Ultrahigh-pressure Eclogite Research Articles

Protoliths and metamorphism of the central Himalayan eclogites: Zircon/titanite U–Pb geochronology, Hf isotope and geochemistry

The high-pressure (HP) eclogites in the central Himalaya provide insights into the metamorphism and exhumation history of crustal material beneath the Tibetan plateau. Due to the paucity of exposure, the nature and timing of the protolith and metamorphism of the eclogites remain poorly known. Here we report zircon and titanite U–Pb ages, bulk-rock and mineral compositions and zircon Hf isotope data on the eclogites from the Thongmön and Kharta areas in the central Himalaya. The eclogites record peak HP eclogite-facies metamorphism at >1.6 GPa, high-temperature to ultrahigh-temperature (HT-UHT) granulite-facies overprinting (ca. 0.88–0.99 GPa and 875–920 °C), and subsequent decompression-cooling retrograde metamorphism (ca. 0.42–0.62 GPa and 800–820 °C). Geochemical data suggest that the eclogite protoliths are most consistent with being seafloor tholeiitic basalts with an E-MORB signature. Inherited magmatic zircon cores from the eclogites give a protolith age of ∼450 Ma and εHf(t) values of + 3.2 to + 7.0. Metamorphic overgrown mantles or some rims of zircon record an early decompression granulite-facies metamorphic stage (∼17.9–15.3 Ma); other metamorphic overgrown rims and metamorphic growth zircons reflect later decompression-cooling retrograde metamorphic stage (∼14.8–13.3 Ma). The titanite U–Pb age (∼15.4 Ma) further indicates the beginning of the later retrograde metamorphism. All these data and observations allow the proposal that in contrast to the ultrahigh-pressure (UHP) eclogites in the western Himalaya, the HP eclogites in the central Himalaya record the long-lived burial and relatively slow exhumation of the Indian continental crust since initial Indo-Asia collision at ∼55 Ma.

Behavior of barium isotopes during high-pressure metamorphism and fluid evolution

Barium (Ba) and its stable isotopes are powerful geochemical tracers for the recycling of subducted materials into the convective mantle. However, this requires a good understanding of the behavior of Ba isotopes during subduction-zone fluid processes. Here we present the first integrated study on Ba isotopes for two eclogite-vein systems (Ganghe and Hualiangting) that formed at high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic conditions from the Dabie orogen, central China. The Ganghe system displays constant δ138/134Ba (−0.01‰ to 0.03‰) in the UHP eclogites but relatively high δ138/134Ba (0.17‰) in the HP vein. In contrast, in the Hualiangting system, δ138/134Ba of the UHP eclogites vary from −0.14‰ to 0.05‰, while δ138/134Ba of the multiple HP veins and the retrograded amphibolites range from −0.17‰ to 0.34‰ and from −0.11‰ to 0.46‰, respectively.The combined results suggest that Ba isotopes can be considerably fractionated during subduction-zone fluid processes. HP-UHP metamorphic dehydration of the eclogites during the early stages of exhumation can produce an isotopically heavy initial vein-forming fluid due to the preferential leaching of epidote enriched with heavy Ba isotopes from the host eclogites. However, such processes have limited effects on the bulk eclogites because the Ba-rich phengite exists as a residual phase. During vein formation, light Ba isotopes are preferentially enriched in minerals relative to the fluids, resulting in the residual fluids with high δ138/134Ba. In addition, the fluids released from the gneisses have negative δ138/134Ba which is different from the eclogites, indicating that Ba isotopes can be used to trace the source of the metasomatic fluids. This study demonstrates that the UHP metamorphic rocks and HP veins can record Ba isotope fractionation during subduction-zone metamorphism and fluid-rock interaction, and recycling of the metamorphic rocks could modify Ba isotopic composition of the mantle wedge and arc volcanics.

Evolution of fluids and melts in deeply subducted continental crust: Insights from an UHP eclogite–vein system in the Dabie terrane, China

Both slab-derived high-pressure (HP) or ultrahigh-pressure (UHP) aqueous fluids and hydrous melts are important agents for material cycling in subduction zones. Although numerous studies have investigated the genesis of both fluid phases, the temporal–spatial relationship and the chemical difference between aqueous fluids and hydrous melts during subduction-zone metamorphism remain mysterious. In this study, we present detailed petrological and geochemical studies on the UHP eclogites and enclosed epidote-quartz veins in the Dabie terrane (eastern China) to understand the compositions and behaviors of fluids/melts in subduction zones. The eclogites consist mainly of garnet, omphacite, and hydrous minerals (epidote, phengite, and paragonite). Two stages of epidote, including epidote inclusions in omphacite (Ep-in) and porphyroblastic epidote (Ep-p), are recognized in the eclogites. The obviously higher contents of strontium (Sr) and light rare earth elements (LREEs) in Ep-p than in Ep-in demonstrate the previous coexistence of lawsonite and Ep-in. The combination of petrographic analyses, epidote Sr isotopes, and phase equilibria modeling indicates lawsonite in the eclogite breakdown at ~2.2–2.7 GPa and 620–750 °C during the initial exhumation, accompanied by the significant release of H2O-rich fluids. This process led to the growth of Ep-p in eclogite and the precipitation of vein omphacite, quartz, and epidote (Ep-v1). On the other hand, many lines of evidence for partial melting in the eclogites are found, including (1) multiphase solid inclusions (MSIs) of Ky + Kfs ± other phases, which represent the in situ products of phengite partial melting; (2) MSIs of Kfs ± Pl ± Qz, which represent the crystallized products of hydrous melts; and (3) veinlets of Kfs ± Di that crosscut the omphacite. Importantly, the MSIs in texture (1) are exclusively hosted in the Ep-p, which formed by lawsonite breakdown. This distinctive phenomenon indicates that the partial melting of phengite was most likely triggered by the injection of H2O from the lawsonite breakdown (i.e., water-fluxed melting). Phase equilibria modeling suggests that the partial melting of phengite occurred at ~1.5–2.0 GPa and 700–750 °C during the further exhumation stage of the eclogites. The occurrence of Ep (Ep-v2) + Pl + Kfs in the veins indicates that melts derived from the eclogites infiltrated the vein system. The compositions of the fluids and melts are quantitatively constrained using the epidote compositions (Ep-v1 and Ep-v2, respectively) and epidote–fluid/melt partition coefficients. The results indicate that the fluids have trace element patterns similar to those of the melts with high contents of large ion lithophile elements (LILEs), Th, U, Pb, and LREEs. Thus, aqueous fluids have the ability to transfer significant amounts of mobile elements from the slab like the hydrous melts, if the appropriate P–T conditions and chemical characteristics of wallrocks can be satisified.Our study reveals multistage metamorphic-fluid/melt evolution in the UHP eclogites, involving metamorphic dehydration and subsequent partial melting during the exhumation of subducted continental crust. We provide petrological evidence linking fluid liberation to the occurrence of partial melting in eclogites. Moreover, this study indicates that the development of vein systems in HP–UHP rocks may provide high-permeability channels that allow the flow and escape of melts formed during later-stage process. We propose that the occurrence of early-stage dehydration events in deeply subducted slabs facilitates the generation of hydrous melts and their transport in subduction zones.

Mafic Archean continental crust prohibited exhumation of orogenic UHP eclogite

The absence of ultrahigh pressure (UHP) orogenic eclogite in the geological record older than c. 0.6 Ga is problematic for evidence of subduction having begun on Earth during the Archean (4.0–2.5 Ga). Many eclogites in Phanerozoic and Proterozoic terranes occur as mafic boudins encased within low-density felsic crust, which provides positive buoyancy during subduction; however, recent geochemical proxy analysis shows that Archean continental crust was more mafic than previously thought, having greater proportions of basalt and komatiite than modern-day continents. Here, we show via petrological modelling that secular change in the petrology and bulk composition of upper continental crust would make Archean continental terranes negatively buoyant in the mantle before reaching UHP conditions. Subducted or delaminated Archean continental crust passes a point of no return during metamorphism in the mantle prior to the stabilization of coesite, while Proterozoic and Phanerozoic terranes remain positively buoyant at these depths. UHP orogenic eclogite may thus readily have formed on the Archean Earth, but could not have been exhumed, weakening arguments for a Neoproterozoic onset of subduction and plate tectonics. Further, isostatic balance calculations for more mafic Archean continents indicate that the early Earth was covered by a global ocean over 1 km deep, corroborating independent isotopic evidence for large-scale emergence of the continents no earlier than c. 3 Ga. Our findings thus weaken arguments that early life on Earth likely emerged in shallow subaerial ponds, and instead support hypotheses involving development at hydrothermal vents in the deep ocean.

How Himalayan collision stems from subduction

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

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

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

New constraints on P–T–t path of high–T eclogites in the Dabie orogen, China

Deciphering the P-T-t path of ultrahigh-pressure (UHP) rocks is crucial for reconstructing the evolution of continental crustal terranes involved in very deep subduction followed by exhumation. Achievieng this goal is especially challenging for UHP terranes which experienced protracted high/ultrahigh temperature (high-T/UHT) exhumation histories, because the widespread anatexis associated to retrograde re-equilibration often obliterate the evidence of UHP metamorphism. In the Dabie-Sulu orogenic belt of central-eastern China, which is the largest UHP terrane in the world, the migmatitic North Dabie complex Zone (NDZ) is the archetype of such UHP terranes experiencing a “slow and hot” exhumation. The NDZ stands out for the widespread anatexis that widely overprinted the traces of eclogite-facies metamorphism, hampering a precise reconstruction of its P-T-(t) evolution.This paper presents the results of a multidisciplinary study integrating P–T pseudosection of phase equilibrium modeling, conventional geothermometry and zircon U–Pb dating with trace element analysis applied on the high–T UHP eclogites from the NDZ. The studied eclogites underwent a multistage metamorphic evolution including post-peak granulite-facies overprinting and subsequent amphibolite-facies retrogression. P–T conditions of the granulite-facies overprinting and the amphibolite-facie retrogression have been estimated at >830 °C, 7–12 kbar and 560–880 °C, 4.3–8.9 kbar, respectively. New zircon U–Pb dating, mineral inclusion and trace element analysis, combined with modeled plagioclase modal variation by pseudosection approach, allowed constraining the ages of prograde metamorphism, eclogite-facies metamorphism, granulite-facies overprinting and amphibolite-facies retrogression of the eclogites at 247–230 Ma, 226–219 Ma, 217–212 Ma and 214–198 Ma, respectively. These results provide new P–T-t estimates and age data for the multiple metamorphic stages during continental collision, contributing to precisely reconstruct the whole P–T–t path of the NDZ.

Newly discovered MORB-Type HP garnet amphibolites from the Indus-Yarlung Tsangpo suture zone: Implications for the Cenozoic India–Asia collision

High pressure (HP) and ultrahigh pressure (UHP) oceanic rocks remain very scarce along the Indus-Yarlung Tsangpo suture zone in the Tibetan Plateau. Such rock types, however, could provide new and important insight into the understanding of the India-Asia collision tectonics and regional metamorphic processes. This contribution focuses on newly discovered HP garnet amphibolites that occur in the North Ayilari range along the Indus suture zone in the Western Himalaya. Petrographic and mineralogical studies as well as pressure–temperature (P–T) calculations show that the garnet amphibolites underwent at least three metamorphic stages. A prograde amphibolite-facies stage at 0.9–1.2 GPa and 605–680°C (M1) was followed by HP granulite-facies metamorphism at 680–750°C and 1.1–1.5 GPa (M2) and cooling to amphibolite-facies metamorphism at 628–650°C, 0.6–0.8 GPa (M3). Zircon and apatite U–Pb dating shows that the garnet amphibolites experienced granulite-facies conditions at 33–32 Ma and a cooling age of 12.5 ± 2.6 Ma at temperatures of 375–550 °C. The garnet amphibolites have light rare earth element (LREE)-depleted distribution patterns, low (87Sr/86Sr)i = 0.70727–0.70833, but high εNd(t) (+8.9 – +9.8) in whole-rock, and high εHf(t) (+13.9 – +16.0) values of zircon, which are similar to those of normal mid-ocean ridge basalt. This confirms that the protolith represents an oceanic slice that dominantly sourced from depleted mantle material. Geochemical, geochronological and geothermobarometrical data demonstrate that the garnet amphibolites represented a part of the Neo-Tethyan ocean, that experienced a profound amphibolite facies metamorphism before reaching HP granulite-facies conditions at ca. 32 Ma as a consequence of crustal thickening during the India–Asian collision. Thus, the studied HP oceanic-type slice represents an important puzzle piece that, together with other continental portions that experienced different P-T conditions such as HP and UHP eclogite and different metapelites and quartzo-feldspathic gneisses, helps to decipher the tectonic evolution of the Western Himalaya orogen, from oceanic subduction to continental collision and subsequent exhumation.

Partial melting of ultrahigh-pressure eclogite by omphacite-breakdown facilitates exhumation of deeply-subducted crust

Results from numerical modelling and experimental petrology have led to the hypothesis that partial melting was important in facilitating exhumation of ultrahigh-pressure (UHP) metamorphic rocks from mantle depths. However, the melting reactions responsible are rarely well-documented from natural examples. Here we report microstructural features and compositional data that indicate in situ partial melting dominated by breakdown of omphacite in UHP eclogite from the Sulu belt, China. Diagnostic microstructures include: (i) the presence of in situ leucosome pockets composed of plagioclase, euhedral amphibole, minor K-feldspar and epidote within host zoisite- and phengite-bearing eclogite; (ii) skeletal omphacite within the leucosome pockets that has a lower jadeite content (25–45 mol.%) than rock-forming omphacite (39–54 mol.%); and, (iii) seams of Na-rich plagioclase that extend along grain boundaries separating phengite, quartz and zoisite, and which commonly exhibit low dihedral angles where they terminate at triple grain-boundary junctions. Major oxide proportions of 57 leucosome pockets, calculated using mineral modes and compositions, yield leucodiorite bulk compositions characterized by intermediate SiO2, high Al2O3 and Na2O, and low K2O contents. In primitive mantle-normalised trace element diagrams, the leucosome pockets show enrichment in large ion lithophile elements, U, Pb, Zr, Hf and Ti, but depletion in Th and Ta, patterns that are similar to those of rock-forming omphacite. Rather than forming predominantly by breakdown of phengite and/or zoisite, as widely proposed in the literature, the leucosome pockets have petrographic characteristics and major oxide and trace element compositions that are consistent with partial melting dominated by omphacite breakdown. Based on conventional thermobarometry, the eclogite was exhumed from pressure–temperature (P–T) conditions of 3.6–3.1 GPa and 900–840°C. Partial melting led to the formation of the leucosome pockets, which equilibrated with the rims of surrounding rock-forming garnet and pyroxene during crystallisation. Conventional thermobarometry using rim compositions yields P–T conditions of 1.6–1.2 GPa and 780–690°C, broadly consistent with calculated phase equilibria and Ti-in-zircon temperatures from zircon overgrowths. Weighted mean ages of ca 217–214 Ma from thin overgrowths on zircon are interpreted to record melt crystallisation. This study provides insight into an overlooked mechanism by which eclogites partially melt during exhumation from UHP conditions, and permits a better understanding of the processes that assist deeply-subducted continental crust to return to shallower depths.

Mesozoic reworking of the Paleozoic subducted continental crust beneath the south-central margin of the North China Block: Geochemical evidence from granites in the Xiaoqinling-Xiong’ershan region

Mesozoic granites are common in the south-central margin of the North China Block, beneath which the North Qinling microcontinent was subducted northwards in the early Paleozoic to subarc depths for ultrahigh-pressure (UHP) metamorphism. Although the continental deep subduction would have produced UHP eclogite facies metamorphic rocks in the North Qinling orogen, it is intriguing whether the Mesozoic granites record reworking of the crustal rocks from the subducted North Qinling microcontinent. This issue is addressed by an integrated study of zircon in-situ U-Pb ages and Hf-O isotopes as well as whole-rock geochemistry for four typical granitic plutons from the Xiaoqinling-Xiong’ershan region in the south-central margin of the North China Block. These plutons are metaluminous to weakly peraluminous, and belong to high-K calc-alkaline to shoshonitic series. They are enriched in LREE and LILE but depleted in HREE and HFSE. In terms of trace element and Sr-Nd-Hf isotope compositions, these granites can be categorized into two groups. Group I granites from the Huashan, Wenyu and Heyu plutons are high in Sr concentrations and Sr/Y and (La/Yb)N ratios without any significant Eu anomalies and thus similar to adakitic rocks, and have more enriched Sr-Nd-Hf isotope compositions. In contrast, Group II granites from the Funiushan pluton have low Sr concentrations and Sr/Y and (La/Yb)N ratios with negative Eu anomalies, and show less enriched Sr-Nd-Hf isotope compositions. Although zircon U-Pb dating for syn-magmatic zircons from the two groups of granite gives consistent emplacement ages of 120 to 137 Ma, relict zircons in them show different U-Pb ages. In detail, relict zircons from Group I granites exhibit Neoarchean to Paleoproterozoic U-Pb ages, with Hf-O isotope compositions similar to those of the ancient basement such as the Taihua Group and the Xionger Group in the south-central margin of the North China Block. In comparison, relict zircons from Group II granites display not only Paleoproterozoic U-Pb ages but also Neoproterozoic and early Paleozoic U-Pb ages. The Paleoproterozoic relict zircons exhibit Hf-O isotope compositions similar to those of the ancient basement in the south-central margin of the North China Block, whereas the younger ones have Hf-O isotope compositions consistent with those of the Neoproterozoic protoliths of metamorphic rocks and early Paleozoic granitoids in the North Qinling orogen. Therefore, Group I granites were derived from partial melting of the ancient basement in the south-central margin of the North China Block, whereas Group II granites were produced by partial melting of a mixing source that was composed of not only the ancient crust from the North China Block but also the juvenile crust from the North Qinling microcontinent. This provides the geochemical evidence for late Mesozoic reworking of the crustal rocks from the North Qinling microcontinent through its northward subduction in the early Paleozoic beneath the south-central margin of the North China Block.

Zircon and titanite behaviors during partial melting of metabasite in the post-collisional stage: Constraints from garnet pyroxenite in the Dabie orogen, China

Abstract An integrated study of petrology, mineral inclusions, U-Pb ages and trace elements in zircon and titanite was carried out for garnet pyroxenites from the Dabie orogen, China. The results provide not only geochemical constraints on zircon and titanite behaviors during partial melting but also petrological insights into anatectic reworking of ultrahigh-pressure metamorphic eclogites in the collisional orogen. The target garnet pyroxenites were transformed from ultrahigh-pressure eclogites produced by the continental deep subduction in the Triassic, and underwent granulite facies metamorphic superimposition in the early Cretaceous under P-T conditions of 746–837 °C and 0.8–1.2 GPa, followed by amphibolite facies retrogression at 737–791 °C and 0.7 GPa. Both zircon and titanite contain two types of metamorphic domains at ca. 131 Ma based on CL images, mineral inclusions, REE patterns and trace element variations. Peritectic zircon domains (D3) contain abundant multiphase crystal inclusions of quartz and K-feldspar and exhibit variably high Nb, Ta, Th, U, P, REE and Y contents. In contrast, anatectic zircon domains (D4) are relatively free of crystal inclusion and display variably low Nb, Ta, Th, U, P, REE and Y contents. Metamorphic and peritectic titanite domains are clearly distinct by a series of differences in petrological and geochemical compositions. There are reduced 176Hf/177Hf ratios for both peritectic and anatectic zircon domains, indicating involvement of Hf-rich minerals in the anatexis of the ancient crust. Therefore, the discrimination between the polygenetic zircon and titanite domains in the metabasites not only provides a powerful means to decipher the nature of crustal anataxis in the collisional orogen, but also places constraints on the formation mechanism of metamorphic, peritectic and anatectic minerals.

Geochemistry of high-pressure to ultrahigh-pressure granitic melts produced by decompressional melting of deeply subducted continental crust in the Sulu orogen, east-central China

The production of granitic melts by decompressional melting of deeply subducted crustal rocks under high-pressure (HP) to ultrahigh-pressure (UHP) conditions is important for the crust-mantle interaction at convergent plate boundaries. However, little is known on the geochemical composition of natural HP to UHP granitic melts from collisional orogens. This problem is partially resolved here through an integrated study of the Yangkou granitic veins in the Sulu orogen, which provide the excellent records of HP to UHP granitic melts from the deeply subducted continental crust. In trace element composition, these granitic veins show enrichment in LREE and LILE but depletion in HFSE with remarkable negative anomalies of Nb, Ta, P and Ti. They have low Sr/Y (5.92–26.3) and (La/Yb)N ratios (12.7–45.6), which are primarily controlled by the solubility of accessory minerals such as apatite and epidote/allanite in anatectic melts. Phengite in the granitic veins have high Si (3.29–3.42 p.f.u.) and Mg (0.16–0.35 p.f.u.) with Mg# of 37.7–68.1 and calculated pressures of 1.7–3.1 GPa, indicating their crystallization under HP to UHP conditions. Geochemical variables such as K2O/Na2O, A/CNK and LREE for whole-rock exhibit a positive correlation with pressure, indicating the significant pressure effect on the mineral stability that in turn controls the composition of anatectic melts. Furthermore, the granitic veins can be divided into two groups based on their formation pressures and geochemical compositions. Group I granitic veins formed at low pressures of 1.7–2.4 GPa and exhibit higher SiO2 and Na2O contents, but lower contents of K2O, MgO, TiO2, P2O5 and most trace elements (e.g., Rb, Ba, LREE) with lower K2O/Na2O ratios and A/CNK values compared to Group II granitic veins that formed at high pressure of 2.6–3.1 GPa. Zircon from the granitic veins give concordant U–Pb ages of 755 ± 10 to 769 ± 9 Ma with steep HREE patterns for relict magmatic cores and 216 ± 4 to 222 ± 4 Ma with steep to flattened HREE patterns for anatectic rims. In comparison to concordant U–Pb ages of 220 ± 3 to 227 ± 3 Ma for newly grown zircon in the host eclogites, it appears that these granitic veins were crystallized from anatectic melts produced by crustal anataxis during the exhumation of the deeply subducted continental crust. The anatectic rims are characterized by the absence of not only oscillatory zoning in CL images but also Eu anomalies and low U contents, indicating their growth from anatectic melts under HP to UHP conditions. The granitic veins show different Sr–Nd–O isotope compositions from the host UHP eclogites, indicating that the former did not originate from partial melting of the latter. This is also supported by the distinct δ18O and εHf(t) values for the Triassic zircon from the granitic veins and eclogites. Instead, there are comparable isotopic compositions between the granitic veins and regional UHP granitic gneisses, indicating that the granitic veins would be derived from partial melting of the underlying UHP granitic gneisses. Group II granitic veins have higher εNd(t) values than Group I ones and the granitic gneisses by 3–5 εNd unit, indicating that the anatectic melts forming Group II granitic veins are in Nd isotope disequilibrium with their source rocks. This can be caused by the behavior of accessory minerals like apatite and epidote/allanite during crustal anataxis at different pressures. The contributions of apatite together with garnet can also account for the increased Hf isotopes in the anatectic zircon. Therefore, the accessory minerals play a key role in constraining the geochemical features of incompatible-element enriched granitic melts during the crustal anatexis under HP to UHP conditions.

Constraints on retrograde metamorphism of UHP eclogites in North Qinling, Central China, from 40Ar/39Ar dating of amphibole and phengite

Coesite- and microdiamond- bearing ultra-high pressure (UHP) eclogites in the North Qinling terrane have been widely retrogressed to amphibolites. Previous geochronological studies on these UHP rocks mainly focused on the timing of peak eclogite facies metamorphism. The Kanfenggou UHP metamorphic domain is one of the best-preserved coesite-bearing eclogite occurrences in the North Qinling terrane. In this study, mafic amphibolites and host schists from this domain were collected for 40Ar/39Ar dating to constrain their retrograde evolution. Two generations of amphibole are recognized based on their mineral parageneses and 40Ar/39Ar ages. A first generation of amphibole from garnet amphibolites yielded irregularly-shaped age spectra with anomalously old apparent ages. Isochron ages of 484–473 Ma and initial 40Ar/36Ar ratios of 3695–774 are obtained from this generation of amphibole, indicating incorporation of excess argon. Second generation amphibole occurs in epidote amphibolites yielded flat age spectra with plateau ages of 464–462 Ma without evidence for excess argon. These ages suggest that the amphibolite-facies metamorphism has taken place as early as 484 Ma and lasted until 462 Ma for the North Qinling UHP metamorphic rocks. Phengite from the country-rock schists yielded 40Ar/39Ar plateau ages of 426–396 Ma, with higher phengite Si contents associated with the older the plateau ages. Based on our new 40Ar/39Ar ages and previous zircon UPb geochronological data, we construct a new detailed pressure-temperature-time (P-T-t) path illustrating the retrograde metamorphism and exhumation rate of the North Qinling eclogites and host schists. The P-T-t path suggests that these UHP metamorphic rocks experienced initial medium-to-high exhumation rates (ca. 8.7 mm/yr) during the Early Ordovician (489–484 Ma), which was mainly derived from buoyancy forces. Subsequently, the exhumation rate decreased gradually from ~0.8 to 0.3 mm/yr from 484 to 426 Ma, which was probably governed by extension and/or erosion.

A New HP–UHP Eclogite Belt Identified in the Southeastern Tibetan Plateau: Tracing the Extension of the Main Palaeo-Tethys Suture Zone

Abstract The Changning–Menglian orogenic belt (CMOB) in the southeastern Tibetan Plateau is an important link between the Longmu Co–Shuanghu suture (LCSS) in the northern Tibetan Plateau and the Chiang Mai–Inthanon and Bentong–Raub sutures in Thailand and Peninsular Malaysia. These belts and sutures are generally regarded as containing the remnants of the oceanic crust of the Palaeo-Tethys that formed by seafloor spreading as a result of the separation of Gondwana- and Eurasia-derived blocks during the Middle Cambrian. In this paper we report the first discovery of abundant unaltered and retrograde eclogites that occur as irregular lenses and blocks in metasedimentary rocks of the CMOB, and these eclogites form an elongate and almost north–south-trending high-pressure (HP)–ultrahigh-pressure (UHP) metamorphic belt that is ∼200 km long and ∼50 km wide. The newly discovered phengite/talc/epidote–glaucophane eclogites, lawsonite–talc–phengite eclogites, dolomite/magnesite–kyanite eclogites and phengite–kyanite-bearing retrograde eclogites have enriched (E-) and normal mid-ocean ridge basalt (N-MORB)-like affinities and mainly positive as well as some negative whole-rock εNd values (–4·34 to +7·89), which suggest an enriched and depleted oceanic lithosphere source for their protoliths. Magmatic zircons separated from the epidote–glaucophane, magnesite–kyanite and (phengite–kyanite-bearing) retrograde eclogites gave protolith ages of 317–250 Ma, which fit well within the time frame of the opening of the Palaeo-Tethys during the Middle Cambrian and its closure during the Triassic. Abundant metamorphic zircons in the eclogites indicate a Triassic metamorphic event related to the subduction of the Palaeo-Tethys oceanic crust from 235 to 227 Ma. Taking into account previous isotopic age data, we now establish the periods of Early–Middle Triassic (246–227 Ma) and Late Triassic (222–209 Ma) as the ages of subduction and exhumation of the Palaeo-Tethyan oceanic crust, respectively. Thermodynamic modelling revealed that the eclogites record distinct HP–UHP peak metamorphic conditions of 23·0–25·5 kbar and 582–610 °C for the phengite–glaucophane eclogites, 24·0–25·5 kbar and 570–586 °C for the talc–glaucophane eclogites, 29·0–31·0 kbar and 675–712 °C for the dolomite–kyanite eclogites, and 30·0–32·0 kbar and 717–754 °C for the magnesite–kyanite eclogites. These P–T estimates and geochronological data indicate that the Palaeo-Tethys oceanic slab was subducted to different mantle depths from 75 km down to 95 km, forming distinct types of eclogite with a variety of peak eclogite-facies mineral assemblages. The eclogites consistently record clockwise metamorphic P–T–t paths characterized by a heating–compression prograde loop under a low geothermal gradient of 5–10 °C km–1, indicating the rapid subduction of cold oceanic crust at a rate of 4·5–6·0 km Ma–1, followed by isothermal or cooling–decompressive retrogression and exhumation at an average rate of 3·2–4·2 km Ma–1. The newly discovered eclogites of the CMOB with their signatures of ocean-crust subduction are petrologically, geochemically and geochronologically comparable with those of the LCSS, providing powerful support for the idea that a nearly 2000 km long HP–UHP eclogite belt extends from the northern Tibetan Plateau to the southeastern Tibetan Plateau, and that it represents the main boundary suture of the Palaeo-Tethyan domain. These results have far-reaching implications for the tectonic framework and complex metamorphic evolution of the Palaeo-Tethyan domain.

Water Content in Garnet from Eclogites: Implications for Water Cycle in Subduction Channels

Garnet from eclogites often shows very heterogenous and extremely high hydroxyl concentration. Eight eclogite samples were selected from the Sulu ultrahigh-pressure terrane and the Sumdo high-pressure metamorphic belt (Lhasa). The mean hydroxyl concentration in pyrope-rich and almandine-rich garnet varies from 54 to 427 ppm H2O and increases with the retrogression degree of eclogites. TEM observations reveal nanometer-sized anthophyllite exsolutions and clinochlore inclusions in water-rich domains in garnet, where anthophyllite is partly replaced by clinochlore. Because of overlapping of the infrared stretching absorption bands for structural OH in garnet and chlorite, it is impossible to exclude contribution of chlorite inclusions to the estimated hydroxyl concentration in garnet. The broad band near 3400 cm−1 is attributed to molecular water and nanometer-sized chlorite inclusions. Anthophyllite exsolutions may be formed by decomposition of hydrous garnet from ultrahigh-pressure eclogites during exhumation. Significant amounts of water can be stored in garnet from massif eclogites in the forms of hydroxyl in garnet and nanometer-sized inclusions of anthophyllite and clinochlore, as well as fluid inclusions. Amphibolite facies retrograde metamorphism can significantly increase both hydroxyl concentration and water heterogeneity in garnet from massif eclogites. These nano-inclusions in garnet provide a window to trace the water cycle in subduction channels.

The exhumation of high- and ultrahigh-pressure metamorphic terranes in subduction zone: Questions and discussions

In terms of petrology, thermomechanical simulation is an important frontier to study the geodynamic process of the exhumation and uplift of high pressure (HP) to ultrahigh pressure (UHP) metamorphic rocks in subduction zones and collision orogenic belts. Based on the recent petrological studies and numerical modellings for the exhumation of HP to UHP metamorphic terranes, the exhumation mechanisms of HP to UHP metamorphic terranes can be roughly summarized into ten types: channel flow, diapiric exhumation, a coexistence mechanism of channel flow and diapiric exhumation, slab breakoff, multi-stage exhumation, divergent plate motion (including slab rollback and the upper-plate divergent motion away from the subducting plate), overthrust exhumation, overpressure mechanism, wedge-like extrusion and microplate rotation. The exhumation of high-density UHP oceanic eclogites is a relative controversial issue. Some of our recent researches on quantitatively determining the exhumation mechanism of UHP oceanic eclogites using thermomechanical and phase equilibrium modelling was introduced in details in this paper. We obtained the 3-D density evolutions of three-type subducted oceanic materials (MORB, serpentine and oceanic sediments) in the P - T space by the methods of phase equilibrium and density calculation. According to the density difference between the metabasic and their surrounding rocks, the exhumed eclogites could be divided into two types. The first category, the self-exhumation eclogites ( ρ MORB ρ mantle), which is driven by their own buoyancy, an example is the coesite-bearing oceanic eclogites from Southwest Tianshan. Another is the carried-exhumation eclogites ( ρ MORB> ρ mantle), which can only be carried back to the surface with the assistance of low-density metasediments and serpentinite due to their negative buoyancy; the coesite-bearing UHP eclogites of Zermatt-Saas in the Western Alps is a typical example. Besides, we have further explored the ultimate self-exhumation depth, exhumation mechanisms, the effect of the transition from high pressure to ultra-high pressure on the exhumation process of oceanic eclogites and the spatial distribution of exhumed HP-UHP metamorphic terranes.

Petrogenesis of leucosome sheets in migmatitic UHP eclogites—Evolution from silicate-rich supercritical fluid to hydrous melt

Recent studies of eclogite in the ultrahigh pressure (UHP) zone of the central Sulu belt in China have shown that phengite remained stable during exhumation from the metamorphic peak to HP eclogite facies conditions. This observation requires that some other source of fluid was involved in the production of melts generated during exhumation from UHP metamorphic conditions. Here, we investigate decimeter- to meter-thick leucosome sheets in retrogressed and migmatitic UHP eclogite in the central Sulu belt. The leucosomes have variable mineral modes and deformation fabrics; some are composite with marginal and interior facies, whereas others are not. They range from granite to trondhjemite, with high abundances of SiO2, K2O + Na2O and Al2O3, similar to hydrous melts, and have major oxide and trace element variations consistent with modal changes in phengite and albite + epidote sensu lato. The leucosomes are enriched in large ion lithophile elements relative to high field strength elements and have rare earth element (REE) patterns enriched in light REE relative to heavy REE, generally with small positive or negative Eu anomalies. Newly-crystallized zircon in the leucosomes records weighted mean ages of c. 223–218 Ma. Early-formed leucosome has Sr-Nd isotope compositions similar to those of nearby unretrogressed UHP eclogites, whereas younger leucosome has Sr-Nd isotope compositions intermediate between the eclogites and surrounding gneisses. Ti-in-zircon thermometry combined with Si-in-phengite barometry indicates crystallization of the leucosomes between ~850 and ~770 °C, over a wide range of pressure (P) from 3.5 to 2.2 GPa, which correlates with age from older (higher P) to younger (lower P). At the metamorphic peak, which may have exceeded 5.5 GPa, the source rocks were likely fluid deficient or fluid absent. During exhumation from UHP conditions, we posit that exsolution of water stored in nominally anhydrous minerals formed a silicate-rich supercritical fluid in eclogite that evolved to a denser, more viscous and more polymerized hydrous melt. Phengite barometry indicates that the early-formed leucosomes crystallized at pressures close to the critical line for the basalt plus water system, possibly by diffusive loss of water to the host eclogite. Infiltration of solute-rich supercritical fluid from the surrounding gneisses blended with melt in the eclogites generating variable Sr-Nd isotope composition intermediate between these end-members, as recorded by leucosomes that crystallized at lower pressures. The youngest leucosomes crystallized at pressures near the critical line for the Ca-granite plus water system creating a low volume of aqueous fluid that precipitated quartz veins associated with the leucosome sheets, particularly at their margins. Subsequently, limited phengite-breakdown melting in the leucosomes is recorded by aggregates of plagioclase + biotite around phengite and thin films and cuspate veinlets and patches of K-feldspar along grain boundaries. Phase equilibrium modelling indicates that this late stage melting occurred at P–T conditions near the transition from HP eclogite to amphibolite facies, with final subsolidus equilibration at 1.0–0.9 GPa and T < 640 °C.

Silicon isotopic fractionation during metamorphic fluid activities: constraints from eclogites and ultrahigh-pressure veins in the Dabie orogen, China

To understand Si isotope fractionation during metamorphic fluid activities in the subduction zone, this study presents the Si isotopic compositions of two high- to ultrahigh-pressure (HP–UHP) eclogite–vein systems from the Ganghe and Hualiangting areas in the Dabie orogen, eastern China. The results show that Si isotopes are significantly fractionated between the veins and their host eclogites in both areas. The δ30Si values of Ganghe eclogites range from −0.50‰ to −0.39‰, higher than that of the studied omphacite−epidote vein (−0.63 ± 0.07‰). The Hualiangting eclogites have δ30Si values of −0.36‰ to −0.29‰, whereas the δ30Si values of the Hualiangting multi-stage veins show greater variation, from −0.45‰ to +0.05‰, revealing significant Si isotope fractionation during metamorphic fluid evolution and vein formation.For the Hualiangting and Ganghe samples, the enrichment of heavy Si isotopes in minerals follows the order of δ30Siphengite (−0.01 to +0.13‰) ≈ δ30Siquartz (−0.14 to +0.10‰) > δ30Siomphacite (−0.63 to −0.33‰) ≈ δ30Siepidote (−0.60 to −0.30‰) ≈ δ30Sikyanite (−0.42 to −0.28‰) > δ30Sigarnet (−0.92 to −0.44‰). The Si isotope fractionation between metamorphic minerals was generally consistent with the equilibrium Si isotope fractionation factors calculated by the first-principles methods. These results suggest that the vein minerals are likely in Si isotopic equilibrium with each other.The δ30Si of the Hualiangting veins are linearly correlated to SiO2 contents with a steeper slope than that of the magma differentiation trend. By also considering the mineralogy of the veins, we conclude that this linear relationship reflects the sequential variation of mineral composition on the Si isotope signature of the veins. The SiO2 content and δ30Si of veins increased with continuing crystallization from the fluid, and the δ30Si of the fluid also increased. The results suggest that Si isotope data can be used to constrain element recycling and metamorphic fluids activities in the subduction zones.

Dawn of metazoans: to what extent was this influenced by the onset of “modern-type plate tectonics”?

The appearance of complex megascopic multicellular eukaryotes in the Ediacaran occurred just when the dynamics of a cooling Earth allowed establishment of a new style of global tectonics that continues to the present as “modern-type plate tectonics”. The advent of this style was first registered in 620 Ma-old coesite-bearing Ultra-High Pressure eclogites within the Transbrasiliano-Kandi mega-shear zone along the site of the West Gondwana Orogeny (WGO). These eclogites comprise the oldest evidence of slab-pull deep subduction capable of inducing continental collisions and producing high-relief Himalayan-type mega-mountains. Life, prior to this time, was essentially microscopic. Yet with increasing Neoproterozoic oxygenation and intensified influx of nutrients to Ediacaran oceans, resulting from the erosion of these mountains, complex macroscopic heterotrophic eukaryotes arose and diversified, taking the biosphere to a new evolutionary threshold. The repeated elevation of Himalayan-type mega-mountains ever since then has continued to play a fundamental role in nutrient supply and biosphere evolution. Other authors have alluded to the influence of Gondwana mountain-building upon Ediacaran evolution, however we claim here to have identified when and where it began.

Decompressional equilibration of the Midsund granulite from Otrøy, Western Gneiss Region, Norway

Abstract The Western Gneiss Region (WGR) of the Scandinavian Caledonides is an archetypal terrain for high-pressure (HP) and ultrahigh-pressure (UHP) metamorphism. However, the vast majority of lithologies occurring there bear no, or only limited, evidence for HP or UHP metamorphism. The studied Midsund HP granulite occurs on the island of Otrøy, a locality known for the occurrence of the UHP eclogites and mantle-derived, garnet-bearing ultramafics. The Midsund granulite consists of plagioclase, garnet, clinopyroxene, relict phengitic mica, biotite, rutile, quartz, amphibole, ilmenite and titanite, among the most prominent phases. Applied thermodynamic modelling in the NCKFMMnASHT system resulted in a pressure–temperature (P–T) pseudosection that provides an intersection of compositional isopleths of XMg (Mg/Mg+Fe) in garnet, albite in plagioclase and XNa (Na/Na+Ca) in clinopyroxene in the stability field of melt + plagioclase + garnet + clinopyroxene + amphibole + ilmenite. The obtained thermodynamic model yields P–T conditions of 1.32–1.45 GPa and 875–970 °C. The relatively high P–T recorded by the Midsund granulite may be explained as an effect of equilibration due to exhumation from HP (presumably UHP) conditions followed by a period of stagnation under HT at lower-to-medium crustal level. The latter seems to be a more widespread phenomenon in the WGR than previously thought and may well explain commonly calculated pressure contrasts between neighboring lithologies in the WGR and other HP–UHP terranes worldwide.