Mafic Eclogites Research Articles

The Geodynamic Significance of Continental UHP Exhumation: New Constraints From the Tso Morari Complex, NW Himalaya

AbstractThe burial and exhumation of continental crust to and from ultrahigh‐pressure (UHP) is an important orogenic process, often interpreted with respect to the onset and/or subduction dynamics of continent‐continent collision. Here, we investigate the timing and significance of UHP metamorphism and exhumation of the Tso Morari complex, North‐West Himalaya. We present new petrochronological analyses of mafic eclogites and their host‐rock gneisses, combining U‐Pb zircon, rutile and xenotime geochronology (high‐precision CA‐ID‐TIMS and high‐spatial resolution LA‐ICP‐MS), garnet element maps, and petrographic observations. Zircon from mafic eclogite have a CA‐ID‐TIMS age of 46.91 ± 0.07 Ma, with REE profiles indicative of growth at eclogite facies conditions. Those ages overlap with zircon rim ages (48.9 ± 1.2 Ma, LA‐ICP‐MS) and xenotime ages (47.4 ± 1.4 Ma; LA‐ICP‐MS) from the hosting Puga gneiss, which grew during breakdown of UHP garnet rims. We argue that peak zircon growth at 47–46 Ma corresponds to the onset of exhumation from UHP conditions. Subsequent exhumation through the rutile closure temperature, is constrained by new dates of 40.4 ± 1.7 and 36.3 ± 3.8 Ma (LA‐ICP‐MS). Overlapping ages from Kaghan imply a coeval time‐frame for the onset of UHP exhumation across the NW Himalaya. Furthermore, our regional synthesis demonstrates a causative link between changes in the subduction dynamics of the India‐Asia collision zone at 47–46 Ma and the resulting mid‐Eocene plate network reorganization. The onset of UHP exhumation therefore provides a tightly constrained time‐stamp significant geodynamic shifts within the orogen and wider plate network.

Sedimentary protolith and high-P metamorphism of oxidized manganiferous quartzite from the Lanterman Range, northern Victoria Land, Antarctica

Abstract. We investigated the mineral assemblage, mineral and bulk-rock chemistry, and zircon U–Pb age of a manganiferous quartzite layer in the Lanterman Range, northern Victoria Land, Antarctica. The mineral assemblage consists primarily of phengite and quartz, along with spessartine-rich garnet, Mn3+ and rare earth element–yttrium (REY)-zoned epidote-group minerals, and titanohematite. Mineral inclusions such as tephroite, rutile and pyrophanite are hosted in porphyroblasts of the latter three minerals and suggest prograde blueschist-facies to low-T eclogite-facies metamorphism (M1). Epidote-group minerals commonly exhibit multiple growth zones of piemontite and/or epidote (M1), REY-rich piemontite (M2), REY-rich epidote (M3), and epidote (M4) from core to rim. Pseudobinary fO2–X diagrams at constant P–T support the stability of an epidote-group mineral-bearing assemblage under highly oxidized conditions during prograde M2 to peak M3 metamorphism. In marked contrast, tephroite-bearing assemblages (M1) are limited to relatively reduced environments and Mn-rich, silica-deficient bulk-rock compositions. Mn nodules have such characteristics, and the contribution of this hydrogenous component is inferred from bulk-rock chemical features such as a strong positive Ce anomaly. The major-element composition of the manganiferous quartzite suggests a protolith primarily consisting of a mixture of chert and pelagic clay. The presence of rare detrital zircons supports terrigenous input from a craton and constrains the maximum time of deposition to be ca. 546 Ma. The lack of arc-derived detrital zircons in the quartzite and the predominance of siliciclastic metasedimentary rocks among the surrounding rocks suggest that the deep-sea protolith was most likely deposited in an arc/back-arc setting at a continental margin. High-P metamorphism associated with terrane accretion during the Ross orogeny took place in the middle Cambrian (ca. 506 Ma), broadly coeval with the metamorphic peak recorded in the associated high-P rocks such as mafic eclogites. Finally, it is noteworthy that the high-P manganiferous quartzite was amenable to exhumation because the paleo-position of the protolith was likely distal from the leading edge of the downgoing slab.

Tracking Garnet Dissolution Kinetics in 3D Using Deep Learning Grain Shape Classification

Abstract The kinetics of fluid-driven metamorphic reactions are challenging to study in nature because of the tendency of metamorphic systems to converge towards chemical equilibrium. However, in cases where mineral textures that reflect incomplete reactions are preserved, kinetic processes may be investigated. Atoll garnet, a texture formed by the dissolution of a garnet’s core, has been described in 2D from thin sections of rocks worldwide. Quantifying the extent of this dissolution reaction requires a sample-wide examination of hundreds of individual grains in 3D. In this study, we quantified the distribution of atoll garnet using micro-computed tomography and grain shape analysis. A convolutional neural network was trained on human-labeled garnet grains for automated garnet classification. This approach was applied to a retrogressed mafic eclogite from the Zermatt–Saas Zone (Western Alps). Pervasive atoll-like resorption preferentially affected the larger porphyroblasts, suggesting that compositional zoning patterns exert a first-order control on dissolution rates. A kinetic model shows that the reactivity of metastable garnet to form atolls is favored at pressure–temperature conditions of 560 ± 30°C and 1.6 ± 0.2 GPa. These conditions coincide with the release of water when lawsonite breaks down during the exhumation of mafic eclogites. The model predicts dissolution rates that are three to five times faster for the garnet core than for the rim. This study shows that deep learning algorithms can perform automated textural analysis of crystal shapes in 3D and that these datasets have the potential to elucidate petrological processes, such as the kinetics of fluid-driven metamorphic reactions.

Zircon trace-element and isotopes (U–Pb, Lu–Hf, δ 18 O) response to fluid-deficient metamorphism of a subducted continental terrane (North Muya, Eastern Siberia)

Abstract The orogenic continental crust of accretionary and collisional belts worldwide is dominated by felsic and metasedimentary rocks, which show variable responses to high-grade metamorphism. Transformation of felsic rocks is commonly limited as compared to that of the enclosed mafic rocks (including eclogites sensu stricto ), which is widely attributed to availability of H 2 O–CO 2 fluids, kinetically controlled growth of high-grade assemblages, and their preferential preservation in metabasites more competent to rehydration. We report on the results of studies of the geochemical behaviour of zircon (trace-element, U–Pb, Lu–Hf and δ 18 O) in three felsic samples (two metagranitoids and one paragneiss), which are spatially (geographically and at the outcrop-scale) juxtaposed with mafic eclogites within the North Muya block (Neoproterozoic Baikalides, northern Central Asian Orogenic Belt). The data imply that metagranitoids and metasediments within the buried continental lithosphere might follow a single subduction-related P – T – t trend, whereas contrasting degrees of mineralogical and zircon transformation were governed by mineral buffer reactions in the absence of external fluids. The latter was significant only in an H 2 O-enriched protolith of metasediments. The formation of 18 O-depleted zircon recrystallization rims together with Mn enrichment of garnet rims indicate a distinct metamorphic stage without or with minor localized fluid infiltration, most likely, related to peak temperature conditions during collision. Supplementary material: The data on the mineral composition, trace-element and U–Pb–Lu–Hf–O isotopic composition of zircon are available at https://doi.org/10.6084/m9.figshare.c.6794043

Petrological study of an eclogite-facies metagranite from the Champtoceaux Complex (La Picherais, Armorican Massif, France)

Abstract. The high-pressure metagranite of La Picherais belongs to the Cellier Unit (part of the lower allochthon of the Champtoceaux Complex; Armorican Massif, western France), where it crops out as an undeformed body embedded within the orthogneisses of the Cellier Unit and is closely associated with numerous mafic eclogite lenses and seldom metahornfels. The petrographic observations of this metagranite reveal the presence of well-developed reaction textures: (1) pseudomorph after plagioclase, (2) garnet and phengite coronae at biotite–plagioclase interfaces, (3) garnet and phengite coronae at biotite–K-feldspar interfaces, and (4) garnet and rutile coronae at ilmenite–plagioclase interfaces, attesting that it underwent high-pressure and low-temperature conditions after the granite intrusion and its cooling. The analysis of the coronae and of a xenolith inclusion found in this granite points to pressure (P) and temperature (T) estimates of P>1.7 GPa and T=600–650 ∘C for the peak of metamorphism. P–T estimates performed on the mafic eclogite collected in the vicinity of the metagranite give values of 2.0–2.2 GPa and 640–680 ∘C, in good agreement with previous estimates made in other places within the Cellier Unit. The La Picherais metagranite is a key example of undeformed high-pressure metagranite allowing the study of the reactivity and degree of transformation of quartzofeldspathic rocks during subduction and constitutes a Variscan equivalent of the Alpine Monte Mucrone or Brossasco–Isasca metagranitoids.

Deciphering the nature and age of the protoliths and peak P−T conditions in retrogressed mafic eclogites from the Maures-Tannneron Massif (SE France) and implications for the southern European Variscides

We present new constraints on the age, nature, and tectonic setting of mafic eclogite protoliths from the Maures-Tanneron Massif, southern Variscan belt. Whole-rock major and trace element geochemistry was combined with zircon dating using 206Pb/238U by LA‒ICP‒MS to improve the understanding of this key-target of the European Southern Variscides. Geochemical data show that protoliths of the mafic eclogites are typical MORBs, while REE and HFSE patterns suggest an E-MORB affinity. However, the geochemical study shows several signs of crustal contamination that increases with the degree of retrogression. A comparison with Sardinian eclogites, which belong to the same Variscan microplate, namely, “MECS” (Maures-Estérel-Corsica-Sardinia), demonstrates that the eclogites are included in migmatites, which is the case for the studied samples, are the most contaminated. The Maures-Tanneron mafic eclogites represent the remnant of an oceanic basaltic crust. Zircon cores display homogeneous Th/U ratios (0.3–0.4), which are consistent with a magmatic origin, and define an age peak at 499.5 ± 2.9 Ma that is interpreted as the most likely emplacement age of the basaltic protolith. This age suggests that this protolith was part of an oceanic floor that was older than the Rheic Ocean and located to the north of the Gondwana active continental margin as predicted by recent unified full plate reconstruction models. Although the studied eclogites are retrogressed, the study of mineral inclusions trapped in garnets combined with thermodynamic modelling yields a P−T range of 17.2–18.5 kbar and 640–660 °C, which is consistent with the standard oceanic subduction palaeo-geotherm. These new data suggest that eclogites recognized in the “MECS” Variscan microplate represent the closure of oceanic domains of different ages (Cambrian or Ordovician).

Isothermal compression of an eclogite from the Western Gneiss Region (Norway)

AbstractIn the Western Gneiss Region in Norway, mafic eclogites form lenses within granitoid orthogneiss and contain the best record of the pressure and temperature evolution of this ultrahigh‐pressure (UHP) terrane. Their exhumation from the UHP conditions has been extensively studied, but their prograde evolution has been rarely quantified although it represents a key constraint for the tectonic history of this area. This study focused on a well‐preserved phengite‐bearing eclogite sample from the Nordfjord region. The sample was investigated using phase‐equilibrium modelling, trace‐element analyses of garnet, trace‐ and major‐element thermobarometry and quartz‐in‐garnet barometry by Raman spectroscopy. Inclusions in garnet core point to crystallization conditions in the amphibolite facies at 510–600°C and 11–16 kbar, whereas chemical zoning in garnet suggests growth during isothermal compression up to the peak pressure of 28 kbar at 600°C, followed by near‐isobaric heating to 660–680°C. Near‐isothermal decompression to 10–14 kbar is recorded in fine‐grained clinopyroxene–amphibole–plagioclase symplectites. The absence of a temperature increase during compression seems incompatible with the classic view of crystallization along a geothermal gradient in a subduction zone and may question the tectonic significance of eclogite facies metamorphism. Two end‐member tectonic scenarios are proposed to explain such an isothermal compression: Either (1) the mafic rocks were originally at depth within the lower crust and were consecutively buried along the isothermal portion of the subducting slab or (2) the mafic rocks recorded up to 14 kbar of tectonic overpressure at constant depth and temperature during the collisional stage of the orogeny.

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

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

Exhumation of deep continental crust in a transpressive regime: The example of Variscan eclogites from the Aiguilles‐Rouges massif (Western Alps)

AbstractMafic eclogites are found in many orogens as lenses embedded in quartzofeldspathic migmatites. These high‐pressure relics are interpreted either as remnants of ancient sutures and thus formed during oceanic subduction or as fragments of lower crust exhumed from the root of orogenic thickened crust. It is critical to distinguish between these two endmember scenarios as the resulting palaeogeographic and geodynamic reconstructions may significantly differ. In this contribution, we investigated eclogite relics from Lac Cornu in the Aiguilles‐Rouges massif, one of the External Crystalline Massifs of the Western Alps. Phase equilibrium modelling suggests that these mafic rocks were buried along a prograde path (M1) from ~600°C/1.2 to 1.6 GPa to peak conditions of ~630–775°C and >1.6 GPa. Zircon rims, with a rare earth element signature typical of eclogite facies zircon (no Eu anomalies, flat HREE spectrum), and rutile were dated by U–Th–Pb laser ablation inductively coupled plasma mass spectrometry (LA‐ICPMS) at c. 335–330 Ma. Prograde deformation has not been identified in the field and is only recognized thanks to the crystallographic preferred orientation (CPO) of inclusions of omphacite and rutile in garnet. Peak pressure conditions were followed by a decompression stage (M2) from ~760°C/1.4 GPa to ~600–650°C/0.9 GPa supported by the breakdown of omphacite into plagioclase–clinopyroxene symplectite and the crystallization of plagioclase–amphibole corona around garnet. The M2 retrogression stage is associated with the development of a main sub‐horizontal planar fabric and the CPO of minerals composing symplectite. This deformation stage is interpreted as the result of horizontal lower crustal flow. The final stage of exhumation (M3) is characterized by the replacement of symplectite and garnet by plagioclase and large euhedral amphibole and by the breakdown of rutile and ilmenite into titanite dated at c. 300 Ma. The CPO of titanite and amphibole are consistent with the development of vertical dextral shear zone in a transpressive regime. The combination of field observations and petrological, microtextural and geochemical analyses suggests that the mafic eclogites preserved in migmatitic rocks of the Aiguilles‐Rouges massif are remnants of a continental lower crust exhumed and juxtaposed with lower‐grade migmatites in crustal‐scale vertical transpressive shear zones.

In situ laser ablation Lu–Hf geochronology of garnet across the Western Gneiss Region: campaign-style dating of metamorphism

The development of in situ laser ablation Lu–Hf geochronology of apatite, xenotime and garnet has opened avenues to quickly and directly date geological processes. We demonstrate the first use of campaign-style in situ Lu–Hf geochronology of garnet across the high- to ultrahigh-pressure Western Gneiss Region in Norway. Mafic eclogites from this region have been the focus of much work, and were clearly formed during continental subduction during the Caledonian Orogeny. However, abundant quartzofeldspathic and pelitic lithologies record a more complex history, with some preserving polymetamorphic age data, and most containing no indication of high-pressure mineral assemblages formed during subduction. Twenty metapelitic and felsic samples spanning 160 lateral kilometres across the Western Gneiss Region have been analysed using garnet Lu–Hf geochronology. The results reveal Caledonian ages for the majority of the garnets, suggesting that some quartzofeldspathic and metapelitic lithologies were reactive and grew garnet during high- to ultrahigh-pressure metamorphism. However, two ultrahigh-pressure eclogite locations, Verpeneset and Fjørtoft, preserve both Caledonian and Neoproterozoic-aged garnets. Despite significant uncertainties on some of the Lu–Hf geochronological ages, laser ablation Lu–Hf efficiently identifies the polymetamorphic history of parts of the Western Gneiss Region, illustrating the effectiveness of this novel analytical method for rapid mapping of metamorphic ages. Supplementary material: All laser ablation Lu–Hf geochronological data for the garnets analysed in this study are available at https://doi.org/10.6084/m9.figshare.c.5715453 Thematic collection: This article is part of the Caledonian Wilson cycle collection available at: https://www.lyellcollection.org/cc/caledonian-wilson-cycle

The Continental Crust beneath the Western Amerasia Basin: Mechanisms of Subsidence

Abstract —The western part of the large Amerasia Basin in the Arctic Ocean comprises the smaller basins of Podvodnikov and Makarov. Judging by the sedimentary structure and the crustal subsidence history, both basins were developed on the continental crust despite their 3–4 km water depths. By the early Miocene, prior to the rapid formation of the basins, the crustal surface had been close to the sea level for a long time. Lithospheric stretching had a minor input to the subsidence, which was rather driven mainly by the prograde metamorphism of gabbro in the lower crust and its transformation into denser eclogite. The mechanism of subsidence associated with the metamorphic transformation from gabbro to eclogite implies that high-velocity eclogite belongs to the lower continental crust metamorphosed under the effect of mantle fluids. This idea undermines the seismic and gravity basin models that commonly attribute mafic eclogite to the sub-Moho lithospheric mantle on the basis of P-wave velocities similar to those in peridotite and interprets the crust beneath the Podvodnikov and Makarov basins as thin continental and oceanic crustal types, respectively.

The impact of a tear in the subducted Indian plate on the Miocene geology of the Himalayan-Tibetan orogen

Abstract The Yadong-Gulu Rift, cutting across the Gangdese belt and Himalayan terranes, is currently associated with a thermal anomaly in the mantle and crustal melting at 15–20 km depth. The rift follows the trace of a tear in the underthrusted Indian continental lithospheric slab recognized by high resolution geophysical methods. The Miocene evolution of a 400-km-wide band following the trace of the tear and the rift, records differences interpreted as indicative of a higher heat flow than its surroundings. In the Gangdese belt, this band is characterized by high-Sr/Y granitic magmatism that lasted 5 m.y. longer than elsewhere and by the highest values of εHf(i) and association with the largest porphyry Cu-Mo deposits in the Gangdese belt. Anomalously young magmatic rocks continue south along the rift in the Tethyan and Higher Himalayas. Here, a 300-km-wide belt includes some of the youngest Miocene Himalayan leucogranites; the only occurrence of mantle-derived mafic enclaves in a leucogranite; young mantle-derived lamprophyre dikes; and the youngest and hottest migmatites in the Higher Himalayas. These migmatites record a history of rapid exhumation contemporaneous with the exhumation of Miocene mafic eclogite blocks, which are unique to this region and which were both heated to >800 °C at ca. 15–13 Ma, followed by isothermal decompression. We suggest that the prominent tear in the Indian lithosphere, sub-parallel to the rift, is the most likely source for these tectono-thermal anomalies since the Miocene.

The Molybdenum isotope subduction recycling conundrum: A case study from the Tongan subduction zone, Western Alps and Alpine Corsica

Molybdenum isotopes have emerged as novel tracers for high-temperature igneous and metamorphic processes. The debate remains to what extent different subducted slab lithologies, such as oceanic crust and marine sediments, contribute to the Mo isotope signature of arc magmas and, hence, exert different controls on the terrestrial Mo cycle. Here we investigate Mo isotope systematics from input to output at the Tongan subduction zone: Arc lavas from different Tongan islands, pelagic sediments and altered oceanic crust (AOC) samples from DSDP site 595/596 on the subducting Pacific plate. For complementary insights into the fate of Mo and its isotopic signatures during prograde subduction metamorphism, we also present data of metasediments and variably altered AOC-type eclogites from the Zermatt-Saas ophiolite, Switzerland and Italy, and the Schistes Lustrés Complex in Alpine Corsica.Manganese oxide-rich pelagic sediments from DSDP site 595/596 show variable, depth-dependent Mo/Mn ratios and Mo isotope compositions controlled by diagenetic reactions. As subducted equivalents, Mn-rich eclogitic metapelites display lower Mo contents and δ98/95Mo ratios compared to their non-subducted protolith. This indicates prominent loss of Mo along with isotope fractionation during early subduction metamorphism. In comparison to unaltered MORB, low temperature seafloor alteration has shifted Mo/Ce and δ98/95Mo in studied AOC samples towards lower ratios, in the range of most mafic eclogites published so far. However, some mafic eclogites show even lower Mo/Ce and δ98/95Mo ratios compared to AOC, likely due to fluid-related Mo loss upon early subduction and preferential incorporation of light Mo into residual rutile.Our data document a prominent loss of isotopically heavy Mo before and upon early subduction metamorphism at shallow depths in the forearc region. Moreover, when prograde rutile crystallizes at ~30 km depth, it fixes the largest fraction of Mo in the subducting material. This creates an “arc Mo-conundrum” as devolatization of the slab at subarc depths is not able to account for the fluid-mobile Mo source responsible for observed higher Mo/Ce and δ98/95Mo in Tongan arc lavas compared to the mantle. As an alternative scenario, Mo mobilization by slab-derived aqueous fluids during the early stages of subduction into the forearc mantle produces serpentinites enriched in Mo with a possible heavy Mo isotopic signature. Mechanical transport and devolatization of forearc serpentinites at subarc regions is a plausible alternative recycling process accounting for the observed Mo systematics in Tongan arc lavas. This is supported by positive covariations of Mo/Ce and δ98/95Mo with elements such as As, Sb, and Cs, which are thought to be mostly released from the subducting material during early stages of subduction. We propose that a multi-stage recycling of metasomatized forearc mantle can be an important process in recycling of Mo and possibly other elements.

Retrograde carbon sequestration in orogenic complexes: A case study from the Chinese southwestern Tianshan

The interaction between ascending carbonic fluids and rocks at shallow depths in orogenic systems plays a crucial role in carbon distribution and fluxes regulation. Here we present field, microstructural, petrological and geochemical results of carbonic fluid–rock interactions affecting exhumed mafic eclogites from the Chinese southwestern Tianshan ultrahigh-pressure (UHP) metamorphic belt. The mafic eclogites recorded two successive, superimposed, metamorphic–metasomatic stages: graphite precipitation along fractures and veins at eclogite facies (Stage#1) and pervasive rock carbonation (i.e., silicate dissolution and carbonate precipitation) at retrograde amphibolite to greenschist facies (Stage#2). This work focuses on Stage#2 carbonation, which consists of the transformation of Stage#1 graphite-bearing eclogites by carbonate + paragonite (± zoisite) + quartz. Carbon and oxygen isotopic data and Deep Earth Water (DEW) thermodynamic modeling suggest an external metasedimentary source for the Stage#2 carbonation. This deep carbon sequestration event can be referred to retrograde, greenschist-facies conditions at about 10 kbar and 450 °C, and redox conditions equal or more oxidized than the quartz–fayalite–magnetite (QFM) buffer. Modeling results also show that the saturation of carbonic fluid with respect to graphite can boost retrograde carbonation processes. Our findings provide new insights into the reactivity of metastable, exhumed metamafic rocks with ascending carbonic fluids in orogenic systems. We conclude that retrograde, fluid-mediated rock carbonation can significantly impact on carbon fluxes from active collisional belts.

Ubiquitous post‐peak zircon in an eclogite from the Kumdy‐Kol, Kokchetav UHP‐HP Massif (Kazakhstan): Significance of exhumation‐related zircon growth and modification in continental‐subduction settings

AbstractU–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 light rare‐earth element (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 to +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.

The 2 Ga eclogites of Central Tanzania: Directly linking age and metamorphism

The ca. 2 Ga retrogressed eclogites in the Usagaran Belt in central Tanzania are among the oldest documented eclogites in the world. As such, they have been used to pinpoint the thermal conditions of the onset of modern style subduction on Earth. Two samples of retrogressed mafic eclogite have been interrogated to reconstruct the metamorphic history of the subducted crust. The samples are several kilometres apart, separated by a mylonitic upper amphibolite-transitional granulite foliation. Lu--Hf garnet geochronology pinpoints eclogite metamorphism at 1994 ± 9 Ma, supported by rutile U--Pb geochronology. Zircon U--Pb geochronology supports subduction metamorphism at ca. 2000 Ma. Mineral equilibria modelling and Zr in rutile thermometry suggests the retrogressed eclogites reached a minimum pressure of 18 kbar at approximately 800 °C, consistent with a warm subduction thermal gradient. These conditions were followed by amphibolite-granulite-facies overprinting. Proposed tectonic models for the formation of the eclogites involve either east-dipping subduction of the Tanzanian Craton margin, or west-dipping subduction of oceanic crust under the Tanzanian Craton. The retrogressed eclogites were likely exhumed via slab breakoff driven buoyancy of the oceanic crust and transferred to shallow crustal levels in a back arc position. This data, combined with data from eclogites from the Eburnian-Transamazonian and Ubende orogens, demonstrate the possible existence of a large subduction system on the margin of west Africa from ca. 2.1–1.9 Ga. This system has led to the preservation of several eclogitic rocks, which suggests that modern style subduction was operating on significant geographic and temporal scales at this time.

Nature and (in-)coherent metamorphic evolution of subducted continental crust in the Neoproterozoic accretionary collage of SW Mongolia

In continental subduction complexes minor volumes of high-pressure mafic rocks (eclogites) often co-exist with much more abundant felsic (granitic) and metasedimentary rocks, which are vital for resolving the origin and metamorphic evolution of subducted continental crust. In SW Mongolia, the Alag Khadny eclogite-bearing accretionary complex (AKC) is assumed to represent either a remnant of oceanic slab, or a continental margin, subducted in the Early Cambrian. Here we present geochronological, geochemical and petrological evidence of subduction records for the three major types of lithologies that host mafic eclogites, including Mesoproterozoic and Neoproterozoic granitic basement and overlying Neoproterozoic continental-margin sediments. Variably deformed, ferroan and peraluminous metagranitoids compose a major part of AKC and are interlayered with eclogites in its southern and eastern margins. They have geochemical features of post-collisional/intraplate high-K calc-alkaline granites. LA-ICP-MS U-Pb zircon geochronology of three distinct metagranite samples show uniform protolith crystallization ages of ca. 0.96 Ga and uncertain re-crystallization in the Late Neoproterozoic or Early Paleozoic metamorphic event, whereas abundant zircon inheritance indicates older, Mesoproterozoic to Paleoproterozoic crustal substrate during granite generation. The existence of Mesoproterozoic crust is highlighted by finding of distinct metagranitoids with the U-Pb zircon crystallization age of ca. 1.6 Ga. Hafnium isotope signatures (TDMC 2.88–1.85 Ga) of zircons from all lithologies preserved the evidence of reworked Neoarchean to Paleoproterozoic crust, similar to that of the Baidrag block (southern Mongolia), for both Mesoproterozoic and Neoproterozoic rocks. Regardless of the specific lithology, the rocks display indicators of high-pressure metamorphic re-equilibration, including garnet (XCa up to 0.65) + epidote + phengite (Si p.f.u. up to 3.56) ± rutile assemblage in metagranitoids, garnet + phengite (Si p.f.u. up to 3.42) in quartz-rich semi-pelites and garnet + phengite (Si p.f.u. up to 3.39) + medium-Mg chloritoid (XMg up to 0.25) + kyanite + rutile in metapelites. Corresponding P-T conditions recovered from different lithologies reveal incoherent subduction of rocks, which could be shallow for granitic basement (1.1–1.4 GPa and 600–670 °C) and clastic metasediments (1.4–1.6 GPa, 570–620 °C), but deeper for metapelites (2.1–2.3 GPa, 500–570 °C). consistent with that of eclogites, The combined data show that the Alag Khadny complex represents a remnant of a rifted Mesoproterozoic to Neoproterozoic (ca. 1.6–0.96 Ga) continental margin consequently metamorphosed under HP conditions during Late Neoproterozoic–Early Cambrian evolution of the southern Central Asian Orogenic Belt. Acquired P-T estimates imply that high-pressure metagranitoids and metasedimentary rocks equilibrated at different depths, but most likely shared a common subduction-related metamorphic evolution.

A case study of in situ analyses (major and trace elements, U-Pb geochronology and Hf-O isotopes) of a zircon megacryst: Implication for the evolution of the Egéré terrane (Central Hoggar, Tuareg Shield, Algeria)

The Tighsi area of the Egéré/Aleksod Terrane (Tuareg Shield) contains mafic eclogites interlayered within anatectic metapelites corresponding to metabasalts coeval with a shallow water marine sedimentation. In this study we present in-situ geochronological (U-Pb) and geochemical (major and trace elements, Hf and O isotopes) analyses from a 2.6 mm zircon megacryst found in a high-pressure kyanite pegmatite enclosed in the metapelites.Oscillatory zoning, HREE-enriched patterns, positive Ce anomalies and the absence of negative Eu anomalies are consistent with crystallization of the megacryst from the anatectic melt. Ti-in-zircon temperatures indicate crystallization at 811 ± 15 °C in an allanite/monazite buffered anatectic melt as evidenced by the low Th/U ratios. Oxygen isotopes yield large intra-grain variations (7.1–12.3‰) with a gradual lowering towards the edge of the grain in contact with secondary feldspathic veinlets. These results indicate fluid-assisted oxygen isotope disturbances, consistent with the low retentivity of O in zircon under wet conditions. Hf isotopes do not display intra-grain variations (mean εHfi = −20.7 ± 1.0) and support production of the leucosome by melting of crustal material. U-Pb analyses of the center of the crystal provide an age of 654 ± 5 Ma (2σ), attributed to post-peak decompression and heating. HP eclogite facies conditions in the Egéré terrane are thus significantly older than HP metamorphism in the western part of the shield (610–625 Ma) in agreement with multiple subduction events along the margins of the Tuareg Shield.The rim of the megacryst, close to feldspathic veinlets, is characterized by a significant decrease of trace elements (U, Y, HFSE), but preserved identical Hf isotope ratios, which is consistent with recrystallization processes. The rim displays a Ti-in-zircon temperature of 717 ± 28 °C and a U-Pb age of 584 ± 6 Ma (2σ) coeval with the climax of batholith intrusion in Central Hoggar. Reheating and softening of the lower/middle crust at that time may have assisted and favored upward viscous flow of basement domes and escape tectonics along lithospheric shear zones. We propose that the final push of the Saharan metacraton in the east was responsible for the observed architecture of the Egéré terrane, where anatectic elongated domes of basement gneisses alternate with HP metasedimentary synforms.

Ultrahigh Pressure Metamorphism and Tectonic Evolution of Southwestern Tianshan Orogenic Belt, China: A Comprehensive Review

AbstractRecently, a huge ultrahigh‐pressure (UHP) metamorphic belt of oceanic‐type has been recognized in southwestern (SW) Tianshan, China. Petrological studies show that the UHP metamorphic rocks of SW Tianshan orogenic belt include mafic eclogites and blueschists, pelitic garnet phengite schists, marbles and serpentinites. The well‐preserved coesite inclusions were commonly found in eclogites, garnet phengite schists and marbles. Ti‐clinohumite and Ti‐chondrodite have been identified in UHP metamorphic serpentinites. Based on the PT pseudosection calculation and combined U‐Pb zircon dating, the P‐T‐t path has been outlined as four stages: cold subduction to UHP conditions before∼320 Ma whose peak ultrahigh pressure is about 30 kbar at 500 °C, heating decompression from the Pmax to the Tmax stage before 305 Ma whose peak temperature is about 600 °C at 22 kbar, then the early cold exhumation from amphibolite eclogite facies to epidote‐amphibolite facies metamorphism before 220 Ma and the last tectonic exhumation from epidote amphibolite facies to greenschist facies metamorphism. Combining with the syn‐subduction arc‐like 333‐326 Ma granitic rocks and 280‐260 Ma S‐type granites in the coeval low‐pressure and high‐temperature (LP‐HT) metamorphic belt, the tectonic evolution of Tianshan UHP metamorphic belt during late Cambrian to early Triassic has been proposed in this paper.

The roles of mechanical mixing and fluid transport in the formation of reaction zones in subduction-related mélange: Evidence from highly siderophile elements

Traverses across amphibole-chlorite schist metamorphic reaction zones at contacts between meta-igneous and ultramafic rocks in high-pressure mélange zones were analyzed for highly siderophile element (HSE) concentrations and 187Os/188Os in two mélange zones (Catalina Schist, Santa Catalina Island, CA, and Attic-Cycladic Complex, Syros, Greece) in order to investigate the relative effects of mechanical mixing and diffusive and advective transport by fluids in subduction-related reaction zones. Results from Catalina Schist traverses show co-varying elevated concentrations of Os, Ir, Ru, Cr, and MgO and inversely correlated initial 187Os/188Os that display irregular patterns across the chlorite-actinolite schist reaction zones. These elements are strongly enriched in peridotite relative to mafic crust, and are relatively fluid immobile. The observations suggest that these components were added via mechanical mixing of mantle peridotitic material with material derived from mafic blocks. In a traverse across a Syros mafic eclogite block with a narrow chlorite schist reaction zone and broader glaucophane-rich reaction zone, the Os, Ir, Ru, and Cr concentrations in the chlorite schist are just barely higher than those in the eclogite block, while the initial 187Os/188Os ratio falls between the eclogite core and adjacent serpentinite. These compositions show a relatively minor metasomatic exchange between eclogite block and serpentinite. A second sample set from Syros was chosen across a contact zone between a meta-tuffite and a serpentinite because previous studies suggest it was not affected by mechanical mixing. This traverse contains a ~1 m wide complex metasomatic reaction zone that is used to study the relative contributions of diffusive and advective transport. The traverse reveals strong mobilization of Os, Ir, Ru and Pt beyond the scale of the outcrop, and minor advective enrichment of Re and Pd. This indicates that all HSE may be mobilized by fluid advection beyond the local scale in some subduction zone mélange zones.