Eclogite Bodies Research Articles

Nd isotope mapping in the Go Home region of Georgian Bay: a misunderstood key to the geology of the Central Gneiss Belt, Grenville Province of Ontario

The Georgian Bay transect forms an important geological section across the deeply exhumed Central Gneiss Belt of the Ontario Grenville Province. Its geological mapping was one of Nick Culshaw’s major research contributions, but the tectonic structure of the southern part of this transect in the Go Home region was never properly established. New Nd isotope data show that the classically recognized Go Home domain should be divided into three structural “decks” with distinct TDM (depleted mantle) Nd model ages. The upper deck consists of the Pine Island and Honey Harbour domains, with TDM ages averaging 1.56 and 1.57 Ga, respectively, which form two isolated units of the Grenvillian allochthonous belt. These are underlain by the newly-recognized Six Mile Lake domain, whose average TDM age of 1.71 Ga is representative of the tectonic duplex entrained elsewhere onto the base of the allochthon. Finally, the lowermost deck yields an average TDM age of 1.87 Ga, which identifies it as part of the Grenvillian parautochthon, exposed here in a tectonic window. The presence of a large retrogressed eclogite body within the allochthonous Honey Harbour domain confirms the exhumation of this unit from the deep crust on a major structural ramp. The coherent boundaries of these domains are consistent with the ramp–flat thrusting model of the Ottawan (Grenvillian) orogeny and limit the action of later extensional tectonics in the southwest Grenville Province.

Coesite-bearing garnet xenocrysts from diatexite in the Nordøyane domain, Western Gneiss Region, Norway: implications for eclogite-melt interaction at ultra-high pressure

Ultra-high-pressure (UHP) metamorphic rocks in the Western Gneiss Region (WGR) of Norway formed in subducted Baltican crust during the Scandian phase of the Caledonian orogeny. In the Nordøyane UHP domain, eclogites locally preserve coesite and microdiamond. Along the north coasts of Haramsøya and Flemsøya, in situ eclogite bodies are surrounded by dioritic “diatexite selvages” containing abundant eclogitic enclaves and xenocrysts. In one location eclogite and diatexite are associated with a quartz diorite dyke that is highly contaminated with eclogitic material. The diatexite matrix and dyke have similar plagioclase + quartz + biotite assemblages with abundant embayed and fragmented garnet and clinopyroxene xenocrysts. Coesite is present in some garnet xenocrysts but has not been found in the adjacent eclogite bodies. Pressure–temperature ( P– T) estimates from in situ eclogites range from 2.2–3.2 GPa and 810–840 °C, spanning the quartz–coesite transition, with P– T estimates from xenocrysts spanning a wider range. Diorites and retrogressed eclogites record amphibolite-facies conditions, ca. 1.0–1.5 GPa and 700–800 °C. The wide range and large uncertainties in P– T estimates reflect high-temperature re-equilibration and a relative lack of P-sensitive assemblages. Differences in texture, composition, and included assemblages indicate that the xenocrysts were not derived from the nearby eclogite bodies. By implication, their dioritic hosts were not locally derived but must have originated from a different, probably deeper, source. The results are integrated with a numerical model for WGR tectonic evolution to assess the sources of the melts and their possible effects on the exhumation of UHP rocks in western Norway.

Eclogite dehydration and melt‐induced embrittlement at high‐pressure conditions

AbstractWe document the complex interaction between metamorphic reactions and deformation behaviour at high‐pressure conditions by combining (micro‐) structural, petrographical and geochemical data from an eclogite body. The well‐equilibrated eclogite fabric is crosscut by mineral veins. Precipitated veins host the eclogitic high‐pressure assemblage with variation in modal composition, indicating that fracturing occurred at eclogite facies conditions. Deflection of the eclogite fabric adjacent to the veins implies ductile reactivation of the veins as flanking structures and strain localization under continuous aseismic shearing. In consequence, the reactivated veins are characterized by undulatory extinction and subgrain formation in quartz indicating crystal plasticity. We show that prograde post‐eclogitization metamorphism resulted in progressive dehydration and melting of the eclogite. Subsequent increase in pore‐pressure‐induced rock failure. The mesoscopic to microscopic characteristics of eclogite‐facies veins indicate a cyclic behaviour of fracturing, vein formation and crystal‐plasticity which demonstrates the strong interaction of chemical and mechanical processes operating at depth.

U–Pb and trace element zircon and apatite petrochronology of eclogites from the Scandinavian Caledonides

The petrochronological records of eclogites in the Scandinavian Caledonides are investigated using EPMA and LA-ICPMS of zircon and apatite for U–Pb geochronology, combined with major and trace element characteristics. Metamorphic zircon from two eclogites from the Lofoten-Vesterålen Complex (Lofoten Archipelago region) collectively yielded a Concordia age 427.8 ± 5.7 Ma and an upper intercept U–Pb age 425 ± 30 Ma. Apatites from the same eclogites provided U–Pb lower intercepts at 322 ± 28 Ma and 354 ± 33 Ma, with the latter also yielding a younger age of 227 ± 24 Ma. Two eclogites from the Lower Seve Nappe (Northern Jämtland) demonstrate different zircon and apatite age records. Metamorphic zircon provided Concordia ages of 467.2 ± 5.9 Ma and 444.5 ± 5.5 Ma, which resolve the age of prograde metamorphism and zircon growth during retrogression, respectively. The lower intercept U–Pb ages of apatites from the same eclogites are 436 ± 18 and 415 ± 25 Ma, respectively. In combination with their geochemical characteristics, they suggest two separate stages of exhumation of eclogite bodies in the Lower Seve Nappe. Zircons from an eclogite from the Blåhø Nappe (Nordøyane Archipelago) yielded a continuum of concordant U–Pb dates from ca. 435 to 395 Ma, which suggests several cycles of HT metamorphism within short intervals. Distinctive trace element characteristics of apatites from the Blåhø Nappe eclogite suggest formation coeval with zircon and garnet during HT metamorphism, but Pb diffusion behaved as an open system until cooling during exhumation of the nappe at 390 ± 12 Ma (lower intercept U–Pb age of apatite). To summarize, this study presents the high potential of coupled zircon and apatite petrochronology of eclogites in resolving their metamorphic evolution, particularly with respect to using trace element characteristics of apatites to constrain the records of their growth, alterations and the meaning of their U–Pb age record.

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.

Formation of serpentinite-hosted talc in a continental crust setting: Petrographic, mineralogical, geochemical, and O, H and Cl isotope study of the Gilów deposit, Góry Sowie Massif (SW Poland)

The Gilów deposit is located in the Góry Sowie Massif of the Central Sudetes (NE Bohemian Massif, SW Poland). Talcose rocks are associated with serpentinite bodies hosted by country migmatitic paragneiss, in an area where paragneiss also hosts amphibolitized eclogite bodies and granitic pegmatite veins. The talcose rocks and wall rocks were subjected to petrographic examination, electron microprobe study of minerals (including monazite Th-U-total Pb chemical dating), bulk rock chemical examination, as well as stable O, H and Cl isotope analysis. The talcose rocks can be used as a carving material and in ceramics, although the presence of tremolite and anthophyllite, and high Cr and Ni contents, preclude application in pharmacy and cosmetics. High Cr (2121.0–2148.4 ppm), Ni (1079–1561 ppm) and Co (57.4–84.8 ppm) contents, low ΣREE (2.13–13.81 ppm), as well as isotopic composition of O and H, classify the talcose rocks as a serpentinite-hosted type of deposit. Location in the strongly folded metamorphic unit, in the intimate vicinity of several faults and mylonitic zones, implies classification along the thrust-bound and fold-related metamorphogenic deposits. The talcose rocks are separated from serpentinites by the metasomatic chlorite schist, which contains two generations of monazite yielding two, different Th-U-total Pb dates. Older (389.8 ± 11.4 Ma) probably dates serpentinites juxtaposition with country paragneiss and chlorite formation. Younger (365.0 ± 18.2 Ma) likely dates amphiboles and talc formation, as overlaps the ages of amphibolite facies, country paragneiss migmatitization and intrusions of granitic pegmatites, and the talcose rocks contain talc-tremolite-anthophyllite assemblage indicative of the low-pressure mid-amphibolite facies. High MgO (24.27–27.21 wt%), Fe2O3 (5.06–5.12 wt%), Cr, Ni and Co contents are inherited from serpentinites. On the other hand, SiO2 (57.30–60.46 wt%), CaO (0.86–4.39 wt%), Al2O3 (1.38–3.04 wt%), TiO2 (0.02–0.04 wt%) and F (197–337 ppm) contents in the talcose rocks are higher than in host-serpentinites, likely due to a metasomatic introduction by fluids derived from migmatitic paragneisses or intruding pegmatites. Introduction of these elements is evidenced by a negative δ37Cl and elevated δ18O of the talcose rocks relative to host-serpentinites that are typical effects of interaction with a felsic crustal material. Interaction with paragneiss- and pegmatite-derived fluids is further evidenced by REE and trace elements patterns of the talcose rocks. The LREE/HREE enrichment, negative Sr and Ti anomalies, and positive Nb-La and Zr-Hf-Sm slopes, of the talcose rocks patterns, are similar as in paragneiss and pegmatite. Serpentinite bodies transformation into the talcose rocks was gradual and associated with increase of δ18O, and decrease of a Cl content, δD and δ37Cl. Pseudomorphic serpentinite, similar to abyssal serpentinites in terms of an isotopic composition (δ18O = +5.7‰, δD = −64‰, δ37Cl = +2.0‰), recrystallized into non-pseudomorphic serpentinite (δ18O = +7.1‰, δD = −48‰, δ37Cl = −0.1‰), which, in turn, was replaced by the talcose rock (δ18O = +8.3‰ to +8.7‰, δD = −52‰ to −48‰, δ37Cl = −2.6‰ to −0.1‰). Moreover, paragneiss- and pegmatite-derived fluids might have interacted with eclogite bodies during amphibolite facies-retrogression. Amphibolitized eclogite has Cl-rich amphibole rims, and negative δ37Cl (−0.5‰) consistent with a Cl-rich crustal fluid.

Detrital zircon U-Pb geochronology of a metasomatic calc-silicate in the Tsäkkok Lens, Scandinavian Caledonides

The Tsäkkok Lens of the Seve Nappe Complex in the Scandinavian Caledonides comprises eclogite bodies hosted within metasedimentary rocks. These rocks are thought to be derived from the outermost margin of Baltica along the periphery of the Iapetus Ocean, but detrital records from the sedimentary rocks are lacking.Many metasedimentary outcrops within the lens expose both well-foliated metapelitic rocks and massive calc-silicates. The contacts between these two lithologies are irregular and are observed to trend at all angles to the high-pressure foliation in the metapelites. Where folding is present in the metapelites, the calc-silicate rocks are also locally folded. These relationships suggest metasomatism of the metapelites during the Caledonian orogenesis. Zircon U-Pb geochronology was conducted on sixty-one zircon grains from a calc-silicate sample to investigate if they recorded the metasomatic event and to assess the detrital zircon populations. Zircon grains predominantly show oscillatory zoning, sometimes with thin, homogeneous rims that have embayed contacts with the oscillatory-zoned cores. The zircon cores yielded prominent early Stenian, Calymmian, and Statherian populations with a subordinate number of Tonian grains. The zircon rims exhibit dissolution-reprecipitation of the cores or new growth and provide ages that span similar time frames, indicating overprinting of successive tectonic events. Altogether, the zircon record of the calc-silicate suggests that the Tsäkkok Lens may be correlated to Neoproterozoic basins that are preserved in allochthonous positions within the northern extents of the Caledonian Orogen.

Brittle Deformation During Eclogitization of Early Paleozoic Blueschist

The Tsäkkok Lens of the Scandinavian Caledonides represents the outermost Baltican margin that was subducted in late Cambrian/Early Ordovician time during closure of the Iapetus Ocean. The lens predominantly consists of metasedimentary rocks hosting eclogite bodies that preserve brittle deformation on the μm-to-m scale. Here, we present a multidisciplinary approach that reveals fracturing related to dehydration and eclogitization of blueschists. Evidence for dehydration is provided by relic glaucophane and polyphase inclusions in garnet consisting of clinozoisite + quartz ± kyanite ± paragonite that are interpreted as lawsonite pseudomorphs. X-Ray chemical mapping of garnet shows a network of microchannels that propagate outward from polyphase inclusions. These microchannels are healed by garnet with elevated Mg relative to the surrounding garnet. Electron backscatter diffraction mapping revealed that Mg-rich microchannels are also delimited by low angle (<3°) boundaries. X-ray computed microtomography demonstrates that some garnet is transected by up to 300 μm wide microfractures that are sealed by omphacite ± quartz ± phengite. Locally, mesofractures sealed either by garnet- or omphacite-dominated veins transect through the eclogites. The interstices within the garnet veins are filled with omphacite + quartz + rutile + glaucophane ± phengite. In contrast, omphacite veins are predominantly composed of omphacite with minor apatite + quartz. Omphacite grains are elongated along [001] crystal axis and are preferably oriented orthogonal to the vein walls, indicating crystallization during fracture dilation. Conventional geothermobarometry using omphacite, phengite and garnet adjacent to fractures, provides pressure-temperature conditions of 2.47 ± 0.32 GPa and 620 ± 60°C for eclogites. The same method applied to a mesoscale garnet vein yields 2.42 ± 0.32 GPa at 635 ± 60°C. Zirconium-in-rutile thermometry applied to the same garnet vein provides a temperature of ∼620°C. Altogether, the microchannels, microfractures and mesofractures represent migration pathways for fluids that were produced during glaucophane and lawsonite breakdown. The microfractures are likely precursors of the mesoscale fractures. These dehydration reactions indicate that high pore-fluid pressure was a crucial factor for fracturing. Brittle failure of the eclogites thus represents a mechanism for fluid-escape in high-pressure conditions. These features may be directly associated with seismic events in a cold subduction regime.

Scapolite pegmatite from the Nordøyane ultra-high pressure domain, Western Gneiss Region, Norway: Partial melting driven by infiltration of mantle-derived fluid

Scapolite, a volatile-bearing framework silicate, is found in pegmatites and leucosomes spatially associated with (ultra)-high pressure ((U)HP) eclogites in the Nordøyane domain of the Western Gneiss Region (WGR), Norway. The field relationships, petrology, and stable isotope geochemistry of scapolite-bearing rocks offer insights into the role of fluids in partial melting of WGR gneisses during the Scandian orogeny. Three types of scapolite pegmatite are distinguished based on mineralogy, texture, and scapolite chemistry. The most abundant Type 1 pegmatites are associated with dioritic to granodioritic migmatites, including local diatexite, developed in dioritic to granitoid orthogneisses. They are zoned from scapolite+quartz cores to hornblende+plagioclase rims, and concentrated in low-strain areas including boudin necks between disaggregated eclogite bodies. They are interpreted as late-stage differentiates of the (grano)dioritic migmatites, which also contain accessory scapolite in leucosomes. Type 2 and 3 pegmatites also contain coarse-grained scapolite but lack hornblende-rich selvages; Type 2 scapolites are texturally and chemically distinct from other varities. Type 1 and 3 pegmatites and host rock migmatites contain meionitic scapolite with high S- and C– contents, whereas Type 2 pegmatites contain marialitic scapolite with high Cl-contents. The pegmatites crystallised at amphibolite-facies conditions, ca. 750–800 °C and 1 GPa. Scapolites in Type 1, 2, and 3 pegmatites and host rock leucosomes share similar δ13C (−7.9 to −2.2‰) and δ34S (−1.1 to 5.7‰) compositions, interpreted to indicate a common source of volatile components. Comparison with literature data suggests a mantle source for the S and C in the scapolite-forming fluids. We propose that infiltration of mantle-derived fluids into the Baltican continental slab during subduction or the early stages of exhumation drove partial melting in subducted Baltican crust.

Oceanic-arc subduction, stagnation, and exhumation: zircon U–Pb geochronology and trace-element geochemistry of the Sanbagawa eclogites in central Shikoku, SW Japan

In the Iratsu and the quartz-bearing eclogite bodies of the Sanbagawa high-pressure type metamorphic belt, southwest Japan, zircon U–Pb dating and trace-element analysis of the mafic gneiss combined with its geologic structure revealed that the protolith basaltic rock constituted the topographic high on a seafloor in relation to intra-oceanic arc magmatism at ca. 195 Ma. Moreover, the metamorphic zircon U–Pb data and the rare-earth element patterns obtained from the subordinated metasedimentary rocks of the Iratsu and the quartz-bearing eclogite bodies indicate that both bodies were subducted from a trench at ca. 120 Ma and underwent the eclogite facies metamorphism between ca. 120 and 90 Ma. This study, combined with previous studies for the Sanbagawa rocks and the Jurassic-Cretaceous accretionary complexes in Japan, identifies the following constraints that led to the tectonic evolution of the Sanbagawa eclogites: 1) the metamorphic unit including the Iratsu and the quartz-bearing eclogite bodies (the Besshi unit) was subducted from a trench at ca. 120 Ma. 2) This unit was stagnated at the depth of the eclogite–facies condition between ca. 120 and 90 Ma. 3) The eclogites in the Besshi unit was exhumed with the younger metamorphic rocks which were subducted at ca. 100–90 Ma (Asemi-gawa unit). 4) The Besshi unit is a high-pressure metamorphic equivalent of the non- or weakly metamorphic Sanbosan accretionary complex and the Mikabu greenstones from a standpoint of age similarity on accretion. The probable mechanism for the stagnation of the Besshi unit at the depth of the eclogite–acies condition needs 1) the detachment of oceanic-arc material from the subducting slab, driven by the resistance against the subduction of the topographic-high part underneath the forearc, and 2) the oceanward movement of the entire arc-trench system, which might have depressed the subduction of the Besshi unit into a deeper depth than its eclogite depth.

Sulfides in Metamorphic Rocks of the Fore Range Zone (Greater Caucasus). A New Type of Mineral Container for Peak Metamorphism Mineral Assemblages

The rocks of the Armovka Formation (the Fore Range zone, Greater Caucasus) have undergone low-grade metamorphism that partially erased information about initial rock formation conditions. We discovered high-pressure mineral inclusions such as omphacite, phengite, garnet, and paragonite enclosed by pyrite and chalcopyrite. Mineral inclusions in sulfides may provide important information about metamorphic pressure−temperature conditions because they are shielded by the host minerals and isolated from significant low-grade overprinting. Calculations performed on phengite inclusions using the phengite Si-content barometry indicate a pressure ranging from 1.7 ± 0.2 to 1.9 ± 0.2 GPa for temperature of 600 ± 40 °C. These data are consistent with estimations obtained for eclogite bodies embedded in rocks of the Armovka Formation. Geothermobarometry of the latest yielded conditions of 680 ± 40 °C and a minimum pressure of 1.6 ± 0.2 GPa to upper pressure boundary at 2.1 GPa. This fact allows us to assume that the metamorphic rocks of the Armovka Formation were immersed in the subduction zone to the conditions of the eclogite facies of metamorphism, forming a coherent subduction complex together with eclogites.

Two high-pressure metamorphic events, Variscan and Alpine, dated by Lu–Hf in an eclogite complex of the Austroalpine nappes (Schobergruppe, Austria)

The Eo-Alpine high-pressure belt in the Austroalpine nappes consists of pre-Mesozoic basement rocks overprinted by eclogite-facies metamorphism during the Late Cretaceous. Parts of this basement were already eclogitized during the Variscan orogeny. Lu–Hf geochronology allowed to identify two high-pressure events in an eclogite body in the Schobergruppe, an Austroalpine basement complex south of the Tauern Window. Two samples from closely neighboring outcrops were studied. Both contain two garnet generations. In one sample, PRI3, garnet belongs almost exclusively to the younger (Alpine) generation with only rare preservation of relic cores. In the other sample, PRI4, Variscan garnet is merely coated by a thin rim of the second, Alpine generation, which is in equilibrium with the high-pressure matrix assemblage. In PRI3, two-point garnet-whole rock ages scatter between ~ 97 and ~ 104 Ma, reflecting Alpine garnet growth with minor contamination by a Variscan component. In PRI4, two-point isochrons yield ages between ~ 300 and ~ 313 Ma. The limited spread in these ages suggests minor contamination by Alpine garnet rims. We propose 97 Ma as the maximum age for Alpine metamorphism, which is close to previously determined ages from other parts of the high-pressure belt, and 313 Ma as a minimum age for Variscan metamorphism. Thermodynamic modelling infers that eclogite-facies conditions were reached in both events; during the Late Cretaceous, these were ca. 1.9 GPa/650 °C. Variscan high-pressure conditions in PRI4 are inferred from the amount of garnet in the sample, which indicates at least 1.6 GPa. We propose that the occurrence of Alpine versus Variscan garnet in eclogites depends on the intensity of Variscan and post-Variscan retrogression: the more prograde, Variscan garnet was removed during retrogression, the more garnet grew during the Alpine cycle.

Timing of metamorphism and exhumation in the Nordøyane ultra-high-pressure domain, Western Gneiss Region, Norway: New constraints from complementary CA-ID-TIMS and LA-MC-ICP-MS geochronology

We present the results of a combined CA-ID-TIMS and LA-MC-ICP-MS U–Pb geochronology study of zircon and associated rutile and titanite from the Nordøyane ultra-high-pressure (UHP) domain in the Western Gneiss Region (WGR) of Norway. The dated samples include 4 eclogite bodies, 2 host-rock migmatites, and 2 cross-cutting pegmatites and leucosomes, all from the island of Harøya. Zircon from a coesite eclogite yielded an age of ca. 413 Ma, interpreted as the time of UHP metamorphism in this sample. Zircon data from the other eclogite bodies yielded metamorphic ages of ca. 413 Ma, 407 Ma, and 406 Ma; zircon trace-element data associated with 413 Ma and 407 Ma ages are consistent with eclogite-facies crystallization. In all of the eclogites, U–Pb dates from zircon cores, interpreted as the times of protolith crystallization, range from ca. 1680–1586 Ma, consistent with Gothian ages from orthogneisses in Nordøyane and elsewhere in the WGR. A zircon core age of ca. 943 Ma from one sample agrees with Sveconorwegian ages of felsic gneisses and pegmatites in the western part of the area. Migmatites hosting the eclogite bodies yielded zircon core ages of ca. 1657–1591 Ma and rim ages of ca. 395–392 Ma, interpreted as the times of Gothian protolith formation and Scandian partial melt crystallization, respectively. Pegmatite in an eclogite boudin neck yielded a crystallization age of ca. 388 Ma, interpreted as the time of melt crystallization. Rutile and titanite from 3 samples (an eclogite and two migmatites) yielded concordant ID-TIMS ages of 378–376 Ma. The results are similar to existing U–Pb data from other Nordøyane eclogites (415–405 Ma). In combination with previous pressure-temperature data from the coesite eclogite, these ages indicate that peak metamorphic conditions of ~3 GPa/760 °C were reached ca. 413 Ma, followed by decompression to ~1 GPa/810 °C by ca. 397 Ma and cooling below ca. 600 °C by ca. 375 Ma. The results are compatible with protracted UHP metamorphism followed by relatively slow exhumation. The question of whether partial melting began at UHP conditions is not resolved by this study.

Clockwise and Anticlockwise P–T Paths of High-pressure Rocks from the ‘La Pioza’ Eclogite Body of the Malpica–Tuy Complex, NW Spain

Clockwise and Anticlockwise P–T Paths of High-pressure Rocks from the ‘La Pioza’ Eclogite Body of the Malpica–Tuy Complex, NW Spain

High-pressure, high-temperature deformation of dunite, eclogite, clinopyroxenite and garnetite using in situ X-ray diffraction

The rheology of eclogite, garnetite and clinopyroxenite in the peridotitic upper mantle was experimentally investigated in a large volume press combined with in situ synchrotron X-ray diffraction techniques to study the impact on mantle convection resulting from the subduction of oceanic lithosphere. Experiments were carried out over a range of constant strain rates (2×10−6–3×10−5 s−1), pressures (4.3 to 6.7 GPa) and temperatures (1050 to 1470 K). Results show substantial strength variations among eclogitic garnet and clinopyroxene and peridotitic olivine. At low temperatures (<1200 K), eclogite is over 1 GPa stronger than dunite. On the other hand, at high temperatures (>1400 K) eclogite is weaker than dunite by 0.2 GPa or more. Garnetite and clinopyroxenite exhibit higher strength than dunite at approximately 1200 K. However, at higher temperature (1370 K), clinopyroxenite is significantly weaker than garnetite (and dunite) by more than a factor of five. We explain these observations by transitions in deformation mechanisms among the mineral phases. In clinopyroxene, high temperature dislocation creep resulting in a strength reduction replaces low temperature twinning. Whereas garnet remains very rigid at all experimental conditions when nominally anhydrous (‘dry’). Microstructural observations show phase segregation of clinopyroxene and garnet, development of a crystallographic and shape preferred orientation in the former but not in the latter, suggesting an overall weak seismic anisotropy. Detection of eclogite bodies in the peridotite-dominated mantle may only be possible via observation of high VP/VS1 ratios. A comparable or weaker rheology of eclogite to dunite suggests effective stirring and mixing of eclogite in the convecting mantle.

Glaucophanic amphibole in the Seba eclogitic basic schists, Sambagawa metamorphic belt, central Shikoku, Japan : implications for timing of juxtaposition of the eclogite body with the non-eclogite Sambagawa schists

Glaucophanic amphibole in the Seba eclogitic basic schists, Sambagawa metamorphic belt, central Shikoku, Japan : implications for timing of juxtaposition of the eclogite body with the non-eclogite Sambagawa schists

K-Ar ages of amphibole-rich metamorphosed veins in eclogite bodies from the Lake Zone, SW Mongolia

K-Ar ages of amphibole-rich metamorphosed veins in eclogite bodies from the Lake Zone, SW Mongolia

Isostasy as a tool to validate interpretations of regional geophysical datasets – application to the mid-Norwegian continental margin

Abstract Isostasy is a well understood concept, yet rarely applied to its full capacity in regional interpretations of crustal structures. In this study, we utilize a recent density model for the entire NE Atlantic, based on refraction seismic data and gravity inversion, to calculate isostatically balanced bathymetry along the mid-Norwegian margin. Since gravity and isostatically balanced elevation are independent observables but both depend on the underlying density model, consistencies and discrepancies point towards model deficits, erroneously interpreted or poorly understood areas. Four areas of large isostatic residuals are identified. Along the outer Vøring Margin, a mass deficit points to more extensive high-density bodies or a shallower Moho than currently mapped. Farther seaward, along the Vøring Marginal High, a mass excess indicates inaccurate mapping of the continent–ocean boundary and surrounding structures. A number of eclogitic bodies along the proximal mid-Norwegian margin have been described in recent publications and their presence is now also confirmed by isostatic calculations. Major elevation and gravity residuals along the transition between the Vøring and Møre margins signify that the structure of this region is poorly understood and modifications to the mapped continent–ocean transition may be required.

Ca-Eskola incorporation in clinopyroxene: limitations and petrological implications for eclogites and related rocks

Clinopyroxene is an essential mineral in eclogitic rocks. It commonly contains minor amounts of the defect-bearing Ca-Eskola (CaEs, Ca0.5□0.5AlSi2O6) component, with higher concentrations generally considered to indicate a high-pressure origin at least within the coesite stability field. Changes in pressure and temperature conditions can lead to exsolution of this component as a free SiO2 phase, which may have a number of petrological implications. This makes it important to understand the factors that maximize CaEs incorporation in clinopyroxene. We have undertaken a series of experiments at high pressures and temperatures (4–10 GPa and 1000–1350 °C) to further investigate the systematics of CaEs incorporation in eclogite-like clinopyroxene and the factors responsible for maximizing CaEs contents. Two simple chemical systems were chosen that allow unambiguous interpretation of the results: (1) CMAS + H2O and (2) two compositions in the NCMAS system. All experimental products contained clinopyroxene and garnet along with either a free SiO2 phase or a silicate melt. Coexisting garnet is grossular-rich, generally with X gr ≥ 0.67. Compositional variations are attributable to the presence or absence of melt and changes in modal amounts of garnet at different pressure–temperature conditions. Even small amounts of H2O lower the solidus temperature and the presence of a melt reduces the SiO2 activity, which destabilizes the CaEs component in clinopyroxene. The CaEs and the Ca-Tschermaks (CaTs, CaAl2SiO6) components in clinopyroxene decrease with increasing jadeite mole fraction, which is also a function of pressure and bulk Al content. Modeling X-ray powder diffraction data yields a molar volume for the CaEs endmember of V CaEs = 60.87(63) cm3, which reasonably agrees with a literature value that was estimated from natural samples. In the presence of coexisting coesite, the CaEs and CaTs do not vary independently of each other, being controlled by the internal equilibrium 2CaEs = CaTs + 3SiO2 (coesite). This relation, observed in simple systems (i.e., CMAS ± Na), is also obeyed by clinopyroxene in more complex, natural analog bulk compositions. An assessment of available experimental data reveals a maximum of 15–18 mol% CaEs in eclogitic clinopyroxene at conditions corresponding to 130–180 km depth. CaEs contents are maximized at high temperatures; i.e., at or near the solidus in the presence of coesite. Thus, this study supports the role of CaEs exsolution in contributing to melt generation during upwelling of eclogite bodies in the mantle, albeit with some caveats. Somewhat higher maximum CaEs contents (~20 mol%) are found in Ca and Al-rich bulk compositions, such as grospydite xenoliths. Such bulk compositions also seem to require the coexistence of kyanite. Other Ca and Al-rich rock types, like rodingites, should have the potential of containing CaEs-rich clinopyroxenes, except that they are SiO2-undersaturated. This emphasizes the further role of bulk composition, in addition to high temperatures, in achieving maximum CaEs contents in high-pressure clinopyroxene.

A review of the NE Atlantic conjugate margins based on seismic refraction data

Abstract The NE Atlantic region evolved through several rift episodes, leading to break-up in the Eocene that was associated with voluminous magmatism along the conjugate margins of East Greenland and NW Europe. Existing seismic refraction data provide good constraints on the overall tectonic development of the margins, despite data gaps at the NE Greenland shear margin and the southern Jan Mayen microcontinent. The maximum thickness of the initial oceanic crust is 40 km at the Greenland–Iceland–Faroe Ridge, but decreases with increasing distance to the Iceland plume. High-velocity lower crust interpreted as magmatic underplating or sill intrusions is observed along most margins but disappears north of the East Greenland Ridge and the Lofoten margin, with the exception of the Vestbakken Volcanic Province at the SW Barents Sea margin. South of the narrow Lofoten margin, the European side is characterized by wide margins. The opposite trend is seen in Greenland, with a wide margin in the NE and narrow margins elsewhere. The thin crust beneath the basins is generally underlain by rocks with velocities of &gt;7 km s −1 interpreted as serpentinized mantle in the Porcupine and southern Rockall basins; while off Norway, alternative interpretations such as eclogite bodies and underplating are also discussed.