 °C/km) through advective and/or conductive heat transfer. Rapid cooling of the wedge from 38 Ma, coeval with the formation of a foreland basin, are interpreted to result from indentation of a promontory of thick Indian crust.

No chemical change during high-T dehydration and re-hydration reactions: Constraints from Erzgebirge HP and UHP eclogite

Meta-basaltic eclogite occurs in the Erzgebirge in three high-pressure (HP) units (Unit 1, 2, and 3) as numerous lenses within high-grade felsic rocks. Units 2 and 3 are common HP units with quartz eclogite, and Unit 1 is an ultra-high pressure (UHP) unit with coesite eclogite, garnetite (metarodingite) and garnet peridotite. The conditions of peak metamorphism increase from Unit 3 (600–650 °C, 20–22 kbar), to Unit 2 (670–730 °C, 24–26 kbar) and Unit 1 (840–920 °C, ≥30 kbar), correlating with a decreasing abundance of hydrous minerals (from >10 vol% in Unit 3 to 0 vol% in Unit 1). In all units, the dominating eclogite type is dark-colored with a composition typical of MORB. A subordinate light type is chemically more variable and has higher contents of Mg, Al, Ca, Cr, Ni, and large ion lithophile elements as well as lower Fe, Zr, Y, and rare earth element concentrations than dark eclogite. Light eclogite formation is interpreted by plagioclase accumulation in a MOR magma chamber.No compositional difference is visible between well-preserved eclogite and samples with variable degrees of post-eclogitic overprint. In addition, dark eclogite from all units is compositionally indistinguishable, implying that prograde dehydration reactions did not modify major and trace element concentrations. This conclusion applies to all reactions in the temperature interval between c. 600 °C (peak T in Unit 3) and c. 900 °C (peak T in Unit 1), including prograde breakdown of calcic amphibole, zoisite, paragonite, and phengite. Together with previous studies on dehydration reactions in blueschist and eclogite at <600 °C (Spandler et al., 2003; Spandler et al., 2004), the present results imply that de-volatilization of the basaltic portion of subducting slabs plays only a minor, if any, role for the enrichment of fluid-mobile elements in the mantle wedge. We infer that most, if not all, of the H2O produced by dehydration reactions between 600 and 900 °C is preserved in eclogite due to the pressure-enhanced capability of garnet and omphacite to incorporate structural water. Incorporation of H2O, produced during the final dehydration stage (c. 800 ± 50 °C, 25–30 kbar), in nominally anhydrous minerals may explain why partial melting obviously did not occur in eclogite.

Гипербазиты как фактор геодинамики по результатам исследований на Таймырском геофизическом полигоне

Seismic data indicate the widespread inversion origin of the largest geological structures of the Taimyr Peninsula and record repeated changes of the direction of tectonic movements. The inversion the Yenisei-Khatanga trough led to the formation of a system of megaarchs over the ancient depocenter containing massive hyperbasite intrusions at the Moho level, which are marked by the gravity maximum. These intrusive processes are an important factor that can initiate inversion and contribute significantly to its development under permanent compressional tectonic regime in the region.According to the geophysical data, the development of troughs in the north of Central and Western Siberia is associated with the formation of eclogites under the depocenters, at the Moho level. Consistent with the electrical resistivity anomalies, eclogites are plastic and overlap mantle hyperbasites of lower density that leads to gravitational buoying up of ultramafic formations. This, along with metamorphic and metasomatic processes, acts as an impulse for the uplift in the axial parts of the troughs. Simultaneously, expansion and acceleration of sagging, due to the displacement of plastic eclogites and crystal-mantle mixture from the penetration zone towards the peripheral parts of the sedimentary basin, result in the uncompensated sediment accumulation and the formation of the Neocomian clinoform complex.

Late Cretaceous calc-alkaline and adakitic magmatism in the Sistan suture zone (Eastern Iran): Implications for subduction polarity and regional tectonics

Abstract The N-S trendingSistan orogen (E Iran) stretches along ~700 km at a high angle compared to other Alpine-Himalayan ranges marking the Neotethyan suture (including the nearby Zagros, Makran or Alborz ranges). Both the geometry and timing of closure of the Sistan ocean are currently debated. We provide geochemical data on Late Cretaceous (~78 ± 8 Ma) magmatic samples collected on the eastern side of the Sistan suture zone. Petrography and major element compositions reveal two coexisting groups: a low-K calc-alkaline series with basaltic to rhyolitic composition, and a set of calc-alkaline intermediate to felsic samples. The low-K calc-alkaline series reflects classical arc magmatism and is characterized by negative anomalies in high field strength elements and positive anomalies in large ion lithophile elements. The calc-alkaline intermediate to felsic samples correspond to high-silica adakites characterized by strong positive anomalies in Sr and higher La/Yb ratios. Sr and Nd isotopic compositions of the low-K calc-alkaline series support partial melting of a DMM-like source contaminated by sediment-derived fluids, consistent with slab-dehydration in a juvenile subduction setting. An additional fraction of slab-derived melt is necessary to model trace element patterns of our adakites. Altogether, results indicate formation of a Late Cretaceous magmatic arc associated with NE-dipping subduction of the Sistan ocean below the stretched continental Afghan margin. The emplacement of adakites postdate the formation of the suture zone eclogites by a few Ma at most. Upwelling of hot asthenosphere following slab break-off would best explain the necessary warming-up of the subduction thermal regime.

The role of garnetization of olivine in olivine-diopside-jadeite system in the ultramafic-mafic evolution of the upper-mantle magmatism (experiment at 6 GPa).

Peritectic mechanisms, controlling fractional ultrabasic-basic evolution of the upper mantle magmatism and genesis of the peridotitepyroxeniteeclogite rock series, are substantiated in theory and experiment. Melting phase relations of a differentiated mantle material are studied with polythhermal section method in the multicomponent olivineclinopyroxene/omphacitecorundumcoesite system with boundary compositions duplicated these of peridotitic and eclogitic minerals. The peritectic reaction of orthopyroxene and melt with formation of clinopyroxene (the opthopyroxene clinopyroxenization reaction) has been determined at a liquidus surface of the ultrabasic olivineorthopyroxeneclinopyroxenegarnet system. As a result of the reaction the temperature-regressive univariant curve olivine + clinopyroxene + garnet + melt is formed. A further evolution of magmatism has experimentally studied at 6 GPa in the ultrabasic-basic olivinediopsidejadeitegarnet system with changeable compositions of the diopsidejadeite solid solutions (controlling the clinopyroxene omphacite mineralogy). Peritectic reaction of olivine and melt with formation of garnet was established on the liquidus surface of the ternary olivinediopsidejadeite system as the mechanism of olivine garnetization and going to the univariant curve omphacitegarnetmelt with formation of bimineral eclogites. Structure of the liquidus surface for the olivinediopsidejadeitegarnet system is inferred, and its role as a physic-chemical bridge between ultrabasic olivinebearing peridotitepyroxenitic and basic silica-saturated eclogitic compositions of the garnetperidotite facies matter. The new experimental physic-chemical results reveal the genetic links between ultrabasic and basic rocks as well as mechanisms of the uninterrupted fractional magmatic evolution and petrogenesis from the olivinebearing peridotitepyroxenitic to silica-saturated eclogite-grospyditicrocks. This provides an explanation for the uninterrupted composition trends for rock-forming components in clinopyroxenes and garnets of the differentiated rocks of the garnetperidotite facieis.