Figure 6 - available via license: Creative Commons Attribution 4.0 International
Content may be subject to copyright.
Diagrams showing compositional variation of garnet, clinopyroxene, amphibole and phengite. (a) Compositional zoning of garnet along the profile shown in Fig. 5a. Primary axis on the left is for Alm, Prp and Grs, whereas the secondary axis on the right is for Sps. (b) Ternary diagram showing composition of clinopyroxene varying from omphacite to diopside. Black circles are for analyzed mineral in 03-57, and blue triangles are for those in 03-59. (c) M4 (Na + K + 2Ca) vs. M1 (Al + Fe 3+ + Ti) diagram showing amphibole chemistry, nomenclature follows Hawthorne et al. (2012); symbols are the same as (b). (d) Mg vs. Si p.f.u. diagram showing the composition of phengite in sample 03-57.
Source publication
Metamorphic textures and a pressure–temperature (P–T) path
of zoisite eclogite are presented to better understand the metamorphic
evolution of the North-East Greenland eclogite province and this particular
type of eclogite. The eclogite contained the mineral assemblage garnet,
omphacite, kyanite, phengite, quartz and rutile at peak pressure. Partia...
Contexts in source publication
Context 1
... 34 mol % pyrope, 24 mol % grossular (+andradite) and 1 mol % spessartine (Alm 40 Prp 34 Grs 24 Sps 1 ). Almandine increases from core to rim to Alm 44 , pyrope increases to Prp 36 at the inner rim and decreases to Prp 31 at the outermost rim, and grossular decreases to Grs 21 at the inner rim and increases to Grs 22 at the outermost rim ( Fig. 6a; Table 3). Spessartine is relatively constant at Sps 1 and increases to Sps 2 at the outermost rim. Grt II has similar composition to the rim of Grt ...
Context 2
... of variable composition is present as inclusions in garnet, in the matrix and in symplectite ( Fig. 6b; Table 4). Inclusions in Grt I are omphacite, with X Na [= Na/(Na + Ca)] up to 0.40. Matrix clinopyroxene varies from omphacite to diopside with a Na-rich core (X Na up to 0.43-0.44) and a Na-poor rim (X Na = 0.16). Clinopyroxene in the symplectite is ...
Context 3
... amphibole is calcic, specifically pargasite and sadanagaite ( Fig. 6c; Table 5). Ferric iron, calculated using the charge balance method (Locock, 2014;Hawthorne et al., 2012), is < 0.40 p.f.u (atoms per formula unit). Zoisite (Table 5) in garnet contains Fe 3+ between 0.12 and 0.14 p.f.u., whereas matrix zoisite shows a slightly larger range, 0.10-0.14. No systematic zoning is seen in ...
Context 4
... 24 at the outermost rim. Spessartine is constant at ∼ 1 mol %. Matrix clinopyroxene is omphacite (X Na up to 0.43) in the core and diopside (X Na = 0.14) in the rim (Table 4). Clinopyroxene in the symplectite is diopside (Fig. ...
Context 5
... is calcic, specifically pargasite and tschermakite ( Fig. 6c; (Fig. 3a). Microstructural evidence suggests that the studied samples were partially melted: the polymineralic inclusions in garnet and graphic amphibole + plagioclase (Fig. 3b) and plagioclase + diopside + Grt II in the matrix (Fig. 3a) represent former melt pockets. Accordingly, this pseudosection was used to constrain peak T . ...
Context 6
... 100 %. The P -T pseudosection was calculated for the range of 1.0-2.5 GPa and 600-950 • C (Fig. 7 and Fig. S1 in the Supplement). Garnet and clinopyroxene are ubiquitous in the modeled phase diagram. The solidus curve shows a posi- Table 6. Representative analyses of minerals in garnet in the zoisite eclogite 03-57. Ph 1 is the pristine phengite (Fig. 6) and Ph 2 occurs as a mineral in polymineralic inclusion in garnet. The dash (−) marks unanalyzed elements, while b.d. marks those below detection limit. Structural formulae for minerals other than phengite were calculated as given in Tables 4 and 5. Phengite is recalculated based on 42 valences for the four-and six-fold cations. This ...
Similar publications
We combine elastic thermobarometry with oxygen isotope thermometry to quantify the pressure-temperature (P-T) evolution of retrograde metamorphic rocks of the Cycladic Blueschist Unit (CBU), an exhumed subduction complex exposed on Syros, Greece. We employ quartz-in-garnet and quartz-in-epidote barometry to constrain pressures of garnet and epidote...
Citations
... This trend is similar to the Cretaceous ages found inland across SW Fennoscandia (Figure 7(a)) and may imply a similar complex thermo-tectonic history is present across NE Greenland. These samples are located within the NE Greenland Eclogite Province [139], where ultra-high-pressure metamorphism is thought to have occurred 365-350 Ma based on analysis of basement zircons [140]. This is significant as AFT samples <3 km from the same zirconbearing rocks produce central ages <80 Ma younger [118], implying a rapid rise through the crust to their present position. ...
The northeast (NE) Atlantic is one of the best-studied geological regions in the world, incorporating a wide array of geological phenomena including extensional tectonism, passive margin development, orogenesis, and breakup-related volcanism. Apatite fission-track (AFT) thermochronology has been an important tool in studying the onshore evolution of the NE Atlantic for several decades. Unfortunately, large regional-scale studies are rare, making it difficult to study geological processes across the whole region. In this work, a compilation of published AFT data is presented from across Fennoscandia, the British Isles, East Greenland, and Svalbard, with the goal of providing an accessible overview of the data and how this vast body of work has improved our understanding of the region’s evolution. Alongside a review of previous literature, interpolated maps of fission track age and mean track length (MTL) highlight regional trends in the data that may result from major first-order processes and areas of low sample density that should be targeted for future study. Additionally, in the absence of metadata required for thermal history modeling, apparent exhumation rate estimates are calculated from available elevation profiles and the timing of major exhumation events inferred from “boomerang plots” of fission track ages against MTL values. Across Fennoscandia, data suggests that the opening of the NE Atlantic and exhumation of the margin have clearly played a major role in the thermal history of the upper crust. The remaining areas of Britain, Ireland, East Greenland, and Svalbard all present more complex trends consistent with a combination of the NE Atlantic’s opening and the interplay between specific bedrock geology of sampling sites and localized geological processes. Areas of low sample density include southern Britain, NE Britain, southeast Greenland, southern Svalbard, and Eastern Fennoscandia, each of which provides the natural laboratory required to answer many unresolved questions.
... This trend is similar to the Cretaceous ages found inland across SW Fennoscandia (Figure 7(a)) and may imply a similar complex thermo-tectonic history is present across NE Greenland. These samples are located within the NE Greenland Eclogite Province [139], where ultra-high-pressure metamorphism is thought to have occurred 365-350 Ma based on analysis of basement zircons [140]. This is significant as AFT samples <3 km from the same zirconbearing rocks produce central ages <80 Ma younger [118], implying a rapid rise through the crust to their present position. ...
The northeast (NE) Atlantic is one of the best-studied geological regions in the world, incorporating a wide array of geological phenomena including extensional tectonism, passive margin development, orogenesis, and breakup-related volcanism. Apatite fission-track (AFT) thermochronology has been an important tool in studying the onshore evolution of the NE Atlantic for several decades. Unfortunately, large regional-scale studies are rare, making it difficult to study geological processes across the whole region. In this work, a compilation of published AFT data is presented from across Fennoscandia, the British Isles, East Greenland, and Svalbard, with the goal of providing an accessible overview of the data and how this vast body of work has improved our understanding of the region’s evolution. Alongside a review of previous literature, interpolated maps of fission track age and mean track length (MTL) highlight regional trends in the data that may result from major first-order processes and areas of low sample density that should be targeted for future study. Additionally, in the absence of metadata required for thermal history modeling, apparent exhumation rate estimates are calculated from available elevation profiles and the timing of major exhumation events inferred from “boomerang plots” of fission track ages against MTL values. Across Fennoscandia, data suggests that the opening of the NE Atlantic and exhumation of the margin have clearly played a major role in the thermal history of the upper crust. The remaining areas of Britain, Ireland, East Greenland, and Svalbard all present more complex trends consistent with a combination of the NE Atlantic’s opening and the interplay between specific bedrock geology of sampling sites and localized geological processes. Areas of low sample density include southern Britain, NE Britain, southeast Greenland, southern Svalbard, and Eastern Fennoscandia, each of which provides the natural laboratory required to answer many unresolved questions.
... Temperature and pressure conditions up to 830°C and 2.4 GPa (W. Cao et al., 2020) were reached during the period of 423-395 Ma (Gilotti et al., 2004;Hallett et al., 2014;McClelland et al., 2016). Further south in the Payer Land gneiss complex (74.5°N), a variety of lithologies in the lowest structural level of the orogen experienced HP granulite-facies metamorphism, with P = 1.4-1.7 GPa at ≈800°C (Elvevold et al., 2003). ...
... HP rocks of the NEGEP are interpreted to lie at the base of thickened Laurentian crust at 400 Ma (Figure 12a), which is estimated to be ≈75 km based on P estimates up to 2.4 GPa (W. Cao et al., 2020). Similar thicknesses have been documented from seismic tomography in the Tibetan plateau (e.g., Gilligan et al., 2015;Xu et al., 2015), where the compositional nature of the thick crust is fiercely debated (Hacker et al., 2000;Schneider et al., 2019;Zeng et al., 2020). ...
Ultrahigh‐pressure (UHP) rocks in North‐East Greenland lie within a larger region of high‐pressure Laurentian crust formed in the overthickened upper plate of the collision with Baltica. Coesite‐bearing zircon dates UHP metamorphism to 365–350 Ma, which formed at the end of the Caledonian collision as a result of intracontinental subduction facilitated by strike‐slip faults that broke the lithosphere. Rutile is the stable Ti‐bearing phase at UHP, while titanite forms on the retrograde path. Trace elements and U‐Pb in titanite were analyzed for six UHP gneisses. Zr‐in‐titanite temperatures range from 764 to 803°C and lie on the isobaric part of the pressure‐temperature path at 1.2 GPa, which fits Ti‐phase stability determined by thermodynamic modeling. Large (>600 μm), zoned titanite preserves three distinct trace element patterns that are due to metamorphism, melting and garnet breakdown. Weighted mean ²⁰⁶Pb/²³⁸U ages range from 347 ± 5 Ma to 320 ± 11 Ma, but age variation as a function of trace element domain for individual samples is not resolvable within uncertainty. Titanite records a prolonged period of exhumation that is also seen in the zircon record, where phengite decompression melting started at ca. 347 Ma, leucosome emplacement accompanied retrograde metamorphism from 350 to 330 Ma; and titanite grew during isobaric cooling from 345 to 320 Ma when the UHP rocks stalled at lower crustal levels. The same transforms that originally break the lithosphere play a significant role in channeling the UHP rocks back to the lower crust via buoyancy driven exhumation, after which time titanite formed.
... Eclogites (≥ 75 % garnet + omphacite; Carswell, 1990) are inherently rather refractory rocks because they contain subordinate quantities of hydrous minerals, depending on composition and the specific pressure-temperature evolution. Partial melting of eclogite therefore typically involves the breakdown of hydrous epidote-group minerals (Cao et al., 2018(Cao et al., , 2020, phengite and/or amphibole (e.g. Schmidt et al., 2004;Cao et al., 2021), but nominally anhydrous minerals such as omphacite may also contribute (Feng et al., 2021). ...
Pristine amphibole-clinozoisite eclogite from within the eclogite type locality (Hohl, Koralpe) of the Eastern Alps in Austria preserves centimetre-thick, concordant, laterally continuous leucocratic segregations of coarse-grained (up to ∼ 1 cm grain diameter) euhedral amphibole-clinozoisite-quartz and disseminated garnet-omphacite-rutile. The segregations locally show selvedges dominated by coarse-grained amphibole at the interface with their host eclogite. Retrogression is limited to thin films of texturally late plagioclase ± amphibole and minor symplectites of diopside-plagioclase partially replacing omphacite. Mineral compositions are largely homogeneous except for clinozoisite, which is significantly enriched in Fe3+, rare-earth and high-field-strength elements in the rock matrix compared to that in segregations. Petrography, mineral chemical data and phase diagram modelling are interpreted in terms of limited melting under high aH2O conditions, at or close to the well-established pressure maximum (21 ± 3 kbar and 680–740°C), followed by melt crystallization near these conditions. Exsolution of melt-dissolved H2O led to the formation of the amphibole-rich selvedges at the leucosome-eclogite interface. Plagioclase ± amphibole/clinopyroxene films formed at lower pressure from final melt vestiges adhering to grain boundaries or from secondary fluid-rock interaction. Natural variability in rock composition and the bulk oxidation state leads to variable mineral modes and calculated high-pressure solidus temperatures for compositional end-members sampled at Hohl. Modelling suggests that oxidized conditions (XFe3+ <0.5) favour hydrated but refractory amphibole-clinozoisite-rich assemblages with a fluid-present solidus temperature of ∼740°C at 20 kbar, whereas more reduced conditions (XFe3+ ∼0.2) yield "true" eclogites (>80 vol % garnet + omphacite) that commence melting at ∼720°C at the same pressure. The interlayering of such eclogites potentially constitutes a fluid source-sink couple under appropriate pressure-temperature conditions, favouring fluid transfer from neighbouring dehydrating layers to melt-bearing ones down gradients in the chemical potential of H2O (µH2O). Phase diagram calculations show that for moderate degrees of fluid-fluxed melting (≤ 10 vol % melt) near the pressure maximum, the observed equilibrium assemblage is preserved, provided the melt is subsequently removed from the source rock. The resulting hydrous melts may be, in part, parents to similar eclogite-hosted pegmatitic segregations described in the eclogite type locality. We suggest that eclogites with a comparable composition and metamorphic history are however unlikely to produce voluminous melts.
... One of the interesting features of the studied eclogites is the presence of large polymineralic inclusions in garnet crystals. Such inclusions have been described from natural samples (e.g., Cao et al., 2020;Lang & Gilotti, 2007) and HT experiments (Perchuk et al., 2005(Perchuk et al., , 2008 and were interpreted as the result of partial melting of primary monomineralic inclusions. In the present case, they may represent former inclusions of amphibole, entrapped during the prograde subsolidus growth of garnet. ...
Two eclogite samples from the Haut-Allier record a prograde evolution from ~20 kbar, 650 °C to 750 °C, 22–23 kbar followed by heating up to 850–875 °C and partial melting. Incipient decompression in high-pressure granulite facies conditions (19.5 kbar, 875 °C) was followed by exhumation to high-temperature amphibolite facies conditions (<9 kbar, 750–850 °C). Following a detailed geochemical, petrological and geochronological investigation using trace-element data and LA-ICP-MS U-Pb dating of zircon, apatite and rutile, the eclogites reveal an Ordovician (c. 490 Ma) rifting event followed by Devonian (c. 370–360 Ma) subduction and Carboniferous (c. 350 Ma) exhumation in this part of the French Massif Central (FMC). The previously proposed Silurian age for the subduction, that strongly influenced many tectonic models, is definitively rejected. In the light of other geological data from the FMC, including the lithological and geochemical zoning of calc-alkaline Devonian volcanism, we propose a southward polarity of the subduction and question the very existence of the so-called Massif Central Ocean. Furthermore, we infer that following subduction, the eclogites were relaminated to the upper plate and exhumed at the rear of the magmatic arc pointing to similarities with the geodynamics of the Bohemian Massif.
The petrochronological record of zircon is particularly complex. Metamorphic zircon with clear eclogitic REE patterns (no Eu anomaly, flat HREE) and inclusions (garnet, rutile, omphacite) shows concordant apparent ages that spread from c. 380 down to c. 310 Ma. This apparent age pattern strongly contrasts with the well-defined age of apatite and rutile of c. 350 Ma. Apparent zircon ages younger than 350 Ma unequivocally testify that zircon can recrystallize outside the conditions of the eclogite facies, which resets the U–Pb while preserving an apparent eclogitic signature. Local fractures filled by analcite, thomsonite, plagioclase and biotite testify to late interaction of the eclogites with alkaline fluids at relatively low temperatures. This interaction, possibly at c. 310 Ma or later, could lead to the recrystallisation of zircon while leaving apatite unaffected.
... Both the breakdown of hydrous minerals (e.g. Cao et al., 2018Cao et al., , 2020Cao et al., , 2021 and H 2 O exsolved from NAMs (e.g. Wang et al., 2020) may contribute to partial melting of eclogite during exhumation. ...
Epidote eclogites embedded in paragneisses from the Austrian Eastern Alps host rare, decimetre- to metre-sized planar pegmatitic segregations (‘pegmatoids’) that consist of a hornblende–plagioclase–epidote–titanite–quartz assemblage. The pegmatoids cut the primary eclogite foliation at a high angle, show abrupt terminations and exhibit numerous microtextures indicating the former presence of melt, whereas the surrounding eclogites and paragneisses lack evidence for dehydration melting. We interpret the pegmatoids as crystallised hydrous melts of intermediate composition derived from a nearby eclogitic source, similar to the eclogite host rock. The host eclogite preserves high-pressure assemblages of garnet–omphacite–epidote–quartz–rutile ± phengite that yield peak P–T estimates of 21 ± 2 kbar and 700 ± 20 ∘C calculated via pseudosection modelling and Zr-in-rutile thermometry. Subsequent near-isothermal decompression occurred in a closed system under fluid-absent conditions that favoured the preservation of garnet–omphacite assemblages in eclogites. Thermodynamic modelling of the metapelitic country rock indicates that the paragneisses remained fluid-absent during early exhumation, but became fluid-saturated at mid-crustal conditions of 7.5–9 kbar and 680–690 ∘C. We suggest that, as the metapelites dehydrated, minor aqueous fluid was released and infiltrated the enclosed eclogites along discordant fractures that formed in response to unloading, further facilitated by steep gradients in μH2O between the fluid-saturated pelitic country rock and fluid-absent eclogite lenses. Eclogite distal from fluid sources/pathways experienced limited retrogression, whereas localised re-equilibration and H2O-fluxed melting is inferred for eclogite affected by extensive fluid infiltration. Zr-in-titanite thermometry on crystals from pegmatoids yields a crystallisation temperature of 697 ± 10 ∘C at 8.5 ± 1.5 kbar, similar to retrograde conditions recorded by the regionally dominant metapelites. Oxygen- and hydrogen isotopic data for eclogite, pegmatoids and paragneiss are consistent with hydrous fluid transfer from paragneisses to eclogites and a small fluid/rock ratio. We suggest that, at conditions that preclude dehydration-melting, even small-scale melting of eclogite requires addition of fluid from a suitable near-field source.
... Many of the existing experimental data (Lambert & Wyllie, 1972;Liu, Jin, & Zhang, 2009;Skjerlie & Patino Douce, 2002) and some natural observations (e.g. Cao, Gilotti, & Massonne, 2020;Cao, Gilotti, Massonne, Ferrando, & Foster, 2019;Miyazaki et al., 2016;Nakamura & Hirajima, 2000;Xia, Zheng, & Zhou, 2008) advocate for partial melting of UHP mafic rocks during their exhumation through HP-MP conditions to crustal depths. Most of the studies linked the incipient melting to dehydration melting of phengite (Cao et al., 2020;Gao, Zheng, & Chen, 2012;Liu et al., 2009), whereas some recent research highlight potentially similar role of zoisite, epidote-group minerals or amphibole at least in some cases (Cao et al., 2019(Cao et al., , 2020 The wet solidus of phengite dehydration melting for coesite and diamond stability fields above 3 GPa, in mafic eclogites is located at > 800 C (Auzanneau et al., 2006;Schmidt et al., 2004;Schmidt & Poli, 2014;Vielzeuf & Holloway, 1988), and thus, if phengite was ever present in the eclogite, the melting curve was most likely reached at both peak P-T and during decompression heating of UHP domains of the Kokchetav Massif (e.g. ...
... Cao, Gilotti, & Massonne, 2020;Cao, Gilotti, Massonne, Ferrando, & Foster, 2019;Miyazaki et al., 2016;Nakamura & Hirajima, 2000;Xia, Zheng, & Zhou, 2008) advocate for partial melting of UHP mafic rocks during their exhumation through HP-MP conditions to crustal depths. Most of the studies linked the incipient melting to dehydration melting of phengite (Cao et al., 2020;Gao, Zheng, & Chen, 2012;Liu et al., 2009), whereas some recent research highlight potentially similar role of zoisite, epidote-group minerals or amphibole at least in some cases (Cao et al., 2019(Cao et al., , 2020 The wet solidus of phengite dehydration melting for coesite and diamond stability fields above 3 GPa, in mafic eclogites is located at > 800 C (Auzanneau et al., 2006;Schmidt et al., 2004;Schmidt & Poli, 2014;Vielzeuf & Holloway, 1988), and thus, if phengite was ever present in the eclogite, the melting curve was most likely reached at both peak P-T and during decompression heating of UHP domains of the Kokchetav Massif (e.g. Shatsky et al., 2015;Stepanov et al., 2014). ...
... Cao, Gilotti, & Massonne, 2020;Cao, Gilotti, Massonne, Ferrando, & Foster, 2019;Miyazaki et al., 2016;Nakamura & Hirajima, 2000;Xia, Zheng, & Zhou, 2008) advocate for partial melting of UHP mafic rocks during their exhumation through HP-MP conditions to crustal depths. Most of the studies linked the incipient melting to dehydration melting of phengite (Cao et al., 2020;Gao, Zheng, & Chen, 2012;Liu et al., 2009), whereas some recent research highlight potentially similar role of zoisite, epidote-group minerals or amphibole at least in some cases (Cao et al., 2019(Cao et al., , 2020 The wet solidus of phengite dehydration melting for coesite and diamond stability fields above 3 GPa, in mafic eclogites is located at > 800 C (Auzanneau et al., 2006;Schmidt et al., 2004;Schmidt & Poli, 2014;Vielzeuf & Holloway, 1988), and thus, if phengite was ever present in the eclogite, the melting curve was most likely reached at both peak P-T and during decompression heating of UHP domains of the Kokchetav Massif (e.g. Shatsky et al., 2015;Stepanov et al., 2014). ...
U‐Pb geochronological, trace‐element and Lu‐Hf isotopic studies have been made on zircons from ultrahigh‐pressure (UHP) mafic eclogite from the Kumdy‐Kol area, one of the diamond‐facies domains of the Kokchetav massif (northern Kazakhstan). The peak eclogitic assemblage equilibrated at >900°C, whereas the bulk sample composition displays LREE and Th depletion evident of partial melting. Zircons from the eclogite are represented by exclusively newly formed metamorphic grains and have U‐Pb age spread over 533–459 Ma, thus ranging from the time of peak subduction burial to that of the late post‐orogenic collapse. The major zircon group with concordant age estimates have a concordia age of 508.1±4.4 Ma, which corresponds to exhumation of the eclogite‐bearing UHP crustal slice to granulite‐ or amphibolite‐facies depths. This may indicate potentially incoherent exhumation of different crustal blocks within a single Kumdy‐Kol UHP domain. Model Hf isotopic characteristics of zircons (εHf(t) +1.5–+7.8, Neoproterozoic model Hf ages of 1.02‐0.79 Ga) closely resemble the whole‐rock values of the Kumdy‐Kol eclogites and likely reflect in‐situ derivation of HFSE source for newly formed grains. The ages coupled with geochemical systematics of zircons confirm that predominantly late zircon growth occurred in Th‐LREE‐depleted eclogitic assemblage, that experienced incipient melting and monazite dissolution in melt at granulite‐facies depths, followed by amphibolite‐facies rehydration during late‐stage exhumation‐related retrogression.
This article is protected by copyright. All rights reserved.
... An underestimation of the peak temperature for eclogites could lead to a wrong assessment of the effect of diffusional homogenization. On the other hand, the outer rim of garnet might equilibrate by intercrystalline diffusion or melting reactions (Gao et al., 2012;Cao et al., 2019Cao et al., , 2020 at high temperatures (> 750 °C). This might be the reason that high temperatures experienced by eclogites were noted in some instances to occur at granulite-facies conditions (Groppo et al., 2015, and several studies considered in Table 1). ...
Eclogites are witnesses of geodynamic processes that are commonly related to subduction of oceanic crust. Information on the part of these processes that refers to the burial of this rock type is rarely published but stored in the eclogitic garnet core and inclusions therein. To better understand general aspects of the burial process, a literature search on the chemical characteristics of garnet in worldwide occurrences of eclogite was undertaken. In most cases extended garnet cores show either a prograde growth zoning with increasing Mg, starting at a few percent of pyrope component, and decreasing Mn contents (type I eclogite) or a (nearly) constant chemical composition frequently with pyrope contents significantly above 10 percent (eclogites of types II and III). Only in minor cases, it is difficult to assign the reported garnet core to an eclogite type. The growth zoning of garnet was thermodynamically modelled for the chemical composition of a basalt following different burial paths. These paths are characterized either by a trajectory along a low geothermal gradient (type I eclogite), as expected for the subducting upper portion of oceanic crust, or a one characterized by nearly isothermal burial at temperatures above 500 °C reaching peak pressures up to 2.1 GPa (type III eclogite), as possibly due to crustal thickening during continent-continent collision, or more (type II eclogite) when basic rocks are tectonically eroded from the overriding continental plate before deep subduction. In addition, diffusion modelling was undertaken on mm-sized garnet demonstrating that the characteristics of the core zoning are not fully obliterated even during residence at temperatures of 800-850 °C within 10 million years. The scrutiny of more than 200 eclogites reported in the literature led to the following result: about half of them are type II eclogites; a third and a sixth can be related to type I and type III, respectively. Among type III are almost all of the few Proterozoic eclogites considered.
To demonstrate the benefit of our study, we link the core zoning of eclogitic garnet from various (ultra)high-pressure terranes in Phanerozoic orogenic belts to the geodynamics shaping corresponding orogens. The eclogites in these belts are dominated by type II. Thus, we propose that some of the material of the lower portion of the overriding continental crust was tectonically eroded by a subducted oceanic plate and brought to great depth. Afterwards, this material was exhumed first in a deep subduction channel and then in an exhumation channel during continent-continent collision where a contact with the upper continental plate was re-established. Furthermore, we suggest that type II eclogite can also occur in extrusion wedges as far as oblique subduction took place.
Eclogite thermobarometry is crucial for constraining the depths and temperatures to which oceanic and continental crust subduct. However, obtaining the pressure and temperature ( P–T ) conditions of eclogites is complex as they commonly display high‐variance mineral assemblages, and the mineral compositions only vary slightly with P–T . In this contribution, we present a comparison between two independent and commonly used thermobarometric approaches for eclogites: conventional thermobarometry and forward phase‐equilibrium modelling. We assess how consistent the thermobarometric calculations are using the garnet–clinopyroxene–phengite barometer and garnet–clinopyroxene thermometer with predictions from forward modelling (i.e. comparing the relative differences between approaches). Our results show that the overall mismatch in methods is typically ±0.2–0.3 GPa and ±29–42°C although differences as large as 80°C and 0.7 GPa are possible for a few narrow ranges of P–T conditions in the forward models. Such mismatch is interpreted as the relative differences among methods, and not as absolute uncertainties or accuracies for either method. For most of the investigated P–T conditions, the relatively minor differences between methods means that the choice in thermobarometric method itself is less important for geological interpretation than careful sample characterization and petrographic interpretation for deriving P–T from eclogites. Although thermobarometry is known to be sensitive to the assumed X Fe ³⁺ of a rock (or mineral), the relative differences between methods are not particularly sensitive to the choice of bulk‐rock X Fe ³⁺ , except at high temperatures (>650°C, amphibole absent) and for very large differences in assumed X Fe ³⁺ (0–0.5). We find that the most important difference between approaches is the activity–composition ( a–x ) relations, as opposed to the end‐member thermodynamic data or other aspects of experimental calibration. When equivalent a–x relations are used in the conventional barometer, P calculations are nearly identical to phase‐equilibrium models (Δ P < 0.1). To further assess the implications of these results for real rocks, we also evaluate common mathematical optimizations of reaction constants used for obtaining the maximum P–T with conventional thermobarometric approaches (e.g. using the highest a Grs ² × a Prp in garnet and Si content in phengite, and the lowest a Di in clinopyroxene). These approaches should be used with caution, because they may not represent the compositions of equilibrium mineral assemblages at eclogite facies conditions and therefore systematically bias P–T calculations. Assuming method accuracy, geological meaningful P max at a typical eclogite facies temperature of ~660°C will be obtained by using the greatest a Di, a Cel, and a Prp and lowest a Grs and a Ms; garnet and clinopyroxene with the lowest Fe ²⁺ /Mg ratios may yield geological meaningful T max at a typical eclogite facies pressure of 2.5 GPa.
A garnet clinopyroxenite from the island of Fjørtoft, Western Gneiss Region (WGR) in Norway, has experienced a complex metamorphic evolution demonstrated by studies involving petrographic observations, mineral chemistry, phase equilibria modelling, and geothermobarometry. This rock, originally an eclogite, documents ultrahigh-pressure (UHP) conditions higher than 3.5 GPa (stage 1), witnessed by Ca-rich garnet inclusions in kyanite. The UHP assemblage was pervasively re-equilibrated at high-pressure (HP) granulite-facies conditions around 1.75 GPa and 870 °C, during which and/or somewhat before Ca-poor garnet, sodian diopside, zoisite, and biotite formed (stage 2a). Dehydration melting of zoisite and biotite (stage 2b) took place at similar pressure-temperature conditions. The produced melt was poor in Si and rich in alkalis and crystallized mainly to analcime + plagioclase + K-feldspar. Oriented composite inclusions of quartz + amphibole + phengite ± garnet, found in clinopyroxene, are interpreted to have resulted from interaction between a fluid (hydrous melt and/or aqueous fluid) and clinopyroxene at the early retrogression stage 3. Stage 1 resulted from deep subduction of the rock. Stage 2 followed after early exhumation probably due to stagnation at a hot and thickened orogenic root. Consequently, other HP granulite-facies metabasites, which are similar to the studied garnet clinopyroxenite and widespread in the northwestern WGR, could have been transformed from former eclogite at this stage, too. In addition, this study provides a direct link between the generation of peralkaline melts and crustal anatexis in collision zones and new insights into the retrograde formation of oriented mineral inclusions in HP-UHP minerals.
