Ultrahigh-pressure Eclogite Research Articles

Extreme geochemical fractionation during mantle melting: Insights from Hf-Nd isotopically ultra-depleted eclogite

Extreme fractionation of elements and isotopes in mafic igneous rocks is abnormal in deciphering the source nature and melting conditions of mafic magmatism. Especially, identification of geochemically ultra-depleted mafic melts and their mantle sources has great bearing on the property of crust-mantle differentiation at plate margins. This is illustrated by extreme Hf-Nd isotope fractionation in ultrahigh-pressure eclogites from the Sulu orogen in east-central China. In addition to the previous finding of ultrahigh εNd(0) values, we report here new data of whole-rock trace elements and Lu-Hf isotopes in eclogites and related rocks from the same region. The present results show extremely high Lu/Hf ratios and abnormally high εHf(0) values of up to 576 for the eclogites, significantly different from the garnet amphibolites and other eclogite-facies metamorphic rocks in the same orogen. This feature is coupled with the ultrahigh εNd(0) values as well as the severe depletion of light rare earth elements (LREE) and high field strength elements (HFSE). Because HFSE and LREE are immobile in aqueous solutions and the effect of melt extraction is insignificant during the continental deep subduction, the extreme fractionation of Lu/Hf and Sm/Nd indicate their origination from a geochemically ultra-depleted mantle source. These eclogites have the depleted mantle Hf model ages of 1.27 Ga to 1.61 Ga, similar to the depleted mantle Nd model ages of 1.39 Ga to 1.67 Ga as previously reported. This suggests that the protolith of the extremely high εHf-εNd eclogites was a kind of mafic igneous rocks derived from fractional crystallization of geochemically ultra-depleted mafic melts, which were produced by partial melting of the highly refractory lithospheric mantle during a series of seafloor spreading initiation-failure cycles at a divergent plate margin after the breakup of supercontinent Columbia in the Early Mesoproterozoic. The mafic igneous rocks were located in a passive continental margin in the Late Paleozoic and experienced deep subduction and exhumation in the Triassic, giving rise to the presently studied eclogites. The ancient geochemical signatures were retained without considerable influence by mantle convection, providing insights into the nature of crust-mantle differentiation during the tectonic transition from supercontinental breakup to seafloor spreading beneath the sub-ridge lithospheric mantle.

Early-paleozoic rapakivi-textured granite from the North Qinling (Central China): implications for crust–mantle interactions in a post-collisional setting

Rapakivi granite is characterized by its unique structure, which has important implications for tectonic settings, magmatic processes, and crust–mantle interactions. In this study, we conducted a combined analysis of the petrography, mineral chemistry, geochemistry, and zircon U–Pb dating and Lu–Hf isotopic compositions of the Niujiaoshan Early Paleozoic rapakivi-textured granite from the North Qinling Belt. Zircon U–Pb dating yielded a crystallization age of 447 ± 7 Ma, which is younger than the ultra-high-pressure (UHP) metamorphic age (~ 500 Ma) but similar to the granulite facies retrograde age (~ 450 Ma) of UHP eclogites and felsic gneisses in the North Qinling Belt. The rapakivi feldspar phenocrysts have ovoid K-feldspar cores, which are rich in mineral inclusions, such as amphibole, biotite, quartz, and plagioclase, indicating early crystallization. The ovoid K-feldspar cores are mantled by oligoclase, whreras the matrix comprises biotite, amphibole, and coarse-grained plagioclase. The amphibole and biotite in the granite are rich in Mg and are indicative of a crust–mantle origin. The εHf (t) values of the zircons range from − 2.04 to + 3.63, suggesting formation via crust–mantle interactions. The rapakivi-textured granite displays high-K meta-aluminous I-type granite affinity, with high SiO2, K2O, and Na2O contents. Based on the geological background and results of this study, we propose that the Niujiaoshan rapakivi-textured granite was formed via the mixing of crustal materials induced by upper mantle magma during the exhumation of the North Qinling UHP metamorphic terrane, which occurred in a post-orogenic setting.

Retrogression of ultrahigh-pressure eclogite, Western Gneiss Region, Norway

Abstract. The Western Gneiss Region (WGR) in western Norway exposes ultrahigh-pressure (UHP) eclogites that occur repeatedly, within an area of high-pressure (HP) eclogites, without evidence of being separated by tectonic shear or ductile flow structures. We studied 10 eclogites from two northern UHP areas and the interjacent HP area to evaluate the significance of this pattern. The orthopyroxene in orthopyroxene-bearing samples has low Al2O3 contents (0.17 wt %–0.37 wt %), provided its grain boundaries were unaffected by partial recrystallisation or replacement. Classical geothermobarometry based on element partitioning between coexisting mineral phases suggests metamorphic conditions within the diamond stability field for the samples from both the HP and UHP areas. The primary clinopyroxene in the associated orthopyroxene-free eclogites contains aligned inclusions of either needle-shaped quartz ± pargasite or lamellar albite, which are absent from the secondary (symplectic) clinopyroxene. Reconstructed mineral compositions of the primary clinopyroxene obtained from grain cross-section surfaces using a scanning electron beam or image processing are non-stoichiometric, and they have higher Ca-Eskola and lower Ca-Tschermak components than the inclusion-bearing host clinopyroxene. The molar ratios of these endmembers are consistent with the needles in the primary clinopyroxene being formed from vacancy-bearing precursor clinopyroxene by the exsolution reaction 2 Ca-Eskola = Ca-Tschermak + 3 quartz during early eclogite-facies retrogression. Further retrogression partially transformed the needle-shaped quartz to irregularly shaped albite within the clinopyroxene and partially transformed both clinopyroxene generations to amphibole that occasionally preserves the needles. The similarity of both the maximum metamorphic conditions and the mineral exsolution microstructures in the eclogites from UHP and HP areas indicates a shared metamorphic history within the stability field of diamond, but a history that diverged during retrogression. Consequently, the alternations of UHP and HP areas in the WGR may have formed by a process that allowed for spatial variations in retrogression efficiency, such as the localisation of strain (recrystallisation) or fluid flow (diffusion) or both, rather than by tectonic stacking of UHP and HP units. Evidence for the UHP metamorphism of WGR crustal rocks is now found from NE to SW along the entire coastal section that covers previously recognised UHP and interjacent areas.

From divergent to convergent plate boundary: A ca. 200 Ma Wilson cycle recorded by ultrahigh-pressure eclogites in the Dora-Maira Massif, Western Alps

Sparse eclogite exposure in accretionary-to-collisional orogens cannot only reveal the sites of ancient subduction zones and plate boundaries, but also elucidate the multi-stage tectonic evolution in the Wilson cycle. The southern part of the Dora-Maira Massif in the Western Alps is well known for its ultrahigh-pressure (UHP) rocks and contains small occurrences of petrographically distinct types of eclogite. Due to the lack of detailed geochronological and geochemical data on these rocks, the nature of their protolith and related geodynamic setting has remained unknown. In the current study, 33 samples from 13 localities were studied by whole-rock major- and trace-element analysis, and a selection of the samples were studied using Sr-Nd-Hf isotopes, as well as high-resolution elemental mapping and U-Pb geochronology of zircon. According to macroscopic appearance, petrography, chemical composition, and principal-component analysis of multi-elements, two major types are distinguished. Light-colored eclogite is phengite-rich, commonly foliated, bears kyanite and quartz/coesite, shows relatively high MgO, K2O, SiO2, εNd(t) (−2.2 to +1.4), and εHf(t) (+3.5 to +7), with a protolith geochemically similar to an enriched-type mid-oceanic-ridge basalt (E-MORB). Dark eclogite generally is massive, rutile-rich, shows higher values of Ti, Fe, P, Nb, and Zr, εNd(t) values between −2.8 and −0.8, εHf(t) values between −6.1 and −3.2, with an oceanic island basalt (OIB)-type protolith composition. Magmatic zircon cores, which are characterized by steep heavy rare earth element (HREE) patterns and negative Eu anomalies, yield consistent protolith ages of ca. 253−252 Ma in both eclogite types. Metamorphic zircon domains with flat HREE patterns and insignificant Eu anomalies yield a younger mean age of ca. 34 Ma, which is the age commonly assigned to eclogite-facies UHP metamorphism. Considering the various geochemical signatures of E-MORB- to OIB-like eclogites and the regional tectonic evolution, their protolith is best explained as rocks, which crystallized from rift-related basaltic magma, associated with the break-up of the Pangea Supercontinent that eventually resulted in the birth of the Piemonte-Liguria Ocean (Alpine Tethys Ocean) between the future Eurasia and Adria plates. Therefore, the UHP eclogites in the Dora-Maira Massif likely fingerprint a multi-stage tectonic evolution from divergent (continental extension, rifting) to convergent (subduction zone) plate boundary, corresponding to the beginning and end of a Wilson cycle. Incidentally, they reveal that mafic rocks in the Alpine basement units may not be polymetamorphic, but may actually consist of post-Variscan products that underwent only Alpine metamorphism.

Exhumation of ultrahigh-pressure eclogite and metarodingite within UHP serpentinites from the Chinese southwestern Tianshan

Extensive research over the past two decades suggests that serpentinite loses its ability to effectively facilitate the exhumation of high-density high-pressure (HP) eclogites at depths of 60–80 km under average subduction zone geotherms. However, at Changawuzi, the western termination of Chinese southwestern Tianshan, eclogite and eclogitic metarodingite are commonly enclosed within ultrahigh-pressure Ti-chondrodite serpentinites (510–530 °C, 37 ± 7 kbar). This metamorphic mafic–ultramafic association, which has undergone a complex evolution from oceanic alteration to deep subduction and subsequent exhumation, presents a unique opportunity to study the incompletely understood mechanisms leading to the exhumation of ultrahigh-pressure (UHP) eclogite in UHP serpentinite.The Changawuzi serpentinite wraps three different rock types: retrograded eclogite, Rodingite I and Rodingite II. Field observations and petrological analyses indicate that the precursor of Rodingite I intruded as vein into the serpentinite protolith prior to subduction, sharing the evolution towards ultrahigh-pressure conditions. According to its composition, the serpentinite originated from a depleted oceanic mantle peridotite, with only minor enrichment in fluid-mobile elements (FME) during oceanic serpentinization. The eclogites have N-MORB like bulk compositions, with lower Si but higher Mg content compared to other eclogites in the Tianshan area, and appear to have been relatively unaffected by oceanic alteration processes. The Changawuzi serpentinite and eclogites likely evolved within a slab setting with limited interaction with seawater. Petrological studies and phase equilibria modeling suggest that the eclogites experienced peak UHP metamorphism at 28–34 kbar and around 460–465 °C. Density measurements yielded relatively low densities of 2.66–2.97 g/cm3 for the host serpentinites, but 3.27–3.63 g/cm3 for Rodingite I. The similarity in the low temperature/ultrahigh pressure conditions, as well as the geochemical characteristics shared by the serpentinite and eclogites, further support the connection between the Changawuzi serpentinites and the interior of the subducting slab. According to our data, two types of UHP eclogitic mafic rocks exhumated with UHP serpentinite. (1) High density Rodingite I veins are embedded within serpentinite and share a common evolution and exhumation history with the host serpentinite. (2) The eclogite and serpentinite tectonically detached from the interior of the subducting slab at different depths and ascended to the plate interface above the slab (subduction channel). These two rock types experienced separate exhumation processes within the subduction channel during the early stages of exhumation. Subsequently, the buoyant serpentinite captured the denser retrograded eclogite at a depth of approximately 80 km, and together they underwent further retrograde metamorphism as they ascended towards the surface. During a later exhumation stage, external fluid induced a second serpentinization process, leading to the formation of Rodingite II within the subduction channel. This study presents a novel exhumation mechanism specific to the Tianshan eclogites and highlights the significant role of deeply subducted serpentinite in the exhumation of eclogites and eclogitic metarodingites from sub-arc depths within an oceanic subduction zone.

Corundum-bearing and spinel-bearing symplectites in ultrahigh-pressure eclogites record high-temperature overprint and partial melting during slab exhumation

Abstract. Corundum- and spinel-bearing symplectites after muscovite were found in ultrahigh-pressure (UHP) eclogites from the Dabie terrane, China. Three types of symplectites were recognized based on their mineral assemblages: (1) symplectitic intergrowths of corundum + plagioclase + biotite after phengite (CPB), (2) symplectitic intergrowths of spinel + plagioclase + biotite after phengite (SPB), and (3) symplectitic intergrowths of spinel + plagioclase after paragonite (SP). The microtextures and mineral assemblages of the symplectites, in combination with the results of thermodynamic modeling on local regions, indicate that these symplectites formed by the breakdown of phengite and paragonite during the granulite-facies metamorphic overprint (770–850 ∘C) of the eclogite at pressures of 0.8–0.9 GPa. Dehydration partial melting reactions occurred during the breakdown of muscovite, which leads to the formation of thin plagioclase films (silicate melts) along grain (garnet, rutile, quartz) boundaries. Mass balance calculations indicate that the development of CPB and SPB symplectites after phengite requires the introduction of Al, Ca, Na, and Fe and loss of Si, Mg, and K. However, the formation of SP symplectites after paragonite requires the input of Mg, Ca, and Fe and removal of Si, Al, and Na. By summarizing the occurrence and growth mechanism of corundum- and spinel-bearing symplectites in global UHP terranes, we find that such symplectites can form by both the subsolidus replacement of an Al-rich anhydrous mineral (kyanite) and the dehydration melting of an Al-rich hydrous phase during high-temperature metamorphism. This study reveals that muscovite-bearing eclogites may experience multiple episodes of partial melting during the slab exhumation, not only at the high-pressure (HP) exhumation stage but also at the lower-pressure metamorphic overprinting stage. Kyanite is a reaction product during the HP partial melting, whereas the low-pressure (LP) melting will consume kyanite. We propose that the occurrence of corundum- and spinel-bearing symplectites after muscovite in eclogites is a potential mineralogical indicator of LP melting in exhumed slabs.

Nitrogen storage capacity of phengitic muscovite and K-cymrite under the conditions of hot subduction and ultra high pressure metamorphism

Nitrogen storage capacity and partitioning between main N hosts have been investigated in N-bearing Al-rich natural pelite at 3.0–7.8 GPa, 825–1070 °C and oxygen fugacity (fO2) from NNO (Ni-NiO buffer)+0.3 to NNO-4.1 log units. Under the conditions of hot subduction, pelite converts into a phase assemblage typical of ultrahigh-pressure (UHP) eclogites. Phengitic muscovite in this assemblage is in equilibrium with a volatile-rich granite-like melt at 3.0 GPa and with supercritical fluid at 5.5–7.8 GPa, and contains, respectively, 115–135 and 759–962 wt ppm NH4+ at fO2 values close to NNO. Both melt and supercritical fluid have H2O and CO2 as predominant volatiles and the NH3/(NH3 + N2) ratio from 0.04 to 0.12. The pattern of nitrogen partitioning between its main hosts in pelite (DNH4Ms-Melt = 0.41–0.58 and DNH4Ms-Fluid = 0.63–2.4) proves that the ammonium exhibits moderately incompatible to compatible behavior within a hot oxidized slab in the pressure range from 3.0 GPa to 7.8 GPa. Therefore, even relatively oxidized sediments containing phengitic muscovite can efficiently transport nitrogen to the mantle at sub-arc depths under the hot subduction conditions. At the same time, the changeover of the N host from biotite to muscovite at the arc depths in combination with subsequent pelite melting and an abrupt decrease in phengitic muscovite abundance in pelite may lead to an avalanche-like N outgassing of the slab. The incorporation of an additional nitrogen source of (NH3 + N2) into pelite reduces fO2 to NNO-3.1 – NNO-4.1 log units and increases both the NH3/(NH3 + N2) ratio in the fluid (up to 0.44–0.86) and the NH4+ concentration in phengitic muscovite (up to 3750–3820 wt ppm). K-cymrite was produced in pelite at P ≥ 6.3 GPa and the bulk nitrogen content 3.2–5.9 wt%. K-cymrite possesses exceptional nitrogen storage capacity: it hosts 1.4–1.6 wt% NH4+, up to 0.5 wt% NH3, and 4–6 wt% N2. The DN-NH4Cym-Ms coefficient is as high as 20. Being stable in sediments subducted to mantle depths, K-cymrite, with its extremely high storage capacity, can act as a huge hidden redox-insensitive nitrogen reservoir in the mantle and thus can be involved in the deep nitrogen cycle.

In Situ High-Pressure Raman Spectroscopic, Single-Crystal X-ray Diffraction, and FTIR Investigations of Rutile and TiO2II

In ultrahigh-pressure (UHP) metamorphic rocks, rutile is an important accessory mineral. Its high-pressure polymorph TiO2II can be a significant indicator of pressure in the diamond stability field. In the present study, in situ high-pressure Raman spectroscopic measurements of natural rutile in UHP eclogite from the main hole of the Chinese Continental Scientific Drilling Project (CCSD) have been conducted up to ~16 GPa. Rutile and recovered TiO2II have also been analyzed via single-crystal X-ray diffraction and FTIR spectroscopy. The results indicate that (1) the phase transition from rutile to baddeleyite-type TiO2 terminates at about 16 GPa under compression at ambient temperature; (2) the metastable TiO2II in the exhumated UHP rocks formed during deep continental subduction can be characterized by a highly distorted octahedral site in the crystal structure. X-ray powder diffraction analyses (with Cu Kα radiation) at ambient conditions are sufficient for identifying the lamellae of TiO2II within natural rutile based on the angles (2θ) of two strong peaks at 25.5° and 31.5°; (3) rutile and recovered TiO2II in the continental slabs can contain certain amounts of water during deep subduction and exhumation. The estimated water contents of rutile in the present study range from 1590 to 1780 ppm of H2O by weight. In the crystal structure of TiO2II, hydrogen can be incorporated close to the long O-O edges (>2.5143 Å) of the TiO6 octahedra. Further studies on the pressure–temperature stability of hydroxyls in rutile and TiO2II may help to understand the transportation and release of water in subducted continental slabs.

The role of continental subduction in mantle metasomatism and carbon recycling revealed by melt inclusions in UHP eclogites.

Subduction is the main process that recycles surface material into the mantle. Fluids and melts derived by dehydration and partial melting reactions of subducted continental crust, a major reservoir of volatiles (i.e., H2O and CO2) and incompatible elements, can substantially metasomatize and refertilize the mantle. Here, we investigate glassy inclusions of silicate melt of continental origin found in Variscan ultrahigh-pressure eclogites to assess the continental crust contribution to mantle metasomatism and the journey of volatiles, carbon in particular, to the deep roots of mountain belts. We argue that the melt preserved in these inclusions is the agent responsible for mantle metasomatism and subsequent ultrapotassic magmatism in the Variscides. We propose that continental subduction can redistribute a substantial volume of carbon in the continental lithosphere, which is subsequently transferred to the continental crust during postcollisional magmatism and stored for a time length longer than that of the modern carbon cycle.

Crystallographic preferred orientations and microtexture of the Himalayan eclogites revealing records of syn-deformation peak metamorphic stage and subsequent exhumation

Electron backscattered diffraction (EBSD) and crystallographic preferred orientations (CPO) studies on Himalayan high- and ultrahigh-pressure eclogites are reported. Subduction-related peak eclogite facies (omphacite, garnet, quartz, rutile) and exhumation-related post-eclogite facies (hornblende, epidote, titanite) minerals were investigated to understand subduction and exhumation-related deformation regimes. Petrographic features from the studied eclogite samples show preferably shape-oriented omphacites coexisting with random to weakly shape-oriented garnets, and overgrown by hornblende and symplectites. Quartz grains are also randomly shape-oriented in the eclogite matrix whereas epidote grains display shape-preferred orientations. Based on EBSD data, an L-type CPO was dominant in omphacites in the studied eclogites, exhibited by the maximum density of [001] axes parallel to the lineation and the high density of poles to the (010) and (110) planes forming girdles perpendicular to the foliation. This CPO likely developed in a constrictional strain regime during the peak eclogite-facies conditions due to dislocation creep and is associated with an intra-crystalline (110) [001] and (110) <110> slip systems. EBSD data of garnets exhibit complex fabric, suggesting they most likely behaved like rigid objects in the presence of omphacite, indicating insignificant plastic deformation. EBSD data of quartz in high-pressure eclogites show irregular CPO but in ultrahigh-pressure eclogites the fabric is relatively strong. EBSD data of rutile show CPO of [001] axes strongly aligned along the lineation, suggesting its deformation was coeval to the omphacite. Titanite, mainly overgrown around rutile, indicates a random CPO different from rutile, suggesting its growth during late-stages. Hornblende in both types of eclogites shows identical fabric to that of omphacite, displaying CPO of [001] axes parallel to the lineation and poles of (010) and (110) planes form girdles perpendicular to the foliation, suggesting topotactic replacement over omphacite during exhumation. The EBSD data, combined with textural features, indicate syn-deformation peak metamorphism as well as preserve records of subsequent exhumation.

Ultrahigh-pressure metamorphism and P-T-t path of the Yangkou eclogites and granitic gneisses from the central Sulu orogenic belt, eastern China, based on phase equilibria modelling

ABSTRACT Coesite-bearing eclogites and garnet-bearing granitic gneisses from the Yangkou area in the middle portion of Sulu ultrahigh pressure (UHP) metamorphic belt records three stages of metamorphism: I) Prograde to peak stage, II) Early retrograde stage, and III) Late retrograde stage. The prograde stage is recorded in the core to mantle in garnet and Si (3.38–3.43) in phengite in eclogite that has the assemblage of Grt-Omp-Amp-Phg-Lws (+ Qtz + Rt + H2O), and the P-T conditions are constrained at 25–27 kbar and 585–595°C. Stage II is constrained by Xgr (0.48–0.55) in garnet from mantle to rim, XAn (0.009–0.011) from mantle to rim in plagioclase of granitic gneiss, and the P–T conditions are estimated to be 29–35 kbar and 865–990°C with the mineral assemblage of Grt-Bt-Pl-Phg-Liq (+ Qtz/Coe). Stage III is constrained by two P-T ranges: mineral assemblage of Grt-Ep-Bt-Pl-Phg-Liq (+ Qtz) and Xgr (0.45–0.72) from mantle to rim in garnet in granitic gneiss constrained an earlier P-T range of 22–23 kbar and 800–820°C, while Xgr (0.34–0.39) and Xpy (0.16–0.18) from mantle to rim in garnet in eclogite in the assemblage of Grt-Omp-Amp-Phg-LL (+ Qtz + Rt + H2O) constrained a later P-T range of 17–18 kbar and 675–690°C. The formation and evolution of these assemblages have important implications for the generation and differentiation of continental crust through the operation of plate tectonics. The data suggest that both mafic and granitic rocks in the Yangkou area subducted to a depth of more than 115 km and underwent UHP metamorphism. The three stages of metamorphism in the study area are consistent with the regional well-established metamorphic evolution history, from UHP eclogite facies, through HP/HT eclogite facies, to amphibole eclogite facies with the timing of 230, 225, and 195 Ma, respectively.

Implication of garnet zoning in high pressure- ultrahighpressure eclogite from Changning-Menglian suture zone, Bangbing area, Southeast Tibetan Plateau

Major and trace element zonation patterns were determined in ultrahigh-pressure eclogite garnets from the Changning-Menglian suture, Bangbing area. Several stages of eclogite metamorphism are recorded by garnet zonation. Cores of garnets are generally darker and inclusion-rich, while rims are brighter and inclusion-poor. Garnets were analyzed for the major element by EPMA and trace element by LA-ICP-MS. The results show that both major elements and rare earth elements in garnet have typical compositional bands. The major element zonation recorded the process of increasing temperature and pressure and the metamorphism of the eclogite from the core to rim, and the rare earth element zonation recorded different mineral chemical reactions during the formation of the garnet and its three stages of growth. Eclogites completely imply the process of rapid exhumation, cooling, and depressurization after short-term peak metamorphism. The contents of some trace elements (such as Y and Sr) in garnet are controlled by major elements and mineral combinations. The formation of new minerals in the process of retrograde metamorphism affects the distribution of trace elements.

Early Paleozoic Continental Arc Mafic Magmatism in the North Qaidam Tectonic Belt: Implications for Subduction of the Proto-Tethyan Oceanic Lithosphere

Abstract The North Qaidam tectonic belt is characterized by the development of high-pressure to ultrahigh-pressure eclogite formed during deep subduction of the continental lithosphere. However, the tectonic processes that occurred prior to continental collision/subduction are relatively poorly studied and this leads to controversy over the evolutionary history of the North Qaidam tectonic belt. In this contribution, we present an integrated study of field observations, petrography, geochronology, and geochemistry (whole-rock major elements, trace elements, and Sr–Nd isotopes as well as zircon Lu–Hf isotopes) of continental arc mafic rocks in the North Wulan metamorphic complex to track Proto-Tethyan oceanic subduction and the nature of metasomatism of the mantle wedge. Zircon U–Pb geochronology demonstrates that continental arc mafic rocks crystallized at ca. 483–472 Ma. Mafic intrusions are enriched in light rare earth elements and large ion lithophile elements but are depleted in high field strength elements; these are typical features of arc-related magmatic rocks. They have relatively radiogenic Sr–Nd–Hf isotope compositions, with relatively high initial 87Sr/86Sr ratios of 0.710363 to 0.719404, low εNdt values of -7.77 to -2.30, variable zircon εHft values ranging from -8 to +2.8, and the single-stage Hf model ages of ca. 1.4–1.0 Ga. These features suggest that the mafic intrusions were sourced from ancient subcontinental mantle that was modified by subducted oceanic slab-derived components. The modified mantle source was generated by the interaction between subcontinental lithospheric mantle peridotite in the overlying mantle wedge and hydrous fluid and felsic melt that were derived from a subducted oceanic slab and seafloor sediments with ancient terrestrial origin in the rutile stability field. Crust-mantle interaction transferred the subducted crustal geochemical signatures to the mantle source during subduction of the Proto-Tethyan oceanic lithosphere. Retreat and rollback of the Proto-Tethyan oceanic slab triggered asthenosphere upwelling and the partial melting of metasomatized and enriched fertile lithospheric mantle to form continental arc mafic magmas in the North Qaidam tectonic belt. Combined with the coeval LP-HT metamorphism, the North Wulan metamorphic complex records the early Paleozoic tectonic evolution of a continental arc-back-arc system, and these continental arc mafic rocks record the subduction of the Proto-Tethyan oceanic lithosphere.

In-situ high-pressure and high-temperature spectroscopic studies of phengite in ultrahigh-pressure eclogite: implications for water transport during ultra-deep continental subduction

In-situ high-pressure and high-temperature spectroscopic studies of phengite in ultrahigh-pressure eclogite: implications for water transport during ultra-deep continental subduction

Signature of Himalayan orogenic features in Brahmaputra River sediments, Bangladesh: Evidence from single-grain heavy mineral chemistry

The present study describes results obtained from the chemistry of detrital heavy minerals i.e. pyroxene, amphibole, biotite, garnet, epidote and Fe-Ti oxides in fluvial sediments of the northern Brahmaputra River (Bangladesh) with an aim to determine conditions of their petrogenesis and provenance. The primary and secondary genera of ferromagnesian minerals occurred in calc-alkaline and peraluminous subduction zone. In which, the garnets are Fe-rich, indicating mostly almandine component (Alm65–Pyp16–Grs8–Sps6 averagely), occurred in medium to high grade metasedimentary rocks in the Lesser Himalaya (LH), along the Main Central Thrust (MCT) and the eastern Himalayan syntaxis. Besides, the fingerprint of omphacite and actinolite owe to ascertain the co-existence of garnet developed in ultrahigh-pressure (UHP) eclogites that may also be drained from the Tso Morari massif. Augite to aegirine-augite pyroxenes emphasizes Fe enrichment in basaltic systems and high to ultrahigh grade metamorphic rocks, which are exposed in the LH, Shillong Plateau, Mikir Hills, South Tibetan Detachment System (STDS), eastern Himalayan syntaxis and Tso Morari massif. Geochemistry and thermobarometry of the primary magmatic amphiboles and biotites manifest the source of granitoid and granodiorite like bodies, and their windows are exposed in the Bomi–Chayu, Gangdese arcs and the western Arunachal Himalaya. Again, metamorphosed Fe-Ti oxide minerals are well-exposed along the NE Lesser Himalaya, where magmatic derivative of Fe-Ti oxide minerals were modified through the diffusional processes in low-grade metamorphism (534–562 °C with 10–22.1–10−21.5fo2). Integrating the aforementioned discussion with the thermochronology, it is evident that the eastern Himalayan syntaxis is the major source of sediment flux, which is carried mostly by the upper Himalayan tributaries i.e. Yigong, Parlung, Dibang and Lohit. Also, the lower Himalayan tributaries i.e. Subansiri and Manas drain the sequestered derivatives dominantly from the Arunachal Himalayan. Tso Morari eclogites (NW Himalaya) have also contribution somewhat of dense minerals to the Tsangpo-Brahmaputra River system. Thus, scrutinizing the fingerprint of single-grain detrital minerals provides key information regarding the source terrains and tectonics of the Himalayan sequences.

Compositional and metamorphic controls on tectonic erosion along a continental subduction-collision zone: Implications from mafic granulites in the northern Sulu orogen

Abstract Subduction erosion has been reported recently in continental subduction-collision zones, and the response of the upper plate remains enigmatic. The Dabie-Sulu orogen is considered to have formed by deep northward subduction of the Yangtze block (lower plate) beneath the North China block (upper plate). However, within the northern Sulu ultrahigh-pressure (UHP) belt, the Haiyangsuo complex has intriguingly been accepted as Neoarchean–Paleoproterozoic metamorphic basement from the North China block, implying subduction-erosion could have occurred. Here, we examined new petrographic, mineral chemical, and published geochronological data from the Haiyangsuo mafic granulites to decipher their multiphase metamorphic evolution. Pressure-temperature (P-T) estimates from pseudosection modeling and geothermobarometry show two major metamorphic events: a medium-pressure granulite-facies event (7.3–8.3 kbar and 830–895 °C) and a later high-pressure granulite-facies event (12.2–16.6 kbar and 800–875 °C). Geochronology and mineral geochemistry link these events to Paleoproterozoic and Triassic metamorphism, respectively. Peak Triassic high-pressure granulite-facies metamorphism was followed by near-isothermal decompression and then near-isobaric cooling. These rocks derive from the upper plate and were metamorphosed at the same time as UHP eclogites, but maximum pressures were substantially lower, and P-T paths did not converge until late-stage exhumation. Evidently, metamorphic basement from the North China block was dragged to lowercrustal to upper-mantle depths (50–60 km) while that from the Yangtze block was subducted deeply, and then they were tectonically juxtaposed during exhumation. These data corroborate thermal-mechanical models that predict tectonic erosion as a major process during continental subduction and collision.

Thallium isotope compositions of subduction-zone fluids: Insights from ultra-high pressure eclogites and veins in the Dabie terrane, eastern China

Dehydration of the subducted slab is a crucial process in the generation of arc magmas. However, the mineralogical controls of slab dehydration remain uncertain. Thallium (Tl) isotopes have emerged as tracers of subducted slab components, but the Tl isotope characteristics of slab fluids are not known. High-pressure metamorphic veins and host eclogites in the Dabie terrane (China) provide important information on the composition and evolution of subduction zone fluids. In this study, we present the first Tl concentration and Tl isotope data for two high-pressure metamorphic epidote-rich veins formed through breakdown of lawsonite, as well as host eclogites and phengite separates, from the Ganghe and Hualiangting areas. The high Tl concentrations in phengite separates and identical Tl isotope compositions between phengites and their host bulk eclogites demonstrate that phengite controls the overall inventory of Tl in the host eclogites. Generally, the vein samples from both areas have much lower Tl concentrations than their host eclogites, suggesting that the fluids from which the veins precipitated were derived from phengite-bearing eclogites that retained Tl in the residue. The Ganghe eclogites display similar Tl isotope compositions compared with Ganghe omphacite-epidote vein. This similarity indicates that the vein-forming fluid was likely derived from the host eclogites, and that Tl isotopes did not fractionate during fluid-release. Three generations of veins in the Hualiangting area record variable Tl isotope values and were likely derived from multi-stage dehydration of heterogeneous host eclogites, some of which may have contained a sediment component. We argue that fluids derived from eclogite dehydration are likely characterized by higher Cs/Tl ratios compared to their host eclogites. Arc magmas that display high Cs/Tl (i.e. >15) that cannot be caused by variations in slab source composition, coupled with relatively low Ba/Th ratios, are likely to directly record the presence of residual phengite at subarc depths.

Electron Backscatter Diffraction Study of Ultrahigh-Pressure Tso Morari Eclogites (Trans-Himalayan Collisional Zone): Implications for Strain Regime Transition from Constrictional to Plane Strain during Exhumation

Abstract The Tso Morari Crystalline Complex (TMCC) of trans-Himalaya (eastern Ladakh, India) contains enclaves of ultrahigh-pressure eclogites that underwent deep burial (≥80 km) and subsequent rapid exhumation during continental subduction, collision, and final accretion of the Indian plate with the Eurasian plate. We present an electron backscatter diffraction (EBSD) study of eight eclogite samples to investigate the deformation mechanism and strain regimes active during peak (HP) metamorphism and subsequent postpeak rapid exhumation of the TMCC. Our study shows that the least retrogressed eclogite exhibits strong linear fabric (L tectonite) characterized by omphacite, having [001] axes parallel to and (110) poles perpendicular to lineation. These features concur with constrictional strain during peak (HP) metamorphism. A transitional planolinear fabric (LS tectonite) is shown by other eclogites that show petrographic evidence of omphacite alteration to amphibole and the presence of lower metamorphic grade minerals like actinolite and chlorite. Characteristics of lattice preferred orientation (LPO) of omphacite and quartz, indicated, respectively, by the LS and B indices, also suggest variation in strain regime from pristine eclogites to their altered counterparts. Based on these results, it is suggested that a constrictional strain regime prevailed during peak (HP) metamorphism in the TMCC due to the buoyant rise of TMCC in response to slab break-off and reverse slab pull during and after the deepest continental subduction. This buoyant rise was also facilitated by compression related to the ongoing India-Eurasia collision. This regime evolved later to plane strain that was superimposed on the UHP rocks at a shallower depth. It is plausibly associated with foliation-parallel extension during exhumation at midcrustal depths. A high-temperature prism c-slip in quartz shown by few samples is interpreted to have formed due to a subsequent granulite facies metamorphic overprint on the eclogite during collisional thickening.

Change in Subduction Dip Angle of the Indian Continental Lithosphere Inferred From the Western Himalayan Eclogites

The occurrence of ultrahigh-pressure (UHP) and high-pressure (HP) rocks in the Himalayan orogen has been conventionally attributed to the different subduction dip angles along the strike. The western Himalayan UHP eclogites point to a steep continental subduction in the Eocene. The present-day geophysical data show low subduction dip angles of the Indian lithosphere beneath southern Tibet and Karakoram, implying that a shift from steep to low-angle subduction probably happened in the western Himalaya. However, the timing and mechanism of such a subduction-angle change are still unknown. Here we present a combined analysis of zircon geochronology and geochemistry of eclogites and gneiss in the Stak massif, western Himalaya. Metamorphic zircons equilibrated with garnet and omphacite show flat heavy rare earth element patterns without Eu anomalies and, thus, yield similar eclogite-facie ages of ca. 31 Ma. The Stak HP eclogite-facie metamorphism is at least 15 Ma younger than those measured in the western Himalayan UHP eclogites, but broadly contemporaneous with other Himalayan HP rocks. Therefore, all the Himalayan HP rocks record higher peak geothermal gradients and younger ages than those of the UHP rocks. Our new data, combined with the magmatic lull observed in the Kohistan–Ladakh–Gangdese arc and with the convergent rate of the Indian plate, suggest a change in subduction dip angle over time. Consequently, we suggest that the entire Indian continental lithosphere experienced an approximately coherent shift from steep to low-angle subduction after the breakoff of the Neo-Tethyan slab since the middle Eocene. This critical change in subduction geometry is interpreted to be responsible for the transition from continental subduction to collision dynamics.

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.